Tribology Letters

Ab initio quantum chemical computations on model perfluoropolyether (PFPE) boundary lubricant film structures are used to study the effect of main chain confinement on dynamic main chain expansion in the direction perpendicular to the underlying surface. The simulations are conducted on single-chain PFPE structures that contain a pendant branch in the middle of the main chain anchored to the underlying surface. Results indicate a substantial increase in the energy required to expand the branched chain compared to an identical but unbranched main chain. Conformational analyses as a function of main chain expansion indicate the removal of low energy torsional pathways in the branched main chain that could otherwise provide low energy pathways for conformational rearrangement to allow the main chain expansion.

Lubrication function of multialkylated cyclopentanes (MACs) greases for the contact TC4/Steel was investigated in detail under the sliding and fretting conditions, and the worn tracks were measured by surface/interface analysis techniques to explore the friction mechanisms. The results demonstrate that these lubricating greases have good friction-reducing function under fretting and sliding conditions, and their friction reduction is superior to wear resistance, especially calcium sulfonate complex grease. Good lubrication function of MACs greases under sliding friction mainly depends on synergies of grease-film and friction-induced chemical reaction film. Fretting behaviors depend on not only grease-film and chemical reaction film, but also fretting wear mechanisms including abrasion, oxidation, and delamination.

Friction between the rake face of a cutting tool and the freshly formed chip surface plays a vital role in influencing both the ease of cutting and the quality of the resultant machined surface. The existence of clean metallic surfaces together with the high local hydrostatic stresses favour the formation of strong adhesion between the cutting tool or insert and the machined component. These adhesive bonds can lead to poor surface integrity although their extent can be limited by the provision of a suitable machining lubricant. In an effort to identify the essential lubricating aspects of fluid activity, as opposed to any role as a coolant, experiments have been carried out involving the orthogonal machining of precipitation‐hardened aluminium alloys, principally Al 2014, in controlled, low‐pressure gas environments in which the feed (i.e., the depth of cut) speed and temperature have been varied while using a variety of tool materials and lubricating species. The results indicate that there can be unexpectedly subtle, but significant, interactions between the metallurgy of the workpiece, the nature of the surface of the tool and the surrounding environment. These are not wholly consistent with conventional theories of vapour phase lubrication in which transport of the lubricant has been assumed to control the effectiveness of the lubricating agent. The implications of these observations for the complex tribological system constituted by the combination of workpiece, tool surface and local environment are discussed.

Progress in the classical field of EHL has for decades been paralyzed by the assumption that shear thinning should be indistinguishable from the shear dependence of the viscosity of a liquid heated by viscous dissipation and that the parameters of this simple shear dependence can be obtained from the shape of a friction curve. In the last few years, by abandoning this assumption and employing real viscosity measured with viscometers, there has been revolutionary progress in predicting film thickness and friction. Now, Spikes and Jie conclude that the previous assumption has as much merit as the use of viscosity measured in viscometers. This suggestion may be popular among those who wish to ignore viscometer measurements in favor of extracting properties from friction curves. However, within the subject article, there are numerous misstatements of fact and misrepresentations by omission, and the recent progress using real viscosity is not acknowledged. The debate has degenerated into a friction curve fitting competition which is not helpful. The great progress of the last few years would not have been possible using the concepts and methods espoused in this article.

The present work is a generic study to examine the effects of the glass-to-rubber transition of resin matrix on the friction and wear characteristics of zirconium oxide (ZrO2) reinforced polybenzoxazine nanocomposites, in relation to the content of ZrO2. The thermal and tribological properties of the nanocomposites were measured by dynamic mechanical thermal analysis (DMA) and friction test, respectively. DMA results revealed that the storage modulus and T g values of the nanocomposites increased with increasing ZrO2 content to 4 wt%, due to the exceptional mechanical strength of ZrO2 particles and the interfacial adhesion between ZrO2 and matrix to restrict the segmental motion of polymer. The friction coefficient (COF) values as a function of applied load (50–750 N) for the nanocomposites under testing temperatures (50, 100, 200, 250, and 300 °C) were measured. Comparable to the pure resin, the nanocomposites possessed relatively higher COF values with the increase of applied pressure under varying temperatures, which resulted from the reinforcement of ZrO2. It is noted that the nanocomposites containing 4 wt% ZrO2 occupied relatively higher modulus and glass transition temperature, resulting in better capability to stabilize the friction coefficient and wear rate under the applied conditions. In addition, the friction mechanism of the nanocomposites were proposed based on the experimental and reference results.

Tribological properties of a material depend on a wide range of parameters/conditions. While some of them are obvious, some are quite subtle. One of these subtle parameters has been identified and reported here. Under sliding conditions, whenever surface layers are formed due to the interaction at the sliding interface (e.g., tribochemical interaction), the resultant tribological response has been conjectured to depend upon the relative length of the pin, parallel to the sliding direction, with respect to the corresponding length of the disc track. The results obtained from the experiments reported in this paper as well as those reported by different researchers in the past, furnish evidence to support this conjecture. It can be considered as a general parameter because of this wide spectrum of applications. It is shown in this paper that this parameter helps in selecting the optimum contact geometry of the pin under various experimental conditions. This might reduce the observed scatter in tribological test results. Such a study leads to the conclusion that, for tribological testing, a square/rectangular contact configuration of pin is preferable to a circular one.

Novel dispersions of thermally reduced graphite oxide (TRGO) in ester oil were prepared by means of high-pressure homogenization. Due to the versatility of the homogenization process, TRGO dispersions with a wide concentration range of 0.01–0.5 wt% could be prepared both in ester oil that contains additives (FGL46 E) and in an additive-free ester oil formulation. The obtained dispersions were characterized with respect to their sedimentation stability and their tribological properties. Therefore, highly resolved Stribeck curves were measured in the boundary region. Compared to benchmark dispersions of carbon black and graphite, our TRGO-containing formulations showed considerably lower friction and wear. The results of this study show the positive influence of TRGO on the friction behavior of ester oil formulations, thereby presenting TRGO-containing ester oils as a promising new family of liquid-based lubricants.

(TiB2–TiC)–Ni/TiAl/Ti functionally gradient materials were prepared by field-activated pressure-assisted synthesis processes. (TiB2–TiC)–Ni composite ceramic, the top layer of the functional gradient materials, was prepared in situ by the combustion synthesis process using Ti and B4C powders as raw materials. Scanning electron microscope (SEM) images of the ceramic layer revealed that the TiB2 and TiC particles in the composite were fine and homogeneously dispersed in the Ni matrix. The friction and wear properties of the (TiB2–TiC)–Ni ceramic were evaluated by sliding against a GCr15 disk at temperatures from ambient up to 400 °C. The experimental results showed that the friction coefficient of the (TiB2–TiC)–Ni ceramic decreased with the increasing testing temperature, load, and sliding speed. However, the loss rate decreased at higher temperature and increased at higher load and higher sliding speed. The wear mechanisms of (TiB2–TiC)–Ni ceramic mainly depend upon thermal oxidation at higher temperature, load, and sliding speed. The worn topography and phase component of the worn surfaces were analyzed using SEM, energy dispersive spectroscopy, and X-ray diffraction. The oxide films of Fe2O3, TiO2, and B2O3 formed during the friction process play an important role in lubrication, which results in a smaller friction coefficient.

Các tính chất hóa học cục bộ của các đỉnh tiếp xúc trong một hệ thống ma sát thực sự có tầm quan trọng quyết định đối với hành vi ma sát vĩ mô thu được. Do đó, các lực bên tác động lên đầu của một kính hiển vi quét lực và ma sát độc lập đã được nghiên cứu như một hàm của hóa học bề mặt được kiểm soát, thực hiện bằng cách điều khiển điện thế của mẫu. Các kết quả thu được cho thấy sự phụ thuộc rõ ràng của hành vi ma sát ở quy mô nano vào những thay đổi trong trạng thái điện hóa của hệ thống.

Liquids confined to molecular scales become anisotropic and often show pronounced self-organization such as layering. Although this effect is well accepted, it is still debated if confinement induces measurable changes of viscous friction. We use molecular dynamics to address this issue by simulating a Lennard-Jones liquid confined between a solid cylinder and an atomically smooth surface. The simulations reproduce the well-established variations of normal force, density, and diffusivity with the distance between wall and cylinder. We find high diffusivity and low density when the numbers of layers is in between integers. This observation seems to contradict most experimental results on the effective damping between atomic force microscope tips and substrates when interpreting them within continuum hydrodynamics used to connect liquid viscosity and diffusivity. This contradiction is resolved by directly extracting the damping that the tip experiences, which we achieve by using the fluctuation-dissipation theorem; as in experiment, we find local minima in the damping near integer numbers of molecular layers and maxima in between. These variations correlate with distinct structural changes in the microscopic order of the fluid. We reconfirm that constitutive equations valid at macroscopic scales cannot be used to interpret confined liquids and finally conclude that viscous friction displays measurable, non-monotonic behavior with the degree of confinement.