An Investigation of Mechanisms in Bonding and Failure of Thermal Spray Coatings

This research focused on investigating the bonding and failure mechanisms of thermal spray coatings, in particular the Cr3C2-NiCr HVOF coating deposited onto 17-4 PH stainless steel. The approach taken involved modelling the macroscale fracture behavior by considering the interfacial geometry, often applied through grit blasting of the substrate, and intrinsic fracture properties of the coating, as measured through micromechanical experiments, and comparing the results with tested macroscale specimens. Intrinsic coating fracture properties were measured through microcantilever (Figure 1) and micropillar splitting experiments (Figure 2). To the fellow’s knowledge, these experimental methods were applied for the first time to a complex composite microstructure. Figure 1: Left) Focused ion beam fabricated microcantilever beam. Right) Fractured microcantilever beam. Black square represents carbide/matrix interfacial fracture. Black circle represents carbide cleavage. White circle represents intergranular matrix fracture. Figure 2: Left) Focused ion beam fabricated micropillar. Right) Fractured micropillar. Note the significant amounts of matrix fracture. The coating interface geometry was characterized by standard statistical analyses of metallographic cross-sections. Inspection of cross-section indentation-induced interfacial cracks allowed for an understanding of the tendency for cracks to follow or deviate from the interface depending on the local interface geometry (Figure 1). Figure 3: Interface crack highlighting propagation into the coating (solid black arrow) rather than following the interface (dashed black arrow) at a region of high local waviness. These factors were used as inputs to an analytical model to predict the macroscale fracture behavior of the coating/substrate system. The model predictions agree very well with experimental results of single edge notch beam (SENB) and double cantilever beam (DCB) macroscale test specimens (Figure 4) (Table 1). The model was adapted from Faber and Evans (1983) fracture model for brittle particulate composites to account for interfacial geometry and calculates the change in global strain energy release rate due to perturbations in the local energy release rate of a tilted and/or twisted crack front. Figure 4: Left) Brazed coating SENB specimen to be loaded in three point bending. Right) Brazed coating DCB specimen with side groove to restrict crack growth to the coating/substrate interface when loaded in tension at the protruding tabs. Table 1: Comparison of experimental results and model predictions KIC (MPam1/2) Sample SENB DCB Model Prediction Grit-Blasted 5.31±1.10 5.3 5.3-5.6 While further testing with different systems and substrate preparation methods is needed to test the robustness of the developed method, the results of this study are promising for the optimization of coating interface fracture toughness based on a given system’s intrinsic properties. 1. This research focused on investigating the bonding and failure mechanisms of thermal spray coatings, in particular the Cr3C2-NiCr HVOF coating deposited onto 17-4 PH stainless steel. The approach taken involved modelling the macroscale fracture behavior by considering the interfacial geometry, often applied through grit blasting of the substrate, and intrinsic fracture properties of the coating, as measured through micromechanical experiments, and comparing the results with tested macroscale specimens. Intrinsic coating fracture properties were measured through microcantilever (Figure 1) and micropillar splitting experiments (Figure 2). To the fellow’s knowledge, these experimental methods were applied for the first time to a complex composite microstructure. Figure 1: Left) Focused ion beam fabricated microcantilever beam. Right) Fractured microcantilever beam. Black square represents carbide/matrix interfacial fracture. Black circle represents carbide cleavage. White circle represents intergranular matrix fracture. Figure 2: Left) Focused ion beam fabricated micropillar. Right) Fractured micropillar. Note the significant amounts of matrix fracture. The coating interface geometry was characterized by standard statistical analyses of metallographic cross-sections. Inspection of cross-section indentation-induced interfacial cracks allowed for an understanding of the tendency for cracks to follow or deviate from the interface depending on the local interface geometry (Figure 1). Figure 3: Interface crack highlighting propagation into the coating (solid black arrow) rather than following the interface (dashed black arrow) at a region of high local waviness. These factors were used as inputs to an analytical model to predict the macroscale fracture behavior of the coating/substrate system. The model predictions agree very well with experimental results of single edge notch beam (SENB) and double cantilever beam (DCB) macroscale test specimens (Figure 4) (Table 1). The model was adapted from Faber and Evans (1983) fracture model for brittle particulate composites to account for interfacial geometry and calculates the change in global strain energy release rate due to perturbations in the local energy release rate of a tilted and/or twisted crack front. Figure 4: Left) Brazed coating SENB specimen to be loaded in three point bending. Right) Brazed coating DCB specimen with side groove to restrict crack growth to the coating/substrate interface when loaded in tension at the protruding tabs. Table 1: Comparison of experimental results and model predictions KIC (MPam1/2) Sample SENB DCB Model Prediction Grit-Blasted 5.31±1.10 5.3 5.3-5.6 While further testing with different systems and substrate preparation methods is needed to test the robustness of the developed method, the results of this study are promising for the optimization of coating interface fracture toughness based on a given system’s intrinsic properties.