1. Abstract

    A short commentary on the two research articles published in Friction in 2021 is presented. Both articles reported experimental results of applications of laser surface texturing technology to the raceway of rolling element bearings. After briefly reviewing the main findings of the articles, the arguable problems and distinctions between the two articles are pointed out.

  2. Abstract

    To understand the effect of abrasives on increasing friction in Cu-based metallic pads under different braking speeds, pad materials with two typical abrasives, titanium carbide (TiC) and alumina (Al2O3), were produced and tested using a scale dynamometer under various initial braking speeds (IBS). The results showed that at IBS lower than 250 km/h, both TiC and Al2O3 particles acted as hard points and exhibited similar friction-increasing behavior, where the increase in friction was not only enhanced as IBS increased, but also enhanced by increasing the volume fraction of the abrasives. However, at higher IBS, the friction increase was limited by the bonding behavior between the matrix and abrasives. Under these conditions, the composite containing TiC showed a better friction-increasing effect and wear resistance than the composite containing Al2O3 because of its superior particle-matrix bonding and coefficient of thermal expansion (CTE) compatibility. Because of the poor interface bonding between the matrix and Al2O3, a transition phenomenon exists in the Al2O3-reinforced composite, in which the friction-increasing effect diminished when IBS exceeded a certain value.

  3. Abstract

    With increasing environmental concerns, the substitution of mineral oil-based cutting fluid has become an urgent issue. Using vegetable soybean oil as base fluid, nanofluid cutting fluids (NFCFs) were prepared by adding different weight concentrations of nanographite particles (NGPs), and their penetration and lubrication performances were studied. A novel simulated tool-chip slit with micrometer-sized geometry was manufactured to evaluate and quantify the penetration rate of the NFCFs by image analysis approach. Moreover, a large number of comparative experiments on the closed-type broaching machine were carried out to compare the performance of the proposed NFCFs and a commercial cutting fluid in terms of cutting force, workpiece surface roughness, and metal chip. It is found that there is an optimal NGP concentration in NFCF for practical cutting applications. When the concentration of NGP is 0.4 wt%, the broaching process lubrication exhibits an ideal mixed lubricate state, resulting in minimal friction resistance, and thus, both the cutting force and chip curling angle reach their corresponding best values. Moreover, the proposed NGP-based vegetable-oil cutting fluid exhibits excellent environment-friendliness and low-cost consumption in the minimal quantity lubrication (MQL) method; this demonstrates its potential for replacing the traditional broaching cutting fluid.

  4. Abstract

    This paper presents an efficient three-dimensional (3D) structural model for bump-type gas foil bearings (GFBs) developed by considering friction. The foil structures are modeled with a 3D shell finite element model. Using the bump foil mechanical characteristics, the Guyan reduction and component mode synthesis methods are adopted to improve computational efficiency while guaranteeing accurate static responses. A contact model that includes friction and separation behaviors is presented to model the interactions of the bump foil with the top foil and bearing sleeve. The proposed structural model was validated with published analytical and experimental results. The coupled elastohydrodynamics model of GFBs was established by integration of the proposed structural model with data on hydrodynamic films, and it was validated by comparisons with existing experimental results. The performance of a bearing with an angular misalignment was studied numerically, revealing that the reaction torques of the misaligned bearing predicted by GFB models with 2D and 3D foil structure models are quite different. The 3D foil structure model should be used to study GFB misalignment.

  5. Abstract

    Laser surface texturing (LST) has been proven to improve the tribological performance of machine elements. The micro-scale patterns manufactured by LST may act as lubricant reservoirs, thus supplying oil when encountering insufficient lubrication. However, not many studies have investigated the use of LST in the boundary lubrication regime, likely due to concerns of higher contact stresses that can occur with the increasing surface roughness. This study aims to examine the influence of LST on the fatigue lifetime of thrust rolling bearings under boundary lubrication. A series of periodic patterns were produced on the thrust rolling bearings, using two geometrically different designs, namely cross and dimple patterns. Base oil ISO VG 100 mixed with 0.05 wt% P of zinc dialkyldithiophosphate (ZDDP) was supplied. The bearings with cross patterns reduce the wear loss by two orders of magnitude. The patterns not only retain lubricant in the textured pockets but also enhance the formation of an anti-wear tribofilm. The tribofilm generation may be improved by the higher contact stresses that occur when using the textured surface. Therefore, in contrast to the negative concerns, the ball bearings with cross patterns were instead found to increase the fatigue life by a factor of three.

  6. Abstract

    Black phosphorus quantum dots (BPQDs), obtained via a typical solution-based top-down method, were used as water-based lubricant additives. BPQDs exhibited remarkable friction reduction and anti-wear properties even at the ultra-low concentration of 0.005 wt%, which reduced the friction coefficient and wear volume of the base liquid by 32.3% and 56.4%, respectively. In addition, the load-supporting capacity of the base liquid increased from 120 N to over 300 N. BPQDs-based additives exhibited a relatively long lifetime at a relatively high load of 80 N. The performance of BPQDs considerably exceeded that of the BP; this may be attributed to their small and uniform particle size, good dispersion stability in water, and high reactivity at the frictional surfaces. The results of the surface wear resistance analysis demonstrated that a robust tribochemical film with a thickness of approximately 90 nm was formed on the rubbing surface lubricated with 0.005 wt% of BPQDs dispersion. Moreover, the film served as a direct evidence of the excellent tribological performance of BPQDs.

  7. Abstract

    In case of lightly loaded radial ball bearings, failure mechanisms other than fatigue such as smearing of raceways due to increased frictional torque and vibrations often prevail. Hence, attempts have been made herein for reducing the frictional torque and minimizing the vibrations of a radial deep groove ball bearing employing surface textures at the inner race. Nanosecond pulsed laser was used to create texture (involving micro-dimples having different dimple area density) on the inner race of test bearings. Using an in-house developed test rig, frictional torque and vibrational parameters were measured at different speeds and light loads (i.e. in vicinity of 0.01C, where C is dynamic load capacity of radial ball bearing). Significant reduction in frictional torque and overall vibrations were found in the presence of micro-dimples on inner race at light loads irrespective of operating speeds. Even without satisfying the minimum load needed criteria for the satisfactory operation, substantial reduction in smearing marks was found on the races of textured ball bearings in comparison to conventional cases.

  8. Abstract

    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.

  9. Abstract

    A unique low-to-high friction transition is observed during unlubricated sliding in metals with a gradient nano-grained (GNG) surface layer. After persisting in the low-friction state (0.2–0.4) for tens of thousands of cycles, the coefficients of friction in the GNG copper (Cu) and copper-silver (Cu-5Ag) alloy start to increase, eventually reaching a high level (0.6–0.8). By monitoring the worn surface morphology evolution, wear-induced damage accumulation, and worn subsurface structure evolution during sliding, we found that the low-to-high friction transition is strongly correlated with distinct microstructural instabilities induced by vertical plastic deformation and wear-off of the stable nanograins in the subsurface layer. A very low wear loss of the GNG samples was achieved compared with the coarse-grained sample, especially during the low friction stage. Our results suggest that it is possible to postpone the initiation of low-to-high friction transitions and enhance the wear resistance in GNG metals by increasing the GNG structural stability against grain coarsening under high loading.

  10. Abstract

    Ethylenediamine with two −NH2 functional groups was used as a critical complexing agent in chemical mechanical polishing (CMP) slurries for a high carbon chromium GCr15 bearing steel (equivalent to AISI 52100). The polishing performance and corresponding mechanism of −NH2 functional groups were thoroughly investigated as a function of pH. It is revealed that, when polished with ethylenediamine and H2O2-based slurries, the material removal rate (MRR) and surface roughness Ra of GCr15 steel gradually decrease as pH increases. Compared with acidic pH of 4.0, at alkaline pH of 10.0, the surface film of GCr15 steel has much higher corrosion resistance and wear resistance, and thus the material removal caused by the pure corrosion and corrosion-enhanced wear are greatly inhibited, resulting in much lower MRR and Ra. Moreover, it is confirmed that a more protective composite film, consisting of more Fe3+ hydroxides/oxyhydroxides and complex compounds with −NH2 functional groups of ethylenediamine, can be formed at pH of 10.0. Additionally, the polishing performance of pure iron and a medium carbon 45 steel exhibits a similar trend as GCr15 steel. The findings suggest that acidic pH could be feasible for amine groups-based complexing agents to achieve efficient CMP of iron-based metals.

  11. Abstract

    High-temperature solid lubricants play a significant role in the hot metal forming process. However, preparing high-temperature solid lubricant is formidably challenging due to the stern working conditions. Here we successfully develop a new type of eco-friendly high-temperature graphite-based solid lubricant by using amorphous silica dioxide, aluminum dihydrogen phosphate, and solid lubricant graphite. The solid lubricating coating exhibits excellent tribological properties with a very low friction coefficient and good wear protection for workpiece at high temperature under the air atmosphere. An array of analytical techniques reveals the existence of solid lubricant graphite in the lubricating coating after the high-temperature friction test. A synergistic effect between the protective surface film and the solid lubricant graphite is proposed to account for such superior lubricating performance. This work highlights the synergistic effect between the protection layer and the lubricant graphite and further provides the insight in designing the high-temperature solid lubricant.

  12. Abstract

    A calculation method for the friction coefficient and meshing efficiency of plastic line gear (LG) pair under dry friction conditions was studied theoretically and experimentally, taking a polyoxymethylene parallel line gear pair (POM PLGP) as an example. Firstly, the geometric and mechanical models of PLGP were built by considering the effects of misalignment and loaded deformation under the actual operating condition. Then, the friction coefficient of POM specimens was obtained via the ball-on-disk experiment, of which the value varies between 0.35 and 0.45 under the experimental conditions. The calculation formula for the friction coefficient of POM LG pair was obtained by fitting the friction coefficient of the POM specimens, and the meshing efficiency of POM LG pair was calculated based on the calculation formula for friction coefficient and the meshing efficiency calculation approach. Finally, the meshing efficiency of POM PLGP specimens was measured using a homemade gear meshing efficiency test rig. The experimental results validated the feasibility of the proposed calculation method for the friction coefficient and meshing efficiency of the plastic LG pair. This study provides a method for the calculation of the friction coefficient and meshing efficiency of plastic gear pairs under dry friction conditions. It also provides the basis for the wear calculation of plastic LG pair under dry friction conditions.

  13. Abstract

    This paper is devoted to an analytical, numerical, and experimental analysis of adhesive contacts subjected to tangential motion. In particular, it addresses the phenomenon of instable, jerky movement of the boundary of the adhesive contact zone and its dependence on the surface roughness. We argue that the “adhesion instabilities” with instable movements of the contact boundary cause energy dissipation similarly to the elastic instabilities mechanism. This leads to different effective works of adhesion when the contact area expands and contracts. This effect is interpreted in terms of “friction” to the movement of the contact boundary. We consider two main contributions to friction: (a) boundary line contribution and (b) area contribution. In normal and rolling contacts, the only contribution is due to the boundary friction, while in sliding both contributions may be present. The boundary contribution prevails in very small, smooth, and hard contacts (as e.g., diamond-like-carbon (DLC) coatings), while the area contribution is prevailing in large soft contacts. Simulations suggest that the friction due to adhesion instabilities is governed by “Johnson parameter”. Experiments suggest that for soft bodies like rubber, the stresses in the contact area can be characterized by a constant critical value. Experiments were carried out using a setup allowing for observing the contact area with a camera placed under a soft transparent rubber layer. Soft contacts show a great variety of instabilities when sliding with low velocity — depending on the indentation depth and the shape of the contacting bodies. These instabilities can be classified as “microscopic” caused by the roughness or chemical inhomogeneity of the surfaces and “macroscopic” which appear also in smooth contacts. The latter may be related to interface waves which are observed in large contacts or at small indentation depths. Numerical simulations were performed using the Boundary Element Method (BEM).

  14. Abstract

    In this study we present a mechanism for the elastohydrodynamic (EHD) friction reduction in steel/steel contacts, which occurs due to the formation of oleophobic surface boundary layers from common boundary-lubrication additives. Several simple organic additives (amine, alcohol, amide, and fatty acid) with different molecular structures were employed as the model additives. It was found that the stronger chemisorption at 100 °C, rather than the physisorption at 25 °C, is more effective in friction reduction, which reaches 22%. What is more, EHD friction reduction was obtained in steel/steel contacts without use of the diamond-like carbon (DLC) coatings with their wetting or thermal effect, which was previously suggested as possible EHD friction reduction mechanism; yet about the same friction reduction of about 20% was obtained here—but with much simpler and less expensive technology, namely with the adsorbed oleophobic surface layers. A small variation in the additive’s molecular structure results in significant changes to the friction, indicating good potential in future EHD lubrication technology, where these additives could be designed and well optimised for notable reduction of the friction losses in the EHD regime.

  15. Abstract

    Polytetrafluoroethylene (PTFE) blended with polyimide (PI) and filled with boron nitride (BN) is prepared through cold pressing and sintering for composites with remarkable wear resistance and reduced coefficient of friction (COF). The characterizations show that BN and PI at different levels, improve the hardness, dynamic thermo-mechanical modulus, thermal conductivity, and tribological properties of PTFE. PI boosts the dispersion and bonding of BN in PTFE. In dry sliding friction of a block-on-ring tribometer, the wear rate and COF of 10:10:80 BN/PI/PTFE reduce to almost 1/300 and 80% of those of pure PTFE, respectively, as the wear mechanism transition from being adhesive to partially abrasive. This occurs only when the additives BN and PI induce a synergistic effect, that is, at concentrations that are not higher than ca. 10 wt% and 15 wt%, respectively. The obvious agglomeration at high percentages of added PI and severe conditions (400 N and 400 rpm) induce strong adhesive failure. The variations in the tensile properties, hardness, crystallization, and microstructure of the composites correspond to different effects. The multiple parameters of the plots of wear and friction are transformed into their contour curves. The mechanism transition maps aid in understanding the influence of various test conditions and composite compositions on the contact surfaces in the space-time framework of wear.

  16. Abstract

    The microstructure, mechanical and micro/nano-tribological properties of the 60NiTi film annealed at different temperature were investigated. The results reveal that annealing as-deposited 60NiTi film at 300, 375, and 600 °C for 1 h leads to structural relaxation, partial crystallization and full crystallization, respectively. Compared with the structurally relaxed structure, the partially crystallized structure exhibits increased hardness but decreased elastic modulus. This is because that the elastic modulus is reduced by Voigt model while the hardness is improved by composite effect. Due to the highest hardness and ratio of hardness to elastic modulus (H/E), the partially crystallized 60NiTi film has the lowest penetration depth and residual depth (i.e., groove depth). Besides, the results also reveal that ductile plowing is the dominant wear mechanism for all the annealed 60NiTi films. Under the condition of the ductile plowing, coefficient of friction and wear resistance are related to penetration depth and residual depth, respectively. Therefore, the partially crystallized 60NiTi film shows the best tribological performance at the micro/nano-scale. The current work not only highlights the important roles of hardness and H/E in improving the micro/nano-tribological properties but also concludes an efficient and simple method for simultaneously increasing hardness and H/E.

  17. Abstract

    A recent systematic experimental characterisation of technological thin films, based on elaborated design of experiments as well as probe calibration and correction procedures, allowed for the first time the determination of nanoscale friction under the concurrent influence of several process parameters, comprising normal forces, sliding velocities, and temperature, thus providing an indication of the intricate correlations induced by their interactions and mutual effects. This created the preconditions to undertake in this work an effort to model friction in the nanometric domain with the goal of overcoming the limitations of currently available models in ascertaining the effects of the physicochemical processes and phenomena involved in nanoscale contacts. Due to the stochastic nature of nanoscale friction and the relatively sparse available experimental data, meta-modelling tools fail, however, at predicting the factual behaviour. Based on the acquired experimental data, data mining, incorporating various state-of-the-art machine learning (ML) numerical regression algorithms, is therefore used. The results of the numerical analyses are assessed on an unseen test dataset via a comparative statistical validation. It is therefore shown that the black box ML methods provide effective predictions of the studied correlations with rather good accuracy levels, but the intrinsic nature of such algorithms prevents their usage in most practical applications. Genetic programming-based artificial intelligence (AI) methods are consequently finally used. Despite the marked complexity of the analysed phenomena and the inherent dispersion of the measurements, the developed AI-based symbolic regression models allow attaining an excellent predictive performance with the respective prediction accuracy, depending on the sample type, between 72% and 91%, allowing also to attain an extremely simple functional description of the multidimensional dependence of nanoscale friction on the studied variable process parameters. An effective tool for nanoscale friction prediction, adaptive control purposes, and further scientific and technological nanotribological analyses is thus obtained.

  18. Abstract

    The contact fatigue of aviation gears has become more prominent with greater demands for heavy-duty and high-power density gears. Meanwhile, the coexistence of tooth contact fatigue damage and tooth profile wear leads to a complicated competitive mechanism between surface-initiated failure and subsurface-initiated contact fatigue failures. To address this issue, a fatigue-wear coupling model of an aviation gear pair was developed based on the elastic-plastic finite element method. The tooth profile surface roughness was considered, and its evolution during repeated meshing was simulated using the Archard wear formula. The fatigue damage accumulation of material points on and underneath the contact surface was captured using the Brown-Miller-Morrow multiaxial fatigue criterion. The elastic-plastic constitutive behavior of damaged material points was updated by incorporating the damage variable. Variations in the wear depth and fatigue damage around the pitch point are described, and the effect of surface roughness on the fatigue life is addressed. The results reveal that whether fatigue failure occurs initially on the surface or sub-surface depends on the level of surface roughness. Mild wear on the asperity level alleviates the local stress concentration and leads to a longer surface fatigue life compared with the result without wear.

  19. Abstract

    The wear interaction of cementite and pearlite in the white cast iron (WCI) was investigated using the two-body abrasive wear test under contact loads of 20, 35, and 50 N. The wear behavior, wear surface morphology, sub-surface structure, and wear resistance were evaluated using X-ray diffraction, microhardness testing, and nano-indentation. The results indicated that when the Cr content was increased from 0 to 4 wt%, there was a significant increase in the microhardness (H) and elasticity modulus (E) of the cementite. This yielded a 15.91%- and 23.6%-reduction in the degree of wear resistance and surface roughness, respectively. Moreover, no spalling and breaking of cementite was observed with increasing Cr content during the wear process, indicating improved wear resistance of the bulk cementite. In addition, the hard phase (cementite) and tough matrix (pearlite) composite structure exhibited a good protective and supporting effect. Thus, it was concluded that the interaction mechanism of the wear phase contributed to the reduction of the wear weight loss of the composite during the wear process. The contribution of the interaction between the hard wear-resistant phase and the tough phase in WCI to the wear resistance decreased with increasing hardness of the pearlite matrix.

  20. Abstract

    Phenomena of friction, wear, and noise in mechanical contacts are particularly important in the field of tribomechanics but equally complex if one wants to represent their exact relationship with mathematical models. Efforts have been made to describe these phenomena with different approaches in past. These efforts have been compiled in different reviews but most of them treated friction, wear mechanics, and acoustic noise separately. However, an in-depth review that provides a critical analysis on their interdependencies is still missing. In this review paper, the interdependencies of friction, wear, and noise are analysed in the mechanical contacts at asperitical level. The origin of frictional noise, its dependencies on contact’s mechanical properties, and its performance under different wear conditions are critically reviewed. A discussion on the existing mathematical models of friction and wear is also provided in the last section that leads to uncover the gap in the existing literature. This review concludes that still a comprehensive analytical modelling approach is required to relate the interdependencies of friction, noise, and wear with mathematical expressions.