Nowadays, Cr 3 C 2 -based cermet coatings by HVOF process are widely recognized for their corrosion and erosion resistance, particularly at high temperatures. These coatings also offer the advantage of being lightweight and exhibiting superior wear, corrosion and cavitation resistance in room-temperature applications. Their lightweight nature and high temperature capability make them an attractive alternative to WC-based alloy coatings and hard Cr plating coatings. The objective of this study is to develop optimal Cr 3 C 2 -NiCr coatings by comparing different feedstock materials, including feedstock with nanocrystalline and/or submicron sized Cr 3 C 2 phases. The focus of the investigation is on understanding the impact of feedstock features such as particle size, morphology, and carbide sizes, as well as sliding abrasive wear conditions (specifically SiC grit size and working load), on the coating properties and sliding wear performance. The results of the study indicate that the sliding wear resistance of the Cr 3 C 2 -NiCr coatings is highly influenced by the features of the Cr 3 C 2 carbides. The presence of nano, submicron and few microns sized carbides in the coatings improves their density and hardness, leading to a significant reduction in wear rates under test conditions. Furthermore, the size of the abrasive SiC grit on the counter surface plays a significant role in determining the sliding wear behavior of these coatings. Based on the analysis of the test data, the mechanisms behind the performance of the Cr3C2-NiCr coatings have been investigated and used to interpret their sliding wear behaviors. A high microhardness in the coating is considered a reliable indicator of high quality, full density, and satisfactory wear resistance. This study has identified and recommended optimized materials for improved coating properties based on the key findings. These findings contribute to the understanding of the relationship between feedstock features, sliding abrasive wear conditions, and the wear rates of HVOF-sprayed Cr 3 C 2 -NiCr coatings.

A common method to combat abrasive wear and prolong the life of a component is to hardface the exposed region by overlay welding. State of the art coatings for these applications consist of a nickel-based ductile matrix with hard tungsten carbide particles embedded in it. An alternative with low environmental impact in combination with high performance to cost ratio is to use iron-based alloys. Critical in affecting the abrasive and impact wear resistance of these alloys is the coating quality e.g. porosity, cracks, dilution from the substrate combined with chemistry, size and volume fraction of the hard phase particles formed during solidification. Selection of the process parameters is critical for producing sound clads with expected properties. This paper focuses on the properties of PTA welded and laser cladded M2, M4 and A11 high speed steel coatings. Clad quality, hardness, abrasive wear resistance and microstructure are presented and interpreted with support of thermodynamic simulations.

A known family of rare-earth oxide (REO) ceramics have recently been found to exhibit intrinsic hydrophobicity, even after exposure to high temperatures and abrasive wear. In this study, thin CeO 2 coatings were developed for hydrophobic applications using suspension high velocity oxy-fuel (SHVOF) thermal spray. It is an efficient method to produce large superhydrophobic surfaces with a unique hierarchically textured structure on a variety of substrates. The use of suspension also enables the process of fine-grained powders to form nanostructured coatings with significant improvement of mechanical and chemical properties for numerous applications. An aqueous suspension with a solid concentration of 30 wt.% sub-micron CeO 2 particles (<200 nm) was used as suspension feedstock. The as-sprayed CeO 2 coating on a stainless steel significantly improved the substrate’s surface hydrophobicity from a low contact angle of 57° to nearly 150°. The surface chemistry of SHVOF thermal sprayed CeO 2 coatings was also investigated by X-ray photoelectron spectroscopy (XPS). It was confirmed that the near-super-hydrophobicity was mainly attributed to its unique hierarchically structured surface.

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.

Iron-based hardfacing alloys are widely used to counteract abrasive and impact wear of industrial components soil in, sand and mineral processing applications. These alloys show a high performance to cost ratio as well as a low environmental impact. The wear resistance of the components hardfaced with these alloys depends on achieved coating microstructure i.e. on the alloys chemical composition, the coating method and process parameters selected. The present work focuses on iron based hardfacing alloys with varying amount of chromium, vanadium, tungsten, molybdenum, boron and carbon deposited by plasma transferred arc (PTA) overlay welding. Weldability, hardness, abrasive and impact wear of the overlays are presented and interpreted through their microstructure. The performance of the iron based overlays is compared with that of nickel-based metal matrix composite coatings with tungsten carbide (MMC) commonly used for hardfacing of parts subjected to severe abrasive wear. The hardness of the iron based overlays investigated ranges between 60 and 65 HRC while abrasive wear is typically below 20 mm 3 (ASTM G65, procedure A). Microstructure consists of different primary precipitated carbides or borides, a martensitic matrix and eutectic structures.

Boiler tube failure is the number one source of forced outages in all coal-fired and biomass-fired power generation plants. It is estimated that plants lose approximately 6% of their power generation annually, due to boiler tube leaks. The major causes for premature tube failure are excessive fireside boiler tube erosion and corrosion caused by impact, abrasive wear, oxidation and molten corrosion of low eutectic alloys.

Metal-matrix composite (MMC) coatings consisting of a nickel based metal matrix, providing toughness and tungsten carbides, providing wear resistance, are used in applications subjected to severe abrasive wear conditions. The aim of this work is to map the influence of type, morphology and amount of tungsten carbides in a nickel based matrix on the wear resistance and microstructure of laser cladded MMC coatings. Abrasive wear, evaluated according to ASTM-G65, is mainly influenced by the volume fraction of the tungsten carbides in the coating. Shape and microstructure of the tungsten carbides have a minor impact on this property, for similar degree of melting or dilution from the substrate material and size of the tungsten carbides. Microstructure analysis shows that the dissolution of the tungsten carbides in the melt pool is influenced by the chemical composition of the liquid metal phase, the microstructure and the amount of tungsten carbides selected.

NiCrBSi alloys are often used in thermal spraying because of their good wear and corrosion resistance even at temperatures over 500°C. Experience has proved these alloys are a good choice for components in the presence of hard particles. The main wear mechanism here is abrasive wear caused by hard particles. Some examples are wear plates exposed to impact sliding; extruders, screw conveyors or mixer parts exposed to grooving; fans, rotor wheel blades or impellors transporting sand/granular material at temperatures over 500°C; or pump parts exposed to fluid containing sand. In spite of such widespread use of NiCrBSi alloys in thermal spraying, their abrasive wear resistance is still not fully understood. In order to better understand, a series of sprayed and fused NiCrBSi coatings with hardness from 36 to 62 HRC were tested for abrasive wear according to ASTM G65–04 norm and the wear volumes achieved are presented. Tribological and metallographic analysis of track wear was done in order to better understand how microstructure and hardness of NiCrBSi coatings influence abrasive wear mechanisms. These results are compared to results previously published.

FeAl intermetallic compound coating was prepared by cold spraying a mechanically alloyed Fe(Al) alloy powder followed by post-spray annealing at 950°C. The high-temperature abrasive wear test was carried out for the annealed cold-sprayed FeAl at a temperature range from room temperature to 800°C. The high temperature abrasive wear of a heat-resistant stainless steel 2520 was performed for comparison. The results showed that the annealing treatment of the as-sprayed Fe(Al) alloy coating at a temperature of 950°C results in the formation of dense FeAl intermetallic compound coating with no particle boundaries. It was found that with the increase of the test temperature the wear rate of the stainless steel increased at the temperature higher than 400°C, while the wear rate of cold sprayed FeAl coating tended to decrease at the temperature higher than 400°C. The high temperature abrasive wear resistance of the cold-sprayed FeAl intermetallic compound coating increased with the increase of the abrasive wear temperature in a temperature range from 400°C to 600°C and changed little in the temperature range from 600°C up to 800°C. The wear resistance of cold-sprayed FeAl coating was higher than that of heat-resistant 2520 stainless steel under 800°C by a factor of 3.

Different post treatment methods are developed up to now to improve the properties of thermally sprayed coatings. In this work, arc sprayed aluminium coatings on aluminium substrates are post-treated by plasma electrolytic oxidation. To estimate the wear resistance of resulting oxide coatings, two abrasive wear tests (ASTM G65 and ASTM C1624) are carried out. Worn surfaces are examined by scanning electron microscopy in order to establish the wear mechanisms. These results of the abrasive wear tests are correlated with the parameters of the PEO process and the hence resulting micro structures of the coatings.

High velocity oxy-fuel (HVOF) sprayed cermet coatings are required in various industrial fields due to their excellent properties, such as combination of wear resistance, corrosion resistance, high hardness, high bonding strength and stability under high temperature. In order to utilize them in the fields, optimization of composition and structure of the coatings are essentially important meaning that both spray powders and spray conditions are key process parameters. In this paper, developed spray powders of cermet materials are introduced for the specific applications, where 1. cavitation erosion, 2. mechanical impact, 3. corrosion by molten alloy and 4. general abrasive wear are major factors that damage the coatings. In order to solve these factors, HVOF coatings of 1. WC/Co/Cr with large WC particle, 2. WC/Cr 3 C 2 /Ni with addition of metal or alloy, 3. MoB/CoCr composed of double boride and 4. WC particle size in WC based cermet, are proposed and these merits are described.

A significant group of steel-making process parts is exposed to high contact pressure, shock abrasive wear and elevated temperature. High productivity repair techniques are necessary because of the large size of the parts. Analysis of coating metallographic investigations, wear and corrosion test results, full-scale tests shows that restoration of base share of these parts is possible by High Velocity Oxygen Fuel / High Velocity Air Fuel (HVOF/HVAF) process. Comparison of manufacture's data has showed that HVAF excels HVOF alternatives noticeably at productivity. At the same time production costs are 2-2.5 times less. With regard to typical steel-making process parts some investigations results and examples of HVAF restoration at Joint Stock Company "Mashprom" are represented.

Three types of cobalt-based cermet coatings were prepared by high velocity oxy-fuel (HVOF) spraying using cobalt- clad TiC-50Co, SiC-50 Co and WC-18Co powders. The microstructure of three coatings was characterized using a scanning electron microscope (SEM). The adhesive strength of the coatings was tested according ASTM C633-79 standard. The hardness of three coatings was measured using a HV-5 Vickers hardness tester. The abrasive wear performance of the coatings was examined by a dry-sand rubber wheel tester according to ASTM G65-61 standard. The results show that the density, thermo physical properties and volume fractions of the solid carbide phases in the spray particle have a significant influence on the adhesive strength of the coatings. The hardness of WC-18Co coating is higher than that of TiC-50Co and SiC-50 coatings and is much lower than WC-17Co coating deposited with sintered-crushed powders. Moreover, the abrasive wear volume loss of the WC-18Co coating is about 60 times higher than that of the WC-12Co coating sprayed by sintered-crushed powder, and greatly lower than that of TiC-50Co and SiC-50 coatings. The wear mechanisms of three coatings are discussed.

For deposition of protective coatings different coating techniques are available. Usually, detailed evaluation of various deposit types and materials is necessary for selection of the best suited coating for specific application fields and demands. Subject of this work are thermally sprayed functional coatings applied as wear (and corrosion) protective layers. Examination of different optimized thermal spray coatings, i.e. HVOF sprayed WC/Co(Cr) and Cr 3 C 2 /NiCr coatings, conventional flame sprayed and fused self fluxing alloy coatings reinforced by hardmetal and APS sprayed oxide Al 2 O 3 /TiO 2 and Cr 2 O 3 coatings, is done in comparison to thick hard chromium platings. Two abrasive wear tests featuring wear by lose abrasive particles are carried out. These impart dry wear conditions according to ASTM G65 (Rubber Wheel test) and wear by abrasive suspensions according to ASTM G75 (Miller test). The work also contains evaluation of newly developed HVOF torch components permitting increased combustion gas, and therefore also particle, velocities concerning the benefit in terms of coating properties. Exemplary evaluation of the new components influence on velocity and temperature of spray particles is carried out by comparative SprayWatch analyses. Both the influence on the coatings microstructure and the wear performance are studied. Coating microstructure is evaluated qualitatively by optical and scanning electron microscopy and the micro hardness HV0.3 is measured. Worn surfaces are studied by SEM in order to deduce wear mechanisms.

Standard materials like WC-Co, WC-CoCr, WC-Ni, and Cr 3 C 2 - NiCr for HVOF spraying exist in various modifications depending on chemistry, carbide size, and production method. They are widely used for wear, erosion, cavitation, and corrosion protection in many industrial fields. But there are a lot of applications in which the surface is exposed to a combination of different wear mechanisms - e.g. corrosion and erosion or abrasion - and usual state-of-the-art coatings do not fully meet the technical demands regarding lifetime under those combined attacks. New materials therefore are needed. In the present study several modified and completely new carbide based materials are investigated. A series of new cermets differing regarding carbide type, carbide size, and metallic binders are compared. The materials were sprayed with the HVOF system DJ 2600 to show the influence of their different chemistry and morphology on the microstructure and properties of the coating in comparison to standard materials. The experiments comprises microstructural examinations as well as abrasive wear, corrosion, and cavitation tests.

The unique wear protection properties of tungsten carbide metal matrix composite materials are resulting in their increasing use in the oilsand industry to combat severe low stress sliding abrasion and various types of slurry abrasion and erosion. Their successful application, mainly in bulk welding and spray coating forms, has extended component service lives, improved reliability and reduced maintenance costs. Increased use of tungsten carbide metal matrix composite hardfacing deposits in oil sands applications is the direct result of understanding carbide thermal degradation and the processes used to deposit these materials. Plasma transferred arc welding (PTAW) has proven to be an effective process for applying these materials. Current and future work on PTAW and other candidate processes to establish the optimum carbide hardfacing method will be reviewed.

WC-Co has been extensively investigated for use in wear resistant coatings for engineering applications. In principal, when the WC particle size decreases in the starting powder, the decomposition of WC increases, and therefore, significant amounts of W 2 C and W 3 C, and even metallic phases, are observed in nanocrystalline WC-Co coatings. The reported increase in hardness of nanostructured materials is generally attributed to the significant decrease in grain size or particle size. However, the presence of brittle, non-WC phases in nanostructured WC-Co coatings leads to sliding and abrasive wear by removal of large plates of the coating. Concurrently, the greater degree of decomposition suffered by the nanostructured powder during spraying leads to a reduction in the volume fraction of the wear-resistant primary WC phase. For the reasons presented above, the present efforts are directed towards the synthesis of a wear-resistant coating using a multimodal WC size distribution of particles in the starting powder. The multimodal distribution is characterized by small WC particle(~50 nm) and coarse WC particles (1.7µm). In addition, the distribution of Co also spanned an order of grain size, hence the name multimodal. The coatings were deposited using HVOF technology.

In this paper, the production of NiCr-TiC powder by SHS, suitable for HVOF spraying, is discussed together with results on the microstructure and coating properties. Compacts for SHS were prepared by mixing elemental Ti and C with pre-alloyed Ni-20wt.% Cr powder to give an overall composition of 35wt.% NiCr and 65wt.% TiC. These were then ignited and a self-sustaining reaction proceeded to completion. Reacted compacts were crushed, sieved, and classified to give feedstock powders in size ranges of 10-45 µm and 45-75 µm. All powder was characterized prior to spraying based on particle size distribution, x-ray diffraction (XRD), and scanning electron microscopy (SEM/EDS). Thermal spraying was performed using both H2 and C3H6 as fuel gases in a UTP/Miller Thermal HVOF system. The resulting coatings were characterized by SEM and XRD analysis, and the microstructures correlated with powder size and spray conditions. Abrasive wear was determined by a modified 'dry sand rubber wheel' (DSRW) test and wear rates were measured. It has been found that wear rates comparable to those of HVOF sprayed WC-17wt% Co coatings can be achieved.

A new low-cost chromium carbide-nickel chrome powder has been developed. Potential applications for these powders include hard-chrome replacement, boiler tubes and turbine engine components. The erosive and abrasive wear properties of the deposited coatings have been found comparable to commercially available nickel-chromium based carbide materials. A significant advantage of these powders is higher deposition efficiency and carbon retention when coatings are deposited using HVOF thermal spray equipment. Results indicate deposition efficiencies up to 50% higher than commercially available carbide powders in the market today. Higher deposition efficiency effectively reduces the application costs allowing these materials to be competitive in a wider range of applications. Powder characteristics and the application costs data are included in this paper. Also discussed are microstructure-property relationships of the various coatings. Data including abrasive slurry wear, hardness, high- and low-angle erosion and superfinished surface finish is reported. Comparisons have been made to commercially available chemical clad and blended CrC-NiCr powders.

Turbine blades in hydropower plants have to withstand the most severe seasonal loading conditions when the river water is heavily loaded with sediments. Welded blade surfaces are considered state of the art, but they cannot withstand the aggressive, eroding and abrasive loading conditions in the major rivers in China. Once a year there is flooding with a maximum amount of sediment in the river water. This is why coating systems with maximum wear resistance must be developed, examined, produced and tested over a maximum period of time. In this article, the wear results of various coating systems examined in a special test facility are described in order to obtain comparable results from several proposed coating systems together with material classification tests. Paper includes a German-language abstract.