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Title: Studies on Structural and Mechanical Properties of Carbon Nanotube Dispersed Tungsten Carbide-Cobalt Nano/Micro Composites
Authors: Kumar, Devender
Supervisor: Singh, Kulvir
Keywords: Tungsten Carbide;Carbon Nanotube;Nanocomposite
Issue Date: 18-Sep-2017
Abstract: The present thesis describes the preparation of the WC-Co nano/micro composite, wherein VC is varied from 0, 2.5, 5, 7.5 and 10 wt.% in a systematic way. The optimized WC-Co-VC nano/micro composites have been selected to study the effects of CNTs on various properties. These samples are characterized by various techniques. The results obtained from various techniques are correlated and supported by the existing and available literature. The results and observations are correlated in terms of different crystalline phases formations, their volume factions, density and microstructural changes with addition of CNT and VC in base composition i.e. WC-Co. These microstructural changes are correlated with the mechanical like hardness, fracture toughness and wear properties. The thesis is divided into five chapters with a list of cited references at the end of thesis. WC-Co nanocomposites, process, properties and their applications are discussed in chapter 1. Alternate materials, which are being used now for some special purpose such as machining, are also discussed. Brief account about all advanced ceramics bear high initial cost apart from limitation of their usage only in specialized machining are also given. Advanced ceramic materials have few advantages over the conventional carbides, still the usage of WC as cutting tool and die materials is the highest among all the materials. Easy manufacturing, relatively lower cost and good reliability are among various factors, which favor the need of development in conventional carbides. The chapter ends with the basic structure, physical, mechanical, thermal and wear properties of WC-Co composites along with properties of its individual constituents. Reviews of the literature related to the various aspects of sintering WC-Co composites containing variable quantities of different grain growth inhibitor are summarized in second chapter. Various conventional and non-conventional methods of sintering WC composites by different researchers is correlated in terms of processing parameters and their effects of mechanical properties. Sintering of most of the WC-Co based cermets is processed through powder metallurgy route. Lot of literature are available in support of processing methods of powder components, which are responsible for final evaluation and testing of mechanical properties of sintered components. Studies related to sintering temperature and holding time under controlled atmosphere is also discussed in detail in chapter 2. It has also been found in the literature that particle size of the raw materials and the sintered product influences the overall performance of the tool materials. Nano/micro particle show better overall performance instead of coarse particles containing WC-Co composites. Efforts are always in V force to control the diffusion of small particles to form large agglomerates. It is also observed that maximum temperature at different stages of sintering and the isothermal holding time largely affects the grain growth. There is sufficient literature in support of the fact that hardness and fracture toughness are equally important but inversely proportional properties for tool and die mateirals. Different modern sintering methods are now days are very popular in terms of controlled grain growth and fast sintering. However, complicated machine set-up, very high initial cost and limited capacity of production are the major limitations of spark plasma sintering and other modern techniques. Few evidences are also observed for the improvement in structural strength of the nanoceramics by adding small amount of CNT’s after surface treatment. Effect of carbon percentage on the phases formed during sintering is also discussed. This chapter completes by giving motivation for the present research work and basis for selecting present compositions. Chapter 3 encompasses the information on the raw materials used, methods of samples preparation and their processing along with characterization and testing techniques used to characterize these samples is presented. WC-Co composites were prepared by pressing the powder into small pellets. The structural properties of the as-prepared ceramic composites were analysed by the X-ray diffraction (XRD), scanning electron microscopy (SEM) with attached energy dispersive spectroscopy (EDS) and field emission scanning electron microscope (FESEM). Microhardness measurement of the polished samples was performed by Vickers indentation method. Fracture toughness was calculated from the crack length produced due to Vickers indentation using Palmquist indentation technique. High temperature application suitability of the sintered materials was evaluated using dilatometer. Wear resistance was estimated using tribometer under the condition of dry sliding friction and alumina as the counter sliding materials. On the selected samples, transmission-electron microscopy (TEM) measurements for confirmation of particle size are also performed. The results obtained from the various characterization techniques are discussed in chapter 4. Different pilot experiments were performed to find the minimum sintering temperature for optimum hardness. Common crystalline phases Co6W6C (η phase), WC, V6C5 and W are present. However, their volume fraction changes with sintering time and VC wt.%. The volume fraction of η phase is highly dependent on sintering time and VC content. Whereas, increase in sintering duration, leads to increase agglomeration of vanadium and cobalt. It has been observed that keeping lower (1100 °C) sintering temperature in combination with long holding time did not contribute much in improving the materials properties. On the other VI hand increasing the sintering temperature with shorter holding duration leads to better hardness. Higher wt.% of VC can also work well as a hardener to increase the hardness of the composite. Idea behind the use of high wt% VC is that the Young's modulus of VC is 422 GPa, which is higher than that of cobalt i.e. 207 GPa and lower than that of WC (696 GPa). Cobalt helps to improve fracture toughness of the ceramic composite. On the other hand, it tends to reduce the hardness. Another composition with 10 wt.% Co was also prepared from the same raw materials to find the hardness and fracture toughness. The results obtained from 10 wt.% Co series show lower values of hardness as well as fracture toughness as compared to 20 wt.% Co series. Effect of carbon nanotubes (CNT) doped WC-20Co composite is also investigated at different sintering temperature. Initially, CNT’s were added in 1, 3, 5, 7, 10 wt.% in WC-20Co-7.5VC. It was observed that higher amount of CNT was degrading mechanical properties due to formation of excess free carbon. Best results in terms of hardness and fracture toughness were obtained for 0.25-0.75 wt.% of CNT, sintered at 1400 °C. Thermal expansion of the materials sintered at 1400 °C is lower than the samples sintered at 1350 and 1300 °C. Even samples containing CNT sintered at 1400 °C shows lower thermal expansion as compared to 1350 °C. It is very clear that addition of CNT’s, improve all the properties in considerable extent. Results obtained from the wear test of the optimized samples, represents an appreciable improvement in the materials of similar composition obtained earlier. Effect of CNT addition in the other physical properties is also evident in the samples containing 0.25 and 0.50 wt.% CNT with similar base composition and processing parameters. Chapter 5 concludes about the important findings of the present study. Different samples are prepared by varying Co, VC and CNT’s with sintering temperature ranging from 1100–1400 °C. Different crystalline phases and their volume fractions change the mechanical properties drastically. The processing parameters are having remarkable effect on the properties. The best sample is observed P53 (WC-20Co-7.5VC-0.75CNT) in respect to hardness, fracture toughness, thermal expansion and wear resistance. Use of nanomaterial as raw materials and high wt.% of cobalt helps to increase the hardness and fracture toughness. High wt.% of VC helps to control the grain growth as well as act as a hardener. Unique findings of the present study are that high hardness and high fracture toughness is obtained in the same samples, which is not reported yet. It have been concluded that 0.75 wt.% of CNT addition can improve the physical and mechanical properties to great extent, even the materials is prepared by conventional liquid phase sintering.
Appears in Collections:Doctoral Theses@MED

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