Toughening mechanism for Ni-Cr-B-Si-C laser deposited coatings

Laser deposited coatings were made from Colmonoy 69 Ni-Cr-B-Si-C alloy and Nb-modified Colmonoy 69 using laser cladding with powder injection. Addition of Nb was done to decrease the structural scale of Cr boride precipitates by providing Nb-rich nucleation agents. The purpose of the study was to evaluate the viability of microstructural refinement as a toughening mechanism for Ni-Cr-B-Si-C alloys. The results show that although a significant refinement of the Cr-rich precipitates while preserving the original level of hardness could be induced in these alloys by a suitable addition of Nb, cracking susceptibility of the deposits was not decreased. This is attributed to the continuous network of hard eutectics providing an easy route for crack growth. The outcome of this work points out that an effective toughening mechanism for Ni-Cr-B-Si-C alloys should include not only refinement of the hard precipitates, but also modification of the eutectic structure.