Armelle Vardelle

Knowledge Concerning Splat Formation: An Invited Review

This paper summarizes our knowledge at the beginning of 2003 about splat formation. First, the analytical and numerical models related to the impact and flattening of single particles on smooth or rough substrates with different tilting are recalled. Then, the different diagnostic methods, including imaging, are briefly described. The last part of the paper is devoted to the results and their discussion. Studies are related to the effect of various parameters on particle flattening. They include the characteristics of particles prior to impact: normal impact velocity, temperature, molten state, oxidation state, etc.; the parameters related to the substrate: tilting angle, roughness, oxide layer composition, thickness and crystallinity, desorption of adsorbates and condensates, wetting properties between impacting particle and substrate, etc.; and, finally, the parameters related to the heat exchange between the flattening particle and the substrate. They depend on previous parameters and control the propagation of the solidification front within the flattening particle, eventually modifying its liquid flow. It is obvious from this review that, if our understanding of the involved phenomena has been drastically improved during the last years, many points have still to be clarified. This is of primary importance because all the coating properties are linked to the particle flattening, splat formation, and layering.

Quo vadis thermal spraying

This paper is devoted to thermal spraying and presents the state of our current knowledge, as well as the following research or development needs: spraying heat sources, i.e., flame, high-velocity oxifuel flame (HVOF), detonation gun (D-Gun), and plasma torches; particle heat and momentum transfer (measurements and modeling), process on-line control, powder morphologies, and injection within the hot jet and reactions with environment; coating formation, i.e., particle flattening and solidification, splat layering, residual stresses, coating microstructure, and properties; and reliability and reproducibility of coatings.

Influence of particle parameters at impact on splat formation and solidification in plasma spraying processes

A measurement system consisting of two high- speed two- color pyrometers was used to monitor the flattening degree and cooling rate of zirconia particles on a smooth steel substrate at 75 or 150 °C during plasma spray deposition. This instrument provided data on the deformation behavior and freezing of a particle when it impinged on the surface, in connection with its velocity, size, and molten state at impact. The results emphasized the influence of temperature and surface conditions on particle spreading and cooling. When the substrate temperature was 150 °C, the splats had a perfect lenticular shape, and the thermal interface resistance between the lamella and the substrate ranged from 10− 7 to 10− 8 W/m2 · K. The dependence of the flattening degree on the Reynolds number was investigated.

Substrate temperature effects on splat formation, microstructure development and properties of plasma sprayed coatings Part I: Case study for partially stabilized zirconia !

In recent years it has been observed that the substrate surface temperature during thermal spray deposition has a profound effect on the morphology of the impacted droplet (splat) and consequently on the microstructure and properties of the deposits. In this set of two papers (one for metal and one for ceramic), the substrate temperature effects have been studied in an integrated manner relating the initial splat formation to microstructure development and eventually to the properties of the deposit. Isolated impacted splats have been obtained on polished steel substrates at two different temperatures (high and low) and these have been analyzed quantitatively for their shape factors and thicknesses. The deposits have been formed nominally at these two different temperatures and their microstructures and properties have been analyzed. The results confirm that there exists a threshold transition temperature for the substrate surface beyond which the splat morphology changes from a fragmented (splashed) to a more contiguous (disk-shaped) morphology. In the case of zirconia this temperature appears to be in the range of about 250‐300°C, which is roughly 10% of the melting temperature of zirconia. It has been further observed that the splat‐substrate and inter-splat contact is significantly improved at higher temperatures, leading to reduced porosity, increased thermal conductivity and strength. These results are assimilated to develop an integrated structure‐property relationship and preliminary arguments are presented as to the reason for such transitions.

Spray parameters and particle behavior relationships during plasma spraying

Using laser anemometry, laser fluxmetry, and statistical two-color pyrometry, the velocity, number flux, and surface temperature distributions of alumina and zirconia particles in dc plasma jets have been determined inflight for various spraying parameters. The flux measurements emphasized the importance of the carrier gas flow rate, which must be adjusted to the plasma jet momentum depending on the arc current, nozzle diameter, gas flow rate, and gas nature. It has also been shown that the particle trajectories depend both on the particle size and injection velocity distributions and that the position and tilting of the injector plays a great role. The particle size drastically influences its surface temperature and velocity, and for the refractory materials studied, only the particles below 45 μm in diameter are fully molten in Ar-H2 (30 vol%) plasma jets at 40 kW. The morphology of the particles is also a critical parameter. The agglomerated particles partially explode upon penetration into the jet, and the heat propagation phenomenon is seriously enhanced, particularly for particles larger than 40 μm. The effects of the arc current and gas flow rate have been studied, and the results obtained in an air atmosphere cannot be understood without considering the enhanced pumping of air when the plasma velocity is increased. The Ar-He (60 vol%) and Ar-H2 (30 vol%) plasma jets, when conditions are found where both plasma jets have about the same dimensions, do not result in the same treatment for the particles. The particles are not as well heated in the Ar-He jet compared to the Ar-H2 jet. Where the surrounding atmosphere is pure argon instead of air (in a controlled atmosphere chamber), he radial velocity and temperature distributions are broadened, and if the velocities are about the same, the temperatures are higher. The use of nozzle shields delays the air pumping and increases both the velocity and surface temperature of the particles. However, the velocity increase in this case does not seem to be an advantage for coating properties.

Experimental investigation of powder vaporization in thermal plasma jets

An experimental study of the vaporization of metallic and ceramic particles in a thermal do plasma jet has been initiated and two series of experiments have been performed: (1) measurement of the vapor concentration within the plasma jet by absorption spectroscopy. (2) Investigation of the vapor cloud surrounding a single particle in flight by emission spectroscopy. The temperature within this cloud is determined by the intensity ratio of two lines which are simultaneously measured. The cloud radius is deduced /torn measurement of the particle velocity by laser doppler anemometry, and the vapor concentration is calculated from the line intensity profile, once the temperature is known. Results on iron and alumina particles injected in argon or argon-hydrogen plasma jets are given and discussed.

Controlling particle injection in plasma spraying

This paper reviews experimental and analytical techniques that examine the efficiency of systems for the injection of powders in plasma jets used in spray coating. The types of injectors, the experimental techniques for observing particle trajectories and distributions, and the mathematical models used to investigate the momentum and heat-transfer phenomena between particles, carrier gas, and plasma jet are described. Experimental data are presented from numerous examples from the plasma spraying of ceramic powders.

Plasma spray: Study of the coating generation

Abstract A plasma sprayed coating is built up particle by particle, resulting in a coating with a layered structure that is highly anisotropic. The spray parameters affect certain factors of the coating, such as the size and distribution of porosity, oxide content, residual stresses, macro and microcracks, factors which have an important influence on the performance and eventual failure of the coating. Many works (modelling and measurements) have been devoted to the particles in flight and how these parameters are influenced by the plasma jets and particle injection macroscopic parameters. However, there is a shortage of published information on the way the particles spread upon impact and solidify on the substrate or the previously deposited layers and how these phenomena are connected to the deposit microstructure. This paper was concerned with expanding our knowledge in this field by concentrating on the following: • • modelling of deformation and solidification of a droplet on a substrate; • • measurement of the splat formation and cooling; • • experimental results and comparison with models.

Reactive thermal plasmas: ultrafine particle synthesis and coating deposition

Thermal plasmas jets produced by d.c. arcs or RF discharges at pressures close to atmospheric pressure are characterized by the high temperatures (between 6000 and 14 000 K ) of heavy species and high velocities (between 100 and 2500 m s’1) of plasma flow. They can be used either for their physical properties, i.e. acceleration and melting of solid particles (in the diameter range 10‐100 mm) to spray thick coatings (from 0.1 mm to a few millimetres), or for their reactive properties. When reagents are introduced in the plasma stream, they are transformed in highly reactive radicals and/or atoms in excited states. These reactive species can form a coating with high deposition rates (~100 m mh ’1) due to the steep gradients in the boundary layer plasma‐substrate: this process is called thermal plasma-assisted chemical vapour deposition (PCVD). Without substrate and using adequate quenching, they can also form ultrafine particles, the size of which is controlled (with diYculty) by temperature during the quenching process. These PCVD processes can be used to form new chemical species on the surface of solid and molten particles in flight in the plasma jets, or on the resulting splats and between the successive passes to produce coatings with dispersed hard phases (reactive plasma spraying). It is also possible to agglomerate reactive particles (e.g. carbon and metal ) a few micrometres in diameter, to achieve exothermic reactions within the particles in flight and spray carbide coatings in air. This paper presents our knowledge in these fields as follows: (1) the main characteristics of plasma d.c. and RF torches, transferred arcs and the injection of reagents and/or quenching gases; (2) the modelling problems of reactive gas flow injection; boundary layer close to the substrate; ultrafine particle synthesis with the corresponding experimental results obtained for PCVD coatings; thin film deposition by flash evaporation and ultrafine particle synthesis; and (3) the reactive plasma spraying achieved either by spraying agglomerated particles (in the 10‐50 mm size range) where self-propagating high-temperature synthesis occurs, or by mixing a reactive gas with the plasma jet.

Influence of the velocity of plasma-sprayed particles on splat formation

The behavior of individual plasma- sprayed particles as they impinge on a surface was monitored using two high- speed two- color pyrometers, one focused 2 mm before the substrate and the other focused on the substrate surface. The influence of the velocity of the impinging particles (determined from the time of flight between the two focused points of the pyrometers) on the deformation and cooling processes was investigated. Results on zirconia particles impacting on a smooth bare steel substrate are presented in terms of apparent flattening time, splat diameter, and cooling rate determined from the pyrometer signals.