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.

Over the last years, interest in thermally sprayed polymer coatings has increased. They have excellent corrosion and chemical resistance properties. Unfortunately, they have some limited properties like scratch and wear resistance. Fillers can be a good solution to improve these properties. In the Thermal Spray Centre of Barcelona, blends of polyamide powder with different contents of boron and silicon nitride have been flame-sprayed and studied. Microstructural characterisation of the composite coatings has been done using Optic and Scanning Electron Microscopies. Wear resistance has been evaluated by means of Ball on Disk (ASTM G99-90) and Rubber Wheel (ASTM G65-91) tests. Salt Spray Fog tests (ASTM B117-90) have been performed in order to evaluate their corrosion resistance.

The present study was carried out within the scope of a project oriented toward the evaluation of applicability of different materials as thermally sprayed coatings for gear flank protection. Investigations were made on APS deposited oxide coatings, such as Cr 2 O 3 and Magnéli phases of different systems, as well as on hardmetal coatings based on WC, Cr 3 C 2 and TiC deposited by HVOF. All coating microstructures were investigated by optical microscopy, with selected samples studied additionally by SEM. Phase composition, hardness, Young’s modulus and fracture toughness were other basic coating properties studied. An oscillating sliding wear test was used as the main tool for preliminary evaluation before more laborious testing. The wear resistance in this test was investigated against 100Cr6 for all materials as well as against sintered silicon nitride and sintered WC-Co for oxide and hardmetal coatings, respectively. Hardmetal coatings based on WC and TiC showed the best results and were selected for further testing.

In magneto-plasma-dynamic (MPD) arcjet generators, plasma is accelerated by electromagnetic body forces. The MPD arcjet generator can produce higher-velocity, higher-temperature, higher-density and larger-area plasmas than those of conventional thermal plasma torches. Two types of MPD arcjet generator were developed for applications to ceramic spray coatings. One generator was installed with a cathode covered with mullite or zirconia ceramics and the other with a titanium cathode. The former was operated with Ar for mullite or zirconia coating by an ablation process of the cathode cover and the latter with N 2 for titanium nitride coating by a reactive process between ablated titanium particles and nitrogen plasma. Silicon nitride reactive spraying was also carried out. The MPD spray process could successfully form dense, uniform and hard ceramic coatings. In titanium nitride reactive spraying, plasma diagnostic measurement and flowfield analysis were conducted. A large amount of N and N + was expected to be exhausted with a high velocity from the MPD generator. Both the electron temperature and the electron number density were kept high at a substrate position compared with those for conventional low-pressure thermal sprayings. A chemically active plasma with excited particles of N + , Ti, Ti + and Ti 2+ was considered to contribute to better titanium nitride coatings. All coating characteristics showed that the MPD arcjet generators had high potentials for ceramic spray coatings.

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.

This paper investigates the stability of molybdenum base silicides, which are located in combustion chambers and in an endothermic environment, for use in radiant tubes for heat treatment. The subject matter was plasma-sprayed molybdenum disilicide, pentamolybdenum trisilicide, hot-pressed molybdenum disilicide, and molybdenum disilicide composites containing SiC and silicon nitride reinforcements. Results of the investigation show that the oxidation resistance of plasma sprayed molybdenum disilicide can be detrimentally effected due to the silicon loss that occurs during the high temperature plasma spray process. 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.

The use of MoSi 2 as a high temperature oxidation resistant structural material is hindered by its poor elevated temperature creep resistance. The addition of second phase Si 3 N 4 holds promise for improving the creep properties of MoSi 2 without decreasing oxidation resistance. The high temperature impression creep behavior of atmospheric plasma sprayed (APS) and hot pressed (HP) MoSi 2 /Si 3 N 4 composites was investigated. Values for steady state creep rates, creep activation energies, and creep stress exponents were measured. Grain boundary sliding and splat sliding were found to be the dominant creep mechanisms for the APS samples while grain boundary sliding and plastic deformation were found to be the dominant creep mechanisms for the HP samples.

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.

The full potential of rolling element bearings operating in specialised conditions such as high speed and corrosive environments are realised using surface coatings. Tungsten Carbide coating by thermal spray HVOF and D-Gim processes are considered for these applications. An experimental approach using a modified four-ball machine simulates the tribological conditions within a rolling element bearing. The fatigue failure modes of the tungsten carbide coating in rolling contact with steel and silicon nitride are examined using conventional surface analysis techniques. The stress fields within the coating are examined using traditional contact theory and residual stress measurement by X-ray diffraction. The residual stress measurements of the pre-test coating, the contacting surface and the fatigue failures are described. Results of residual stress relating to orientation, failure depth, coating thickness are discussed along with the fatigue failure mode.