For the first time, the influence of gas mixture on first damage resistance of a plasma nitrided DIN 18MnCrSiMo6-4 bainitic steel was investigated. Samples were nitrided at 500 °C with three different N2-H2 gas mixtures, containing 5, 24, and 76 vol.% N2. Samples were characterized concerning the resulting roughness, microstructure, compound layer’s phase composition, residual stresses in the diffusion zone, and surface hardness. Tribological ball-on-flat tests were carried out in reciprocal mode using zirconia as ball material for friction coefficient and the compound layer resistance until the first damage. The test results were evaluated statistically by analysis of variance (ANOVA). As the amount of nitrogen in the gas mixture decreases, the ε-Fe2-3(C)N content in the compound layer decreases. A γ’-Fe4N monophasic compound layer was achieved at 5 vol.% N2 gas mixture. The diffusion zone as expected presented compressive residual stresses with the highest values near the surface. In the tribological tests, better results were obtained for 5 and 24 vol.% N2 in the gas mixture as higher amounts of γ’-Fe4N were formed. The 76 vol.% N2 gas mixture led to a brittle behavior, due to the biphasic compound layer (γ’-Fe4N and ε-Fe2-3(C)N) with a predominant content of ε-Fe2-3(C)N.

In this study, the sliding wear of a DIN 18MnCrSiMo6-4 continuous cooling bainitic steel plasma nitrided with a nitrogen rich gas composition was investigated. To evaluate the influence of processing time and temperature on mechanical and microstructural characteristics of nitrided layer, the samples were nitrided at 400, 450, 500 and 550 °C for 3, 6 and 9 h. The produced nitrided layers were characterized concerning the microstructure, phase composition, microhardness and surface roughness. The samples were tested by ball-on-flat reciprocating dry sliding for friction coefficient and wear analysis. The tests were stopped after a given damage criteria involving the rapid growth of the friction coefficients and wear. The correlation of the different treatment parameters and resulting case depths and surface hardness with sliding distance at the time of microcracks formation or delamination of the surface layer was evaluated statistically by the analysis of variance (ANOVA). The plasma nitrided samples at 550 °C showed better wear performances in the ball-on-flat tests than the other groups investigated, since these samples have a thicker compound layer and diffusion zone higher than the other conditions investigated. In general, the beginning wear is slower because of the hardest region of the compound layer.

AbstractSeveral studies regarding superficial treatments of Nitinol (NiTi) shape-memory have been developed aiming to the improve corrosion resistance and to block the Ni release to adjacent tissues. The necessary heat treatment to achieve the shape memory effect normally occurs at temperatures between 500 and 600 °C. However, titanium oxide (TiO2) is formed on the NiTi surface during the shape memory process heat treatment. In this work the effects of the heat treatment time on the surface characteristics of the formed NiTi oxide, at temperatures that promote the shape-memory (530 and 570 °C), were evaluated. The TiO2 layers which were obtained were evaluated by X-ray diffraction (XRD), energy dispersive X-ray (EDX), scanning electron microscope (SEM), thermogravimetric analysis (TGA), wettability and roughness. The results show that by increasing the exposure time at the temperature of 570 °C the formation of a thicker oxide is promoted, with less superficial roughness and of a hydrophobic nature. According to the literature, these characteristics indicate that the obtained oxide layer has properties that accelerat the osseointegration process.