The Study On High Temperature Oxidation Resistance Of Coating On GH586 Nickel Base Superalloy Prepared By Pack Cementation Process

Nickel base superalloys have excellent high temperature properties,widely used in the manufacture of high-temperature parts in the aerospace and industrial gas turbines.In high temperature oxidation and corrosive environment,the superalloys ar e easy to be oxidated and corroded.Chromium element plays an important role in the growth of oxide scale,which could be used to protect the superalloy.In the long term oxidation and corrosion of superalloy,chromium element is consumed to produce the protective oxide scale.Adding alloy elements like aluminium and chromium are positive to improve the operating temperature,but it could have the negative influence on the high temperature mechanical properties.Two kind of coatings are prepared by pack cementation process on nickel base superalloy GH586.The first coating is Y-Cr-Al coating prepared by one step pack cementation process,and the second coating is prepared by two step pack cementation process on the Y-Cr-Al coating.After that,high temperature oxidation test are conducted and the high temperature resistance are discussed.The following conclusion can be drown:(1)One step pack cementation experiment result shows that it could obtain the coatings with variety organization and performance when different Y2O3 in the pack powder.When there is no Y2O3 in the pack powder,too much pores are shown in the coating,the thickness of the coating is 10μm,and the hardness of the coating reaches 598 HV.When the Y2O3 increased to 1%,the coating becomes compact and dense,the thickness of the coating reaches 42μm,and the hardness of the coating goes up to 615 HV.When the Y2O3 increased to 2%,the thickness of the coating reach 44μm,the coating is still compact and dense,the Cr-rich precipitate are founded in the coating,the hardness of the coating goes up to 698 HV.When the Y2O3 increased to 3%,the thickness of the coating decreases greatly to 24μm,but the coating is still dense and compact,the Cr-rich precipitates are still shown in the coating,the hardness of the coating goes up greatly to 928 HV.(2)The high temperature oxidation tests are conducted to test the oxidation resistance of the Y-Cr-Al coating at 1000℃,and then the high temperature oxidation resistance of the coatings are analyzed.It can be found that all the coatings are positive to improve the oxidation resistance of the alloy.Y2O3 addition in the pack is positive to improve the high temperature oxidation resistance of the coating.When the Y2O3 addition is 2%,the coating have the best high temperature oxidation resistance,and the oxide scale consist of fine and dense α-Al2O3.While the Y2O3 is 3%,the high temperature oxidation resistance decrease a little,and the oxide scale consist of bigger α-Al2O3.(3)Two step pack cementation prepared coatings could be obtained on the surface of first step pack cementation coating.After second step pack cementation,the dimension of alloy grain remains unchanged,which is still 10-25μm.The two step pack cementation prepared Al coating、Y-Al coating and Y-La-Al coating present different hardness,reaching 685 HV,774HV and 767 HV,respectively.Furthermore,all the coating are dense and compact,the thickness of the coating reaches 285μm、200μm and 190μm,respectively.(4)The high temperature oxidation tests are conducted to test the oxidation resistance of two step pack cementation prepared coatings at 1000℃,it can be found that all the two step pack cementation prepared coatings have better high temperature oxidation resistance compared with one step pack cementation prepared coatings.The weight gain of two step pack cementation prepared Al coating,Y-Al coating and Y-La-Al coating after 150 h oxidation at 1000℃ are 0.65mg/cm2,0.6mg/cm2 and 0.32mg/cm2,respectively.After oxidation,all the coatings consist of β-Ni Al phase,which can provide enough protection for the substrate.The oxide layers consist of θ-Al2O3 and α-Al2O3,which present the condition of transition of θ-Al2O3 to α-Al2O3.Compared with two step pack cementation prepared Al coating,the two step pack cementation prepared Y-Al coating and Y-La-Al coating shows better high temperature oxidation resistance.