Graphite is useful in high-temperature applications in many engineering fields, such as heat-treating, brazing, and sintering industries. As the operation becomes severe, carbon experiences degradation leading to failure. In this study, a protective coating of W and Mo as the intermediate layer by air plasma spraying on graphite substrate was investigated to find a better intermediate layer. Their performance was explored as a bonding layer in a protective alumina-YSZ ceramic topcoat. X-ray diffraction and scanning electron microscope were used to observe the cross-section of coatings and the difference in the bonding characteristics between W and Mo, respectively. W was found inferior to Mo as a bonding performance over 1450 °C in view of carbide formation against the thermodynamic data. It seems to be related to the formation of a barrier layer as oxide during air plasma spraying.

One of the main levers to reduce CO2 emissions in cars and trucks is mass and friction reduction, which is often achieved through the use of special coatings. The aim of the present work was to develop metal-ceramic-lubricant composite coatings with the best combination of wear, seizure, fatigue, and thermal resistance. Metal-based coatings incorporating hard particles and solid lubricants were cold sprayed onto steel substrates and the relationship between coating microstructure and tribology was studied. To meet the demanding tribological requirements of heavily loaded engines, the interfaces between the different components were optimized by selecting appropriate feedstock powders and assessing a wide range of process parameters. Alumina-reinforced bronze composite coatings were made from powders with different morphologies. Aggregated ceramic powders were found to be more beneficial in terms of wear than massive powders, and graphite was found to be effective for reducing seizure.

High-velocity oxyfuel (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 their friction and wear behaviour. HVOF technology was used for the integration of solid lubricants to achieve strong cohesion between particles while minimizing thermal decomposition. Coating microstructure and composition were measured and correlated to the results of tribological and corrosion tests. The integration of the solid lubricant greatly reduced friction and wear volume at room temperature, but the lubricating effect was highly dependent on atmosphere and temperature. Cr3C2-NiCr with hBN, however, tends to exhibit more stable wear resistance over a wider temperature range and can be used at temperatures beyond 450 °C.

Chemical composition differences between the feedstock powder and the final coating of a series of composite abradable coatings were investigated. Graphite filler material mass distributions in the coating, overspray powder and burned power is calculated. A preliminary mechanism of graphite loss during the deposition process is established. It is found that the graphite content in the coating is significantly lower than that in the feedstock powder. Over 70% graphite in the feedstock powder is lost during the deposition process. Melting and shrinkage of the nickel shell of the nickel cladded graphite particle as flying through the flame, which resulted in the exposure of the graphite core to the flame and substrate, is the main reason for graphite loss and chemical composition change between the feedstock powder and the final coating. A random manner of particle structure transformation in the flame and its reactions with the spray environment is concluded as an important reason for the poor process repeatability of abradable coatings.

The microstructure of thermal barrier coatings (TBC) plays an important role in the thermal cycling behavior of TBCs. In this study, ceramic coatings with different pore structures were prepared by atmospheric plasma spraying (APS). Graphite with different morphology was used as pore former to adjust the pore structure of the coatings. Then, the thermal cycling behavior of TBCs with different structure was characterized. By depositing a porous 8YSZ layer on the conventional 8YSZ layer, the thermal cycling life of TBCs can be improved. However, when the porosity of the porous layer increased to about 30%, the porous layer detached from the under layer after several cycles. An elastic energy model was applied to explain the thermal cycling behavior of TBCs

In this study, suspension plasma spraying is used to produce cast iron coatings that benefit from a graphite structure. In order to increase the graphite content, different hydrocarbons in the form of liquid suspension (hexane and toluene) and gas precursor (methane) were injected into the plasma stream along with iron powder. Besides promoting the formation of a soot carbon structure, liquid hydrocarbon injection also prevents in-flight particle oxidation, which is a major concern when spraying metals. In addition, it has been observed that using a shroud during spraying significantly increases the amount of soot carbon in cast iron coatings, which can be transformed into graphite by post annealing.

The goal of this study is to find applicable spray conditions for producing tungsten (W), zirconium carbide (ZrC), and W-ZrC cermet layers. In the experiments, W and ZrC powder mixtures were fed into the plasma of a water-stabilized plasma gun and coatings approximately 1 mm thick were sprayed on graphite substrates. Pure W and pure ZrC were deposited under similar conditions. Microhardness, surface roughness, XRD, XRF, dilatometry, and spectroscopic techniques were used to characterize the coatings. The resulting coatings were found to be hard with a high elastic modulus.

Cast iron coatings containing solid lubricant of graphite are an attractive candidate for wear resistant applications of an aluminum alloy substrate. It is difficult to generate a graphite structure in coatings sprayed with as-atomized cast iron powder which contains a few graphite owing to their high solidification rate. Although a graphite structure is remained in coatings sprayed with fully annealed cast iron powder, graphite carbon becomes lower than that in the annealed powder because of the in-flight oxidation and dissolution of graphite into molten iron. The present study is focused on an increase in graphite carbon through flying droplet diagnostic at a certain spray distance, that is, in-situ measurements of droplet temperature and velocity. Water-atomized cast iron powder which was annealed at 900°C for 3.6ks, was supplied as a spray material. The fully annealed powder was plasma-sprayed onto an aluminum alloy substrate, as well as carrying out flying droplet diagnostic. The amount of graphite carbon can be estimated by flying droplet temperature and velocity, which are controlled by spray parameters such as plasma gas flow rate and plasma current. It is confirmed that droplet velocity exhibits stronger influence on graphite carbon compared with droplet temperature. High velocity causes an increase in graphite carbon, so that it is possible to fabricate graphite-graded cast iron coatings with high amount at the surface and low at the interface.

In order to improve gas turbine performance it is possible to decrease back flow gases in the high temperature combustion region of the turbo machine reducing shroud/rotor gap. Thick and porous TBC systems and composite CoNiCrAlY/Al 2 O 3 coatings made by Air Plasma Spray (APS) and composite NiCrAlY/graphite coatings made by Laser Cladding were studied as possible high temperature abradable seal on shroud. Oxidation and thermal fatigue resistance of the coatings were assessed by means of isothermal and cyclic oxidation tests. Tested CoNiCrAlY/Al 2 O 3 and NiCrAlY/graphite coatings after 1000 hours at 1100°C do not show noticeable microstructural modification. The oxidation resistance of new composite coatings satisfied Original Equipment Manufacturer (OEM) specification. Thick and porous TBC systems passed the thermal fatigue test according to the considered OEM procedures. According to the OEM specification for abradable coatings the hardness evaluation suggests that these kinds of coatings must be used with abrasive tipped blades. Thick and porous TBC coating has shown good abradability using tipped blades.

Graphitization behavior of water-atomized cast iron powder at each thermal spraying step, such as droplet flight, droplet impingement and splat layering, was successively examined. Both as-atomized cast iron powder and coatings sprayed with the powder contain no graphite structure owing to their rapid solidification. A short period of pre-annealing at 1173 K allows the formation of graphite structure in the cast iron powder, in which there exist precipitated graphite of 3.58 mass%. The microstructure observation exhibits that pre-existed pores in the as-atomized powder strongly affect the precipitating sites of graphite, that is, mainly inside the individual powder instead of the surface. However, marked reduction in graphite structure occurs to coatings sprayed with the pre-annealed powder because of in-flight burning and dissolution into molten iron. In-process post-annealing at 773 K for 60 s reveals the formation of graphite structure resulted from the decomposition of iron based metastable carbide in splats and coatings sprayed with the as-atomized powder. Chemical analysis demonstrates that graphitization level of post-annealed cast iron coatings is higher than that of coatings sprayed with the pre-annealed powder. Precipitated intersplat graphite structure of 1.68 mass% appears in cast iron coatings when introducing methane as a powder feeding carrier gas which is liable to decompose in plasma flame. The resultant coatings with graphite structure embedded in hard matrix are anticipated to offer superior wear resistance in comparison to centrifugally cast iron containing flaky graphite of 1.76 mass%.

In order to overcome the disadvantage of local carburizing of steel components in contact with light-weight graphite or carbon fiber reinforced ceramic racks alumina based thermal spray coatings are produced as diffusion barriers with improved life time compared to rapidly degrading alumina or boron nitride pastes. The powder flame sprayed coatings are also capable to prevent damage by excess filler material in high temperature brazing processes effectively. Besides graphite also C/C racks are coated with pure alumina, Al 2 O 3 -TiO 2 and Al 2 O 3 -Cr 2 O 3 . Conventional powder flame spraying is applied in order to provide a low-cost solution for realization of diffusion barriers. Coatings are characterized by means of optical microscopy and SEM with regard to the interface to the substrates and their porosity. Coated racks are used in field tests for case hardening of steel components. The life time of thermal spray coatings is compared to alumina and boron nitride based pastes. Comparative liquid metal corrosion tests are carried out with NiCr7Si4.5B3.1Fe3 filler at 1,050 °C.

Water atomized cast iron powder of Fe-2.17C-9.93Si-3.75Al (in wt%) were deposited onto an aluminum alloy substrate by atmospheric DC plasma spraying to improve its tribological properties. Pre-annealing of the cast iron powder allows to precipitate considerable amounts of graphite structure in the powder. However, significant reduction in graphitized carbon in cast iron coatings is inevitable after plasma spraying in air atmosphere due to the in-flight burning and the dissolution into molten iron droplets. Hexagonal boron nitride (h-BN) powders which have excellent lubricating properties like graphite were incorporated to the cast iron powder as solid lubricant by sintering process (1300 °C) to obtain protective coatings with low friction coefficient. The performance of each coating was evaluated using ring-on-disk type wear tester under paraffin base oil condition in air atmosphere. Conventional cast iron liner which has flaky graphite embedded in pearlitic matrix was also tested in similar conditions in order to make a comparison. Sections of worn surfaces and debris were characterized and wear behaviour of plasma sprayed coatings are discussed.

The present work aimed at demonstrating the effectiveness of graphite containing plasma sprayed coatings to reduce friction coefficient in dry or starving lubrication conditions. Operative, environmental and economic considerations were important driving factors. Graphite particles were mechanical mixed with a cermet powder containing a fine dispersion of chromium carbide particles in a nickel-chrome metallic matrix. The coatings material was deposited by Atmospheric Plasma Spray onto steel substrates. The microstructure of the obtained coating was characterised by XRD and SEM/EDS. A strip draw apparatus, simulating the stamping process of steel sheets, was used to test the coatings. Several operative conditions were reproduced, by modifying the load and the amount of lubricant used. For comparison, uncoated samples were tested in the same conditions. The comparison of the results of strip draw tests carried out on coated and uncoated samples, shown that the use of graphite based self-lubricant coatings, deposited by plasma spray, allowed to decrease drastically the amount of lubricant.

A new process has been developed to incorporate graphite particles in a stainless steel coating during its formation. Four means have been tested to inject the graphite particles outside the plasma jet and its plume : graphite suspension, a graphite rod rubbed on the rotating sample, powder injection close to the substrate with an injector or an especially designed guide. The latter process has been shown to be the most versatile and the best controllable one. It allows to incorporate uniformly between 2 and 12 vol % of graphite particles (2-15 µm) within the plasma sprayed stainless steel coatings. A 2 vol % seems to give the best results : a low decrease (6%) of the coating hardness and the best results in dry friction studied with a pin on disk set up. In this case, depending on the sliding velocity (0.1 to 0.5 m/s) and loads (3.7 to 28 N) the dry friction coefficient against a 100C6 pin is reduced by a factor between 1.5 and 4 compared to that obtained with plasma sprayed stainless steel.

The aim of this study is to achieve, on aluminum alloy substrates, coatings with higher hardness and better dry tribological behavior than that of the substrate. Using atmospheric plasma spraying and a vibratory fluidized bed powder feeder, coatings consisting of a stainless steel matrix with embedded graphite particles were successfully deposited. The dry coefficient of friction of the composite layer is comparable with that of a commonly used stainless steel surface. SEM examinations of polished cross-sections reveal a lamellar structure with randomly distributed graphite inclusions. Paper includes a German-language abstract.

Fundamental aspects of a plasma sprayed cast iron coating on an aluminum alloy substrate are investigated in the present study: focusing on the effects of preheat substrate temperature (T S ) and chamber pressure (P C ) on the splat morphology, the adhesive strength of splats, the formation of a reaction layer and graphite. Splash-type splats appear at low T S but disk and star-shaped splats arise at high T S . Deformed substrate ridges, mainly due to the slight surface melting, are formed adjacent to the splat periphery at high T S . At low T S , pores are observed at the splat/substrate interface, which cause a decrease in the adhesion of splats. In contrast, a reaction layer composed of iron, aluminum and oxygen is ready to form at high T S . The amount of graphitized carbon increases in cast iron splats with T S . At a low P C of 26.3 kPa, disk-type splats are in the majority at a constant T S of 473 K. As P C increases, star-shaped splats appear along with disk splats. The flattening ratio of disk splats decreases with the increase of P C , because of a decrease in the kinetic energy and temperature of molten droplets. An interfacial oxide layer composed of iron, aluminum and oxygen is ready to form at high P C . The number of pores intensively increases with P C , which leads to a decrease in the adhesive strength of splats. The amount of formed graphite in cast iron splats slightly increases with P C , however, that of a rapidly solidified phase of Fe-Si-C decreases because of lowering of the solidification rate.

In order to expand the fields of application and to improve the performance of graphite (Cg), it is necessary to reduce its permeability towards of oxygen and to limit its reactivity and especially its oxidation. It is, therefore, essential to protect it from the environment through the use of ceramic coatings. Adhesion between ceramic coatings and graphite is controlled by the mechanical stresses in the coatings and the thermodynamic work of adhesion. Different metal-graphite systems were examined which showed that the adhesion particularly depended on the thermal expansion coefficient mismatch between the two materials and on metal carbide stability. Thus, the role of the addition on the graphite surface of elements such as Cr, Mo, Al, Si, O on the adhesion of metals or ceramics to graphite has been identified.

Composite self-lubricating coatings were developed using high-energy plasma spraying (HEPS). These coatings would be potentially used in high contact pressure rolling/sliding systems. The coatings are based on a steel coating deposited by high energy plasma spraying using wire feedstock. Solid lubricants such as graphite and soft metal were investigated. Twin roller rolling/sliding tests were performed at 5% and 35% creep and contact loads of 700 N to 1700 N on a 5 mm contact face. Reduced friction, compared to a steel coating-steel or 1080 wrought steel couple was observed under these rolling-sliding contact conditions.