Silicon carbonitride, a new hard material and its relation to the confusion about ‘harder than diamond’ C 3N 4

Abstract

The claim by Cohen (Science 261 (1993) 307) that powerful computational tools allow us to predict ‘properties of substances even before we have created them’ was made in conjunction with the claim of special properties for a hypothetical phase, C 3N 4. Among such properties was hardness, and it was asserted that, the covalent form of C 3N 4 could be ‘harder than diamond.’ This assumption contradicted what chemists have known since 1816 in their experimentation with carbon nitrides. Never was there a single hint of the existence of a covalent, single bond C–N network. In the last decade some 400 papers have been written about this non-existent material of dubious significance (R.C. DeVries, Mater. Res. Innovat. 1 (1997) 161). No C 3N 4 material has ever been made and the claims on both the chemical composition and crystal structure are clearly in error. The impact of such exaggerated claims on the scientific enterprise cannot be ignored. In contrast, we report herein on a related but real hard material, silicon carbonitride, with the α-Si 3N 4 crystal structure modified by the introduction of carbon atoms. Synthesis of this Si–N–C crystalline material was possible by using a CH 4/H 2/N 2 microwave plasma etching of solid Si. Films on Si, SiC, Si 3N 4 and diamond, as well as crystal agglomerates of a few mm 3 volume, have been prepared. This phase possesses a micro-hardness lower than cubic boron nitride and a band gap of 3.8 eV. The present experiments indicate that only 6 at.% of C have been incorporated into α-Si 3N 4. We might suggest that ‘first principles’ calculations be undertaken to explain the limited solubility of carbon in the α-Si 3N 4 phase.