Preparation And High Temperature Corrosion Behavior Of Aluminized Nanocrystalline Coating

Application of nanocrystalline coating with same composition on the surface of single crystal superalloy can avoid the formation of harmful brittle phases arising from the interdiffusion of the coating and the substrate,meanwhile significantly improve high temperature oxidation resistance of the substrate alloy.However,hot corossion resistance of the nanocrystalline coating should be improved.In present work,a novel aluminized nanocrystalline coating was prepared,and the oxidation and hot corrosion behavior of the coating was investigated.A nanocrystalline coating with the same composition of DD98 M alloy was deposited on a DD98 M single crystal superalloy using magnetron sputtering technique,and the thickness of the coating was about 25 microns.Then AlSi coating with thickness of about 5-15 microns was deposited on it using multi-arc ion plating technique.The as-prepared nanocrystalline+Al-Si composite coatings were vacuum annealed at 750?,870? and 1080?,respectively,and the influences of annealing temperature and time,the thickness and Si content of the AlSi coating on the microstructure of the coatings after annealing were investigated.The coatings formed after annealing all contain three layers.The outer layer was loose and porous.XRD results showed that it was Ni₂Al₃ when anealed at 750? and?-NiAl when anealed at 870? and 1080?.The middle layer was composed of?-NiAl phase with precipitations rich in Cr and Si.The inner layer was composed of??-Ni₃Al phase with precipitations rich in W and Ta.The variation on thickness and Si content of AlSi coating has little effect on the microstructure of the composite coating formed after anealing.Dense aluminized nanocrystalline coating with double layers was produced after the loose and porous outmost layer was stripped by sandblasting.The outer layer of the aluminized nanocrystalline coating was composed of?-NiAl phase with precipitations rich in Cr and Si.The inner layer was composed of??-Ni₃Al phase with precipitations rich in W and Ta.After oxidation at 1050?,mixed oxides such as NiO,?-Al₂O₃,Ta_0.8O₂,CrTaO₄and NiAl₂O₄ formed on the surface of the DD98 M alloy,and the oxide film cracked and spalled seriously.A thin?-Al₂O₃ layer with a small amount of NiAl₂O₄ formed on the surface of the nanocrystalline coating.A pure?-Al₂O₃ film formed on the surface of the aluminized nanocrystalline composite coating.Nanocrystalline coatings and composite coatings significantly improve the oxidation resistance of the alloy.When exposed to Na₂SO₄+25%K₂SO₄ at 900?,corrosion resistance of the alloy was improved by the preoxidized nanocrystalline coating and significantly improved by the aluminized nanocrystalline coatings.When exposed to Na₂SO₄+25%NaCl at 900?,catastrophic corrosion occurred for the preoxidized nanocrystalline coating after short time.The three aluminized nanocrystalline coatings with different Si content significantly improve the corrosion resistance of the alloy.The higher the Si content of the coating is,the better the corrosion resistance is.