Grain size effect on self-mated CVD diamond dry tribosystems

Chemical vapour deposited (CVD) diamond coatings are important for tribological applications due to their unique combination of properties. Previous work demonstrated that silicon nitride (Si3N4) excels as a substrate for diamond coatings due to its low thermal expansion coefficient mismatch relative to diamond films resulting in a significant adhesion improvement. In this study, dense Si3N4 substrates were fabricated by pressureless sintering and diamond coated by microwave plasma chemical vapour deposition (MPCVD). The deposition time varied between 1 and 10 h in order to investigate the effect of the diamond grain size and film thickness on the tribological behaviour of self-mated CVD diamond coatings on Si3N4. Reciprocating dry sliding ball-on-flat (BOF) wear tests were performed in air up to 16 h, at room temperature, with normal applied load ranging from 10 to 105 N. The stroke and frequency of the sliding motion were kept constant with values of 6 mm and 1 Hz, respectively. Several characterisation techniques (scanning electron microscope (SEM), atomic force microscope (AFM), micro-Raman) were used to identify the prevailing surface damage mechanisms. After a very short running-in regime, with high friction coefficients, a steady-state regime is reached, characterized by extremely low friction values (μ ∼ 0.03). A mild wear mode was achieved for the longer runs, with wear coefficient values around 10−8 mm3 N−1 m−1. The larger grain sized and thicker coatings present smaller compressive residual stresses (below 1 GPa) due to a better in-depth accommodation of the contact pressure. This delays film delamination to much higher applied loads (105 N) than the thinner, small grain sized coatings, grown for 1 h that fail in sliding under 35 N of normal load.