Study On Surface Nanocrystallization And Nitriding Of Titanium And Ferrous Alloys

Surface nanocrystallization as pretreatment technique seems to be appropriate for lowering nitriding temperature of steel materials efficiently as well as obtaining satisfactory surface properties. However, further study is required to understand the behaviour of this lower temperature nitriding process. Additionally, studies on lower nitriding temperature of titanium alloys by means of surface nanocrystallization are rather limited. The nitriding temperature of titanium is very high, which may induce serious deterioration of the substrate. Hence, systematic studies on lower nitriding temperature are essential to traditional nitriding process of titanium. Besides, the reduction in brittleness of nitriding layer on steel materials is difficult in traditional nitriding process, and the fabrication of a surface nitrided layer with high hardness and excellent toughness presents a major challenge to researchers.In this work, the lower temperature nitriding behaviour of titanium with nanostructured surface layer and its surface properties were investigated. Besides, some analysis methods were used to investigate the effectiveness of this combined treatment which is applied to improve the surface properties of titanium clad composite sheet and the biocompatibility of titanium implants. Additional, the overall tensile property of pure iron by surface nanocrystallization and nitriding were studied to supplement the shortage of previous study. At last, in order to reduce the surface brittleness of the nitrided steel, a combined treatment of ion sputtering and surface nanocrystallization was used. The main results are as follows:1. A nanometer-grained surface layer without porosity and contamination was fabricated on the pure titanium by means of surface mechanical attrition treatment (SMAT). The average grain size in the top surface layer is about 12nm. The SMAT sample with a surface nanostrutured layer was found to be thermally stable below 500℃. After isothermal annealing at 500℃ for 150min, a slight grain growth occurred in the surface layer. After isothermal annealing at 500℃ for 240min, obvious abnormal grain growth occurred in the surface layer, which indicates that the excellent thermal-stability disappeared.2. The pretreatment of surface nanocrystallization seems to be appropriate for lowering nitriding temperatures of titanium efficiently. The experimental results clearly showed that a much thicker compound layer (approximately 10μm) was formed on the SMATed sample compared with that of the coarse-grained sample during the nitriding at lower temperature (approximately 550℃). The XRD and TEM results indicated that the nitrided layer on the SMAT sample is composed of rianostructured ε-TiN and γ-Ti2N phases with grain size in the range 10-50nm. Additionally, it can be observed that a large amount of particle-like nitrides (approximately 150-300nm) precipitated beneath the compound layer. The surface hardness of the nitrided SMAT sample is much higher than those of the sample without SMAT. Besides, the wear resistance is greatly enhanced for the nitrided SMAT sample.3. Combined treatment of SMAT and nitriding not only provides an effective approach to reduce the nitriding temperature of titanium. But also significantly improves the surface properties of titanium clad composite sheet. The technology of lower temperature nitriding by means of SMAT seems to be appropriate for reducing nitriding treatment temperature of the titanium layer in Ti/Al clad sheet to ensure the integrity of aluminium layer. Additionally, in order to solve the problem of different nitriding temperature of titanium layer and steel layer on the Ti-steel clad sheet, the SMAT was employed to reduce the nitriding temperature to 550℃, which is similar to the nitriding temperature of steel.4. The biocompability of titanium implants were improved by means of SMAT and lower temperature nitriding. The osteoblast-like cells growing and adhesion on the nitriding SMAT sample showed the better morphology, attachment, proliferation, ALP and OC expression in comparison with the original sample. Moreover, the longer working life of the titanium implant can be obtained by this combined treatment which is attributed to the improved surface hardness and wear property.5. Thickness of the compound layer and surface hardness of the nitrided SMAT Fe sample are much larger than those of the sample without SMAT. The nitrided layer on the SMAT sample is composed of nanostructured Fe2-3N phase with grain size in the range 10-40nm. Additionally, it can be observed that a large amount of particle-like nitrides with grain size of range from tens to hundreds nanometer precipitated beneath the compound layer. The tensile test results showed that the higher strength as well as better plastic can be obtained on nitrided SMAT sample in comparison with the nitrided coarse-grained sample. The result of the tensile test revealed that yield strength of the nitrided SMAT sample is 390MPa, and ultimate strength can reach 450MPa, moreover, the elongation to fracture is 44%.6. The brittleness of nitrided layer on 20CrMo steel can be improved by the combined treatment of plasma sputtering and SMAT. The results showed that the compound layer with 12um thickness formed by nitriding treatment can be decomposed by argon sputtering treatment. The surface layer of the sputtered sample is composed of diffusion structure with relative lower hardness. By means of SMAT, a deformed diffusion layer of about 100μm thick can be formed on the sputtered sample in which the top surface layer contains nanocrystallines with random crystallographic orientations, and the average grain size is about 20-30nm. It is noted that a nanostructured diffusion surface layer with satisfactory hardness and wear property can be on the treated sample relative to that of the conventional nitrided sample. Especially, the toughness of the surface layer was greatly improved by this combined treatment.