High-frequency induction heated sintering of High-energy ball milled TiC0.5N0.5 powders and mechanical properties of the sintered products

Commercial TiC_0.5N_0.5 powders were high-energy ball milled for various durations and consolidated without binder using the high-frequency induction heated sintering method (HFIHS). The effect of milling on the sintering behavior, crystallite size and mechanical properties of TiCN powders were evaluated. A nanostructured dense TiCN compact with a relative density of up to 98% was readily obtained within 3 min. The ball milling effectively refined the crystallite structure of TiCN powders and facilitated the subsequent densification. The sinter-onset temperature was reduced appreciably by the prior milling for 10 h from 1170 °C to 820 °C. Accordingly, the relative density of TiCN compact increased as the milling time increases. The microhardness of sintered TiCN was linearly proportional to the density while its toughness did not show any correlation with the crystalline size or density. It is clearly demonstrated that a quick densification of nano-structured TiCN bulk materials to near theoretical density could be obtained by the combination of HFIHS and the preparatory high-energy ball milling processes.     

Effect of Fe2O3 addition on consolidation and properties of 8 mol% yttria-stabilized zirconia by high-frequency induction heated sintering (HFIHS)

Dense nanophase 8 mol% yttria-stabilized zirconia was sintered by high-frequency induction heated sintering (HFIHS) within 6 min from 8YSZ nanopowder prepared by co-precipitation method. Sintering was accomplished under the combined effects of an induced current and mechanical pressure. Highly dense 8YSZ with relative density of up to 96% was produced under simultaneous application of a 60-MPa pressure and the induced current. The effects of Fe₂O₃ additions on the sintering behavior, mechanical properties and ionic conductivities of the 8YSZ were investigated.     

溶胶凝胶法制备锆-钴高温防氧化粉体

在调节剂柠檬酸铵的作用下,采用溶胶凝胶法制备了Y稳定的Zr-Co粉体,经高温烧结制得致密陶瓷薄膜。利用场发射扫描电镜（SEM）表征了Zr-Co粉体的形貌特征和烧结后的陶瓷薄膜结构,热重（TG／DTA）分析了Zr-Co粉体烧结过程的失重和相转变吸放热过程,X射线衍射（XRD）分析跟踪了不同温度段烧结体系的组成变化趋势,在此基础上对粉体烧结后陶瓷薄膜的防氧化效果进行了分析,结果表明：温度对粉体的颗粒形貌和晶相结构有较大的影响,高温烧结的粉体性质稳定,经过1200℃烧结后的陶瓷膜层具有很好的致密性,具有很好的防氧化效果。     

Rapid consolidation of nanocrystalline Al2O3 reinforced Ni–Fe composite from mechanically alloyed powders by high frequency induction heated sintering

Nano-powders of Ni–Fe and Al₂O₃ were made from NiO and FeAl powders by high-energy ball milling. Nanocrystalline 5Ni_0.6Fe_0.4–Al₂O₃ composite was consolidated by high frequency induction heated sintering (HFIHS) method within 2 min from mechanically alloyed powders of Al₂O₃ and Ni–Fe. The average grain size and mechanical properties of the composite were investigated.     

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.     

In vitro cellular response and hydrothermal aging of two-step sintered Nb2O5 doped ceria stabilized zirconia ceramics

Niobia (1 mol. %) doped Ceria Stabilized Zirconia (NbCSZ) powders were synthesized using a co-precipitation method. The synthesized powders were uniaxially compacted and sintered in air. The two-step sintering method was adopted to sinter the samples, and the sintering schedule was optimized based on density, grain size, the phase present, and the hardness of the sintered sample. It was observed that the two-step sintering method effectively suppressed the grain growth of NbCSZ samples and helped in achieving a finer grain size of 1.57 μm along with the hardness of 1195 HV10 and optimum fracture toughness value 6.20 MPa m^1/2. The Low-Temperature Degradation (LTD) behavior of the sintered samples was estimated through an accelerated hydrothermal aging test, which revealed that the samples are highly resistant to LTD and shown no phase change even after 150 h of study. Moreover, the cytocompatibility of the NbCSZ was tested by culturing MG63 cells on the samples for 7 days. The NbCSZ was found to be highly biocompatible as evident from cell viability and metabolic activity assay.     

PS/化学气相沉积CF选区激光烧结多指标正交试验参数的优化

针对选择性激光烧结(SLS)工艺中聚苯乙烯(PS)粉末烧结件精度不高且强度不能满足实际需求的问题,将经过化学气相沉积法处理的碳纤维(CF)粉末、PS粉末、分散剂和抗静电剂进行机械混合,制备了PS/气相沉积CF复合粉末。以Z向尺寸相对误差、致密度和弯曲强度为实验指标,先采用极差分析法进行单个指标的优化,再利用综合平衡法对多指标正交试验进行整体参数优化分析。结果表明:复合粉末粒度主要分布在30.63~142.00μm;对于PS粉末烧结件,化学气相沉积CF粉末比普通的CF粉末具有更好的改善尺寸精度、填充和增强的效果;烧结件的最佳工艺参数组合为:预热温度为80℃,激光功率为27 W,扫描速度为1 800 mm/s和扫描间距为0.30 mm。此时烧结件的Z向尺寸相对误差为5.47%,致密度为0.626 g/cm3,弯曲强度为10.68 MPa。相比于纯PS粉末烧结件,分别提高了24.55%,30.42%,136.28%。     

Effect of Processing Parameters on the Mechanical and Microstructural Behavior of Ultra-Fine Al2O3– (ZrO2+8%Mol Y2O3) Bioceramic, Densified By High-Frequency Induction Heat Sintering

High-frequency induction heat sintering (HFIHS) is a comparatively new technique that consolidates metals and ceramics very rapidly to full density. In this work, superfast densification behavior and the attendant microstructural features of Al₂O₃–(ZrO₂+8% mol Y₂O₃) composites processed by HFIHS were investigated. The effects of processing parameters such as sintering temperatures, pressures, and heating rate, on the mechanical and microstructural properties were studied. The results indicated that HFIHS was effective in the preparation of fine-grained, nearly fully dense Al₂O₃–8YSZ ceramics from the powder with a smaller particle size by optimizing the overall processing parameters.     

Observation of Toughness Improvement of the Hydroxyapatite Bioceramics Densified Using High-Frequency Induction Heat Sintering

A novel nanocomposite has been introduced to improve the fracture toughness of hydroxyapatite (HAp) bioceramics. Composites of HAp/NiO with 1–5 wt% of electrospun NiO nanofibers were developed and studied. The mixtures were consolidated very rapidly to full density by high-frequency induction heat sintering (HFIHS). The role of NiO nanofibers in enhancing the toughness, hardness, and microstructure properties of the HAp bioceramic was investigated. Field emission scanning electron microscopy was used to examine the morphology of the bioceramic composites and fracture surfaces to reveal the dominant toughening mechanisms. The results showed that the sintering behaviors, toughness, and hardness of the resulting composites were significantly enhanced by the inclusion of NiO nanofibers. The fracture toughness of the heat sintered HAp/NiO electrospun nanofiber composites was 1.8 times higher, as the concentration of NiO electrospun nanofibers reached 5 wt%.     

Low temperature sintering of PZT powders coated with Pb5Ge3O11 by sol–Gel method

Low temperature sintering of PZT powders was investigated using Pb₅Ge₃O₁₁(PGO) as a sintering aid. PZT powders with 150 nm particle size were coated with PGO which was prepared from precursor solutions of Ge(OiPr)₄ and Pb(NO₃)₂ by sol–gel method. 1 wt% PGO-added PZT powders were densified at 750°C for 2 h to sintered bodies with the relative density of approximately 95%. An addition of PGO improved the sinterability of PZT powders with a reduction of sintering temperature by about 300°C. Dielectric and piezoelectric properties of PGO-added PZT ceramics sintered at ≦950°C were superior to those without PGO additives. However, a higher sintering temperature above 1000°C deteriorated the dielectric and piezoelectric properties of PGO-added PZT ceramics. This may be attributed to the change of microstructure involving the formation of solid solution between PZT and PGO. The 1 wt% PGO-added PZT bodies sintered at 750°C exhibited an electromechanical coupling factor, Kp, of about 56%.     

LaCoO3 ceramics obtained from reactive powders

The aim of the present study was to establishing the correlation between the structure and properties of the LaCoO₃ powders obtained by aqueous sol–gel method with citric acid and their sintering behavior in order to obtain fully densified ceramics with perovskite structure. Two types of cobalt and lanthanum reagents were used in synthesis, namely nitrates and acetates. The sintering was realized at temperatures ranging between 800 and 1200 °C for 2 h. The sintered samples were investigated by classical ceramic methods (shrinkage, density, porosity) and by structural and morphological investigations: XRD, SEM, AFM and XPS. The electrical properties of the samples were determined by impedance spectroscopy. The ceramics obtained with powders starting with acetates have presented a lower sintering ability as compared with the samples obtained from powders starting with nitrates. LaCoO₃ ceramics with best properties was obtained from powders starting with nitrates sintered at 1100 °C.     

Effect of sintering temperature on the morphology,crystallinity and mechanical properties of carbonated hydroxyapatite (CHA)

Effect of sintering temperature on the physical and mechanical properties of synthesized B-type carbonated hydroxyapatite (CHA) over a range of temperature in CO₂ atmosphere has been investigated. The B-type CHA in nano size was synthesized at room temperature by using a direct pouring wet chemical precipitation method. The synthesized CHA powders were subsequently consolidated by sintering treatment from 800 to 1100 °C. The sintered CHA samples were evaluated using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrometry, X-ray fluorescence (XRF), carbon-hydrogen-nitrogen-sulfur-oxygen (CHNS/O) elemental analyzer, Field emission scanning electron microscopy (FESEM), and Vicker's indentation technique. The results obtained from XRD and FESEM indicated that the synthesized B-type CHA powders were nanometer in size. The crystallinity and crystallite size of the sintered CHA samples were increased due to increasing sintering temperature. The heat treatment between 800 °C and 1000 °C has resulted in coarsening and increased hardness of the sintered CHA samples. However, these properties began to deteriorate when sintering beyond 1100 °C due the formation of calcium oxide.     

Densification and characterization of rapid carbothermal synthesized boron carbide

Submicrometer boron carbide powders were synthesized using rapid carbothermal reduction (RCR) method. Synthesized boron carbide powders had smaller particle size, lower free carbon, and high density of twins compared to commercial samples. Powders were sintered using spark plasma sintering at different temperatures and dwell times to compare sintering behavior. Synthesized boron carbide powders reached >99% TD at lower temperature and shorter dwell times compared to commercial powders. Improved microhardness observed in the densified RCR samples was likely caused by the combination of higher purity, better stoichiometry control, finer grain size, and a higher density of twin boundaries.     

Evaluation of the sinterability,densification behaviour and microhardness of spark plasma sintered multiwall carbon nanotubes reinforced Ti6Al4V nanocomposites

Structural and industrial demands for lightweight engineering materials with exclusive properties have been rising in recent decades for automobile and aerospace applications. This has encouraged various innovations in materials engineering communities to synthesis advanced engineering materials using improved fabrication technique such as spark plasma sintering (SPS). In this study, titanium-based nanocomposites were synthesized by reinforcing Ti6Al4V reinforced with (0.5, 1.0 and 1.5 wt%) multiwall carbon nanotubes (MWCNT) powders. The starting powders were blended by shift-speed ball milling. Thereafter, SPS technique was used to consolidate the admixed powders by employing the following sintering parameters; sintering rate, 100 °C/min, compressive pressure, 50 MPa, holding time, 10 min and sintering temperatures of 900–1100 °C. The influence of MWCNT additions on the sinterability, densification behaviours and microhardness of the sintered nanocomposites were investigated. The results revealed that the densification of the sintered nanocomposites was in the range of 97.51–99.61% which decreased with an increase in concentration of the MWCNT. Meanwhile, the densification and microhardness improved tremendously with an increase in sintering temperatures.     

新型MAX相陶瓷Mo2Ga2C的放电等离子烧结

Mo2Ga2C是一种新型MAX相，该材料粉体已经可以被稳定的制备。但是Mo2Ga2C粉体不容易被烧结为致密的块体。本文采用放电等离子烧结技术(SPS)高温处理Mo2Ga2C粉体，通过对制备样品的物相组成和微观结构的表征，研究Mo2Ga2C的烧结性能。SPS烧结Mo2Ga2C 的最佳工艺参数为：烧结温度700℃，保温时间20min，轴向压力30MPa。在此条件下SPS烧结Mo2Ga2C样品相对密度达到71.81%。延长保温时间比升高烧结温度对Mo2Ga2C的致密化有更明显的助益；而增大轴向压力对样品的致密化产生负影响。相对于热压烧结，SPS可以在较低的温度快速制备Mo2Ga2C样品，但是制备的样品的致密度较低。     

Electrochemical Synthesis and Sintering of Nanocrystalline Cerium(IV) Oxide Powders

Nanocrystalline CeO₂ powders were prepared electrochemically by the cathodic electrogeneration of base, and their sintering behavior was investigated. X-ray diffraction and transmission electron microscopy revealed that the as-prepared powders were crystalline cerium(IV) oxide with the cubic fluorite structure. The lattice parameter of the electrogenerated material was 0.5419 nm. The powders consisted of nonaggregated, faceted particles. The average crystallite size was a function of the solution temperature. It increased from 10 nm at 29°C to 14 nm at 80°C. Consolidated powders were sintered in air at both a constant heating rate of 10°C/min and under isothermal conditions. The temperature at which sintering started (750°C) for nanocrystalline CeO₂ powders was only about 100°C lower than that of coarser-grained powders (850°C). However, the sintering rate was enhanced. The temperature at which shrinkage stopped was 200°-300°C lower with the nanoscale powder than with micrometer-sized powders. A sintered specimen with 99.8% of theoretical density and a grain size of about 350 nm was obtained by sintering at 1300°C for 2 h.     

Microstructures and grain boundaries of cubic boron nitrides

Two types of chemically pristine polycrystalline cubic boron nitrides (c-BN) are sintered at different conditions using the starting cubic and hexagonal BN powders and their microstructures and grain boundaries are investigated systematically by transmission electron microscopy. The two c-BN samples are found to exhibit a number of twins inside their grains and have a similar grain size, despite their huge differences in the grain size of the starting powders and sintering conditions. Twin width for the c-BN sintered from hexagonal BN is significantly smaller than that for the c-BN sintered from c-BN. Grain boundaries in the two samples can be atomically abrupt without any amorphous or secondary layers and oxygen is detected merely at the grain boundaries of the c-BN sintered from the c-BN powders. Such microstructural differences have a direct impact on mechanical behaviors of the c-BN, as the Vickers hardness of the c-BN sintered from hexagonal BN powders is found to be higher than that of the c-BN sintered from the c-BN powders.     

Sintering of binderless WC–Mo2C hard materials by rapid sintering process

High-frequency induction heated sintering (HFIHS) is a new rapid sintering method which was developed recently for the fabrication of ceramics and composites. This method combines a short time high-temperature exposure with pressure application. In this work, we reported results on the sintering of binderless WC–x wt.%Mo₂C (0 ≤ x ≤ 6) hard materials using ultra fine powders of WC and Mo₂C. A complete densification of the materials was achieved within 1 min. The relative densities of the composites were about 99% for an applied pressure of 60 MPa and an induced current for 90% output of total capacity. The Vickers hardness decreased and the fracture toughness increased with increasing the Mo₂C content.     

Precoarsening to Improve Microstructure and Sintering of Powder Compacts

MgO and Al₂O₃ were sintered by two types of processes: a conventional isothermal sintering and a two-step sintering consisting of an initial low-temperature precoarsening treatment before conventional isothermal sintering. The final microstructure from two-step sintering can be more uniform and finer than that of compacts sintered conventionally. A narrow-size-distribution alumina powder was sintered under constant-heating-rate conditions, with and without a precoarsening treatment, and the results were compared. The differences between two-step and conventional processing were clarified by experiments on precoarsened and as-received ZnO powders. These compacts were precoarsened at 450°C for 90 h with virtually no increase in the overall density. The resulting grain size was 1.7 times the starting one, but the standard deviation of the precoarsened powder size distribution was smaller than that of the asreceived powder. Precoarsened compacts sintered to nearly full density showed improved homogeneity. The sintering stress of the precoarsened ZnO was approximately 0.8 that of the as-received one. A computational model has been used with two components of coarsening to describe the differences in pore spacing evolution between the precoarsened and the as-received system. The benefit of two-step sintering is attributed to the increase in uniformity resulting from precoarsening. The increased uniformity decreases sintering damage and allows the system to stay in the open porosity state longer, delaying or inhibiting additional coarsening (grain growth) during the final stage of densification. Two-step sintering is especially useful for nonuniform powder systems with a wide size distribution and is a simple and convenient method of making more uniform ceramic bodies without resorting to specialized powders or complicated heat schedules.     

Sintering of Nanosized MnZn Ferrite Powders

The sintering and microstructural evolution of nanosized MnZn ferrite powders prepared by a hydrothermal method were investigated. The microstructure of sintered ferrite compacts depends strongly on the strength of the agglomerates formed during the compacting of nanosized ferrite powders. It was found that at 700°C the theoretical density of sintered compacts can almost be reached, while above 900°C an increase of porosity was identified. The formation of extra porosity at higher sintering temperatures is caused mainly by the oxygen release which accompanies the dissolution of relatively large grains of residual alpha-Fe₂O₃ in the spinel lattice.