Chromium carbide-nickel chromium coatings produced by HVOF spraying are widely used for high temperature wear and erosion resistant applications. Examination of the literature shows that whilst the mechanical properties of these coatings have been widely investigated, there has been little research into the physical processes occurring during HVOF spraying of this system, such as carbide dissolution, liquid-metallic phase oxidation, decarburisation and rapid solidification. The purpose of the present work has been to perform a systematic characterisation of the chromium carbide-nickel chromium system in both the initial powder and as-sprayed states with a variety of spraying conditions using optical, scanning and transmission electron microscopy, electron microprobe and X-ray diffraction. The presence of amorphous and nanocrystalline phases has been demonstrated. The nanocrystalline structures tend to be Ni rich, with the amorphous phases rich in Cr. Carbides of the form Cr 3 C 2 were found to be dissolved slightly during spraying, increasing the Cr and C contents of the liquid metallic phase. There was no evidence of chromium carbide oxidation.

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.

The wear of piston rings in large marine two-stroke diesel engines is a major maintenance cost. Applying coatings with good oxidation, corrosion resistance and high temperature strength, can lower the total maintenance cost. In the past nickel aluminide with chromium carbide have been applied to pistons by thermal spraying. Using laser cladding a suitable microstructure can be formed while at the same time avoiding cracks and bonding issues. In this report powders and coatings were manufactured in order to be able to investigate the dry-sliding wear behavior. Material with three levels of carbides was atomized. Wear test samples were manufactured by laser cladding. The dry sliding wear-mechanism maps are generated by using block on ring test setup where coated blocks slide against cast iron rings. All alloys exhibited regions of plasticity-dominated wear and oxidational wear with a transition region in-between. The carbide-containing alloys showed lower friction and wear in comparison to the carbide free nickel aluminide alloy.

An important problem for the petrochemical industry is the behaviour of materials in aggressive environments, when hydrogen sulphide, carbon dioxide and sand, which contribute to corrosion erosion of the surface, are present. Generally, the use of hard materials such as thermal sprayed tungsten carbide and chromium carbide reduces this problem. Cemented carbides are quite suitable for this purpose: they are composite materials of pure carbides with binder metal alloys of low melting point and high ductility; the selection of the binder metals depends mostly on its ability to wet the surface of the carbide particles to ensure secure coating adhesion. Among the cemented carbides, namely tungsten carbide cobalt-chromium based (WC/CoCr) is considered as the standard for application to ball valve bodies and seats in the petrochemical field, while chromium carbide nickel-chromium based (Cr 3 C 2 /NiCr) is suitable for particular applications. Inconel 625 is also used in this field and usually applied by welding. This paper addresses the characterization of corrosion behavior of HVOF coated samples of WC/CoCr, Cr 3 C 2 /NiCr and Inconel 625 in aggressive environments, and in particular ferric chloride test according to standard ASTM G48-92 and H 2 S/CO 2 test based on NACE standards has been carried out. According to the test results, WC/CoCr based coatings show the best behavior both in terms of corrosion, thus confirming to be very versatile and useful for the application in petrochemical field.

This paper investigates the microstructure, bond adhesion, and high temperature and thermal fatigue performance of plasma-sprayed chromium carbide, nickel-chromium coatings after continuous exposure at high temperature inside a combustion chamber that simulates the extreme conditions existing in different industrial applications. Experiments on high temperature behavior under oxidative and neutral atmospheres and thermal fatigue tests are carried out in a test combustion chamber. Finally, the adhesion between the substrate and the coating layer is assessed using tensile tests. The paper discusses the results with special consideration of the microstructure development through the heat treatment. Paper includes a German-language abstract.

The appropriate selection of bulk materials and coatings of valve components, is an important factor for the economic success of oil and gas production activities in petrochemical field. Materials and coatings are important because particle erosion and surface wear is associated to corrosion by hydrogen sulphide during oil and gas flow. The wear of high pressure valves of gas system will lead to pollution, safety problem and cost increases. The most popular solution of these problems is the deposition of hard material like tungsten carbide or chromium carbide by thermal spray. Particularly these coatings are deposed by HVOF (High Velocity Oxygen Fuel) to obtain a very high hardness with excellent cohesion and adhesion. Tungsten carbide cobalt-chromium based coating, chromium carbide nickel-chromium coating as well as Inconel 625 are adopted actually in the specifications of the industrial petrochemical companies and their behavior and wear, erosion and corrosion properties are reported in literature. This paper addresses the study and surface analysis and characterization of alternative coatings such as NiAl and composite material WC / intermetallic compounds containing mainly Ni, Cr, Co and Mo. The best parameters to produce these coatings has been found by implementing a DOE and the obtained coatings have been systematically submitted to corrosion and functional tests based on the determination of the behaviour of the thermal spray coatings in an atmosphere of H 2 S and CO 2 [1] and to wear and erosion test according to ASTM G75-95; removed material weight and usured surface damages have been determined. Furthermore the coatings have been completely characterized before and after the tests from the point of view of the structure (porosity, coating cohesion and adhesion, hardness, wear) and of the surface properties by means of a prototype 3- dimensional stylus micro-topography surface analysis system. Their corrosion and functional behaviour have been finally compared with the behaviour of the above mentioned coatings applied at present as standard in the petrochemical sector. The results state that WC/intermetallic compound could be a good substitute of IN625 for certain kind of application where good antierosion behaviour is requested.

This paper investigates the oxidation that occurs during the flight movement of a powder particle and during the spatter solidification in the thermal spray process. The effects of oxidation on droplet flattening, on the mechanical and thermal interactions between spatter and substrate, on spatter morphology, on porosity, and on adhesion are studied. The influence of wetting and oxygen dissolution is analyzed. The experimental results show that during High Velocity Oxy-Fuel spraying of the chromium carbide-nickel-chromium powder, the relative mass of chromium oxide in the coating is about 4.95%. The theoretical results agree well with the experimental observations. Paper includes a German-language abstract.

This paper analyses the influence of specific coating parameters such as robot velocity, spray distance and part cooling on the risk of crack formation within Chromium- Carbide / Nickel-Chromium coatings. To understand the effect in more detail, metallographic investigations were conducted. These also provide sufficient data to examine other important coating characteristics such as porosity, mechanical stresses and homogeneity. As an additional analytical method Element Mapping is utilised to show the level of oxidation and its impact on the coating microstructure. The methods X-ray diffraction (XRD) and In-situ coating property (ICP)-Sensor are used to investigate the development of stresses in different coatings. With the information from all these examinations a concept was derived to achieve thick, crack-free wear protective coatings.

In this paper, the residual stresses in high velocity oxy-fuel-sprayed chromium carbide/nickel-chromium coatings are measured using a bending technique "Almen" type measured. The influence of the most important spray parameters (including total gas flow rate, fuel-oxygen ratio, spray distance, thermal effects of the process, and particle speed) on the generation of internal stresses is examined. The influences of the process factors and material properties are studied, for example the "hammering" of the coating by impacting particles and the lack of conformity of the coefficient of thermal expansion of the coating/substrate. The results show that all residual stresses were caused by pressure and were very sensitive to the thermal effects of the process. Paper includes a German-language abstract.

The present work has the purpose of comparing different thermal spraying techniques, namely axial plasma spray, standard air plasma spray and high velocity oxygen flame (HVOF), for depositing metal matrix composites, in this case chromium carbide nickel-chromium based. The quality of the coatings deposited by these three techniques has been assessed in terms of structural characteristics (porosity, oxide concentration, unmelted particles presence, etc.) and of mechanical characteristics (hardness, adhesion, etc.) as well as surface morphology. A specific efficiency test has been carried out to compare the three examined technologies. The results of the present study indicate that, against a slightly decrease in the quality of the film in terms of structural and mechanical properties, axial plasma sprayed coatings can be sprayed with a higher efficiency in comparison to the traditional technologies.

Thermal spray coatings are widely used for erosion resistance, but the relationship between the microstructure of the coatings and their erosion resistance is not well understood. In this paper the performance of several commonly used coatings at ambient and elevated temperatures is reviewed in light of the coatings' structure and compared with a new coating. Two high temperature industrial applications, solid particle erosion in steam turbines and alumina-based erosion have been chosen to illustrate the significance of a coating's structure on its performance.

Agglomerated and sintered Cr 3 C 2 -25%NiCr powders possess excellent flow ability and appearance that have been extensively applied to resist abrase and erosion in high temperature applications such as power boiler and turbine blade. Microstructure of Cr 3 C 2 -25%NiCr coatings were observed through scanning electronic microscope (SEM), and bond strength and microhardness of coatings were measured by tensile shearing test and Vickers hardness test. It is indicated that ultrafine Cr 3 C 2 -25%NiCr coatings have some outstanding properties to traditional Cr 3 C 2 - 25%NiCr coatings by plasma sprayed.

Cr 3 C 2 -NiCr coatings are commonly used to provide abrasion and erosion wear resistance on the surface of components, in particular for corrosive and atmospheric high-temperature environments. For these classical and new applications the knowledge of the thermophysical properties is highly important. In the present work the dependence of the heat conductivity on temperature of two HVOF-sprayed Cr 3 C 2 -25NiCr-coatings prepared by a liquid-fuelled HVOF-process from two different feedstock powders from room temperature up to 700 °C was determined. Thermal diffusivities, density functions, specific heat capacities and coefficient of thermal expansion (CTE) were measured in order to compute the heat conductivity for the coatings. All measurements were performed twice (as-sprayed and after a first thermal cycle) in order to take into account the structural and compositional changes. XRD and FESEM studies were performed in order to characterize the phase compositions and microstructures in the as-sprayed and heat-treated states. Heat conductivities (average of the two coatings) ranging from about 11 W/(mK) at 50°C up to about 20 W/(mK) at 700°C were determined. Differences between the two coatings were clearly detectable. The heat conductivity of the Cr 3 C 2 -NiCr coatings is significantly lower than determined previously for a WC-17%Co coating.

Fiber-reinforced polymer composites are an important class of structural materials, offering high strength-to-weight ratios and high rigidities. For many applications, however, their wear resistance is less than desirable. Wear-resistant thermal spray coatings have the potential to improve the surface properties of fiber-reinforced polymer composites, although some require the application of a bond coat to achieve sufficient adhesion. The present study was conducted to find acceptable bond coat materials and compare their performance. Materials such as polyamides, polyimides, polyether-ether-ketone, or simply aluminum or nickel were found to be suitable bond coats for many composite substrates.

Tungsten carbide-12 wt.% cobalt (WC-12Co) coatings and chromium plating are used to provide wear resistant surfaces in gas turbine applications. These treatments provide surfaces with hardnesses greater than 60 Rockwell C. In addition, a surface finish better than 8 microinches RMS is required for optimum performance. To achieve this surface finish, diamond grinding is required. The diamond grinding step adds considerable cost to the product and economical benefits could be achieved if more conventional grinding techniques were incorporated. A program was initiated to develop an alternative thermal spray coating, with a target hardness lower than 60 Rockwell C, but high enough to provide the wear resistance required. Spray development was conducted on five commercially available materials using the Diamond Jet 2600 high velocity oxy-fuel process. Laboratory evaluation included coating microstructure, macro- and microhardness, bond strength, salt spray corrosion, and cyclic compression tests.

This paper presents the electrochemical characterization of a chromium carbide-NiCr coating applied using high-speed flame spraying. It examines the behavior of the complete system, the steel, the steel coating, and the coating immersed in NaCl solution alone. The paper discusses electrochemical measurement methods such as the measurement of polarization resistance, anodic polarization, and open circuit potential. The tests are compared with each other and with results from metallographic examinations. The structure was characterized by light and scanning microscopy. In addition, an analysis of the residual water was carried out using an ICP technique. The paper also includes a study of the various mechanisms that could affect the behavior of such coating types in a corrosive environment. Paper includes a German-language abstract.

A hybrid technique that combines plasma spraying and metal diffusion treatment was developed to improve the density, hardness, and adhesion of carbide cermet coatings. In this study, which was conducted to assess the process, Cr3C2-NiCr was deposited on stainless steel substrates by atmospheric plasma spraying. The test specimens were then embedded in a powder mixture of chromium, alumina, and ammonium chloride and diffusion treated. Cross-sectional SEM imaging, X-ray diffraction, and microhardness testing found the resulting surface layer to be extremely dense with outstanding hardness and exceptional bonding strength.

Thermal spray coatings are very effective in combating wear and corrosion in many applications. New thermal spray processes and coating compositions continue to be developed with concomitant improvements in the performance of the coatings and their use in new applications. Nonetheless, the thermal spray coatings are not without competition from other coating and overlay processes and materials. This brief review considers the microstructures and the wear and corrosion resistance of a number of alternative coatings to thermal spray coatings, including physical vapor deposition, chemical vapor deposition, electroplating, autocatalytic, and laser cladding.

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.

To protect structural and functional materials against influence of the severe environments, various coating technologies can be used. This study considers the use of high velocity oxy-fuel process the coatings available for applications based on deposition using the continuous detonation spray process. The properties of the continuous detonation sprayed WC-NiCr and Cr 3 C 2 -NiCr cermet coating are investigated through the mechanical, corrosion and wear resistance test. The test results are also compared with the properties of the duplex phase stainless steel substrate and the atmospheric plasma sprayed coatings with the same powders. The corrosion resistance was examined by electochemical method, corrosion potential monitering. Furthermore, the erosion wear resistance was examined by the abrasive and cavitation erosion wear test. The experimental results exhibited that the anti-corrosive and anti-erosive properties of the continuous detonation sprayed coatings are superior to that of their plasma sprayed coatings. It was found that the WC-NiCr cermet coating appears to be more effective than the Cr 3 C 2 -NiCr cermet coating in abrasive erosion environment, whereas the Cr 3 C 2 -NiCr cermet coating is more effective in cavitation erosion environment.