This work deals with the flattening of alumina molten particles, called droplets, on stainless steel substrates either smooth or blasted and preheated at different temperatures. The blasted surface roughness has been limited to Ra= 1.4 µm to image the flattening droplet. Besides flattening and splat cooling, the wettability of melted millimeter-sized alumina drops on the same substrates was measured. The transition temperature, Tt, has been shown to be different between smooth and rough surfaces. For a smooth surface, Tt, is 170°C, and at 200°C 100% of disk shaped splats are obtained. For the rough surface, Tt is close to 300°C with porous splats, becoming almost dense at 450°C. Close to alumina melting temperature, wettability does not vary with the substrate preoxidation, which may not be the case when the temperature is much over the melting one as in plasma spray conditions.

Atmospheric plasma and oxy-acetylene flame were used to spray alumina-titania micrometer sized particles with respectively 13 wt.% and 45 wt.% of TiO 2 (AT-13 and AT-45). Plasma spraying was also used to spray nanometer-sized- agglomerated particles (AT-13). The enthalpy of spray guns was varied to achieve coatings with different phases and structural characteristics. The influence of the different structural characteristics and the phases of coatings on their hardness and tribological behavior was then studied. The wear resistance was determined by dry elastic contact between an alumina ball, 6 mm in diameter, and the polished coated discs. The ball was moved at a linear speed of 0.1 m/s under a load of 5 N during 20,000 cycles. Drilling tests between a steel drill bit, 12.5 mm in diameter, and the coating surface were also carried out in order to determine the wear resistance under plastic contact. The wear test results showed that AT-13 coatings were more resistant than the AT-45 ones, which was due to the presence of α and γ alumina, phases presenting a high mechanical resistance. On the contrary the resistance of AT-45 coatings, consisting of Al 2 TiO 5 and Al 6 Ti 2 O 13 brittle phases of low hardness, was poorer. Under elastic contact the reduction of the wear resistance of coatings elaborated by flame spraying was not obvious, but under plastic contact the plasma sprayed coatings were more resistant than those deposited by oxy-acetylene flame.

Plasma spraying allows melting totally or partially micrometer sized particles, which flatten in about one is onto the substrate to build the coating by layering resulting solidified splats. The coating adhesion is essential and depends mainly on the behaviour of first lamellae in contact with the substrate. But in the plasma spray process about 108 particles/sec impact onto the substrate, and thus it is difficult to understand the role of the different spray parameters onto the coating quality. In order to get a better understanding of phenomena involved, it is necessary to study a single lamella formation. The experimental set-up is composed of a fast (50ns) two-colour pyrometer and an imaging system, comprising two fast (1 to 10 µs) CCD cameras triggered by the velocity signal of the particle in flight prior to its impact. This work is focused on alumina particles flattening onto stainless steel (304L) substrates preheated at different temperatures during different times.

Suspension plasma spraying (SPS) is able to process a stabilized suspension of nanometer-sized feedstock particles to form thin (from 20 to 100 µm) coatings with unique microstructures. The void network architecture of these ceramic coatings is a challenge to be characterized and quantified using commonly used techniques due to small sizes involved. Nevertheless, the discrimination of these pore architectures in terms of size and shape distribution, anisotropy, specific surface area, etc., is critical for the understanding of processing, microstructure, and properties relationships. USAXS (Ultra-Small Angle X-Rays Scattering) appeared as a suitable measurement technique allowing discriminating the void size distribution over a large range (up to four orders of magnitude). Results indicate that as-sprayed SPS coatings exhibit unusual porous architecture: 1) average void size is about the same than the feedstock one; i.e., nanometer sizes with multimodal void size distribution; 2) about 80% of the voids exhibit characteristic dimensions smaller than 30 nm; 3) the total void content varies between 13 to 20% depending upon considered operating parameters. In-situ annealing measurements were performed as they proved to deliver more relevant results compared to ex-situ measurements: even at temperatures as low as 800°C, the microstructure transforms - while the total void content does not change significantly. Indeed, it has been demonstrated that the smallest voids (equivalent diameters smaller than 50 nm) coalescence was the predominant mechanism and that it was more sensitive to temperature than time.

Alumina-titania plasma spray coatings are widely used for their tribological performances. The combination of these two ceramics in a particular mix percentage permits to manufacture coatings with better wear resistance in comparison to those made of pure alumina. Suspension plasma spraying permit to manufacture sub-micrometer structure coatings very fine structure thanks to precursors which have an initial size of 10 to 300 nm. The use of a liquid feedstock, aqueous or alcoholic, allows the use of nanometer particles directly without the need to agglomerate them to obtain conventional nanostructured micrometer-sized powders. This study aims at studying Al 2 O 3 and Al 2 O 3 -TiO 2 coatings made from aqueous and alcoholic suspensions produced by suspension plasma spraying. Microstructures and phase evolutions are considered. Manufactured coatings present different architectures depending of operating parameters and feedstock particle sizes; the lower the particle diameter, the thinner the microstructure. Phases composition are discussed and compared to conventional micrometer-sized structure Al 2 O 3 and Al 2 O 3 -TiO 2 coatings.

An experimental set-up has been developed, at the SPCTS Laboratory, to produce fully melted, millimeter-sized, ceramic or metallic drops with impact velocities up to 10 m/s. Such impact velocities allow reaching impact Weber numbers, close to those of the plasma spray process (We = 2300). A fast camera (4000 image/s) combined to a fast pyrometer (4000 Hz), allows following the drop flattening. For studding the flattening at the micrometer scale, a DC plasma torch is used to melt micrometer sized alumina particles (around 45 μm). The experimental set-up is composed of a fast (50 ns) two-color pyrometer and two fast CCD cameras (one orthogonal and other tangential to the substrate). The flattening of millimeter and micrometer sized particles is compared. First are studied impacts of alumina drops (millimeter sized) with impact velocities up to 10 m/s. Then are considered micrometer sized alumina particles (about 45 μm in diameter) sprayed with a DC plasma torch. A correlation has been found between both flattening scales and, in spite of the lower impact velocity at the millimeter scale, ejections are also found at the micrometer scales. This work shows that to compare phenomena at the two different scales it is mandatory to have Weber numbers as close as possible in both cases.

Large (3 x 3 x 0.05 m 3 ) refractory pieces (as the ones used for examples in smelters or incinerators) do not sustain regular glazing in a kiln, mostly due to high associated costs. Still, glass coatings could find use on such pieces due to their physical properties (durability, chemical inertia, tightness, etc.). Thermal spraying, using oxyacetylenic flame in particular, appears as a cost-effective solution permitting to circumvent the aforementioned disadvantages. This study aims at evaluating the quality of two types of coatings in terms of permeability. The first type considered coatings (resulting from a previous optimization of the spray operating parameters) sprayed directly on the substrates whereas the second one considered an additional brass underlayer manufactured by twin-wire electric arc spraying. The wettability of the glaze on the refractory substrate and on the brass underlayer was studied to comprehend the coating structural attributes (thickness, porosity, crazing, etc.) as well as their effects on the permeability. A specific measuring device was developed to assess permeability.

Al 2 O 3 -13 % wt TiO 2 thermal sprayed coatings exhibit good wear resistance under tribological conditions, thanks to their high toughness. Alumina/titania coatings with two different structures, and similar titania content, have been elaborated by atmospheric plasma spraying (APS). The aim was to compare the effect of the structure on hardness and wear resistance. As feedstocks were used Al 2 O 3 -13 wt % TiO 2 powders with two different structures: micrometer sized ones, with two size distributions (5 – 30 μm and 15 – 45 μm), and agglomerated nano-meter sized particles (grain diameters between 200 to 500 nm) with a size distribution between 10 and 45 μm. Coatings were sprayed onto low carbon steel (XC38) substrates with several spray parameters, the plasma forming gas used being mixtures of Ar and H 2 (45/15 and 53/17 SLM, respectively). Results show that, with the tribological conditions used in this study, the wear resistance of coatings principally depends of their hardness and not very much on the starting powder morphologies.

Intermediate temperature solid oxide fuel cells include in their design a solid electrolyte layer, usually made of yttria-stabilized zirconia, that acts as an ionic conductor through which oxygen ions diffuse. This layer must be as thin as possible to limit ohmic losses yet have a low leakage rate corresponding to a low level of connected stacking defects such as microcracks. Suspension plasma spraying (SPS) appears to be a viable method for manufacturing such layers and is used in this study to produce gastight coatings that with further improvements may meet the requirements of SOFCs. The paper describes the setup and optimization of the SPS process and the methods used to evaluate the solid electrolyte layers.

This review presents recent developments in suspension and solution precursor thermal spraying and demonstrates some of the tools available to characterize plasma-liquid interactions and the coatings produced. It compares and contrasts the two methods with conventional thermal spraying routes and identifies areas needing improvement.

One of the goals of this study is to better understand how suspension plasma spraying parameters, particularly plasma gas mixtures, influence layer formation. Another goal is to produce finely structured layers of Al 2 O 3 -ZrO 2 with a wide range of architectures. To that end, a simple theoretical model is used to describe the operating conditions of the plasma torch and the influence of spraying parameters is expressed in terms of the shape and size of spray beads.

In this study, nickel-zirconia cermet layers are produced by solution precursor plasma spraying (SPPS) and compared with suspension plasma sprayed (SPS) coatings of similar content. Although nickel is uniformly distributed in both coatings, its presence in the suspension caused problems with the SPS process. With the SSPS process, precursor solutions are fragmented into droplets in which Ni, Zr, and Y are intimately mixed, resulting in very fine microstructure without the problems encountered with the SPS process. It was also found that plasma gas enthalpy and spray distance have predominant effects on in-flight pyrolysis of the elements, and that plasma gas mixture has an impact on porosity as well as the oxidation state of the nickel.

Suspension plasma spraying facilitates the production of thick coatings structured at the submicron or even nanometer scale. Due to the large volume fraction of internal interfaces, nanostructured coatings tend to be superior to their microstructured counterparts. Suspension plasma sprayed oxide ceramics, for example, have higher coefficients of thermal expansion, lower thermal diffusivity and hysteresis, higher hardness and toughness, and better wear resistance. In this work, Y-PSZ thermal barrier coatings are manufactured by means of SPS using two commercial submicron powders with different particle size distributions. By varying spray parameters, several coating architectures and thicknesses were achieved. The coatings were subjected to a series of thermal and isothermal shocks in order to assess the effect of particle size distribution, layer thickness, and substrate roughness on thermomechanical behavior.

Numerous works have shown that decreasing the scale of coating structure leads to an improvement in tribological behavior. Suspension plasma spraying has proven particularly effective at producing coatings with submicron even nanoscale structure, while maintaining the versatility of thermal spraying. This paper examines the dry sliding behavior of several ceramic oxide composite coatings produced by suspension plasma spraying. The structural scale and the effect of composition are studied as well.

This paper reviews various powder treatments and particle alteration processes and evaluates their effect on the microstructure and properties of thermal spray coatings. It discusses the benefits and drawbacks of thermal plasma treatments for powders, the use of self-propagating high-temperature synthesis (SHS), and different ways nanopowders are produced and sprayed. It covers several spraying methods and a wide range of materials, including ceramics, metals and alloys, cermets, and composites. It also covers mechanical alloying and powder milling processes and addresses the potential risks of inhaling nanopowders.

The aim of this work is to investigate the effect of substrate surface chemistry (e.g., oxidation and atom diffusion) on the flattening of a single millimeter-sized alumina drop. To that end, a new technique to produce such drops with different impact velocities has been developed. It consists of a rotating crucible heated by a transferred plasma arc and a piston that controls substrate velocity and, as a result, the impact velocity of the drop. A fast camera working in concert with a fast pyrometer precisely records drop flattening and cooling. This system makes it possible to study interface phenomena, such as desorption and wettability, as well as the effects, at impact, of the kinetic energy or Weber number of the flattening drop.

The aim of this study is to quantify the pore network architecture of thick zirconia coatings deposited by suspension plasma spraying of nano-sized particles. It is shown by FESEM fractography that the coatings exhibit a granular structure with a lamellar layer at the substrate interface and that the average void diameter is orders of magnitude smaller than the resolution of conventional SEM systems.

In the present work, the effect of coating toughness on the wear resistance of alumina-titania coatings was studied. Two parameters were modified: Al 2 O 3 -TiO 2 ratio and particle size distribution. Coatings were obtained by atmospheric plasma spraying on a carbon steel disc. Coating toughness was measured by indentation tests, while wear resistance was determined by ball-on-disc measurements. The results show that coatings with higher toughness exhibit better wear resistance.

The objective of this study is to compare the tribological properties of alumina coatings structured at two different scales, a micrometric one manufactured by atmospheric plasma spraying (APS) and a nanometric one manufactured by suspension plasma spraying (SPS). Coatings architectures were analyzed and their friction coefficient in dry sliding mode measured. Sub-micrometric-sized coatings present a lower friction coefficient compared to micrometric- sized one, thanks to their higher cohesion and smaller structural feature characteristic sizes.

Suspension plasma spraying (SPS) is a fairly recent technology that 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. 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, in particular the spray patterns influence.