COHESION IN PLASMA-SPRAYED COATINGS - A COMPARISON BETWEEN EVALUATIONMETHODS

The integrity, and especially, the cohesive strength of alumina plasma -sprayed coatings prepared from a range of precursor powders of differ ent size and crystallinity has been investigated by several different methods. Several chromia, and alumina-titania coatings, as well us a s intered bulk alumina, have also been studied for comparison and refere nce purposes. Methods of characterisation have included assessment of the coatings resistance to open tribe-system wear by dry panicle erosi on and abrasion on the one hand, and their resistance to mechanical fa ilure during controlled scratch testing and tensile fracture in four-p oint bending, on the other. Controlled scratching was carried out on b oth top-and cross-sections, with the cross-sectional scratching done i nside a scanning electron microscopy (SEM). Bending was carried out, i n ambient conditions, using test equipment small enough to be placed i n the SEMI in order to observe the progressive cracking behavior and f ailure of the coatings. Some correspondence was found between most of the techniques/methods studied, but erosion and abrasion with large, h ard particles and scratch testing on both top surfaces and cross-secti ons provided the best correlation. It is concluded that these an the m ost promising methods found so far to rank the cohesion characteristic s of plasma sprayed coatings. (C) 1998 Elsevier Science S.A.