Improved Adhesion of Plasma-Sprayed Zirconia Coatings on Textured Alumina Substrates

Abstract

Coating adhesion by thermal spraying method requires sufficient surface roughness of the substrate. The scale of the roughness must be adapted to the spread particle thickness size to optimize their mechanical attachment after spreading and cooling, particularly in the case of plasma spraying. Grit blasting is the most common process used to create the surface asperities required for coating bonding. However, to further increase adhesion, the use of laser texturing for metallic substrates is beneficial and is already well documented in literature. In the case of ceramic substrates such as alumina, grit blasting with corundum particles is no longer effective in creating a roughness of a few micrometers. Laser texturing therefore appears to be a potential candidate for generating adhesion in coatings. In this work, adhesion mechanisms of YSZ coatings produced by atmospheric plasma spraying (APS) on a textured alumina bulk substrate were investigated. The influence of substrate surface texturing by two different lasers (pulsed and continuous) on YSZ monolayer was studied. The lasers create an adapted surface profile to generate adhesion of YSZ coatings. For the pulsed laser, the depth of the holes must reach 100 µm, while for the continuous laser, the arithmetic mean surface roughness (Sa) value must exceed 25 µm. YSZ monolayer reaches a maximum adhesion of 4.8 MPa. Al₂O₃ and Y₂O₃ materials were selected as potential bond coats to increase YSZ topcoat adhesion on textured surfaces. The use of a bond coat with the same surface profile can improve adhesion, particularly with yttria, to reach a maximum value of 13.2 MPa. Surface texturing combined with the use of a bond coat is an appropriate approach for creating adhesion of a ceramic coating to a ceramic substrate.