Residual Stress in Thermal Spray Coatings

9414537舍德勒热喷涂层是一项重要的“使能”技术，被广泛用于提高各种应用中材料的性能，包括航天器、燃气轮机、化学反应器、磨机和轧辊、桥梁和医疗假体。金属、金属陶瓷和陶瓷涂层可防止磨损、腐蚀和热降解。热喷涂工艺的一个主要限制是涂层在加工过程中由于快速凝固和热膨胀系数失配而产生的残余应力。残余应力通常随着涂层厚度的增加而增加。当这些应力的大小超过涂层的附着力或结合强度时，后者会因与基材脱粘、剥落或破裂而失效。然而，对于如何控制和最小化这些残余应力，人们的理解并不完整。因此，为了沉积更厚的涂层并提高其耐腐蚀性和耐磨性，必须对关键工艺参数和组成相对残余应力的影响有基本的了解。该项目有三个目标，所有这些都将增加对热喷涂技术工艺科学的了解和理解：1)使用X射线应力分析和X射线拉伸测试技术，将无损但相对简单的“阿尔门”弯曲测试校准到已知的应力绝对值。2)确定了关键工艺参数(如颗粒速度)对涂层残余应力的影响。3)了解材料体系和增强相(包括其组成和体积分数)对残余应力的作用。将对高速燃油燃烧喷涂和等离子喷涂涂层进行分析；分析方法将包括残余应力测量的X射线应力分析；涂层表征的光学显微镜、显微硬度和X射线衍射；以及最终涂层的滑动磨损测试，试图将残余应力与涂层性能联系起来。这项工作有望开发一种非破坏性的方法来直接测量热喷涂涂层中的残余应力，这将是评估涂层质量和性能的有效工具。这项工作还将通过更好地了解工艺参数和第二相粒子在涂层中产生和/或保持残余应力中所起的作用，从而扩大热喷涂工艺科学基础。优化这些条件最终将允许沉积性能更好的更厚的涂层，从而克服热喷涂涂层扩展到更多潜在应用的主要障碍。

9414537 Schadler Thermal spray coatings, an important "enabling" technology, are widely used to enhance the performance of materials in a diverse range of applications, including space vehicles, gas turbines, chemical reactors, mills and rolls, bridges and medical prostheses. Metal, cermet, and ceramic coatings provide protection against wear, corrosion and thermal degradation. A major limitation in thermal spray processing is the residual stresses which develop in coatings during processing due to rapid solidification and coefficient of thermal expansion mismatches. Residual stresses generally increase as a function of coating thickness. When the magnitude of these stresses exceeds the adhesive or cohesive strength of the coatings, the latter fails, either by debonding from the substrate, spalling or cracking. There is, however, an incomplete understanding of how to control and minimize these residual stresses. In order to deposit thicker coatings and improve their corrosion and wear resistance, therefore, a fundamental understanding of the effect of key processing parameters and constituent phases on the residual stresses must be obtained. The project has three goals, all of which will result in increased knowledge and understanding of the processing science in thermal spray technology: 1) To calibrate a nondestructive, but relatively simple, "Almen" bend test to known absolute values of stress using x-ray stress analysis and x-ray tensile testing techniques. 2) To determile the effect of key process parameters (such as particle velocity) on residual stresses in coatings. 3) To understand the role of material system and reinforcing phases, including their composition and volume fraction, on residual stresses. Both High Velocity Oxy-Fuel (HVOF) combustion spray and plasma spray coatings will be analyzed; analysis methods will include x-ray stress analysis for residual stress measurements; optical microscopy, microhardness and x-ra y diffraction for coating characterization; and sliding wear testing of the final coatings to attempt to relate residual stress to coating performance. The work promises to develop a non-destructive means for directly measuring residual stresses in thermal spray coatings which will be an effective tool for evaluating coating quality and performance. The work will also expand the thermal spray processing science base by providing a better understanding of the roles which processing parameters and second phase particles play in generating and/or retaining residual stresses in coatings. Optimizing these conditions will eventually allow thicker coatings, with improved properties, to be deposited, thus overcoming a major barrier to expansion of thermal spray coatings to many more potential applications.