L. Wang

Superhydrophobic surfaces for the reduction of bacterial adhesion

As an important research area, the development of antibacterial materials has attracted extensive interest from researchers. Typical antibacterial materials involve the use of biocides and antibacterial metallic ions, such as Ag+, as well as killing by highly reactive species, such as hydroxyl radical, hydrogen peroxide and superoxide produced by the photocatalysis of TiO2. However, the intensive usage of biocides has a growing concern in the increase of bacterial resistance and cross-resistance to antibiotics and antibacterial Ag+ depending on its dissolution property may have potential implications on human health and environment. Currently TiO2 is mainly activated with UVA light and research on visible light photocatalysis is still under development. Recently, a new scheme using superhydrophobicity has raised more attention and interests especially for its ability in reducing bacterial adhesion. This paper provides a detailed review on the basics, recent developments, existing challenges and future perspectives of superhydrophobic surfaces especially in reducing bacterial adhesion.

Wear performance of oil lubricated silicon nitride sliding against various bearing steels

Abstract The selection of bearing steel surfaces for use with silicon nitride rolling elements within hybrid bearings is critical to the performance and life of such components, which have potential applications in advanced high speed aircraft. The wear and friction performance of these combinations is a major factor currently being considered for the next generation hybrid bearings. This paper reports on hybrid bearing contacts that have lubricated Si 3 N 4 elements, which have been loaded against various bearing steels under pure sliding contact conditions on a fully instrumented pin-on-disc wear test rig. The wear and friction performance of Si 3 N 4 has been compared to a baseline case of bearing steel M50 ball sliding against a M50 disc. Both hybrid and steel on steel contacts were lubricated by an aircraft engine oil Mobil Jet II. Wear mechanisms were determined by post-test analysis of the pin wear scars, disc wear surface and wear debris using optical microscopy, surface profilometry and FEG-SEM (scanning electron microscopy). The wear rates of Si 3 N 4 sliding against different bearing steels are ranked by performance and related to their wear mechanisms, hardness and microstructure. Typical sliding contact wear mechanisms were found for the steel on steel combination while Si 3 N 4 sliding against steel showed that transgranular and sub-micron-cracking mechanisms predominate. Evidence of material transfer (steel onto the silicon nitride) was found. Friction values for the various combinations are also reported and found to be substantially lower ( μ =0.04) than bearing steel on bearing steel combinations ( μ =0.17). The disc and pin wear was monitored on-line by an electrostatic wear sensor, LVDT and laser displacement probe, a friction strain gauge, and an infrared thermometer. Correlations between wear rate and charge generation/level, friction, contact temperature, and disc hardness are presented.

Tribological design constraints of marine renewable energy systems

Against the backdrop of increasing energy demands, the threat of climate change and dwindling fuel reserves, finding reliable, diverse, sustainable/renewable, affordable energy resources has become a priority for many countries. Marine energy conversion systems are at the forefront of providing such a resource. Most marine renewable energy conversion systems require tribological components to convert wind or tidal streams to rotational motion for generating electricity while wave machines typically use oscillating hinge or piston within cylinder geometries to promote reciprocating linear motion. This paper looks at the tribology of three green marine energy systems, offshore wind, tidal and wave machines. Areas covered include lubrication and contamination, bearing and gearbox issues, biofouling, cavitation erosion, tribocorrosion, condition monitoring as well as design trends and loading conditions associated with tribological components. Current research thrusts are highlighted along with areas needing research as well as addressing present-day issues related to the tribology of offshore energy conversion technologies.

Electrostatic charge generation associated with machinery component deterioration

Detection and monitoring increased levels of electrostatic charge, produced as a result of machinery component deterioration, have the potential for the basis of an oil system component condition monitoring system. As increases in electrostatic charge level may be associated with the earliest stages of oil film/contact breakdown and the onset of wear, then charge monitoring may be used to identify a wide prognostic window over which component condition may be determined. The ability to monitor component deterioration from an early stage, and hence predict when replacement is required, will enable greater flexibility in terms of maintenance planning and equipment availability. This paper reviews the state of the art of using the electrostatic technology for monitoring component condition and deterioration, discusses the possible electrostatic charge generation mechanisms at the oil-component surface interface and the influence of oil chemistry under wear conditions. Results are included from simple tribometer and component rig tests.

Challenges and developments of self-assembled monolayers and polymer brushes as a green lubrication solution for tribological applications

Self-assembled monolayers (SAMs), after originally being investigated due to their functions in changing surface wettability, have been significantly developed over the years. Many types of SAMs have been developed on a variety of substrates. However their formation mechanism, rate and quality are found to be influenced by many factors. A range of SAMs including single- and multi-component are included in this review with focus on the nano and macro tribological properties. More recently, surface initiated polymer brushes, i.e. macromolecular assemblies attached to a substrate, have emerged to be an alternative and promising method for surface modification. The ability to tether these macromolecules to tribological contacts is key to their resistance to shear under loaded contacts. This review also covers atom transfer radical polymerisation (ATRP) and the role of this technique in developing new lubrication solutions. Particular care has been taken to include the development of lubrication solutions for silicon nitride due to the importance of this material as an engineering ceramic. This paper reviews the state-of-the-art development of SAMs and polymer brushes especially the potential opportunities and challenges in applying them in tribological contacts as a lubrication solution.

Real-time monitoring of wear debris using electrostatic sensing techniques

Abstract In this article, electrostatic charge sensing technology has been used to monitor adhesive wear in oil-lubricated contacts. Previous work in this area using FZG gear wear rig and pin-on-disc tribometers demonstrated that ‘precursor’ charge events may be detected prior to the onset of scuffing. Possible charging mechanisms associated with the precursor events were identified as tribocharging, surface charge variation, exo-emissions, and debris generation. This article details tests carried out to investigate the contribution of wear debris. Tests were carried out on a modified pin-on-disc rig using a sliding point contact and fitted with electrostatic sensors, one of which monitored the disc wear track and the other the disc surface just outside the wear track. Baseline tests used mild wear conditions with no seeded particles added to the entrained lubricant, whereas the high wear tests entrained seeded steel particles into the contact to promote wear. The wear debris produced dynamic charge features and the overall charging activities are directly related to the wear rate (i.e. charging levels increase with increasing wear rate). There appears to be a link between the net volume loss and the charge levels, relating charge directly to the increasing rate of debris production. Wear debris due to natural wear produced positive dynamic charge features, whereas debris from the seeded tests produced negative dynamic charge features. The polarity of the charge on debris is thought to depend on which charging and wear mechanism is predominant.

Use of structured surfaces for friction and wear control on bearing surfaces

Surface texturing with purposely made regular micropatterns on flat or curved surfaces, as opposed to random roughness inherited from machining processes, has attracted significant attention in recent years. At the 2013 World Tribology Congress in Turin alone there were over 40 presentations related to surface texturing for tribological applications, from magnetic hard discs and hydrodynamic bearings to artificial joints. Although surface texturing has been reported being successfully applied in industrial applications such as seals, pistons, and thrust pad bearings, the demand for robust design is still high. Etsion has recently reviewed the modeling research mainly conducted by his group Etsion I (2013 Friction 1 195–209). This paper aims to review the state-of-the-art development of surface texturing made by a wider range of researchers.

Formation mechanisms of white etching cracks and white etching area under rolling contact fatigue

The formation of white etching cracks in the 1 mm zone beneath the contact surface in steel rolling element bearings causes a premature wear failure mode called white structure flaking. The formation drivers of white etching cracks are contested, as are the initiation and propagation mechanisms of the cracks. Hydrogen diffusion into bearing steel sourced from the hydrocarbon lubricant or water contamination and transient operating conditions have been suggested as formation drivers. Extensive work has been conducted at Southampton to further understanding of white structure flaking and this paper summarises these evidences and the conclusions made. Serial sectioning has been used to map subsurface wear volumes of wind turbine gearbox bearings from service and large-scale test rigs, test specimens/bearings from laboratory under hydrogen charged conditions and non-hydrogen charged conditions. The process involves polishing of cross sections of test specimens/bearings at ∼3–5 µm material removal intervals typically over hundreds of slices, and this was used to map white etching cracks in their entirety for the first time. Serial sectioning has allowed a comprehensive investigation of the initiation and propagation mechanisms of white etching cracks and thresholds for their formation with respects to concentration of diffusible hydrogen, contact pressure and number of rolling cycles. From these studies it has been found that white etching cracks can form by subsurface crack initiation at inclusions under hydrogen charged and non-hydrogen charged conditions; hence it has been confirmed that this is one mechanism of WEC formation. Small/short sized sulfide inclusions, globular manganese sulfide + oxide inclusions and small globular oxide inclusions between ∼1 µm and 20 µm in diameter/length predominated as crack initiators. In addition, detailed focused ion beam/transmission electron microscopic studies have been conducted to enhance the understanding of butterfly crack and white etching area formation mechanisms.

Feasibility of using electrostatic monitoring for oil lubricated ceramic to steel sliding contacts

Abstract Previous work has shown that electrostatic charge signals can be used to detect the onset of wear in lubricated tribocontacts. Preliminary investigations have shown the viability of this system when tested on a laboratory-based pin-on-disc rig, a reciprocating laboratory wear rig [1] , [2] , [3] and in an FZG gear scuffing rig [4] , [5] . These preliminary experiments have indicated several charging mechanisms could be involved within highly stressed lubricated contacts, namely tribocharging, exo-emissions, surface charge variations and debris generation [6] , [7] . This paper details further studies looking at the levels of charge generated by lubricated hybrid (silicon nitride sliding on bearing steel) contacts. Correlation between the charge levels genreated within hybrid contacts under fully lubricated conditions and the resulting wear rates is investigated. Results from the Pin-on-Disc test for silicon nitride against various bearing steels contacts, lubricated by Mobil Jet II, are presented and compared to steel on steel contact performance. The materials are ranked based on their performance during the wear tests. The wear mechanisms of hybrid contacts are discussed based on the on-line multi-sensor monitoring results and the post-test wear surface observation by 3-D laser profilometry measurements.

Electrostatic monitoring of the effects of carbon black on lubricated steel/steel sliding contacts

The aim of this work is to understand better the relationship between the wear of sliding tribo-contacts and soot contamination in diesel lubricants, as modelled by the presence of carbon black. The work examined sensitivities between charge and wear induced by carbon black-contaminated lubricants. Previous research, using a pin-on-disc (POD) tribometer [ [1] , [2] , [3] ], has shown correlations between wear, friction and temperature with electrostatic charge detected by sensors mounted a distance from the contact but focused on the disc wear track. These correlations were found for both steel/steel and steel/Si3N4 contacts lubricated by uncontaminated base oils and commercial aviation oils. The present test programme has been carried out on a pin-on-disc tribometer which was instrumented with Linear Variable Differential Transformer (LVDT), temperature and friction transducers, as well as an electrostatic wear site sensor. The test conditions of 5m/s sliding velocity and a contact stress of 2.05 GPa were chosen to match test conditions used previously. Results from a preliminary test programme which varied the detergents, dispersants and carbon black in lubricants are discussed. Comparisons between the wear rates, friction, disc surface temperature, oil conductivity, oil chemistry, and electrostatic charge are presented for the various Group 1 oil formulations. These studies suggest designs for a systematic matrix to understand how charged additive and/or contaminant species influence wear and performance over time. Analysis using optical spectroscopy and 3-D profilometry of the worn surfaces of the pin and disc were used to identify the wear mechanisms.