Indentation tests to determine the fracture toughness of nickel phosphorus coatings

In this paper, Vickers indentation was used to study the resistance to cracking of electroless nickel phosphorus coatings. For low indentation loads, cracking initiates only at the tips of the indent (primary cracking). It was shown that these cracks are of the Palmqvist type. For higher loads, new cracks, for which initiation sites are located on the edges of the indent, begin to form (secondary cracking). It was found that this change in the cracking mechanism occurs when the primary cracks reach the interface with the substrate. This result shows that the interface resistance to cracking is higher than the cohesion of the coating. In addition, it was possible to apply the same formulae whatever the cracking process, if the total number of cracks was divided by 4, like for the first cracking process. Values of fracture toughness Kc=1.5 MPa m1/2 for a 300 °C treatment and 2.1 MPa m1/2 for a 600 °C treatment were found, independent of the coating thickness.