2. Thesis and Dissertations

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Author: search.filters.author.M.R., Ramesh

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    Mechanical and Dynamic Behavior of Additively Manufactured Polymer Nanocomposites
    (National Institute of Technology Karnataka, Surathkal., 2024) Kumar, Sumodh; M.R., Ramesh; Doddamani, Mrityunjay
    Rapid production of high quality components without any additional tools is the key to cost reduction in industrial applications. The present work deals with the additive manufacturing of polymer nanocomposite and their graded variants using fused filament fabrication (FFF) technology based 3D printing process. High Density Polyethylene (HDPE) is used as a matrix, and functionalized MWCNTs are used as a filler material in this work. The development of nanocomposite (NC) with lightweight functionalized MWCNTs serves the purpose of reduction in weight with enhanced properties. The functionalized MWCNTs (0.5%, 1%, 3%, and 5% by weight) are blended with the HDPE to develop functionalized MWCNTs/HDPE NCs in the form of pellets, which are further characterized through scanning electron microscopy (SEM), melt flow index (MFI), and thermogravimetric analysis (TGA). Then, these developed NCs are utilized to extrude NC filaments for 3D printing. The extruded NC filaments are characterized for quality and printability, which are further utilized in 3D printing of NC and functionally graded nanocomposite (FGNC) samples. The 3D printed NCs and FGNCs are comprehensively characterized for prints quality through various techniques such as SEM, rheology, XRD, and density, and finally subjected to the tensile, flexural, compression, hardness, impact, mechanical buckling, free vibration, and thermal buckling. Also, the waste NC filament is investigated for its recycling potential. The SEM analysis revealed the uniform distribution of the functionalized MWCNTs in the HDPE in the developed nanocomposites, extruded nanocomposite filaments, and the 3D printed samples, confirming the suitability of the processing parameters used in the blending, extrusion, and 3D printing. It also revealed the seamless and strong layer bonding in the 3D printed NCs and FGNCs. The rheology findings also confirm the uniform distribution of the fillers qualitatively. It is observed that complex viscosity (η*) for the NCs slightly increases as the functionalized MWCNTs content increases up to 0.5%, and then dramatically increases for 5% (almost two orders) at the low frequency by maintaining a steeper graph, confirming the good dispersion of the MWCNTs in the HDPE matrix. The complex viscosity (η*), storage modulus (E'), and loss modulus (E") of the 3D printed NCs and FGNCs increase while the damping factor (𝑇𝑎𝑛𝛿) decreases as the functionalized reinforcement content rises. The MFI of the developed NCs (H0.5-H5) showed a decreasing trend with increase in the MWCNTs content, confirming the inclusion of the fillers in the matrix. The MFI decreases in the 14.52%-51.60% range for the H0.5- H5 NCs compared to the HDPE. The thermal stability of the developed NCs increases with the addition of the filler. The highest thermal degradation temperature is observed for H5 NC (479.07 ºC). The density of the filaments and the specimens decreases with the increase in the MWCNTs content. The highest density reduction of 16.06% and 15.58%, respectively, is observed in the H5 filament and the respective H5 print compared to the pure HDPE. The density of the FGNCs was also found to decrease with layer gradation. The FGNC-2 exhibited the highest weight-saving potential of 12%. 𝑇𝑀𝑒𝑙𝑡 of the filaments and the specimens increases with an increase in the MWCNT content, while 𝑇𝐶𝑟𝑦𝑠𝑡 increases up to 0.5% and 1% for the filaments and the specimens, respectively, then decreases. 𝛼𝐶𝑟𝑦𝑠𝑡 is higher for the filaments and the specimens than the neat HDPE. The XRD results also revealed the higher crystallinity of the NCs than the pure HDPE. A tensile study of the extruded filaments, NC, and FGNC prints revealed that the modulus and strength increase with the addition of the MWCNTs. The highest tensile modulus and strength are observed for the H5 NC filament, which is 105% and 30% higher than the pure HDPE filament. For the printed NC specimens, the modulus increases by 15.89%, 19.41%, 44.87%, and 81.43% in the H0.5, H1, H3, and H5 NC specimens, respectively, compared to the pure HDPE specimens. The highest modulus is registered for the H5 NC specimen (~81.5%). Further, it is noted that the tensile modulus of the printed specimens is higher than the respective filaments. The tensile modulus of the printed H-H5 NC specimens increased by 15.42%, 13.97%, 13.96%, 15.47%, and 2.38% compared to the respective filaments. The highest tensile strength (UTS) is registered for the H5 (16.6 MPa), 12.16% higher than the pure HDPE. It is noted that the H5 NC specimen has the highest modulus and strength among all the NC specimens, 81.43%, and 12.16%, respectively, higher than the HDPE. Further, the FGNCs are also tested for tensile. It is seen that their modulus and strength increase with the layer gradation. The enhancements in the moduli are 55.15% and 90.41% for the FGNC-1 and the FGNC-2 compared to the pure HDPE, respectively. Moreover, the FGNCs exhibited higher moduli than their respective homogeneous NCs, showing the potential to replace the NCs. The FGNCs showed higher strength than the respective homogeneous NCs, 23.75% and 37.12%, respectively, higher strength than the HDPE are noted for the FGNC-1 and FGNC-2. The tensile properties of the NCs are compared with the existing composites, where it is found that the 3D printed NC sample exhibited the highest tensile strength compared to the other fillers reinforced HDPE composites. The printed NCs and FGNCs are also investigated for the flexural responses. It is observed that the flexural modulus and strength of the NC specimens increase with the MWCNTs increase. The highest modulus and strength, 24.71% and 22.23% are noted for the H5 NC print compared to the neat HDPE. It is noticed that the specific strength and modulus of the printed NC specimens increase with the filler loadings. The H5 specimen showed the highest specific modulus and strength, 47.62% and 44.73% higher than the pure HDPE. For the FGNCs, the flexural moduli and strength are also noted to increase with the layer grading. The highest flexural strength and moduli are exhibited by the FGNC-2, which is 28.57% and 26.83% higher than the pure HDPE. The FGNC-2 also exhibited the highest specific strength and modulus, 46.16% and 44.14%, respectively, higher than the pure HDPE. The flexural behavior of the NCs and the FGNCs was also studied numerically, and the experimental findings were found to match well with the FEA results. The compression, hardness, and impact studies are also conducted on the printed NCs and FGNCs, which revealed an increasing trend of compressive modulus and yield strength with the increased MWCNTs content. The highest compressive modulus and yield strength are found for the H5 NC, which are, respectively, 44.89% and 9.28% higher than the pure HDPE. In the FGNCs, the layer gradation also notes a similar increasing trend. The FGNC-1 and FGNC-2 exhibited 35.75% and 61.14% higher modulus than the pure HDPE with the highest of the FGNC-2. The yield strengths of the FGNC-1 and the FGNC-2 are 8.89% and 11.56% higher than the pure HDPE with the maximum FGNC-2. The hardness and the impact strength of the NCs and FGNCs also increase with the filler content. The highest hardness and impact strength are noted for the H5 NC among all the NCs, while in the case of the FGNCs, are for the FGNC-2. The FGNC-2 exhibited the highest hardness and impact strength, respectively 76.80% and 119.99% higher than the pure HDPE. Mechanical, thermal, and dynamic loading causes instability, leading to the failure of the structure. Therefore, the printed NCs and FGNCs are also investigated for mechanical buckling, free vibration, and thermal buckling behavior. Results revealed that the NCs and the FGNCs buckling strength increases with the increased MWCNTs content. The highest critical buckling load (𝑃𝑐𝑟) is noted for the H5 NC among the NCs, which is 79.03% (by DTM) and 79.13% (by MBC method) compared to the HDPE. The 𝑃𝑐𝑟 of the FGNC-1 and the FGNC-2, calculated from DTM and MBC methods, are 54.38% and 91.34% higher than the pure HDPE. It is noted that the H5 NC and FGNC-2 displayed the highest buckling strength among the 3D printed NCs and FGNCs. The 𝑃𝑐𝑟, calculated from DTM, MBC, and VCT methods, exhibited good agreement. The natural frequency of the NCs and the FGNCs increases with the MWCNTs loading while decreases with increasing compressive load. Their damping factor showed a decreasing trend with the filler loading while observed to be increasing with a rise in the compressive load. The experimental and numerical findings are observed to be in very good agreement. The property chart reveals the superior performance of the H5 NC and the FGNC-2 compared to thermosetting composites. Thermal buckling studies of the NCs and the FGNCs under various heating conditions revealed that the critical buckling temperature (𝑇𝑐𝑟) and the deflection due to heating is very sensitive to heating type. The 𝑇𝑐𝑟 is noted as the highest in case-3 and the lowest in case-2. The maximum deflection is observed in case-2, while no significant difference is observed in case-1 and case-3. The 𝑇𝑐𝑟 of the NCs and FGNCs increases, while the deflection decreases due to the addition of the MWCNTs and their gradation. The highest 𝑇𝑐𝑟 is observed for the H5 NC (11.16% higher than the pure HDPE) and FGNC-2 (19.06% higher than the pure HDPE). The lowest deflection is observed for the H5 NC (53.33% lower than the pure HDPE) and the FGNC-2 (73.34% lower than the pure HDPE), indicating enhanced thermal stability. It is seen that the H5 NC exhibited the superior performance among all the NCs, while the FGNC-2 showed the best performance between the FGNC-1 and FGNC-2. Moreover, the FGNCs exhibited the superior performance to all the homogeneously reinforced NCs. In addition to comprehensive studies on the 3D printed NCs and FGNCs, this work also considered the study on the recycling potential of the waste polymers, which is increasing in the environment and causing many hazardous environmental problems. In this study, the waste extruded nanocomposite filaments that get wasted during filament extrusion are collected and examined for recycling potential. The waste filaments of the functionalized MWCNTs/HDPE NCs (H1 composition) are recycled to obtain the useful NC filaments for utilization in 3D printing. These NC filaments are further tested to check their potential for 3D printing. The samples are 3D printed using the recycled filaments, and subjected to density, XRD, and tensile tests to examine their recycling potential. The recycled filaments and the respective prints showed enhanced density, crystallinity, and tensile properties with respect to the extrusion cycle. After third extrusion cycle, no enhancement in the properties is found. The tensile strength and modulus of 1x, 2x, and 3x prints are 63.82%, 67.11%, and 67.76%, and 45.63%, 55.34%, and 97.81%, respectively, higher than the W/UR print. The highest tensile strength and modulus are observed for 3x print, which is 67.76% and 97.81%, respectively, higher than the W/UR print. This study revealed that the H1 NC filament can be recycled (extruded) three times to achieve property enhancement. The H1-3x print exhibited superior tensile strength and modulus among all the recycled prints. Through such an approach, the environmental pollution due to the plastic waste generated by 3D printing industries can be substantially lowered, in addition to expanding the filament material options for the FFF community. A similar route can also be explored for the other NCs (0.5%, 1%, 3%, and 5 wt.%), increasing application areas across various engineering fields such as structural, marine, aerospace, and automobile. The efforts presented in this research exhibited the potential of the 3D printed NCs and the FGNCs to be utilized as integrated and jointless functional components in a wide spectrum of engineering applications such as marine, automobile, aerospace, construction, defense, electronics, and naval in addition to increasing the novel materials options for 3D printing industries.
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    Elevated Temperature Sliding Wear Behavior Of Cocrnitimox And Cocrnitiwx High Entropy Alloys Processed Using Mechanical Alloying and High-Velocity Oxy-Fuel Spray
    (National Institute Of Technology Karnataka, Surathkal., 2024) Addepalli, Syam Narayana; Joladarashi, Sharnappa; M.R., Ramesh
    Maraging steels, widely used in the aircraft landing gear components were subjected to wear due to the harsh working conditions. Surface modification of these components by the deposition of advanced coating materials prolong their life. High entropy alloys (HEA) are a contemporary class of materials with multiple primary elements having applications in different fields, owing to their exceptional mechanical and physical properties. Therefore the curent research is aimed at enhancing the wear performance of maraging steels, by the deposition of HEA coatings. CoCrNiTiMox and CoCrNiTiWx (x: molar ratio; x= 0.5, 1, 1.5) HEAs were processed by mechanical alloying of pure metal powders for further application as feedstock in the High velocity oxy-fuel (HVOF) technique. The phase and microstructural transformation of the ball milled powders is investigated in detail by optimizing the milling time and speeds. The milling process is extended for 50 h and milled powder samples were collected at regular intervals of 10, 20, 30, 40 and 50 h to characterize the samples for their suitability to deposit using thermal spray techniques. The milled powders were characterized with respect to the phases, particle morphology, chemical homogeneity, particle size and crystallite sizes. Based on the characterization studies, the powders milled at a speed of 200 rpm for 10 h were selected as feedstock for HVOF deposition. After the deposition of coatings, the microstructural and mechanical characterization of coatings were performed. The phases and microstructure of the deposited HEA coatings were determined by X-ray diffraction (XRD) and scanning electron microscope (SEM). The microhardness of the coating was determined by using a vickers indenter on the coatings cross-section, with a load of 300 g and a dwell time of 15 s. The deposited coatings fracture toughness was determined by using the Evans and Wilshaw’s approach. The tribological behaviour of CoCrNiTiMox and CoCrNiTiWx HEA coatings at elevated temperatures was studied extensively using a Pin-on-Disc tribometer. The deposited coatings exhibited a lamellar structure and good mechanical bonding with the substrate. The porosities of CoCrNiTiMox and CoCrNiTiWx HEA coatings, as calculated using ImageJ software, were found to be in the range of 1-2%. i The mechanical performance of the CoCrNiTiMox and CoCrNiTiWx HEA coatings revealed superior values, when compared to other HEA coatings. The microhardness of CoCrNiTiMo0.5, CoCrNiTiMo, and CoCrNiTiMo1.5 HEA coatings were found to be 841±62 HV0.3, 927 ± 45 HV0.3 and 952±23 HV0.3, respectively. On the other hand, the microhardness of CoCrNiTiW0.5, CoCrNiTiW, and CoCrNiTiW1.5 HEA coatings were found to be 863±52 HV0.3, 951 ± 38 HV0.3 and 1025±39 HV0.3, respectively. The fracture toughness of CoCrNiTiMo0.5, CoCrNiTiMo, and CoCrNiTiMo1.5 HEA coatings were found to be 2.89 ± 0.31 (Mpa m1/2), 3.26 ± 0.25 (Mpa m1/2) and 3.79 ± 0.35 (Mpa m1/2) respectively. Likewise, the fracture toughness of CoCrNiTiW0.5, CoCrNiTiW, and CoCrNiTiW1.5 HEA coatings, were found to be 3.22 ± 0.26 (Mpa m1/2), 3.54 ± 0.32 (Mpa m1/2) and 3.87 ± 0.3 (Mpa m1/2) respectively. Further, it can be witnessed that the as-sprayed HEA coatings exhibited a steady increment in the mechanical properties with an increment in the molar fraction of Molybdenum and Tungsten. The specific wear rate of CoCrNiTiMo HEA coating dropped by 70.5%, declining from 17.34 ± 2.8 x10-6 mm3/N-m to 5.1 ± 1.6 x10-6 mm3/N-m, while CoCrNiTiW dropped by 76.3%, decreasing from 15.8 ± 3.7 x10-6 mm3/N-m to 3.73 ± 2.1 x10-6 mm3/N-m, with an increase in the temperature from RT to 600 °C. The wear rates of coatings exhibited a significant reduction at elevated temperatures, owing to the formation of TiO2, CoMoO4, NiO tribofilms for CoCrNiTiMo, and TiO2, CoWO4, WO3 oxides for CoCrNiTiW. Further, the CoCrNiTiMo1.5 HEA coatings offered better wear resistance, as compared to CoCrNiTiMo0.5 HEA coatings, at any temperature and loading condition, due to the increment in the molar fraction of Molybdenum. Additionally, the CoCrNiTiW1.5 HEA coatings exhibited superior wear performance, when compared to all the six compositions in the current research. The investigation of worn surfaces showed a transformation in wear mechanisms from adhesive and abrasive wear at room temperature to oxidative wear at elevated temperatures.