An Integrated Assessment of Process-Microstructure-Property Relationships for Thermal-Sprayed NiCr Coatings

Abstract

Process-microstructure-property relationships have been systematically investigated and quantified for a large process window of thermal-sprayed Ni-20 wt.%Cr coatings. Detailed monitoring of particle state, coating formation, and multifunctional characterization has been performed providing a framework to not only examine the product coating, but also concurrently their evolution dynamics. Several distinct microstructures resulted from this expanded process window and shed light into the effects of in-flight particle state, nature of the interfaces, impact-induced peening, strain hardening, oxide content, on coating properties notably hardness, residual stress, elastic modulus, electrical and thermal conductivity. Nine processing conditions from five different thermal spray torches provided a wide range of particle velocities from 150 to 800 m/s and temperatures from 1800 to 2400 °C. Correlation between particle states and evolving stress obtained via in situ monitoring of coating deposition indicated increment of compressive stress at high particle kinetic energies, as well as enhanced strain hardening via peening. Hardness, therefore, showed strong dependency on the residual stress evolution. Elastic modulus was found to be strongly dependent on densification and intersplat bonding, whereas electrical and thermal conductivities were found to be more sensitive to defects in the intersplat interfaces (oxides, interlamellar porosity). In comparison to bulk properties, elastic modulus, and thermal conductivity of the sprayed coatings were generally lower, while electrical conductivity can approach the bulk value. Coating hardness exceeds the bulk property in most cases owing to the strain hardening during impact. Hardness was the most sensitive property to the process condition.