Issue 7

S. Bagheri Fard et alii, Frattura ed Integrità Strutturale, 7 (2009) 3-16 ; DOI: 10.3221/IGF-ESIS.07.01 9 Chemical reaction kinetics Gaseous nitriding is one of the most widely used surface modification techniques to improve the surface hardness, anticorrosion properties and wear resistance of metallic materials by formation of a surface nitrided layer. However nitriding processes are performed at relatively high temperatures (550°C-600°C) for a long duration and may induce serious deterioration of the substrate in many families of materials. It has been experimentally demonstrated that chemical reaction kinetics are greatly enhanced when the grain size is significantly reduced to the nanometer scale. Since mechanically induced nanostructures store a large excess energy in the grain boundaries and grain interior in the form of non-equilibrium defects, which constitute an extra driving force for the nitride formation process that may further facilitate their chemical reactivity [89-92]. Investigations have reported that surface nanocrystallization of elemental iron samples with a purity of 99.95 wt. % specimens via SMAT performed in an apparatus in which steel balls (8 mm in diameter) vibrated by a generator with a frequency of 3 kHz repeatedly stroke the sample surface, greatly enhances the nitriding kinetics and reduces the activation energy for the diffusion of nitrogen significantly. It has been found that the nitriding temperature of iron processed by SMAT can be reduced to 300 °C, which is at least 200 °C below the conventional nitriding temperatur e [93]. In another experiment stainless steel balls (with a mirror like surface and a diameter of 8 mm) struck an iron sample with a purity of 99.95 wt.% at the bottom of a cylinder-shaped vacuum chamber attached to a vibration generator (50 Hz) within 60 min, the grains in the surface layer were effectively refined into the nanometer scale. The sample was protected by a high-purity Argon atmosphere during the SMAT to avoid oxidation. The experimental evidence confirmed that the mechanically induced surface nanocrystallization of Fe created a considerable amount of stored energy in the surface layer that constituted an effective driving force for the nitriding process at low temperatures [94]. The reduced nitriding temperature is of considerable importance seeing that it may allow for the nitriding of material families (such as alloys and steels) and work-pieces that cannot be treated by conventional nitriding. Wear, coefficients of friction and scratch resistance The process of microstructure refinement also has proved to lead to an enhancement of the wear, friction and scratch resistance [44, 73, 88,93-95] . The coefficients of friction and penetration curves for iron SMAT treated samples (stainless steel shots with a diameter of 8 and the vibration generator of 50 Hz within duration of 60 min) were measured in an experiment. Results showed that the coefficient of friction of the treated sample (0.38±0.06) was considerably smaller than that of the original sample (0.52±0.03). The nanoscratch experiments were repeated several times with very consistent results, indicating enhanced wear and friction resistance of the surface layer after SMAT and nitriding [94]. In another experiment the hardness on top surface nanostructured layer of SMAT treated pure Fe bulk samples, reaches a value as high as about twice that of the coarse-grained matrix. Also the wear and friction measurements on a SMAT low- carbon steel sheet showed that the wear volume loss is lower than that of the untreated original one. As it is clear in Fig. 8, the friction coefficient values at different applied loads for the as-treated sample are evidently smaller (about a half) than those of the original sample [73]. (a) (b) Figure 8 . Variations of the wear volume loss with load (a) and variations of the coefficient of friction with load (b) for the SMAT and the original low-carbon steel sample s [73]. 0 0,05 0,1 0,15 0 2 4 6 8 10 Load (N) Wear Volume Loss (mm^3) as-treated original 0 0,1 0,2 0,3 0,4 0 2 4 6 8 10 Load (N) Friction coefficient as-treated original