Preparation Of Ceramic Multilayer Film By Sol-Gel Method

Ceramic multilayer film prepared by sol-gel method can improve the oxidation resistance of metal substrate.In this paper,alumina sol and silica sol were prepared.Stainless steel was selected as the substrate.Firstly,alumina sol was coated by dip-pull method,then alumina sol and silica sol were alternately coated to the base material,at last alumina coating and alumina & silica composite coating were prepared by heat treatment.High temperature cyclic oxidation experiment through 100 hours at 900? was carried out by discontinuous weight method.The high temperature oxidation resistance of the film was studied through the analysis of oxidation kinetics curve,sample surface morphology and XRD diffraction analysis.The film with the best high temperature oxidation resistance was selected,and the high temperature oxidation resistance mechanism was summarized.At the same time,the effect of concentration of sodium carbonate and the times of coating on the high temperature oxidation resistance of alumina film were studied.In order to prepare a stable sol solution with suitable viscosity and pH value,the viscosity of the solution was kept at around 2.0cp,the pH value was controlled at around 5,and the most suitable temperature was 65 ?.With the concentration of sodium carbonate increasing,the viscosity of the sol increase and the film will be thicker.And the adhesion between the film and the substrate was not good so that the scale will easily peeled off during cyclic oxidation and its high-temperature oxidation resistance was worse.The experimental data showed that the coating on the surface of the sample can significantly improve its high temperature oxidation resistance compared with the blank sample.For the alumina single coating,the coating prepared with the parameter ratio of aluminum nitrate concentration 40%-sodium carbonate concentration 20% was better than the coating prepared by the process with aluminum nitrate concentration 40%-sodium carbonate concentration 25%.For the following reasons: with the increase of the concentration of sodium carbonate,the viscosity of the colloid increases and the fluidity of the colloid was poor.the adhesion of film with the matrix was low that leads to the spallation of the coating in the process of cyclic oxidation thus the substrate was oxidized.The composite coating can significantly improve the oxidation resistance of the substrate,especially the adhesion of the oxide film on the sample.Among these,the film with two layers of silica-alumina composite coating had the best high temperature oxidation resistance.After calculation,the weight gain per unit area of the coating sample was 0.321 of the blank sample,and the weight of peeling off per unit area was 0.365 of the blank sample.The coating prepared by sol with different compositions had a critical value of coating times.In other words,the number of coating times was not more is better.This phenomenon was affected by factors such as sol viscosity,coating thermal expansion coefficient,compactness and adhesion.In general,the oxidation resistance of alumina single coating was the best,but when the concentration of sodium carbonate was high,the coating times had not obvious effect on the oxidation resistance of the substrate.The composite coating of silica-alumina can effectively improve the density and adhesion of the coating,reduce the difference of thermal expansion coefficient between the coating and the substrate.The coating with two layers had the best high temperature oxidation resistance.When the critical value of coating times was exceeded,the high temperature oxidation resistance of each coating became worse.XRD spectrum shows that the two elements Cr and Ni in stainless steel appear their oxide forms in the high temperature oxidation environment.The coating reduced the oxidation rate of the substrate,yet the protective effect gradually disappears in the oxidation process of 100 hours.