Nanocomposite layers of ceramic oxides and metals prepared by reactive gas-flow sputtering

Abstract

Thin nanocomposite layers of ceramic oxides in conjunction with a second metallic phase were prepared by reactive gas-flow sputtering. The two nanogranular phases were dispersed into each other by co-deposition from two separate targets. TiO₂ and Al₂O₃ were chosen as ceramic phase, respectively, which was obtained by sputtering one of the metals and reacting with oxygen. In order to investigate the influence of a hard and a ductile metal, either tungsten or copper was chosen as the second metallic phase. Highly compact thin films could be prepared as was revealed by SEM. A low hydrogen content of 1–2 at.% was measured by SIMS indicating a low sample porosity and an effective residual gas displacement from the surface of the growing film. X-Ray diffraction revealed the TiO₂-derived films to contain rutile-phase grains of average grain size of less than 10 nm. Mechanical and tribological properties of the 3- to 4-μm thick coatings were determined by a micro-indenter operating with a Vickers-type diamond and by a pin-on-disk tester. The plastic microhardness HU_pl amounted up to 24.1 GPa in case of TiO₂-derived nanocomposite films and up to 14.8 GPa in case of Al₂O₃-based films. Also, with respect to tribological properties TiO₂:Cu and TiO₂:W coatings were superior over Al₂O₃-derived ones. A wear coefficient of 0.35×10⁻⁶ mm³/Nm was obtained for optimised TiO₂ films against an alumina sphere under unlubricated sliding conditions. In lubricated pin-on-disk tests wear coefficients on the order of 0.1×10⁻⁶ mm³/Nm were obtained for both metal-doped TiO₂ and Al₂O₃ films.

Introduction

Hard coatings for wear and abrasive protection are mainly fabricated from nitride compounds and carbon-based compounds [1]. Their usage has enabled prolonged lifetimes of a large variety of sliding mechanical assemblies like engine working parts, rolling elements such as bearings, cutting tools and others [2]. For many applications in air or under humidity it would be desirable, however, to dispose of hard coatings that better withstand oxidation or corrosion under chemical attack. Oxides would accordingly represent an ideal class of materials for these purposes since they are stable at high temperatures and in chemically aggressive environments. Various oxide systems are currently under investigations with many activities focussing on the Al–O system [3], [4] and the Zr–O system [5]. Two strategies might be applied to overcome the brittleness of ceramic oxides, which is the main obstacle for their use in tribological applications: one approach makes use of the change of mechanical properties with decreasing grain sizes down to the nanometer range [6], while in a second approach the dispersion of a second metallic phase into the granular ceramic material is used. In this work, we report on experiments that make equally use of both approaches by preparing nanogranular dispersions of metal–oxides and metals by virtue of the gas-flow sputtering (GFS) technique [7], [8]. This deposition technique appears well suited for the preparation of nanogranular particles and coatings, since it operates in a pressure range where high collisional probabilities between the active species leads to the formation of nanometer-sized particles already in the gas–plasma phase [9]. The GFS process may be equally applied to the deposition of a large variety of metallic and metal–oxide coatings and high deposition rates of some 10 μm/h may be obtained for coatings of metal oxides. The technique is well suited for industrial applications, since it requires only a comparatively simple vacuum equipment and can be up-scaled to large areas [10]. For the study on nanocomposites presented here, TiO₂ and Al₂O₃ were chosen for the oxide phase, while Cu and W were chosen for the metallic matrix phase. The focus of this work was on the region of high oxide concentrations combined with less amounts of the metallic matrix phase. It will be shown that metal–oxide–metal coatings with high mechanical hardness and advantageous tribological properties can be prepared by the GFS technique.