Surface chemistry-controlled tribological behavior of silicon and diamond

Abstract

SEM tribometric experiments were performed with Si(100) vs. Si(100) interfaces in mode-rate vacuum to 850°C. The results are compared with similar tests previously completed with fine-cauliflowered PCD (PCDfcf) mated against itself, and polished C(100)-textured polycrystalline diamond (PCDC(100)) sliding against Si(100). All data agree with a hypothesis connecting the thermal desorption of adsorbates and wear with the generation of dangling bonds on the sliding surfaces. Linking of the counterfaces by the free radicals appears to be the main cause of high adhesion and friction. The high friction can be drastically reduced by dissociative chemisorption of certain passivating gaseous species condensing at sufficiently low surface temperatures. Strong circumstantial evidence continues to mount for the incremental reduction in high temperature friction being caused by surface reconstruction. Deconstruction of the sliding surfaces and the reemergence of high friction eventually occurs on discontinued heating, until the adsorbates chemisorb on the cooled surfaces. There, the friction drops to a level determined by the characteristic shear strength of the interfaces and the wear-induced increase in the real area of contact. The maximum friction measured at high temperatures in vacuum, indicative of the most intensive interaction of dangling bonds at the interface, scaled only approximately with the 1.8 times strength of the C-C versus the Si-Si bonds. The 1.6 experimental ratio is lower than the theoretical, reflecting the broad distribution of dangling bond energies (densities of surface trap states) for PCD and even for polished Si(100). The wear rate of Si(100) sliding against itself is about four-orders-of-magnitude higher (~ 2 × 10-12 m3/(Nm)) than that of unpolished PCDfcf vs. itself (4 × 10-16 m3/(Nm)) or rough and unpolished PCDC(100) wearing its polished version (8.5 × 10-16 m3/(Nm)).