Cast nickel aluminides, alloyed with combinations of Cr, Pt, and Hf were tested in l-h cycles under hot corrosion conditions at 950°C, and in oxygen at 1150°C. In addition to these model alloys, a cast alloy resembling the composition typically found in commercial platinum aluminide coatings was tested. Hot corrosion tests clearly demonstrated that Cr rather than Pt was the alloying addition needed to decidedly improve hot corrosion resistance of NiAI. Pt-containing alloys showed no improvement in hot corrosion resistance as compared to rapidly attacked Hf-doped NiAI. However, as little as 2 at.% Cr retarded the rate of hot corrosion attack significantly. Hot corrosion resistance improved with increasing Cr content, with preoxidation prior to hot corrosion testing resulting in further improvement for low Cr additions only. At 1150°C, Cr degraded oxidation resistance; both oxide scale formation rate and susceptibility to scale spallation of Cr-containing NiAI+Hf were increased relative to simple Hf-doped NiA1. The addition of Pt to undoped NiAI improved oxidation resistance by improving spallation resistance, however, Pt was less effective than reactive element doping with Hf. The cast platinum aluminide coating composition exhibited poor hot corrosion and oxidation resistance relative to Cr-containing and Hf-doped NiA1, respectively. In essence, the present results on cast aluminides indicated that hot corrosion resistance and exceptional scale spallation resistance appears to be mutually incompatible goals for coating performance.
Aluminide coatings, deposited either by pack cementation or by chemical vapor deposition techniques (CVD), have been applied to gas turbine vane and blade airfoils since about 1970 I. Over 80% of all coated first stage blade airfoils are estimated to be coated by these techniques, indicating the enormous market share captured by these coatings. More recently, aluminide * Part of this work was performed while the author was on sabattical at Oak Ridge National Laboratory. coatings were recognized as suitable bond coats for thermal barrier coatings (TBCs), in particular electron beam physical vapor deposited (EB-PVD) TBCs. The need for improved adherence of the protective oxide scale, in particular if used in TBC applications, has led to contemporary use of platinum-modified aluminide coatings for high performance gas turbine components, although the mechanism by which Pt exerts this beneficial effect on scale adhesion is not well understood. Early experiments by Jackson and Rairden 2 indicated that platinum remains most concentrated at the coating-gas interface, thereby retarding diffusion of certain refractory elements to the coating-alumina scale interface and thus improving oxidation resistance. Schaeffer et al. 3 pointed out that Pt additions improved oxidation and hot corrosion resistance of coated superalloys, although the superiority of platinum aluminides was not fully attributed to a Pt-related adherence effect. Rather Pt changed the growth mechanism of the coating from outwardly growing without any Pt to inwardly growing when Pt was present. Improved isothermal and cyclic oxidation resistance was believed to be mainly attributed to a purity effect with Pt retarding diffusion of deleterious elements to the coating-alumina interface. Improvement of hot corrosion resistance by Pt additions compared to conventional aluminide coatings was attributed to Pt beneficially affecting scale adherence and cracking resistance 3. Haynes et al. 4 and Zhang et al. 5 recently observed the following influences of Pt on scale adhesion on CVD coatings on a Ni-base superalloy: (I) mitigation of the detrimental effects of high sulfur levels, (2) drastic reductions
