When compared with conventional thermal spraying processes, thermal spraying of suspensions allows to produce coatings with outstanding properties in terms of microstructure, surface topography, and phase compositions, as well as mechanical, electrical or tribological requirements. The use of suspensions as feedstock results in an almost unlimited flexibility in terms of chemical composition of the sprayed coatings. Moreover, thermal spraying of suspensions is a promising technique for processing expensive raw materials. Zn 2 TiO 4 coatings are only one example where the high costs of blended oxide powders as feedstock material hinders the market introduction, whereas outstanding electrical properties and photocatalytic activity of thermally sprayed Zn 2 TiO 4 coatings are of great interest for various industrial applications. In this work, single oxide ZnO and TiO 2 raw materials as well as a Zn 2 TiO 4 feedstock powder were used to develop tailored aqueous suspensions suitable for thermal spraying. To follow the formation of the compositions in the system ZnO-TiO 2 , differential thermal analysis (DTA) and thermal gravimetry (TG) measurements were performed. Preparation routes of stable suspensions with low sedimentation rates, low viscosity and good flowability are discussed. Exemplary microstructures and phase compositions of sprayed coatings are shown. In all sprayed coatings, the Zn 2 TiO 4 phase has been formed during Suspension High Velocity Oxygen Fuel Spraying (S-HVOF). This work demonstrates the potential to develop appropriate cost-efficient suspension feedstocks from single oxide raw materials to obtain Zn 2 TiO 4 coatings.

Thermally sprayed WC-Co coatings provide excellent wear resistance and corrosion protection under heavy loads, but their application usually involves additional grinding and polishing steps, which can be 3-4 times costlier than the spraying process itself. There is thus the motivation to develop a process that produces smooth, near-net-shape carbide coatings. This contribution is an investigation of WC-12Co coatings obtained by suspension HVOF spraying. Significant work was devoted to the development and characterization of water-based hardmetal suspensions synthesized from commercially available WC and Co powders. The suspensions produced were sprayed using the HVOF process, and the resulting coatings were evaluated based on microstructure, hardness, and phase composition.

During suspension plasma spraying, the evaporation of liquid from the solution precursor alters the composition of the working gases thereby changing their thermal transport properties. This aim of this work is to better understand how aqueous calcium-phosphate, used in the synthesis of hydroxyapatite, affects thermal transport in Ar-H 2 plasma gas mixtures. Transport properties of the working gases were determined before and after injection of the precursor solution using T&TWinner, a free computational tool for thermochemistry. The results show that a significant increase occurs in the thermal conductivity of the Ar-H 2 gas mixture after the injection of the calcium-phosphate solution, but there is little change in momentum transfer between the working gases and solution droplets based on viscosity calculations. Although the software predicts an increase in the heating ability of the Ar-H 2 plasma jet, the absence of fully melted splats in the coatings suggests that it is not enough to melt HA particles.

In this work, a numerical model is developed and used to investigate changes that occur to ceramic suspensions in a plasma jet. The model accounts for atomization processes, evaporation of the fluid phase, and melting of the solid phase. Attention is also given to changes in the physical properties of the suspension mixture as the liquid phase evaporates. The findings clearly show that inclusion of a valid viscosity model is essential to understanding particle trajectories and breakup patterns. The validity of the model is evaluated for tests cases involving YSZ suspensions sprayed at different velocities, injection angles, and injector configurations. Particles for each test case are captured on substrates placed 8 cm away from the nozzle and particle counts and spatial distributions are compared.

The paper analyzes different working gases and their mixtures for possible application in cold gas spraying (CGS). The gases considered include Ar, CO 2 , and steam, alone and in combination. Typical pressures and temperatures were used, along with CFD simulation software, to calculate gas velocity profiles along the axis of a convergent-divergent nozzle. The profiles are compared with that of nitrogen, the standard gas for cold spray, flowing under the same conditions. Velocity and temperature profiles were also calculated for copper particles in the various gas flows as well as nitrogen. Differences in temperature and velocity are explained based on sonic velocities, viscosities, and thermal conductivities of the respective gases.

Understanding the particle injection into the gas flow issuing from an APS torch is necessary to optimize the spraying parameters. In order to solve numerically this task, the distribution of gas velocity and temperature at the torch outlet is required. In this work this is achieved by developing a model which not only delivers the solution for the electrically charged gas flow within the torch, but also includes the thermodynamical condition of minimum entropy production. This additional condition fixes the size of the electric arc inside the torch, whose radius is particularly responsible for the form of the calculated velocity and temperature profiles at the torch nozzle. The velocity and viscosity of the gas flow near the torch outlet mainly control the trajectory of particles injected into the gas flow. For the typical gas mass flow and torch power used in APS, the resulting temperatures at the gas core are slightly above the ionization temperature of the gas species. The radial location of the viscosity maximum corresponding to the ionization temperature is calculated, since this maximum strongly influences the particle trajectory. Finally, the influence of plasma fluctuations on the heat transfer to the injected particles is discussed.

This work is an attempt to understand the physical mechanisms lying on the basis of using DC torch modulation technology for spraying. The influence of the plasma disturbances created by means of arc current modulation and an influence of these disturbances on thermodynamic parameters and dynamic viscosity of the modulated plasma jet are carried out. Thus, the phenomena providing modulation effects, both as in the plasmatron itself as well as in the mechanisms influencing on the plasma technology parameters have become understandable. It was shown, that inserting hydrocarbons into the air plasma jet conduces to transition of the torch with vortex stabilization of the arc to the laminar spraying mode with a high coefficient use of powder. The obtained physical estimations are used for explanation of the coatings sprayed characteristics. In the article are described shortly the advantages and shortcomings of Plasma Spraying Technology, HVOF, Cold Spraying, Plasma Spraying with Modulation, Detonation Technologies. Each technology has important features like high velocity of spraying particles, melted spraying material and non - melted spraying material. However, these technologies have rigid requirements in regard with a size of spraying powder or wire. In one case it is possible to neglect these requirements (in a certain degree) in the case of Plasma Technology with Modulation which is a combination of all advantages of the above mentioned technologies and was applied to military and space technique.

The suspension plasma spraying (SPS) technique implemented on DC plasma spray guns is a complex thermal spraying process. In order to gain a better understanding of this deposition technique a systematic “splat” study using a shutter mechanism and the line-scan test was conducted varying liquid feedstock properties (viscosity and surface tension) and injection parameters (stream velocity and mass-loading). Splat morphology revealed the degree of particle agglomeration within the droplet formed from the liquid/plasma interaction, as well as their impacting velocity and heating history. The droplet formation was correlated to the liquid feedstock injection velocity and its viscosity. A simple model was developed to explain the experimental results correlating suspension properties to suspension droplet fragmentation mechanism.

Thermal spray has traditionally been used for depositing metallic, carbide and ceramic coatings, however, it has recently been found that the high kinetic energy of the High Velocity Oxy-Fuel (HVOF) thermal spray process also enables the solventless processing of high melt viscosity polymers, eliminating the need for harmful, volatile organic solvents. A primarily goal of this work was to develop a knowledge base and improved qualitative understanding of the impact behavior of polymeric particles sprayed by the HVOF combustion spray process. Numerical models of particle acceleration, heating and impact deformation during HVOF spraying of polymer particles have been developed. A Volume-of-Fluid (VoF) computational fluid mechanics package, Flow3D®, was used to model the fluid mechanics and heat transfer during particle impacts with a steel substrate. The radial temperature profiles predicted using particle acceleration and heat transfer models were used as initial conditions in Flow3D® together with a temperature-dependent viscosity model to simulate polymer particles with a low temperature, high viscosity core and high temperature, lower viscosity surface. This approach predicted deformed particles exhibiting a large, nearly hemispherical, core within a thin disk, and was consistent with experimental observations of thermally sprayed splats made using an optical microscope.

It has been said that plasma-sprayed ceramics particles are often supercooled before the impact on substrate. Some numerical models of the droplet impact actually included the supercooling effects. However, there is no report that has experimentally confirmed the effects on splat morphology. Therefore, in this research, we have mainly investigated the supercooling effects on splat morphology as well as splat microstructure. To achieve this, we developed an in-situ measurement technique utilizing radiation from a melt particle to monitor the impact of single particle successively under plasma spraying. The system was able to identify each single particle, which enabled us to correlate the splat morphology with impact velocity and thermal history of each particle during the impact. Yttria-stabilized zirconia powders were sprayed onto quartz glass substrate by the argon-hydrogen dc-rf hybrid plasma under atmospheric pressure. Waveforms of emissions and thermal history obtained during the impact were precisely analyzed. Especially, we closely examined thermal history during particle spreading to find the recalescence. In addition, splat morphologies were examined statistically in relation to their thermal histories. Based on the measurement, we also evaluated the viscosity of zirconia, cooling rate, and thermal contact resistance experimentally.

A two-color imaging pyrometer system is employed to examine twin wire arc spray gun used in Spray Tooling Process. Key aspects analyzed in this study include particle temperature, particle velocity, and particle distribution. The influence of process parameters such as gas flow rate, type of gas (N 2 , and air), voltage and current, as well as, spray cap design has been studied. Influence of a super atomizer called the arc jet has also been investigated. The measurements were taken at 80 mm, 160 mm and 200 mm from the gun tip. Finally, the influence of the material on the spray characteristics has been examined. The material list includes 0.8% carbon steel, 0.8% Carbon steel with B, stainless steel, Molybdenum, Ni, Ni-Al and Copper. The type of gas used for atomization significantly influences the particle temperature and velocity. Lowest temperature is observed with nitrogen gas and the highest temperature with the arc jet. Arc jet also produced particles with significantly higher velocity. The flow rate did influence the particle velocity where as temperature wasn’t affected noticeably. The wire material has considerable impact on the particle temperature and velocity. Lower melting point alloys showed higher temperature whereas the velocity has complex dependence on the density and viscosity of the material. Cap type affects the temperature and velocity of the particle. As cap opening increases the temperature and velocity decrease.

Ultra-high molecular weight polyethylene (UHMWPE) has remarkable properties in the bulk state and has substantial potential for use as a protective coating on metals. However, the molecular architecture responsible for these exceptional properties also causes difficulties in the formation of coatings by flame spraying. This paper studies two UHMWPE materials with molecular weights of 2 million and 6 million. The flow of splats for each UHMWPE and blends of selected polyethylenes were characterized and a model developed for the flow of these polymers with respect to polymer composition, viscosity and thermal spray parameters. The model was applied to the polyethylene system and the experimental results show that controlling the composition and the process parameters is essential for the deposition of high-quality coatings.

In the present paper mathematical model of the deformation behavior of a liquid spherical particle upon its impingement onto a solid surface, including flattening and simultaneous solidification is developed. Particle-substrate interactions are investigated for typical thermal spray process. Numerical simulation for the complete Navier-Stokes equations is based on the finite-difference method on rectangular mesh in cylindrical coordinates. The energy equation is solved for both particle and substrate regions using the adjoint conditions for the temperature. In this paper main attention is paid to investigation of the temperature in contact of the particle with substrate. In connection with the oxide films effect on the surface substrate taking onto account thermal resistance of oxide is simulated. Heat transfer process in particle and substrate has been modeled by 2-D problem of heat conduction with influencing hydrodynamic processes into molten particle. Particle solidification and the movement of the solidification front have been described by means of one-dimensional Stefan problem. Numerical results for the heat transfer process and the effect of some important processing parameters such as particle diameter, viscosity, oxide films and temperature of plasma on the flattening and solidification of a single liquid particle have been discussed. Numerical algorithms were realized in the form of applied programs complex.

The gasturbine industry is performing utmost efforts to allow increased operation temperatures to improve engine efficiency by a reduced cooling air/fuel-ratio. The reason is to save fuel and at the same time lower emission rates. A possible solution to improve the insulation of the Thermal Barrier Coating (TBC) is followed by the strategy of developing reliable thick TBCs. To be successful concerning the coating quality and to apply thick TBCs at reasonable costs and spray time it is necessary to apply the coating consistently by high deposition rates. The plasma spraying of thicker coatings leads to long spray periods which demands for a process control to keep the process parameters within the tolerances during the entire spray procedure. A major property is the control of substrate temperature with pyrometer systems and to define the process tolerances. Furthermore, a high deposition efficiency is of prime importance to shorten spray times. The optimisation of the gas composition in terms of viscosity and thermal conductivity (SPRAL 22) enables to increase dramatically this efficiency by 30% to 70%. Moreover, in order to reduce thermal expansion mismatch between substrate and ceramic top coat, a high amount of porosity is beneficial to lower young's modulus of the ceramic top. This can be achieved by adding a polymer powder to the yttria partly stabilised zirconia powder. It also leads to an increase in deposition efficiency.