The influence of alumina substrate temperature and phase structure : columnar gamma phase, columnar alpha phase and granular alpha phase on splat formation and crystal growth has been studied by SEM and Atomic Force Microscopy. X Ray Diffraction at low angle has allowed to obtain informations on phase structure of layered splats according to substrate phase structure and coating thickness. Column sizes of splats are correlated to a ID model of splat cooling showing the influence of substrate thermal properties and splat thickness on crystal growth kinetic. Finally, coatings adhesion-cohesion values function of spraying parameters are in good agreement with splat morphology and microstructural evolution.

The previous studies indicate that the fabrication of metal matrix composites (MMCs) by cold spraying is effective and promising. When light materials, such as SiC and Al 2 O 3 , were used as reforcements, it is difficult to obtain a high volume fraction of hard phase in the composite just through the simple powder mixture. Therefore, in this study, a Ni-coated Al 2 O 3 powder, which was produced through hydrothermal hydrogen reduction method, was employed aiming at increasing the volume fraction of ceramic particles in the deposited composite coating. It was found that a dense Ni-Al 2 O 3 composite coating could be deposited with the Ni-coated Al 2 O 3 powder under the present spray conditions. X-ray diffraction analysis indicated that the composite coating had the same phase structures as the feedstock. The volume fraction of Al 2 O 3 in the composite was about 29±6%, which is less than that in the feedstock (nominal: 40-45%) due to the rebound of some Al 2 O 3 particulates upon kinetic impacting. The microhardness of the composite coating was about 173±33Hv 0.2 .

The HVOF spraying process has recently been considered for the deposition of dense alumina coatings for dielectric coatings in semiconductor applications and as a turbine blade tip coating in aeroengines. However, due to the lower flame temperature of the HVOF process compared to the plasma spray process it is necessary to have good control over the key process parameters to achieve the correct coating characteristics. The work reported presents the results of a design of experiment study carried out on a TopGun HVOF system used to prepare coatings of alumina. The influence of several key parameters on coating characteristics such as porosity, alumina phase type, microhardness, surface roughness and adhesion have been determined. The parameters varied included oxygen and hydrogen fuel gas flow rates, and spray distance. Based on the results of these investigations recommendations are made on the control of key parameters and the range of coating characteristics that can be expected.

Alumina and stabilized Zirconia were plasma sprayed in air using the water-stabilized plasma torch. In the case of Alumina two different stand-off distances were applied at spraying. Nd-YAG laser was then used for additional treatment of plasma sprayed coatings. The laser was maintained in a quasi-continual regime and defocused from the surface to increase the treated coating's area. Energy density was varied together with the laser scanning velocity to ensure variance in thermal history of the treated surfaces. Microhardness, surface roughness and slurry abrasion resistance (SAR) were measured before and after the laser treatment. Results vary in dependence on the laser treatment's parameters. When the treatment results in substantial changes of the structure (up to a complete re-melting of the surface), enhancement of all measured properties was proven. It is demonstrated that the extent of change of mechanical properties can be correlated with optical properties of coating materials at the laser wavelength. Microstructural aspects of the laser treatment are discussed as well, especially at the boundary between the laser-annealed layer and the basic coating's microstructure. It is pointed out that laser overheating due to use of an extremely high energy density can cause delaminating of the coatings.

Alumina-titania coatings deposited by air plasma spraying (APS) are widely used to protect components against wear at low temperatures. It is known that microstructures formed by the post-laser remelting of as-sprayed coatings exhibit a densification but also numerous macrocracks due to the rapid cooling and thermal stresses. By using the laser-assisted air plasma spraying (LAAPS), the laser beam interacts simultaneously with the plasma torch in order to increase coating surface temperature and possibly superficially remelt the coating. As a result, the microstructure is partially densified and macrocracks, which are generally produced in the post-laser irradiation treatment, can be inhibited. In addition, this hybrid spraying can be done without the post-treatment of coating. In this paper, LAAPS was performed to improve the mechanical properties of Al 2 O 3 -13%TiO 2 coatings. The coating microstructure was characterized by SEM and X-ray diffraction. The mechanical characterization was done by hardness measurements, erosive wear tests and abrasion wear tests. Results showed that laser assistance may induce: (1) the disappearance of vertical and horizontal macrocracks due the laser irradiation in the coatings for a laser irradiation density lower than 34 W.mm-2, (2) an important decrease of the amount of microcracks in the deposited splats, (3) the partial transformation of the metastable γ-Al 2 O 3 phase in the equilibrium α-Al 2 O 3 phase, (4) a hardness increase of 11%, (5) an improvement of the erosive wear resistance by 12% and (6) an improvement of the abrasive wear resistance by 38%.

As already shown 3 years ago, the preoxidation of smooth (Ra < 0.05 µm) low carbon steel substrates in a furnace under a CO 2 rich atmosphere at atmospheric pressure allows the formation of a wustite (Fe1-xO) layer which improves significantly the adhesion (> 55 MPa) of alumina coatings in spite of the rather low roughness (0.10 µm < Ra < 1.00 µm) of the oxidized surface. This contribution is devoted to a more precise study of the wustite layer and its interface with the alumina layer by X-ray diffraction (XRD), Mossbauer spectroscopy and scanning electron microscopy (SEM). Firstly the substrate was oxidized under different temperatures and durations in order to control the oxide layer thickness and structure. Secondly the substrate samples were preoxidized during 15 minutes at 1273 K under CO 2 atmosphere and, afterwards, preheated by the plasma jet in air just before coating. In this case the analysis was focused both on the alumina splat formation and the interface between splat and the oxide layer. Only a non-continuous alumina layer (a few splats) was sprayed: this allowed surface analysis down to the substrate through the alumina layer and the interface. This method avoids any modification of the searched information by a complex specimen preparation as required in the case of transmission electron microscopy (TEM) for example. For the steel surface preheated in CO 2 atmosphere, before spraying, SEM observations and XRD patterns showed the presence of a continuous oxide layer formed by wüstite crystals with an average size of 1-5 µm. After deposition, splats consisted of transitional alumina (γ phase) but the underlayer was no longer pure wüstite. XRD and Mossbauer identified magnetite at the surface of the oxide scale in contact with alumina. This can probably be considered as the result of a partial topotactic transformation of wustite into magnetite, since no morphological change of the oxide layer has been observed. It has been established that this transformation is a consequence of the pre-heating treatment, and not due to any reaction with alumina. It is worth noting that, under these conditions, γ alumina has a spinel structure analogous to that of the magnetite phase with which it was in contact: the alumina structure was possibly induced by that of the magnetite underlayer.

Compound coatings of MCrAlY and alumina shows a sandwich structure with good wear-resistance and intensity at elevated temperature, the coating are usually applied to the furnace roll in modern continuously annealing line or continuously galvanizing line, to prevent the pickup forming on the roll surface. Coatings were prepared by the detonation spraying and the effect of spraying parameter of the denotation gas composition on the microstructure of coatings was investigated. The percentages of MCrAlY and alumina in the coating were determined by the composition of detonation gas mixture. Alumina content in the coating increased with the reduction of the nitrogen dilution in the gas mixture, which resulted in the hardness enhancement of the coating. The microstructure of coatings is different from that prepared by thermal plasma spraying or HVOF.

Plasma-sprayed alumina-waste glass composite coatings on ceramic substrates were produced. Two kinds of alumina powders, different alumina volume fractions, and two glass powders particle size distributions were tested. Post-process thermal treatments were performed. The coatings were characterized by SEM, XRD, Vickers microhardness, fracture toughness, abrasion resistance tests. Coatings superior to traditional tile glazes were obtained with as high as 50 vol.% of waste glass. Fine glass powders (<45µm) must be employed to achieve adequate toughness. A low-cost spray-dried alumina can be used instead of the expensive commercial powders. The thermal treatment enhances the coating properties. A FEM thermo-mechanical simulation was performed. Elastic modulus calculations show a definite coating anisotropy (higher mechanical properties in the longitudinal direction). Compressive residual stresses in the alumina and tensile ones in the glass are developed after the thermal treatment. Crack propagation studies based on Griffith model show cracks initiating from larger pores and propagating easily through the glass, thus explaining the coating toughening achieved through the employment of finer glass powders. Cracks are stopped by alumina; this effect is enhanced in the thermally treated coatings. The numerical and experimental (from indentation fracture toughness test) crack propagation patterns are in good agreement.

Plasma spraying of ceramic nano-powders suspended in a liquid carrier medium is an emerging technology, which allows the formation of thinner coatings with microstructures more refined than conventional plasma spraying. An external injection system, where the suspension enters the plasma jet radially, is installed on a F4-Sulzer Metco dc torch for the production of nanostructured Al 2 O 3 and ZrO 2 coatings. The effect of injection parameters, such as initial droplet diameter, droplet velocity and suspension flow rate is studied. The suspension droplets are continuously generated through an exchangeable micron-sized nozzle with a superimposed pulse of variable ultrasonic frequency. The heat transfer from the plasma to the liquid feed is optimized at high droplet velocity, moderate counter-current injection angle and flow rates not exceeding a threshold value, which depends on the plasma enthalpy and the latent heat of the suspension medium. A significant effect of initial droplet size (220 – 500 µm) or solid concentration (5 – 15 %) is not observed. In-flight particle states are measured for different plasma conditions, and are related to the resulting microstructures by SEM and XRD. High particle temperatures give rise to a refinement in crystallite size, while the particle velocities govern the deposition efficiencies and porosity levels. The results show that the particles follow closely the gas flow in the free stream, as well as in the stagnation boundary layer close to the substrate by virtue of their limited inertia. The prominent difference in microstructure between highly porous alumina and very dense zirconia coatings is explained in terms of particle impact velocities, which are simulated for typical operating conditions as a function of particle size and free-stream gas velocity.

This study assesses the quality of flame-sprayed alumina coatings produced from recently developed alumina cord using argon and compressed air as atomizing gases. Coatings of different thicknesses were deposited on aluminum substrates and then analyzed using optical microscopy, X-ray diffraction, and resistivity measurements. The coatings, particularly those sprayed with argon, had fine microstructure and higher surface and volume resistivity than flame-spray coatings made from alumina cord in the past. They were also found to have higher alpha phase content than plasma-sprayed coatings, regardless of the atomizing gas used. The effect of humidity and the possible formation of aluminum hydroxides are also addressed.

Alumina matrix composites reinforced with metal thin wire (Inconel-600) were successfully fabricated by plasma spray forming. The atmospheric plasma sprayed matrix layers and wire layers arranged by filament-winding technique were piled up alternately. Though the matrix and the wire were partially bonded only on the side which sprayed particles came flying to, a solid structure was obtained by this technique. Spraying in one direction perpendicular to the substrate made peculiar V-shape pores around the wires, but tilting the torch was effective to reduce the pores. The flexural strength of composite did not increase in spite of some crack deflections on the fracture surface. Owing to the wire pullout, however, the composite exhibited a remarkably higher apparent fracture energy than that of monolithic alumina ceramics.

This paper is devoted to the study of alumina splats formation and the adhesion/cohesion of alumina coatings on oxidized 1040 steel substrates. When preheating with the plasma torch, a duplex oxide layer is formed with an upper hematite and an under magnetite. Measurements have been performed on oxide layers with 470 nm average thickness corresponding to preheating times of about 10 min. In these conditions, the relative thickness of each sub-layer can be controlled by monitoring the heating rate and the preheating temperature. When the hematite upper layer is thick, it is mostly broken by the droplet impact and correspondingly the resulting coating has a poor adhesion (≈ 34 MPa) the rupture occurring within the oxide layer. When the hematite upper layer is thin, the droplet impact breaks only partly the oxide layers and the rupture of the coating occurs both within the oxide layer and the substrate surface (≈ 40 MPa).

The effect of an alumina shell on stainless steel particles used in plasma spraying has been studied. The mean size of the injected particles is about 65 nm and the thickness of the alumina shell is 3 µm. The composite powder is plasma sprayed using a PTF4 type plasma gun with an internal injection 3 mm upstream of the nozzle exit. The results show that without preheating the substrate splats are extensively fingered and become circular when the substrate surface is preheated over 200°C. EDS analysis of the distribution of the various elements shows that the alumina either uniformly covers the stainless steel splat or is distributed in pieces over the surface. This behavior has been explained by collecting particles in flight and analyzing them. A composite stainless steel/alumina coating sprayed on a rough stainless steel substrate preheated to 400°C has been examined and compared with a pure stainless steel coating. Both hardness and cohesion are improved for the alumina coated particles due to the random distribution of alumina within the steel matrix.

The plasma-spray process is specified by the associated processing parameters, where these influence the properties of the resultant deposits. This article describes the preparation and processing of composite powders for use in thermal spraying by mixing high purity zircon and alumina powders. The spheroidized powder were obtained by high energy ball milling and rapid solidification from the molten state during plasma spraying. The article discusses the processes involved in spray drying and plasma spheroidization, describing thermal analysis and mullitization kinetics in the spheroidized alumina/zircon mixtures.

For utilization of free-standing ceramic parts produced by plasma spraying it is very important to know the temperature dependence of the linear thermal expansion coefficient and its relation to the porosity of the structure. Zircon ZrSiO4 and gray alumina (96 wt % AI2O3) were plasma sprayed by the water stabilized plasma gun WSP PAL 160. Samples of both materials were cut from thick coatings with respect to their orientation to the gun axis during the gun's horizontal spraying cycling with a constant speed. Thermal expansion coefficients and the differential thermal analysis were performed using SETARAM complex measuring system (up to 1750 °C), the density/porosity was measured by several techniques, such as Archimedean weighing, helium pycnometry, etc. It was found that both, the porosity and the thermal expansion coefficient, change for different locations in the thick deposit due to the varying trajectories of individual particles/droplets in the plasma stream. Measured data for deposits are then compared with data for bulk ceramics. The dependence of the thermal expansion coefficient on porosity in a given location was determined and its general applicability for free-standing plasma spraying is then discussed in the paper.

The effect of the chamber pressure and the spray distance on the splat formation of the plasma sprayed spherical alumina powder was investigated. The velocity and the surface temperature of the sprayed particles were measured by DPV2000. The particle temperature was changed in wide range by the chamber pressure; a solid core remained in the sprayed particle at 30 kPa and the surface temperature reached to the boiling point of alumina over 90 kPa. At the higher temperature, the splashing behavior was explained by the model of liquid film stability. However, in the cooling stage after coming out from the plasma flame, the inhomogeneous temperature in the sprayed alumina particle led to the complex splashing. The temperature distribution in the molten particle is very important to consider the splat formation.

Three kinds of nanostructured alumina coatings were prepared by air plasma spraying. As-sprayed coatings were characterized by SEM and XRD. Their surface roughness, porosity and microhardness were also examined. Results indicated that the smaller the size of the starting powders, the better the properties of the coating. The differences in the phase composition of the as-sprayed coatings could be used to explain the differences observed in coating properties.

In order to obtain nano-structured ceramics composite coating for high temperature application, pre-mixed Al 2 O 3 /Y 2 O 3 powders were plasma-sprayed in this study. Plasma spraying of spray-dried Al 2 O 3 /Y 2 O 3 powder resulted in the formation of amorphous coating of metastable Al 2 O 3 -Y 2 O 3 solid solution. After the heat treatment, α-Al 2 O 3 /YAG nano-structured composite coating was successfully obtained via eutectic reaction between Al 2 O 3 and Y 2 O 3 . It was possible to control the sizes of Al 2 O 3 and YAG particles widely by heat treatment with proper condition. Hardness of the coatings showed close relationship with their microstructure.

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.

For electrical or thermal insulation, the porosity of an air plasma sprayed (APS) coating is an important property to control. Moreover in aggressive environment the interconnected porosity is responsible for the substrate corrosion. To solve, at least partially, this problem, deposition by mutitechniques (APS and a PECVD) was used to close interconnected and opened porosities. In this study, titanium alloy (TA6V) substrates were coated by alumina using either one or both deposition processes. Electrochemical characterization technique was used to evaluate the open porosity in alumina coatings. It consists of evaluating the polarization resistance of the reference sample surface (uncoated substrate) and to compare it to coated ones. After different tests for selecting the electrolyte solution, the influences of different parameters (thickness and relative position) of each deposition process on coating porosity were examined.