Adhesion and Residual Stress Characterization of HVOF Sprayed Inconel 718 Coatings

Licentiate thesis, 2008

Thermal spray technology in general is nowadays recognized as an industrial process, mature enough to apply coatings on different substrate materials that protect parts against wear, corrosion, heat, or combinations of these. The quality demands on coating homogeneity and adhesion strength are, nowhere more than in the automotive and aeronautic industries, continuously increasing. This was the major reason why the High-Velocity-Oxy-Fuel process was invented. However, adhesion strength is still a limitation of thermal spray processes, specifically in the case of thick coatings, and is a critical property which, sometimes over relatively short time periods, can result in coating spallation. The focus of this study is on High-Velocity-Oxy-Fuel (HVOF) coatings for aerospace applications, where the requirements on coating performance are the toughest. Improving the coating adhesion is, in the aerospace context, necessary to increase permissible repair thicknesses. Therefore, understanding the relationship between coating thickness and residual stress levels – particularly in the interface between coating and substrate on the one hand and coating adhesion on the other – is a prerequisite for any bond strength improvement, and, consequently forms the aim of the current research. The chosen application was the aeronautic repair of Inconel 718 parts comprised of the same material, which is an application with high requirements in terms of adhesion strength. One early finding in the investigation was that, in order to establish the relationship between coating thickness, the residual stress state and adhesion strength, new techniques, methods and algorithms needed to be developed. A method that modifies the standard tensile strength evaluation method ASTM-C633 was developed alongside a method for determining adhesion shear strength. It was also found that determination of residual stresses by destructive (Modified Layer Removal Method) and non destructive (Neutron Diffraction) methods is a tricky task when a material is to be coated with the same material. The different techniques, methods and models that have been developed will provide valuable tools in future work to evaluate and understand relationships between coating thickness, residual stress states and adhesion strength as well, additionally, for studying ways in which process parameters can be optimized for enhanced adhesion strength in HVOF sprayed coatings.