Wet Contact Conditions Research Articles

DEM simulation of single sand grain crushing in sanded wheel–rail contacts

In railways, low adhesion conditions lead to problems in braking and traction. Sanding increases adhesion by blasting sand grains towards the wheel–rail contact. Which physical mechanisms increase adhesion during sanding is poorly understood, as research in this field has been almost exclusively experimental. This work is the first step in developing a DEM model describing the sanding process. Previous experimental studies involve single grain crushing tests focusing on initial breakage and repeated breakage under realistic wheel–rail contact pressures of 900 MPa. Tests on two types of rail sand under dry and wet contact conditions, showed different spread of fragments as well as the formation of clusters of solidified fragments at high stresses. The developed DEM model captures the observed behaviour for both types of sand, and both contact conditions. Combining experimental research and DEM modelling aims to improve the understanding of adhesion increasing mechanisms in sanded wheel–rail contacts.

Sanded Wheel–Rail Contacts: Experiments on Sand Crushing Behaviour

In railway operation, the sanding process is used to overcome low adhesion conditions in the wheel–rail contact. In the literature, previously conducted research has been experimental, e.g., measuring adhesion coefficients (ACs) under different contact conditions (dry, wet, …) or applying different sands. Under dry conditions, sanding can reduce measured ACs, while under wet conditions different types of rail sand can leave ACs unchanged or increase adhesion. Despite active research, the physical mechanisms causing the change in ACs under sanded conditions are still poorly understood. A possible remedy is the development of advanced models of sanding including local effects. As a basis for such a model, this study presents experimental results concerning single grain crushing behaviour of two types of rail sand under dry and wet contact conditions. Firstly, initial breakage behaviour is investigated with focus on the particle fragments’ size and spread as only fragments within the running band are available to influence the AC during roll-over. Secondly, single grain crushing tests are conducted under realistic wheel–rail load showing the formation of solidified clusters of sand fragments, as well as their size and thickness. This information is important for understanding mechanisms and for future physics-based modelling of the sanding process in wheel–rail contacts.

Friction and Wear Pattern of Silica-Reinforced Styrene-Butadiene Rubber (SBR) in Sliding Contact with a Blade Indenter

This study investigated the friction and wear pattern of silica-reinforced Styrene-Butadiene Rubber (SBR) in sliding friction with a steel blade indenter. The experiments were conducted using a pin-on-disc tribometer at various applied loads and examined under dry and wet contact conditions. Analysis was focused on investigating the coefficient of friction and length of wear pattern spacing. Related to coefficient of friction identification, the abrasion theory was applied here. In addition, the stick-slip theory to identify the wear pattern spacing was also applied. Results of the experiments show that the overall coefficient of friction (COF) decreases along with the increasing applied loads. The COF in wet conditions is much lower at the beginning of sliding time than the COF in dry conditions. The wear pattern spacing increases with increasing loads. However, it seems that there is no significant difference in pattern spacing between the dry and wet contact condition. In general, the experimental results agree qualitatively with the analytical results.

Developing Novel Poly(Lactic-Co-Glycolic Acid) (PLGA) Films with Enhanced Adhesion Capacity by Biomimetic Mushroom-Shaped Microstructures

The adhesion strength of biomimetic mushroom-shaped microstructures made of a biocompatible and biodegradable copolymer, PLGA, is experimentally studied. The adhesive microstructures were successfully produced using a two-step molding technique. The adhesive experiments were performed on a flat and smooth glass substrate using different normal preloads and under both dry (in air) and wet (underwater) contact conditions. Regardless of contact conditions, mushroom-shaped microstructures generate much higher adhesion strength compared to a flat smooth reference sample made of the same PLGA material. Furthermore, PLGA mushroom-shaped microstructures exhibited superior adhesion strength using agar substrate as a representative material for biological tissue. Surface texturing indicates a good approach to modify adhesive properties PLGA without additional excipients. The effect of surface texturing is more pronounced under wet contact condition. The concept of using biomimetic adhesive surfaces made of biocompatible and biodegradable polymers is very promising for potential applications in drug delivery, tissue engineering and biomedical field.

Tribological Behavior of Micro Coir Filler Reinforced Polymer Composite under Dry, Wet, and Heated Contact Condition

ABSTRACT This article speculated the wear and frictional performance of treated micro coir filler reinforced polymer composite. The surface of the coir filler is modified by the alkali treatment and the composite is prepared with a variation of coir filler reinforcement. The investigation of wear test is steered under dry sliding contact, wet and heated contact state using the pin-on-disk multi-tribometer counter to a stainless steel disc. Experiments 1were conducted considering three different parameters, namely, different normal load (5–40 N), sliding velocity 100 cm/s, and sliding distance (0–2000 m). The results showed that the wear behavior of coir filler-reinforced composite is highly dependent on operating parameters and filler reinforcement. The specific wear rate is lesser in wet contact conditions compared to dry and heated contact conditions. Moreover, lower coir filler loading presented enhanced wear presentation related to the higher filler loading composite. From microscopic images, some micro and macro cracks, debonding, and plastic deformation are witnessed on the worn surfaces of the composite.

Effect of Intermetallic and Secondary Phases on Dry and Wet Sliding Wear Behavior of Super Duplex Stainless Steel

The purpose of this investigation was to quantify the effects of intermetallic and secondary phases on the sliding wear behavior of super duplex stainless steel 2507 against bearing steel type EN-31 under both dry and wet contact conditions. Phases were precipitated by suitable heat treatment in the range of 475–875 °C. A pin-on-disc technique was used to investigate the wear characteristics in the wet environment of 3.5% NaCl solution, 1 M HCl solution, 10−2 M sodium thiosulfate solution, and artificial ocean water. These media were chosen to closely match the actual service environment of super duplex stainless steel 2507. Pins were slid against a disc of EN-31 steel for distances of 1,000, 1,500, 2,000, 2,500, and 3,000 m. Results indicate that precipitation of the sigma phase gives the best wear resistivity, followed by precipitation of chi and R phases and then precipitation of the α′ phase. The experimental investigations reveal that hydrogen in acidic medium accelerates the reaction kinetic compared to the neutral medium because of the different cathodic reactions. Progressive wear was closely observed using the change in the sliding pin’s contact face profile and corresponding weight loss data. Scanning electron microscopy was used to study the wear mechanism.

Rolling Contact Fatigue and Wear Behavior of High-Performance Railway Wheel Steels Under Various Rolling-Sliding Contact Conditions

An experimental investigation was carried out to study and compare the response to cyclic loading of the high-performance railway wheel steels ER8 EN13262 and SUPERLOS®. Rolling contact tests were performed with the same contact pressure, rolling speed and sliding/rolling ratio, varying the lubrication regime to simulate different climatic conditions. The samples, machined out of wheel rims at two depths within the reprofiling layer, were coupled with UIC 900A rail steel samples. The wear rates, friction coefficients and hardness were correlated with the deformation beneath the contact surface. The crack morphology was studied, and the damage mechanisms were identified. The distribution of crack length and depth at the end of the dry tests was analyzed to quantify the damage. The main difference between the steels lies in the response of the external samples to dry contact: SUPERLOS® is subjected to a higher wear and lower friction coefficient than ER8, and this reduces the density of surface cracks that can propagate under wet contact conditions. The analysis of feedback data from in-service wheels confirmed the experimental results.

Influence of wet contact conditions on the multidirectional fretting behavior of Polyetheretherketone and composites

Polyetheretherketone (PEEK) polymers are increasingly used in tribological applications where fretting damage occurs. Fretting wear results from the repeated sticking and slipping of mutually loaded contacts. In this study, a custom built multi-axis tribometer is used to replicate fretting of filled and unfilled PEEK in a pin-on-flat configuration in both dry and wet conditions. The performance of the PEEK based polymers is shown to depend on the shear stresses generated when adhesive contacts are broken during slip. Unfilled PEEK experiences frictional heating that leads to softening of the wear surface. The softened material can be easily sheared from the bulk and leads to cratering and fracture of the samples. The extent of damage depended strongly on MW. The buildup of heat is prevented when water lubrication is used and wear is also reduced. Lubricating fillers such as PTFE and graphite lower the frictional forces and reduce the tractive shear stresses. Filled samples however were susceptible to softening from water that exacerbated fretting damage. The fretting performance appears strongly tied to the static friction response on the material. Static friction measurement along with FEA analysis was conducted to better understand the observed phenomena. A stop-go experiment was used to capture the friction response due to the shearing of adhesive contacts.

Direct nanomaterial-DNA contact effects on DNA and mutation induction

The toxicity of nanomaterials has been well known, but mechanisms involved have been little known. This study was aimed at looking at direct interaction between nanomaterials and naked DNA for some fundamental understanding. Two different types of nanomaterials, carbon nanotubes (CNTs) and tungsten trioxide (WO3) nanoplates, were simply mixed with naked DNA plasmid, respectively, in two different contact modes, dry or wet (in solution), for varied time periods. DNA topological forms were analyzed for changes using gel electrophoresis and fluoro-spectrometry. The nanomaterial-contacted DNA was transferred into bacteria Escherichia coli (E. coli) cells for mutation observation. Certain types and degrees of DNA damage were observed, such as single strand break and double strand break, and bacterial mutation was confirmed. The DNA damage increased with the contacting time in an exponential manner and increased more rapidly in the initial stage for the wet contact. The nanomaterials-contacted DNA transferred bacteria had about less than 10% survival but almost 100% mutation for the surviving cells. The CNTs were more offensive than the metal oxide nanomaterials. The mutation spectrum from the DNA sequencing analysis showed that DNA point mutation was dominated by transversion, which was dominated by guanine changes in the wet contact condition while by cytosine changes in the dry contact condition. The point mutation occurrence in the wet contact was more than in the dry contact, confirming the wet contact more active and thus dangerous than dry contact. This experiment, although as a model study, revealed that direct simple contacts between nanomaterials and DNA could cause DNA changes and thus induce mutations which might potentially lead to cancers, diseases and genetic changes. This could be a mechanism for nanomaterial genotoxicity to the cells and also provided a caution to applications in using nanomaterials for DNA delivery.

EPOXY COMPOSITE BASED ON KENAF FIBERS FOR TRIBOLOGICAL APPLICATIONS UNDER WET CONTACT CONDITIONS

In the current work, kenaf fibers reinforced epoxy (KFRE) composite was developed. Adhesive wear and frictional characteristics of KFRE were investigated under wet contact conditions at different fiber orientations, considering different applied loads (50–200 N) and sliding distances (0–30 km). Scanning electron microscopy (SEM) was used to observe the damages on the worn surfaces. The results revealed that fiber orientation plays a main role in determining the wear and frictional performance of the composites. Kenaf fibers, in normal orientation (N-O), enhanced the wear performance of the composite by about 35%–57%. It appears that the damages on the surface of the composite (N-O) were less than the other orientations (P-O and AP-O). Compared to previous work, KFRE has good potential of replacing glass fibers in tribological applications.

Dynamic adhesive forces in rough contacting bodies including normal and sliding conditions

Using a high-resolution, high-dynamic bandwidth capacitive force transducer and two piezoelectric actuators, adhesive pull-off forces between nominally flat rough silicon surfaces were measured under various dynamic conditions in normal and tangential directions and environmental humidity levels. The upper specimen approached and retracted with a constant velocity in the vertical (normal) direction, while the lower specimen started moving in the horizontal (tangential) direction during the middle of the contact. The experiments were performed under 35 and 60% relative humidity conditions. It was found that sliding of the contacting surfaces led to a significant reduction in pull-off forces under low-humidity contact conditions, whereas it caused higher pull-off forces under partially wet contact conditions. Comparing the effects of sliding velocity and sliding distance on the measured pull-off force values, it was found that the sliding distance played an important role in the increase in pull-off forces.

Tribological Characteristics of Sustainable Fiber-Reinforced Thermoplastic Composites under Wet Adhesive Wear

The friction and specific wear rate of sustainable kenaf fiber–reinforced polyurethane composites were investigated against stainless steel counterface and under wet contact conditions. The new composites were evaluated at different applied loads (50–80 N), sliding distances (up to 2.7 km), and fiber mat orientations. Scanning electron microscopy (SEM) was used to observe the damage features on the worn surfaces. The results revealed that sustainable kenaf fibers assisted in enhancing the wear and frictional performance of the polyurethane thermoplastic composite by about 59 and 90%, respectively. Operating parameters and mat orientation controlled the wear and the frictional behavior of the composite. Better wear performance was exhibited at high loads and when the fiber mats were oriented perpendicularly to the sliding direction. Observations of the worn surfaces revealed different features of damage such as microcracks, fiber tearing, fiber detachment, and delamination. However, there was no trace of fiber pull-out in any of the tested conditions.

Investigations on wear and frictional properties of kenaf fibre polyurethane composites under dry and wet contact conditions

In this paper, an investigation was conducted to understand the wear and frictional properties of kenaf fibre reinforced polyurethane composites. The effect of applied loads (30 to 80 N), sliding distance (up to 2.7 km), and fibre orientation [i.e., parallel (P–O), anti–parallel (AP–O) and normal (N–O)] were determined under dry and wet contact sliding. Specific wear rates for all fibre orientations improved under wet contact except parallel orientated composites. Presence of water in wet sliding has significantly reduced the friction coefficient compared to dry sliding besides acting as a cooling agent. A scanning electron microscope (SEM) was used to study the worn surfaces and wear mechanism encountered. The predominant wear mechanism observed under dry sliding was micro–cracks, plastic deformation and thermo–mechanical deformation. Meanwhile, the worn surfaces under wet contact are very minimal, namely micro–ploughing at elevated sliding distances.

Effect of betelnut fibres treatment and contact conditions on adhesive wear and frictional performance of polyester composites

The current work is an attempt to use treated betelnut fibres (in mat form) as reinforcement in polyester composites. The fibres were obtained from Alor Star in the state of Kedah, Malaysia. Wear and frictional characteristics of T-BFRP composite were investigated against polished stainless steel counterface under dry/wet contact conditions using a BOD machine. The tests were conducted at 2.8 m/s sliding velocity, different applied loads (5–200 N) and sliding distances (0–6.72 km). Different orientation of the fibre mats with respect to the sliding direction of the counterface was considered; anti-parallel (AP), parallel (P) and normal (N). The worn surface morphology was studied using a scanning electron microscope (SEM). An optical microscope was used to observe the wear track surface on the counterface, and the modifications on the counterface roughness were studied. This work revealed that the wear and frictional performance of the composite were enhanced under wet contact conditions by about 54% and 95% compared to the dry. Specific wear rate under wet test for AP-O (2.5 ± 0.01 × 10 −8), and P-O (2.3 ± 0.01 × 10 −8), was low compared to the dry test (AP-O: 5.1 ± 0.01 × 10 −8; P-O: 15 ± 0.5 × 10 −8). The composite exhibited high wear performance in AP-O compared to P-O followed by N-O under both dry/wet contact conditions.

Polyester composite based on betelnut fibre for tribological applications

A polyester composite based on betelnut fibres was fabricated and its adhesive wear and frictional performance studied using a block on disk machine at different applied loads and sliding distances at 2.8m/s sliding velocity under dry/wet contact conditions. SEM was used to study worn surface morphology. The results revealed that betelnut fibre reinforced polyester (BFRP) composite had better wear and frictional performance under wet contact condition compared to dry. The wear mechanism of the BFRP composite was predominated by micro and macro-cracks in the polyester regions and debonding of fibres.

Wet adhesive wear characteristics of untreated oil palm fibre-reinforced polyester and treated oil palm fibre-reinforced polyester composites using the pin-on-disc and block-on-ring techniques

This article presents an investigation on the wear and friction characteristics of oil palm fibre-reinforced polyester (OPRP) composites sliding against a polished stainless steel counterface under wet contact conditions. Two different types of OPRP composites were fabricated, which were based on treated and untreated oil palm fibres (treated oil palm fibre-reinforced polyester (T-OPRP) and untreated oil palm fibre-reinforced polyester (UT-OPRP), respectively). The experiments were conducted using two different techniques, pin-on-disc (POD) and block-on-ring (BOR), integrated into the same tribo-machine. The tests were conducted at different rotational speeds (500 and 700 r/min) and 50 N applied load for different durations (10-60 min). The specific wear rate ( Ws) and the friction coefficient were presented as a function of sliding distance. The morphology of the worn surfaces was observed using scanning electron microscopy (SEM) and the damage features were characterized. The results revealed that treating oil palm fibres has a significant effect on the wear and frictional performance of OPRP composites. Treating the oil palm fibres enhanced the wear properties of polyester by about 35-52 and 65-75 per cent in the case of the POD and BOR techniques, respectively. The observations on the worn surfaces showed various features of the damages such as debonding and breakage of fibres in the UT-OPRP composite.

ADHESIVE WEAR AND FRICTIONAL BEHAVIOR OF MULTILAYERED POLYESTER COMPOSITE BASED ON BETELNUT FIBER MATS UNDER WET CONTACT CONDITIONS

In the current study, a multilayered polyester composite based on betelnut fiber mats is fabricated. The adhesive wear and frictional performance of the composite was studied against a smooth stainless steel at different sliding distances (0–6.72 km) and applied loads (20–200 N) at 2.8 m/s sliding velocity. Variations in specific wear rate and friction coefficient were evaluated at two different orientations of fiber mat; namely parallel (P–O) and normal (N–O). Results obtained were presented against sliding distance. The worn surfaces of the composite were studied using an optical microscope. The effect of the composite sliding on the stainless steel counterface roughness was investigated. The results revealed that the wear performance of betelnut fiber reinforced polyester (BFRP) composite under wet contact condition was highly dependent on test parameters and fiber mat orientation. The specific wear rate performance for each orientation showed an inverse relationship to sliding distance. BFRP composite in N–O exhibited better wear performance compared with P–O. However, the friction coefficient in N–O was higher than that in P–O at lower range of applied load. The predominant wear mechanism was debonding of fiber with no pullout or ploughing. Moreover, at higher applied loads, micro- and macrocracking and fracture were observed in the resinous region.

ADHESIVE WEAR PERFORMANCE OF CFRP MULTILAYERED POLYESTER COMPOSITES UNDER DRY/WET CONTACT CONDITIONS

The tribo-performance of a new engineering composite material based on coconut fibers was investigated. In this work, coconut fibers reinforced polyester (CFRP) composites were developed. The tribo-experiments were conducted by using pin-on-disc machine under dry and wet sliding contact condition against smooth stainless steel counterface. Worn surfaces were observed using optical microscope. Friction coefficient and specific wear rate were presented as a function of sliding distance (0–0.6 km) at different sliding velocities (0.1–0.28 m/s). The effect of applied load and sliding velocity was evaluated. The results showed that all test parameters have significant influence on friction and wear characteristics of the composites. Moreover, friction coefficient increased as the normal load and speed increased, the values were about 0.7–0.9 under dry contact condition. Meanwhile, under wet contact condition, there was a great reduction in the friction coefficient, i.e. the values were about 0.1–0.2. Furthermore, the specific wear rates were found to be around 2–4 (10-3) mm3/Nm under dry contact condition and highly reduced under wet condition. In other words, the presence of water as cleaner and polisher assisted to enhance the adhesive wear performance of CFRP by about 10%. The images from optical microscope showed evidence of adhesive wear mode with transition to abrasive wear mode at higher sliding velocities due to third body abrasion. On the other hand, optical images for wet condition showed less adhesive wear and smooth surfaces.

Wear and friction characteristics of CGRP composite under wet contact condition using two different test techniques

This paper presents an investigation on wear and friction characteristics of chopped strand mat (CSM) glass fibres 450 g/m 2 reinforced polyester (CGRP) composite under wet contact condition against polished stainless steel counterface. The experiments were conducted using two different techniques, Pin-on-Disc (POD) and Block-on-Ring (BOR), which were integrated in a newly developed machine that could carry both techniques running simultaneously against identical counterface characteristics. CGRP composite was tested in two different orientations (parallel and anti-parallel), i.e. P-O and AP-O. The effects of test duration (5–30 min), rotational speed (500 and 700 rpm) and applied load (30–100 N) on specific wear rate and friction coefficient were evaluated. Worn surface morphologies of the composite were observed with a scanning electron microscopy (SEM) and the damage features were characterized. The results revealed that CSM glass fibres orientations and test parameters have a significant influence on the wear and frictional characteristics of the composite in both POD and BOR techniques. CGRP in AP-O showed better wear and frictional performance compared to P-O. When the composite was tested using POD, abrasive wear nature was observed on the worn surface in both orientations. Meanwhile, using BOR technique, smoother surface was observed in AP-O. Presence of water assisted to cool the interface and polish the composite surface in the case of BOR. On the other hand, in POD, rough surface was observed on the composite surface in both P-O and AP-O. Additionally, the counterface roughness was increased at all tested condition and highly pronounced in POD when the CGRP was tested in P-O.

Fretting Wear Behavior of Cu–Al Coating on Ti-6Al-4V Substrate under Dry and Wet (Lubricated) Contact Condition

The fretting wear behavior of Cu–Al coating was investigated with and without fatigue load under the dry and wet (lubricated) contact conditions. The Cu–Al coating was plasma deposited on titanium alloy, Ti-6Al-4V. Fretting regime was determined from the shape of fretting hysteresis loop. Fretting regime changed from partial slip to total (gross) slip at ∼15 μm of the applied relative displacement, and this transition point was independent of fatigue loading and contact surface (lubricated versus dry) conditions. Wet contact condition reduced frictional force during cycling, as evidenced by the lower-tangential force. Wear analysis using the accumulated dissipated energy approach did not show any effect of contact surface condition. In other words, the relationship between the accumulated dissipated energy and wear volume showed a linear relationship, and it was independent of loading and contact surface conditions, as well as of the fretting regime. Further, the relationship between the wear depth and accumulated dissipated energy did not show any effect of loading and contact surface conditions, as well as of the fretting regime up to instant when the maximum wear depth was equal to the coating thickness.