Hard Asperities Research Articles

Effect of graphene content on the tribological and corrosion behavior of high-speed-laser-clad high-entropy-alloy composite coatings

To repair oil drill pipe joints that have failed owing to dry wear and tribocorrosion, graphene-reinforced CoCrFeMo0.5NiTi0.5 high-entropy alloy composite coatings (HEACCs) were developed through high-speed laser cladding. The HEACC containing 1.5 wt% graphene exhibited dry wear and tribocorrosion rates that were 59.18 % and 32.20 % of those observed in the high-entropy alloy coating, respectively. The HEACC exhibited a corrosion current density of 2.475 × 10−7 A/cm2. The HEACC containing 1.5 wt% graphene underwent progressive damage during dry wear owing to the combined effects of abrasive wear, which created furrows; adhesive wear, which led to flaking; and oxidative wear, which formed oxide layers. During tribocorrosion, chloride ions exacerbated surface damage caused by hard abrasives and asperities, intensifying corrosive–abrasive wear interactions.

Braking Performance of Fe-Based and Cu-Based Powder Metallurgy Pads Paired with C/C-SiC Disk on Full-Scale Dynamometer

In this study, the friction performance of copper-based (Cu-PM) and iron-based (Fe-PM) powder metallurgy pads was systematically compared and analyzed by performing a braking test paired with a C/C-SiC disk on a full-scale flywheel brake dynamometer, including coefficient of friction (COF), wear rate (Wr), and worn surface under dry and wet tests. The test procedure simulated the braking conditions on high-speed trains with initial brake speeds (IBS) from 50 km/h to 200 km/h, as well as under various brake clamping forces (FB). During dry tests, both pads exhibited high levels of COF. The mean value of average COF (µm) for the Cu-PM pad was 0.371, which was slightly higher than that of 0.358 from the Fe-PM pad. The stability of µm over the IBS ranged from 68% to 82% for Cu-PM pads under different FB, while it was from 82% to 85% for Fe-PM pads. However, during the wet tests, the mean value of µm dropped to 0.207 for Cu-PM pads, and with the minimum µm of 0.062; whereas, they were 0.344 and 0.188 for Fe-PM pads, respectively, showing a significant difference in the friction performance between the two pads. Additionally, the Wr of the Cu-PM pad was 0.110 cm3/MJ during dry tests, which was 17.3% lower than that of 0.133 cm3/MJ observed for the Fe-PM pad. After the brake test with the Cu-PM pad, the friction film primarily covered the Si/SiC-rich region of the disk, effectively inhibiting the plowing of SiC hard asperities onto the pad surface. However, the friction film predominantly covered the fiber-rich region of the disk after the test with the Fe-PM pad, and the exposure of hard Si/SiC-rich asperities would continuously plow the pad during braking and resulted in furrows on the surface of the Fe-PM pad. In terms of wear mechanisms, adhesion wear was the primary mechanism on the contact surfaces of the Cu-PM pad and the C/C-SiC disk, while abrasion dominated the interaction between the Fe-PM pad and the C/C-SiC disk. This study provides valuable insights for the application of C/C-SiC brake disks on trains.

Tribological performance of the aluminium based syntactic foams – Influence of sliding distance

In the paper presented, the Syntactic foams were made by the process of stir casting and were analysed for their performance in tribology. Study was done at microscopic level to analyse the quality of the samples prepared and the results produced showed that the stir casting process helps in producing rational dispersal. The distance of sliding varied from ranges between 500 m and 2000 m at a constant load of 25 N and sliding speed of 640 RPM. The results indicated that the rate of wear was found slightly higher after some distance when it was compared with the results in the initial stages. The reason behind this might include hard and sharp surface asperities. When the asperities in the starting stages wore off, the subsequent wearing required some significant amount of energy. The friction coefficient remained same even after the sliding distance was growing. In order to observe the worn surfaces to define wear mechanisms, the scanning electron micrographs were studied.

Abrasive Wear Behaviour of Organophosphate

Wear is very important phenomena which play a vital role while designing any component. It directly affects the life of the component. Abrasive wear phenomena are simply caused by the passage of relatively hard particles or asperities over a surface. Abrasive wear simply includes division of two surface parts by abrasion of other mating surface that is situated between the friction areas. A systematic study of abrasive wear of Silicon has been carried out using a two body pin-on-disc wear machine. The objective of this study is to evaluate the abrasive wear of organophosphate by grinding it at different load at different rpm. In this thesis a study of abrasive wear of organophosphate at different speed were given to analyze the possibility of wear. A review of abrasive wear behavior of organophosphate using different wear rate speed has been discussed in this thesis. Mainly focus on the varying wear condition during varying rpm. as we all know that wear is very important parameter which directly affects the life of a component. The goal of this study was to evaluate abrasive wear only. Key Words: Abrasive wear, Behaviour of organophosphate, Wear rate

Evolution of morphology, microstructure and hardness of bodies and debris during sliding wear of carbon steels in a closed tribosystem

Debris generated during sliding wear of steels have an important role on tribosystem's behavior. However, the evolution of morphology, microstructure and hardness of debris and solid bodies, and their role on friction and wear is not quite evident, especially for closed tribosystems. Interrupted pin-on-disk wear tests were carried out on low, medium, and high carbon steel tribopairs, while an adaptation of the test apparatus effectively prevented debris ejection from wear track, simulating a closed tribosystem. The experiment was designed according to a repeated measures ANOVA model for multiple dependent variables that includes running-in duration, wear rates, surface hardness and their ratios, and debris size and shape. Plasticity-dominated severe wear took place in all cases, generating two types of metallic debris: flake-type via adhesion and delamination processes and, after sufficient sliding distance, cutting-type due severe shear deformation caused by the abrasive action of hard asperities. Debris were, at least, 20% softer than worn pins and disks, and provided some lubricating effect when adhered, sheared and rolled between sliding surfaces. Low carbon steel wear generated more soft and adherent debris, resulting in prolonged running-in periods. Finally, steel carbon content did not affect wear mechanisms, debris morphology (size and shape), worn surface hardness and wear rates in closed tribosytems, when a debris-flow controlled process prevails.

Enhancing the wear performance of Ti-6Al-4V against Al2O3 and WC-6Co via TiBn layer produced by boriding

Abstract In this study, mechanical and tribological properties of the borided dual-phase α + β type Ti6Al4V titanium alloy were examined. For this purpose, Ti6Al4V alloy samples were borided for 6 h at a temperature of 1100 °C by the powder-pack boriding process. As a result of boriding, a boride layer consisting of TiB2 with a thickness of max ∼25 µm and TiB phases with a thickness of max ∼10 µm was obtained on the Ti6Al4V sample surfaces. As a result of the boride layer’s nanoindentation tests carried out using the Berkovich indenter, it was found to have an elastic modulus of 534.255 GPa and a hardness of 36.537 GPa. Wear tests were carried out using the pin-on-disc method under a load of 10 N and with a sliding distance of 1000 m. Whereas the dominant type of wear in non-borided samples was abrasive wear, oxidative mild wear was generally observed in borided samples. In borided samples, as a result of becoming of surface smoother by hard asperities breaking and increasing the actual contact area, the friction coefficients increased. It was determined that with boriding, the wear performance of Ti6Al4V alloy improved ∼46.8 times against the Al2O3 counterpart and ∼4.57 times against WC-6Co counterpart.

Coupled evolution of piston asperity and cylinder bore contour of piston/cylinder pair in axial piston pump

The wear condition of the piston/cylinder pair is crucial to the performance and reliability of the axial piston pump. The hard piston surface, the soft cylinder bore surface, and the interface oil film affects each other during the wear process. Specifically, in the mixed lubrication region, the geometry of the hard piston surface asperity directly affects the wear of soft cylinder bore surface, while the asperities may deform or even degrade when penetrating and sliding against the cylinder bore. So far, there is no suitable method to simulate their coupled evolution. This paper proposed a wear process simulation model considering the real-time interaction between the elasto-plastic deformation of the piston surface asperity, the wear contour of the cylinder bore, and the lubrication condition of the interface. An offline library of the elasto-plastic constitutive behavior of the asperity based on the finite element method (FEM) is established as a part of the simulation model to precisely analyze the deformation and degradation of the asperity and quickly invoke them in the numerical wear process simulation. The simulation and experimental results show that the piston asperity and the cylinder bore contour converge to a steady state after running-in for about 0.5 h. The distribution of the simulated asperity degradation and wear depth is also verified by the experiment.

Mechanical Abrasion by Bi-layered Pad Micro-Asperity in Chemical Mechanical Polishing

Pad asperities in chemical-mechanical polishing (CMP) provide necessary forces for mechanical abrasion. This article investigates the abrasive behaviour of polishing pads at the asperity contact scale. A contact mechanics model predicts that compliant and soft asperities or rigid and hard asperities may solely achieve either large contact area or high indentation depth respectively, whereas bi-layered asperities can enable both the enlarged contact and deep abrasion. Hemispherical pad micro-asperities with precise dimensions, including the new bi-layered design, were fabricated using thermal reflow and micro-replica molding techniques and their polishing behaviours were experimentally compared using a pin-on-disk polishing setup.

TiC–NiCr thermal spray coatings as an alternative to WC-CoCr and Cr3C2–NiCr

TiC-based hardmetal coatings containing 25 or 40 vol% Ni-20 wt%Cr matrix (hereafter TiC–25NiCr and TiC–40NiCr) were obtained by High Velocity Oxygen-Fuel (HVOF) and High Velocity Air-Fuel (HVAF) spraying, starting from high-energy ball milled feedstock powders. These coatings are intended as critical raw materials-free solutions against wear and corrosion.HVOF-sprayed coatings contain some more oxide inclusions than do HVAF ones, but, irrespective of the deposition conditions, TiC–40NiCr coatings are usually somewhat harder (800–900 HV0.3) than TiC–25NiCr ones. They also exhibit lower wear rates in ball-on-disc sliding tests against Al2O3 at room temperature. A hard asperity can indeed penetrate slightly deeper into TiC–25NiCr, as it deforms inelastically through microcracking. Bigger abrasive grooves are thus produced.The wear resistance of TiC–40NiCr coatings compares favourably to that of a Cr3C2-25% (NiCr) reference, and even approaches that of WC-10 wt%Co-4wt.%Cr. TiC–40NiCr coatings are also more corrosion resistant than both reference materials when tested by electrochemical polarization in a 3.5% NaCl solution.At 400 °C, to the contrary, TiC–25NiCr coatings exhibit better sliding wear resistance, whilst more severe abrasive grooving and adhesive tearing affect TiC–40NiCr samples. TiC–NiCr coatings are also unaffected by the transverse macro-cracking that was found to compromise the usefulness of WC-CoCr at 400 °C.

Influence of hardmetal feedstock powder on the sliding wear and impact resistance of High Velocity Air-Fuel (HVAF) sprayed coatings

The present work aimed to clarify how the characteristics of WC-CoCr hardmetal feedstock powders, namely the grain size of the WC carbides and of the binder and the compressive strength of the sintered aggregates, affect the dry sliding wear and impact resistance of coatings deposited by High Velocity Air-Fuel (HVAF) spraying.Ball-on-Disc tests, which mimic a sliding wear process in the presence of hard asperities as it may occur e.g. in hydraulic seal joints or papermaking components, resulted in mild wear through near-surface microscale plastic flow, the exact nature of which was significantly affected by WC size. Finite element simulations of a single-asperity sliding process indeed showed that large WC grains concentrate contact stresses, thus undergoing very localised deformation. It is experimentally seen that repeated deformation of the carbide grains resulted in their cracking and pull-out. Uniformly distributed, fine carbides allowed the matrix to take on some stress, thus undergoing more homogeneous plastic flow. Block-on-Ring tests elicited adhesive wear as it may happen e.g. in metal-to-metal contacts (e.g. petrochemical valves). This could be effectively restrained by low matrix mean free path. Cyclic impact resistance of coarse-grained coatings was better than that of fine-grained ones, because of better large-scale cohesive strength.

Nanostructured CVD Tungsten Carbide Coating on Aircraft Actuators and Gearbox Shafts Reduces Oil Leakage and Improves Durability

Wear and corrosion of aircraft actuator parts, helicopter gearbox rotating shafts and wear of seals working against these parts can lead to oil leaks which require expensive maintenance and increase the risk of failure. The Hardide® nanostructured chemical vapor deposition (CVD) Tungsten Carbide coating was tested on metal parts working against seals as an approach to reduce oil leakage from gearboxes and actuators and increase maintenance intervals. The coating protects metal pistons and shafts from abrasion and corrosion, keeping their surface roughness parameters within optimum ranges for longer; this reduces seal wear and makes the whole unit more durable and reliable. The 50-100 micron-thick CVD Hardide coating can be applied uniformly on internal and external surfaces and has enhanced fatigue and anti-galling properties. The coating has high wear resistance outperforming Hard Chrome by 14 times and Thermal Spray WC-Co (12%) by three times. The fine-grain coating nanostructure wears uniformly so even worn Hardide coating shows no hard micro-grain asperities which are abrasive for seals. The coating is free from porosity and from Cobalt binder and is an effective barrier against corrosion. As a result the coating keeps the optimal seal-friendly surface finish for longer even in abrasive and corrosive environments. The coating was qualified by Airbus as an environmentally friendly replacement for Hard Chrome plating.

An Analytical Model of Mechanistic Wear of Polymers

This paper proposes a wear model for polymers based on so-called mechanistic processes comprising both low cycle fatigue and abrasive wear mechanisms, which are prominent in polymer–metal sliding interfaces. Repeated elastic contact causes localized fatigue, whereas abrasive part is an anticipatory outcome of plastic contacts by hard metal asperities on to soft polymer surface. Further, presuming adhesive interactions in elastic–plastic contacts, asperity contact theories with necessary modifications were analyzed to assess load and separation for their subsequent use in elementary wear correlations. Both Gaussian and Weibull distributions of asperity heights were considered to include statistics of surface microgeometry. Finally, volumetric wear was written in terms of roughness parameters, material properties, and sliding distance. Validation was conducted extensively, and reliability of the formulation was achieved to a large extent. Experimental part of this work included several pin-on-disk tests using polyether ether ketone (PEEK) pins and 316L stainless steel disks. Disks with different roughness characteristics generated by polishing, turning, and milling were tested. Experimental results agreed well with predictions for the polished surface and with some deviations for other two surfaces. Further, fatigue to abrasive wear ratio was identified as an analytical tool to predict prevailing wear mechanism for polymer-metal tribo-systems. After examining the considered cases, it was both interesting and physically intuitive to observe a complete changeover in wear mechanisms following simply an alteration of roughness characteristics.

Investigation of micro-abrasion characteristics of thin metallic coatings by in-situ SEM scratch test

The aim of this study was to elucidate the micro-abrasion characteristics of thin metallic coatings through in-situ visualization of the scratch formation process. Pure silver (Ag), pure copper (Cu), and Ag/Cu composite coatings with thicknesses of a few hundred nanometers were deposited on silicon substrates using a magnetron sputtering system. A custom-built pin-on-reciprocator-type tribotester installed inside of a scanning electron microscope (SEM) was utilized to perform the experiments. A single crystal diamond tip was used as the tool in order to represent a hard single asperity. Micro-abrasion mechanisms of the three different coatings were compared by analyzing the friction and scratch characteristics. The friction force measured during the scratch test was directly correlated with the micro-abrasion mechanism. It was found that different chemical and mechanical properties of the coatings led to the formation of different amounts of debris and burrs. Depending on the type of coating, different micro-abrasion mechanisms could be identified.

Nanoindentation Response of Scratched Polymeric Surfaces

The effects of imposed strains on the polymeric surfaces during scratching on the material deformation below the visible surface have not been reported in the literature. The major concern for the polymeric surfaces is the problems related to the effective sectioning for imaging/scanning unlike metal and ceramics. This article describes an experimental qualitative methodology, based on nanoindentation data, to analyze subsurface deformations of polymers resulting from scratch deformations. Poly(styrene), a brittle polymer, poly(methylmethacrylate), a ductile polymer, and poly(etheretherketone), a semicrystalline polymer, were selected for the present study. Nanoindentation responses of the scratched poly(styrene), the scratched poly(methylmethacrylate), and the scratched poly(etheretherketone) surfaces were analyzed with emphasis on the detection of subsurface crazing damage. The polymers were scratched using a 900 conical indenter on a pendulum sclerometer. The scratched polymeric surfaces were assessed using scanning electron microscopy (SEM). The polymeric surfaces were observed to be deformed by a well-known ductile ploughing mechanism. The deformed polymeric surfaces were indented using an MTS Nanoindenter. The data show that the hard asperity scratching initiates subsurface damage, which may tentatively lead to the development of subsurface voidage or crazing in certain areas of the deformed polymers, particularly within the base of the scratch groove. Major conclusions of the work are that the nanoindentation of damaged polymeric surfaces provides a qualitative methodology to estimate the subsurface damage and craze formation. This methodology is important in the context of polymers where conventional effective sectioning of the damaged surface to analyze the subsurface deformations might not be possible.

An in vitro study of the wear mechanism of a leucite glass dental ceramic

Ceramics and glass-ceramics are materials of choice for dental crowns due to their attractive hardness, biocompatibility, etc. However, a major problem with their usage is the observed high wear of either the opposing dental enamel or both the enamel and ceramic itself. Although these issues have been known for some time, the responsible wear mechanism(s) are not clearly identified. Accordingly, an in vitro investigation was undertaken to identify the mechanism(s) responsible for the wear of a popular leucite glass-ceramic (LGC) dental prosthesis material when opposing both itself and human dental enamel. Reciprocating sliding wear tests were carried out with LGC or enamel cusp sliding on LGC flat-surface samples with varying surface roughness. The wear loss was quantified using profilometry and the wear scar surface and subsurface were analysed using electron microscopy techniques to reveal the underlying wear mechanism. The present results revealed that the dominant wear mechanism for LGC, when opposed by LGC or enamel, was micro-abrasion due to lateral crack formation/extension. Leucite crystals in the ceramic׳s microstructure appear to act as hard asperities and their loading/unloading during sliding contact caused the formation and extension of lateral cracks and, consequently, wear of the LGC dental material. For human enamel, the dominant wear mechanism was delamination.

Friction and Wear of Ti<sub>3</sub>SiC<sub>2</sub>-Ag/Inconel 718 Tribo-Pair under a Hemisphere-on-Disk Contact

Room temperature friction and wear of Ti3SiC2-Ag sliding against Inconel 718 with a hemisphere-on-disc configuration were investigated in air. The effects of Ag content and TiAlN coating on Inconel 718 substrate were also included. Ti3SiC2/Inconel 718 tribo-pair showed high friction coefficient (0.6) and severe wear due to pullout of Ti3SiC2 grains was observed at a sliding speed of 1 m/s. Ti3SiC2-Ag composites had better tribological behavior than that of monolithic Ti3SiC2 in sliding against Inconel 718. At a sliding speed of 0.01 m/s, Ti3SiC2-Ag/Inconel 718 tribo-pairs exhibited moderate friction coefficient (0.32-0.4). At a sliding speed of 1 m/s, severe wear was not observed for Ti3SiC2-15vol.%Ag and Ti3SiC2-20vol.%Ag composites although the tribo-layer was not rich in Ag. When Ti3SiC2-Ag composites mated with TiAlN coating on Inconel 718 substrate, moderate friction coefficient (0.29-0.36) and low wear rate (10-6 mm3N-1m-1) were obtained at 0.01 m/s. A transition from mild wear to severe wear of Ti3SiC2-Ag composites at 0.1 m/s can be attributed to the ploughing effect by hard asperities on TiAlN coating.

Evaluation of nanotribological behavior of amorphous polystyrene: The macromolecular weight effect

Tribological testing of polymers is of prime importance in many industrial applications. Silicon nitride AFM tips have been used to mimic the contact between amorphous polystyrene surfaces and a hard asperity, which is useful in understanding of how a multitude of asperities behave in a macroscopic contact. In this study, the adhesion force and the friction force of four PS molecular weights were measured and the average contact pressure was calculated by using the JKR contact theory. The nanotribological behavior of polystyrene showed a dependence on macromolecular weight with varying applied normal force and sliding velocity. The study indicates that the length of polymer chains noticeably influences the tribological behavior of amorphous polystyrenes. Mechanisms governing such behavior differences were ascribed to energy dissipating modes.

Micro-scratching of polyethylene terephthalate: Mechanisms of wear debris generation

The functional use of polymers is often limited by polymer wear debris particles. This motivates research on the microscopic origin of such debris. The effects of individual hard asperities on the wear of a polymer surface can be tested in micro-scratching experiments. Based on micro-scratching with conical tips, “wall” formation and low-cycle fatigue wear at scratch intersections have been suggested to constitute a major source of debris particles. This proposition and other mechanisms of wear debris generation have been investigated in the present work on polyethylene terephthalate (PET). PET is a ductile polymer for which significant debris generation due to low-cycle fatigue may be expected. A micro-scratcher with a silicon cubic corner tip and different attack geometries were used. A variety of continuous scratching track patterns and repeated cycling of these patterns with up to 40 passes were explored to investigate effects at both, scratch intersections and end-of-track points. The micro-scratch patterns on PET were characterized with scanning electron microscopy and atomic force microscopy. Lumpy corner protuberances, known to occur in PMMA at scratch intersections, have not been observed for PET. While the formation of “walls” at intersections has been confirmed, no wear debris is generated and the “walls” appear to be diminished with increased cycling. Instead, the repeated compression and merger of end-of-track pileups at points of directional change result in long extrusions of PET debris chains. This debris generation mechanism may depend on the geometry of the tip and may be least significant with cylindrically symmetric, conical tips. Under the experimental conditions explored, the debris chain extrusions appear to be the dominant sources of wear debris particles. A sideward component of motion in the directional change at an end-of-track point favors the severing of an extrusion. The severing results in detached debris clusters that can indeed be observed for PET in the vicinity of end-of-track points. The number of merged PET particles in a debris cluster tends to be equal to the number of scratch track passes before the detachment of the extrusion.

Stability criteria for a pyramidal shaped asperity ploughing through a plastically deforming substrate

In two body abrasion processes hard asperities plough through a soft surface. If the asperities can resist the forces that act on it, scratches will develop in the soft material. If the asperities cannot withstand these forces, they will break off and not cause direct abrasion damage. The same is the case for galling, where lumps develop on one of the surfaces because of material transfer. These lumps will abrade the counter surface, if the lumps are strong enough to withstand the forces that act on it. In order to describe these phenomena, simple criteria are desired to describe the mechanical stability of asperities and lumps.In this work, an analytical model is presented for the mechanical stability of asperities. In the analysis, a pyramidal asperity shape will be assumed. Given the pyramidal asperity shape, several cases will be studied: the load is carried by a pyramid with a triangular base, a pyramid with a triangular base and an extended backside and the case where a crack has developed. Based on these models stability criteria of ploughing pyramidal asperities will be developed. Important results of the model will be discussed in the context of abrasion and adhesive wear processes.

Molecular dynamics simulation of severe adhesive wear on a rough aluminum substrate

Severe adhesive wear on a rough aluminum (Al) substrate is simulated by a hard Lennard-Jones asperity impacting an Al-asperity at high speeds using molecular dynamics (MD). Multiple simulations investigate the effects of variations in the inter-asperity bonding, the geometric overlap between two asperities, the relative impact velocity and the starting temperature. The effect of these experimental variables on degree of adhesive wear and the temperature profiles are discussed, and a design of experiments method is used to help interpret the results. The results indicate that increasing the inter-asperity bonding, the geometric overlap and the starting temperature of two asperities will substantially increase the wear rate, while raising the impact velocity slightly decreases the wear rate. It is observed that the deformation mechanism involves local melting and the formation of a liquid like layer in the contact area between two asperities, and the amorphous deformation of the Al-asperity.