|Criteri di scelta di materiali per impieghi tribologici nel reattore a fusione ITER
|Tipo di pubblicazione
|Articolo su Rivista peer-reviewed
|Anno di Pubblicazione
|Buttol, P., Ceschini L., Damiani C., and Palombarini G.
Design of ITER (International Thermonuclear Experimental Reactor) involves critical problems concerning selection, testing and validation criteria of materials for components operating under severe environmental conditions. This paper presents the main results of a study aimed at selecting materials for tribological components of the ITER divertor. The main functions of the divertor are to control the purity of plasma and to exhaust helium and solid particles. The divertor is constituted by 60 cassettes (about 20 t each) realised in AISI 316L stainless steel. A remote replacement of cassettes is designed at 1 year intervals to allow maintenance operations. The operating and maintenance conditions are reported in Tables II and III. The cassettes are displaced by movers running on rails by wheels. The movers, in turn, are driven by a rack-and-pinion mechanism. In emergency conditions, the movement must be performed under sliding conditions; therefore, the cassettes must be equipped with sliding pads. Due to the operating conditions of ITER, the materials for the pad-rail couple must display low magnetic permeability, low-friction and adhesion, high wear resistance, as well as good thermal, mechanical and irradiation resistance,. The operating and maintenance conditions, in particular temperature and pressure (UHV), suggest the use of dicalcogenides solid lubricants, such as MoS 2 or WS 2, applied as thin films on sliding surfaces or incorporated into composite materials. The sputtering technique allows to obtain lubricant films wear resistant and adherent to the substrate which, especially in vacuum, also display a very low friction coefficient,. The use of a self-lubricating composite material coated with a solid lubricant could be a solution allowing lubrication also in the case of thin film damage. Relevant properties of some candidate materials for pads are reported in Tab. V. The following amagnetic alloys were considered for the rails: AISI 316L (as reference material for ITER structural components), PH austenitic steels (17-10 P, HNM, AISI 660), Mn steels (Hadfield) and Ni-based superalloys (Inconel 625, 718 and X-750, Nimonic 80A). The AISI 316L and 660, and Inconel 718 were selected for testing, coupled with the following candidate materials for the pads: two DEVA™ (Glacier) sintered metals with WS 2 dispersed into the matrix, and an Al-bronze. The experimental apparatus and preliminary results are reported in Figures from 3 to 6. The results of tests carried out using samples coated with sprayed MoS 2 are also presented. The best results were obtained in any case using MoS 2films, with friction coefficients <0.1. Among the pad materials sliding against uncoated rail materials, the CuSnPb+WS 2 DEVA™ composite displayed very low coefficients (∼0.05), while the NiFeCu+WS 2 DEVA™ composite and the Al-bronze gave unsatisfactory results (0.4-0.5).
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