A study of ion-beam-deposited B-C-N ternary system

Abstract

In this study, we concentrated on two important groups of materials in the boron-carbon-nitrogen (B-C-N) ternary system. They are (i) cubic BN and C-doped BN films, and (ii) boron and B-rich BNX films. Both of them were deposited by dual ion beam deposition. Materials of the first group are envisaged because they may be in a cubic phase constructed of strong covalent tetrahedral bonds, and so have very high hardness. Materials of the second groups are constructed of network of B-based icosahedral atomic clusters with great rigidity. Thereby all of them are of primary interest and application potential for being used as surface protective coating materials. For the C-doped BN films, the C content is varied from 3.0 to 11.8 at.%. Films with low C contents (<= 6,2 at.%) are dominated by the cubic zinc-blende structure, and so have very high hardness and elastic modulus. For the film with the lowest C content (3.0 at.%) as an example, H and E are as high as 41.6 and 303.7 GPa, respectively. However, the film peels off quickly after deposition due to the existence of high internal stress. Associated with the high hardness, the critical load of damage of the film under scratching is the highest (> 50 mN). The friction coefficient u depends on the normal load L in the form of u, = a Lb, where a and b are constants. Because of the high internal stress, the scratch track continues to extend and propagate after it is generated by scratching. Increase in the incorporation of C has the effect of lowering the volume fraction of the cubic phase, while a hexagonal graphite-like phase grows instead. As a consequence, both H and E drop, and finally reach 8.6 and 98.8 GPa at a C content of 11.8 at.%. The adhesion of the films on substrates is greatly improved, because the internal stress in these films is lowered. A carbon content of about 7 at.% is found to be appropriate for a film to have both satisfactory mechanical properties and good adhesion on substrate. Accompanied by the reduction of internal stress, the scratch track produced on a film with high C content propagates much slower as time proceeds. For the B-rich BNX films, the N content was increased from 4.1 to 40.2 at.%. The films were found to be solid admixtures of a B-rich phase constructed of icosahedral atomic clusters, and a graphite-like boron nitride phase. The fraction of the B-rich phase drops with increasing N content, and that of the graphite-like boron nitride phase grows instead. The hardness of the films rises first, and reaches the maximum value of 18.8 GPa at a N content of 20.2 at.%. The hardness-to-modulus ratio (H/E) and the critical load of the films also increase, indicating stronger wear resistance of the films. This result is explained by assuming that some N-B-N chains are formed and located interstitially among icosahedral clusters. The chains enhance the cross-linking between the icosahedral clusters producing a higher hardness of the films. If the N content is further increased (> 20.2 at.%), the film hardness drops accordingly as a result of the increase in the fraction of the graphite-like boron nitride phase. These films are found to be more insulating, because of the higher electrical resistivity of the graphite-like structure. The friction coefficient u of all these films depends on the normal load L in nanoscratch tests in the form of u = a Lb, where a and b are constants. The internal stress of the films is found to increase monotonically with increasing N content.