Influence of Metal Bond Coat Thickness on Adhesion Strength of APS Ceramic Coatings

This paper investigates the influence of metal bond coat (MBC) thickness on the adhesion strength of air plasma spray (APS) ceramic coatings for extreme service ball valve applications. The study was conducted using three different ceramic coatings (Cr₂O₃, Cr₂O₃-Al₂O₃, and TiO₂) applied on three different substrates (Titanium G12, Inconel 625, and 2507 Duplex stainless steel) with varying thicknesses of MBC. Tests were performed for adhesion strength (ASTM C633), metallography (ASTM E1920), and percent porosity (ASTM E2109 Method B). The results showed that the adhesion strength of the ceramic coatings was not reduced until the MBC thickness exceeded 0.060 in. The failures observed were either cohesive in the ceramic, cohesive in the adhesive, or a combination of both. It was concluded that a recommended MBC thickness limit of 0.030 in. can be established based on the experimental data.