Plasma Spray Deposition on Inclined Substrates: Simulations and Experiments

In the plasma spray coating process, the coating’s profile and overall thickness are dependent on the number of overlapping traverses of the torch, the shape of the particle spray plume, the spatial distribution of the in-flight parameters of the particles within, and the orientation of the substrate. In this paper, a semi-empirical methodology for predicting three-dimensional deposits by the plasma spray process is developed. It comprises of three stages: first, spatial distributions of the in-flight parameters of multi-sized particles within the spray plume are determined by Computational Fluid Dynamics simulations. The size and shape parameters of the splats formed when individual droplets impact and spread out are obtained by experiments. Finally, a computer program is developed to integrate the particle parameters distribution and the empirical splat geometric data to generate a three-dimensional profile representing the deposit. The procedures predict the deposition volumes and thicknesses for different substrate inclinations with good agreement to experimentally sprayed deposits.     

Influence of process parameters on the deposition footprint in plasma-spray coating

This paper presents an investigation of the influence of plasma spray process conditions on the in-flight particle behavior and their cumulative deposition to form a coating on the substrate. Three-dimensional computational fluid dynamics (CFD) analyses were performed to model the in-flight particle behavior in the plasma-spray process and their deposition on the substrate. The plasma spray was modeled as a jet issuing from the torch nozzle through the electrical heating of the arc gas. In the model, particles were injected into the plasma jet where they acquired heat and momentum from the plasma, some got melted and droplets were formed. By means of a droplet splatting model, the particle in-flight data generated by the CFD analyses were further processed to build up an imaginary three-dimensional deposition profile on a flat stationary substrate. It is found that the powder carrier gas flow rate influences the particle distribution on the substrate by imparting an injection momentum to the particles that were directed radially into the plasma jet in a direction perpendicular to the plasma jet. The larger sized particles will acquire higher injection momentum compared with the smaller sized particles. This causes particle distribution at the substrate surface that is elliptical in shape with the major axis of ellipse parallel to the particle injection port axis as illustrated in Fig. 1. Larger particles tend to congregate at the lower part of the ellipse, due to their greater momentum. The distribution of particle size, temperature, velocity, and count distribution at the substrate was analyzed. Further, based on the size and the computed particle temperature, velocity histories, and the impact sites on the substrate, the data were processed to build up a deposition profile with the Pasandideh-Fard model. The shapes of deposition profiles were found to be strongly driven by the segregation effect.     

Thermal spray shape deposition

This paper describes a new spray-forming process based on thermal spray shape deposition. Shape deposition processes build three-dimensional shapes by incremental material buildup of thin, planar crosssectional layers. These processes do not require preformed mandrels and can directly build three-dimensional structures of arbitrary geometric complexity. The basis for the thermal spray approach is to spray each layer using a disposable mask that has the shape of the current cross section. Masks can be produced from paper rolls, for example, with a CO₂ laser. In addition to applications for rapid prototyping, this approach makes possible the fabrication of composite structures and integrated electronic/mechanical assemblies that are not feasible with conventional manufacturing technologies.     

往复扫描喷射沉积过程中雾化器运动的研究

雾化器运动是雾化器往复扫描喷射沉积过程中一个非常重要的因素。研究了雾化器往复扫描喷射沉积过程中雾化器的运动，总结了不同的喷射距离下，雾化器喷射密度函数的分布以及雾化器在扫描过程中运动速度v与沉积锭坯半径R之间的关系。在雾化器往复扫描喷射沉积过程中，根据对雾化器运动的研究进行数值模拟，实现了喷射沉积过程的工艺优化，为进一步研究喷射沉积过程提供了理论依据，对于工业生产也具有指导意义。     

The influence of plasma spraying regime and initial substance injection location on the structure of deposited coatings

Atmospheric pressure non-equilibrium plasma spray technology has been developed, tested and successfully employed for deposition of catalytic and tribological coatings. Aluminum hydroxide based coatings doped with carbon particles up to 50 μm of thickness were deposited on the stainless steel substrates.Results of experimental investigation on structural characteristics of plasma sprayed coatings in dependence of prevailing external factors are presented. The effect of plasma source operating regime and injection location of precursor dispersed particles (DP) on the morphology and properties of metal oxide coatings was investigated. It has been determined the relation between microstructure of coatings and arc current in plasma source.Significant differences in size and shape of coating grains were observed during deposition process when metal oxide dispersed particles were placed into the reactor channel and directly into the plasma torch discharge chamber together with the reacting gas. The plasma spray pyrolysis process has occurred in some particular regimes which have been successfully applied in the synthesis of micron- and submicron-sized uniform shaped coatings with narrow size pores and controlled surface morphologies.     

往复式喷射沉积管坯制备中喷射高度的闭环控制

分析了往复式喷射沉积制备大壁厚管坯的工艺原理,研究了喷嘴喷射高度在线检测及闭环控制方法及技术。喷射高度控制系统包括漏包提升执行机构、沉积层厚度在线测量、基于PLC的喷嘴高度控制。提升执行机构采用伺服电机驱动的丝杠螺母机构,针对沉积层间断增长的特点,采用间断提升控制方式;研究了沉积层厚度在线测量方法,分析了收集基底形状误差对测量及控制精度的影响并提出多点测量方案。理论分析表明,采用三点测量法可消减基底形状误差的影响。对不同内径及壁厚的管坯进行了喷射实验,喷射高度累积误差低于5%,较好地满足了大壁厚管坯制备对稳定的喷射高度的要求。     

Deposition of Electrically Conductive Coatings on Castable Polyurethane Elastomers by the Flame Spraying Process

Deposition of metallic coatings on elastomeric polymers is a challenging task due to the heat sensitivity and soft nature of these materials and the high temperatures in thermal spraying processes. In this study, a flame spraying process was employed to deposit conductive coatings of aluminum-12silicon on polyurethane elastomers. The effect of process parameters, i.e., stand-off distance and air added to the flame spray torch, on temperature distribution and corresponding effects on coating characteristics, including electrical resistivity, were investigated. An analytical model based on a Green’s function approach was employed to determine the temperature distribution within the substrate. It was found that the coating porosity and electrical resistance decreased by increasing the pressure of the air injected into the flame spray torch during deposition. The latter also allowed for a reduction of the stand-off distance of the flame spray torch. Dynamic mechanical analysis was performed to investigate the effect of the increase in temperature within the substrate on its dynamic mechanical properties. It was found that the spraying process did not significantly change the storage modulus of the polyurethane substrate material.     

A Method to Predict the Thickness of Poorly-Bonded Material Along Spray and Spray-Layer Boundaries in Cold Spray Deposition

Multi-traverse CS provides a unique means for the production of thick coatings and bulk materials from powders. However, the material along spray and spray-layer boundaries is often poorly bonded as it is laid by the leading and trailing peripheries of the spray that carry powder particles with insufficient kinetic energy. For the same reason, the splats in the very first layer deposited on the substrate may not be bonded well either. A mathematical spray model was developed based on an axisymmetric Gaussian mass flow rate distribution and a stepped deposition yield to predict the thickness of such poorly-bonded layers in multi-traverse CS deposition. The predicted thickness of poorly-bonded layers in a multi-traverse Cu coating falls in the range of experimental values. The model also predicts that the material that contains poorly bonded splats could exceed 20% of the total volume of the coating.     

Effect of Process Inputs on Coating Properties in the Twin-Wire Arc Zinc Process

Relationships between process inputs and coating properties were characterized using a twin-wire arc torch spraying zinc. Specifically, standoff distance, primary and secondary atomizing gas pressures, and arc current were varied in order to determine their effects on deposition efficiency, surface roughness, coating porosity, and spray particle size. Process associations were investigated using an analysis of variance with a design of experiments approach with the intent of determining which spray parameters affect each of the aforementioned coating properties. The associations found are consistent with other studies of the twin-wire arc spray process and provide a framework for selecting process operating conditions based on desired coating properties. Such a specific outline has not been previously available.     

Al2O3-ZrO2 Finely Structured Multilayer Architectures from Suspension Plasma Spraying

Suspension plasma spraying (SPS) is an alternative to conventional atmospheric plasma spraying (APS) aiming at manufacturing thinner layers (i.e., 10-100 μm) due to the specific size of the feedstock particles, from a few tens of nanometers to a few micrometers. The staking of lamellae and particles, which present a diameter ranging from 0.1 to 2.0 μm and an average thickness from 20 to 300 nm, permits to manufacture finely structured layers. Moreover, it appears as a versatile process able to manufacture different coating architectures according to the operating parameters (suspension properties, injection configuration, plasma properties, spray distance, torch scan velocity, scanning step, etc.). However, the different parameters controlling the properties of the coating, and their interdependences, are not yet fully identified. Thus, the aim of this paper is, on the one hand, to better understand the influence of operating parameters on the coating manufacturing mechanisms (in particular, the plasma gas mixture effect) and, on the other hand, to produce Al₂O₃-ZrO₂ finely structured layers with large varieties of architectures. For this purpose, a simple theoretical model was used to describe the plasma torch operating conditions at the nozzle exit, based on experimental data (mass enthalpy, arc current intensity, thermophysical properties of plasma forming gases, etc.) and the influences of the spray parameters were determined by mean of the study of sizes and shapes of spray beads. The results enabled then to reach a better understanding of involved phenomena and their interactions on the final coating architectures permitting to manufacture several types of microstructures.     

A stochastic model to simulate the formation of a thermal spray coating

We present a three-dimensional, stochastic model of thermal spray coating. It is capable of predicting coating porosity, thickness, roughness, and the variation of these properties with spray parameters. The model assigns impact properties to molten droplets landing on the substrate by generating random values of process parameters, assuming that these properties follow normal distributions with user-specified means and standard deviations. We prescribed rules to calculate splat sizes after droplet impact and their interaction with each other. Porosity is assumed to be solely due to the curl-up of the splats as a result of thermal stresses. We use a Cartesian grid to define the computational domain and to track the shape and position of the deposited coating. The surface of the coating and the location of pores within it are specified using a variable known as the “volume fraction,” defined as the fraction of the volume of a computational cell occupied by coating material. Results are given for the variation of coating porosity, thickness and roughness with varying particle speed, size, and spraying gun speed. Predicted trends agree with experimental observation.     

Trajectory Generation and Coupled Numerical Simulation for Thermal Spraying Applications on Complex Geometries

For high process reproducibility and optimized coating quality in thermal spray applications on complex geometries, atmospheric plasma spraying and high-velocity oxygen fuel torches are guided by advanced robot systems. The trajectory of the torch, the spray angle, and the relative speed between torch and component are crucial factors which affect the coating microstructure, properties, and, especially, the residual stress distribution. Thus, the requirement of high-performance thermally sprayed coatings with narrow dimensional tolerances leads to challenges in the field of robot-assisted handling, and software tools for efficient trajectory generation and robot programming are demanded. By appropriate data exchange, the automatically generated torch trajectory and speed profile can be integrated in finite element method models to analyze their influence on the heat and mass transfer during deposition. Coating experiments assisted by online diagnostics were performed to validate the developed software tools.     

In Flight Properties of W Particles in an Ar-H<Subscript>2</Subscript> Plasma

An in-flight properties measurement performed on W particles, injected into thermal plasma generated by an inductively coupled RF plasma torch, is presented. The measured surface properties of the particles along the centerline of the plasma plume are expressed by means of temperature and velocity maps, within the domain formed by individual particle’s diameters and their distances from the torch exit. The influence of some of the processing parameters (plate power, carrier gas flow rate, spray chamber pressure) on particle properties is discussed for both individual particles and the resultant integral spray plume characteristics. The results so obtained appear to confirm the suitability of the RF plasma process for the deposition/production of W coatings/deposits.     

Surface preparation and thermal spray in a single step: The PROTAL process—Example of application for an aluminum-base substrate

Thermal spray techniques can fulfill numerous industrial applications. Coatings are thus applied to resist wear and corrosion or to modify the surface characteristics of the substrate (e.g., thermal conductivity/thermal insulation). However, many of these applications remain inhibited by some deposit characteristics, such as a limited coating adhesion or pores or by industrial costs because several nonsynchronized and sequential steps (that is, degreasing, sand blasting, and spraying) are needed to manufacture a deposit. The PROTAL process was designed to reduce the aforementioned difficulties by implementing simultaneously a Q-switched laser and a thermal spray torch. The laser irradiation is primarily aimed to eliminate the contamination films and oxide layers, to generate a surface state enhancing the deposit adhesion, and to limit the contamination of the deposited layers by condensed vapors. From PROTAL arises the possibility to reduce, indeed suppress, the preliminary steps of degreasing and grit blasting. In this study, the benefits of the PROTAL process were investigated, comparing adhesion of different atmospheric plasma spray coatings (e.g., metallic and ceramic coatings) on an aluminum-base substrate. Substrates were coated rough from the machine shop, for example, manipulated barehanded and without any prior surface preparation. Results obtained this way were compared with those obtained using a classical procedure; that is, degreasing and grit blasting prior to the coating deposition.     

基于TIG堆焊技术的低碳钢零件精密快速成形

建立了基于TIG堆焊技术的熔焊快速成形系统,包括焊接电源、TIG焊枪、送丝机构、数控机床、工控机和电压电流采集卡等.TIG焊机进行堆焊成形,数控机床控制焊机的行走并进行后续的切削加工,工控机采集焊接过程中的成形数据,进行反馈控制.针对低碳钢TIG堆焊成形过程中热量积累导致工件边缘塌陷的问题建立了电压反馈模糊控制系统.结果表明,在电压反馈模糊控制系统的辅助下进行堆焊可获得成形良好的墙体工件.进行了圆筒形状低碳钢零件的熔焊快速成形,在TIG堆焊的基础上进行了数控切削加工,获得了高精度的成形工件.     

Atmospheric pressure barrier torch discharge and its optimization for flexible deposition of TiO2 thin coatings on various surfaces

This paper reports on the preparation of titanium (IV) oxide films via the improved atmospheric pressure barrier torch discharge (BTD) deposition. This approach is a modification of the atmospheric pressure glow discharges (APGDs) ranking among plasma enhanced chemical vapor deposition (PECVD) techniques. These methods are based on plasma–chemical reactions of the precursors' vapors occurring in the active plasma environment. The layers are produced in terms of heterogeneous recombination reactions of the high active species on the supporting surface. The major treated topic comprises the influence of the used support on the physical properties of the layers. A set of three different supports was used including quartz slides (non-conducting, dielectric), silicon discs (semi-conducting) and polished Ni sheets (conducting). Crystallographic structure, surface roughness, surface wettability and the film thickness were assessed and used as a set of physical properties to be discussed and compared for each of the films and mutually. In parallel the qualitative analysis of the emission spectra of the barrier torch discharge during the deposition process was also presented. Different conductive connection of plasma stream with the substrate crucially influences the temperature of this substrate. It has a direct effect mainly on the crystallinity and morphology of the films and also on the plasma parameters. This knowledge might be used as a tool for the optimization of deposition conditions. Photocatalytic functionality of the layers was quantified in a simple test based on the photocatalytic oxidation of Rhodamine B (C₂₈H₃₁ClN₂O₃) under UV radiation.     

冷喷涂沉积机理及其装备的研究进展

冷喷涂是近几年基于空气动力学发展起来的新型表面改性技术。冷喷涂技术在较低的温度下进行,相比热喷涂有很多优势,成为研制开发非晶、纳米及其他温度敏感材料的有效手段,在工业及国防领域有着重要的应用前景和价值。简要介绍了冷喷涂技术的原理、特点以及在保护涂层、功能涂层、近净成形、零件修复等方面的应用。涂层沉积机理的研究对冷喷涂技术的研究具有重要的理论意义,对工艺参数的优化以及优质涂层的制备具有重要的指导作用。冷喷涂装备对涂层质量和喷涂效率的提高至关重要。冷喷涂装备使冷喷涂技术的研究从理论研究到实验研究过渡,最终由实验室研究向工业应用过渡。详细阐述了冷喷涂涂层沉积机理及其研究进展。系统阐述了冷喷涂装备(真空冷喷涂、激光辅助冷喷涂、脉冲气体冷喷涂、激波风洞冷喷涂等)的工作原理及研究现状。     

Identification of Desirable Precursor Properties for Solution Precursor Plasma Spray

In solution precursor plasma spray chemical precursor solutions are injected into a standard plasma torch and the final material is formed and deposited in a single step. This process has several attractive features, including the ability to rapidly explore new compositions and to form amorphous and metastable phases from molecularly mixed precursors. Challenges include: (a) moderate deposition rates due to the need to evaporate the precursor solvent, (b) dealing on a case by case basis with precursor characteristics that influence the spray process (viscosity, endothermic and exothermic reactions, the sequence of physical states through which the precursor passes before attaining the final state, etc.). Desirable precursor properties were identified by comparing an effective precursor for yttria-stabilized zirconia with four less effective candidate precursors for MgO:Y₂O₃. The critical parameters identified were a lack of major endothermic events during precursor decomposition and highly dense resultant particles.     

New attachment for controlling gas flow in the HVOF process

During the decade, the high-velocity oxyfuel (HVOF) process proved to be a technological alternative to the many conventional thermal spray processes. It would be very advantageous to design a nozzle that provides improved performance in the areas of deposition efficiency, particle in-flight oxidation, and flexibility to allow deposition of ceramic coatings. Based on a numerical analysis, a new attachment to a standard HVOF torch was modeled, designed, tested, and used to produce thermal spray coatings according to the industrial needs mentioned above. Performance of the attachment was investigated by spraying several coating materials including metal and ceramic powders. Particle conditions and spatial distribution, as well as gas phase composition, corresponding to the new attachment and the standard HVOF gun, were compared. The attachment provides better particle spatial distribution, combined with higher particle velocity and temperature. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), 5–8 May, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

Modeling of heat flow and solidification during atomization and spray deposition processing

A mathematical model of the spray deposition process, based on heat flow analysis during solidification of droplets, as well as that of the spray deposit, is presented. The heat flow during cooling of droplets is analyzed in five distinct stages. A one-dimensional heat transfer model, using a finite difference method, is used to calculate the temperature of the deposit. The results indicate that the cooling rate of a wide size range of droplets of Al-4.5 Cu alloy in the spray varies from 10³–10⁵°C s⁻¹ in contrast to a slow cooling rate of 1–10°C s⁻¹ of the spray deposit. The spray enthalpy on the deposition surface increases linearly with the melt superheat. In contrast, the atomization gas pressure does not have a significant influence on the enthalpy of the spray in this process. The cooling rate of the deposits predicted from the model compares well with those obtained by the measurements.