Estimation of titanium oxide layer thickness on thermally oxidized NiTi alloy based on color variations

This paper studies the estimation of titanium oxide (TiO2) thickness on a thermally oxidized nickel titanium (NiTi) alloy based on its surface color. The NiTi plate specimens were subjected to oxidation at temperatures from 450 °C to 900 °C for different duration to induce the formation of oxide layers of different thicknesses. The oxide thickness of the specimens oxidized from 650 °C to 900 °C was measured by means of scanning electron microscopy. A fitting curve was generated from the measured thicknesses, and further extrapolated to predict the oxide thickness for specimens heated at lower temperature of 450 °C to 600 °C. The oxide thickness estimation was done using thin‐film optical interference, with reference to the observed color of the oxidized surface. The thin‐film optical interference approach showed accurate measurement of oxide thickness as compared to the extrapolated data, highlighting the applicability of this measurement technique. Thermal oxidation of NiTi alloy at low oxidation temperature range produces surface colors depending on the thickness of titanium oxide (TiO2) layer formed, owing to the thin‐film optical interference. The thickness of the titanium oxide layer can be estimated based on its color, provided that the refractive indices of TiO2 and NiTi are known.