Suspension plasma spraying (SPS) is increasingly studied to produce finely structured coatings with dense and columnar microstructures for promising thermal barrier coatings especially in aerospace application. However, this process involves many parameters and complex phenomena with large spans of time and space scales in many physical mechanisms, like droplet break-up, liquid droplet evaporation, and various physical phenomena occurring within the suspension droplet, making it difficult to master. Especially, understanding the interactions of liquid drop submitted to plasma with the submicronic suspended particles is essential for material process optimization and control. For SPS understanding, a meaningful modelling of suspension treatment requires a prior analysis of these physical mechanisms and their characteristic times. This study details the different phenomena, their significance and characteristic timescales as well as the selection of the main governing forces acting between the different continuous and discrete phases (plasma, liquid, submicronic particles). We explore associated mechanisms: droplet breakup, carrier liquid evaporation, convective mixing and submicronic particle diffusion within the droplets. These mechanisms involve mass and heat transfer, that should condition particle agglomeration morphology before melting.

An optimized powder/suspension based atmospheric Plasma Spray (PS) process, using a Triplex Pro 210 TM torch, was implemented to elaborate Cu:TiO2 surface coatings on stainless steel. Nanometric Degussa P25 TM powder was prepared in a water-based suspension and co-sprayed with a Cu spheroidal powder. The bacterial reduction, evaluated with 1h-exposure to Escherichia Coli (E. Coli), was two times higher for the Cu:TiO2 coating compared to the bare stainless steel substrate. Since the coatings obtained by plasma spray are relatively porous, their antibacterial efficacy was compared to smooth Ag and Cu doped titanium nitride (TiN) films obtained by physical vapor deposition technique (PVD). For the same exposure time, the PVD smooth coatings showed a much lower antibacterial efficacy proving the topography effect on bacterial adhesion.

In the last 15 years, the cold spray process has demonstrated a great efficiency for the deposition of metallic powders. In this case, the consolidation of coatings is achieved thanks to the high kinetic energy of unmelted particles exhibiting a ductile behaviour. Dealing with ceramics, cold spray is also of great interest because one can expect properties not reachable with classical thermal spray technologies thanks to lower involved temperatures. However, cold spray of ceramics still remains challenging because of the ceramics intrinsic brittleness. Here, in the specific case of hydroxyapatite and to overcome this brittleness issue, we investigate the role of an intermediate PEEK layer between the substrate and the deposit. We highlight how this sublayer previously deposited by FS or air APS spraying can help improving the consolidation of the coating and its growth.

One of the main levers to reduce CO2 emissions in cars and trucks is mass and friction reduction, which is often achieved through the use of special coatings. The aim of the present work was to develop metal-ceramic-lubricant composite coatings with the best combination of wear, seizure, fatigue, and thermal resistance. Metal-based coatings incorporating hard particles and solid lubricants were cold sprayed onto steel substrates and the relationship between coating microstructure and tribology was studied. To meet the demanding tribological requirements of heavily loaded engines, the interfaces between the different components were optimized by selecting appropriate feedstock powders and assessing a wide range of process parameters. Alumina-reinforced bronze composite coatings were made from powders with different morphologies. Aggregated ceramic powders were found to be more beneficial in terms of wear than massive powders, and graphite was found to be effective for reducing seizure.

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 study, a hybrid plasma spraying process is used to produce particle-reinforced metal-matrix composite coatings on 316 stainless steel. Two injectors are mounted at the output of the plasma gun, one feeding a nickel-base alloy powder, the other feeding a suspension of alumina nanoparticles. Different feed rates, suspension compositions, and alumina particle contents are used and their effects on microstructure, microhardness, porosity, adhesion, and wear behavior are assessed.

In nuclear plants, the replacement of hardfacing Stellite, a cobalt-base alloy, on parts of the piping system in connection with the reactor has been investigated since the late 60’s. Various Fe-base or Ni-base alloys, Co-free or with a low content of Co, have been developed but their mechanical properties are generally lower than that of Stellites. The 4th generation nuclear plants impose additional or more stringent requirements for hardfacing materials. Plasma transferred arc (PTA) coatings of cobalt-free nickel-base alloys with the addition of sub-micrometric or micrometric alumina particles are thought to be a potential solution for tribological applications in the primary system of sodium-cooled fast reactors. In this study, PTA coatings of nickel-base alloys reinforced with alumina particles were deposited on 316L stainless steel substrates. The examination of coatings revealed a refinement of the microstructure. Under the conditions of the study, the addition of alumina particles did not improve the micro-hardness of coatings but improve their resistance to abrasive wear.

The phenomena occurring after injection of water-ethanol suspension of fine hydroxyapatite powder are simulated numerically. The mathematical modeling starts with the calculation of the map of velocity and temperature of working plasma gases. The map is calculated by taking into account the evaporation of the liquids included in the suspension. The suspension is injected through a mechanical injector into the anode-nozzle of the SG-100 torch. The plasma was generated with the use of working gases composed of 45 slpm of Ar and 5 slpm of H 2 and with the electric power input of 30 kW. The initial droplets of suspension were supposed to be spherical with a diameter equal to that of the injector, i.e., 500 µm. The trajectory of suspension was calculated until the evaporation of liquids. Then, the simulation of the movement and heating of solid hydroxyapatite (HA) started. The HA powder was home synthesized and exhibited a bimodal size distribution with two maxima around 3 and 10 µm. The equations describing the momentum and heat transfer from hot gas to the solids took into account the small size of solid particles. In particular, the thermophoresis force, as well as, the drag coefficient modified for non-continuum effect were used in the calculation of the trajectory of small particles. Similarly, the non-continuum effect was considered in the calculation of heat transfer. The obtained trajectories were tentatively correlated with the microstructure of the suspension plasma sprayed HA coatings.

Coatings, a few-millimeter thick, are widely used to protect new mechanical parts against abrasion and erosion or rebuild worn parts. The plasma transferred arc process is a commonly used process to deposit such coatings. It makes it possible to bring about a metal bath inside which melted powders are introduced to form an alloyed coating between the feedstock material and substrate material with metallurgical adhesion. The main parameters of the process are the arc current intensity, plasma and shrouding gas flow rates, distance between the cathode tip and piece, velocity of plasma torch displacement; they all have a notable effect on the produced coating. This study investigates the plasma behavior and properties of the clad by using a design of experiments. The properties of the coating are the dilution level, porosity, and efficiency of material deposition, heat flux transferred to a water-cooled calorimeter, and the hardness in the clad and the substrate to estimate the thermally affected area.

Plasma generated by an SG-100 torch was applied to a spray suspension formulated with the use of ZrO 2 +8 wt% Y 2 O 3 (8YSZ) solid phase. The solids had a mean size of about 4.5 μm and were obtained by milling of commercial Metco 204 NS powder. The suspension was formulated with 20 wt% solid phase, 40 wt% water and 40 wt% ethanol. The plasma spray parameters were optimized with the electric power equal to 40 kW, working gases composition Ar 45 slpm and H 2 5 slpm, spray distance varying from 40 to 60 mm, and torch scan linear speed varying from 300 to 500 mm/s. Coatings with thicknesses ranging from 51 to 106 μm were sprayed onto stainless steel substrates. The porosity of the samples was found from the image analysis of metallographically prepared cross-sections of the samples to be in the range of 8 to 12%. Thermal diffusivity was measured with the use of the commercial NanoFlash system in the temperature range from room temperature to 523 K. The measurements were made with the use of the coatings sprayed on the substrate, and a 2-layer numerical model was developed to determine thermal diffusivity of the coatings. The diffusivity was in the range from 0.196 × 10 -6 to 0.352 × 10 -6 m 2 /s in room temperature depending on the spray parameters. The obtained data were then associated with the literature data of density and specific heat and experimental porosity to find thermal conductivity, which was in the range of 0.47 to 0.86 W/(mK) at room temperature, depending on the spray run.

Plasma generated by an SG-100 torch was applied to spray suspension formulated with the use of ZrO 2 +8 wt% Y 2 O 3 (8YSZ) and ZrO 2 +24 wt% CeO 2 +2.5 wt% Y 2 O 3 (24CeYSZ) as solid phases. The solids have the mean size of about 4.5 µm for 8YSZ and 3.9 µm 24CeYSZ and were obtained by milling of commercial powders Metco 204 NS and Metco 205NS, respectively. The suspensions were formulated with the use of 20 wt% solid phase, 40 wt% water, and 40 wt.% ethanol. The plasma spray parameters were optimized by keeping constant: (i) the electric power at 40 kW (ii) the working gases composition 45 slpm of Ar and 5 slpm of H 2 . On the other hand, the spray distance was varied from 40 to 60 mm and torch linear speed was varied from 300 to 500 mm/s. The coatings were sprayed onto stainless steel substrates to reach the thickness ranging from 70 to 110 µm (8YSZ) and about 70 µm (24CeYSZ). The coating microstructures were analyzed with the use of a scanning electron microscope. Mechanical properties were tested with the use of indentation and scratch tests. The indentation test was carried out with various loads ranging from 100 to 10,000 mN to determine elastic modulus and Martens microhardness. Young’s modulus of the coatings was in the range 71 to 107 GPa for 8YSZ and 68 to 130 GPa for 24CeYSZ. Scratch tests were conducted to determine the scratch macrohardness.

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.

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.

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.

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 study aims to better understand stress fields in composite coatings produced by plasma spraying. To that end, Al 2 O 3 -TiO 2 coatings are deposited under conditions that result in architectures differing in pore content, crack density, and crack orientation. SEM images of the coatings are divided into discrete stress domains that are analyzed by finite elements. FEA simulations show that network architecture has a significant influence on stress fields and that secondary phases have a particularly negative effect. The paper also proposes a generic method for stress analysis based on representative volume elements and points out its advantages and limitations.