Diamond-like carbon sintered compacts formed by spark plasma sintering

A spark plasma sintering (SPS) method was utilized for the novel production of diamond-like carbon (DLC) compacts. Two amorphous carbon powders with different particle sizes (45 μm and 24 nm diameter) were employed as starting materials for the sintering experiments. The carbon powders were sintered using a SPS system at various sintering temperatures and holding times. The structural properties of the sintered compacts were evaluated using X-ray diffraction (XRD) analysis and high-resolution transmission electron microscopy (HRTEM). Disk-shaped compacts were obtained by sintering the powder with a particle diameter of 45 μm, although the compacts were very brittle and easily broken. However, sintering of the 24 nm diameter powder particles at temperatures of 1473 to 1573 K with a holding time of 300 s led to the successful production of sintered compacts without breakage. Reflection peaks related to graphite structure were observed in XRD patterns of the compacts sintered from the 24 nm diameter particles. HRTEM analysis revealed that the compacts sintered at 1473 K with a holding time of 300 s had an amorphous structure and consisted of 34% sp³ carbon bonding. Evaluation of the structural properties indicated that sintered compacts with DLC structure could be created by the SPS method with 24 nm diameter amorphous carbon particles.     

Microstructure and mechanical properties of hot-pressed B4C/TiC/Mo ceramic composites

Boron carbide (B₄C)/TiC/Mo ceramic composites with different content of TiC were produced by hot pressing. The effect of TiC content on the microstructure and mechanical properties of the composites has been studied. Results showed that chemical reaction took place for this system during hot pressing sintering, and resulted in a B₄C/TiB₂/Mo composite with high density and improved mechanical properties compared to monolithic B₄C ceramic. Densification rates of the B₄C/TiC/Mo composites were found to be affected by additions of TiC. Increasing TiC content led to increase in the densification rates of the composites. The sintering temperature was lowered from 2150 °C for monolithic B₄C to 1950 °C for the B₄C/TiC/Mo composites. The fracture toughness, flexural strength, and hardness of the composites increased with increasing TiC content up to 10 wt.%. The maximum values of fracture toughness, flexural strength, and hardness are 4.3 MPa m^1/2, 695 MPa, and 25.0 GPa, respectively.     

In-situ fabrication of TiC-Fe3Al cermet

Titanium carbide has high hardness, resistance to oxidation and abrasion while iron aluminide has proper ductility as well as good strength and excellent oxidation resistance up to high temperatures. Therefore, it can be expected TiC-iron aluminide cermet to have excellent mechanical properties as a cutting tool and a wear-resistance material. In this study, mechanical milling and hot press sintering processes were used to manufacture in-situ TiC-Fe₃Al cermet, whose microstructure and mechanical properties were examined according to the changes in volume fraction of TiC and milling time. After 48 h of milling each mechanically alloyed powder crystallized in a TiC and Fe₃Al biphasic material. The milled powder was hot-pressed at 1250 ℃ and 50 MPa for 30 min to obtain sintered bodies also consisting of only TiC and Fe₃Al phases. The hard phase, TiC, had a size of 100–300 nm with overall uniform distribution decreasing as the volume fraction of TiC increased. The hardness of each sintered body showed a linearly increasing tendency according to the increase in TiC content, the hardness for 90 vol% TiC cermet being as high as 1813Hv. On the other hand, the bending strength was 1800 MPa and 1780 MPa when TiC volume fraction was 50% and 70%, respectively, while it showed an abrupt decrease up to 580 MPa at 90% TiC volume fraction. Fe₃Al phase is effective to toughening of TiC-Fe₃Al cermet and the volume fraction of Fe₃Al phase significantly influences the bending strength of the cermet.     

Examination of nanocrystalline TiC/amorphous C deposited thin films

The relationship between structural, chemical and mechanical properties of nanocrystalline TiC/amorphous C (TiC/a:C) thin films was studied. Thin films were deposited by DC magnetron sputtering on oxidized silicon (Si/SiO₂) substrates in argon at 25 °C and 0.25 Pa. The input power of the carbon target was kept at constant value of 150 W while the input power of the titanium target was varied between 15 and 50 W.It was found that all thin films consist of a few nanosized columnar TiC crystallites embedded in carbon matrix. The average size of TiC crystallites and the thickness of the carbon matrix have been found to correlate with Ti content in the films. The mechanical properties of the films have been strictly dependent on their structure. The highest values of the nanohardness (∼66 GPa) and Young's modulus (∼401 GPa) were observed for the film with the highest TiC content which was prepared at the largest input power of Ti target.     

Sintering,mechanical, electrical and oxidation properties of ceramic intermetallic TiC–Ti3Al composites obtained from nano-TiC particles

The paper discusses the development of a new material system for interconnect application in Solid Oxide Fuel Cells (SOFC) based on TiC–Ti₃Al. Nano-sized TiC powders utilized in this research were synthesized using carbon coated TiO₂ precursors from a patented process. The pressureless sintering of TiC–Ti₃Al in a vacuum was applied at temperatures between 1100 °C and 1500 °C and content of Ti₃Al was varied in the range of 10–40 wt%. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for phase evaluation and sintering behavior. Relative density increased markedly with increasing sintering temperature because of grain growth and formation of the Ti₃AlC₂ secondary phase. Dense products (>95% TD) were prepared from nanosized TiC powders with 10 and 20 wt% Ti₃Al, but with about 8 to 10% porosity for 30 and 40 wt% Ti₃Al. The mechanical properties were determined from Vickers hardness and fracture toughness calculations. Vickers hardness decreased and fracture toughness increased with increasing Ti₃Al content. The electrical conductivity and oxidation behavior of TiC–Ti₃Al composites were investigated to evaluate the feasibility for SOFC interconnect application. The electrical conductivity measurements in the air at 800 °C for 100 h were made using the Kelvin 4-wire method.     

Titanium carbide based composites for high temperature applications

Titanium carbide based composites with nickel alloys and iron alloys are currently used in high performance applications where wear and corrosion are the main sources of material failure. For high temperature critical applications, however, the metallic binders nickel and iron limit the use of TiC-based composites. Hence, new binder systems which have good high temperature properties need to be developed in order to extend the use of TiC-based composites. Silicides and aluminides are potential binder systems with their good high temperature corrosion and mechanical properties. In this study, two binder systems, Fe–25 at% Si and Fe–40 at% Al have been selected, and were processed with reaction sintering of elemental Fe and Si, or Fe and Al powders, with 65 wt% TiC and 80 wt% TiC powders at temperatures at around 1410–1430°C under vacuum. X-ray diffraction analysis show TiC and Fe₃Si phases in the TiC-iron silicide composites, whereas TiC, Fe₃Al₂ and Fe₃AlC_0·5 phases were observed in TiC–iron aluminide composites. Differential thermal analysis of the samples shows that liquidus temperatures of the iron-silicide and iron-aluminide binders were around 1265 and 1425°C, respectively. Vickers microhardness values of 1100–1470 kg mm⁻² and 3-point bending strengths of 600–775 MPa were obtained in these high density reaction sintered TiC–iron silicide and TiC-iron aluminide composites.     

Pressureless sintering of titanium carbide doped with boron or boron carbide

Titanium carbide ceramics with different contents of boron or B₄C were pressureless sintered at temperatures from 2100 °C to 2300 °C. Due to the removal of oxide impurities, the onset temperature for TiC grain growth was lowered to 2100 °C and near fully dense (>98%) TiC ceramics were obtained at 2200 °C. TiB₂ platelets and graphite flakes were formed during sintering process. They retard TiC grains from fast growth and reduced the entrapped pores in TiC grains. Therefore, TiC doped with boron or B₄C could achieve higher relative density (>99.5%) than pure TiC (96.67%) at 2300 °C. Mechanical properties including Vickers’ hardness, fracture toughness and flexural strength were investigated. Highest fracture toughness (4.79 ± 0.50 MPa m^1/2) and flexural strength (552.6 ± 23.1 MPa) have been obtained when TiC mixed with B₄C by the mass ratio of 100:5.11. The main toughening mechanisms include crack deflection and pull-out of TiB₂ platelets.     

Electrical and thermal response of silicon oxycarbide materials obtained by spark plasma sintering

In order to obtain dense silicon oxycarbide (SiOC) materials that maintain the properties of glass, non-conventional spark plasma sintering was used to sinter SiOC powders from 1300 to 1700 °C and with 40 MPa of pressure. The concurrence of electrical current, high pressure and low vacuum while the material is being heating produces a dense SiOC-derived material composed of a SiO₂ glassy matrix reinforced with SiC nanowires grown in situ, graphene-like carbon and turbostratic graphite. SiOC materials with high electrical and thermal response are obtained as a result of this new processing technique. Electrical resistivity undergoes an extraordinary decrease of five orders of magnitude from 1300 (1.0 × 10⁵ Ω m) to 1700 °C (0.78 Ω m), ranging from insulate to semiconductor material; and thermal conductivity increases by 30%, for these sintering temperatures.     

Effect of temperature and heating rate on the sintering performance of SiC-Al2O3-Dy2O3 and SiC-Al2O3-Yb2O3 systems

Samples of SiC+10 vol%(Al₂O₃+Dy₂O₃) and SiC+10 vol%(Al₂O₃+Yb₂O₃) mixtures were obtained by cold isostatic pressing and sintered for one hour in a dilatometer at 1800 °C and 1900 °C, applying heating rates of 10, 20 and 30 °C/min. The results of the complete sintering cycle indicated that the heating rates do not significantly influence the shrinkage, but that temperature and total sintering time may be relevant factors. The compacts sintered at 1900 °C shrank on average 9% more than those sintered at 1800 °C, and it was found that the sintering time can be reduced by 40–50% at faster heating rates. The maximum shrinkage rates occurred at temperatures lower than those of the sintering thresholds for the two mixtures, two temperatures and three heating rates. It was also found that after formation of the liquid, the mechanisms of particle rearrangement and solution-precipitation were not as fast as reported in the literature, even at high heating rates, for example 30 °C/min, but they are responsible for much of the shrinkage occurring throughout the sintering cycle.     

Ceramic TiC/a:C protective nanocomposite coatings: Structure and composition versus mechanical properties and tribology

The relationship between the structure, elemental composition, mechanical and tribological properties of TiC/amorphous carbon (TiC/a:C) nanocomposite thin films was investigated. TiC/a:C thin film of different compositions were sputtered by DC magnetron sputtering at room temperature. In order to prepare the thin films with various morphology only the sputtering power of Ti source was modified besides constant power of C source. The elemental composition of the deposited films and structural investigations confirmed the inverse changes of the a:C and titanium carbide (TiC) phases. The thickness of the amorphous carbon matrix decreased from 10 nm to 1–2 nm simultaneously with the increasing Ti content from 6 at% to 47 at%. The highest hardness (H) of ~26 GPa and modulus of elasticity (E) of ~220 GPa with friction coefficient of 0.268 was observed in case of the film prepared at ~38 at% Ti content which consisted of 4–10 nm width TiC columns separated by 2–3 nm thin a:C layers. The H3/E2 ratio was ~0.4 GPa that predicts high resistance to plastic deformation of the TiC based nanocomposites beside excellent wear-resistant properties (H/E=0.12).     

Sputtered nanocrystalline ceramic TiC/amorphous C thin films as potential materials for medical applications

The relationship between structural behaviour of sputtered TiC/amorphous C (TiC/a:C) thin films and corrosion properties was measured in three various pH solutions (0.5 M NaCl (pH=6); 0.1 M HCl (pH=1); and 0.1 M NaOH (pH=13)). The ~400 nm thick nanocomposites were deposited by DC magnetron sputtering on different substrates (Ti6Al4V alloy and CoCrMo alloy) in argon at 25 °C and 0.25 Pa with 150 W input power of carbon target and 50 W input power of titanium target. The structure and composition of nanocomposites were investigated by Transmission and Scanning Electron Microscopy. In both samples the structural investigations confirmed columnar structure of TiC/a:C films with 25–50 nm sized cubic TiC. These columns were separated by 2–3 nm thin amorphous carbon layers. TiC/a:C /Ti6Al4V alloy implant material showed better corrosion resistance than the TiC/a:C/CoCrMo alloy in 0.5 M NaCl solution based on results of the Electrochemical Impedance Spectroscopy. For both samples, the 0.1 M NaOH solution was the most corrosive media.     

Some observations on filter pressing and sintering of yttria-stabilized zirconia nanopowder

The aim of this work was investigation of the sintering behaviour of a material prepared by filter pressing of an yttria-stabilized zirconia powder with grain sizes of about 8 nm. The water suspension of the powder was filter-pressed under 5 MPa. The early shrinkage of the filter-pressed sample, during its non-isothermal sintering was attributed to removal of water layers adsorbed on the powder surface. The observation of the microstructure evolution in samples heat-treated at different temperatures was performed. Pore growth during sintering was related both to presence of agglomerates, and to pore coalescence. Particle arrangement in the material was very uniform, which led to uniform densification of the material. Heat treatment of the sample for 30 min at 1200°C resulted in the material of 99.96% relative density, and grains within nanometric range.     

Development and evaluation of TiB2–AlN ceramic composites sintered by spark plasma

Titanium diboride (TiB₂) is a ceramic material with high mechanical resistance, chemical stability, and hardness at high temperatures. Sintering this material requires high temperatures and long sintering times. Non-conventional sintering techniques such as spark plasma sintering (SPS) can densify materials considered difficult to sinter. In this study, TiB₂–AIN (aluminum nitride) composites were sintered by using the SPS technique at different sintering temperatures (1500 °C, 1600 °C, 1700 °C, 1800 °C, and 1900 °C). x-ray diffraction was used to identify the phases in the composites. mechanical properties such as hardness and indentation fracture toughness was obtained using a vickers indenter. Different toughening mechanisms were identified, and good densification results were obtained using shorter times and lower temperatures than those previously reported.     

Consolidation of binderless nanostructured titanium carbide by high-frequency induction heated sintering

The rapid sintering of nanostructured TiC hard materials in a short time was investigated with High-Frequency Induction Heating Sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and prohibition of grain growth in nanostructured materials. A dense nanostructured TiC hard material with a relative density of up to 99% was produced with simultaneous application of 80 MPa pressure and induced current of output of total power capacity (15 kW) within 4 min. The effect of the ball milling times on the sintering behavior, grain size and mechanical properties of binderless TiC was investigated.     

Sub-micrometre grained UO2 pellets consolidated from sol gel beads using spark plasma sintering (SPS)

UO₂ beads from the sol supported precipitation method were calcined at a low temperature in order to obtain porous micro-beads, composed of nanometric particles. The sintering behaviour of the beads in spark plasma sintering was investigated. The powder had a good sinterability and the final grain size of the pellets could be tailored by varying the processing conditions, in order to resemble the microstructure of the traditionally fabricated UO₂ pellets (i.e. grains of several µm size), or to achieve sub-micrometre size as observed in the high burnup structure. Dense UO₂ pellets with a grain size as small as 300 nm were obtained by sintering at 835 °C without dwell time, whereas 3 µm grained pellets were obtained at 1000 °C and a 5 min dwell time.     

Properties and rapid consolidation of nanostructured TiC-based hard materials with various binders by a high-frequency induction heated sintering

The densification of hard TiC–10 vol.% binder (Co, Ni, Fe) materials was accomplished within 2 min using a high-frequency induction heated sintering (HFIHS) method. The advantage of this process is not only rapid densification to almost the theoretical density but also the prevention of grain growth of the nano-structured materials. Highly dense TiC–binder (Co, Ni, Fe) composites with a relative density of up to 99.9% were obtained within 2 min by HFIHS under 80 MPa. The average grain size of TiC in the TiC–10 vol.% Ni composite was approximately 44 nm. The hardness and fracture toughness of the dense TiC–10 vol.% binder (Co, Ni, Fe) composite produced by HFIHS were also investigated.     

Athermal and thermal mechanisms of sintering at high heating rates in the presence and absence of an externally applied field

In order to establish the relative contributions of thermal and athermal mechanisms to densification in the absence of an extrinsic sintering pressure, nanometric powder compacts were sintered with and without applied fields using varied heating rates from 50 °C/min up to 800 °C/min. The relative contribution of the thermal and athermal mechanistic contributions to the densification behavior of two model dielectric ceramics, hydroxyapatite and zinc oxide, is evaluated in the context of the current leading theories of field-assisted sintering mechanisms. The effects of elevated heating rates in nanometric, dielectric ceramics are found to be minimal in the absence of a field. However, in the presence of an applied field there appears to be a synergistic effect with heating rate.     

Copper-doped BaZrO3 crucibles for YBCO single crystal growth

The corrosion resistance of BaZrO₃ to molten barium cuprates is controlled by secondary phases, and requires high chemical and phase purity, as well as high density for sustained flux containment. The high sintering temperatures required for solid-state derived powders is a significant obstacle inhibiting more widespread use of high density BaZrO₃. We have investigated the use of Cu²⁺ doping to enable practical sintering temperatures whilst still producing corrosion resistant ceramics. Low levels of copper addition (0.2 wt.% CuO equiv.) produced densities of >97% theoretical using sintering temperatures as low as 1400 °C, as long as the dopant was added after all powder calcination steps, i.e. immediately before forming/sintering. Corrosion resistance at 0.2 wt.% CuO equiv. was equal to un-doped materials fabricated using the same powder processing method.     

Formation of multi-walled carbon nanotubes by Ni-catalyzed decomposition of methane at 600–750 °C

Ni-catalyzed decomposition of methane at high temperatures was examined by using a thermogravimetric apparatus. The catalyst (10 wt.% Ni supported on spherical alumina) gave quite a high carbon nanotube (CNT) yield at the temperatures below 680 °C. At > 700 °C, however, carbon formation rate decreased with increasing the reaction temperature. Temperature-programmed reaction also showed the maximum CNT growth rate at ~ 690 °C. This result ruled out the possibility that the apparent negative activation energy is caused by sintering of Ni particles. Detailed examination on the kinetic expression led us to a conclusion that the dissolution of carbon atoms formed by dissociation of methane into bulk of the nickel particles is the rate-determining step at high temperatures, while methane adsorption is the rate-determining step at lower temperatures. This idea also explains the fact that the carbon yield drastically decreased at high temperatures. The CNTs formed at these temperatures had thinner walls than those formed at lower temperatures. The latter fact also supports the idea that the solubility of carbon in the nickel particles decreases at high temperatures.     

Dense zircon (ZrSiO4) ceramics by high energy ball milling and spark plasma sintering

The addition of sintering additives has always been detrimental to the mechanical properties of sintered ceramics; therefore, methods to reduce or, as in this case, eliminate sintering additives are usually relevant. In this paper, dense zircon ceramics were obtained starting from mechanically activated powder compacted by spark plasma sintering without employing sintering additives.The high energy ball milling (HEBM) of starting powder was effective to enhance the sintering kinetics. The structural changes of the zircon powder introduced by the HEBM were evaluated. The phase composition and the microstructure of bulk zircon material were analyzed by SEM (EDAX) and XRD. The Vickers hardness and the fracture toughness were evaluated as well.Fully dense materials were obtained at 1400 °C with a heating rate of 100 °C/min, 10 min soaking time and 100 MPa uniaxial pressure. The zircon samples sintered at temperatures above 1400 °C were dissociated in monoclinic zirconia and amorphous silica. The dissociation was detrimental for the mechanical properties. Unlike conventional sintering methods (hot pressing, pressureless sintering) SPS permitted to overcome the dissociation of the zircon material and to obtain additive free, fully dense zircon ceramic with outstanding mechanical properties.