Effect of Vanadium Addition on the High-Temperature Friction Behavior in Nanostructured Al–Cr–V–N Films Prepared by UBMS

In recent year, vanadium-doped tribological films have become available as possible candidates for self-lubrication at high temperatures. In this work, quaternary Al–Cr–V–N films were deposited onto silicon wafer and WC-Co substrates by an unbalanced magnetron sputtering using high purity (99.99%) CrAl2 and V targets with argon-nitrogen reactive gases. EPMA results revealed that vanadium atoms can incorporated from 0 to 13 at.% into the films. The maximum hardness value was ~32 GPa at vanadium content of 7.1 at.% in the Al–Cr–V–N films. The high-temperature tribometer was used to analysis the friction characteristics of the films with elevated temperature. As a result of the high temperature friction test after heating up to 700 °C, the average friction coefficient decreased from 0.62 to 0.35 with increasing of vanadium contents in the Al–Cr–V–N films. It is concluded that the reduction of the friction coefficient is attributed to the formation of V2O5, which is a Magnéli phase that acts as a lubrication at high temperature.

Multiscale Numerical and Experimental Analysis of Tribological Performance of GO Coating on Steel Substrates

Herein, nano-tribological behaviour of graphene oxide (GO) coatings is evaluated by a combination of nanoscale frictional performance and adhesion, as well as macroscale numerical modelling. A suite of characterisation techniques including atomic force microscopy (AFM) and optical interferometry are used to characterise the coatings at the asperity level. Numerical modelling is employed to consider the effectiveness of the coatings at the conjunction level. The macroscale numerical model reveals suitable deposition conditions for superior GO coatings, as confirmed by the lowest measured friction values. The proposed macroscale numerical model is developed considering both the surface shear strength of asperities of coatings obtained from AFM and the resultant morphology of the depositions obtained from surface measurements. Such a multi-scale approach, comprising numerical and experimental methods to investigate the tribological behaviour of GO tribological films has not been reported hitherto and can be applied to real-world macroscale applications such as the piston ring/cylinder liner conjunction within the modern internal combustion engine.

Tribological Film Formation in Hydraulic Motors

This paper presents an investigation of the tribological films formed in hydraulic motors. Hydraulic motors convert the fluid power energy produced by positive displacement pumps into rotary motion. Earlier research found that the efficiency of this energy transformation can be enhanced by reducing boundary friction. In order to study the nature of the boundary films formed in an orbital motor, a prototype ashless hydraulic fluid was evaluated in a low-speed high-torque dynamometer. The resulting tribofilm was probed via Energy Dispersive X-Ray Spectroscopy. The results reveal that increasing the hydraulic system temperature raised the relative phosphorus level of the tribofilm.

Adhesion, Friction and Mechanical Properties of CF3- and CH3-Terminated Alkanethiol Monolayers: A Comparative Study

Self-assembled monolayers (SAMs) have received considerable recent attention as molecular-level lubricants in, for example, micro-electro-mechanical systems (MEMS).[1] Of particular interest as tribological films have been SAMs terminated by fluorocarbon groups, because of their inert nature and enhanced thermal stability. Surprisingly however, fluorocarbon films were shown to actually produce higher coefficients of friction (relative to CH3-terminated films) in atomic force microscopy (AFM) studies. Subsequent work has concluded that the increased van der Waals radius of the fluorine groups (∼45%) causes a steric disruption of the order of the molecular surface giving rise to an increased friction.[2] We present results from a direct comparison of the adhesive, mechanical and frictional properties of SAMs terminated by CF3 and CH3 groups using both Interfacial Force Microscopy[3] (IFM) and the AFM. IFM results are shown for a two micron tungsten tip interacting with C16 alkylthiol molecules assembled on Au(111) single-crystal surfaces. AFM results involve a ∼20 nm tip interacting with the same two molecules assembled on Au films deposited on mica surfaces. A direct AFM comparison is accomplished by using a “nanografting technique”.[4]

Understanding the Tribological Chemistry of Chlorine- and Sulfur- and Phosphorus-Containing Additives

Chlorine- and sulfur- and phosphorus-containing compounds are commonly added to the base fluid to synthesize lubricants used under extreme-pressure (EP) conditions. Analyzing the resulting tribological films on iron reveals that chlorinated hydrocarbons thermally decompose forming a layer that consists of iron chloride (FeCl2) or carbide (Fe3C), and that dialkyldisulfides react to form FeS and Fe3C. Alkyl phosphates thermally decompose on iron oxide to form alkyl and alkoxy, as well as POx species, on the surface. The alkyl and alkoxy species thermally decompose on heating to evolve gas-phase products and deposit carbon onto the surface. The POx species rapidly diffuse into the oxide forming a film that consists of a carbonaceous layer covering a phosphate film. The tribological properties of evaporated and reactively grown thin films have been investigated in ultrahigh vacuum. This strategy eliminates contamination and allows films of known composition and structure to be grown on well-characterized substrates. Three tribological regimes are identified depending on film thickness. In the first regime, an initial rapid decrease in friction is found when a film that is a few nanometers thick (corresponding to a monolayer) covers the surface. The friction coefficient increases once again in the second regime as the film becomes thicker, due to the increased contact area between the film and the rough tribotip, and the behavior is well described by a modified Greenwood-Williamson model. A third regime is found when the film becomes thicker than the interfacial roughness, where the surfaces are completely separated by the film. Finally, measuring the friction coefficients of thin halide films deposited onto various substrates, where the local pressure at the asperity tips depends on the substrate hardness, shows that the shear strength of the “monolayer” films depends on pressure.