This work assesses the challenges of preparing dense technical ceramic substrates for thermal spraying and evaluates the capabilities of laser ablation in comparison with sandblasting. Sintered Si3N4 and AlN substrates were prepared by both methods and surface roughness was measured before and after treatment. Alumina coatings were deposited by suspension-HVOF and atmospheric plasma spraying, and coating cross-sections were analyzed by optical microscopy and SEM. Sandblasting had little or no effect on surface roughness and cracks were observed in coating cross-sections at the near-surface region of the substrate. Laser ablation, on the other hand, significantly increased surface roughness for both ceramics, producing hole patterns that are shown to vary with laser power and pulse timing. In the case of plasma spraying, the best coatings were achieved when the holes in the substrate were less than 100 µm in depth. With suspension sprayed coatings, the best results were obtained on substrates with deeper (> 100 µm) holes.

In magneto-plasma-dynamic (MPD) arc jet generators, plasma is accelerated by electromagnetic body forces. Silicon nitride reactive spraying was carried out using an MPD arc jet generator with crystal silicon rods and nitrogen gas. Because higher-velocity, higher-temperature and higher-density and larger-area plasmas are produced with the MPD arc jet generator than those with conventional thermal plasma torches, nitriding of silicon can be enhanced. A dense and uniform β-Si 3 N 4 coating 30 µm thick was formed after 200 shots at a repetitive frequency of 0.03 Hz with a discharge current of 9 kA and a substrate temperature of 700 °C. The Vickers hardness reached about 1300. Furthermore, silicon carbide and aluminum nitride sprayings were conducted with the same spraying system. Surface modification is under study with lots of chemically reactive gases. All results showed that the MPD arc jet generator had high potential for spraying and surface modification.

Non-oxide ceramics, such as silicon nitride, have a unique combination of high strength, toughness, wear resistance, thermal and chemical stability. However, the use of these materials as thick protective coatings on engineering components has been severely restricted by their decomposition behavior. Silicon nitride, for instance, does not melt but decomposes at ~1900oC and so thermal spraying of pure silicon nitride powder is impracticable. A limited amount of research has been carried out on depositing silicon nitride in various metallic or ceramic matrix materials but none have produced adequate coating microstructures or coating properties. This paper concerns the design of oxide matrix systems for silicon nitride composite coatings. A quantitative model is developed for the viscous flow of two-phase feedstock particles on impact with the substrate and is applied to the deposition of silicon nitride – ceramic matrix coatings. A number of matrix systems are investigated including a series of yttria-alumina and yttria-alumina -silica compositions. The research shows that the oxide matrices successfully protect the silicon nitride from decomposition but that the matrix composition and particle loading have a critical influence on splat flow and coating quality.

Silicon nitride and sialons are very attractive materials for thermal spaying, but the high temperatures of spray processes lead to their decomposition instead of melting. Therefore, the use of these materials as protective coatings has been very restricted. Nevertheless, researchers have tried to provide silicon nitride-based coatings using metallic or oxide binders. Oxide binder additions to silicon nitride have been quite successful. In this paper, mixtures of silicon nitride and oxides were prepared for the thermal spraying of silicon nitride-based materials by using a detonation gun. Powders for the spraying were prepared through mixing, sintering, crushing and sieving. To get an oxide binder of low melting point, three components of oxides, Al 2 O 3 -ZrO 2 -TiO 2 , were selected; the ratio of oxides was determined to have a low melting point. When the sintering temperatures were below 1400°C, phases of the powders and coating layers were composed of α-Si 3 N 4 and oxides and any of sialon phases were not found. By sintering at the temperatures between 1400 and 1600°C in a nitrogen gas environment, χ(chi)-sialon (Si 6 Al 10 O 21 N 4 ) and β’-sialon (Si 3 Al 3 O 3 N 5 ) were formed. The ratio of β’-sialon increased as the sintering temperature increased. TiO 2 was transformed to a nitride, TiN. During the spraying procedure χ-sialon was decomposed to amorphous binder, but β’-sialon was not totally decomposed. Finally a coating layer composed of tetragonal-zirconia and β’-sialon was made.

In this paper, a quantitative model of the viscous behavior of two-phase particles hitting a substrate is used to optimize a plasma spraying process for silicon-nitride composite layers. The model is derived from the observed behavior of Si 3 N 4 -YAS (Y 2 O 3 -Al 2 O 3 -SiO 2 ) layers and provides a basis for further study of ceramic-matrix composite layers. Paper includes a German-language abstract.

Oxide-bonded silicon nitride (OBSN) powders have been developed to address thermal spray problems associated with high temperatures. This paper examines how such powders perform when applied via detonation gun (DGS) and atmospheric plasma spraying (APS) with axial powder injection. All coatings were characterized using optical microscopy and X-ray diffraction with additional tests being performed on DGS coatings. For the first time, relatively dense Si3N4-rich coatings with an oxide binder phase were produced, and some of the DGS coatings were found to be sufficiently wear resistance for industrial use.

Thermal spraying of silicon nitride has been considered impossible because the high temperatures involved lead inevitably to decomposition/oxidation of the material. To address these issues, improved silicon nitride-based powders were developed, two of which have been tested as reported in this paper. The powders were applied using low pressure plasma spraying (LPPS) and the resulting coatings characterized based on microhardness, adhesion, and cohesion strength. Phase transformations of the powders during spraying were also investigated and preliminary optimization strategies by statistical variation of plasma spray parameters were tested.