Plasma spraying at very low pressure (50-200 Pa) is significantly different from atmospheric plasma conditions (APS). Applying powder feedstock it is possible to defragment the particles into very small clusters or even to evaporate the material. As a consequence, the deposition mechanisms and the resulting coating microstructures could be quite different compared to conventional APS liquid splat deposition. Thin and dense ceramic coatings as well as columnar-structured strain-tolerant coatings with low thermal conductivity can be achieved offering new possibilities for application in energy systems. To exploit the potential of such a gas phase deposition from plasma spray-based processes, the deposition mechanisms and their dependency on process conditions must be better understood. Thus, plasma conditions were investigated by optical emission spectroscopy. Coating experiments were performed, partially at extreme conditions. Based on the observed microstructures, a phenomenological model is developed to identify basic growth mechanisms.

An adapted HVOF system has been computationally investigated in order to test the effects of injecting a cooling gas on both the gas phase dynamics and particle behaviour through the system. An existing liquid-fuelled HVOF thermal spray gun is modified by introducing a centrally located mixing chamber. The gas phase model incorporates liquid fuel droplets which heat, evaporate and then exothermically combust within the combustion chamber producing a realistic compressible, supersonic, turbulent jet. The trajectory of each discrete phase powder particle is tracked using the Lagrangian approach, with the inclusion of heating, melting and solidification through each particle. The results obtained give an insight to the complex interrelations present between the gas and particle phases, and demonstrates the usefulness of this modelling approach in aiding the development of thermal spray devices.

The combustion assisted thermal spray systems are being used to apply coatings to prevent surface degradation. They offer a highly attractive way to modify the surface properties of the substrate to extend the product life. The quality of combustion assisted thermal spray coating depends greatly on the flow behavior of reacting gases and particle dynamics. The present study investigates the effect of gas phase and its interaction with particles through the nozzle of a thermal spray gun by developing a comprehensive mathematical model. The objective is to develop a predictive understanding of various design parameters of combustion assisted thermal spray systems. The model was developed by considering the conservation of mass, momentum and energy of reacting gases. The particle dynamics was decoupled from the gas phase dynamics since the particle loading in the spray process is very low. The developed model was employed to investigate the influence of various design parameters on the coating quality of thermal spray process.

This paper reports on the synthesis of SiC material through the decomposition of silanes in a thermal high frequency (HF) plasma. The process is based on thermal plasma technology for chemical deposition from the gas phase and on suspension plasma spray technology, in which a liquid or suspension is injected axially and atomized in the plasma flame. The liquid silane then decomposes, and forms SiC with some gaseous by-products such as HCl. Various plasma parameters were varied, for example the plasma power level, the plasma gas composition, the chamber pressure, and the silane composition. The paper also presents first investigations into the elementary and phase composition as well as the morphology of the powders and coatings. Paper includes a German-language abstract.

The system B-C-N contains the hardest known materials like diamond, cubic boron nitride and boron carbide, which also show excellent chemical resistance. The oxidation resistance is shifted to higher temperatures in comparison to pure diamond. But pure BCN coatings cannot be produced by conventional thermal spray processes, as the materials lack both a liquid phase and sufficient ductility to permit deposition. Conventional VPS equipment is successfully applied in Thermal Plasmajet CVD processes for high deposition rate synthesis of diamond coatings. The feasibility of SiCN or boron carbide synthesis by this method has also been proven. The use of liquid precursors results in outstanding deposition rates and improved operational safety. Methylized borazine is applied for synthesis of BCN coatings in thermal plasma jets. The use of single source precursors is advantageous with concern to the homogeneity of the coating forming species stoichiometry. For long-term storage cooling is necessary, but also under ambient conditions the precursor shows sufficient stability. Plasma gun nozzles with different diameter and design are applied and evaluated with concern to the resulting coating properties. Deposition rates of up to 1,500 µm/h have been achieved with homogeneous coating thickness and morphology on areas with 50 mm diameter. No porosity is detected in SEM investigations on cross sections and fracture surfaces show a fine columnar coating morphology. XRD investigations point at an amorphous structure. Only for very high substrate temperatures the formation of crystalline boron carbide B8C and h-BN or graphite phases is detected. Oxygen contamination results in boric acid formation and therefore has to be avoided carefully. During coating deposition on mild steel substrates the formation of boride and nitride reaction zones is observed. VPS sprayed nickel or molybdenum interlayers permit to inhibit the evolution of reaction zones. Thereby BCN coatings with thicknesses of up to 10 µm are deposited without local delamination. Space resolved emission spectroscopic analyses are carried out in order to detect coating forming and intermediate species. As Thermal Plasmajet CVD is a pure gas phase deposition process, the control of the space resolved emission permits easy process control.

A model for oxidation of molybdenum particles during plasma spray deposition is developed. The diffusion of metal an-ions or oxygen cat-ions through a thin oxidized film, chemical reactions on the surface, and diffusion of oxidant in gas phase are considered as possible rate-controlling mechanisms with controlling parameters as the temperature of the particle surface, and local oxygen concentration and flow field surrounding the particle. The deposition of molten particle and its rapid solidification and deformation is treated using a Madejski-type model, in which the mechanical energy conservation equation is solved to determine the splat deformation and one-dimensional heat conduction equation with phase change is solved to predict the solidification and temperature evolution. Calculations are performed for a single molybdenum particle sprayed under the Sulzer Metco-9MB spraying conditions. Results show that the mechanism that controls the oxidation of this droplet is the diffusion of metal/oxygen ions through a very thin oxide film. A higher substrate temperature results in a larger rate of oxidation at the splat surface, and hence, a larger oxygen content in the coating layer. Compared to the oxidation of droplet during m-flight, the oxidation during deposition is not weak and can become dominant at high substrate temperatures.

Spattering is an unavoidable phenomenon in the selective laser melting (SLM) process, which can cause various printing defects and harmful powder recycling. Since the size of powder spattering is too small at the micron level, it is difficult to investigate the entire dynamic spattering process experimentally. The comprehensive understanding of the intricate dynamics of powder spattering during the SLM process remains incomplete. Therefore, we develop a new multiphase flow model to study the transient dynamic behaviors of the gas phase and powder spattering, which agrees well with the experimental observation result. It is the first time that the whole transient dynamic process of powder motion from starting to move induced by the vapor jet to falling to the substrate wall and stopping completely was observed. Powder spattering motion dynamics induced by metal vapor jet and argon gas flow, as a function of time, laser parameters, and location, are presented. The moving speed, total amount, and dropping distribution on the substrate of powder spattering that varies with laser parameters are quantified.

In this work, a CFD model is built to investigate gas flow and in-flight particle characteristics in HVOF spraying with hydrogen as the fuel. Gas flow is solved for a particle-free jet, combustion is represented using a simple eddy-dissipation model, and a time-averaged fluid flow (k-ε) model is used to account for turbulence. Acceleration and heating of individual particles are modeled in a "snapshot" of the gas flow. Particle acceleration is shown to be governed by drag and particle heating by conduction and radiation between particulates and gas phases. Modeling results agree well with experimentally obtained data and observations.

To use the manifold possibilities that arc spraying offers to deposit wear resistance layers, the knowledge of the particle formation and their characteristics are necessary. The work is focused on studying the particle trajectories during arc spraying with cored wires. Different cored wires under various spraying parameters are investigated by means of a high speed camera. Particle properties in-flight, such as velocity and temperature are determined. Correlation between particle trajectories and particle characteristics at different spraying conditions are established. At the same time, the particle-laden gas flow is simulated numerically and the computed solutions are used to illustrate the utility of the proposed CFD model and compared with experimental results. The employed mathematical model represents a system of macroscopic conservation laws for the continuous gas phase and for the gas-solid mixture. This formulation makes it possible to circumvent the numerical difficulties associated with the implementation of a (potentially ill-posed) two-fluid model. The discretization in space is performed using a high-resolution finite element scheme based on algebraic flux correction in terms of local characteristic variables. The artificial diffusion operator is constructed on the discrete level and fitted to the local solution behavior using a multidimensional flux limiter of TVD type.

In a DC plasma spray torch, the plasma-forming gas is the most intensively heated and accelerated at the cathode arc attachment due to the very high electric current density at this location. A proper prediction of the cathode arc attachment is, therefore, essential for understanding the plasma jet formation and cathode operation. However, numerical studies of the cathode arc attachment mostly deal with transferred arcs or conventional plasma torches with tapered cathodes. In this study, a 3-D time-dependent and two-temperature model of electric arc combined with a cathode sheath model is applied to the commercial cascaded-anode plasma torch SinplexPro. The model is used to investigate the effect of the cathode sheath model and bidirectional cathode-plasma coupling on the predicted cathode arc attachment and plasma flow. The model of the plasma-cathode interface takes into account the non-equilibrium spacecharge sheath to establish the thermal and electric current balance at the interface. The radial profiles of cathode sheath parameters (voltage drop, electron temperature at the interface, Schottky reduction of the work function) were computed on the surface of the cathode tip and used at the cathode-plasma interface in the model of plasma torch operation. The latter is developed in the open-source CFD software Code_Saturne. It makes it possible to calculate the flow fields inside and outside the plasma torch as well as the enthalpy and electromagnetic fields in the gas phase and electrodes. This study shows that the cathode sheath model results in a higher constriction of the cathode arc attachment, more plausible cathode surface temperature distribution, more reliable prediction of the torch voltage, cooling loss, and more consistent thermal balance in the torch.

Diamond films have been deposited on WC - 6% Co hard metal tools by the DC plasma jet CVD synthesis. The parameters of the process (gas composition, temperature of the gas phase and the substrate, process pressure) as well as of the substrate surface (material, pretreatment) are related to the diamond film growth. For machining abrasive materials the hard and wear resistant diamond coatings must adhere good to the substrate. The wear behaviour of thin diamond films on hard metals under cavitation treatment has been examined. Thus the conditions of diamond synthesis have been varied especially concerning the coating duration and the process pressure and engineering. The cavitation test reacts more sensitive to coating defects of pm size than the conventional testing methods (scratch test, indenter method) and considers the microstructure of the material. Paper text in German.

Two common concerns in dc plasma torches are stability of plasma jet and anode erosion. The challenge is how to get a stable plasma jet with minimal anode erosion. This study tackles this question by using an external axial magnetic field applied to a cascaded plasma torch. A 3D, time-dependent model of the torch is used to predict the value of the magnetic field and its effect on heat flux to the anode as well as plasma jet stability. The model couples the gas phase and electrodes, making it possible to follow anode temperature evolution. For specific operating conditions, the model predicts an azimuthal self-magnetic field induced by electric arcing and the subsequent effect of an external field on arc attachment and anode wall temperature. This approach is expected to provide a better understanding of arc behavior in dc plasma torches and facilitate the control of anode erosion.

In the Plasma Spray-Physical Vapor Deposition (PS-PVD) process, the vapor atom of feedstock material is one deposition unit of the columnar structure coating. It is reported that the gas phase may be transformed into cluster when the powder feeding rate increases from small to large or the sedimentation distance increases from a certain distance to another distance. In order to understanding the variation of vaporized coating material in free plasma jet, the gaseous material capacity of plasma jet must be fundamentally understood. In this work, the thermal characteristics of plasma were firstly measured by optical emission spectrometry (OES). The results show that the free plasma jet is in the local thermal equilibrium due to a typical electron number density from 2.1×1015 to 3.1×1015 cm -3 . In this condition, the temperature of gaseous zirconia can be equal to the plasma temperature. A model was developed to obtain the vapor pressure of gaseous ZrO 2 molecules as a two dimensional map of jet axis and radial position corresponding to different average plasma temperatures. The overall gaseous material capacity of free plasma jet was further established. At a position of plasma jet, clusters may form when the gaseous material exceeds local maximum gaseous material capacity.

In this study, YSZ coatings are deposited by plasma spray-physical vapor deposition using a shroud to limit expansion of the plasma jet and increase its heating ability. Optical emission spectroscopy shows that the shroud significantly increases the evaporation of YSZ particles in the jet, resulting in coatings with a hybrid columnar structure. SEM examination of coating surfaces and cross-sections reveal micro and nanoscale features and, in each case, the mechanisms of formation are discussed.

This paper provides an overview of chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes and the advantages they offer physical vapor deposition for the application of friction and wear coatings for micromechanical assemblies and components. It explains how hard and solid lubricant phases can be applied by these non-line-of-sight deposition methods, achieving nanoscale conformality and coating uniformity on buried surfaces and interfaces. It also discusses inherent disadvantages and explains how plasma excitation can be incorporated in either process to overcome material limitations.

This study compares two methods for modeling the breakup of droplets during suspension plasma spraying. One is based on Taylor analogy breakup, the other on Kelvin-Helmholtz Rayleigh Taylor breakup. A three-dimensional model with two-way coupling is used to simulate flow within the plasma plume and interactions between suspension droplets, and a Reynolds stress model is used to simulate gas field turbulence. After breakup and vaporization, the solid suspended particles are tracked through the domain to determine the characteristics of coating particles. The numerical results are validated against experiments using high-speed imaging.

High Velocity Oxygen Fuel Process (HVOF) involves supersonic two-phase flow of gas-solid particles. Two kinds of shocks are formed in a typical HVOF process. Adjustment of the over-expanded flow to the atmospheric pressure at the exit of the nozzle results in formation of shock diamonds while high speed flow impingement on a substrate creates bow shock. The latter is found to be responsible for deviation of the injected particles from their trajectories near the substrate which significantly reduces the chance of some particles landing on the substrate. An attempt is made to study the behavior of particle trajectory as it interacts with the bow shock formed near the substrate. The strength and location of bow shock was found to vary for different substrate geometries and stand-off distances. In this work, various particle sizes impinging on substrates with various configurations (flat, concave and convex) are simulated and the effect of shock diamonds and bow-shock, on particle trajectory is studied.

The two-phase flow properties of copper particle laden nitrogen are measured and compared to computational fluid dynamic calculations, determining the achievable degree of consistency between model and reality. Two common, commercial nozzles are studied. A two-way coupled Lagrangian scheme along with the RSM turbulence model is used to track the particles and to model the interactions between the gas and the particulate phase. Significant agreement is found for the geometrical gas flow structure, the resulting particle velocities, and the dependence of the two-phase flow on the particulate phase mass loading. The particle velocities decrease with increasing mass loading, even for modest powder feed rates of less than 3 g/s. The velocity drop occurs even when the gas flow rate is kept constant. Adiabatic gas flow models neglecting the energy consumption by the particles are thus inaccurate, except for very dilute suspensions with low technical relevance. For the cases modeled, the experiments evidence the high predictive power of the chosen approach.

Computational Fluid Dynamics (CFD) is used to model the Shock-wave Induced Spray Process (SISP). SISP utilizes the kinetic and thermal energy induced by a moving shock-wave to accelerate and heat powder particles, similar to Cold Gas-Dynamic Spraying (CGDS), where the particles impact the substrate and deform plastically to produce a coating. Individual powder particles reach the substrate at different velocities and temperatures depending on their location within the unsteady flow regime. The critical velocity correlated to particle impact temperature and a CFD model are used to predict whether a particle traveling within this unsteady flow regime will bond to the substrate upon impact or bounce off. This information is then used to predict if a coating can be formed under a specific set of spray conditions.

In the last decade 3D-CFD has been successfully established for three-dimensional simulations of fluid flow, mixture formation, combustion and pollutant formation in internal combustion engines. In direct injected engines, the accuracy of simulation results and hence their contribution to design analysis and optimization strongly depends on the predictive capabilities of the models adopted for simulation of the injector flow, spray formation and propagation characteristics. The original KIVA-3V code uses the droplet collision algorithm of O’Rouke for calculating collisions in Lagrangian spray simulation. The modified KIVA-3V code includes no-time-counter (NTC) method which is proposed by David P. Schmidt (2000). The NTC method of calculating spray droplet collisions is both faster and more accurate than the current standard method of O’Rouke. In this work an original and a modified version of KIVA-3V multi-dimensional CFD code is used to simulate the spray resulting from the injection of diesel fuel. Numerical results from original and modified KIVA-3V are compared to experimental data and also results obtained from Fire code.