Thermomechanical characterization of B4C vacuum plasma sprayed coatings on stainless steel tubular substrates

Abstract

Boron carbide coatings have attracted increasing interest because of their promising properties. Their utilization for thermonuclear plasma-facing components and, in particular, as armour for the separate tubular first wall [1] could contribute to limiting the radiating power losses due to impurity particles and simultaneously protect the structural material from excessive thermal fatigue. In view of such applications, a characterization of the coatings with respect to the properties affecting their therm0mechanical behaviour in the presence of thermal shocks must be performed [1]. Feasibility tests carried out recently on B4C [2] have shown that vacuum plasma spraying (VPS) is a promising technique for producing thick coatings (up to 2 mm). Another technological aspect to be assessed is whether B4C can be deposited on an AISI 316L stainless steel tubular substrate of significant length (350 mm) and diameter (33.7 mm) without damage or detachment. The coatings were deposited on a 400 ~m thick cermet bond coat of molybdenum and B4C with a composition ranging from pure Mo on the stainless steel to pure B4C. The pure B4C coatings on the bond coat reached layer thicknesses of 500, 1000 and 1500 ktm. The absence of microcracks at the stainless steel/ bond coat interface and inside the B4C layer was first assessed by optical metallographic examinations, but later scanning electron microscope (SEM) examinations showed a significant amount of porosity (Fig. 1). Pure B4C ring specimens (Fig. 2) obtained by cutting away the stainless steel and the bond coat and grinding all of the external surfaces were used to