Investigation on wear and corrosion properties of multi-layered graphene reinforced titanium alloy (Ti64) composite

Owing to excellent mechanical properties, Titanium (Ti) alloys have been of great interest in recent years. However, due to their poor wear resistance application of these alloys is restricted in the areas involving wear and friction. To address such challenges, in the current work, multilayer graphene (MLG) reinforced titanium matrix composites have been fabricated via the spark plasma sintering process. The content of MLG in Ti6Al4V powder is varied between 0% and 1.2%. The present work explores the influence mechanism of MLG addition in Ti6Al4V by investigating fabricated nanocomposites’ microstructure, nanohardness and wear behaviour. Microstructural study reveals an in-situ reaction between a carbon source from MLG and Ti from the Ti6Al4V to form a secondary phase TiC. Moreover, due to the formation of TiC, an increase in the nanohardness value of sintered nanocomposites Ti6Al4V/1.2 wt.% MLG (5.39 GPa) was recorded when compared to fabricated Ti6Al4V alloy (3.23 GPa). Dry sliding wear tests were performed under loads of 3–7 kg. It was observed that Ti6Al4V/1.2 wt.% MLG nanocomposite shows the maximum wear resistance across various sliding velocities with a minimum wear rate of 15 × 10−6 g/m. It is evident from the results obtained that MLG plays a vital role in improving the microstructural, mechanical, and tribological properties of sintered nanocomposites.

The pioneering work related to the spark plasma sintering (SPS) technique, also known as pulsed electric current sintering (PECS), started in around 1906 when the first direct current (DC) resistance sintering (RS) apparatus was developed. Later, a similar process was developed and patented in the 1960s. The present-day SPS which is now widely used for sintering metals and ceramics was introduced by the Sumitomo Coal Mining Co., Ltd. of Japan in 1990. Since then this technique which is based on the idea of using plasma generated by an electric discharge machine for sintering has attracted immense attention in the area of powder technology and development of the composites. The SPS process can be considered as a modified hot pressing process where a pulsed electric current is passed through a graphite die and the specimen is heated by the Joule heat transferred from the pressing die. In SPS, the compaction and sintering stages are combined in a single operation. Due to the pulsed electric current and the spark plasma effect, the SPS process is capable of introducing simultaneous rapid heating and cooling rates; accompanied with high pressure, this process can realize nearly full densification at a relatively lower temperature, within a very short time. In the past, materials were confined only to the monolithic form, but for better physical, chemical and tribological properties, composite materials have evolved. Metal matrix nanocomposites (MMNCs) have evoked keen interest in recent times because of their excellent structural and functional properties and have the potential to replace the existing materials in a wide range of applications. MMNCs refer to materials where rigid nanosized reinforcements, typically having size <100 nm, are embedded in ductile metals or alloys which act as the matrices providing attractive physical and mechanical properties such as high specific modulus, strength-to-weight ratio, fatigue strength, temperature stability and wear resistance. The properties of the MMNCs can be designed and custom-made as per the requirement of the application. MMNCs received much attraction as compared to the metal matrix composites (MMCs) due to the size and strength of the nanometric reinforcements. Apart from the nanofillers, a fine grain size of the matrix could also contribute to the improvement in the properties of the composites. As conventional processing techniques require a long holding time at high sintering temperatures, which can damage the structure of the nanofillers, processes like the SPS are the ideal routes for development of such composites. Also during conventional sintering, abnormal grain coarsening becomes particularly severe which in turn implies that achieving a very high level of densification and maintaining a fine grain size is very difficult. Attaining a uniform dispersion of the nanofillers in the composites is not easy using liquid-processing techniques due to the difference in the densities of the nanofiller, and the matrix and the non-wettability at the interface between them lead to a heterogeneous structure that affects the overall properties of the nanocomposites. By SPS, prevention or reduction in grain growth, to maintain the nanostructure of the matrix, is possible through careful control of consolidation parameters, particularly heating rate, sintering temperature and time. Due to the short period of sintering, grain growth can be restricted, and materials having submicron-sized or nanosized microstructures having enhanced properties can be developed. As the SPS technique requires very short sintering time, it is ideally suited for the development of nanocomposites reinforced with carbonaceous nanofillers like graphene or its derivatives and carbon nanotubes (CNTs), as short sintering time is essential for preserving their structural integrity and intrinsic properties. A wide variety of materials like metals and alloys, ceramics, composites, cermets etc. can be successfully developed by the SPS process. However, it should be noted that although SPS presents many advantages as compared to other conventional sintering techniques, it also has a few limitations. One of the major drawbacks of the SPS process is the heterogeneity of temperature field during the temperature cycle, resulting in heterogeneous microstructures in the sintered samples. Also, SPS allows only simple symmetrical shapes and is expensive as it requires a pulsed DC generator. The focus of this chapter will be on the fundamentals of the SPS technique, the kinetics of densification and grain growth during SPS and how the SPS technique could be effectively used to develop metal matrix nanocomposites (MMNCs). The chapter provides a detailed overview of the SPS process, the current state of research in the area of MMNCs developed by the SPS and its future.

The influence of different sintering temperatures during Spark Plasma Sintering (SPS) process on microstructure of La-Ca-Sr-Mn-O ceramics has been studied. The powders of La0.67Ca0.33-xSrxMnO3 (x = 0.33; 0.03) (LCSM) perovskite were prepared by milling of the stoichiometric amounts of the starting materials - lanthanum oxide (La2O3), calcium oxide (CaO), strontium carbonate (SrCO3) and manganese oxide (MnO2), and subsequently calcinated twice. After the second calcinations the LCSM powders were treated by SPS method at four different temperatures (1000°C, 1150°C, 1200°C and 1250°C), at uniaxial pressure of 50 MPa in a vacuum. The microstructure characterizations were done by polarized light microscopy and scanning electron microscopy. The microstructural observations showed that increasing sintering temperature leads to an increase of grain size. The energy dispersive spectral (EDS) analysis confirmed that higher sintering temperatures cause changes in the phase composition of the investigated LCSM perovskite materials. The benefits of the LCSM samples preparation by SPS process are discussed.

Ni-free Ti-based bulk metallic glass with potential for biomedical applications produced by spark plasma sintering

Gas-atomized Ni52:5Nb10Zr15Ti15Pt7:5 metallic glassy alloy powders were consolidated by a spark plasma sintering (SPS) process. The densification behavior during the SPS process as well as the structure, thermal stability and mechanical properties of the sintered specimens were investigated. The glassy alloy powders were densified rapidly when the temperature exceeded about 740 K. The density of the sintered specimens increased with an increase in sintering temperature. The specimens with full densification and no crystallization were obtained by the SPS process at a sintering temperature of 773 K with a loading pressure of 600 MPa. The sintered specimens exhibit high-strength and can meet large-size requirement. [doi:10.2320/matertrans.ME200817]

Spark plasma sintering of a lunar regolith simulant: effects of parameters on microstructure evolution, phase transformation, and mechanical properties

Microstructures of binderless tungsten carbides sintered by spark plasma sintering process

Consolidation of titanium tri-aluminide by spark plasma sintering: K.Kobayashi et al. (National Industrial Research Inst., Nagoya, Japan.) J. Jpn Soc. Powder Powder Metall., vol 44, no 6, 1997, 554–559. (In Japanese.)

Oxides are generally non-flammable, durable and non-toxic materials; high safety and reliability in a battery system is assured by the use of oxide as an electrolyte instead of the highly-reactive non-aqueous liquid. To develop an oxide-based all-solid-state lithium-ion battery (ASS-LIB) for applications such as electric vehicles, the formation of strong interfacial contact between the electrolyte and the electrode powder is desirable, which can be achieved through powder technology. In oxide-based ASS-LIB, the good contact interfaces should be prepared by a simple powder sintering process, and also produces little ion-blocking impurities.[1] A Li2O-Al2O3-TiO2-P2O5 (LATP) solid electrolyte with NASICON-type structure possesses high bulk conductivity of over 10-4 S cm-1 at R.T. and high electrochemical stability (applicable potential range: 2.6−6 V vs Li/Li+)), which is a suitable candidate for assembling ASS-LIB with high safety and chemical stability. However, the poor interfacial contact between electrode and solid electrolyte is one of the general problems for showing the electrochemical activity of ASS-LIB. Spark plasma sintering (SPS) would be a useful tool for designing the ASS-LIB since dense ceramics can be sintered at shorter time with suppressing the formation of by-products at the interface. Actually, Aboulaich et al. have successfully measured the charge-discharge profiles of Li3V2(PO4)3 and LiFePO4 electrodes at ASS-LIB assembled with Ge-based Li1.5Al0.5Ge1.5(PO4)3 electrolyte by this SPS technique.[2] However, most electrode materials produce impurities by contact with the solid electrolytes containing Ti4+ ions as LATP after sintering at high temperature (900oC).[3] In this presentation, the ASS-LIBs using LATP electrolyte were assembled by SPS with tungsten carbide die, instead of conventional carbon die, which could be applied high pressure over 200 MPa and processed at low temperature below 300 °C, resulting in suppressing the formation of by-products at the interface. Samples for AC impedance measurement were prepared by SPS of Au/LATP powder (average particle size: 0.2 μm)/Au. The sintering temperature, applied pressure and time at SPS were 300 °C, 600 MPa and 1 min for the densification. During the SPS process, the shrink of the LATP pellet was observed at around 200 – 250 °C, which was obviously lower than the densification temperature of LATP at conventional sintering in furnace. The increase of neck region at the grain-boundary of LATP particles could be also observed by cross-sectional SEM image of LATP pellet. As a result, total Li-ion conductivities for LATP pellet was 2.2 x 10-5 S cm-1 at 30 °C. Composite electrode powder was prepared from a mixture of 50 wt% carbon-coated LiFePO4 and 50 wt% LATP electrolyte. Au/composite electrode powder/ LATP powder was assembled by SPS process at the same condition for the sample of AC impedance measurement. Lithium foil was used as a reference/counter electrode. A poly(ethylene oxide)-based polymer electrolyte film was inserted between the lithium foil and the LATP electrolyte separator to prevent the reduction of Ti4+ ion in LATP by contacting with the lithium metal. Electrochemical charge-discharge test was performed at a constant current of 10 μA cm-2. The ASS-LIB shows initial charge-discharge profile which was similar to the liquid electrolyte case, and the discharge capacity was 75 mAh g-1 at 28 °C and 131 mAh g-1 at 60°C, respectively. No impurity peak was observed in the powder XRD pattern of the LiFePO4-LATP composite electrode after SPS process, due to the low sintering temperature. Therefore, the low-temperature and high-pressure SPS process with tungsten carbide die would be one of the superior techniques for assembling ASS-LIBs with suppressing the formation of by-products and reducing grain-boundary resistance at electrode/electrolyte and electrolyte/electrolyte interfaces.

Mechanical properties of SiC/Ti-15V-3Cr-3Sn-3Al composites by spark plasma sintering

We investigated consolidation behavior of gas-atomized Cu50Zr45Al5 metallic glassy alloy powders by a spark plasma sintering (SPS) process. Density of the sintered samples increased with an increase in sintering temperature. The nearly full density samples without crystallization could be attained by the SPS process at sintering temperature of 693 K under pressure of 600 MPa. The produced samples exhibited high-strength and met large-size requirement. The SPS process makes it possible to fabricate the large-size bulk metallic glasses without limitation of dimensions and alloy system.

Ti-6Al-4V compact bulk was fabricated by Spark plasma sintering (SPS) with initial pressure of 1.7 MPa, holding pressure of 50 MPa, heating rate of 100 °C/min, and holding time of 5 min at different sintering temperature. The fracture morphology of the specimen sintered at different temperatures was observed to investigate the sintering mechanism. It can be concluded that there are four stages in the SPS process: activation and rearrangement of particles, connection of particles, growth of sintering neck and bulk deformation. The high-quality bulk compact can be obtained when the above mentioned four sintering stages proceed in turn and are all fully completed. The compact bulk has the best mechanical properties when the sintering temperature was 1050°C. The relative density of the bulk Ti-6Al-4V exceeds 99.5%. The tensile strength and the elongation of Ti-6Al-4V obtained by SPS process are 901 MPa and 13.9%, respectively.

The effect of slip casting and spark plasma sintering (SPS) temperature on the transparency of MgAl2O4 spinel

Nanostructured Fe‐18Cr‐2Si alloys were developed by a combination of mechanically alloying (MA) and spark plasma sintering (SPS) process. SPS was carried out in vacuum at three different temperatures (900, 1000, and 1100 °C) with a fixed holding time of 10 min and an applied pressure of 50 MPa. Potentiodynamic polarization (PDP), linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS) were used to study the effect of sintering temperature on the electrochemical properties of newly developed nanostructured ferritic Fe‐18Cr‐2Si alloy. XRD results showed that, after sintering, crystallite size increased with increasing sintering temperature and maximum crystallite size was 23.64 nm. The results showed that with increasing sintering temperature, corrosion resistance was increased in terms of pitting potential (Epit), passive current density (ip), and polarization resistance (Rp). The improved corrosion resistance was found to be closely related with the densification of the sintered alloys.

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