Lubricity of zinc oxide thin films: Study of deposition parameters and Si as an additive

Zinc oxide preferentially crystallizes into a wurzite structure and has a unique set of properties. There have been numerous studies on doped zinc oxide thin films as an optical coating or as a semiconductor material. However, very little work has been reported on its tribological properties. Recent reports from this laboratory revealed that ZnO has good potential for controlling friction and wear. ZnO has an open structure and favorable coordination number, which permits zinc to freely move to different positions in the crystal lattice and to accommodate external atoms as substitutes. The nature of the substitution and the concentration of Zn interstitials may be used to control tribological performance. In this work, thin films of zinc oxide were deposited by pulsed laser ablation while silicon was added simultaneously by magnetron sputtering. The effects of deposition geometry and oxygen partial pressure on stoichiometry and microstructure were evaluated. It was found that the angle of deposition and oxygen partial pressure control coating texture. Depositions normal to the sample surface, along with 10 mtorr of oxygen, produced strong (002) texture. These conditions were selected for Si-doping studies. The tribological characteristics of Si-doped coatings were evaluated at both room and high temperature. Addition of Si around 7-8% gave a coefficient of friction of about 0.2 at 300 degreesC, decreasing to 0.13 around 500 degreesC.