Study On Preparation And Performance Of Glass-based Oxidation Resistance Coating On Molybdenum Electrodes

The molybdenum has excellent performances,especially good mechanical properties and erosion-resisting characteristics at quite a high temperature.These fine performances enable it to obtain widespread application as electrode material in glass industries.In the process of roasting kiln,however,molybdenum is extremely easy to oxidize.When the temperature climbs to 750℃,it is oxidized to MoO₃ with yellow smog.This worsens the operating environment,and seriously influences the life of electrode material and furnace.Due to the severe oxidation problem that occurs when molybdenum is heated up to high temperature in air,this research is to develop one glass-based coating used for protecting molybdenum electrodes from oxidation in the heating up process.The coating is a suspension consisted of barium silicate glass powder,refractory material of Cr₂O₃, organic sticking agent and solvent of terpineol.It is made over the bare surface of molybdenum electrode through spread coating.Comparing with the process technologies of vacuum or inert gas,CVD and solid phase diffusion,this method has more advantages, such as simpler processing technic,lower cost and better protective effect.The oxidation resistance property of coating was systematically researched through addition quantity of Cr₂O₃ and pretreating craft of coating in the article.First of all,on the basis of synthetically analyzing literatures,a kind of frit used in ceramic coating was chosen to be the basic glaze of the research.Through thermodynamics calculation,the standard free energies of formation of any component in basic glaze were received.The result was no reaction between molybdenum and any ingredient of the basic glaze in the temperature range of roasting kiln.After the basic glaze was ensured,the painting thickness of coating and the addition contents of Cr₂O₃ were discussed in chapter three.When the painting thickness of coating was about 300μm,the performance of coating was better.Through EDS and the test of oxidation resistance property,the results indicated that the binding force between the coating and the molybdenum substrate was good,and it was hard to find the crack and flakes.When the ratio of basic glaze,Cr₂O₃,and deflocculant was 14:8:1,the coating's thermal expansion coefficient was 6.1×10^-6/℃closed to 6.7×10^-6/℃,the thermal expansion coefficient of molybdenum substrate(0～1200K).Therefore the thermal expansion coefficient of coating matched with the molybdenum substrate so well that it was difficult to bring the crack.In Chapter four it mainly reported the influence of different pretreating crafts over the oxidation resistance property of coating.The value of weight loss of molybdenum sample prepared by the craft D was just 4.59mg/cm²,and the coating's surface covered with compact and homogeneous glass.The samples prepared by the craft D were carried on the test of oxidation resistance property.The results indicated that at 1000℃and keeping 120min,the weight loss rate of sample prepared by the craft D was only by 7.85% comparing with that of the sample untreated;and at 1200℃and keeping 90min,the value was still by 8.90%.It shew that the oxidation resistance character of coating was improved evidently.After ensuring the best pretreating craft,data fitting was introduced according to five values of weight loss by oxidation at 1000℃.The results showed that the relation between the heat treatment time(x) and the value of weight loss of sample(η) was linear relationship.And the one-dimensional linear regression equation was(?) =7.3932-7.3742x. With the regression equation,the weight loss rate at different times could be extrapolated.According to the test results,the protection mechanism of glass-based oxidation resistance coating was discussed in chapter five.The coating's surface covered with compact glass.The Ba/Cr,Mo and O compound came into being in the joint between coating and molybdenum substrate according to XRD and EDS analysis.The inert glass membrane and the reaction membrane were able to reduce the diffusion of oxidized gas to the molybdenum substrate effectively.In the end,it was drew a conclusion that the good protection effect of glass-based oxidation resistance coating on molybdenum electrodes owed to the reacting and melting protective mechanism and the inertial screen protective mechanism.