Improved passivating Cr2O3 scales for thin film high temperature PdCr strain gages

Abstract

Overcoat protection schemes for thin-film sensors have typically focused on inhibiting the growth of native oxides formed on the sensor surface, rather than on improving the passivating nature of these native oxides. In this work, 730Åsputtered Cr overcoats and heat treatments in varying partial pressures of oxygen enhance the passivating nature of native Cr2O3 formed on PdCr thin-film strain gages. Results of dynamic strain tests are presented and the implications in using this approach are discussed. PdCr gages with sputtered Cr overcoats withstood 12,000 strain cycles of 1100 με during 100 h of testing at a temperature of 1000°C in air. Gage factors of 1.3 with drift rates as low as 0.1 ω h★ were achieved for devices having a nominal resistance of approximately 100 ω. TCRs ranging from +550 ppm °C★ to + 798 ppm°C★ were realized depending on the overcoat scheme used and the thermal history of the film. Possible mechanisms for an anomaly in the electrical characteristics of these films at 800°C and improvements in stability due to the use of overcoats are presented.