Research On The Preparation And Properties Of Coating On Molybdenum

Abstract:Molybdenum, a refractory metals, that is characterized by their extremely high melting points of2610℃, good thermal conductivity, low coefficient of thermal expansion, high strength, and good deformation ability, has widely been applied in point light source, aerospace, heating element nuclear industries and so on. But cold shortness, low strength in low temperature and poor oxidation resistance in high temperature seriously restrict its application. Molybdenum sample starts to oxidize at300℃in air. When the temperature is higher than600℃, the product of MoO3causes rapid volatilization. To improve the oxidation resistance of the molybdenum sample, two kind of coating technique can be use to make the films on the molybdenum substrate.Firstly, Stainless steel thin films were deposited by DC magnetron sputtering on the molybdenum substrate. X-ray diffraction, transmission electron microscopy, atomic force microscope and nano-indentation were used to investigate the phase, surface topography, physical, mechanical properties and oxidation resistance of the films.Secondly, Si-Al coating, Si coating and Al coating were prepared by packing cementation method. The antioxidation properties at750to1050℃in the air were tested by the use of XRD, SEM, TEM, EPMA and EDS. The influence of different temperature and time on the thickness, composition, morphology, phase compositions and oxidation resistance of the coating was studied. The kinetic equations of the coating were established during oxidation process at1000℃. The conclusions are as follows:(1) Stainless steel thin films were deposited by DC magnetron sputtering on the molybdenum substrate. The results show the coat spreaded smoothly and high bonding strength by making on the substrate of0.005μm roughness in the negative bias of75V for two hour. The anti-oxidation test indicated that the coating could protect the molybdenum pole and the weight loss of the specimen for1.23mg/(cm2*h) at800℃. (2) Using the pack cementation method prepare Al-Si, Si and Al coats. Studies have shown that the coats preparation at1000℃for5hours have high coating thickness and dense, uniform composition, better bonding strength, excellent oxidation resistance. In static oxidation test of750～1050℃, coating preparation at1000℃for5hours is with a relatively good antioxidant properties. Si-Al coating forming the molten SiO2film and non-molten state A12O3oxide film on the surface, has a good oxidation resistance; Si coating forms the molten SiO2film that has poor oxidation resistance at750～850℃; Al coating has a good oxidation resistance at750～950℃, however, it would lose its protective function at1000℃.(3)In static anti-oxidation experiments at1000℃for4h, Si-Al coating weight increment is accord with the parabolic curve relationship that the weight increment of the coating for1.23mg/(cm2*h) for4h.Si coatings have good oxidation resistance at1000℃with that the weight loss of the coating for0.051mg/(cm2*h) for4h; Al coating is weight increment in the early stage and weightlessness, gradual failure for4h.(4)Al-Si coating system overcomes the shortcomings of "pest" that Si coating at the low temperature (750～850℃),and compared with Al coating, it has longer life and anti-oxidation performance at the high temperature.