Damage and Degradation Assessment of Stage 1 Bucket Coatings in a 1,100° C-Class Gas Turbine

According to increasing the inlet temperature of gas turbines, the role of coating is getting more and more important for the bucket life. It is, therefore, very important to understand the actual phenomena of the bucket coatings under the operation. Four different kinds of new and serviced stage 1 bucket coatings for 1,100°C-class gas turbines, such as a platinum-aluminide diffusion coating and overlay-type coatings of CoCrAlY, over-aluminized CoCrAlY and CoNiCrAlY, were selected and evaluated nondestructively and destructively at every inspection times. The platinum-aluminide and over-aluminized CoCrAlY coatings suffered from heavy cracking that propagated from the root of the airfoil to the tip during operations, but the CoCrAlY and CoNiCrAlY coatings did show little or no cracking. The degrees of oxidation and microstructure degradation were also different not only coating by coating, but also the locations of the airfoil surface due to the surface temperature distributions. To understand precisely the actual phenomena, hardness and 3-point bend tests at room and elevated temperatures were conducted by using the new and serviced bucket coatings and figured out there were good correlation between the hardness and the strain to cracking, which imply higher hardness coating at certain temperature has more sensitive to the cracking. The degrees of the coating crack conditions of the serviced buckets were the same as these fundamental test results. The effects of microstructure degradation on the hardness and the strain to cracking were also evaluated to understand why the cracking propagate to the higher temperature and lower centrifugal stress portion of mid-span and tip of the airfoil. The test results showed the microstructure degradation reduced the hardness and also reduced the sensitivity to the cracking, but still enough sensitivity to cause cracking. Formation of oxide layer was also found to enhance the cracking sensitivity by the further microstructure observation of the serviced buckets.Copyright © 2006 by ASME