Development And Evaluation Of Pt Modified Mcraly Coatings

In order to achieve a high thrust-weight ratio,the operating temperature of aero-engines continues to increase and hot sections are faced with an extreme operating environment.The MCrAlY overlay coatings have been widely used in hot sections of aero-engines.The consumption of Al in the MCrAlY coating leads to the failure of coating.In this paper,the Pt modified MCrAlY coating,Pt modified gradient MCrAlY coating and Pt modified MCrAlY+AlY composite coating were fabricated.The XRD,SEM,EDS,TEM and EPMA were used to analyze the microstructure of the coating and its evolution or degradation under high temperature oxidation or hot corrosion conditions.The Pt+NiCrAlYSi coating and NiCrAlYSi+Pt coating were prepared by combining electroplated Pt and arc ion plating NiCrAlYSi.The main phase of the Pt+NiCrAlYSi coating were ?'-Ni3Al,?-NiAl and ?-Cr phase.For the Pt+NiCrAlYSi coating,the Pt mainly distributed in the inner part of the coating and substrate.No trace of Pt was observed in the outer part of the coating.In contrast,the Pt mainly distributed in the outer part of NiCrAlYSi+Pt coating and no Pt diffused to the substrate.After isothermal oxidation at 1100 ?,The weight gains of the NiCrAlYSi coating,Pt+NiCrAlYSi coating and NiCrAlYSi+Pt coating were 1.81 mg/cm2,1.68 mg/cm2 and 1.37 mg/cm2,respectively.The NiCrAlYSi+Pt coating had the lowest oxidation rate among three kinds of coatings.Although the continuous oxide scales formed on the surfaces of all the coatings,the morphologies of Al2O3 particles were obviously different.The needle or whisker-like Al2O3 particles can be observed on the NiCrAlYSi coating.The coarse nodule-like Al2O3 particles were formed on the Pt+NiCrAlYSi coating.With the increasing of oxidation time,the NiAl2O4 spinel phases were observed on the NiCrAlYSi coating and Pt+NiCrAlYSi coating,while the Al2O3 was still the only oxidation product on the NiCrAlYSi+Pt coating.After exposure of 300 h,the intensities of NiAl2O4 spinel phases for the NiCrAlYSi coating and Pt+NiCrAlYSi coating became stronger.However,only particularly weak NiAl2O4 spinel diffraction peaks were detected on the NiCrAlYSi+Pt coating.According to the spalled oxides weight of three kinds of coatings after the cyclic oxidation,the resistance of alumina scale spallation was in the order of NiCrAlYSi+Pt coating,Pt+NiCrAlYSi coating and NiCrAlYSi coating.It was more helpful to improve the oxidation resistance of the NiCrAlYSi coating that the Pt layer electroplated on the surface of NiCrAlYSi coating.The gradient NiCrAlYSi coating and Pt modified gradient NiCrAlYSi coating were fabricated on a Ni-base superalloy using arc ion plating(AIP)?electroplated Pt and subsequent chemical vapour deposition(CVD).Both gradient coatings were compsed of three layers:a pure ? phase outer layer,a middle layer of ? phase with some precipitates and a bottom layer with a high density of precipitates phase enriched in Cr.According to results of high temperature oxidation tests,Pt modified gradient NiCrAlYSi coating had the best oxidation resistance performance.After oxidation tests,the accumulation of deformation causes the surface rumpling.Therefore,the volume reduction causing by ? phase to ?' phase transformation increased the morphology instability and resulted in surface rumpling during oxidation.After 10 h hot corrosion in the the mixed salt of 25 wt.%K2SO4+75 wt.%Na2SO4,most of the Al2O3 particles were in nodule-shaped or needle-shaped morphology on the surface of aluminized coating.However,for Pt modified aluminized coating,only nodule-shaped Al2O3 particles were observed.After hot corrosion for 120 h in the 25 wt.%NaCl + 75 wt.%Na2SO4,the serious internal sulfidation and oxidation were found in both coatings.According to EDS results and thermodynamic calculation,the internal oxide was Al2O3 and internal sulfide was CrS for both coatings.The NiCrAlYSi+Al coating,NiCrAlYSi+AIY coating and Pt modified NiCrAlYSi+AlY composite coating were fabricated by using arc ion plating(AIP),electroplated Pt and subsequent vacuum annealed.For three kinds of coatings,the distribution of Al and Cr were similar.The A1 exhibited a downhill gradient distribution from outer layer to inner layer.The Cr mainly distributed outside and inside the coating with less distribution in the middle region.The isothermal oxidation rate at 1100 ? was in the order of NiCrAlYSi+Al coating,NiCrAlYSi+AIY coating and Pt modified NiCrAlYSi+AlY composite coating.According to the results of cyclic oxidation at 1200 ?,the Pt modified NiCrAlYSi+AIY composite coating had the best alumina scale spallation resistance,the resistance of alumina scale spallation for the NiCrAlYSi+Al coating was the worst.The pinning effect in the alumina scale played an important role in enhancing the resistance of oxide scale spallation for the Pt modified NiCrAlYSi+AIY composite coating.After hot corrosion in the 25 wt.%NaCl+75 wt.%Na2SO4 for 120 h,the internal sulfidation and oxidation in the Pt modified NiCrAlYSi+AlY composite coating was the slightest among three kinds of coatings.The addition of Pt can improve the hot corrosion resistance of the coating.