Diamond-like carbon (DLC) coatings are widely used as protective and self-lubricating surfaces in metal-metal contacts. Their frictional behavior is governed by the formation and evolution of carbon-rich transfer layers (TLs), which can be tailored through functionalization with carbon nanomaterials. Recent studies have shown that graphene sheets (GSs) and nanodiamonds (NDs) act synergistically to achieve ultra-low friction in microrough (similar to 0.2 mu m) metal-DLC contacts under dry N-2 at a 1 N load. Here, we probe how this lubrication mechanism evolves with increasing load from 1 to 10 N-corresponding to local contact pressures up to similar to 11-16 GPa-respectively, in dry N-2 and humid air conditions. Ball-on-disk experiments are performed on an industrial hydrogenated DLC coating sliding against stainless-steel. In dry N-2, GS-ND functionalization yields a low and stable coefficient of friction across the entire load range, reaching a minimum of about 0.05. In humid air, higher friction levels are observed across all loads (CoF similar to 0.10-0.15), accompanied by oxidation-driven modifications of both wear debris and the counterface contact region, with oxygen content increasing by more than a factor of three compared to dry N-2. Detailed microscopy and spectroscopy analyses indicate that enhanced lubricity in dry N-2 arises from TLs incorporating GSs, NDs, and nanoscroll-like structures, whereas humid air promotes interfacial amorphization and oxidation, leading to load-insensitive friction and boundary lubrication effects through physisorbed water molecules.
The poor tribological and mechanical performance of Al alloys hinders their use in practical applications where low COF and high durability are required. This study examined and evaluated a novel laser-sintered Ni-Cr coating to improve the load-carrying capacity and tribological performance of an Al alloy (Al 6061) substrate. The authors demonstrate that laser sintering cycle count is a decisive process variable governing coating dispersion, microstructural consolidation, and tribological performance in Ni-Cr coatings fabricated via Selective Laser Sintering (SLS). Increasing the laser cycle count progressively refined the surface morphology, improved coating dispersion, and strengthened interparticle bonding. As a result, the average durability after three cycles was seven times that after one laser cycle, accompanied by markedly improved COF. To further improve durability and load-carrying capacity, Ti3SiC2 was introduced into the Ni-Cr coating. The coating containing 10 wt% Ti3SiC2 exhibited a 20-fold increase in durability, extending the time to failure to approximately 70,000 s (700 m) while maintaining a low coefficient of friction (similar to 0.48) compared with the coating containing no Ti3SiC2. The greater durability of the Ni-Cr-10wt%Ti3SiC2 coating in this novel study was attributed to improved adhesion to the substrate, better particle distribution during sintering, and greater load-carrying capacity. While further process changes do not yield feasible samples, this study showed that surface properties can be improved within the available small-process regime. Overall, laser sintering of a Ni-Cr-10wt%Ti3SiC2 coating shows promise as a means to improve the tribological and mechanical performance of Al 6061. This study should aid researchers and other stakeholders in fabricating well-adhering, durable, and tribotactic composite coatings on Al6061 and similar material systems.
Spiral-groove dry gas seals are widely used in various rotating machinery, and their performance prediction is of great significance for structural design and operational optimization. Existing studies still face several limitations, including the limited fidelity of numerical simulations, the insufficient number of experimental samples, and the restricted generalization capability of models based on a single data source. To address these issues, this study constructed a multi-source data system integrating numerical simulation data and experimental data, and systematically compared four representative data fusion methods, namely the uncertainty-weighted fusion algorithm, TrAdaBoost, MFDNN, and CoKriging, with analysis of their applicability and predictive performance. The results show that multi-source data fusion can effectively exploit the complementary advantages of different data sources and improve the prediction accuracy of dry gas seal performance. In terms of the comparison of data fusion methods, all four methods achieved good results for the groove-depth problem; however, for the spiral-angle and groove-number problems, which exhibit stronger nonlinear characteristics, clear differences were observed among the methods. Among them, TrAdaBoost showed the best overall performance, followed by MFDNN, then CoKriging, while the uncertainty-weighted method was relatively weaker. In terms of seal performance, the influence of groove depth on seal performance was relatively direct; the spiral angle is recommended to be controlled within 10-14 degrees, and the groove number within 12-16, so as to balance opening force and leakage rate. This study can provide a reference for the rapid performance prediction and parameter optimization of spiral-groove dry gas seals.
To address the problems of insufficient precision reserve, limited rotational speed, and excessive temperature rise in high-speed ultra-precision machine tool spindle bearings, the influence of frictional power loss on the thermo-mechanical behavior of the bearing system was investigated. Firstly, based on the analysis of the heat source of the bearing, the friction power consumption model of the bearing assembly is established, and the analysis of the bearing temperature field is realized by studying the heat energy transfer. Secondly, the test bench is built for experimental verification. Finally, through the study of thermal-mechanical coupling performance, the influence of different rotational speeds on bearing stress and life is analyzed. The results show that the friction power consumption generated by the spin sliding of the bearing rolling element accounts for the largest proportion, accounting for 31% of the total friction power consumption; the increase in bearing speed will increase the bearing temperature. At 55,000 r/min, the highest temperature at the rolling element is close to 75 degrees C, followed by the inner ring up to 68 degrees C, and the lowest outer ring temperature is 57 degrees C. The temperature has a great influence on the bearing performance. Under the same working conditions, the equivalent stress is increased by 21%, the contact pressure is increased by 25%, and the fatigue life of the bearing is reduced by 5.6%. Bearing performance is significantly affected by thermodynamic behavior.
To elucidate the role of environmentally friendly oxide additives in a molybdenum disulfide (MoS2)-based solid lubricant, this study investigates the tribological behavior of a MoS2-TiO2 coating deposited via a spray-bonding process and compares it with a commercial Sb2O3-containing formulation (Everlube 620C). Interfacial characteristics and wear-related mechanisms were systematically analyzed using scanning electron microscopy (SEM), focused ion beam (FIB), Raman spectroscopy, and X-ray diffraction (XRD). The MoS2-TiO2 coating exhibited a higher steady-state coefficient of friction (0.35-0.45) and wear compared to the baseline. Its wear behavior was governed by fracture-induced three-body abrasion, driven by the hard and brittle nature of TiO2, which promotes stress concentration at particle-matrix interfaces, crack initiation, particle pull-out, and debris generation. These processes suppress the formation of a desirable MoS2-rich tribo/transfer film, leading to deformation-dominated friction. Overall, the findings indicate that the intrinsic mechanical properties and interfacial behavior of TiO2 limit its effectiveness as an additive in MoS2-based coatings, highlighting the importance of additive selection and compatibility in achieving optimal tribological performance. Notably, this study was performed at an additive volume fraction equivalent to that of Sb2O3 in Everlube 620C, serving as a foundation and indicating that further optimization of TiO2 particle size and concentration is required to achieve comparable performance.
