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.

[Construction and identification of recombinant Mycobacterium smegmatis vaccine expressing Cysticercus cellulosae cC1 antigen].

OBJECTIVE: To construct recombinant Mycobacterium smegmatis vaccine expressing Cysticercus cellulosae cC1 antigen. METHODS: The recombinant pET28a-cC1 plasmid was extracted and double digested by Xho I and BamH I restriction enzymes, and shuttle plasmid pMV261 was extracted and double digested by Hind III and BamH I restriction enzymes. Both fragments were modified by Klenow fragment to form blunt end, then the large fragments of cC1 and pMV261 plasmid were purified and ligated by T4 ligase enzyme. The recombinant pMV261-cC1 plasmid was constructed and sequenced. Then the pMV261-cC1 plasmid was transformed into Mycobacterium smegmatis by the electrotransformation method. The recombinant cC1-Mycobacterium smegmatis was induced by heat and identified by the Western blotting method with the sera of cysticercosis patients. In addition, the growth states of the Mycobacterium smegmatis and the recombinant cC1-Mycobacterium smegmatis were compared and the growth curves were drawn. RESULTS: The restriction enzyme and sequencing results showed that the recombinant pMV261-cC1 plasmid was successfully constructed. After heat induction, a 40 kD band was showed by PAGE analysis of cC1-Mycobacterium smegmatis. The Western blotting results showed that the sera of cysticercosis patients could recognize the 40 kDa band, which suggested that cC1 protein was expressed in Mycobacterium smegmatis. Compared with the Mycobacterium smegmatis, the recombi- nant cC1-Mycobacterium smegmatis showed no significant difference in proliferation characteristics. CONCLUSION: The recombinant cC1-Mycobacterium smegmatis vaccine has been successfully constructed.

[Cloning and efficient prokaryotic expression of soluble stage-specific antigen cC1 from Cysticercus cellulosae].

OBJECTIVE: To clone the coding gene of the stage-specific antigen cC1 from Cysticercus cellulosae and express high levels of soluble cC1 in E.coli. METHODS: The cC1 gene was amplified from Cysticercus cellulosae by RT-PCR and cloned into pMD18-T vector, followed by subcloning into the prokaryotic expression plasmid pET28a. The recombinant plasmid was transformed into E.coli BL21(DE3) and the expression conditions were optimized. The expressed product was purified by Ni(+)-affinity chromatography, analyzed by high-performance liquid chromatography (HPLC), and identified with SDS-PAGE and Western blotting. RESULTS: The fragment length of the amplification product by RT-PCR was 1056 bp. Comparison of the amplified gene sequence with the cC1 gene in Genbank identified a samesense point mutation at 423 position in the gene cloned into the expression plasmids. After a 6-h induction with 0.05 mmol/L IPTG at 37 degrees celsius;, the expression of the 40 kd soluble fusion protein exceeded 60% of the total bacterial protein, and the fusion protein was recognized by Cysticercus-infected human sera. The purity of the fusion protein was about 94% after purification by affinity chromatography. CONCLUSION: The stage-specific antigen cC1 from Cysticercus cellulosae has been successfully cloned and the soluble protein efficiently expressed in E.coli, which provides the basis for its further study and application.