Cobalt-chromium-molybdenum alloys are commonly used for biomedical applications such as dental implants and joint implants. Once the material is implanted into the body it is exposed to the corrosiveness of biological fluids and, in some cases, to mechanical loading that can lead to the combined action of wear and corrosion; better known as tribocorrosion. The effect of four different simulated body fluids on the tribocorrosion behaviour of a CoCrMo alloy has been investigated. The degradation of the studied CoCrMo alloys due to tribocorrosion shows a great dependence on the chemical composition of the media. Phosphate-buffered saline (PBS)-based solutions tend to show higher mass loss than the solutions prepared with distilled water. Phosphates present in PBS tend to accumulate on the surface of the alloy and change its tribological performance. In addition, proteins show a lubricating effect reducing the coefficient of friction of the system in the boundary lubrication regime.

Nanodiamonds (NDs) were investigated as reinforcement for ultra high molecularweight polyethylene (UHMWPE). Dry and wet mixing with planetary ball milling was compared and analysed by scanning electron microscopy (SEM), differential scanning calorimerty (DSC), X-ray diffraction (XRD) and contact angle measurements. The composites were mixed from one to four hours to study the dispersion of the nanoparticles. It was concluded that wet mixing is more effective at distributing nanodiamonds in comparison to dry mixing. It could also be concluded that dry mixing increases the temperature by 20°C more than wet mixing which resulted in a more distinct welding process of the UHMWPE powder.

Abrasive wear is largely involved in many industries processes, and can cause serious problems and economic loss. A number of theoretical models and numerical models have been established to study wear phenomena. However, simulation and prediction of wear at large scale are seldom presented. Sliding abrasive wear of steel plates from interaction with granular material is here studied with numerical simulations. Abrasive wear of unloading of two different dumper body geometries are studied with the smoothed particle hydrodynamics method coupled to the finite element method. These numerical tools are of interest as they can reproduce interaction between solid and granular material. Wear pattern on the dumper bodies obtained from numerical simulation shows a reasonably good correspondence to experimental measurements. An advanced analysis tool that takes into account both the actual material flows, coupled with wear calculation model would be a new tool to design and optimise handling equipment against wear.

The use of surface coatings is emerging as one of the most important approaches in reducing friction and wear in various tribological applications. Even though single layer coatings have a wide range of applications, the performance of the single layer alone may not always be adequate to meet the desired tribological property requirements. Hence, coatings consisting of multilayers to meet different property requirements in demanding applications are required. In this study, the tribological properties of a graded composite multilayer coating, with a specific layer sequence of MoS2/Ti‐MoS2/TiBN‐TiBN‐TiB2‐Ti deposited on tool steel substrate, have been investigated at temperatures of 40 and 400°C respectively. The experimental results from the tests at 40°C have shown that the friction coefficient value ranges between 0�02 and 0�034. It was found that the deposition parameters influenced the friction and durability of the coatings. Higher substrate bias was found to result in higher friction, and the coating deposited at high substrate bias and low N2 flow showed the lowest durability. The friction coefficient and durability of the coatings were found to be highly dependent on temperature. At high temperature, the friction coefficient increases almost threefold, and the durability decreases significantly.

One of the tribological requirements on engine bearing material is its ability to safely embed contaminant particles onto its surface and minimise damage to both the bearing and crankshaft surfaces. In this work, a journal bearing test rig that operates under constant load has been employed to investigate the embeddability behaviour of selected multi-layered Pb-free engine bearing materials at three different rotational speeds using engine oil contaminated with SiC particles. Experimental results have shown that third-body abrasive wear is influenced by the lubricant film thickness. There was also difference in embeddability of the different materials. Bismuth-based overlay and MoS2 containing polyamide-imide-based overlay-coated materials show higher wear compared to tin-based overlay and a polyamide-imide-based composite overlay-coated material. Steel counter surfaces sliding against bismuth-based overlay and MoS2 containing polyamide-imide-based overlay exhibited higher wear than those sliding against tin-based overlay and polyamide-imide-based composite overlay. 

The use of solid lubricants is an effective way to control friction and wear in applications where traditional lubricants such as oils and greases cannot be used. MoS2 is a popular solid lubricant which has been widely used in many applications, especially space applications in view of its good performance in vacuum. Recent developments in physical vapour deposition technology have led to the development of sputtered MoS2 films doped with different metals to improve their durability and reduce the detrimental effects of oxidation and humidity on their tribological performance. In this work, a MoS2-Ti coating deposited on a hot forming tool steel substrate has been studied at ambient and elevated temperatures. The objective was to investigate how the friction and durability of the MoS2-Ti coating are affected after exposure to elevated temperatures. The results have shown that low friction values of ∼0·02 were obtained at room temperature and low relative humidity of 25%. An increase in relative humidity to 40% led to an increase in friction by almost 100%. There was a very significant degradation in frictional characteristics as well as durability after the MoS2-Ti coating was exposed to 400°C. The wear of the counter surface also increased when sliding against the MoS2-Ti coating exposed to elevated temperatures. This has been attributed to interaction of the counterbody with hard abrasive molybdenum oxides formed on the MoS2-Ti coating due to exposure to elevated temperatures.

Abrasive wear in industrial applications such as mining, materials handling and agricultural machinery constitutes a large part of the total wear. Hardened high strength boron steels are known for their good wear resistance and mechanical properties, but available results in the open literature are scarce. This work aims at investigating how different quenching techniques affect the two-body abrasive wear resistance of hardened high strength boron steels. Furthermore, the wear as a function of depth in thicker hardened high strength boron steel plates has also been studied. The material characterisation has been carried out using microhardness, SEM/energy dispersive spectroscopy and three-dimensional optical surface profilometry. The results have shown that water quenched and tool quenched high strength boron steel had similar wear resistance. The main wear mechanisms appear to be microcutting combined with microfatigue. Workhardening during the abrasion process has been found to affect the abrasive wear.

Tribological systems working under severe conditions like high pressures, sliding velocities and temperatures are subjected to different phenomena such as wear, oxidation and changes in mechanical properties. In many cases, there are several mechanisms occurring simultaneously.The predominating type(s) of wear mechanism(s) presented will depend on the materials in contact, operating parameters and surrounding environment. In this work, high temperature tribological studies of boron steel sliding against tool steel were conducted using a pin-on-disc machine under unlubricated conditions at five different temperatures ranging from 25 to 400C, three different loads: 25, 50 and 75 N (contact pressures of 2, 4 and 6 MPa respectively) and a sliding speed of 0.2 ms-1. Scanning electron microscopy/energy dispersive spectroscopy and X-ray techniques were used for analysing the resulting damage and tribolayers of the worn surfaces. Additionally, hardness measurements were carried out in a special hot hardness rig in the same temperature range as that used in pin-on-disc tests. The results have shown that for a given load, the wear rate of boron steel decreased as the temperature increased, reaching itslowest value at 400C at 50 N. In the case of the tool steel, it could be observed that at 200C and above, the wear rate decreased as the load increased. This behaviour is consistent with the formation of a protective oxidised layer initiated at 100C. At higher temperatures, such layers become more pronounced. The obtained data were finally used to construct a friction and wear mechanism map for this material pair that takes temperature and pressure into account.

Tribological studies were carried out with tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings, varying in thickness and roughness, using two different contact configurations lubricated with seven types of hydraulic oils. Tribopair of cast iron and ta-C coated steel were tested in both non-conformal and conformal, unidirectional sliding contacts. The friction and wear results were mainly affected by the thickness of the coating in the non-conformal contact and the surface roughness of the coating in the conformal contact. Tests done with mineral base oil containing rust inhibitor in the non-conformal contact and with Polyalphaolefins and synthetic ester base oils in the conformal contact resulted in the lowest friction while that with mineral base oil containing zinc resulted in high friction and counterface wear. The results highlight the interdependence of contact configuration, lubricant chemistry, coating’s surface morphology and coating’s thickness in determining the tribological behaviour of ta-C coatings under boundary lubrication.

Since abrasion is a common cause of seal failures, understanding the mechanisms of abrasion of an elastomer in the presence of lubricants is of importance in sealing applications. In this research a block on ring configuration was used to study the influence of lubrication on the two body abrasion of several commonly used sealing elastomers (two acrylonitrile butadiene rubbers, an acrylic rubber and a fluoro rubber). The friction force and the abrasive wear of the samples were measured and the worn surfaces and wear particles were investigated using an optical microscope. The tear strength of the elastomers before and after immersion in monoester oil as well as the oil absorption has been measured. Both scratches (parallel to the direction of sliding) and ridges (perpendicular to the direction of sliding) were observed on the worn surfaces of nitrile rubbers but the surfaces of acrylic and fluoro rubber were characterised by scratches only. The worn surfaces of nitrile rubbers were defined with more continuous ridges at lower sliding velocity and the presence of a lubricant in the contact reduced the continuous ridges. Examination of the wear particles shows that the wear particles (particularly for acrylic rubber) under dry sliding condition were aggregated, but the lubricant dispersed the wear particles and prevented aggregation. In most cases, abrasive wear of the elastomers under lubricated condition is higher than that under dry condition. The results show that the friction coefficient increased with increasing sliding velocity and decreased with contact pressure. Apart from the fluoro rubber, the friction coefficient as well as the tear strength of the elastomers decreased significantly in the presence of lubricant, particularly for acrylic rubber.

Mechanical components in tribological systems exposed to elevated temperatures are gaining increased attention since more and more systems are designed to operate under extreme conditions. In hot metal forming, the effect of temperature on friction and wear is especially important since it is directly related to process economy (tool wear) and quality of the produced parts (friction between tool and workpiece). This study is therefore focused on fundamental understanding pertaining to the tribological characteristics of prehardened hot work tool steel during sliding against 22MnB5 boron steel. The tribological tests were carried out using a high temperature reciprocating sliding friction and wear tester under a normal load of 31 N (corresponding to a contact pressure of 10 MPa), a sliding speed of 0·2 m s−1 and temperatures ranging from 40°C to 800°C. It was found that friction coefficient and specific wear rate decreased at elevated temperature because of formation of compacted wear debris layers on the surfaces.

The unique biocompatible and wear resistant nature of ultrahigh molecular weight polyethylene (UHMWPE) makes it a suitable material for load bearing applications in total joint replacements (TJR). However, oxidation induced wear is a common cause limiting the life span of implants. Hence, Vitamin-E, a common antioxidant, is added to prevent the wear loss due to oxidation of UHMWPE. In addition, Hydroxyapatite does improve the toughening mechanisms occurring in the polymer matrices. Hence, in this study, a novel hydroxyapatite (HA) nanoparticles reinforced Vitamin-E doped UHMWPE nanocomposites (UHMWPE-E/HA) were developed here in this work. The HA concentration was varied between 0.5 - 3 wt.% and optimum wt% is reported. The mechanical and thermal properties were investigated thoroughly using an array of characterizations. Particularly, a considerable improvement in fracture toughness (K_IC) was obtained. Detailed examination of fractured surfaces was performed to understand the effect of HA reinforcement on fracture toughness of UHMWPE-E/HA nanocomposites.

Metal-on-metal joint replacements are considered as an alternative to metal-onpolyethylene implants, specially in case of young patients who require a safe and long-term performance of the device. The reduction of wear particles is a key factor in order to improve the life time of the implant in the human body. Metals have excellent properties that may increase the long-term success of the artificial joint replacement. However, corrosion of the metallic implant leads to an increase of the ion levels in the body of the patient. Metallic ions may produce a host response that can induce a catastrophic failure of the implant. This review initially focuses on the consequences that the degradation of the metals used in orthopaedic implants have for the health of the patient, and the different biological reactions that lead to the failure of the implant. Parameters that affect the release of particles and ions into the body are discussed as well. Special ttention is given to the tribology, corrosion and tribocorrosion behaviour of metal-on-metal implants. Finally, an overview of mathematical models that have been used to model the behaviour of the implants are also presented.

Carbon based polymer composites have been suggested as an alternative to conventional ultra high molecular weight polyethylene (UHMWPE) in total joint replacements. The aim of this study was to investigate the use of graphene oxide (GO) as reinforcement of UHMWPE. Under optimised mixing conditions, the prepared UHMWPE/GO composite showed an enhanced thermal stability compared to conventional UHMWPE. Also, the present study has shown the potential of ball milling as a processing method for synthesising UHMWPE/GO composites to be used in load bearing implants.

Joint replacements have considerably improved the quality of life of patients with joints damaged by disease or trauma. However, problems associated with wear particles generated due to the relative motion between the components of the bearing are still present and can lead to the eventual failure of the implant. The biological response to wear debris affects directly the longevity of the prosthesis. The identification of the mechanisms by which cells respond to wear debris and how particles distribute into the human body may provide valuable information for the long term success of artificial joints. During the last few decades, orthopaedic research has been focused on predicting the in vivo performance of joint replacements. However, the exact relationship between material physicochemical properties and inflammatory response has not been fully understood. Laboratory wear simulators provide an accurate prediction of implant wear performance. Though, particles generated from such wear simulators require validation to compare them with particles extracted from peri-implant tissues. This review focuses initially on the current status of total joint replacements (hard on soft and hard on hard bearings) as well as on the tribological behaviour of the potential materials currently under investigation. Then, the correspondence between particles observed in vivo and those generated in vitro to predict the cellular response to wear debris is discussed. Finally, the biological effects of the degradation products generated by wear and corrosion are described

In recent years, drilling extended reach wells have become more and more common in the petroleum industry to optimise the oil and gas production. Extended reach wells are defined as wells that have two times more horizontal step out than true vertical depth. High friction (frictional torque) and drag are two of the mechanical limiting factors while drilling longer horizontal wellbores. There are numerous methods and tools developed to lower the drillstring friction. Drilling non-circular wellbores is a new concept with potential to minimise the mechanical friction by reducing drillstring and sidewalls contact area. However, this will cause an increase of contact pressure owing to the reduced contact area between drillstring and the formation. This article presents results obtained from an experimental study pertaining to the friction behaviour using a pin on disc set-up with steel pin and granite disc in the presence of water and oil based lubricants. These tests have been designed to represent frictional contact conditions between a rotating steel drillstring and the wellbore wall at different contact pressures. Test results show that the friction coefficient decreases with an increase of contact pressure in wet condition for both water and oil based lubricants. It is also observed that the friction factor increased by adding sand and phyllite particles to the water based lubricant. Considering the wear scars, the friction coefficient shows reduction with increased contact pressure in all tests with and without particles for both water and oil based lubricants.

Ball on disc tribometer and atomic force microscopy (AFM) were used to analyse the effect of base oil polarity on the friction behaviour of steel-steel contacts lubricated with base oil + zinc dialkyldithiophosphate (ZDDP) solutions. Understanding the lubrication properties of the first chemisorbed layer of additives on work pieces yields important information for the optimisation of lubricant formulation, in particular with regard to the type of additive and amount needed. To characterise the influence of base oil polarity, two reference base oils [hexadecane (non-polar) and diethylenglycol (polar)] were blended with different concentrations of C4-ZDDP, and the solutions were tested. A monolayer of base oil/additive solution was deposited on an ASI 52100 steel plate and scanned in AFM contact mode under various rubbing times and applied load conditions. An AFM technique was developed to estimate the microscopic values of friction coefficients showing how the oil polarity contributes to the differences in friction behaviour of the solution due to the addition of ZDDP. With different base oils [(hexadecane (non-polar base oil) and diethylenglycol (polar oil)] the authors observed significant different friction behaviours (in micro scale and nano scale) due to the addition of ZDDP compared to the base oil alone. This observation may be attributed to the contribution of base oil to transport the ZDDP additive onto the surface which will be discussed in more details in the paper. These results display the importance of base oil polarity on the friction behaviour of formulated lubricants containing additives