2004 Volume 45 Issue 5 Pages 1788-1793
The durability of thermal barrier coatings (TBCs) at gas turbine has been improved substantially by instituting more strain tolerant zirconia-ceramic top coatings with vertical cracks artificially, such as ZrO2-Y2O3 plasma-sprayed coating with purposely segmentation. Under the necessity of TBCs with thermal shock resistance and durability at high temperature, we have developed the top coating of 2CaO·SiO2-10∼30 mass%CaO·ZrO2 (C2S-10∼30%CZ) with many vertical microcracks that is formed with the spraying process only, namely one vertical microcrack does not pierce through the sprayed layer but remains within a single flattened ceramic particle of the top coating. In order to discuss the mechanism of the vertical microcracking, the flattening-solidification behavior of the C2S-10∼30%CZ sprayed splat was investigated. Microcrack was observed within the splat of C2S-10∼30%CZ particle after solidification on the polished SUS304 stainless steel.
It is assumed that spraying particle liquefied in plasma flame collides and strongly adheres on the substrate or the accumulated particles and change the phase from liquid to brittle solid due to rapid cooling, at the same time, the tensile stress is generated on the external surface of the particle and fine vertical microcrack occurs in the particle which could not withstand this stress.
From strong correlation between the vertical microcracking and amorphous like behavior of the C2S-10∼30%CZ coating, the liquid phase of C2S-10∼30%CZ particle sublimated some CaO in plasma flame is considered to take an important role in vertical microcracking formation due to shrinkage during the cooling and solidifying process.
