This paper presents the results of a study on the tribological properties of TiC-based coatings deposited by HVOF spraying. Four powder feedstocks consisting of (Ti,Mo)(C,N) hardmetal with Ni and Co binders were prepared by agglomeration and sintering. The feedstocks differ in composition and particle size distribution, the latter being optimized for fuel type and equipment requirements. Coating specimens are evaluated based on microstructure, hardness, bonding strength, and friction and wear behavior. The results are presented and correlated with spray parameters, equipment differences, and feedstock characteristics.

Different feedstock powder compositions of the alumina-chromia system were deposited on steel substrates by various methods, including conventional plasma spraying, three-anode plasma spraying, and HVOF. The powders used for plasma spraying had particle sizes of -38+10 µm and for HVOF spraying -25+5 µm and -25+10 µm. The coatings were evaluated by their microstructure, phase composition, and corrosion, wear, and electrical properties. The study shows that wear properties depend strongly on the spray process and that coatings obtained by HVOF spraying have dense structures and excellent wear behavior. Coatings produced by the three-anode plasma process, despite their higher porosity, were found to be harder than conventional plasma coatings and can be sprayed with higher feed rates. The coating properties do not appear to have a linear dependence on chromia content.

In this study, iron-based coatings are deposited on stainless steel substrates by HVOF and HVAF spraying and are evaluated based on SEM examination, hardness measurements, and corrosion and wear testing and by comparison with WC-CoCr and CrC-NiCr reference coatings. The results indicate that corrosion resistance is insufficient if the coating is not fully dense and has open porosity. During spraying, the particles must be totally melted and rapidly solidified to achieve uniform coating composition. Open porosity and nonuniform distribution of alloy elements, particularly chromium, is seen to induce crevice corrosion in iron-based coatings.

Thermal spray coatings are widely used in sealed constructions where a specific seal material is sliding on the coating surface in aqueous conditions. Such applications are often highly corrosive and therefore limit the lifetime and increase the wear of the seal and the coating. In this work special test equipment was manufactured to study corrosion and wear performance of the specific test materials used under chlorine containing conditions at low pH-values. In the test procedure, a rotating seal material was pressed with certain pressure against the thermal spray coating material in the test solution and the wear of the seal and the weight loss of the coating material was measured during the test. Coating samples for the tests were prepared using HVAF, HVOF, twin wire arc spray and atmospheric plasma thermal spray techniques. Corrosion resistant stainless steel material EN1.4404 was tested as a reference together with the thermal sprayed coatings. Altogether four different seal materials were tested and the seal material was found to affect the weight loss of the tested base material and also to the coated material during the corrosion wear tests. Pure graphite seals were seen to accelerate the wear rate of both bulk stainless steel material and the thermal spray coatings, as compared to the other seal materials involved in the tests. The HDPE polymer seal gave the lowest weight loss in the tests. Clear differences in corrosion wear resistance were found between tested thermal spray coatings. Clearly the best performance was achieved with plasma sprayed chromium oxide coating compared to the other tested coatings when glass fibre reinforced teflon was used as the seal material.

Alumina is a relatively low-cost material and easily processable by thermal spraying into wear and corrosion resistant coatings. However, thermally sprayed alumina coatings show inferior corrosion resistance versus chromia coatings, particularly in low and high pH electrolytes. Further, alumina possesses decreased mechanical properties in the as-sprayed state. In the present study, the effect of chromia addition on the properties of the plasma and HVOF sprayed alumina coatings were studied. Pure alumina powder and four different Al 2 O 3 -Cr 2 O 3 powders were prepared in two different atmospheres and produced to coatings with APS and HVOF methods. Phase composition of the powders and coatings were studied by X-ray diffraction. Electron microscopy was used for the examination of the microstructure of the powders and coatings. The mechanical properties of the coatings were studied by hardness and abrasion resistance tests. The chemical properties of the coatings were examined by corrosion exposure tests. Results indicate that with chromia addition it is possible to improve the mechanical and chemical properties of the thermally sprayed alumina coatings.

In order to produce plasma sprayed coatings with best economical, technical and environmental qualities several factors need to be taken into consideration. These include properties of the powder feedstock, equipment reliability, optimized process parameters and stability of the spray process. In case of plasma spray guns with conventional and frequently used radial powder feeding configurations, proper and constant powder feeding during spraying plays an important role. Once optimized spray parameters need to be preserved and optimal spray conditions for best coating quality and highest deposition efficiency should also be kept constant during possible changes of the spray parameters, e.g. due to electrode wear, powder quality variations etc. Various spray process monitoring tools are available and useful for this purpose; especially if they are cost-effective to be adopted by industrial thermal sprayers. In the present work, a conventional 40 kW plasma spray process equipped with a typical radial single point powder feeding was studied with the robust and industrially viable “SprayWatch G” system with an on-line monitoring CCD camera. The plasma spray torch was run under slightly varying spray conditions typical to normal industrial spraying; the spray process was monitored, and various actions, e.g. powder carrier gas flow rate adjustment, were realized in order to restore the most optimum conditions. Special attention was paid on finding correlations between the temperature, brightness, shape, and direction of the spray plume and the properties, e.g. deposition efficiency and physical properties of the sprayed alumina coatings.

The dry sliding wear behaviour of two HVOF-sprayed Fe-Cr-Ni-Si-B-C (Colferoloy) alloy coatings was studied by ball-on-disk tests performed at room temperature (against alumina and 100Cr6 steel balls), at 400 °C and at 700 °C (against alumina balls only). HVOF-sprayed Ni-Cr-Fe-B-Si-C and Cr 3 C 2 -NiCr layers were also tested for comparison. Under all test conditions, the wear rate of the Colferoloy coatings is lower than that of the Ni-Cr-Fe-B-Si-C coating but larger than that of the Cr 3 C 2 -NiCr cermet. Specifically, at room temperature, the Colferoloy coatings exhibit a combination of mild abrasion, delamination and tribo-oxidative wear against alumina, whereas, against steel, they undergo very limited delamination with negligible wear loss. By contrast, the Ni-Cr-Fe-B-Si-C coating suffers larger wear against steel and undergoes more severe abrasive grooving against alumina. Although the Colferoloy and Ni- Cr-Fe-B-Si-C coatings possess similar microstructure and micro-hardness, their scratch behaviours, which depend on cracking resistance and plastic deformability, differ, thus explaining the micromechanical reason for the different wear mechanisms. At 400°C and 700°C, all of the metal alloy coatings are softened and suffer more severe abrasive grooving; by contrast, the behaviour of the Cr 3 C 2 -NiCr layer at 700 °C is controlled by the formation and delamination of an oxidised layer.

In twin wire arc spraying process it is possible to use feedstock wires of two different compositions at the same time. As a result of this procedure it can be achieved composite coatings called also as pseudo alloys with modified physical properties. In this study nickel and cobalt based super alloy materials were arc sprayed with pure molybdenum wire to tailor corrosion and wear resistance of the coatings. Coatings for the tests were sprayed using two different twin wire Sulzer Metco arc-spraying units, Smart Arc and OSU 300, operating with suitable spray parameters to produce coatings of good quality. It was already known that these twin wire configurations are producing coatings with differing microstructures. Coating sprayed with the OSU system was clearly finer in structure and one purpose of this study was to measure the effect of the micro structural size on the corrosion and wear properties of the final coatings. Microstructures of the coating materials were studied and analyzed from cross-sectional specimens. Volume fraction of pure molybdenum in the coating matrices was evaluated with simple line method and according to the results volume fraction of pure molybdenum metal is over 50 volume-% in all of these tested composite coatings and higher in materials sprayed with OSU unit. Also the microstructure of the coatings was seen to be finer when OSU was used as was expected. Wear resistance was measured with modified ASTM G65 rubber wheel sand abrasion wear test and corrosion resistance was tested in low pH values and chlorine containing environment according to the ASTM G48 corrosion testing standard. Corrosion testing was carried out at room temperature 22°C and also at higher 50°C temperature. Molybdenum addition is clearly improving the abrasion wear resistance of the tested coating systems. At room temperature also the corrosion resistance is getting better with molybdenum addition but at higher temperature this effect is not so clear.

In order to improve both corrosion and wear resistance of standard plasma sprayed alumina coatings chromia alloyed feedstock powders and high velocity oxygen fuel (HVOF) spraying as an alternative process were used in this work. Alumina and alumina-chromia powders with different chromia contents (27% and 50%-mass., but with different amount of (Al,Cr) 2 O 3 solid solution formed) were deposited by TopGun HVOF spraying and atmospheric plasma spraying (APS). The coatings were evaluated by optical microscopy, microhardness measurements, XRD and SEM. Abrasion wear resistance of the coatings was tested with the rubber-wheel abrasion test. Corrosion characteristics of the coatings were evaluated by exposure tests at 85 °C for 310 hours at low and high pH conditions. The amount of dissolved elements in solution, as analysed by ICP, was found to be a direct indication of the corrosion resistance of the coatings. Coatings deposited by TopGun HVOF spraying resulted in a denser microstructure, higher microhardness and significantly improved abrasion wear resistance as compared to corresponding coatings deposited by APS. For the tested conditions, corrosion resistance of both APS and HVOF-sprayed alumina coatings was low but significantly improved by chromia addition for both spray processes. However, the content of chromia addition alone does not allow to draw conclusions on the effectiveness of this measure.

Thermally sprayed cermet coatings are widely used in pulp and paper industry and also in other applications of such corrosive conditions where chlorine is present. In these conditions corrosion resistance and wear resistance of the coatings are the most important characteristics. Corrosion resistance can be improved by sealing the corrosion sensitive base material surface under the coating from chlorine containing environment. Test materials in this work were thermally sprayed WC-10Co4Cr, Cr 3 C 2 -25NiCr and Sanicro 28 coatings prepared using HVAF and arc spray techniques. The degradation of the coatings and base material surface was characterized using micro hardness tests, optical and scanning electron microscopy, and also by conventional weight loss measurements. Wear resistance of the coatings was evaluated using modified rubber-wheel abrasion tests. In this work ASTM G48 corrosion testing standard was utilized in corrosion testing of these corrosion resistant thermally sprayed coatings. In demanding chlorine containing corrosion conditions it is essential to understand where the corrosion is concentrating in the structure. In the studied corrosion conditions Sanicro 28 and Cr 3 C 2 -25NiCr coatings were heavily corroded inside the coating material. Corrosion resistant Cr 3 C 2 -25NiCr cermet coating was corroded also between the coating and the base material and the adhesion between the base and the coating was lost in some of the specimens after one day exposure. Unexpectedly HVAF sprayed WC-10Co4Cr coating was giving the best performance of the coatings tested in this work.

To investigate the properties of thermal sprayed glass-composite coatings, experimental measurements have been carried out. The coating demands such as thermal, electrical or mechanical resistance need to be optimised along with stress-minimizing procedures in order to reach high-quality surface coatings. The relevant properties aging, oxidation resistance and bond strength to the substrate are evaluated. Different material combinations were sprayed using the VPS, LPPS and HVOF processes, which combine several parameter sets and substrates. The results of the investigations show that coating properties such as conductivity, thermal expansion, porosity and surface roughness can be well directed influenced by controlling the mixture of the powder material, the pretreatment of the powder and particle size distribution.

Microscale wear mechanisms in thermally sprayed hard coatings were examined. A test procedure for examining microwear with various abrasives was developed. Different abrasivity of kaolin, precipitated calcium carbonate and titania was found to affect wear mechanisms. Fine-particle abrasion caused the surface to loose its gloss and smoothness. Coatings subjected to fine-particle abrasion were examined with optical gloss measurements and scanning electron microscopy. It was shown that in tungsten carbide based hard coatings the microscale wear was governed by preferential wear of the soft binder phase. However, also coating defects like pores or poorly adhered splats tended to provide nucleation points for microscale wear. It was found that coatings that performed well in dry-sand rubber wheel abrasion or wet abrasion tests did not necessarily have good microwear resistance. The results showed that different abrasives had effect on the wear phenomena and wear rate of hard coatings. Coatings also behaved differently and novel modifications in the composition affected the wear behaviour. In conclusion, the results provided deep understanding of the role of various abrasives in the wear phenomena of thermally sprayed hard coatings.

Thermally sprayed hard coatings, including tungsten carbide and chromium carbide cermets and other hard metallic materials, were studied in two types of wear tests. Surfaces of the coatings were worn by coarse and hard quartz sand in a rubber-wheel dry abrasion wear test, and by fine and soft kaolin abrasive in a wet slurry abrasion wear test. The aim of the work was to study how the surfaces retain their high polished finish and gloss, and the type of wearing of different coatings and materials. The results showed that coatings with hard tungsten carbides were worn preferentially by removal of the the binder material. Cermet coatings with softer chromium carbides, and with another types of uniform microstructures showed more uniform wear and better retained their glossy finish.

The paper deals with wear properties of various advanced oxide ceramic coatings deposited by plasma and HVOF spray processes. Several types of ceramic oxide coatings on the base of Al 2 O 3 , Cr 2 O 3 and TiO 2 are studied in the work. The coatings are characterised by their wear properties in slurry abrasion wear test with fine abrasives (kaolin) and in dry abrasion conditions with coarse abrasives (quartz sand). The surfaces of the wear tested coatings were studied by SEM. The results show that coatings with high homogeneity and minor porosity, especially HVOF sprayed and some plasma sprayed coatings perform extremely well in both of the wear tests used in the study. In the test with hard abrasive and high load, the coatings with lower interlamellar strengths seem to be more prone to wear than coatings with higher homogeneity, e.g. HVOF sprayed oxide ceramic coatings. The results can be explained on basis of type of the coating material and the coating microstructure.