LCF Life of As-Deposited and Annealed NiCr-Y Coatings for Oxidation and Hot Corrosion Protection of Disk Alloys

Increasing temperatures in aero gas turbines is resulting in oxidation and hot corrosion attack of turbine disks. Since disks are sensitive to low cycle fatigue (LCF), any environmental attack, and especially hot corrosion pitting, can seriously degrade the life of the disk. Application of metallic coatings is one means of protecting disk alloys from this environmental attack. However, the presence of a metallic coating can degrade the LCF life of a disk alloy. Therefore, coatings must be designed which are not only resistant to oxidation and corrosion attack, but do not significantly degrade the LCF life of the alloy.Three different NiCr-Y coating compositions (29, 35.5, 44 wt.% Cr, all with 0.1 wt.% Y) were applied at two thicknesses by Plasma Enhanced Magnetron Sputtering (PEMS) to two similar Ni-based disk alloys. One Ni-35.5Cr-0.1Y coating also received a thin ZrO2 overcoat. The coated samples were also given a short thermal anneal in a low PO2 environment to encourage bonding of the coating and substrate as well as initiating formation of a chromia scale. Without further environmental exposure, the LCF life of coated and uncoated samples was evaluated at 760°C in air. The LCF lifetime of all coated samples was less than that of uncoated samples. The LCF life scaled with the Cr content and the high-Cr, thin coating showed the highest LCF life. Pre and post-test characterization of the various coatings, including identification of crack initiation sites, will be presented and the effect of the coating on the LCF life discussed.