Novel structured coatings by means of gas tunnel type plasma spraying

Plasma spraying system with high precise, has been expected for smart thermal processing. Plasma Spray Gas tunnel type plasma spraying technique has come under attention with regard to thermal barrier coatings, corrosion and wear resistance coatings and biomaterial coatings because of its superior advantages such as high energy density plasma compared to other conventional plasma jets under various operating conditions. For example, 8YSZ (8%Y₂O₃+ZrO₂) coating formed has a high hardness layer at the surface side, which shows the graded functionality of hardness. In this study, gas tunnel type plasma spraying was employed to produce 8YSZ+La₂Zr₂O₇ composite coatings for thermal barrier applications, reactive plasma sprayed TiN coatings for tribological applications and HA coatings for bio-medical applications. These kinds of novel structured material coatings were deposited by gas tunnel type plasma spraying under optimum spraying conditions. The performance such as the structural, mechanical properties, electrochemical corrosion and apatite layer formation behaviour of these coatings was investigated and the results are discussed. The obtained results show that the gas tunnel type plasma spraying has high energy density and also high efficiency as compared to the conventional plasma spraying system and hence the following results were obtained. 1) 25% 8YSZ+La₂Zr₂O₇ composite coatings had superior corrosion resistance against Na₂SO₄ compared to all other coatings. 2) The increase of the nitrogen gas flow rate in the plasma gas (Ar/N₂) and the spraying distance can significantly increase the nitride phases in the gas tunnel type reactive plasma sprayed TiN coatings. 3) Hydroxyapatite layer formation was found on the surface of the HA coatings produced by gas tunnel type plasma spraying, after 10 days of immersion in SBF solution.