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

高温合金一词用于描述镍或钴基合金，通常在工作温度超过540°C时使用。这些合金还含有高铬、中等钨和/或钼以及少量碳或硅。由于其独特的化学成分和显微组织，这些合金在高温下具有优异的耐磨性、机械强度和蠕变性能，良好的表面稳定性，以及耐腐蚀和抗氧化性能。近年来，随着工业应用中许多机械零件的使用温度和磨损/腐蚀严重程度的提高，例如燃气轮机的热区零件，以及涂层沉积技术的进步，高温合金越来越多地被用作表面涂层，以保护能源和航空航天工业中的机械在高温环境下免受磨损和腐蚀的侵蚀。然而，由于现有测试设备和方法的限制，这些涂层的高温力学性能很难评估，因此很少有报道。这些性能的缺乏极大地限制了这些合金的应用。为此，本研究以细观力学理论为基础，建立了高温硬度和断裂韧性模型，用于评价高温合金涂层的强度。在模型开发中，将采用基于高温微压痕的方法，并结合微断裂力学理论。将建立硬度和断裂韧性之间的关系，以揭开高温合金涂层断裂/失效的神秘面纱。此外，本研究还试图以较少的试错来推进高温合金涂层的设计和制造，从而节省实验成本，缩短设计和制造周期。提出的高温合金涂层高温硬度测定和断裂韧性评定方法可推广到陶瓷、金属陶瓷复合材料等多种涂层体系。

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.