S. Dallaire

Temperature evolution of plasma-sprayed niobium particles impacting on a substrate

Abstract A fast two-color pyrometer is used to monitor the temperature evolution of individual plasma- sprayed niobium particles after their impact on a substrate. The experimental setup permits rejection of thermal events corresponding to in-flight particles intersecting the pyrometer field of view, ensuring that only particles impacting on the substrate surface are examined. Cooling rates as high as 10 8 Ks -1 are measured when particles are sprayed on steel and alumina substrates. An interruption of the rapid cooling process and a transient temperature increase are observed near the niobium melting point. Experimental results are interpreted with the help of a numerical thermal flow model taking into account undercooling and the recalescence effects.

Impacting particle temperature monitoring during plasma spray deposition

Instrumentation for monitoring the thermal history of individual plasma sprayed particles as they impact on a substrate is described. A double-wavelength fibre optic temperature sensor is focused on a small spot on the substrate surface to record the cooling rate of particles impacting on this region. Discrimination against in-flight particles intersecting the pyrometer field of view is obtained by a second fibre optic sensor viewing the same spot at an angle and working in coincidence with the first sensor. Typical recorded thermograms are presented and interpreted with reference to a numerical thermal propagation model.

The importance of particle size distribution on the plasma spraying of TiC

Abstract The particle size distribution of the spray material is recognized as an important parameter in plasma spraying, although it is still a subject of controversy. However, it stands to reason that all the particles impinging on the substrate must be in a molten state. Overheating of small particles leads to vaporization and loss in efficiency. Inadequately heated coarse particles result also in reduced efficiency and in poor quality coatings. Titanium carbide is a material of considerable interest, mostly in the hard surfacing industry and for high thermal flux protection. For TiC coatings prepared by plasma spraying, attempts have been made to produce dense coatings. Previous studies on gaseous environments and spraying parameters always resulted in porous deposits. This study assesses the influence of the particle size distribution on deposition efficiency and coating density. Commercial TiC powders were classified in different fractions and sprayed under selected spraying conditions. The results indicate that both the size and particle size distribution of powders play a decisive role with respect to deposition efficiency and coating density.

Protection of graphite electrodes from oxidation

Abstract In high temperature environments graphite has excellent physical properties. However, when it is exposed to oxidizing atmospheres it does not form solid oxides that could provide protection against further attack. Thus, the problem of protecting graphite electrodes against oxidation in air above 1000 °C has not yet been solved satisfactorily. This paper is concerned with a study to reduce oxidation of graphite electrodes with an inexpensive protective coating. The proposed coating was not designed to be inert to oxygen but to lower the graphite loss noticeably during a steel melting operation. This coating is composed basically of two layers that become modified when the coated graphite part is used. The first layer deposited onto graphite acts as a diffusion barrier for carbon. This layer also has to withstand oxidation because pores within graphite could contain oxygen. The second layer behaves as an oxygen retention material. The plasma spraying technique was deliberately selected to perform the coating. Nevertheless, the process chosen was found to avoid noxious reactions that could occur between the coating components. Coated graphite samples were first tested at 1600–1700 °C in air under isothermal as well as under cyclic conditions. After it had been exposed for 6 h at 1700 °C, the coating did not spall off and still provided protection to the graphite. Furthermore, industrial graphite electrodes were coated and tested in a steel making plant. The coating was found to reduce the graphite loss significantly.

Plasma-sprayed TiC protective coatings for fusion devices: Coating fabrication criteria☆

Abstract Plasma-sprayed TiC coatings are of interest as protective coatings for surfaces exposed to high energy fluxes in fusion devices. Thick, dense coatings are required primarily to withstand erosion and sputtering for long times whereas less dense coatings seem preferable for preserving a high thermal shock resistance. This study was carried out to outline the fabrication criteria needed to obtain thick TiC coatings which can withstand severe thermal shocks. TiC coatings with different microstructures (porosities) and thicknesses, plasma-sprayed under atmospheric and inert atmosphere conditions, were exposed to pulsed electron beam thermal shocks. Different microstructures were obtained after spraying TiC powders of various particle size distribution and morphology. The porosity of sprayed coatings ranges from 17% to 29%. The results obtained after thermal exposure indicate that the thermal shock resistance decreases as the TiC coating porosity increases. They also indicate that coatings obtained by spraying a narrow band powder into an inert gas enclosure contain some porosity that could help them to withstand spallation.

Microstructural investigation of RBa2Cu3O7−δ (R=Y, Gd, Nd)

Abstract RBa 2 Cu 3 O 7−δ perovskites, where R is Y, Gd and Nd, were synthesized using the same experimental conditions. The resistivity curves of the three compounds were similar but their microstructures were different. Secondary phases (CuO, BaCuO 2 ) were formed in the compounds containing Y and Gd while no such phases were detected in the Nd perovskite material.

Analytical transmission electron microscopycharacterization of plasma spray synthesized TiB2Fe coatings

Cermet coatings containing TiB2 crystals dispersed in an iron-based matrixhave been obtained by the plasma spray synthesis process developed at the Industrial Materials Research Institute. The main feature of this technique is the combination in a one-step operation of both the synthesis and the deposition process. The resultant coatings are thick and possess a good abrasion resistance. n nIn order to elucidate the fine microstructure of these TiB2Fe coatings obtainedthrough a rapid solidification process and to understand their good abrasion-wear behavior, a transmission electron microscopy study has been undertaken. n nEnergy-dispersive spectroscopy using an ultrathin window detector andelectron energy loss spectroscopy (EELS) techniques, both allowing the detection of relatively light elements, were used. For the range of compositions investigated, EELS was more sensitive for the localization of boron and offered a better spatial resolution. n nThe results have shown that coatings obtained by the plasma spray synthesisprocess contain TiB2 particles ranging from 10 to 700 nm in size dispersed in a matrix of iron and FeTi. The unusual microstructure of these coatings is probably responsible for their good wear resistance.

Interaction of plasma-sprayed YBa2Cu3Ox coatings with alumina substrates☆

Abstract Superconducting YBa 2 Cu 3 O x coatings can be obtained by plasma spraying. Since the as-sprayed coatings do not have an appropriate crystalline structure and are not superconducting, a thermal treatment must be carried out to crystallize them in the appropriate YBa 2 Cu 3 O x phase. During heat treatment, reactions between the substrate and coating occur and, in some cases, may prevent superconducting properties from being obtained. In the present study, YBa 2 Cu 3 O x coatings were deposited onto alumina substrates by plasma spraying and heat treated under flowing oxygen at 950 °C for various periods of time. The modification in coating microstructure was investigated after different heat treatments. A degradation mechanism of the superconducting coatings is proposed.

THERMAL SHOCK EVALUATION OF PLASMA – SPRAYED TiC THICK COATINGS BY MEANS OF THE TEST LIMITER FACILITY OF THE TOKAMAK DE VARENNES

The Tokamak de Varennes has an auxiliary limiter facility which allows various materials to be exposed to plasma discharges. Thick, plasma-sprayed TiC coatings (200–500 μm) have been evaluated The factor which limits the thickness is presently the thermal shock resistance. Samples whose stoichiometry and micro-structure were analysed have been pre-tested using a 15 kW electron beam system. The results were used to optimize the deposition of coatings on hemispherical heads which were tested in the tokamak. With plasma currents of 200 kA, the heads have absorbed 30 kW during 0.7 sec leading to a power deposition of about 15 MW/m2. The coatings tested so far indicated an optimum thickness of about 400 μm which is still too thin for long-term operation. For this thickness, only microcracks (segmentation of the surface) are observed. For thinner coatings, the Inconel substrate was melted, whereas for thicker ones spalling inside the coatings was observed. No contamination of the core plasma by Ti has been observed, although the production of Ti from the surface has been observed, and Ti has been detected on collector probes placed on the vessel wall. AES evaluation of the exposed surface of a TiC flake has shown no significant difference in Ti and C content when compared to the reverse (unexposed) side.

Interaction of RBa2Cu3Ox (RY or Nd) coatings with alumina and zirconia substrates

Abstract As-deposited YBa2Cu3Ox coatings by plasma spraying are not superconducting because of their inadequate crystalline structure and low oxygen content. A postdeposition heat treatment in oxygen is required to restore the appropriate superconducting YBa2Cu3Ox structure. During heat treatment, deterimental reactions between coatings and substrates may occur and lead to the degradation or destruction of the coating superconducting properties. In the present paper, interactions of RBa2Cu3Ox (Rue5fcY, Nd) coatings with alumina and zirconia substrates are examined. The modifications of the coating electrical properties and microstructure are studied using X-ray diffraction, energy dispersive X-ray analysis and resistivity measurements. Coating degradation is shown to occur by diffusion of the barium atoms out of the coating leading to the formation of Y2BaCuO5 and CuO in yttrium-based coatings, and to the formation of nonstoichiometric Nd1+yBa2−yCu3Ox and CuO in neodymium-based coatings. The coating degradation is more important on alumina substrates than on zirconia substrates for both yttrium- and neodymium-based coatings.