Diffusion barrier coatings for CMC thermocouples

A platinum:silicon carbide thermocouple has been developed to measure the surface temperature of ceramic matrix composites (CMC) with high resolution. Platinum was deposited by rf sputtering onto a SiC-SiC CMC substrates coated with a dielectric, such that the SiC-SiC CMC was one thermoelement and the platinum film was another thermoelement comprising the Pt:SiC(CMC) thermocouple. The purpose of the dielectric was to electrically isolate the platinum leads from the SiC-SiC CMC. The thermoelectric output, hysteresis and drift of the Pt:SiC(CMC) thermocouples were measured at temperatures ranging from 600 °C to 1000 °C. The thermoelectric powers generated by the Pt:SiC thermocouples were an order in magnitude greater than conventional Pt:Pd or Type K thermocouples. Thermoelectric powers as large as 250 μV/K were reported for these thermocouples, as compared to thermoelectric powers of 10 μV/K reported for Pt:Pd and Type K thermocouples. The results presented within show that the Pt:SiC(CMC) thermocouples exhibit excellent stability at high temperatures, relatively low drift rates, and little hysteresis during thermal cycling. However, the Pt:SiC junctions were prone to oxidation effects as well as the formation of platinum silicides at high temperature, which can compromise the junction and lead to excessive drift. Therefore, a number of diffusion barrier coatings were applied to the Pt:SiC junctions in an attempt to improve stability and lower drift in this promising new class of thermocouples.