Hot pressing of hafnium diboride aided by different sinter additives

Experiments were conducted by hot-pressing to densify HfB₂ commercial powders using HfSi₂ (5 vol%) or B₄C (7 vol%) as sinter additives. The former was very effective as sintering enhancer: 15 min at 1,600 °C was a sufficient condition to achieve near full density. The occurrence of a liquid phase sintering, which substantially enhanced densification during hot-pressing, was found out. The HfB₂–B₄C powder mixture, hot-pressed at 1,900 °C for 40 min, achieved a relative density of 94%. Aside the key role of sintering enhancer, B₄C also allowed the development of a uniform microstructure, preventing an excessive growth of the diboride matrix (2 μm mean size). On the contrary, thanks to the liquid phase that sustained substantial transfer of mass during heating, the HfSi₂-doped composition had an uneven and more obvious grain growth (4 μm mean size), with faceted diboride grains up to 10 μm.     

Reactive processing of titanium carbide with titanium

The reactive sintering of titanium carbide with titanium metal was studied using mechanical mixtures of fine-grained powders heated in vacuum above the TiC-Ti eutectic temperature. Mixtures with bulk compositions of TiC_0.94 to TiC_0.63 yielded nonstoichiometric carbide with less than 0.5 wt% residual titanium metal after sintering, while residual metal was observed at higher titanium concentrations. The effects of time, temperature, and composition on Mohs hardness, final porosity and final grain-size were determined using a Box-Wilson experimental design. The experimental ranges studied were sintering times of 10 to 100 min, sintering temperatures of 1650 to 1850° C, and compositions from TiC_0.94 to TiC_0.58. Over these experimental ranges, the effects of time and temperature were small compared with those of composition. The Mohs hardness increased approximately linearly from two to nine with increasing percentage of titanium metal in the starting powder. The average grain size ranged from 15 to 70μm, increasing with increasing time and temperature. For bulk compositions TiC_0.94 to TiC_0.70 grain growth was largely due to the conversion of titanium to substoichiometric carbide which grows epitaxially on the carbide grains. Substantial grain growth occurred for higher metal concentrations. The open porosity decreased from 28% to 16% as the amount of titanium metal in the starting powders was increased. Both the grain growth and the densification during reactive sintering of titanium-titanium-carbide mixtures were analysed in terms of a sintering model adapted from Kuczynski. A factor which empirically describes the behaviour of the system over a range of compositions was incorporated into the equations proposed by Kuczynski. Microstructural evidence and the activation energies for grain growth and densification all indicate that the rapid reaction between titanium metal and titanium carbide to form substoichiometric carbide occurs via short-circuit diffusion of carbon out of the carbide grains along Ti₂C platelets. Low sintered densities are attributed to the rapid formation of a solid titanium-carbide skeleton which prevents significant particle rearrangement in the eutectic liquid. Solution-precipitation processes do not appear to contribute significantly to the densification in this system.     

Microwave sintering of titanium diboride

Using a 2.45 GHz, 6 kW microwave furnace adapted for inert gas sintering, titanium diboride (TiB₂) can be rapidly microwave-sintered to >90% of theoretical density with sintering temperatures of 1900 to 2100 °C and soak times of 30 min or less. Densification behaviour with low-level additives was evaluated; 3 wt% chromium diboride (CrB₂) was an excellent sintering aid-grain growth inhibitor. A special covering system was required to produce oxidefree TiB₂. Specimen surface and interior temperatures were determined with a hole experiment. Comparison with conventional sintering indicates that microwave sintering of TiB₂-3 wt% CrB₂ occurs at lower temperatures (i.e., 200 °C lower) and can yield material with improved hardness, grain size, and fracture toughness.A portion of the material in this article was presented at a Symposium on Microwave Processing of Ceramics, 91st Annual Meeting of the American Ceramic Society, Indianapolis, Indiana, April 1989.Operated for the U.S. Department of Energy by Martin Marietta Energy Systems, Inc., under contract DE-AC05-84OR21400.     

粉末粒径对热压烧结碳化硼致密化及力学性能的影响

采用平均粒径分别为3.5 μm、1.5 μm和200 nm的碳化硼粉体为原料经1850℃热压烧结制备了碳化硼陶瓷, 研究了粉体粒径对陶瓷烧结致密化过程及其性能的影响。根据保温时间对线收缩率的影响及热压初期的塑性流动机理, 得出了不同粉体间烧结初期的激活能差。结果表明: 在相同工艺条件下, 随着粉体平均粒径的减小, 粉体的扩散激活能降低, 致密化初始温度降低, 而且完成塑性流动所需时间也会明显缩短, 致密化速率加快, 致密度增大; 碳化硼陶瓷的显微结构与力学性能亦随着粉体粒径的减小而改善; 1850℃保温1 h后, 平均粒径为200 nm的粉体制备的碳化硼陶瓷相对密度可达90.5%, 硬度为(17±1.8) GPa。     

Densification process of borosilicate glass powders under hydrothermal hot-pressing conditions

Borosilicate glass powders containing Li₂O over 3.3 wt% were densified by the addition of water under hydrothermal hot-pressing conditions at 25 MPa below 300° C. The initial shrinkage proceeded by a viscous flow mechanism. The glass powders with a high concentration of network-modifying oxides had high densification rates. At low temperature, the glass powders were gently densified after an induction period in which shrinkage was very slow. During the induction period, the glass powders reacted with water and lithium in the glass structure was dissolved in water.     

Densification and microstructural evolution during laser sintering of A356/SiC composite powders

This article reports experimental results on laser sintering of A356 aluminum alloy and A356/SiC composite powders. Effects of scan rate, sintering atmosphere, hatch spacing, and SiC volume fraction (up to 20%), and particle size (7 and 17 μm) on the densification were studied. The phase formation and microstructural development were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). Laser sintering under argon atmosphere exhibited higher densification compared to nitrogen. A faster sintering kinetics was observed as the scan rate decreased. Except at a low SiC content (5 vol%), the composite powders exhibited lower densification kinetics. The densification was improved when finer SiC particles were utilized. Microstructural studies revealed directional solidification of aluminum melt to form columnar grains with inter-columnar silicon precipitates. In the presence of SiC particles, aluminum melt reacted with the ceramic particles to form Al₄SiC₄ plates.     

Reactive sintering mechanism of Ti + Mo<Subscript>2</Subscript>C and Ti + VC powder compacts

TiC reinforced Ti-matrix composites have been synthesized successfully by reactive sintering of Ti-1.5%Fe-2.25%Mo (wt%) powder compacts with addition of Mo₂C and VC particles. The reactions for the formation of TiC particles start at 600 °C, but the distribution of TiC particles and the densification behavior in the two compacts are significantly influenced by the metal carbides (Mo₂C or VC). The compact with addition of Mo₂C has a relative density of 98% after sintering at 1300 °C for 1.5 h, but TiC particles are agglomerated in the Ti matrix. The compact with addition of VC has a relative density of about 91% after sintering at 1300 °C for 1.5 h, but TiC particles distribute more homogenously in the Ti matrix. Different TiC particle distribution and densification behaviors are attributed to the reaction rates between Ti and metal carbides and the subsequent diffusion process.     

Effect of metal oxide precursor on sintering shrinkage, microstructure evolution and electrical properties of silver-based pastes

Since the shrinkage behavior of silver-based films has been correlated with the characteristics of oxide additives used, the relative role of metal oxides and metal-organic precursors in sintering shrinkage and microstructure evolution of silver films was investigated and compared in this work. Films with an oxide powder additive exhibit two-stage shrinkage behavior in contrast to one-stage continuous shrinkage, which occurs in silver films with metal-organic precursors, added. Furthermore, metal-organic precursors are less effective than metal oxide powders in reducing shrinkage of silver-based films. That can be reasonably explained that metal-organic precursors can be effectively decorated around silver powder to inhibit the sintering densification. The Zr-based organic precursor among the metal-organic precursors exhibits optimal retardation in sintering densification of silver film, which is probably interrelated to refractory characteristics of ZrO₂. The unique conductivity and grain growth of silver film with 1.0 wt% tungsten-organic precursor added was possibly due to the partial dissolution of W into Ag.     

Sintering properties of submicron TiC powders from carbon coated titania precursor

The sintering behavior of submicron titanium carbide (TiC) synthesized from carbon coated titania (TiO₂) precursor was investigated in TiC-Ni system. The densification was examined as functions of initial carbon content (30.95–34 wt.%) and Ni content (3–20 wt.%). The sintered density of TiC-Ni was markedly decreased with increased carbon content in the precursor. The amount of Ni had a relatively small influence on the densification of submicron TiC-Ni cermet compared with TiC (commercially available HCS)-Ni cermets. The results show that submicron TiC with only 3 wt.% Ni can be sintered to densities above 95% TD in flowing Ar+10H₂ at 1500°C and below. The improvements in densification result from the capillary force increase since it is inversely dependent on the particle size. With decreased Ni content, the Vickers hardness increased and the fracture toughness decreased, as expected. However, the sufficient densification cannot be achieved for commercial HCS TiC powder sintered with Ni (<10 wt.%) under the same conditions. Therefore, both the Vickers hardness and fracture toughness decreased as the Ni content decreased. This was due to the increase of porosity in the sintered samples containing commercial TiC powder.     

Investigation in the sintering of Y2O3 powders in the temperature range 1000 to 1400° C

This paper is devoted to a study of the sintering of two Y₂O₃ powders in the temperature range where only minor densification occurs. Two powders have been examined; one powder, Y₂O₃-A, was obtained by decomposition of hydroxide, because earlier examinations showed [11] that use of this powder resulted in the highest densities of samples in the sintering temperature range from 1300 to 1900° C. The second powder, Y₂O₃-D, was purchased externally. In order to ensure that the pores in the Y₂O₃-A compacts closed as late as possible, the heating rates up to the appropriate temperatures (1000 to 1400° C) were varied in the range 0.013 to 6° C sec^–1. The results obtained show that the heating rate in this temperature range, for the powder obtained by decomposition of hydroxide, is of primary importance in the densification of the material, and that cessation of shrinkage was not observed in the period of 240 min.     

Preparation of Ti<Subscript>3</Subscript>AlC<Subscript>2</Subscript> by mechanically activated sintering of 3Ti/Al/2C/0·2Sn

The mechanically activated sintering process was adapted to synthesize titanium aluminum carbide (Ti₃AlC₂) at low temperature. A mechanically induced self-propagation reaction occurred by mechanical alloying of 3Ti/Al/2C powder mixtures. In addition to powder products, a large amount of rigor granules with a size of 0.5 ∼ 10 mm were produced. Fine powders containing Ti₃AlC₂, Ti₂AlC and TiC were obtained. The granules composed of Ti₃AlC₂, Ti₂AlC and TiC. Adding Sn may remove Ti₂AlC and enhance the synthesis of Ti₃AlC₂. After Sn was added, the products only contained Ti₃AlC₂ and TiC. The Ti₃AlC₂ content of the powders and granules were 75 wt% and 88 wt%, respectively. The mechanically alloyed products were pressureless sintered at 900–1300°C for 2 h. Sintering of these products at 900 ∼ 1200°C yields samples containing over 95 wt% Ti₃AlC₂. The sintered powder compacts with high purity Ti₃AlC₂ had a fine organization. The lath Ti₃AlC₂ of the granules had a length of 10–20 μm.     

Rapid fabrication of in situ TiC particulates reinforced Fe-based composites by spark plasma sintering

TiC particulates reinforced Fe-based composites have been fabricated using ferrotitanium and carbon black powders with the combination of in situ and spark plasma sintering (SPS) technique. The sintering and densification behaviors were investigated. The results show that when the composite was sintered at 1150 °C for 5 min, the maximum relative density and hardness are 99.2% and 83.2 HRA, respectively. The phase evolution during sintering indicates that the in situ reaction occurs evidently between 850 °C and 1050 °C. The microstructure investigation demonstrates that with the rapid in situ SPS technique, fine TiC particulates with a size of ~ 1 μm are homogeneously distributed in the matrix.     

Constitutive modeling of alumina sintering: grain-size effect on dominant densification mechanism

In the present work, alumina powders with the initial grain sizes of 0.9 and 7.0 μm, respectively, were sintered at different temperatures. Constitutive laws for densification were employed to model the sintering process of alumina ceramics. Based on the constitutive laws employed and the experimental results obtained, the dominant densification mechanism was identified and the effect of grain size on dominant densification mechanism was discussed. The activation energy for densification was also evaluated. In the investigated sintering temperature range, interface reaction was identified as the controlling process in sintering of alumina powders with the initial grain size of 0.9 μm, while grain-boundary diffusion was identified as the dominant process in sintering of alumina powders with the initial grain size of 7.0 μm. The activation energies for densification of the finer and coarser grain size alumina ceramics were determined as 342 and 384 kJ mol⁻¹, respectively, which provided a strong support on the densification mechanism investigation.     

Influence of stoichiometry and impurity on the sintering behaviour of barium titanate ceramics

The influence of stoichiometry, i.e. Ba/Ti ratio, and impurity on the densification of BaTiO₃ were investigated. The BaTiO₃ powders were prepared by conventional calcination of BaCO₃ and TiO₂. The stoichiometric ratios (Ba/Ti) were in the range 0.99 to 1.005. Impurity effects on the sintering behaviour were investigated with different purities of raw powders. The sintering behaviour of BaTiO₃ has been studied extensively but an understanding of stoichiometric effects on densification is still incomplete. An excess of TiO₂ lowered the onset temperature of sintering (initial state of sintering — 3% shrinkage). These results indicate that stoichiometric variation of BaTiO₃ affects the initial state of sintering. The rate of densification for a Ti-rich sample was considerably faster than that for a Ba-rich sample. It was a so-called activated sintering. The TiO₂ excess reacts with BaTiO₃ to form Ba₆Ti₁₇O₄₀, which forms with BaTiO₃ a eutectic melt at 1320 °C. The liquid phase, however, enhanced grain growth, not densification.     

Study of the sintering properties of plasma synthesized ultrafine SiC powders

A study on the sintering of ultrafine SiC powders synthesized from elemental Si and CH₄ using radio frequency (r.f.) induction plasma technology is reported. The powder had a particle size in the range of 40 to 80 nm and was composed of a mixture of α and β-SiC. It was subjected to pressureless sintering in an induction furnace in the presence of different sintering aids. With the addition of B₄C (2.0 wt% B) by mechanical mixing, the powders could only be partially densified, with the highest value of 84.5% of theoretical density being achieved at 2170 °C for 30 min. Through the use of “in-flight” boron doping of the powder during the plasma synthesis step (1.65 wt % B), the ultrafine powder obtained could be densified to above 90% of its theoretical density at 2050 °C for 30 min. The addition of oxide sintering aids (7.0 wt % Al₂O₃ + 3.0 wt % Y₂O₃) by mehanical mixing produced sintered pellets of 95% of theoretical density at 2000 °C for 75 min. The Vicker’s microhardness of the sintered pellets in this case was as high as 31.2 GPa. In order to improve our understanding of the basic phenomena involved, extensive microstructural (scanning electron energy microscopy: SEM), physical (shrinkage, weight loss, porosity, hardness) as well as chemical analysis (prompt gamma neutron activation analysis (PGNAA), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA)) was carried out. This helped establish a relationship between the properties of the as-synthesized powder and their sintering properties. The influences of sintering temperature, sintering time, additive concentration, and powder purity on the densification behaviour of the plasma-synthesized powders was investigated. The results were compared with data obtained using commercial powder. This revised version was published online in November 2006 with corrections to the Cover Date.     

A new synthesis reaction of Ti3SiC2 from Ti/TiSi2/TiC powder mixtures through pulse discharge sintering (PDS) technique

Ti/TiSi₂/TiC powder mixtures with molar ratios of 1:1:4 (M1) and 1:1:3 (M2) were first employed for the synthesis of Ti₃SiC₂ through pulse discharge sintering (PDS) technique in a temperature range of 1100–1325 °C. It was found that Ti₃SiC₂ phase began to form at the temperature above 1200 °C and its purity did not show obvious dependence on the sintering temperature at 1225–1325 °C. The TiC contents in M2 samples is always lower than that of the M1 samples, and the lowest TiC contents in the M1 and M2 samples were calculated to be about 7 wt% and 5 wt% when the sintering was conducted at the temperature near 1300 °C for 15 minutes. The relative density of the M1 samples is always higher than 99% at sintering temperature above 1225 °C, indicating a good densification effect produced by the PDS technique. A solid-liquid reaction mechanism between Ti-Si liquid phase and TiC particles was proposed to explain the rapid formation of Ti₃SiC₂. Furthermore, it is suggested that Ti/TiSi₂/TiC powder can be regarded as a new mixture to fabricate ternary carbide Ti₃SiC₂. Received: 5 September 2001 / Accepted: 11 September 2001     

Influence of reinforcement and processing on steel-based composites: Microstructure and mechanical response

Steel matrix composite reinforced with 2–4 vol.% titanium diboride particles was fabricated successfully by powder metallurgy route through hot pressing method. Influence of sintering parameters on densification was investigated by measurement of density of resultant composites. Microstructural analysis of hot-pressed materials was performed. Hardness and deformation behavior under constant load were evaluated by conducting microhardness and nanoindentation tests. The addition of titanium diboride proved to be effective for enhancement of hardness and strength. Composite with 4 vol.% titanium diboride sintered at 1100°C resulted in improved hardness and elastic modulus which could be related to Orowan strengthening resulting from homogeneous distribution of fine titanium diboride particles in steel matrix. The results indicate that proposed method is economically feasible to process steel matrix composites with improved properties. A comparatively lower temperature and pressure offers better control of interface kinetics and microstructure.     

Abnormal grain growth enhanced densification of liquid phase-sintered WC–Co in support of the pore filling theory

This study examines the effect of grain growth on densification during liquid phase sintering of compacts with faceted grains. Two kinds of WC powders with different sizes were used to produce WC–Co alloys. Large pores of ~5 μm size were generated in 95WC–5Co (wt%) using spherical Co particles of the same size. The overall sintering behavior was observed by measuring grain growth and densification as a function of sintering time at a sintering temperature of 1350 °C. When the WC powder was fine (0.4 μm), large pores disappeared upon filling of pores by liquid with the formation of abnormal grains. On the contrary, when the WC powder was large (4.2 μm), grain growth is not observed, and large pores remained intact even after a long period of sintering (24 h). These observations confirm that densification during final stage liquid phase sintering occurs via filling of pores by liquid as a result of grain growth. This finding is consistent with the model of densification predicted by the pore filling theory.     

Microstructure evolution and densification behavior of TiC/316L composite powders during cold/warm die compaction and solid-state sintering: 3D particulate scale numerical modelling and experimental validation

To identify the microstructure evolution and densification behavior of TiC/316L composites in powder metallurgy (PM) process, 3D particulate scale numerical simulations were conducted to reproduce the cold/warm compaction and solid-state sintering of TiC/316L composite powders with corresponding physical experiments being carried out for model validation. The effects of compaction parameters and sintering temperature on the densification behavior of TiC/316L composite powders were systemically investigated. The particle deformation and morphology, stress/strain and microstructure evolutions, and grain size distribution in the whole process were characterized and compared to further illustrate the densification behavior and the underlying dynamics/mechanisms. The results show that compared with the cold compaction, the warm compaction can not only achieve higher relative density, smaller and more uniform equivalent stress, and weaker spring back effect, but also improve the friction condition among powder particles. The plastic deformation of 316L particles is the main densification mechanism during compaction. In the solid-state sintering of TiC/316L compacts, the densification is mainly indicated by shrinkage and vanishing of large residual pores along with the growth of the sintering necks, accompanied by the particle movement and growth along the boundary regions. Meanwhile, the particle displacement and grain size distribution are more uniform in the warm compacted TiC/316L component. Moreover, the equivalent (von Mises) stress in 316L particles is smaller than that in TiC particles.     

Sintering densification of titanium hydride powders

The sintering densification behaviors of titanium hydride are investigated at different compaction pressures, compared with pure titanium. The results show that the shrinkage and densification of the TiH₂ specimens after sintering are obviously higher than those of pure Ti. It is also found that the densification mechanism of the TiH₂ is mainly attributed to the volume change in the TiH₂ sintering process, and the surface cleaning effect of the released H atoms. In addition, it is also validated experimentally that the released H atoms are accompanied by the formation of H₂O, which reduces the oxide film on Ti surface, increases the chemical activity of the Ti surface, and promotes the sintering densification.