Thermal-sprayed coatings have been extensively used in aerospace with the main purpose to overcome critical challenges such as abrasive wear, corrosion, and erosion under high temperatures and pressures. Such protective coatings can also play a crucial role in optimizing the efficiency of gas turbine engines and therefore in reducing fuel consumption and CO 2 emissions. CuAl-based thermal sprayed coatings are commonly employed in tribological interfaces within gas turbine engines to improve the fretting wear resistance. These coatings are typically deposited by more traditional thermal spray techniques such as Air Plasma Spray (APS), which can result in high amounts of oxidation within the coating. The main purpose of this study is to critically evaluate lower temperature deposition techniques such as High Velocity Oxygen Fuel (HVOF). More specifically, commercially available Cu-10Al powders were deposited by APS and HVOF and compared in terms of their microstructural, mechanical properties, and tribological behavior at various temperatures. The results showed that the friction coefficient for both coatings was equivalent at room temperature while it was lower for the APS coating at high temperature. Similarly, the specific wear rates showed little difference between the different deposition processes at room temperature while the APS coating had a lower wear rate at elevated temperature when compared to the HVOF coating. The differences in the friction and wear behavior were attributed to differences in the interfacial processes.

Composite coatings using mixed alloy matrices reinforced with carbon-based solid lubricants as feedstock materials were prepared by atmospheric plasma spraying. The aim of the present study was to investigate the tribological characteristics of such coatings exploring potential benefits of CNTs as nano-additive to reduce friction and wear, improving lubrication conditions during operation in tribosystems, such as piston ring – cylinder liner systems. The chemical composition of feedstock materials and the thermal spray parameters during coatings deposition are correlated to friction coefficient and wear rate using pin-on-disk measurements. The developed coatings hybrid behaviour is studied. Co-based cermet as well as metal alloy anti-wear performance along with the promoted lubrication conditions during operation is revealed. The dependence of the developed coatings quality and performance on the characteristics of the feedstock powder is thoroughly discussed.

Self-fluxing alloys are an established thermal spray system in case of superimposed tribological and corrosive loads. A dense coating with high bonding strength can be formed by fusing. Such coating system represent the state of the art in valve technology. Diamond-like carbon (DLC) top coatings are used for friction-reduction. As an alternative approach, this study focuses on the possibility of incorporating solid lubricants in self-fluxing alloy coatings. This allows for higher local stress and failure tolerance as well as a reduced process chain. Molybdenum disulfide (MoS 2 ) was studied as solid lubricant in the self-fluxing alloy NiCrBSiFe. In this preliminary study, the optimization of the MoS 2 content with up to 10.0 wt% was performed in spark plasma sintered (SPS) bulk materials. The wear behavior under oscillating wear conditions was investigated. Besides the decrease in coefficient of friction (COF), the wear resistance was increased by incorporating MoS 2 . Furthermore, the distribution of the solid lubricants within the SPS bulk material and the influence of the production route were analyzed.

Recent studies have shown that graphene can improve the tribological performance of materials by lowering the coefficient of friction and increasing wear resistance. In this present work, the authors evaluate an inductively coupled plasma process that synthesizes graphene nanoflakes in-flight and uniformly deposits them on metallic substrates. The quality of the graphene flakes was characterized and coating surface friction was measured using a ball-on-three-plates tribometer. Test results showed that graphene nanoflake coatings reduced the coefficient of friction of steel from 0.6 to less than 0.2.

Due to good performance in abrasive and sliding wear and enhanced oxidation behavior, coatings based on Co-Cr-W alloys are widely used in industrial applications, where the material is exposed to high temperature. Within the scope of this study, a Co-based alloy similar to commercial Stellite 6, which additionally contains 20.6 wt.% of vanadium, was deposited by Twin Wire Arc Spraying (TWAS). Multi-criteria optimization using statistical design of experiments (DoE) have been carried out in order to produce adequate coatings. The produced coatings have been analyzed with respect to their tribological behavior at elevated temperatures. Dry sliding experiments were performed in the temperature range between 25°C and 750°C. Oxide phases were identified in the investigated temperature range by X-ray diffraction (XRD) using synchrotron radiation. The V-doped Stellite-based coating possesses a reduced coefficient of friction (COF) of about 0.37 at elevated temperatures (above 650°C), which was significant lower when compared to conventional Stellite 6 coating that serves as reference. In contrast, both produced coatings feature a similar COF under room temperature. X-ray diffraction reveals the formation of cobalt vanadate and vanadium oxides above 650°C. The formation of vanadium oxides exhibits the ability of self-lubricating behavior, thus leading to enhanced tribological properties.

NiCrBSi is a material generally used in wear-resistant coatings. In order to improve the tribological properties of atmospheric plasma-sprayed NiCrBSi coatings, Molybdenum (Mo) was incorporated into the NiCrBSi coatings to reduce the friction coefficient and wear rate under dry and oil-lubricated conditions. In this paper, Mo-NiCrBSi composite coatings with Mo content of 5, 10, 20 and 30 wt.% were deposited on stainless steel substrates respectively by atmospheric plasma spray. X-ray diffraction, optical microscopy and scanning electron microscopy equipped with energy dispersive spectroscopy were utilized to investigate the phase structure and surface morphology of the composite coatings. Reciprocating friction tests were conducted to measure the friction coefficients and 3D optical microscopy was used to depict the wear track profiles. The results showed that the 30 wt.% Mo-NiCrBSi coating exhibits the best tribological performance. In addition, MoO 2 and MoS 2 films were formed in the friction process under dry condition and oil-lubricated condition respectively.

Wear-resistant cobalt–based alloy (Stellite 12) coatings deposited by plasma transferred arc (PTA), commonly used to protect critical mechanical components in harsh environments, were modified by addition of hard ceramic particles (TiC) and solid lubricant compounds (MoS 2 and CaF 2 ) to improve the overall tribological performance. In this preliminary study, microstructural, microhardness and tribological analyses were carried out to assess: a) the feasibility of PTA deposition of thermally sensitive phases characterised by very low density; b) the effect of the addition of a mixture of soft and hard phases on the coating hardness; c) the effect of the modified composition in terms of wear resistance; d) the effect of the addition in terms of lubrication (friction coefficient and produced heat). Results showed that: a) an appropriate pre-consolidation of feedstock materials can be effective in preserving the heat-sensitive phases within the microstructure of PTA deposits; b) the addition of a total amount of 5% wt. of solid lubricants and reinforcing carbides produced a limited decrease in the coating hardness (about 13%) and an evident improvement in terms of friction coefficient but, on the other hand, a remarkable reduction (about 30%) in wear resistance. Further investigation will be addressed to optimize the composition of modified feedstock to counteract the softening effect of lubricant phases without depressing the self-lubrication behaviour.

This study assesses the effect of machine hammer peening (MHP) and carbide grain size fraction on the friction and wear behavior of arc-sprayed WC-W 2 C FeCMnSi coatings. SEM examination shows that post-treatment by MHP compresses the coating, reducing both thickness and porosity, particularly in coatings with ultrafine carbides. The treatments also cause cracking, however, especially in carbide phases. Ball-on-disk tests were carried out on as-sprayed and treated samples to determine sliding wear and friction properties, and dry sand rubber wheel tests were used to evaluate abrasion resistance. SEM and EDX analyses before and after wear testing show how coating microstructure and grain size correlate with the friction and wear test results obtained and the given surface treatments.

Cryomilled Cu-Al powder and the Cu/Al mixture were sprayed onto aluminum substrate using the cold spraying process. This study focused on the wear properties of the nanocrystalline (NC) Cu-Al coating in comparison to its coarse-grained (CG) Cu/Al counterpart. The results showed that the as-sprayed deposit presented a dense microstructure. Investigations on the worn surface of the NC coating revealed that the plastic deformation with grooves and some debris were prominent with no visible cracking. Nanocrystalline Cu-Al coating showed a good wear resistance with a low friction coefficient. The enhancement of the wear properties of the NC Cu-Al was attributed to the grain refinement and the superior hardness.

This paper presents the results of a study on the static friction properties of (Ti,Mo)(C,N)-Ni hardmetal coatings deposited by HVOF spraying. Coating samples were sprayed on quenched and tempered CrMo steel friction rings using an experimental feedstock powder. Friction surfaces were characterized based on the geometry and distribution of prominent peaks and static coefficient of friction was measured. Test results show that the hardmetal coatings have good potential for use in frictionally engaged joints, but more work is needed to establish a correlation between coating properties and friction behavior.

Thermal spraying produces coatings with relatively rough surfaces compared to other deposition methods. In this work, NdFeB coatings were deposited on stainless steel by plasma spraying at various standoff distances. Some of the coatings were also annealed. Surface roughness profiles of as-sprayed and heat-treated coatings were measured by contact profilometry and analyzed via statistical methods. The effect of standoff distance and annealing on roughness is discussed along with the significance of measurement direction and evaluation length.

This study investigates the static friction properties of HVOF-sprayed Cr 3 C 2 -NiCr coatings. Measurements of the static coefficient of friction (CoF) of as-sprayed coatings show their potential for use in frictionally engaged joints. The form, orientation, and geometric characteristics of Cr 3 C 2 -NiCr friction surfaces are assessed as well and slipping curves are determined. The results show a standard deviation in the static CoF depending on nominal contact pressure, but it is not yet possible to establish a correlation with coating properties such as carbide grain size and geometrical parameters such as coating roughness.

This study investigates the sliding wear behavior of HVOF sprayed coatings derived from conventional, fine, and nanostructured WC-Co powders. The results show that WC-Co coatings produced from fine and nanostructured feedstocks have significantly higher wear resistance and lower friction coefficients than coatings derived from conventional sized powder. This is attributed to scaling effects in the microstructure and phase evolution of the coating material as explained in the paper.

Gate valves used in oil and gas production undergo stringent qualification before going into service. During qualification there is no external lubrication, leaving contact surfaces susceptible to friction evolution and wear. The work presented in this paper was carried out to better understand the changes that can occur during qualification and where and when the limit for mild wear and stable friction is reached. Ni-Cr alloy gate valve components were coated with WC-CoCr by HVOF spraying and dry sliding wear tests were conducted in nitrogen and in air. The coatings were then evaluated by means of SEM, EDX, and XRD analysis, nanoindentation and surface roughness measurements, and compression tests on micropillars milled out by FIB. Similar tests and analyses were performed on gate valves returned from the field. Examination of the valves that had been in service revealed the presence of oxygen rich layers on polished surfaces due to opening and closing of the gate. Such layers were also observed in coating samples following tribological testing. Initial surface roughness was found to play a role in the development of the oxygen rich layers as well as friction evolution.

This work evaluates the tribological properties of conventional and nanostructured Al 2 O 3 -13TiO 2 coatings obtained by atmospheric plasma spraying. The structure and composition of the composite coatings and powders were analyzed by SEM, TEM, and x-ray diffraction. Nanoindentation and ball-on-disc tests were conducted and surface topography was examined by noncontact 3D profiling. Coating samples of both types were polished and their friction coefficients were measured. The coefficient of friction for nanostructured coatings was 0.51, while that of conventional coatings was 0.62.

Up to now no coating systems are marketable in the field of direct hot extrusion, which provide both surface protection of the parts being in contact to the billet (i.e. container and die), and a significant reduction of the frictional losses being induced by the billet passing along the container walls. To dispense the use of lubricants and to enhance the usable forming capacity of the process, different oxide ceramics were given in one suspension and plasma sprayed. The aim is to reach a mixing of the feedstock to obtain deterministic solid solutions of the oxide phases which show a reduction of their coefficient of friction under dry sliding conditions. To reach this goal the high surface-to-volume ratio of feedstock with primary particle sizes below 100 nm was used. By means of X-ray diffraction it could be proven, that the desired phases could be synthesized. The coatings showed a considerable lowering of their frictional coefficient in tribological testings against 100Cr6 in the region of the operation temperatures for the hot extrusion of aluminium alloys. Besides the experimental work the fundamentals of the mixing process of different oxides regarding crystallographic aspects are discussed.

To meet new regulations and specifications for internal combustion engines, new approaches to significantly decrease fuel consumption and emissions are needed. The deployment of tribologically functional coatings applied by supersonic flame spraying represent a promising technology for achieving these targets. Thermally sprayed coatings can help in improving efficiency of internal combustion engines by reducing the internal friction and improving the durability and wear resistance of the engine’s cylinder wall thereby facilitating extreme engine downsizing concepts. Thermal spraying is also capable of processing highly corrosion resistant materials like alloys and ceramics to enable the safe utilization of biofuels in modern combustion engines. In addition, specific surface structure of thermal spray coatings, including their intrinsic porosity, shows the benefit of reducing the friction by sustaining hydrodynamic friction even in spots with low relative movement, e.g. top and bottom dead center. On top, the open surface porosity can reduce the oil consumption and thereby decrease the polluting emissions of internal combustion engines. The thermally sprayed coatings were applied using HVOF and HVSFS processes deploying various materials, including novel nanostructured powders. The coated cylinders and engines have been compared to state-of-the-art components with respect to friction coefficient, wear and oil consumption.

Nanostructured materials are of particular scientific interest because of their physical and mechanical properties, which are superior to those of conventional materials. They are more widely used in various industrial applications mainly due to decreasing production costs. The work is concerned with a study of the tribological properties of a HVOF sprayed composite of nanostructured WC12Co mixed with nanostructured Fe 3 O 4 , having the properties of solid lubricant. The coatings were sprayed by means of a Hybrid Diamond Jet system. A T-01 ball on disc tribological tester was used to study their resistance to wear and determine the coefficient of friction on the basis of friction force obtained in the course of continuous measurement at a set load. Result of investigations were compared with properties of coatings sprayed with standard WC12Co/ Fe 3 O 4 . The microstructures and compositions of the nanostructured powders and coatings were analyzed by scanning electron microscopy, transmission electron microscopy, and energy dispersive X-ray analysis. Their phase composition was studied using a Bruker D8 Advance diffractometer.

The HVOF sprayed wear resistant hardmetal coatings with favourable sliding properties are suitable for increasing the lifetime of sliding applications, such as pistons of combustion engines, pumps and other hydraulic devices. In practice, the coatings face the problem of their interaction with other media, in the case of sliding wear usually lubricants. In the paper, the friction properties of five different HVOF sprayed coatings are evaluated by pin-on-disk test according to ASTM G-99 under dry and lubricated conditions and lubricated block-of-ring test according to ASTM G77. Several types of lubricants designed for combustion engines were used to compare their influence on coatings sliding wear behavior. Based on the results, the suitability of coatings for the application on the engines parts is discussed and the effect of counterpart material and different types of lubricants on the coefficient of friction and coatings wear rate is analyzed. It was confirmed, that the CrC-based coating are more suitable for the application under the condition corresponding to combustion engines, e.g. elevated temperature and steel counterpart, than the WC-based coatings. From the group of CrC-based coatings, the superior behavior was observed at the CrC-CoNiCrAlY coating, the matrix material of which offers further enhancement of the sliding wear behavior.

The combination of thermally sprayed hard coatings with a polymer based top coat leads to multilayered coating systems with tailored functionalities concerning wear resistance, friction, adhesion, wettability or specific electrical properties. The basic concept is to combine the mechanical properties of the hard base coating with the tribological or chemical abilities of the polymer top coat suitable for the respective application. This paper gives an overview of different types of recently developed multilayer coatings and their application in power transmission under dry sliding conditions. State of the art coatings for dry sliding applications in power transmission are mostly based on thin film coatings like DLC or solid lubricants, e.g. MoS 2 . A new approach is the combination of thin film coatings with combined multilayer coatings. To evaluate the capability of these tribological systems, a multi-stage investigation has been carried out. In the first stage the performance of the sliding lacquers and surface topography of the steel substrate has been evaluated. For this purpose case-hardened steel substrates were laser textured and coated with different sliding lacquers. In the following stage thermally sprayed hard coatings were tested in combination with different sliding lacquers. For this test stage steel samples were coated with oxide ceramics, metal alloys and hard metals by high velocity flame spraying (HVOF) and high velocity suspension flame spraying (HVSFS). After a grinding process several types of sliding lacquers were applied by air spraying on the coated specimens. Wear resistance and friction coefficients of combined coatings were determined using a twin disc test-bed.