Sintering study of nanocrystalline tungsten carbide powders

WC powder with an average grain size of 6 nm was obtained after high energy ball milling under protective gas atmosphere. The kinetics of densification was studied during sintering the powder in a dilatometer up to 1450 °C. The microstructure was investigated by TEM and high resolution SEM after various stages of sintering. The green density of the specimens was 45%. Three stages of sintering were defined: (a) rearrangement of particles at low temperature (850 °C) without grain or particle growth, (b) neckformation between powder particles at 1000–1250 °C and initial grain growth at 1200 °C, (c) pore elimination accompanied by massive grain growth at 1300–1450 °C.     

Consolidation of nanoscale iron powders

The consolidation behavior of two types of nanoscale iron powders-vacuum condensed (nanograins in nanoparticles) and ball-milled (nanograins in microparticles), was studied. The consolidation of two microscale powders, atomized and ground, was also characterized for comparison. Consolidation techniques investigated were cold closed die-compaction, cold isostatic pressing (CIPing), and after CIPing, sintering or hot isostatic pressing (HIPing). The mechanical properties, density, and microstructure of the resulting compacts were found to depend on the original powder type and its consolidation history. Significant differences were found between the microscale and nanoscale powders. An additional reason, besides the dissimilarity in grain size, for the differences observed relates to the fact that the nanograin powders contained significant amounts of oxygen, which ultimately resulted in a distinctly two-phase bulk microstructure. The vacuum condensed powder achieved satisfactory green strength on CIPing, and high hardness (440 Hv) on low temperature sintering. While unnecessary for complete consolidation, HIPing at 500 °C was found to be beneficial and compacts of this powder thus treated were found to have a hardness of 520 Hv and high compressive yield strength (1800 MPa). For ball-milled powders, HIPing was found to be essential for achieving effective consolidation: ball-milled material, which remained friable after CIPing and sintering at 580 °C, achieved exceptionally high hardness (820 Hv) when HIPed at 580 °C and 175 MPa. The ductility was greatly improved when HIPed at temperatures between 700 °C and 850 °C, while preserving its relatively high strength. The behavior of these nanoscale powders can be understood by invoking the usual densification, particle bonding, and grain growth mechanisms. Optimization of these processes may result in unique mechanical properties of ball milled powders.     

Sintering behaviour of highly agglomerated ultrafine zirconia powders

The sintering behaviour of highly agglomerated ultrafine zirconia powders can be described by a combination of mechanisms such as neck formation and shrinkage, fissure formation and growth, pore growth, grain growth, pore-rearrangement shrinkage, and pore entrapment. There exist two optimum sintering temperatures: one is due to the competition between neck formation and shrinkage, fissure formation and pore growth; the other is due to the competition between pore shrinkage and pore entrapment, both resulting from grain growth. It is also found that an increase of green density, which is caused either by a different consolidation pressure, a different preparation method, or a different calcination temperature, results in a decrease of sintered bulk density. This can be explained by the state of agglomeration and the uniformity of powder packing.     

Densification of sintered lead zirconate titanate by hot isostatic pressing

The effects of hot isostatic pressing (HIPing) on sintered lead zirconate titanate are presented. Densities up to 98% were obtained by HIPing for 1 h at 1300°C with argon gas pressures of either 20.7 or 138 MPa. The microstructural changes observed after HIPing, and the rapid initial kinetics for densification and pore shrinkage, indicate that prssure-enhanced grain rearrangement and solution-precipitation processes are primarily responsible for densification. The persistance of large voids after HIPing suggests that it may be impossible to completely eliminate gross processing-related defects in lead zirconate titanate by HIPing.     

Processing of grain-size functionally gradient bioceramics for implant applications

This paper reports work on the processing of functionally gradient alumina bioceramics with a continuously decreasing grain size across the thickness, with the view of ultimately utilizing high-quality nano/ultrafine powders only at the surface of an implant to provide superior wear and mechanical properties. A model of disc geometry is used to examine the feasibility of producing this brand of materials. Wet processing/ball milling and sequential slip casting procedures were used to de-agglomerate alumina powders and deposit green layers of varying particle sizes from 50 to 250 nm. Both pressure-less sintering and hot pressing were evaluated as high temperature sintering/consolidation processes. The results indicate that pressure-less sintering may not be suitable. Hot pressing, however, achieved very promising results producing near fully dense product with a grain size that gradually changes across its thickness.     

Pore size distribution,grain growth,and the sintering stress

The effects of a pore size distribution and of the pore separation on the sintering stress is examined using a simple model. The sintering stress is found to be proportional to the mean of the pore sizes weighted according to the Voronoi cell pertaining to each pore, rather than to the simple pore size average. Large heteropores are shown to have little effect on the mean effective sintering stress. Decreases in pore coordination number of such pores, resulting from grain growth can significantly increase the stress intensification factor. The near-constancy of the sintering stress, observed experimentally for many powders over a wide range of sintered densities, does not directly follow from the simple model. It is argued that this constancy results from pore shrinkage, due to densification, which is compensated by pore growth due to coarsening.     

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 of AA2024-SiC nanostructured metal matrix composites

Nanostructured metal matrix composites (NMMCs) in large-dimension billets were fabricated by hot isostatic pressing (HIPing) of cryomilled powders consisting of AA2024 alloy reinforced by 25 wt.% SiC particles. Microstructure of the bulk nanostructured composites and cryomilled powders was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In addition, mechanical properties of the bulk nanocomposites were also addressed.     

Densification,microstructure, and properties of W-Mo-Cu alloys prepared with nano-sized Cu powders via large electric current sintering

This study rapidly fabricated a novel W-Mo-Cu alloy by large current electric field sintering at a relatively low temperature, and the effects of the powder size of Cu on the densification, microstructure, and properties were comprehensively investigated. The particle size of Cu did not influence the phase type but significantly affected the densification, microstructure, and properties. XRD and TEM results showed that the alloy contained three new phases aside from W, Mo, and Cu phases, i.e., Mo-W ordered phase, Mo-Cu solid solution, and Cu_0.4W_0.6 intermetallic compound. Copper powders with smaller sizes were beneficial to improving the distributional homogeneity of elements and the sintering densification. Therefore, the alloy prepared with 100 nm Cu powders had a denser and more homogeneous microstructure and better comprehensive properties than that prepared with 5 μm Cu powders. Overall, the W-Mo-Cu alloy prepared with 100 nm Cu powders at 980 °C proposed the best comprehensive properties, and its relative density can reach 98% approximately.     

超细/纳米粉末改进Ti(C,N)基金属陶瓷性能研究进展

综述了近几年超细或纳米粉末改进Ti(C,N)基金属陶瓷性能的方法,简要分析了含超细或纳米粉末Ti(C,N)基金属陶瓷的致密化问题.总结了真空烧结 热等静压处理和放电等离子烧结的特点,并分析了微波烧结和等离子活化烧结制备Ti(C,N)基金属陶瓷的可能性.     

掺杂ZnO对SnO_2电极烧结性能的影响

通过共沉淀法制备了掺氧化锌的二氧化锡超细粉体,平均粒径为20nm,并制得氧化锡电极样品.主要研究了在氧化锡电极中添加不同含量的ZnO对电极烧结性能的影响情况,分析电极的力学性能、电学性能以及微观结构,结果表明,一定量ZnO掺杂有利于提高SnO2电极的烧结致密化,同时也提高了SnO2电极的导电性.     

氧化锌铝(ZAO)靶材的制备及性能研究

采用冷等静压成型(CIP)、低真空烧结和热等静压(HIP)烧结技术制备ZAO靶材,采用排水法、SEM、XRD、光谱发射法(ICPOES)分析了2种烧结方法制备的ZAO靶材的密度、结晶状态和组织成分.结果表明,ZAO细粉经压力造粒后,CIP成型压坯相对密度可达75%以上,该压坯经低真空高温烧结,相对密度达到95%,而采用HIP低温烧结的靶材可达到98%以上,靶材结晶完整,组织成分均匀.     

Spark plasma sintered tantalum carbide: Effect of pressure and nano-boron carbide addition on microstructure and mechanical properties

TaC and TaC-1 wt.% B₄C powders were consolidated using spark plasma sintering (SPS) at 1850 °C and varying pressure of 100, 255 and 363 MPa. The effect of pressure on the densification and grain size is evaluated. The role of nano-sized B₄C as sintering aid and grain growth inhibitor is studied by means of XRD, SEM and high resolution TEM. Fully dense TaC samples were produced at a pressure of 255 MPa and higher at 1850 °C. The increasing pressure also resulted in an increase in TaC grain size. Addition of B₄C leads to an increase in the density of 100 MPa sample from 89% to 97%. B₄C nano-powder resists grain growth even at high pressure of 363 MPa. The formation of TaB₂/Carbon at TaC grain boundaries helps in pinning the grain boundary and inhibiting grain growth. The effect of B₄C addition on hardness and elastic modulus measured by nanoindentation and the indentation fracture toughness has been studied. Relative fracture toughness increased by up to 93% on B₄C addition.     

Mo-8wt%Cu纳米复合粉末的制备和烧结

采用机械合金化法制备出Mo-8wt%Cu超细复合粉末,并对由该复合粉末所制得的压坯进行了液相烧结,利用SEM、XRD等分析手段对复合粉末的特性和烧结体的组织进行了表征和观察,实验结果表明,该方法制备的Mo-8wt%Cu超细复合粉末颗粒细小,平均粒径在300nm左右,高能球磨后的复合粉末由Mo-Cu过饱和固溶体相和Cu相组成,而且两相的晶粒度达到纳米级,其中Mo-Cu过饱和固溶体相的晶粒约为106nm,复合粉末具有很高的烧结特性,经高温烧结后合金致密度达到98.5%以上,而且金相组织分布均匀。     

Effect of particle size distribution on sintering

A sintering model, taking into account the effect of particle size distribution and the effect of grain growth, was developed previously. Experimental data from the sintering of high-purity alumina were used to testify this model. The sintering was carried out at 1500 °C in air for various lengths of time. The results agreed with the prediction of the model. The powders with a narrower starting particle size distribution exhibited a lower sintering rate prior to the occurrence of grain growth, but a higher densification rate after the grain growth took place. The grain size/density trajectory was applied to reveal the effect of particle size distribution on sintering behaviour. It is suggested that powders with a narrower size distribution are preferable.     

无压烧结制备高致密度AlN-BN复合陶瓷

以低温燃烧合成前驱物制备的比表面积为17.4m²/g的AlN粉末和市售BN粉末为原料, 利用无压烧结工艺制备AlN-15BN复合陶瓷, 研究了复合陶瓷的烧结行为以及制备材料的性能, 结果表明: 由于AlN粉末的烧结活性好, 复合材料的烧结致密化温度主要集中在1500～1650℃之间, 在1650℃烧结后, AlN-15BN复合陶瓷的相对密度可达95.6%. 继续升高烧结温度, 材料的致密度变化不大, 热导率继续增加. 在1850℃烧结3h后, 可以制备出相对密度为96.1%, 热导率为132.6W·m^-1·K^-1, 硬度为HRA64.2的AlN-15BN复合陶瓷. 提出了高比表面积的AlN粉末促进复合陶瓷烧结的机理, 利用XRD, SEM等手段对烧结体进行了表征.     

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.     

Preparation and properties evaluation of zirconia-based/Al2O3 composites electrolytes for solid oxide fuel cell systems

The compaction behaviour of ultrafine yttria-doped zirconia powders (6–8 nm) without and with alumina additions (0 to 20 wt%) has been studied. From the pore size distribution and using isothermal and nonisothermal techniques, the sintering behaviour of zirconia compacts in the temperature range 800–1500 °C was studied. It was found that alumina additions (up to 10 wt%) enhanced the zirconia compacts' densification process and, above that alumina content, that process was retarded. Alumina additions did not affect the grain grown process in tetragonal zirconia samples. However, this was strongly hindered in the fully stabilized zirconia ones. The results were compared with those obtained in the same experimental conditions on a commercial zirconia powder.     

SDC电解质致密化研究

以共沉淀-喷雾干燥法制备的Ce0.8 Sm0.2 O1.9(SDC)粉体为原料,模压成型后高温烧结获得SDC电解质陶瓷片.研究模压成型过程中加压时间、压力大小以及烧结温度对烧结体致密度的影响,利用XRD和SEM分别对不同烧结温度获得的烧结体结构和表面形貌进行分析.研究表明,压力30MPa、加压时间30min后获得的坯体,随着烧结温度的升高,烧结体致密度呈上升趋势,烧结温度达到1450℃时进入烧结后期,烧结体具有较高的致密度.此外,通过测定烧结过程中坯体收缩率,对SDC电解质陶瓷片的烧结动力学进行了研究,从而确定SDC电解质致密化的烧结温度为1300～1500℃.     

Low-temperature sintering of zinc oxide varistors

High-field varistors in the system ZnO-CoO-MnO-Bi₂O₃ were fabricated using powders prepared by a previously developed coprecipitation process. Following calcination, the powders were compacted and densified by conventional pressureless sintering at temperatures below 750° C in air, The effects of sample green density, sintering temperature, and grain-growth inhibitor on densification and microstructure development were investigated. Addition of aluminium at the 125 p.p.m level was used to inhibit grain growth. Samples with densities >0.98 theoretical and grain sizes <1m were fabricated by high-pressure cold-isostatic pressing followed by sintering at 730° C. For comparison, typical commercial varistor devices have grain sizes of about 20 m and switching fields of approximately 2 kV cm^–1 after sintering at 1200 to 1400° C. As a result of the fine grain size, our high-field varistors had switching fields of 45 kV cm^–1 at a current density of 10 A cm^–2. Consistent with earlier work on extremely high-density varistors (>0.98 theoretical) prepared from similar powders, nonlinearity coefficients of about 10 were measured for current densities between 2.5 and 10 A cm^–2.