Preparation And Properties Of Graphite Oxidation Resistant Coatings

Graphite materials,because of their advantages such as low density,high specific strength,good thermal shock resistance and chemical stability,have been widely used in various fields.In the production process of stirring and degassing of aluminum liquid,degassing rotor used by the material is graphite with a carbon content of 99.9%or more.However,graphite materials have a fatal flaw.When it is used at the atmosphere of oxygen,they are easily oxidized above 500? which greatly limits their scope of application.Therefore,it is especially vital to restrain the oxidation behavior of graphite materials in high temperature environments.At present,the main method solving this problem is to apply a protective coating on the graphite surface.In this paper,Ni coating was prepared on graphite substrate by electroplating method,then the Aluminium coating was prepared on the surface of the Ni coated graphite by packing method,and impregnated coating was prepared on the surface of the Al coated sample by the method of dipping phosphate and borate.These coated samples were tested in air at 700?and 800? for oxidation.Besides,the high temperature oxidation resistance of Ni coating,Aluminium coating and dip coating were researched.The results were as follows:(1)Under suitable technological conditions,Ni coating,Aluminium coating and dipping coating were successfully prepared by electroplating,packing and dipping methods,respectively.The experimental results indicated:when the samples were oxidized in the air at 700? and 800?,the weight loss rate from bottom to top was dipping coating,Aluminium coating,Ni coating and substrate.(2)When electroplating Ni coating is oxidized,the phase on the surface is mainly NiO.In addition,Ni and C are also exist.because carbon can reacts with oxygen and NiO is also converted into Ni by C.Carbon is always in the process of consumption,so the anti-oxidation effect of Ni-plated coating samples is not outstanding.(3)When Aluminium coating is oxidized,Al2O3 will be formed on the surface,which is beneficial to improving the oxidation resistance.However,the content of Al2O3 is relatively less and the coating is unsound.So it is hard to prevent the oxidation of carbon absolutely.(4)The impregnation coating generates Al(PO4)after oxidation,which has excellent thermal stability and can protect the substrate effectively in high temperature environment.Its antioxidant properties have been further improved.