Abstract. Residual strains in air plasma sprayed (APS) and high velocity oxy-fuel (HVOF) alumina (Al 2 O 3 ) coatings were investigated by means of neutron diffraction technique. Results of this investigation conclude that through thickness residual strain in the APS coating was mainly tensile whereas the HVOF coating had both compressive and tensile residual strain. Further analysis of Vickers indentation fracture behaviour using acoustic emission (AE) technique concluded that the nature and magnitude of these residual strain fields had a direct effect on the fracture response of two coatings during the indentation process.
IntroductionThermally sprayed alumina (Al 2 O 3 ) coatings are used in industry for thermal and electrical resistance applications. Measurement of residual strains in these coatings is critical for modeling and designing improved components. Non-destructive through thickness residual strain measurements in thermal spray coatings is possible via the high penetration depth achieved by the neutron diffraction technique [1][2]. The influence of coating process parameters on the residual stress fields of Al 2 O 3 coatings has been a topic of research of previous investigations [3]. Recent advancements in high velocity oxy-fuel (HVOF) technique has made it possible to deposit much finer alumina powders at relatively lower temperatures than those achieved by air plasma spraying (APS) process [4]. This has the effect of limiting phase transformations during coating deposition, which along with the relatively higher velocity and lower temperature of HVOF process can influence residual stress behavior. This paper therefore aims to investigate the through thickness changes in the residual strain profile of finer powder and conventional Al 2 O 3 coatings deposited by the HVOF and APS processes, respectively. Further analysis interlinking fracture behavior and residual strain is also presented.
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