Properties of low friction anti-seize coatings for fusion applications

Remote maintenance in fusion machines such as JET (Joint European Torus) and ITER (International Thermonuclear Experimental Reactor) relies on sliding interfaces such as bolted joints. Experience in JET, where removal torques much higher than installation values with uncoated bolts is commonplace, led to the installation of experimental bolted assemblies in 2015: the first of its kind in JET. These assemblies included some 660B stainless steel ITER Blanket-specific bolts with a solid sputtered coating of MoS₂ as part of an ITER-funded project known as CHEF (Coating and Humidity Experiments on Fasteners). CHEF also includes two ex-vessel activities. The first is a characterisation of the coating fundamental properties including outgassing, friction, wear and sensitivity to humidity: on flat sample discs. The second is the measurement of the coating performance on bolted assemblies subjected to a sequence of tests: initial tightening cycles, thermal vacuum exposure, environmental exposure (high humidity and high temperature), repeated thermal vacuum exposure, and final tightening cycles. This sequence approximates to a possible operation cycle of ITER bolted joints which receive an accidental humidity exposure during operation.