Mcraly Coating Protective Performance Of Nickel-base Single Crystal Alloy Studied

Becauses of the excellent high temperature mechanical performance, the single crystal superalloy have been chosen as the primary aeroengine materials. Howerver, a protective coating must be performed due to its poor high temperature oxidation resistance and hot corrosion resistance to molten salt.In this paper, the NiCoCrAlYTa coating was sprayed on the substrate of the single crystal superalloy by low pressure plasma spray technique, and then the sprayed coating was vacuum heat treated. The phase composition, microstructure, adhesive strength and toughness of sprayed coating and heat treated coating were characterized by using the XRD, SEM, EDS, EMPA and micro-hardness tester analysis equipment. The high temperature oxidation resistance and the hot corrosion resistance in the Na₂SO₄ melted salt and 75%(wt%)Na₂SO₄+25%(wt%) melted salt were investigated. The results show that:The sprayed coat possesses dense and even micro-structure with very low porosity. The sprayed coating is mostly composed byγ-Ni andβ-NiAl, also with a few content ofγ' -Ni₃Al. After heat treat, mutural elemental diffusions between the coating and single crystal superalloys take place, which revealed the good adhesive strength for the treated coating.The oxidation resistance for coated single crystal superalloy at 900℃is much better than that of single crystals without coating. The single crystals without coating present great weight lost after 100h at 900℃, while the coated single crystal exhibited excellent oxidation resistant, because a dense protective film was formed after 200h at 900℃, 1000℃and 1100℃. The oxidation kinetics curve tends to be logarithm shape at 900℃, a near-parabolic law at 1000℃, and cube curve at 1100℃. The mass gain was below 0.003mg/cm . The oxidation films present as different crystal structure at various temperature.Effectively protective films did not form in the single crystals at the Na₂SO₄ molten salt and the 75%(wt%)Na₂SO₄+25%(wt%) molten salt, which lead to occurance of internal oxidation and sulfidation,and finally the breakdown and exfoliation of superalloy in a very short time. NiCoCrAlYTa coated superalloy present good corrosion resistance at the Na₂SO₄ molten salt and the 75%(wt%)Na₂SO₄+25%(wt%) molten salt., due to the formation of continuous and dense Al₂O₃ films. But the latter reveal that crack, slight internal oxidation and internal sulfidation take place in mixed molten salt corrosion because of the strong penetration and corrosion of Cl^-.