Effect of substrate variables on the interface fracture energy of thermal spray coatings

The effects of substrate material, substrate roughness and substrate thickness on the interface fracture energy (G_C) of thermal spray coating/metallic substrate interfaces are determined. Air Plasma Sprayed (APS) coatings of Yttria Stabilized Zirconia (YSZ) and Alumina, as well as NiCrAlY are investigated with different substrate materials (stainless steel-SS, mild steel-MS), roughness and thickness. Modified cantilever beam bending with Digital Image Correlation (DIC) based tracking of crack length is used for high-throughput measurements. A 46 % increase in G_C is seen due to an increase of 73 % substrate roughness (R_a) of substrate for YSZ/SS, while a 60 % increase in G_C is seen due to an increase in 166 % R_a of substrate for Alumina/MS system. Interface fracture energy G_C of YSZ/MS is found to be ∼75 J/m², compared to ∼50 J/m² for the Alumina/MS for similar coating/substrate thickness as well as substrate roughness. For the same coating type and thickness of YSZ, G_C decreased to 60 J/m² when the substrate was changed to SS, even with an increase in R_a. This could be attributed to the significant effect of residual stress, originating from thermal expansion mismatch, with higher compressive residual stresses increasing the propensity for interfacial delamination. There is no effect of substrate thickness on G_C, independent of whether the substrate is machined before or after the deposition. Mechanisms of these changes are discussed in the context of total interface fracture area and crack closure stresses.