Spark plasma sintering of high-toughness ZrO2 materials doped with yttrium

Zirconia powders doped with yttrium prepared by special liquid-phase precipitation method were sintered by spark plasma sintering (SPS) to obtain high performance samples.The microstructure,phase composition,and mechanical properties of the samples were studied.The results of X-ray diffraction (XRD),Raman spectrum,and transmission electron microscope (TEM) show that the phase is tetragonal.The powders with large surface area and high sintering activity,due to small crystallite size,could be densified at 1100℃.The highest relative density of the sample obtained at 1300℃ is higher than 99% (the tetragonal phase is 6.08 g/cm3).The Hv and Kic are 13.76 GPa and 15.4 MPa.m1/2,respectively.     

Spark plasma sintering of TiC particle-reinforced molybdenum composites

In order to improve the recrystallization resistance and the mechanical properties of molybdenum, TiC particle-reinforcement composites were sintered by SPS. Powders with TiC contents between 6 and 25 vol.% were prepared by high energy ball milling. All powders were sintered both at 1600 and 1800 °C, some of sintered composites were annealed in hydrogen for 10 h at 1100 up to 1500 °C. The powders and the composites were investigated by scanning electron microscopy and XRD. The microhardness and the density of composites were measured, and the densification behavior was investigated. It turns out that SPS produces Mo–TiC composites, with relative densities higher than 97%.The densification behavior and the microhardness of all bulk specimens depend on both the ball milling conditions of powder preparation and the TiC content. The highest microhardness was obtained in composites containing 25 vol.% TiC sintered from the strongest milled powders. The TiC particles prevent recrystallization and grain growth of molybdenum during sintering and also during annealing up to 10 h at 1300 °C. Interdiffusion between molybdenum and carbide particles leads to a solid solution transition zone consisting of (Ti_1 −x Mo_x)C_y carbide. This diffusion zone improves the bonding between molybdenum matrix and TiC particles. A new phase, the hexagonal Mo₂C carbide, was detected by XRD measurements after sintering. Obviously, this phase precipitates during cooling from sintering temperature, if (Ti_1 −x Mo_x)C_y or molybdenum, are supersaturated with carbon.     

Spark plasma sintering on mechanically activated W-Cu powders

Mechanically activated W-Cu powders were sintered by a spark plasma sintering system (SPS) in order to develop a new process and improve the properties of the alloy. Properties such as density and hardness were measured. The microstructures of the sintered W-Cu alloy samples were observed by SEM (scanning electron microscope). The results show that spark plasma sintering can obviously lower the sintering temperature and increase the density of the alloy. This process can also improve the hardness of the alloy. SPS is an effective method to obtain W-Cu powders with high density and superior physical properties.     

Spark plasma sintering behavior of pure aluminum depending on various sintering temperatures

We have successfully fabricated high-density pure aluminum (Al) bulk by means of a spark-plasma-sintering (SPS) process. The relative density of Al was enhanced as the sintering temperature of the SPS process increased. During the SPS process for pure Al power, the Al oxide layer on the surface of the Al particle was partially broken by the microplasma and applied pressure. The microstructures of the spark-plasma-sintered compacts obtained at various temperatures were observed by optical microscopy, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy. We believe that the pinning effect, rapid heating cycle, and applied pressure played an important role in restraining the particle growth despite the increase in sintering temperature. It is feasible that the employed SPS process could be very useful to achieve fully densified Al compact.     

WC-7Co硬质合金放电等离子烧结工艺

采用放电等离子烧结(spark plasma sintering,SPS)技术制取WC-7Co硬质合金。研究了烧结温度、烧结压力对烧结WC-7Co硬质合金力学性能的影响,探讨了最佳烧结热压比,分析了粉末烧结致密化过程和晶粒长大机制。结果表明,WC-7Co硬质合金在1150℃烧结时,随着压力的增加,烧结致密性呈现先增加后降低的变化趋势,在30 MPa时可获得最佳烧结致密性。在升温速率为100℃/min,保温时间为5 min,烧结温度为1150℃,热压比为38℃/MPa的工艺条件下,利用SPS技术可制备组织致密、综合力学性能良好的WC-7Co硬质合金。     

Spark plasma sintering consolidation of nanostructured TiC prepared by mechanical alloying

Spark plasma sintering technique was used for the consolidation of nanostructured titanium carbide synthesized by mechanical alloying in order to avoid any important grain growth of the compact materials. The TiC phase was obtained after about 2 h of mechanical alloying. Towards the end of the milling process (20 h), the nanocrystalline powders reached a critical size value of less than 5 nm. Some physical and mechanical properties of the consolidated carbide were reported as a function of the starting grain size powders obtained after different mechanical alloying durations. The crystalline grain size of the bulk samples was found to be increased to a maximum of 120 nm and 91 nm for carbides mechanically alloyed for 2 h and 20 h respectively. The Vickers hardness showed to be improved to about 2700 Hv for a maximum density of 95.1% of the bulk material.     

Spark plasma sintering of ZrB2 powders synthesized by citrate gel method

The densification behavior of nanocrystalline zirconium diboride (ZrB₂) powders with nickel (5 vol%) is reported by spark plasma sintering (SPS) technique. SPS experiments were performed at 1600 and 1900 °C with 65 MPa pressure and 1 min holding time. A maximum relative density around 95% was obtained after SPS processing of ZrB₂ at 1900 °C while the density of ZrB₂ sample sintered at 1600 °C reached 88% of the theoretical density. Hardness and fracture toughness values are 11 GPa and 4.11 MPa m^1/2 for the sample sintered at 1600 °C and 13.7 GPa and 2.65 MPa m^1/2 for the sample sintered at 1900 °C, respectively.     

Spark plasma sintering of a commercially available granulated zirconia powder—II. Microstructure after sintering and ionic conductivity

The microstructure of TZ3Y zirconia samples sintered by spark plasma sintering was investigated using transmission electron microscopy. The results of these observations were used to confirm the mechanisms involved in the control of densification. For the second time, the ionic conductivity of some samples obtained by SPS was investigated as a function of temperature. The results were compared with the best results found in the literature and discussed.     

Spark plasma sintering of a commercially available granulated zirconia powder: Comparison with hot-pressing

A commercially available granulated TZ3Y powder has been sintered by hot-pressing (HP). The “grain size/relative density” relationship, referred to here as the “sintering path”, has been established for a constant value of the heating rate (25 °C min^?1) and a constant value of the macroscopic applied pressure (100 MPa). It has then been compared to that obtained previously on the same powder but sintered by spark plasma sintering (SPS, heating rate of 50 °C min^?1, same applied macroscopic pressure). By coupling the analysis of a sintering law (derived from creep rate equations) and comparative observations of sintered samples using transmission electron microscopy, a hypothesis about the densification mechanism(s) involved in SPS and HP has been proposed. Slight differences in the densification mechanisms lead to scars in the microstructure that explain the higher total ionic conductivity measured, in the temperature range 300–550 °C, when SPS is used for sintering.     

Spark plasma sintering of pure TiCN: Densification mechanism,grain growth and mechanical properties

This study aims to disclose the densification mechanism and grain growth behaviors during the spark plasma sintering (SPS) of undoped TiCN powder. The SPS experiments were performed under temperatures ranging from 1600 °C to 2200 °C and a fixed pressure of 50 MPa. The sintering mechanisms were described in different models according to two grain growth behaviors: densification without grain growth at low temperatures (1600–1700 °C) and grain growth without apparent densification at higher temperatures (1800–2200 °C). At the constant grain stage, a creep model is applied to describe the densification process. In addition, the effective stress exponents, n, are calculated, indicating that the densification can be attributed to both grain boundary sliding (n = 1.5) and dislocation climbing (n = 3.13 or n = 4.29). During the second stage of sintering, the grain growth model reveals that the grain-growth is controlled by grain boundary diffusion. In addition, the Vickers hardness varies from 4326 Hv to 6762 Hv when the density ranges from 90% to 96.3%.     

Spark plasma sintering of W-10Ti high-purity sputtering target: Densification mechanism and microstructure evolution

The densification mechanism and microstructure evolution of W-10Ti sputtering target prepared by spark plasma sintering (SPS) method at a temperature ranges from 900 to 1600 °C, with dwelling time of 6 min and fixed pressure of 30 MPa were investigated. Densification occurs mainly at low temperatures (900 to 1300 °C), while grain growth occurs at high temperatures (1400 to 1600 °C). The creep model has been used to reveal the densification process. The effective stress exponent n is calculated systematically, which indicates that the densification process is mainly due to the particle rearrangement (n < 1), grain boundary diffusion (n = 1–2), and dislocation climbing (n = 3.77 or 4.14). In addition, the apparent activation energy Q_d is calculated to be 119.30 and 271.79 kJ/mol when the effective stress exponent n is equal to 1 and 2, respectively. It is also found that the microstructure of W-10Ti alloys is greatly affected by the sintering temperatures. The solution between W and Ti significantly improves with the increase of the sintering temperature. The solubility of W in βTi(W) exceeded the eutectoid point (28.97 wt% W) and the eutectoid structure (βW(Ti) + αTi) forms in cooling process when the temperature is up to 1300 °C. With the temperature increasing to 1500 °C, the composition of the βTi(W) phase is located in the miscibility gap of the (βTi(W), βW(Ti)) system, which tends to decompose in to βTi(W) and βW(Ti) phases.     

Spark plasma sintering of ZrB2-based composites co-reinforced with SiC whiskers and pulverized carbon fibers

Spark plasma sintering (SPS) process was used to preparation of ZrB₂-based composites co-reinforced with SiC whiskers as well as various amounts of pulverized carbon fibers. The effects of C_F content on microstructure and mechanical characteristics of ZrB₂–SiC_w–C_F composites were scrutinized. Although all composites approached high densification, a fully-dense sample was fabricated by the addition of 2.5 wt% C_F. The growth of ZrB₂ grains was remarkably inhibited in C_F-reinforced composites. No in-situ formed phase was detected by XRD; however, trace of nano-ZrC clusters was observed in SEM fractographs of ZrB₂–SiC_w–C_F samples. The formation of such nano-sized ZrC refractory phase was also proved by thermodynamics. The hardness of composites slightly decreased from 21.9 to 19 GPa with increasing the C_F content. Reversely, the fracture toughness values enhanced from 4.7 to 6 MPa.m^½ with increasing the amount of pulverized carbon fibers.     

In situ joining of dissimilar nanocrystalline materials by spark plasma sintering

Joining of nanocrystalline materials will be a great challenge. In this paper, a reaction synthesis-based in situ joining technique was developed for joining dissimilar nanocrystalline materials by use of the spark plasma sintering (SPS) process. The joining technique combines the nanocrystalline material processing and joining operations into a single process.     

Spark plasma sintering of (Ti0.9W0.1) C powders prepared by mechanical alloying

Nanocrystalline (Ti_0.9W_0.1)C powder with a diffraction crystallite size of about 10 nm was synthesized by mechanical alloying. The formation of (Ti_0.9W_0.1)C carbide was detected by XRD measurements and microscopic observation. The sintering of these powders by a spark plasma sintering (SPS) at different temperatures were also studied. The results show that the maximum hardness was obtained for more relative density materials, meanwhile, the grain size is large. The micro-hardness and the relative density of the powder milled for 10 h and sintered at 1200 °C for 5 min under 100 MPa reach, respectively, 2978 HV and 98.35%.     

Economic and ecological aspects of plasma surface engineering

For new developments such as plasma surface technologies, it is first necessary to prove their scientific possibilities and limits. Therefore it is mainly questions of process understanding and process handling, quality of the substrate and coating as well as its interactions which are considered and discussed.

To use these technologies in industry it is also necessary to consider ecological aspects as well as economic and marketing aspects. For a scientist at first these aspects are not as important.

An overview of different economic aspects of the application of plasma surface technologies is presented by considering the following: investment and running costs of plants; costs of process development for special applications; comparison of different technologies; certainty and reproducibility of the coating technologies; cost and profit balance of coated work pieces; costs of marketing.

Recently, the importance of the ecological aspects of industrial technologies has increased greatly. In most cases these questions are closely linked to the economic aspects. Nearly all ecological problems can be solved and handled, but the costs for this are increasing greatly. Although the laws currently differ in different countries, in future the laws everywhere will become more strict. The main concerns of the application and handling of a plasma assisted process are for example the use or production of toxic media, security of plants for the staff and for the environment, and handling of used coated material.     

Phase formation of MgB2 superconducting materials fabricated by spark plasma sintering

Much research on MgB₂ has been carried out because MgB₂ has a higher transition temperature (T_c) of 39 K than that of other metallic superconductors and because the bulk form of MgB₂ has exhibited high current density. In this study, Mg powder of less than 10 μm and B powder of less than 3 μm with equivalent MgB₂ composition were mixed simply under argon atmosphere. In order to consider the effect of a pinning element on the superconducting properties, activated carbon of 5 at.% was added to mixed powders. The MgB₂ bulk was fabricated with mixed powders in graphite molds at the various temperatures by spark plasma sintering. The formation of the MgB₂ phase was confirmed with Differential Thermal Analysis (DTA) at 550 °C. The relative density of sintered MgB₂ was 97 %, which increased as the sintering temperature increased. The sintering proceeded initially in the solid state and then by liquid phase sintering with increasing temperature without abnormal grain growth. In the Physical Property Measurement System (PPMS) result, the Tc was about 37 K in the carbon-added sintered sample. The 300 nm size MgB₂ grains of hexagonal shape were formed after spark plasma sintering, but the MgB₄ phase did not produce precise T_c.     

火花等离子烧结技术制备的WC/Co纳米硬质合金

研究了火花等离子烧结工艺与YG10、YG12两种纳米硬质合金性能的关系.然后采用火花等离子烧结技术制备了硬质合金功能梯度材料,该材料由纳米WC/10%Co、纳米WC/12%Co、微米WC/15%Co混合粉以及不锈钢圆片烧结而成.显微硬度压痕显示该材料各层间的应力较小.     

Recent development in reactive synthesis of nanostructured bulk materials by spark plasma sintering

As a relatively novel sintering technique, spark plasma sintering (SPS) has been used extensively over the past decade to prepare a wide variety of materials, e.g., ceramics, composites, cermets, metals and alloys. Many applications of the SPS technique are the fabrication of nanostructured materials using nanosize powdered precursors as starting materials. This article provides a review of research activities that concentrate on the development of the SPS reaction sintering (SPS-RS) to produce dense nanostructured materials, which indicate that it is possible to synthesize and compact dense bulk materials with controlled sub-micron or even nanoscale grain sizes by the use of the SPS technique.     

Spark plasma sintering and in vitro study of ultra-fine HA and ZrO2–HA powders

Spark plasma sintering (SPS) was employed to sinter RF suspension plasma sprayed HA ultra-fine powders and ZrO₂–HA nano-composite powders. The powders were sintered at 1000 °C for 5 min at 11.1 MPa and 1100 °C for 5 min at 11.1 MPa, respectively. After sintering, the samples were ground and polished for subsequent indentation and microscopy studies. The as-sintered compacts of the HA–CaP powders were studied in vitro by immersion in simulated body fluid (SBF). The in vitro studies indicated that a bio-active apatitic layer was formed as early as 1 week after immersion. Optical microscopy and SEM investigation revealed negligible porosity and dense microstructure suggesting liquid phase sintering to have taken place. Phase composition was calculated with the aid of XRD and the Rietveld method. The results indicated that the mechanical properties of the as-sintered compacts were improved in the presence of nano-ZrO₂. The Young’s modulus increased to 130 MPa and the fracture toughness was 1.6 MPa m^1/2 for ZrO₂ loading lower than 3 vol.% indicating greater enhancement of properties than that suggested by the rule of mixtures.