Hardening gradients have been introduced to heavy-duty gears in order to enhance their bearing capacity, but this has led to an increase in the complexity and difficulty of fatigue analysis. To explore ways to mitigate this, in this paper a contact fatigue model of an 18CrNiMo7-6 gear pair with a hardness gradient was established, and the Brown-Miller-Morrow multiaxial fatigue criterion was adopted to calculate the contact fatigue lifespan under different contact pressures. The gear contact fatigue limit was then determined by fitting the S-N curve. The accuracy of this gear contact fatigue limit prediction method was validated experimentally. Furthermore, the effects of case hardening depth (CHD) and surface hardness (SH) on contact fatigue lifespan and limits were explored, and we found that the contact fatigue limit of the typical carburized gear was more affected by SH than CHD. In addition, we also examined which hardness gradients were more beneficial for the fatigue performance of gears with atypical hardening gradients.

Using nanoadditives in lubricants is one of the most effective ways to control friction and wear, which is of great significance for energy conservation, emission reduction, and environmental protection. With the scientific and technological development, great advances have been made in nanolubricant additives in the scientific research and industrial applications. This review summarizes the categories of nanolubricant additives and illustrates the tribological properties of these additives. Based on the component elements of nanomaterials, nanolubricant additives can be divided into three types: nanometal-based, nanocarbon-based, and nanocomposite-based additives. The dispersion stabilities of additives in lubricants are also discussed in the review systematically. Various affecting factors and effective dispersion methods have been investigated in detail. Moreover, the review summarizes the lubrication mechanisms of nanolubricant additives including tribofilm formation, micro-bearing effect, self-repair performance, and synergistic effect. In addition, the challenges and prospects of nanolubricant additives are proposed, which guides the design and synthesis of novel additives with significant lubrication and antiwear properties in the future.

Although grease can effectively lubricate machines, lubrication failure may occur under high speed and heavy load conditions. In this study, Mn₃O₄/graphene nanocomposites (Mn₃O₄#G) were synthetized using a hydrothermal method as lubricant additives. The lubrication properties of compound grease with Mn₃O₄#G nanocomposite additive under heavy contact loads of 600-900 N (3.95-4.59 GPa) were investigated. First, the nanocomposites were dispersed into L-XBCEA 0 lithium grease via successive electromagnetic stirring, ultrasound vibration, and three-roll milling. Compound grease with additives of commercial graphene (Com#G) was also investigated for comparison. Tribological test results revealed that the trace amounts of Mn₃O₄#G (as low as 0.02 wt%) could reduce the coefficient of friction (COF) of grease significantly. When the concentration of Mn₃O₄#G was 0.1 wt%, the COF and wear depth were 43.5% and 86.1%, lower than those of pure graphene, respectively. In addition, under the effect of friction, the microstructure of graphene in Mn₃O₄#G nanocomposites tends to be ordered and normalized. Furthermore, most of the Mn₃O₄ transformed into Mn₂O₃ owing to the high temperature generated from friction. Using the Ar gas cluster ion beam sputtering method, the thickness of the tribofilm was estimated to be 25-34 nm. Finally, the improvement of the lubrication properties was attributed to the synergistic effect of the adsorbed tribofilm, i.e., the graphene island effect and the filling effect of Mn₃O₄#G.

Impregnated graphite has attracted considerable attention and has been widely used as an ideal friction material in many fields. However, the influence of the friction temperature on its tribological properties has not been clearly studied; furthermore, the evolution mechanism of transferred tribofilm is unknown. In this study, the tribological properties of impregnated graphite were investigated at different friction temperatures, and the evolution of the carbon-based tribofilm was also determined. The results revealed that the tribological properties significantly improved with an increase in friction temperature. The friction coefficient and wear depth of impregnated graphite reduced by 68% and 75%, respectively, at a high temperature of 160 ℃ compared with those of non-impregnated graphite. The significant properties of the impregnated graphite can be attributed to a transferred carbon-based tribofilm with an ordered structure induced by the friction temperature, which uniformly and stably adsorbs on friction interfaces. This study provides an important basis for designing graphite-based friction materials with improved properties suited for industrial applications.

In this study, the tribological behaviors of graphene as a lubricant additive for steel/copper and steel/steel friction pairs were compared. For the steel/copper friction pair, the graphene sheets remarkably decreased the coefficient of friction and wear scar depth under low loads, but these slightly increased under high loads. The steel/steel friction pair showed excellent tribological properties even under high loads. Severe plastic deformation on the copper surface reduced the stability of the graphene tribofilm because of a rough copper transfer film on the steel during the running-in period. The results provide a better understanding of the mechanism of graphene as a lubricant additive.

It is well known that groove texture with a careful design can be used to enhance the load-carrying capacity of oil film under the conditions of hydrodynamic lubrication. In this study, a general parametric model was developed, and agenetic algorithm-sequential quadratic programming hybrid method was adopted to obtain the global-optimum profile of the groove texture. The optimized profiles at different rotating speeds are all chevrons. The numerical analysis results verified the effect of the optimization. In addition to the numerical optimization, experiments were conducted to validate the superiority of the optimized results.The experimental results show that the optimized groove texture can efficiently reduce the coefficient of friction (COF) and the temperature rise of the specimen. In particular, the optimized groove textures can achieve stable ultra-low COF values (COF < 0.01) under certain conditions.