Boundary Lubrication

Purpose This paper aims to address the influence of tribofilms and running-in on failures and friction of gears. The operation regime of gears is increasingly shifted to mixed and boundary lubrication, where high local pressures and temperatures occur at solid interactions in the gear contact. This results in strong tribofilm formation due to interactions of lubricant and its additives with the gear flanks and is related to changes of surface topography especially pronounced during running-in. Design/methodology/approach Experiments at a twin-disk and gear test rig were combined with chemical, structural and mechanical tribofilm characterization by surface analysis. Pitting lifetime, scuffing load carrying capacity and friction of ground spur gears were investigated for a mineral oil with different additives. Findings Experimental investigations showed a superordinate influence of tribofilms over surface roughness changes on damage and friction behavior of gears. Surface analysis of...

Purpose This paper aims to address the influence of tribofilms and running-in on failures and friction of gears. The operation regime of gears is increasingly shifted to mixed and boundary lubrication, where high local pressures and temperatures occur at solid interactions in the gear contact. This results in strong tribofilm formation due to interactions of lubricant and its additives with the gear flanks and is related to changes of surface topography especially pronounced during running-in. Design/methodology/approach Experiments at a twin-disk and gear test rig were combined with chemical, structural and mechanical tribofilm characterization by surface analysis. Pitting lifetime, scuffing load carrying capacity and friction of ground spur gears were investigated for a mineral oil with different additives. Findings Experimental investigations showed a superordinate influence of tribofilms over surface roughness changes on damage and friction behavior of gears. Surface analysis of...

Many attempts have been made in recent centuries to investigate friction, adhesion, lubrication, and wear. Most of the experimental approaches and theories were based on macroscopic experiments, such as tensile and indentation tests. For a long time, only the bulk properties of the materials were considered.Late in this century a new term was created combining all of the above-mentioned properties which deal with the science of interacting material interfaces in relative motion: tribology. The state of the art of science today reveals that processing in nature depends strongly on interfaces that cannot be described only by bulk properties. Tribologists realize they must study the sliding surfaces by analytical surface-science tools. With the surface force apparatus developed by J.N. Israelachvili and D. Tabor, we have a surface analysis tool that provides new insight into the field of macroscopic sliding contact of lubricated systems.After Amontons' laws were established as a fi...

Many attempts have been made in recent centuries to investigate friction, adhesion, lubrication, and wear. Most of the experimental approaches and theories were based on macroscopic experiments, such as tensile and indentation tests. For a long time, only the bulk properties of the materials were considered.Late in this century a new term was created combining all of the above-mentioned properties which deal with the science of interacting material interfaces in relative motion: tribology. The state of the art of science today reveals that processing in nature depends strongly on interfaces that cannot be described only by bulk properties. Tribologists realize they must study the sliding surfaces by analytical surface-science tools. With the surface force apparatus developed by J.N. Israelachvili and D. Tabor, we have a surface analysis tool that provides new insight into the field of macroscopic sliding contact of lubricated systems.After Amontons' laws were established as a fi...

Low-speed friction experiments were conducted under boundary lubrication in a pin-on-disk tester. The 304 stainless steel disk had smooth areas alternating with areas of coarse surface texture consisting of indents or macroscopic grooves, 0.3–0.4 mm in size and in area fractions varying between 25 and 70%. The 3.2 mm flat pin was also SS304. The coarse texture has detrimental effects. For each pattern the friction coefficient is greater than for the smooth areas. It becomes independent of load as boundary conditions set in. In comparing various patterns it is noted that this friction coefficient increases with pressure or with the amount of area removed to form the texture. It is suggested that partial removal of surface area to form the texture reduces the number of trapped liquid patches, which otherwise can provide low-friction load support. The coarse texture may also conduct lubricant away from the contact area.

The frictional behaviors of a variety of fatty esters (methyl palmitate (MP), methyl laurate (ML), and 2-ethylhexyl oleate (EHO)) and oleic estolide esters (methyl oleic estolide ester (ME) and 2-ethylhexyl oleic estolide ester (EHE)) as additives in hexadecane have been examined in a boundary lubrication test regime using steel contacts. Critical additive concentrations were defined and used to perform novel and simple Langmuir analyses that provide an order of adsorption energies: EHE ≥ ME > EHO > MP > ML. Application of a general adsorption model indicates slight cooperative adsorption of EHE, ME, and EHO. MP and ML data require larger attractive interaction terms (α ≤ −2.3) to be adequately fit. Irrespective of ester functionality increasingly negative adsorption energies appear to correlate with molecular weight. This suggests that multiple site coverage and multiple adsorptive interactions are likely for each of the esters studied.

The paper focuses on the coefficient of friction (COF) of a magnetorheological fluid (MRF) in the wide range of working conditions across all the lubrication regimes—boundary, mixed, elastohydrodynamic (EHD), and hydrodynamic (HD) lubrication, specifically focused on the common working area of MR damper. The coefficient of friction was measured for MR fluids from Lord company with concentrations of 22, 32, and 40 vol. % of iron particles at temperatures 40 and 80 °C. The results were compared with a reference fluid, a synthetic liquid hydrocarbon PAO4 used as a carrier fluid of MRF. The results show that at boundary regime and temperature 40 °C all the fluids exhibit similar COF of 0.11–0.13. Differences can be found in the EHD regime, where the MR fluid COF is significantly higher (0.08) in comparison with PAO4 (0.04). The COF of MR fluid in the HD regime rose very steeply in comparison with PAO4. The effect of particle concentration is significant in the HD regime.

Despite the widespread use of solid-form drug delivery within the pharmaceutical industry, tablets remain challenging to formulate because their properties depend strongly on the powder composition and details of the compaction process. Powder compaction simulations, using the finite element method (FEM) in combination with the density-dependent Drucker-Prager Cap model, can be used to aid the design process of pharmaceutical tablets. Parametrisation is typically carried out manually and requires experimental data for each powder considered. This becomes cumbersome when considering different ratios of component powders. An automated parameterisation workflow was developed and validated using experimental powder mixtures of microcrystalline cellulose and dibasic calcium phosphate dihydrate. FEM simulations reproduced experimental compaction curves with a mean error of 2.5% of the maximum compaction pressure. Moreover, a mixing methodology was developed to estimate parameters of mixtures using only pure-component parameters as input. The experimental compaction curves of mixtures were predicted with a mean error of 4.8%.

This paper describes a theoretical investigation of the rheological effects of couple stress fluids on the performance of narrow porous journal bearings. A most general modified Reynolds equation is derived for narrow porous journal bearings using the Stokes constitutive equations for couple stress fluids. The fluid in the film region and in the porous region has been modelled as a couple stress fluid. The analysis takes into account velocity slip at the porous interface using the Beavers‐Joseph criterion. A closed‐form expression for field pressure is obtained for narrow journal bearings. Eigen‐type expressions for field variations are obtained. The dimensionless load‐carrying capacity, attitude angle, and coefficient of friction are presented for different operating parameters. It is observed that narrow porous journal bearings with couple stress fluids as lubricant show a significant increase in load‐carrying capacity with reduced coefficient of friction as compared to the Newton...

In this study, ~3.5 m thick multilayer TiAlN, AlTiN, and AlCrN coatings were deposited on the H13 steel surface by Cathodic Arc Physical Vapor Deposition (CAPVD) method. The tribological performance of the coatings were evaluated by a tribometer at dry and boundary lubrication conditions. Then, coating surfaces were investigated by optical microscope, optical profilometer and atomic force microscope (AFM) to evaluate the morphological changes, wear volumes and tribofilm thickness. Also, Scanning electron microscopy (SEM/EDX) and X-ray photoelectron spectrometry (XPS) analysis were applied to coating surfaces for the tribochemical evolution of the tribofilm. Results showed that AlCrN coating performed the best tribological behavior at dry and lubricated conditions, when compared to TiAlN and