Wear Behaviors of Stainless Steel and Lubrication Effect on Transitions in Lubrication Regimes in Sliding Contact

The wear behavior of AISI304 stainless steel was investigated under dry, water-, and oil-lubricated conditions. A block-on-disk wear test was conducted in this work, since the test conditions could be controlled easily. For oil-lubricated contact, a significant amount of thin and elongated cutting chip-like debris was observed. This is attributed to the high lubricating effect of oil. Strain-induced martensitic (SIM) transformation was observed for all AISI304 blocks after the wear test, while AISI304 consisted of a single γ-austenite phase prior to the wear test. The Stribeck curve and the corresponding lubrication regimes were also considered to explain the wear behaviors and lubrication effect of AISI304. In comparison to the dry or water-lubricated conditions, which fall in the boundary lubrication regime at a low rotation speed, it is considered that the high viscosity of the oil-based lubricant causes the lubrication condition to enter the mixed lubrication regime early at a lower speed, thus reducing the specific wear rate over the 100–300 rpm range.

Tribological Behavior of Novel CNTs-Based Lubricant Grease in Steady-State and Fretting Sliding Conditions

The tribological behavior of novel 7.5 wt% carbon nanotube-based lubricant greases in PAO (polyalphaolefin) oil with and without 1.0 wt% MoS2, together with several other commercial greases such as calcium, lithium, were studied. The test results showed a marked reduction of frictional coefficient achieved by the CNTs based grease samples with an average benefit of around 30% compared to conventional greases. The steady state test under 1.00 GPa average contact pressure in a mixed lubrication regime and the fretting test showed the best results in terms of friction reduction obtained by CNTs greases. Steady state tests at higher average contact pressure of 1.67 GPa proved to have a lower friction coefficient for CNTs grease containing MoS2; otherwise CNTs grease without MoS2 showed an average value of CoF comparable to calcium and lithium greases, both in a boundary and a mixed regime. The protection against wear, a considerable decrease (−60%) of reference parameter was measured with CNTs grease with MoS2 (NLGI 2) in comparison with the worst conventional grease and −22% in comparison with the best conventional grease. The data indicated that our novel carbon nanotube greases show superior tribological properties and will have promising applications in the corresponding industry.

Lubrication efficiency of crankpin bearing using square-cylindrical textures of partial textures optimized by genetic algorithm

Purpose This paper aims to propose an optimal design of the partial textures in the mixed lubrication regime of the crankpin bearing (CB) to maximize the CB's lubrication efficiency. Design/methodology/approach Based on a hybrid model between the slider-crank-mechanism dynamic and CB lubrication, the square-cylindrical textures (SCT) of partial textures designed on the CB’s mixed lubrication regime are researched. The effect of the density distributions of partial textures on CB’s lubrication efficiency is then evaluated via two indices of increasing the oil film pressure (p) and decreasing the frictional force (Ff) of the CB. The SCT’s geometrical dimensions are then optimized by the genetic algorithm to further improve the CB’s lubrication efficiency. Findings The results show that the SCT of partial textures optimized by the genetic algorithm has an obvious effect on enhancing CB’s lubrication efficiency. Especially, with the CB using the optimal SCT of partial textures (4 × 6), the maximum p is significantly increased by 3.7% and 8.2%, concurrently, the maximum Ff is evidently reduced by 9.5% and 21.6% in comparison with the SCT of partial textures (4 × 6) without optimization and the SCT of full textures (12 × 6) designed throughout the CB’s bearing surface, respectively. Originality/value The application of the optimal SCT of partial textures on the bearing surface not only is simple for the design-manufacturing process and maximizes CB’s lubrication efficiency but also can reduce the machining time, save cost and ensure the durability of the bearing compared to use the full textures designed throughout the CB’s bearing surface.

Tribological study of multi-scale textured parallel sliding contacts by considering a mixed lubrication regime and mass conservation condition

Surface texturing is a viable technique to enhance the tribological performance of sliding interacting contacts. Single-scaled surface textures exhibit better tribological performance only at hydrodynamic lubrication regime (fluid film pressure) but not in mixed lubrication regime where fluid film pressure and asperity contact pressure co-exists. In most of the machinery with the increase in load and/or decrease in speed, there is a shift of lubrication regime from hydrodynamic to mixed lubrication. To address this, the present work proposed a multi-scale (a combination of shallow and deep) textures concept. A numerical model is developed to study its effect on the tribological characteristics of parallel sliding contacts by considering mixed lubrication regime and mass-conservative cavitation condition. It has been observed that multi-scaled textures exhibit superior results in comparison with single-scaled textures. Moreover, improved tribological characteristics are observed when shallow surface textures are placed first towards the fluid inlet flow.

Influence of the running-in process on the working ability of contact surfaces in lubricated sliding conditions

The influence of the running-in process operating parameters on tribological properties of the block-on-disc samples in lubricated sliding conditions is analyzed and discussed in detail. Different running-in regimes are achieved by varying the normal load and sliding speed. After the running-in period, during which the operating parameters are varied, all samples are placed in a working regime under the same set of operating conditions. At the end of the running-in period, as well as at the end of the working period, an analysis of the changes in the surface roughness, microhardness, wear rate, and coefficient of friction is performed. Less desirable properties in terms of wear rate and steady-state coefficient of friction are noticed for the samples that were run-in with the operating conditions which were the same as the working regime operating conditions. In the defined test conditions, it is shown that the intensity of normal load applied during the running-in process has a dominant influence on the amount of wear and coefficient of friction value. It was also shown that the running-in process can significantly improve the roughness of the initially rough contact surfaces. The results of experimental testing indicate that the variation of the operating parameters during the running-in process can be used to improve the working ability of the sliding contact surfaces under the mixed lubrication regime.

The effects of DLC-coated journal on improving seizure limit and reducing friction under engine oil lubrication

Internal combustion engines have been improved markedly in recent years through efforts to conserve resources, reduce emissions and improve fuel efficiency. In this regard, the authors have been working to reduce friction and improve the seizure properties of the crankshaft main journal and main bearing. These mechanical components of internal combustion engines incur large friction losses. In order to reduce friction, journals have been coated with a diamond-like carbon (DLC) coating, which has been reported to reduce friction in the fluid lubrication regime in recent years. Another current issue of journals and bearings is the need to improve seizure resistance. Therefore, these properties were evaluated for material combinations of aluminium alloy bearings and DLC-coated journals, which have low affinity. The results revealed that friction was reduced under a fluid lubrication regime and seizure resistance was improved under a mixed lubrication regime.

Adjoint optimization of surface textures in mixed lubrication regime

In this work we assess the applicability of the adjoint optimization technique for determining optimal surface topographies of two surfaces in relative motion in presence of a thin lubricant films that can cavitate. Among the existing numerical tools for topology optimization in engineering problems, the adjoint method represents a promising and versatile technique, which can also be applied to the field of full film tribology. In particular, the design of surfaces with complex textures can thoroughly benefit from this method, as it allows dealing with a large number of degrees of freedom at low computational cost. We show that this optimization method can be successfully applied to cavitating lubricant flows such as in pin-on-disc tribometers, giving the possibility to extend the results also to other typical applications such as journal and slider bearings. It is shown that the adjoint method can optimize the whole gap height distribution point by point in a more efficient way than traditional optimization approaches and parametric studies. In particular, thanks to the sensitivity analysis the adjoint method is able to find the placement and depth profile of each texture element.

Tribological performance of textured parallel sliding contact under mixed lubrication condition by considering mass conservation condition and couple-stress parameter

To enhance the tribological performance of mechanical parts, one of the reliable methods is surface topography modification, in which the surface of one/both interacting contacts were to be modified. Surface texturing is one of the surface modification techniques. In the present work, a numerical code is developed to address the effect of texture shape (elliptical and triangular), size and distribution (parallel and zigzag) on the tribological performance parameters (minimum film thickness, percentage of hydrodynamic load from the total generated pressure and frictional coefficient) under mixed lubrication regime for a known value of load support. In the present analysis, the mass conservative, i.e. Jakobson-Floberg-Ollson (JFO) cavitation condition and couple stresses of lubricant are considered. In addition, surface irregularities are considered by using the flow factors of Patir-Cheng model. The results show that texture shape has a significant effect, whereas the texture distribution has a slight effect on the tribological performance parameters. Moreover, the couple stress of lubricant has a prominent effect on the tribological performance.

Lubrication mechanisms of hexagonal boron nitride nano-additives water-based lubricant for steel–steel contact

This study intends to explore the lubrication mechanism of hexagonal boron nitride nano-additive. Synergistic analysis comprising worn surface observation, surface wettability testing, and the Stribeck curve principle is used to test this water-based lubricant on steel–steel contact. Distilled water and 0.1–5.0 vol.% hexagonal boron nitride nano-additive is used to prepare a mixture using sonification technique. A viscometer is employed to determine the viscosity of the nanolubricant. A four-ball tribometer is employed to determine the tribological characteristics and lubrication performance. Hamrock and Dowson equations are used to determine the minimum film thickness needed for lubrication. Surface morphology characteristics are inspected using energy-dispersive X-ray spectroscopy, scanning electron microscopy, surface tension meter, and profilometer. The efficacy of the lubricant as friction and wear-reduction additive is determined to have a mixed lubrication regime with the optimum concentration of 1.0 vol.% hexagonal boron nitride. Protecting film, mending effect, rolling effect, and polishing effect have been recommended as the lubrication mechanisms. Increasing the addition of hexagonal boron nitride additives may lead to a change in the lubrication regime from mixed to hydrodynamic, where agglomeration is observed in the nanoparticles, and an increase in friction is observed.

Applying load-sharing method to the sliding contact in the presence of nano-lubricants

The main goal of this study is to present a model to investigate the effect of nano-particles’ weight fraction on the friction coefficient of rough contact in the mixed-lubrication regime. Experimental testing involves pin-on-disk measurements of the friction coefficient with CuO nano-particles added to engine oil. Theoretical analyses involve developing a method for treating an EHL line contact with provision for surface roughness that takes into account the load-carrying capacity of surface asperities, lubricant, and nano-particles. Results show that theoretical and experimental results for friction coefficients are in good agreement. A parametric study is conducted to investigate effect of load, the geometry of the nano-particles, and their mechanical properties as well as their weight fraction on the friction coefficient.