Key Engineering Materials Vol. 527

Abstract: The aim of the current study was to elaborate and compare abrasive wear resistance of thick coatings deposited with different hardfacing technologies. To produce metal matrix composite (MMC) coatings commercial iron and nickel based powders with recycled hardmetal content of 40 vol. % were studied. For deposition technologies plasma transferred arc (PTA) hardfacing, high-velocity oxy-fuel (HVOF) spraying were used. The microstructure of produced thick coatings was examined, including the distribution of hard phase and homogeneity of metal matrix. Micromechanical properties, including hardness and elastic modulus of features were measured by universal hardness measurements. Furthermore, behavior of coatings subjected to abrasive rubber-wheel wear (ARWW) and impact wear (AIW) tests were studied. Wear resistance of experimental PTA hardfacings at low velocity ARWW and AIW tests notably exceeds that of HVOF sprayed coatings. Wear mechanism dominating at abrasive wear in most cases is the removal of metal matrix due to lower hardness. Assignment of hardmetal waste as initial material can significantly decrease the cost of production, improve the mechanical characteristics of coatings and consequently increase their wear resistance. Results indicate, that the choice of matrix for the same reinforcements can also be as an important factor for combating abrasive wear. Fe-based thick coating, produced by PTA is more wear resistant compared to the Ni-based ones.

Abstract: Basic mechanical and wear properties of a commercial copper based composite Glidcop were studied. A Glidcop AL-60 grade (with 1.1 wt.% Al2O3) was used as the initial material. It was further treated by the Equal Channel Angular Pressing (ECAP) process in order to induce massive plastic deformation and to achieve very fine grained microstructure. Both, as-received and ECAP-ed materials were then characterized and the results compared. Hardness and elasticity modulus of the experimental materials were measured by instrumented indentation. Tribological properties were studied by pin-on-disk technique in dry sliding against a steel ball at a various temperatures from room temperature up to 600 °C. For all systems the coefficient of friction and specific wear rates were evaluated. Worn surfaces were studied by scanning electron microscopy and level of oxidation was measured using EDX spectrometry. It was found that above 200 °C the coefficient of friction decreased by about 50 %. The wear resistance with increasing temperature increased due to formation of harder oxide rich surface layer. Damage mechanisms were identified and their relationship with structural characteristics was inferred.

Abstract: The development of polyimide (PI) composites, particularly for use in aerospace and tribological applications, has gained importance over the past decades. However, there have been scarce studies on the ultraviolet (UV) or atomic oxygen (AO) irradiation characteristics of the polyimide composites, the understanding of which may aid in expanding its application in space environment. To study the irradiated effects of UV and AO on the tribological behaviour of the PI composites, glass fibers (GF)-reinforced PI composites filled with Talc were irradiated by UV or AO in a ground based simulation system. Glass fibers (GF) reinforced PI composites filled with talc were fabricated by means of a hot press molding technique. The volume contents of the talc (10, 20, 30 %) were chosen to study the effects of filler content on the tribological behavior of the composites, while the proportion of the glass fiber was kept at 15 vol %. To contrast the different effects of UV and AO irradiation on the tribological properties of the composites, experiments without irradiation and after UV or AO irradiation were conducted. The specimens of the composites were irradiated with UV for a period of 4h, while AO irradiation chosen for 6h. Dynamic mechanical analysis (DMA) and thermal gravimetric analysis (TGA) measurements were conducted. The friction and wear behaviors of the composites, rotating against GCr15 steel balls, were investigated on a ball-on-disk test rig. at room temperature and at a rotating speed of 0.1256 m/s and a load of 1N. Experimental results revealed that the composites exhibited high modulus and wear rate values with increasing talc content, but low coefficients of friction (COF). After AO irradiation, the COF of the composites increased, but UV irradiation had no obvious effect on the tribological property of the composites. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) study of the composite surface showed that the chemical composition of the surface changed after UV irradiation because of the photooxidation and rough surface or even mountain-like structures were formed after AO erosion. The experiments indicated that the different space environments had an important effect on the tribological properties of the polymer composites. It is expected that this study may help expand the application of the polyimide composites in the field of space.

Abstract: Lead containing bronzes are widely used as bearings, bushings and sliding plates. Lead is a heavy metal and known to be harmful to people and environment, but currently no feasible substitute material exists in many engineering applications. The ecological considerations in recent years have led to restrictions by the European Commission on the use of lead for example in the bearings used in cars (The Directive on the end-of-life of vehicles). However, due to the lack of substitutes for lead in many application areas, the ban has been given an exemption. The aim of the ECOBEARINGS project was to evaluate, whether any metallic or composite bearing material has similar tribological and mechanical properties as the currently used standard lead bronzes. In heavy machine engineering, lead bronze alloys containing up to 10-20 % of Pb are commonly used as sliding bearings such as thrust and journal bearings. In lead bronzes, lead has a function of a solid lubricant especially during the start of the operation of the bearing. Additionally, lead bronzes form a soft sliding surface where small contamination particles can be embedded in the material without causing an abrasive effect. A variety of different lead-free bearing solutions are already commercially available, but there are many applications where the emergency lubricating properties of lead cannot be substituted yet. It has been proposed that other solid lubricants such as graphite or bismuth could possibly have similar tribological characteristics as lead in bronze bearing materials. In this work, bronze alloys containing solid lubricants such as bismuth and graphite were produced by casting and spray forming. The alloys were characterized and tribologically tested in boundary lubricated and dry sliding conditions. The results were compared with the testing results of lead bronze. On the basis of the results, it was noted that different solid lubricants have very special tribological characteristics. This information can be used in the further development of ecologically sound bearing materials.

Abstract: An abrasive wear analysis of the hardened surface layer in high austenitic manganese steel is presented to determine the effects of plastic strain and strain rate. The pre-deformation levels used in this study represent the typical surface layers resulting from the abrasive contact of Hadfield type steels. The effect of dynamic loading used in the pre-straining is accounted for alongside with various plastic strain levels. A friction study of the surface was included to determine the localization of work hardening and the ability of the microstructure to resist abrasion.

Abstract: Wear tests were conducted using a pin-on-disc tester. In the experiment, a steel disc of hardness 40 HRC was put in contact with a steel pin of hardness 64 HRC with spherical end. Disc samples were prepared in order to obtain very similar values of the Sq parameter of one-process and two-process isotropic surfaces. Height of one–process disc surfaces, characterized by the Sq parameter was in the range 0.5 – 6 µm, but of two-process textures 1- 4.5 µm. Dry and lubricated tests, using different contact conditions were carried out. During testing, the friction force was monitored as a function of time. Wear of disc was measured after the test using white light interferometer. Tests under boundary lubrication condition were done using L-AN 46 oil.

Abstract: Current paper handles the comparison of impact wear and sliding wear properties of the hard PVD single layer TiN and Ti(C,N), multilayer (Ti,Al)N and nanocomposite FiVIc® coatings on the plasma nitrided low-alloy 42CrMo4 steel. All the studied coatings demonstrated a relatively high impact wear resistance at the low (104) and medium (105) number of impacts, however, all the studied coatings vanished at the high number of impacts (106). Most extensive wear among the coatings during the sliding wear test was observed for the (Ti,Al)N coating, the FiVIc® showed the least extensive wear; the most extensive wear of the counterbody (hardened steel ball) was registered for the (Ti,Al)N coating, the lowest – for the FiVIc® and Ti(C,N) coatings. The principle wear mechanism of coatings was tribooxidation and mild abrasion, of the counterbody – plastic deformation

Abstract: Abrasive wear into embedded abrasive (Al₂O3) of Fe–C–Si–Mn–Cr–B layers welded by an electric arc has been studied. The amounts of carbon (0.45–2.92%), chromium (1.1–18.7%) and hardness (73–82 HRA) of welded layers have no impact on wear. Due to great differences in hardness of welded layers and abrasive, the metal surfaces are intensively wearing due to microcutting. It is determined that the harder the layers, the greater their resistance to wear. Alloying of a medium content carbon layer (0.45% C) with boron of 0.51% reduces the wear up to 27%. Minimal amounts of carbon 0.45%, chromium 1.1%, boron 0.5% guarantee stable and high resistance of welded layers to wear.