Combination of DLC coatings and EP additives for improved tribological behaviour of boundary lubricated surfaces

The aim of the present investigation was to obtain some further understanding of the mechanism responsible for low-friction behaviour of W-containing DLC coatings (W-DLC) when lubricated with EP additivated oil. Boundary lubricated wear and friction tests were performed under reciprocating sliding motion using a high frequency test rig and a contact pressure of 1.5 GPa. Additionally, some of the tests were performed in a load-scanning reciprocating test rig, with the contact pressure being in the range from 2.4 to 5.6 GPa. The influence of concentration of a sulphur-based EP additive on the friction behaviour was investigated.This investigation showed that W-DLC coatings greatly improve the tribological properties of boundary-lubricated surfaces, especially when pairing coated and uncoated steel surfaces. The improved tribological behaviour was found to be governed by the gradual formation of a WS₂ type tribofilm on the steel counter-face or on revealed steel substrate. The friction level depends on the additive concentration.     

The effect of chemical structure of basefluids on antiwear effectiveness of additives

The effect of the chemical structure of a lubricant on the complexity of friction and wear under boundary conditions is studied using a pin-on-disc machine and a four-ball wear tester. The two test units are used to compare the friction and wear characteristics of four hydrocarbon fluids. Three vegetable oils and three additives are also evaluated in the four-ball wear tester to demonstrate the importance of chemical structure of the base fluids and the additives on the effectiveness of the additives in controlling friction and wear.     

Investigation of anti-wear additives for low viscous synthetic esters: Hydroxyalkyl phosphonates

Various synthetic esters are widely applied as lubricating fluid to reduce friction and wear at tribological contact. Among them low viscous synthetic esters are expected to improve fuel efficiency by minimizing the fluid friction. These low viscous esters are composed of short-chain fatty acids. Therefore, low viscous synthetic esters are inherently polar molecules. Since efficiency of anti-wear additives decreases with increase of polarity of the base oil, new additive technology is requested.In this work, hydroxyalkyl phosphates [P(O)(OCHRCH₂OH)₃], and hydroxyalkyl phosphonates [P(OH)_n(OCHRCH₂OH)_3−n, where n=1,2] are proposed as novel anti-wear additives for polar synthetic esters. The anti-wear additives are evaluated under the boundary conditions. The additives prevent wear in polar esters, in which conventional anti-wear additives do not work at all. Interestingly, effects of substituent in additive molecule on anti-wear properties are found. Alkyl and aryl derivatives reduce wear remarkably, whereas allyl derivatives exhibit poor results. It is speculated that the anti-wear inefiiciency of allylic compounds is due to auto-oxidation of the additives.A facile preparative method for hydroxyalkyl derivatives characterizes the present additive system. They are prepared in situ by simply mixing phosphonic acid and substituted epoxides. Flexiblity of lubricant design can be made possible by the present additive system.     

The effect of branching on slip and rheological properties of lubricants in molecular dynamics simulation of Couette shear flow

Molecularly thin liquid films of alkane in extreme conditions in a thin film lubrication regime have been investigated. To get an insight into the effects of molecular architecture in the behaviour of these thin lubricant films we have studied six different molecules, mainly isomers of C₃₀. In this work the effect of branching on rheological properties and behaviour of lubricant film is examined. Our study shows viscosity and normal stress effects depend on the degree of branching. Dynamics of the molecules and their orientation are also affected by the degree of branching. A weaker layering near the wall is observed for branched molecules. Slip between the wall and lubricant film also was larger for branched molecules. Branched molecules had less tendency to change their orientation under the flow. The results obtained here could be helpful in designing new lubricants at the molecular level.     

Preparation and characterization of spark-anodized Al-alloys: Physical, chemical and tribological properties

Aerospace applications and energy saving strategies in general boosted the interest and the research in the field of light weight materials, typically on alloys based on aluminium. Aluminium itself does not provide sufficient mechanical strength for structural parts, but there exists a lot of recently developed alloys containing silicon, copper, magnesium, zinc or manganese in various combinations and compositions exhibiting excellent mechanical properties. These alloys require surface treatments or coatings to withstand corrosive ambient conditions. Among those treatments known as chromate replacements, plasma oxidation processes were used for different applications, especially if the surfaces have to face mechanical load or severe environmental conditions. In this work, specimens of different aluminium alloys have been plasma oxidized by micro-arc treatment in silicate and phosphate solutions. The ceramic coatings were characterized with respect to phase composition, micro-hardness and corrosion stability. In addition, the tribological performance of the coatings was investigated using a ball-on-disc tribometer with reciprocating motion against sintered alumina ball. The typical worn surfaces of the Al substrate and the ceramic coatings were observed by a scanning electron microscope. Applying same wear conditions, the wear rates in different depth of the coatings are nearly similar. However, in a defined depth of the coatings, wear rate gradually decreases with wear duration. During friction process, a-stable transfer layer consisting of oxides was formed on the tribo-contact area of the coatings. The friction coefficient in a steady friction state is in the range of 0.8.     

The polarity of metallic surfaces in the context of the corrosive and scuffing wear control

The concept of the metallic surfaces polarity in the context of scuffing performance is postulated and elucidated in the presented paper. The machining by grinding and surface treatment by burnishing is applied to control introducing changes in surface polarity and acid/base component of surface free energy is used for their quantitative determination. A clear relationship between the acid/base component of surface free energy and an activation of scuffing for the steel–cast iron friction pairs lubricated by oils with the surface-active sulphur-based additives is found. Obtained results are commented and clarified; thanks to the negative-ion concept of extreme pressure action of organo-sulphur compounds. Additionally, surface reactivity investigations are performed in order to determine the influence of acid/base component of surface free energy on the corrosion wear. It recognised a clear relationship between the acid/base component of surface free energy and the mass decrement of steel surfaces in the hydrochloric acid environment. On the basis of both parts of the investigations (scuffing and reactivity tests), an optimal surface polarity is determined for steel–cast iron friction pairs lubricated by lubricants with surface-active sulphur-based additives.     

The combined effects of ZDDP, surface texture and hardness on the running-in of ferrous metals

The effects of surface characteristics including roughness, lay direction and hardness of rubbing pairs on the antiwear performance of secondary short chain ZDDPs under a boundary lubrication condition are studied experimentally. The antiwear performance of the ZDDPs is evaluated by the duration of running-in periods recorded in wear tests of specially prepared specimens. A running-in period is defined as the time interval from the beginning of a wear test to the time at which the contact resistance between the rubbing pair approaches infinity. The shorter running-in period yields the better antiwear performance. The wear tests were conducted with a reciprocating sliding contact made by flat-on-flat specimens which were made from grey cast iron, quenched medium carbon steel and bearing steel. Two lay directions of the surface texture, namely, parallel and perpendicular to the sliding direction were ground, which made four possible combinations in a rubbing action. The CLA roughness of the specimens ranged from 0.35 μm to 0.04 μm. It is found that to obtain a shorter running-in period and to enhance the antiwear performance of ZDDPs, the following rules should be obeyed. If the hardness numbers of the two rubbing members are near the same, the combination of their lay directions should be both in parallel but perpendicular to the sliding direction and, the roughness values should be smaller than 0.09 μm. If the hardness number of one member is much greater than that of the other, the soft member should be the smaller one and the surface of the hard member should be as smooth as possible. Under any circumstance, the smaller members should be chamfered.     

The effects of iron particles on the friction and wear mechanisms of ceramics in ethanol

For the combinations of an Si₃N₄ pin and five kinds of ceramic disk (SiC, Si₃N₄, Al₂O₃, ZrO₂, TiC), a friction and wear test was carried out in ethanol and in ethanol containing iron particles (1 wt.%, average diameter d = 200 nm, D = 12 μm under cohered condition) under a load in the range 5.88–11.50 N, at a sliding velocity of 0.138–0.196 m s⁻¹. A topographical analysis was also performed on the microasperities of the wear surfaces to estimate the behavior of the iron particles, and the degree of surface damage. As a result, the following facts were found. (1) The addition of iron particles in ethanol decreased both the wear rates of SiC and TiC disks and the mating pins, and also decreased the wear rate of the Al₂O₃ disk but increased that of the mating pin. The addition increased the wear rates of both ZrO₂ and Si₃N₄ disks and the mating pins. (2) The average coefficients of friction with the addition of iron particles were greater than those without iron particles. (3) The wear rates of pin and disk depended on the topographies of wear surfaces and the wear index Γ.     

Frictional behaviour of imidazolium sulfate ionic liquid additives under mixed slide to roll conditions: part 2 — influence of concentration and chemical composition of ionic liquid additive

Current work presents the investigation of frictional behaviour of ionic liquid lubricant mixtures under mixed slide to roll ratio. On the contrary to the previous study, which focuses on determination of the most suitable ionic liquid additive for identical ionic liquid weight concentration in lubricant mixture, this work had two scopes. The first one was to determine the optimal chemical composition of ionic liquid additive by investigating the lubricant mixtures with identical molar ratio, and the second one, to optimise additive concentration for certain ionic liquid structure. The changes were observed by two means. Namely, in frictional behaviour, ionic liquid concentration plays significant role. On the contrary, for the mixtures with identical molar concentration, the chemical structures with longer alkyl substituent do not always exhibit improvement. Experiments also revealed correlation between ionic liquid structures, concentration and wear. Atomic force microscopy (AFM) analysis of worn surfaces confirmed the above statements. Copyright © 2015 John Wiley & Sons, Ltd.     

The use of secondary ion mass spectrometry coupled with image analysis to identify and locate chemical elements in soil minerals: The example of phosphorus

The mobility andbioavailability of elements in soils and sediments largely depends on their distribution on the diverse inorganic and organic constituents. This work addresses the example of phosphorus (P) associated to goethite and calcite, that is, to the major minerals involved in the retention of P in soils and sediments in calcareous environments. Synthetic goethite (FeOOH) and calcite (CaCO₃) were reacted with P prior to being analysed by dynamic secondary ion mass spectrometry (SIMS). Powdery samples were embedded in resin, cut in thin sections, and imaged with a Cameca IMS 4F ion microscope used in scanning mode with a primary ion beam of caesium that produced negatively charged secondary ions (?) (Cameca, Cedex, France). Carbon, O, P, and calcium (Ca) were directly imaged at m/z 12, 16, 31, and 40, respectively, while Fe was imaged via the polyatomic ion FeO^? ion at m/z 72. The SIMS data were treated by image analysis procedures. The visual comparison of images and the scatterplot method showed that P strongly interacted with goethite, probably following an adsorption process, and was thus evenly distributed at its surface. Conversely, P was not evenly distributed at the surface of calcite which rather suggests a precipitation process, and the scatterplot method confirmed a poor relationship between P and Ca. For the goethite‐calcite mixture, visual examination suggested that P occurred as clusters which were largely associated with calcite, whereas a statistical analysis of the various images showed that the distribution of P was largely related to that of goethite particles. This work confirms the potential contribution of iron oxides in the retention of P in calcareous environments and shows that coupling image analysis to sensitive analytical techniques such as SIMS is a powerful approach for providing quantitative information on the location of elements at low bulk concentrations.