Residual Stress in Thermal Spray Coatings

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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.