Characterization of Ni-based coatings on carbon steel by electron microscopy

• Electroplating and pack cementation successfully developed NiCoCrAl coatings.
• The hardness and surface morphology may be influenced by the Co content.
• The temperature of the coating process affects the deposition of the coating.
• Most phases are simply shown by electron diffraction and HR-XTEM images.
• We report a detailed description of the coated layer.

Coatings formed from NiCoCrAl were deposited by a combination of electroplating and pack cementation on a carbon steel substrate. The effect of Co concentration and temperature on the oxidation and hardness properties of the carbon steel was studied. The microstructure and morphology of the coatings were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microhardness of the coated samples was evaluated by a Vickers microhardness tester. FeNi(5%)CoCrAl coated sample exhibited better oxidation and hardness properties than that of FeNi(1%)CoCrAl coated sample. The microstructure and phase constitution of these coatings developed at 800 °C were similar. Samples coated at 800 °C and 1000 °C consisted of three and two layers, respectively. The formation of an intermetallic layer in all the coatings was confirmed. The γ-(Ni,Fe), β-(Ni,Al) and ζ hexagonal structures were identified in the coated layers of all samples, while orthorhombic Al3Ni was only identified in the samples coated at 800 °C. An analysis of the correlations between the structure, hardness properties, oxidation behaviors, and phase formation is also discussed.