High Normal Loads Research Articles

Mechanical and tribological properties of Zr-based bulk metallic glass for sports applications

The microstructure, and mechanical and tribological properties of zirconium-based bulk metallic glass (Zr-BMG) were investigated for golf club heads. The Zr-BMG had a fully amorphous structure without any detectable crystalline phases. The hardness and Young's modulus of the Zr-BMG were relatively high even though they were lowered by about 36% and 28% respectively by the increased indentation depth from 200 to 1000nm due to strain softening during the nanoindentation test. The tribological behaviour of the Zr-BMG was investigated by sliding against alumina (Al2O3), 100Cr6 steel or polymethyl-methacrylate (PMMA) balls under normal loads of 1–3N. The wear of the Zr-BMG tested against the Al2O3 and 100Cr6 balls became higher with higher normal loads while a higher wear resistance of the Al2O3 ball than that of the 100Cr6 ball caused a higher wear of the Zr-BMG. A much lower wear resistance of the PMMA ball than that of the Zr-BMG resulted in a preferential wear of the PMMA ball without generating any apparent wear of the Zr-BMG during the testing. The Zr-BMG tested against the Al2O3 ball exhibited the highest wear under all the normal loads due to the highest wear resistance of the Al2O3 ball among the counter balls.

Scratch behaviour of graphene alumina nanocomposites

The scratch resistance behaviour of alumina-graphene nanoplatelet (GNP) (0.5, 2 and 5 vol.-%) composites was investigated using a Rockwell indenter with normal applied loads ranging from 1 to 200 N. The alumina-GNP composites behaved differently during scratch testing depending on the normal applied load. The coefficient of friction of the composites did not change much at low normal loads but increased with increasing amount of GNP in the alumina matrix for high normal loads. The addition of GNP contributed to improved scratch resistance of alumina nanocomposites only for low loads below ∼97 N. This correlates with the mechanical properties of the composites. As the applied load increased, the scratch resistance of the GNP composites decreased due to the presence of weakly bonded grain boundaries in the alumina matrix, which enhanced chipping of material.

Study of the anisotropic frictional and deformation behavior of surfaces textured with silver nanorods

In this study, we investigated the anisotropic frictional and deformation behavior of a nanostructured thin film (NSTF) consisting of silver (Ag) nanorods tilted at an average angle of 65° to the surface normal and compared to that of a continuous thin film of Ag. The NSTF demonstrated significant frictional anisotropy with the coefficient of friction (COF) being ~45% higher against the tilt direction than that of along the tilt direction. Furthermore, the NSTF sample exhibited lower COF than the continuous thin film at low applied normal load and higher COF at higher normal loads. In addition, the deformation of the nanorods increased with increasing normal loads and is shown to be also dependent on the direction of the scratch.

Effects of electrolytes on the growth behavior, microstructure and tribological properties of plasma electrolytic oxidation coatings on a ZA27 alloy

In this study, plasma electrolytic oxidation (PEO) was successfully applied on a ZA27 alloy to produce continuous and dense ceramic coatings. The effects of the electrolytes (silicate, aluminate and aluminate/borate composite solutions) on the coating growth behavior, microstructure, phase compositions, and tribological properties were investigated. ZnAl2O4, Al2O3, and CuAlO2 can be found in all the three coatings. In addition, the coating produced with silicate solution contains ZnSiO4. During the process of coating, the Al element preferred to react with electrolytes to form the ceramic coatings. The addition of borate solution can accelerate the coating growth rate and be beneficial to produce a denser coating. Coatings produced with silicate and aluminate/borate electrolyte have a structure with porous outlayer, dense layer, and inner barrier layer, whereas the coating produced with aluminate electrolyte only shows a dense monolayer structure. The coating produced with aluminate/borate composite electrolyte achieved the best wear performance, lowest friction coefficient, and highest wear resistance among the three coatings under both low and high normal loads.

Combined Effects of Structural Transformation and Hydrogen Passivation on the Frictional Behaviors of Hydrogenated Amorphous Carbon Films

Tribological behaviors of hydrogenated amorphous carbon (a-C:H) films under single asperity contact are investigated by molecular dynamics (MD) simulations. Hydrogen concentration and normal load are found to play essential roles in the frictional behavior of a-C:H films. With low hydrogen concentration, the a-C:H film shows high adhesion and friction even at very low normal loads (1.75 nN). The sp3-to-sp2 rehybridization is observed in the a-C:H films with all studied hydrogen concentrations, which is greatly enhanced with increasing normal load. At high normal loads, formation of nanocrystalline graphene-like lamellar structures is observed locally, usually beneath the instantaneous contact area, acting as a lubricating agent, demonstrating local graphitization due to combined effects of the compression and shear process. With high hydrogen concentration, the friction shows a linear increase along with the normal load and higher load bearing capability. Hydrogen atoms accumulate on the film surface duri...

Non-Newtonian Inertia Squeeze Film Characteristics in Rectangular Stepped Plates

A study of non-Newtonian inertia squeeze film in rectangular stepped plates has been presented in this paper. Applying the momentum integral method incorporating the micro-continuum theory of non-Newtonian fluids, a non-Newtonian inertia lubrication equation is derived. It is found that the fluid inertia effects yield in a higher normal load capacity as well as a longer squeeze film time as compared to the non-Newtonian stepped squeeze film in the absence of fluid inertia forces.

Preparation and evaluation of tribological properties of oil-soluble rice-like CuO nanoparticles

Purpose – The purpose of this paper is synthesis of oil-soluble non-spherical nanoparticles modified with free phosphorus and sulphur modifier and investigation of their tribological properties as environment-friendly lubricating oil additives. Design/methodology/approach – To study the effect of morphology of nanoparticles on their tribological properties, rice-like CuO nanoparticles were synthesized. To improve the solubility of CuO nanoparticles in organic media, the in-situ surface modification method was used to synthesize these products. The morphology, composition and structure of as-synthesized CuO nanoparticles were investigated by means of transmission electron microscopy, X-ray powder diffraction, thermogravimetric analysis and Fourier transform infrared spectrometry. The tribological properties of as-synthesized CuO nanoparticles as an additive in liquid paraffin (LP) were evaluated with a four-ball tribometer. The morphology and elemental composition of worn steel ball surfaces were analysed by X-ray photoelectron spectroscopy. Findings – It has been found that as-synthesized CuO nanoparticles with rice-like morphology have an average size of 7 and 15 nm along the shorter axle and longer axle, respectively, and can be well-dispersed in LP. Tribological properties evaluation results show that as-synthesized CuO nanoparticles as additives in LP show good friction-reducing, anti-wear and load-carrying capacities, especially under a higher normal load. Originality/value – Oil-soluble rice-like CuO nanoparticles without phosphorus and sulphur were synthesized and their tribological properties as lubricating oil additives were also investigated in this paper. These results could be very helpful for application of CuO nanoparticles as environment-friendly lubricating oil additives, owing to their free phosphorus and sulphur elements characteristics.

Apparent friction and cohesion of a partially wet granular material in steady-state shear

The shear resistance of glass beads with wetting liquid incorporated (polyethylene glycol of molecular weight of 400g/mol) is experimentally and theoretically investigated in a large range of both liquid content (from 0.007 % to 20 % in volume) and normal loading, in a steady-state flowing situation. It is observed that the behaviour is not purely frictional (shear stress proportional to the normal stress) except for the lowest liquid content (up to 0.1 %) for which it is observed that the friction coefficient of wet granular material is slightly higher than the dry material. At high normal loading, an apparent cohesion is observed and the behaviour may be interpreted as frictional, with the same friction coefficient plus an additional normal stress due to the capillary forces between the beads. At low normal loading, the apparent cohesion is less and may be qualitatively explained by the diminution of broken capillary contacts during shearing when the porosity of the granular bed increases. This result is corroborated by a theoretical approach estimating the capillary forces in play.

Tribochemical mechanism of amorphous silica asperities in aqueous environment: a reactive molecular dynamics study.

Reactive molecular dynamics (ReaxFF) simulations are used to explore the atomic-level tribochemical mechanism of amorphous silica (a-SiO2) in a nanoscale, single-asperity contact in an aqueous environment. These sliding simulations are performed in both a phosphoric acid solution and in pure water under different normal pressures. The results show that tribochemical processes have profound consequences on tribological performance. Water molecules could help avoid direct adhesive interaction between a-SiO2 surfaces in pure water under low normal load. However, formation and rupture of interfacial siloxane bonds are obviously observed under higher normal load. In phosphoric acid solution, polymerization of phosphoric acid molecules occurs, yielding oligomers under lower load, and tribochemical reactions between the molecules and the sliding surfaces could enhance wear under higher load. The bridging oxygen atoms in silica play an important role in the formation of interfacial covalent bonds, and hydrogen is found to have a weakening effect on these bonds, resulting in the rupture during shear-related loading. This work sheds light on tribochemical reactions as a mechanism for lubrication and wear in water-based or other tribological systems.

Damage mechanisms in stainless steel and chromium carbide coatings under controlled environment fretting conditions

Fretting is of a serious concern in many industrial components, specifically, in nuclear industry for the safe and reliable operation of various component and/or system. Under fretting condition small amplitude oscillations induce surface degradation in the form of surface cracks and/or surface wear. Comprehensive experimental studies have been carried out simulating different fretting regimes under ambient and vacuum (10−9MPa) conditions and, temperature up to 400°C. Studies have been carried out with stainless steel spheres on stainless steel flats, and stainless steel spheres against chromium carbide, with 25% nickel chrome binder coatings. Mechanical responses are correlated with the damage observed. It has been observed that adhesion plays a vital role in material degradation process, and its effectiveness depends on mechanical variables such as normal load, interfacial tangential displacement, characteristics of the contacting bodies and most importantly on the environment conditions. Material degradation mechanism for ductile materials involved severe plastic deformation, which results in the initiation or nucleation of cracks. Ratcheting has been observed as the governing damage mode for crack nucleation under cyclic tangential loading condition. Further, propagation of the cracks has been observed under fatigue and their orientation has been observed to be governed by the contact conditions prevailing at the contact interface. Coated surfaces show damage in the form of brittle fracture and spalling of the coatings. Existence of stick slip has been observed under high normal load and low displacement amplitude. It has also been observed that adhesion at the contact interface and instantaneous cohesive strength of the contacting bodies dictates the occurrence of material transfer. The paper discusses the mechanics and mechanisms involved in fretting damage under controlled environment conditions.

Tribological characteristics and tribo-chemical mechanisms of Al–Mg–Ti–B coatings under water–glycol lubrication

In this work, the tribological characteristics and tribo-chemical mechanisms of the water–glycol lubricated Al–Mg–Ti–B coatings under various normal loads were studied by pin-on-disk unidirectional sliding test, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS). The water–glycol lubricated Al–Mg–Ti–B coatings showed a steady-state friction coefficient as low as 0.02 and this value was not influenced by the applied normal loads. As the normal loads increased, the friction coefficients of the Al–Mg–Ti–B coatings decreased more rapidly. Examination of the wear tracks of water–glycol lubricated Al–Mg–Ti–B coatings by SEM and EDX shows that the sliding wear process was dominated by a polishing effect. XPS results indicate that significant surface boron oxidation of Al–Mg–Ti–B coatings occurred during sliding in a water–glycol lubricated environment; such effect was more pronounced at higher normal loads. However, there is no clear evidence of the presence of boric acid in XPS spectra at all applied normal loads. This is thought to be due to the unstable nature of boric acid and in situ surface characterization of the water–glycol lubricated Al–Mg–Ti–B coatings needs to be done to clarify this issue.

Modelling of ploughing-cutting transition regime during abrasive manufacturing process

Abrasive manufacturing process is widely used in the automotive manufactures (polishing of valves, crankshafts, camshafts, etc.). To understand the abrasive wear mechanisms during abrasive manufacturing process, a multi-asperity abrasive wear model is proposed. The latter combines different deformation modes of the surface with abrasive wear mechanisms, which depend on a degree of penetration DP (ratio between the groove depth and the half-width of the groove produced by each abrasive grain). This study showed that the wear volume per unit sliding distance increases with the total normal load. At low load, the wear volume increases slowly and the main wear mechanism is ploughing without resulting in material removal. At higher normal load, the wear rate increases with increasing the degree of penetration, leading to the transition in dominant wear mode from ploughing to wedging and cutting.

Microstructural characterization and mechanical property of Fly Ash/Al-25Mg composites

Fly ash/A1-Mg composites are fabricated by powder metallurgical method. The morphology and structure of fly ash/A1-Mg composites are characterized by scanning electron microscope (SEM) and X-ray diffraction, respectively. The influences of different fly ash content on the friction and wear behavior of the composites are investigated at a constant sliding velocity of 400 r/min and the worn mechanism of composites is discussed. The results indicate that the friction coefficient is steadily lower than that of Al alloy matrix at the lower fly ash content and loads. For the fly ash/A1-Mg composites, the wear mechanism is characterized as abrasive wear and adhesive wear under small normal load and at low fly ash content, and it is characterized as delamination wear and abrasive wear transferred onto the counterpart under high normal load and at high fly ash content.

Long-Duration Frictional and Wear Performance of the Diamond/DLC Bilayered Film under Water-Lubricating Condition

In the present study, the long-duration frictional and wear performance of the MCD/DLC (Micro-crystalline Diamond / Diamond-like Carbon) bilayered film are investigated under water-lubricating conditions. All friction tests are carried out on a rotation “ball-on-plate” tribotester where the MCD/DLC is slid against with a Φ 6.0 mm Si3N4ball and the whole sliding contact is immersed in deionized water during the sliding process. A full factorial experimental plan is conducted with four sliding velocities ranging from 0.126 to 0.503 m/s and four normal loads from 2 to 8 N. The duration of each sliding process is 24 h. For the sake of comparability, conventional MCD and NCD (nanocrystalline Diamond) films are also adopted under each sliding condition. The results show that the stable coefficient of friction (COF) of MCD/DLC film is ranging from 0.025 to 0.12 under the water-lubricating condition, comparable with the NCD film but much lower than that of single-layered MCD film; in contrast, the top-layered DLC film does not show beneficial effect on enhancing the sliding stability of single-layered diamond films. Moreover, its specific wear rate is estimated at the level of 10-8mm3N-1m-1, higher than that of MCD or NCD films. The sliding interface is under boundary lubrication condition, high normal load causes more prominent mechanical interactions between two contacted surfaces and thus produces a smoother and cleaner equilibrium sliding interface, which finally results in the decreasing tendency of stable COF as the load rises. Comparatively, the sliding velocity does not exhibit evident influence on the stable COF of the MCD/DLC film.

Laboratory evaluation of governing mechanism of frictionally connected MSEW face and implications on design

This paper presents a laboratory evaluation of purely frictionally connected geotextile and concrete facing block of Mechanically Stabilized Earth Wall (MSEW) systems. The study focuses on investigating the governing failure mechanism along the wall face, as determined from the pullout of reinforcement in between the facing blocks (herein referred as pullout mechanism) and sliding of the blocks over the geotextile, where the reinforcement stays stationery (herein referred as direct shear mechanism). A total of seventy-two tests were performed to investigate the effect of laboratory specimen size, difference in geotextile reinforcement, and repeatability of the test results. Overall, the results showed that at lower normal loads, sliding of the blocks over the geotextile reinforcement along the wall face is more likely to occur before the pullout of the geotextile in between the blocks. At higher normal loads, this order is reversed and pullout of the geotextile appears to occur first. The test results also indicated that the size of the specimen tested in the laboratory frictional connection evaluation has an effect on the measured connection strength.

Lubricating properties of silica/graphene oxide composite powders

The silica/graphene oxide (GO) composites powders of different compositions were synthesized by hydrothermal method. Composites were dispersed in ethylene glycol keeping the additive’s concentration constant (0.125 wt%). The lubricating properties of the dispersions were studied in steel–steel contact by pin-on-disc method. Tests were conducted at a low speed (0.008m/s) and a high normal load of 68.9N. Composite with 10 wt% of GO in silica matrix showed reduction of coefficient of friction in ethylene glycol by 38% and reduction of disc wear coefficient by 31% compared to pure ethylene glycol. Silica/GO composites provided lower friction and wear rate in ethylene glycol lubrication compare to pure silica and pure GO.

A Wear Damage Assessment of High Temperature Forging Tool

High temperature forging tools are highly damaged by wear shearing under cyclic loading which reduces the life of tools. In real conditions, depending upon the tool areas, the level of wear can change. The surfaces of tools can be treated by cobalt-based hardfacing using different welding processes. This study focuses on tribological damages of Stellite 21 deposited by MIG process. Wear tests are carried out at room and high temperature on a ring on disc tribometer under high normal load. Different testing conditions are examined. The surface plastic strain due to the friction shear stresses is demonstrated by different methods like SEM observations, micro-hardness measurements and XRD analyses. More particularly, it is shown that the initial (200) crystallographic preferred orientation due to the welding process is modified into (111) crystallographic preferred orientation due to shear stresses regardless the loading. Moreover, a relationship between the gradient of the plastic shear strain at the friction subsurface and the level of the micro-hardness measurement has been established. In regard of the results, when the shear strain exceeds a threshold, the micro-hardness measurement is stabilised. The ultimate value could be induced by the stacking fault energy that is considered as weak in Stellites.

Fretting wear of Mg–Li–Al based alloys

The fretting wear behavior of newly developed Mg–Li–Al based alloys LAT971 and LATZ9531 is investigated by using reciprocating fretting wear testing under dry condition with varying number of cycles (10–104), normal load (1–10N), oscillation frequency (1–9Hz), and displacement amplitude (80–200µm). The worn surface is characterized by scanning electron microscopy to reveal wear mechanisms governing under each condition. Overall, results show that LATZ9531 elicits consistent performance, lower coefficient of friction (~0.50) than conventional AZ31 (~0.69) and LAT971 (0.60). But, under the condition of high normal load (of 10N), LAT971 shows least wear volume loss (~0.05mm3) as well as lower coefficient of friction (~0.26) compared to that of LATZ9531 or conventional AZ31. In addition to central cracking in LATZ9531, mainly adhesive and delamination wear mechanism are noticed. Under frequency variation LAT971 shows enhanced performance especially at high frequency (~9Hz). However, with increase in slip amplitude (~200μm), the major wear mechanism is observed to be abrasive and oxidative wear, wherein LATZ9531 shows least wear volume. Friction hysteresis of increasing slip amplitude shows increase in energy consumption during fretting wear, and its shape confirms that in spite of change in stroke lengths, fretting is occurring in the gross slip regime.

Unoxidized graphene/alumina nanocomposite: fracture- and wear-resistance effects of graphene on alumina matrix.

It is of critical importance to improve toughness, strength, and wear-resistance together for the development of advanced structural materials. Herein, we report on the synthesis of unoxidized graphene/alumina composite materials having enhanced toughness, strength, and wear-resistance by a low-cost and environmentally benign pressure-less-sintering process. The wear resistance of the composites was increased by one order of magnitude even under high normal load condition (25 N) as a result of a tribological effect of graphene along with enhanced fracture toughness (KIC) and flexural strength (σf) of the composites by ~75% (5.60 MPa·m1/2) and ~25% (430 MPa), respectively, compared with those of pure Al2O3. Furthermore, we found that only a small fraction of ultra-thin graphene (0.25–0.5 vol%, platelet thickness of 2–5 nm) was enough to reinforce the composite. In contrast to unoxidized graphene, graphene oxide (G-O) and reduced graphene oxide (rG-O) showed little or less enhancement of fracture toughness due to the degraded mechanical strength of rG-O and the structural defects of the G-O composites.

Effect of the Normal Load on the Release of Aerosol Wear Particles During Abrasion

A study highlighting the aspect of the generation of aerosol wear particles during abrasion is presented. The substrate chosen is a masonry brick which is reinforced with TiO2 nanoparticles. This is done using a pin on plate arrangement. The material removal mechanism via fracturing is first understood. The parameter chosen for the study is the normal load. The formed aerosols are then characterized by their number concentration, particle size distribution, individual particle shape, size and chemical composition. Having irregular shapes, the aerosol wear particles have unimodal size distributions with 5–7 % (in mass) of Ti content. The size mode increases with the increase in normal load. However, at higher normal loads, while there is an unexpected increase in the wear mass, the maximum concentration of the aerosol particles saturates. During the whole study, no free nanoparticles of TiO2 were found.