Development of high-temperature hardness and fracture toughness models for superalloy coatings based on micromechanics theory

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The term superalloy is used to describe nickel- or cobalt-based alloys that are generally employed when operating temperatures are beyond about 540ºC. These alloys also contain high chromium, moderate tungsten and/or molybdenum, and minor carbon or silicon. Because of their unique chemical compositions and microstructures, these alloys possess excellent wear resistance, mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. In more recent years, with the increasing of operation temperature and wear/corrosion severity for many mechanical components in industrial applications, for example, the hot-section components in gas turbine engines, and the advance of coating deposition technologies, superalloys are more often used as surface coatings for protection of machinery in energy and aerospace industry against wear and corrosion attacks in high-temperature environments. However, due to the limitations of the existing testing facilities and methodologies, the high temperature mechanical properties of these coatings are difficult to assess, and hence have been rarely reported. The lack of these properties has significantly limited the application of these alloys. To this end, the proposed research is aimed to develop the high temperature hardness and fracture toughness models for evaluating or assessing the strength of superalloy coatings based on micromechanics theory. High-temperature microindentation-based approaches will be employed, combined with microfracture mechanics theory, in the model development. The relationship between hardness and fracture toughness will be established to demystify the mechanisms that govern the fracture/failure of superalloy coatings. Furthermore, this research also attempts to contribute to the advance of design and fabrication of superalloy coatings with less trial and-error, thus saving the experimental cost and shortening the design and fabrication cycle. The proposed methods for high temperature hardness determination and fracture toughness assessment of superalloy coatings can be extended to a range of coating systems such as ceramics, metal ceramic composites, etc.