Influences Of Several Coatings On High Temperature Oxidation And Hot Corrosion Behaviors Of Single Crystal Superalloy DD98M

DD98M alloy is a second generation single crystal nickel-based superalloy without Re addition,which is developed on the basis of DD98 alloy by adjusting the level of alloying elements such as Ti and Al.Owing to have a combination of attractive strong points such as low cost and high strength,it is expected to be used as turbine blade material of advanced aircrafts.The working environment of gas turbines is extremely harsh,so the materials must exbihit both mechanical strength and excellent corrosion resistance at elevated temperatures.In this thesis,influences of Ni-27Cr-11Al-0.5Y and Ni-19Co-27Cr-11Al-0.5Y-0.2Hf-0.1Si(wt.%)coatings deposited by arc ion plating,thermal barrier coatings(TBCs)with 8wt.%Y2O3-ZrO2 ceramic top coat deposited by electron beam physical vapor deposition and Ni-27Cr-11Al-0.5Y bond coat deposited by arc ion plating and sputtered nanocrystalline coating with the same composition as DD98M alloy deposited by magnetron sputtering on high temperature oxidation and hot corrosion behaviors of single crystal alloy DD98M were investigated,using thermo-gravimetric analysis(TGA),X-ray diffraction(XRD),scanning electron microscopy(SEM)equipped with energy dispersive spectroscopy(EDS),electron probe micro-analyzer(EPMA)and transmission electron microscopy(TEM).The main research results are summarized as follows:Complex oxides of NiO,?-Al2O3,Ta0.8O2,CrTaO4 and spinel(Ni,Co)Cr2O4 formed on the surface of DD98M alloy when oxidized at 900°C-1100°C.At lower temperature(900?-950?)and in a short term,the oxide scales formed on DD98M were adherent,dense and protective.With the oxidation temperature and time increasing,the cracking and the spallation of the scales became serious.Inner nitridation occured and black faceted-like AIN particles and gray particle-like and strip-like TiN particles formed in the surface layer of the alloy.After exposure for a short term in molten Na2SO4+25wt.%K2SO4 and Na2SO4+25wt.%NaCl salts at 900°C,catastrophic corrosion of DD98M alloy occurred and the corrosion products formed on DD98M alloy were thick and spalled seriously.A continuous and dense a-Al2O3 formed on the surface of the NiCoCrAlYHfSi-coated and NiCrAlY-coated DD98M alloy when oxidized at 900?-1050°C.The adherence of the oxide scale formed on the NiCoCrAlYHfSi coating was much better than that on the NiCrAlY coating.The application of the NiCrAlY and NiCoCrAlYHfSi coatings greatly improved the oxidation resistance of DD98M alloy.At 1100°C,the oxide scales formed on the two types of MCrAlY coatings were cracked and spalled seriously and led to the depletion of aluminium in the coatings.The unitary a-Al2O3 was not able to form any more.Inter-diffusion occurred between the two types of MCrAlY coatings and the substrate,which led to the formation of an inter-diffusion zone(IDZ)and a secondary reaction zone(SRZ)in the surface layer of DD98M alloy.Coating composition has a significant effect on the formation of the IDZ and SRZ.Thermal exposure temperatLure also has influences on the development of the SRZ.The IDZ of the NiCrAlY coated specimen were composed of the y phase and the Al and Ta-rich ?'phase.The y' phase in the IDZ accommodated most of the inward diffusing aluminum,so the SRZ formation was suppressed when oxidized at 1050? and 1100?.However a SRZ with?-TCP still formed when oxidized at 950? and 1000?.The IDZ of the NiCoCrAlYHfSi coated specimen was a single y phase.A large amount of ?-TCP precipitated in the SRZ of the NiCoCrAlYHfSi coated specimen when oxidized at 950°C-1050°C,but the amount of?-TCP decreased when oxidized at 1100?.In molten Na2SO4+25wt.%K2SO4 salt at 900?,continuous and dense a-Al2O3 oxide scale formed on the two types of MCrAlY coatings after 100h corrosion,which greatly improved the corrosion resistance of DD98M alloy.In molten Na2SO4+25wt.%NaCl salt at 900?,?-Al2O3 oxide scale formed on the NiCoCrAlYHfSi coating and complex oxides scale formed on the NiCrAlY coating after 100h corrosion.Two types of MCrAlY coatings improved the corrosion resistance of DD98M alloy in molten Na2SO4+25wt.%NaCl salt and the corrosion resistance of the NiCoCrAlYHfSi coating was better than that of the NiCrAlY coating.TBCs greatly improved the oxidation resistance of DD98M alloy when oxidized at 900?-1050?.After 500h oxidation,neither cracking nor spallation of TBCs was observed and a thin thermally grown oxide(TGO)layer formed between the yttria stabilized zirconia(YSZ)top coat and the NiCrAlY bond coat.The thickness of the TGO increased with the oxidation temperature and time increasing.However,the YSZ layer locally spalled and led to the failure of TBCs after 300h oxidation at 1100°C.When exposed in molten Na2SO4+25wt.%K2SO4 and Na2SO4+25wt.%NaCl salts at 900°C for 100h,an adherent and dense TGO was formed between the YSZ top coat and the NiCrAlY bond coat.TBCs greatly improved the corrosion resistance of DD98M alloy in the sulfate and chloride salts.DD98M sputtered nanocrystalline coating possesses a typical columnar structure and the width of the columnar grains ranges from 30 to 100 nm.The coating is only composed of y phase and exhibited an apparent[111]texture.The oxide scale formed on the sputtered nanocrystalline coating was adherent and it was mainly composed of ?-Al2O3 when oxidized at 900?-1100?.No obvious inter-diffusion was observed between the coating and the substrate during the thermal exposure and the SRZ formation was effectively suppressed.Nanocrystallization promoted the selective oxidation of aluminum at the initial stage of the oxidation and the formation of unitary alumina external scale,and enhanced the adhesion of the oxide scale,thus improved the oxidation resistance of the DD98M alloy.The sputtered coating greatly improved the corrosion resistance of DD98M alloy in molten Na2SO4+25wt.%K2SO4 salt and the pre-oxidation treatment of the sputtered coating led to an even better effect.However,in molten Na2SO4+25wt.%NaCl salt,the corrosion resistance of DD98M alloy was not significantly improved by the nanocrystalline coating and the pre-oxidized coating.