T.J. Harvey

A review of experimental techniques to produce a nacre-like structure

The performance of man-made materials can be improved by exploring new structures inspired by the architecture of biological materials. Natural materials, such as nacre (mother-of-pearl), can have outstanding mechanical properties due to their complicated architecture and hierarchical structure at the nano-, micro- and meso-levels which have evolved over millions of years. This review describes the numerous experimental methods explored to date to produce composites with structures and mechanical properties similar to those of natural nacre. The materials produced have sizes ranging from nanometres to centimetres, processing times varying from a few minutes to several months and a different range of mechanical properties that render them suitable for various applications. For the first time, these techniques have been divided into those producing bulk materials, coatings and free-standing films. This is due to the fact that the materials application strongly depends on its dimensions and different results have been reported by applying the same technique to produce materials with different sizes. The limitations and capabilities of these methodologies have been also described.

Effect of oil quality on electrostatic charge generation and transport

This paper presents an experimental investigation into the effect of lubricating oil quality on tribocharging. The work forms part of a larger investigation into the causes of electrostatic charging events detected prior to severe scuffing during base oil lubricated wear experiments. A variable speed spinning disc charger was used for measurements of tribocharging, whilst controlling the oil viscosity, temperature, ageing and disc surface roughness. The results are discussed with reference to interactions between the double layer and fluid boundary layers and shear stress which could lead to charge separation at the disc-oil interface. Viscosity and surface roughness have been shown to affect the tribocharging of a base oil in a direct relationship, while temperature effects indicate a maxima due to transportation-recombination competition. Ageing experiments indicate an increase in conductivity, which will eventually lead to an increase in charging ability. Charge sign inversion is discussed in relation to specific adsorption of charged species (additives) on the disc surface, the presence of oppositely charged impurities and the differential of adsorption and/or diffusion rates for the negatively and positively charged species within the oils.

Investigation of electrostatic charging mechanisms in oil lubricated tribo-contacts

Fundamental studies using a spinning disc tribocharging rig have shown that disc surface roughness, oil temperature and oil composition have an effect on the charging ability of lubricating oils. Further investigations into the effect of oil composition on the charging nature have employed the oil droplet rig to investigate a range of base and formulated oils. Experimental results indicated that charging in base oils is controlled by levels of impurities, such as sulphur, and aromatic content. For formulated oils it has been determined that the additives are the major contributor to the charging nature and any effect that impurities and aromatics have is negligible. Studies into the sensitivity of the electrostatic sensor system to surface charge has shown that surface transformations that occur during scuffing in tribocontacts can lead to contact potential differences detectable by this system.

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.

Electrostatic charge monitoring of unlubricated sliding wear of a bearing steel

Previous papers [O.D. Tasbaz, H.E.G. Powrie, R.J.K. Wood, Electrostatic monitoring of oil lubricated contacts for early detection of wear, in: Proceedings of the International Conference on Condition Monitoring, University of Wales Swansea, 12–15 April 1999; Wear 230 (1) (1999) 86; H.E.G. Powrie, O.D. Tasbaz, R.J.K Wood, C.E. Fisher, Performance of an electrostatic oil monitoring system during FZG Scuffing Test, in: Proceedings of the International Conference on Condition Monitoring, University of Wales Swansea, UK, 1999, pp. 155–174; Condition Monitor 177 (2001) 6] have shown that electrostatic sensing technology can be used to successfully monitor adhesive wear processes in oil-lubricated contacts. This work has shown that ‘precursor’ charge signals can be detected prior to the onset of adhesive wear. The possible charging mechanisms involved have been identified as tribocharging, surface charge variations, triboemission and debris generation. This paper details experimental work focused on monitoring the unlubricated wear performance of a bearing steel sliding against a bearing steel using electrostatic sensing technology. Tests were carried out using a modified and instrumented pin-on-disc wear test rig with conditions designed to produce sliding wear in the mild oxidational wear regime. Test results have shown that there is a direct correlation between both the wear rate and coefficient of friction with the magnitude of charge detected. The influence of the periodical formation and delamination of oxidation products is also presented. The charge detected was found to be predominantly negative. The dominant wear related charging mechanism was thought to be surface charging as a result of contact potential difference.

Development of synergy model for erosion- corrosion of carbon steel in a slurry pot

Abstract A semi-empirical model (model 1) has been developed for synergy of corrosion dominated, non-passivating, erosion–corrosion of carbon steel in a slurry pot, developed from a stripping and recovery model to yield an expression for enhanced corrosion due to erosion damage revealing the underlying oxide–free surface. Dynamic Hertzian contact mechanics is used to model damage during erodent particle impact, as well as the incorporation of squeeze film and collision efficiency effects. The model also accommodates the effect of the erodent deforming the surface leading to increased corrosion activity. Experimental erosion data appear to be consistent with classical erosion behaviour as predicted by the kinetic energy of the erodent model. The experimental erosion–corrosion data show a more complex relationship due to the action of corrosion and its interaction with erosion, ranging from corrosion dominant to an equal influence of erosion and corrosion. Comparisons between modelled and experimental data indicate the nascent surface has a corrosion activity ranging from 30 to 100 times that of the unaffected surface, depending on experimental parameter studied. An empirical approach (model 2) indicates that experimental synergy correlates with a medium synergy level. This model (model 1) represents a step towards modelling the complex interactions of erosion and corrosion.

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.

The effects of diesel contaminants on tribological performance on sliding steel on steel contacts

This article presents the findings from a parametric study examining the effect of four contaminants (soot, oxidation, moisture, and sulphuric acid) at varying levels (four for each). It was observed that all contaminants and contaminant levels reduce the conductivity of the oil. Oxidation and soot contaminants produced large increases in viscosity. The wear rate was mainly influenced by acid and soot additions, while the coefficient of friction was increased by all contaminants and contaminant levels. The steady-state charge levels changed for some contaminants. The best correlation of steady-state charge with the other measured tribological parameters of wear rate, friction, and temperature is seen for the series of oxidized oils. The multi-contaminated oil (L4 × 4) shows remarkably little degradation in tribological performance. Analysis of the wear mechanisms shows that soot and oxidation produced abrasion and polishing wear, respectively, while sulphuric acid and moisture produced corrosive wear.

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 boundary and mixed lubrication

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 and in an FZG gear scuffing rig. These preliminary experiments have indicated several charging mechanisms could be involved, namely tribocharging, surface charge variations, debris generation and exo-emissions. This paper details further studies looking at the levels of charge generated under boundary and mixed lubrication for base oils and formulated lubricants using a pin-on-disc facility. Results will be presented that show major differences in charge levels between base oil and formulated oil and the dynamic nature of charge within the wear track. The onset of scuffing will be related to surface contact potential differences and phase transformations. Precursor charge events prior to catastrophic metal/metal interaction within the contact are identified and their use for early detection of wear and boundary film failure discussed. Charge sign inversion is discussed in relation to space charge caused by additive adsorption.