Engineering structural components inevitably suffer from fretting wear on the mating surface under service conditions, leading to material removal, surface degradation, and accelerated fatigue failure. This study conducted surface dimple texturing using multi-spot overlapping laser shock. To improve the fretting wear performance, the laser shock was combined with a diamond-like carbon (DLC) coating deposition. In recent decades, surface texturing for use in regulating friction performance has been a focus of numerous studies. During the friction and wear processes, textured dimples contribute to a reduction in wear by storing debris and by lowering the actual contact area between friction pairs. Laser shock has the advantages of a high loading pressure, deeper plastic deformation layer, improved controllability, and ease of automation, which means that laser shock has broad application prospects in fields such as aerospace, automotive manufacturing, and marine engineering. Laser shock peening (LSP) effectively avoids thermal damage to materials via lasers, achieves surface shaping through local plastic deformation, and hardens the treated surface via the introduction of residual compressive stress. Based on the LSP effect, laser shock surface dimple texturing is beneficial in improving resistance to fretting wear. DLC coatings have several excellent characteristics, such as high hardness, elastic modulus, a low friction coefficient, and high wear resistance. However, because of the difference in the thermal expansion coefficient between the coating and substrate, residual compressive stress exists inside the coating, and coating peeling during service is a challenging problem encountered by DLC coatings. Prior to the preparation of surface coating, surface peening methods to increase the hardness of the substrate can improve the adhesion between the coating and substrate, and the presence of textured dimples can reduce the contact area and store lubricants. Combining laser shock surface dimple texturing with DLC coating is beneficial in improving the fretting wear properties. Currently, a single-spot laser shock is commonly used for laser shock surface dimple texturing and is widely applied in macroscopic friction and wear. The degree of residual compressive stress between adjacent single spots is also relatively low, and even residual tensile stress occurs, which is not conducive to improving the fatigue performance. In engineering components such as dovetail joints, the fretting contact parts not only undergo fretting wear, they also experience fatigue failure. Multi-spot overlapping LSP can achieve 100% full coverage, which is beneficial in improving fretting damage performance. To further improve fretting wear resistance, textured dimple arrays with a regular arrangement can be constructed by taking advantage of the overlapping effect of laser shock to achieve laser shock surface dimple texturing. Accordingly, investigating the fretting wear behavior of titanium alloys treated with laser shock surface dimple texturing and DLC coating composite modification is of great significance for providing a reliable experimental basis for enhancing fretting wear resistance.

This study initially applied square-spot overlapping LSP on the surface of a titanium alloy to construct surface textural dimples and prepared textured dimple arrays with different texture densities by adjusting the overlapping rate. Prior to the laser shock dimple-texturing treatment, aluminum foil was used as the absorption layer and a 1-mm-thick layer of flowing deionized water was used as the constraint layer. The treatment was performed using a neodymium glass (Nd∶YAG) solid pulse laser based on the given laser shock parameters and paths on the samples. A DLC coating was then applied to the surface of the laser shock textured dimples using physical vapor deposition (PVD) technology with a coating thickness of approximately 3.5 μm. Several samples with combined laser shock dimple texturing and DLC coating were obtained. Surface microhardness and residual stress were measured using an HV-1008 Vickers hardness tester and an XL640 X-ray stress tester, respectively. A fretting fatigue device was used to perform fretting fatigue experiments with constant fretting cycles on the as-received, LSP-20%, LSP-30%, and LSP-40% coated samples. The surface wear morphology and elemental composition of the surface wear areas of the experimental samples were analyzed via laser confocal microscopy (OLS5100), scanning electron microscopy (SEM, Hitachi SU5000, Japan), and energy dispersive spectroscopy (EDS, Oxford spectrometer). Finally, the fretting wear characteristics of the as-received and laser shock dimple-textured samples with DLC coating were analyzed, and the effect of the laser shock dimple-texturing treatment on the fretting wear behavior of the composite-modified titanium alloy was investigated.

In the presence of DLC coatings, the surface microhardness is significantly increased for both the as-received and laser shock dimple-textured samples. However, the residual compressive stress on the surfaces of the samples is not weakened by the DLC coating [Fig.4(e)]. Under fretting fatigue conditions, the DLC coatings peel off from the fretting contact edge of the as-received coated sample, but the surface coating of the laser shock dimple-textured coated sample shows less wear damage (Fig. 5). Under plane?plane contact, the surface-textured dimple arrays reduce the contact area. The macroscopic wear morphologies reveal that wear debris from the grinding part covers the surfaces of the composite-modified samples. As the overlapping rate increases, the size of the textured dimples is increased to help reduce the contact area, thereby decreasing the wear debris covered at higher overlapping rates. Indentation test results show that the adhesion of the surface coating is poor for the as-received sample, and severe peeling of the DLC coating occurs locally during fretting wear. Laser shock dimple texturing introduces a work-hardening effect and improves the bonding between the DLC coating and the substrate without obvious wear and peeling of the surface coating. The presence of the textured dimples reduces the contact area and alleviates the wear of the DLC coating. At higher overlapping rates, the size of the textured dimple is larger and the texture density is higher, further reducing the wear area of the DLC coating and exhibiting better wear resistance. For the laser shock dimple-textured samples with DLC coatings, the fretting process mainly involves the transfer of adhesive Ti, from which a layer of wear debris is observed on the surface. As the size and density of the textured dimples varies, the fretting contact area changes, and the surface textural dimple characteristics affect the fretting wear behavior of the DLC coating.

This study investigated the effects of laser shock dimple texturing on the fretting wear behavior of DLC coatings. Textured dimple arrays were constructed on the surface of a Ti6Al4V titanium alloy via a square-spot overlapping laser shock process. This was followed by the deposition of a DLC coating. The morphologies and different levels of microhardness and residual stress of the textured dimpled surfaces were characterized. The fretting wear behaviors of the as-received, LSP-20%, LSP-30%, and LSP-40% coated samples were compared and analyzed under fretting fatigue conditions. The following primary conclusions were drawn: 1) Square spots with a side length of 4 mm were used for multi-spot overlapping laser shock; the textured dimple arrays with dimensions of 0.8 mm×0.8 mm, 1.2 mm×1.2 mm, and 1.6 mm×1.6 mm were prepared on the surfaces of titanium alloy samples by adjusting the laser overlapping rates. The corresponding textured dimple densities were 6%, 18%, and 45%, and the depth of the textured dimple was approximately 8 μm. 2) Following laser shock dimple-texturing treatment, the microhardness of the textured dimple surface is increased by 30%, and the residual compressive stress level reaches 660 MPa. In addition, the application of the DLC coating is found not to reduce the residual compressive stress on the surface of the textured dimples. 3) The DLC coating peels off on the fretting contact edge of the as-received sample, where the length of the damaged area is greater than 2 mm and the wear depth is close to 30 μm. However, the surface coating of the laser shock dimple-textured sample shows less wear damage. Thus, laser shock dimple texturing improves the wear resistance of the DLC coating. 4) The fretting wear behavior of DLC coatings is affected by surface-textured dimple characteristics. Material transfer causes the adhesive Ti to be distributed across the contact surfaces of the composite-modified samples. The adhesive Ti covering the surface decreases as the density of the textured dimples increases. Thus, surface-textured dimples play a role in reducing the contact area. The combination of DLC coating deposition and laser shock dimple texturing significantly improves the fretting wear resistance of titanium alloys.