Issue 7

S. Bagheri Fard et alii, Frattura ed Integrità Strutturale, 7 (2009) 3-16; DOI: 10.3221/IGF-ESIS.07.01 10 The nanostructured surface layer of pure titanium SMAT treated in air at room temperature with stainless steel (balls 3 mm in diameter, for 30 min, at a vibration frequency of 20 kHz), also revealed a lower friction coefficient, almost 50% smaller than that of the untreated titanium. And approximately 50% higher scratch resistance at the level of the top nanolayer than its value in the bulk. It can be noticed that the variations of the scratch resistance are very similar to that of the hardness [88] . In another experiment, standard nanoscratch tests have been performed using particular nanoindenters on elemental iron plates (with a purity of 99.95 wt. %) which were SMAT treated: 8 mm diameter steel balls vibrated by a generator with a frequency of 3 kHz. Repeated nanoscratch experiments indicated that the wear and friction resistance of the surface layer treated by SMAT were greatly enhanced [93]. Micro-scratch tests were also performed to evaluate the wear resistances of sandblast-annealed brass samples. The wear resistance of the brass was considerably improved by nanocrystallization. It was also revealed that with an increase in the annealing temperature; the scratch resistance was lowered significantly. The difference in volume loss reached one order of magnitude larger, when the grain size changed from 20 nm to 80 nm. The increase in the wear resistance by nanocrystallization is consistent with the associated improvement in the mechanical behavior of the material [44]. Corrosion There are some results indicating that the corrosion resistance can also be markedly improved by shot peening methods [43, 44, 46,54,55, 93] . Jiang et al. carried out corrosive immersion tests on sand blasted 35A commercially pure titanium specimens (treated with SiO 2 particles of 200–300 µm in diameter and compressed air pressure of about 300 psi followed by a recovery treatment below 300 °C, for 30 min with subsequent air cooling).The results indicated that in the surface nano-crystalline layer, the high density of grain boundaries was beneficial to the formation of a thin passive film, which could restrict the movement of metal ions from metal surface to the solution, thus minimizing corrosion and improving polarization behavior of the sandblast-annealed titanium [46]. The effect of air blast shot peening on corrosion resistance in surface nanocrystallization of 1Cr18Ni9Ti stainless steel was also investigated by polarization curves and pit corrosion tests. Shot peening was carried out by a flow of stainless steel balls with a diameter of 0.8 mm under 0.5 MPa for 5 min. It was reported that compared with the as-received coarse crystalline counterpart, the passive film on the surface of shot peened sample is easier to form and is more stable. Shot- peening-induced surface nanocrystallization can markedly enhance the overall and local corrosion resistance of steel in chlorine–ion-contained solution [55]. Fig. 9 shows potentiodynamic polarization curves obtained in 3.5% NaCl solution for shot-peened and as received samples. It is demonstrated that shot peening significantly improved the polarization behavior of stainless steel, not only markedly decreasing anodic current density and passivation-maintaining current density, but also having the tendency of shifting cathodic current density to lower value, and slightly lowering the free corrosion potential. Additionally, shot peening induced a considerably enlarged passive region of stainless steel and raised the breakdown potential of passive film, and for as-received reference samples, there was no remarkable passive region in polarization curve. Figure 9 : Potentiodynamic polarization curves of shot-peened and as-received samples of 1Cr18Ni9Ti stainless steel obtained in 3.5% NaCl solution [55].