Thermally sprayed 75Cr3C2-25NiCr coating is applied on structural components to improve corrosion, oxidation and erosion behaviour at high temperature. In order to understand the microstructural and mechanical behaviour of coating a long term investigation was carried out at 650 °C. The Objective of the present work is to study the effect of long term heat treatment (i.e. 650 °C/ 3000 h) on various characteristics of the detonation gun sprayed 75Cr3C2-25NiCr coating. Hardness and nanoindentation studies were carried out on Cr3C2-NiCr coating after fixed time intervals. The morphological and microstructural features of the coated surface were characterized using scanning electron microscope (SEM). The phases formed during the long term exposure were identified by x-ray diffraction (XRD) technique. The coating showed the presence of various Cr, Ni, based oxides, Fe and Ni based spinels. Vanadium, niobium, and molybdenum showed maximum tendency to diffusion from substrate to the coating. Hardness of the coating reached a maximum value of 870 HV after 2160 h of exposure. The hardness of coating observed an initial drop in hardness due to carbide coarsening which was recovered by precipitation of carbides from the solid solution. © 2019 IOP Publishing Ltd.