Ultrafine MoSi 2 powders have been synthesized from commercial MoSi 2 powders by using an Ar-H 2 induction plasma. Reactor pressure and plate power were taken as the experimental parameters to optimize the phase as well as the size distribution of ultrafine MoSi 2 powders. The powders were collected from porous metal fibers. They were composed of both metastable hexagonal structure (β-MoSi 2 ) and stable tetragonal structure (α-MoSi 2 ) with small levels of Mo 5 Si 3 and free silicon.

It has been shown that high velocity oxy-fuel (HVOF) thermal spray coatings with good wear resistance can be produced from Ni(Cr)-TiC powders manufactured by self-propagating high temperature synthesis (SHS) reactions. In the present work the process was expanded to include additions of Mo and W with the objective of modifying the carbide phase in an attempt to increase the wear resistance further. The effect of changing the matrix, i.e. substituting Fe for Ni, and changing the ceramic phase from TiC to TiB 2 was also examined. The feedstock powder and resultant coatings are characterised in terms of x-ray diffraction analysis and scanning electron microscopy while the coating properties are measured by microhardness and dry sand rubber wheel (DSRW) abrasive wear testing. The results show that Fe(Cr)-TiB 2 and Ni(Cr)-(W, Ti)C coatings have wear rates comparable to that of conventional Cr 3 C 2 -NiCr coatings produced from sintered and crushed powder, but further improvements are needed to achieve the wear resistance of WC-Co coatings.

Scientist and engineers show an increasing interest in metal foams. Recent activities are not only limited to the different techniques which already exist to produce and characterise metal foams, but also focus on further steps that these fairly new materials have to pass before entering mass production. Metal foams do not possess a high strength; to increase their mechanical properties and to improve the surface finish, thermally sprayed coatings can be applied. A metal foam is mainly characterised by its alloy composition, its density and the size and shape of the cells and pores. The quality of the coating depends on the coating material, the chosen process, the preparation of the surface, spraying parameters and the adhesion of the coating. The main focus of this paper is to reveal the necessary requirements to realise a sound composite made of a thermally sprayed coating onto metal foam. Basic investigations are carried out and during a later stage of the project parameters are to be identified which allow to describe the properties of the new composite material. The priority within this project will be on aluminium foams.

Zirconia is effective in improving the fracture toughness to a number of ceramics when introduced as a reinforcement either in the form of particulate, dispersed phase, or whiskers because of its unique tetragonal-monoclinic (t → m) transformation. In this paper, the authors attempt to prepare ZrO 2 reinforced Mullite by plasma spraying mixtures of zircon and alumina. Pre-mixed powders of zircon (ZrSiO 4 ) and alumina are injected into a dc plasma jet. The plasma sprayed particles are collected in distilled water and analyzed. The dissociation of zircon in thermal plasma is a well-established fact. The present investigation aims to utilize the plasma dissociation of zircon to produced ZrO 2 -toughened mullite from a zircon + alumina mixture. The results indicate that the plasma sprayed powders consist of zirconia, amorphous SiO 2 , zircon, and alumina. It was found that ball milling and plasma spraying could yield fine grained, even amorphous and chemically homogeneous, composite powders. Recrystallization of amorphous phases and formation of mullite occurred at about 1000 °C in plasma sprayed powders. This value is more than 500 °C lower than the formation of mullite in as-milled powders. Uniform coatings with good structural integrity were obtained by plasma spraying. The relative quantity of mullite in coatings after heat treatment is about 4 times as much as that obtained in the spheroidized powders. Preheat treatment of the spheroidized powder promoted dissociation of zircon. Zirconia remained as tetragonal under 1000 °C in the sprayed coatings.

As ceramic materials, the use of hydroxyapatite (HA) in clinical applications is severely limited by its intrinsic poor mechanical properties. The incorporation of some bioinert ceramics is believed to be a way to improve the mechanical reliability of HA matrix. HA coatings with titania addition were produced by using high velocity oxy-fuel (HVOF) spraying process in the present study. The mechanical properties of the as-sprayed coatings in terms of adhesive strength, shear strength and fracture toughness were investigated aiming to reveal the reinforcing effect of the titania addition in HA coatings. Qualitative phase analysis through X-ray diffraction (XRD) showed that mutual chemical reaction between TiO 2 and HA occurred during coating formation, from which CaTiO 3 was resulted. Totally unmelted titania powders were observed which suggests that the mutual reaction locates at HA/TiO 2 splats' interface. Significant influence of coating microstructure on mechanical properties was revealed. As the content of titania in HA coatings reached 20vol%, the adhesive strength decreased largely. As the content of titania reached 30vol%, the further augmentation of the adhesive strength of is possibly resulted from the improved coating microstructure. The fracture toughness exhibited the values of 0.48 Mpa ⋅ m ½ 0.60 Mpa ⋅ m ½ and 0.67 Mpa ⋅ m ½ for pure HA coating, 10vol% TiO 2 blended HA coating and 20vol% TiO 2 blended HA coating, respectively.

The main application of HVOF processes is the production of wear and/or corrosion protective coatings based on cermets (e.g. WC-Co). To reduce the weight of the sprayed volume, to increase the oxidation resistance and to reduce the costs of consumables SiC based cermets are of particular interest. The outstanding mechanical, thermal and chemical properties of SiC have made this material attractive for thermal spray applications. Thermal spraying of pure SiC is impossible, because of its decomposition at elevated temperatures. The development of suitable alloys as metallic binders enables the wetting of the SiC particles with reduced reactivity between the matrix and the hard particle during the thermal spray process. In the present work starting from the metallurgical concept of the metallic matrix to the production route from the atomization of the metallic matrices, the spray drying and sintering as well as the processability of the composites is discussed. The coatings are characterized concerning porosity and SiC content using optical and scanning electron microscopy, digital image analysis, X-ray diffraction and micro hardness testing. The wear resistance of the coatings is determined by oscillating wear test. Microstructure and performance of the SiC composite coatings are discussed with respect to technical applications.

ZrB 2 -SiC composites are considered a class of promising materials for aerospace applications such as nose and leading edges of re-entry vehicles. Results on such materials obtained by hot isostatic pressing have confirmed their high resistance to the oxidation at temperature up to 2000°C. Ongoing work has shown that such materials can be obtained in the form of coatings by means of Plasma Spraying techniques. On this regard, the most critical aspect was correlated to the decomposition of the SiC phase at a temperature quite lower than the melting point of ZrB 2 . Experimental evidence indicated that such decomposition can be avoided when a proper methodology of preparation of the starting powders is adopted, and if suitable thermal spraying parameters are selected. In any case, high temperature oxidation testing (up to 1800°C) confirmed the outstanding behaviour of this materials obtained by plasma spraying. This paper is focussed on preliminary studies of oxidation behaviour for plasma sprayed ZrB 2 -SiC composites suitable for thermal protection shields.

Various low-temperature abradable coatings for applications reaching 482°C were evaluated with the purpose of being applied onto the compressor spacers of an 11 MW land-based gas turbine engine. The coating is intended to reduce the compressor rotor tip-clearance, and is expected to increase the compressor efficiency by a range of 0.5 to 1%. This study evaluated physical and metallurgical characteristics, abradable and oxidation-resistance properties of plasma-sprayed AlSi-BN, NiCrFeAl-BN and quasicrystal AlCuFeCr, and flame-and plasma-sprayed NiCrFeAl-BN.

Recent developments in the field of plasma sprayed ceramic coatings at Shanghai Institute of Ceramics (SIC) are presented. Nano-titania and nano-tungsten carbide coatings were prepared. Their structure and properties were detected. The super hard B 4 C coating was deposited by APS. The physical and mechanical and anti-irradiation properties of B 4 C were measured. Wollastonite coating was deposited and its bioactivity has been tested. The results obtained indicated that (1) nano-titania coating possessed porous structure and unique electric properties; (2) nano-WC-Co coating exhibited notable wear resistance; (3) B 4 C coating was excellent irradiation resistance and (4) the carbonate-containing hydroxyapatite was formed on the surface of wollastonite coating, which indicated that this coating has excellent bioactivity.

Ceramic oxides can be deposited by the High Frequency Pulse Detonation process leading to coatings with unique properties as result of simultaneous melting and high velocities of the sprayed particles. In this paper, several Al 2 O 3 based powders have been HFPD sprayed and the resulting coatings characterized. For this purpose, microstructural evaluation, XRD phase analysis and functional behavior (dielectric strength and wear resistance) have been tested.

NiCrBSi spray and fuse types of coatings are routinely applied on various jobs where metallurgical bond of the coating to the substrate is desired. These coatings require a subsequent fusing operation at temperature of about 1050°C. In the present study NiCrBSi coatings were formed by the HVOF process and by the conventional spray and fuse route. The coatings were characterized for their microstructure, hardness and porosity. The aim of this study was to see whether HVOF spraying NiCrBSi spray and fuse powder could cause instant fusing of the coating without the need for additional fusing operation. XRD study of the coatings was also carried out to find out the difference between the spray and fused NiCrBSi coatings and the HVOF sprayed coatings. The HVOF spraying of NiCrBSi was carried under different set of parameters to determine the effect of spraying parameters on the properties of coating.

In thermal spray, it is well established that tailoring the powder characteristics is of major importance to achieve reproducible coatings on a microstructural and chemical point of view. Among the techniques developed to produce thermal spray powders, spray drying has proved to be the most versatile process. The spray drying method consists in spraying a slurry containing finely dispersed particles of the materials to be agglomerated. However, in order to prepare specific thermal spray powders, two steps have to be mastered: the slurry stability and the spray drying operating conditions. The present study was focused on the relationships that exist between the slurry rheology, the powder morphology and the coating properties. This work was performed on a model material namely Al 2 O 3 . In a first part, the effects of the surfactant percentage and pH on the stability of the suspensions were determined. The evolution of the viscosity of the slurries versus the amount of binder was measured. In a second part, these slurries were used to prepare spray-dried powders. The effects of some process parameters such as atomizing air pressure and slurry feed rate on the granule characteristics (morphology, density, particle size distribution, and powder flow ability) were investigated. Finally, some coatings were deposited using the APS technique on steel substrates from the non-sintered spray-dried powder previously realized. The coating morphology and the crystallographic structure were evaluated as a function of the spraying conditions.

Titanium suboxides form a class of ceramic materials possessing such technically interesting properties as electrical conductivity and solid lubrication. Consequently, these materials have a high potential for application as thermally sprayed coatings. In this paper the preparation and characterisation of two spray powders of different composition by agglomeration and sintering with a narrow range of the value n in Ti n O 2n-1 is described. Powders were characterised by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), thermogravimetric measurements (TG), helium pycnometry, nitrogen adsorption and mercury intrusion techniques. The sprayability was tested by plasma spraying. The coating structures were studied by optical microscopy, XRD, TG and TEM. Although the powders were only partially oxidized as a result of the spray process, the crystallographic structure was changed significantly, according to XRD and TEM investigations. As an alternative method of preparation of titanium suboxide spray powders, the reduction of a fused and crushed spray powder with hydrogen is described. Powder particle shape and size distribution are not changed in this process.

The present work has the purpose of comparing different thermal spraying techniques, namely axial plasma spray, standard air plasma spray and high velocity oxygen flame (HVOF), for depositing metal matrix composites, in this case chromium carbide nickel-chromium based. The quality of the coatings deposited by these three techniques has been assessed in terms of structural characteristics (porosity, oxide concentration, unmelted particles presence, etc.) and of mechanical characteristics (hardness, adhesion, etc.) as well as surface morphology. A specific efficiency test has been carried out to compare the three examined technologies. The results of the present study indicate that, against a slightly decrease in the quality of the film in terms of structural and mechanical properties, axial plasma sprayed coatings can be sprayed with a higher efficiency in comparison to the traditional technologies.