Key Engineering Materials Vol. 674

Abstract: Basalt reinforced composites are quite recently (during last 20 years) developed materials having low density, high specific strength, good frictional, heat and chemical resistance. Natural mineral based fibres are potential alternatives to glass fibres for their strength and to carbon fibres for their lower cost. In order to use basalt reinforced composites for lightweight applications, it is necessary to perform wear characterisation. Basalt fibre, powder and scales reinforced, unsaturated polyester and epoxy resin composites were fabricated with various ratios of basalt and polymeric matrixes. The tribological behaviour of basalt reinforced composites was determined according to the ASTM G132 standard test method for pin abrasion testing. Results showed that type and content of reinforcement have a significant influence on the mechanical and tribological properties of the composites. Scanning electron microscope images are given to illustrate the wear mechanism of composites.

Abstract: In this article by the example of two abrasive materials from oxide of aluminum and nitride of boron, the processes developing in a thin surface layer between a grain of abrasive and the material, processed by microcutting with speeds of 40 and 160 m/s, the preforms from armko-iron, steels R18, 45, H18N10T and alloy titanium VT6 are investigated, using the scanning electron microscopy method.

Abstract: During the irradiation of high molecular weight poly (tetrafluoroethylene) (PTFE) in presence of oxygen perfluoroalkyl (peroxy) radicals and functional groups are formed which allow chemical coupling reactions (cc = chemical compatibilized) with oils and plastics. The micropowder resulting from the irradiation of PTFE are used in base oils to improve the tribological properties significantly if oil molecules are covalently linked to primary PTFE particles in the oil dispersion. These oil-PTFE-cc-dispersions show primarily anti-wear (AW) properties. The use of additional reactive groups (e.g. phosphite groups) in the oil gives the dispersions extreme pressure (EP) properties additionally. This article demonstrates the usefulness of the oil-PTFE-cc-dispersions in rolling bearings and in ropes using phosphite-modified PTFE products as additive in lubricants. The investigations are completed by wear tests with different contact geometries. A model is shown to explain the effect of phosphite groups on 100Cr6 metallic surfaces.Keyword s: PTFE, high performance lubricant, oil-PTFE-cc dispersion

Abstract: The paper presents the results of wear tests of shot-peened and non-shot-peened cast steels with the use of an especially designed test rig simulating real operating conditions of chain wheels. The chain wheels subjected to tests were operated with the use of loose quartz abrasive. The studies involved the determination of strength and plastic properties, hardness distributions, microstructure and linear wear of the selected cast steels. Based on the results obtained, the following was found: the abrasive wear of cast steel chain wheels increased after shot peening.

Abstract: The present study addresses the influence of the gradient microstructure of nanocrystalline TiAlSiN coatings on their tribological behaviour. Cathodic arc deposition was applied to elaborate such coatings, with a total thickness of 3.5 μm, onto stainless steel substrates. Their microstructure has been characterised via Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS) and has been reported in detail previously. Since the main application of TiN-based coatings is the enhancement of the anti-wear resistance of metallic substrates, this work is focused on the tribological performance of gradient TiAlSiN coatings under dry sliding conditions. For this purpose, tests were carried out in a ball-on-disc apparatus, using an alumina ball as counterbody. The influence of the sliding velocity on the evolution of the friction coefficient and the wear lifetime of the gradient coatings has been evaluated in comparison to those of TiN coatings of the same thickness, tested under the same experimental conditions. It was found that the gradient microstructure results in an increase of the coatings’ mean lifetime by an average factor of three.

Abstract: The erosive wear resistance of manual arc welded hardfacings with low-carbon or stainless steel matrix, varied WC grain size (0.23-0.61 mm) and varied WC content (max. 40 wt. %) was studied. Electrodes were produced by JSC “Anykščių varis“ company (Lithuania). Testing was performed according to GOST 23.201-78 standard using the Centrifugal Accelerator of Kleis (CAK). Test parameters were as following: room temperature; silica sand with particles size 0-0.6 mm; particle impact velocity – 10, 30, 50 and 80 ms-1; impact angles – 30° and 90°.It was found that there is only minor (usually not more than 2 times) effect of WC grain size and content on erosive wear of studied hardfacings. There is strong effect of velocity on wear rate. The graph showing the effect of abrasive particle‘s kinetic energy on wear rate is provided. It was found that the wear rate increases 2.8 times faster than kinetic energy of abrasive particles. The wear resistance of coatings could be improved by addition of WC when hardfacing is tested with impact angle of 30°. However, for impact angle of 90° the addition of WC into hardfacing has no effect or is even detrimental (leading to the increase in wear rate). The hardfacings with WC grain size in the range from 0.14 to 0.27 mm have the lowest wear rates during testing with impact angle of 30°. A discussion of the wear mechanisms for the hardfacings is provided, based on data and observations obtained by using scanning electron microscopy.

Abstract: In the present study, the influence of the volume fraction of graphene on the tribological properties of copper matrix composites was examined. The composites were obtained by the spark plasma sintering technique in a vacuum. The designed sintering conditions (temperature 950°C, pressing pressure 50 MPa, time 15 min) allowed obtaining almost fully dense materials. The tribological behaviour of copper-graphene composite materials was analysed. The tests were conducted using a CSM Nano Tribometer employing ball-on-plate tribosystem. The friction and wear behaviour of copper-graphene composite materials were investigated. An optical microscope, interferometer, and scanning electron microscope were used to analyse the worn surfaces. In friction zone, the graphene acts as a solid lubricant, which results in the increase in the content in the composites positively influencing the tribological characteristics of the steel- Cu-graphene composite.

Abstract: Fe-based hardfacings with high vanadium content become more important for industrial applications because of low production and material costs, combined with high abrasion resistance. The aim of this work is to gain a deeper fundamental understanding of heat management impact of a Fe-based hardfacing alloy on resulting wear properties related to real field conditions. In this work a FeVCrC alloy was deposited by plasma transferred arc welding technology varying solidification kinetics due to active heat management. In-situ thermal couple measurements during processing were performed to determine t_8/5 cooling-off time relevant for precipitation formation. Microstructural investigation was done by microscopy (e.g. OM, SEM), X-ray diffraction as well as macroscopic hardness, respectively. Wear performance was characterized by using the ASTM G65 dry-sand rubber-wheel procedure A for 3-body abrasion under low-stress. In addition 2-body impact/abrasion behavior was evaluated utilizing continuous impact abrasion test (CIAT). For quantitative wear description 3D microscopy and mass loss determination were done after the test. Additional SEM investigations on the worn surface and on the tribologically stressed near-surface region were applied to deepen the fundamental wear understanding.Results showed that the t_8/5 cooling-off time is strongly influenced by solidification conditions due to thermal substrate properties. Nevertheless t_8/5 cooling-off time is not necessarily correlated to wear behavior. Significant variations of micro-and macro structural parameters were obtained for dilution with substrate, precipitation content and distribution, whereas macro hardness was obtained on a relatively constant level. Two-body impact/abrasion wear behavior showed plastically deformed zones where the fine precipitations act in a ductile way. Under 3-body abrasion conditions precipitations of hard phases play a dominant role for providing high wear resistance related to microstructural features like amount, distribution and size of precipitations. It can be claimed that no direct correlation of 2-body and 3-body wear rates with deposition hardness of hardfacing has been observed.

Abstract: The aim of the paper was to investigate the temperature effect on the wear mechanism of AlCrN coated components. The coating was deposited by Physical Vapour Deposition process (PVD) on WC/Co substrate. Tribological tests were performed in sliding conditions using high temperature T‑21 tribotester, produced by ITeE-PIB Radom. The tests were performed in a ball-on-disc configuration (Si₃N₄ ceramic ball), under dry friction conditions at room temperature, 600°C and 750°C. An optical microscope, interferometer, and scanning electron microscope were used to analyse the worn surfaces. Following this study, it was found that wear resistance of the coating AlCrN tribosystem depended on the temperature. The biggest wear was reported at room temperature. At 600°C the intensity of wear of the coating was 4-fold lower, and at 750°C wear was 6-fold lower that at room temperature. High temperature wear resistance of AlCrN coating involves creating protective oxide layer. Performed analysis of structure the surface layer, showed a much higher content of oxygen in wear scar than outside. At high temperatures, friction additional intensified oxidation process thus the amount of oxygen in surface layer increased with temperature. Oxide layer, Al₂O₃ and Cr₂O₃ probably, created at high temperature was a barrier to further oxidation of the coating and had very high wear resistance at high temperature.

Abstract: Polyimide (PI) composites have been widely used in a space science due to extraordinary properties, such as excellent mechanical and electrical properties, good thermal stability and chemical inertness, as well as high wear resistance. However, atomic oxygen (AO), as one of the main radiated constituents in low earth orbit, had an important influence on the structrural and tribological properties of the polyimide matrix. To investigate the mechanism of AO erosion on polyimide, MoS₂/Al₂O₃/PI composites were fabricated by means of a hot-press molding technique and irradiated by AO in a ground-based simulation system. The chemical composition change of the irradiated surface was examined by X-ray photoelectron spectroscopy (XPS). Then, the friction and sliding wear behavior against GCr15 steel balls were evaluated in a ground-based simulation facility using ball-on-disk tribology test rig. The worn morphologies and radiated surfaces of the materials were observed by Scanning electron microscope (SEM) to reveal the wear mechanism. Experimental analysis indicated that oxidation induced by AO irradiation and degradation of PI molecular chains on the composite’ surface results in change in chemical composition and formation of “carpet-like” structures. Affected layer, gradually formed during the process of irradiation, plays an important role for wear performance of the materials increasing friction coefficient and wear rate. Incorporation of Al₂O₃ nanofibers and MoS₂ nanoparticles is shown to be favourable for AO resistance, which is helpful for improvement in wear resistance of the PI.