Oxidation and Thermal Cycle Behavior of Yb2Si2O7-c-AlPO4-SiCw-Mullite Multilayer Coatings

The mullite ceramic coating has excellent oxidation resistance in static environments, but it is easy to generate micro-cracks during the thermal cycling process due to the low toughness. Therefore, a novel cristobalite aluminum phosphate particle (c-AlPO4) and SiC whisker toughened mullite middle coating (c-AlPO4-SiCw-mullite, ASM) were prepared on the SiC coated carbon fiber reinforced SiC composites (C/SiC) by a novel sol–gel combined with air spraying method. Meanwhile, the ytterbium disilicate (Yb2Si2O7) top coating was prepared on the surface of the ASM middle coating to avoid excessive oxidation of SiC whiskers (Fig. 1). Results show that the bi-layer ASM/SiC coating with 20 wt.% of c-AlPO4 showed the most excellent oxidation resistance, the weight loss rate after the oxidation in ambient air for 210 h at 1773 K was merely 3.00 × 10–5 g cm−2 h−1. After 16 thermal cycles (240 min) between 1773 K and room temperature in burner rig tests, the weight loss of mullite/SiC coating coated samples was up to 38.49 × 10–3 g cm−2, while the weight losses of SM/SiC and ASM/SiC coatings coated samples were reduced to 14.09 × 10–3 g cm−2 and 5.59 × 10–3 g cm−2, respectively. However, although c-AlPO4 particles further improved the service life of SM coating in high-temperature environment, the overflow of POx gas aggravated the formation and expansion of cracks in the Yb2Si2O7 outer coating and caused the service life of overall Yb2Si2O7/ASM/SiC coating to be slightly lower than that of Yb2Si2O7/SM/SiC coating. This study guides the design of modified bi-layer and tri-layer EBCs with long service life in high-temperature and complex environments.