Natural tourmaline as an efficient alternative to ceramic-type material for plasma spraying

• Published in: Journal of the South African Institute of Mining and Metallurgy 2018-04, Vol.118 (4), p.387-393
• Main Authors: Ctibor, P., Nevrlá, B., Pala, Z., Vrtiška, L.
• Format: Article
• Language: English
• Subjects: Annealing; Argon; Ceramic coatings; Ceramic powders; Ceramics; Crystal defects; Hematite; Mechanical analysis; Microhardness; Mullite; Optical microscopy; Plasma spraying; Powder spraying; Process parameters; Quality management; Reflectance; Substrates; Tourmaline; X-ray diffraction
• ISSN: 2225-6253; 0038-223X; 2411-9717
• DOI: 10.17159/2411-9717/2018/v118n4a9

Expensive synthetic materials with top-quality properties are often used to prepare coatings deposited by plasma spraying. Such high quality is not needed for a variety of end-user applications. We selected natural mineral tourmaline as an inexpensive alternative material and sprayed it successfully, forming a ceramic-like coating. The experimental powder for spraying was as-mined tourmaline schorl, and was only crushed and sieved to the appropriate size for plasma spraying with a hybrid water-argon stabilized plasma torch (WSP-H). Deposits on metallic and ceramic substrates, as well as self-supporting plates and samples annealed in air at 1 000°C, were further studied by optical microscopy, X-ray diffraction, thermal and mechanical analysis, and optical reflectance tests. The results showed that the coatings are highly porous (44 ± 8% in the most extreme case) and partly amorphous tourmaline is always formed by plasma spraying. The crystallinity depended on the process parameters. During subsequent annealing, tourmaline decomposed to a mullite-like phase and haematite; this process was accompanied by a microhardness increase, in spite of retained microstructure defects. Plasma spraying of a material with structurally bonded water and with incongruent melting, where both factors are considered as major disadvantages, was demonstrated.