•Reinforcements’ properties exert an important influence on tribological performance of POM.•Obvious synergy of aramid particles with SiO2 nanoparticles and with PTFE was identified.•Tribological properties are closely related to the structures of transfer films.•Reinforcements’ properties play an important role in formation of transfer film.Series of polyoxymethylene (POM) composites filled with three kinds of micro-sized reinforcing fillers, i.e. short carbon fibers (SCF), aramid particles (AP) and short glass fibers (SGF), with addition of polytetrafluoroethylene (PTFE) and/or silica nanoparticles were prepared. The tribological properties and transfer film structures were comprehensively investigated. It was revealed that mechanical properties of the micro-sized reinforcing fillers influenced significantly the tribological mechanisms of PTFE and silica nanoparticles in POM hybrid composites in aspect of transfer film formation. In comparison to SCF and SGF, AP played a better synergetic role with PTFE and SiO2 nanoparticles in enhancing the tribological performance. It is assumed that tribo-physical and -chemical actions occurring on the reinforcements’ asperities are important for tribological behaviors of the composites.

•Hexagonal nanoparticles improve sliding performance of PPS composite in diesel.•Presence of only limited quantity diesel results in best tribological performance.•Tribo-chemical reactions of PPS with steel and AlN nanoparticles were identified.The roles of various fillers, i.e. short carbon fibers (SCF), graphite, hexagonal structured tungsten disulfide (WS2) and aluminium nitride (AlN) nanoparticles, on the tribologial performance of polyphenylene sulphid (PPS) were explored with low-sulfur diesel as lubricant. It was demonstrated that the nanoparticles, especially AlN, remarkably improved the tribologial performance of PPS composites under diesel lubrication conditions. Moreover, presence of only very limited quantity of diesel results in the best tribological performance. It was manifested that the tribological performance was closely dependent on the tribofilm formed on steel counterface. Tribo-chemical reactions of PPS with both the steel counterpart and AlN nanoparticles were identified, as deemed important for generation of a robust tribofilm.

Dependence of the tribological behaviors of polyimide- and polyetheretherketone-based composites on pv (pressure × speed) factors is investigated in air ambience. It is demonstrated that the hybrid composites filled with nanosilica/carbon fibers/graphite exhibit ultralow friction and wear under extreme conditions. In particular, the friction coefficients of the hybrid nanocomposites at 40 MPa m s−1 are in the range of 0.03–0.04, which are even lower than those obtained with poly alpha olefin lubrication. Moreover, the friction coefficients are lower than those of carbon fibers reinforced polymer composites ever reported in literatures. In order to reveal the underlying mechanisms of ultralow friction and wear, tribochemistry and tribofilms' nanostructures are comprehensively analyzed. It is identified that chelation of polymeric molecular radicals with steel counterface occurs enhancing tribofilm's bonding strength. Striking orientation of the molecules of remnant polymer in tribofilm is indicative that the film exhibits an easy-to-shear characteristic under extreme pv conditions. Nanosilica released onto sliding interface and iron oxide particles abraded from the counterface are thereafter tribosintered into a compact layer, which accounts for the high load-carrying capability of the tribofilm.

•The hydration of h-BN nanoparticles plays an important role to form a stable boundary film on the sliding contact surfaces.•H3BO3 and B2O3 molecules are arrayed in a closely packed outmost layer of the boundary film.•The basal planes of H3BO3 and B2O3 are aligned parallel to the sliding direction.•The aligned H3BO3 and B2O3 basal planes are crucial for the high tribological performance of the POM nanocomposite.Development of high-performance polymer composites exposed to water lubrication conditions is of increasing interests for numerous applications, where high durability and reliability are demanded. However, formation of a protective boundary reaction layer on the rubbing surfaces can be problematic with the presence of water. The roles of hexagonal boron nitride (h-BN) nanoparticles on the tribological performance of polyoxymethylene (POM) and POM composite reinforced with short carbon fibers were investigated. It is identified that the addition of low loading h-BN greatly improves the tribological performance, e.g. wear resistance of POM is enhanced by one order of magnitude. Moreover, h-BN and carbon fibers play a synergetic role in enhancing the wear resistance. Tribo-chemistry and nanostructures of the boundary film were comprehensively investigated. It is revealed that H3BO3 and B2O3 generated as products of tribo-chemical reactions are arrayed in a closely packed outmost layer of the boundary film and exert an important influence on the tribological performance. Our work gives the evidence that the basal planes of H3BO3 and B2O3 are aligned parallel to the sliding direction, leading to low friction and wear.Download high-res image (334KB)Download full-size image

•Two PEEK composites filled respectively with short carbon fibers and nano-silica are assembled side-by-side.•The assembled sample exhibits a much enhanced tribological performance in comparison to the individual composites.•The assembled sample showed a performance comparable with the hybrid nanocomposite.•When the assembled sample is slid against steel, a nanostructured tribofilm is generated.•A new design concept of tribo-engineering components by assembling composites with specific functionalities is proposed.In the present work, two polyetheretherketone (PEEK) composites, i.e. conventional composite filled with short carbon fibers (SCF) and nanocomposite filled with SiO2 nanoparticles, were mechanically assembled side-by-side using adhesive. The tribological behavior of the assembled sample was investigated using a Pin-On-Disc tribometer with the two composites' surfaces rubbing in succession along the same wear track on a steel counterpart. It was demonstrated that the assembled sample exhibited a greatly enhanced tribological performance in comparison to those obtained when the composites ran respectively. A synergetic role of the conventional composite and the nanocomposite was therefore identified. Moreover, it was manifested that the assembled sample showed a tribological performance comparable to the hybrid nanocomposite filled with combined SCF and SiO2 nanoparticles. The tribological mechanisms dominating the synergetic effect of the two composites were analyzed based on direct comparisons of tribofilm morphologies. It is expected that the present work provides a new design concept for tribo-enginnering components, i.e. assembling individual composites with specific functionalities.Download high-res image (216KB)Download full-size image

•Addition of as-synthesized g-C3N4 enhanced dramatically the wear resistance and friction coefficient of PEEK.•Thermally etched g-C3N4 led to lower friction coefficient owing to destruction of inter-planar hydrogen bonds.•Hydroxylated g-C3N4 further improved the wear resistance as a result of enhanced interlayer hydrogen bonds.•Significant fraction of g-C3N4 transferred onto the counterface leading to generation of a load-carrying tribofilm.Graphitic carbon nitride (g-C3N4) was synthesized and the tribological behaviors of g-C3N4 reinforced poly-ether-ether-ketone (PEEK) were investigated. It was demonstrated that the addition of as-synthesized g-C3N4 (AS-CN) enhanced dramatically the wear resistance of PEEK, and meanwhile the friction coefficient was increased. Moreover, interlayer hydrogen bonds (H-bonds) of g-C3N4 played an important role in the tribological characteristics of PEEK composites. In comparison to AS-CN, thermal-etched g-C3N4 led to a lower friction coefficient owing to destruction of the H-bonds. On the contrary, hydroxylated g-C3N4 (h-CN) possessed strengthened interlayer H-bonds and a denser stacking structure, and hence further enhanced the wear resistance. Compared to carbon fibers, conventionally employed for developing anti-wear polymer composites, h-CN was identified to be significantly more effective for improving the wear resistance of PEEK, i.e. a very low specific wear rate of 4.0 × 10− 7 mm3/Nm was achieved by simply filling with 10 vol% h-CN. Characterizations of tribofilms' nanostructures manifested that significant fraction of g-C3N4 transferred onto the counterface and led to generation of a load-carrying tribofilm. It is expected that the output of the present work will pave a route for developing binary and eco-friendly anti-wear materials.Download high-res image (311KB)Download full-size image

Tribological behaviors of various polyimide (PI) composites when rubbing with medium carbon steel (MCS35) and NiCrBSi, were comprehensively investigated. When the conventional PI composite filled with carbon fibers and graphite was concerned, the carbon-based tribofilm formed on NiCrBSi surface resulted in obvious friction- and wear-reduction. However, no lubricating tribofilm was formed on MCS35 surface. Chelation of polymeric molecular radicals with the metallic counterparts was identified on the worn surfaces. Theoretical calculations corroborated that the Ni-based metal–organic compound showed a higher stability than the Fe-based one. With respect to the sliding of the hybrid nanocomposites containing silica or h-BN nanoparticles, the nanoparticles released onto the interface significantly mitigated tribo-oxidation of metallic counterparts, and were finally tribo-sintered into a compact layer after being mixed with remnant polymer and tribo-oxidation products. The effect of counterpart material was less pronounced for the tribological mechanisms of the nanocomposites than for the conventional composite.Download high-res image (87KB)Download full-size image

The tribological behaviors of various oxide nanoparticles, i.e. Bi2O3, CuO, SiO2 and ZrO2, added into a carbon fibers reinforced polyether-ether-ketone were comprehensively investigated. It was demonstrated that nanoparticle types played an important role in the tribological performance. When sliding took place at a low FV (load × speed) condition, the addition of CuO and ZrO2 nanoparticles led to the formation of patch-like tribofilms increasing friction and wear. However, at FV factors ranging from 30 to 300 N m/s, the hard nanoparticles, i.e. SiO2 and ZrO2, resulted in dramatic improvement of the tribological properties. Moreover, nano-ZrO2 was significantly more effective than nano-SiO2 for enhancing the tribological performance. Hard nanoparticles released onto the sliding interface removed the tribo-oxidation layer on steel counterface. Hereafter, they were “tribo-sintered” into an oxide-based tribofilm having a high load-carrying capability. However, the soft nanoparticles, i.e. Bi2O3 and CuO, did not help to form a robust and lubricating tribofilm.

•Hybrid soft-hard nanoparticles added into SCF-reinforced PEEK significantly shortened the running-induration and improved its tribological performance.•Robust and slippery tribo-films consisting of hybrid functional nanoparticles were generated.•Physical and chemical actions the sliding interface can be tuned by filling hybrid nanoparticles with distinct functionalities.•This work can pave a route for developing new self-lubrication composites subjected to extreme running conditions.Tailoring a robust and slippery tribo-film on polymer-metal interface is a potential approach for improving the tribological performance. In this work, hybrid soft-hard nanoparticles were compounded into polyetheretherketone (PEEK) reinforced with carbon fibers. Effects of nanoparticle combinations on tribological performance of the composites and tribo-film’s functionality were explored. It was demonstrated that hybrid oxide nanoparticles, i.e. Bi2O3-SiO2 and CuO-SiO2, accelerated formation speed of tribo-films. The soft nanoparticles, i.e. Bi2O3 and CuO, having low-melting points expedited tribo-sintering of wear products on the interface. Moreover, combination of WS2-SiC nanoparticles resulted in a “slippery” tribo-film withstanding harsh rubbing conditions. Hard species endowed the tribo-film a high load-carrying capability, while the soft ones imparted the tribo-film an easy-to-shear characteristic. Striking stretching and re-orientation of PEEK molecules in tribo-film were identified and corroborated the “slippery” characteristic. It is expected that this work can pave a route for developing new self-lubrication composites under extreme running conditions.Download high-res image (190KB)Download full-size image

•Tribological properties of PI composites under oil lubrication were studied.•SGF and ZnS led to a synergetic role in friction reduction and wear resistant.•Short glass fibers imparted their high abrasion resistance to the composite.•ZnS led to complex tribo-chemical reactions on the worn surface.•A tribofilm was formed on the counterface when being slid against ZnS/SGF/PI.In response to the challenges posed by friction and wear of tribo-components exposed to boundary and mixed lubrication conditions, there is increasing interest in the development of polymer-based, self-lubricating composites. This investigation addressed the effect of various fillers (i.e. short carbon fibers (SCF), short glass fibers (SGF), ZnS sub-micrometer particles and combinations of respective fibers with ZnS particles), on the friction and wear of polyimide (PI) when lubricated by poly-alpha-olefins (PAO). A sliding plate-on-ring geometry, using GCr15 steel rings, was used. It was demonstrated that the reinforcing fibers greatly improved the composites’ wear resistance. SGF was observed to be more effective than SCF in respect to friction- and wear-reduction. Moreover, an obvious synergism between SGF and ZnS was identified. The hybrid composite ZnS/SGF/PI exhibited excellent tribological performance under oil lubrication. With respect to the hybrid formulation, the glass fibers imparted their high abrasion resistance to the composite; while ZnS led to complex tribo-chemical reactions. A stable tribofilm consisting of transferred materials and various tribo-chemical products improved the boundary lubrication capability of the sliding system.Download high-res image (289KB)Download full-size image

The formation of a tribo-film on the counterface plays an important role on the tribological performances of polymer subjected to mixed and boundary lubrication conditions. However, when freshwater is used as a lubricant, the formation of a tribo-film usually is hindered. In order to overcome this disadvantage, PEEK/α-FeOOH nanocomposites were developed and their tribological performances were studied under water lubrication conditions in the present work. It was demonstrated that the inclusion of α-FeOOH nanoparticles (NPs) into the PEEK matrix improves significantly the tribological performance of the matrix. The nanostructures and properties of the tribo-films formed on the steel counterface were comprehensively studied. It was identified that the addition of α-FeOOH NPs promotes the formation of a lubricating tribo-film which covers the entire counterface. Based on the investigations on tribo-films, we deem that the α-FeOOH NPs act as precursors for the dehydration reaction promoting the formation of a tribo-film which consists of α-Fe2O3 and transferred PEEK. It is assumed that the enhanced tribological performance is related to the possibly high load-bearing capability and “easy-to-shear” characteristic of the tribo-film.

•Counterpart materials exert an important influence on the tribological performance of epoxy composites.•Variation of counterpart materials leads to distinctly different transfer film structures.•A close relationship between the tribological behaviors and transfer film structures was revealed.The dependence of the friction and wear of epoxy (EP) composites materials on counterpart materials, such as standard bearing steel, medium carbon steel and chrome-plating (Cr), was investigated. The conventional composite filled with short carbon fiber (SCF) and graphite shows the highest tribological performance when rubbing against Cr, whereas, the hybrid nanocomposite (EP filled with SCF, graphite and silica nanoparticles) exhibits the lowest friction and wear when sliding against the standard bearing steel. The role of nanoparticles in the tribological performance is distinctly different when sliding against with various counterpart materials. It is demonstrated that counterpart materials exert an important influence on material transfer, tribo-oxidation and mechanical mixing of wear products, resulting in the different formation mechanisms of transfer film.

•Short carbon and glass fibers enhance greatly wear resistance of epoxy.•Graphite reduces friction but increases wear of carbon fiber reinforced epoxy.•Silica nanoparticles improve tribological properties of epoxy composite.•Tribological behavior is closely related to tribofilm formation.The aim of this work is to investigate the tribological behaviors of epoxy (EP)composites under water lubrication conditions at both varying and constant sliding speeds. In particular, the roles of reinforcing fillers, i.e. carbon and glass fibers, solid lubricants and SiO2 nanoparticles, on the friction and wear properties of EP were investigated. It is demonstrated that both the reinforcing fibers significantly enhance the wear resistance of EP. Under mixed and boundary lubrication conditions, the addition of SiO2 nanoparticles into EP conventional composite filled with carbon fibers and graphite reduces the friction and wear. It is revealed that the tribological performance of the fiber-reinforced EP composites is mainly attributed to the high abrasion resistance of the fibers and the tribofilm formation.

•Tribological behaviors of various hybrid EP nanocomposites were investigated.•No synergy between CNT and silica nanoparticles is identified.•SCF and silica play a synergetic role in enhancing tribological performance.•Addition of silica nanoparticles into SGF-reinforced EP exerts a negative effect.•Tribological mechanisms of silica are dependent on nature of reinforcing fillers.The effects of silica nanoparticles on the tribological behaviors of epoxy (EP) composites reinforced with carbon nanotubes (CNT), short carbon fibers (SCF) and short glass fibers (SGF) were comprehensively investigated and compared. It was revealed that the tribological mechanisms of nanoparticles were distinct in EP composites filled with various reinforcements. Silica enhanced the wear resistance of CNT-reinforced EP, whereas no synergy between CNT and silica was identified. Nevertheless, SCF and silica played a synergetic role in enhancing the tribological performance. However, the addition of silica into SGF-reinforced EP exerted a negative effect. Owing to distinct tribo-physical and chemical actions between rubbing reinforcements and counterface, the presence of silica nanoparticles on the interface generated transfer films with different structures and functionalities.

•Tribological behaviors of short glass fibers (SGF) reinforced epoxy (EP) composites when lubricated with base oil were investigated.•SGF significantly reduces both the friction coefficient and the wear rate of EP under oil lubrication conditions.•The formation of a tribofilm on the metallic counterface and the transfer of materials between composite and counterpart play an important role on the tribological performance.The tribological performance of short glass fibers (SGF), solid lubricants and silica nanoparticles filled epoxy (EP) composites was investigated under oil lubrication conditions. It is demonstrated that the addition of SGF greatly reduces the friction and wear of EP. However, further addition of solid lubricants and silica nanoparticles does not change obviously the friction and wear. It is identified that the high tribological performance of SGF reinforced EP is related to the high load carrying capacity and abrasion resistance of SGF. The nanostructure of the tribofilm was comprehensively characterized. It is deemed that the tribofilm plays an important role in the tribological performance by avoiding the direct rubbing of the sliding pairs exposed to boundary and mixed lubrication conditions.

•Effect of counterface topography on tribofilm of epoxy nanocomposite was studied.•Close relationship between the topography and composite׳s performance is revealed.•Optimum topography leading to formation of nanostructured tribofilm was identified.The topographic effect of steel counterface, finished by mechanical grinding with Ra ranging from 0.01 to 0.95 µm, on the structure and functionality of the tribofilm of a hybrid nanocomposite, i.e. epoxy matrix filled with monodisperse silica nanoparticles, carbon fibers and graphite, was systematically investigated using a pin-on-disc sliding contact geometry. The nanostructure of the tribofilm was comprehensively characterized by using combined focused ion beam and transmission electron microscope analyses. It was identified that oxidation of the steel surface, release, compaction and tribosintering of silica nanoparticles and deposition of an epoxy-like degradation product as well as fragmentation of carbon fibers are main mechanisms determining the structure and functionality of the tribofilm. The size of roughness grooves determines the type and size class of wear particles to be trapped at the surface. An optimum groove size leading to a maximum of surface coverage with a nanostructured tribofilm formed mainly from released silica nanoparticles was identified.

In order to avoid occurrence of severe seizure of motion components exposed to mixed and boundary lubrication, e.g., in engine and transmission systems, to replace metal–metal friction pairs by polymer–metal pairs provides a potential solution. In the present work, the tribological behaviors of short carbon fiber (SCF)-reinforced epoxy (EP) composites when lubricated with polyalphaolefin base oil were investigated. It was identified that the running-in tendency and friction coefficient of the composites show a close dependence on the lubrication regimes. SCF improves greatly the wear resistance of EP although they increase slightly the friction coefficient. It was demonstrated that further addition of monodispersed SiO2 nanoparticles into the EP composite filled with SCF and graphite leads to higher wear resistance. The reinforcement of SCF and the formation of a high-performance tribofilm on the surface of steel counterpart play an important role on the tribological properties of EP-based materials.

•Counterface topography influences formation of the tribofilms of PEEK composites.•A close relationship between structure and property of the tribofilms was revealed.•A nanostructured tribofilm benefits best the tribological performance.The effect of steel counterface topography on the formation mechanisms of nanostructured tribofilms of polyetheretherketone (PEEK) hybrid nanocomposites was studied. Three types of surface finishes with mean roughness Ra ranging from nano- to micro-scale were investigated. Tribo-sintering of nanoparticles, oxidation of counterface steel and compaction of wear debris are identified to be competing factors dominating the formation and function of the tribofilms. Counterface topography played an important role on the competing factors, and thereby influenced significantly the final structure, the load-carrying capability and the lubrication performance of the tribofilms. It was disclosed that a thin tribofilm, which mainly consists of silica nanoparticles and which forms on the counterface with a sub-micron roughness, benefits best the tribological performance of the composites.

•Tribology behavior of PEEK materials with diesel and oil lubrication was studied.•Formation of a tribofilm plays an important role on the tribology behavior.•PEEK materials exhibit much higher performance than a low alloy steel.In modern industries, more and more mechanical components are exposed to mixed and even boundary lubrication conditions, inducing fast wear and even scuffing of the motion systems. In order to enhance the lifetime and reliability of the motion systems, replacing metal–metal friction pairs by metal–polymer ones can be one of the most effective approaches. The present work focuses on tribological behavior of pure polyetheretherketone (PEEK) and a formulated PEEK composite lubricated with diesel and engine oil. It was demonstrated that in mixed and boundary lubrication regimes the structure of PEEK materials affect significantly the tribological performance. Formation of a tribofilm on the surface of metallic counterbody plays an important role on the tribological behavior of the PEEK-based materials.