Inefficient systemic loss of energy in machinery can be minimized by a reduction of friction and wear in mechanical components [1
]. For example, the indispensable cogs and bearings in the industrial machine are of interest, with the hope of bridging the gap between our desire for energy and adverse environmental effects [3
]. Tribology design and surface engineering technologies are sought after for improving the effectiveness and reducing friction losses in mechanical systems. Available literature indicates that surface texturing is one potential strategy to improve energy efficiency and decrease waste disposal and emissions [2
]. Textured surfaces, with some intricate microstructures (pits, craters, and grooves), have gained widespread acceptance in tribology because of their ability to achieve the micro-hydrodynamic bearing effect, acting as reservoirs for the continuous supply of lubricants, and trapping of wear debris by eliminating or reducing plowing effects of the working surfaces [5
Among the surface texture fabrication techniques, laser surface texturing (LST) has become an established manufacturing method [7
], owing to its advantages of being extremely fast, clean to the environment, and provides excellent control on shape and size of micro-structures [8
]. It has been proved that LST is valid for tribological applications in mechanical face seal [12
], thrust bearings [14
], cutting tools [16
], cams/tappets [19
], drill bit [20
], and piston rings [9
]. LST is also used for the reduction of friction (or stiction) in magnetic storage devices [23
] and for the minimization of wear and mechanical losses for micro-electro-mechanical system (MEMS) devices [24
]. However, LST is deficient in the new challenges of improving the performances of tribological systems. Importantly, it fails to timely exhibit good transitional process from boundary (and dry) lubrication to full-film lubrication, or vice-versa, though the true state of the tribological system is complicated and filled with uncertainties. An important but little-known problem is the friction and wear mechanisms of textured surfaces with different lubrication regions, as well as effective measures to improve the transitional behavior in various lubrication regimes. LST can increase the range of hydrodynamic lubrication regime in the Stribeck curve [25
]. Meanwhile, it is stated that the bulges at the edge of the dimple need to be optimized to have the positive effect of LST on lubrication regime transitions [27
]. The friction coefficient is an important parameter for tribological performance, followed by wear loss, friction force, efficiency, and reliability, which are all critical to the operation of a special tribological system. In addition, it is necessary to understand the detailed wear and durability of laser-textured surfaces for different regions and working conditions.
Sputtered carbon coatings have been reported to have excellent tribological properties and, therefore, have the potential to be used as a hard solid lubricant and wear-resistant coatings [28
]. The coating film is recommended for laboratories and plants for their overt properties, such as chemical stability, morphology, and anti-friction. Specifically, coated surfaces have been amenable to playing an active role in dry friction cases for several decades [29
]. There is limited literature about the exaltation of tribological performances using appropriate coating film, under mixed film lubrication and boundary lubrication. Though some investigations have shown that carbon film is perfect and can offer a series of attractive properties for many applications [31
]. Arslan et al. [34
] reported two approaches to generate low-wear surfaces with diamond-like carbon (DLC) coating by laser surface texturing. Their results suggested that the tribological performance of a cylinder on a coated plate tribo-pair could be enhanced. Yasumaru et al. [35
] investigated the tribological properties of DLC films’ nanostructured surfaces by femtosecond laser ablation, where a MoS2
layer on the nanostructured surface improved the friction properties. The creation of small and shallow cavities with a laser lithography technique on a DLC layer has allowed a significant reduction of the friction coefficient of DLC/steel contact [36
]. Recently, an interesting investigation by Ding et al. [37
] on carbon film with micro-dots has suggested that the influences of laser textures on the tribological performance of amorphous carbon film are strongly dependent on the friction pairs. For instance, LST has not improved the tribological performance of the friction pairs consisting of Si3
balls, whereas it is effective for the reduction of friction for the steel ball. Further, the DLC/LST composite specimens can obtain a low friction coefficient, which has been accounted for by the combined action of dimple-induced graphitizing transformation [38
Previously conducted tribological studies have mostly focused on the suitable room temperature conditions for human beings to the detriment of outdoor industrial equipment (such as ships, kowtows, and wind turbines) that operates below room temperature. For extreme low-temperature conditions, the tribological researches focus on space and cryogenic aspects, which does not encourage technology development and theoretical progress. In the present research work, the tribological properties of the textured surface with thin carbon coating was compared with three different surfaces (smooth surface, laser-textured surface, and carbon-coated surface) under starved and lubricated conditions at below room temperature (8–10 °C). The result provided fundamental insights to guide the design and analysis of surface textures for wind turbine bearings, precision bearings, and precision actuators in places like northern China, Scandinavia, Alaska, and northern Russia, where temperatures often remain below room temperature for weeks or months during winter.
In this study, the laser-textured surface with DLC coating displays different tribological behaviors during the friction and wear test, and these behaviors are related to the bulk materials and micro-pits and the DLC-coated layer. Firstly, laser surface texturing can be seen as a partial laser machining on the surface of carbon steel by laser ablation gasification, accompanying laser transformation hardening (also known as laser quenching) [49
]. The heating effects of the laser beam need some attention. The heat is absorbed rapidly (10−1
s) in a small area on the surface of the workpiece, and the temperature rises sharply [50
]. The solid phase transformation occurs between the melting point of the material and the critical temperature of austenite transformation. After self-quenching, martensite is obtained, and the transformation hardening of the workpiece surface is realized, resulting in the complete hardening of the 45 steel surface with partially quenched areas. The microstructure consists of fine needle-like martensite [51
], although material structure, heating, and cooling speed are important factors for the transformation of hardening behavior [52
]. This results in improved strength and hardness of the surface. Specifically, due to the steep temperature field distribution, there is a sharp transition from the laser hardened layer to the matrix and the second characteristic of local diffusion of carbon, and the hardness drop is as high as HV400–600 [50
]. Besides the laser nitriding, other recombination actions might well exist during the surface texturing to improve the tribological properties for the nitrogen as the assist gas during LST processing. The surfaces of the dimples also contribute to improving the tribological properties, especially for the contribution of low and steady friction [53
]. Importantly, the hardness of hydrogenated DLC films, which is typically around 10–20 GPa depending on the sp2
ratio and the hydrogen content, will act as anti-friction and anti-abrasion film [54
]. Generally, TK8869, as a target material for carbon coatings, tends to be amorphous carbon material and is accepted as a soft coating for anti-friction and anti-abrasion. Therefore, the DLC carbon coating is adaptive to the boundary lubrication and mixed lubrication conditions for controlling friction and wear. Conclusively, emphasis should be placed on the effect of surface texture and coating on the surface friction properties. Regulation of the friction coefficient and the expansion of the Stribeck curve are available for the novel surface, treating with combined LST and DLC coating. After the tribological experimental data by Kalin et al. [55
], the friction coefficients obtained are analyzed, and the results are shown in Figure 15
. As shown, the friction coefficients increase initially with the Stribeck parameter and then decrease with a further increase of the parameter in the measurable range. LST and DLC are effective in regulating the behaviors of friction couples under different working conditions. Specifically, LST can expand the contact parameters for hydrodynamic lubrication and induce the friction transition on the Stribeck curve [27
]. Meanwhile, DLC coating exhibits comfortable friction properties under boundary lubrication by forming a low friction film on the surface. Additionally, the friction properties can be highly improved by using extreme-pressure and anti-wear additives [56
]. In this study, the experimental results, by considering the compound influence of LST and DLC coating, are far from that of the literature [55
]; they may be accounted for the clear difference in the initial surface roughness and lubrication oil. However, compared with the LST surfaces, friction transitions occur on the Stribeck curve of the novel design LST surfaces with DLC coating.
Wear mechanisms of LST surface with carbon coating is complicated for the given full-range lubrication and friction conditions. After the transformation of hardening through the LST process, the occurred grain refinement can improve the plastic deformation resistance and fracture strength, increase the stress caused by cracks, and increase the wear resistance [50
]. Meanwhile, the DLC film is acknowledged for the low friction coefficient, good wear resistance, and high hardness [58
]. However, the composite response of LST and DLC film is not usually comfortable during the friction and wear behavior by some working conditions, such as contact pressure, temperature [19
], texture geometry [36
], and lubricated medium [59
]. Further, the combined investigation of LST and DLC film is far from the universal conclusion for different engineering applications, although these works have been conducted by enough research teams [19
]. Note that the order of surface texturing and DLC coating is different, and most of these investigations are conducted on the textured surface of DLC coating samples. For carbon steel samples, micro-cracking that results from tensile stress tends to occur within the smooth surface during sliding motion [64
]. As known, micro-cracks are usually a bad sign of delamination wear, which consists of several basic steps, such as plastic deformation of the subsurface layer, subsurface crack nucleation, propagation, and generation of loose wear sheets [65
]. Obviously, it is a fact that subsurface deformations, void elongation, and crack formation in medium-carbon steel are detrimental to the mating part and should be avoided by taking any surface protection measures. Thus, surface treatments, such as coating and texturing, are better methods of improving the tribological performance and properties of mechanical components [34
], resulting in a clear difference in the worn surfaces after friction and wear experiments (see Figure 10
and Figure 11
After the LST, the hardness of the surface is enhanced by the laser action, while the substrate material of the counterpart is soft. Thus, both the laser-textured and non-textured zones of the contact surfaces of the friction couple will be in a mess due to frequent plowing and heating effects during the friction process. This results in the active surface been in the inhomogeneous state without harmonized interaction with each other, leading to heat accumulation, which enhances oxidation and friction. Lastly, as oxidation is exacerbated, the sheet scuff peels off, causing serious wear and tear. Conversely, apart from the solid film for the friction reduction of coating surfaces under the lean oil condition, soft carbon coatings undergo the action of plowing and squeezing mechanisms, and the surface roughness changes by the process of scratching and penetration concurrently. This generates new debris, which brings some additional mechanisms of friction and wear [67
]. Differently, hard coating usually affects the load-carrying capacity through coating strength and substrate deformation [68
]. Throughout the contact, the reduction of the actual area and the interlocking materials affects chiefly the surface roughness, while the asperity fatigue leads to the generation of wear debris. The bearing capacity and wear resistance of friction pairs under lubrication conditions can be improved by the hydrodynamic lubrication, micro-oil pool effect, and micro-lubrication of LST technology. At the same time, the tribological properties of friction couples under dry friction, and the boundary lubrication conditions can be enhanced by DLC coating. Through the coupling and synergy of these two mechanisms, comprehensive tribological properties of friction pairs can be improved, and the working conditions and application fields of specific friction pairs can be expanded. Thereafter, the lubrication design principle of the textured surface with carbon coating is illustrated and shown in Figure 16
The wear traces shown by SEM photographs indicate different wear mechanisms for the different friction pairs with various treated surfaces. Specifically, the textured surface with coating presents oxidative and adhesive wear mechanism under starved lubrication conditions. The oxidation is not an incomprehensible result because amorphous carbon film cannot hold stable inertia at a fairly high temperature. The flashing temperature may get to hundreds Centigrade owing to the micro-contact with a heavy load during the dry friction condition. Thus, local carbonization occurs at the actual zone rather than the apparent region. Additionally, carbon film has certain mobility during micro-plowing and micro-cutting, which are micro-abrasive mechanisms of generating wear and material migration. Superficially, it is undesirable to improve the abrasion resistance of the working surface with micro-pits and carbon film. However, the truth of the matter is quite different because the periodic temperature rise may be higher in starved friction. Consequently, the carbon film is softened occasionally, and sound solid lubrication might well be realized, with no great loss of the bulk material and the counterpart after friction. Moreover, diamond and diamond-like carbon cannot produce protective oxide layers with oxidation and volatile CO2
. Additionally, for diamond-like carbon, H2
O is formed, which will emerge at the oxidation temperature [46
], and a water-mediated hydrogen diffusion mechanism may exist on the oxide surface at a normal temperature or less.
Surface roughness is an important factor in scuffing failure, whether it occurs under conditions of boundary lubrication or mixed lubrication [48
]. In this study, the surface of the bulk materials is not very smooth with some initial defects, which might have affected the result of the test. The micro-pits array of the laser-textured surfaces contains an error in depth and profile induced by the laser pulses. In addition, it is not easy to design the thickness of the carbon film to get an excellent tribological performance. A fairly thin film would lose its effects in dry and starved conditions, whereas a thick film would rather weaken the interface characteristic of the textured surface contact resistance and heat transfer. Moreover, the friction coefficients for several surfaces under starved conditions are not available, which are constrained by the weak resistance to shock for torque sensors. Consequent upon these problems and oversights, friction and wear behaviors carried out are quantitative to an extent, accompanied by qualitative analysis. This result could not be set aside without a grave responsibility for tribological performances and behaviors. Surface oxidization and van der Waals attractive energy over the carbon structures with nanoscale conformity [69
] lead to a novel, critical, and unfolding surface treatment, yet some sagging problems with physicochemical actions demand further study.
This study illustrated the tribological performances of a laser-textured surface combined with carbon coatings compared with three typical surfaces (smooth surface, textured surface, and coated surface). The advantages of the textured surface with coating were demonstrated by surface characterization and tribological tests on a ring-on-ring rig. The analyzed results made us understand the relationship between material properties, surface topographies, and tribological behavior, as outlined below.
(1) Engineering surfaces after texturing and coating could be characterized using classical roughness and Abbott–Firestone parameters. Worn regions were isotropic in the horizontal (parallel to the sliding direction) and vertical directions. Changes in surface topography of worn traces were related to the wear behaviors and friction mechanism.
(2) Laser-textured surfaces with carbon coating significantly improved the friction and wear properties of the 45 steel under lubricated conditions, by consistently maintaining a steady-state friction coefficient. Comparing to the smooth surface, the stable friction coefficient under lubricated conditions (medium speed and load) of the textured surface, coated surface, and textured and coated surface were reduced by 17%, 41%, and 59%, respectively.
(3) The trend of temperature rise of the textured surface with coating was not consistent with that of the friction coefficient under different working conditions, which has accounted for the significant difference in the influence of the degree of working conditions (i.e., speed and load) on the friction coefficient and temperature rise.
(4) Under lubricated conditions, the plowing friction was weakened by the viscous shearing of lubricating oil, and the micro-cracks in the perpendicular direction of the sliding motion were present. The textured surface with a coating underwent the plowing friction accompanied by corrosion and adhesive wear under the conditions of high-speed and light load.
(5) Diamond-like carbon films, combined with microscopic bearing and pooling effects of surface textures, could significantly improve the hydrodynamic lubrication performance, whereas the available solid lubrication was available by the formation of low friction film under dry friction conditions. The additional effect provided a basis and scheme for lubrication adaptive design under complex working conditions.