High Velocity Oxygen Fuel Process Research Articles

Effect of Cr3C2 distribution on the wear and corrosion properties of HVOF-sprayed WC–12Co/Cr3C2–25NiCr composite coatings

In this study, experimental ball-milling mixed feedstock powder (BMP) and spray-drying feedstock powder (SDP) were prepared, and the WC–12Co/Cr3C2–25NiCr coatings were sprayed with high-velocity oxygen fuel (HVOF). The microstructures, phase compositions, as well as wear and corrosion resistance of the two coatings were explored. The results indicated that SDP at a 250 °C inlet temperature and 2 wt% binder had a uniform elemental distribution, excellent fluidity, and apparent density. The SDP coating was also more uniform and denser than the SMP coating. In addition, an amorphous phase formed in the SDP coating during the HVOF process. The SDP coating exhibited a uniform microstructure, better wear property, and wear rate of 5.5 × 10−14 m3/(N∙m), which is 29% lower than that of the BMP coating. Furthermore, the SDP coating showed superior corrosion resistance in a 36 wt% HCl solution, with a corrosion current of about 2.26 × 10−3 A∙cm−2 and corrosion rate of 26.57 mm/a, which is 81.8% lower than that of the BMP coating. The molecular dynamics simulation indicated that the uniform distribution of Cr3C2–25NiCr substantially reduced the adsorption energy of HCl for WC–12Co.

Prediction of In-Flight Particle Properties and Mechanical Performances of HVOF-Sprayed NiCr-Cr3C2 Coatings Based on a Hierarchical Neural Network.

High-velocity oxygen fuel (HVOF) spraying is a promising technique for depositing protective coatings. The performances of HVOF-sprayed coatings are affected by in-flight particle properties, such as temperature and velocity, that are controlled by the spraying parameters. However, obtaining the desired coatings through experimental methods alone is challenging, owing to the complex physical and chemical processes involved in the HVOF approach. Compared with traditional experimental methods, a novel method for optimizing and predicting coating performance is presented herein; this method involves combining machine learning techniques with thermal spray technology. Herein, we firstly introduce physics-informed neural networks (PINNs) and convolutional neural networks (CNNs) to address the overfitting problem in small-sample algorithms and then apply the algorithms to HVOF processes and HVOF-sprayed coatings. We proposed the PINN and CNN hierarchical neural network to establish prediction models for the in-flight particle properties and performances of NiCr-Cr3C2 coatings (e.g., porosity, microhardness, and wear rate). Additionally, a random forest model is used to evaluate the relative importance of the effect of the spraying parameters on the properties of in-flight particles and coating performance. We find that the particle temperature and velocity as well as the coating performances (porosity, wear resistance, and microhardness) can be predicted with up to 99% accuracy and that the spraying distance and velocity of in-flight particles exert the most substantial effects on the in-flight particle properties and coating performance, respectively. This study can serve as a theoretical reference for the development of intelligent HVOF systems in the future.

Microstructural and wear properties of WC-12Co cemented carbide fabricated by direct energy deposition

The microstructural and wear properties of WC–12Co (wt.%) cemented carbide bulk material manufactured by additive manufacturing direct energy deposition (DED) process were investigated and compared to those of WC-12Co coating material fabricated by high-velocity oxygen fuel (HVOF) thermal spray process. The feedstock powder, DED and HVOF materials were analyzed using scanning electron microscopy/energy dispersive x-ray analysis (SEM/EDAX), X-ray diffraction (XRD) and an electron back-scattered diffraction (EBSD) technique. WC and α-Co phases were observed in the initial WC-12Co cemented carbide powder feedstock. After the DED process, WC phase growth and M12C phase were identified. However, the amount of M12C phase decreased after the HVOF process. The DED material had relatively higher porosity (BD: 11.58%; PD: 10.23%) as compared with HVOF (porosity: 1.66%). The hardness of the HVOF sample was measured as 1429 ± 49 HV, which was much higher than that of the DED (BD:1373 ± 68 HV, SD1: 1352 ± 62 HV) sample, due to the existence of porosity in the DED sample. A pin-on-disc tribometer was used to measure wear rates and coefficients of friction. The wear-worn surfaces were analyzed using SEM with an energy-dispersive spectrometer. The wear test results indicated that the DED material exhibited a lower wear rate (higher wear resistance) and a lower friction coefficient than those of the HVOF material. The difference in the wear behaviour of the DED and HVOF materials could be explained in terms of differences in WC size, the extent of reaction and decarburization during spraying, and the formation of the high oxide content.

Effect of HVOF processing parameters on [formula omitted] hard coatings deposited on AISI 4140 steel

High Velocity Oxygen Fuel (HVOF) thermal spray coating is a deposition process that is widely employed at present to improve surfaces and the remanufacturing of components subject to severe wear, offering an alternative to reduce the cost of spare parts and used as a substitute for electro-chromium, also generating greater thickness than chemical vapor deposition and physical vapor deposition. This study evaluates the influence of the projected application parameters for Cr3C2-20(Ni20Cr) coatings on AISI 4140 steel using the HVOF technique. To do so, a 24 factorial experimental design was used to evaluate the effect on the thickness, porosity, surface hardness, and microhardness in the cross-section, produced by four of the main operational factors in the HVOF process: the fuel/oxygen (F/O) ratio entering the combustion chamber, powder flow, the roughness of the substrate before deposition, and spray gun speed. The morphology of the Cr3C2-20(Ni20Cr) powders and the microstructure of the coatings were studied through a scanning electron microscope and energy dispersive spectroscopy, while the composition were analyzed by X-ray diffraction. It was determined that: (i) the F/O ratio is the operation parameter with the most significant influence on the properties studied; (ii) the gun speed has a significant effect on the thickness generated and porosity; and (iii) the powder flow and roughness of the substrate have a greater effect on the surface hardness and microhardness. Finally, it was determined that coatings with great thickness, low porosity, and great hardness, ideal for the recovery of parts, can be achieved from a F/O ratio of 0.45 and a powder flow with the system feeder rotating at 12 rpm, applied to a substrate with a roughness of Ra=18μm, combined with a spray gun speed of 5 mm/s.

Oxidation Behavior of NiCoCrAlY Coatings Deposited by Vacuum Plasma Spraying and High-Velocity Oxygen Fuel Processes

To reduce the formation of detrimental complex oxides, bond coatings in the thermal barrier coatings for gas turbines are typically fabricated using vacuum plasma spraying (VPS) or the high-velocity oxygen fuel (HVOF) process. Herein, VPS and HVOF processes were applied using NiCoCrAlY + HfSi-based powder to assess the oxidation behavior of the bond coatings for both coating processes. Each coated sample was subjected to 50 cyclic heat treatments at 950 °C for 23 h and cooling for 1 h at 20 °C with nitrogen gas, and the weight change during the heat treatment was measured to evaluate the oxidation behavior. After the oxidation test, the coating layer was analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The VPS coating exhibited faster weight gain than the HVOF coating because the alumina particles generated during the initial formation of the HVOF coating inhibited oxidation and diffusion. The VPS coating formed a dense and thick thermal growth oxide (TGO) layer until the middle of the oxidation test and remained stable until the end of the evaluation. However, the HVOF coating demonstrated rapid weight loss during the final 20 cycles. Alumina within the bond coat suppressed the diffusion of internal elements and prevented the Al from being supplied to the surface. The isolation of the Al accelerated the growth of spinel TGO due to the oxidation of Ni, Co, and Cr near the surface. The as-coated VPS coating showed higher hardness and lower interfacial bonding strength than the HVOF did. Diffusion induced by heat treatment after the furnace cyclic test (FCT) led to a similar internal hardness and bonding strengths in both coating layers. To improve the quality of the HVOF process, the densification of the coating layer, suppression of internal oxide formation, and formation of a dense and uniform alumina layer on the surface must be additionally implemented.

Thermal Spraying of MCrAlY Overlay Coating Using New Ethanol-Fueled HVOF Gun

The MCrAlY overlay coatings are widely used for high-temperature protection of hot section part of gas turbines and jet engines. This type of coatings are usually thermally sprayed using APS (Atmospheric Plasma Spraying), LPPS (Low Pressure Plasma Spraying) as well as HVOF (High Velocity Oxygen Fuel) methods. In present article the newly developed ethanol based HVOF gun was used for production of this type of coatings. The stainless steel 18-8 type was used as a base material. The AMDRY 386 (Oerlikon-Metco) NiCrAlY powder was used for coatings production. In the research different oxygen (400, 500, 600 NLPM) and ethanol (16.5, 18.3, 21.3, 23.6 and 26.6 dm3/h) flow ratio were selected for experimental processes. The powder feed ratio was also changed during process. After deposition the microstructural assessment using Scanning Electron Microscopy and chemical composition analysis using EDS method were conducted. The obtained results showed that coating was above 100 μm thick depending on the process parameters. The low concentration of pores and oxides was also observed on coatings cross-section. Using of ethanol HVOF gun enables to form good quality MCrAlY coatings with 50% reduction of oxygen consumption in comparison with conventional HP/HVOF torch using kerosene such as JP 5000. The other benefit of its using is lower CO2 emission and lower concentration of carbon in coating in comparison with classic JP 5000 HVOF gun. The ethanol HVOF is a promising technology and might be considered as an replacement of LPPS and HVOF process for production of MCrAlY type of coatings.

Feature-based modeling for industrial processes in the context of digital twins: A case study of HVOF process

Industrial process modeling is currently undergoing a fundamental transformation, leading towards interconnected close-loop twins of models, i.e., the parametrically-controlled real-world physics model, and its corresponding digitalized virtual system model. Between these models, a highly time-sensitive mutual validation mechanism and a coherent and consistent control strategy are applied, which demand complex virtual-and-real world model-based information mapping and synchronization. Thus, this research proposes a semantic conceptual framework for industrial process modeling in the context of digital twins. Based on a hierarchical structure of digital twins, this framework modularizes the modeling process in terms of the semantic information modules of physics in the real-world phenomena, and clarifies inter-module associations and near-real-time data transmission coupled with virtual analysis in CAE environment so that the time-sensitive phenomenon information objects distributed on virtually-separated sub-level physics models can be supported for representing the real-world process comprehensively. Advanced feature concept is adopted to construct the digital models as the basic compositions of any virtual industrial process. The related feature definitions are extended in this work so that the common characteristics in the concept of digital twins could be generically and concisely represented. To validate the modeling method, the digital model of a prototyped High-velocity Oxygen-fuel (HVOF) coating process system was constructed as the case study. The data from the nozzle trajectory, the flame, and the in-flight particle behavior, and the transient thermal performance of the coating layer and substrate were synchronously incorporated for simulating the transient phenomena of the substrate component temperature and coating thickness distribution on the substrate surface. The final simulation result validates that the feature-based digital model is able to comprehensively reflect the real-world scenario on the virtual side. To illustrate how the developed model provides meaningful feedback to the real-world control, a general digital twin setup of the model is given at the end of the case study.

RESEARCH ON TRIBOLOGICAL BEHAVIOR OF Cr3C2-25NiCr-COATED BRAKE DISC

This research presents the results of a comparative study to determine the friction and wear behavior of the high-velocity oxygen fuel (HVOF)-sprayed Cr3C2-25NiCr-coated brake disc. The friction and wear behavior of Cr3C2-25NiCr-coated brake disc (CD) and the sliding against the commercial brake pad were examined on a laboratory-scale disc-pad dynamometer and the findings were compared with those of reference brake disc (UCD). The coating was deposited by means of the HVOF process on a grey cast iron (GCI) substrate. Braking tests were performed according to the SAE-J2430 test standard. The as-sprayed coating was characterized for their microstructure and phase constitution by SEM/EDS and XRD. The results reveal that the coefficient of friction (COF) of CD and UCD varies between 0.42–0.57 and 0.44–0.62, respectively. The Cr3C2-25NiCr-coated brake disc was found to have exhibited a comparable COF and lower wear loss to the reference brake disc. The addition of a ceramic phase to the brake disc coating played an important role in reducing the COF to an acceptable range and improving the sliding wear resistance of the brake pair.

A parametric simulation model for HVOF coating thickness control

High-velocity oxygen-fuel (HVOF) thermal spraying is a coating process involving multidisciplinary aspects, e.g., fuel–oxidant combustion, flame–particle jet, particle deposition, mass and heat transfer, and even robotic kinematics. Like most coating processes, in HVOF processes, coating thickness is a significant property determining the coating performance; hence, this property should be accurately controlled during the process. In view of green, smart, and digital manufacturing, the coating thickness prediction model is demanded to produce high-quality coatings efficiently. This paper presents an approach to parametrically simulate the coating thickness in HVOF processes via an integrated numerical model. Firstly, an axisymmetric computational fluid dynamics (CFD) model is constructed to compute the behaviors of the fuel–oxidant combustion, flame–particle jet, and particle deposition distribution. Secondly, based on the particle distribution in a 2D axisymmetric model, a 3D single coating thickness profile model is developed by constructing a circular pattern using the axis of the nozzle. Further, this profile is smoothened by a Gaussian model, and its mathematical expression is obtained. Finally, a numerical model couples spray paths with the mathematical expression to model the coating thickness distribution on a substrate surface under industrial scenarios. At the end of this paper, to verify the proposed model’s effectiveness, four sets of operating parameters with a single straight path were experimentally implemented. The width and height of the bead-like shape coating were in good agreement with the simulated results. The normalized root-mean-square errors of the cross-sectional profile heights were around 10%.

Implementation of Artificial Neural Networks for Forecasting the HVOF Spray Process and HVOF Sprayed Coatings

In the high velocity oxygen fuel (HVOF) spray process, coating properties are sensitive to the characteristics of in-flight particles, which are mainly determined by process parameters. Obtaining a comprehensive multi-physical model or analysis of the HVOF process remains challenging because of the complex chemical and thermodynamic reactions that occur during the deposition procedure. This study proposes to develop a robust methodology via the artificial neural networks (ANN) to solve this problem for the HVOF sprayed coatings under different operating parameters. Two ANN models were developed and implemented to predict coating’s performances (microhardness, porosity and wear rate) and to analyze the influence of operating parameters (stand-off distance, oxygen flow rate, and fuel flow rate) while considering the intermediate variables (temperature and velocity of in-flight particles). A detailed procedure for creating and optimizing these two ANN models is presented in this work, which encodes the implicitly physical phenomena governing the HVOF process. Results show that the developed implicit models can satisfy the prediction requirements and clarify the interrelationships between the spraying conditions, behaviors of in-flight particles, and the final coating performances, resulting in providing better control of the HVOF sprayed coatings.

Effect of powder manufacturing process on characteristics of nanostructured MCrAlY coatings: dry vs. wet ball milling

Metallic MCrAlY coatings have been widely utilized to protect the high-temperature materials operating in aggressive conditions of gas turbines. However, with more demands on the turbine inlet temperature rise for efficiency gains, there is a need to further improve the high-temperature performance of the MCrAlY coatings. A possible way to meet this challenge is by microstructure modification. The aim of this study is to produce nanocrystalline MCrAlY powders via wet and dry mechanical milling techniques and deposit the obtained feedstock powders by high-velocity oxygen fuel (HVOF) spraying method. A comprehensive characterization and comparison of the different powder processing techniques and the corresponding coatings were studied. It was established that the nano-scaled MCrAlY feedstock powder with low contamination levels could be achieved by mechanical milling. Moreover, the powder samples were well-deposited by the HVOF process and the correlation between powder properties and coating characteristics was investigated.

A feature-based model for optimizing HVOF process by combining numerical simulation with experimental verification

High velocity oxygen-fuel (HVOF) thermal spraying is an advanced thermal spraying technology involving a controlled complex physicochemical phenomenon process with parameters. Optimizing the process while considering interwoven multidisciplinary aspects is a non-trivial research challenge. This paper proposes a close-loop optimization model for the process based on a sophisticated numerical simulation method on in-flight flame/particle behaviors. The advanced feature-based CAD/CFD integration technology is used to construct a theoretical framework in order to further develop a core multiple-view and yet integrated model covering numerical modeling, numerical analysis, and parameter optimization, where the simulation intent can be associated with the optimization intent. A prototype environment has been developed to model the HVOF system configurations, working conditions, and then derive optimal process parameters. To validate the modeling method, an actual HVOF spray system depositing WC-12Co coating was modeled. The optimal process parameters generated by the model were implemented in the HVOF spray system and its coating properties were tested experimentally to verify the model effectiveness.

Development of Al2O3-based hybrid ceramic matrix composite coating to mitigate the erosive wear of advanced steel

Steels are susceptible to surface- and subsurface-related degradations (e.g. corrosion and wear) depending upon the working environment; however, desired properties can resolve these issues. The authors have selected the high-velocity oxygen-fuel process to develop alumina-based composite coatings on ASME SA 387 GR22 CL2 steel. The coating material chosen comprises ceria and hexagonal boron nitride as the reinforcements. The metallurgical characteristics of coatings are recorded using X-ray diffraction and scanning electron microscopy. Mechanical properties are mapped using a nano-indenter; and an air jet erosion test rig is used to evaluate erosion wear response. Improvements in the mechanical properties and erosive wear resistance of the hybrid coating are attributed to microstructural changes and the generation of new phases. The values of [Formula: see text], [Formula: see text], and [Formula: see text] are used to develop the mechanistic understanding of coatings’ erosion behavior. Splat cracking and splat detachment are the observed erosion wear mechanisms.

Wear Resistance of Boron-Modified Supermartensitic Stainless Steel Coatings Produced by High-Velocity Oxygen Fuel Process

The wear resistance of boron-modified supermartensitic stainless steel coatings produced by high-velocity oxygen fuel (HVOF) was investigated through pin-on-disk measurements. It was shown that addition of boron leads to the formation of an interconnected and rigid boride network delimiting the grain boundaries of the martensitic matrix. A very refined structure was formed as result of the high cooling rates imposed to the molten alloy during the HVOF process. The specific wear rates of the HVOF coatings were about tenfold lower than the boron-free supermartensitic stainless steel, lying in the order of 10−5 mm3/N m. The refined boride skeleton along the HVOF coating was found to be effective to reduce the materials’ removal from the exposed softer martensitic matrix. While the supermartensitic stainless steel master alloy and the mild steel substrate displayed severe adhesive wear, the HVOF coatings exhibited mild delamination wear at low sliding velocities (10 and 20 cm/s) and abrasive wear at the highest tested velocity (40 cm/s). The studied boron-modified supermartensitic HVOF coatings are an interesting approach to protect the surface of inexpensive steel substrates against wear.

Evaluation of mechanical properties of WC-Co and CrC coatings obtained from powder and flexible cored wire feedstock materials

This paper assesses the microstructure, porosity, microhardness (Hv ) and fracture toughness (KC) of cemented carbides coatings deposited by High-Velocity Oxygen Fuel (HVOF) and Flame (FSFC) spray processes. Three types of coatings were applied for each process from similar feedstock materials in powder and cored wire form. Measurements of fracture toughness were conducted with the Vickers indentation method in the cross section of the deposited layers. An image processing system was applied for the analysis of porosity, showing a globular morphology of the pores in both processes. The experimental results allowed corroborating that the coatings applied by HVOF using powder feedstock have lower porosity than those achieved with the Flame Spray of cored wire material (FSFC). In HVOF and FSFC processes, the porosity of the coating could be an important factor, but when kept in low values, the decrease in Hv and KC are mainly related to the microstructural changes in the coating. The results also allow considering the FSFC technique as a feasible alternative particularly in applications such as internal part of industrial pipes where it becomes difficult to use HVOF due to accessibility problems.

Preparation and Performance of Electroless Nickel on HVOF (High-Velocity Oxygen Fuel) Sprayed Inconel 625 Nickel Coating for Corrosion Protection Applications

High Velocity Oxy-Fuel (HVOF) spraying is one of the preferred surface engineering technologies that can offer advantages such as user-friendliness and cost-efficiency in mass production of coatings for various applications. While the main applications of the HVOF process are related to wear, corrosion resistance can be one of the important desired features of the surface engineering process. However, the porosity of HVOF sprayed coatings is usually a problem when coatings are used in corrosion applications. Low carbon steel substrates were High-Velocity Oxygen Fuel (HVOF) sprayed with Inconel 625 and then coated with a thin film of electroless nickel (electroless nickel plating). Reference sample without electroless nickel was sprayed at the same time. Characterization of the coatings was made by X-Ray diffraction analyses, microstructural surveys, cross-section and corrosion tests. Results showed that sequential application of HVOF spraying and electroless coating processes provided the multi-layered coating consisting of an inner inconel 625 based layer and an outer Ni-P layer. Electroless Ni-P also caused the remarkable increasing in the corrossion resistance as compared to the as-HVOF sprayed state.

Characterization and Corrosion Resistance of Boron-Containing-Austenitic Stainless Steels Produced by Rapid Solidification Techniques.

The composition of a commercial duplex stainless steel was modified with boron additions (3.5, 4.5, and 5.5 wt.%) and processed by rapid-quenching techniques: Melt-spinning, copper-mold casting, and high-velocity oxygen fuel (HVOF). Spray deposition was also used to produce alloys as the process may induce rapid-solidified-like microstructures. These processing routes led to microstructures with distinguished corrosion resistance. Among the alloys with different boron contents, the 63.5Fe25Cr7Ni4.5B composition enabled the production of fully amorphous ribbons by melt-spinning. The cooling rate experienced during copper-mold casting, high-velocity oxygen fuel, and spray deposition did not ensure complete amorphization. The crystalline phases thereby formed were (Fe,Cr)2B and (Fe,Mo)3B2 borides in an austenitic-matrix with morphology and refinement dependent of the cooling rates. Fully amorphous 63.5Fe25Cr7Ni4.5B ribbons exhibited outstanding corrosion resistance in chloride-rich alkaline and acid media with negligible corrosion current densities of about 10−8 A/cm² and a broad passivation plateau. Although the specimens of the same composition produced by HVOF process and spray deposition exhibited lower corrosion resistance because of intrinsic porosity and crystalline phases, their corrosion behaviors were superior to those of AISI 1045 steel used as substrate with the advantage to be reinforced with hard borides known to be resistant against wear.

Microstructural and Wear Properties of WC-based and Cr<sub>3</sub>C<sub>2</sub>-based Cermet Coating Materials Manufactured with High Velocity Oxygen Fuel Process

Microstructural and Wear Properties of WC-based and Cr<sub>3</sub>C<sub>2</sub>-based Cermet Coating Materials Manufactured with High Velocity Oxygen Fuel Process

Production and Corrosion Resistance of Thermally Sprayed Fe-Based Amorphous Coatings from Mechanically Milled Feedstock Powders

Mechanically milled FeCrNbB feedstock powders from commercial precursors were used to produce amorphous coatings through two different industrial thermal-spraying techniques: high-velocity oxygen fuel (HVOF) and flame spraying. Microstructure, thermal behavior, and hardness of the coatings and their corrosion resistances in acidic and alkaline chloride-rich media were comparatively studied. HVOF process was effective to produce ~ 200-µm-thick highly amorphous coatings with hardness over than 700 HV0.3 and low porosity (~ 5 pct). Flame-sprayed ~ 220-µm-thick coatings were nanocrystalline, composed of α-Fe, Fe2B, FeNbB, and Fe2O3 phases and presented hardness of 564 HV0.3 and ~ 10 pct porosity. Electrochemical measurements indicated that HVOF coatings exhibit higher corrosion resistance than flame-sprayed ones thanks to the higher amorphous content and lower porosity resulting from the former processing route. Electrochemical impedance spectroscopy results demonstrated that amorphous HVOF Fe60Cr8Nb8B24 (at. pct) coatings are interesting to protect mild steels such as the API 5L X80 against corrosion in chloride-rich environments.

Corrosion resistance of metalized layers on steel parts in ventilation mill

Corrosion behavior of metalized layers, obtained by Plasma Transferred Arc (PTA) process and by High-Velocity Oxygen Fuel (HVOF) process with the purpose to improve the wear resistance of vital parts of ventilation mill in a thermal power plant, has been tested. The test is performed using three electrochemical techniques, in a solution containing chloride and sulfate ions. It is shown that the steel surface (base metal) dissolves uniformly, without pitting or other forms of local dissolution. Morphology of metalized layers surface indicates that dissolution is non-uniform, but it still can be considered as general corrosion. The corrosion rate of base metal and metalized layer obtained by PTA process is rather low, while the corrosion rate of the metalized layer obtained by HVOF process is much higher. Also, the difference in corrosion potentials between the base metal and the HVOF layer is pretty high but slightly less than maximum allowed difference (prescribed by the standard), to avoid excessive galvanic corrosion. The values of corrosion rate obtained by different electrochemical techniques are in excellent agreement.