Effect of Different Mixing Methods on Fe-MWCNTs MMC Fabricated by Conventional Powder Metallurgy

Abstract

Multi walled carbon nanotubes (MWCNTs) are considered as a promising reinforcement due to their unique physical and mechanical properties. The properties like light weighthigh strength, superior thermal and electrical properties of MWCNTs has high potential for reinforcement in the conventional materials. Researchers are interested in multi-walled carbon nanotubes (MWCNTs) reinforced metal matrix composites (MMCs) since they can be utilised as functional materials with fascinating thermal and electrical properties as well as structural applications due to their high specific strength. The aim of the current work is fabrication of multi-walled carbon nanotubes reinforced ironbased MMCs by conventional powder metallurgy (PM) route. High to low volume content MWCNTs reinforced five different compositions such as Fe-21, Fe- 4, Fe- 2, Fe-1 and Fe- 0.5 vol. % MWCNTs were chosen for fabrication of the composites. The composites were fabricated by various processes, parameters and atmospheres to investigate the effect on composite powder, microstructure and properties of the sintered composite. MWCNTs were dispersed in iron matrix by planetary milling for long time and short time, turbula mixing, sonicating and mixing in pestle-mortar. For long milling time (12 and 10 h) composite, in every 2 hours of milling, small amount of powder was picked up for characterization. The final synthesis powders and every 2 h milled Powders were characterized by XRD analysis, SEM/FESEM, TEM, particle size analysis and Raman spectroscopy. After successful synthesis, composite powders were cold compacted in a uni-axial hydraulic press and then sintered at three different temperatures of 900, 1200 and 1300 °C for 2 hours under argon and hydrogen gas atmosphere in a tubular furnace. After successful fabrication of composite, finally properties like density, hardness and compression strength of sintered composites were measured by Archimedes’ principle, Vickers hardness tester and UTM respectively. The tribological properties, electrical conductivity and corrosion behavior of composite were also studied. For long milled composite, the effect of milling conditions (wet and dry milling) and volume percentage of MWCNTs on phase evolution, size and morphology of composite powder and also on sintered composites have been studied. The wet milling and dry milling were conducted in toluene and argon gas atmosphere respectively. It has been found from FESEM analysis that 21 vol. % MWCNTs of 12 hours wet milled composite provides uniform distribution of MWCNTs in iron matrix. In case of other composition and long-time milling, MWCNTs are destructed into nano scale amorphous powder and embedded into the iron powder. After 10 h milling, the FESEM pictures demonstrate that the final Fe powder has spherical in shape. From phase analysis of the milled powder, it has been observed that ferrite, austenite, cementite and other iron-carbide metastable phases are evolved during milling. These metastable phases were formed due to mechano-chemical reaction between iron powder and carbon of MWCNTs. Crystallite size, lattice parameter, lattice strain and dislocation density were also derived from XRD data using FWHM. Due to high impact force of balls on powder during milling, it is found that increasing the milling time reduces the crystallite size and increase in lattice parameter, lattice strain and dislocation density. Particle size analysis was done for the milled powders and an average particle size of 7 μm was obtained after 10 hours of wet milling for Fe-2 vol. % MWCNTs. For Fe-21 vol. % MWCNTs, it has been noted that the average particle size drops from 100 to 13 μm after 10 hours of milling. Moreover, the wet milled powders are fine (average size d50 = 10 μm) and exhibit narrow size distribution whereas dry milled powders (average size d50 = 90 μm) show wide range binomial distribution. The internal morphology of milled powder was investigated by using transmission electron microscopy (TEM). By using Raman spectroscopy, the structural stability of MWCNTs was investigated. The morphology of sintered samples was observed by optical microscope and SEM/FESEM. It has been observed that clear grain and grain boundaries are formed for the composite sintered at 1300 C. At high temperature, enhanced particle-particle bonding occurred due to high rate of diffusion. The XRD patterns of sintered samples show the presence of high intensity sharp peaks of -Fe with a very less intensity of austenite and very weak peaks of iron oxide. Hence, the metastable iron carbides and austenite phases disappeared and iron oxide (Fe3O4) was formed a long with ferrite after consolidation at all temperatures. For high (21) vol. % Fe-MWCNTs composite, the maximum relative density, hardness and compressive strength values were found in Fe-21 vol. % MWCNTs wet milled, sintered at 1300 C for 2 hours composite and reported to be 92 %, 450 VHN and 525 MPa respectively. For all compositions, it has been found that density, hardness and compressive strength of composites increase with increase in sintering temperature due to improved particle-particle bonding. Among low volume (X=0.5, 1, 2 & 4) content MWCNTs reinforced composite, Fe-1 vol. % MWCNTs reinforced wet milled iron composite sintered at 1300 C for 2 h has achieved the maximum relative density, Vickers hardness and compressive strength having 90 %, 350 VHN, 800 MPa respectively. The optimum properties achieved at Fe-1 vol. % MWCNTs is due to proper dispersion of MWCNTs inside the iron matrix. For short milled and without milled iron-MWCNTs composite, it is found that no appreciable destruction or distortion of MWCNTs are found after 20-minute milling or turbula mixing. Moreover, no solid solutions (ferrite and austenite) or iron carbides phases were evolved during the processing of the composite powder. For short milled and without milled powder, it has been observed that MWCNTs are stable; retain their structure after milling and consolidation. In short milling and without milling; both iron powder and MWCNTs are absorbed less mechanical forces compared to long milling. The maximum density, hardness and compressive strength of 86 % relative density, 170 VHN and 425 MPa respectively were found in Fe-4 vol. % MWCNTs composite when consolidated in argon gas at 1300C for 2 hours. From short milled and without milled iron-MWCNTs composite, some samples were selected for non-lubricated sliding wear, electrical conductivity and corrosion study. It has been found that Fe-0.5 vol. % MWCNTs composite sintered at 1300C in argon exhibits higher wear resistance than other composites. The electrical conductivity of the composites increases with increase in MWCNTs amount up to 2 vol. %, beyond that conductivity decreases. Iron-0.5 vol. % MWCNTs metal matrix composite sintered in H2 at 1300 C shows better corrosion resistance than other composites.