HARD COATING MATERIALS

Compounds and Properties

Hard coating materials can be divided into three categories, depending on the nature of the bonding. The first includes the ionic hard oxides of Al, Zr, Ti, etc. Next are the covalent hard materials exemplified by the borides, carbides, and nitrides of Al, Si, and B, as well as diamond. Finally, there are the metallic hard compounds consisting of the transition metal borides, carbides, and nitrides. Typical mechanical and thermal property valúes for important representatives of these three groups of hard materials are Usted in Table 12-1. The reader should be aware that these data were gathered from many sources (Refs. 1-6) and that there is wide scatter in virtually all reported property valúes. Differences in processing (e.g., CVD, PVD, and sintering of powders), variations in structure (e.g., grain size, porosity, density, defects) and composition (e.g., metal-nonmetal ratio, purity), to-gether with statistical error in measurement, contribute to the uncertainties. Perusal of this tabulated information leads to the following broad conclusions:

1. All of these compounds have extremely high hardnesses. This can be appreciated by noting that heat-treated tool steel has a hardness of about Hu = 850. Hardness is the most often quoted material property of hard coatings. Therefore, Section 12.3 has been specially reserved for an extensive discussion of the concept of hardness, the technique of measure-ment, and the significance of its magnitude in coatings.

2. These compounds have very high melting points and decomposition temper-atures. For example, the decomposition temperatures of TaC, HfC, and diamond exceed the melting point for tungsten (MP = 3410 °C).

3. The modulus of elasticity is lowest for the ionic solids. In comparison, only the stiffest metáis have modulus valúes overlapping those of the oxides Usted.

4. The linear thermal expansión coefficient generally increases in going from the covalent to metallic to ionic hard compounds. Metáis tend to have thermal expansión coefficients that are higher by approximately a factor of two or more than these hard compounds.

5. The thermal conductivity of the hard metallic and covalent compounds is comparable to that of the transition metáis and their alloys. Good metallic electrical conductors have proportionally higher thermal conductivities. Ceramic oxides are the poorest thermal conductors.

The last two properties have important implications for the properties and performance of coatings. An important source of coating residual stress is the thermal contribution generated by the difference in expansión between coating and substrate. The illustrative problem dealing with TiC on steel is worth reviewing and indicates the magnitude of possible effects.

The susceptability to cracking of coatings subjected to varying temperature histories is an important limitation to the performance of thermal coatings. To see how thermal cracking can occur, consider the rapid cooling of a high-tem-perature component. The surface coating contracts more than the interior, which is still relatively hot. As a result, the surface forces the interior into compression and is itself stretched in tensión.