Offset bending for interfacial toughness of plasma sprayed ceramic coatings

A modified three-point bending technique has been proposed to measure the interfacial toughness of thick plasma sprayed ceramic coatings. Four different ceramic materials (titania, yttria stabilized zirconia, alumina, and yttria) on steel substrate were investigated resulting in a range of toughness values. Plasma sprayed coatings were prepared on grit blasted substrates with two different roughnesses and subsequently a notch in the centre of the coated beam was fabricated using a SiC saw. A two-step three-point bend test was conducted: first, the notched sample was loaded with a small offset to create pre-crack. The pre-cracked specimens were further loaded with offset 0.5 to measure interfacial toughness. Crack propagation manifested as a pop-in in the load-deflection plot. Subsequently, finite element analysis was used to calculate interfacial toughness, G_c, corresponding to the pop-in load. The visualization of crack propagation after pop-in load suggested that the crack had propagated along the interface in all four coatings. The widely accepted tensile adhesion test (ASTM C633) was separately performed on the same four coatings to compare the results obtained from the offset bending test. A correlation between interfacial toughness measured using offset bending and adhesion strength measured using the tensile adhesion test has been proposed.