Lubricity of zinc oxide thin films: Study of deposition parameters and Si as an additive
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J.S. Zabinski Air Force Research Laboratory, Wright Patterson Air Force Base, OH 54533 (Received 5 March 2001; accepted 10 September 2001)
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 °C, decreasing to 0.13 around 500 °C.
I. INTRODUCTION
Oxides have good potential as high-temperature lubricants due to their inherent chemical stability at elevated temperatures. Numerous attempts have been made to design and apply oxides of refractory metals as solid lubricants.1–3 However, these materials generally perform poorly at room or low temperature. Thin film engineering at the nanoscale has been suggested earlier to tame the natural brittleness of oxides. Gleiter suggested nanoscale crystallinity as a possible mechanism to increase plasticity.4 Schiotz and coworkers demonstrated that plastic deformation in nanocrystalline ceramics could be due to grain boundary sliding with only slight dislocation in the grains.5 Nanostructured materials have played a paramount role in innovative research in the last decade. Researchers have demonstrated that nanocrystals and structures have unique properties that may be controlled by selection of particle size and manipulation of defects.6 –8 In most cases, doping gives rise to point defects that change electrical or optical properties. However, under favorable energetic or thermodynamic conditions, these point defects
can transform into linear or surface defects. Some point defects can migrate by normal diffusion and segregate to grain boundaries. Changes in the chemistry and structure of the grain boundaries may affect mechanical and tribological characteristics. Zinc oxide has a hexagonal crystal structure and open crystal structure, which permit
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