This study assesses the influence of atmospheric plasma spraying parameters on splat stacking and porosity formation in bioglass coatings prepared from commercial powders. Coating samples were deposited on stainless steel substrates using spraying parameters established through numerical simulations. Different Ar-H 2 mixtures were used as the forming gas, and plasma current and spraying distance were varied. Coating microstructure and phase composition were determined by SEM and XRD analysis. Although numerical simulations for each parameter set predicted a suitable Sommerfeld number for proper splat stacking, Na 2 O and P 2 O 5 volatilization occurred during spraying, promoting the formation of porosity in the coatings. Denser coatings were obtained, however, by adjusting the gas mixture ratio, plasma current, and spraying distance such that enthalpy of the plasma jet is sufficient to overcome the glass transition temperature of the powder and at the same time avoid the evaporation of volatile oxides.

In surface science of functional oxides, Titanium dioxide (TiO 2 ) is one of the most investigated crystalline systems either in rutile or anatase phases. Rutile phase is widely used in microelectronic, tribological applications and in the conversion of solar energy. Anatase phase is used in self-cleaning, antifogging, photo-catalytic and biomedical technologies. This work focuses on studying the required process conditions to obtain TiO 2 targets by APS onto metallic substrates using commercial TiO 2 powders (Oerlikon Metco in Switzerland) with suitable physical and chemical properties for technological and medical applications as PAPVD coatings. APS targets were compared to sintered ones. The raw powders were characterized by laser diffraction, SEM, XRF and XRD while the characteristics of the APS-deposited targets as well as the sintered ones were determined by SEM and XRD to identify the constituent phases. This work allowed confirming the advantages and limitations of both processes in terms of grain size, chemical composition, microstructural homogeneity and density in order to choose the best option to manufacture targets for PAPVD coatings for technological and medical applications.

In this study, bioactive glass powders were synthesized from four different types of oxides (SiO 2 , P 2 O 5 , CaO and MgO). These oxides were mixed, melted, milled and sieved to produce powders with two chemical compositions of the 31SiO 2 -11P 2 O 5 -(58-x)CaO-xMgO system. The powders were plasma sprayed onto AISI 316L stainless steel and Ti6Al4V titanium alloy substrates using a F4MB Sulzer Metco gun. The physical and mechanical properties of coatings, as well as their bioactivity were evaluated. The bioactivity tests were carried out exposing the surface of coatings to simulated body fluid (SBF) during 1, 9 and 15 days. The thickness and hardness of apatite layer produced on the surface of each coating during bioactivity tests were evaluated. The results indicate that the thickness of apatite layer formed during 15 days in SBF is between 31 and 51 µm and its hardness is between 1.5 and 1.9 GPa according to the chemical composition of feed stock powder used to manufacture the coatings. Additionally, the harness of bioglass coatings decreased around 26% after to expose them to SBF.

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.

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.

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.

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.

In this study, micro and nanostructured alumina coatings were plasma sprayed on steel substrates using different combinations of spraying parameters. Test results show that the nanostructured coatings have lower hardness, a lower friction coefficient, and higher wear resistance than microstructured alumina coatings.

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.

Intermediate temperature - solid oxy-fuel cells (IT-SOFCs) include in their design a solid electrolyte layer made of yttria-partially stabilized zirconia (Y-PSZ), an ionic conductor, through which oxygen ions diffuse. This layer needs to fulfill several characteristics among which a low leakage rate corresponding to a non-connected pore network and a low level of stacking defects such as microcracks or globular pores. Moreover, the thickness of this layer needs to be as low as possible (about 20 µm) in order to limit ohmic losses. Suspension plasma spraying (SPS) appears as a potential technological route to manufacture such layers structured at micrometric or sub-micrometric scales. In SPS, a stabilized suspension, made of a liquid, solid particles and a dispersant, is injected within the plasma flow. The liquid is very quickly fragmented and then vaporized and the individual particles, or the particle agglomerates, depending on the average size and morphology of the solid feedstock, are heated and simultaneously accelerated towards the substrate surface where they impact, spread and solidify, analogously in a first approximation to larger particles, to form a layer. The architecture of the layer is very closely related to plasma operating parameters (from which derive plasma flow stability), from the suspension characteristics, in particular the feedstock particle size distribution and from the suspension injection parameters. This work aims at presenting recent developments made to optimize some of these operating parameters to maximize the electrolyte layer characteristics.

When spraying ceramic particles with a low thermal conductivity such as zirconia using Ar-H 2 direct-current (d.c.) plasma jets where the heat transfer is important, heat propagation phenomena take place with the propagation of melting, evaporation or even solidification fronts. Most models neglect these heat propagation phenomena assuming the particle as a lumped media. This work is aimed at developing a model coupling the effect of heat propagation with the particle dynamic within plasma jets. It uses an adaptative grid in which the coordinates of the phase change fronts are fixed. It allows minimizing the calculation costs (approximately 10 seconds on PC under windows XP against 1hour with an enthalpy model). Such calculations are illustrated for dense and porous agglomerated zirconia as well as iron particles which evaporation in an Ar- H 2 (25 vol %) plasma is important.

This work is concentrated on plasma sprayed cermet coatings consisting of stainless steel (SS) (17 wt % Cr and 12 wt % Ni) and chromium oxide (Cr 2 O 3 ). These powders were sprayed simultaneously, however being injected separately to account for the drastic difference in their thermo-physical properties. Chromium oxide was injected internally and stainless steel externally. The plasma parameters (arc current, hydrogen vol %, mass flow rate) were optimized as well as the injector positions. Coatings were achieved with different mass ratios of SS and Cr 2 O 3 . All exhibited a lamellar structure with well distributed Cr 2 O 3 and SS lamellae. They were then systematically characterized by their phase content (XRD), composition (EDS), Vickers micro hardness, morphology (SEM), slurry and dry wear resistances. Finally the best dry linear abrasion resistance was obtained for the pure chromium oxide coatings, while the best slurry wear resistance corresponded to coatings with a mass ratio SS/ Cr 2 O 3 of 56/44.

The aim of this paper is to study the influence of plasma spray set-up parameters, such as the hydrogen percentage in the plasma forming gas and arc current, as well as powder injection parameters: the carrier gas flow rate and injected particle size on the melting, vaporization and velocity of particles of a Fe-Cr based alloy and stainless steel AISI 316L. Particles were collected in flight in a cylinder where they were quenched by argon jets to prevent there flattening at impact and chemical interaction with their environment. After collection, particles were analyzed by SEM, EDS and X-ray diffraction. When the injected particle size was between 60 and 100 µm, particles collected in flight exhibit complex shapes with a mean diameter of 40 µm, and the higher was the percentage of hydrogen the smaller was the resulting mean diameter. This probably corresponds to an inefficient cooling by the argon jets resulting in particles impacting at the bottom of the collector and partially exploding. When the injected particle size was between 10 and 35 µm, collected particle were nearly spherical with almost the same mean diameter, but then the oxidation changed with spray parameters. Measurements of temperature and velocity of the in-flight particles during plasma spraying were done using the Spray Watch (Oseir, Finland) equipment. Relationships between in-flight particle parameters and characteristics of the collected particles were found. Abstract only; no full-text paper available.

The purpose of this work is to optimise the chemical composition of perovskite coatings prepared by injecting a suspension of submicrometric LaMnO 3 perovskite particles (d 50 =~ 1 µm) in a direct current (d.c.) plasma jet. The perovskite powder composition, the particle size and the plasma parameters were modified in order to diminish the manganese evaporation. The process consists in mechanically injecting a well dispersed stable suspension of submicrometric perovskite particles in a dc plasma jet. In the process, large suspension droplets (~300 µm) are sheared into tiny ones (a few µm) by the plasma jet flow. Then the solvent is evaporated and the particles melt resulting in perovskite droplets of about 1 µm impacting on the substrate, the coating resulting from their layering. Such coatings are to be used as cathodes for the SOFCs (Solid Oxide Fuel Cells). Best results were obtained by injecting a stable suspension containing a 10 mol% MnO 2 doped perovskite powder with 3 µm particle size in an Ar plasma forming gas and 300 A of current intensity.

The numerical forecast constitutes an interesting way for plasma spraying to minimize the number of experiments to achieved the optimum spraying conditions. Many computational codes have been developed to predict the properties of the plasma jet (velocity, temperature) and the particles behavior within (temperature, velocity, melting state). According to the particle injection orthogonally to the plasma jet, the models have to be 3D. However, such codes need several hours if not several days of calculations to obtain the results of one condition. This is the main drawback of the existing sophisticated codes. The computing time is not compatible with industrial needs. Various clever numerical methods were developed in the past to simulate 2-D parabolic gas flows for laminar boundary layers or jets. For example, the Genmix 2-D axi-symmetric algorithm developed by Spalding and Patankar, and known as the Bikini method requires a very low-cost memory and computing time. This algorithm makes it possible, when using the proper thermodynamics and transport properties of plasma gases and the whole equation of Boussinesq-Oseen-Basset and taking into account the thermophoresis and non continuity effect for small particle, to predict in a fast and rather realistic way, the velocity and temperature fields of the plasma jet.

A perovskite suspension was air plasma sprayed (APS) with a direct current (dc) plasma jet. The process conditions including the standoff distance, plasma forming gases and their ratio influence on the perovskite deposits and splats characteristics were investigated. Particularly the molten state of particles, phases obtained, morphology and composition of splats were studied. The process consists in injecting a well dispersed stable suspension of submicrometric particles. The suspension was produced by attrition milling LaMnO 3 perovskite powder (mean particle size of 0.8 µm). The plasma forming gases influences drastically the obtained phases in deposits. When the plasma gas mixture contains hydrogen the heat transfer is too high and the perovskite phase is decomposed but when the plasma forming gases is Ar/He or only Ar, just a very little quantity of perovskite is found to be decomposed in the plasma jet. With Ar/He, perovskite splats are well melted, but with argon there is a high percentage of semi-melted and non-melted particles.

Stainless steel particles have been covered with an alumina shell by the mechanofusion process in order to reinforce stainless steel coatings by uniformly distributed alumina particles. Two stainless steel particle size distributions (PSD) in the range of 65 µm and 120 µm were tested. It was found that the mechanical energy input induced a spherical shape of the final composite particles with a controlled shell thickness (3 µm and 2 µm respectively) without forming new phases that usually take place during the mechanofusion process. The new spherically-shaped composite particles were sprayed in air with a D.C. plasma torch working with an Ar/H 2 mixture as plasma forming gas. At mid-flight, two types of composite particles were detected : the first case corresponded to well molten particles where all the alumina shell has flowed to the tail of the particle ; the second case was related to particles which still retained some evidence of the alumina shell uniformly distributed around the stainless steel core. When the mechanofused particles were sprayed onto a cold smooth substrate (stainless steel 316L, Ra<0.05 µm), the resulting splats were extensively fingered and became disk shaped when the substrate surface was preheated over 300°C. However, alumina was either spread exactly on the stainless steel splat corresponding to well molten particles or dispersed in fingers and frozen over the surface of the stainless steel splat corresponding to particles covered by the broken alumina shell. An important effect of fine particle size on in-flight droplet behavior is detected because the center of gravity is more decentred than that of coarse particles influencing the deposit build-up. The composite stainless steel/alumina coatings sprayed on a rough stainless steel substrate (Ra = 6.7 ± 0.3 µm) preheated to 200 °C are compared to those of pure stainless steel. Hardness and adhesion/cohesion of deposits formed with fine particles were found to be improved comparatively to a pure stainless steel deposit. However, when coarse particles are used, the value of hardness is decreased and works is in progress to understand this phenomenon.

A new process has been developed to incorporate graphite particles in a stainless steel coating during its formation. Four means have been tested to inject the graphite particles outside the plasma jet and its plume : graphite suspension, a graphite rod rubbed on the rotating sample, powder injection close to the substrate with an injector or an especially designed guide. The latter process has been shown to be the most versatile and the best controllable one. It allows to incorporate uniformly between 2 and 12 vol % of graphite particles (2-15 µm) within the plasma sprayed stainless steel coatings. A 2 vol % seems to give the best results : a low decrease (6%) of the coating hardness and the best results in dry friction studied with a pin on disk set up. In this case, depending on the sliding velocity (0.1 to 0.5 m/s) and loads (3.7 to 28 N) the dry friction coefficient against a 100C6 pin is reduced by a factor between 1.5 and 4 compared to that obtained with plasma sprayed stainless steel.

The aim of this study is to achieve, on aluminum alloy substrates, coatings with higher hardness and better dry tribological behavior than that of the substrate. Using atmospheric plasma spraying and a vibratory fluidized bed powder feeder, coatings consisting of a stainless steel matrix with embedded graphite particles were successfully deposited. The dry coefficient of friction of the composite layer is comparable with that of a commonly used stainless steel surface. SEM examinations of polished cross-sections reveal a lamellar structure with randomly distributed graphite inclusions. Paper includes a German-language abstract.