Role of sliding-induced tribofilms on fracture of particles in aluminium-silicon alloys

Mechanical properties of tribofilms formed on the surfaces of silicon particles in Al-Si alloys when placed in sliding contact against a steel counterface at 100 °C under boundary lubrication condition were studied using instrumented nano-indentation techniques and cross-sectional TEM. With an average thickness of ∼82 nm, the tribofilms had a hardness of 3.45±0.17 GPa and an elastic modulus of 116.30±6.64 GPa. Deformation and fracture characteristics of tribofilm-covered silicon particles were investigated using micro-indentation techniques and compared with those without tribofilms. The increase in Weibull modulus, m, from 1.2 (for silicon particles without tribofilm) to 4.3 (for silicon covered with tribofilm) showed that the tribofilm decreased the probability of lateral crack propagation that caused chipping fracture in silicon. The extent of pressure-induced phase transformation of silicon particles with a diamond cubic crystal structure to Si-III (bcc) and Si-XII (rhombohedral) was reduced due to a decrease in the 'effective' elastic modulus of tribofilm-covered silicon. It was also shown that silicon's R-curve behaviour (lateral crack stability) was improved due to reduction in the driving force that was available for crack growth in tribofilm-covered silicon.