Investigation of silicon wear against non-porous and micro-porous SiO2 spheres in water and in humid air

Tribochemical wear, a method to achieve controlled material removal without residual damage on substrates, plays a very important role in super-smooth silicon surface manufacturing. By using nonporous SiO2 spheres and micro-porous SiO2 spheres, the wear of silicon substrates was comparatively investigated in DI water, humid air (50% RH) and dry air. The wear behaviors presented entirely different cases at the same load in DI water and humid air: (a) less material removal of silicon against non-porous SiO2 spheres than micro-porous SiO2 spheres in water and (b) more serious wear of silicon substrate against non-porous SiO2 spheres in humid air. When the wear tests were operated in dry air, no obvious damage was incurred on the silicon surface against the non-porous SiO2 spheres but slight wear was observed against micro-porous SiO2 spheres under the given conditions. Raman results revealed that a hydrolysis reaction was involved in the tribochemical wear of the silicon substrate and the micro pores in SiO2 spheres could accelerate this process. The corresponding analysis suggests an exponential dependence of wear rate on contact stress, which is consistent with the stress-assisted chemical kinetics model. Although with much lower elastic modulus, micro-porous SiO2 spheres caused a larger wear rate of silicon substrate than non-porous SiO2 spheres at the same contact pressure both in water and humid air. The results indicate that the micro-porous SiO2 spheres can promote the tribochemical reaction due to the storage of water molecules in micro pores.