Preparation And Properties Of Anti-oxidation Coating On Mo-Cr-Al Alloy

With the continuous improvement of the speed of aerospace vehicles,higher requirements are put forward for the comprehensive performance of materials such as lightweight,high temperature oxidation resistance and high temperature mechanics.Molybdenum is an ideal aerospace material with high melting point and good high temperature strength.However,its poor high temperature oxidation resistance and high density restrict its engineering application.Therefore,it is urgent to develop low density molybdenum alloy materials with excellent high temperature oxidation resistance.In this paper,based on the actual demand of low density and high temperature oxidation resistance,pure Mo and Mo-30Cr-10Al alloy sheets were prepared by powder metallurgy method through alloy system design,and silicide coatings were prepared on the surface of the matrix alloy by embedding infiltration method,and rare earth Y was introduced to modify the silicide coatings.The phase composition,microstructure and element distribution of the coating were analyzed by XRD,SEM and EDS.The growth process,high temperature oxidation resistance behavior and rare earth Y modification mechanism of the coating were studied.（1）Si-containing coatings were prepared on pure Mo and Mo-30Cr-10Al alloys by double-layer coating infiltration.The density of Mo-30Cr-10Al alloy sheet after Si infiltration is 6.91 g/cm³,which is 23.9%lower than that of pure Mo 9.09 g/cm³.The microhardness at room temperature is 695.4,which is 112.7%higher than that of pure molybdenum.After siliconizing on pure Mo,Mo Si₂layer,Mo₅Si₃layer and Mo matrix are formed from outside to inside.After siliconizing on Mo-30Cr-10Al alloy,the coating structure is from outside to inside.The outermost layer is（Mo,X）Si₂,which is a kind of Mo Si₂,the second outer layer is（Mo,X）₅Si₃,which is a solid solution phase of Mo₅Si₃,X is Cr and Al,and the innermost layer is the alloy matrix.The adhesion between the substrate and the coating was determined by the scratch method.The adhesion between the coating and the Mo substrate was 7.2N,and the adhesion between the Mo-30Cr-10Al substrate and the coating was 9.7N.After cyclic oxidation at 1300℃for 100 h,the oxidation weight gain of pure Mo was 3.62mg/cm²,and that of Mo-30Cr-10Al alloy was 5.59 mg/cm².This shows that after adding Cr and Al elements,the material not only reduces the density,but also improves the hardness,and maintains excellent oxidation resistance.（2）With the increase of Si infiltration time,the coating thickness of Mo-30Cr-10Al alloy increases continuously,and the coating thickness reaches 570μm after second Si infiltration for 40 h.Cyclic oxidation was carried out at 1300°C in atmospheric environment.The weight gain of the first Si infiltration for 20 h was first increased and then began to lose weight after 90 h.The weight gain of the second Si infiltration for 20 h was first increased and then began to lose weight after 70 h.The oxidation weight gain of the second Si infiltration for 40 h was always within 100 h,and the oxidation weight gain was 5.2 mg/cm²at 100 h.The Kp values of the three are Kp（20h）>Kp（20h+20h）>Kp（20h+40h）,indicating that the extension of Si infiltration time can improve the oxidation resistance stability of Mo-30Cr-10Al alloy coating.（3）The anti-oxidation coating with thickness of 657μm can be obtained by Si and Y co-permeation on Mo-30Cr-10Al alloy,which is 15.2%higher than that of 570μm without Y addition.The bonding force between the coating and the alloy substrate after adding Y is 16.7 N,which is greater than 14.3 N without Y;the weight gain of Y modified by air circulation oxidation at 1300°C for 100 h was 4.99 mg/cm²,and the weight gain without Y was 6.02 mg/cm²,and the Kp（0.97）after adding Y was less than that without Y（0.99）.In summary,the preparation of anti-oxidation coating on the surface of Mo-30Cr-10Al alloy by Si and Y co-deposition is beneficial to improve the high temperature oxidation stability of the alloy.