High-velocity Oxy-fuel Thermal Spray Process Research Articles

The Effect of Deposition Rate on Microstructural Evolution in WC-Co-Cr Coatings Deposited by High-Velocity Oxy-Fuel Thermal Spray Process

Tungsten carbide-reinforced cermet composites are widely used as wear- and erosion-resistant coatings due to their combination of hardness, strength, toughness and thermal properties. In this study, WC-Co-Cr coatings were deposited using high-velocity oxy-fuel thermal spray technology under different kinematic spray parameters. The microstructure of coatings was investigated using scanning electron microscopy and transmission electron microscopy techniques, and elemental/phase analysis was completed using x-ray diffraction, electron energy loss spectroscopy and x-ray energy-dispersive spectroscopy. The findings indicated the presence of W2C and W6Co6C in addition to WC and chromium oxide (Cr2O3) in a nanocrystalline cobalt-rich matrix. The newly developed phases (W2C and W6Co6C) were formed as a result of decarburization/oxidation of tungsten carbide (WC). They were found as individual particles (0.2-0.4 µm) embedded in the metallic binder phase as well as surrounding existing WC particles. Nonetheless, increasing the coating deposition rate, i.e., reduction of coating build-up per pass, increased the coating hardness from 1516 ± 124 to 1713 ± 106 HV0.3 and led to a higher degree of phase transformation reactions in the coating.

Investigation of operating parameters on high-velocity oxyfuel thermal spray coating quality for aerospace applications

The coating quality by the high-velocity oxyfuel (HVOF) thermal spray process is greatly dependent on the operating parameters chosen during the operation. Depending on the application, the operating conditions are manipulated to enhance the desired coating property. Enhancement of multiple properties necessitates optimization of the operating conditions. This entails a comprehensive analysis of several coatings deposited at different operating conditions. The design of experiments is an effective method that helps in identifying optimum operating conditions with a reduced number of experiments. In the present study, a 24 factorial design approach is used to establish the relationships between four operating parameters (coating thickness, fuel/oxygen ratio, spraying distance, and powder injection rate) and three coating properties (roughness, microhardness, and contact angle). The results obtained are utilized in identifying the optimum parameters that produce the best coating quality. Furthermore, the study also compares the coating quality obtained by the HVOF thermal spray process with that by chrome plating. The results show that the HVOF thermal spray process produces superior coating quality and it can be a promising environmentally friendly candidate for replacing chrome plating.

Improving Impact Resistance of High-Velocity Oxygen Fuel-Sprayed WC-17Co Coating Using Taguchi Experimental Design

The impact resistance of the WC-17Co coating as a function of high-velocity oxy-fuel thermal spraying process parameters was studied and presented in this paper. Design of experiments using Taguchi method and ANOVA were used for optimizing process parameters including grit type, spray distance, oxygen flow rate, carrier gas flow rate, powder feed rate, substrate preheat temperature and coating thickness to attain the maximum impact resistance in the coating. A falling mass impact tester apparatus was designed and fabricated for measuring the impact energy that was absorbed by each coating before failure. After each test, the cracks of each coating were observed under an optical microscope. According to the results, grit type was the most influential factor on increasing the impact resistance of the coatings and the effects of carrier gas flow rate, powder feed rate and substrate preheat temperature on impact resistance of the coatings were found to be negligible. The result of confirmation test showed that Taguchi method was a useful approach in predicting optimum parameters.

Microstructural, Hyperfine, and Magnetic Properties of FeSiBCuNb Deposits

Mechanically alloyed Fe73.5Si13.5B9Nb3Cu1 powders were used as feedstock materials for depositing nano/amorphous coatings by high-velocity oxy-fuel thermal spraying process. Morphology, structure, microstructure, hyperfine, and magnetic properties have been investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Mossbauer spectrometry, and vibrating sample magnetometer. The coating structure consists of bcc Fe(B)-type, bcc Fe(Si)-type, and amorphous phases. The crystallite sizes of Fe(Si) and Fe(B) solid solutions increase after spraying to about (72–82) and (48–81) nm, respectively. The Mossbauer spectrometry results reveal the presence of Fe2B boride. The squareness ratio Mr/ Ms and the coercivity values of the coatings decrease with increasing the number of passes.

Development of a semi automated dual feed unit to produce FGM coatings using the HVOF thermal spray process

The application of functionally graded materials (FGMs) is quite difficult, but thermal spray processes like Plasma spray have demonstrated their unique potential in producing graded deposits, where researchers have used twin powder feed systems to mix different proportions of powders. FGMs vary in composition and/or microstructure from one boundary (substrate) to another (top service surface), and innovative characteristics result from the gradient from metals to ceramics or non-metallic to metals. The present study investigates an innovative modification of a high velocity oxy-fuel (HVOF) thermal spray process to produce functionally graded thick coatings. In order to deposit thick coatings, certain problems have to be overcome. Graded coatings enable gradual variation of the coating composition and/or microstructure, which offers the possibility of reducing residual stress build-up with in coatings. In order to spray such a coating, modification to a commercial powder feed hopper was required to enable it to deposit two powders simultaneously which allows deposition of different layers of coating with changing chemical compositions, without interruption to the spraying process. Various concepts for this modification were identified and one design was selected, having been validated through use of a process model, developed using ANSYS Flotran finite element analysis. In the current research the mixing of different proportions of powders were controlled by a computer using LabVIEW software and hardware, which allowed the control and repeatability of the microstructure when producing functionally graded coatings.

High-Temperature Behavior of a High-Velocity Oxy-Fuel Sprayed Cr3C2-NiCr Coating

High-velocity oxy-fuel (HVOF) sprayed coatings have the potential to enhance the high-temperature oxidation, corrosion, and erosion-corrosion resistance of boiler steels. In the current work, 75 pct chromium carbide-25 pct (nickel-20 pct chromium) [Cr3C2-NiCr] coating was deposited on ASTM SA213-T22 boiler steel using the HVOF thermal spray process. High-temperature oxidation, hot corrosion, and erosion-corrosion behavior of the coated and bare steel was evaluated in the air, molten salt [Na2SO4-82 pct Fe2(SO4)3], and actual boiler environments under cyclic conditions. Weight-change measurements were taken at the end of each cycle. Efforts were made to formulate the kinetics of the oxidation, corrosion, and erosion-corrosion. X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM)/energy dispersive spectroscopy (EDS) techniques were used to analyze the oxidation products. The coating was found to be intact and spallation free in all the environments of the study in general, whereas the bare steel suffered extensive spallation and a relatively higher rate of degradation. The coating was found to be useful to enhance the high-temperature resistance of the steel in all the three environments in this study.

Application of Acoustic Emission for Monitoring the HVOF Thermal Spraying Process

This research aims to characterise and quantify the acoustic emission (AE) generated during the high velocity oxy-fuel (HVOF) thermal spraying process, recorded using piezoelectric AE sensors. The HVOF process is very complex involving high temperature turbulent flow through a nozzle with entrained particles, the projection of these particles, and their interaction with the target surface. Process parameters such as gun speed, oxy-fuel pressure and powder specification affect various characteristics of the coating, including thermal residual stresses; the lamellar microstructure and the topology and geometry of pores, all formed when the fused powder hits the surface, forming “splats”. It is widely acknowledged in the thermal spray industry that existing quality control techniques and testing techniques need to be improved. New techniques which help to understand the effects of coating process parameters on the characteristics of the coating are therefore of value, and it was anticipated that recording the AE produced when the fused particles contact the surface would aid this understanding. As a first stage, we demonstrated here that AE associated with particle impact can, in fact, be discerned in the face of the considerable airborne and structure-borne noise. In order to do this, a new test method using a masking sheet with slits of varying size was developed. Thermal spraying was carried out for a range of spray gun speeds and process parameters. The AE was measured using a broad band AE sensor positioned on the back of the sample as the spot was traversed across it. The results show that the amplitude and energy of the AE signals is related to the spray gun speed, powder used and the oxy-fuel pressure. Using a simple geometrical model for particle impact, the measured AE was found to vary with the energy and number of particles impacting on the sample in a predictable way.

HVOF coatings as an alternative to hard chrome for pistons and valves

Automotive manufacturers have specified chromium plating for decades because of its appearance, wear and corrosion resistance, however chromium plating cause effects on human health because of the use of substances in the galvanic process whose toxicological features have not always been recognised. The improvements of the high-velocity oxy-fuel thermal spray process allow the chromium coating replacement with a comparable or superior surfaces and more environment friendly. The present study describes and compare the mechanical and tribological properties of HVOF CrC75 (NiCr20) 25 coatings sprayed from three different agglomerated feedstock powders with various powder size distributions. These results have been compared with conventional hard chromium plating. The objective of the present work is applying these new HVOF coatings in piston rings and valve stems applications. Furthermore, the studied HVOF coatings are produced with fine-powders in order to avoid the blasting and regrinding operations necessary when plasma spray coatings are used. The coating microstructures were characterised by BSE–SEM microscopy. Differences in 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 pin on disc tribometer 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 CrC–NiCr coating, obtained with the lowest feedstock powder size, presented the best wear resistance under all the studied conditions. The Fine CrC–NiCr coatings have demonstrated superior performance to hard chrome with regard to mechanical and tribological properties, and they can be proposed as an alternative to hard chrome coatings.

Aerodynamic study on supersonic flows in high-velocity oxy-fuel thermal spray process

To clarify the characteristics of gas flow in high velocity oxy-fuel (HVOF) thermal spray gun, aerodynamic research is performed using a special gun. The gun has rectangular cross-sectional area and sidewalls of optical glass to visualize the internal flow. The gun consists of a supersonic nozzle with the design Mach number of 2.0 followed by a straight passage called barrel. Compressed dry air up to 0.78 MPa is used as a process gas instead of combustion gas which is used in a commercial HVOF gun. The high-speed gas flows with shock waves in the gun and jets are visualized by schlieren technique. Complicated internal and external flow-fields containing various types of shock wave as well as expansion wave are visualized.

Alternative to chromium: characteristics and wear behavior of HVOF coatings for gas turbine shafts repair (heavy-duty)

Hard chromium plating is usually used to restore to original dimensions the worn surfaces of gas turbine shafts. However, such a technology presents harmful effects on the environment and the public health and it exhibits, moreover some intrinsic technical limitations. HVOF (high-velocity oxy-fuel) thermal spraying process appears as more environmentally friendly than chromium plating process but exhibits also lower potential production costs when compared to hard chromium deposits. In such a way, HVOF process appears as an alternative to hard chromium plating for shafts repair, by reducing the frequency of maintenance operation and repair and by deferring the need to fabricate or to buy replacement parts for used engines of previous generation. The purpose of this study was to investigate and to compare microstructural properties, wear resistance, and potentials of HVOF sprayed Tribaloy©-400 (T-400), Cr 3C 2–25%NiCr and WC–12%Co coatings for a possible replacement of hard chromium plating in gas turbine shafts repair. It was shown that thermal spray coatings exhibit the adequate properties compared to electrodeposited hard chromium coatings.

Alternative to chromium: characteristics and wear behavior of HVOF coatings for gas turbine shafts repair (heavy-duty)

Hard chromium plating is usually used to restore to original dimensions the worn surfaces of gas turbine shafts. However, such a technology presents harmful effects on the environment and the public health and it exhibits, moreover some intrinsic technical limitations. HVOF (high-velocity oxy-fuel) thermal spraying process appears as more environmentally friendly than chromium plating process but exhibits also lower potential production costs when compared to hard chromium deposits. In such a way, HVOF process appears as an alternative to hard chromium plating for shafts repair, by reducing the frequency of maintenance operation and repair and by deferring the need to fabricate or to buy replacement parts for used engines of previous generation. The purpose of this study was to investigate and to compare microstructural properties, wear resistance, and potentials of HVOF sprayed Tribaloy©-400 (T-400), Cr3C2–25%NiCr and WC–12%Co coatings for a possible replacement of hard chromium plating in gas turbine shafts repair. It was shown that thermal spray coatings exhibit the adequate properties compared to electrodeposited hard chromium coatings.

Analysis of particle behavior in High-Velocity Oxy-Fuel thermal spraying process

This paper analyzes the behavior of coating particle as well as the gas flow both of inside and outside the High-Velocity Oxy-Fuel (HVOF) thermal spraying gun by using quasi-one-dimensional analysis and numerical simulation. The HVOF gun in the present analysis is an axisymmetric convergent-divergent nozzle with the design Mach number of 2.0 followed by a straight passage called barrel. In the present analysis it is assumed that the influence of the particles injected in the gas flow is neglected, and the interaction between the particles is also neglected. The gas flow in the gun is assumed to be quasi-one-dimensional adiabatic flow. The velocity, temperature and density of gas in the jet discharged from the barrel exit are predicted by solving Navier-Stokes equations numerically. The particle equation of motion is numerically integrated using three-step Runge-Kutta method. The drag coefficient of the particle is calculated by linear interpolation of the experimental data obtained in the past. Particle mean temperature is calculated by using Ranz and Marchalls’ correlation for spherical particles. From the present analysis, the distributions of velocity and temperature of the coating particles flying inside and outside the HVOF gun are predicted.

Mechanical and tribological properties of high velocity oxy-fuel thermal sprayed nanocrystalline CrCNiCr coatings

One of the most important use of high velocity oxy-fuel (HVOF) thermal spray coatings is for wear resistance. In this work, the characteristics of nanocrystalline CrCNiCr coating and their effect on the mechanical and tribological properties of the material have been investigated. The objective of this study is the replacement of hazardous hard chromium plating technology used today in industry for an efficient and clean HVOF technology, using micro- and nanocrystalline CrCNiCr coatings. Commercially available, CrCNiCr powder was mechanically treated, in order to obtain nanopowders. Later, the HVOF thermal spray process was used to produce conventional and nanocrystalline CrCNiCr coatings. The coating microstructures were characterised by SEM and optical microscopy. Differences in roughness have been determined by profilometry. The ultra-microindentation technique was applied to measure the hardness and the elasto-plastic properties of the coating. In order to evaluate the friction and wear properties of different substrate materials, a pin-on-disc method has been used.

Production of free standing objects by high velocity oxy-fuel (HVOF) thermal spraying process

The feasibility of manufacturing free-standing thin walled objects by HVOF thermal spraying process is investigated. By this method hollow cylindrical objects have been produced using different metallic materials. Coating materials in the form of powder were sprayed on pre-shaped cores. Some releasing agent was used to separate the free standing body from the pre-shaped cores. These free standing objects were characterised for the mechanical properties. The parameters measured includes hardness, porosity and surface roughness.

High-velocity oxyfuel thermal spray coatings for biomedical applications

Plasma spraying is used to produce most commercially available bioceramic coatings for dental implants; however, these coatings still contain some inadequacies. Two types of coatings produced by the high- velocity oxyfuel (HVOF) combustion spray process using commercially available hydroxyapatite (HA) and fluorapatite (FA) powders sprayed onto titanium were characterized to determine whether this relatively new coating process can be applied to bioceramic coatings. Diffuse reflectance Fourier transform infrared (FTIR) spectroscopy, x- ray diffraction (XRD), and scanning electron microscopy (SEM) were used to characterize the composition, microstructure, and morphology of the coatings. The XRD and FTIR techniques revealed an apatitic structure for both HA and FA coatings. However, XRD patterns indicated some loss in crystallinity of the coatings due to the spraying process. Results from FTIR showed a loss in the intensity of the OH∼ and F∼ groups due to HVOF spraying; the phosphate groups, however, were still present. Analysis by SEM showed a coating morphology similar to that obtained with plasma spraying, with complete coverage of the titanium substrate. Interfacial SEM studies revealed an excellent coating-to-substrate apposition. These results indicate that with further optimization the HVOF thermal spray process may offer another method for producing bioceramic coatings.