Effect of Mo and Co additions on the microstructure and properties of WC-TiC-Ni cemented carbides

In this work, the effects of 1.0 wt.% additions of Mo and Co on the microstructure and properties of WC-TiC-Ni cemented carbides were investigated using scanning electron microscope, mechanical properties tests, corrosion resistance and abrasion resistance tests. The results show that 1.0 wt.% Mo addition can refine the WC grains and increase the hardness. Moreover, with the addition of minor Mo, the corrosion resistance and abrasion resistance of alloys improved significantly. The addition of 1.0 wt.% Co can inhibit the growth of WC grains, improve the density and hardness slightly, and enhance the abrasion resistance of cemented carbides. However, the minor Co has negative effect for the corrosion resistance.     

Change in relative density of WC-Co cemented carbides in spark plasma sintering process

The relative density of WC-Co cemented carbides during spark plasma sintering (SPS) was analyzed.Based on the change in displacement of the ram in the SPS system,the relative densities in the sintering process can be achieved at different temperatures.The results indicated that densification of the samples started at near 900℃,the density rapidly reached its maximum at the increasing temperature stage,in which the temperature was lower than the sintering temperature of 1200℃,and most of the densification took place in the stage.Besides,the theoretcal values were consistent with the experimental results.     

VC, Cr3C2 doped ultrafine WC-Co cemented carbides prepared by spark plasma sintering

The influence of the addition of 0.3, 0.5 and 0.7 wt.% VC on the density, microstructure and mechanical properties of WC-Cr₃C₂-11 wt.% Co with 0, 0.2, 0.4 and 0.6 wt.% Cr₃C₂ hard metals prepared by spark plasma sintering (SPS) at a temperature of 1200 °C (5 min, 40 MPa) was investigated. Microstructure analysis revealed that the WC grain size in the sintered hard metals was strongly influenced by the VC and Cr₃C₂ content. With the addition of inhibitors and the increased amount of Cr₃C₂, the density is reduced, and on the contrary, the addition of VC as an inhibitor contributes to promoting the densification. The combined addition of Cr₃C₂ and VC could strongly reduce the WC grain growth to about 350 nm. Observation suggests that the fracture of WC-Co cemented carbide is brittle and intergranular. The amount of added VC/Cr₃C₂ should be controlled in a certain range. Samples with an appropriate proportion of VC/Cr₃C₂ added exhibit higher hardness which can be up to 1938 HV₃₀. Toughness, too, can reach 16.34 MPa m^1/2.     

Effect of ZrO2 content on microstructure and mechanical properties of W alloys fabricated by spark plasma sintering

The ultra-fine-grained ZrO₂-reinforeced W composites were obtained by hydrothermal synthesis method and spark plasma sintering. The precursors of WO₃ and Zr(OH)₂ were evenly mixed in aqueous solutions via the liquid-liquid doping, a hydrothermal method. The addition of ZrO₂ could significantly refine W particles, and thus the ultra-fine and well-mixed W-ZrO₂ composite powders were obtained after reduction, with the average size of 2 μm for W particles and ~20 nm for Y₂O₃-stabilized ZrO₂ particles. The relative density of the sintered samples increases from 92.8% to 98.9% with increasing ZrO₂ content. The W-1.5wt%ZrO₂ alloy possesses the prominent properties of 538.8 HV for microhardness, and 1628 MPa for compressive strength when compressed to 21% strain-to-failure at room temperature and a strain rate of 1.7 × 10⁻³ s⁻¹. The high temperature compressive tests shows that the existence of peak stress is not observed in the true stress-stain curves, indicating that the dynamic recovery is the main cause of deformation softening during deformation.     

Ultrafine WC-0.5Co-xTaC cemented carbides prepared by spark plasma sintering

Ultrafine (below 500 nm) tungsten carbide (WC) - 0.5 wt% cobalt (Co) cemented carbides were prepared by spark plasma sintering (SPS), containing varied tantalum carbide (TaC) contents, at 1500 °C under 50 MPa. The sintering behavior and microstructure of these materials were investigated. It was found that adding TaC could get a rapid shrinkage period during sintering and make a finer microstructure with a more narrowing range of the grain-size distribution. Adding inadequate or excessive amount of TaC, however, not only lower the sinterability but also get a result against the finer microstructure. Moreover, the (Ta,W)C solid-solution phase (formed by the part dissolution of WC in TaC grains) was mainly gathered at the WC grain boundary, and Co film was found. Benefiting from the ultrafine and homogeneous microstructure, these materials maintained excellent hardness and improved fracture toughness.     

放电等离子烧结时间对高密度W-7Ni-3Fe合金组织性能的影响

利用放电等离子烧结技术制备高密度W-7Ni-3Fe合金,研究了烧结保温时间对合金致密度、物相、显微组织以及力学性能的影响。结果表明,在1200℃烧结5~14 min后,合金均能实现充分致密化,保温时间对相对密度影响较小。合金中的W晶粒随保温时间的延长开始尺寸变化不大,烧结11 min以上才明显长大,但大多数W晶粒尺寸仍小于5μm。烧结时间超过8min,合金中新出现一种灰色的富W组织。随保温时间延长,合金的洛氏硬度下降不大,然而抗弯强度却明显上升。合金弯曲断口形貌在较短保温时间以沿晶断裂为主,粘结相的延性撕裂和W晶粒的解理断裂随烧结时间延长逐渐增多。     

Effect of Al2O3 ceramic binder on mechanical and microstructure properties of spark plasma sintered WC-Co cermets

This study investigates how the partial replacement of Co with Al₂O₃ ceramic binder has an effect on the sintering behaviour, microstructure, and final mechanical properties of WC-Co cermets via spark plasma sintering. To examine this, three batches (WC-6 wt%Co, WC-3 wt%Co-3 wt%Al₂O_3, and WC-6 wt%Al₂O₃) were mixed through high energy ball mill, and sintering was carried out at temperatures of 1350 °C and 1600 °C. The results showed nearly full dense WC-Co cermets at different temperatures. It was shown that WC-6 wt%Al₂O₃, in comparison to reference WC-6 wt%Co cermet, not only led to the rise in sintering temperature from 1350 °C to 1600 °C, but also reduced its strength and toughness. But replacing some part of Co with alumina (WC-3 wt%Co-3 wt%Al₂O₃) exhibited the combination of high strength (1095 MPa), hardness (17.62 GPa), and fracture toughness (19.46 MPa·m^1/2).     

深冷处理对WC-Co硬质合金组织和性能的影响

对YG8和YG25两种WC-Co硬质合金不同时间深冷处理后的力学性能和疲劳性能进行了研究,利用X射线衍射方法分析了深冷前后合金的相变和残余应力变化,通过扫描电子显微镜（SEM）观察了材料的断口形貌和断裂方式。结果表明：深冷处理能有效提高了WC-Co硬质合金的硬度、强度、耐磨性和疲劳寿命。深冷处理时间是最主要的工艺参数,对YG8合金来说,2 h为深冷处理最佳工艺时间,而YG25则为8 h。其性能变化的主要原因是深冷处理导致硬质合金表面残余应力的变化和Co粘结相发生马氏体相变。     

Microstructure and mechanical properties of ultrafine WC–Ni–VC–TaC–cBN cemented carbides fabricated by spark plasma sintering

Ultrafine WC–Ni–VC–TaC cemented carbides with different amounts of cubic boron nitride (cBN) were fabricated by spark plasma sintering, and the microstructure and mechanical properties of the as-prepared cermets were investigated. Scanning electron microscopy observations showed that the size of WC grains in the cermet samples was 0.2–0.4 μm. After the addition of cBN, the samples were still quite dense with the highest relative density of almost 98% when the addition fraction of cBN was 50 vol.%, although some micropores might exist in the samples. X-ray diffraction results indicated that no phase transformation of cBN was detected. The relative density and hardness of the cemented carbides increased with the addition fraction of cBN, but their strength decreased. When the fraction of cBN increased from 0 up to 50 vol.%, the hardness of the samples increased from 2100 to 3200 HV, but the flexural strength decreased from 1950 to 1250 MPa.     

Mechanical properties and microstructure of Fe3Al intermetallics fabricated by mechanical alloying and spark plasma sintering

Fabrication technology and mechanical properties of the Fe3Al based alloys were studied by spark plasma sintering from elemental powders （Fe-30Al, volume fraction, %） and mechanically alloying powders. The mechanically alloying powders were processed by the high-energy ball milling the elemental mixture powders with the milling time of 5, 8 and 10 min, respectively. The spark plasma sintering process was performed under the pressure of 50 MPa at 1 050 ℃ for 5 min. The phase identification by X-ray diffraction presents the Fe reacts with Al completely during the processing time. The samples are nearly full density （e.g. the relative density of sinter of raw powder is 99%）. The microstructure was observed by TEM. The mechanical properties were tested by three-point bending at room temperature in air. The results show that the mechanical properties are better （e.g. bend strength of 1 500 MPa ） than those of the ordinary Fe3Al casting.     

Effect of direct current patterns on densification and mechanical properties of binderless tungsten carbides fabricated by the spark plasma sintering system

Binderless tungsten carbide materials (bWCs) were fabricated by the spark plasma sintering (SPS) system. Ultrafine WC powders with adjusted oxygen contents and C/W atomic ratios were used as raw materials. Constant and pulsed direct current patterns (constant DC and pulsed DC) were chosen as the power supplies. The results indicate that for WC starting powders with either low (0.31%) or high (0.95%) oxygen contents, a relative density larger than 99.0% can be reached by pulsed DC at 1820 °C. Nevertheless, the severely oxidized WC powders cannot be well-densified by constant DC. A high degree of densification of bWCs facilitates the collaborative improvement of the toughness and hardness. The existence of W₂C facilitates the improvement of the hardness at the high expense of the toughness. The existence of graphite phase is substantially detriment to the toughness. The grain coarsening facilitates the improvement of the toughness with sacrificed hardness. The related mechanism is discussed.     

放电等离子烧结ZrB_2-YAG-Al_2O_3复相陶瓷的氧化性能

通过共沉淀法获得包覆式Al2O3-Y2O3/ZrB2复合粉体并对其进行放电等离子烧结来提高ZrB2陶瓷的烧结致密度和高温抗氧化能力。研究表明:通过引入YAG-Al2O3制备的陶瓷和纯ZrB2陶瓷相比,在相同氧化条件下得到的氧化层厚度有所变薄,说明通过引入YAG-Al2O3可以改善ZrB2陶瓷的抗氧化性能。在相同氧化条件下,引入Al2O3越多的陶瓷氧化层厚度越小。     

Densification and microstructure evolution of W-TiC-Y2O3 during spark plasma sintering

Spark plasma sintering (SPS) is one of the methods used to achieve the low-temperature densification of refractory metal materials. In this study, powder prepared through a wet chemical method was consolidated via SPS at 1100 °C, 1200 °C, 1350 °C, 1600 °C, and 1800 °C to obtain a high-performance W-TiC-Y₂O₃ composite material. Densification was studied by analyzing the densification curve and changes in the microstructure of the samples. This process could be divided into three stages: the bonding stage, the sintering neck growth stage, and the shrinkage and spherification stage of closed pores. Surface diffusion and grain boundary diffusion played different roles in densification. The density, grain size, and Vickers hardness of the tungsten material increased significantly as temperature increased. This study evaluated the sintering process and provided a basis for obtaining high-performance tungsten materials through SPS.     

WC-Co硬质合金结构参数对烧结表面残余应力的影响

通过建模分析和实际测量,对WC-Co硬质合金结构参数与烧结表面残余应力之间的关系进行了研究。结果表明,硬质合金的结构参数对烧结表面残余应力有很大的影响。相同WC粒径条件下,随Co相体积分数的增大,硬质合金烧结表面残余应力的数值增大。当Co相体积分数固定时,增大WC颗粒的粒径,可明显降低硬质合金烧结表面的残余应力。实际测量结果与分析结论相吻合。     

Influence of consolidation process and sintering temperature on microstructure and mechanical properties of near nano- and nano-structured WC-Co cemented carbides

In this paper the influence of the consolidation process and sintering temperature on the properties of near nano- and nano-structured cemented carbides was researched. Samples were consolidated from a WC 9-Co mixture by two different powder metallurgy processes; conventional sintering in hydrogen and the sinter-HIP process. Two WC powders with different grain growth inhibitors were selected for the research. Both WC powders used were near nanoscaled and had a grain size of 150 nm and a specific surface area of 2.5 m²/g. Special emphasis was placed on microstructure and mechanical properties; hardness and fracture toughness of sintered samples. Consolidated samples are characterised by different microstructural and mechanical properties with respect to the sintering temperature, the consolidation process used and grain growth inhibitors in starting powders. Increasing sintering temperature leads to microstructure irregularities and inferior hardness, especially for samples sintered in hydrogen. The addition of Cr₃C₂ in the starting powder reduced a carbide grain growth during sintering, improved microstructural characteristics, increased Vickers hardness and fracture toughness. The relationship between hardness and fracture toughness is not linear. Palmqvist toughness does not change with regard to sintering temperature or the change of Vickers hardness.     

Mechanical properties of β-SiC fabricated by spark plasma sintering

The consolidation of SiC nanopowder synthesized by the mechanical alloying method was subsequently accomplished by spark plasma sintering of 1700 °C for 10 min under an applied pressure of 40 MPa. The SiC sintered compact with relative density of 98% consisted of nano-sized particles smaller than 100 nm. This phenomenon resulted in the ordering process of stacking disordered structure formed by mechanical alloying. In this work, the effect of grain size and relative density on the mechanical properties were studied. The mechanical properties of sintered compacts were evaluated and compared with the reference samples fabricated from the commercial SiC powder (β-SiC, 0.3 μm, IBIDEN Co., Gifu, Japan) with sintering additive (B–C mixture). The Vickers hardness and bending strength of those sintered compacts increased with the increment of the density. However, the mechanical properties were lower than those of reference samples in case of lower density, even though the mechanical property was close to that of reference sample in case of higher density. This phenomenon was considered for the difference of bond strength between grains because those sintered compacts were fabricated without any sintering additives, while those reference samples were fabricated by accelerating the grain bonding with a sintering additive of B–C mixture. In other words, those results indicated that the effect of sintering additive affected on mechanical properties directly. This paper was presented at the International Symposium on Manufacturing, Properties, and Applications of Nanocrystalline Materials sponsored by the ASM International Nanotechnology Task Force and TMS Powder Materials Committee on October 18–20, 2004 in Columbus, OH.     

WC-Co硬质合金微观结构随机分布模型与弹性性能预报

基于“随机法”构建了WC-Co硬质合金微观结构的二维参数化模型,使用移动窗口法确定了微观结构代表性体积单元的尺寸,通过有限元模拟拉伸实验计算了材料的弹性性能.该模型考虑了微观结构的拓扑参数,反映了微观结构中平均粒径、长径和短径、形心位置和取向角分布的随机性特征,并实现了Co相的体积分数的控制.结果表明,采用移动窗口法确定的代表性体积单元包含约120个晶粒;在拉伸实验中晶粒形状及其位置分布对各向异性的影响较小.弹性性能模拟结果和采用Golovchan模型计算及实验结果吻合较好,从而验证了该方法能够有效预测WC-Co硬质合金的弹性性能.     

Fabrication and mechanical properties of WC-Co-Al2O3 nanocomposites by spark plasma sintering

Small amounts of nanocrystalline Al2O3 particles were doped in WC-Co nanocrystalline powders to study their reinforcing effects, and spark plasma sintering technique was used to fabricate the WC-Co-Al2O3 nanocomposites. Experimental results show that the use of Al2O3 nanoparticles as dispersions to reinforce WC-Co composites can increase the hardness, especially the transverse rupture strength of the WC-Co hardmetal. With addition of 0.5%(mass fraction) Al2O3 nanoparticles, the spark plasma sintered WC-TCo-0. 5Al2O3 nanocomposites exhibit hardness of 21.22 GPa and transverse rupture strength of 3 548 MPa. The fracture surface of the WC-TCo-0.5Al2O3 nanocomposites mainly fracture with transcrystalline rupture mode. The reinforcing mechanism is maybe related to the hindrance effect of microcracks propagation and the pinning effect for the dislocations movement, as well as the residual compressive strength due to the Al2O3 nanoparticles doped.     

Consolidation and properties of ultrafine binderless cemented carbide by spark plasma sintering

Owing to the absence of metal binder, binderless cemented carbides have higher wear, corrosion, and oxidation resistance. WC-0.3VC-0.5Cr3C2 powders with an average particle size of 200nm and a little amount of active element were consolidated by spark plasma sintering. The sintered microstructure revealed that the average WC grain size was 0.24μm, which was almost consistent with the initial fine powder. The results of XRD showed that W2C phase was formed. Nearly complete densification of ultrafine binderless cemented carbide was achieved by sintering at 1400℃ for 120s under 50MPa. The resulting hardness and the fracture toughness were 28.18 GPa and 6.05MPa·m1/2, respectively.