| Thermal spray coatings represent a category of surface enhancement processes whereby a material is melted and applied to a base material (substrate) to form a new surface deposit. The most common reasons for applying a thermal spray coating are to improve the wear, corrosion, or the thermal insulative properties of the base component. A thermal spray coating applied with the intent of improving wear resistance may have a deleterious effect on the service life of the component. Many thermal spray coatings fail while in-service because of cracks or debonding. A worse case occurs when fatigue failure leads to the whole component having a shorter than expected service life. Residual stresses are inherent in thermal spray coatings because the application process involves large temperature gradients in materials with different mechanical properties. Residual stresses can influence coating performance and service life. Therefore, there is interest in evaluating residual stresses and improving coating performance.; The goals of this work are (1) to develop a method for determining residual stresses, (2) to establish linkages between residual stresses and performance, and (3) to investigate methods for controlling residual stresses in thermal spray coatings. Through-thickness residual stress distributions are evaluated by an experimental technique, the Modified Layer Removal Method (1). Coating performance data are generated for bond strength and fatigue life. Linkages between residual stresses and coating performance are established. For example, in a specific jet engine application, an appropriate magnitude of compressive residual stress results in the longest fatigue life compared to other stress levels. Coating process selection, spray parameter control, substrate preheating, and post-heat treatment are investigated as possible residual stress control methods. As an example, a relation is established between preheating the substrate and coating residual stresses. The final research study incorporates the control of residual stresses through spray parameter adjustment and examines the effects on flexural fatigue life. Residual stress control strategies and methods for designing thermal spray coated components are presented in the conclusions. |
