Effects of Residual Stress on Critical Strain for Macroscopic Crack Formation on Thermal Spray Coatings
Koichi AKITA and Shogo TOBE
Abstract:A new evaluation method of mechanical strength of thermal spray coatings using an acoustic emission (AE) measurement during bending loading was proposed. Plasma thermal spray molybdenum coatings deposited on steel specimens were used in this study. In-situ microscopic observations on the cross-section of the coating and acoustic emission measurements during bending loading were performed and the microscopic fracture process was investigated. Microscopic cracks grew from initial microscopic cracks and pores at the early stage of loading. Microscopic cracks were joined and a macroscopic crack formation was started at a certain bending strain. The macroscopic crack propagated towards to the substrate. Finally, the interface cracking and delaminating were occurred. The cumulative AE event rapidly increased when the macroscopic crack formation started. Therefore, the critical strain for the macroscopic crack formation of the thermal spray coating can be determined by the rapidly increasing point of the cumulative AE event. The effect of the residual stress of the coating on the critical strain for the macroscopic crack formation was investigated. The specimens having five kinds of the residual stress levels were prepared by in-situ bending during spraying. The residual stresses of coatings were measured using a synchrotron radiation. The critical strain for the macroscopic crack formation became higher on the specimen having higher compressive residual stress. That is the compressive residual stress increases the mechanical strength of the thermal spray coating. Key Words:Plasma spray coating, Molybdenum, Mechanical strength, Acoustic emission, Crack, Fracture process, Residual stress, X-ray diffraction, Synchrotron radiation