碳化烧结一步法制备WC-10Co硬质合金的摩擦磨损性能

以W、C、Co、VC、Cr_(3)C_(2)为原料,按照WC-10Co-0.9VC-0.3Cr_(3)C_(2)进行配料,采用等离子球磨和碳化烧结一步法制备了WC-10Co硬质合金。试验结果表明,WC-10Co硬质合金平均晶粒度为0.25μm,致密度为99.2%,硬度为92.3 HRA。WC-10Co硬质合金摩擦系数随着载荷的增加而减小,平均摩擦系数在0.48~0.57之间。与商用YG8硬质合金相比,通过碳化烧结一步法制备的WC-10Co合金具有更短的“跑和期”,更低的摩擦系数以及更高的耐磨损性能。模拟结果表明,碳化烧结一步法所制备的WC-10Co合金亚表层受磨球影响引起的Hamilton最大接触应力为1 563.9 MPa,远低于商用YG8硬质合金的1 847.1 MPa。

烧结碳化新工艺硬质合金的切削试验

对采用烧结碳化新工艺制取的硬质合金刀片进行了切削试验。结果表明，该刀片在车削奥氏体不锈钢、球磨铸铁和冷硬铸铁时，其耐磨性较好，与常规工艺生产的ＹＧ６刀片相当。

SPS原位碳化合成WC-6Co硬质合金

以钨粉、钴粉、炭黑、有机碳为原料,研究了W、Co和C混合粉末在放电等离子烧结系统中碳化反应快速烧结制备WC-6%Co硬质合金的原位碳化烧结工艺。系统研究了配碳量、烧结温度、保温时间和加压方式对硬质合金的显微组织结构和性能的影响。结果表明:碳含量为理论值1.2倍时合金物相为纯WC和Co相,无脱碳相,也未发现游离碳;烧结温度在1 250℃时,WC晶粒均匀且无异常长大;保温时间设置为5 min时,合金性能较为理想;在W、Co和C混合粉末烧结初期施加30 MPa压强,待温度升到800℃时再将压强加到50 MPa制备的合金孔隙较少,致密化程度高。本工艺较传统制备工艺具有流程短,成本低的特点,可为硬质合金工业生产提供参考。

低压烧结温度对一步法制备超细晶WC-Co基硬质合金组织及性能的影响

采用等离子球磨技术制得W-C-10Co-0.9VC-0.3Cr_3C_2纳米复合粉体,并利用单向模压成型法将其压制成生坯,再经低压烧结一步法制备成硬质合金。研究表明,等离子球磨3h所获得的复合粉体呈片层状形貌,并且成分分布均匀。在1 380℃及1 400℃烧结时,由于等离子球磨的特殊作用,VC、Cr_3C_2对WC晶粒长大抑制作用突显。1 380℃烧结制备的硬质合金,致密度为99.2%,WC平均晶粒尺寸为250nm,硬度和横向断裂强度分别为92.3HRA和2 443 MPa,具有最佳的WC晶粒尺寸与致密度配合,以及最佳的综合力学性能。

Effect of pre-calcination for modified CaO-based sorbents on multiple carbonation/calcination cycles

In the present work,the effect of pre-calcination on carbonation conversion and cyclic stability of modi fied CaObased sorbent was investigated by thermogravimetric analyzer(TGA).The modi fied CaO-based sorbents with CaAc_2 as precursor were respectively doped with different elements(Mg,Al,Ce,Zr and La).The speci fic surface area,pore volume and pore size distribution were tested by N_2 isothermal adsorption measurements.The phase compositions of sorbents were characterized by X-ray diffraction(XRD).The results showed that the cyclic stabilities of the sorbents were improved by pre-calcination.The pre-calcination was conducted at 900°C for 5 h in air by the muf fle furnace.With pre-calcination,the cyclic stabilities of sorbents could be as high as 96% after 22 cycles,such as CaO-Al,CaO-Ce and CaO-La.After contact with air,the carbonation conversions of spent sorbents with pre-calcination suddenly increased by about one-sixth due to the change of channel structure by hydration.Both the cyclic stability of sorbent and the durability of reactivation were related to the structural stability of sample,especially the stability of mesopores between 2 nm and 5.5 nm.The present work also provided an easy and low-cost method for reactivating the spent CaO-based sorbents.

Phase composition, transition and structure stability of functionally graded cemented carbide with dual phase structure

The phase composition, phase transition and phase structure transformation of the wire-cut section of functionally graded WC-Co cemented carbide with dual phase structure were investigated by XRD phase analysis. It is shown that the composition of η phase in the core zone is Co_3W_3C (M_6 C type). The structure of cobalt based solid solution binder phase is fcc type. At the cooling stage of the sintering process, the phase transition of η phase, i.e. M_6C→M_12C and the martensitic phase transition of the cobalt based solid solution binder phase, i.e. fcc→hcp are suppressed, which facilitates the strengthening of the alloy. Because the instantaneous temperature of the discharge channel is as high as 10 000 ℃ during the wire cutting process, the processed surface is oxidized. Nevertheless, the oxide layer thickness is in micro grade. In the oxide film, η phase is decomposed into W_2C and CoO, and cobalt based solid solution binder is selectively oxidized, while WC remains stable due to the existence of carbon containing liquid organic cutting medium.

Effect of sintering atmosphere on microstructure and properties of TiC based cermets

The effect of the sintering atmospheres (vacuum, N_2, Ar) on the microstructures and properties of the TiC based cermets was studied using XRD, SEM/BSE and energy dispersive spectrometer. Compared with the alloy sintered in vacuum, the carbon content of the specimen sintered in N_2 and Ar is lower by 0.5%; and the nitrogen content is higher by 0.3% when sintered in nitrogen. The central part of the ring structure may be carbide with either a high W or Ti content. The ring structures are (Ti, W, Ta, Mo, Co, Ni)C solid solutions with different metallic elements and distributions. The composition of the binder phase is (Co, Ni) solid solution with different Ti, W, Ta, Mo, C contents. The structures are uniform for the cermets sintered in vacuum and the properties are the best. When sintered in Ar or N2, the O_2 and N2 in the atmosphere take part in the sintering reaction to break the carbon balance in the cermets to form a shell structure and defects, which results in poor density, microhardness (HV) and transverse rupture strength (TRS).

Investigation of compaction and sintering behavior of SiC powder after ultra-fine treatment

Silicon carbide ceramics were prepared with SiC powder treated by the fluidized bed opposed jet mill as raw materials, and the effects of the ultra-fine treatment mechanism on the compaction and sintering behavior of SiC ceramics were investigated. The results showed that the compacts had higher density and microstructure homogeneity when the sintering temperature of the compact was decreased; and that the surface microstructure, densification and mechanical properties of the sintered body could be ameliorated obviously.

Microstructure and mechanical properties of WC-Co,WC-Co-Cr_3C_2 and WC-Co-TaC cermets fabricated by spark plasma sintering

Spark plasma sintering was successfully used to produce WC-Co cermets with the addition of Cr3C2and TaC grain growth inhibitors. The spark plasma sintered compacts were investigated by scanning electron microscopy, X-ray diffraction analysis, density measurements, hardness tests, fracture toughness tests and elastic modulus tests. The results were comparedwithan inhibitor-free WC-Co cermet consolidated under the same process parameters. By using Cr3C2and TaC additives, it is possible to improve the hardness and fracture toughness of WC-Co cermets, but Cr3C2has better grain growth inhibition propertythan that of TaC. The best combination of hardness （HV30（2105±38）） and fracture toughness （（8.3±0.2）MPa·m^1/2） was obtained by the WC-5Co-2Cr3C2 cermet.

Enhancement of the activity and durability in CO oxidation over silica-supported Au nanoparticle catalyst via CeO_x modification

The supported Au nanoparticles have been regarded as promising catalysts for CO oxidation but still suffer from unsatisfactory catalytic activity and durability.Herein,we show a simple and efficient strategy to simultaneously enhance the catalytic activity and durability for CO oxidation.Key to the success is modification of the supported Au nanoparticle catalyst with nanosized CeOx to construct abundant Au‐CeOx interface.Owing to the maximized interfacial effect on the Ce‐Ox‐modified Au nanoparticles,the concentration of positively‐charged Au species(Auδ+)was remarkably improved,leading to enhanced catalytic activities in the oxidation of CO.Importantly,the stability of Au nanoparticles is remarkably increased by CeOx modification,exhibiting good durability in a continuous test of CO oxidation at higher temperatures.

Sintering and microstructure of silicon carbide ceramic with Y_3Al_5O_(12) added by sol-gel method

Silicon carbide (SiC) ceramic with YAG (Y3Al5O12) additive added by sol-gel method was liquid-phase sintered at different sintering temperatures, and the sintering mechanism and microstructural characteristics of resulting silicon carbide ceramics were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and elemental distribution of surface (EDS). YAG (yttrium aluminum garnet) phase formed before the sintering and its uniform distribution in the SiC/YAG composite powder decreased the sintering temperature and improved the densification of SiC ceramic. The suitable sintering temperature was 1860 °C with the specimen sintered at this temperature having superior sintering and mechanical properties, smaller crystal size and fewer microstructure defects. Three characteristics of improved toughness of SiC ceramic with YAG added by sol-gel method were microstructural densification, main-crack deflection and crystal ‘bridging’.

Densification and grain growth kinetics of boron carbide powder during ultrahigh temperature spark plasma sintering

Dense B;C material was fabricated using spark plasma sintering(SPS), and the densification mechanisms and grain growth kinetics were revealed. The density, hardness, transverse flexure strength and toughness of samples were investigated and the model predictions were confirmed by SEM and TEM experimental observations. Results show that SPSed B;C exhibits two sintering periods: a densification period(1800-2000 °C) and a grain growth period(2100-2200 °C). Based on steady-state creep model, densification proceeds by grain boundary sliding and then dislocation-climb-controlled mechanism. Grain growth mechanism is controlled by grain boundary diffusion at 2100 °C,and then governed by volume or liquid-phase diffusion at 2200 °C.

W粉粒径和烧结温度对一步法制备WC-10%Co硬质合金组织性能的影响

等离子球磨“碳化烧结一步法”制备WC-Co硬质合金有利于板状晶WC的形成和形态控制。本文进一步研究了等离子球磨W-C-Co复合粉末的组织演变,着重考察原始W粉粒径和烧结温度对WC-10%Co硬质合金组织、性能的影响。结果表明,等离子球磨使W颗粒显著呈片状,并增加其中位错等缺陷,提高粉末中的变形储能,同时增加了W/C反应界面,均有利于WC板状晶的生成;随着原始W粉粒径增加,等离子球磨所制备的层片状聚集体的片径越大,其生成的板状WC晶粒也越大,板状WC晶粒的定向排列程度也越高;随着烧结温度增加,WC晶粒的长径比和板状WC晶粒的定向排列程度有所提高。当原始W粉粒径为2.5μm、烧结温度1440℃时,所制备的WC-10%Co硬质合金样品垂直于压制方向截面的横向断裂强度、硬度和断裂韧性分别为3542 MPa、14.896 GPa、16.73 MPa·m^(1/2);平行于压制方向截面的硬度和断裂韧性为13.975 GPa、15.06 MPa·mm^(1/2)。

等离子球磨技术在制备WC-Co硬质合金中的应用

本文简述了高强韧WC-Co硬质合金的发展方向,介绍了等离子球磨新技术的基本原理和方法,总结了近年来基于等离子球磨技术的"碳化烧结一步法"在制备高性能WC-Co硬质合金中的进展。等离子球磨制备硬质合金表现出3点优势:(1)显著提高了W与C反应的活性,极大地降低了WC的合成温度,有望采用"碳化烧结一步法"制备WC-Co硬质合金,简化制备工艺流程,实现节能降耗;(2)等离子球磨有利于形成板状结构的WC晶粒,并且能够较方便地控制WC的形态,为设计和调控WC-Co硬质合金的组织创造了很大的空间;(3)利用等离子球磨方法制备WC-Co硬质合金适于普通烧结的规模生产技术,所得到的材料具有优异的强韧力学性能。

In situ investigation of SiC powder's microwave sintering by SR-CT technique

Microwave sintering is being developed as a novel technique for the preparation of dense structural ceramics,but the mature theory has not been established due to the technical difficulties.The synchrotron radiation X-ray computed tomography(SR-CT) technique was introduced for the first time into the study of microwave sintering to in-situ observe the microstructure evolution of silicon carbide(SiC) material in this paper.By applying the SR-CT technique,the reconstructed 2D and 3D images of the specimen were obtained and the double logarithm curve of mean neck size and time(Ln(x)-Ln(t)) were obtained from these reconstructed images.Various sintering phenomena including sintering neck growth during microwave treatment were observed from the reconstructed images.Furthermore,the differences in microstructure evolution and sintering kinetics between microwave and conventional sintering were analyzed based on the reconstructed images and the Ln(x)-Ln(t) curve.1) The sharp surface of grains near the contact region distinctly grew blunt and the sintering neck growth between these grains were obviously observed at the early stage.Besides,the larger particles grew faster than smaller ones.The main reason for these phenomena may be the micro-focusing effect of electric fields.2) During each of the three sintering stages,the sintering kinetics curve of double logarithm relationship between mean neck size and time shows a good linear relationship,but at the middle stage the slope of the curve increases dramatically,which is quite larger than conventional sintering.The preliminary interpretation for these extraordinary phenomena has been discussed in details.

Microstructure and mechanical properties of liquid phase sintered silicon carbide composites

Silicon carbide (SiC) composites were prepared by hot-press sintering from α-SiC starting powders with BaAl2Si2O8 (BAS). The effects of additives on densification, microstructure, flexural strength, and fracture behavior of the liquid phase sintered (LPS) SiC composites were investigated. The results show that the served BAS effectively promotes the densification of SiC composites. The flexural strength and fracture toughness of the SiC composites can reach a maximum value of 454 MPa and 5.1 MPa·m1/2, respectively, for 40% (w/w) BAS/SiC composites. SiC grain pullout, crack deflection, and crack bridging were main toughening mechanisms for the sintered composites.

Dense and pure high-entropy metal diboride ceramics sintered from self-synthesized powders via boro/carbothermal reduction approach

Equimolar quinary diboride powders,with nominal composition of(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2,were synthesized by boro/carbothermal reduction(BCTR)of oxide mixtures(MOx,M=Ti,Hf,Zr,Nb and Ta)using B4 C as source of B and C in vacuum.By adjusting the B4 C/MOxratios,diboride mixtures without detectable MOxwere obtained at 1600℃,while high-entropy diboride(HEB)powders with particle size of<1μm was obtained at 1800℃.The phase,morphology and solid solution evolution process of the HEB powders during the BCTR process were comprehensively investigated.Although X-ray diffraction pattern indicated the powders synthesized at 1800℃ were in a single-phase Al B2 structure,elemental mappings showed that(Ta,Ti)-rich and(Zr,Nb)-rich solid solution coexisted in the HEB powders.The distribution of niobium and zirconium atoms in HEB was unable to reach uniform until the HEB powders were spark plasma sintered at 2000°C.(Ti0.2 Hf0.2 Zr0.2 Nb0.2 Ta0.2)B2 ceramics with a relative density of 97.9%were obtained after spark plasma sintering the HEB powders at 2050℃ under 50 MPa.Rapid grain growth was found in this composition when the sintering temperature was increased from 2000 to 2050℃,and the averaged grain size increased from 6.67 to 41.2μm.HEB ceramics sintered at 2000℃ had a Vickers hardness of 22.44±0.56 GPa(under a load of 1 kg),a Young’s modulus of^500 GPa and a fracture toughness of 2.83±0.15 MPa m1/2.This is the first report for obtaining high density HEB ceramics without residual oxide phase,benefiting from the high quality HEB powders obtained.

A new sediment type of coated grain: Oolitic sinter

As a special sedimentary grain type, the coated grain(with an ooid model) has been known for two centuries due to its fascinating special fabric and genesis developments. The leading factors in forming the coated grain consist mainly of:(1) microorganism movement field;(2) chemical sedimentary effect;(3) hydrodynamic force environment and topography condition;(4) abundant core material supply;(5) embedding condition; and(6) humic acids condition in water medium. With the development of the coated grain genesis, the single factor theory cannot reasonably explain the exact formation of the surface sediment of coated grain. Here, we find a new way to study the coated grain on the basis of traditional research methods. The Wenquan area on the northeast edge of the Qiangtang Basin is one of the few areas where the coated grain is developing, and is a rare "natural laboratory" for the study of the coated grain and the thermal spring sediment. The oolitic sinter of the area has the triaxiality modality of pea polymer, and is obviously different from the karst travertine and the normal lacustrine ooid. We found that the hot spring water in the Wenquan area has higher partial pressure of CO2(PCO2) and saturation index of the calcite(SIc) than normal. Macrocosmically, the oolitic sinter is shaped like a pea, and its grains and gap fillings are light yellow. Microcosmically, the sinter grain forms six types of fundamental lamina, and those six types are developed to be four grain types with different combinations. The C-axis of the mineral grain of sinter cement(calcite) is normal to the lamina face, and grows on it with distinct generation formations. In short, the grain type of oolitic sinter is the oncoid, with the grain development caused by the factors such as the shallow water of strong hydrodynamic force, the special hydrochemistry condition, and the extensive algae activities(diatom).