Stabilization of Ceramic Strain Gages to Temperatures beyond1500°C

摘要

Thin film strain gages based on indium-tin-oxide (ITO) are being developed to measure static anddynamic strain at temperatures approaching 1500°C. These ceramic strain gages exhibit excellentoxidation resistance and high temperature stability, surviving tens of hours of strain testing attemperatures as high as 1530°C in air. A thickness dependence of the active ITO strain elements on thehigh temperature stability and piezoresistive response of the ceramic strain gages was observed. ThickITO strain elements exhibited a relatively large piezoresistive response at temperatures approaching1500°C and beyond. In addition to the ITO thickness, it was determined that aluminum doping of theITO strain elements had a profound effect on the high temperature stability of these ceramic strainsensors. Electron spectroscopy for chemical analysis (ESCA) depth files showed that interfacialreactions between ITO and aluminum were responsible for its enhanced stability via the inter-diffusionof aluminum and indium at these temperatures, which produced a very stable ITO/Al_2O_3 solid solution.To determine the nature of the interfacial reaction product, ITO films were deposited onto high purityAl_2O_3 and AlN surfaces and were thermally cycled to 1500°C. The reason for including AlN as part ofthis study was to eliminate sources of oxygen transport to the interface, so that aluminum-indiuminteractions alone could be studied. ITO films were deposited on Al_2O_3 and AlN surfaces by rfsputtering. In some instances, platinum was deposited on the surface prior to the ITO deposition. ESCAdepth profiles showed that an interfacial reaction had occurred between the ITO and the Al_2O_3 and/orAlN. This observation was based on the presence of two new indium-indium peaks at 448.85 and456.40eV. These peaks correspond to the indium 3d5 and 3d3 binding energies, the values of which aresignificantly higher than these associated with stoichiometric indium-tin-oxide films. The results of ourefforts to improve the high temperature stability of ITO strain gages are presented as well as thepiezoresitive response of selected ITO strain gages in the temperature range 25- 1550°C .