On morphometric uniformity of diamond micron powders

The paper presents the findings of characterization of initial diamond powders and products of their flotation separation, analysis of the adequacy of mean values of morphometric characteristics and assessment of powder uniformity by a system-criterion method. The separation is demonstrated to provide a significant improvement of powder uniformity in dimensional characteristics and shape index. The flotation products have been found to feature the best uniformity which explains their higher abrasive ability. The flotation separation method has proved to be a suitable tool for improving morphometric uniformity of diamond micron powders, and the proposed system of computer-aided analytical methods has been demonstrated to offer an efficiency means for quantitative analysis of morhometric characteristics of the powders.     

Cohesion of lactose powders at low consolidation stresses

The flow characteristics of a powder system are known to be influenced by particle size distribution, particularly the content of fine particles, and interparticle forces. This paper reports an investigation that has identified and quantified links between physical properties, viz size distribution, bulk density and particle density, and cohesion in compacted beds of powder. An annular shear cell was used in the determination of the cohesion of cohesive and free-flowing milled lactose powders at low consolidation stresses in the range 0.31–4.85 kPa and under ambient conditions. Following consideration of the compaction and shearing processes, it was postulated and confirmed that cohesion could be expressed as a function of powder surface area per unit volume and dimensionless preconsolidation stress. It was shown that care is needed in the measurement of surface–volume mean diameter when applying correlations developed from the experimental data.     

Shock compaction of cubic boron nitride powders

C-BN powders with different grain sizes were dynamically compacted by explosive shock loading using approximate peak pressures from 33 to 77G Pa. The density and the microhardness of the resulting c-BN compacts were strongly dependent upon the grain size of the c-BN powders used as the starting materials. The best c-BN compacts, with 98% of the theoretical density and microhardness of 51.3G Pa, were obtained from the coarse c-BN powder (40 to 60m). In the compacted fine c-BN powder (2 to 4m) conversion of the c-BN to low density forms of BN at a residual temperature degraded the interparticle bonding significantly. X-ray line-broadening analysis of the compacted c-BN powders indicated that the residual lattice strain increased with the increase in grain size of the starting powder, while the crystallite size was independent of the grain size.     

Shock synthesis and synthesis-assisted shock consolidation of suicides

Shock-induced chemical synthesis and synthesis-assisted consolidation of high-temperature materials (suicides) were investigated. Niobium, molybdenum, and titanium powders mixed with silicon powders were chosen as reactant materials for shock-induced synthesis of silicides. In parallel experiments, these reactant materials were also respectively mixed with inert intermetallic compound powders of NbSi₂, MoSi₂, and Ti₅Si₃ in different proportions and were shock consolidated. Shock processing was carried out using a modification of the experimental set-up developed by Sawaoka and Akashi. The shock waves were generated in the materials by the impact of a flyer plate at a velocity of 2 km sec^–1. An explosive plane-wave generator was used to initiate the main explosive charge to accelerate the flyer plate. The passage of shock waves of sufficient pressure and temperature induced a highly exothermic and self-sustaining reaction between reactant materials. The shock-synthesized intermetallic compounds and the heat of reaction enhanced bonding between inert matrix materials. The proportion of reactant powder mixtures blended with inert intermetallic materials plays a very important role in the synthesis-assisted consolidation process. Characterization of compacts was done by optical microscopy, scanning electron microscopy, and X-ray diffraction. A preliminary analysis of shock-induced chemical reactions is conducted; it predicts a 30% increase in shock pressure and shock-wave velocity over those in unreacted powders. For shock synthesis, the profuse formation of voids indicates that melting of the material occurred; in contrast, unreacted regions did not exhibit porosity.     

Hot isostatic consolidation and transformation of sigma phase powders

Abstract

Superferritic stainless steel compositions containing 38%Cr and at least 2% each of Ni and Mo can be converted to sigma phase by heat treatment and then to powder by mechanical attrition. This powdered sigma phase has been experimentally processed by powder metallurgical techniques to produce green compacts which were consolidated in a hot isostatic press. Depending on composition, several of the powdered alloys were converted back to ferrite during hipping. The mechanism of the consolidation process, the nature of the microstructures produced, and the prognosis for its industrial exploitation are examined.     

Strength of diamond powders synthesized in the Mg-Zn-B-C system

The strength of diamond powders of grit size 125/100 μm synthesized in the Mg-Zn-B-C system (reaction mixtures having different boron concentrations) has been investigated. It has been shown that the distribution of strength by fracture load is correctly described by the Weibull function. The strength index of diamond powders and confidence intervals has been defined. It has been found that the strength index dependence on boron concentration in the range from 1 to 40 at % is described by the function having the minimum at 20%.     

Dynamic consolidation of rapidly solidified titanium alloy powders by explosives

Consolidation of rapidly solidified titanium alloy powders employing explosively generated shock pressures was carried out successfully. The cylindrical explosive consolidation technique was utilized, and compacts with densities in the range 97 to 100% were produced. Better consolidation (with more interparticle melting regions and less cracking) was achieved by using a double tube design in which the outer tube (flyer tube) was explosively accelerated, impacting the powder container. Optical and transmission electron microscopy observations were carried out to establish microstructural properties of the products. It was observed that consolidation is achieved by interparticle melting occurring during the process. The interior of the particles in Ti-17 alloy exhibited planar arrays of dislocations and twin-like features characteristic of shock loading. Two dominant types of microstructures (lath and equiaxed) were observed both in Ti-662 and Ti-6242 + 1% Er compacts, and very fine erbia (Er₂O₃) particles were seen in the latter alloy. The micro-indentation hardness of the consolidated products was found to be higher than that of the as-received powder material; and the yield and ultimate tensile strengths were found to be approximately the same as in the as-cast and forged conditions. The ductilities (as measured by the total elongation) of the shockcompacted materials were much lower than those of the cast or forged alloys. Hot isostatic pressing of the shock-consolidated alloys increased their ductility. This enhancement in ductility is thought to be due to the closure of existing cracks. These excellent mechanical properties are a consequence of strong interparticle bonding between individual powder particles. It was also established that scaling up the powder compacts in size is possible and compacts with 50, 75, and 100 mm diameter were successfully produced.     

Full density consolidation of pure aluminium powders by cold hydro-mechanical pressing

Gas atomized pure Al powders were successfully consolidated into full density at room temperature by newly developed cold hydro-mechanical pressing (CHMP). The effects of the configuration of pressures on the relative density and microstructure of consolidated specimens were investigated. The full density consolidation of metal powders at room temperature resulted from severe shear deformation of the particles which ruptured the surface oxide layers. Our works demonstrate that the novel CHMP provides an economical approach to consolidate particulate material into high quality bulk preforms for engineering applications.     

Interaction of diamond with ultrafine Fe powders prepared by different procedures

We have studied the interaction of synthetic diamond crystals with ultrafine Fe powders during catalytic diamond gasification in a hydrogen atmosphere at 900°C. The Fe powders were prepared by three procedures: reduction of Fe₂O₃ nanopowder; evaporation using an ELV-6 electron accelerator, followed by condensation; and reduction of ferric chloride. The surface-processed diamond crystals were examined by electron microscopy. The results indicate that ultrafine powders produced by the first two procedures cause predominantly lateral etching of diamond. The Fe particles prepared by the third procedure penetrate into the bulk of diamond crystals and produce etch pits and “tunnels,” thereby markedly increasing the specific surface area of the crystals.     

Effect of activators on the properties of nickel coated diamond composite powders

Nickel coated diamond composite powders were fabricated via a newly developed direct electrodeposition technique. The effects of activators on the coating of diamond were firstly investigated and diamond grinding wheels were then prepared from Ni-coated diamond composite powders with different activators. The microstructural characterizations of this composite powders were finally conducted by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction, and the mechanical and tribological properties of as-prepared diamond grinding wheels were also measured. There are changes in microstructures and properties of the composite powders with activators. The activator concentration also has an influence on the morphologies and phase structures of the Ni coating on diamond particles.The composite powders with more compact coating of nickel can be prepared by adding 1 g dm~(-3) or more AgNO_3 as an activator to electrodeposit nickel on diamond. The mechanical and tribological properties of diamond grinding wheels were significantly improved when the coating phase structure of Ni crystal grew with(111) plane orientation on the surface of diamond particles. The wheels made from nickel coated diamond composite powders possessed the advantages of easy preparation and outstanding tribological properties. Therefore, Ni coated diamond composite powders exhibit a great potential to be extensively applied in diamond cutting and grinding tools.     

Special features of sintering diamond powders of various dispersions at high pressures

The density, porosity and hardness of diamond polycrystals sintered from diamond powders of various grit sizes have been studied as functions of sintering length and temperature. Based on the derived dependences, a hypothesis has been formulated that capillary forces oppose to the compaction of diamond particles through the external action. The degassing of a statically synthesized diamond nanopowder is shown to double the hardness of polycrystals sintered from this nanopowder.     

Consolidation of mixed diamond and cobalt granule powders by magnetic pulsed compaction

Magnetic pulsed compaction (MPC) was introduced to consolidate mixed diamond and cobalt (Co) granule powders for the production of drilling segments. The diamond-Co samples prepared by MPC at the compacting pressure of 4 GPa showed a high sintered density of 99.6% as well as a high green density of 86.4%. A fine and homogeneous microstructure and a high hardness were also observed in the sintered bulk. Finally, a perforating test revealed that a higher drilling speed of 11.71 cm/min and a longer tool life of 7.96 m were obtained for the drilling segments prepared by the MPC process, whereas the values for those fabricated by a conventional process were 10.15 cm/min and 6.55 m, respectively. This property improvement of the MPCed segments was attributed basically to the enhanced green density and the homogeneous distribution of the diamonds.     

Deformation processes during high-pressure sintering of the diamond powders produced by catalytic synthesis

Deformation structures formed in diamond grains during polycrystalline sample sintering at 7.7 GPa were studied using TEM. A number of deformation features were observed in diamond during sintering in the temperature range 700–2500 °C. Based on these data a sequence for the structure formation processes in diamond grains under P-T treatment was ascertained.     

Physico-mechanical properties and structure of diamond polycrystalline composite materials produced from variously dispersed powders

A nanostructural diamond composite material having grains of average size 0.08 μm and hardness corresponding to the hardness of diamond composite materials with grains of average size 30 μm has been produced using nanotechnologies of powder materials. Temperature dependences of hardness of nanostructural diamond composites of various dispersions have been compared. The thermostability of a diamond composite has been shown to depend not only on the composition of a sintering aid but on the size of diamond powders as well. Our findings have indicated that the material obtained holds promise in finishing products of nonferrous metals and alloys instead of natural diamonds.     

Specific surface area determination methods, devices, and results for diamond powders

A review of available methods for specific surface area determination for various dispersed materials is presented. Fundamental differences between the currently used devices and procedures of the analysis of external and total specific surface, including those applicable to powders of superhard materials and refractory compounds, are discussed. Some data on specific surface of synthetic diamond grits, micron powders, and submicron powders are provided for practical applications.     

NiAl-Al2O3 intermetallic matrix composite prepared by reactive milling and consolidation of powders

Reactive milling of nickel oxide and aluminium powders corresponding to the stoichiometric reaction 3NiO + 5Al resulted in the formation of intermetallic matrix composite NiAl-Al₂O₃, with 28 wt% of alumina. Prolongation of the milling process allowed obtaining the microstructure with nanosize range of crystallites of both phases, as showed XRD measurements and TEM observations. The refinement of microstructure was accompanied with an increase of lattice strain as a result of ball milling. The particles size and morphology changed from several tens of micrometers and polyhedron shape observed immediately after the reaction took place, to several micrometers and spherulitic shape after long-term milling. Two consolidation techniques of nanocomposite powders were applied: explosive compaction and hot-pressing under high pressure. Both methods allowed obtaining the samples of high density (up to 99% of theoretical one) and microhardness above 13 GPa. Simultaneously, a nanocrystalline structure of the material was preserved.     

Effects of the synthesis of intermetallic compounds on the growth and consolidation of detonation diamond nanocrystals

The synthesis of intermetallic compounds belonging to the nickel-aluminum system in the presence of detonation nanodiamond (DND) particles leads to a significant change in the structure of consolidated nanocrystals. The broadening of (111), (220), and (311) X-ray diffraction reflections usually decreases by an order of magnitude, which indicates that the size of nanocrystals increases in approximately the same proportion. This effect significantly depends on the state of the surface of diamond nanoparticles. The X-ray data suggest that aluminum present in the initial metal-diamond mixture actively interacts with oxygen adsorbed on the surface of particles, which leads to the formation of Al₂O₃ oxide. The interaction between particles via cleaned interfaces at high temperatures leads to the activation of recrystallization processes.     

Formation of metal–TiN/TiC nanocomposite powders by mechanical alloying and their consolidation

Fe–TiN, Ni–TiN, and SUS316–TiC nanocomposite powders were prepared by ball-milling Fe–Ti, Ni–Ti, and SUS316–TiC powder mixtures in a nitrogen or argon gas atmosphere. Fe–63vol.% TiN and Ni–44–64vol.% TiN milled powders were dynamically compacted by use of a propellant gun to produce bulk materials of nanocrystalline structure and having grain sizes between about 5 and 400 nm. SUS316–2.8–5.6vol.% TiC milled powders were consolidated by hot isostatic pressing (HIP) to produce bulk materials having grain sizes between about 100 and 400 nm. The possibility of using fine-dispersed TiN/TiC particles to pin grain boundaries and thereby maintain ultra-fine grained structures of grain sizes down to the nanocrystalline scale has been discussed.