In situ monitoring of cracking behaviors of plasma-sprayed coatings by the laser acoustic emission technique
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Makoto Watanabe and Seiji Kuroda Hybrid Materials Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
Kaita Ito Department of Materials Engineering, The University of Tokyo, Tokyo 153-8902, Japan; and Innovative Materials Engineering Library, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan (Received 7 April 2009; accepted 19 June 2009)
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 3045 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. I. INTRODUCTION
Important in thermal spray technology is to ensure the reliability and the reproducibility of fabricated coatings. There are several nondestructive evaluation techniques, such as ultrasonic technique,1–3 eddy current,3,4 and neutron scattering,5–7 to name a few, that are capable of examining the coating thickness, microstructure, and defects in the coating and/or at the interface. However, because most of these techniques can be used only after the fabrication process, additional steps, time, and costs to apply them directly into the industrial fabrication process are needed. Thus, in situ monitoring is more desirable. Thus far, there are several techniques that can perform in situ monitoring of the thermal spray process. One typical technique is the diagnostics of the sprayed particles by measuring their temperature and velocity. Several devices such as DPV-2000 (Tecnar Automation, St. Bruno, QC, Canada) and Spraywatch (Oseir, Tampere, Finland) are commercialized for this purpose. Because these systems are very effective in monitoring the distribution of the particle temperature and velocity and a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0364
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J. Mater. Res., Vol. 24, No. 10, Oct 2009 Downloaded: 27 Mar 2015
their trajectories, they can be adapted to improve the reproducibility of coating thickness, microstructures, and properties needed for various industrial applications. In addition, it is useful to detect some unexpected troubles during spraying such as spitting, in which a deposit of exce
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