Adhesion testing of thermally sprayed and laser deposited coatings

Commercial coatings were deposited on low-alloyed steel tubes (Fe1Cr0.5Mo) by arc spray, HVOF (high velocity oxy fuel) and laser cladding. The adhesion strength was tested with two methods: acoustic emission and a combination of four point bending and metallography. The agreement between the results obtained from the two different experimental techniques is very good. Laser coatings showed no delamination for strains up to 15%, while coatings deposited with the arc spray and HVOF processes delaminated in the strain intervals 1.4–1.9% and 0.8–1.8%, respectively. The suggested delamination mechanism is the initial formation of a radial crack in the coating after which the coating/substrate interface comes under an increased tension load and fractures. Arc sprayed coatings of Metcoloy 2 (Fe13Cr) mixed with the binder 80Ni20Al show a strongly improved adhesion strength if the splat size is sufficiently large. The delamination interval increases to 10.5–11.5%. However, for small splats the effect is eliminated.