Four-point bending tests of thermally produced WC-Co coatings

The use of hard coatings is limited by their brittleness and cracking when they are submitted to loading. In this study, a four-point bend test combined with acoustic emission measurements was used to measure the curvature when WC-Co plasma (atmospheric plasma sprayed) and HVOF (high velocity oxy fuel) sprayed coatings break. The coatings were deposited onto a 30NCD16 beam of rectangular cross-section. The heterogeneous beams were tested with the coating in tension. The APS and HVOF coatings are compared. The influence of the state of the surface before spraying and the influence of a post-heat treatment were studied. The role of residual stresses determined by a step-by-step hole drilling method, is also discussed. The results presented show the capacity of the four-point bend test to characterize the toughness of a coating on its substrate.