Mechanical, tribological, and stress analyses of ion-beam-deposited boron-rich boron nitride films with increasing N con
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Mechanical, tribological, and stress analyses of ion-beam-deposited boron-rich boron nitride films with increasing N content K.F. Chan, C.W. Ong,a) and C.L. Choy Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, People’s Republic of China
R.W.M. Kwok Department of Chemistry, The Chinese University of Hong Kong, Shatin, Hong Kong, People’s Republic of China (Received 1 February 1999; accepted 26 July 1999)
Boron (B) films and B-rich BNx films with different N contents (4.1–40.3 at.%.) were deposited by dual ion-beam deposition. The films consist of a B-rich phase constructed of icosahedral atomic clusters and a graphitelike boron nitride phase. The films with N content 艋20.3 at.% is dominated by the B-rich phase. Their hardness rises with increasing N content to reach a maximum value of 18.8 GPa. The hardness-to-elastic modulus ratio (H/E) and the critical load of the films also increase, showing stronger wear resistance of the films. These results can be explained if some N–B–N chains are formed at the interstitial sites in the network of the B-rich phase, which cross-link different icosahedral atomic clusters in the B-rich phase and strengthen the rigidity of the structure. For the films with higher N contents, the volume fraction of the graphitelike boron nitride phase becomes higher, and the hardness drops as a consequence. However, the change in the H/E ratio is rather mild. This implies that the wear resistance of the films is not altered and explains why the critical load of the films remains almost unchanged. In addition, the friction coefficient of all the films depends on the normal load L in the form of ⳱ aLy, where a and y are numerical parameters and are insensitive to the change in the N content. Furthermore, compressive stress was found to increase from about 0.12 to 1.7 GPa when the N content increased from 4.1 to 40.3 at.%.
I. INTRODUCTION
Boron (B)-rich solids generate a class of superhard materials constructed of a network of B-based icosahedral atomic clusters. For instance, a rhombohedral unit cell of ␣-rhombohedral B (␣–B) comprises B icosahedral clusters at its vertices. A boron icosahedral cluster is constructed of 12 B atoms located at the vertices of an icosahedron (denoted as B12).1 Based on this framework, a number of binary B-rich solids are generated by adding a second type of atoms (e.g., C, N, or O) into boron.2– 6 For example, B4C contains C-B-C chains located at the interstitial sites surrounded by icosahedral atomic clusters. The chains cross-link the clusters and result in a greater hardness. A similar cross-linking effect is proposed to occur in B-rich boron nitride by N–B–N chains, so that the material is also expected to be very hard.2,3,6
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http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 10, Oct 1999 Downloaded: 17 Mar 2015
However, the mechanical and tribological properties of B-ri
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