Laminar Plasma Research Articles

Hardening mechanism and thermal-solid coupling model of laminar plasma surface hardening of 65 Mn steel

In the present work, the laminar plasma surface hardening method is employed to enhance the service life of metal components fabricated from 65 Mn steel. The mechanical and wear behaviors of the laminar plasma surface hardened 65 Mn steel were analyzed. The martensite transition transformation of the temperature of the laminar plasma-hardened 65 ferrite Mn steel was determined by a thermal-solid coupling model. Based on the orthogonal experimental results, the optimal hardening parameters were confirmed. The scanning velocity, quenching distance and arc current are 130 mm/min, 50 mm and 120 A, respectively. The pearlites and ferrites are transformed into martensites in the hardened zone, while the ratio of martensite in the heat-affected zone decreases with the increase in the hardening depth. Compared to the untreated 65 Mn steel, the average hardness increases from 220 HV0.2 to 920 HV0.2 in the hardened zone and the corresponding absorbed power increases from 118.7 J to 175.5 J. At the same time, the average coefficient of friction (COF) decreases from 0.763 to 0.546, and the wear rate decreases from 5.39×10−6 mm3/(N·m) to 2.95×10−6 mm3/(N·m), indicating that the wear resistance of 65 Mn steel could be significantly improved by using laminar surface hardening. With the same hardening parameters, the depth and width of the hardened zone predicted by the thermal-solid coupling model are 1.85 mm and 11.20 mm, respectively, which are in accordance with the experimental results; depth is 1.83 mm and width is 11.15 mm. In addition, the predicted hardness distributions of the simulation model are in accordance with the experimental results. These results indicate that the simulation model could effectively predict the microstructure characteristics of 65 Mn steel.

Microstructure and corrosion resistance of novel rare-earth-zirconia doped Y2O3 plasma-sprayed coating

Y2O3 coatings are widely applied in semiconductor etching machines to protect the inner walls of aluminum alloys. This study reports the preparation of yttria-based coatings on aluminum alloy substrates using atmospheric laminar plasma spraying (ALPS) methods. In this study, four yttria-based coatings were designed and tested for their thermal performance, plasma etching resistance, and resistance to laser ablation. The rare-earth-zirconia (RE-ZrO2) doped Y2O3 coating exhibited the best thermal insulation performance, with a thermal conductivity of approximately 0.6 W m−1 k−1 at 600 °C. Plasma etching experiments demonstrated that more rare-earth fluorides were generated on the surface of the RE-ZrO2 doped Y2O3 coating, which weakened the plasma energy. Finally, the lowest etching rate was achieved. Laser ablation experiments demonstrated that the depth of the ablation pit on the surface of the RE-ZrO2 doped Y2O3 coating was shallow, indicating good laser ablation resistance. These results indicate that rare-earth-doped yttria-based coatings provide excellent corrosion protection against plasma etching and laser ablation.

An investigation into crack initiation and extension mechanism of laminar plasma quenched-tempered rail material with different quenched zones

Laminar Plasma Quenching Tempering (LPQT) is an innovative surface treatment technique for rails, which effectively enhances both their fatigue resistance and wear resistance. This study examines the crack initiation and propagation mechanisms in LPQT-treated rail materials with distinct quenched zones using double-disc testing and metallographic analysis. The findings indicate that the crack initiation and propagation process in LPQT rails with different quenched zones is characterized by four stages. Moreover, the grain size within the spot-quenched zone of LPQT rails is notably smaller, which results in a reduced resistance to RCF crack propagation. Consequently, the crack propagation in point-quenched LPQT rail material is more intricate, with a more severe damage scenario.

Mechanism and improvement of the rolling contact fatigue of the surface layer with heterogeneous microstructures of the rail steel treated by laminar plasma jet

Surface layer with heterogeneous microstructures (SLHM), characterized by hardened spots discretely embedded in a soft substrate, can be prepared by laminar plasma quenching (LPQ) to improve the wear resistance of the railway rail. However, severe rolling contact fatigue (RCF) cracks and spalling would occur in the hardened spots that only consist of quenched microstructure. This study analyzed the RCF damage mechanism of the LPQ prepared hardened spots by contact simulation. To improve the RCF resistance while maintaining the high wear resistance of the treated rail, a novel SLHM, whose discrete hardened spots are surrounded by transitional zone with the hardness lower than that of the quenched zone but higher than that of the substrate, was proposed and prepared by laminar plasma quenching-tempering (LPQT). Then, twin-disc tests and contact simulations were conducted to investigate the wear and RCF resistances of the LPQT treated rail steel. Results showed that the cause of the severe RCF cracks and spalling at the entry side of the LPQ prepared hardened spots was that the cyclic tensile stress at the entry side was always higher than that at the exit side under the cyclic rolling contact loading. The high tensile stress at the entry side of the LPQ prepared hardened spots resulted from the elastic deformation induced by tensile plastic deformation of the neighboring substrate. When the thickness of the transitional zone was about 0.1 mm, the generation of severe RCF cracks and spalling was significantly inhibited in the LPQT prepared hardened spots because the transitional zone decreases the cyclic tensile stress at the entry side of the quenched zone. Besides, the wear loss of such LPQT treated rail steel was only increased by 18.1 % compared with that of the LPQ treated rail steel, indicating that the wear resistance of the LPQT treated rail steel was not significantly reduced. These findings are promising for designing the microstructure of the surface layer to obtain wear-resistant rail with high RCF resistance.

Study on rolling contact fatigue crack initiation and propagation in U75V rail treated by laminar plasma

Surface discrete hardening by laminar plasma (HLP) enhances rail wear resistance by rapidly quenching surfaces to form discrete martensitic zones. In comparative field tests on a North China freight railway, HLP-treated U75V rails supported twice the total passing weight before reaching wear limits, compared to untreated rails. However, HLP treatment increases susceptibility to rolling contact fatigue (RCF) cracks, including interface cracks from mechanical differences, impact cracks from wheel-rail contact, deformation cracks from cumulative deformation, and matrix cracks extending from interfaces. The ratchet effect and seesaw mechanism, due to plastic deformation accumulation and quenching area warping, are crucial for crack initiation and propagation. This research advances understanding of HLP-treated rails' tribological behavior, emphasizing the necessity for customized maintenance strategies.

Modeling of the Arc Characteristics inside a Thermal Laminar Plasma Torch with Different Gas Components

For low costs, numerical simulation is an effective method to reveal the internal mechanisms inside a thermal plasma torch. Various simulation models for studying the inside or outside characteristics of thermal plasma torches have been built and discussed. However, to reveal the inside mechanisms of thermal plasma torches under various working conditions to support the materials processing, more attention should be paid to building precise models of laminar plasma torches. Thus, based on the user-defined function (UDF) and user-defined scalar (UDS) of ANSYS Fluent software, the assumptions, governing equations, boundary conditions, and solving method were discussed in detail, and a corresponding numerical model of a homemade laminar plasma torch was first built. For verifying the effectiveness of the proposed numerical model and studying the influence of the gas components on the arc characteristics, the working conditions and experimental setups were introduced in sequence. Finally, the numerical and experimental results of the homemade laminar plasma torch were obtained and discussed in detail. The study results show that: ① The axial temperature of the plasma torch could be divided into three sections along the axis: peak temperature area (10 mm < x < 20 mm), stable temperature area (20 mm < x < 62 mm) and decrease temperature area (62 mm < x < 95 mm). Under the same input conditions, when pure argon gas was used, the peak temperature at the outlet was reached at approximately 7590 K, while for pure nitrogen and 50%Ar + 50%N2, the corresponding peak temperatures were 6785 K and 7402.2 K, respectively. ② The axial velocity of pure nitrogen is much higher than that of pure argon and 50%Ar + 50%N2, while that of pure argon and 50%Ar + 50%N2 has little difference. In addition, when nitrogen gas was used, the peak velocity at the outlet reached 185 m/s, whereas, for argon gas and 50%Ar + 50%N2, the corresponding peak velocities were 146 m/s and 169 m/s, respectively. ③ The simulated arc voltage trends under different working conditions are well in accordance with the experimental arc voltage trends.

From simulation to reality: CFD-ML-driven structural optimization and experimental analysis of thermal plasma reactors

Thermal plasma reactors offer an environment of high temperature, enthalpy, and reactivity, making them highly efficient for solid waste treatment and promising for clean energy production from municipal and industrial waste. Optimal geometrical parameters of the reactor can enhance waste treatment and reactor performance. This study presents a comprehensive analysis of 11 key geometrical parameters of a thermal plasma reactor. Utilizing CFD Fluent software, numerical simulations were conducted to generate a dataset. Subsequently, a predictive model focusing on the average temperature in the core melt zone was trained using six Machine Learning (ML) algorithms. The Particle Swarm Optimisation (PSO) algorithm optimized the hyperparameters of the Gradient Booster Regression (GBR) model, which was combined with a Genetic Algorithm (GA) to identify the reactor's optimal geometrical parameters. A DC arc plasma torch-solid waste thermal plasma reactor treatment system was established on this basis. The study also explored the effects of gasification coefficient, reaction temperature, and thermal plasma jet mode on system performance. Findings indicate that the PSO-GBR model achieved the highest prediction accuracy, with the temperature in the core reaction zone reaching 3621 K. The deviation between numerical simulations and machine learning predictions was a mere 1.3%. Enhancing syngas yield and energy efficiency is achievable by controlling reaction temperature and increasing the gasification coefficient. A laminar plasma jet mode, at equal power, provides a more effective reaction environment. The accuracy and reliability of the machine learning-driven regression model and optimization results are significant in guiding the optimal design of plasma reactors and advancing waste-to-energy conversion processes.

Analysis of rolling contact fatigue and wear characteristics of aging-treated laminar plasma strip quenching rails

The application of laminar plasma discrete quenching (LPQ) technology to enhance the wear resistance of railway tracks has been a notable advancement. Nonetheless, it has been observed that LPQ-treated rails experience a reduction in rolling contact fatigue (RCF) resistance. Our previous study found that aging treatment can improve the toughness of laminar plasma spot quenched rails. However, the microstructure as well as the mechanical properties of strip quenching are significantly different from those of spot quenching. To tackle this concern, the study delves into the wear and RCF damage mechanisms of aging-treated laminar plasma strip quenching (ALPQ) rails. The findings reveal that the aging process induces the precipitation of supersaturated solid solutions within the hardened layer, consequently elevating the RCF resistance of ALPQ rails. Following appropriate aging treatment, nearly complete elimination of fatigue cracks on the rail surface is achieved.

Effect of inhomogeneity on crack initiation for wear-resistant U75V rail treated by laminar plasma

The surface discrete hardening of laminar plasma (HLP) is an effective method of improving wear resistance for rails. While fracture failures on as-treated rail heads are still unavoidable and the related crack mechanism is unclear. As-treated U75V rail and untreated standard U75V rail were installed simultaneously on railway for comparison. The wear form and cracking mechanism of the rails after service are studied in detail. The results show that wear resistance of HLP-treated rail is improved dramatically compared with untreated ones. The wear depth of HLP-treated rail serviced for 9 months is still less than that of standard rail serviced for only 3 months. Central ablation induces overheated structures and hot cracks, resulting in cracks within the quenched area. Decarburized layer is the main reason for a ring-shaped reticular ferrite existing at the quenched area edge. Crack initiation at the interface is facilitated by the presence of reticulated ferrite and a non-uniform microstructure formed at the surface edge of the quenched area. These cracks propagate solely within the quenched area, preventing their propagation into the interior of the HLP-treated rail.

Arc Column Oscillation: A New Mode of Arc Fluctuation in Laminar Plasma Torch

Arc Column Oscillation: A New Mode of Arc Fluctuation in Laminar Plasma Torch

Laminar plasma quenching-tempering: A rapid surface heat treatment technique for controllable modification of the rail steel

Plasma selective quenching is promising in enhancing the rail's wear resistance by preparing martensitic hardened zones discretely embedded in the substrate on the surface layer. However, severe cracks and spalling occur in the hardened zone during the rail service, which may be related to its microstructure and hardness. To modify the microstructure and hardness of the hardened zone, a novel laminar plasma quenching-tempering (LPQT) technique that consists of performing laminar plasma quenching (LPQ) and laminar plasma tempering (LPT) successively was proposed to obtain the hardened zone with desired microstructure and hardness. LPQ is firstly performed on the rail surface to prepare martensitic hardened zones with high hardness. LPT is then performed on each hardened zone to modify its microstructure and hardness. The numerical simulation of LPT was established to determine the processing parameters of LPT for accurately controlling the tempering temperature of the hardened zone. The effects of the tempering temperature on the microstructure and hardness of the hardened zone were investigated. Results showed that with the increase of the tempering temperature from Ms temperature to austenitizing temperature, the hardness of the hardened zone was decreased from 800 HV to 450 HV as the acicular martensite in the hardened zone precipitated carbides and was transformed into the tempered martensite. Besides, the total processing time of LPQT for treating a hardened zone is normally <2 s. Application of LPQT in modification of the rail steel showed that it helped to eliminate the severe cracks and spalling in the LPQ treated hardened zone while its wear resistance was still improved compared with that of the untreated rail steel. Overall, LPQT is a rapid and controllable surface heat treatment technique for modifying the microstructure and hardness of the rail surface, which is promising to be used to balance the wear and RCF resistances based on the service condition of the rail.

Comparison of chemical composition and wear of iron and nickel aluminide coatings applied by plasma spraying

Given such properties as wear resistance, corrosion resistance, heat resistance, contact and cyclic strength, iron and nickel aluminides can be used as coatings in friction pairs for various purposes. The object of research is the process of obtaining plasma-sprayed coatings from powders based on iron and nickel aluminides. The task of research is the determination and comparison of the structure, chemical composition, and wear resistance of coatings from powders of iron aluminide and nickel aluminide, obtained by the method of atmospheric plasma spraying. To reduce the oxidation of powders during spraying, the conditions for generating a laminar plasma jet were created. Such conditions were provided by the specially developed structure of the electric arc plasma gun and the operating parameters of coating spraying. As a result of the correct choice of spraying modes, completely melted splats were formed during the collision of powder particles with the surface. The resulting coatings had a layered microstructure with microcracks and peeling. The porosity and characteristics of the delamination of the coatings depend on both the modes and the spraying technology. Tribological tests of the coatings were carried out under conditions of dry metal-on-metal sliding friction in a pair with high-speed steel. It is shown that the wear of nickel aluminide coatings under these conditions is 2–2.5 times lower than that of iron aluminide coatings. The coefficient of friction of iron aluminide coatings is slightly lower than that of nickel aluminide coatings. The wear of samples made of 30CrMnSi steel is 3–4 times higher than samples with coatings. The difference in the wear of the coatings is explained by the more intense oxidation of iron aluminide in the friction process. Conclusions were drawn regarding the possibility of using the investigated coatings in various friction pairs of structural elements, in particular in the automotive industry

Identification of elastoplastic properties of materials with gradient residual stresses using a co-simulation method and nanoindentation experiments

Nanoindentation technology has many advantages in the acquisition of elastoplastic properties of certain small-scale materials (coatings, films, etc.) due to its small size and non-destructive properties. However, because of the microscopic scale involved, test results are also more susceptible to disturbances by internal residual stress of the specimen. To address these issues, this paper develops a co-simulation method for combining the optimization algorithm with finite element simulation in order to obtain the true elastoplastic properties of unknown materials using nanoindentation testing. This method is suitable for any surface treated materials, and the reliability of the method is also verified by the case of laminar plasma quenched rail.

Surface treatment of rail to enhance rolling contact fatigue and wear resistance: Combined spot laminar plasma quenching and tempering method

The railway industry faces the challenge of improving the fatigue and wear resistance of rails simultaneously. Laminar plasma quenching (LPQ) can enhance wear resistance but may result in severe rolling contact fatigue (RCF) damage. This study proposes a combined LPQ and tempering method to address this issue and evaluates its effect on rail RCF damage and wear resistance using twin-disc tests. The findings demonstrate that tempering diminishes the grain density and mitigates the lattice distortion in the quenched zone of LPQ rails, thereby enhancing the RCF resistance in that zone. This treatment leads to a substantial reduction in surface fatigue cracks and spalling of the rails and a wear resistance improvement of over one-fold compared to ordinary rails. Thus, this rail surface treatment approach effectively improves both the fatigue and wear resistance of rails simultaneously.

Characterization of Fe-Based Layers Deposited by Laminar Plasma Cladding on Low-Carbon Steel

Characterization of Fe-Based Layers Deposited by Laminar Plasma Cladding on Low-Carbon Steel

Study of annular coaxial powder feeding effect on the characteristics of laminar plasma jet and atmospheric cluster deposition

In this paper, by means of numerical simulation and experimental analysis, the performance of the engulfment based annular coaxial powder feeding method and the particle behaviors were studied in the laminar plasma spraying process. First, numerical simulation methods are used to analyze the flow state of the laminar plasma jet and its interaction with the powder particles. Then, the laminar plasma spray was applied to deposit coatings based on the annular coaxial powder feeding method, and the performance of this method was evaluated by observing the coatings. The numerical simulation results show that the powder particles can be fed into the laminar plasma jet without affecting the flow state of the plasma jet, and can be sufficiently heated. The experimental results are consistent with the numerical simulation results. The surface morphologies of the coatings indicates that vapor phase deposition and liquid phase deposition occur simultaneously during the spraying process, and the laminar plasma spray - physical vapor deposition process has the potential to be applied in atmospheric environments.

Effect of material non-uniformity and residual stress on wear and transient rolling contact behaviour in laminar plasma quenched (LPQ) rails

Laminar plasma discrete quenching of rail surfaces significantly improves the wear resistance of quenched rails by forming a discrete hardened layer on the rail surface, while also introducing residual compressive stress. However, in practice, laminar plasma quenched (LPQ) rails demonstrate numerous fine cracks and spalling at the edges of the hardened layer, potentially threatening the safety of railway traffic. However, most previous studies have focused on the microstructure of LPQ materials and two-disc tests, with few of them including a finite element analysis of the mechanical properties of the quenched rails. In this paper, an ANSYS/LSDYNA explicit finite element (FE) model of the transient rolling contact between the quenched rail and the wheel is developed for the rolling contact behaviour and wear characteristics of the LPQ rail. The innovation of the model is the fact that it takes the inhomogeneous material properties between the substrate and the hardened layer of the rail surface into account, as well as the real residual stress. It was found that the inhomogeneous material properties of the substrate and the hardened layer may be the main cause of micro-cracking at the edges of the hardened layer. Residual compressive stress within the hardened layer has a non-negligible effect on the rolling contact characteristics of the rail. The research methodology in this paper further develops the FE model for LPQ rails, while the results obtained will be important for controlling rail damage.

Determining the elastic–plastic properties of materials with residual stress included using nanoindentation experiments and dimensionless functions

Residual stresses are widespread in materials and significantly affect their mechanical properties. In this paper, with the impact of residual stress taken into account, a method was proposed to reveal the elastic–plastic properties of materials, and the correctness of the method is verified using the example of laminar plasma quenched (LPQ) materials. Firstly, nanoindentation experiments were conducted on the LPQ materials. Then, the dimensionless functions corresponding to different residual stresses were characterized. Finally, the impact of residual stress on the dimensionless functions and nanoindentation response was discussed. The research results show that the hardened layer is made of isotropic material and that the hardened layer's yield stress and hardening index are obtained. There is a linear relationship between the residual stress and dimensionless functions.

A single-step plasma method for rapid production of 2D, ferromagnetic, surface vacancy-engineered MoO3−x nanomaterials, for photothermal ablation of cancer

A rapid, clean plasma-chemical technique is demonstrated here, for cost-effective, synthesis of surface vacancy engineered, 2D, molybdenum-oxide nanomaterials, during a one-step, integrated synthesis-hydrogenation process for biomedical applications. A laminar plasma beam populated with O and H radicals impinges on a molybdenum target, out of which molybdenum-oxide nanomaterials are very rapidly generated with controlled surface O vacancies. 2D, dark-blue coloured, nano-flake/ribbon like MoO3−x is produced maximum up to 194 g h−1, the core of which still remains as stoichiometric molybdenum-oxide. These nanomaterials can get heated-up by absorbing energy from a near-infrared (NIR) laser, which enable them as photothermal therapy (PTT) candidate material for the invasive precision therapy of cancer. The surface defects endows the products with robust ferromagnetism at room temperature conditions (maximum saturation-magnetization: 6.58 emu g−1), which is order of magnitude stronger than most other vacancy engineered nanomaterials. These nanometric metal-oxides are observed to be perfectly compatible in animal physiological environment and easily dispersed in an aqueous solution even without any pre-treatment. The MoO3−x nanomaterials are stable against further oxidation even under prolonged atmospheric exposure. In vitro experiments confirm that they have ideal efficacy for photothermal ablation of human and murine melanoma cancer at relatively lower dose. During in vivo PTT treatments, they may be manipulated with a simple external magnetic field for targeted delivery at the malignant tumours. It is demonstrated that commensurate to the neutralization of the malignant cells, the nanomaterials themselves get self-degraded, which should get easily excreted out of the body.

Investigation on spheroidization of refractory tungsten powders by laminar DC plasma torch

The spherical tungsten powders were prepared by a low-power DC laminar plasma torch. At a discharge current of 50 A, the tungsten powder fed downstream cannot be spheroidized by using Ar discharge, while a small number of spherical particles can be observed with upstream feeding. When N2 is added to the working gas, the input power is increased to maintain the same discharge current, which leads to the increase of plasma jet length, thereby expanding the effective high temperature region. Optical emission spectroscopy measurements show that the electron excitation temperature of N2-Ar mixed discharge is higher than that of argon discharge. In addition, compared with argon plasma, N2-Ar mixed gas plasma has larger thermal conductivity due to the molecular dissociation. Therefore, the heat is more efficiently transferred from the plasma to the tungsten particles. The improvement of plasma jet and heat transfer characteristics by introducing nitrogen into the discharge contributes to the increase of spheroidization rate. EDS analysis shows that the oxygen content in the tungsten particles decreases after spheroidization, which is caused by the evaporation of tungsten oxide during plasma treatment.