Critical assessment and demonstration of high-emissivity coatings for improved infrared signal quality for Taylor–Quinney coefficient experimentation

Abstract

Over the infrared (IR) spectrum metals have relatively low emissivity. Considering that modern studies focused on determining the Taylor–Quinney coefficient rely on IR-based measurements to quantify temperature rise, there is a need to improve measurement certainty. This is of particular concern during the earliest stages of deformation when small temperature rises result in a low IR-signal to noise ratio. Herein an economical approach to improving signal quality is presented based on the deposition of a thin high emissivity coating on the material substrate (i.e., specimen) of interest. Coating material and coating thickness selection are founded on both heat transfer and electromagnetic performance. A maximum allowable thickness of 1μm was determined for the coating to ensure the recorded temperature of the coating closely matches the temperature of the deforming substrate (i.e., specimen) during the sub-millisecond duration of a split-Hopkinson (or Kolsky) pressure bar experiment. Using the transfer matrix method, the spectral emissivity of coatings made from commonly available metallic deposition sources with increasing thicknesses was calculated. Theoretical predictions identified a 250 nm thick Ti coating as a promising candidate for increasing surface emissivity without the coating acting as a thermal barrier. Confirmation of the coating layer’s ability to substantially improve IR signal amplitudes was examined experimentally for two low emissivity materials, specifically OFHC copper (Cu) and Magnesium (Mg) alloy AZ31B. IR detector calibration experiments showed a four to six times improvement in signal amplitude compared to native Cu and Mg surfaces. Additionally, the calibration curves of coated specimens converge exhibiting the same behavior allowing for a unified calibration curve to be used regardless of substrate material. Split-Hopkinson pressure bar experiments supported by IR thermography were conducted on coated specimens and Taylor–Quinney coefficients are reported. Taylor–Quinney coefficients obtained, are compared to literature values when appropriate, and show positive agreement to previously reported values. Real-time high-speed imaging and post-mortem analysis showed robust adhesion between the substrate to the thin Ti layer up to 16% strain.