1. Introduction
In recent years, laser-assisted surface modification technologies with high precision and efficiency are widely used in the production of alloy parts. This is because the heat input is small and only the vicinity of the laser irradiated area can be modified locally with minimal damage or deformation [1]. Especially in the case of steel materials widely used for sliding components such as bearings, high-precision heat treatment without any removal is required. To improving the hardness of steel materials, control of carbon content and appropriate temperature control are effective.
In this presentation, a novel method of surface modification for SCM440 by carbonization using short pulse laser irradiation in the atmosphere of PAO4 lubricating oil is proposed. When the pulse laser beam is focused on target high-temperature, high-pressure condition will be generated in the irradiated area and the lubricating oil will be ablated into particles such as atoms, molecules, ions, or radicals [2]. Then ablated particles are reacted with heated and form carbide on surface.
2. Experiment and result
The 900 ps laser pulses at 1064 nm wavelength were delivered by a microchip laser with the repetition rate of 20 Hz. Laser pulses were focused to SCM440 surface by a plano-convex lens with a focal length of 60 mm. To ensure the laser fluence is below the ablation threshold, laser pulses with the pulse energy of 0.5 mJ was irradiated onto the SCM440 plate with a certain defocus distance at normal incidence. The beam diameter on the SCM440 plate was set as 100 μm. The SCM440 plates were fixed to a 2-axis stage and scanned in a reciprocating motion to modify 10mm square area. The effect irradiated pulse number N was set as 1, 2, 4, 8, 16 and 32 by controlling the scanning speed of laser. The hardness of SCM440 surface after the laser irradiation, was evaluated by nano-indentation.
Figure 1(a) shows the hardness of each SCM440 surface before and after the laser irradiation under conditions of different effective irradiated pulse number. The hardness of modified surface increased by approximately 7~8 Gpa compared to before irradiation.
The tribological properties of the irradiated surfaces are investigated with ball-on-disk reciprocating friction tests in PAO4 oil. Figure 1(b) shows transition of friction coefficient. In case of N=1, stable low friction could keep longer than unirradiated surface. However, surface of N=16, 32 have shorter lifetime. It is known that long heating times and high temperatures in the heating cause embrittlement. In this case, the input heat propagates over wider and reheat the previously irradiated area. The reheat causes the embrittlement of the modified layer.
The modified layer was observed by TEM. A thin flake sample for TEM was prepared using FIB. Before using FIB, SCM440 surface is coated by Au layer by vacuum deposition method. Figure 1(c) is a cross-sectional image of a TEM sample irradiated with N=32. This shows the formation of a modified layer with a thickness of about 200 nm on the surface. Figure 1(d) is the diffraction pattern of region (1) of Fig. 1(c). This shows the existence of amorphous Fe5C2 and Fe3C. The localized superheated area on the sample surface, combined with the rapid cooling facilitated by the oil atmosphere, led to the formation of an amorphous state after irradiation.
3. Conclusion
Based on these results, the effectiveness of this method as a localized surface hardening treatment for steel materials using carbonization phenomena has been demonstrated. Furthermore, by using this method, it will become possible to irradiate the material in the environment of machine tools, even with lubricating oil adhered after machining processes.
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