Reaction sintering of diamond using a binary solvent-catalyst of the Fe-Ti system

Reaction sintering of diamond was investigated using a starting mixed powder of purified natural graphite and a binary solvent-catalyst of the Fe-Ti system under high pressure (7G Pa) and temperature (1700°C) conditions for e treatment time of 1 to 15 min. Diamond sintered compact of about 100% conversion ratio from graphite todiamond was obtained with the binary solvent-catalyst content: 11.4 to 17.0 vol% (30 to 40wt%) Fe and 6.6 to 7.7vol% (10wt%) Ti. The sintered compact having the bulk density of 4.1 to 5.5g cm^–3, consisted of diamond phase and metal carbide (Fe₃C and TiCx) phase. The Vickers microhardnesm (under 1000 g load) mfthesintered diamond phase was >8OO0, while that ofthe metal carbide phase was 1000 to 2000. The transformation from graphite to diamond proceeded in a short time (< 1 min), which was followed by a particle joining between the formed diamond grains, when the densification would be attained at the reaction time of 15 min by pooling out the melt of carbon and solvent-catalyst.     

Effects of added diamond powder on the reaction sintering behaviour of diamond in the graphite-nickel system

Polycrystalline diamond sintered compact was prepared under high pressure and temperature conditions (7 GPa, 1700°C, 10 to 30min) from the starting material with the composition of the system 50 wt% carbon (diamond + graphitized pitch coke (GPC))-50 wt% Ni. The effects of added diamond powder on the microstructure of the sintered compact and reaction sintering behaviour were investigated. The grain size of diamond in the sintered compact decreased remarkably from 20 to 40m to 2 to 3m on addition of 10 to 20 wt% diamond powder (grain size: 1m) to the GPC-Ni system. The grain size can be controlled by that of the added diamond powder. A sufficient supply of carbon from GPC plays an important role in the formation of a covalently bonded compact of diamond. The grain growth of the formed diamond is depressed by the coexistence of diamond powder, which controls the solubility of carbon in the metal-carbon system and also the grain growth process by solution-reprecipitation.     

Effect of high-pressure pre-treatment of starting carbon on diamond formation

Four starting carbons differing in crystallinity and grain size were pre-treated with or without nickel at 3 GPa and 1800° C or at 6 GPa and 1700° C. Diamond synthesis from carbons pre-treated and then further treated in vacuum was carried out at 8 GPa and 1700° C. Pre-treated carbons with or without nickel, which were fully or partly graphitized, changed a little or did not convert to diamond at 8 GPa and 1700° C. Diamond did form from the pre-treated carbons after treatment in a vacuum at 1000° C. Diamond formation, even from the graphitized carbons, was found to be inhibited mainly by gases adsorbed on the treated carbon during the pre-treatment under high pressure.     

Effect of ambient pretreatment of graphite and solvent-catalyst on diamond formation

Diamond was formed from purified natural graphite under high pressure and temperature conditions (7 G Pa, 1700° C) using a solvent-catalyst in the unary (Fe) or binary (Fe-Ti) system. The effect of an ambient pretreatment of the starting mixed powder (graphite and solvent-catalyst) was investigated in relation to the formation and grain growth of diamond. An initial desorption of adsorbed water vapour or harmful gases from the starting powder in vacuum (2 × 10^–5 torr) at higher temperatures (>400° C) was required in order to increase the conversion ratio from graphite to diamond. The subsequent ambient pretreatment at 1000° C in different atmospheres was found to affect the grain growth process of diamond. The depression of grain growth was confirmed in both cases of pretreatments in vacuum (2 × 10^–5 torr) and in an argon atmosphere (1 × 10^–3 or 760 torr). The diamond grains were discrete in the vacuum pretreatment, while a particle joining between the diamond grains was promoted in the argon pretreatment. The pretreatment in an N₂ atmosphere (1 × 10^–3 or 760 torr) tended to accelerate the grain growth of diamond.     

Effects of crystallinity of starting carbons on diamond formation in presence of nickel under high pressure and high temperature condition

The processes of graphitization and diamond formation of several carbons in the presence of nickel were investigated under 8 GPa at temperatures up to 1800° C. Diamond was formed easily from graphitized pitch coke which had a well-developed graphitic structure and in less amount from glassy carbon preheated at about 3000° C which was partly graphitized. On the other hand, pitch coke and glassy carbon, preheated at about 2000° C and not graphitized, did not transform to diamond but remained graphitized even in the diamond stable region. Diamond from graphitized pitch coke and glassy carbon preheated at about 3000° C grew to form by direct bonding.     

Pressureless sintering of metal-bonded diamond particle composite blocks

Metal-bonded diamond particle composite blocks were fabricated by pressureless sintering, using multi-phase copper based alloys as the bonding materials. The processing sequences included ball-milling the diamond particles and metallic powders, uniaxially pressing the milled powder mixtures into green compacts, and sintering the green compacts in vacuum. The bonding materials, constituted of Cu, Sn, Ti, Mo and TiC, were prepared by blending various elemental and prealloyed powders. Addition of Ti as an active element effectively enhanced the interfacial cohesion strength, by developing an intermediate layer between the diamond particles and the matrix phase. This resulted in the observation that the failure mode of the composite blocks in a bending test was predominantly cleavage of diamond particles instead of pull-out of diamond particles.     

Structure formation of the superhard carbon ceramics in sintering powders of the carbon amorphous phase in a mixture with nanocrystalline diamond at high static pressures

The results have been discussed of studying the structure and hardness of polycrystals produced from nanodispersed powders of mixtures of the amorphous carbon phase and nanocrystalline diamond. The initial powders have been synthesized by high-temperature shock compression and sintered at a pressure of 13 GPa and temperatures from 1100 to 2000°C. The sintering has been accompanied by the transformation of amorphous carbon into diamond. As the diamond content of the sample has been increased from 10 to 100%, the hardness increased from 45 to 80 GPa.     

Phase composition of composite materials produced by high-pressure reaction sintering in the cubic boron nitride—diamond—aluminum system

X-ray diffraction analysis has been used to study the phase composition of composite materials produced by high pressure—high temperature (4.2 GPa, 1750 K) sintering of cBN and Al powders with diamond added to the reaction mixture. It has been shown that as a result of the reaction sintering depending on the relationship among the mixture components, in parallel with cBN and diamond, the composite materials may contain aluminum nitride, diboride, carboboride and carbide as well as solid solutions of boron and/or carbon based on the crystalline lattices of Al, AlN, and cBN. A possibility is shown of dispersion hardening of a composite providing the diamond content is below the threshold percolation. Along with diamond an increase in the resistance to abrasive wear of composites is responsible by the Al₃BC phase, which is located at the phase boundaries.     

Effect of starting powders on the sintering of nanostructured ZrO2 ceramics by colloidal processing

Abstract

The effect of starting powders on the sintering of nanostructured tetragonal zirconia was evaluated. Suspensions were prepared with a concentration of 10 vol.% by mixing a bicomponent mixture of commercial powders (97 mol.% monoclinic zirconia with 3 mol.% yttria) and by dispersing commercially available tetragonal zirconia (3YTZ, Tosoh). The preparation of the slurry by bead-milling was optimized. Colloidal processing using 50 μm zirconia beads at 4000 rpm generated a fully deagglomerated suspension leading to the formation of high-density consolidated compacts (62% of the theoretical density (TD) for the bicomponent suspension). Optimum colloidal processing of the bicomponent suspension followed by the sintering of yttria and zirconia allowed us to obtain nanostructured tetragonal zirconia. Three different sintering techniques were investigated: normal sintering, two-step sintering and spark plasma sintering. The inhibition of grain growth in the bicomponent mixed powders in comparison with 3YTZ was demonstrated. The inhibition of the grain growth may have been caused by inter-diffusion of cations during the sintering.     

Effect of starting powders on the sintering of nanostructured ZrO2 ceramics by colloidal processing

The effect of starting powders on the sintering of nanostructured tetragonal zirconia was evaluated. Suspensions were prepared with a concentration of 10 vol.% by mixing a bicomponent mixture of commercial powders (97 mol.% monoclinic zirconia with 3 mol.% yttria) and by dispersing commercially available tetragonal zirconia (3YTZ, Tosoh). The preparation of the slurry by bead-milling was optimized. Colloidal processing using 50 μm zirconia beads at 4000 rpm generated a fully deagglomerated suspension leading to the formation of high-density consolidated compacts (62% of the theoretical density (TD) for the bicomponent suspension). Optimum colloidal processing of the bicomponent suspension followed by the sintering of yttria and zirconia allowed us to obtain nanostructured tetragonal zirconia. Three different sintering techniques were investigated: normal sintering, two-step sintering and spark plasma sintering. The inhibition of grain growth in the bicomponent mixed powders in comparison with 3YTZ was demonstrated. The inhibition of the grain growth may have been caused by inter-diffusion of cations during the sintering.     

Effect of the carbon isotope composition on the properties of diamond

Parameters of the interatomic interaction potential for ¹²C and ¹³C carbon isotopes in the crystal lattice of diamond have been determined. Based on these data, the isotope dependence of the properties of diamond such as the Debye temperature, molar heat capacity, thermal expansion coefficient, vacancy formation energy, self-diffusion activation energy, surface energy, and longitudinal sound velocity is described. This approach is used for estimating a change in the bulk compression modulus of lithium crystals upon the passage from ⁷Li to ⁶Li. As the temperature increases, the isotope dependence of the heat capacity at constant volume vanishes; at a certain temperature, the isotope dependence of the thermal expansion coefficient changes from growth to decay. The expected isotope dependence of the parameters of phase transitions is predicted. It is shown that carbon condensates formed upon deposition from the gas phase must be enriched with the heavy isotope.     

Effects of the size of nano-copper catalysts and reaction temperature on the morphology of carbon fibers

In this study, carbon fibers with different morphologies, including coiled carbon nanofibers and straight carbon fibers, were obtained by the chemical vapor deposition using a Cu-catalytic pyrolysis of acetylene at 250 °C. The influences of nano-copper catalyst particle size and the reaction temperature on the morphology of carbon fibers were investigated. Under the same reaction condition, coiled carbon nanofibers generally were synthesized using nano-copper catalyst with smaller particles size, and bigger copper particles are apt to produce straight carbon fibers. With decreasing of reaction temperature to 200 °C, straight carbon fibers were obtained, instead of coiled carbon nanofibers at 250 °C. The product was characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and X-ray powder diffraction (XRD).     

Effect of zirconia additions on the reaction sintering of aluminium titanate

Yttria-stabilized zirconia was added to a chemically prepared mixture of alumina and titania. The effect of the zirconia on the microstructure and resultant properties was studied following reaction sintering to form aluminium titanate. An increase in mechanical strength was observed with little effect on the excellent thermal properties of the aluminium titanate. This was attributed to generation of extra microcracks by the transformation of the zirconia phase and the unusual microstructure produced by the presence of zirconia.     

Effects of sintering temperature and addition of Fe and B4C on hardness and wear resistance of diamond reinforced metal matrix composites

The effects of sintering temperature and addition of Fe instead of Co into the matrix composition on the mechanical properties of diamond-reinforced MMC’s have been studied. Diamond-reinforced MMC’s based on Fe-Co compositions with and without boron carbide (B₄C) have been processed. Three different matrix composites (with different Fe/Co ratios) have been produced with and without B₄C at a pressure of 25 MPa and sintered in N₂ at various temperatures (800, 900, and 1000°C). After sintering, mechanical properties of the resultant composites have been studied and the results discussed. Addition of B₄C has been found to improve the hardness and wear resistance of the composites. Optical microscopy, SEM and EDS have been used to examine the microstructure and surface of the synthesized composites.     

Behaviour of cobalt infiltration and abnormal grain growth during sintering of diamond on cobalt substrate

The infiltration behaviour of molten cobalt into a diamond powder compact was examined when the latter was placed on a cobalt disc and held at high pressure of 5.8 GPa and high temperature of 1350 to 1500° C. The larger the grain size of the starting diamond powder and the higher the holding temperature, the more easily cobalt infiltrated into the diamond compact. The infiltration is considered to occur because of the negative pressure in the voids formed between diamond grains. Although diamond powder was consolidated in this process of cobalt infiltration, abnormal grain growth was also observed in the boundary between cobalt and diamond compact because of the dissolution and precipitation process of the compact into molten cobalt.On leave from Chengdu University of Science and Technology, Chengdu, China.     

Hydrogen induced microstructural variation in diamond and diamond like carbon thin films

Hydrogen plays a crucial role in the growth of micro-crystalline diamond (MCD) and diamond like carbon (DLC) thin films grown by plasma assisted chemical vapour deposition (PACVD) processes. It selectively etches graphite phase and helps in stabilizing the diamond phase. The presence of various hydrocarbon species in the plasma and their reaction with atomic, excited or molecular hydrogen on the substrate surface decide the mechanism of diamond nucleation and growth. Several mechanisms have been proposed but the process is still not well understood. Control of hydrogen and other deposition parameters in the PACVD process leads to deposition of yet another class of materials called diamond like carbon. By varying the concentration of hydrogen it is possible to produce purely amorphous carbon films on the one hand and amorphous hydrogenated carbon films (with as high as 60% hydrogen) on the other. Very hard, optically transparent and electrically insulating films characterize the diamond like behaviour. The proportion of hydrogen and its bonding with carbon or hydrogen in the film can be varied to obtain very hard to very soft films which could be optically transparent or opaque. The microstructure of these films have been investigated by a large number of techniques. The results show interesting situations. This paper reviews the work on the role of hydrogen on the growth, structure and properties of MCD and DLC thin films.     

Effects of the TiC and sintering process on the TiC-steel composite

In this paper, the composite of low alloy steel reinforced with TiC particles was prepared by conventional powder metallurgy process. The effects of the combined carbon content of TiC powder, sintering temperature and heating rate on the composite were studied. The results showed that the selected TiC powder has a combined carbon content of 17.91?wt-% and the optimal sintering process is heating from 600 to 1440°C at a rate of 1°C?min^?1 and holding for 1?h at 1440°C. The composite after heat treatment has excellent mechanical properties with density of 6.45?g?cm^?3, hardness of 68–69 HRC and TRS of 1763?MPa, respectively, and will be used as wear-resistant parts, assembly fixtures, moulds, etc.     

不同反应气源对制备纳米金刚石膜的影响

为确定两种典型的反应气源对制备纳米金刚石膜的影响,分别以CH4/Ar/H2及CH4/N2混合气体作为反应源,用微波等离子体化学气相沉积(MWPCVD)法制备纳米金刚石薄膜.XRD和Raman分析表明两种气源条件下得到的膜材均为金刚石多晶膜,但用CH4/N2气为反应源沉积的膜材中非金刚石相成分明显更多;AFM和SEM对照分析证实所有膜层的平均晶粒尺寸及表面粗糙度均在几十纳米量级,但CH4/N2气源沉积的膜中容易形成异常长大的晶粒,不利于表面质量的提高.研究结果表明,以CH4/Ar/H2混合气体作为反应气源可制备物相组成纯度更高、表面形态更为优越的纳米金刚石膜.     

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.