Surface and Coatings Technology, 61(1993) 67—71 67 Thermal diffusivity of plasma-sprayed tungsten coatings Christian Moreaua Philippe Fargier-Richard’~ Robert G. Saint~Jacquesb and Paolo Cieloa ~Industrial Materials Institute, National Research Council Canada, 75 boulevard de Mortagne, Boucherville, Qud. J4B 6Y4 (Canada) 1’Institut National de Ia Recherche Scientifique-Energie et Matériaux, 1650 Montée Ste-Julie, C.P. 1020, Varennes, Qué J3X 1S2 (Canada) 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 factorial 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. 1. Introduction erosion by hydrogen such as carbon-based materials. One of the main disadvantages of these high Z materials Plasma spraying is a versatile technology for coating is the important radiation energy loss associated with a large variety of substrates with any material that can the presence of partially ionized impurities of these be melted without decomposition. The technique may materials in the plasma. However, the ionization distance be highly automated and robotized, assuring good repro- in the plasma is shorter for tungsten than for lighter ducibility of the coating properties [1, 2]. Plasma spray- elements, favoring a rapid redeposition of the sputtered ing is currently under investigation as a coating atoms on the divertor plates, and thus low plasma technique to protect and repair surfaces exposed to the contamination by these sputtered impurities [7]. plasma in fusion reactors [3—5].In such an environment As discussed above, the divertor plates will be exposed the exposed surfaces are submitted to intense bombard- to severe thermal loads during the reactor operation, so ment of particles and heat fluxes, leading to severe the thermal properties of materials used on these plates erosion and damage. For example, the divertor plates of are critical parameters and must be determined. the future machines (ITER and NET) will have to Tungsten coatings are also used in ingot molds owing sustain, in normal operation, heat fluxes of the order of to their good resistance to molten metals [8], and 10 MW m 2 and will be eroded by sputtering at a rate characterization of their thermal properties for this ranging from 5 to 230 mm a year depending on the application is also useful. Plasma-sprayed coatings are nature of the exposed surfaces [3, 4]. Moreover, during built by the successive accumulation of molten or par- plasma disruptions (off-normal operating conditions), tially molten droplets spreading on the substrate surface the heat load can reach 10 MJ m2 deposited in 0.1 ms, and forming thin lamellae. The thermal contact between leading to severe localized damage of the divertor plates. these lamellae is not, in general, perfect and is limited Thus the possibility of repairing the exposed parts in by the presence of thin pores or secondary phases (e.g. the reactor using plasma spraying is an attractive solu- oxides and nitrides) at the interface between the lamellae tion to this tremendous materials problem. [9, 10]. Thermal resistance at the lamella—substrate Carbon-based materials (e.g. graphites, boron-doped interface has been estimated by different workers to graphites and carbon-fiber composites), beryllium and range from negligible values to values of the order of refractory metals such as tungsten and molybdenum and 10-s m2 KW 1 [10— 13]. Recently, thermal resistance their alloys are currently considered as potential candi- was determined from direct measurements of the lamella dates for the plasma-exposed divertor surfaces [3, 4]. cooling rate by optical pyrometry during and immedi- Tungsten and tungsten alloys have a low sputtering ately after the impact. For example, a thermal resistance yield (less than 10.6 compared with 2 x l0~ for ber- of 3 x iO~m2 K W1 was found at the lamella—lamella yllium at 75 eV) [6] and are not subject to chemical interface in molybdenum coatings [14] and higher resist- Elsevier Sequoia