Plasma sprayed zirconia-base coatings are of general interest as thermal barrier coatings in diesel and gas turbine engines. However, residual pores and cracks in these coatings decrease the mechanical properties. Therefore the improvements to ZrO2- 25CeO2-2.5Y2O3 (CYSZ) coatings by SiC impregnation were studied. The effect of the nano-SiC sealing on the wear properties was also studied. The coatings were produced by an atmospheric plasma spraying process. The CYSZ coatings were sealed by repeated polycarbosilane (PCS) solution infiltration and pyrolysis. Through thermogravimetry analysis, the PCS precursor solution started to decompose at 100oC and finished at 800oC. After 800oC pyrolysis process, the SiC phase was amorphous. DSC analysis revealed that the crystallization of the amorphous silicon carbide started at about 1030oC and finished at 1100oC. The mean grain size determined using the Sherrer and Wilson formula from the breath of the SiC(220) line was not larger than 5 nm. Under dry condition sliding wear against sintered ZrO2-8Y2O3 (YSZ), weight losses were markedly decreased by SiC sealing. The sealing resulted in the wear transition, which corresponded to the change in the mechanism from a brittle fracture mode causing sever wear to a plastic deformation mode causing mild wear. The friction coefficient was also decreased by both the surface smoothing effect of the SiC sealing and the possible excess carbon of the non-stoichiometric SiC due to SiC sealing. During sliding wear of unsealed and sealed coatings, a stress induced tetragonal to monoclinic martensitic phase transformation, which deteriorates wear resistance, was not observed.

Keywords: Air plasma spray, CYSZ, PCS sealing, Nano SiC, Porosity, Wear resistance