A secondary acetylene after burner attached with an HVOF gun has been developed to produce high quality coatings required of ceramics. The secondary burner adds heat energy, enough to make deposition, to fine ceramic particles heated and accelerated by an HVOF gun. The energized particles are still not fully melted but just softened or partially melted on the surface and accelerated at as high as jet flame velocity. The coatings produced by this way present extremely unique characteristics, quite unlike from plasma coatings or D-gun coatings. The phase transformation in coatings is very little observed. The porosity is negligible. The wear resistance and the density are very close to those of the bulk. This paper will present significant features of HVOF ceramic coatings in comparison with plasma coatings.

In the current study, the tribological properties of TiC-based coatings paired with polycrystalline alumina under unlubricated sliding conditions were investigated in order to demonstrate the technological and engineering potential of such coatings. (Ti, Mo)(C, N)-Co coatings were prepared from an agglomerated and sintered spray powder by HVOF spraying using JP-5000 equipment. Cr 3 C 2 -NiCr coatings were studied for comparison. Sliding wear tests were performed over the temperature range from 23°C to 800°C with sliding speeds in the range 0.3-3 m/s, a wear distance of 5000 m and a normal force of 10 N. Wear rates of coatings and sintered alumina counterparts were measured separately. Compared with Cr 3 C 2 -NiCr coatings, (Ti, Mo)(C,N)-Co coatings showed significantly lower total wear rates, corresponding to those found in the region of mixed/boundary lubrication. With few exceptions, the coefficients of friction were found to be lower for (Ti, Mo)(C,N)-Co coatings than for Cr 3 C 2 -NiCr coatings. After tests were performed, the coating microstructures were studied by optical microscopy and SEM. The oxide scales formed on the coating surfaces were investigated by SEM and X-ray diffraction. Comparison of the total wear rates of the couples consisting of sintered alumina and a (Ti, Mo)(C,N)-Co coating with those of other alumina-ceramic and hardmetal-hardmetal tribological systems demonstrates the immense potential of TiC-based coatings for sliding wear applications.

Molybdenum disilicide (MoSi 2 ) is a suitable material for high temperature applications especially because of its excellent high temperature oxidation resistance. For several high temperature applications MoSi 2 shows high potential to be used as a protective coating. The oxidation behaviour of HVOF sprayed MoSi 2 coatings is studied at 1500 °C. The oxidation tests are carried out in a simultaneous thermogravimetric device and the mass change is measured in dependence on the oxidation time. The microstructure of the coatings before and after oxidation is examined by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXS). The mass of the coating increases according to a parabolic function. During the oxidation test the microstructure changes significantly from a typical thermal spray coating microstructure with lamellae, pores and a phase mixture of MoSi 2 and Mo 5 Si 3 to a two phase system with sharply separated grain boundaries. On the surface of the coating a silicon dioxide layer with a thickness of less than 10 µm is formed.

Improved HVOF spraying with a gas shroud has been developed to fabricate environmental barrier coatings of corrosion resistant alloys such as HastelloyC. For such coatings, control of oxidation of the powder material during spraying is very important and the gas shroud has been effective to lower oxygen content to 0.19mass%. In the present study, further reduction of oxygen content to 0.063mass% was achieved by changing the composition of combustion gas by introducing nitrogen into the combustion chamber. This value is almost comparable to the oxygen content 0.042mass% of the feedstock powder but the porosity of the coating increased. Introduction of nitrogen to the combustion chamber lowered the temperature of the spray particles in flight while maintaining their high velocity. Another coating with 0.14mass% was obtained with open porosity below 0.1vol% by changing the mixing ratio of nitrogen, which exhibited improved environmental barrier property in artificial seawater.

Environmental protection and worker safety measures against hard chrome plating are increasing and leading many industries to adopt alternatives. HVOF thermal spray technology for applications of tungsten carbide based coatings have proved to be more environmentally friendly and effective than hard chrome. In this study, the results of wear tests for WC-12%Co sprayed coatings deposited onto AFNOR 25CD4 low carbon steel are compared to those for hard chrome. It was shown that WC-12%Co coatings exhibit the adequate tribological properties compared to those of electrodeposited hard chrome.

This research delivers an integrated performance of generic coating-substrate systems under impact loading, and comprehends the understanding of underpinning failure mechanisms. This investigation thus benchmarks the coating design process for high impact stress applications. Repeated severe impacts to the coatings often result in poor performance by cracking and delamination from the coating-substrate interface. The durability and delamination resistance of coatings depend on the choice of coating and substrate materials, coating deposition process and service conditions. The design of overlay thermal spray coatings thus requires an optimization of these parameters. A thorough understanding of the underpinning failure mechanisms is thus critical for future coating developments. This investigation thus provides an insight to the role coating and substrate properties on the impact resistance of coated materials, and maps the relationship between the impact resistance of WC-Co and WC-CrC-Ni coatings on a variety of substrates. Results indicate that coating’s delamination resistance during impact loading not only depends upon the hardness and roughness of the substrate material, but more importantly, substrates with higher work-hardening coefficient indicate a higher delamination resistance during impact loading.

The use of polymer matrix composites [PMC's] in the gas flow path of advanced turbine engines offers significant benefits for aircraft engine performance, but their useful lifetime is limited by their poor erosion resistance. HVOF and flame sprayed polymer/cermet functionally graded coatings based on a polyimide matrix filled with varying volume fractions of WC-Co are being investigated to improve the erosion and oxidation resistance of polymer matrix composites. A study of the coating's effectiveness as erosion barriers was accomplished through a statistical analysis of the results of solid particle erosion testing of coated and uncoated PMC samples using a design of experiments [DoE] approach. Three coating systems and a control were evaluated in a randomized test matrix. The coatings were tested at room temperature and 250 °C, using an alumina erodent impacting the coatings at a speed of 100 m/s at angles of 20° and 90°. Erosion volume loss at 250 °C was approximately twice than at room temperature, but the maximum erosion volume loss did not exceed 0.30 mm 3 at the elevated temperature. In general, as the angle of incidence of the eroding material increased from 20 degrees to 90 degrees the volume loss increased.

The laser-flash method is used to determine the thermal diffusivity of HVOF sprayed WC-Co(Cr) and Cr 3 C 2 -Ni20Cr as well as APS sprayed Cr 2 O 3 and electroplated hard chromium coatings in the temperature range between RT and 600°C. Additionally bond and/or corrosion protective coatings like Ni5Al, Ni20Cr and 316L are characterized taking into account the different manufacturing methods twin wire arc spraying, HVCW and HVOF. With respect to the application example of drying rollers in paper industries the Taber-Abraser wear test is applied to evaluate the wear resistance. Finally the coatings are characterized concerning their corrosion resistance by salt fog test and by exposure to humid SO 2 environment. For WC-CoCr feedstock the effect of carbide size and micro hardness on thermal, wear and corrosion properties are studied. WC-CoCr coatings with maximum micro hardness and fine carbides show the best thermal conductivity. The use of coarse carbide feedstock permits manufacturing of coatings with the highest resistance against dry abrasive wear, but the protective function depends severely on the processing conditions.

Reactive thermal spraying, in which thermodynamically stable compounds are formed by expected in-process reactions, has attracted considerable attention as a result of wide availability to in-situ composite coatings. Such inprocess reactions differently proceed in HVOF and plasma spraying because of differences in the flame temperature and speed. In the present study, composite powder of SiO 2 /Ni/Al-Si-Mg was deposited onto an aluminium substrate to fabricate in-situ composite coatings by both spraying methods. The coating hardness sprayed with Al-Si-Mg core powder increases with silicon and magnesium content, whereas the coatings by HVOF spraying show higher hardness than those by plasma spraying. In the present reactive spraying, the exothermic reaction of SiO 2 with molten Al-Si-Mg alloy leads to composite materials of MgAl 2 O 4 , Mg 2 Si and Al-Si matrix. Moreover, a rapid formation of aluminide (NiAl 3 ), which is introduced by an exothermic reaction of plated nickel with Al-Si-Mg core powder, enhances the reduction of SiO 2 especially in HVOF spraying. A series of in-process reactions mainly proceed during splat layering on a substrate, instead of during droplet flight even in DC plasma spraying. Plasma sprayed composite coatings become much harder due to the great progress of in-process reactions.

Rietveld analysis was used to measure the phase content of twenty-four coatings sprayed with a multimodal WC-12Co feedstock using three HVOF spray guns. Different correlations were investigated involving, on the one hand, the thermal spray history (particle temperature and velocity) and the mechanical properties of the deposits (microhardness and abrasion resistance), and, on the other, the phase structure of the tested coatings. The results show an inherent effect of the spraying temperature, regardless of the HVOF process used, on the generation of sub-carbide species, mainly W 2 C, W and an amorphous-nanocrystalline phase. In particular, it was found that decarburization of the WC to produce the amorphous/nanocrystalline and tungsten phases is beneficial in enhancing the mechanical properties as long as there is no large amount of tungsten present in the coating. A linear relationship relates this phase with the presence of the amorphous-nanocrystalline phase, independently of the spraying process used in this study. It was also shown that metallic tungsten precipitated in a nanocrystalline structure only with the Diamond Jet HVOF process.

This paper analyzes the behavior of coating particles as well as the gas flow in a High-Velocity Oxy-Fuel (HVOF) gun by using numerical simulation. Special attention is paid to the particle behavior in turbulent boundary-layer inside the barrel.

One of the most important uses of HVOF thermal sprayed CrC-NiCr coatings is for wear resistance. In this work Cr 3 C 2 75-NiCr25 coatings were obtained by high-velocity oxy-fuel system from three agglomerated feedstock powders with various powder size distributions (- 30, -10 and –5 µm). The powders were agglomerated, sintered, plasmafused and crushed, in order to increase their density. The coating microstructures were characterised by SEM microscopy. Differences in coating roughness have been determined by profilometry. The ultra-microindentation technique was applied to measure the hardness and the elasto-plastic properties of the coatings. Experiments using a tribometer (pin on disc configuration) under lubricated and dry conditions have been performed in order to evaluate the friction and wear properties of the different coatings. It was found that the coatings obtained with the lowest feedstock powder size presented the best sliding wear resistance under all the conditions. This fact could be explained in terms of differences of cohesion between the carbide particles and the binder phase. The lower feedstock powder presents a lower carbide particle size that involves a better distribution and cohesion of the chromium carbide with the NiCr binder phase. This fact leads a minor production of “third body” hard particles in the wear tests that influenced quite considerably in the final wear rate of the studied coatings.