Effects of spray distance on the microstructure and mechanical properties of reactive plasma sprayed TiCN coatings

Abstract

The objective of this study is to evaluate the effects of spray distance on the microstructure and mechanical properties of reactive plasma sprayed titanium carbonitride (TiCN) coatings under nitrogen plasma gas. For this purpose, five different distances were adopted, and the morphology (single splats, surface and cross section) and composition (phase and chemical) of as-sprayed coatings were investigated. Mechanical properties of bonding strength, hardness, elastic modulus as well as indentation toughness of coatings were systematically evaluated. Results show that all of coatings consist of TiCxN1−x (0 ≤ x ≤ 1), TiOx, amorphous graphite and CNx phases, and the amount of ternary TiCN increases as the spray distance increases. Typical microstructure of plasma sprayed coatings containing fully melted particle, partially melted particles, un-melted particles and defects (pores and cracks) are observed. As the spray distance increase, the surface morphology and splats shape change from uneven and irregular ones to relatively flat and regular “disc splats” due to the improved dwell time and sufficient melting in plasma jet. In this case, the maximum hardness about 17.8 ± 1.9 GPa, elasticity modulus about 214.5 ± 18.3 GPa and bonding strength about 24.2 ± 2.1 MPa are obtained due to the relatively high TiCN content, favorable indentation toughness and compact microstructure. As the spray distance further increases to 150 mm, irregular “fingers splats” appear, and in this case the mechanical properties of coating are worsened. Therefore, a significant influence of spray distance on the microstructure and mechanical properties of as-sprayed TiCN coating is found.