Cohesion in plasma-sprayed coatings - A comparison between evaluation methods

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