Formation Mechanism of Microstructure in NiCr Coatings by Plasma Spray Melting

It is crucial to understand the formation mechanisms of coatings produced by plasma spray melting for achieving target microstructures through process control. NiCr coating evolution was systematically investigated in this study by regulating standoff distance and power input, elucidating the origins of lamellar, spherical, and sandwich structures. A tripartite particle-level analysis─encompassing in-flight behavior, spreading-solidification behavior, and multilayer stacking─reveals the coating growth mechanism. Results show that coatings become more uniform with reduced porosity as standoff distance increases (optimal range: 20-24 mm), while progressive thinning accompanies this extension. Higher power accentuates lamellar structures, whereas excessive power induces coating discontinuity. The formation of internal spherical structures depends primarily on particle characteristics and melting behavior. Disk-shaped, splattered-edge, or shrinkage-rimmed splats result from the effects of liquid-phase contraction and solidification contraction. The coatings form through layer-by-layer stacking, with their spreading morphologies being governed by substrate temperature gradients, cooling rates, particle residence time. The metallurgically bonded coatings prepared by PSM achieve a bonding strength of up to 68 MPa, representing an improvement of over 25% compared to nonmetallurgically bonded coatings. This finding further validates the feasibility of the PSM technology and provides a crucial foundation for its future application in developing high-performance functional coatings.