Research On Hot Corrosion Of Novel Abradable Coatings Used In Aero-engines

With the rapid development of aviation industry,advanced propulsion engines with high efficiency and low fuel consumption are the objectives of the design and manufacture of aircrafts.Abradable coatings are employed in these gas turbine engines to reduce the gap clearance between rotating and stationary parts,i.e.the function of an abradable seal is to wear preferentially when rotating blades come into contact,and then to minimize the over-tip clearance,reduce fuel consumption and improve the overall efficiency of the engine.Naval aircrafts are always exposed to the marine environment where entrained salt in the wet air and high humidity are present.Generally,NaCl enters the turbines as an aerosol and deposits on the blades and porous coatings.Solid NaCl may accelerate the hot corrosion rates of the coatings at elevated temperature.But the current research of aero-engine corrosion is mainly focus on salt spray corrosion of the low-pressure compressor and hot corrosion of the high-pressure turbine.However,the hot corrosion of high-pressure compressor caused by solid NaCl is lack of attention,and little is known about the corrosion mechanism of coatings in naval aircrafts by NaCl alone especially at temperatures below its melting point(801℃).And the NaCl-induced hot corrosion process can not be explained by the sulfidation-oxidation mechanism and the salt fluxing mechanism.Thus it is momentous significance to research the abradability and NaCl-induced hot corrosion mechanism of abradable coatings which will be used in the marine environment.The abradablity,oxidation and hot corrosion resistant properties of NiCrAl-BN and CoCrAl-BN were investigated in the study.Then a systematic study was carried out on the hot corrosion mechanism caused by solid NaCl and water vapor.The effect of Mo and Al addition on the oxidation and hot corrosion resistance of Ni-based alloys was also studied.At last,NiCrMoAl-BN coating with good hot corrosion resistant property was fabricated successfully.The main research contents and conclusions are as follows:(1)Tribological behavior indicated that CoCrAl-BN had better abradablity so that Ti6Al4V blade wore less during high speed wear tests.The wear mechanisms of CoCrAl-BN were cutting and rupture.While the main wear mechanisms of NiCrAl-BN were smearing,crushing,adhesive transfer and melting.The NiCrAl-BN showed a better resistance to oxidation at 500℃-800℃ with higher oxidation activation energy which was 260 kJ·mol-1.And the oxidation activation energy of CoCrAl-BN was 116 kJ·mol-1.Ni2CrO4 formed on the surface of NiCrAl-BN,while CoO formed on the CoCrAl-BN coating.Ni2CrO4 could hinder the migration of oxygen and metal ions more efficiently than CoO.When the abradable coatings both exposured to the solid NaCl at 750℃,the oxide film of coatings was spalling,resulting in severe weightlessness.(2)The hot corrosion of Ni,Cr,and 80Ni20Cr in the presence of NaCl and water vapor at 750℃ was studied.The results indicated that no significant oxidation or corrosion effect of Ni was found in this case.However,a severe damage caused by the synergistic effect of NaCl deposit and water vapor on Cr or NiCr was found.TG-DSC and thermodynamics calculations were shown that Cr could react with NaCl or water vapor and form corrosive Cl2 or HCl,accelerating the corrosion of alloy by chlorination and oxidation.The element Ni in NiCr alloy was accelerated corroded by means of reactions among Cr and solid NaCl as well as water vapor.NaCl formed low melting temperature eutectics with Na2CrO4,CrCl2 and CrCl3 during corrosion.Under the cover of molten salt,the oxide film dissolves,resulting in accelerated corrosion.(3)The oxidation and hot corrosion of porous NiCrMo alloys with different Mo addition at 750℃ under cyclic procedure were investigated in order to elucidate the effect of Mo addition on oxidation and hot corrosion in the presence of NaCl.When the alloys exposure to air at 750℃,Mo accelerates the oxidation of Ni-based alloys due to M0O3 destroys the film stability.Without Mo addition,Cr2O3 formed on the surface.The oxide film consisted of NiO,NiCr2O4 and Cr2O3 with Mo addition.The hot corrosion tests showed that when the amount of Mo addition was more than 9.0%,the oxide film which consisted of NiO-NiCr2O4-Cr2O3 exhibited better matching ability with substrates.With the increase of Mo addition,the Kic of the oxide films increased.And the higher Kic of oxide film could be prevented cracking by thermal stress more effectively.(4)The effects of Al on the electrochemical properties of porous NiCrMo alloy and the hot corrosion resistance caused by NaCl deposited were investigated.The electrochemical tests showed that the addition of Al deteriorated the electrochemical properties of the alloys.With the increase of A1 content,the corrosion current density increased and the corrosion potential gradually decreased as well.Hot corrosion tests by NaCl suggested that the oxide films formed during the hot corrosion of the alloys showed good combination with the matrix without voids or cracks.And due to the selective oxidation of Al,the oxidation of Ni-based alloys could be reduced effectively.(5)Based on the experimental results and theoretical analysis mentioned above,NiCrMoAl-BN composite powder was prepared by hydrogen reduction,solid alloying,hydrochloric acid pretreatment technologies.And NiCrMoAl-BN abradable coating with excellent salt spray corrosion resistant and NaCl-induced hot corrosion resistant was fabricated by atmospheric plasma-spray.