R.G. Saint-Jacques

The relationship between the microstructure and thermal diffusivity of plasma-sprayed tungsten coatings

Tungsten and tungsten alloy coatings are candidate materials for plasma facing components of divertor plates in future fusion reactors. In normal operation, the sprayed coatings will be submitted to intense heat fluxes and particle bombardment. This work investigated the relationship between the microstructure of plasma-sprayed tungsten coatings and their thermal diffusivity as determined by the laser flash method. The microstructural investigation was carried out on copper-infiltrated coatings. Such a preparation technique permitted the measurement of the total true contact area between the lamellae within the tungsten coatings. The spraying atmosphere was found to strongly influence the interfacial contact between lamellae and coating thermal diffusivity.

Thermal diffusivity of plasma-sprayed tungsten coatings

Abstract Refractory metals such as tungsten and molybdenum are currently considered as potential materials for the plasma-exposed divertor surfaces in future tokamak reactors. Tungsten and tungsten alloys are of interest owing to their low sputtering yield and good resistance to chemical erosion by hydrogen compared with carbon-based materials. In the present paper, the influence of the spraying parameters on the thermal diffusivity of tungsten coatings plasma sprayed on copper coupons was investigated. Experiments were carried out using a Taguchi fractional approach, and the thermal diffusivity of plasma-sprayed coatings was determined using the flash method. The results show that the spraying atmosphere is the dominant factor affecting the coating thermal diffusivity; the gun transverse velocity and the powder feed rate have a smaller influence. The arc gas and power were found to have no significant effect on the thermal diffusivity of the sprayed coatings.

Behaviour of plasma-sprayed TiC coatings under H and He irradiation

Abstract The effects of H and He irradiation on plasma-sprayed TiC coatings have been studied. The effects were studied by SEM and TEM microscopy, and by H depth-profiling with the ERD technique. Samples prepared by CVD were also implanted for comparison. In the case of H, bubbles have been observed during an in-situ implantation inside the TEM. In order to understand the absence of hydride, H depth-profiling was performed. The H saturation concentration is about 20 at.% or slightly more (the uncertainty is due to the effect of the porosity of the coatings on the ERD technique). Helium bubbles are observed above 10 15 He/cm 2 . Blisters are formed on polished samples above 10 17 He/cm 2 . In contrast, the high roughness of non-polished samples prevents blister formation. CVD samples behave essentially like the polished samples.

Enhanced resistance of plasma-sprayed TiC coatings to thermal shocks

In previous tests the maximum observed TiC coating thickness which showed good resistance to 0.7 s thermal shocks was 400 μm. For this thickness, only microcracks perpendicular to the surface (segmentation) were observed, whereas for thicker ones, spalling inside the coatings occurred. With a combination of substrate surface modification (macroroughening or spraying a bondcoat) and preheating to 375°C before the thermal shocks, it has been possible to completely avoid the delamination during heating and to promote the resistance to delamination on cooling. This allows a doubling of the thickness of thermal shock resistant coatings up to 1mm. Hemispherical coated limiters were tested in TdeV (Tokamak de Varennes) with plasma currents of 210 kA and have absorbed 20 kW during 1.2 s associated with a power deposition factor of about 10 MW m −2 s 0.5 .

Characterization of Si nanocrystals by different TEM-based techniques

Silicon nanocrystals (Si-nc) embedded in SiO(2) matrix and obtained by ion implantation (50keV, 1.0x10(17)Si/cm(2)) were characterized by means of three different transmission electron microscopy (TEM) techniques: Dark Field (DF), Scanning TEM Annular Dark Field (STEM-ADF) and Z contrast. The strengths and weaknesses of each technique for the characterization of the Si-nc were evaluated and discussed. DF imaging, which has the best contrast, was chosen to give the average Si-nc size evaluated to 5.6nm. On the other hand, STEM-ADF, which is only sensitive to the crystalline phase, provided an evaluation of the Si-nc density of 3.27x10(17)nc/cm(3). Finally, comparison between the STEM-ADF and Z contrast imaging permitted to evaluate the amorphous phase remaining after the annealing to around 12%.

Étude de déformations induites par précipitation d'ions H au cours du recuit du systéme Ag-H implanté

Abstract Surface deformation of low-temperature (15 K) H-implanted thin Ag films ( ~ 70 nm) with fluences above 4 × 1017H/cm2 (i.e. H/Ag ~ 0.35) was observed by annealing at ~ 100 K at a speed of ~ 2 K/s. By TEM and SEM studies, one can identify this deformation with blister formation in the samples. Using the gas-driven blistering model and the stress-driven model, the following mechanism was proposed: By annealing, the H atoms dissolved in the host lattice at low temperature precipitate very rapidly at ~ 100 K in the form of overpressurized microbubbles. This induces strong stress in the thin films and hence blistering. The application of the stress-driven model formula in this particular case leads to a satisfactory result.

High heat load testing of plasma sprayed W coatings

The influence of five spraying parameters on the thermal shockresistance of plasma sprayed tungsten coatings was evaluated with a pulsed electron beam gun. The pulse duration was 0.2 s and the absorbed power density 60 MW/m2. Two series of samples were tested. Both were plasma sprayed in controlled inert atmosphere, one at atmospheric pressure (AP) and the other at low pressure (LP). The porosity seems to be a positive factor for thermal shock resistance: the cracks are more numerous and thinner in less dense specimens. Moreover, the coating thickness is a crucial factor. Indeed, the 100 μm thick coatings (LP and AP) showed no delamination whereas 1 mm thick AP coatings suffered edge delamination.

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.