Molybdenum Boride Cermets. The molybdenum borides MoB and Mo 2 B have less thermal stability than the previously discussed metal borides, but their electrical properties, hardness, and wear resistance are very good. When cemented with nickel, these cermets have excellent corrosion resistance, for example, to dilute sulfuric acid (Ref 79). Nickel-bonded molybdenum boride exhibits interesting behavior in two areas: First, if the composition corresponds to the compound molybdenum-nickel boride (Mo 2 NiB 2 ), if the cermet contains Mo 2 B in addition to Mo 2 NiB 2 , or if a low-melting, intermetallic binder containing chromium boride and nickel is used, cutting tool materials can be produced from the composition that are comparable to commercial WC tool tips for machining brass, aluminum, and cast iron (Ref 90, 91). Second, the Mo 2 NiB 2 -type composition has thermal expansion characteristics that closely match those of the refractory metals and a favorable melting temperature; these properties make it ideal for use as a high-temperature braze for molybdenum and tungsten, without risk of excessive grain growth or embrittlement of the primary metal structure (Ref 79, 92). When used in rod form with shielded arc welding equipment, this cermet is suitable for brazing electronic components in applications such as vacuum tubes and magnetrons. Recently, a molybdenum boride cemented with an iron-base binder phase alloyed with nickel and chromium has shown promise as a cutting tool material (Ref 93). This cermet exhibits good mechanical properties coupled with excellent wear and corrosion resistance. In specific tool applications, such as extrusion dies for hot copper and tools for can making, this boride cermet has performed better than cemented carbides. The role of nickel in the Mo 2 FeB 2 cermet and the effect of varying its content up to 10 wt% in the Fe-5B-44.4Mo composition have also been investigated, mainly as part of a study of the corrosion resistance potential of the material (Ref 94). The nickel enters only into the iron-base binder phase, which changes with increasing nickel content from ferritic to martensitic to austenitic. The martensitic binder phase at 2.5% Ni gives the cermet a transverse rupture strength of 2.24 GPa (325 ksi) and a hardness of 86.9 HRA. References cited in this section 75. L. Kaufman, E.C. Clougherty, and J.B. Berkowitz- Mattuck, Oxidation Characteristics of Hafnium and Zirconium Diboride, Trans. AIME, Vol 239 (No. 4), 1967, p 458-466 76. R. Kieffer and F. Benesovsky, Hartmetalle, Springer-Verlag, 1965, p 475-479 77. R. Steinitz, Borides Part B: Fabrication, Properties and Applications, in Modern Materials, Vol 2, Academic Press, 1960, p 191-224 78. C.E. Halcombe, Jr., "Slip Casting of Zirconium Diboride," Report Y- 1819, U.S. Atomic Energy Commission, 28 Feb 1972 79. J.L. Everhart, New Refractory Hard Metals, Mater. Methods, Vol 40 (No. 2), Aug 1954, p 90-92 80. J.D. Latva, Selection and Fabrication of Ceramics and Intermetallics, Met. Prog., Vol 82 (No. 4), Oct 1962, p 139-144, 180, 186 81. L. Kaufman and E.V. Cl ougherty, Investigation of Boride Compounds for High Temperature Applications, Metals for the Space Age, Springer-Verlag, 1965, p 722-758 82. L. Kaufman and E.V. Clougherty, "Investigation of Boride Compounds for Very High Temperature Applications," Report RTD-TDR-63-4096, Part 1, U.S. Air Force Materials Laboratory, Dec 1963 83. E.V. Clougherty, R.L. Pober, and L. Kaufman, Synthesis of Oxidation Resistant Metal Diboride Composites, Trans. AIME, Vol 242 (No. 6), 1968, p 1077-1082 84. E.V. Clougherty et al., "Research and Development of Refractory Oxidation Resistant Diborides," Report AFSC-ML-TR-68-190, U.S. Air Force Materials Laboratory, Part 1, Oct 1968; Part 2, Vol 1-7, Nov 1969- June 1970; Part 3, May 1970 85. H.M. Greenhouse, R.F. Stoops, and T.S. Shevlin, A New Carbide-Base Cermet Containing TiC, TiB 2 and CoSi, J. Am. Ceram. Soc., Vol 37 (No. 5), 1954, p 203-206 86. E.T. Montgomery et al., "Preliminary Microscopic Studies of Cermets at High Temperatures," U.S. Air Force Report WADC-TR-54-33, Part 1, April 1955, Part 2, Feb 1956 87. Gradient Ceramic/Metals Made by Advanced Methods, Adv. Mater. Proc., Vol 132 (No. 4), Oct 1987, p 20 88. S.J. Sindeband, Properties of Chromium Boride and Sintered Chromium Boride, Trans. AIME, Vol 185, Feb 1949, p 198-202 89. I. Binder and D. Moskowitz, "Cemented Borides," PB 121346, Office of Technical Services, U.S. Department of Commerce, 1954-1955 90. R. Steinitz and I. Binder, New Ternary Boride Compounds, Powder Metall. Bull., Vol 6 (No. 4), Feb 1953, p 123-125 91. I. Binder and A. Roth, An Evaluation of Molybdenum Borides as Cutting Tools, Powder Metall. Bull., Vol 6 (No. 5), May 1953, p 154-162 92. A. Blum and W. Ivanick, Recent Developments in the Application of Transition Metal Borides, Powder Metall. Bull., Vol 7 (No. 3-6), April 1956 93. K. Takagi, S. Ohira, T. Ide, T. Watanabe, and Y. Kondo, New P/M Iron- Containing Multiple Boride Base Hard Alloy, in Modern Developments in Powder Metallurgy, Vol 16, Metal Powder Industries Federation, 1985, p 153-166 94. K. Takagi, M. Komai, T. Ide, T. Watanabe, and Y. Kondo, Effect of Ni on the Mechanical Properties of Fe, Mo Boride Hard Alloys, Int. J. Powder Metall., Vol 23 (No. 3), 1987, p 157-161 Other Refractory Cermets The nitrides, carbonitrides, and silicides of certain transition metals have gained importance for specific uses in operations involving high temperatures. The main mode of application for these refractory cermets, however, is in the form of coatings, such as TiN and TiC-TiN in various ratios for high-speed cutting tools or MoSi 2 for surface protection of molybdenum against high-temperature oxidation. In a very few cases, these compounds are used as solids, either in the pure state or cemented with a lower-melting metallic phase. Carbonitride- and Nitride-Based Cermets. Titanium nitrides and titanium carbonitrides have been found suitable for use as the hard phase for tool materials (Ref 95). The best binder is an alloy of 70Ni-30Mo, and optimum hardness, in the 1000 to 2000 HV range, is obtained with 10 wt% binder. The hardness increases progressively with the TiC component of the solid solution. The same trend prevails for the hardness of a cermet containing 14 wt% binder: The values increase from about 1400 to 1900 HV for the straight cemented TiC composition. Transverse rupture strength does not follow any trend; the best values reach about 1300 MPa (188 ksi) for a 10 wt% binder composition with a 72-to-18 TiN-TiC ratio and a 14 wt% binder material with a 69-to-17 TiN-TiC ratio. This compares with 1070 and 1275 MPa (155 ksi and 185 ksi), respectively, for the straight TiC cermets with 10 and 14 wt% binder. The hardness of titanium nitride alone cemented with 10% of the 70Ni-30Mo alloy has a hardness level of about 1050 HV and a transverse rupture strength of about 785 MPa (115 ksi). Titanium carbonitride cermets for tool applications are discussed in greater detail in the section "Titanium Carbonitride Cermets" in this article. Combinations of nitrides and borides, with or without metallic binder, can also be fabricated into tools. A mixture of 60 wt% tantalum nitride (TaN) and 40 wt% ZrB 2 has been hot pressed into tool bits that have performed very well at very high cutting speeds (Ref 96). Nitride products based on the metalloids boron and silicon, like their carbide counterparts, have gained some significant commercial uses since their early development in the 1950s and 1970s, respectively. The normal hexagonal crystal lattice of boron nitride (BN) can be converted to a cubic crystal form by reacting boron powder with nitrogen at a minimum temperature of 1650 °C (3000 °F) while simultaneously applying pressure in excess of 7000 MPa (1000 ksi) with the aid of special press tools adopted from the manufacture of synthetic diamond. The product is extremely hard and is considered to be one of the best electrical insulators known, especially at high temperatures up to about two-thirds of its melting point, that is, in the vicinity of 2730 °C (4950 °F) (Ref 45, 97). Cermets exhibiting excellent cutting performance have been achieved by bonding carefully graded particles of the superhard cubic boron nitride with cobalt or similar hard metal binders. Hot pressing is the preferred method of powder consolidation, and tool bits made in this manner outperform tungsten carbide tips by a factor of two-to-one and better (Ref 98). The nitride of silicon and its combination with different oxides, notably Al 2 O 3 (known as the SiAlONs), as well as the different silicon ceramics based on silicon carbide, belong to the increasingly important new class of refractory materials known as structural ceramics. Additives of these cermets are nonmetallic and serve mainly to control the sintering mechanism. They do not contribute to a strengthening of the hard particle structure in the sense of a metallic binder. In fact, they cause a weakening of the grain-boundary network at high temperature in many systems. Therefore, these silicon ceramics are considered to lie outside the material classification for cermets. Silicide Cermets. The metallic silicides have found commercial use only in isolated instances. This is due chiefly to the extreme brittleness of these compounds and to the concomitant problems encountered when they are fabricated into solid objects. Because of its outstanding high-temperature oxidation resistance, and its favorable coefficients of thermal expansion and electrical resistance, molybdenum disilicide (MoSi 2 ) is an important material for heating elements. Poor resistance to mechanical and thermal shock is the major deficiency of molybdenum disilicide and limits the applications of this material to simple cylindrical or rectangular shapes. Additions of metallic elements to remedy this handicap have been only partially successful, and MoSi 2 cermets with nickel, cobalt, and platinum binder metals are still too brittle for fabrication into complex shapes (Ref 99). High-temperature bearings have been made experimentally by infiltrating molten silver into hard matrices containing MoSi 2 , tungsten disilicide (WSi 2 ), or vanadium disilicide (VSi 2 ); these bearings have shown good antifriction behavior against steels at elevated temperatures (Ref 100). Graphite- and Diamond-Containing Cermets. Materials that contain a combination of carbon in the form of graphite or diamond with metals constitute a border region for cermets and are usually not designated as such. However, because the carbon and metallic components are most often intimately mixed and uniformly distributed in the microstructure, they are pertinent to this discussion. Graphite-metal combinations for electrical contact applications basically fall into two types of materials. For metallic brushes used in motors and generators, the metallic phase consists of copper or bronze; in the case of sliding contacts involving relatively low rubbing speeds and light contact pressure, the metallic phase is silver. In brushes, the graphite particle content may spread over a wider range, from 5 to 70 wt%. A typical binary composition contains 70% Cu and 30% graphite. To improve wear and bearing properties, many brushes also contain up to 10% Sn and/or Pb and up to 12% Zn (Ref 101). The graphite content in the silver contact composition generally ranges between 2 and 50 wt%. Graphite-containing metallic friction materials for brake linings and clutch facings have a predominantly metallic matrix to utilize a high thermal conductivity. This property permits rapid energy absorption, making this type of material suitable for service under a more severe wear and temperature environment than that which is possible for organic, resin-bonded asbestos friction elements. The most important contribution of a cermet-type lining material in aircraft brakes probably has been an increased energy capacity without additional weight or the use of a larger unit (Ref 102). The friction coefficient of these cermets is tailored to the requirements of the particular application, principally by varying the ratio of a friction-producing ceramic to the graphite, which acts as a solid lubricant. The metallic matrix phase is essentially a bearing alloy containing 60 to 75 wt% Cu and 5 to 10% each of tin, lead, zinc, and/or iron. Graphite content falls within the 5 to 10% range, and the ceramic, mainly SiO 2 with the possibility of some Al 2 O 3 additions, amounts to 2 to 7% (Ref 103). Cermets composed of diamond, varying in size from coarse splinters to fine dust inside a metal matrix, are used for grinding, lapping, sawing, cutting, dressing, and trueing tools. The size of the diamond is important for the efficiency of the tool; although finish improves as the grain or grit size becomes finer, the cutting speed is slower. For dressing tools, 5 to 35 diamond splinters are embedded per carat with a size of approximately 1 to 2.5 mm (0.04 to 0.1 in.). For rough grinding, the grit size is in the range of 0.15 to 0.5 mm (0.006 to 0.02 in,); for fine polishing, it falls between 0.05 and 0.15 mm (0.002 and 0.006 in.). Even finer diamond powder is used in combination with tungsten carbide for specialized applications such as polishing plane surfaces of hard metal tools or finishing the rolls for Sendzimir-type mills. Typical compositions of these tools contain 12 to 16 wt% diamond dust embedded in a tungsten carbide matrix cemented with 13% Co (Ref 104). Other metallic bonding substances are based on copper, iron, nickel, molybdenum, or tungsten. Examples for copper matrices are bronzes with 10 to 20% Sn or 2 to 4% Be, which can be strengthened by precipitation hardening, and a 47Cu-47Ag-6Co alloy. Bonding metals suitable for somewhat higher-temperature service include iron-nickel, iron-nickel- chromium, and iron-tin-antimony-lead alloys; Permalloy; and nickel alloys containing 2 to 8% Be. Refractory metal-base matrices are alloys of the molybdenum-copper, molybdenum-cobalt, or tungsten-nickel-copper types and tungsten-nickel- iron heavy alloys (Ref 104). In general, the bond materials must be selected with consideration of lowest possible processing temperatures to avoid the possible transformation of the diamond to graphite. References cited in this section 45. B.C. Weber and M.A. Schwartz, Container Materials for Melting Reactive Metals, in Cermets, Reinhold, 1960, p 154-158 95. R. Kieffer, P. Ettmayer, and M. Freudhofmeier, About Nitrides and Carbonitrides and Nitride- Based Cemented Hard Alloys, in Modern Developments in Powder Metallurgy, Vol 5, Plenum Press, 1971, p 201-214 96. F.C. Holtz and N.M. Parikh, Developments in Cutting Tool Materials, Eng. Dig., Vol 28 (No. 1), 1967, p 73, 75, 99 97. Borazon Man Made Material Is Hard as Diamond, Mater. Methods, Vol 45 (No. 5), 1957, p 194, 196 98. N.J. Pipkin, D.C. Roberts, and W.I. Wilson, Amborite A Remarkable New Cutting Material from De Beers, Ind. Diamond Rev., June 1980, p 203-206 99. R. Kieffer and F. Benesovsky, Hartmetalle, Springer-Verlag, 1965, p 487-489 100. R.H. Baskey, An Investigation of Seal Materials for High Temperature Applications, Trans. Am. Soc. Lub. Eng., Vol 3 (No. 1), 1960, p 116-123 101. F.V. Lenel, Powder Metallurgy, Metal Powder Industries Federation, 1980, p 556 102. R.H. Heron, Friction Materials A New Field for Ceramics and Cermets, Ceram. Bull., Vol 34 (No. 12), 1955, p 295-298 103. F.V. Lenel, Powder Metallurgy, Metal Powder Industries Federation, 1980, p 485 104. C.G. Goetzel, Treatise on Powder Metallurgy, Vol 2, Interscience, 1950, p 171-174 Superabrasives and Ultrahard Tool Materials T.J. Clark, G.E. Superabrasives; and R.C. DeVries, G.E. Corporate Research and Development Center (Retired) Introduction THE PRINCIPAL superhard materials are found as phases in the boron-carbon-nitrogen-silicon family of elements (Fig. 1). Of these, the superhard materials of commercial interest include silicon nitride (Si 3 N 4 ), silicon carbide (SiC), boron carbide (B 4 C), diamond, and cubic boron nitride (CBN). Silicon nitride provides the base composition for the important category of SiAION ceramics, which are used in structural applications (see the article "Structural Ceramics" in this Volume) and as high-speed cutting tool materials (see the article "Ceramics" in Machining, Volume 16 of ASM Handbook, formerly 9th Edition Metals Handbook). Fig. 1 The carbon-boron-nitrogen- silicon composition tetrahedron showing the principal known superhard materials: the diamond form of carbon, cubic BN, SiC, and B 4 C. Polycrystalline aggregates of diamond and SiC as well as Si 3 N 4 are also commercially available. The carbides of the metalloids boron and silicon (B 4 C and SiC in Fig. 1) are also of considerable industrial significance and enjoy such diverse applications as superhard tools and electrical resistor heating elements. These compounds are processed and used both with or without metallic binder phases. When these two metalloid carbides are used without a metallic binder phase, the resultant material most likely falls within the material group of ceramics. If silicon carbide (SiC) and boron carbide (B 4 C) are used with a metallic binder phase, then the resultant material is considered a cermet (see the article "Cermets" in this Volume). This article focuses exclusively on the superhard materials consisting of either diamond or CBN. The other commercially significant materials in Fig. 1 are discussed in the above-mentioned articles of ASM Handbook. Additional information on the superhard nitrides and carbides can be found in Ref 1 and 2. Information on possible new hard materials is available in Ref 3. The focus of this article is further restricted to synthesized diamond and CBN. The latter does not occur in nature, and the former commands 90% of the industrial diamond market. These materials will be treated in terms of the forms in common use: diamond or CBN grains (looser or bonded) and sintered polycrystalline diamond or CBN tools. References 1. Ceram. Bull., Vol 67 (No. 6), 1988 2. P. Schwarzkopf and R. Kieffer, Refractory Hard Materials, Macmillan, 1953 3. A.Y. Liu and M.L. Cohen, Prediction of New Low Compressibility Solids, Science, Vol 245, 1989, p 841-842 Synthesis of Diamond and Cubic Boron Nitride The basic objective in the synthesis of diamond and CBN is to transform a crystal structure from a soft hexagonal form to a hard cubic form. In the case of carbon, for example, hexagonal carbon (graphite) would be transformed into cubic carbon (diamond). Synthetic CBN and diamond are produced either as crystalline grains or as sintered polycrystalline products. The synthesis of CBN or diamond grit can be achieved by static high-pressure high-temperature (HPHT) processing or by dynamic (explosive) techniques. The HPHT method, despite high equipment investment costs, is the predominant technique for producing synthetic diamond and CBN. In addition, diamond is also synthesized under metastable conditions (see the section "Low-Pressure Synthesis of Superhard Coatings" in this article). High-Pressure High-Temperature Synthesis. The bulk of synthetic CBN and diamond is made by subjecting hexagonal carbon or boron nitride to high temperatures and high pressures with large special-purpose presses or with the commonly used mechanical device known as the uniaxial belt (Ref 4). By the simultaneous application of heat and pressure, hexagonal carbon or boron nitride can be transformed into a hard cubic form. This requires strenuous pressures and temperatures, as illustrated in the graphite-diamond and hexagonal BN-cubic carbon nitride equilibrium diagrams (Fig. 2, 3). Fig. 2 Pressure- temperature diagram showing the stability regions of diamond and graphite and the role of the solvent/catalyst in lowering the synthesis conditions Fig. 3 Equilibrium diagram for HBN and CBN It is possible to directly convert graphite to diamond, but very high pressures are required, and the properties of the resultant product are difficult to control. In commercial practice, the required conditions for diamond synthesis can be reduced by the use of solvent/catalysts such as nickel, iron, cobalt, and manganese or alloys of these metals (Ref 5, 6). Figure 4 shows an example of a metal-carbon system at 5.7 GPa (57 kb), where a stable diamond plus liquid region exists. Even with solvent/catalysts it is necessary to simultaneously sustain a pressure of about 5 GPa (50 kb)and a temperature of about 1500 °C (2700 °F), for periods ranging from minutes to hours, to make the variety of products in common use today. Fig. 4 Nickel- carbon system at 5.7 GPa (57 kb) showing the stability regions of diamond (d) and graphite (g) in equilibrium with liquid (l + d, l + g). a, austenite. Source: Ref 7 Conditions are similar for the synthesis of CBN, but the reactants are usually alkali, alkaline earth metals, or compounds. Cubic boron nitride can be grown from a variety of solvent/catalysts, including metal systems similar to those used for diamond synthesis (Ref 8). Because the pressure-temperature conditions for the conversion of hexagonal boron nitride (HBN) to CBN are less severe than those for the conversion of graphite to diamond, some sintered polycrystalline products are synthesized by the direct process under static conditions; however, most commercial monocrystalline CBN is made by a solvent/catalyst process. Explosive Shock Synthesis. The direct conversion of graphite to diamond, or HBN to CBN, can be done on a commercial scale using explosive shock techniques (Ref 9). The process is relatively simple but produces only fine-grain materials, which are principally used as polishing powders or as possible source materials for sintering into polycrystalline products. Low-Pressure Synthesis of Superhard Coatings. The history of diamond synthesis under metastable conditions (plasma- assisted, chemical vapor deposition, or physical vapor deposition coating processes) goes back at least to the late 1950s and perhaps even earlier. The efforts of Russian (Ref 10) and Japanese (Ref 11) scientists in the period from 1975 to 1985 made this technique feasible for limited commercial applications. The potential exists to make films or sheets of polycrystalline and single-crystal diamond at temperatures of about 900 °C (1650 °F) and at pressures of less than 1 atmosphere (0.1 MPa). A limited amount of information exists (Ref 12) on grinding or machining applications of these materials. Some films have been made for x-ray windows, speaker diaphragms, and wear surfaces. Synthesis of Polycrystalline Diamond and Polycrystalline Cubic Boron Nitride. It is possible also to produce polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN) by sintering (or binding) many individual crystals of diamond or CBN together to produce a larger polycrystalline mass. It is commercial practice to enhance the rate of sintering by the addition of a metal second phase (Ref 13). In addition, the whole mass must again be maintained in the cubic region of the respective temperature-pressure phase diagram to prevent the hard cubic crystals from reverting to the soft hexagonal form. By such high-temperature high-pressure sintering techniques, it is possible to obtain a mass of diamond or CBN in which randomly oriented crystals are combined to produce a large isotropic mass. An immense range of polycrystalline products can be made of diamond or CBN. Changes in grain size, the second phase employed, the degree of sintering, the particle size distribution, and the presence or absence of inert ceramic, metallic, or non-metallic fillers are examples of factors that have profound effects on the mechanical, physical, and thermal properties of the final product. By careful formulation it is possible to tailor material properties for particular applications. References cited in this section 4. H.T. Hall, Ultra-High-Pressure, High-Temperature Apparatus: The "Belt," Rev. Sci. Instrum., Vol 31 (No. 2), 1980, p 125-131 5. H.P. Bovenkerk, F.P. Bundy, H.T. Hall, H.M. Strong, and R.H. Wentorf, Jr., Preparation of Diamond, Nature, Vol 184, 1959, p 1094-1098 6. R.J. Wedlake, Technology of Diamond Growth, in The Properties of Diamond, J. Field, Ed., Academic Press, 1979 7. H.M. Strong and R.E. Hanneman, Crystallization of Diamond and Graphite, J. Chem. Phys., Vol 46, 1967, p 3668-3676 8. R.C. DeVries and J.F. Fleischer, Phase Equilibria Pertinent to the Growth of Cubic Boron Nitride, J. Cryst. Growth, Vol 13/14, 1972, p 88-92 9. P.S. DeCarli, Method of Making Diamond, U.S. Patent 3,238,019, March 1966; and P.S. DeCarli and J.C. Jamieson, Formation of Diamond by Explosive Shock, Science, Vol 133, 1966, p 1821-1822 10. B.V. Spitsyn, L.L. Bouilov, and B.V. Derjaguin, Vapor Growth of Diamond on Diamond and Other Surfaces, J. Cryst. Growth, Vol 52, 1981, p 219-226 11. S. Matsumoto, Y. Sato, M. Tsutsumi, and N. Setaka, Growth of Diamond Particles from Methane- Hydrogen Gas, J. Mater. Sci., Vol 17, 1982, p 3106-3112 12. B. Lux and R. Haubner, Low Pressure Synthesis of Superhard Coatings, Int. J. Refract. Met. Hard Mater., Vol 9, 1989, p 158-174 13. R.H. Wentorf, Jr. and W.A. Rocco, Diamond Tools for Machining, U.S. Patent 3,745,623, July 1973 Properties of Diamond The crystal structure of diamond and the lattice structure of graphite are shown in Fig. 5. The conversion from graphite to diamond is accompanied by a 26% decrease in volume. For diamond, all the lattice sites are occupied nominally by carbon, but boron and nitrogen can be substituted for carbon in amounts in the parts-per-million range. Synthesized diamond usually has metal, metal carbide, and graphite inclusions; however, some of the metal may be on defect or interstitial sites and thus may not be visible. Fig. 5 Arrangement of carbon atoms in diamond and graphite. The arrows indicate the transformation of graphite to diamond at HPHT conditions and the reverse transformation at low pressures and high temperatures (LPHT). Diamond oxidizes in air above about 600 °C (1100 °F) and back converts into a poorly graphitized form (as indicated by the reverse arrow in Fig. 5) upon heating in the absence of air. The reaction rate for the transformation back into graphite is dependent on conditions, but it is a significant factor at temperatures about 750 °C (1400 °F) in many practical applications. These phenomena impose critical limitations on the use and fabrication of bonded-abrasive tools. Diamond is chemically inert to inorganic acids, but upon heating it reacts readily with carbide-forming elements such as iron, nickel, cobalt, tantalum, tungsten, titanium, vanadium, boron, chromium, zirconium, and hafnium. Controlled reactivity is important in forming metal bonds, but that same reactivity can limit the use of diamonds in cutting and grinding applications. The thermal conductivity of some near-perfect diamond crystals can be as high as 5× that of copper at room temperature. Less-perfect materials still have a high conductivity, and this has to be taken into consideration before use in many applications. In terms of electrical conductivity, diamond is an electrical insulator unless doped with boron or, as in some commercial materials, mixed with a metal phase. Diamond is the hardest practical material known (Table 1). The hardness of single-crystal diamond varies as a function of orientation, but this is important only in single-point tools and in the polishing of gemstones, diamond microtome blades, and diamond surgical knives. Table 1 Properties of selected hard materials Density Compressive strength Coefficient of thermal expansion Thermal conductivity g/cm 3 lb/in. 3 Hardness, HK GPa 10 6 psi mm/mm/°C × 10 -6 in./in./°F × 10 -6 W/m · K cal/°C · cm · s Diamond (C) 3.52 0.127 7000-10000 10 1.5 4.8 2.7 2100 5.0 Cubic boron nitride 3.48 0.126 4500 7 1 5.6 3.1 1400 3.3 Silicon carbide (SiC) 3.21 0.116 2700 1.3 0.19 4.5 2.5 42 0.10 Alumina oxide (Al 2 O 3 ) 3.92 0.142 2100 3 0.435 8.6 4.8 33 0.08 Tungsten carbide (WC- Co, 6%) 15.0 0.542 1700 5.4 0.78 4.5 2.5 105 0.25 Although hard, diamond is a brittle material and breaks on impact, primarily by cleavage on the four (111) planes. Toughness or friability can be varied considerably for synthesized grains. Thus, it is possible to make a very friable material for some grinding operations and a very tough material for stonecutting. The presence of defects and second phases can be manipulated to influence the fracture properties of synthesized diamond. This control is not available from most natural stones. Most natural diamonds are essentially octahedral in shape as grown. Irregularly shaped fragments can be obtained by crushing and selection. Synthesized diamond can be reproducibly grown as cubes, cubooctahedrons, and octahedrons (Fig. 6). The cubooctahedral shapes are generally preferred for stone sawing, but they are not always appropriate for grinding. [...]... information on tool fabrication and applications is available in the article "Ultrahard Tool Materials" in Machining, Volume 16 of ASM Handbook, formerly 9th Edition Metals Handbook Polycrystalline diamond tool blanks are useful in the machining of nonferrous and nonmetallic materials (Tables 4 and 5) and are commercially available in a variety of shapes and sizes (Fig 12) An important variable for the... applications and surface finish Hardness, toughness Capstans and draw blocks, pulleys and sheaves, guides, rolls, dies Pulp and paper High-speed paper manufacturing Abrasion and corrosion resistance Slitting and sizing knives, stock-preparation equipment Machine tool and process tooling Machine components and process tooling Hardness, high stiffness-toweight ratio, low inertial mass, and low thermal... monolithic ceramics and ceramic-ceramic composites Chemically, structural ceramics include oxides, nitrides, borides, and carbides Many processing routes are possible for structural ceramics and are important because the microstructure, and therefore the properties, are developed during processing General properties and uses of structural ceramics are reviewed first Ceramic processing is described and the relationship... 0.1-0.3 0.004-0. 012 Sintered tungsten carbide 20-40 65-130 0.15-0.25 0.006-0.010 Aluminum alloys Polycrystalline cubic boron nitride (PCBN) tool blanks are useful in the machining of iron, steel, and cobalt- and nickel-base alloys (Tables 4 and 7) This makes them complementary to rather than competitive with the PCD tool blanks They are generally not recommended for use with superalloys or steels that... iron Superalloys Nickel-base superalloys Cobalt-base superalloys >35 HRC Stellite, AiResist, Haynes Cr, W Yes No Yes No Iron-base superalloys >35 HRC A-286, Incoloy Cr, Ni, Mo Yes No Yes No Carbide/Oxide-base materials >35 HRC UCAR LA-2, LC-4 Al2O3, Cr2O3 WC No Yes No Yes Metal-base materials >35 HRC Stellite, Hastelloy Mo, Ni, Cr, Co, Fe Yes No Yes No Sand or permanent cast alloys Mold cast alloys 40-145... described and the relationship of processing, microstructure, and properties presented Specific structural ceramic materials, including composites, are presented This article concludes with a discussion of future direction and problems with structural ceramics Uses and General Properties of Structural Ceramics Industrial uses, required properties, and examples of specific applications for structural ceramics... shapes, and qualities (Table 2) Diamond or CBN grains can be used as loose abrasives, as bonded abrasives in grinding wheels and hones, and as bonded abrasives in single-point applications such as turning tools, dressers, and scribes Loose Abrasive Grains Lapping and polishing constitute two major applications of both natural and synthetic loose abrasive grains Of the synthetic abrasive powders, alumina and. .. control, dimensional control, excellent wear properties, and high strength The fine-grain microstructure and good mechanical properties lend the Y-TZP as a candidate material for knife-edge applications, including scissors, slitter blades, knife blades, scalpels, and so forth However, compared to Mg-PSZ, Y-TZP is more expensive, has a lower fracture toughness, and is not nearly as flaw tolerant There are... such as for the sawing of marble, granite, and concrete These grains are in the 20 to 60 mesh size (850 to 250 μm) range Nearly all of the common resin bonds are thermosetting resins, and most of the thermosetting resins are phenolic resins Resin powders and a solvent, such as furfural, are mixed with superabrasive particles and a filler, such as silicon carbide, and then placed in a mold containing the... Polishing As with lapping, aluminum oxide and silicon carbide are widely used synthetic abrasives for polishing They are harder, more uniform, longer lasting, and easier to control than most natural abrasives Aluminum oxide grains are very angular and are particularly useful in polishing tougher metals, such as alloy steels, high-speed steels, and malleable and wrought iron Silicon carbide is usually . Aluminum alloys Sand or permanent cast alloys Mold cast alloys 40-145 HB A356, A390 Si, Cu, Mg No No No Yes Die cast alloys 65 -125 HB A360, 380, 390 Si, Cu, Zn No No No Yes Wrought alloys. blanks are useful in the machining of nonferrous and nonmetallic materials (Tables 4 and 5) and are commercially available in a variety of shapes and sizes (Fig. 12) . An important variable for the. Ceramics" in this Volume) and as high-speed cutting tool materials (see the article "Ceramics" in Machining, Volume 16 of ASM Handbook, formerly 9th Edition Metals Handbook) . Fig. 1 The