Boris B. Bokhonov

Carbon uptake during Spark Plasma Sintering: investigation through the analysis of the carbide “footprint” in a Ni–W alloy

In the majority studies of materials produced by Spark Plasma Sintering (SPS), the powders are consolidated in graphite dies using graphite punches and protective graphite foil. As a result, carbon uptake by the sintered material can occur. In this study, a Ni–15 at% W alloy was studied in this context for the first time and was chosen as a suitable model system, in which tungsten forms stable carbides whereas nickel does not, but offers a medium for carbon diffusion into the interior of the compact. In a disk-shaped 3 mm-thick compact Spark Plasma Sintered at 900 °C, carbon uptake resulted in the formation of tungsten carbide WC particles ranging from 0.2 to 2 μm in the subsurface layers of the compact (within distances 50–100 μm from the interface with the foil). The size of the WC particles varied with distance, smaller particles forming in the vicinity of the interface—in the area in which the nucleation was favoured at high carbon concentrations. However, it was not only the subsurface layer that was affected by the presence of carbon: particles of Ni2W4C were found at depths greater than 100 μm from the interface and throughout the volume of the compact. The distribution of the submicron WC particles and particles of Ni2W4C corresponded to a network of boundaries between the agglomerates of the Ni–W powder that was consolidated into a compact. These boundaries offered paths for faster diffusion of carbon from the foil when compared with the volume of the agglomerates. The carbide subsurface layer dramatically changed the interdiffusion behaviour of the sintered material in a pair with aluminum due to a significantly reduced concentration of tungsten capable of diffusing within a metallic phase.

Towards a better understanding of nickel/diamond interactions: the interface formation at low temperatures

We report the formation and development of the interface between diamond and nickel in partially densified compacts obtained from powder mixtures by Spark Plasma Sintering, hot pressing and conventional sintering at 700 and 900 °C – temperatures, which are well below the melting point of nickel and even below that of the nickel–graphite eutectic. The nickel particles sintered among themselves and formed joints with facets of the diamond crystals. Most of these joints fractured cohesively leaving Ni-containing patches on the diamond facets. The microstructure of the patches adhered to the diamond surface in compacts sintered at 900 °C and their geometry and orientation relative to the edges of the diamond facets suggest that the formation and development of the nickel/diamond interface are associated with melting and solidification of the melt according to certain directions of the diamond crystalline lattice. A possible explanation of the formation of a liquid at such a low temperature is contact melting of a metastable eutectic between nickel and diamond.

Formation of Aluminum Particles with Shell Morphology during Pressureless Spark Plasma Sintering of Fe–Al Mixtures: Current-Related or Kirkendall Effect?

A need to deeper understand the influence of electric current on the structure and properties of metallic materials consolidated by Spark Plasma Sintering (SPS) stimulates research on inter-particle interactions, bonding and necking processes in low-pressure or pressureless conditions as favoring technique-specific local effects when electric current passes through the underdeveloped inter-particle contacts. Until now, inter-particle interactions during pressureless SPS have been studied mainly for particles of the same material. In this work, we focused on the interactions between particles of dissimilar materials in mixtures of micrometer-sized Fe and Al powders forming porous compacts during pressureless SPS at 500–650 °C. Due to the chemical interaction between Al and Fe, necks of conventional shape did not form between the dissimilar particles. At the early interaction stages, the Al particles acquired shell morphology. It was shown that this morphology change was not related to the influence of electric current but was due to the Kirkendall effect in the Fe–Al system and particle rearrangement in a porous compact. No experimental evidence of melting or melt ejection during pressureless SPS of the Fe–Al mixtures or Fe and Al powders sintered separately was observed. Porous FeAl-based compacts could be obtained from Fe-40at.%Al mixtures by pressureless SPS at 650 °C.

Inter-particle interactions in partially densified compacts of electrically conductive materials during spark plasma sintering

Spark Plasma Sintering (SPS) is normally carried out to obtain consolidated materials of low residual porosity. SPS can also be used for the preparation of partially densified (porous) materials. Partial densification is achieved during SPS by using relatively low sintering temperatures or pressureless conditions. For a pressureless assembly, short punches are used; in addition, sintering without the upper punch is possible. It was shown that the number of contacts between the nickel and synthetic diamond particles was higher in the compacts partially sintered by SPS than in those cold-pressed and vacuum-annealed at the same temperature. During pressureless SPS, conditions for non-uniform current distribution can be realized causing non-uniformities in the microstructure of the compacts. It was shown that SPS of metals without the upper punch produces porous gradient structures. Electric current passing through the porous compacts and interfaces between the compacts and the punches/foils under pressureless conditions induces specific local effects evidenced by the morphology evolution of the particles and microstructure of the sintered material in those areas of the compacts.

In situ formation of metal-ceramic composite coatings by detonation spraying of titanium

We have studied the phase formation and concomitant microstructure evolution in the coatings deposited by detonation spraying of a titanium powder. By varying the O2/C2H2 ratio, detonation products of different compositions were produced, which determined the course of the phase evolution. The carrier gas also played an important role in the phase formation of the coatings. It was found that the phase composition of the coatings forms as a result of nitridation, oxidation and carbon capture depending on the O2/C2H2 ratio and the nature of the carrier gas. The in situ formed metal-ceramic coatings possess unique microstructures and present interesting objects for studying their mechanical behavior. Detonation spraying of titanium can be suggested as a convenient way of forming metal-ceramic coatings with a controlled phase composition resulting from the reactions of titanium with the spraying environment of variable chemistry.

Surface modification of synthetic diamond with tungsten

In the present study, tungsten-containing coatings on diamond particles were obtained by chemical vapor deposition using tungsten carbonyl as a precursor. It was found that tungsten reacts with diamond, which results in the formation of tungsten carbide WC crystallizing along certain crystallographic directions of the diamond crystal. Morphological studies of the coated diamonds have shown that the {111} facets of diamond are more reactive than the {100} facets.

Structural and mechanical characterization of detonation coatings formed by reaction products of titanium with components of the spraying atmosphere

Structural characterization of detonation deposits formed by reaction products of titanium with the components of the spraying atmosphere showed that ceramic-based coatings of unique microstructures—consisting of alternating layers of different compositions—can be formed. For the first time, mechanical characteristics of the coatings formed by reaction-accompanied detonation spraying of titanium were evaluated. It was found that high-yield transformation of titanium into oxides and nitrides during spraying can result in the formation of coatings with high fracture resistance and interface fracture toughness. The hardness of the coatings measured along the cross-section of the specimens was higher than that on the surface of the coatings, which indicated mechanical anisotropy of the deposited material. In terms of mechanical properties, coatings formed by the reaction products appear to be more attractive than those specially treated to preserve metallic titanium.