Comment on Experiment Fundament of Vepreks nc-TiN/a-Si$lt;inf$gt;3$lt;/inf$gt;N$lt;inf$gt;4$lt;/inf$gt; Model and Exceed Diamond Hardness

Abstract

Since hardness exceeding that of diamond was reported by Vepreke, TiN/Si3N4 nanocomposite film has at- tracted much attention from the scientific community and resulted in a ballooning number of publications over the past 15 years. This paper commented on the Veprek's microstructure model and superhardness of this kind of films from experimental fundamental aspects. In term of the microstructure, the model of nc-TiN/a-Si3N4 proposed by Veprek, in which equiaxed nanocrystalline TiN (nc-TiN) were embedded in an amorphous Si3N4(a-Si3N4) matrix, was short of sufficient experimental evidence. Direct transmission electron micrographs observations showed that the TiN nanocrystals still had a columnar morphology instead of equiaxed morphology. Si3N4 interface tissue had a thickness of about 0.5-0.7 nm and existed in the crystalline state. Low-energy coherent interfaces are formed between Si3N4 and neighbouring elongated TiN grains. As far as the preparation technology, the hardness exceed diamond has not been repeated by others in this material up to now. Veprek attributed it not only to the lack of a sufficiently high temperature and partial pressure of nitrogen, but also to the unavoidable oxygen in preparation. However, those critical conditions were imposable to satisfy in technology. For the samples, although Veprek declared their films received the hardness exceed diamond (up to 138.9 GPa), they have not confirmed by any others, and moreover, these samples have disap-