Mechanical, tribological, and stress analyses of ion-beam-deposited boron-rich boron nitride films with increasing N content

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-r ich phase constructed of icosahedral atomic clusters and a graphitelike bor on nitride phase. The films with N content less than or equal to 20.3 at.% is dominated by the B-rich phase. Their hardness rises with increasing N co ntent 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 stro nger 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-ri ch phase, which cross-link different icosahedral atomic clusters in the B-r ich phase and strengthen the rigidity of the structure. For the films with higher N contents, the volume fraction of the graphitelike boron nitride ph ase becomes higher, and the hardness drops as a consequence. However, the c hange in the H/E ratio is rather mild. This implies that the wear resistanc e of the films is not altered and explains why the critical load of the fil ms remains almost unchanged. In addition, the friction coefficient mu of al l the films depends on the normal load L in the form of mu = aL(y), where a and y are numerical parameters and an insensitive to the change in the N c ontent. 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.%.