Relationships between Film Chemistry, Structure, and Mechanical Properties in Titanium Oxide
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Relationships between Film Chemistry, Structure, and Mechanical Properties in Titanium Oxide M. Pang, D.E. Eakins, M..G. Norton, and D.F. Bahr Mechanical and Materials Engineering, Washington State University, Pullman WA 99164-2920 ABSTRACT Titanium oxides were grown anodically to selected final potentials on grade II polycrystalline titanium under different anodization rates. XPS and RBS results show that the oxide consists of primarily TiO2 with a non-stoichiometric oxide/metal interface, with the slower growth rate associated with a thicker layer at the interface. Characterization using TEM reveals that the structure of the oxide evolves from a primarily amorphous phase to islands of crystallites in an amorphous matrix, to an entirely crystalline phase by increasing the polarization potential. Slower growth rates tend to remain crystalline at higher potentials. The mechanical strength of oxide films extracted from load-depth data by nanoindentation varies dramatically for oxide films grown by different rates at 9.4 V, and to a lesser extent at lower potentials. The variation of film strength is associated with both compositional and structural characteristics. INTRODUCTION The extensive application of thin oxide films as wear-resistant materials necessitates a thorough understanding of their mechanical performance. The development of nanoindentation techniques allows the investigation of the mechanical response of surface layers in the nanometer regime. Experimentally, upon contact, thin oxide films can sustain pressures on the order of the theoretical strength prior to their brittle fracture in the case of iron, tungsten, and aluminum [1, 2]. Numerous models have been developed to predict thin film strength [3, 4] in the spirit of macroscopic mechanical analysis by employing such mechanical parameters as Young’s modulus (E), and hardness (H). However, the mechanical behavior of thin oxide films is complex and depends on film morphology, composition and microstructure. While the compositional and structural characterization of titanium oxides is well established [5, 6], the correlation of film mechanical behavior, chemistry, and structure is still poorly understood. In this paper, anodically grown titanium oxide on titanium was chosen as it is a useful engineering system. The variation of film strength and the evolution of film structures are examined by altering film formation conditions, i.e. the film growth rate. Anodization allows in situ nanomechanical testing at various stages of film growth. EXPERIMENTAL PROCEDURE A grade II polycrystalline titanium was vacuum annealed to achieve a primarily α (HCP) phase grain structure. The sample was then sectioned, ground to 600 grit, and electropolished in a mixture of 60% H2SO4, 25% HF and 15% glycerine at approximately 30 V. The surface titanium oxide was grown by anodic polarization using an EG&G 173 potentiostat and 175 Universal Programmer. A Teflon electrochemical mini-cell that can fit into the sample stage of the nanoindenter system was utilized. A Pt wire was
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