Alumina based oxidation protection coatings

Abstract

Many modern applications are pushing materials to their limit. An industry longing for lighter, harder, more durable and more resistant materials is motivating research and development of novel materials and coatings. Many of whom, suffer from degradation and oxidation at high temperatures. Alumina coatings proved to be a promising candidate to overcome those problems of high temperature applications. Due to the chemical inertness, high hot hardness and oxidation resistance alumina thin films drew interest as protective coatings against wear and corrosion. While the stable α phase needs high deposition temperatures above 1000 C and is primarily limited to chemical vapour deposition (CVD) the metastable γ Al2O3 can be deposited at temperatures below 650 C in a physical vapour deposition (PVD) process. The transformation from the γ-phase to the α-phase is accompanied by a reduction in volume leading to cracking and coating failure. In thin films, γ Al2O3 appears to transform to α Al2O3 at higher temperatures up to 1200 C. Hence it is possible to use γ Al2O3 as a protective coating up to 1000 C. Over the last two decades rising interest in crystalline alumina thin films led to many publications and the implementation of many different sputter deposition techniques. This thesis aims for investigations on different alumina thin films for the potential use as diffusion barrier and protective coating for a broad spectrum of applications. All coatings were deposited in a reactive magnetron sputtering process using a 99.99 % pure aluminium target in Ar/O2 atmosphere with an oxygen content between 10 and 35 %. The effect of different deposition parameters and doping elements on the coating properties was studied using scanning electron microscopy (SEM), nanoindentation and X-ray diffraction (XRD). Four coatings, that showed the most promising morphology and hardness, were annealed at 900 C in ambient air for 100 hours and their oxidation protection behaviour was investigated. To ensure a proper bond between the substrate and the protective coating it is important to reduce the formation of substrate oxidation products at the interface. Since many materials are very sensitive to oxygen at elevated temperatures the deposition process itself must not raise the oxygen level above a material specific threshold. The damaging effect of the sputter process was investigated using energy dispersive X-ray spectroscopy (EDX) and secondary ion mass spectrometry (SIMS). To reduce oxidation of the substrate during the PVD process depositions at more conserving sputtering parameters were investigated.