Chromium carbide (Cr 3 C 2 ) High velocity thermal spray coating Carbide based thermal spray coatings are routinely applied to mitigate erosion under industrial conditions. However, the mechanism of erosion response under aggressive high velocity impact conditions remains unclear. In this work Cr 3 C 2 -25%NiCr thermal spray coatings were eroded at an impact velocity of 150 m/s by 20-25 µm alumina grit. Coatings were deposited by High Velocity Air Fuel (HVAF) and High Velocity Oxygen Fuel (HVOF) thermal spray techniques to generate a range of coating quality spanning that applied industrially. In Part 1 of this two-part series, the mechanism of erosion as a function of coating composition and microstructure variation is discussed. The HVOF coating underwent significant in-flight dissolution of the carbide phase. The erosion response of the supersaturated NiCr matrix was characterised by brittle cracking and fracture. The HVAF coating retained a high carbide content with minimal phase dissolution. However, the rapid solidification of the matrix material made the coating prone to brittle interphase cracking during impact. On a larger scale, splat based erosion mechanisms played a significant role, especially in the HVOF coating. The mechanisms of impact response of these coatings were dependent upon the depth of erodent penetration and could not, therefore, be extrapolated from erosion testing at lower velocities.