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.     

Thermomechanical properties of TiC particle-reinforced tungsten composites for high temperature applications

Tungsten and tungsten alloys are widely used in high temperature environments where arc ablation or mechanical deformation and damage are the main sources of materials failure. For high temperature critical applications in thermomechanical environments, however, the low strength limits the use of tungsten and tungsten alloys. Hence, new tungsten based materials with good high temperature thermomechanical properties need to be developed in order to extend the use of tungsten. TiC particle-reinforced tungsten based composites (TiC_p/W) were fabricated by hot pressing at 2000 °C, 20 MPa in a vacuum of 1.3×10⁻³ Pa. The composites were examined with respect to their thermophysical and mechanical properties at room temperature and at elevated temperature. Vickers hardness and elastic modulus increased with increasing TiC content from 0 to 40 vol.%. The highest flexural strength, 843 MPa, and the highest toughness, 10.1 MPa m^1/2, of the composites at room temperature were all obtained when 20 vol.% TiC particle were added. As the test temperature rose, the flexural strength of the TiC_p/W composites firstly increased and then decreased, except in the monolithic tungsten. The highest strength of 1155 MPa was measured at 1000 °C in the composite containing 30 vol.% TiC particles. The strengthening effect of TiC particles on the tungsten matrix is more significant at high temperatures. With the addition of TiC particles, the thermal conduction of tungsten composites was drastically decreased from 153 W m⁻¹ K⁻¹ for monolithic W to 27.9 W m⁻¹ K⁻¹ for 40 vol.% TiC_p/W composites, and the thermal expansion was also increased. The new composites are successfully used to make high temperature grips and moulds.     

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.     

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.     

不同结合剂原料放电等离子烧结制备TiC/Ti3SiC2复合结合剂金刚石复合材料

分别以Ti/Si/2TiC混合粉体和Ti₃SiC₂单相粉体作为结合剂原料,采用放电等离子体烧结技术合成了TiC/Ti₃SiC₂结合剂金刚石复合材料,探讨不同的结合剂原料和保温时间对TiC/Ti₃SiC₂结合剂金刚石复合材料的物相构成、微观形貌以及磨削性能的影响。结果表明:采用Ti/Si/2TiC为结合剂原料,保温1 min时,会形成较多量的Ti₃SiC₂,Ti₃SiC₂基体与金刚石结合良好,二者之间没有孔隙;当保温5 min时,Ti₃SiC₂发生分解,基体主相转变为TiC,同时有一定量的Si,金刚石表面被侵蚀,形成凹凸不平的表面。采用Ti₃SiC₂为结合剂原料时,Ti₃SiC₂基体发生严重的分解,生成TiC和Si;金刚石与基体间存在一个过渡层,厚度约15 μm。Ti/Si/2TiC为结合剂原料保温1 min时试样的磨耗比值最大,为1 128。单相Ti₃SiC₂为结合剂的2个试样的磨耗比值约为100左右。      

Spark plasma sintering for multi-scale surface engineering of materials

Recently, significant progress has been made in understanding the effect of multi-scale microstructural features, including nano-, micro-, and macro-features, on the properties of materials. Controlling the length scale of micro-structural features provides tremendous opportunities for enhancing the properties of materials, including extraordinary strength and hardness, unprecedented damage from tribological contacts, and improvements in a number of functional properties of the materials. Spark plasma sintering (SPS) process which combines the effects of uniaxial pressure and pulsed direct current is becoming increasingly important for the processing of bulk shapes of amorphous and nanostructured materials. These materials can also be good candidates for high-performance coatings. This article presents a review of our ongoing efforts to use SPS to produce engineered coatings of amorphous and nanostructured materials for various applications, including structural, tribological, and biomedical applications.     

Microstructure characteristics of SiC particle-reinforced aluminum matrix composites by plasma spraying

Aluminum with 55 and 75 vol.% SiC powders were ball milled as plasma spray feedstock. The feedstock was deposited onto a graphite substrate to form a freestanding composite by air plasma spraying. The microstructure characteristics of the sprayed composite were investigated by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The SiC volume fraction and porosity in the sprayed composites depend on plasma spray conditions. The silicon phase was formed in the sprayed composites in some plasma spray conditions, and its amount was related to the input of electrical power into the plasma spray. The mechanism of silicon formation was studied. In the sprayed composites, no reaction products could be observed in the Al/SiC interface. Impurity materials from ball media, stainless steel, and ZrO₂ reacted with aluminum and silicon to form complex compounds during plasma spray deposition.     

Oxidation behavior of TiC particle-reinforced 304 stainless steel

TiC particle-reinforced 304 stainless steels were prepared using a new developed in situ technology and exhibited the uniform distribution of TiC particles in the matrix. The oxidation behavior of 304SS-2TiC and 304SS-6TiC (all in weight percentage) was compared with that of 304SS at 850 °C in air for 96 h using thermogravimetry analysis. For 304SS, the rate of weight gain was very slow initially, but accelerated suddenly to a very high level, forming breakaway oxidation. The addition of TiC particles to 304SS resulted in no breakaway oxidation and maintained a low oxidation rate in the whole reaction time investigated. Examination of oxide scale morphology and cross-section analysis by scanning electron microscopy and optical microscopy showed a significant scale spallation and a deep oxide penetration in the case of 304SS, but a rather continuous, dense and adherent oxide layer formed on the surface of TiC particle-reinforced alloys. XRD analysis revealed the presence of Cr₂O₃ together with spinel-type oxides in the oxide scale. For TiC-containing alloys, fine TiO₂ was also found on the surface and the amount of this oxide increased with TiC addition. The TiC addition developed finer matrix structure before oxidation, which accelerates chromium diffusion. As a result, scale adherence was improved and oxidation resistance was increased.     

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。     

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.     

Corrosion behavior of TiC particle-reinforced 304 stainless steel

TiC particle-reinforced 304 stainless steels were prepared using a new developed in situ technology and their corrosion behavior was compared with that of 304SS in 5 wt.% HCl solution. As compared to 304SS, E_corr, E_pit and E_rp values had shifted to a more negative region in 304SS containing TiC, indicating faster corrosion rate by TiC addition. The addition of TiC particles to 304SS resulted in no rapid pit propagation but maintained a high corrosion rate in the whole immersion time investigated.     

Densification, microstructure, and fracture behavior of TiC/Si3N4 composites by spark plasma sintering

TiC/Si3N4 composites were prepared using the β-Si3N4 powder synthesized by self-propagating high-temperature synthesis (SHS) and 35 wt.% TiC by spark plasma sintering. Y2O3 and A12O3 were added as sintering additives. The almost full sintered density and the highest fracture toughness (8.48 MPa·m1/2) values of Si3N4-based ceramics could be achieved at 1550℃. No interfacial interactions were noticeable between TiC and Si3N4. The toughening mechanisms in TiC/Si3N4 composites were attributed to crack deflection, microcrack toughening, and crack impedance by the periodic compressive stress in the Si3N4 matrix. However, increasing microcracks easily led to excessive connection of microcracks, which would not be beneficial to the strength.     

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.     

Densification behaviour of pure molybdenum powder by spark plasma sintering

Pure molybdenum was sintered with SPS under various temperatures, external pressures and heating rates. The microstructure of the specimens representing the different sintering conditions was investigated by classical metallographic methods. The relative density, the microhardness and the chord length distribution were measured. Linear shrinkage, depending on time or temperature, was calculated from piston travel, which was recorded during sintering process. These results show that the main part of consolidation takes place during fast heating up. The densification behaviour is controlled mainly by sintering temperatures and applied pressure. The molybdenum powder was successfully consolidated by SPS in very short times. A relative density of 95% was reached by sintering temperatures of 1600 °C and external pressure of 67 MPa.     

放电等离子烧结钼的组织和性能

研究了放电等离子烧结（SPS）钼的致密化,组织和硬度,与其它烧结方法相比,SPS是一种时间短,温度低的先进快速烧结法。利用SPS在比CIP－S和热压烧结（HP）低180－500℃下进行烧结,保温3min,得到钼的相对密度是95．2％－97．9％,硬度比其它方法烧结的高出HV20－HV75。SPS烧结钼的密度呈现随烧结温度的升高而增加的趋势。但是其硬度有所下降,SPS在较低温度下烧结的钼的硬度较高,原因是其晶粒较细,SPS烧结钼的断口呈沿晶断裂,属于脆性断裂。     

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.     

热压烧结法制备C_f/TiC/Cu复合材料的组织及性能

采用热压烧结法制备Cf/TiC/Cu复合材料,研究Cf/TiC/Cu复合材料的界面反应原理及微观形貌,以及碳纤维(Cf)含量对复合材料密度、强度等性能的影响。结果表明:Cu-C-Ti三元体系在低于1100℃时,溶解在铜液中的钛原子与碳纤维接触发生反应,在碳纤维表面形成以TiC为主相的过渡层。该过渡层靠近铜液的一侧可能覆盖着一层钛铜化合物膜,TiC通过该膜层与铜紧密结合在一起,改善铜与碳纤维的界面结合,因此有利于提高Cf/TiC/Cu复合材料的性能。在钛含量不变的情况下,随碳纤维含量(质量分数)的增加,材料性能有所降低,当碳纤维含量为5%时,Cf/TiC/Cu复合材料的综合性能最好,其电阻率低达0.054μΩ·m,平行于压力方向的抗弯强度为237.90MPa,垂直于压力方向的抗弯强度为237.44MPa。     

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.