| With the entrance to 21st century, the resource and environment problems have become the key problems in the survival and development of human beings. Therefore in the long term, the trends of the industrial development all over the world can be reduced to the saving of energy and resource, and what is more, the pursuit of natural production. The austenite stainless steel has been widely used because of its good mechanical property and the corrosion resistance performance. However, due to its low hardness and poor wear resistance, the use of austenite stainless steel is restricted under the application of friction. Through several years of numerous researchers'active explorations, great progress has been achieved on the surface treatment technique and method for enhancing the wear-resistant, corrosion resistance and oxidation resistance of the austenite stainless steel. And one of methods is to prepare one layer of compact oxide passive thin film on the material surface. This thin film can slow or prevent the further oxidation of material, and then enhances its anti-high temperature attrition and the corrosion properties.Previous reports about metal surface or metal film surface oxidation by pulsed laser were concerned with the resolution of laser beam and its thermal field, which are all important in laser oxidation. However, microstructure and oxidation kinetics of the oxides are seldom studied. In our study, scanning electron microscope (SEM), field emission scanning electron microscope (FESEM), high resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are used to investigate the microstructure and mechanism of the growth about the different oxides on the stainless steel surface. The results show that the oxidation behaviors of oxides are influenced significantly by pulsed laser parameters. Through the experiment, we find that the different oxidation kinetics have played important roles in the formation of the surface oxide microstructure and metal oxide grain growth. Eventually, it leads to the formation of different oxide morphologies. The relationships among the oxide microstructure, microhardness, wear and corrosion on the stainless steel surface have also been studied. The main results are as follows:1. Study of the process parameters on laser melting oxidation stainless steel surfaceAssume that the temperature field of the pulsed laser acting on the metal surface is non-steady and transient, as well as the laser can be used as a surface heat source, so it can simulate and analysis the temperature field on the surface of the stainless steel during laser melting oxidation with different laser energy densities using simple mathematical models and the heat conduction law. The temperature field analysis indicates that the maximum temperatures of the stainless steel surface are far more than the melting point of the stainless steel Tm (1371~1398°C) when it is irradiated with the lower laser energy density (4.30×106 J/m2 ~ 6.20×106 J/m2) and smaller pulsed width (0.2 ms ~ 0.6 ms). (Under this condition, the laser power density is very high because laser power density is equal to the ratio of the laser energy density and pulsed width.) Then, it will induce strong melting and evaporation at the surface of stainless steel after laser irradiation. When the laser energy density is higher (1.90×107 J/m2 ~ 3.52×107 J/m2) and pulsed width is larger (10 ms ~ 20 ms) (the laser power density is lower), the maximum temperatures of the stainless steel surface are slightly higher than the melting point of the stainless steel. Thereafter, it will follow the melting and solidification process on the surface of the stainless steel which will have an important impact on the morphologies and properties of the stainless steel. If we continue to increase the laser energy density, the surface temperatures of the stainless steel will substantially increase which will be likely to make the sample surface gasification. Then, it will have an unfavorable effect on the surface of the stainless steel.In order to verify the practicality of the different laser processing parameters used in the mathematical model of the temperature field, the melting oxidation treatment of the stainless steel is carried out by the Nd:YAG pulsed laser beam. From the surface morphology observed by FESEM, it can be seen that: (1) When the pulsed width is smaller than 1 ms, there appear a lot of craters after each pulse, at the same time the stainless steel surface becomes rougher and is not extremely smooth; the XRD analysis indicates that the stainless steel surface does not produce oxide when the laser energy density is lower (4.30×106 J/m2 ~ 6.20×106 J/m2) and the pulsed width is smaller (0.2 ms ~ 0.6 ms). When the laser energy density is increased to 7.00×106 J/m2 (pulsed width is 1.0 ms), there form a small amount of oxides on the stainless steel surface. (2) When the pulsed width is larger than 1 ms, the craters are not found basically, and the specimen surface becomes smoother and there is no cracks, impurities and other defects; from the XRD analysis, it can be seen that the peak intensity ofγ-Fe the stainless steel first strengths then weakens, but the peak intensities of oxides (e.g. Cr2O3, Fe2O3 and MnO2) gradually strengthen with the increasing of the laser energy density. (3) From the optimized parameter of the laser energy density 3.52×107 J/m2 and pulsed width 20 ms, it can be observed that the cross-section of the stainless steel sample can be divided into three regions, i.e., melting zone, heat affected zone and the matrix. Then, the sample is carried out deep corrosion and it can be clearly seen that the cross-section is composed of oxide layer (about 4~8μm), melting layer and the matrix.2. The oxides microstructure on the stainless steel surface by laser meltingThe morphologies in the central region and edge of the laser spot, the formation mechanism of microstructure evolution and the kinetic and thermodynamic behavior of the different morphologies on the surface of the stainless steel are discussed from the optimized parameter of the laser energy density 3.52×107 J/m2 and pulsed width 20 ms. Combination of XRD and XPS analysis, it can be seen that the main products on the surface of the stainless steel after laser melting oxidation treatment are the oxides of Cr2O3, Fe2O3 and MnO2. From the analysis of the temperature field during the laser melting, the temperature curve in one laser spot on the stainless steel surface can be obtained under the same laser energy density. With the increasing of the distance from the center r, the highest temperature in one laser spot on the stainless steel surface gradually decreases from 2165°C (r≈0μm) to 1503°C (r≈200μm). In the subsequent rapid cooling process, the highest solidification velocity in the central region of the laser spot can be up to 89 m/s; while in the edge, it is the smallest which is approximate to 0. Due to the great differences in the solidification velocity between the central region and the edge in one laser spot, it induces the difference on the morphologies between the central region and the edge.From the high magnification FESEM micrographs in accordance with the different distances from the center r in one laser spot, it can be seen that there are nano-particles in the central region. With the increasing of the r, there appear the hexagonal morphologies in the edge which are different from the nano-particles in the central region. From the XRD and EDS analysis, it can be speculated that the nano-particles in the central region are mainly Fe2O3. From the XRD and EDS analysis, it can be known that the hexagons in the edge are mainly Cr2O3. The hexagons of Cr2O3 in the edge show that the metal atoms diffuse from the inside of the oxide layer to the surface with the rich of the Cr elements, which will be favorable to the rich of the Cr elements in the oxide film. Hence, it will help to form the dense oxide film. The TEM analysis shows that the microstructure changes dramatically and nano-particles as well as the hexagonal structures have been found. In addition, the amorphous phase has been also found in the TEM analysis because of rapid cooling rate.3. The surface layer microhardness and the friction and wear properties on the stainless steel surface by laser melting oxidationThe surface microhardness and cross-section microhardness, the wear loss, wear coefficient and wear morphologies are investigated after the laser melting oxidation treatment with different laser processing parameters.The results show that the microhardness on the surface and the cross-section has been obviously enhanced. The laser surface melting oxidation treatment with different laser parameters will induce the wear loss on the surface of stainless steel to reduce and the friction coefficient to decrease. In the range of these experimental parameters, with the increase of the applied load and sliding friction distance, the wear mechanism will change from the mild oxidation wear and abrasive wear mechanisms to severe oxidation wear, abrasive wear and delamination wear mechanism. After the laser melting oxidation treatment with different laser processing parameters, the adhesion and plastic flow on the worn surface of stainless steel significantly reduce. There mainly have three kinds of wear debris during the wear process: flake wear debris, massive wear debris and stripe wear debris.4. The corrosion properties on the stainless steel surface by laser melting oxidation The effects of the laser melting oxidation treatment on the corrosion resistance of the stainless steel surface are analyzed by adopting the chemical immersion corrosion weight loss and the electrochemical experimental methods. At the same time, the mechanism enhancing the corrosion resistance of the stainless steel surface after laser melting oxidation is also discussed.In the chemical immersion corrosion experiments, there appear a lot of etch pits on the untreated surface of the stainless steel. While after the laser melting oxidation treatment, the immersion corrosion resistance of stainless steel has a certain degree of improvement. In the electrochemical corrosion experiments, the corrosion current density after laser melting oxidation treatment on the surface of the stainless steel is much lower than that on the un-treated surface. The corrosion current density decreases with the laser energy density increasing in the range from 1.90×107 J/m2 to 3.52×107 J/m2. After the laser melting oxidation treatment, in the initial corrosion, there present the oxide film damage process and the filling process of corrosion products on the stainless steel surface under the erosive effects. And in the later corrosion, it is the anodic dissolution process of the fine grains on the subsurface. It may include the following several processes about the improvement of the corrosion resistance on the stainless steel surface after laser melting oxidation treatment: clean and smooth surface; homogenization and refining of the subsurface microstructure; low melting point, low gasified temperature impurity volatility and gasification, and so on.In a word, the research of the present study lies not only in the further investigation and broadening on the present laser oxidation technique, but also in the more embedded comprehension of the traditional oxidation technique. At the same time, it provides the reference ideas for the studies of in-situ oxidation on the metal material surface to fabricate oxides which are more prevailed internationally. Therefore, the work in this paper is of rather available values in theoretical and practical applications.
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