In situ monitoring of cracking behaviors of plasma-sprayed coatings by the laser acoustic emission technique

Multichannel acoustic emission (AE) measurements by four laser interferometers were developed and applied during a plasma-spray coating process that is known as being a high-temperature process and an extremely noisy environment in both mechanical and electrical domains. The AE signals could be successfully detected during the cooling period after the deposition, and it was clearly indicated that a higher preheating temperature resulted in the improvement of bonding between splats and/or at the interface of the coating and the substrate. The maximum principal stress generated during the deposition process was calculated by the transient heat and stress analysis and the obtained AE events. The critical stress for crack initiation of the alumina coating on a steel substrate with an NiCr bond coat layer was estimated as 30∼45 MPa. The developed techniques were shown to be a potential tool for in situ monitoring of the thermal spray process, by which an increase in the reliability of thermal spray coatings can be expected.