Structure, hardness and mechanical properties of magnetron-sputtered titanium-aluminium boride films

Abstract

TiAlB films nominally 2 μm thick were produced by simultaneous sputtering from TiAl and TiB2 targets onto Si-(110) and AISI316 stainless steel substrate materials at a temperature of 170°C. The influence of different B/Al ratios on the structure, hardness and mechanical properties of coatings containing 32 at.% Ti was investigated, over a range of chosen compositions between B/Al=1 and B/Al=10. All coatings showed a very dense microstructure, and evidence of Al, Al3Ti, Ti and TiB2 phase-bonding was observed in X-ray diffraction and X-ray photoelectron spectroscopy analyses of selected coatings. Correlation of compositional and mechanical data showed that film hardness approximately trebled (from 12 to 35 GPa) and elastic modulus approximately doubled (from 185 to 340 GPa) with increasing B/Al ratio over the selected range. Reciprocating-sliding wear tests with both SAE52100 and WC-6% Co counterface materials indicated that the lowest coating wear occurred at B/Al ratios greater than 2, although the wear rate increased slightly at B/Al ratios greater than 4, indicating a gradual reduction in coating toughness as the boron content exceeded 50%. All coatings with B/Al≥2 exhibited lower wear than untreated AISI316 substrates and particularly in the case of the WC-6% Co counterface material (for which the contact forces were higher), they were significantly better than commercial TiN of a similar thickness. These results demonstrate the advantages which relatively low modulus hard coatings can provide – particularly on softer substrate materials which provide limited load support.