Preparetion And Evaluation Of Modified MCrAlY (M=Ni,NiCo) Coatings

In this paper,the NiCrAlYSc coating,NiCoCrAlYHfZr coating and the gradient NiCoCrAlYHfZr coating were prepared by arc ion plating.The XRD,SEM/EDS,TEM and EPMA were used to study the coating microstructure as well as the evolution and degradation process of the coating under high temperature oxidation or hot corrosion conditions.The main results are as follows:The NiCrAlYSc coatings with different Sc content were prepared on DD26 by arc ion plating technology.All the coatings were composed of ?'/? phase and ?-NiAl phase after annealing.During isothermal oxidation at 900?.,the weight gain of 0.2 wt.%Sc-modified coating was the smallest and a dense ?-Al2O3 scale was formed on its surface,while the surface of the other three coatings were covered by needle-like oxides.Under the condition of isothermal or cyclic oxidation at 1100?,there was no obvious scale spallation on the surface of the 0.2 wt.%Sc-modified coating.On the contracy,the other three coatings showed different degrees of spallation and some NiAl2O4 could be detected.Small addition of Sc in the coating can improve the scale adhesion,thereby improving the oxidation resistance of the coating.But addition of Sc had a limited improvement in the hot corrosion performance.In Na2SO4+K2SO4 mixed salt,all the coatings entered into the accelerated corrosion stage after 100 h,but 1.0 wt.%Sc-modified coating performed better hot corrosion resistance in the stable corrosion stage due to a uniform and continuous Al2O3 scale was formed on its surface,while in the mixed salt containing NaCl,0.4 wt.%Sc-modified coating showed strongest resistance for the hot corrosion,which had the lightest degree of internal oxidation and internal sulfidation.The NiCoCrAlY coating and NiCoCrAlYHfZr coating was prepared on the fourth-generation superalloy DD91 by using arc ion plating technology.The two coatings consisted of ?'-Ni3Al,?-(Ni,Co)Al and a small amount of NiCoCr phase,and ? phase was dispersed in the coating.According to the results of isothermal oxidation at 1100? and cyclic oxidation at 1150?,the NiCoCrAlYHfZr coating exhibited better protection ability.The addition of Hf and Zr was beneficial to improve the scale adhesion,therefore the high temperature oxidation resistance of the coating was enhanced.In addition,the elements interdiffusion occured in both coating during the high temperature oxidation,but a thinner secondary reaction zone was formed in the NiCoCrAlYHfZr coating,the size and quantity of the TCP phases precipitated in this area were significantly smaller than those in NiCoCrAlY coating.After hot corrision in the mixed salt of 75 wt.%Na2SO4+25 wt.%K2SO4 at 900 ?,there was no obvious internal oxidation and internal sulfidation in both coatings,so they both had good corrosion resistance.The presence of NaCl in the mixed salt intensified the hot corrosion of the two coatings,but the oxide scale on the surface of the NiCoCrAlYHfZr coating effectively hindered the development of internal oxidation and internal sulfidation,thus improving the hot corrosion resistance of the coating.Gradient NiCoCrAlY coating and gradient NiCoCrAlYHfZr coating were prepared on DD91 by arc ion plating.The Al-rich ? phase occupied the whole outer layer of the two coatings,while the inner layer consisted of ?' phase and ? phase.It can be seen clearly that there is little difference in oxidation rate and mass gain between the two coatings during isothermal oxidation at 1100?,but the oxide scale of gradient NiCoCrAlY coating spalled seriously after oxidation for 1000 h.In the process of cyclic oxidation at 1150?,the gradient NiCoCrAlYHfZr coating showed better resistance to scale spallation.During high temperature oxidation,the ?phase in the coating gradually transformed into the ?' phase due to the consumption of Al.In addition,there is a higher Al concentration gradient between the coating and the substrate,so the mutual diffusion is more serious.During the hot corrosion of Na2SO4+K2SO4,the oxide scale on the surface of gradient NiCoCrAlY coating was obviously thickened and contained a large amount of sulfide,while the oxide scale of gradient NiCoCrAlYHfZr coating was still pure Al2O3 and the coating showed a lower weight gain.The results showed that the two kinds of coatings suffered more serious hot corrosion in Na2SO4+NaCl mixed salt.The gradient NiCoCrAlYHfZr coating had a lower corrosion rate,indicating that gradient NiCoCrAlYHfZr coating can provide better protection against hot corrosion attacks than gradient NiCoCrAlY coating.