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.

Knowledge concerning splat formation : An invited review

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.

Quo vadis thermal spraying

Atmospheric plasma sprayed thick thermal barrier coatings (TBCs), comprising of 1.5 mm thickness yttria stabilized zirconia (YSZ) coating, have been developed for increasing thermal protection of combustor applications. Different segmentation crack densities of the YSZ coating were created by controlling the deposition conditions. It was found that the substrate temperature played a dominant role in determining the segmentation crack density. The density was found to increase with the increase of substrate temperature and liquid splat temperature. High passage thickness, if achieved by low plasma gun speed, contributed to improve the segmentation crack density. The density could not be effectively improved by increasing the powder feed rate, although a high passage thickness was obtained in this case. The pores, mainly consisting of delaminations with the radii range between 0.02 and 1 μm, prevent the segmentation cracks from propagation. The coatings sprayed at high substrate temperature show an excellent intersplat bonding and often even continuous columnar grains through several splats. Additionally, horizontally branching cracks were also formed together with the segmentation cracks when a high lamellar thickness was used during the spraying. Thermal cycling tests showed that the coating with a segmentation crack density of approximately 3.6 mm−1 had a lifetime of more than 1700 cycles at 1238 °C (surface)/938 °C (bond coat), indicative of excellent thermal shock resistance. Failure of the TBCs occurred by chipping in the surface layer of YSZ coating, which is different from the traditional interface delamination failure occurred with thinner TBCs.

Atmospheric plasma sprayed thick thermal barrier coatings with high segmentation crack density

This article presents what is our present knowledge in plasma spraying of suspension, sol, and solution in order to achieve finely or nano-structured coatings. First, it describes the different plasma torches used, the way liquid jet is injected, and the different measurements techniques. Then, drops or jet fragmentation is discussed with especially the influence of arc root fluctuations for direct current plasma jets. The heat treatment of drops and droplets is described successively for suspensions, sols, and solutions both in direct current or radio-frequency plasmas, with a special emphasize on the heat treatment, during spraying, of beads and passes deposited. The resulting coating morphologies are commented and finally examples of applications presented: Solid Oxide Fuel Cells, Thermal Barrier coatings, photocatalytic titania, hydroxyapatite, WC-Co, complex oxides or metastable phases, and functional materials coatings.

Parameters Controlling Liquid Plasma Spraying: Solutions, Sols, or Suspensions

From the pioneering works of McPherson in 1973 who identified nanometre-sized features in thermal spray conventional alumina coatings (using sprayed particles in the tens of micrometres size range) to the most recent and most advanced work aimed at manufacturing nanostructured coatings from nanometre-sized feedstock particles, the thermal spray community has been involved with nanometre-sized features and feedstock for more than 30 years.Both the development of feedstock (especially through cryo-milling, and processes able to manufacture coatings structured at the sub-micrometre or nanometre sizes, such as micrometre-sized agglomerates made of nanometre-sized particles for feedstock) and the emergence of thermal spray processes such as suspension and liquid precursor thermal spray techniques have been driven by the need to manufacture coatings with enhanced properties. These techniques result in two different types of coatings: on the one hand, those with a so-called bimodal structure having nanometre-sized zones embedded within micrometre ones, for which the spray process is similar to that of conventional coatings and on the other hand, sub-micrometre or nanostructured coatings achieved by suspension or solution spraying. Compared with suspension spraying, solution precursor spraying uses molecularly mixed precursors as liquids, avoiding a separate processing route for the preparation of powders and enabling the synthesis of a wide range of oxide powders and coatings. Such coatings are intended for use in various applications ranging from improved thermal barrier layers and wear-resistant surfaces to thin solid electrolytes for solid oxide fuel cell systems, among other numerous applications.Meanwhile these processes are more complex to operate since they are more sensitive to parameter variations compared with conventional thermal spray processes. Progress in this area has resulted from the unique combination of modelling activities, the evolution of diagnostic tools and strategies, and experimental advances that have enabled the development of a wide range of coating structures exhibiting in numerous cases unique properties. Several examples are detailed. In this paper the following aspects are presented successively (i) the two spray techniques used for manufacturing such coatings: thermal plasma and HVOF, (ii) sensors developed for in-flight diagnostics of micrometre-sized particles and the interaction of a liquid and hot gas flow, (iii) three spray processes: conventional spraying using micrometre-sized agglomerates of nanometre-sized particles, suspension spraying and solution spraying and (iv) the emerging issues resulting from the specific structures of these materials, particularly the characterization of these coatings and (v) the potential industrial applications.Further advances require the scientific and industrial communities to undertake new research and development activities to address, understand and control the complex mechanisms occurring, in particular, thermal flow?liquid drops or stream interactions when considering suspension and liquid precursor thermal spray techniques. Work is still needed to develop new measurement devices to diagnose in-flight droplets or particles below 2??m average diameter and to validate that the assumptions made for liquid?hot gas interactions.Efforts are also required to further develop some of the characterization protocols suitable to address the specificities of such nanostructured coatings, as some existing 'conventional' protocols usually implemented on thermal spray coatings are not suitable anymore, in particular to address the void network architectures from which numerous coatings properties are derived.

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Engineering a new class of thermal spray nano-based microstructures from agglomerated nanostructured particles, suspensions and solutions: an invited review

This article is devoted to the study of the adhesion/cohesion (A/C) of alumina coatings sprayed on four different substrates: aluminium alloy, titanium alloy, cast iron and mild steel. Fused and crushed alumina particles (−45 + 22 μm) were sprayed with a custom-made torch with a nozzle of internal diameter 7 mm working with an arc current of 600 A and a plasma gas mixture of 45 slm Ar, 15 slm H 2 , which resulted in a voltage of 64 V and a thermal efficiency of 52%. The particles were injected internally and were almost fully molten (less than 3% α-phase in the collected powders after their passage in the plasma jet). The influence of substrate roughness and temperature before (preheating), during and after spraying was systematically studied. R a values were 6–8, 10–13 and 14–21 μm and the substrates were preheated to temperatures T p of 170, 200, 300, 350, 380 and 500 °C. The most important results were the following: 1. • for cold substrates A/C increased with R a up to a maximum of 20 MPa 2. • for substrates whose expansion mismatch with alumina was less than 4.10 −6 K, the highest A/C values (50–60 MPa) were obtained with T p between 350 and 500 °C provided the substrates were not oxidized 3. • at these temperatures the highest values of A/C were obtained with the lowest R a 4. • when particle velocity and surface temperature upon impact decreased, for example by load effect, A/C values decreased too

Influence of substrate roughness and temperature on the adhesion/cohesion of alumina coatings

This paper examines how the grit blasting process influences the surface roughness of different sub-strates, the grit residue, and the grit erosion. The influence of grit blasting conditions on induced sub-strate residual stresses is also discussed. Aluminum alloy, cast iron, and hard steel were blasted with white alumina grits of 0.5,1, and 1.4 mm mean diameters. Grit blasting was performed using either a suction-type or a pressure-type machine equipped with straight nozzles made of B4C. The influence of the follow-ing parameters was studied: grit blasting distance (56 to 200 mm), blasting time (3 to 30 s), angle between nozzle and blasted surface (30°, 60°, 90°), and blasting pressure (0.2 to 0.7 MPa). The roughness of the substrate was characterized either by using a perthometer or by image analysis. The grit residue remain-ing at the blasted surface was evaluated after cleaning by image analysis. The residual stresses induced by grit blasting were determined by using the incremental hole drilling method and by measuring the de-flection of grit-blasted beams. Grit size was determined to be the most important influence on roughness. The average values of Ra and Rt and the percentage of grit residue increased with grit size as well as the depth of the plastic zone under the substrate. An increase of the pressure slightly increased the values of Aa and Rt but also promoted grit breakdown and grit residue. A blasting time of 3 to 6 s was sufficient to obtain the highest roughness and limit the grit breakdown. The residual stresses generated under the blasted surface were compressive, and the depth of the affected zone depended on the grit diameter, the blasting pressure, and the Young’s modulus of the substrate. More-over, the maximum residual stress was reached at the limit of the plastic zone (i.e., several tenths of a mil-limeter below the substrate surface).

Alumina grit blasting parameters for surface preparation in the plasma spraying operation

Splat size and shape- factor distributions for plasma- sprayed alumina particles on various substrates were studied using a setup derived from the line- scan test. Direct- current (dc) and radiofrequency (rf) plasma torches were used to study the influence of particle velocity at impact. The influence of substrate temperature prior to spraying also was studied. Splats collected on smooth substrates kept below 100 °C were extensively fingered and had poor substrate contact. When the substrates were heated to 300 °C before spraying, the splats became disk- shaped and their substrate contact was very good. Similar results were obtained for rough substrates. Coating adhesion decreased with particle velocity and was lower for the dc plasma torch when using larger particles, which did not melt as well as smaller ones. Melting and adhesion were much improved with the rf torch.

Comparison of plasma-sprayed alumina coatings by RF and DC plasma spraying

Suspension plasma spraying (SPS) is able to process sub-micrometric-sized feedstock particles and permits the deposition of layers thinner (from 5 to 50 μm) than those resulting from conventional atmospheric plasma spraying (APS). SPS consists in mechanically injecting within the plasma flow a liquid suspension of particles of average diameter varying between 0.02 and 1 μm, average values. Upon penetration within the DC plasma jet, two phenomena occur sequentially: droplet fragmentation and evaporation. Particles are then processed by the plasma flow prior their impact, spreading and solidification upon the surface to be covered. Depending upon the selection of operating parameters, among which plasma power parameters (operating mode, enthalpy, spray distance, etc.), suspension properties (particle size distribution, powder mass percentage, viscosity, etc.), and substrate characteristics (topology, temperature, etc.), different coating architectures can be manufactured, from dense to porous layers. Nevertheless, the coupling between the parameters controlling the coating microstructure and properties are not yet fully identified. The aim of this study is to further understand the influence of parameters controlling the manufacturing mechanisms of SPS alumina coatings, particularly the spray beads influence.

Suspension Plasma-Sprayed Alumina Coating Structures: Operating Parameters Versus Coating Architecture

Solution Precursor Plasma Spraying (SPPS) is a thermal spray process where a feedstock solution is heated and pyrolized to form fine (i.e., ≪1000 nm) molten particles that deposit onto a substrate to form a layer. The benefits of implementing the SPPS process include, among others: (i) the possibility to create unique microstructures at nanometer scale without the injection feeding problems usually associated to powder feeders and delivery cables and (ii) rapid exploration of novel precursor compositions. In this study, preparation and characterization of porous anode layers with homogeneous Nickel distribution and nanometer sized microstructure are considered for solid oxide fuel cell (SOFC) application. Once the solution is injected, the droplets go through several chemical and physical changes and impact the substrate in different states, from fully molten one to unpyrolized one. The effects of some spray parameters, such as the spray distance and the plasma flow mass enthalpy, on the layer architecture and composition were investigated. The results show that dense or porous layers can be manufactured depending on the operating parameters.

Alain Grimaud

Papers

Influence of substrate roughness and temperature on the adhesion/cohesion of alumina coatings

Alumina grit blasting parameters for surface preparation in the plasma spraying operation

Comparison of plasma-sprayed alumina coatings by RF and DC plasma spraying

Suspension Plasma-Sprayed Alumina Coating Structures: Operating Parameters Versus Coating Architecture

Elaboration of Porous NiO/8YSZ Layers by Several SPS and SPPS Routes