W.W.F. Chong
University of Southampton
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Featured researches published by W.W.F. Chong.
Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology | 2013
W.W.F. Chong; Mircea Teodorescu; Homer Rahnejat
Accurate prediction of friction is crucial for design and efficient operation of many devices, comprising various contacts. In practice, contacting surfaces are rough and often wet. There are several mechanisms, which contribute to friction, including viscous shear of a coherent fluid film, as well as that of a thin adsorbed layer of boundary active molecular species. Additionally, adhesion and elastoplastic deformation of asperities on counterface surfaces may occur. Traditional friction models are based on statistical representation of surface topography as well as description of boundary shear films based on the theoretical lubricant film Eyring shear stress. The study reports a more realistic friction model than the traditional ones, which do not take into account the wet nature of the asperities. The fluid–surface interaction is a main contribution of the article, not hitherto reported in literature. It is shown that ignoring the effect of surface wetness can lead to the over-estimation of boundary friction and under-estimation of contact load-carrying capacity.
Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology | 2013
W.W.F. Chong; S Howell-Smith; Mircea Teodorescu; Nick Vaughan
The article proposes a mathematical model, which predicts the frictional performance of an internal combustion engine compression ring in the vicinity of the top dead center region. It accounts for the blow-by induced inter-ring pressures drop and for the cavitation region at the trailing edge of the contact. The model is used to predict the behaviour of both new and worn compression rings for wide open throttle operating conditions. It is shown that the wear of the ring profile increases the oil film thickness decrease the frictional losses and significantly reduces the extent of the cavitation region.
Faraday Discussions | 2012
W.W.F. Chong; Mircea Teodorescu; Homer Rahnejat
The formation of low shear strength surface-adhered thin films mitigates excessive friction in mixed or boundary regimes of lubrication. Tribo-films are formed as a consequence of molecular chemical reactions with the surfaces. The process is best viewed in the context of a lubricant-surface system. Therefore, it is usually surmised that the adsorption of lubricant molecular species to the contact surfaces is underlying to the formation of ultra-thin lubricant films. The paper considers contact between smooth surfaces at close separation. This may be regarded as the contact of a pair of asperity summits, whose dimensions, however small, are far larger than the size of fluid molecules within the conjunction. In such diminishing separations the constraining effect of relatively smooth solid barriers causes oscillatory solvation of fluid molecules. This effect accounts for the conjunctional load capacity but does not contribute to mitigating friction, except when molecular adsorption is taken into account with long chain molecules which tend to inhibit solvation. The paper presents an analytical predictive model based on the Ornstein-Zernike method with the Percus-Yevick approximation of a narrow interaction potential between conjunctional composition. The predictions confirm the above stated physical facts in a fundamental manner.
Journal of Physics D | 2011
W.W.F. Chong; Mircea Teodorescu; Homer Rahnejat
The physics of molecularly thin fluid films formed between surface features at close range is investigated. It is found that the interplay between discrete lubricant drainage from such contacts and localized contact deflection plays an important role both on the load carrying capacity of these asperity level conjunctions as well as on friction. Small spherical molecules tend to solvate near assumed smooth surfaces of asperities at nano-scale. Their discrete drainage at steadily decreasing gaps adds to the viscous friction of any bulk lubricant film. However, at the same time the generated solvation pressures increase the load carrying capacity. Conversely, long chain molecules tend to inhibit solvation, thus showing a decrease in the load carrying capacity, whilst through their wetting action reduce friction. Consequently, real lubricants should comprise molecular species which promote desired contact characteristics, as indeed is the case for most base lubricants with surmised properties of certain additives. The methodology presented underpins the rather empirical implied action of surface adhered films. This is an initial approach which must be expanded to fluids with a more complex mix of species. If applicable, this could also be an alternative (potentially time saving) approach to Monte Carlo simulations for molecular dynamics.
International Journal of Engine Research | 2014
W.W.F. Chong; Mircea Teodorescu; Homer Rahnejat
The key driving forces in engine development are fuel efficiency and emission levels. These aspects are particularly poignant under vehicle idling or low crawling motions in typical city driving. Under these conditions, the parasitic frictional losses are exacerbated and the emission levels are especially high. A key engine sub-system is the valve-train system. Although it accounts for only 2–3% of the overall engine losses, it is the highest loaded conjunction in the engine, thus limiting the opportunity for lowering the lubricant bulk viscosity. The paper presents detailed tribology of the cam–tappet contact, subjected to a mixed thermo-elastohydrodynamic regime of lubrication. In particular, the frictional behaviour of the conjunction is investigated under the stringent North American emission testing city cycle. Such a comprehensive approach has not hitherto been reported in the literature. The predictions show good conformance with vehicle frictional assessments in industry. It further demonstrates that under the aforementioned cycle, the highest power losses occur mainly as the result of lubricant film viscous shear at low sliding speeds and below the lubricant limiting Eyring shear stress.
Surface Topography: Metrology and Properties | 2015
W.W.F. Chong; Homer Rahnejat
Understanding the scale-dependence of friction is increasingly viewed as a critical quest. With progressively thinnerfilms, mixed and boundary regimes oflubrication have become commonplace. Therefore, at the micro-scale a greater need for mitigating friction is desired in order to improve operational efficiency of many machines and mechanisms. Furthermore, there is a growing tendency to use low friction hard wear-resistant advanced coatings to guard against wear. In parallel, there has been much attention paid to lubricant rheology and formulation. However, only in recent times there has been an emerging view of lubricant-surface combination as a system. In this perspective it is essential to relate the observed and measured friction at component level to the underlying interactions in micro/nano-scales. This is the approach inthis paper. Observed phenomenon at micro-scale are related back to the activation energies of lubricant-surface system, providing in particular results for surface modified Ni-SiC coated specimen in combination with formulated lubricants, the combination of which represent the lubricant-surface system of choice in cylinders of high performance race engine. The nano-scale conjunction of an AFM tip with lubricated surface engineered specimen, subjected to various conjunctional loading and sliding kinematics is investigated. It is shown that the measured frictional characteristics can be adequately described in terms of activation energies in line with the Eyrings thermal activation model for cases of fairly smooth asperity tip contact conjunctions.
Journal of Physics D | 2012
W.W.F. Chong; Mircea Teodorescu; Homer Rahnejat
An analytical method based on statistical mechanics is proposed to predict ultra-thin adsorbed films of physical fluids with molecular diversity formed on smooth surfaces. The model is representative of molecular interactions at the smooth summits of surface asperities in the nano-scale. At this physical scale the constraining effect of the solid barriers promotes discretization of the fluid volume into molecular layers. These layers are usually ejected from the contact in a stepwise manner. The integrated effect of intermolecular forces as well as their interactions with the contiguous surfaces is responsible for the discontinuous drainage of the fluid. However, at the same time, the adsorption energy of the molecular species strives to form a molecular monolayer upon the boundary solids. The net result of these complex interactions is an ultra-thin adsorbed film, whose shear characteristics depends on a competition between the repulsive solvation pressure and the energy of molecular adsorption. It is shown that very thin low shear strength films are formed in this manner. This would depend on the molecular concentration and the wall adsorption energy. An important implication is that boundary adherent films should be viewed as a result of surface-fluid combination for which the choice of concentration and fraction content of particular species are crucial.
Key Engineering Materials | 2015
W.W.F. Chong; Miguel De la Cruz
The paper introduces an alternative approach to predict boundary friction for rough surfaces at micros-scale through the empirical integration of asperity-like nanoscale friction measurements. The nanoscale friction is measured using an atomic force microscope (AFM) tip sliding on a steel plate, confining the test lubricant, i.e. base oil for the fully formulated SAE grade 10w40. The approach, based on the Greenwood and Tripp’s friction model, is combined with the modified Elrod’s cavitation algorithm in order to predict the friction generated by a slider-bearing test rig. The numerical simulation results, using an improved boundary friction model, showed good agreement with the measured friction data.
Key Engineering Materials | 2015
W.W.F. Chong; Homer Rahnejat
At microscale, friction is better understood fundamentally through hydrodynamic and elastohydrodynamic lubrication. However, the mechanisms governing friction at nanoscale remains a subject of interest. With the emergence of small-scale devices such as Microelectromechanical Systems (MEMS) and Nanoelectromechanical Systems (NEMS), there is a need to improve on the fundamental understanding of friction at diminishing gaps. Therefore, the paper investigates the friction of a simple fluid (n-hexadecane 99%) using an atomic force microscope. The measurements are interpreted using modified Eyring’s thermal activation energy approach in order to examine the effect of molecular solvation at the assumed smooth summit of asperities. It is found out that solvation for a sliding contact could be observed through the shear stress activation volume due to generated thermal energy, which indicates the movement of the fluid molecules into and out of the contact.
ASME 2014 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference, IDETC/CIE 2014 | 2014
S.J. Chidlow; W.W.F. Chong; Mircea Teodorescu
This paper proposes a hybrid (semi-analytic) solution for determining the contact footprint and subsurface stress field in a two-dimensional adhesive problem involving a multi-layered elastic solid loaded normally by a rigid indenter. The subsurface stress field is determined using a semi-analytic solution and the footprint using a fast converging iterative algorithm. The solid to be indented consists of a graded elasticity coating with exponential increase of decay of its shear modulus bonded on a homogeneously elastic substrate. By applying the Fourier Transform to the governing boundary value problem, we formulate expressions for the stresses and displacements induced by the application of line forces acting both normally and tangentially at the origin. The superposition principle is then used to generalize these expressions to the case of distributed normal pressure acting on the solid surface. A pair of coupled integral equations are further derived for the parabolic stamp problem which are easily solved using collocation methods.Copyright