To evaluate the adhesive strength is important because the adhesive strength is one of the basic properties of thermal spray coatings. The standard tensile test method “ISO 14916” and the standard shear test method “EN 15340” are often used to evaluate the adhesive strength of thermal spray coatings. On the other hand, the indentation test methods such as “ISO 20267” have some advantage to evaluate the interfacial fracture toughness as the adhesive strength of thermal spray coatings. In this paper, a novel indentation test method is proposed to evaluate the interfacial fracture toughness from the critical indentation load for the interfacial crack initiation. To detect the critical indentation load, in this study, the acoustic emission (AE) technique was used. Some AE parameters obtained during the indentation test were investigated by using the clustering technique which is one of the machine learning methods. The experimental results showed that the AE signals measured during the indentation test could be categorized into three types; the first was due to the plastic deformation to generate the impression, the second was by the interfacial crack propagation, the third was from the final spallation cracking. It revealed from the results of clustering analysis and the in-situ observation that the cumulated AE energy measured during the indentation load was the effective parameter to detect the critical indentation load for the interfacial crack initiation. The apparent interfacial fracture toughness could be evaluated from the critical load for the interfacial crack initiation detected by using the AE technique.

View full abstract