Strain induced diffusivity effects in indium-tin-oxide strain gages

The effect of strain level on gage factor was investigated from room temperature to 1000°C and the piezoresistive response was established for a number of ITO thin film strain sensors tested in both tension and compression. A new zirconia based laminated substrate capable of being strained to 2000με prior to fracture, was used to evaluate the response of the ITO strain gages at strain levels approaching 1000με. When tested in tension at temperatures up to 1000°C, the gage factor was marginally dependent on strain and rapidly approached a steady state value of 4.0. However, when the same ITO strain gages were tested in compression, the gage factor was considerably less stable, i.e. gages exhibited much greater variability as a function of temperature. This lead us to postulate that there may be a strain induced diffusivity effect (oxygen diffusion effect) that might be responsible for the observed trends in gage factor at higher strains. To study this effect, high purity alumina coatings were deposited onto the ITO strain gages, which were then tested to 1000με. Preliminary results indicate that protective coatings may mitigate the enhanced oxygen diffusivity at high strain levels and thus, stabilize the piezoresistive response, and in some instances actually increasing the piezoresistive response. The results of this study are presented and the implications on high temperature strain measurement are discussed