Counter Surface Research Articles

A Novel Tribometer Designed to Evaluate Geological Sliding Contacts Lubricated by Drilling Muds

Abstract Great interest in improving lubricity, or reducing friction, of drilling muds used for horizontal oil well drilling is motivated by increasing the horizontal reach that can be attained by a single drilling site. However, there are a limited number of commercially available devices that can be used to evaluate novel drilling mud solutions under sliding conditions that accurately replicate those encountered in the field, and those that are available are often prohibitively expensive. Here, the design of a low-cost lubricity meter, or tribometer, is documented. The purpose-built tribometer is capable of varying rotating speeds, applied normal loads, temperature, and counter surface materials. In particular, the counter surface of the tribometer can be either a steel surface, as is often used in the industrial lubricity meters available in corporate laboratories, or a geological core specimen taken from the drill site. The novel instrument was then used to evaluate four commercially available water-based drilling fluid lubricant additives, dissolved in distilled water, for both a steel-on-steel contact and a steel-on-rock contact. The steel-on-steel contact shows that the tribometer replicates the results of tests typically conducted in drilling fluid labs, thus verifying the performance of the newly developed tribometer. Additional results show that the friction and performance of the lubricant depend significantly on the materials used: steel-on-steel contacts show much lower friction than steel-on-sandstone contact. Finally, a weak dependence on the applied load is shown for a number of lubricant additives examined.

Validity of Amontons\u2019 law for run-in short-cut aramid fiber reinforced elastomers: The effect of epoxy coated fibers

Friction between two contacting surfaces is studied extensively. One of the known friction theories is Amontons’ law which states that the friction force is proportional to the normal force. However, Amontons’ law has been found to be invalid for elastomers. In the present study, the validity of Amontons’ law for short-cut aramid fiber reinforced elastomers is studied. Two types of fillers are used to reinforce the elastomers, namely highly dispersible silica and short-cut aramid fibers. Short-cut aramid fibers with two different surface treatments are used, namely non-reactive fibers with standard oily finish (SF-fibers) and fibers treated with an epoxy coating (EF-fibers). A pin-on-disc tribometer is used to investigate the frictional behavior of the composites in sliding contact with a granite counter surface. The results show that, after the run-in phase, Amontons’ law is valid for those composites that are reinforced by short-cut aramid fibers (without reinforcing filler, i.e., silica) if the contact pressure is below a threshold value. However, once the contact pressure exceeds this threshold value, Amontons’ law will be invalid. The threshold contact pressure of the composites containing EF-fibers is higher than of the composites containing SF-fibers. The composites that are reinforced by silica and short-cut aramid fibers do not follow Amontons’ law.

Development of a highly transparent and flexible touch sensor based on triboelectric effect

Wearable devices are inevitable in future lifestyles. Sensors, especially touch sensors, are fundamental requisites and key driving forces for related technologies. Moreover, the requirement of standalone operation without an external power supply is increasing in several fields due to a wide range of functionalities. We developed a triboelectric effect-based self-powered touch sensor (TESS) with highly transparent and flexible characteristics. The current film type of a TESS exhibits a high light transmittance of 89% and can function as a sensor on highly curved states. The electrical signals generated from a TESS under various conditions, such as bent/unbent state, material of the counter surface, and attached state to human skin, were analyzed. The TESS can be used to measure the magnitude and exact moment of the contact and detachment of other materials while attached to other substrates. This study could significantly increase the applicability of self-powered touch sensors for flexible electronic devices and related technologies.

Oxidation Effects on wear Resistance of SS-304 and SS-316 Austenitic Stainless Steels

Present manuscript deals with the sliding wear behaviour of SS- 304 and SS-316 with and without oxidation prior to wear test. The dry sliding wear performance of the steel samples was noted in as received conditions and then after oxidizing the sample at 600°C for 6 hours. The variations in hardness of both the steels before and after oxidation were estimated. The dry sliding wear tests were carried out using pin on disk technique at different sliding velocity and different loads, by using hardened steels counter surface. The worn surfaces were characterised by using FE-SEM/EDS analysis to ascertain the mechanism of wear in the samples. From various characterisation results and observations of volume loss, the effect of prior oxidation was analysed for the both stainless steels. It has been found that the oxidation leads to the formation of some protective oxides which leads to increase in the hardness of the samples due to which the wear performance was improved compared to the as received alloys.

Fabricating Laser-Induced Periodic Surface Structures on Medical Grade Cobalt–Chrome–Molybdenum: Tribological, Wetting and Leaching Properties

Hip-implants structured with anti-bacterial textures should show a low-friction coefficient and should not leach hazardous substances into the human body. The surface of a typical material used for hip-implants, namely Cobalt–Chrome–Molybdenum (CoCrMo) was textured with different types of laser-induced periodic surface structures (LIPSS)—i.e., low spatial frequency LIPSS (LSFL), hierarchical structures consisting of grooves superimposed with high spatial frequency LIPSS (HSFL) and Triangular shaped Nanopillars (TNP)—using a picosecond pulsed laser source. The effect of LIPSS on the wettability, friction, as well as wear of the structures, when slid against a polyethylene (PE) counter surface and biocompatibility was analyzed. Surfaces covered with LSFL show superhydrophobicity and grooves with superimposed HSFL, as well as TNP, show hydrophobic behavior. The coefficient of friction (CoF) of LIPSS against a polyethylene (PE) counter surface was found to be higher (ranging from 0.40 to 0.66) than the CoF of (polished) CoCrMo, which was found to equal 0.22. It was found that the samples release cobalt within biocompatible limits. Compared to polished reference surfaces, LIPSS cause higher friction of CoCrMo against PE contact. However, the wear of the PE counter surface only increased significantly for the LSFL textures. For these reasons, it is concluded that LIPSS are not suitable for a heavily loaded metal-on-plastic bearing contact.

Oxidation Effects on wear Resistance of SS-304 and SS-316 Austenitic Stainless Steels

Present manuscript deals with the sliding wear behaviour of SS- 304 and SS-316 with and without oxidation prior to wear test. The dry sliding wear performance of the steel samples was noted in as received conditions and then after oxidizing the sample at 600°C for 6 hours. The variations in hardness of both the steels before and after oxidation were estimated. The dry sliding wear tests were carried out using pin on disk technique at different sliding velocity and different loads, by using hardened steels counter surface. The worn surfaces were characterised by using FE-SEM/EDS analysis to ascertain the mechanism of wear in the samples. From various characterisation results and observations of volume loss, the effect of prior oxidation was analysed for the both stainless steels. It has been found that the oxidation leads to the formation of some protective oxides which leads to increase in the hardness of the samples due to which the wear performance was improved compared to the as received alloys.

Effects of tempering and heat strengthening on hardness, indentation fracture resistance, and wear of soda lime float glass

AbstractAnecdotal and empirical evidence suggests that heat strengthening treatments compromise the wear and scratch resistance of soda lime silicate (SLS) glass. This can be problematic when trying to clean inorganic debris from the glass surface which can result in scratches degrading the optical clarity of the glass surface. Interestingly, past work using a ball‐on‐flat tribometer showed that SLS has peculiar wear behaviors as a function of relative humidity (RH). In dry conditions, inorganic counter surfaces abrade SLS. In contrast, in high humidity conditions, SLS substrate exhibits wear resistance for a variety of inorganic counter‐surface materials. The focus of this fundamental study is to investigate the hardness, indentation fracture resistance (IFR), and wear resistance of SLS float glass with various thermal treatment histories—as‐received (exposed to ambient lab air for about a year), annealed, heat strengthened, and thermally tempered—as a function of humidity using Vickers indentation and ball‐on‐flat tribometer. Several hypotheses were considered, and it was proposed that Si‐O bond parameters, specifically strained bonds, may play an important role in the mechanochemical wear behavior of soda lime glasses at high humidity conditions.

Correlation of Tribological Behavior and Fatigue Properties of Filled and Unfilled TPUs

For a long service time, fatigue has been a typical problem that mechanical sealing materials face. How does it relate to tribological performance? In this study, filled and unfilled thermoplastic polyurethanes (TPUs) were investigated. Dumbbell and faint wait pure shear (FWPS) specimens were used to characterize the fatigue properties and crack growth rate of TPUs, respectively. Additionally, to identify the impact of temperature on fatigue tests, the tests were conducted at room temperature and 80 °C. Different tribological tests were conducted to investigate their tribological properties. Fracture surfaces from fatigue tests and worn surfaces from tribological tests were analyzed using a scanning electron microscope (SEM). Two wear models were verified to correlate between fatigue and tribological properties; one of the models is better for rough counter surfaces, while the other is advantageous if the counter surface is smooth.

Effects of flyash addition on the dry sliding tribological behavior of aluminum composites

Flyash is an industrial waste obtained from thermal power plants. The mixing of flyash particles in the air may cause damage to the environment. In this paper, attempts have been made to utilize the waste flyash particle to fabricate an aluminium composite. Three aluminium composites with fly-ash (5 wt%–10 wt%) were fabricated using a stir casting with vortex technique. Wear and friction behavior of the aluminium flyash composites was evaluated using a linear reciprocating tribometer at 0.38 Mpa pressure and 10 Hz frequency against a mild steel counter surface. It is established that flyash contents enhanced the wear and friction behavior of the aluminium metal composite. The aluminium-flyash composite with 10 wt% flyash particles yields the lowest amount of wear, friction coefficient value and maximum hardness among the fabricated materials. The scanning electron microscope (SEM) images also indicated the lower microcutting and wear wedges on the 10 wt% flyash composite.

Hydrophobic Reduced Graphene Oxide-Based Ni Coating for Improved Tribological Application

Electrodeposited nickel-graphene oxide (GO) composite coating was produced by pulse electrodeposition method followed by a functionalization with octadecyl amine (ODA) molecules to convert the embedded hydrophilic GO to hydrophobic reduced GO-ODA. Morphology, compositional, and tribological properties were investigated for the functionalized Ni-GO-ODA composite coating. FE-SEM and AFM microscopic studies show the homogeneous presence of embedded GO-ODA particles in the Ni matrix. XRD, EDS, and Raman spectra confirmed the successful functionalization of GO to GO-ODA. Post-functionalization, the water contact angle of the coating shows 111.8°, thereby confirming its hydrophobic nature. Tribological experiments were conducted by using a linear reciprocating ball on flat sliding microtribometer with varying load and sliding speed at ambient dry sliding condition. Experimental results showed that the wear and friction resistance properties were significantly enhanced after functionalization of GO with ODA. With increasing load, the counter surface penetrates into the bulk of the coating and exposes the embedded particles, which further under shear built a surface film which reduces friction. The wear and failure mechanisms of these coatings subjected to tribological testing were also investigated. This work provides a novel approach to design durable functionalized nano-coatings for tribological applications in various industries such as automotive, aerospace, locomotive and renewable energy technologies.

A fractal view on running-in process: taking steel-on-steel tribo-system as an example

Purpose Running-in is a transient process prior to steady state and of great importance for mechanical performance. To reveal the fractal behavior in the running-in process, the steel-on-steel friction and wear tests were performed. Design/methodology/approach The friction coefficient, friction temperature, friction noise and vibration were recorded, and the surface profile of lower sample was measured on line. The signals and profiles were characterized by correlation dimension and box-counting dimension, respectively. Findings The signals have the consistent fractal evolvement law, that is, the correlation dimension increases and tends to a stable value. The box-counting dimension of one surface becomes close to that of the other surface. The running-in process can be interpreted as a process in which the fractal dimension of friction signals increases, and the counter surfaces spontaneously adapt to and modify each other to form a spatial ordered structure. Originality/value The results reveal the running-in behavior from a new perspective.

Examinations on PEEK wear debris accumulation over counter surfaces in room and vacuum sliding environments

Abstract Wear debris on sliding tracks can often be a significant contributing factor to the friction and wear behaviors of polymer–metal sliding systems. This paper disseminates insightful examinations on the variegated patterns of PEEK wear debris and their mobility/stability on the sliding tracks on counter surfaces of 316L steel discs. Sliding experiments were performed both in an air-conditioned room and in a moderate to high vacuum (25-10 mPa) environments. Counter surface discs were produced by four commercial machining processes: polishing, turning, milling, and grit-blasting. Thereby, different types of counter surface topography were obtained. Light optical microscopic images on wear tracks were recorded to visualize and inspect PEEK wear debris pattern. The polished surface produced discrete and short debris fragments; whereas, grit-blasted counter surface generated thick and continuous debris patches. An attempt was made to explain such dramatically different patterns of debris accumulation as influenced by disc counter surface micro-geometry. Under vacuum sliding conditions, the polished surface produced significant reduction in friction and wear of PEEK, while the grit-blasted surface showed quite opposite response. Turned and milled surfaces exhibited medial behaviors. In addition, milled and grit-blasted surfaces produced discolored debris of PEEK under vacuum condition. Physicochemical changes of PEEK debris under vacuum conditions thus speculated as being the reason behind this observation. Overall, PEEK transfer layer deposition pattern, their quality, and their impact on system's tribological functionalities were found to be strongly influenced by the counter surface topographical characteristics and the sliding environments.

Surface modification of Al6061-SiC surface composite through impregnation of graphene, graphite & carbon nanotubes via FSP: A tribological study

Fabricating high wear resistance Al matrix composite without sacrificing its ductility and thermal properties is a critical issue in the designing of Al-based composites. In present study, a detailed wear mechanism of Al-SiC surface composite modified by the impregnation of Graphene, Graphite & carbon nanotubes through friction stir processing (FSP) is documented. The results revealed that with SiC and graphene nanoplatelets (GNP) reinforcements the friction coefficient and specific wear rate decreased by ~34% and ~50% as compared to as-received Al6061 alloy respectively. On the other hand, combination of SiC and carbon nanotubes (CNT) reinforcements severely deteriorates the wear resistance of the composite. The layered structure, large specific surface area and the wrinkled morphology of the graphene flakes are the primary reasons for the increased wear resistance. The exfoliation of GNP to few layered graphene (FLG) under the effect of plastic flow stresses during FSP also contributes significantly to the improvement of surface properties. The abrasion is identified as the dominant wear mechanism in Al-SiC-Graphite & Al-SiC-GNP hybrid composite. Whereas, delamination due to adhesion with the counter surface is the leading mechanism in Al-SiC-CNT hybrid composite. Various characterization strategies such as microstructural characterization through SEM, interfacial study through TEM and phase analysis through XRD corroborate the results. Finally, GNP is identified as the best reinforcement among the carbon family for improvement in wear resistance of Al6061-SiC surface composite.

Friction and wear mechanism of short-cut aramid fiber and silica reinforced elastomers

Important phenomena during sliding contact of elastomeric materials are friction and wear. Wear reduction of elastomers can be achieved by minimizing the propagation of cracks in the elastomer during sliding contact. Adding fillers like silica and fibers is a way to reduce the propagation of cracks and as a result reduction of wear. In the present study, the wear processes of short-cut aramid fiber reinforced elastomers as a function of sliding distance and their relation to friction are investigated. Two different types of systems are considered, i.e. (1) elastomers reinforced by solely short-cut aramid fibers and (2) elastomers reinforced by short-cut aramid fibers and silica. A pin-on-disc tribometer and a microscope are used to analyze the friction and wear mechanisms of the elastomeric composites in sliding contact with a granite counter surface. The results show that the coefficient of friction of the composites consists of different stages, these stages are influenced by the wear processes during sliding. For elastomers which are reinforced by short-cut aramid fibers and silica, a higher energy input is needed to achieve all stages since the presence of silica in the elastomer matrix increases the resistance of matrix particle detachment. A general friction behavior of short-cut aramid fiber and silica reinforced elastomers is proposed.

Tribological property enhancement of 3Y-TZP ceramic by the combined effect of CaF2 and MgO phases

Calcium fluoride (CaF2) often provides enhanced lubricating properties into ceramic composites when combined with other metal oxides to form some lubricating substrate. Present work unveils the superiority of small amount of CaF2 (1 wt%) as solid lubricant with the addition of metal oxide such as magnesium oxide (MgO) into 3 mol% yttria stabilized tetragonal zirconia polycrystals (3Y-TZP) ceramic matrix. With addition of 0.5 wt% MgO in CaF2/3Y-TZP ceramic composite, the mean coefficient of friction (μ) was down to ∼42% and the subsequent wear rate (k) was reduced to ∼31% as compared to pure 3Y-TZP due to the presence of smooth tribo layer at the contacting interfaces. The tribological tests were performed against a hard-abrasive silicon carbide (SiC) counter surface under dry sliding condition. The wear mechanisms and elemental distributions in the worn surface were studied by field emission scanning electron microscopy (FESEM) coupled with energy dispersive X-ray (EDS) setup. X-ray diffraction (XRD) analysis was carried out to identify different phases present in test specimens as well as in wear track. The presence of chemical states in the tribo layer was investigated by means of X-ray photoelectron spectroscopy (XPS) analysis.

Atomistic Insights Into Lubricated Tungsten/Diamond Sliding Contacts

The reinforcement of coatings with diamond particles results in superior tribological performance for automotive applications. In addition to improving the coating’s bulk properties, sliding of diamond on metallic counter bodies contributes to improved tribological performance. Therefore, in order to design better diamond reinforced coatings, it is imperative to understand the atomistic mechanisms at sliding metal/diamond interfaces. Here, we investigate the interfacial tribo-chemical mechanisms leading to low friction in lubricated tungsten/diamond sliding contacts by combining reactive atomistic simulations with on-line tribometry experiments linked to chemical analysis. Reactive classical molecular dynamics simulations reveal the dehydrogenation of hexadecane lubricant molecules between tungsten/diamond contacts by proton transfer from the hexadecane to octahedral sites of the tungsten surface. Subsequent chemisorption of the radicalized hexadecane on dangling C-bond sites of the diamond surface leads to the formation of low-density hydrocarbon films, which significantly lower frictional resistance in the tribo-contact. Quasi-static density functional theory calculations confirm the classical molecular dynamics results and reveal that radicalized hydrocarbon molecules can also bond via C-O bonds on a WO3 layer covering the tungsten counter surface. The on-line tribometry experiments confirm the reduction of friction under hexadecane lubrication and ex situ chemical analysis by means of XPS, AES and EELS provide evidence of the formation of a carbon-rich tribofilm on the diamond and tungsten-oxide surfaces as predicted by the atomistic simulations.

A Comparison of Friction Measurements of Intact Articular Cartilage in Contact with Cartilage, Glass, and Metal

The goal of this study was to develop a method of friction testing utilizing cartilage counter surfaces with a complete subchondral bone plate and compare the results to the cartilage on glass and metal (steel) counter surfaces. Articular cartilage surfaces with the underlying subchondral bone intact were not isolated through plug removal. Friction testing was completed using a tribometer (n=16). The coefficient of friction (COF) was measured between the proximal articular surfaces of the second carpal bone when brought into contact with the articular surface of the distal radial facet. The COF of the distal radial facet was obtained with glass and metal counter surfaces. Cartilage-cartilage interfaces yielded the lowest COF when a normal force of 5N and 10N was applied. No statistically significant increase in COF was noted for any combination when an increased normal force was applied (10N), although an increase was observed when glass and metal was in contact with cartilage. COF significantly increased when comparing the cartilage counter surface to metal under an applied load of 5N (p=0.0002). When a 10N load was applied, a significant increase in the COF was observed when comparing the cartilage counter surface to both the glass and metal counter surfaces (p=0.0123 and p < 0.0001 respectively). Results have shown that the described methodology was accurate, repeatable, and emulates physiologic conditions when determining the friction coefficient. The determined COF of cartilage against cartilage is significantly lower than cartilage against metal or glass.

The Formation of a Modified Surface Layer on Elastomeric Materials

Surface modification of an elastomer may be formed during sliding contact with a rigid counter surface. This alteration leads to a change of mechanical properties at the surface and as a result a change in frictional behavior. Therefore, investigations related to the formation of a modified surface layer on elastomers and its effect on friction are of importance. In the present study, the formation of a modified surface layer on elastomer reinforced by silica is studied. Sliding friction is performed using a pin-on-disc tribometer. Several parameters are varied, namely contact pressure, velocity, and roughness of the counter surface. The existence of a modified surface layer is investigated by using a scanning electron microscope. The results show that the existence of a modified surface layer depends on the competition between the formation rate of the layer and the wear rate. The formation of the layer depends on the contact pressure, velocity, and sliding distance. A general formulation to calculate the volume of formation is proposed. Furthermore, a map of the formation of a modified surface layer is developed.

Friction in the contact between skin and a soft counter material: Effects of hardness and surface finish

The interaction behaviour of skin with a counter surface depends strongly on the surface roughness of the counter surface. For relatively hard surfaces this effect is described in various literature, but for soft, or compliant, materials this is much less studied. Inside the contact, the protuberances on the surface will deform substantially. In order to gain insights into the effect of surface roughness and hardness on the frictional behaviour between skin and a soft counter surface a range of experiments were performed using artificial skin and various silicone compounds which are commonly used in medical devices that interact with the human skin. Using these results, a ‘friction map’ was created that shows the friction behaviour as a function of the elastic modulus and the surface roughness. When the surface roughness is increased the friction coefficient decreases due to the reduction in the real area of contact, which weakens the adhesion between the two surfaces. A minimum coefficient of friction was observed at a surface roughness of approximately 4 µm. For the softest compounds tested there was minimal effect of surface roughness on friction because the roughness protuberances inside the contact will be flattened. Silicone compounds with increased hardness showed a larger sensitivity of the friction to the surface roughness, because these harder surface roughness protuberances are more resistant against deformation. The friction map provides a tool when designing products that require certain frictional properties: for products that are required to adhere to skin a smooth and soft material is recommended, whereas for products that require a low coefficient of friction a harder compound with a surface roughness of approximately 4 µm is recommended.

Embeddability behaviour of some Pb-free engine bearing materials in the presence of abrasive particles in engine oil

ABSTRACTOne 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.