An accurate and reliable wear analysis requires detailed knowledge of the tribological conditions of the studied system. In this work, a numerical model which can quantify wear and is applicable to hydraulic motors is developed. Detailed tribological knowledge can be acquired through strategic experimental testing and numerical simulations. The model is constructed to include the effect on wear from varying lubricant film thickness. The development of the wear model includes consideration of wear observed in the Scanning Electron Microscopy (SEM) analysis of tested motors. The model is of the Archard type, in which the k-value is estimated from experiments, after considering the effect of lubrication. The contact pressure is the solution to a lubrication model that governs both the hydrodynamics of the lubricant film and the direct contact between the rough surfaces. To validate the model, a hydraulic motor is run at different operating conditions and the apparent wear depth is analysed after the tests. Numerical simulations mimicking the same configuration are performed and the predicted wear depths are compared to the experimental results. Similarities and differences are discussed and it is evident that a clear correlation exists between the wear predicted with the model and the measurement data of the apparent wear in the hydraulic motor. There are also discrepancies because of the model simplicity and the uncertainty in the specifications of the tested system. The results imply that wear analysis using numerical simulations aid the understanding of wear in machinery. The combined knowledge of physical conditions on different important scales enables in-depth analysis with numerical tools which cannot be achieved through experimental investigations alone. Furthermore, the numerical model can be refined leading to better wear predictions.

Detailed understanding of wear processes is required to improve the wear resistance and lifetime of machine components. Atomic force microscopy (AFM) is used to measure surface height profiles with high precision, before and after a wear experiment. The distribution and depth of wear on steel surfaces is then calculated using a relocation method. A numerical investigation of wear based on Archard's equation is conducted on the same measured surfaces. A good correlation was found between the model and experiment for wear larger than a hundred nm. The wear mechanisms considered in the numerical simulation was thus found to be the cause of the majority of the wear. On the scale of tens of nm the correlation was limited, but the measured wear was still analysed in detail.

Wearing-in of a machine component can increase the conformity between contacting pairs and smoothen the surface topography. A two scale model, combining the wearing-in effects, resulting in changes in the surface topography, with the wear that occurs on the component, is presented. The geometry of the components are represented with measured coordinates. Wear leads to changes of the geometry, which has an effect on several tribological conditions, such as contact forces, relative velocities and conformity. Due to the wear on the topography scale, the load sharing is also affected. The model is applied to orbital hydraulic motors. The wear depth predicted with the model, is qualitatively in good agreement with the wear depth recorded in experiments.

The heart of gerotor motors is a gear-set. The gear-set consists of an inner gear which is rotating and orbiting in contact with an outer gear. Wear in these contacts is investigated experimentally and with a numerical implementation of an Archard based wear model in combination with a load sharing concept. The model utilizes the symmetry of the motor and is based on a three-scale approach to estimate the wear on the gears. The global model calculates contact forces, relative surface velocities and contact radii in the contacts between inner and outer gear. The calculations performed on the local scale are used to collect information about the influence of the surface roughness on lubricant film thickness. The wear depth is calculated on a semi-local scale, involving only one tooth on the outer gear. In partial elastohydrodynamic lubrication, load is carried by the part of the conjunction where there is direct contact between the mating surfaces and by the lubricant pressure. In the wear model, wear only occurs as a direct consequence of contact between the mating surfaces. Experimental results are compared with the model predictions for equivalent running conditions. The wear predicted by the model agrees with the experimental results. For this reason, it is concluded that wear in the gerotor motor is dominated by the wear mechanisms which are considered in the tribological model.

A costly effect, caused by the relative motion of machine components in contact, is wear. Solid surfaces come into contact when the lubricant film thickness is thin, resulting in locally high pressures and high wear rates. Components and machines will, due to this wear, eventually lose their functionality. Machines are often lubricated, meaning that there is a sharing of load between lubricant and asperities in the interfaces between machine components. The roughness of the surfaces influences the likelihood that asperities collide. When asperities collide, protective layers grow on the solid surfaces due to chemical constituents of lubricants. These chemical constituents are called additives and some additives can, under boundary lubrication conditions, interact with surfaces to form the protective layers, also called tribofilms. The formed layers and the formation of layers have previously been studied separately. Numerical simulations are often used to model phenomena such as wear. In this thesis it is asked whether it is possible to model wear on different length and time scales, and to model the interplay between these length and time scales? Can we model the chemical effects on the nano scale, in combination with contact mechanical effects due to roughness on the micro scale? Can we learn something from numerical simulation of the small changes in roughness during a wear process of a mixed lubrication contact in a component?Several numerical simulation methods have been used. Contact mechanics problems are treated through the boundary element method. A model based on Arrhenius equation is used to include chemical effects in a chemo-mechanical simulation of the growth of tribofilm. An equation for the lubricant film thickness is combined with contact mechanical behaviour to estimate load sharing. A method to include the effect of wear on the load sharing is included in the model. Archards wear equation is used to simulate wear at several length scales. A model to determine the load distribution between gears in an orbital type hydraulic motor is developed.Three wear test rigs are utilized. An orbital type hydraulic motor test rig is used to validate wear models for the gear set. A ring-on-ring test is used to experimentally simulate steady state conditions of wear on the topography scale. A sliding reciprocating ball-on-disk test rig is used to evaluate the wear in a contact.Topography measurements and microscopy are used to quantify and qualitatively locate wear. The methods for topography measurements are stylus profilometry, vertical scanning interferometry and atomic force microscopy. The microscopy techniques which are used to locate and classify wear scars, are confocal microscopy and scanning electron microscopy.Numerical simulation results are compared with measurements of the wear in a reciprocating ball on disk rig. Wear on the nano-meter scale on a rough surface is simulated by means of numerical methods, and wear depths from the numerical simulations are compared with wear depths measured by Atomic Force Microscopy. A model for tribofilm growth due to chemical reactions on the nano-scale is applied to model a wear process on a surface topography. Wear is numerically investigated in a gear set in an hydraulic motor and the results are compared with results from experimental investigations. A two scale model is applied to the gear set, in which wear effects the geometry of the gears and the small scale surface topography.In Paper A, ball on disk wear is simulated numerically and by experimental methods. In Paper B, dry contact wear on the topography scale is considered. Roughness features on the scale of 100 nm, were similarly worn in experiment and simulation. In Paper C, a tribochemistry model is constructed in order to evaluate the growth of tribofilms on rough surfaces. The numerical model exhibits similar behaviour as other authors have observed when conducting experiments involving lubricant additives.A multi scale wear model is applied to study wear in hydraulic motors in Paper D-F. The measured wear depths agree well with the wear depths predicted by numerical simulation. The location of the majority of the wear is the same in the numerical simulations and the experiments. Refinements of the methods to determine force distribution as well as the influence of roughness, improves the predictive capabilities of the multiscale wear models.Further improvements of multi scale wear modelling are suggested, and some interesting future research topics are suggested.

During the running-in period of a boundary lubricated contact, the surface roughness will be dramatically changed, involving large plastic deformation and wear on the asperity summits. This paper presents a running-in model. The model is based on the Boundary Element Method (BEM) and will focus on the investigation of large plastic deformations. The BEM model is based on the minimization of the complementary potential energy and deals with the contact at interfaces. The BEM model used in this paper previously adopted a linearly elastic- perfectly plastic material model and was frictionless, therefore a correction factor is needed for the model to improve its ability to account for high friction and large plastic deformation. The correction factor is made by improving the yield criterion and is derived with the assistance of the Finite Element Method (FEM). Both the friction and the strain hardening mechanism taken from the real material are implemented in the FEM model; the load and the contact area are required to match in both simulations. With the correction factor, results are promising for the current model to simulate the running-in process.

Wear is a consequence of nature which becomes costly if uncontrolled. Basic wear protection is provided by lubrication which will decrease the severity of the contact between asperities. If the conditions of a contact are such that there can be no hydrodynamic lift off by the oil and most of the contact occurs in between such asperities, the protection is provided by chemically reacted layers, sometimes as thin as just a few nanometers.In such cases where wear is governed by the most basic wear mechanisms, analytical models and numerical simulation tools have been developed and used to predict the extent of wear. Few of these models concider the interplay between contact mechanics and wear mechanisms. Wear modelling must keep improving.The goal for this work is to examine the predictive efficiency of current models and initiate construction of reliable models for the chemical growth of wear reducing layers. To achieve this, numerical simulations of contact mechanics are used in Paper A to calculate the wear of contact surfaces and in Paper B as a basis for conditions of chemical growth.The contact mechanics model is based on a solution to Boussinesq’s problem applied to equations for the potential energy by Kalker. The method takes the contact’s surface topographies and substrate material properties as input and outputs elastic and plastic deformation, contact pressure and contact area. The numerical implementation is efficiently evaluated by means of FFT-accelerated techinques. The wear is usually treated as a linear function of contact pressure and in this case the Archard wear equation constitute a feasible approximation. This equation is implemented in the present contact mechanics model to approximately predict the extent of wear, in boundary lubricated contacts, by means of numerical simulations.The chemistry of lubricant additives is discussed. Using chemical theory for adsorption as by Arrhenius, the molecular perspective of antiwear additives is explored. Mechanical properties of tribochemical antiwear layers are taken into account in the developed method. The results in Paper A from wear simulations and comparison with an experiment shows the usefulness of wear equations of geometrical contact mechanics. The chemical model in Paper B for tribofilm growth is applied to rough surfaces allowing comparison of the synergy between contact mechanics and chemistry fordifferent surface contacts. The results show how tribofilms grow on rough or smooth surfaces. The model can be used to compare chemical acitivity for different surface designs.

A model for tribofilm growth is developed. The model is used in combination with numerical contact mechanics tools to enable evaluation of the combined effects of chemistry and contact mechanics. The model is tuned with experimental data and is thereafter applied to rough surfaces. The growth of the tribofilm is evaluated for 3 different contact cases and short-term tribofilm growth behaviour is analyzed. The results show how tribofilms grow in patches. The model is expected to be used as a tool for analysis of the interaction between rough surfaces.