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.

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.

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.

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.

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.

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.

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.

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.

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.

HVOF sprayed WC/Co(Cr) and Cr 3 C 2 /Ni20Cr composite coatings have gained high acceptance in many industrial applications for protection of components against wear. The achieved coatings have quite good corrosion resistance for use of chromium containing matrices. Present research in the field of PTA-Surfacing resulted in the development of high chromium and high vanadium containing iron based hard alloys with simultaneous improvement of abrasive wear and corrosion resistance. These properties of the PTA-Surfaced coatings were studied and it was found that the newly developed alloys have nearly same wear resistance and improved corrosion resistance compared to Co-based alloys reinforced with Fused Tungsten Carbides (FTC). One major advantage of high chromium and high vanadium containing iron based coatings is machinability by turning and milling processes. These features make it attractive to be used in thermal spraying. The absence of substrate melting in HVOF-spraying is advantageous, as the coatings preserve the properties of the alloy due to prevention of dilution with substrate material in contrast to PTA-Surfacing. High chromium and high vanadium containing iron based atomized powder was used for HVOF spraying and deposition efficiency was measured. The sprayed coatings were studied metallographically by optical microscopy, SEM, XRD and micro-hardness measurements. Later, abrasive wear and corrosion properties of the coatings are investigated.

This work assesses the effect of heat treatment on the oxidation resistance of blended-powder CrC-NiCr coatings produced by HVOF spraying. The as-sprayed coating phases oxidized independently with NiCr rapidly forming nickel oxide, which was subsequently consumed by a solid-state reaction with Cr 2 O 3 . The NiO formed two distinct morphologies: large bulbous oxides on thin regions of exposed alloy and thin, flat oxide layers on large alloy regions. Heat treatment led to sintering of the splats and diffusion of Cr from the carbide phase into the NiCr alloy, increasing Cr to the point where Cr 2 O 3 dominated the oxide composition from the earliest exposure time. The implications of the change in oxidation mechanism and oxide morphology are discussed.

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.

Cr 3 C 2 -25%NiCr, Stellite 6, NiCrBSi and Hastelloy C-276 coatings were deposited on substrate material P91 by HP/HVOF (High Pressure / High Velocity Oxygen Fuel) thermal spraying technology. The resistance against high temperature corrosion was evaluated exposition of coatings to corrosive-aggressive environment in the form of molten salts mixture with composition of 60% V 2 O 5 and 40% Na 2 SO 4 at temperature of 750 °C. Further, coatings were exposed to cyclic conditions. After the corrosion tests, all coatings were analyzed using scanning electron microscope (SEM), and analysis of elemental composition (EDX). Alloys-based coatings showed very similar corrosion mechanism in the selected aggressive environment and the same can be stated about cermet coatings. The obtained results prove that HVOF deposited coatings can replace current surface protection of components in power equipment such as nitriding.

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.

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.

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.

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.

This article investigates composite powder materials based on double chromium and titanium carbides with nickel-chromium binder produced using self-propagating high-temperature synthesis. It focuses on the hypersonic velocity oxygen fuel coatings from the synthesized powders. Laboratory tests were focused on the solid particle erosion which occur in energy production systems such as fluidized bed combustors, advanced pulverized cool boilers, and entrained coal gasifiers. Tests were conducted at elevated temperature in a blast nozzle type of tester using bed or fly ashes retrieved from the operating CFB boilers. It was observed that, when adjusting carbide phase composition and chromium content in titanium carbide solid solution, one can control the oxidation kinetic and wear resistance of the material. Fine-grained structure and high cohesion strength of the composite materials formed during synthesis provide their excellent elevated temperature erosion performance in a wide range of test conditions.

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.

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.