Bonding mechanism from the impact of thermally sprayed solid particles

Power particles are mainly in solid state prior to impact on substrates from high velocity oxy-fuel (HVOF) thermal spraying. The bonding between particles and substrates is critical to ensure the quality of coating. Finite element analysis (FEA) models are developed to simulate the impingement process of solid particle impact on substrates. This numerical study examines the bonding mechanism between particles and substrates and establishes the critical particle impact parameters for bonding. Considering the morphology of particles, the shear-instability–based method is applied to all the particles, and the energy-based method is employed only for spherical particles. The particles are given the properties of widely used WC-Co powder for HVOF thermally sprayed coatings. The numerical results confirm that in the HVOF process, the kinetic energy of the particle prior to impact plays the most dominant role in particle stress localization and melting of the interfacial contact region. The critical impact parameters, such as particle velocity and temperature, are shown to be affected by the shape of particles, while higher impact velocity is required for highly nonspherical powder

Numerical study of molten and semi-molten ceramic impingement by using coupled Eulerian and Lagrangian method

Large temperature gradients are present within ceramic powder particles during plasma spray deposition due to their low thermal conductivity. The particles often impinge at the substrate in a semi-molten form which in turn substantially affects the final characteristics of the coating being formed. This study is dedicated to a novel modeling approach of a coupled Eulerian and Lagrangian (CEL) method for both fully molten and semi-molten droplet impingement processes. The simulation provides an insight to the deformation mechanism of the solid core YSZ and illustrates the freezing-induced break-up and spreading at the splat periphery. A 30 μm fully molten YSZ particle and an 80 μm semi-molten YSZ particle with different core sizes and initial velocity ranging from 100 to 240 m/s were examined. The flattened degree for both cases were obtained and compared with experimental and analytical data

Sliding wear behaviour of WC-Co reinforced NiCrFeSiB HVOAF thermal spray coatings against WC-Co and Al2O3 counterbodies

© 2020 The Authors NiCrFeSiB alloys reinforced with WC-Co are potentially useful composite coating materials for use in applications in which resistance to sliding wear, hot corrosion and high temperature is required. Furthermore these materials offer an advantage over WC-Co and WC-CoCr coatings in applications where a more ductile coating is required. A powder feedstock containing a 50/50 mixture of WC-Co/NiCrFeSiB was sprayed by a HVOAF (high velocity oxy-air fuel) thermal spray torch, which was developed by Monitor Coatings Castolin Eutectic for internal diameter applications, with two sets of spray parameters with the overall gas flowrate entering the torch changed. The powder feedstock and sprayed coatings were characterised using SEM imaging, XRD and measurement of mechanical properties such as microhardness and indentation fracture toughness. The specific wear rates of the coatings were measured when testing the coatings against WC-Co and Al2O3 counterbodies and it was determined that the coating sprayed at the higher gas flowrate wore out less against both counterbody materials, due to its superior microhardness. Tests against the Al2O3 counterbody led to increased material loss of both coatings in comparison to testing against WC-Co. This was due to the wear of the Al2O3 ball throughout the test leading to an increase in contact area between the coating and counterbody

Application of HVOF WC-Co-Cr coatings on the internal surface of small cylinders: Effect of internal diameter on the wear resistance

Due to the restrictions and mediocre performance of current methods of coating complex shaped parts in which line of sight processes currently struggle, the development of new coating methods is essential, with High Velocity Oxy-Fuel (HVOF) thermal spray coatings being a good candidate. In this study, a new compact High Velocity Oxy-Air Fuel (HVOAF) thermal spray torch designed to coat internal surfaces was traversed within cylindrical pipes of internal diameters (IDs) of 70 mm, 90 mm and 110 mm and a WC-10Co-4Cr coating was applied with a commercially available powder feedstock. Powder and coating microstructures were analysed using SEM/EDX and XRD. Fracture toughness and microhardness of the coatings were measured, and dry sliding wear performance was investigated at two loads: 96 and 240 N. It was found that the coating sprayed at 90 mm (medium ID) had a lower specific wear rates at both test loads due to the highest fracture toughness and microhardness; whereas, the coating sprayed at 110 mm (high ID) showed the highest specific wear rates at both low and high conditions due to poor fracture toughness

Internal diameter HVOAF thermal spray of carbon nanotubes reinforced WC-Co composite coatings

© 2021 The Authors A novel process, relatively fast and scalable compared to existing ones, has been used to incorporate carbon nanotubes (CNT) 0.5 wt% onto commercial WC-Co thermal spray powder. Nano enabled WC-Co/CNT coatings were obtained by internal diameter HVOAF thermal spray at three different spray powers (48, 41 and 34 kW). A general increase in microhardness and decrease in fracture toughness has been found when adding CNT. Also, carbide retention index upon spray was improved by lowering the process temperature and adding CNT, reaching 99% in the low power case with CNT. The interplay of these features has shown an overall better wear performance in the medium power case without CNT; in fact, the addition of CNT has improved the wear performance at high power conditions (reduction of coefficient of friction by half) while reducing the wear performance in medium and low power. A tailored choice of CNT concentration can offer enhanced mechanical or tribological properties according to the needs for different applications. This novel process for nanoparticles incorporation opens the way for the production of large batches of readily usable nano-enabled powder

Modeling the effects of concentration of solid nanoparticles in liquid feedstock injection on high-velocity suspension flame spray process

This paper presents the effects of the concentration of solid nanoparticles in the liquid feedstock injection on the high-velocity suspension flame spray (HVSFS) process. Four different concentrations of solid nanoparticles in suspension droplets with various droplet diameters are used to study gas dynamics, vaporization rate, and secondary breakup. Two types of injections, viz. surface and group, are used. The group-type injection increases the efficiency of droplet disintegration and the evaporation process and reduces the gas cooling. The initiation of the fragmentation process is difficult for small droplets carrying a high concentration of nanoparticles. Also, smaller droplets undergo rapid vaporization, leaving clogs of nanoparticles in the middle of the barrel. For larger droplets, severe fragmentation occurs inside the combustion chamber. For a higher concentration of nanoparticles, droplets exit the gun without complete evaporation. The results suggest that, in coating applications involving a higher concentration of nanoparticles, smaller droplet sizes are preferred

Microstructural evaluation of thermal-sprayed CoCrFeMnNi0.8V high-entropy alloy coatings

The aim of this work is to improve the understanding of the effect of the cooling rate on the microstructure of high-entropy alloys, with a focus on high-entropy alloy coatings, by using a combined computational and experimental validation approach. CoCrFeMnNi0.8V coatings were deposited on a steel substrate with high velocity oxy-air-fuel spray with the employment of three different deposition temperatures. The microstructures of the coatings were studied and compared with the microstructure of the equivalent bulk high-entropy alloy fabricated by suction casting and powder fabricated by gas atomization. According to the results, the powder and the coatings deposited by low and medium temperatures consisted of a BCC microstructure. On the other hand, the microstructure of the coating deposited by high temperature was more complex, consisting of different phases, including BCC, FCC and oxides. The phase constitution of the bulk high-entropy alloy included an FCC phase and sigma. This variation in the microstructural outcome was assessed in terms of solidification rate, and the results were compared with Thermo-Calc modelling. The microstructure can be tuned by the employment of rapid solidification techniques such as gas atomization, as well as subsequent processing such as high velocity oxy-air-fuel spray with the use of different spray parameters, leading to a variety of microstructural outcomes. This approach is of high interest for the field of high-entropy alloy coatings

Computational and experimental investigation of the strain rate sensitivity of small punch testing of the high-entropy alloy CoCrFeMnNi

The suitability of determining the strain rate sensitivity (SRS) of the CoCrFeMnNi high-entropy alloy (HEA) by small punch (SP) testing has been assessed at displacement rates ranging from 0.2 to 2 mm∙min−1. The stress was found to increase as the displacement rate was raised from 0.2 to 2 mm∙min−1, whereas the plastic strain distributions were similar in all cases. However, for a higher displacement rate of 10 mm∙min−1, the sample was found to exhibit a drop in strength and ductility attributed to casting defects. The strain-rate sensitivity exponent (m) was found to be 0.1387 whilst the Finite Element Analysis (FEA) simulations predicted a slightly smaller value of 0.1313. This latter value is closer to m = 0.091 obtained from nanoindentation strain rate jump tests since the results are insensitive to the presence of small casting defects. The relationship between the experimental and the empirically derived predicted properties from the SP tests revealed a high level of agreement for maximum stress properties. The properties predicted at 2 mm∙min−1 (R2 = 0.96) offered a stronger fit than at 0.5 mm∙min−1 (R2 = 0.92)

Effect of particle and carbide grain sizes on a HVOAF WC-Co-Cr coating for the future application on internal surfaces: microstructure and wear

The use of nanoscale WC grain or finer feedstock particles are possible methods of improving the performance of WC-Co-Cr coatings. Finer powders are being pursued for the development of coating internal surfaces, as less thermal energy is required to melt the finer powder compared to coarse powders, permitting spraying at smaller stand - off distances. Three WC-0Co-4Cr coatings, with two different powder particle sizes and two different carbide grain sizes, were sprayed using a high velocity oxy-air fuel (HVOAF) thermal spray system developed by Castolin Eutectic - Monitor Coatings Ltd., UK. Powder and coating microstructures were characterised using XRD and SEM. Fracture toughness and dry sliding wear performance at three loads were investigated using a ball–on-disc tribometer with a WC–Co counter body. It was found that the finer powder produced the coating with the highest microhardness, but its fracture toughness was reduced due to increased decarburisation compared to the other powders. The sprayed nanostructured powder had the lowest microhardness and fracture toughness of all materials tested. Unlubricated sliding wear testing at the lowest load showed the nanostructured coating performed best; however at the highest load this coating showed the highest specific wear rates with the other two powders performing to a similar, better standard

Numerical modelling the bonding mechanism of HVOF sprayed particles

During high velocity oxy-fuel (HVOF) thermal spraying, most powder particles remain in solid state prior to the formation of coating. A finite element (FE) model is developed to study the impact of thermally sprayed solid particles on substrates and to establish the critical particle impact parameters needed for adequate bonding. The particles are given the properties of widely used WC-Co powder for HVOF thermally sprayed coatings. The numerical results indicate that in HVOF process the kinetic energy of the particle prior to impact plays the most dominant role on particle stress localization and melting of the particle/substrate interfacial region. Both the shear-instability theory and an energy-based method are used to establish the critical impact parameters for HVOF sprayed particles, and it is found that only WC-Co particles smaller than 40 ?m have sufficient kinetic and thermal energy for successful bonding.<br/