Preparation And Performance Investigation Of Pt-modified Aluminide Coatings

For modern gas-turbine engines,the Turbine Entry Temperature(TET)has been increased continuously to meet the strategy of both increasing the power-output efficiency and reducing greenhouse emissions(e.g.,CO2,SO2 and NOx etc.).In order to enhance the resistance against high temperature oxidation/hot corrosion and prolong the life time of engine components working under higher temperatures,Pt-modified nickel aluminide coatings as well as a novel Hf-doped(Ni,Pt)Al coating were prepared by the electro-plating of Pt and Pt/Hf,and successive high-temperature-low-activity"above-pack" aluminization treatment.Firstly,the current piece of work systematically investigated the effect of aluminization parameters on microstructure of the acquired(Ni,Pt)Al coating.A multiple linear regression fitting method was used to establish the relationship between the final coating thickness and the aluminization parameters.The equation obtained after multiple linear regression was confirmed to be reliable in predicting the coating thickness after verifications by R-testing,F-testing as well as T-testing.The high temperature oxidation performance of(Ni,Pt)AI coating with different thickness of initial Pt layer at 1100? showed that the coating of 5 ?m Pt possessed the lowest oxidation rate,strongest scale adhesion and lowest maginitude of surface rumpling,indicating the best oxidation resistance in contrast to plain NiAl and(Ni,Pt)Al coatings with 2 and 8 ?m initial Pt platings.In the current study,an assumption was proposed explain the difference of surface rumpling,in which an interaction existed among phase transformation,creep deformation and scale spallation of the coating itself during the high temperature exposures.The oxidation resistance and performance of(Ni,Pt)Al coating are determined by the aspects mentioned above.Hot corrosion test of(Ni,Pt)Al coating with different thickness of initial Pt plating in the mixed salt of Na2SO4/NaCl(75:25,wt./wt.)at 900 ? indicated that a competition existed between the growth and the dissolution of oxide scale.Due to higher Pt content,samples with 8 ?m Pt have lower AI activity,leading to a thinner oxide scale during the initial stage of hot corrosion,which entitled S and Cl atoms diffuse through the oxide more easily than through the thicker oxide scale that formed on 5 ?m Pt samples.As a result,severer internal oxidation and sulfidation as well as higher dissolving rate of the oxide scale occurred in 8 ?m Pt samples.The dense and adherent oxide scale formed via pre-oxidation treatment at 1100 ? could slow down the diffusion of S and Cl atoms from diffusing inwardly and result in a smoother oxide scale/coating interface due to less internal oxidation and internal sulfidation.A Hf-doped(Ni,Pt)Al coating was prepared by firstly co-depostion of Pt-Hf composite layer and subsequent aluminisation treatments.Distribution of Hf was detected in the outer(Ni,Pt)AI as uniform solid solution with relatively low content(<0.1 at.%)and gathering between the outer(Ni,Pt)Al and interdiffusion zone(IDZ)as the form of Hf-rich belt.This unique structure of the Hf-doped(Ni,Pt)Al coating revealed superior oxidation resistance in the tests of both isothermal and cyclic manners oxidation test when compared with normal Pt-modified NiAl coatings.As indicated by the experimental results,the oxidation rate,the scale spallation tendency and the rumpling extent of(Ni,Pt)Al coating were drastically decreased by the modification of Hf-doping.Besides,the internal Hf-rich belt could partly act as the diffusion barrier to inhibit the element interdiffusion between the substrate and the coating,which in turn reduced the consumption rate of Al and therefore slowed down the coating degradation rate.In the hot corrosion test,a negative effect of Hf on the hot corrosion resistance was found because of an easier access S and Cl from the molten salts to the coating interior for Hf-doped(Ni,Pt)Al coating in comparison of Hf free(Ni,Pt)Al,where two main reasons may account for this phenomenon.Firstly the addition of hafnium delayed the transformation of transient oxide to stable oxide,which results in a less compact oxide film staying on the coating for longer time and S as well as Cl atoms could transport through the voids and gaps between needle-like AI2O3 easily.Besides,a thinner oxide scale formed on Hf-doped(Ni,Pt)AI due to further decrease of oxidation rate by Hf addition,resulting in a less effective barrier for S and Cl atoms diffusing.Chlorine in the molten salt could react with Hf and form volatile HfCl4,which accelerated the depletion of Hf in the coating.Thus Hf-doped(Ni,Pt)Al coating was not suitable serving in corrosive environment at low temperature,but competent in application at higher temperature that requires excellent oxidation performance and prolonged service life.