NITRIDES

Nitrides are less stable than the oxides, carbides and sulfides, and their use in air at elevated temperature is limited because of their tendency to oxidize. However, in several instances, the oxide film is protective and deterioration is slow. Despite their limitations, nitrides have interesting properties and are sure to find many specialized uses as technology becomes more complex.

Stable Nitrides Aluminum Nitride

Aluminum nitride is conveniently prepared by an electric arc between aluminum electrodes in a nitrogen atmosphere. Crucibles of the pressed powder, sintered at 1985°C, are resistant to liquid aluminum at 1985°C, to liquid gallium at 1316°C, and to liquid boron oxide at 1093°C. Aluminum nitride has good thermal shock resistance and is only slowly oxidized in air (1.3% converted to Al2O3 in 30 h at 1427°C). It is inert to hydrogen at 1705°C but is attacked by chlorine at 593°C.

Aluminum nitride is an excellent substrate for creating wide-band-gap semiconductors for wireless communications and power-industry applications. Since aluminum nitride withstands very high temperatures, this substrate material can be used for microelectronic devices on jet engines. Such substrates also would improve the production of blue and ultraviolet lasers that could be used to squeeze a full-length movie onto a CD. Aluminum nitride crystals have also been grown in a tungsten crucible at 2300°C.

Boron Nitride

The crystal structure of boron nitride is similar to that of graphite, giving the powder the same greasy feel. The platy habit of the particles and the fact that boron nitride is not wet by glass favors use of the powder as a mold wash, e.g., in the fabrication of high-tension insulators. It is also useful as thermal insulation in induction heating.

Boron nitride can be hot-pressed to strong ivory-white bodies that are easily machined. Hot-pressed boron nitride is stable in air to about 704°C. From 704 to 982°C the rate of oxidation increases moderately. It is also stable in chlorine up to 704°C. However, at 982°C it is attacked rapidly.

Like commercial graphite, hot-pressed boron nitride is anisotropic. Thermal expansion parallel to the direction of pressing is ten times that in the perpendicular direction. The ratio for modulus of rupture is 2:1.

Although boron nitride resembles graphite in many respects, it differs uniquely in electrical characteristics, having high resistivity and high dielectric strength even at elevated temperatures (see Table N.4). This feature, combined with easy machinability has led to extensive use in high-temperature electronics.

Boron nitride is available as -325-mesh powder.

A cubic form of boron nitride (Borazon) similar to diamond in hardness and structure has been synthesized by the high-temperature, high-pressure process for making synthetic diamonds. Any uses it may find as a substitute for diamonds will depend on its greatly superior oxidation resistance.

TABLE N.4

Properties of Boron and Silicon Nitrides Compared with Graphite and Alumina

Property	Boron Nitride	Graphite	Silicon Nitride	Alumina
Melting point, °F				3722
Specific gravity
Crystal			3.18	3.96
Body			1.5-2.7	2.6-3.9
Hardness, DPHa			1100	2800
Modulus of rupture (room temp.), 1000 psib
			1-20	38
Coefficient of thermal expansion (avg at 70-1800°F)/°F x 106b
			1.37	4.3
Thermal conductivity (room temp.), Btu-in./ft2 h/°Fb
			130	20-30
Electrical resistance, ^-cm
At room temp.			1013	1016
At 900°F			1013	1012
Dielectric constant			9.4	12.3

Cutting tool materials like mixed ceramics or CBN cutting tools are already available for hard machining. To identify the proper cutting tool material, one must analyze the application.

In case of cutting interruptions, CBN cutting tools will be the appropriate choice. Continuous cuts allow the use of mixed ceramics or coated mixed ceramics for better efficiency. When producing a gear wheel, for example, turning with a CBN-tipped insert reduces the cost per wheel by more than 60%, compared to grinding. At the same time, disposal costs for grinding sludge vanish, because hard turning does not require coolant.

Silicon Nitride

Silicon nitride is most easily prepared by direct reaction of nitrogen at about 1316°C with finely divided elemental silicon (<150 mesh), either as loose powder or as a slip-cast or otherwise preformed part. Conversion of silicon particles to the nitride Si3N4 is accompanied by the growth of a felt of interlocking needles in the void space between particles. Despite an overall porosity of 15 to 25%, silicon nitride bodies are effectively impervious in many applications because of the microscopic size of the pores.

Although silicon nitride is not machinable in its final form except by grinding, the partially converted body can be machined by conventional methods after which conversion can be completed without dimensional change.

Silicon nitride is indefinitely resistant to air oxidation up to 1649°C, but begins to sublime at about 1925°C. It is not attacked by chlorine at 899°C or hydrogen sulfide at 982°C nor by the common acids. Because of a low coefficient of thermal expansion, resistance to thermal shock is relatively good.

Uncoated and coated silicon nitride cutting tools dominate the high-performance end of gray cast iron machining. They typically offer metal removal rates at least three times higher than coated carbide grades.

The newly developed cutting tool combines a 6% cobalt substrate with a 10-^m-thick, medium-temperature TiCN/Al2O3/TiN coating. Medium-temperature chemical vapor desposition TiCN coatings show a reduced tendency for the forming of eta-phase at the interface between coating and carbide substrate.

Recently developed silicon nitride cutting tools have a substantially improved fracture resistance. Because of their insufficient chemical wear resistance, however, they have a limited use in machining nodular cast irons, mainly in areas of severe cutting interruptions at higher speeds (>400 m/min).

Titanium and Zirconium Nitride

Titanium and zirconium nitrides for use in refractory bodies are most conveniently prepared by treating the corresponding metal hydrides with ammonia at 1000°C.

Sintered TiN can be heated to a bright red heat with only superficial oxidation, and then plunged into water without cracking; ZrN is less resistant to oxidation.

Combination coatings involving both chemical vapor deposition and physical vapor deposition technologies provide the wear-resistance advantages of chemical vapor deposition TiCN coatings with the compressive residual stress advantages of physical vapor deposition TiN coatings. The net result is improved wear and chipping resistance. These coatings increase the speed capabilities of carbide cutting tools in titanium turning by a factor of 2.

Eliminating coolants can turn an easy-to-machine material into a difficult drilling problem, when using standard cutting tools. The introduction of TiAlN coatings represents a significant step toward dry drilling.

The goal in all dry machining is to develop cutting tools with higher resistance to thermal load and fatigue. Cermet tools may be one of the most suitable materials for these applications.