Comment on Experiment Fundament of Veprek’s nc-TiN/a-Si3N4 Model and Its “Exceed Diamond Hardness”

doi:10.15541/jim20140313

Abstract

Abstract:

Since hardness exceeding that of diamond was reported by Vepreke, TiN/Si₃N₄ nanocomposite film has attracted 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 disappeared now.

Key words: TiN/Si3N4 films, super hardness, microstructure

CLC Number:

TQ174

LI Ge-Yang. Comment on Experiment Fundament of Veprek’s nc-TiN/a-Si₃N₄ Model and Its “Exceed Diamond Hardness”[J]. Journal of Inorganic Materials, 2015, 30(1): 1-8.

Figures/Tables 7

Fig. 1 Illustration of nanostructure and limited growth of a nanocrack in nanocrystalline/amorphous composite films[21]

Fig. 2 Low magnification (a) and high magnification (b) cross- sectional TEM images of ~5at% Si nanocomposite film[26]

Fig. 3 Cross-sectional HRTEM image of TiN/Si3N4 multilayers (a) and hardness of TiN/Si3N4 vs thickness of Si3N4 (b)[29]

Fig. 4 Hultman microstructural model of nanocomposite films[33],Si3N4 tissue is amorphous (lSi3N4>0.7 nm)(a) and part of tissue forms epitaxial pseudo-crystal (lSi3N4≤0.5-0.7 nm)(b)

Fig. 5 A typical HRTEM plane-view image of nanocomposite coatings [26]

Fig. 6 Illustrations of a planar TiN/Si3N4 interface in pure (a) and oxygen contaminated (b) systems[50]

Fig. 7 Two indentation SEM images of ultra-hardness composite films provided by Veprek. The hardness is only 51 GPa[58] compares with the claimed 138.4 GPa[8] (a) and the hardness is over 100 GPa[27] (b) computed by the scale on the SEM images

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Comment on Experiment Fundament of Veprek’s nc-TiN/a-Si₃N₄ Model and Its “Exceed Diamond Hardness”

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