The adhesion strength of thermal sprayed coatings is very important because it dominates the total performance of coatings such as reliability and fatigue life. Therefore, it is essential to establish reliable evaluation methods for adhesion strength, which is significantly affected by the spray conditions even for the same combination of substrate and coating materials. The residual stress (including the thermal stress) is one of key factors that can dominate the adhesion strength. However, the correlation between the adhesion strength and residual stress is still unclear. The purposes of the present study are firstly to establish reliable experimental method to evaluate the adhesion strength and secondly to reveal the relationship between the adhesion strength and the residual stress.

SUS316 steel coatings were deposited on low carbon steel substrates by HVOF spraying. Two types of samples were fabricated. One is the cylindrical shape for the adhesion test and the other has the rectangular dimension for the residual stress evaluation. The tensile adhesion tests were carried out by modifying the conventional test method. The circumferential interface cracks were introduced by putting carbon thin layer on the substrate surface before spraying, and the edge of the substrate was masked to prevent undesirable deformation during spraying. The interface fracture toughness was estimated from the load at failure and the width of carbon area in radial direction (corresponding to the crack length) by finite element analysis. The residual stress was evaluated by measuring the relaxed strain at the substrate surface for reduction of the coating thickness. Thinning of the coatings was performed by polishing.

It was made possible to evaluate the adhesion strength of SUS316 steel coatings with good reproducibility for the samples deposited under various spray conditions. The correlations between adhesion strength and residual stress will be discussed.