A study of the densification mechanisms during spark plasma sintering of zirconium (oxy-)carbide powders

Zirconium oxycarbide powders with controlled composition ZrC_0.94O_0.05 were synthesized using the carboreduction of zirconia. They were further subjected to spark plasma sintering (SPS) under several applied loads (25, 50, 100 MPa). The densification mechanism of zirconium oxycarbide powders during the SPS was studied. An analytical model derived from creep deformation studies of ceramics was successfully applied to determine the mechanisms involved during the final stage of densification. These mechanisms were elucidated by evaluating the stress exponent (n) and the apparent activation energy (E_a) from the densification rate law. It was concluded that at low macroscopic applied stress (25 MPa), an intergranular glide mechanism (n ? 2) governs the densification process, while a dislocation motion mechanism (n ? 3) operates at higher applied load (100 MPa). Transmission electron microscopy observations confirm theses results. The samples treated at low applied stress appear almost free of dislocations, whereas samples sintered at high applied stress present a high dislocation density, forming sub-grain boundaries. High values of apparent activation energy (e.g. 687–774 kJ mol^?1) are reached irrespective of the applied load, indicating that both mechanisms mentioned above are assisted by the zirconium lattice diffusion which thus appears to be the rate-limiting step for densification.     

Nano-sized zirconium carbide powder: Synthesis and densification using a spark plasma sintering apparatus

Nano-sized zirconium carbide powder was synthesized at 1600 °C by the carbothermal reduction of ZrO₂ using a modified spark plasma sintering (SPS) apparatus. The synthesized ZrC powder had a fine particle size of approximately 189 nm and a low oxygen content of 0.88 wt%. The metal basis purity of the synthesized powder was 99.87%. The low synthesis temperature, fast heating/cooling rate and the effect of current during the modified SPS process effectively suppressed the particle growth. Using the synthesized powder, monolithic ZrC ceramics with high relative density (97.14%) were obtained after the densification at 2100 °C for 30 min at a pressure of 80 MPa by SPS. The average grain size of the densified ZrC ceramics was approximately 9.12 μm.     

Identification of microstructural mechanisms during densification of a TiAl alloy by spark plasma sintering

This work aims at identifying, by coupled scanning and transmission electron microscopy (SEM and TEM) observations, the densification mechanisms occurring when an atomized Ti-47Al-1W-1Re-0.2Si powder is densified by spark plasma sintering (SPS). For this purpose, interruptions of the SPS cycle have been performed to follow the evolution of the microstructure step by step. The powder particles exhibit a classical dendritic microstructure containing a large amount of out-of-equilibrium α phase. During heating-up, the microstructure undergoes successive transformations. At T = 525-875 °C the α phase transforms into γ. The γ phase formed is supersaturated in W and Re. It de-saturates for T above 875 °C by discontinuous precipitation of W and Re-rich B2 phase. Densification takes place for T between 900 °C and 1150 °C by plastic deformation of the powder particles. TEM observations show that the repartition of the plastic deformation is correlated to the dendritic microstructure, and that dynamic recrystallization mechanisms occur. Microstructural phenomena directly resulting from the high currents involved in the SPS process have not been observed.     

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

Dense B₄C 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 B₄C 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.     

放电等离子烧结制备超细WC基硬质合金

采用纳米碳化钒(V8C7)粉末作为晶粒抑制剂及放电等离子烧结(SPS)方式制备超细WC基硬质合金.X射线衍射结果表明:超细WC基硬质合金主要由WC和Co3C两相组成,随着温度的升高,WC的衍射峰逐渐向小角度偏移.扫描电镜结果表明:SPS和纳米V8C7粉末对超细WC基硬质合金的微观组织具有重要影响.SPS使超细WC基硬质合金在较低温度下(1200℃)实现致密化;纳米V8C7粉末可以有效抑制超细WC基硬质合金中WC的晶粒长大,1200℃时WC的晶粒尺寸约500 nm.力学性能结果表明:1200℃时超细WC基硬质合金具有较高的性能(相对密度99.5%,洛氏硬度93.2,断裂韧性12.5 MPa·m1/2).     

放电等离子烧结制备立方氮化硼聚晶

采用放电等离子烧结工艺(spark plasma sintering,简称SPS)在氮气气氛中制备了以Si3N4-AlN-Al2O3-Y2O3系为基的立方氮化硼聚晶(polycrystalline cubic boron nitride,简称PcBN).烧结工艺参数为:加热速率300 ℃/min,初始压力30 MPa,保温时间5 min,烧结温度分别为1250 ℃、1350 ℃和1450 ℃.利用X射线衍射分析(XRD)和扫描电子显微镜(SEM)对样品的物相构成和试样新鲜断口的微观形貌进行了分析和观察,同时利用显微硬度测试仪测试了样品的显微硬度.实验结果表明,Si3N4-AlN-Al2O3-Y2O3-BN系聚晶材料可以在非常短的时间内致密化,样品的相对密度可达95%以上.采用SPS快速烧结工艺,样品中的超硬磨料cBN依然保持立方结构.随着烧结温度的提高,样品的硬度不断增加,PcBN的显微硬度为28～48 GPa.结合剂组份可与立方氮化硼牢固地结合在一起.放电等离子烧结工艺可成为制备PcBN刀具材料的一种新的制备方法.     

放电等离子烧结纳米硬质合金的研究

采用放电等离子烧结 (SPS)和普通真空烧结两种烧结工艺烧结 92WC - 8Co纳米硬质合金。放电等离子烧结 ,在 115 0℃的烧结温度、4.5kN压力下保温 5min ,烧结体就完全致密 ,其合金中的WC晶粒度小于 2 0 0nm ,硬度可达到 94.2HRA。真空烧结达到完全致密 ,烧结温度需 140 0℃ ,保温时间 30min ,WC晶粒度为 (30 0 40 0 )nm ,硬度最高为 93HRA。结果表明 :放电等离子烧结硬质合金的温度显著降低 ,烧结时间大大缩短 ,有效地抑制了WC晶粒的长大。SPS还显著降低微孔等缺陷 ,制品性能也大大提高。     

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.     

The effect of particle size on the densification kinetics of tungsten powder during spark plasma sintering

The influence of particle size on the densification kinetics of tungsten powder during spark plasma sintering was investigated. The densification rate of tungsten powder in the intermediate sintering stage decrease with increasing particle size, resulting in a delay in the sintering stages of coarse powder. The isothermal densification kinetic behaviors of tungsten powder show that the densification of tungsten powder can be divided into two kinetic stages: a low-stress exponent segment (n = 1.5) and a high-stress exponent segment (n = 3 or 4). With increasing of particle size, n increases from 3 to 4, and the activation energy decreases from 304 to 254 kJ/mol for the high-stress exponent segment. This is because the densification mechanism has a tendency to change from diffusion creep to dislocation creep or dislocation glide as the particle size increases. The evolution of the activation energy exactly matches the transformation of the deformation mechanism, indicating that the densification activation energy does not reflect a barrier to densification, but rather a barrier to deformation with different deformation mechanisms.     

Enhancement of densification and sintering behavior of tungsten material via nano modification and magnetic mixing processed under spark plasma sintering

In the present study, the influence of nano additives (Ni, Fe) and different mixing (turbular and magnetic) on the densification, microstructure and micro-hardness of the tungsten material under spark plasma sintering is analyzed. After turbulent mixing the nanoparticles are distributed widely in the W interparticle gaps but after magnetic mixing the nanoparticles are distributed not only on the gaps of the W particles but also on the broken surfaces. Ni incorporated tungsten materials achieved the maximum density of 98.3% at 1400 °C (turbular mixing) and 97.9% at 1300 °C (magnetic mixing). Fe incorporated tungsten material showed density of 97.7% at 1600 °C and 97.2% at 1400 °C after turbular and magnetic mixing. The influence of nanoparticles in the densification process was explained by Laplace force, boundary slip and Agte-Vacek effect. The microstructural analysis showed that nano-modification reduced the degree of porosity, and provides a compact material at low temperatures. X-ray fluorescence analysis reveals that magnetic mixing shows more uniform distribution of nanoparticles than turbular mixing. The nanoparticles incorporation increased the micro hardness of tungsten material. Hence, it is clear that magnetic mixing and nano modification greatly improved the densification and sintering behavior of the tungsten material.     

掺碳碳化硼活化烧结及其动力学

研究了在中位径为０．４２μｍ的碳化硼微粉中添加１％～４％碳为活化剂的烧结过程。研究了烧结坯的密度、抗弯强度、晶粒度与掺碳量、烧结温度、保温时间的关系。在一定烧结温度以上,少量碳的掺入明显提高了烧结活性,烧结坯密度明显提高,在本实验条件下最佳掺碳量为１％～２％,最佳烧结温度为２１６０～２２００℃。研究了掺碳碳化硼烧结动力学,得出其物质迁移机制主要为晶界扩攻。最后研究了其相组成,掺入的碳除溶解于碳化硼     

Fabrication of WC-Co/(Ti,W)C graded cemented carbide by spark plasma sintering

The WC-Co/(Ti, W)C graded cemented carbide was prepared by spark plasma sintering. The substrate is WC-8Co, and the hard layer is (Ti, W)C solid-solution. The effects of sintering temperature and holding time on the microstructure and properties of graded cemented carbide were analyzed. The hard layer is mainly formed by dissolving WC in the Co-phase and then by solid-solution reaction with TiC. As the sintering temperature increases, the migration rate of WC increases. When the holding time is 5 min, the thickness and the W content of the (Ti, W)C solid-solution hard layer increases with the increasing of sintering temperature. The thickness of the (Ti, W)C solid-solution can reach 51 ± 2 μm at the sintering temperature of 1700 °C for the holding time of 5 min. The hardness of hard layer surface increases first and then decreases with the increasing of sintering temperature. The Vickers hardness is the highest at 1600 °C, which can reach HV_0.221.53GPa. As the holding time increases, the thickness of the solid-solution hard layer increases, but the rate of growth decreases. As the thickness increases, the difference in the W element concentration between the solid-solutions of the same pitch decreases along the layer depth direction, and W element concentration in the entire hard layer increases. The oxidation behavior of graded cemented carbide at 400 °C and 600 °C was investigated. The (Ti, W)C hard layer has superior oxidation resistance relative to the WC-Co substrate.     

Reaction spark plasma sintering of niobium diboride

Niobium diboride (NbB₂) is synthesized and consolidated by the spark plasma sintering technique. Elemental reactants such as niobium (Nb) and boron (B) were subjected to two stage heat treatment, initially at 1200 °C for synthesis and followed by densification at the temperatures in the range of 1700 °C to 1900 °C. High dense NbB₂ (~ 97.7%ρ_th) is obtained at 1900 °C after 15 min holding period. Load application during heat treatment stage is found to improve the sinterability of the niobium diboride compacts. Hardness, elastic modulus and indentation fracture toughness of the high dense NbB₂ are measured as 20.25 GPa, 539 GPa and 4 MPa m^1/2 respectively.     

The activated sintering of WCu composites through spark plasma sintering

Tungsten-copper composites of network structure are increasingly being studied because of their unique mechanical, thermal and electrical properties. Chemical plating and mechanical alloying were used to prepare tungsten-copper alloys with different compositions in this investigation. Samples obtained through mechanical alloying method and sintered by spark plasma sintering (SPS) technique at 1030 °C, 60 MPa exhibited excellent performance. Ni element and Cr element were added to the composite as activating elements to improve the interface wettability of tungsten‑copper. The experimental results showed the Cr element contributes more to the densities, tensile strength, flexural strength and thermal properties of the composites than Ni element. Besides, the effect of two different ball milling ways on the properties of the composites was also studied. The sample W20Cu (15Cr), which was prepared by Cu(Cr) ball milling, showing a network structure, has the best performance.     

SPS制备TiAl基合金微观组织演变研究

以等离子旋转电极雾化球形Ti47Al2Cr2Nb0.2W预合金粉末为原料,采用SPS技术在不同烧结温度下（1100℃、1200℃、1250℃、1300℃）制备TiAl基合金,结合XRD、SEM及TEM等检测手段,对预合金粉末致密化过程中显微组织演变进行了系统研究以及不同烧结温度对力学性能的影响。结果表明：当烧结温度超过1200℃时,组织内存在少量高温残留B2相,且随着烧结温度的升高,β相（B2相）含量有所增加;烧结温度在1100℃时合金具有最高的压缩断裂强度、拉伸断裂强度和断裂应变,分别为2367MPa、600MPa和2.25%;α2相、β相和γ相存在如下晶体学关系： ∥ ∥ , ∥ ∥ ;1300℃烧结样品中有形变诱导α2→γ相变以及形变孪晶产生。     

Taguchi analysis on the effect of process parameters on densification during spark plasma sintering of HfB2-20SiC

Field assisted sintering (FAST) has emerged as a useful technique to densify ultra high temperature ceramics like HfB₂-20SiC to a high density at relatively low temperatures and shorter times. The effect of various process variables on the densification during spark plasma sintering of HfB₂-20SiC was studied using Taguchi analysis. The statistical analysis identified sintering temperature as the most significant parameter affecting the densification of HfB₂-20SiC material. A density of 99% was achieved on sintering at 2373 K for 8 min at 30 kN pressure and heating rate of 100 K/min.     

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.