Characterisation of thermal barrier coatings and ultra high temperature composites deposited in a low pressure plasma reactor

A low pressure plasma process working at 600-800 Pa was used to deposit from aqueous solution ZrO₂-4 mol% Y₂O₃ (Yttria partially stabilized Zirconia-YpSZ) layers and stacks of Ta₂O₅/YpSZ layers for use as thermal barrier coatings (TBC). The observation of the cross section revealed a high porosity. The thermal diffusivity of the layers (1 × 10⁻⁷ m² s⁻¹) was measured by a laser flash technique and compared with values obtained on air plasma sprayed material (3 × 10⁻⁷ m² s⁻¹). The plasma reactor were also used to deposit ZrB₂-ZrO₂-SiC layers used as Ultra High Temperature Composite (UHTC) from aqueous solutions of zirconyl and Boron nitrates containing suspensions of SiC. Layers up to 100 μm thick were obtained on SiC substrates. XRD was used to study the crystallinity of the layer. The presence of ZrB₂ and SiC phases was confirmed after the deposition. XRD analysis showed that heat treatment at 1073 K under oxidizing conditions led to the loss of ZrB₂ and the appearance of ZrO₂ phases. To understand the behaviour of the layers to interaction with atomic oxygen (combustion for TBC and spacecraft re-entry phase for UHTC), we have measured the atomic oxygen recombination coefficient to determine the number of adsorption sites on the surface of the coatings. This was accomplished by using a low pressure plasma reactor coupled with optical spectroscopic measurements as a diagnostic technique.