Study On High Temperature Oxidation And Hot Corrosion Resistance Of Co/Pt Modified Aluminide Coatings

Aluminide coatings are widely used as high-temperature protective coatings for aero-engine turbine blades.However,with the continuous increase of the inlet temperature of aero-engine turbines,the service environment of turbine blades is becoming more and more severe,and ordinary aluminide coatings can no longer meet the requirements of ultra-high temperature and heat resistance.In this thesis,aluminide coating,Pt-modified aluminide coating,Co-modified aluminide coating and Co/Ptaluminide coating were prepared on the surface of nickel-based single crystal superalloy substrate by electroplating and high temperature vapor aluminizing method.The microstructure,high temperature oxidation and hot corrosion behavior of the four coatings were systematically observed,revealing that Co,Ptand Co/Ptsynergistic effect on the high temperature oxidation and hot corrosion resistance of aluminide coatings influence mechanism.The main results are as follows:The prepared NiAl,NiPtAl,NiCoAl and NiCoPtAl coatings are composed ofβ-NiAl phase outer layer（OZ）and interdiffusion zone（IDZ）.All four kinds of coatings are diffusion-type coatings,which are dense and well bonded to the substrate.The content of Al element in the four coatings is sufficient（14.5～18.9 wt.%）,and the coating thicknesses are28.2μm,43.8μm,28.6μm,and 44.3μm,respectively.Among them,the NiPtAl coating and the NiCoPtAl coating are slightly thicker,mainly because of the high temperature.During the vapor phase aluminizing process,Ptpromotes the diffusion of Al and Ni.After constant temperature oxidation at 1100℃ for 300 h,the weight gain of NiPtAl coating,NiCoPtAl coating and NiCoAl coating increased in turn,and the thickness of oxide film（α-Al₂O₃）of the three coatings increased in turn,among which the oxide film of NiPtAl and NiCoPtAl coating Dense and complete,and the oxide film of NiCoAl coating has obvious peeling.The analysis shows that Ptreduces the growth rate of the oxide film of the aluminide coating and improves the adhesion of the oxide film;while Comakes the oxide film of the aluminide coating grow rapidly to form protection,but the adhesion of the oxide film to the substrate is insufficient.In the later stage of constant temperature oxidation,the oxide film is thick and easy to peel off;under the synergistic effect of Co/Pt,anα-Al₂O₃film with good adhesion,high density and moderate thickness is formed on the surface of the NiCoPtAl coating,so it has good high temperature resistance.Oxidation properties.After 100 h hot corrosion of sulfate Na₂SO₄/K₂SO₄（75:25,wt./wt.）at 900℃,the NiCoPtAl coating did not peel off obviously,and the degree of internal sulfide and internal oxidation was not obvious.The addition of Co/Ptmade the aluminide coating resistant to sulfuric acid Salt thermal corrosion performance is improved.The microstructure observation shows that the addition of Copromotes the formation of oxide film at the initial stage of hot corrosion,and the diffusion rate of S element in Cois lower than that in Ni.Therefore,the addition of Coeffectively alleviates the sulfidation/oxidation cycle reaction during the corrosion process and improves the hot corrosion performance of the aluminide coating.Under the synergistic effect of Co/Pt,the NiCoPtAl coating forms a dense oxide film,which improves the adhesion and self-healing property of the oxide film,inhibits the diffusion of sulfate and products in the coating,and makes the coating exhibit excellent performance.Good resistance to sulfate hot corrosion.After 100 h hot corrosion of mixed salt Na₂SO₄/NaCl（75:25,wt./wt.）at 900℃,the NiAl coating and NiCoAl coating show obvious peeling and cracking phenomenon,the oxide film of NiPtAl coating and NiCoPtAl coating is relatively complete,and there is no Clearly fall off.The microstructure observation shows that the addition of Copromotes the formation of oxide film at the initial stage of hot corrosion,but with the increase of corrosion time,Coreacts with Cl^-in the coating to form volatile CoCl₂,which makes the coating resistant to hot corrosion.sharply decreased.Under the synergistic effect of Co/Pt,the oxide film in NiCoPtAl coating can effectively inhibit the diffusion of Na₂SO₄/NaCl corrosion salt in the coating,so that the coating still exhibits good hot corrosion resistance in high temperature Cl^-containing environment.