Hardest carbide

Hardness. Table 6.5 shows that carbides of Group VI are not as hard as the carbides of Groups IV and V (see Ch. 4, Table 4.6 and Ch. 5, Table 5.6). This may reflect the lower strength oftheirM-C bonds. The hardness decreases with temperature as shown in Fig. 4.6 (Ch. 4). Alone among the refractory carbides, WC retains its hardness up to approximately 800°C and, above 400°C, it is the hardest carbide. 

The introduction of carbon vacancies produces a marked reduction in the microhardness in the Group 4 cubic carbides. A maximum has been observed in the TaC phase and if the latter behavior is typical of Group 5, the high value for VCq.ss (see Table 80) could result because the phase boundary composition happens to fall near the maximum hardness value. If NbC has a maximum, a composition near NbCo.9 could be the hardest carbide. Clearly, there are theoretical and practical reasons to pursue this observation further. 

When a steel is cooled sufficiendy rapidly from the austenite region to a low (eg, 25°C) temperature, the austenite decomposes into a nonequilihrium phase not shown on the phase diagram. This phase, called martensite, is body-centered tetragonal. It is the hardest form of steel, and its formation is critical in hardening. To form martensite, the austenite must be cooled sufficiently rapidly to prevent the austenite from first decomposing to the softer stmeture of a mixture of ferrite and carbide. Martensite begins to form upon reaching a temperature called the martensite start, Af, and is completed at a lower temperature, the martensite finish, Mj, These temperatures depend on the carbon and alloy content of the particular steel. 

Cementite, the term for iron carbide in steel, is the form in which carbon appears in steels. It has the formula Fe C, and thus consists of 6.67 wt % carbon and the balance iron. Cementite is very hard and britde. As the hardest constituent of plain carbon steel, it scratches glass and feldspar, but not quart2. It exhibits about two-thirds the induction of pure iron in a strong magnetic field, but has a much lower Curie temperature. 

Carbides and diamond [7782-40-3] are imbedded as small particles in a binder and are used to cut tooth stmcture. Diamond chips are the hardest and most effective abrasive for tooth enamel. 

Tantalum carbide (TaG) is one of the hardest substances known. This compound represents its oxidation state of +4 for tantalum. 

Why diamond works so well is because of its two most desirable properties, namely. (I) it is the hardest substance known to science, and t2) it is very- transparent to optical radiation as well as to x-rays. Compared to diamond, tungsten carbide, which was used in older pressure generating devices, has a much lower compressive strength and, further, it is opaque to radiation. 

Titanium carbide, TiC, is made by the action of carbon black on titanium dioxide at 2000 °C. It is the most important hard metallic material after tungsten carbide, and in fact is the hardest of all the metal carbides with a hardness rating of 9 on the Mohs scale - diamond is 10. In itself it is too brittle to be used pure but when mixed with the carbides of tungsten, tantalum and niobium it delivers great strength. 

Boron forms a compound with carbon, B C. This substance, boron carbide, is the hardest substance known next to diamond, and it has found extensive use as an abrasive and for the manufacture of small mortars and pestles for grinding very hard substances. 

Aluminum is a constituent of many minerals, including clay (ka-olinite), mica, feldspar, sillimanite, and the zeolites. Some of these minerals are discussed under the chemistry of silicon, in Chapter 31. Aluminum oxide (alumina), occurs in nature as the mineral corundum. Corundum is the hardest of aU naturally occurring substances except diamond it scratches all other minerals, but is itself scratched by diamond, and also by the artificial substances boron carbide, and silicon carbide, SiC. Corundum and impure corundum (emery) are used as abrasives. 

Titanium carbide is by far the hardest material available for the treatment of surfaces. 

Boron also forms important compounds with two other elements, carbon and nitrogen. Boron carbide (B4C) and boron nitride (BN) are important compounds because of their hardness. In fact, boron nitride may be the hardest substance known. Both compounds have very high melting points 4,262°F (2,350°C) for boron carbide and more than 5,432°F (3,000°C) for boron nitride. 

Boron carbide (B4C) is one of the hardest known materials with excellent properties of low density, very high chemical and thermal stability, and high neutron absorption cross-section. Bulk B4C is conventionally synthesized by high temperature (up to 2400 °C) reactions, such as the carbothermal reduction of boric acid or boron oxide. Nanocrystalline B4C was solvothermally synthesized in CCI4 at 600 °C (Reaction (32)). 

Silicon carbide, or carborundum, SiC, is one of the hardest substances known and is used as an abrasive. It has the structure of diamond with half of the carbons replaced by silicon. It is prepared industrially by reduction of sand (Si02) with carbon in an electric furnace. 

Titanium carbide, TiC, is the hardest of the known metal carbides. It can be made by heating titanium(IV) oxide, Ti02, with carbon black to 2200 °C. (Carbon black is a powdery form of carbon that is produced when vaporized heavy oil is burned with 50% of the air required for complete combustion.)... 

Sodium borohydride is marketed in powdered or pellet form, and in solution, for use in fuel cells. Boron nitride can withstand temperatures of up to 650°C (1,202°E) when subjected to high pressures and temperatures, it forms cubic crystals whose hardness rivals that of diamond. Boron carbide, produced by reacting coke and boric acid at 2,600°C (4,712°E), is a highly refractory material and one of the hardest substances known. It has both abrasive and abrasion-resistant applications, and is used in nuclear shielding, see ALSO Davy, Humphry Gay-Lussac, Joseph-Louis Nuclear Chemistry. 

In the absence of experimental thermochemical evidence about the strength of the metal-carbon bonds in metal carbonyl carbide systems, we can turn to the binary compounds formed between transition metals and carbon for information about the last point, the strength of metal-carbon bonds to core carbon atoms. Transition metal carbides are important. They include, in substances such as tungsten carbide, WC, some of the hardest substances known, and the capacity of added carbon to toughen metals has been known since the earliest days of steel-making. Information about them is, however, patchy. They are difficult to prepare in stoichiometric compositions of established structure and thermochemistry the metals we are most interested in here (osmium, rhenium, and rhodium) are not known to form thermodynamically stable binary phases MC and the carbides of some other metals adopt very complicated structures. Enough is, however, known about the simple structures of the carbides of the early transition metals to provide some useful pointers. 

Nitrides and carbides are also considered among the hardest materials. Table 5.2 shows data for the measured Vickers microhardness for these compounds. Such measurements are performed using a diamond indenter with square geometry. The indenter is forced towards the surface of the material and the diagonal of the microindentation is measured. In all cases, carbides and nitrides are significantly harder than the pure metals and are also comparable or superior to that of ceramic materials. 

Silicon carbide, SiC, is one of the hardest materials known. Surpassed in hardness only by diamond, it is sometimes known commercially as carborundum. Silicon carbide is used primarily as an abrasive for sandpaper and is manufactured by heating common sand (silicon dioxide, Si02) with carbon in a furnace. 

Boron carbide is among the hardest materials yielding only to diamond and boron nitride. It is also one of the most corrosion-resistant compounds at room or moderate temperatures. When considering the corrosion resistance of boron carbide materials, it is important to remember that they are rarely stoichiometric, with the carbon content varying from 9.88 to 23.4% [96] Many of them contain free carbon or sintering aids. Thus their behavior depends on the chemical composition. 

Besides diamond and cBN, the well known boron carbide B4C is among the hardest materials and has been comprehensively reviewed by F. Thevenot [103], In the present chapter, the latest developments concerning the binary and ternary systems B-N, boron carbide nitrides (B-C-N), and boron suboxides are discussed. Other hard materials based on boron are described by R. Telle et al. in Part III. 

---