A Study Of Fabrication Method And Tribological Performance Of Micro Dimple Based On Laser Shock Processing

Wear induced by friction is the main reason for the failure of mechanical devices. Therefore, friction control, improvement of lubrication status and wear reduction have become the urgent demand for the saving of energy and raw material as well as the comprehensive improvement of operation efficient and service life of mechanical systems. Surface texturing like micro-dimple array helps improve tribological performance and prolong service life of friction pairs. LPT(Laser Peen Texturing) is a new method for micro-dimple array fabrication which is based on the mechanical effect of laser shock processing. However, the method of this process and the tribological performance of micro-dimple array are not clear now. In this study, the applicability of LPT, the effect of process parameters on the geometrical and physical characteristics of dimple array and the tribological performance of dimple array were systematically investigated, which is helpful for the selection of processing parameters of LPT. The main contents of this paper are as follows:(i) Study of process mechanism and method of LPT.By the combination between the forming process analysis of laser-induced plasma and the simulation analysis of dynamic response process of metal under laser shock pressure, the mechanism of LPT was studied. Meanwhile, the plastic deformation and residual stress were predicted. By investigating the geometrical and dimensional conformity of micro dimples fabricated by LPT, it was found that LPT shows good process stability and applicability, which is an important basis for the LPT process analysis. Based on LPT, a new method for dynamic hardness measurement of material under high strain rate was developed.(ii) Investigation of LPT process.Through experimental method, the effect of LPT process parameters on geometrical and dimensional characteristics, dimple surface roughness, micro-hardness of plastic deformation layer and grain size were systematically investigated.It was found that, dimple diameter, dimple depth, dimple aspect ratio, the hardening of plastic deformation layer and the refinement degree of grain all increase with laser power density and repeated shock number, but become saturated gradually. With the increase of laser spot diameter, the dimple diameter increase correspondingly which is always less than the laser spot diameter. There exists a threshold of laser spot diameter. When laser spot diameter is larger than the threshold, the dimple depth, hardening of plastic deformation layer and refinement degree of grain decrease with the increase of laser spot diameter, while the aspect ratio decreases with it. When smaller than the threshold, due to the plasma shielding effect, the dimple depth, hardening of plastic deformation layer and refinement degree of grain increase with the increase of laser spot diameter. But when the laser spot diameter is smaller than the threshold, because both the depth and the diameter of dimples have the trend to become larger with the increase of laser spot diameter, it is hard to conclude whether the aspect ratio will become larger or smaller.Variation of the dimple surface roughness induced by LPT depends on the original surface roughness of the material and the LPT parameters. If the original surface roughness of the material is relatively large, at the same time the laser power density and repeated shock number are relatively small, the roughness of dimple surface will be smaller than that of original surface. Otherwise, the roughness of dimple surface will be larger than that of original surface.(iii) Tribological performance of micro-dimple array under the condition of hydrodynamic lubrication.The effect of micro dimple density, dimple aspect ratio and Reynolds number on the film pressure distribution, dimensionless average load capacity, dimensionless average film shear force and dynamic pressure coefficient were investigated by utilizing 3D CFD(Computational Fluid Dynamics) simulation. It was found that under the condition of hydrodynamic lubrication, the reason that micro dimple can improve tribological performance is wedge effect and eddy current effect. Under the parameters given in this study, there exists an optimum micro dimple density and an optimum micro dimple aspect ratio under which the optimum dynamic pressure performance could be obtained. Moreover, the optimum micro dimple aspect ratio decreases with the increase of Reynolds number, while the optimum micro dimple density does not change with Reynolds number.(iv) Tribological performance test of micro-dimple array under the condition of starved lubrication.By experimental method, the friction and wear characteristics of dimple array with different parameters under starved lubrication were investigated in detail. Through studying the effect of dimple density on friction coefficient, maximum load capacity and failure time, it was found that the friction performance do not vary monotonically with dimple density. There might exist an optimal texture density at which the textured surface exhibits the best friction performance. It is because of the contact pressure of the solid-solid area and the lubricating effect of the solid-liquid area. By studying the effect of dimple depth on friction performance, it was found that the friction performance becomes better with the increase of dimple depth. Furthermore, through the morphology and EDS(Energy Dispersive Spectrometer) analysis of worn surface, it is found that specimen textured by LPT could reduce abrasive and adhesive wear effectively.In summary, the process mechanism of LPT was investigated by the combination between the forming process analysis of laser-induced plasma and the study of dynamic response of metal under laser shock pressure. The applicability of LPT process as well as the effect of LPT process parameters on the geometrical, dimensional and physical characteristics of dimple was investigated by LPT process tests. By utilizing 3D CFD simulation and tribological performance test, the friction and wear-resistant performance of dimple array under the conditions of hydrodynamic lubrication and starved lubrication, respectively. These results offer guidelines for the parametric optimization of dimple array fabricated by LPT.