Adhesion strength of HVOF sprayed Inconel 718 coatings

Doctoral thesis, 2011

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 meet new customer demands. Therefore, understanding the relationship between coating adhesion strength and residual stress levels – through the deposit thickness on the one hand and particularly at the interface between coating and substrate 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. If, in this application, bond strength can be enhanced then permissible repair thickness can be increased. 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, in the sense that gluing is replaced by brazing, was developed alongside a method for determining adhesion shear strength. It was also found that determination of residual stresses by neutron diffraction is a tricky task when a material is to be coated with the same material. Algorithms for the determination of residual stress levels in the coating-substrate interface therefore had to be specifically developed. Residual stresses determined by the Layer Removal Method (MLRM) were compared with the Neutron Diffraction ones in order to be validated. Tentative simulation models were also developed, in order to be able to separate different types of residual stresses, such as thermal mismatch stresses and peening stresses, to be able to predict residual stresses both for single particle impacts and for layer-by-layer coating formation. The developed techniques, methods and models provide valuable tools to evaluate and understand relationships between coating thickness, residual stress states and adhesion strength as well. Additionally, statistical approaches based on the previously developed characterization tools were presented for studying ways in which process parameters can be optimized for enhancing adhesion strength of HVOF sprayed coatings.