The Study Of Preparation And Insulation Perfoemance Of Micro-arc Oxidation-enamel Composite Coating On Titanium Alloys

Ceramic layers formed by microarc oxidation on pure titanium and titanium alloys show high adhesive strength,high corrosion resistance and electric insulating properties.However,the insulation performance under wet conditions of microarc oxidation ceramic coatings degrades,thus affecting the applications of titanium alloy parts under special conditions such as galvanic corrosion.On the other hand,enamel coating are oxidation resistant and corrosion resistant at elevated temperature.Whereas,due to some factors like the thermal mismatch between enamel coating and metal substrate,it is prone to forming cracks and thus to failure in the long-term services.Therefore,a combination of microarc oxidation and enamel process may prepare a composite coating with excellent performance on titanium alloys,which is crucial to enhance the lifetime of titanium parts under severe and complicated conditions including high temperature,high pressure and corrosive circumstance.In the experimental section,ZTi60 titanium alloys were used as substrates for coating preparation.Three kinds of coatings were prepared on sample surface,namely,microarc oxidation ceramic layer,enamel layer and composite coating.On the basis of performance evalutions of the three kinds of coatings,the effects of the microstructure of microarc oxidation ceramic layers and the sintering process for enamel coating preparation on the performance of composite coatings were investigated.Furthermore,SEM and XRD were employed to study the microstructure and phase composition of these coatings.Also,the thickness,insulation performance and adhesion of the composite layer were studied by using eddy current thickness gauge,voltage tester and coating adhesion tester,respectively.The results show as follows:(1)For the three kinds of coatings,the microarc oxidation ceramic layers show optimal adhesive strength,the enamel layers present the lowest adhesive strength,and the adhesive strength of the composite coatings is between the above-mentioned both coatings.In addition,the microarc oxidation ceramic layers show the lowest insulating performance,but the composite coatings are the optimal.(2)The microarc oxide ceramic layer on ZTi60 substrates is mainly composed of TiO2 phase,and meanwhile the enamel layer and the composite coating are composed of Al2O3,Ti02,SiO2 and so on.(3)With the increase of frequency and duty cycle for microarc oxidation process,the adhesive strength of the composite coatings increases and then decreases.It is further found that the adhesive strength of the composite coating is closely related to the microstructure of microarc oxide ceramic layer.When the micropore size of the ceramic layer is slightly larger than that of glaze slurry,the composite coating shows a more excellent adhesive strength.The optimal frequency is 600 Hz,and the optimal duty ratio is 10%.(4)The insulating performance increases with the thickening of the composite coating.The dielectric strength of the composite coating increases gradually with the increase of frequency,but decreases with the increase of duty cycle.(5)With the increase of sintering temperature,the adhesive strength of the composite coating increases but then decreases,while the insulation performance keeps a tendency of gradual increase.With the extension of holding time,the adhesive strength of the composite coating gradually decreases,but the insulation performance gradually increases.The suitable sintering process is the sintering temperature of 850 ? and the holding time of 50 min.