Diffusion barrier coatings for CMC thermocouples
Date of Original Version
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.
Surface and Coatings Technology
Rivera, Kevin, Matt Ricci, and Otto Gregory. "Diffusion barrier coatings for CMC thermocouples." Surface and Coatings Technology 336, (2018): 17-21. doi:10.1016/j.surfcoat.2017.09.032.