NEW DEVELOPMENTS IN THE FIELD OF SUPERHARD COATINGS

As a result of covalent bonding, the four elements boron, carbon, nitrogen and silicon can form superhard materials (Vickers hardness of more than 4000), such as diamond, highly tetrahedral amorphous carbon (ta-C and ta-C: H), crystalline and amorphous silicon carbide, cubic boron nitride and, possibly, the hypothetical carbon nitride beta-C3N4. Lately, fast progress has been achieved in chemical vapour deposition of diamond for coating of tools, as well as in the low temperature preparation of ta-C, ta-C:H, a-SiC:H and c-BN. This second group of superhard materials is produced by subplantation of energetic particles (of the order of 100 eV) into the initially amorphous deposits. This leads to high stress values and high thermal spike temperatures close to the surface. Depending on the mobilities of the atoms in the network and on the quenching temperature (about 10(14) K s(-1)), the material may form nanocrystallites (c-BN) or remain amorphous (ta-C), with clustering of sp(3) and sp(2) bondings. However, coatings with more than 93% c-BN have been produced by sputtering of h-BN and argon ion plating at room temperature, without using an ion beam arrangement. Various attempts to prepare and identify beta-C3N4 are reported. For this material, a higher hardness is predicted than that for diamond.