Dec. 16, 2024
In the world of engineering and manufacturing, where precision and efficiency are paramount, the role of wear resistance plastic has evolved far beyond their conventional image. With the advent of advanced materials and innovative techniques, plastics have emerged as versatile champions in combating mechanical wear, enhancing durability, and optimizing performance. At the heart of this transformation lies a remarkable property &#; the low coefficient of friction and wear resistance &#; a characteristic that not only inhibits mechanical wear and damage but also serves as a catalyst for a multitude of benefits.
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Industries spanning from automotive to aerospace, from machinery to medical devices, are embracing the transformative potential of low friction plastics. By mitigating wear and tear, these materials play a pivotal role in inhibit mechanical wear and damage, prevent contamination, reduce noise, and minimize maintenance needs.
Industrial processes and production greatly rely on the importance of low-friction plastics, primarily due to their exceptional ability to withstand wear and abrasion. To enhance these qualities even further, the inclusion of high-performance plastic introduces an extra layer of functionality. Let&#;s explore several noteworthy high-performance plastics ideally suited for industrial applications, PEEK, PPS, PI, and PTFE.
PTFE seals for dry runningWear resistance pertains to a material&#;s capacity to withstand gradual surface volume loss caused by mechanical actions like repetitive rubbing, sliding, or scraping.
Materials with wear-resistant qualities diminish friction between interacting surfaces, enabling components to preserve their shape and integrity over extended periods, especially in scenarios involving contact between load-bearing interfaces.
Wear is the direct result of the same processes that causes friction &#; the movement of the asperities on the surfaces over one another.
The principles underpinning wear, friction, and lubrication fall within the domain of tribology &#; the scientific and engineering discipline concerned with studying interactions between surfaces in relative motion.
Within our products, an extensive range of high performance plastic is available, designed to curtail wear either through their inherent low-friction attributes or self-lubricating properties. These materials mitigate mechanical wear while consistently upholding performance benchmarks within applications.
A CONSIDERATIONS-Which factors influence wear resistance and wear rate?
While wear may appear to be simple there are a number of different mechanisms that can cause wear, like adhesion, abrasion, fatigue, erosion, corrosion, which Briefly summarized in the following three aspects:
Contact Method
Working Environmental conditions
Load
Predicting wear in actual materials is a challenging task due to the multitude of variables involved. Consequently, providing precise estimates for the wear of a particular plastic is often only feasible through indicative values, unless the plastic&#;s performance under specific application conditions has been empirically evaluated. The following diagram presents approximate wear values for various plastics when in contact with dry steel.
Friction arises when there is resistance to the movement between two surfaces. The coefficient of sliding friction is a key factor in describing friction. It measures the strength of the frictional force compared to the normal force. A greater coefficient of sliding friction indicates a stronger frictional force. Therefore, the lower the coefficient of friction, the more smoothly two surfaces can slide against each other.
Various sliding and wear mechanisms come into play based on the specific sliding conditions. In the context of plastics, abrasion and adhesion take on significant roles. Abrasion involves the wear and removal of material from softer sliding partners due to the rough surfaces of harder sliding components. On the contrary, adhesion relies on surface bonds, where surface roughness and polarity are key factors determining whether adhesive wear takes place.
Coefficients of friction are commonly presented as values in tables; however, these values are always approximations due to the diverse array of factors influencing the coefficient of friction (material pairing, surface conditions, lubrication, temperature, humidity, wear, normal force, etc.). Thus, the practical coefficient may deviate from model test results. It&#;s essential to always incorporate the system parameters from model tests at the very least.
The most accurate outcomes arise from tests conducted under real-world conditions. Yet, it&#;s important to acknowledge that the ratios between the test results and actual applications can vary.
In numerous engineering scenarios, materials with low friction and high wear resistance are sought, such as thermoplastics. In comparative applications, high performance plastics generally exhibit a lower coefficient of friction than metals. Furthermore, advanced plastic materials frequently possess self-lubricating attributes, rendering them well-suited for prolonged usage and load-bearing situations.
When contrasted with metals or other interacting surfaces, wear-resistant high performance plastics deliver numerous advantages in high-friction scenarios:
As previously mentioned, industries demand low-friction plastics to enhance operational efficiency. These specialized materials offer impressive wear resistance. Furthermore, high-performance plastic not only provide corrosion resistance but also contribute to weight reduction and decreased heat and friction. These attributes translate into extended part lifespan and reduced maintenance expenses.
PTFE, also recognized as polytetrafluoroethylene or Teflon®, stands as a semi-crystalline fluoropolymer known for its exceptional thermal stability, chemical resistance, and high melting point. Often utilized as an additive to polymers like polyamide, polyacetal, polyester, polycarbonate, and TPE, PTFE imparts remarkable sliding capabilities, electrical resistance, and a non-stick surface.
Nonetheless, PTFE exhibits relatively low mechanical strength and a higher specific gravity in comparison to other plastics. The incorporation of additives such as glass fiber, carbon, or bronze can enhance its mechanical properties. PTFE is prominently employed in chemical plant engineering and applications requiring high chemical stress with sliding components.
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PEEK, also referred to as polyetheretherketone, stands as a distinctive semi-crystalline engineering thermoplastic renowned for its excellent chemical compatibility. Capable of functioning at temperatures up to 480°F with a melting point around 646°F, PEEK materials excel in hot water or steam environments, maintaining impressive flexural and tensile strength even in challenging conditions. Components manufactured from PEEK material exhibit reduced weight, enhanced strength, and prolonged durability under harsh circumstances.
PI, or polyimide, emerges as a non-melting high-temperature polymer renowned for its robustness, dimensional stability, and resistance to creep even at temperatures exceeding 500°F. Its suitability for demanding friction and wear scenarios stems from its low wear rates, ability to function in unlubricated conditions, and high pV-rates. PI finds application in industries such as vacuum, space, and semiconductor due to its high purity and minimal outgassing.
PPS, or polyphenylene sulfide, constitutes a semi-crystalline high-temperature thermoplastic with exceptional chemical resistance. Exhibiting robust mechanical strength, even at temperatures surpassing 392°F, PPS boasts commendable dimensional stability and impressive electrical properties. This material sees use in diverse sectors including electronics, automotive, medical, mechanical engineering, and the chemical industry.
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Wear&#;resistant coatings generally enhance the surface durability in a desirable way. They have been successfully deposited on substrates by different techniques including physical vapor deposition (PVD),[ 15 ] chemical vapor deposition,[ 16 ] hybrid physical&#;chemical vapor deposition,[ 17 ] and thermal spray methods.[ 18 ] Comprehensive summaries on microstructures,[ 19 ] characterization methods,[ 14k ] and selection criteria[ 14t ] of wear&#;resistant coatings can be found in previous review works.
To achieve the high wear resistance of coatings, several strategies have been carried out including i) the ion bombardment treatment that applied prior to the coating deposition, which improves the adhesion strength due to ion&#;beam&#;induced interface mixing,[ 20 ] ii) multiple doping of elements for strengthening the integrated coating performance, such as hardness and surface quality,[ 21 ] and iii) fabrication of composite coatings with a gradient structure that efficiently enhances bonding strength at the interface between the coating and substrate.[ 22 ]
The hardness of coatings is strongly related to their compositions and microstructures.[ 23 ] For instance, it is determined by the resistance of the coatings to bond distortion and dislocation formation and propagation, which in turn depend on the number of obstacles such as grain and column boundaries, second&#;phase particles, and solutes in the coatings. Such relation has promoted coating designs, which can be achieved by tailoring microstructures. It has been clear that for nanocrystalline materials with a grain size range of 2&#;3 nm, the volume fraction of interfaces can approach 50%. The constitution of interfaces is important in determining the mechanical properties of coatings. The coatings with supersaturated phases embedded are for wear&#;resistant applications because several pseudo&#;binary nitrides or carbides show large miscibility gaps and can be fabricated by vapor deposition techniques to yield effectively&#;quenched supersaturated solid solutions that are ready for age hardening.[ 23 ] The coatings with optimized nanostructures and interfaces showed extremely high hardness even after thermal treatments at &#; &#;.
Coating hardness could also be increased by spinodal decomposition or by the formation of precipitates that happens in several ternary, quaternary, and multinary transition metal nitrides due to their miscibility gaps.[ 24 ] In high&#;speed cutting applications, the so&#;called self&#;lubricating coatings such as diamond&#;like carbon or MoS2 coatings are actually low&#;friction protective layers formed during wear sliding.[ 25 ] The better performance of these coatings can be achieved by including Mo&#;containing compounds and further doping to match the lubricants used in rubbing surfaces by tuning the surface energy or to match the stiffness of the mechanical components.[ 26 ] Through Ti,[ 27 ] Cr,[ 28 ] Zr,[ 29 ] Ni,[ 30 ] Ti&#;Si,[ 31 ] or Sb2O3 [ 32 ] doping, MoS2 coating hardness can be increased owing to a distortion of its crystal structure, which leads to improved wear resistance. Au doping of the MoS2 coating has a different enhancement mechanism. The good load&#;supporting ability of Au nanoparticles enables MoS2 shearing, resulting in the superior tribological behavior of the Au&#;doped MoS2 coating compared to that of the bare MoS2 coating.[ 33 ]
Systems with more types of elements such as ternary or quaternary systems show strong segregation in the two binary compounds with thermodynamically driven &#;compositional modulation,&#; thus resulting in isotropic coatings with enhanced mechanical and anti&#;wear properties.[ 34 ]
The approach of self&#;organization during sliding is useful in guiding the design of coatings or other wear&#;resistant materials.[ 35 ] Remarkable microstructural changes within the surface occur as the coatings adapt to friction and wear. Such changes are responses to the external impact, based on the principle developed by I. Prigogine,[ 36 ] which states that the second thermodynamics law cannot eliminate the possibility of highly organized dissipative structures being formed in an open tribo&#;system. The self&#;organized structures are formed during the running&#;in period of friction; their early formation can efficiently reduce the wear volume of coatings. Fox&#;Rabinovich[ 35 , 37 ] pointed out that the self&#;organization easily occurs for coatings with non&#;equilibrium states, which could be realized by the high&#;energy ion impacts in modern techniques of surface engineering such as physical vapor deposition. The non&#;equilibrium states corresponding to the complexity of coatings can be tailored by methods such as element doping, the formation of solid solution and binary or ternary compounds, and the employment of multi&#;layer structures. The future design of coatings should further reduce their complexity and non&#;equilibrium states to make them sustain more external impacts and adapt to varying working conditions.
Recently, nanocomposite coatings have attracted intensive interest due to their exceptional physico&#;mechanical properties meeting the requirements at specific working conditions, such as water lubrication and electrical contact.
Nickel&#;based nanocomposite coatings have potential in applications at multiple scales due to their promising corrosion and wear resistance, good ductility, and superior electrical properties.[ 38 ] For traditional metallic materials, wear resistance is related to hardness, according to the Archard theory, which depicts that wear volume V is inversely proportional to the hardness at a given wear condition.[ 39 ] Ni&#;P nanocomposite coatings were reported to show high hardness of 250&#; HV at ambient conditions.[ 40 ] It could be further improved to &#; HV by heat treatments at 200&#;300 °C for 30&#;40 weeks.[ 41 ] Such improvement is due to the uniform dispersion of nickel boride formed during the low&#;temperature treatments and the iron boride formed within the composite coatings.
In the past two decades, SiC, carbon nanotubes (CNTs), ferrites, nanodiamonds, and oxide nanoparticles (e.g., CeO2, TiO2, and Al2O3) have been proven to be the promising nanoadditives for the development of nanocomposite coatings.[ 40 , 42 ] Compared to SiC nanoparticles, CNTs were demonstrated to be a good additive for the enhancement of the wear resistance of Ni&#;based coatings.[ 43 ] They could be packed into the pores of Ni&#;based coatings to form a dense passivation layer with high wear resistance. Nanocomposite coatings including CrBN, CrSiCN, CrBCN, and TiN&#;/CrN&#;based coatings have attracted intensive interest in water applications[ 44 ] and have been reported to show good tribological properties at water lubrication conditions.[ 45 ]
Many studies on wear&#;resistant coatings focused on the development of unique structures, for example, the core&#;shell structure,[ 46 ] amorphous/nanocrystalline,[ 47 ] and gradient multilayer coatings.[ 48 ] A polymer coating with the core&#;shell structure of polymethylmethacrylate wrapped polytetrafluoroethylene shows a low friction coefficient of 0.069 and wear rate of 1.04 × 106 mm3 N&#;1. Voevodin et al.[ 47 ] fabricated nanocrystalline WC/amorphous DLC composite coatings with a biphasic structure composed of 1 ± 3 nm sized amorphous particles and 5 ± 10 nm sized nano&#;crystal grains. The hardness of the WC/DLC coatings was much higher than that of the metal&#;doped DLC coatings. The superior strength, high corrosion, and wear resistance of metallic glass make it a promising material for coatings. Sahasrabudhe et al.[ 49 ] fabricated Fe&#;based amorphous coatings on a Zr substrate using the laser engineered net shaping technique. The coatings consisted of crystalline phases were embedded in the amorphous matrix. The content of the amorphous phase increased after the laser heat treatment, resulting in an increase of coating hardness up to nearly 22%. After wear tests in a 3.5% NaCl solution, the wear rate of the amorphous coatings was observed to reduce by 96% compared to that of the Zr substrate. The enhancement in the wear resistance of Zr was attributed to the over 800% improvement in hardness of the Zr substrate.
The coating design with a nanocrystalline/amorphous structure is efficient in achieving excellent mechanical and tribological properties. [ 50 ] The amorphous phase can solve the problems of lattice misfits between two different polycrystalline coating materials with the random orientation of the grains. Since the lattice misfit initiates crack formation and propagation, it needs to be thin enough and form a three&#;dimensional skeleton with high elastic modulus to reduce such initiation and achieve the better anti&#;wear behavior of the amorphous phase. The friction coefficient of DLC coatings can reach a very low value owing to their chemical inertness and small contact area as a result of their high elastic moduli.[ 51 ] The nanocrystalline phase should have a nanoscale size fitting the stability limit of the crystalline phase, which has notable tribological behavior as the nanocrystalline grains improve its hardness.
The low&#;temperature deposition technique is efficient to avoid interdiffusion&#;induced decreases in hardness and thus improves the coating wear resistance. The nanocrystalline/amorphous structure has been employed for the fabrication of several coatings including nc&#;TiN/a&#;Si3N4, nc&#;TiC/a&#;C, and nc&#;TiCN/a&#;SiCN, which improve its strength efficiently.
Existing references regarding amorphous coatings well cover research fields of fabrication processes,[ 52 ] microstructures,[ 53 ] mechanical behavior, corrosion,[ 54 ] and wear properties.[ 55 ] The wear behavior of amorphous coatings depends on the residual stress and adhesion of coatings, which are related to the difference in the thermal expansion coefficient between the coating and the substrate. The fracture toughness of coatings is a critical factor, which relates to the crack initiation and propagation during mechanical loading. Further experimental and numerical studies on adhesion, residual stress, and fracture toughness in different amorphous coating systems are required to obtain deep insights into anti&#;wear mechanisms by controlling these critical factors.
Hydrogen&#;free DLC coatings showed a superior anti&#;wear behavior compared to hydrogenated DLC:H coatings at the water lubrication condition because delamination occurred with the latter coatings in the presence of hydrogen. [ 56 ] DLC coatings with a higher content of sp2&#;hybridization would be promising for water lubrication. Using the a&#;CNx coatings, the wear rate was maintained at an order of magnitude of 10&#;7&#;10&#;8 mm3 (Nm)&#;1 with water lubrication. Compared to the DLC and a&#;CNx coatings, the TiN&#;/CrN&#;based coatings showed a higher wear rate of 10&#;6&#;10&#;7 mm3 (Nm)&#;1 with water lubrication but exhibited a better anti&#;wear behavior under corrosive conditions owing to the formation of a passive layer during the sliding process.
Our previous studies reported the effect of sputtering ion beam energy on the bonding structures, anti&#;corrosion behavior, and mechanical and tribological properties of amorphous carbon films fabricated by the ion beam sputtering method within an argon ion beam energy range of 1&#;3 keV. [ 57 ] Improved adhesion was obtained in the films fabricated at high ion beam energy, resulted from the collective effect of the low residual stress within the films, film graphitization, and mixed interface. The wear resistance of amorphous carbon films increased with the increase of ion beam energy. A critical parameter of hardness/elastic modulus, H/E, was demonstrated to be a suitable factor to evaluate coating wear properties.
The wear resistance and friction coefficient of coatings depend on their hardness and thickness values, respectively. The internal residual stress of coatings induced by fabrication and post&#;treatment processes has an important effect on the wear resistance. The residual stress of a&#;CNx coating can reach as high as 5 GPa after a PVD sputtering process, resulting in weak adhesion to the substrate. A Ti+C/a&#;CNx gradient multilayer coating with a hardness of 19 GPa was prepared by Liu et al.[ 48 ] using ion&#;beam&#;assisted magnetron sputtering. The gradient layer existed between the CNx layer and the substrate. After a thermal treatment, the Ti+C layer could alleviate the internal residual stress of the coating as a result of the formation of misfit dislocations.
Two&#;dimensional (2D) materials including transition metal dichalcogenides and graphene are very attractive as the material of novel wear&#;resistant coatings.[ 3 , 58 ] The typically used graphene&#;based materials have high mechanical strength, good lubricity, and thermal stability, thus acting as promising coating candidates for vehicles, especially for airplanes and ships by providing light weight and high wear resistance under shear forces.[ 59 ]
Graphene&#;based materials can mitigate mechanical failures of coatings through surface enhancement and stress transfer. The high resistance to crack initiation and deflection as well as crack branching and bridging have been determined as important strengthening mechanisms of graphene&#;based materials. Graphene has been proven to be an effective solid lubricant,[ 58 , 60 ] allowing it to be a good candidate in tribological applications at both the nano&#; and the macroscale. Typically, a small amount of graphene effectively improves the wear resistance of polymer coatings.[ 61 ] Meanwhile, as a monolayer material, graphene showed extraordinary anti&#;wear behavior originated from interactions with hydrogen bonds to form sp3 carbon. [ 62 ] Nevertheless, challenges in the weak bonding strength between the metallic&#; or ceramic&#;matrix and graphene&#;based materials hinder practical applications of the latter materials in the fabrication of metallic and ceramic wear&#;resistant coatings.
Newly emerging MXenes,[ 58c ] such as Ti3C2Tx&#;nanosheets, have shown ultra&#;high wear resistance and good lubrication behavior under dry conditions resulted from their graphite&#;like structure with low shear strength.[ 63 ] A 2.3&#;fold reduction in friction and a 2.7&#;fold reduction in wear have been observed in the Ti3C2Tx&#;nanosheet modified steel surface compared with the bare steel under a contact pressure of 0.8 GPa and at relative&#;low humidity. Ti3C2Tx&#;nanosheets show excellent anti&#;friction and anti&#;wear properties at moderate contact stress and low humidity conditions. Ti3C2/nanodiamonds composite MXene coatings exhibit extremely high wear resistance during sliding against a polytetrafluoroethylene counterpart owing to integrated effects of protective polytetrafluoroethylene for the 2D structure of Ti3C2, rolling ability of nanodiamonds within Ti3C2 inter/inner layers, and formation of the tribofilm.[ 64 ]
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