Abstract Since 2004; when it was first isolated; graphene has been the subject of great attention both in scientific research and in various industrial applications; thanks to its excellent physical; chemical and mechanical properties. Recently; studies have shown graphene as a new solid lubricant that improves the tribological performances of materials by lowering the coefficient of friction and increasing the wear resistance. However; the industrial-scale manufacture of graphene has been limited by the lack of a fast; inexpensive and effective coating method for large surfaces. As part of our work; we have developed a versatile inductively coupled plasma technology process in which graphene nanoflakes (GNFs) were synthesized in-flight and uniformly deposited on metallic substrates. The coatings and the quality of the graphene flakes were characterized; and the macroscale tribological performance of the GNFs coating was studied using a ball-on-three-plates tribometer. The results showed that the GNFs coating reduced the coefficient of friction (COF) from 0.6 to below 0.2 compared to an uncoated substrate. The innovative approach to GNFs coating preparation provided in this work is expected to have diverse and important applications in the industry.

Abstract High Velocity Oxy Fuel (HVOF) sprayed coatings of Cr3C2- NiCr containing solid lubricants such as nickel cladded graphite and hexagonal boron nitride were successfully developed and characterised with the aim of optimizing the frictional and wear behaviour; particularly in the field of industrial valves. HVOF coating technology was applied for the integration of solid lubricants as an alternative to atmospheric plasma spraying in order to achieve strong cohesion between particles; but simultaneously to minimize thermal decomposition. Microstructure and composition of these HVOF coatings were characterized and correlated to the results of tribological and corrosion tests. The abrasive wear test in the temperature range of 30 to 650 °C was carried out with the ball-on-disk setup. Self-mated linear contact setup was also utilized to examine the coatings in the practiceoriented tribological system. Corrosion properties were studied through electrochemical measurement in artificial seawater. Through the solid lubricant integration; the friction and wear volume could significantly be reduced at room temperature by minimizing particle pull outs and adhesive wear. Yet; due to the extrinsic property of the solid lubricants; their lubricating effect was highly dependent on the atmosphere and temperature. Cr3C2-NiCr with hBN tends to exhibit more stable wear resistance at a broader spectrum of temperatures; and also allows the utilization of the coating at temperatures higher than 450 °C. The application of HVOF is a promising alternative for the integration of the solid lubricants to reduce friction coefficient and wear.

Abstract For components that are required to function in sliding or rubbing contact with other parts, degradation often occurs through wear due to friction between the two contacting surfaces. Depending on the nature of the materials being used, the addition of water as a lubricant may introduce corrosion and accelerate the degradation process. To improve the performance and increase the life of these components, coatings may be applied to the regions subject to the greatest wear. These coatings may be engineered to provide internal pockets of solid lubricant in order to improve the tribological performance. In the present study, coatings containing a solid lubricant were produced by thermal spraying feedstock powders consisting of a blend of tungsten carbide-metal and a fluorinated ethylene-propylene copolymer-based material. The volume content of this Teflon-based material in the feedstock ranged from 3.5 to 36%. These feedstocks were deposited using a high velocity oxy-fuel system to produce coatings having a level of porosity below 2%. Sliding wear tests in which coated rotors were tested in contact with stationary carbon-graphite disks identified an optimum level of Teflon-based material in the feedstock formulation required to produce coatings exhibiting minimum wear. This optimum level was in the range of 7-17% by volume and depended on the composition of the cermet constituent. Reductions in mass loss for the couples on the order of 50% (an improvement in performance by a factor of approximately two) were obtained for the best-performing compositions, as compared to couples m which the coating contained no solid lubricant.

Abstract Quasicrystals are materials whose structure cannot be understood within classic crystallographic methodology. Quasiperiodic structures have a long-range orientation order but lack transitional periodicity. This non-periodic structure gives quasicrystals a unique range of physical properties including: poor thermal conductivity; excellent crystallographic stability up to their melting point; can be thermally sprayed to display either wear resistant or abradable characteristics; possess excellent release properties (low coefficient of friction); and, they are non-work hardenable. This effort describes parametric conditions utilized to deposit coatings of identical parent metal chemistry but with vastly different end properties.

Abstract Amorphization induced by sliding wear and consequent wear resistance have been investigated in relation with the microstructure of Fe-Cr-B alloy spray coatings. The Fe-Cr-B spray coated layer exhibited much higher wear resistance and significantly lower friction coefficient in comparison with that of low carbon steel substrate thanks to the amorphous surface film formed during the dry sliding wear. Electron microscopy on the cross-section of the coated layer exhibited intra-particle segregation associated with rod-shape Cr rich (Cr,Fe)xB particles in the matrix of Fe-Cr solid solution phase. From the observations using TEM and EDS, Fe-Cr solid solution phase with super-saturated B and Si content was confirmed to be the phase which mainly contribute to the crystalline-to-amorphous transition induced by sliding wear. The formation of oxide inclusions seems to impede the crystalline-to-amorphous transition by lowering the solute content in Fe-Cr solid solution phase. Keywords : microstructure, wear-resistance, Fe-Cr-B alloy, amorphization, detonation gun

Abstract Different oxides layers have been studied by friction in natural sea water medium under the same conditions (Cr 2 O 3 , Al 2 O 3 , Al 2 O 3 + Cr 2 O 3 ). The evolution of different parameters have been analyzed: friction coefficient, electrochemical potential, degradation of the layers in the contact, impedance spectroscopy. The main result observed is concerned by the cracking of the coatings under stresses, in such a way interconnection paths are rapidly present between the substrate and the sea water medium through the layers. The coating impregnation process by epoxy, before the tribocorrosion tests, improve the protection of the substrate against the corrosion.

Abstract For applications in which two contacting surfaces are in constant motion relative to each other, materials that are both wear resistant and non-abrasive are often required. Such attributes become even more important when the moving contact occurs with no liquid lubricants present to facilitate sliding. In the present study several WC-based coatings deposited using the HVOF process and containing one or more metal constituents as the binder (or matrix) phase were evaluated to determine their performance under conditions cf sliding wear. Image analysis of the coatings indicated a level of porosity of less than 1%. Hardness measurements found that values for the Vickers microhardness number were in the range of 1100-1500. For the wear tests, the test couple consisted of a coated ring (thrust washer type design) rotating against a stationary carbon disk. For each test, the contact load, speed of rotation and duration were controlled. During the test, the temperature of the carbon disk and the torque were recorded using a data acquisition system. This data was used to determine the coefficient of friction for each couple which, together with the results of measurements of weight change, provided a measure of the comparative performance of the various coatings. The preliminary results indicated that the values for the coefficient of friction for the various couples ranged from 0.15 to 0.29. The three coating compositions consisting of lONi-WC, 12Co-WC and 17Co-WC were found to out-perform other WC-based materials in these sliding wear tests.

Abstract Protective coatings adapted for titanium alloys which come into frictional contact with one another are described. Among the many coatings investigated, the best ones are cobalt based. The coatings are sprayed only on one of the rubbing surfaces. During rubbing, a small part of the coating transfers to the unprotected titanium alloy surface. The rubbing pair is thus essentially composed of two cobalt alloy-based surfaces. This leads to low coefficient of friction and little or no damage to the rubbing surfaces. The coatings find particular application in the protection from adhesive and fretting wear, galling and seizure of gas turbine and jet engine parts or the like made from titanium alloys.

Abstract Gas atomized Al 63 Cu 25 Fe 12 powders of varying size fractions were plasma sprayed to study the relationships between coating microstructure and tribological behavior. After spraying, annealing was performed on one of the coatings. Abrasion and sliding wear tests were performed on the coatings. The results indicate a correlation between the abrasion resistance and hardness of the coatings. Furthermore, sliding wear tests reveal lower coefficients of friction for the as-sprayed coatings compared to the annealed coatings.