Thermoelastic Coating Characterization using a Superelastic NiTi Alloy

Temperature changes occurring in solids as a result of external stressing (thermoelasticity) can be used as the heat source in thermographic infrared inspections [1]. Some materials, for example most metals, are highly reflective in the infrared and consequently require an emissivity enhancing coating to facilitate an infrared inspection. Understanding the effect such a coating has on the thermoelastic response from the material is a fundamental part of interpreting the results, and has spawned an effort to characterize the effect of the coating on a thermoelastic response [2,3]. A theoretical examination of the thermoelastic response from a substrate/coating configuration based upon [4] has led to the identification of three distinct response regimes. The coating thickness and the stressing frequency determine the response regime for a given coating. For high frequencies and thick coatings the response generated from the substrate material is damped out within the coating, and a small coating thermoelastic response is revealed. This response is excited by strain continuity across the substrate/coating bondline, earning the label “strain witness response.” Operating within the strain witness regime had been a problem until the recent introduction of a superelastic NiTi alloy as the substrate material. An introduction to this NiTi alloy and its application to coating characterization precedes a description of the experiment and presentation of theoretical and experimental results.</p