We have presented an application of the modified Moving Particle Semi-implicit (MPS) method for squeeze film flows. In addition to calculating the flow field of a squeeze film using the full Navier-Stokes equations, this method has the advantage of being meshless, which gives it the capability of analyzing dynamic and/or highly transient squeeze films by discretizing the domain as a series of particles and numerically analyzing inter-particle interactions. Although past literature has indicated the MPS method in its original form to be unstable in terms of its calculation of pressure, a modified algorithm was implemented to provide agreement between the numerical results and the analytical solutions.

The tendency of smooth surfaces to stick spontaneously to each other is becoming a serious problem, with: a) the increasing quality in surface finish for many components and systems, b) on miniaturization in mechanical components, and c) in demanded precision of positioning of parts in high-end equipment machines and systems. Surfaces tend to be made smoother in order to gain flatness or in order to fulfill the need for more precise and reproducible positioning of parts. Adhesion or even sticking of the surfaces is a major showstopper for these applications. There are several measures that can be taken in order to reduce spontaneous adhesion. Quantification of the effectiveness of the chosen solution is most often done using an AFM with probes varying from 1 nm to 8 micron of contact diameter. A serious disadvantage in measuring adhesion by sharp tips is the wear of the tips. Sharp tips wear easily, resulting in undefined contact areas. When the real area of contact is not well defined, the quantification of the adhesion force is not significant. In the current study results of AFM measurements from literature with different tip diameters of colloidal probes are compared with measurements we performed using AFM cantilevers with a plateau tip and using probes from large spheres using an alternative setup (UNAT). These methods give results that are in good agreement with values found in literature. Large contacting surface enhance the quality of the measured adhesion values. Another part of the study deals with a deliberately roughening of smooth surfaces to minimize (spontaneous) adhesion. Good agreement has been found with existing results. For the use of larger surfaces it is important that the surfaces to be tested are extremely clean. Particles on smooth surface do influence the measurements quite easily. Especially for larger areas, the possibility of encountering particles on the surface are more likely, when particles are present. For the measurements in this study a lot of care has been taken therefore to remove contamination: particles as well as contamination of organic origin.

Discrete Model is based on the description of the physical state (velocity, position, temperature, magnetic moment, electric potential ..) of a large number of discrete elements that form the media to be studied. It is not basedon a continuous description of the media. Thus, it is particularly well adapted to describe media evolution driven by discontinuous phenomena like the description of multi fracturation followed by debris flow like wear study. Recently, the use of discrete model has been widened to face problem encountered with complex rheological behavior and/or multi-physical behavior. Multi-physical problems face complex mathematical formulation because of the mixing of different families of differential equations when continuous approach is chosen. With the discrete model, each particle has a physical state and state evolution is due to local physical particle interaction: it is often much simple to write. From the beginning of the years 2000, this method has been widely applied for the study of tribological problems. Wear has been investigated[1], thermo-mechanical behavior of a contact[2], subsurface damage due to surface polishing[4]… Recently, it has been shownhow quantitative results can be obtained with this method[4]. To help and promote research in this area, a platform GranOO has been developed under a C++ environment and is distributed under thefree GPL license. The goal of this platform is to offer to researchers, especially in the tribology domain, a numerical and easy to use environment to facilitate the access to Discrete Element simulations. The main aspect of this platform ispresented in the extended abstract and some applicationsgiven as examples. Details can be found on url www.granoo.org.

Water lubricated rubber bearings are used from many years in ship building and other industry because of their advantages such as: long serviceable life, low coefficient of friction, long life of shaft sleeve, good vibration damping, good abrasion resistance, low maintenance required, grit particles and other abrasives are expelled through the grooves [1,2,3,4].

The cold spray process consists of coating build-up by sequential impact, deformation and bonding of many particles. Therefore, formation and properties of a deposited layer are not only affected by the impact behavior of a single particle, but also by subsequent impact events. To investigate the material behavior under such conditions, impact of multiple particles in cold spray was studied here by the finite element method. Effects of high strain rates and temperature on material yield and failure criteria were considered. Particle conditions prior to impact were derived from fluid dynamics calculations. To predict sticking behavior of the particle, an interfacial cohesive strength parameter was defined between the particle and the substrate. The effects of temperature and particle positioning were examined for three particle impacts. In addition, simulations involving 100 consecutive particle impacts were carried out, and findings were compared with experimental observations. Results showed that subsequent impacts have a large effect on the previously impacted particles for cohesion, degree of deformation, and residual stresses.

Understanding granular flows has always been important for predicting natural phenomena such as rockslides and soil erosion, as well as industrial processes such as coal-based fossil fuel systems and solids processing. As such, it becomes important to understand granular flows from both a classical granular flow and tribological perspective. Inherently important in the study of granular flows is the study of the individual particle level interactions, which define the global behavior of the flow. The current work examines both the coefficient of restitution (COR) and coefficient of friction (COF) for various material combinations. COR and tribological experiments are performed on various sphere and plate (disk) materials, such as low carbon steel, tungsten carbide (WC), and NITINOL 60.

The properties of metallic alloys can be substantial modified by the addition of a second phase particles. This is especially noticeable when hard particles are incorporated in a relatively soft matrix, often resulting in improved mechanical and tribological performance. This paper presents the results of our study on mechanical and tribological performance enhancement of 6061 Aluminum alloys by incorporation of B 4 C particle via Friction stir processing (FSP). Unidirectional ball on flat friction and wear tests were conducted with a base material, friction stir processed 6061-Al and 6061-Al doped with B 4 C particles via FSP against 52100 bearing steel balls under dry sliding conditions. The incorporation of particles not only reduced friction by 30% but also reduced wear by 2 orders of magnitude compared to unprocessed base and FSP material without particles incorporation. FSP alone without particles addition did not have a significant effect on the tribological behavior of the tested aluminum alloy.

In the present investigation, the tribological performance of aluminum micro (100 to 200 μm particle size) and nano (47 nm particle size) composites was studied using a three pin-on-disk tribometer under dry conditions. As a basis for comparison, the tribological performance of aluminum alloys was also studied. The pins made of these materials were then slid against a steel disk under ambient conditions. Tests were conducted at a sliding velocity of 1.58 m/s for a normal load of 30 N. The worn surfaces of the pins were analyzed using a scanning electron microscope. Based on the experiments, it was observed that the nano-composites significantly outperformed all of the other materials with respect to friction levels. It was also discovered that the nano-composites exhibited the best wear performance among the composites investigated. The size of the reinforcement particle trapped at the interface and the hardness ratio of the mating materials were specifically found to play an important role in determining the friction and wear performance of the materials investigated.

An in-house solver was created to simulate contact mechanics and hydrodynamic lubrication using smoothed particle hydrodynamics (SPH), SPH is a meshfree, particle-based method that can be used to solve continuum problems in solid and fluid mechanics. In this approach, the problem domain is represented by particles that move according to prescribed governing equations. Because smoothed particle methods have been shown to have the advantage of being able to model large deformations without concern of degradation in numerical accuracy, they have received recent interest for application to complex problems in tribology—the exploration of which is the focus of this study.

Nickel-titanium based superelastic materials are emerging as candidates for rolling element-bearing applications [1]. When properly prepared, these unique intermetallics are hard, exhibit excellent tribological properties and are intrinsically corrosion immune [2]. In addition, recent investigations have revealed that, unlike traditional bearing steels, superelastics can endure much higher levels of recoverable elastic strain during compressive deformation [3]. This behavior enables bearings that are more resilient to load induced damage such as raceway denting and from the ingestion of hard particles. Despite these positive attributes, these alloys differ significantly from conventional steels and these differences must be carefully considered to achieve successful applications. These differences include reduced elastic modulus, high hardness and enhanced resistance to indentation loads. The current paper introduces nickel-titanium based superelastic bearing materials and compares their properties to current bearing materials. General bearing design practices and manufacturing processes are also examined to identify and explore the challenges and opportunities for making resilient rolling element bearings utilizing this new class of superelastic materials.

Polishing of a nominally flat aluminum substrate using a fluid jet, carrying abrasive particles, was investigated. The effects of jet orientation and polishing duration were studied, on a fixed spot on the aluminum substrate. Stylus profilometry was used to examine the final surface along different directions. It was seen that the final erosion contour is strongly dependent on the jet orientation. The surface roughness was found to be dependent on the slurry parameters. On average, the average roughness (R a ) of the aluminum surface was reduced from 430 nm to 139 nm. Best polishing was obtained with the jet orientation of 30°. The process was simulated by using computational fluid dynamics (CFD), and it was concluded that dynamic pressure is the main parameter affecting erosion. Results apply to erosion as well as polishing of metal surfaces.

Impact of a single copper particle in cold gas dynamic spray is simulated by finite element method by including the effects of high strain rates and temperature on material plasticity and failure. In order to predict stick behavior of the particle, cohesive forces that act between the particle and the substrate are included in the model by defining an interfacial cohesive strength parameter. Effect of this parameter on the deformation and stick/rebound behavior of the particle is studied. It is found that significant particle deformation, large area of contact between particle and substrate is needed to generate enough cohesive force to absorb the rebound energy of the particle and achieve sticking. As the impact velocity and assumed interfacial cohesive strength increases, particle is more likely to stick on to the substrate. Critical velocities in the same range with experimental results are predicted.

This presentation describes ongoing research performed on a simple shear cell apparatus, previously described [1]. As a complement, discrete particle simulations and continuum models have been used to predict normal and shear forces in the ongoing experiments. The trends and orders-of-magnitude of the models and experiment are in basic agreement. Theoretical models used are constructed with basic principles, rather than curve fitting, to obtain effective properties of the mixture such as viscosity or conductivity. The experiment itself serves to determine the effects of shear rate, packing fraction, particle size and film thickness on the load carrying normal stress. Additionally, the frictional shear stress can be investigated. The working particulate medium within the apparatus consists of glass of aluminum spheres, poly-dispersed over four size increments, all less than 1.00 mm diameter. The upper annular disk is held stationary in a rotational sense by a force transducer, and applies predetermined normal stress values which vary according to a system of interchangeable counterweights. The lower transfer surface and the sidewalls of the annular ring are rotated by applied mechanical torque. Experimental trials consist of shear initiation, after which the trough velocity, film thickness, supporting load, and frictional torque are measured. From these measurements one can calculate the average shear rate, the average load-carrying normal stress, the average frictional shear stress, and the solids volume fraction. Such third body granular flow may apply to some solid lubrication mechanisms, and to applications such as smart clutches and dampers. The continuum theory presented is unique in that it addresses solid-like behavior and its transition to fluidized behavior. The discrete particle dynamics rely on the conceptual models of Iordanoff and colleagues [2]. Our findings are that the two theoretical predictions agree reasonably with the experimental results, suggesting validity of the approach. These results are promising, and may be used to further develop high level predictive models. Furthermore, similar methods of small scale experimental particle simulation can be used to develop simpler more usable continuum approaches.

Granular flows continue to be a complex problem in nature and industrial sectors where solid particles exhibit solid, liquid, and gaseous behavior in a manner which is often unpredictable locally or globally. In tribology, they have also been proposed as lubricants because of their liquid-like behavior in sliding contacts, and due to their ability to carry loads and accommodate surface velocities. The present work attempts to model a granular Couette flow in an annular shear cell using a lattice-based cellular automata (CA) computational modeling approach. The CA framework has the flexibility to employ rule-based mathematics and/or first-principle physics to rapidly model physical processes. The model developed in this work incorporates dissipative effects due to friction between particles and boundaries, in addition to particle spin. This new model also includes a rigorous treatment of boundary-particle interactions. Additional work has been done to account for the transfer of momentum through particle chains, allowing for the modeling of transitional granular flows, which consist of both kinetic and contact flow regimes.

Particulate contaminated lubricants can be detrimental to any tribosystem. Debris can originate from the external environment, be unintentionally introduced during fabrication, or be generated internally during operation. These particles, if not carefully filtered out, can have serious implications on the integrity of a tribosystem. This study focused on the abrasive wear observed from four-ball wear tests in the presence of lubricant with particulate contaminants. The contaminants studied were silicon dioxide sand particles and self generated AISI E-52100 steel particles. The particles were submerged in an industrial EP gear oil. The effects of particle size, composition, and concentration on abrasive wear of sliding interfaces was analyzed.

Particle based modeling approaches, such as the discrete element method (DEM) approach, require the definition of accurate contact (collision) models. An essential parameter within these models is the coefficient of restitution ( e ), which defines the ratio of post-collision to pre-collision relative velocity during the collision of two materials. In this study, e of various steel-material combinations is predicted through both physical experiments and explicit finite element modeling of a falling sphere colliding with a stationary plate, and examined against a theoretical formulation of e . Experiments are performed on various sphere materials including steel, brass, chrome steel, tungsten carbide, aluminum, polybutadiene, and nitinol 60, Experimental results for metals colliding with steel, match what is predicted by theory, as they show a decreasing trend in e with increasing impact velocities. Additional experimental results for polybutadiene rubber show no velocity dependence, remaining constant across the impact velocities examined; this matches with theory. Modeling results, obtained via the explicit finite element method (FEM) approach, are compared against experimental results for verification. Current explicit FEM results show very good quantitative agreement with experimental results for various materials (brass, steel, tungsten carbide, and chrome steel) impacting steel, but do not display the proper decreasing trend in e for increasing impact velocity, over the range of impact velocities examined (∼2 m/s–3.2 m/s). However, for simulations at impact velocities above this range, a decrease in e is witnessed. Current and future work focuses on refining the models to correctly display this qualitative trend within the current range of experimental impact velocities, as well as performing experiments on a wider range of impact velocities.

Asperity failure due to repetitive sliding is a common process of wear particle formation. Linear elastic fracture mechanics and the finite element method (FEM) were used to analyze asperity cracking due to sliding against another rigid asperity. The maximum ranges of the tensile and shear stress intensity factors (SIFs) were used to determine the crack growth direction and the dominant mode of fracture. Simulations of repetitive sliding showed a strong dependence of the wear particle size and wear rate on the direction and rate of crack growth. The maximum ranges of tensile and shear SIFs were used to determine the dominant mode of crack growth. The effects of asperity interaction depth, sliding friction, initial crack position, crack-face friction, and material properties on crack growth direction, dominant fracture mode, and crack growth rate are discussed in the context of FEM results. It is shown that the asperity interaction depth and sliding friction exhibit the most pronounced effects on the crack growth direction and growth rate. A transition from shear- to tensile-dominant mode of crack growth was observed with the increase of the asperity interaction depth and/or sliding friction coefficient. Crack-face opening, slip, and stick mechanisms are discussed in the light of crack mechanism maps constructed for different asperity interaction depths and sliding friction coefficients.

Lubricating grease is commonly used for lubricating ‘sealed and greased for life’ bearings. This grease lubricates the rolling contacts. It also provides an additional sealing function to protect the bearing against ingress of contamination. The sealing function of lubricating grease in the vicinity of the seal lip contact has been studied experimentally. The effects of the lubricant rheology on the migration of ingress particles has been examined. In grease, experimental results reveal that contaminant particles consistently migrate towards the sealing contact where the shear rate reaches its highest value. In contrast, for a Newtonian base oil and a non shear thinning elastic fluid, it has been observed that the migration effect takes place in the opposite direction, and brings particles away from the sealing contact. It is concluded that the sealing function of grease in the vicinity of the sealing contact is due to the fluid rheology and more specifically to the shear thinning behaviour of the lubricant.

We present preliminary results from a study of the friction of surfaces with nanoscopic roughness. Surfaces with asperities of a chosen height and average spacing were formed by self-assembly of nanometer-sized solid particles on a flat substrate. The adsorption density of particles was investigated with Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM), and the suitability of these substrates for friction experiments at different length scales was evaluated. Initial results from friction experiments with a nanoparticle-covered surface in non-adhesive contact with an atomically flat surface in a surface forces apparatus (SFA) are presented.

Erosion due to solid particles has a detrimental effect on the performance of components used in many critical applications. The problem becomes more acute at high temperatures. Thermal sprayed ceramic coatings are widely used in such situations wherein the present scenario considers better coatings. The ceramic composite coatings of alumina-zirconia were developed and their response to particle erosion and high temperature sliding wear and are experimentally evaluated. Based on a brittle erosion model, parametric influence on the variation of erosion rate was mathematically modeled adopting regression analysis and its validity is verified through a set of confirmation tests. Alumina-zirconia exhibits better performance with higher wear resistance and lesser coefficient of friction, at higher temperature, compared to plain alumina or alumina-titania.