Conventional methods for generating Gaussian random surfaces, including the moving average (MA) time series model with nonlinear conjugate gradient method (NCGM), two-dimensional (2-D) digital filter method, and spectral representation method (SRM), are implemented with a wide range of autocorrelation length and truncation length values of the autocorrelation function (ACF). The ACF, power spectral density function (PSDF), and essential roughness parameters of the simulated surfaces are calculated and compared. Based on the simulation results, the mechanism of the truncation length of ACF affecting the simulated surfaces can be summarized as that the step formed by truncating ACF is not sufficiently small, thus resulting in non-negligible errors in the corresponding PSDF. Such errors will be propagated to the simulated surfaces. A conservative criterion is proposed to avoid the adverse effects of truncating ACF: make the autocorrelation length less than 8% of the surface dimensions and the truncation length at least seven times autocorrelation length. The results show that the MA model with NCGM overestimates the PSDF values of simulated surfaces in the high-frequency region, meaning significant high-frequency noise in the simulated surfaces. The 2-D digital filter method and the SRM have almost the same performance, and both methods are better than the MA model with NCGM when the criterion of truncating ACF is fulfilled. The SRM generates rough surfaces with the smallest standard deviation in terms of the roughness parameters, ACF, and PSDF in most cases, meaning that it can generate accurate surfaces at every single simulation and is more stable and efficient. Therefore, the SRM is the most recommended method among the three methods studied.

The prediction of lubrication performance is required to be the basement of friction optimization for marine engines. This paper simulates the lubrication performance of marine engines based on statistical models which have the advantages of fast, efficient, and macroscopic fault location. Boundary lubrication exists in the piston ring-cylinder liner (PRCL) of two-stroke marine engines because of the harsher load, lower speed, and larger structure. It has been proposed that there would be tribofilm under boundary lubrication which has a significant influence on the contact. To understand the boundary lubrication, it is necessary to study the lubrication regime transition. In this paper, firstly, the coefficient of friction curve combined with the thickness ratio embodies the lubrication regime transition process of two-stroke engines under work conditions. However, the phenomenon that the coefficients under boundary lubrication are smaller than that of other regimes shows the non-objectivity of this curve. Therefore, the Stribeck curve is introduced for objectively evaluating the transition. Then, the calculation of asperities contact pressure under boundary lubrication, which Wen proposed, is introduced into the classic Greenwood-Williamson model, the problem that the original model cannot reflect the boundary lubrication regime in the form of the Stribeck curve is improved. Finally, the results are compared before and after modifying the model to verify this study's practicability. It provides more precise asperities contact pressure for the tribofilm growth calculation from the perspective of the Stribeck curve under the PRCL statistical model in future work.

The piston ring-cylinder liner (PRCL) is one of the most important parts of marine diesel engines and contributes 25% to 50% of total friction loss. The lubrication simulation analysis of the PRCL system is a challenging task. Complete understanding and precise prediction of lubrication loads is a key to understanding the friction behavior of PRCL systems as the accuracy of the friction prediction depends upon precise prediction of lubrication loads. Therefore, this paper focuses on the gas pressure calculation which is the primary source of lubrication loads. The procedure presented combines the advantages of two mainstream methods to predict loads in the PRCL system. The result is a significant reduction in the computation time without compromising on accuracy. Firstly, a comparison of both approaches is presented which suggests that each technique has its limitations (one is time-bound, and one is accuracy-bound). Then, the results from both calculation methods are verified against literature and a parametric study is performed to identify the key structural parameters of PRCL system that affect the calculation efficiency. Finally, a correlation coefficient is introduced into the analysis to combine the two approaches which then identifies the conditions under which the use of the faster method becomes invalid and replaces it with the more accurate approach. This ensures optimum performance of the calculation procedure by switching between the fast and the accurate method depending upon the accuracy requirement under given conditions, thereby, simplifying the dynamic and lubrication model of PRCL systems. The study has direct implications for the tribological design of the PRCL interface.

Experimental results have confirmed that parallel rough surfaces can be separated by a full fluid film. However, such a lift-off effect is not expected by the traditional Reynolds theory. This paper proposes a deterministic mixed lubrication model to understand the mechanism of the lift-off effect. The proposed model considered the interaction between asperities and the micro-elastohydrodynamic lubrication (micro-EHL) at asperities within parallel rough surfaces for the first time. The proposed model is verified by predicting the measured Stribeck curve taken from literature and experiments conducted in this work. The simulation results highlight that the micro-EHL effect at the asperity scale is critical in building load-carrying capacity between parallel rough surfaces. Finally, the drawbacks of the proposed model are addressed and the directions of future research are pointed out.

Ashless dialkyldithiophosphate (DDP) antiwear additives are good candidates to replace the widely used metallic DDPs such as zinc dialkyldithiophosphate (ZDDP), which are less environmentally friendly. A newly designed insitu tribological rig was utilised to perform in-situ synchrotron X-ray absorption spectroscopy (XAS) in order to examine the decomposition reactions of two types of DDPs; acidic and neutral. The tribological experiments showed that the two DDP additives decomposed to form protective tribofilms on the steel surface, which provided better antiwear protection than ZDDP regardless of the tribofilm thickness. The neutral DDP formed a thinner tribofilm (about 33 nm) than ZDDP (about 41 nm), whereas the tribofilm of the acidic DDP had a much lower thickness (<7 nm) but more superior antiwear protection. The two DDPs also provided lower friction coefficient (<0.1) than the 0.12 provided by ZDDP. The XAS experiments suggest that the DDPs decompose to form initially iron sulphate, which is quickly reduced to sulphide before forming the phosphate layers of the protective tribofilm. These layers consisted initially of iron phosphate of short chains but as rubbing continued organic phosphate with long chains started to form.

The effect of relative humidity on the decomposition reaction of zinc dialkyldithiophosphate (ZDDP) additive under boundary lubrication regime has been investigated using Raman spectroscopy and atomic force microscopy (AFM). The ZDDP tribofilms were formed using the pin-on-disc tribometer at 30 and 90% relative humidity, which was controlled using a custom-made humid chamber. The results suggest that relative humidity does not change only the length of the phosphate chains but also their affinity towards the available cations. At low relative humidity, zinc pyrophosphate to polyphosphate chains were formed compared to predominantly shorter iron orthophosphate chains at high relative humidity. In addition, relative humidity altered the structure of the mature tribofilm. At low relative humidity, the tribofilm had a patchy structure with average thickness of 140 nm and fractal dimension of 2.17. However, at high relative humidity, the tribofilm had a thin dendritic structure with average thickness of 85 nm and fractal dimension of 2.34. This means that the structure is more complex with a larger number of nesting self-similar structures at all scales. This is significant in making the transition from descriptive towards quantitative information of the tribofilm structure.

The application of the spectral representation method in generating Gaussian and non-Gaussian fractal rough surfaces is studied in this work. The characteristics of fractal rough surfaces simulated by the spectral representation method and the conventional Fast Fourier transform filtering method are compared. Furthermore, the fractal rough surfaces simulated by these two methods are compared in the simulation of contact and lubrication problems. Next, the influence of low and high cutoff frequencies on the normality of the simulated Gaussian fractal rough surfaces is investigated with roll-off power spectral density and single power-law power spectral density. Finally, a simple approximation method to generate non-Gaussian fractal rough surfaces is proposed by combining the spectral representation method and the Johnson translator system. Based on the simulation results, the current work gives recommendations on using the spectral representation method and the Fast Fourier transform filtering method to generate fractal surfaces and suggestions on selecting the low cutoff frequency of the power-law power spectral density. Furthermore, the results show that the proposed approximation method can be a choice to generate non-Gaussian fractal surfaces when the accuracy requirements are not high. The MATLAB codes for generating Gaussian and non-Gaussian fractal rough surfaces are provided.

The effect of water on the transient kinetics of the decomposition reaction of zinc dialkyldithiophosphate (ZDDP) additive has been investigated using X-ray photoelectron spectroscopy (XPS). Water was found to have a long-lasting detrimental effect on ZDDP triboreactive film layers by changing their composition depending on their proximity to the metal surface. Particularly, water impeded the polymerisation reaction in the top layers, which results in the formation of short zinc (thio)phosphate chains. The most probable explanation for this was related to the formation of a single or multiple bridges between water molecules and PO2− groups of the decomposed ZDDP. This caging effect, which does not fade away as water evaporates, can suppress the chemical reactivity of these groups to a great extent, which in turn can hinder their polymerisation into long phosphate chains.

In recent years, several theoretical models to predict the triboreactive film thickness of zinc dialkyldithiophosphates (ZDDP) have been developed. Although these models are not complete and are approximate in nature, they provide a framework which can be used to evaluate the factors impacting the tribofilm growth. In this paper, rough surfaces with different roughness parameters were numerically generated and used to calculate the tribofilm growth. The simulation results show that lower negative skewness, higher kurtosis, and larger autocorrelation length values give thicker ZDDP tribofilms with skewness and autocorrelation values having a greater effect on the tribofilm growth. Based on the simulation results, it is found that these parameters influence the tribofilm growth by changing the contact ratio which eventually changes the dynamic growth of tribofilm and wear of substrate. The results presented in this paper have bigger implications for industry in designing surface textures that promote the anti‐wear action.

It has been long speculated that the good antiwear properties of the nano-thin triboreactive interfaces might have a rheological origin that can greatly influence their friction, lubrication and antiwear characteristics, which account for nearly one quarter of the worldwide total energy consumption. However, the measurement of the nanoscale viscosity of such tribological interfaces is still a challenging task. This is mainly due to their ultralow thickness, i.e. typically < 150 nm, high viscosity and reactivity, which make the currently used bulk, interfacial or micro-gap rheological techniques inadequate. Here we demonstrate two methods, i.e. creep and squeeze flow, that can be used to quantify the viscosity of triboreactive films in-situ as they form and ex-situ after formation. Films generated from the zinc and ashless dialkyldithiophosphate (ZDDP and DDP) antiwear additives were analysed as model systems because of their industrial and academic importance. The results confirm that the formed tribofilms behave as molten glass with an average viscosity ranging from 2 x 10(11) to 7 x 10(12) Pa s. During its formation, the molten glass showed rich intrinsic rheological properties that allowed them to maintain local order on the nanoscale through the motion and reconfiguration of single and multiple patches within the formed film, which can significantly predetermine its superior antiwear properties. The findings of this study open future opportunities for optimizing the nano-flowable glass to efficiently control the lubrication of tomorrow's engines without the need of any environmentally harmful oil additives.

The ability to simulate mixed lubrication problems has greatly improved, especially in concentrated lubricated contacts. A mixed lubrication simulation method was developed by utilizing the semi-system approach which has been proven to be highly useful for improving stability and robustness of mixed lubrication simulations. Then different variants of the model were developed by varying the discretization schemes used to treat the Couette flow terms in the Reynolds equation, varying the evaluation of density derivatives and varying the contribution of terms in the coefficient matrix. The resulting pressure distribution, film thickness distribution, lambda ratio, contact ratio, and the computation time were compared and found to be strongly influenced by the choice of solution scheme. This indicates that the output from mixed lubrication solvers can be readily used for qualitative and parametric studies, but care should be taken when making quantitative predictions.

Tribological performance of mating surfaces can be improved by developing surfaces with predefined textures and it is crucial to understand their role in modifying the performance and action of boundary lubrication additives. An isotropic texture T1, intermediate texture T2 and anisotropic texture, T3, were imported into our mixed simulation framework. It was found that T3 outperforms by providing the most feasible conditions for the activation of the lubricant additive, molybdenum dithiocarbamate (MoDTC) i.e. generating higher pressure peaks (shear), making more lubricant (reactant) available and providing greater load bearing area (greater reaction probability). Hence, facilitating the formation of functional boundary films. T2 seems to generate highest pressure peaks but its performance is worst due to its inability to sustain lubricant within the contact.

Most power transmitting components operate under mixed lubrication conditions. Concentrated pressures and smaller lubricant film thickness may cause surface and subsurface stresses to exceed the material yield limit causing permanent geometrical changes. A model was developed to include elastoplastic behaviour within a deterministic unified mixed lubrication framework. Model details are presented and the model is validated against published simulation data. A parametric study to address the effect of material yielding on the contact parameters is performed. It is found that the model successfully produces all the key features of the PEHL contact. The model provides a valuable tool to analyse the PEHL contacts with minimal increase in computational effort and complexity.

Mixed lubrication is a contact condition when the total load is carried by both the fluid lubricant and the solid contacting asperities. The aim of this study is to couple tribochemistry with lubrication. A recent semi-deterministic tribochemical model of tribofilm growth is integrated in a deterministic mixed lubrication model. The model considers the variable hardness of the tribofilm and enables the study of lubrication and tribochemistry and their mutual interaction. Results from the current model are compared against the previously published results. The model can be easily adapted to actual experimental conditions and geometries. The model can be used beyond pure boundary lubrication conditions to monitor tribofilm growth under mixed lubrication conditions.

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A new mechanism for the action of antiwear tribofilms is proposed. The antiwear action of ZDDP additive is believed to be mainly due to the formation of tribofilms that reduce wear by chemical action. In this study, a mixed lubrication model is developed and tribofilm growth integrated into this model to simulate the effects of tribofilms on lubrication. The dynamic evolution of the contacting surfaces due to plastic deformation, wear and tribofilm growth continuously change the lubrication characteristics inside the contact. It is observed that the growth of tribofilm roughens the contact and increase contact severity. It was found that this roughness increase also helps to entrain more lubricant, resulting in thicker lubricant films. Therefore, the plot of the evolution of film thickness ratio (h(central)(t)/R-q(t)) shows that the lubrication regime is improved by the presence of tribofilm. Therefore, not only the chemical presence but the physical presence of the tribofilm on the surfaces also helps to improve contact performance by retaining more lubricant and improving the lubrication regime.

Despite the ubiquitous use of the zinc dialkyldithiophosphate (ZDDP) as an antiwear additive, no complete information is yet available on its exact decomposition reactions and kinetics to form triboreactive protective films on contacting surfaces. This hinders the replacement of ZDDP with more environmentally friendly additives of similar antiwear capabilities. Using a multitechnique approach, this study shows that before the formation of a phosphate-rich protective film, the decomposition of ZDDP proceeds by forming intermediate zinc sulfide and sulfate species, which can be mechanically mixed with the iron oxides on the rubbing steel surfaces. The mixed sulfur-oxide layer can play different vital roles including binding the subsequently formed phosphate layers with the metal surface. These layers consist mainly of zinc thiophosphate of initially short chains, which are formed due to the excess concentration of metal oxide on the surface. As the concentration of the oxide decreases in the subsequent layers, the short chains start to polymerize into longer ones. The polymerization process follows first-order reaction kinetics with two distinctive phases. The first one is a fast transient burst phase near the steel surface, whereas the second phase dominates the formation process of the layers away from the substrate and is characterized by slow kinetics. The findings of this study provide new insights into the decomposition mechanisms of the currently most widely used antiwear additive and open future opportunities to find green alternatives with similar superior antiwear properties.

The reaction kinetics of zinc dialkyldithiophosphate (ZDDP) and ashless DDP antiwear additives were studied using in-situ single asperity AFM experiments. The results show that the ZDDP molecules decompose under high temperature and contact pressure following first order reaction kinetics (n=0.71±0.14), whereas the DDP molecules follow a more complex fractional order (n=0.52±0.07). The fractional order indicates that the decomposition process includes more side and intermediate reactions that consume part of the additive to form species, e.g. possibly volatile products, other than the antiwear phosphate glass on the contacting surfaces. In the case of ZDDP, the formation rate of the phosphate glass follows an equal exponential dependence on both temperature and contact pressure. However, in the case of DDP, the rate increases exponentially only over contact pressure and linearly over temperature. The findings of this study advance our understanding of the currently widely-used P-based antiwear additives and open future opportunities to develop new green alternatives with similar antiwear capabilities.

Atomistic simulations based on the static lattice model are performed to calculate the equilibrium and growth morphologies of CdS polymorphs. Morphologically important surfaces are optimized to calculate their structural and energetical properties such as surface and attachment energies. A common feature of all the nonpolar CdS surfaces is the outward movement of their anions and the inward movement of their cations. The relaxation of surfaces is critically important as it changes the surface and attachment energies significantly. The {11 (2) over bar0} surface has the lowest surface energy (0.58 J/m(2)) for the wurtzite phase of CdS, whereas {110} surface has the lowest surface energy (0.62 J/m(2)) for the zincblend phase of CdS. The {10 (1) over bar0}, {12 (3) over bar0}, and {1 (1) over bar 00} surfaces of wurtzite CdS all have the same surface energy value (0.60 J/m(2)), which is very close to that of {11 (2) over bar0} surface. Therefore, all these surfaces appear in the equilibrium morphology of the wurtzite CdS. The equilibrium morphology of the zincblend CdS is completely dominated by the {110} surface. The growth morphology of the wurtzite CdS consists of {10 (1) over bar0}, {1 (1) over bar 00}, {0001}, and {000 (1) over bar} surfaces. The growth morphology of the zincblend CdS is found to be identical to its equilibrium morphology and, therefore, includes only the {110} surface.