Influence of the Matrix Material and Tribological Contact Type on the Antifriction Properties of Hybrid Reinforced Polyimide-Based Nano- and Microcomposites.

This paper addresses peculiarities in the formation and adherence of a tribofilm on the wear track surface of antifriction PI- and PEI-based composites, as well as a transfer film (TF) on a steel counterface. It is shown that during hot pressing, PTFE nanoparticles melted and coalesced into micron-sized porous inclusions. In the PEI matrix, their dimensions were much larger (up to 30 µm) compared to those in the PI matrix (up to 6 µm). The phenomenon eliminated their role as effective uniformly distributed nanofillers, and the content of 5 wt.% was not always sufficient for the formation of a tribofilm or a significant decrease in the WR values. At the loaded content, the role of MoS2 and graphite (Gr) microparticles was similar, although filling with MoS2 microparticles more successfully solved the problem of adhering to a PTFE-containing tribofilm in the point tribological contact. This differed under the linear tribological contact. The higher roughness of the steel counterpart, as well as the larger area of its sliding surface with the same PTFE content in the three-component PI- and PEI-based composites, did not allow for a strong adherence of either the stable PTFE-containing tribofilm on the wear track surface or the TF on the steel counterpart. For the PEI-based composites, the inability to shield the steel counterpart from the more reactive polymer matrix, especially under the conditions of PTFE deficiency, was accompanied by multiple increases in the WR values, which were several times greater than that of neat PEI.

Role of Testing Conditions in Formation of Tribological Layers at Line Contacts of Antifriction CF-Reinforced PI- and PEI-Based Composites.

High-strength PI and PEI polymers differ by chemical structure and flexibility of the polymer chains that ensure lower cost and higher manufacturability of the latter. The choice of a particular polymer matrix is of actuality at design of antifriction composites on their basis. In this study, a comparative analysis of tribological behavior of PI and PEI- based composites was carried out with linear contact rubbing. The neat materials, as well as the two- and three-component composites reinforced with chopped carbon fibers, were investigated. The third components were typically used, but were different in nature (polymeric and crystalline) being solid lubricant fillers (PTFE, graphite and MoS2) with characteristic dimensions of several microns. The variable parameters were both load and sliding speed, as well as the counterface material. It was shown that an improvement of the tribological properties could be achieved by the tribological layer formation, which protected their wear track surfaces from the cutting and plowing effects of asperities on the surfaces of the metal and ceramic counterparts. The tribological layers were not formed in both neat polymers, while disperse hardening by fractured CF was responsible for the tribological layer formation in both two- and three component PI- and PEI-based composites. The effect of polymer matrix in tribological behavior was mostly evident in two-component composites (PI/CF, PEI/CF) over the entire P⋅V product range, while extra loading with Gr and MoS2 leveled the regularities of tribological layer formation, as well as the time variation in friction coefficients.

Effect of Transfer Film on Tribological Properties of Anti-Friction PEI- and PI-Based Composites at Elevated Temperatures.

The structure, mechanical and tribological properties of the PEI- and PI-based composites reinforced with Chopped Carbon Fibers (CCF) and loaded with commercially available micron-sized solid lubricant fillers of various nature (polymeric-PTFE, and crystalline-Gr and MoS2) were studied in the temperature range of 23-180 (240) °C. It was shown that tribological properties of these ternary composites were determined by the regularities of the transfer film (TF) adherence on their wear track surfaces. The patterns of TFs formation depended on the chemical structure of the polymer matrix (stiffness/flexibility) as well as the tribological test temperatures. Loading with PTFE solid lubricant particles, along with the strengthening effect of CCF, facilitated the formation and fixation of the TF on the sliding surfaces of the more compliant PEI-based composite at room temperature. In this case, a very low coefficient of friction (CoF) value of about 0.05 was observed. For the more rigid identically filled PI-based composite, the CoF value was twice as high under the same conditions. At elevated temperatures, rising both CoF levels and oscillation of their values made it difficult to retain the non-polar PTFE transfer film on the sliding surfaces of the PI-based composite. As a result, friction of the ceramic counterpart proceeded over the composite surface without any protecting TF at T ≥ 180 °C. For the sample with the more flexible PEI matrix, the PTFE-containing TF was retained on its sliding surface, providing a low WR level even under CoF rising and oscillating conditions. A similar analysis was carried out for the less efficient crystalline solid lubricant filler MoS2.

Experimental-FEM Study on Effect of Tribological Load Conditions on Wear Resistance of Three-Component High-Strength Solid-Lubricant PI-Based Composites.

The structure, mechanical and tribological properties of the polyimide-based composites reinforced with chopped carbon fibers (CCF) and loaded with solid-lubricant commercially available fillers of various natures were investigated. The metal- and ceramic counterparts were employed for tribological testing. Micron sized powders of PTFE, colloidal graphite and molybdenum disulfide were used for solid lubrication. It was shown that elastic modulus was enhanced by up to 2.5 times, while ultimate tensile strength was increased by up 1.5 times. The scheme and tribological loading conditions exerted the great effect on wear resistance of the composites. In the tribological tests by the 'pin-on-disk' scheme, wear rate decreased down to ~290 times for the metal-polymer tribological contact and to ~285 times for the ceramic-polymer one (compared to those for neat PI). In the tribological tests against the rougher counterpart (Ra~0.2 µm, the 'block-on-ring' scheme) three-component composites with both graphite and MoS2 exhibited high wear resistance. Under the "block-on-ring" scheme, the possibility of the transfer film formation was minimized, since the large-area counterpart slid against the 'non-renewable' surface of the polymer composite (at a 'shortage' of solid lubricant particles). On the other hand, graphite and MoS2 particles served as reinforcing inclusions. Finally, numerical simulation of the tribological test according to the 'block-on-ring' scheme was carried out. Within the framework of the implemented model, the counterpart roughness level exerted the significantly greater effect on wear rate in contrast to the porosity.

UHMWPE-Based Glass-Fiber Composites Fabricated by FDM. Multiscaling Aspects of Design, Manufacturing and Performance.

The aim of the paper was to improve the functional properties of composites based on ultra-high molecular weight polyethylene (UHMWPE) by loading with reinforcing fibers. It was achieved by designing the optimal composition for its subsequent use as a feedstock for 3D-printing of guides for roller and plate chains, conveyors, etc. As a result, it was experimentally determined that loading UHMWPE with 17% high density polyethylene grafted with VinylTriMethoxySilane (HDPE-g-VTMS) was able to bind 5% glass fillers of different aspect ratios, thereby determining the optimal mechanical and tribological properties of the composites. Further increasing the content of the glass fillers caused a deterioration in their tribological properties due to insufficient adhesion of the extrudable matrix due to the excessive filler loading. A multi-level approach was implemented to design the high-strength anti-friction 'UHMWPE+17%HDPE-g-VTMS+12%PP'-based composites using computer-aided algorithms. This resulted in the determination of the main parameters that provided their predefined mechanical and tribological properties and enabled the assessment of the possible load-speed conditions for their operation in friction units. The uniform distribution of the fillers in the matrix, the pattern of the formed supermolecular structure and, as a consequence, the mechanical and tribological properties of the composites were achieved by optimizing the values of the main control parameters (the number of processing passes in the extruder and the aspect ratio of the glass fillers).

Effect of Various Type of Nanoparticles on Mechanical and Tribological Properties of Wear-Resistant PEEK + PTFE-Based Composites.

The mechanical and tribological properties of polyetheretherketone (PEEK)- and PEEK + PTFE (polytetrafluoroethylene)-based composites loaded with and four types of nanoparticles (carbonaceous, metallic, bimetal oxide, and ceramic) under metal- and ceramic-polymer tribological contact conditions were investigated. It was found that loading with the nanofillers in a small content (0.3 wt.%) enabled improvement of the elastic modulus of the PEEK-based composites by 10-15%. In the metal-polymer tribological contact, wear resistance of all nanocomposites was increased by 1.5-2.3 times. In the ceramic-polymer tribological contact, loading PEEK with metal nanoparticles caused the intensification of oxidation processes, the microabrasive counterpart wear, and a multiple increase in the wear rate of the composites. The three component "PEEK/10PTFE/0.3 nanofillers" composites provided an increase in wear resistance, up to 22 times, for the metal-polymer tribological contact and up to 12 times for the ceramic-polymer one (with a slight decrease in the mechanical properties) compared to that of neat PEEK. In all cases, this was achieved by the polymer transfer film formation and adherence on the counterparts. The various effects of the four types of nanoparticles on wear resistance were determined by their ability to fix the PTFE-containing transfer film on the counterpart surfaces.

Effect of Adhesion on Mechanical and Tribological Properties of Glass Fiber Composites, Based on Ultra-High Molecular Weight Polyethylene Powders with Various Initial Particle Sizes.

The aim of this study was to assess the effect of adhesion between the non-polar, ultra-high molecular weight polyethylene (UHMWPE) matrix and the glass fiber fillers of various lengths treated with the commercially available "KH-550" agent, on the mechanical and tribological properties of the UHMWPE-based composites. The motivation was to find the optimal compositions of the polymer composite, for the compression sintering manufacturing of lining plates for the protection of marine venders and construction vehicles, as well as transport equipment. It was shown that the initial powder size at equal molecular weight determined the distribution patterns of the glass fibers in the matrix, and, as a consequence, the mechanical and tribological properties of the composites. Based on the obtained experimental data and the results of the calculation by a developed computer algorithm, control parameters were determined to give practical recommendations (polymer powder size and glass fiber length), for the production of the UHMWPE-composites having specified mechanical and tribological characteristics. The "GUR4022 + 10% LGF" composite, loaded with the chopped 3 mm glass fibers treated with the "KH-550", was recommended for severe operating conditions (high loads, including impact and abrasive wear). For mild operating conditions (including cases when the silane coupling agent could not be used), the "GUR2122 + 10% MGF" and "GUR2122 + 10% LGF" composites, based on the fine UHMWPE powder, were recommended. However, the cost and technological efficiency of the filler (flowability, dispersibility) and polymer powder processing should be taken into account, in addition to the specified mechanical and tribological properties.

Increasing Wear Resistance of UHMWPE by Loading Enforcing Carbon Fibers: Effect of Irreversible and Elastic Deformation, Friction Heating, and Filler Size.

The aim of the study was to develop a design methodology for the UltraHigh Molecular Weight Polyethylene (UHMWPE)-based composites used in friction units. To achieve this, stress-strain analysis was done using computer simulation of the triboloading processes. In addition, the effects of carbon fiber size used as reinforcing fillers on formation of the subsurface layer structures at the tribological contacts as well as composite wear resistance were evaluated. A structural analysis of the friction surfaces and the subsurface layers of UHMWPE as well as the UHMWPE-based composites loaded with the carbon fibers of various (nano-, micro-, millimeter) sizes in a wide range of tribological loading conditions was performed. It was shown that, under the "moderate" tribological loading conditions (60 N, 0.3 m/s), the carbon nanofibers (with a loading degree up to 0.5 wt.%) were the most efficient filler. The latter acted as a solid lubricant. As a result, wear resistance increased by 2.7 times. Under the "heavy" test conditions (140 N, 0.5 m/s), the chopped carbon fibers with a length of 2 mm and the optimal loading degree of 10 wt.% were more efficient. The mechanism is underlined by perceiving the action of compressive and shear loads from the counterpart and protecting the tribological contact surface from intense wear. In doing so, wear resistance had doubled, and other mechanical properties had also improved. It was found that simultaneous loading of UHMWPE with Carbon Nano Fibers (CNF) as a solid lubricant and Long Carbon Fibers (LCF) as reinforcing carbon fibers, provided the prescribed mechanical and tribological properties in the entire investigated range of the "load-sliding speed" conditions of tribological loading.

Material Design Methodology for Optimized Wear-Resistant Thermoplastic-Matrix Composites Based on Polyetheretherketone and Polyphenylene Sulfide.

The main goal of this paper is to design and justify optimized compositions of thermoplastic-matrix wear-resistant composites based on polyetheretherketone (PEEK) and polyphenylene sulfide (PPS). Their mechanical and tribological properties have been specified in the form of bilateral and unilateral limits. For this purpose, a material design methodology has been developed. It has enabled to determine the optimal degrees of filling of the PEEK- and PPS-based composites with carbon microfibers and polytetrafluoroethylene particles. According to the results of tribological tests, the PEEK-based composites have been less damaged on the metal counterpart than the PPS-based samples having the same degree of filling. Most likely, this was due to more uniform permolecular structure and greater elasticity of the matrix. The described methodology is versatile and can be used to design various composites. Its implementation does not impose any limits on the specified properties of the material matrix or the reinforcing inclusions. The initial data on the operational characteristics can be obtained experimentally or numerically. The methodology enables to design the high-strength wear-resistant composites which are able to efficiently operate both in metal-polymer and ceramic-polymer friction units.