Section Page Number
I-Introduction 3
A-Definition of High Performance Ceramics 3
B- Categories of Ceramics 4
C- Structure 5
D-General Properties of High Performance Ceramics 6

II-Materials
A-Boron Carbide (B4C) 10
B-Boron Nitride (BN) 13
C-Tugsten Carbide (WC) 16
D-Titanium Diboride (TiB2) 20
E-Alumina (Al-sO3) 25
F-Zirconia (ZrO2) 29
G-Silicon Carbide (SiC) 36

III-Applications 41

IV-Manufacturing  46

èII-Material Profile:

  1 .  Introduction
  2 .  Mechanical  properties
  3 .  Preparation and processing
  4 .  General features physical, chemical
  5 .  Environment interaction: oxidation and corrosion
  6 .  Applications: list of applications

èIII-Applications:
  1 .  Heat Resistant and refractory properties
  2 .  Cutting tools
  3 .  Abrasives and wear resistance
  4 .  Military
  5 .  Aerospace
  6 .  Electronic
  7 .  Geological (mining, tunneling, quarrying)
  8 .  Others: Corrosion resistance

èD- Manufacturing:

Survey of Hot pressing, cold forming  and sintering
 

HIGH PERFORMANCE CERAMICS'

A-DEFINITION:
High performance ceramics are a group of ceramics that characterizes this age. They are ceramics with incredibly light weight, high hardness, non-corrodable, high melting points, high price and advanced applications.
Introducing these new ceramics to mankind was a direct output of the start of space era. Several problems faced the scientists. The most important one was the problem of increase in the space capsule temperature as it penetrates the atmosphere when it comes back from outer space. Scientists were forced to search for new materials that can bear these very high temperatures. After being successful in the aerospace field, it transferred to aviation and aeronautics. They proved a very high durability and accountability in withstanding severe conditions, because of their high thermal stability versus environment interaction, and their light weight.
Next, these group of ceramics transferred to the field of cutting tools, and being used as abrasive materials. This group has materials with very high hardness numbers; close to the diamond hardness number. The need for such materials in cutting tools was due to the advancement in the field of NC and CNC machines. These machines, when first introduced, had lower speeds and poor cutting quality. When motors became capable of producing high speeds, need for hard materials became a must. Thus a category of these materials was introduced, which is: carbides.
These materials have shown great importance in the military field as armors. In our project, we will tackle a group of these materials, and their applications.

1-Materials made of clay:
Among the most ancient manufactured articles. They have played a vital role in human civilization. Clay is made up of fine, plate-like crystals (usually from 1 to 10 microns) of hydrated alumino-silicates. The plate-like form of clay crystals reflects the molecular layer structure of the silicon-oxygen and aluminum-oxygen groups in the clay compounds.

2-Traditional white-wares and porcelains:
They contain at least three starting materials--clay, feldspar, and silica sand. The most common ceramic articles of pottery, porcelain, brick, and pipe form complex mixtures of several different solid phases after firing. Most of the traditional white-wares and porcelains  have a smooth, polished surface due to the presence of the glassy phase.
3-"High technology (performance)" ceramics:
New types of materials that surpass earlier ceramics in strength, hardness, light weight, or improved heat resistance. The are composed of particles of absolutely uniform size. They are far less vulnerable to fracture or thermal shock than ordinary ceramics.

4-Ferrites compounds:
Iron oxides containing other elements such as Ni, Mn and Co. These compounds are magnetic but do not conduct electricity as do the magnetic metals. Some new metallic ceramics are superconductors. At relatively high temperatures (77 K, -200 °C), they conduct electrical current without the resistance produced by copper and other common conductors

5-Amorphous ceramics:
Substances that lack the usual crystalline ceramic structure. They are used to make complex shapes and thin ceramic films.

6-Glass ceramics:
Glass ceramics are formed when uniform crystals are grown by treating with controlled heat. Glass ceramics have higher strength, chemical durability, and electrical resistance than ordinary glass. They have with low thermal expansions, giving good resistance to thermal shock.

D-GENERAL FEATURES OF CERAMICS
 Ceramics are made from inorganic, non-metallic chemicals processed at high temperatures. They are one of the three main types of engineering materials besides metals and plastics. The general features of ceramic are well-known to almost all those who have ceramics in their homes. When someone drops something heavy onto a ceramic tile in his/her kitchen then the immediate observation is that the tile breaks. This means that the ceramic is exhibiting its brittleness despite the fact that ceramics are hard materials. Another feature of ceramics are their high resistance to heat. This property has made possible their uses as crucibles and in making floor and wall tiles to thermally insulate a house. Many also are electrically insulating materials or have different electrical properties from the metals. For example. They are now used as superconductors due to their high electrical conductivity at very high temperatures. This special type of ceramics is called Electro-ceramics. These Electro-ceramics can have an induced current that will go on perpetually and so in effect the Electro-ceramics have zero resistance, they have found wide applications such as in levitation trains. All ceramics have very high melting temperatures. This allows for their use as boats in metal and alloy production and in crucibles. Another major aspect of ceramics are their wear-resistant, corrosion resistant properties. These allow for their use as abrasives and in other applications that are liable to severe wear and corrosion like the tips of drilling rigs.

A-INTRODUCTION:
Since the first successful preparation of pure boron by the French chemists Joseph Gay-Lussac and Baron Louis Thénard in 1808 and independently by the British chemist Sir Humphry Davy, scientists have been working hard to get the best benefit of its properties.
During and after WW II, research escalated in boron field, especially with advances in the "new" material category at that time, which was ceramics materials. Boron compounds, such as: boron carbide and boron nitride have found for themselves a variety of useful application as abrasives, armors, cutting tools and heat resistant components. This is because of the excellent properties they demonstrate as heat-resistant materials, wear resistant in addition to useful range of mechanical, thermal and physical properties.
B-MATERIAL PROFILE:

1- Basic Information:
The following table includes the basic information about the element Boron (B)
Property Value
Boron Symbol B
Natural Abundance  38th in earth crust
Density  2.35 g/cc
Atomic Number 5
Mass number 10.811
Periodic table group III A or 13
Melting point 2330°C
Boiling point 3650°C
Crystal structure Rhombohedral/Icoshedral
Hardness +9.3 (Moh’s scale)

2-Metallurgy, ores and preparation:

Pure Boron is an amorphous powder. However, a crystalline form can be prepared. Boron is extracted from one of the following ores:
  1 . Mineral Borax=sodium tetraborate: Na2B4O7.10H2O
  2 . Boric Acid: H3BO3
  3 . Ulexite: NaCaB5O9.8H2O
  4 . Colemanite: Ca2B6O11.5H2O
  5 . Kernite: Na2B4O7.4 H2O
  6 . Boracite: Mg7Cl2B16O30

The crystalline boron is prepared by reducing one of its oxides and then dissolving pure amorphous boron powder in molten aluminum and slowly cooling. It can be prepared using a reducing agent:
 

3-General chemical aspects:

Despite being in the same group of Aluminum, boron shows non-metallic chemical and physical properties, similar to carbon and silicon. However, boron is an electric conductor, like carbon. Thus, boron is considered a semi-metallic element.
  Boron exists in period 2, group 13 (IIIA) of the periodic table, with valence of 3. The electron configuration of boron is 1s2, 2s2, 2p1.
  Boron does not react with water , hydrochloric acid, or hydrofluoric acid, and it is unaffected by air in room temperature. However, it is reacts at red hot to form boron oxide (B2O3). Under the same conditions, it reacts with nitrogen forming boron nitride (BN). With metals, it form metals borides, such as: magnesium boride (Mg3B2), and Titanium diboride (Ti B2).
  Boron has a crystalline Icoshedral appearance(with 20 equilateral triangles faces, and 12 vertices). Moreover, it has another allotropic amorphous form of an Rhombohedral shape.

4-General properties for Boron-Based (boron-rich) solids:

There is a group of materials that are called boron based materials, and mostly they are ceramic materials. They are different modifications for the elementary boron. They exhibit close similarities in their mechanical, physical and chemical properties because they have the same crystal structure (Icosahedral), which is the structure with 12 faces. We can list a group of generalities of properties between all these material. In our scope we will mainly concentrate mainly on two major materials, which are: boron carbide and boron nitride.

These boron based ceramics have:
 Ø  Very high melting temperatures (2000è4000 K). Boron carbide has a m. p.  of 2900 K.
 Ø  Very high hardness at ambient temperatures. This is mainly attributed to their crystal structure, that is closely similar to the crystal structure of Diamond, which is the hardest existing material. Boron carbide and boron nitride rank respectively second and third in Moh’s scale of hardness, after diamond.
 Ø  They have low densities in comparison to other materials. For example; boron carbide density is 2.5 g/cc.
 Ø  They have very small thermal expansion coefficients.
 Ø  As indicated above concerning elemental boron, boron rich ceramics have high resistance to thermal attack. Therefore they are corrosion resistant materials.
 Ø  Applications of boron-rich solids with respect to their electronic properties have been missing, though the properties are very favorable, because they allow use under extreme conditions (high temperatures, high pressures, strong abrasive loading, chemically aggressive surrounding), which are not accessible for the most other materials.
 Ø  Other properties: semi-conductor behavior, and radioactive absorption.

What seems really very promising concerning this category of materials is that they can work in very extreme conditions, that can not be withstood by other materials. In addition, with the advances in the field of materials science, they offer materials scientists a chance to tailoring materials in the way they want.
In order to be able to tailor boron-based ceramics, there are some important facts that should be considered concerning their overall properties:

  1 . Being similar in crystal structure (Icoshedral), which is a very complicated network with lots of holes, they can accommodate foreign atoms. Not only that, they can form solid solutions with different materials, and will develop considerable improvement in their mechanical  and physical properties.
  2 . Since boron based compounds are allotropic (having more than one crystal structure), it was detected that some of them have different crystal structures existing in one sample. This means that by manipulating such a percent of some of these structures, we can develop different properties.
  3 . Recent developments in the field of boron based ceramics proved that metals could be used to dope boron compounds. This will improve the physical and mechanical properties.
  4 . The method of tailoring boron based ceramics is not at all difficult, because it is mainly related to the method of working, production and preparation. Hot pressing has become a very important method of working. In addition melting and chemical reaction still possible and very controllable.

5-Available fields of applications for boron-rich ceramic materials:

 Ø  Cutting tools and their coating.
 Ø  Abrasives (grinding, lapping, polishing)
 Ø  Heat resistant shields in aerospace applications.
 Ø  Fiber reinforcement of polymers.
 Ø  Thermocouples.
 Ø  Shields and bullet proof shields for military applications.
 Ø  Anti-Oxidant in refractories.
 Ø  Shields in nuclear reactors.
 Ø  Nozzles
 Ø  Ceramic composites
 Ø  Tool and die fabrication.
    

4-Chemical Properties:

 ·  Stable in contact with dilute and concentrated acids and alkali
 ·  Inert to most organic compounds
 ·  Attacked slowly in mixtures of hydrofluoric/sulfuric acid or hydrofluoric/nitric acid. Reaction accelerates with finer particle size.
 ·  Resists attack by water vapor at 200-300°C.
 ·  Corrosion increases with temperature and reduced particle size.
 ·  Dissociates rapidly in contact with molten alkali and acidic salts to form borate.

With all the previously mentioned properties, boron carbide has emerged lately in materials technology as an excellent substitute for other strengthening agents, such as: silicon carbide, for applications that requires high wear resistance and high stiffness. Nowadays, boron carbide is used as a reinforcing element for aluminum and titanium.

5-Uses and Applications:

The applications if boron carbide are listed as follows:
  1 -  Wear resistant components.
  2 -  Armor tiles in military purposes; it was first used in Vietnam war as a light hard bullet proof armor for helicopters and tanks. It is used also as armor for vital equipment. For example, it was used by NASA to protect the shield of the space shuttles. Its low weight and high hardness has resulted in boron carbide being a material of preference for personnel armor and to protect certain vital equipment.
  3 -  Abrasives: lapping and polishing powders.
  4 -  Raw material: in preparing other boron compounds, notably titanium diboride, which is another very hard material with variety of wear and corrosion resistance applications.    2TiO2+B4Cà2TiB2+4CO
  5 -  It represents an alternative to SiC (silicon carbide) for applications where a high stiffness or a good wear resistance is required.
  6 -  Inserts for spray nozzles and bearing liners and wire drawing guides. This  takes advantage of its abrasion resistance.
  7 -  Because of its boron content and resistance to high temperatures, boron carbide is used as a shield for neutrons in nuclear reactors.
   

1-Introduction:
Boron nitride is a inorganic material with low reactivity and several applications. It is one of the hardest man-made materials. It has several applications because it shares a wide range of material properties; thermal, electrical, mechanical and physical and  mechanical. It has variety of combination of these properties that made it available for different applications. Material engineers find the unusual combination of electrical insulation and high thermal conductivity, in addition to the excellent thermal shock, useful in a variety of electronic and electrical applications. With excellent refractory qualities, chemical inertness, molten material indestructibility plus easy machinability, fabrication into shapes for high performance in difficult operating conditions results in longer life components. It is non - toxic and is machined very easily.

2-Physical Properties:

Boron nitride has three different crystal structures
  1 . Alpha BN: Hexagonal (HBN)
  2 . Beta BN: Cubic (CBN)
  3 . Pyrolitic (PBN).
Therefore, we have to distinguish between the three types of boron nitride. They differ in uses, applications, and properties.
èProperties of Alpha BN
Property Value or description
Crystal structure Hexagonal
Density 2.28 g/cc
Name White graphite
Melting temperature 2700°C
Ductility soft

èProperties of Beta BN
Property Value or description
Crystal structure Cubic
Density 3.48 g/cc
Name Cubic Boron Nitride (CBN)
Oxidation temperature 1100°C
Hardness (Mohs) 10.00-
Lattice Constant (A°) 3.615
Melting temperature 3027°C
Thermal conductivity 160.6 W/m.°K

Applications of BN:
èHBN
 Ø  Releasing molds.
 Ø  High temperature lubricants
 Ø  Insulating filler material in composites
 Ø  Additive in silicone oils and resins.
 Ø  Filler for tubular heaters and neutron absorbers
 Ø  Hot pressed HBN and titanium diboride powders are machined into evaporation boats or flash boats which are used to coat plastic films or TV tubes.
 Ø  Hot pressed HBN billets are machinable ceramics.
  In general, uses of hexagonal BN tend to concentrate on utilizing its high thermal conductivity, ease of machining, excellent electrical insulating characteristics, inertness and non-toxicity. Some of the markets served by BN are microwave tubes, plasma arc insulation, crucibles, low friction seals, high temperature fixture and as a prototype material from which parts can be machined to final shape.
 
  CBN
 Ø  High temperature lubricants
 Ø  Mold release agents
 Ø  Insulating filler material in composites,
 Ø  Filler for silicone rubber
 Ø  Additive in silicone oils and resins,
 Ø  Filler for tubular heaters and neutron absorbers
 
  3-Preparation and manufacturing processes:
 
  Beta phase crystals are formed by subjecting the alpha phase to extreme pressure and heat in a process similar to that used to produce synthetic diamonds. Melting of either phase is possible only with a high nitrogen overpressure. The alpha phase decomposes above 2700°C at atmospheric pressure and at approximately 1980°C in vacuum.
 
  Alpha phase BN is manufactured using hot pressing or pyrolytic deposition techniques. These processes cause orientation of the hexagonal crystals resulting in varying degrees of anisotropy . There is one pyrolytic technique that forms a random crystal orientation and an isotropic body, however, the density is only 50% to 60% of theoretical. Both manufacturing techniques yield high purity (greater than 99%) boron nitride. The major impurity in the hot pressed materials is boric oxide which tends to hydrolyze in the presence of water degrading dielectric and thermal shock properties. The addition of CaO to tie up the borate minimizes the water absorption. Hexagonal hot pressed BN is available in a variety of sizes and shapes while the pyrolytic hexagonal material is currently available in thin wall, generally less than l mm, geometry only.
 
  4-Chemical properties:
 
 ·  BN will oxidize above 1100°C, forming a thing boric acide layer on its surface that prevents further oxidation as long as it coats the BN.
 ·  BN is stable in reducing atmospheres up to 1650?C. However, it starts decomposing at above 1500 ?C.
 ·  High thermal conductivity, ease of machining, excellent electrical insulating characteristics, inertness and non-toxicity.
èBORODIZING PROCESS

One of the most important applications for boron-based ceramics is the process of borodizing (boriding). Borodizing is a thermo-chemical surface treatment process in which boron sprays or powders are added to the surface of tools, molds and process equipment, hence increasing service life.
This process is mainly applied to extrusion dies for the ceramic industry, oil field drilling tools, textiles machinery, aircraft shields and for the engine parts.  In this process the surface of the material is transformed into metallic boride layer. The boride layer is formed by the diffusion of boron into the base metal at high temperature. Thus, it is not a normal coating or plating; the reaction between boron compounds and the metallic surface provide a very strong, hard surface.
Borides in general have very high hardness (check with Titanium diboride). What seems very interesting that in these borodized metals can replace very expensive tools. For examples, borodized cutting tools that are made of steel can replace other sharp tools. In addition, they are adequate for such an application (cutting tools); they can withstand temperatures up to 648 °C.
In the process of Borodizing, boron carbide is used as boron source. The parts to be processed are placed in contact with boron carbide powder. Then, the piece is placed in a furnace. Boron diffuses in the work piece to form borides. What adds to that process is that boronized parts can be heat treated, quenched, and worked without any effect on the obtained properties.

 Tungsten also known as Wolfram and hence its symbol (W) is one of the transition element metals in the periodic table. It has an atomic number of 74 and an atomic weight of 183.9. The true discoverer is either the Swedish chemist Carl Wilhelm Sheele or the Spanish D’Elhuyar brothers Juan Jose and Fausto. The main Tungsten ores are wolframite and scheelite. It has one of the highest melting temperatures in all the materials known to man as well as a high electrical resisitivity and thermal conductivity. All these properties were the chief reasons that Thomas Edison’s light bulb with a cotton filament was replaced later on by a tungsten filament and till this day the light bulb has the same filament. It has other uses in television and X-ray tubes and cutting tools.

 Tungsten Carbide is a ceramic with the symbol WC, it was developed in the 1920’s for wear-resistant dies to draw incandescent-lamp filament wire. Earlier efforts to manufacture the WC-W2C eutectic alloy was unsuccessful because of its inherent brittleness, therefore the researchers diverted their attention to powder metallurgy techniques. At the present time, these powder metallurgy techniques are being further developed and refined to reduce the manufacturing costs and improve performance.

 Tungsten carbide is a mixture of tungsten, carbon, and cobalt. Tungsten carbide is harder than most steel, has greater mechanical strength, transfers heat quickly, and resists wear and abrasion better than other metals. Among the materials that resist severe wear, corrosion, impact or abrasion, tungsten carbide is superior. The application of tungsten carbide on industrial wearing surfaces has been proven to greatly enhance the performance factors for a whole spectrum of industrial applications. The working lives of many kinds of machinery can be greatly prolonged by the surface coating of wear-prone materials with tungsten carbide. Various industries state that the lives of certain parts of machinery can be extended five times by coating with tungsten carbide. It has wide applications in construction, coal mining, cement production, rock crushing and agricultural industries. It is also very useful in rebuilding worn parts. Therefore as one can see, that most of its applications utilize the high hardness and abrasive abilities.
 
 
 

2-MECHANICAL PROPERTIES

 Tungsten carbide has a density of 15.63 g/cm3. It is a very hard material with a hardness no. of 1700-2400 kg/mm2 on the Knoop scale. Hardness of a material is most commonly indexed by two scales, the Mohs scale which concerns dynamic hardness, meaning hardness by cutting and rates the materials according to results when a softer material is scratched by a harder one. There is also the Knoop scale which is based on the degree of penetration of a soft material by a hard one. It is a static test. Tungsten carbide also has a Young’s modulus of 668 GPa a tensile strength of 344 MPa and a compressive strength of 2683-2958 MPa. It has a melting point of 2777°C.
Below is table of the mechanical properties of tungsten carbide.
PROPERTY: VALUE
Density 15.63 g/cm3
Melting Point 2777°C
Hardness on the Knoop scale 1700-2400 Kg/mm2
Young’s Modulus at 100°C 668 GPa
Tensile Strength at 25°C 344 MPa
Compressive Strength at 25°C 2683-2958 MPa
Compressive Strength at 100°C 1404 MPa
Thermal Conductivity at 100°C 86 W/mK
Thermal Conductivity at 1000°C 45 W/mK
Expansion Coefficient at 100°C 4.79 ´ 10-6/°C
Expansion Coefficient at 1000°C 5.09 ´ 10-6/°C
 

3-PREPARATION, MANUFACTURING AND PROCESSING:

 The micro-hardness and abrasive power of tungsten carbide has been determined for various conditions of reduction and carbidization. It was found that they both increase with rise a in the reduction and carbidization temperatures. This increase in micro-hardness and abrasive power of high-temperature tungsten carbide can be attributed to the greater mobility of the tungsten and the carbon atoms, this leads to healing of the macro- and micro-defects in the grains during reduction and carbidization, especially in the case of tungsten carbide produced at temperatures close to the re-crystallization temperature. In the production of tungsten carbide by the reduction of the oxides, evaporation of the volatile compound (oxygen) from the chemical compound (WO3) causes the appearance of submicroscopic cracks and voids. When carbide is made from the metallic powder, the same amount of porosity is retained.
 Since the defectiveness of WC must be accompanied by a corresponding hardness characteristic, which falls with an increase in the number of defects, the micro-hardness and the abrasive power of WC have been determined in relation to the reduction and carbidization conditions. The hardness measurements were made either directly on grains of carbide powder or on specimens produced by hot-compaction followed by annealing to remove the internal stresses set up as a result of the compaction process.
 The abrasive power of the materials is then measured. In this method the carbide grains being investigated are held between rotating steel and glass disks and a certain amount of glass is found from the latter in a predetermined time. The change in abrasive power, like that of the micro-hardness is determined by the reduction and the carbidization conditions, reaching a maximum value when the tungsten and the tungsten carbide are produced at temperatures close to the re-crystallization temperatures and when the WC particles with a more perfect crystal lattice are obtained.
 The reduction in the number of defects in tungsten carbide with rise in the temperature and the corresponding increase in its hardness and abrasive power can be ascribed to the greater mobility of the tungsten and tungsten carbon atoms, which results in an increase in the pycnometric weight of tungsten carbide and in the healing of the micro- and macro- defects in the grains.
 Most cemented carbides are manufactured by a powder metallurgy process consisting of tungsten carbide powder production, powder consolidation, sintering and post-sintering forming. Tungsten carbide powder generated by a carburization process, is mixed with a relatively ductile matrix material (cobalt, Nickel or iron) and paraffin wax in either an attrition or ball mill to produce a composite powder. Spray drying yields uniform, spheroidized particles that are 100 mm to 200mm in diameter.  The powder is consolidated into net and near-net-shape green compacts and billets by pressing and extrusion. Pressed billets can also be machined to shape before sintering. The density of the green compacts is around 45 to 65% of the theoretical.
The green parts are then de-waxed at a temperature between 200°C and 400°C and then they are pre-sintered between 600 and 900°C to impart adequate strength for handling.
An alternative technology is a combination sinter-HIP process that combines de-waxing, pre-sintering, vacuum sintering, and low-pressure HIP, to speed up the overall cycle time. Microstructure is enhanced in the sinter-HIP process because of the collapse of material into voids rather than plastic deformation of binder.
 
 

4-GENERAL FEATURES:

 Tungsten carbide has a melting point of  3143K and a density of 15.63g/cm3. It also has a re-crystallization temperature of 1650K it has an atomic weight of 93.65 amu. It is chiefly obtained from the ores of wolframite and scheelite which are the ores of tungsten. It is one of the hardest materials on the face of the earth and it is very wear resistant and corrosion resistant. Thermal Conductivity at 100°C is 86 W/mK,
Thermal conductivity at 1000°C is 45 W/mK. The Expansion Coefficient at 100°C is 4.79 ´ 10-6 /°C and the Expansion Coefficient at 1000°C is 5.09 ´ 10-6/°C

5-ENVIRONMENTAL INTERACTION:

 It has been established that the relative wear resistance of various metals is a fundamental characteristic of their working properties, since it enables us to determine the resistance of the materials to failure in the heavily deformed state. It is natural to associate the relative wear resistance with the bonding forces in the crystal lattice, which are usually characterized by the modulus of normal elasticity or by the square of the coefficient of lattice rigidity.
Extending the life of equipment and machinery by significantly reducing the wear on the machinery and parts is the goal of maintenance engineers and technicians. Gigantic efforts are made to improve an overall company’s performance by means of extending the useful life of operating plant and equipment’s. The application of tungsten carbide onto industrial wearing surfaces has been demonstrated to measurably enhance performance factors for a whole spectrum of industrial applications.
 Tungsten carbide is both a practical and efficient means of extending the life cycle of wearing machinery and their parts. Adhered to surfaces as a wear protective coating, the material has been found to greatly extend the life of equipment. Abrasive or destructive wear on un-coated alloy steel parts, results in less than desirable performance and reductions in cost. Protecting wearing surfaces with tungsten carbide has the following benefits:
  1 . Significant increase in the life of consumable parts.
  2 . Reduction of localized wear patterns (e.g. cutting edges) .
  3 . Change out costs for parts minimized.
  4 . Measurably lower material processing costs.

  A variety of industries report that the life of certain parts can be extended about five times when coated with tungsten carbide. The material is widely used in                              the construction, coal mining, cement production, rock crushing and agricultural industries. As an example, pipe elbows that wear out in little over a  year can last beyond five years if hard-surfaced with crushed tungsten carbide. Coating working surfaces greatly increases operational life. Tungsten carbide is also a very effective material for rebuilding worn parts.

6-APPLICATIONS

 Tungsten carbide can be used for a wide variety of applications. It has many applications that utilize the corrosion resistant property such as wear plates, drawing dies, wear parts for wire wearing machines. There are other applications that make use of its high hardness such as punches, bushings, dies, cylinders, discs, rings and intricate shapes as well as performs and blanks. There are other minor applications such as rusticator blades, sander nozzles, air jets, sander guns.
Tungsten carbide is also used primarily and extensively for making drilling tips tunneling , mining and quarrying purposes, i.e. for most geological activities. Tungsten carbide is also made into tiles for  wear and abrasion resistance.

1-INTRODUCTION:

 Titanium is one of the strongest metals on the face of this earth. It has an atomic weight of 47.9 amu and it is in the transition metal area of the periodic table. It was discovered in 1791 by the British William Gregor, but it was the German Martin Heinrich who rediscovered it and named it titanium. Its main ores are menachanite and rulite. It is also very useful as it is very light. It is lighter than steel and more rigid than aluminum and it has a high resistance to corrosion. However the only drawback is that it is quite expensive due to the difficulty in removing all the oxygen from its ore. Titanium is used in aircraft, compressor blades and in missiles and space capsules.
 Titanium diboride is made of titanium combined with boron. It is one of the refractory borides. It is well-known for its stiffness and hardness, furthermore in great contrast to most conventional ceramics it is electrically and thermally conductive Its symbol is TiB2. It has an HCP crystalline structure.
TiB2 has a very high purity and it is very stable even in hydrochloric acid and hydroflouric acids which are two of the most corrosive acids. It also has superb densification properties and very high tensile and compressive strengths and hardness. Not to mention its excellent wettability and stabilty. It is produced by vacuum arc-casting followed by either hot-pressing or pressureless sintering Titanium diboride is the only compound that is stable in the Ti-B phase diagram. Titanium diboride has many applications as a corrosion resistant material as in crucibles and cutting tools in addition to some military applications.

2-MECHANICAL PROPERTIES:

 The diborides of the transition metals offer a combination of attractive properties including high specific strength, high specific modulus, high hardness and a high melting point with good oxidation resistance to 1400°C.
Regarding titanium diboride it has a very high strength at very high temperatures and it has a high elastic modulus and a high compressive strength. In addition it has excellent wettability and stability in liquid metals such as Aluminium and Zinc. It has a hardness superior to tungsten carbide and its fracture toughness is even greater than that of silicon nitride and the stiffness-to-weight ratio is excellent. Its mechanical properties are the following:
Property: Value:
DensityMelting PointLattice ParameterComposition weight of boronCoefficient of linear expansionVickers Hardness No.Modulus of elasticityFlexural strengthFracture toughnessCompressive strengthPoisson’s ratio 4.5 g/cm32980°Cc = 3.23 A a = 3.228 A31.12%6.4 in/in/°C ´ 10-63000 kgf mm510-575 GPa350-575 MPa5-7 MPa m1/297´ 103 psi0.18-0.2

3-PREPARATION, MANUFACTURING AND PROCESSING:

The most common production process for large quantities of TiB2 is by reacting titania (TiO2) with carbon and boron carbide (B4C) or Boric Oxide (B2O3) as follows
  2 TiO2 + C + B4C ® 2 TiB2 + 2CO2
  2 TiO2 +5C+ 5B2O3 ® 2 TiB2 + 5CO2
The purity of the synthesized powder is primarily defined by the purity of the raw materials. Generally, several different grades of TiO2, carbon, B4C and B2O3 can be used for the production of TiB2, depending on the required particle size, purity and price requirements of the synthesized powder.
Vacuum Arc-Casting is used to produce a 100% dense, single-phase titanium diboride. Graphite hearths are used, the molten titanium diboride wets the graphite and exhibits excellent fluidity, shapes are produced both by gravity and tilt-pour casting methods.
Sintered parts of titanium diboride are usually produced by either hot pressing or pressureless sintering, although hot isostatic pressing HIP also has been used. Quite a number of different sintering methods and sintering aids are used to produce fully dense parts of titanium diboride.
 Hot pressing of titanium diboride is taken place at temperatures greater than 1800°C under vacuum or 1900°C in an inert atmosphere such as argon. However hot pressing is expensive and the net-shape fabrication is not possible, hence the required shape still must be machined from the hot-pressed billet. Some usual sintering aids used for hot-pressed parts include iron, nickel, cobalt, carbon, tungsten, and tungsten carbide.
 Pressure-less sintering of titanium diboride is a cheaper method for the production of net-shaped parts. Due to the high melting point of titanium diboride sintering temperatures in excess of 2000°C are often required to promote sintering.
 However, recently a group of researchers at the Georgia Institute of Technology have patented a cost reducing method of manufacturing titanium diboride by employing a high-energy chemical reaction. The method called HTS i.e. high temperature synthesis uses powdered metal, either magnesium or aluminum, and powders of titanium oxide and boron oxide. The materials are mixed and placed in a high-temperature crucible. This mixture is the ignited and the self-sustaining reaction produces titanium diboride particles dispersed within a matrix of alumina or magnesium oxide. This latter compound is the leached leaving titanium diboride particles  each about 0.5mm in diameter. Moreover the sub-micron particles allow the material to be molded to near net-shape, thus reducing costs.

4-GENERAL FEATURES:

 Titanium diboride has a room temperature resistivity of 15´ 10-6 W cm and a thermal conductivity of 25 W/mK. It also provides excellent resistance to chemical reactions and thermal shock and thermal stability and high operating temperatures. In addition it resists most chemical reagents and has excellent wettability and stability in liquid metals such as Aluminium and Zinc.
 The main producers of titanium diboride are Advanced Refractory Technologies Inc., Advanced Ceramics Corp. And Cerac (USA), H.C. Stark Co. And Electroschmeltzwerk Kempten in Germany, Denka in Japan and Borides Ceramics and Composites in the UK The world wide production of titanium diboride in 1997 was 80 metric tons. Its typical price is around $35-$12 /Kg depending on the grade and the quality and purity.
 
 
 

5-ENVIRONMENTAL INTERACTION:

 Titanium diboride has a very good oxidation resistance till about 1300°C and 1400°C. Titanium diboride is known to oxidize parabolically to a solid compound of TiO2. It also is relatively very corrosion resistant to most chemical reagents that combat it.
Below is a table describing its chemical resistance to reagents:
Reagents:Acids     -    concentratedAcids     -    diluteAlkalisHalogens                                    Metals Performance:FairGoodFairGoodGood
Therefore as one can see, it has very good potential applications as a corrosion resistant material.

6-APPLICATIONS:

  Titanium diboride was originally developed to make lightweight armor for the US army tanks. It also has many commercial applications such as nozzles, seals, cutting tools, dies, wear parts due to its corrosion resistance and also molten metal crucibles and electrodes. It is used in crucibles because it has very high melting temperature and it chemical reactivity is low. Another use is in metallizing boats, again due to the similar reason as crucibles. Currently there is great excitement in the scientific research field concerning titanium diboride as it has been proved that titanium diboride and ZrB2 have great  potential as electrodes in Aluminum reduction cells. This is primarily due to the previously mentioned fact that titanium diboride  is strongly wetted by and only slightly soluble in Aluminum. Therefore this could heavily reduce the costs of manufacturing Aluminum and with this decrease comes along a reduction is most light-industry costs where Aluminum is chiefly employed.

CONVENTIONAL CERAMICS

B-Properties
Aluminum is the lightest structural metal and is highly ductile, capable of being cast, rolled, stamped, drawn, machined, or extruded. Moreover, it is corrosion resistant, heat reflective, and an excellent conductor of electricity. Although aluminum is soft and has relatively low tensile strength in its pure form, it can be made much harder and stronger if alloyed with copper, magnesium, or zinc. Aluminum is more widely used than any other metal except iron and steel.

Property Value
Symbol Al
Atomic Number 13
Atomic Weight 26.9815
Group in Periodic Table IIIA
Density at 32°F (0°C) 2.699
Boiling Point 3,272°F (2,467°C)
Melting Point 1,219.46°F (660.37°C)
 
 
 

C- Production of Alumium:
In 1886 Charles Martin Hall of the United States and Paul-Louis-Toussaint Heroult of France developed independently a method, still used today, of reducing alumina in which alumina is dissolved in molten cryolite and is decomposed electrolytically.
The first step in treatment is to remove impurities from the ore. This refining process turns bauxite into aluminum oxide, or alumina. , powdered bauxite is mixed with hot caustic soda (sodium hydroxide). In large pressure tanks, the hydrated aluminum oxide of bauxite forms a solution of sodium aluminate. The impurities remain in solid form and are filtered out as "red mud." . As it cools, crystals of aluminum hydroxide appear. Kilns heat the crystals white hot and drive off the chemically combined water, leaving pure alumina. Alumina is reduced to pure aluminum by electrolysis. In the electrolytic cell used in making aluminum, the alumina is dissolved in a bath, or electrolyte. Then a strong electric current is passed through the solution. The action reduces the alumina and deposits nearly pure aluminum on the bottom of the bath. When enough has accumulated, the molten aluminum is tapped and cast.

D-Uses and Applications:
Pure aluminum metal is utilized in
  1 . The manufacture of appliances and food and beverage packaging, principally in the form of foil wrappings and cans.
  2 . Electronic components
  3 . Reflectors
  4 . Utensils
  5 . It is also converted into a powder that can be mixed with other substances to produce metallic paints, rocket propellants, flares and solders.

A- Introduction:
Alumina, or aluminum oxide, Al2O3, is the compound from which commercial aluminum is produced. It occurs in nature as both corundum--and as ruby, sapphire, and several other gemstones--and as an important constituent of bauxite, which is mined and refined to produce a purified, calcined alumina in the form of a fine white powder. This powder is smelted to manufacture aluminum products.

B- Properties:
Alumina occurs in two crystalline forms. Alpha alumina is composed of colorless hexagonal crystals; gamma alumina is composed of minute colorless cubic crystals with specific gravity about 3.6 that are transformed to the alpha form at high temperatures. Alumina powder is formed by crushing crystalline alumina; it is white when pure. Dense alumina microstructures with grain sizes of about 0.5 my-m are common products of the grinding industry

C-General and Mechanical Properties:
Alumina is characterized by a:
 ·  high melting point,
 ·  high hardness and high mechanical strength, although mechanical strength is reduced at temperatures above 1000oC.
 ·  Due to the relatively large coefficient of thermal expansion, thermal shock resistance is reduced.
 ·  Alumina is an electrically insulating material, with a high electrical resistivity, increasing with purity.
 ·  Good chemical stability of alumina, leads to a high corrosion resistance.
 ·  It is insoluble in water and only slightly soluble in strong acid and alkaline solution
 ·  Mechanical resistance to particle breakage is an important property of alumina powders
 ·  Strong particles minimize the problem of dust generation during transport and processing.
 

Property   Units
   99.9%Al2O3
Density  g/cm3 3.92
Mechanical Properties:
Flexural Strength  20oC MPa 350
 800oC MPa 250
Compressive Strength  MPa 2500
Modulus of Elasticity  GPa 350
Hardness  R45N 84
  Hv0.3 1700
Fracture Toughness (KIc)  MPam1/2 4.5
Thermal Properties:
Max. Use Temp.  oC 1725
Thermal Expansion Coeff.  x 10-6/oC 8.5
Thermal Conductivity  W/mK 28
Thermal Shock Resistance  oC 200
Electrical Properties:
Resistivity 25oC Ohmcm >1014
 300oC Ohmcm -
 500oC Ohmcm 1012

D-Production of Alumina:
Most aluminum produced today is made from bauxite. First discovered in 1821 near Les Baux, France (from which its name is derived), bauxite is an ore rich in hydrated aluminum oxides, formed by the weathering of such siliceous aluminous rocks as feldspars, nepheline, and clays. During weathering the silicates are decomposed and leached out, leaving behind a residue of ores rich in alumina, iron oxide, titanium oxide, and some silica. Ores contain at least 45 percent alumina and no more than 5 percent to 6 percent silica.
The Bayer Process of separating alumina from the bauxite ore was patented in 1888 and is still used today.

  1 . Bauxite is pulverized by being mixed with soda ash and lime in a ball mill.
  2 . Water is added to turn the mixture into a slurry,
  3 . The slurry is drained from the ball mills into tanks or digesters.
  4 . In these tanks, which are heated by the injection of live steam, the alumina contained in the slurry is liquefied, then poured into settling tanks. Solids--largely sand, iron, and other elements that do not dissolve--move downward while a coffee-colored liquor remains on top.
  5 . Cleared of all solids, the liquid is pumped into large, open- topped vats, or precipitators, up to six stories high. There the liquid is agitated, and minuscule alumina crystals begin to form. Agitation causes the crystals to adhere to each other as they slowly sink to the bottom of the precipitators, and they become slightly larger than grains of sugar.
  6 . The crystals are then pumped into settling tanks and washed again to remove the soda ash and lime solution that was added at the beginning of the process.
  7 . The final step is to drive off the remaining moisture by passing the alumina, which now resembles white mud, through kilns that heat it to more than 1,000 deg C (1,830 deg F). The sugarlike alumina, now dry and about 99 percent pure, pours out of the lower end of the tilted kiln and is stored in silos, ready to go into the reduction cells to make the metal.

D- Uses and Applications:
The available fields of applications for Alumina are:
  1 . abrasives
  2 . high-temperature refractories
  3 . ceramics and glass
  4 . Heated alumina has a porous structure that easily absorbs moisture and vapors and is therefore used to dehydrate liquids and gases.
  5 . Aluminum sulfate, or activated alumina (the product of alumina, or clay, or bauxite, with sulfuric acid) is important in paper manufacture as a color binder and a filler.
  6 . Other alumina compounds produce Alums and are used for waterproofing fabrics and as the antiperspirant in commercial deodorants.

A-Introduction:
Zirconium is a grayish white lustrous metal, an element of the second series of transition metals. It has the symbol Zr, an atomic weight of 91.22, and an atomic number of 40. The name comes from the Arabic "zargun", meaning gold color, describing the gemstone now known as zircon. Impure zirconium was first isolated by Jons Jakob Berzelius by heating a mixture of potassium zirconium fluoride with potassium in an iron tube. Zirconium occurs in abundance in S-type stars and has been identified in the Sun and meteorites.
Zircoium is never found in nature, it occurs chiefly as a silicate in the mineral zircon and as an oxide in the mineral baddeleyite, which is found in commercial quantities in Brazil. Zirconium ores also contain the element hafnium, a metal with properties similar to those of zorconium.
 
B-Properties:
Zirconium has a density of 6.506 g/cm-3, a boiling point of 4,377 º C, and a melting point of 1,852 º C. Zirconium is superconductive at low temperatures. Zirconium combines readily with oxygen, hydrogen and nitrogen at high temperatures. Zirconium has a low neutron absorption cross section and a high resistance to the corrosive environments.

Symbol Zr
Atomic number 40
Atomic weight 91.22
Group in periodic table IVb
Boiling point 6,471  F (3,577  C)
Melting point 3,375  F (1,857  C)
Specific gravity 6.51
 

C-Production
The pure metal is produced commercially by reduction of the chloride with magnesium (the Kroll process).

D-Uses and Applications:
Typical applications of Zirconium include:
  1 . The production of pipes and jackets for fuel elements.
  2 . Zirconium and its tin-iron-nickel-chromium alloy Zircaloy are widely used by the nuclear industry.
  3 . Along with niobium, zirconium is used in the construction of magnets with potential applications to the generation of electrical power.
  4 . Zirconium is also used in the manufacture of procelain, steel, certain nonferrous alloys and refractories.
  5 . Zirconium is used in vacuum tubes to remove traces of gases because it combines readily with oxygen, hydrogen and nitrogen at high temperatures.

F-ZIRCONIA ZrO2
 

A-Introduction:
The title of the first scientific paper to highlight the possibilities offered by the ‘transformation toughening’ mechanism which occurs in certain zirconia ceramics was : ‘Zirconia - Ceramic steel’. Since 1975, considerable research, development, and marketing effort has been expended on this single material which offers the traditional ceramic benefits of hardness, wear resistance and corrosion resistance, without the characteristic ceramic property of absolute brittleness. To use zirconia to its full potential, the properties of the oxide have been modified extensively by the addition of cubic stabilising oxides. These can be added in amounts sufficient to form a partially stabilised zirconia (PSZ) or to form a fully stabilised zirconia which has a cubic structure from its melting point to room temperature.

b) General Mechanical properties:

Zirconia based materials are caracterized by a:

  1 . high strength,
  2 . high fracture toughness,
  3 . high hardness,
  4 . wear resistance,
  5 . good frictional behavior.
  6 . non-magnetic.
  7 . electrical insulation.
  8 . low thermal conductivity.
  9 . corrosion resistance in acids and alkalis.
  10 . modulus of elasticity similar to steel.
  11 . coefficient of thermal expansion similar to iron.
 

 ZrO2
Density  g/cm3 5.6
Mechanical Properties:
Flexural Strength  20oC Mpa 545
 800oC Mpa 354
Compressive Strength  Mpa 1700
Modulus of Elasticity  Gpa 205
Poissons Ratio   0.31
Hardness  Hv0.3 1120

Fracture Toughness (KIc)  MPam1/2 6
Thermal Properties:
Max. Use Temp.   oC 1000
Thermal Expansion Coeff.  x 10-6/oC 10
Thermal Conductivity  W/mK 2.5
Thermal Shock Resistance  oC 375
Specific Heat Capacity  J/kgK 400
Electrical Properties:
Resistivity 20oC Ohmcm 1010
 400oC Ohmcm 5x1010
 1000oC Ohmcm -
 

A- Un-stabilized (Pure) Zirconia
1- Introduction and properties:
Pure Zirconia has a low thermal shock resistivity, a high melting point (2,700° C) and a low thermal conductivity. Its polymorphism, however, restricts its widespread use in ceramic industry. During a heating process, zirconia will undergo a phase transformation process. The change in volume associated with this transformation makes the usage of pure zirconia in many applications impossible.

2-Preparation:
Zirconia is usually produced from the zircon, ZrSiO4. To produce zirconia from zircon, the first step is to convert zircon to zirconyl chloride. It can be done by:
Zircon (ZrSiO4) + NaOH
¯ Melting
Na2ZrO3
¯+ HCl
ZrOCl2 8H2O
 There are two methods are used to make zirconia from the zirconyl chloride: thermal decomposition and precipitation.
  A. Thermal decomposition method: Once the zircornyl chloride (ZrOCl2 8H2O) is heated to 200° C, it starts dehydration and becomes dehydrated ZrOCl2. The ZrOCl2 decomposes into chlorine gas and becomes zirconia at a much higher temperature. Zirconia lumps obtained from the calcination then undergo a size reduction process into the particle size range needed. This method is associates with low production cost. However, it is not easy to produce zirconia powders with high purity and fine particle size by the method.
  B. Precipitation method, on other hand, uses chemical reactions to obtain the zirconia hydroxides as an intermediate. Its processing can be described as following:
ZrOCl2 8H2O
¯
Solution
¯+NH4OH
Precipitated intermediates Zr(OH)4
¯Wash
Cl--free Precipitate
¯Filtration
Wet powders Zr(OH)4
¯Freezing Dry (Liquid N2)
Dry Powder Zr(OH)4
¯Calcination
Zirconia Powder ZrO2
  By this method, the grain size, particle shape, agglomerate size, and specific surface area can be modified within certain degree by controlling the precipitation and calcination conditions. Furthermore, its purity is also easier to be controlled. For the applications of zirconia in the slip casting, tape casting, mold injection and so forth, particle size and specific surface are important characteristics. Well-controlled precipitated zirconia powder can be fairly uniform and fine. Particle size can be made less than 1 micrometer.

3-Uses and applications:
Pure Zirconia is used as
  1 . an important component of lead-zirconia-titanate electronic ceramics
  2 . an additive to enhance the properties of other oxide refractories
 

B- Partially Stabilized Zirconia (PSZ)
1- Introduction
Addition of some oxides, such as CaO, MgO, and Y2O3, into the zirconia structure in a certain degree results in a solid solution, which is a cubic form and has no phase transformation during heating and cooling. This solid solution material is termed as stabilized zirconia, a valuable refractory.

2- Structure and Phases of PSZ:
Partially stabilized Zirconia is a mixture of zirconia polymorphs, because insufficient cubic phase-forming oxide (stabilizer) has been added and a cubic plus metastable tetragonal ZrO2 mixture is obtained. A smaller addition of stabilizer to the pure zirconia will bring its structure into a tetragonal phase at a temperature higher than 1,000 ° C, and a mixture of cubic phase and or tetragonal phase at a lower temperature. Therefore, the partially stabilized zirconia is also called as tetragonal zirconia polycrystal (TZP) . PSZ is a transformation-toughened material. The pure zirconia particles in PSZ can metastabily retain the high-temperature tetragonal phase. The cubic matrix provides a compressive force that maintains the tetragonal phase. Stress energies from propagating cracks cause the transition from the metastable tetragonal to the stable monoclinic zirconia. The energy used by this transformation is sufficient to slow or stop propagation of the cracks.

3- Properties:
  1 . Partially Stabilized Zirconia can withstand extremely high temperatures
  2 . The low thermal conductivity ensures low heat losses
  3 . The high melting point permits stabilized zirconia refractories to be used continuously or intermittently at temperatures of  2,200°C in neutral or oxidizing atmospheres. Above 1,650° in contact with carbon, zirconia is converted in to zirconium carbide.
  4 . Zirconia is not wetted by many metals and is therefore an excellent crucible material when slag is absent. It has been used very successfully for melting alloy steels and the noble metals.

4- Preparation
In order to achieve the requirement of the presence of cubic and tetragonal phases in their microstructure, stabilizers (magnesia, calcia, or yttria) must to be introduced into pure zirconia powders prior to sintering. Stabilized zirconia can be formed during a process called in-situ stabilizing. Before the forming processes, such as molding, pressing or casting, fine particles of stabilizer and monoclinic zirconia are well mixed. Then the mixture is used for forming of green body. The phase conversion is accomplished by sintering the doped zirconia at 1700° C. During the firing (sintering), the phase conversion takes place.
High quality stabilized zirconia powder is made by co-precipitation process. Stabilizers are introduced during chemical processing, before zirconium hydroxide's precipitation.
ZrOCl2 8H2O
¯+Stabilizer (Y2O3, for example) + HCl
Solution
¯+NH4OH
Co-precipitated intermediates Zr(OH)4 + Y(OH)3
¯Wash
Cl--free Precipitate
¯Filtration
Wet powders Zr(OH)4 + Y(OH)3
¯Freezing Dry (Liquid N2)
Dry Powder Zr(OH)4 + Y(OH)3
¯Calcination
Stabilized Zirconia Powder ZrO2 + Y2O3
 
A cubic (or tetragonal) phase zirconia is formed during calcination of chemically precipitated intermediates. These powders have chemically higher uniformity than in-situ stabilizing powder and can be used in applications such as refractories, engineering ceramics and thermal barrier coatings.

5-Uses and Applications:
PSZ refractories are rapidly finding application as
  1 . Setter plates for ferrite and titillate manufacture
  2 . Matrix elements and wing tunnel liners for the aerospace industry.
  3 . Heat engine components, such as cylinder liners, piston caps and valve seats.
  4 . Oxygen sensors and solid oxide fuel cells

C- Fully Stabilized Zirconia
1- Introduction:
Generally, addition of more than 16 mol% of CaO (7.9 wt%), 16 mol% MgO (5.86 wt%), or 8 mol% of Y2O3 (13.75 wt%), into zirconia structure is needed to form a fully stabilized zirconia. Its structure becomes cubic solid solution which has no phase transformation from room temperature up to 2,500 ° C.

2- Preparation:
The sintering kinetics, such as shorter sintering time, lower sintering temperature and denser specific gravity of sintered body can be greatly enhanced by small particle size, large specific surface area and desired particle size distribution . However, the physical properties of powders also depend upon individual application. Forming processes such as tape casting and extrusion sometimes need a smaller specific surface area to enhance dispersion of the powders when mixed with solvents. Special efforts during the preparation of powders are needed to control initial particle size distribution, agglomeration, and calcination
A. Particle size: Particle size is mostly determined by the agglomerate's size that are formed in the early stages of powder preparation and during the precipitation. There is no or little effect on the particle size during drying or calcination stage, even calcinated at different temperature or for different time periods.
B. Crystallite Size: Crystallite size is determined during calcination due to the crystal growth. The calcination temperature has more significant effect on final crystal size than calcination time.
C. Specific Surface Area: As crystallite size, the specific surface area is strongly influenced by the calcination parameters, especially by calcination temperatures.

3- Uses and Applications:
As a good ceramic ion conducting materials, fully yttria stabilized Zirconia (YSZ) has been used in oxygen sensor and solid oxide full cell (SOFC) applications. The SOFC applications have recently been attracting more worldwide attention, due to their high energy transfer efficient and environment concerns.

B)Properties and Occurrence
Silicon is prepared as a brown amorphous powder or as grey-black crystals. It is obtained by heating silica, or silicon dioxide (SiO2), with a reducing agent, such as carbon or magnesium, in an electric furnace. Crystalline silicon has a hardness of 7, compared to 5 to 7 for glass. Silicon melts at about 1410° C, boils at about 2355° C, and has a relative density of 2.33. The atomic weight of silicon is 28.086.
Silicon is not attacked by nitric, hydrochloric, or sulfuric acids, but it dissolves in hydrofluoric acid, forming the gas, silicon tetra-fluoride, SiF4. It dissolves in sodium hydroxide, forming sodium silicate and hydrogen gas. At ordinary temperatures silicon is impervious to air, but at high temperatures it reacts with oxygen, forming a layer of silica that does not react further. At high temperatures it also reacts with nitrogen and chlorine to form silicon nitride and silicon chloride, respectively.
Silicon constitutes about 28 per cent of the earth's crust. It does not occur in the free, elemental state, but is found in the form of silicon dioxide and in the form of complex silicates. Silicon-containing minerals constitute nearly 40 per cent of all common minerals, including more than 90 per cent of igneous-rock-forming minerals. The mineral quartz, varieties of quartz (such as carnelian, chrysoprase, onyx, flint, and jasper), and the minerals cristobalite and tridymite are the naturally occurring crystal forms of silica. Silicon dioxide is the principal constituent of sand. The silicates (such as the complex aluminum, calcium, and magnesium silicates) are the chief constituents of clays, soils, and rocks in the form of feldspars, amphiboles, pyroxenes, micas, and zeolites, and of semiprecious stones, such as olivine, garnet, zircon, topaz, and tourmaline.

C)Generals uses  of silicon
Silicon is used in the steel industry as a constituent of silicon-steel alloys. In steel making, molten steel is deoxidised by the addition of small amounts of silicon; ordinary steel contains less than 0.03 per cent of silicon. Silicon steel, which contains from 2.5 to 4 per cent silicon, is used in making the cores of electrical transformers because the alloy exhibits low hysteresis. A steel alloy, known as duriron, containing about 15 per cent silicon, is hard, brittle, and resistant to corrosion; duriron is used in industrial equipment that comes in contact with corrosive chemicals. Silicon is also used as an alloy in copper, brass, and bronze.
Silicon is a semiconductor, in which the resistivity to the flow of electricity at room temperature is in the range between that of metals and that of insulators. The conductivity of silicon can be controlled by adding small amounts of impurities, called dopants. The ability to control the electrical properties of silicon, and its abundance in nature, have made possible the development and widespread application of transistors and integrated circuits used in the electronics industry.
Silica and silicates are used in the manufacture of glass, glazes, enamels, cement, and porcelain, and have important individual applications. Fused silica, a glass made by melting quartz or hydrolyzing silicon tetrachloride, is characterized by a low coefficient of expansion and high resistance to most other chemicals. Silica gel is a colorless, porous, amorphous substance; it is prepared by removing part of the water from a gelatinous precipitate of silicic acid, SiO2·H2O, which is formed by adding hydrochloric acid to a solution of sodium silicate. Silica gel absorbs water and other substances and is used as a drying and de-colourising agent.
Sodium silicate, Na2SiO3, an important synthetic silicate, is a colorless, water-soluble, amorphous solid that melts at 1088° C. It is prepared by reacting silica (sand) and sodium carbonate at a high temperature or by heating sand with concentrated sodium hydroxide under pressure. The aqueous solution of sodium silicate, called water glass, is used for preserving eggs; as a substitute for glue in making boxes and other containers; as a binder in artificial gemstones; as a fireproofing agent; and as a binder and filler in soaps and cleansers. Another important silicon compound is the silicon-carbon compound carborundum, which is used as an abrasive.
Silicon monoxide, SiO, is used as a coating to protect other materials, the outer surface oxidising to the dioxide SiO2. Such layers are applied also as components of interference filters.
SILICON CARBIDE
Properties of silicon carbide:
 Silicon carbide is a polymorph material, (i.e. it favors more than one structure, SiC can favor up to 20 different structures). These structures change according to the amount of temperature and pressure that are applied to the material when it is being formed.
 Silicon carbide has long been recognized as an ideal material for applications where superior attributes such as hardness and stiffness, strength at elevated temperatures, high thermal conductivity (can withstand temp.> 2000°C), low coefficient of thermal expansion and resistance to wear and abrasion, are of primary value. Because it is a lightweight (its density is close to that of aluminum) material it has a higher advantage over other materials.
Until recently, the only commercially available silicon carbide has been sintered or reaction bonded hexagonal alpha silicon carbide. One issue design engineers have had to contend with is the fact that the traditional powder consolidation process produces two-phase materials, in other words some reaction bonded SiC contains as much as 40% free silicon.

Types of SiC and impurities added to it:
Some impurities are added to the silicon carbide structure, examples of these impurities are silicon dioxide (SiO2), iron (Fe), aluminum (Al), and carbon (C). These are added to compromise the true performance of the silicon carbide.
There are different types of silicon carbide products, the properties of each type is set according to the application that it is going to be used in. The names that given to silicon carbide are given according to the process that is used to manufacture the carbide. Some known processes are electrically conductive sintered alpha silicon carbide, black silicon carbide, green silicon carbide, and CVD silicon carbide.

A-Electrically conductive sintered alpha silicon carbide:
It is a dense type of SiC, it has superior resistance to oxidation, corrosion, wear, and to chemical attacks. The single phase SiC also has high strength, and good thermal conductivity.

Properties Data
Density (g/cm3) >3.05
Vickers hardness (at 1 Kg) 2400
Flexural strength (MPa)Room temperature800oC1200oC1500oC 395403405400
Young’s modulus (GPa) 390
Compression strength (MPa) 4100
Fracture toughness (MPa) 4.5
Thermal conductivity (W/m oK) 100-120
Thermal expansion (10-6/ oK) 4.0
Specific heat (J/g oK) 0.6-0.7
Electrical resistivity (ohm-cm) 0.5-20
Bend creep @ 1000oC and 200MPa  for 1000hrs No detectable creep

The process used to prepare this type of carbide is done by hot pressing, cold forming, and by electrically conductive sintering.
This carbide is wear and corrosion resistance, and can withstand chemical attacks.
General applications for this type of carbide are grinding media, seal rings, high power resistors, induction heating susceptor, and as burners and crucibles.

B-Black silicon carbide:
It is composed of 98.5% premium grade, medium high density, high intensity magnetically treated SiC. Most impurities are removed from the carbide. What is special about this type of SiC is available in a very large number of grit sizes ranging from #8 to #280.

Typical chemical analysis Properties
SiC 98.5%
SiO2 0.5%
Si 0.3%
Fe 0.08%
Al 0.1%
C 0.3%
Ball mill toughness: On crude SiC, 55 min. finished product is much higher but not tested on a routine basis.
Apparent specific gravity: Course sizes: 3.20g/cm3, #500 grit: 3.14g/cm3(Pycnometer method)
Hardness 100gm (KNOOP scale) 2580
Melting point Sublimes at approx. 2600oC
Particle shape Blocky and sharp
The process used to prepare this type of carbide is done by hot pressing, cold forming, and by using high intensity magnetically treated SiC grain.
This carbide is wear and corrosion resistance, and can withstand chemical attacks.
Because the particles shape of the carbide is blocky and sharp it can be used for polishing, lapping, blasting, compounds, and vitrified and resinoid wheels, primarily for grinding and finishing non-ferrous and non-metallic materials. It is also used for refractories, composites, wire sawing, non-skid, tumbling and many other applications.
 

C-CVD Silicon Carbide: -
 CVD SiC is a unique type of SiC because it has a purity of <99.9995%, it is homogeneity, chemically and oxidation resistant. It is thermally stable, is very cleanable and polishable, and is dimensionally stable.

PROPERTIES  TYPICAL VALUES (AT RM TEMP.)
Crystal structure FCC (beta phase polycrystalline)
Sublimation temp.(o C) ~2700
Grain size (in microns) 5-10
Density (gm/cm3) 3.21
Hardness (Kg/mm2)-Knoop (500g load)-Vickers (500g load) 25402500
Chemical purity 99.9995%
Flexural strength, 4 point (MPa/Ksi)Mil Std 1942 B -   at room temp. -   at 1400oC       470/68  575/84
  Weibull parameters  Modulus, m                   Scale factor, beta (MPa/Ksi)     11.45  462/66
  Fracture toughness, KIC values  -micro-indention (MN m^-1.5)  -controlled flow (MN m^-1.5)     3.3  2.7
  Elastic modulus (GPa/10-6 psi)  -sonic  -4 point fracture     466/68  461/67
  Coeff. of thermal expansion (1/K) -   at room temp.-at 1000oC 2.2*10-64.0*10-6
Heat capacity (J/Kg.K) 640
Thermal conductivity (W/m.K) 300
Poisson’s ratio 0.21
Polishability (optical profilometer) <3 angstroms RMS
Electrical resistivity 1-50 ohm/cm
 
 

Degrees(OC) -140 -100 0 200 500 700 1000 1200
Specific heat(J/Kg.K) 175 301 574 952 1134 1189 1251 1295
Thermal conductivity(W/m.K)  396 485 333 221 137 110 78 63
Thermal expansion coefficient (1/K * 10-6) 0.4 0.08 1.9 3.7 4.6 4.9 5.0 5.1
Elastic modulus (GPa) - - 460 457 450 440 435 422
Flexural strength (MPa) 460 465 470 480 500 515 540 555

The process used to provide these properties is a process called bulk chemical vapour deposition  (CVD) process. This process beta SiC is better than the normal processes that are usually used (sintering or reaction bonding).
The applications that can be used with this type of SiC are similar to its old applications but the difference is that it provides better quality in its performance.
This SiC can be used in a number of industries like the automotive industry, semiconductor processing industry, wear components such as pump seals, the mirror optics industry, information storage industry, and many other industries that are in demand of a tough and wear resistant material.

D- Green Silicon Carbide: -
 Green silicon carbide is an extremely hard manmade material that processes very high thermal conductivity. It is also able to maintain its strength at elevated temperatures (it is 7.5 times stronger alumina at these elevated temperature). Green silicon carbide doesn’t melt but it sublimes at ~ 2815.5oC.
PHYSICAL PROPERTIES
Hardness (Knoop) 2600
Hardness (Moh’s) 9.4
Melting point (oC) 2600
Specific gravity (gm/cm3) 3.2
Particle shape Blocky, sharp
Color  Green

TYPICAL CHEMICAL ANALYSIS
SiC 99.5%
SiO2 0.2%
Si 0.03%
Fe 0.04%
C 0.1%
Green silicon carbide is made be adding silica sand (to provide the silicon) and coke (to provide the carbon) and they are bonded together into very complex shapes.
General applications of green SiC are in aerospace, blasting, coatings, composites, refractories, compounds, kiln furniture, and is used as an abrasive as honing stones, lapping, polishing, sawing silicon and quartz, and in grinding wheels.

Ceramics serve a wide range of applications, from basic pottery to substrates in electronic packaging. Refractory materials, in turn, serve their own range of applications. For example refractory products are used in the linings of boilers, incinerators, glass furnaces, ceramic kilns and metallurgical processing vessels. Those refractory products which are used in aluminum, steel and other metallurgical applications are designed to meet very specific requirements for a wide variety of melting, transport, treatment and forming processes.

   "High technology" ceramics are new types of materials that surpass earlier ceramics in strength, hardness, light weight, or improved heat resistance. For example, ceramic powders can now be made from particles of absolutely uniform size. When sintered, these powders produce ceramics that are far less vulnerable to fracture or thermal shock than ordinary ceramics. Added to a matrix of metal or ceramic, thin ceramic fibers increase the tensile strength of the material. New super-hard ceramics make excellent cutting tools and bearings. Advanced Zirconia ceramic offers ideal solutions for many difficult applications where wear, abrasion, impact corrosion and high temperatures course conventional materials fail.

     A broad range of machinable and fully-dense ceramic materials based on ceramics like alumina, alumina-silicates, boron nitride, glass ceramics, magnesium oxide and zirconium phosphate are used for applications in which high temperature insulation, thermal shock resistance and high dielectric strength are required. Applications include many fields like aerospace in which they are used as gas nozzles, thermal insulators and space mirrors. In the automotive field they are used as diesel port liners, turbine nozzles, seals and shrouds. In the electrical field they are used as connector housings, heater supports and resistor supports. In the electronics field they are used as electrical insulators, vacuum tube structures, microwave housings and capacitor insulators. In the heat-treating field they are used as induction heat tubes, kiln furniture and hot forming dies. In the metallurgical field they are used as molten metal crucibles, troughs and thermocouple sheaths. In the petrochemical field, they are used as high-temperature corrosion and wear resistant components.
In the plastics field, they are used as hot die parts for thermoplastic equipment.
Alumina is used to manufacture high-strength alumina bolts, nuts and washers in various metric sizes. These fasteners are electrical insulators, non-magnetic and resistant to chemical corrosion and high temperature oxidation. They are ideal replacements for plastic, stainless steel and other exotic fasteners as they will not rust, seize or melt even in molten steel.
      Heat conductive ceramics like beryllium and silicon carbide are used to produce specific laser tube components. Ceramic components include bore gain segments, baffles, high voltage feed-thrust  ceramics, as well as brazed vacuum assemblies for cathode and anode envelopes. Ceramic seals are used in ion tubes.   Other applications for heat conductive ceramic components are heat sinks, heat exchangers, and other heat transmission devices.

      Potting and casting materials based on alumina, magnesia, silica, Zirconia and silicon carbide ceramics are used to fabricate high temperature parts and tooling, and encapsulate various types of electrical components which require high dielectric strength and volume resistivity. These materials offer unique properties with respect to operating temperatures, thermal conductivity and dielectric and mechanical strength. Hydraulic-setting ceramics are used in the production of small components such as temperature probes, electrical feed-thrust as well as large heat treating fixtures, induction coils and crucibles. Chemical setting ceramics are used to pot small devices such as temperature sensors, gas igniters and high intensity lights. Other applications for high temperature ceramic potting and casting materials in the electrical field are heating elements, PTC devices, ballast resistors, rheostat resistors, high-intensity lights and halogens. Applications in the metallurgical field are encapsulating RF coils, furnace carriers, sintering boats, heating element holders, welding jugs and standoffs.
      Alumina, zirconia, graphite, magnesia and silica are used to manufacture high temperature ceramic adhesives. They are used for bonding ceramics, metals, glass, graphites, textiles and composite materials used in design, process and maintenance applications. They are used in non-structural, high temperature applications and for coating, sealing and potting small components used throughout industry. These materials exhibit high thermal, chemical and electrical resistance. Typical applications in the electrical field are halogen bulbs, resistance heaters, fiber optics and gas igniters. In the mechanical field, they are used as catalytic converters, ceramic-to-ceramic bonding, ceramic-to-metal bonding, gasketing/sealing, radiant heaters, refractory and textile insulation, sagger plates and thread locking. Typical applications in the sensors and instruments field are gas chromatographs, high vacuum components, liquid metal inclusion counters, mass spectrometers, oxygen analyzers, strain guages and temperature probes.
     High temperature ceramic-metallic pastes are used to repair defects in cast aluminum, cast iron, steel and stainless steel. Formulated using the most advanced inorganic-ceramic technology, these advanced materials resist high-temperatures. Applications are widespread and found typically in the aerospace, automotive, foundry, heat treating, incineration and power generation industries. Typical applications are afterburners, boilers, castings, exhaust stacks, flanges, furnaces, headers, incinerators, manifolds, molds, dies and ovens.

     Alumina, alumina silicate and silicon carbide and other ceramics are used to manufacture ceramic fixtures for semiconductor applications like dicing blocks, clip and clamp rings, insulators and fixtures. Alumina is used to produce  ceramic sputtering targets which are used in wear coatings, dielectric coatings and barrier coatings. It is also used to produce ceramics fixtures for wafer processing and handling. Typical applications for these are dimensionally stable wafer handler arms/end effects, polishing blocks for wafer manufacturing, vacuum assisted wafer clamping, electrostatically assisted wafer clamping and mechanically assisted wafer clamping. Alumina is also used in manufacturing ceramics for semiconductor processing chambers  which are used in lithography, implementation, CVD, PVD and flat panel display. It is also used in manufacturing ceramics used in the field of lasers as it is used in plasma tubes, wave guides, large complex parts and cavities and heat plates. It is also used to manufacture ceramic lapping components and ceramic metallizing and glazing components.

     As industrial awareness of advanced materials increases, the number and variety of application areas for advanced ceramics increases rapidly. In general engineering field many thousands  of engineering components have benefited from advanced ceramic solutions for wear, corrosion and thermal resistance, providing considerable lifetime increases over conventional metal components. Although it is not always the optimum design solution, frequently, advanced ceramics can be used as direct replacements for existing designs.  Typical components include wear plates and thermal barriers, bearings for high speed and high stiffness spindles, bushes, gears and many others. As many highly-advanced ceramics are used as a direct replacement material for top of the range metals such as tool steels and stiletto, this allows more applications to emerge. Typical applications of advanced ceramic components are bearings, bushes,
Wear plates, drive shafts, gears, weld pins and valves. The advantages of this are that advanced ceramic components offer a more cost effective solution than a top of the range metal because of significant reductions in downtime are possible and reductions in spare part consumption.
      In the chemical and process industry, there is a never ending quest for increasing pumping efficiencies longer lifetimes and the ability to deal with more hazardous liquids and solids. Therefore, advanced ceramics are  playing an increasing role in this field. Advanced ceramics are now used as pump shafts, seats, bearing surfaces, gears and even complete pump bodies. Many of these components are found leading edge applications such as chemical industry valve sets or oil field flow control devices. In pumping components, the excellent erosive wear and corrosion resistance of Zirconia has led to their use in several pump components, typical parts subjected to high  stress, such as shafts, couplings or thrust plates. Centrifugal pumps are available which have all major parts in advanced ceramics materials, with Zirconia being used for the shaft, rotor cover and can. Typical application areas for such pumps are in sludge pumps and process pumps for the chemical industry. Advanced ceramic components are also used in petrochemical and process industries as valves, seats, nozzles etc..
Rotary seals is one of the first uses for technical ceramics like alumina in wear applications.  For rotary pumps the ceramic ring is fitted to a metal cup, then attached to the shaft, where it is spring loaded against a carbon-phenolic ring. The properties sought are sliding wear and corrosion resistance. Sintered silicon carbide is now being considered due to its longer lifetime, better acid resistance and pressure-velocity(p-v) ratio, however, the cost is often three times that of alumina.

    With steel-like strength, high hardness and an ultra-fine grain size, zirconias are finding an increasing number of applications as blade edges and cutting tools. In addition to the very clean cutting characteristics, they display much more greater lifetime than conventional materials.
The advantages of this are:
  1 . Reduction in downtime due to blade changeovers
  2 . Non-metallic
  3 . Non-magnetic
  4 . Highly corrosion resistant

      Advanced ceramics are finding an increasing applications within the electronics industry, whether it is as alumina substrates or Zirconia screwdrivers. Aluminas and Zirconia are frequently used in electronic applications as precision insulators and trimming rf capacitors. They are also use in making screwdrivers for the tuning of sensitive electronic devices. A wide range of advanced ceramic substrates are available in materials from alumina and aluminum nitride through to Zirconia, both fully and partially stabilized. A wide range of insulators are produces, many as ceramic to metal brazed assemblies.
    With ultra high melting points advanced ceramics are often the only material of choice for high temperature applications. The melting temperatures for alumina, Zirconia and silicon carbide respectively are 2050 C, 2700 C, 2300 C. The interatomic bonding of these materials provides an excellent platform for high temperature operations, well above the regime of super alloys or other metallic. Alumina is used to produce furnace roof hangers. These components operate as load bearing supports for furnace roof insulation. Zirconia tubes are used extensively in optical fiber production, both as inspection devices and for thermocouple protection. Tn these applications the material experiences temperatures in excess of 2000 C.

        Applications within the tube industry call for materials with good wear resistance and good surface properties, such that the quality of the manufactured product is not compromised by contact with the tooling. Seaming rolls, wire dies, guides and forming tools in alumina Zirconia and silicon nitride have recorded performances in terms of many thousands of hours and meters of metal. By a careful manipulation of microstructure and material selection for metal\ceramic combinations, specific solutions are frequently derived for individual components and application areas. They are used to produce bending and expanding tools which are highly resistant to fracture and have a good resistance to cold welding. These tools are used for bending and expanding aluminized steel tubes. The extended tool life helps to reduce set up and maintenance costs during the shaping process. They are used as mandrels for expanding copper tubes. These mandrels have an improved tool life and can lead to a reduction or even elimination of lubricants. They are also used to make welding rolls which are unaffected by inductive or magnetic fields during longitudinal seam welding of tubes. This reduces customary excessive roll wear. They are also used  to make metal extrusion dies which have an increased life of approximately ten to fifteen times over steel, including reduced stress cracking is claimed for aluminum and copper/brass extrusion using  these dies.

     Applications within  the wire industry call for materials with both good wear resistance and good surface properties, such that the quality of the drawn product is not compromised by contact with dies, cones, pulleys etc. these guides and rolls can outlast a conventional high speed steel by a factor of >20. Depending on the shape and complexity of the part the price penalty is rarely more than a factor of 4. Consequently the cost benefit analysis is very favorable, without taking account of the effect on machine downtime. They display bending strengths similar to the yield strength of low alloy steels. With  a sub micron grain size and near-zero porosity, these advanced ceramics can be finished to display the highest degrees of surface finish, polish and precision. Unlike the brittle behavior displayed by conventional alumina materials, these advanced ceramics can withstand severe impacts and mechanical shocks. In acid or alkali mediums, they display excellent resistance to the most hazardous of environments. Their advantages are as follows :
1)Minimize possible damage to the wire surface during drawing
2)Assurance of wire surface quality due to the improved sliding characteristics of ceramic materials.
3)Improved production reliability, especially for thin wires, due to the reduced adhesion between wire and pulley.
4)Reduced abrasion leading to improved quality and increased tool and die life.

     Zirconia  is used to produce milling media of a high density, high toughness and super hard media which has significantly better milling efficiencies compared with other media such as alumina or soda glass. Typical applications are high strength and high toughness products such as piezoelectric materials, magnetic materials and dielectric materials. They are also used to manufacture wear and corrosion resistant products like coatings, textile, pigment dispersions, ink and dyestuffs. They are also used in the manufacture of pharmaceuticals and food stuffs. They are also used in the manufacture of electronic ceramics, refractory ceramics and advanced ceramics.

 
  Over the last twenty years there has been a considerable increase in the use of ceramic materials for implant devices. Due to excellent combinations of strength and toughness together with bio-inert properties and low wear rates, some advanced ceramics are now displacing alumina in applications such as femoral heads for total hip replacements. Other applications which would benefit from a Zirconia implant include knee joints, shoulders, phalanges joints and spinal implants.

In slip casting, a suspension of ceramic powder, usually in water, is poured into a mold made of plaster of Paris. Water is absorbed by the mold, and a hard lining on the mold wall is built up; excess liquid is poured out of the mold. Using slip casting, a number of complex shapes can be made economically, since the cost of the molds is low.
 After forming, the ceramic ware must be carefully heated for a few hours at about 100°-200° C (about 200°-400° F) to remove excess water or binder. The rate of drying must be carefully controlled so that warping and defects do not form as the sample shrinks. After drying, the article is fired at a high temperature (800 °-2000 ° C / 1500 ° – 3500 ° F) to sinter or bind together the individual crystals of the ceramic powder into a solid, coherent mass. The higher the firing temperature, the more dense and less porous the material becomes. A wide range of properties in ceramics is possible with different firing temperatures and times. Many ceramic materials are harder than metals, and while brittle in tension, demonstrate great compressive strength.
In solid-state sintering, individual particles join together in an increasingly dense mass as continuos pores are formed, and finally only isolated pores remain.
Particles in the original powder lead to more rapid sintering. A more dense material
is formed at longer times and at higher temperature, since fewer and smaller pores remain after these treatments.
In hot pressing, a sample is heated to the firing temperature and pressed at the same time. This process is expensive because special dies, usually of graphite, are needed; but it allows production of materials that could be sintered only at much higher temperatures without simultaneous pressing.

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