Research On Preparation And Mechanism Of Al₂O₃ Gradient Coating On 316L Stainless Steel Substrate

In the first part of this dissertation, conception, principle and history of functional gradient coatings, tritium permeation barriers, double glow plasma surface alloying and hot dipping were overviewed. The advantages and shortcomings of the typical tritium permeation barriers coating, such as oxides, aluminum-based coatings, titanium-based coatings and silicide coatings on stainless steel surface, were discussed in detail respectively. On the basis of these, double glow plasma surface alloying and hot dipping, as new methods used to alumetize and oxidize, were put forward to prepare tritium permeation barriers.Alumetized coating have been prepared by double glow plasma surface alloying. Firstly, the processing parameters such as type of source polar, work temperature, voltage of source polar and voltage of workpiece polar were determined according to groping experimentation. Secondly, the other processing parameters such as work air pressure and distance between workpiece polar and source polar were optimized according to thickness of coating, aluminium content of surface and quality of surface. Microstructure, composition and phase were studied by scanning electron microcopy(SEM), energy dispersion X-ray spectrum(EDS), X-ray diffraction(XRD) and glow discharge spectrometry(GDS). The optimal processing parameters are as follows: workpiece polar voltage is 450V, source polar voltage is 950V, distance between the two polar is 30mm, work air pressure is 75Pa, work temperature is 980℃, duty ratio of impulsing power source 0.3. On this condition, the average thickness coating with good quality is 20μm while the composition of the alumetized coating from surface to interior is gradient distribution.In the other hand, alumetized coating have been prepared by hot dipping and subsequent high temperature diffusion. The optimal processing parameters are dipping temperature 750℃, time10 minutes, diffusion temperature 950℃and time 4 hours.The average thickness of coating is 122μm. The coating is compact, gradient distribution, uniform and has good adherence with the substrate.As-prepared Alumetized coating was oxygenated by rudimental O2 in 98.5%N2 as source. Oxygenated at 900℃for 4 hours, thickness of oxide is 15μm. The result of X-ray photoelectron spectroscopy (XPS) shows that oxygen has preferred to combine with aluminium. But alumina is in gradient distributions in the oxide film except surface.Comparing with the hot-dip coating, the one fabricated by double glow plasma surface alloying has more excellent properties of thermal shock resistance and oxidation resistance.