Formation mechanism of supersonic plasma-sprayed nanostructured composite ceramic coatings

• The formation mechanism of microstructure of NCs is thoroughly discussed.
• The formation mechanism of micro-defects is comprehensively concluded.
• Schematic diagram is used for analyzing formation mechanism of NCs.
• Reducing gap in particle diameter and changing tap density of particle can improve performances.
• Increases of velocity and tap density of particle are effective ways to improve quality of NCs.

In this paper, nanostructured composite ceramic coatings (NCs) were prepared by supersonic plasma spray technology, and their surface, cross-section and fracture morphologies were observed by scanning electron microscope (SEM). The phase composition and high resolution morphology are analyzed by X-ray diffraction (XRD) and transmission electron microscope (TEM). In the final, the formation mechanisms of microstructure and micro-defects were concluded. The results show that a great deal of metastable γ-Al2O3 phase and a few amount of Al2TiO5 phase were generated after the spraying. Fully melted regions (FM regions) and partially melted regions (PM regions) are interlocked and piled up without obvious boundaries. The bimodal distributional microstructure is formed in the form of lamellae, which is relative to position of particles injected into plasma jet, melting points about composite phase and thermal conductivity of particles in the process of spraying. Reducing the gap in particle diameter and changing tap density of agglomerated particles can improve effectively performances of NCs. The increases of velocity and tap density of particles are the effective ways to reducing micro-defects and improving quality of NCs.