Re(de)construction-induced friction signatures of polished polycrystalline diamond films in vacuum and hydrogen

Abstract

Pin-on-flat SEM tribometry was performed with polished, mostly C(100)-textured and acid-cleaned polycrystalline CVD diamond films heated to 950°C then cooled to room temperature. Testing in~1.33×10-3 Pa = 1× 10-5 Torr vacuum was followed by similar experimentation in 13 to 40 Pa (0.1to 0.3 Torr) partial pressures of 99.999%-pure H2. In vacuum, all tests showed the characteristic step function-with-trough coefficient of friction (COF) signatures previously hypothesized as footprints of wear- and thermal desorption-induced generation, re(de)construction and passivation of the danglingσ bonds on the interacting surfaces. In hydrogen, all wear tracks exhibited stepfunction-like COF curves caused by adsorbate de(re)sorption on heating and cooling. A distinct re(de)construction COF trough obtained at the highest temperatures could be duplicated during repeated sliding in the same track on a large number (but not all) of the wear paths. The repeatable, incremental reduction in COF at the onset of heating and its substantial reduction on final cooling are attributed to tribocatalytically enhanced dissociative chemisorption of molecular hydrogen. The wear rates of the polished diamondon the pin tip, as controlled by the progressively reduced unit stresses caused by the enlargement of the wear scar, are between3.9×10-16 and 2.6×10-16m3/(N m) in PH 2, in good agreement with previous data.