Fabrication Of Uniform And Crack-free ZrO₂ Films By Sol-Gel Process

In recent years, the film technique of material surface is a new technology of giving a number of excellent characteristics to the surfaces of metals and non-metals. As an important part of material science, it has been widely emphasized and its application field is extended continuously. The thermal expansion coefficient of ZrO₂ is adjacent to the stainless steel. The ZrO₂ coating has good chemistry stability and rigidity. The stainless steel has antiseptic quality, so we choose it as substrate in our experiments , which can combine the better mechanical properties of metals with the better chemistry stability of ceramic films.ZrO₂ films can be obtained through many methods such as physical vaporous deposition (PVD), chemical vaporous deposition (CVD), electrochemistry and Sol-Gel method. Sol-gel method has become one of the most popular and important techniques. It has been used to prepare different types of new materials intensively, as well as in the formation of powders, sheets, fibers and thin and thick films for various advanced technological applications. In the protection of erode environment technique, sol-gel coat technique has the most potential application. More and more scientists pay attention to this, because thin films derived from sol-gel technology have the advantage of simpleness, low temperature application, cost effectiveness and versatility in coating different substrate materials and complex geometries.In our experiment we prepared the crack-free and uniform ZrO₂ films on the surface of stainless steel by sol-gel method. Spinning coating method was applied. Films were formed through coating, drying , sintering. Scanning Electron Microscopy (SEM) was used to analyze the morphology of the films. X-ray diffraction was used to investigate the microstructure of the films. DTA studies were carried out to analyze the energy change and phase transition behavior of thefilms. Based on sol-gel method, precursor composition, times of coating, different contents of activator, sintering temperature and laser radiation parameter are fitted together, we changed the parameters of these processes and obtained the optimal films.Because of three different polymorphs of ZrO₂: monoclinic (m-ZrO₂), tetragonal (t-ZrO₂) and cubic (c-ZrO₂), we adopted Y₂O₃ as the stabilizing agent of ZrO₂ to avoid ZrO₂ phase transformation that can lead to crack.Sol particle size is very small and sol has high chemical activity, large surface area, existing with many unsaturated bonds. After aging a period, some particles grow fast and others increase slowly, so they are unstable and tend to agglomerate together forming new larger particles. The activator PVA based on its nature, keeps the sol particles separated in the solution because of steric hindrance mechanism or electrostatic stabilization mechanism or both stabilizing nano-cluster to improve the stability of sol.Drying is the most important and difficult stage, which affects the morphology, crack, micropore, isothermal adsorption of the films, So Choosing a good drying method is a keystone of this paper. Supercritical drying method request rigid condition and low temperature and high temperature drying can not obtain uniform films, so they are not suitable for our experiment. In our experiment we adopted classification-drying method. The solvent was discharged fast at the initial drying stage at 80°C, at the same time particles closed up and the sol formed network. The system became dense and viscous. Sol lost the most part solvent, then was dried at 50°C, 30°C slowly we obtained uniform films.Gel transformed from sol is an open framework structure and is nanoporous. In order to obtain compact films we coated, dried and sintered again and again and the sol was filtered into the porous gel network and combined with it, after coating each time the micro-cracks and nanopores were filled, the films became more compact.Lasers were found to be a useful and rapid method to process films. Laser irradiated the films that had been sintered at 300°C and the films that are composed of ZrO₂ sol and ZrO₂ nano-powders. Laser irradiation offered some advantages such as rapid melting and high solidification rates, which leaded to structure that was more homogenous and fine. Also, it facilitated good adherence between the metal substrate and the coating.