Environmental Resistant Coatings for High Temperature Mo and Nb Silicide Alloys
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Environmental Resistant Coatings for High Temperature Mo and Nb Silicide Alloys J.H. Perepezko University of Wisconsin-Madison, Department of Materials Science and Engineering 1509 University Ave., Madison WI 53706 ABSTRACT The challenges of a high temperature environment impose severe material performance constraints in terms of melting point, oxidation resistance and structural functionality. In metallic systems the multiphase microstructures that can be developed in the Mo-Si-B system and Nb silicide alloys offer useful options for high temperature applications. Since the alloy compositions that exhibit the lowest oxidation rate will most likely not yield optimum mechanical properties performance, it is important to develop robust and compatible oxidation resistant coatings. An effective strategy to achieve the necessary environmental resistance is based upon the use of an integrated Mo-Si-B based coating that is applied by a pack cementation process to develop an aluminoborosilica surface and in-situ diffusion barriers with self-healing characteristics for enhanced oxidation resistance. The environmental performance requires resistance not only to high temperature oxidation, but also resistance to water vapor, CMAS (calcia-magnesia-aluminosilica) attack, hot corrosion and thermal cycling. Under these extended environmental conditions the Mo-Si-B based coating exhibits robust performance. INTRODUCTION In the search for high temperature materials with capabilities beyond the limit of current Ni base superalloys the choices of alternative materials are limited by numerous performance criteria [1]. One empirical guideline, the Johnson relation, would require that the melting temperature, Tm be above 2500°C [2]. Another issue is creep, the slow deformation of a material subjected to stress. Lower creep rates translate into higher maximum operating temperatures and creep rates tend to be lower for materials with higher Tm. There are only a limited number of ceramics, intermetallic compounds, and refractory metals that keep their strength at high temperatures and satisfy this initial Tm criterion. As single components, the ceramic and intermetallic phases, which have good oxidation resistance and low density, suffer from severe embrittlement problems and flaw sensitivity at low temperatures that make them unacceptably prone to failure. Composite design strategies have addressed some of the limitations, and silicon carbide matrix composites are being considered for temperatures up to 1200°C [3]. Of the refractory metals, Mo and Nb form alloys that satisfy many of the requirements for engine applications, but they suffer from a severe drawback of poor oxidation resistance. The oxide layer that forms on Nb, Nb2O5, does not offer protection from further oxidation, and Mo forms an oxide, MoO3, that is volatile above about 700°C [4]. However, silicon- containing alloys (silicides) of these metals show some oxidation resistance. The route forward for Mo and Nb alloys borrows from lessons learned in the development of nickel s
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