Sensing, Control, and In Situ Measurement of Coating Properties: An Integrated Approach Toward Establishing Process-Property Correlations

Abstract

Over the last decade there has been an explosion in terms of available tools for sensing the particle spray stream in thermal spray processes. This has led to considerable enhancement in our understanding of process reproducibility and reliability. Despite these advances, the linkage to coating properties has continued to be an enigma. This is partially due to the complex nature of the build-up process and the associated issues with measuring properties of these complex coatings. In this paper, we identify critical issues in processing-structure-property relations particularly with respect to the linkage to particle properties. Our goal is to demonstrate an integrated strategy, one that combines particle state sensing, with process mapping and extracting coating properties in situ through the development of robust and advanced curvature-based techniques. These techniques allow estimation of coating modulus, residual stress and, non-linear response of thermal sprayed ceramic coatings all within minutes of the deposition process. Finally, the integrated strategy examines the role of process maps for control of the spray stream as well as tailoring properties of thermal spray coatings. Examples of such studies for yttria-stabilized zirconia thermal barrier coatings are discussed.

Optimization and Characterization of High Velocity Oxy-fuel Sprayed Coatings: Techniques, Materials, and Applications

“…These differences with single particle and ensemble sensors may lead to different results when measuring temperature and velocity. Comparison with single and ensemble sensors has shown good relation for ceramic and metallic coatings when velocity was measured [12]. However, average particle temperature measured by single and ensemble sensors did not correlate for metallic materials.…”

“…However, average particle temperature measured by single and ensemble sensors did not correlate for metallic materials. The reason for this might be that high-temperature ceramic materials have higher total radiated intensity due to higher temperature and higher emissivity, but metals are influenced by oxidation and change of emissivity [12]. Therefore the results of diagnostic tools need careful consideration.…”

“…The process-structure-property relations can be presented by process maps, which can be used as design tool for coating processing. Process maps are interrelationships among the process variables and output responses [12]. The process mapping optimization tool has been widely applied for plasma spray process [12,14], but it can be successfully used for HVOF process as well [10,15].…”

“…Process optimization is used for process development by parameter improvement, tracking instabilities and examining reproducibility, and that way for improved coating manufacturing. A strategy for coating design can be obtained by careful use of process maps, because process parameters can be linked to coating characteristics and hence understanding of the process-microstructure-property relationships is possible [12]. Through process optimization it is possible to produce reliable and reproducible coatings with designed performance together with predictable life of operation [15].…”

“…Melting index (MI) also takes into account the time particles stay under the influence of flame and particle size. It can be expressed as [12]: (1) where T = measured particle surface temperature [K], D = particle size [m], and Δt fly particle in-flight time assuming constant acceleration of particles [s]. In the MI formula the Δt fly = 2L/v, where L = spray distance [m], and v = particle velocity [m/s].…”