Multiple strengthening mechanisms of cold-sprayed cBNp/NiCrAl composite coating

The cBNp/NiCrAl composite coating with 40 vol.% cBN was deposited by cold spray using a mechanically alloyed composite powder. The microstructure of the as-sprayed coating was characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that cBN particles in the size ranges of both nanometers and submicrometers were uniformly dispersed in NiCrAl alloy matrix. The XRD and TEM results revealed that the phase and nanocrystalline structures of the NiCrAl matrix were completely retained from the powder to coating during cold spraying. The hardness test results show that the dense composite coating exhibited an increased Vickers hardness of 1175 Hv compared with 280 Hv of the annealed NiCrAl matrix phase. The high hardness of the composite coating is attributed to the strengthening effects of particle dispersion, work hardening and crystal refinement. The contributions of the individual strengthening mechanisms, including particles dispersion, work hardening and crystal refinement, to hardness were theoretically estimated. The estimated hardness of the composite coating agreed well with the tested value. The work hardening strengthening is the most significant mechanism followed by crystal refinement effect and dispersion strengthening.