Thermal Spray Process Research Articles

Mathematical modelling and validation of the mechanical properties of HVOF-sprayed WC-12Co coatings considering the phase composition and defects generated at different process parameters

The thermal spraying process involves many machine-based parameters or independent variables (fuel flow rate, oxygen flow rate, powder feed rate, stand-off distance and many others in the HVOF process). Any combination of these parameters produces only two measurable responses, namely, particle temperature and particle velocity. In this current work, first, the machine-based spray parameters are varied in specific ways to produce different particle temperatures at near-constant velocities and vice-versa. This approach has reduced the number of independent variables to only two. These two factors further influence the in-flight particle reactions, thereby affecting the phase composition and the microstructural defects in the as-sprayed coatings. The X-ray diffraction patterns of the as-sprayed coatings revealed the presence of W2C, W, Co3W9C4, and amorphous Co-W-C phases apart from the WC phase. The W2C and W originated owing to the decarburization of the WC phase. On the other hand, the dissolution of the carbide in molten binder led to the formation of Co3W9C4 and the amorphous Co-W-C phase. The nano-hardness of both the carbide and binder phases increased with an increase in particle temperature owing to a higher degree of decarburization and carbide dissolution respectively. The porosity was the most significant micro-structural defect present in the as-sprayed coatings. An increase in either particle temperature or velocity reduced the porosity present in the as-sprayed coatings. The mechanical properties (microhardness, elastic modulus and indentation fracture toughness) tend to improve as porosity decreases. A comprehensive mathematical model was proposed to predict the mechanical properties of as-sprayed coatings as a function of composition and defects. Finally, the process maps of mechanical properties were plotted in temperature-velocity space.

The important factors for structure and mechanical properties in the repair of iron base metal component by thermal spray and welding processes

Cast iron is widely used in the manufacturing industry due to its high strength and wear resistance properties. However, cast iron's brittle nature results in frequent failure of cracks during formation or use. Among the repair methods that can be used are thermal spray and the welding process. Even though both welding and thermal spray have been implemented for various metal repairs processes, however very limited technical reports as well as scientific papers are found for this topic. Therefore, the optimum condition of metal repair by both processes is still needed to be explored. The present works focus on the comparison between thermal spray and welding method for cast iron repairs purposes. In the experiment of thermal spray process focus has been given in optimizing spraying distance on microstructure and hardness properties. On the other hand, in the welding experimental works focus has been given on the influence of groove design on microstructure and hardness. Each research variable is carried out to obtain optimal crack repair results. It was observed that thermal spray process produces less Heat Affected Zone (HAZ) area compared to the welding process therefore having less critical area. The highest hardness value of thermal spray method is 101.33 shown by 30 cm spraying distance. Meanwhile, the highest hardness value of HAZ area of welding method is 600 HV shown by specimen A. It was obtained that from the present experimental works, thermal spray process produces better results than welding process. However, the value of the specimen hardness produced by the welding and thermal spray method depends on the type of coating material used

Comparison on erosion performance of HVOF sprayed NiCrSiBCFe-WC-Co and NiCrSiBCFe-Cr3C2NiCr coatings

Hard coatings are used to improve the wear resistance of various engineering materials while maintaining the mechanical properties of the base material. In this work, the high-velocity oxy-fuel (HVOF) thermal spray process was used to deposit NiCrSiBCFe(65)-WC-Co(35) and NiCrSiBCFe(65)-Cr3C2-NiCr(35) coatings on SA213-T17 boiler steel to improve the erosion wear resistance of the base material. The erosion wear behaviours of the specimens were investigated using an air jet erosion tester at 30 and 90° impact angles and 400 ± 25 °C temperature. The surface morphologies of the as-deposited and eroded samples were examined using the Scanning electron microscope. The NiCrSiBCFe-WC-Co ceramic coating deposited on SA213-T17 boiler steel exhibited superior erosion resistance compared to NiCrSiBCFe-Cr3C2-NiCr coating and boiler steel under all the tested conditions. This enhanced performance can primarily be attributed to the higher hardness of the NiCrSiBCFe-WC-Co coating due to the presence of harder phases such as WC, W2C and NiCo2O4.

Numerical Analysis of Quenching Stress in Thermal Spray Process Using SPH Method

Numerical Analysis of Quenching Stress in Thermal Spray Process Using SPH Method

Synthesis of a novel AlBeSiTiV light weight HEA coating on SS316 using atmospheric plasma spray process

High Entropy Alloys (HEAs) are currently a subject of significant research interest in the fields of materials science and engineering. They are rapidly evolving due to their exceptional properties, and there is considerable focus on expanding their application potential by developing HEA coatings on various substrate materials. This area of study holds promise for advancing technology and innovation in diverse industries. In this study, a novel equiatomic AlBeSiTiV Light Weight HEA was synthesized via mechanical alloying and was sprayed on the substrate SS316 by the thermal spray process. The microstructural characterization revealed that synthesized HEA had a major FCC phase and the average coating thickness was observed to be 150 μm. The average microhardness was measured to be 975 ± 13 HV for the coating which was five times than the substrate. The coated samples' wear resistance was found out using a pin-on-disc apparatus by varying the wear process parameters and Taguchi's L27 Orthogonal Array was used to interpret the parametric influence on wear rate. ANOVA and regression analysis revealed applied load to be the most significant factor followed by distance and velocity. The major wear mechanisms observed were adhesion abrasion and oxidation, and the formation of tribolayer was observed at higher velocity and distance.

High Temperature Corrosion Resistant and Anti-slagging Coatings for Boilers: A Review

AbstractHigh temperature corrosion and slag deposition significantly reduce the thermal efficiency and lifespan of biomass-fired boilers. Surface modification with protective coatings can enhance boiler performance and prevent commercial losses due to maintenance and damage. This review focuses on the development of corrosion-resistant coatings (CRCs) and anti-slagging coatings (ASCs) over the past decade. CRCs are explored through thermal spray processes that include arc spray, atmospheric plasma spray (APS), high-velocity oxygen fuel (HVOF), detonation gun (D-gun™), and cold spray. Studies on alloys, ceramics, and ceramic–metal composites are summarised, highlighting the high temperature corrosion prevention mechanisms and discussing new coating materials. ASCs are reviewed in the context of advancements via thermal spray and slurry spray methods. The mechanisms for slag reduction, testing methods to evaluate ASC effectiveness, and the necessary architecture for preventing slag deposition are examined. A lab-based rig simulating fly ash deposition onto water-cooled coating coupons for anti-slagging investigations is also presented. Further research is needed to develop and evaluate materials for ASCs effectively. Graphical Abstract

Effect of Heat Treatment on Mechanical Properties and Corrosion Response of HVOF Sprayed High Entropy Alloy Coatings

Surface and sub-surface related degradation of steels can be minimized using suitable surface coatings. High entropy alloys (HEA) are prominent and emerging materials among many coating materials. The current study investigates the effect of heat treatment of HEA coating on mechanical, metallurgical, and corrosion properties. The HEA coatings on SS304 steel were deposited using a High-Velocity Oxy-Fuel (HVOF) thermal spray process. The developed coatings were furnace heat treated at 700 °C, 900 °C, and 1100 °C, respectively, and their performance was benchmarked with the as-sprayed coatings. The metallurgical, mechanical, and microstructural analyses were performed using X-ray diffraction (XRD), Nanoindentation, Scratch test, and Field Emission Scanning Electron Microscope (FESEM) techniques. The corrosion response of the as sprayed and heat-treated coatings were recorded using a Potentiostat. The results indicated that as-sprayed coatings consisted of a single-phase BCC solid solution; however, the single-phase changed to a dual dual-phase system after heat treatment (BCC+FCC). The 900 °C heat-treated HEA coating exhibited superior mechanical and corrosion properties. But those characteristics started diminishing when the heat treatment temperature exceeded 900 °C. The introduction of the new FCC phase softened the coating, thereby leading to the evolution of microcracks in the coating. These micro-cracks acted as channels for electrolyte diffusion and further corroded the coatings. The current study surmised that HVOF-sprayed HEA coating should not be heat treated at above 900 °C.

Thermal spray to embed optical fibers for the monitoring and protection of metallic structures

In the framework of using fiber optics (FO) for structural health monitoring, a true challenge is to fix the fiber onto structures guaranteeing both protection for the former and an effective adhesion on the latter. This work proposes a method to obtain such result via thermal spray technique on metallic structures, allowing its use in the most severe conditions of corrosion and wear. Since the transmission medium between the structure and the sensitive part of the optical fiber is represented by the fiber coating, three differently coated fibers were used on C-40 steel substrate: polyacrylate, polyimide and ORMOCER. In addition, the use of a primer to improve the bond on the substrate was evaluated. The adhesion between FO and metallic coating is evaluated through optical microscopy (OM) and scanning electrons microscopy (SEM) analysis. The functionality is also verified with both thermal and mechanical tests to calibrate the measuring accuracy. The results indicate that the best combination is that of the polyimide fiber, a zinc primer and aluminum coating. The proven qualities are the adhesion at the interface between the metallic coating and the fiber optics, and the preservation of the structural integrity of the fiber itself and its coating, and a precise measurement of strain acquired by fiber Bragg grating sensors (FBGs). The use of the thermal spray process is thus proved to be a solution for the optical fiber and substrate interaction, since it preserves the integrity of the optical fiber, due to the low temperature of the process, adding the protection that the metallic coating offers as well.

A conceptual model for evaluating eco-efficiency of thermal spraying processes

Thermal spraying (TS) is one of the main processes for obtaining surfaces with the desired protective properties in various industrial applications. TS is an energy-intensive treatment required to heat the application material and consumes different resources. To assess the environmental impact of TS, it becomes necessary to integrate an approach that jointly analyses and evaluates the economic and environmental variables influencing the system. The concept of eco-efficiency (EE) added to the TS process allows for assessing the environmental and economic condition through the survey and application of eco-indicators. The lack of an EE evaluation model for TS processes was identified based on literature searches. Thus, the overall objective of this work is to propose a conceptual model to evaluate the EE of TS treatment, selecting environmental and economic indicators considered more impactful in the process. The model developed consists of three main steps: (i) the input and output indicators (environmental and economic) are identified by applying the Analytic Hierarchy Process (AHP) method; (ii) the structure to be employed in the model is defined; and (iii) the Data Envelopment Analysis (DEA) model is applied to define the EE evaluation form. The proposed model consists of clear and easy-to-follow steps for evaluating the EE of spraying processes, filling the gap found in the literature. The use of DEA allowed the integration of the environmental and economic indicators obtained from the TS processes to generate important insights for evaluating EE. The results prove the model's effectiveness in identifying the EE results for each analysed unit of the TS process. The model has provided an evaluation consistent with the existing studies, and the EE scores were assessed according to twenty-one decision-making units (DMUs) allowing the identification of the most eco-efficient DMUs concerning TS processes.

Effect of Thickness on the Residual Stress Profile of an Aluminum Cold Spray Coating by Finite Element Analysis

This research investigates the influence of thickness on residual stress profiles in aluminum cold spray coatings using finite element analysis (FEA). Residual stress is a critical factor that impacts coating adhesion, fatigue life, and susceptibility to delamination in thermal spray processes. Despite its acknowledged importance, predictive analysis of these stresses on a layer-by-layer basis remains relatively unexplored. This study introduces an innovative numerical methodology to analyze the progression of residual stresses across various deposition efficiencies (10%, 40%, 60%, and 100%) and layer thicknesses, thereby enhancing predictive accuracy for cold spray coatings. The findings demonstrate that the number of deposited layers significantly affects residual stress profiles in both coatings and the substrate, with compressive residual stress predominating in the coatings and deeper tensile stress predominating in the substrate. Residual stress behavior near the last deposited layer aligns with the expected peening effect. Discrepancies in substrate stress distributions may arise from variations in deposition parameters and unconsidered temperature effects. While the model generally aligns with theoretical and some empirical data, observed discrepancies underscore the need for further validation. This study lays the groundwork for informed decision-making for cold spray processes by providing insights into stress management, thereby contributing to enhancing coating integrity and performance.

Al2O3 Coatings for Protection of Stainless Steel 316L against Corrosion in Zn-Al and Zn-Al-Mg

The production and quality of automotive-grade galvanised steel are affected by the limited service life of the pot roll bearings used in continuous galvanising lines. The journal bearings are subjected to severe degradation as they react with the molten Zn bath, and coatings can provide corrosion protection to the bearing materials. This research investigates the performance of Al2O3 coatings applied via the HVOF thermal spray process to stainless steel 316L substrates. Immersion tests were conducted in baths of different compositions, namely GI (Zn-0.3 wt.% Al) and ZMA (Zn-1.5 wt.% Al-1.5 wt.% Mg). Material characterisation after testing showed evidence of coating degradation after 1 week, as the coating tended to crack and detach from the substrate, allowing the molten Zn to attack the underlying steel. The coefficient of thermal expansion of Al2O3 and steel was measured, and a difference of 13 × 10−6 K−1 was found, leading to the development of cracks in the coatings. Zn penetration through cracks was determined to be the main failure mechanism of the Al2O3 coatings, which otherwise remained inert to Zn-Al. Conversely, the coatings immersed in Zn-Al-Mg reacted with the Mg in the molten metal bath, showing that changing bath composition affected the performance of the coatings in molten Zn alloy.

Development of TiNbMoMnFe multi-principal element alloy coating using mechanical alloying and HVOF thermal spray

Multi-Principal Element Alloys (MPEA) were designed to possess good strength, ductility, wear and corrosion resistance, and biomedical compatibility. There are many applications where failure is associated with surface-related degradation, and suitable coatings can mitigate them. This study focuses on the development of MPEA coatings using the HVOF thermal spray process and explores their mechanical and tribological responses. In the present study, the transitional and refractory elements-based feedstocks were prepared by a mechanical alloying process using a high-energy ball mill for 5 h, 10 h, and 15 h milling time. The molar ratio of the developed coating was TiNbMoMnFeOX (1:1:1:1:1:X, where X = 0.48 to 0.62). Thus, prepared feedstock powders and coatings were characterized to record their metallurgical, mechanical, and tribological responses wherever it is applicable. The used scientific tools include X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), field emission scanning electron microscope (FESEM), universal tribometer, and nanoindenter set-up. The results showed that mechanical alloying led to the formation of MPEA, and the 15 h milled MPEA powder coating expressed superior performance. Furthermore, the process exhibited significant microstructural changes in the alloy system, and dual (TiNb and TiMo) and ternary (TiNbMo) BCC phases were developed during the process. These BCC phases were responsible for the coating's improved mechanical and tribological responses. The solid solution hardening of the ternary phase (TiNbMo) was the most dominant mechanism for improving the mechanical and tribological properties.

Corrosion Behaviour of a Cr2O3 Coating on Mild Steel in Synthetic Mine Water

The Cr2O3 coating on the surface of ASTM A516 Grade 70 mild steel substrates was developed using the thermal plasma spraying process for protection against corrosion and wear. The microstructural behaviours for both coating and substrate were analysed using SEM and XRD techniques. The corrosion behaviours of the coatings and substrate in synthetic mine water with varying pH values (6, 3, and 1) were evaluated according to ASTM standards for potentiodynamic polarisation measurements. Tafel plots were drawn to determine the corrosion rates. Vickers hardness of the coatings and substrate were measured. The Cr2O3 coating exhibited cracks due to the solidification and cooling process, as well as some pores between the top and bonding layers caused by unmelted or partially melted particles. The corrosion tests revealed that a decrease in pH levels led to increased corrosion rates in both samples. The Cr2O3 coating demonstrated superior corrosion resistance, ranging from 0.036±0.003 mm/year to 0.110±0.004 mm/year, compared to the mild steel substrate, which ranged from 0.262±0.021 mm/year to 0.336±0.026 mm/year, across all pH values. Moreover, it exhibited significantly greater hardness (1260±77 HV3) than the mild steel substrate (180±14 HV3). The lower corrosion rates and higher hardness of Cr2O3 coating than the mild steel substrate make it a suitable coating in applications where corrosion resistance and high hardness properties are essential.

Effect of Heat Treatment on the Electrochemical and Tribological Properties of Aluminum-Bronze Coatings Deposited Used the Thermal Spraying Process

The corrosion and wear resistance of aluminum-bronze coatings deposited by thermal flame spraying were investigated after a 10 h heat treatment at 500 °C in a nitrogen atmosphere. The coatings were characterized by SEM, EDS, XRD, and XRF. Corrosion resistance was evaluated by Tafel and EIS tests, while wear resistance was assessed using a ball-on-disc test. Results showed the heat treatment compacted the coating microstructure, increased oxide content, and improved splat bonding through diffusion mechanisms. This led to enhanced corrosion resistance, evidenced by reduced corrosion current density in electrochemical tests, resulting from densification impeding electrolyte penetration. Heat treatment also increased wear resistance, as indicated by lower wear rates in ball-on-disc tests, attributable to increased hardness, reduced friction coefficients, and annealing-induced microstructural changes. Overall, the heat treatment optimizes the anticorrosive and tribological properties of thermally sprayed aluminum-bronze coatings via favorable alterations in composition, morphology, and physical characteristics. The research provides a new understanding of how thermal spray parameters and post-deposition heat treatment can be utilized to enhance the corrosion and wear resistance of aluminum-bronze coatings for marine applications.

Influence of coating microstructure on the corrosion behavior of Inconel 625 coatings fabricated by different thermal spraying processes

Inconel 625 coatings with dense microstructures (porosity <2 %) were fabricated on AISI 304L stainless steel substrates by flame spraying (F), twin-wire arc spraying (A), plasma spraying (P), and high-velocity oxygen fuel spraying (H). The quantity of oxide within the coating layers followed the descending order of F > A > P > H, with Cr2O3 identified as the predominant oxide in the Inconel 625 coating layers. The corrosion resistance of the Inconel 625 coating specimens decreased in the order of H > A > P > F. The outstanding corrosion resistance of the Inconel 625 coating prepared by high-velocity oxygen fuel spraying was attributed to the compact coating layer with a very low oxide content. It is suggested that the localized galvanic corrosion around the oxide/matrix interface may deteriorate the corrosion resistance of the coating layer, which is the primary factor contributing to the low corrosion resistance of the F specimen. Although the P specimen exhibited a lower oxide content than the A specimen, it demonstrated inferior corrosion resistance. This phenomenon can be attributed to the significant formation of microcracks within the P specimen, creating penetration tunnels for the corrosive solution and consequently reducing its corrosion resistance.

Development of fine WC-NiCr powder coatings by optimising HVOF spray parameters

Thermal spray processing parameters need to be optimised to mitigate decarburisation and oxidation of carbide-based coatings that causes unacceptable brittleness. In the present work, a theoretical model was developed to optimise kerosene fuelled high-velocity oxy-fuel (HVOF) flame characteristics and spray parameters for producing high-quality WC-NiCr coatings using finer feedstock of particle size distribution of – 30 + 5 μm. The model suggested an oxygen-to-kerosene ratio of 3.3 and a shorter torch barrel to avoid decarburisation in coatings. In total four parameter sets suggested by the theoretical model were selected to spray the fine-cut powder using a 100 mm long torch barrel. The developed theoretical framework was successful to provide optimised set of HVOF spray parameters to deposit high-quality fine carbide coatings with less decarburisation. Also, the coatings deposited using the optimised set of parameters exhibited the best performance in terms of low porosity, inter-splat cracks, brittleness, as-spray surface roughness, and corrosion resistance. Hence, the optimised parameters can be used to produce a finer as-sprayed finish, thereby demonstrating the potential in reducing grinding efforts.

Investigating the corrosion resistance of Zn and Al coating deposited by arc thermal spraying process

Carbon steel is commonly used as a structural material in offshore windmill towers. The bases of these towers, which are located in tidal and submerged zones, often face corrosion challenges in marine environments. In this study, protective Zn and Al coatings were applied to S355J2 + N carbon steels by twin-wire arc spraying. The corrosion resistance of these coatings was evaluated by immersing them in various saline solutions, including NaCl solutions, sea salt solutions, and seawater. Corrosion tests were conducted for up to 1600 h. The results indicated that the corrosion rates of the Zn coatings and the accumulation of corrosion products on the Al coatings increased with higher concentrations of the testing solutions. The corrosion products of Zn coatings tested in sea salt solutions or seawater were mainly gordaite (NaZn4(SO4)(OH)6Cl(H2O)6), whereas those tested in NaCl solutions were mainly simonkolleite (Zn5(OH)8Cl2H2O). After testing in 5 wt% sea salt solutions for 1600 h, the Zn coatings exhibited interlayer decohesion owing to the formation of an oxide layer within the laminar structure. By contrast, the Al coatings displayed a well-mixed microstructure, ensuring strong adhesion and reducing susceptibility to interlayer decohesion in saline solutions. Consequently, in the current study, the Al-coated specimens demonstrated superior performance to that of the Zn-coated specimens in saline solutions.

Conformable Shear Mode Transducers from Lead‐Free Piezoelectric Ceramic Coatings: An Innovative Ultrasonic Solution for Submerged Structural Health Monitoring

AbstractShear mode‐guided ultrasonic waves are highly regarded for submerged or subterranean structural health monitoring (SHM), owing to their non‐dispersive feature and minimized acoustic energy loss when in contact with liquid or solid. High‐performance shear mode ceramic ultrasonic transducers with robustness and cost‐effectiveness are highly demanded for underwater or underground SHM applications, especially in harsh environments. However, the implementation of discrete shear mode piezoelectric ceramic ultrasonic transducers is hindered by the inconsistency with manual installation, lack of conformability on curved surfaces, and unreliable acoustic coupling between the transducers and the structure. Here, direct‐write conformable shear mode ultrasonic transducers made from piezoelectric lead‐free ceramic coatings, which are in situ produced on steel structures by a scalable thermal spray process, are proposed. The obtained lead‐free lithium‐doped potassium sodium niobate (KNN‐LN) ceramic coatings exhibit a high effective shear piezoelectric strain coefficient (d24, f) above 60 pm V−1 in a broad frequency range from 100 Hz to 200 kHz. The resulting conformable shear mode KNN‐LN ceramic coating transducers successfully showcase the functions of exciting and detecting stable shear mode ultrasonic wave signals with operation temperature exceeding 200 °C and demonstrate reliable capability in defect detection in both air and liquid environments.

The Influence of Plasma Spraying Parameters on Microstructure and Porosity of Bronze-Polyester Coatings for Plain Bearings Applications

The plasma sprayed bronze coatings are widely used for repairing of plain bearing used in different applications. This type of coating was not deeply analyzed in state-of-art publications. In presented article we fill this gap in the case of plasma spraying process. The influence of power current (300/500/700A) and hydrogen flow (0/4/8 NLPM) on microstructure and thickness of aluminium bronze-polyester coating was investigated. The Thermico A60 plasma torch was used for thermal spray process of coating on flat carbon steel samples (grade S355). The Metco 604NS was sprayed with 20g/min powder feed rate. The obtained results showed the presence of local large pores formed by burning of polyester in plasma plume. This gap makes role of oil pockets in bearings. The obtained coatings were characterized by large deviation in thickness in range 200-350 micrometers. It might be concluded that in requires additional milling process after deposition.

Preparation and Characterization of Multilayer NiTi Coatings by a Thermal Plasma Process.

The deposition of multilayer coating of NiTi is carried out by a thermal plasma spraying process on a stainless steel substrate. The deposition of melted NiTi particles creates an adhesion layer on the substrate with the subsequent formation of multilayer coating with a certain thickness. Six layers of coating are created to achieve a certain thickness in terms of the sprayed sample. This paper aims to investigate multilayer NiTi coatings created through a thermal plasma process. The key variable feed rate was considered, as well as its effect on the microstructure characteristics. The shape memory effect associated with the coating properties was analyzed in detail. The variable feed rate was considered one of the most important parameters in the thermal plasma spraying process due to its ability to control the quality and compactness of the coating structure. The coatings were characterized by examining their microstructure, thermal, chemical, and microhardness. The indent marks were made/realized along the cross-section surface for the analysis of crack propagation resistance and wear properties. The coating's surface did not display segmentation crack lines. Nevertheless, the cross-sectional surfaces showed evidence of crack lines. There were eutectic zones of the interlamellar structure observed in the structure of the coating. The plasma-sprayed samples from thermo-mechanical analysis of the hysteresis curve provide strong confirmation of the shape memory effect.