2. Thesis and Dissertations

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    Experimental Investigation of 3d Printed Syntactic Foam Composites
    (National Institute of Technology Karnataka, Surathkal, 2019) Patil, Balu.; Doddamani, Mrityunjay
    Polymer matrix composites can reduce the structural weight and result in improved fuel efficiency and performance in transportation applications. Thermoplastic matrix composites have been used for semi-structural and engineering applications. In addition to the ease of fabrication using a wide range of forming processes, thermoplastic polymers are recyclable, which is the strong driving force for their current and future applications. Rapid production of high quality components is the key to cost reduction in industrial applications. The present work is the first attempt of manufacturing syntactic foams, hollow particle filled lightweight composites using thermoplastic based fused filament fabrication /fused deposition modeling (FFF/FDM) 3D printing process. High Density Polyethylene (HDPE) is used as the matrix material and fly ash cenospheres as the filler. Development of syntactic foams with cenospheres serves dual purpose of beneficial utilization of industrial waste fly ash and reduction in the component cost. Hollow fly ash cenospheres are blended with HDPE to form cenosphere/HDPE blend and is extruded to filament form and finally fed through 3D printer for printing ecofriendly lightweight syntactic foams. Prior to filament development, thermal degradation, melt flow index (MFI) and rheological properties of cenosphere/HDPE blend are studied. MFI decreased by 39.29, 60.54 and 70.51% with increasing cenospheres content of 20, 40 and 60 vol. % respectively. Rheology study of cenosphere/HDPE blend revealed complex viscosities values are maximum at a lower frequency but decreases with an increasing frequency indicating shear thinning behaviour. Both storage and loss modulus showed an increasing trend with filler content and frequency. Single screw extruder parameters are optimized to develop ecofriendly syntactic foam filament with minimum cenosphere fracture and to obtain homogeneous mixing of constituents. The optimized parameters are used for manufacturing syntactic foams filament with 20, 40 and 60 vol.% cenosphere in HDPE matrix. Further, recycling potential of foam filament is also studied. Density of H40 (HDPE with 40 vol.% ofcenospheres) foams increased in up to two extrusion passes (2X) due to cenosphere breakage and porosity consolidation. Tensile properties of developed filaments are carried out to assess its viability into 3D printer. Tensile modulus and yield strength of neat HDPE filaments increased with each extrusion pass. Specific modulus of 3D printed H40-2X and 3X are 1.6 and 2.6 times higher than the respective filaments, however, fracture strain decreases by up to 40%. For first time extruded (1X) filament with addition of cenosphere density reduces due to intact cenosphere and void formation during extrusion, making it a 3 phase foam material. The void content and weight saving potential increases with increase in filler content and their values are higher for 3D prints than respective filament. Higher filler loading increases filament modulus by 7.72-12.79% as compared to HDPE. Among the foam filaments, H20 composition registered the highest ultimate strength (10.30 MPa) and strain at break (26.20%). Differential scanning calorimeter and X-ray diffraction analysis of neat HDPE and foam filaments crystallinity is used to assess the parametric optimization of 3D printing process. It is observed that addition of cenosphere reduced crystallinity of HDPE. HDPE and foam filaments exhibit lower crystallinity as compared to respective printed material. Coefficient of thermal expansion (CTE) of 3D printed HDPE and its foam is studied to understand warping and shrinkage phenomenon occurring during printing. It is observed that filler addition in HDPE matrix reduces CTE remarkably. Warpage of the specimen is reduced with filler content and print quality is further improvised by optimizing printer speed, layer thickness, print temperature and cooling conditions. Tensile tests are carried out on filaments and printed samples. Cenospheres addition resulted in improved tensile modulus and decreased filament strength. Tensile modulus of printed foams increases with filler content. 3D printed HDPE and foams modulus is better than respective feedstock material (filament). Tensile properties of 3D printed HDPE and foams are compared with injection molded samples. 3D printed HDPE registered higher tensile modulus and fracture strength compared to injection molding. Flexural test is conducted on 3D printed sample in two configurations (topand bottom face of print subjected to the load). Results obtained from both configurations reveals that second configuration has shown better flexural modulus and strength. Neat HDPE print did not show any fracture below 10% strain. Flexural modulus increases with cenosphere content. Highest modulus is exhibited by H60 which is 1.56 times better than neat HDPE print. Raster gaps in 3D prints lowers flexural modulus and strength as compared to fully dense injection molded sample. Quasi-static and regular strain rate compressive response is investigated on prints. Compressive behaviour of 3D printed foams follow similar trend in quasi-static and regular compressive mode as reported in fully dense injection molded two-phase foams. Modulus of neat HDPE is higher for all strain rates as compared to foams. Yield strength shows an increasing trend with strain rate. Highest specific compressive modulus and yield strength is observed for H60 and H20 respectively at 0.1 s-1 among foams. Further, HDPE matrix syntactic foam prints are characterized for their viscoelastic properties by dynamic mechanical analysis. Tests are conducted over 30-125°C temperatures. Storage and loss modulus increase with increasing volume fraction of cenospheres, with a slight difference between HDPE, H20 and H40 vol.%, at all temperatures. Storage modulus decreased with increasing temperature for neat HDPE and foam prints. Storage and loss modulus decrease with increasing temperature in the range of 30-125°C, while Tan δ increases. Structureproperty correlations of all the investigated properties are presented with the help of exhaustive SEM images to understand underlying mechanisms. Property maps for selected test conditions are presented for comparative analysis between FFF/FDM based 3D printing of eco-friendly lightweight syntactic foam prints and other processing routes used for thermoplastics. This work is an effort towards making wide material choices availability for FFF based 3D printing industries. Finally, the potential for using the optimized parameters of 3D printing is demonstrated by printing several industrial components as a deliverable of of this work.
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    Experimental Investigation of Cenosphere Reinforced HDPE Syntactic Foam Composite
    (National Institute of Technology Karnataka, Surathkal, 2016) B. R., Bharath Kumar; Doddamani, Mrityunjay
    Polymer matrix composites can reduce the structural weight and result in improved fuel efficiency and performance in transportation applications. Thermoplastic matrix composites have been used for semi-structural and engineering applications. In addition to the ease of fabrication using a wide range of forming processes, thermoplastic polymers are recyclable, which are the strong driving forces for their current and future applications. Rapid production of high quality components is the key to cost reduction in industrial applications. The present work is the first attempt of manufacturing syntactic foams, hollow particle filled lightweight composites, using an industrial scale Polymer Injection Molding (PIM) process. High Density Polyethylene (HDPE) is used as the matrix material and fly ash cenospheres as the filler. Development of syntactic foams with cenospheres serves dual purpose of beneficial utilization of industrial waste fly ash and reduction in the component cost. Pressure and temperature used in PIM are optimized to minimize cenosphere fracture and obtain complete mixing of cenospheres with HDPE. The optimized parameters are used for manufacturing syntactic foams with 20, 40 and 60 wt.% cenosphere without any surface treatment initially. With increasing cenosphere content, density and tensile strength reduce and modulus increases. A theoretical model based on a differential scheme is used to estimate the properties of cenospheres by conducting parametric studies because of inherent difficulties in direct measurement of cenosphere properties. Further, the influence of cenosphere surface treatment, functionalization of HDPE and blending method on tensile properties are investigated. Cenospheres are treated with silane and HDPE is functionalized with 10% dibutyl maleate. Tensile test specimens are cast with 20, 40 and 60 wt.% of cenospheres using injection molding. Modulus and strength are found to increase with increasing cenosphere content for composites with treated constituents. Highest modulus and strength were observed for 40 and 60 wt.% untreated mechanically mixed and treated brabender mixed cenospheres/HDPE blends, respectively. These values are 37 and 17% higher than those for virgin andfunctionalized HDPE. Theoretical models are used to assess the effect of particle properties and interfacial bonding on modulus and strength of syntactic foams. Brabender mixing method provided highest ultimate tensile and fracture strengths, which is attributed to the effectiveness of brabender in breaking particle clusters and generating the higher particle-matrix surface area compared to that by mechanical mixing method. Theoretical trends show clear benefits of improved particle-matrix interfacial bonding in the strength results. Effect of surface treatment and blending method on flexural properties is dealt next. Flexural test specimens are cast with 20, 40 and 60 wt.% of cenospheres using PIM. The flexural modulus and strength are found to increase with increasing cenosphere content. Particle breakage increases with the cenosphere content and the measured properties show increased dependence on processing method. Untreated constituents blended by mechanical mixing provide the highest benefit in flexural modulus. Modulus of syntactic foams is predicted by two theoretical models. Bardella-Genna model provides close estimates for syntactic foams having 20 and 40 wt.% cenospheres, while predictions are higher for higher cenosphere content, likely due to particle breakage during processing. The uncertainty in the properties of cenospheres due to defects contribute to the variation in the predicted values. Untreated constituents blended by mechanical mixing route as observed in tensile and flexural characterization registered higher tensile modulus and better flexural performance. Thereby, characterization of cenosphere/HDPE syntactic foams synthesized by mechanical mixing route for untreated constituents is dealt in the subsequent investigations. Quasi-static and high strain rate compressive response is investigated later. Thermoplastic matrix syntactic foams have not been studied extensively for high strain rate deformation response despite interest in them for lightweight underwater vehicle structures and consumer products. Quasi-static compression tests are conducted at 10-4, 10-3 and 10-2 s-1 strain rates. Further, a split-Hopkinson pressure bar (SHPB) is utilized for characterizing syntactic foams for high strain rate compression.The compressive strength of syntactic foams is higher than that of HDPE resin at the same strain rate. Yield strength shows an increasing trend with strain rate. The average yield strength values at high strain rates are almost twice the values obtained at 10-4 s-1 for HDPE resin and syntactic foams. Further, HDPE matrix syntactic foams are characterized for their viscoelastic properties by dynamic mechanical analysis. Tests are conducted over 35-130°C temperatures and 1-100 Hz frequency range and combined using the time-temperature superposition principle to generate a set of isothermal master curves. Storage and loss modulus increase with increasing weight fraction of cenospheres, but with little difference between 40 and 60 wt.%, at all temperatures. The sensitivity of storage modulus to weight fraction of cenospheres increases with increasing frequency. Storage and loss modulus decrease with increasing temperature in the range of 35- 130°C, while tan δ increases. The Williams-Landel-Ferry (WLF) constants are a linearly increasing function of cenosphere weight fraction. Structure-property correlations of all the investigated properties are presented with the help of exhaustive SEM images to understand underlying mechanisms. Finally, the potential for using the optimized parameters of injection molding process is demonstrated by casting several industrial components as a deliverable of this work.
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    Experimental Investigation of Cenosphere Epoxy Syntactic Foam Composites
    (National Institute of Technology Karnataka, Surathkal, 2018) Shahapurkar, Kiran; Doddamani, Mrityunjay; Mohan Kumar, G. C.
    Polymer matrix composites provide lower weight structures and result in improved efficiency and performance in transportation applications. Thermosetting polymers when used with suitable hollow reinforcing constituents, higher specific properties can be achieved that cater to variety of applications. Development of syntactic foams with cenospheres serves dual purpose of beneficial utilization of industrial waste fly ash and reduction in the component cost in addition to weight reduction. In the present study, LAPOX L-12 epoxy resin is used as the matrix material and fly ash cenospheres (hollow microspheres) in as received and silane modified conditions are used as filler. Manual stirring method is employed for developing cenosphere/epoxy syntactic foams with as received and surface treated cenospheres in 20, 40 and 60 volume %. With increasing cenosphere content, density of untreated and silane treated foams decreases. Influence of cenosphere surface treatment and volume fraction of cenospheres in epoxy matrix on compression, quasi-static compression, flexural, tensile, dynamic mechanical analysis, wear and erosion properties are investigated in this work. Effect of arctic conditions on the compressive and flexural response of cenosphere/epoxy syntactic foams is dealt to understand the behavior of foams under extreme conditions. Samples are conditioned under arctic environment at a temperature of -60°C. Compression and flexural tests are then conducted at room temperature as well as at in-situ -60°C on the conditioned samples and compared against unconditioned samples tested at room temperature. For the case of unconditioned samples, compressive strength decreased whereas compressive modulus increased with increasing cenosphere volume fraction for both surface modified and as received cenospheres. For the arctic conditioned samples, a reduction in compressive modulus and significant increase in strength is observed for untreated and treated syntactic foams as compared to their unconditioned counterparts. Increase in flexural modulus is noted while a decrease in flexural strength is observed as compared to neat resin at room temperature with increasing filler content for both untreated and treated cenosphere reinforced syntactic foams. For the case of arcticexposed samples, an apparent increase in flexural modulus is observed as compared to room temperature tested cenospheres/epoxy syntactic foams. In addition, an apparent increase in the flexural strength is noted under arctic environment. Room temperature quasi-static compressive response is investigated at different strain rates. The energy absorption of syntactic foams increases with increase in cenosphere content. Compressive modulus of untreated and treated syntactic foams is observed to be higher than that of neat epoxy sample at the same strain rate. Silane treated foams exhibit higher modulus. Yield strength of untreated and treated foams decreases as compared to neat epoxy. Tensile modulus of both untreated and treated syntactic foams increases with increase in cenosphere volume fraction as compared to neat epoxy. Strength values of syntactic foams show decreasing trend as compared to neat epoxy. Treated syntactic foams registered better results as compared to untreated ones. Storage modulus increases with increasing cenosphere content and decreases with increasing temperature. Loss modulus of syntactic register lower values as compared to neat epoxy, while damping is noted to be increasing. Syntactic foams with treated cenospheres reveal higher values of damping for all the volume fractions. Treated syntactic foams render higher stiffness and damping as compared to untreated syntactic foams and neat epoxy at elevated temperatures. Wear rate decreases with increasing cenosphere content at all the tested conditions. Specific wear rate decreases significantly with increasing applied loads. Further, coefficient of friction decreases with higher filler loading and filler surface modifications. Wear debris is analysed further and disc temperature is also reported. Erosion behavior is studied at room temperature for 30 to 90° impact angles and 30 to 60 m/s velocities. Results show a strong dependence of impact angle and velocity on erosion rate of syntactic foams. With increasing cenosphere content erosion rate decreases for all impact angles. Erosion rate decreases with increasing impact angle and with decreasing velocity. Structure-property correlations of all the investigated properties are presented with the help of exhaustive SEM images to understand underlying mechanisms. Finally, the potential of using the evaluated properties are presented in the form of property map. These property maps provide guidelines toindustrial practioners and researchers in selecting appropriate materials based on the envisaged applications.
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    Studies on Elevated Temperature Tribological Behavior of Fly Ash Based Plasma Spray Coatings
    (National Institute of Technology Karnataka, Surathkal, 2018) Mathapati, Mahantayya; M R, Ramesh; Doddamani, Mrityunjay
    Material behavior at elevated temperature is becoming an increasing technological importance. Components working at higher temperatures like in land based gas and steam turbines, power generation boiler tubes, hot sections of aero engine, propulsion bearings, materials processing and internal combustion engines are subjected to surface friction, wear, oxidation and hot corrosion conditions. Service conditions of such components in elevated temperature environments may compromise their mechanical properties resulting in the reduced life cycle. Components working in such adverse conditions demand suitable surface modification techniques like thermal spray coatings that are widely adopted in similar situations. Plasma spray coating processes belong to the family of thermal spraying techniques and are widely used in many industries to protect the components against erosion, oxidation and wear. Thermal energy is utilized in this process to deposit a wide variety of materials including finely divided metallic and non-metallic materials. Higher temperatures utilized in these processes enable the use of coating materials with very high melting points like ceramics, cermets, and refractory alloys. The present work explores the possibility of using fly ash based plasma spray coatings for high temperature applications. The proposed coatings are investigated for their resistance to erosion, oxidation and wear under laboratory conditions. Commercially available Cr3C2-25NiCr, NiCrAlY, WC-Co, fly ash cenospheres, MoS2, CaF2 and CaSO4 are used as coating feedstock in the present investigation. Six types of coatings namely Cr3C2-NiCr/Cenosphere, NiCrAlY/WC-Co/Cenosphere, Cr3C2-NiCr/Cenosphere/MoS2/CaF2, Cr3C2-NiCr/Cenosphere/MoS2/CaSO4, NiCrAlY/WC-Co /Cenosphere/MoS2/CaF2 and NiCrAlY/WC-Co /Cenosphere/MoS2/CaSO4 are deposited on MDN 321 steel substrate (Midhani Grade). Coatings are characterized using Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD). Further, microstructure and mechanical properties (microhardness, adhesion strength, erosion, oxidation, and wear) have been characterized to evaluate their potential for hightemperature application. For the chosen spray parameters, seemingly dense laminar structured coatings (six types as mentioned earlier) with a thickness in the range of 350-400 m having porosity lower than 5 % has been achieved. Erosion behavior of MDN 321 steel, Cr3C2-NiCr/Cenosphere, and NiCrAlY/WCCo/Cenosphere coatings are investigated at elevated temperatures using solid particle erosion test (ASTM G76-13) set up at 200, 400, 600 °C with 30 and 90° impact angles using alumina erodent. Erosion resistance of both the coatings is observed to be higher than the substrate for the test temperatures chosen and noted to be more prominent at lower impact angle and higher temperature. Both the coatings exhibited a brittle mode of material removal through brittle cracking and chipping. NiCrAlY/WC-Co/Cenosphere coating reported better erosion resistance as compared to Cr3C2-NiCr/Cenosphere coating which may be attributed to plastic deformation of the NiCrAlY matrix due to the ductility of the matrix and hard WC-Co reinforcement to resist the matrix plow thereby reduces the erosion loss. Cyclic oxidation behavior of MDN 321 steel, Cr3C2-NiCr/Cenosphere and NiCrAlY/WC-Co/Cenosphere coatings are further carried out at 600 °C for 20 cycles. Each cycle consisted of heating at 600 °C for 1 hour, followed by 20 minutes of cooling in air. The thermogravimetric technique is used to approximate the kinetics of oxidation of substrate and coatings. Both the coatings reported lower weight gain as compared to the substrate. NiCrAlY/WC-Co/Cenosphere coating registered less weight gain as compared to Cr3C2-NiCr/Cenosphere coating which is attributed to the excellent oxidation resistance of NiCrAlY and formation of CoWO4 along with NiO and Cr2O3 oxides on the coating surface. Influence of solid lubricants on Cr3C2-NiCr/Cenosphere and NiCrAlY/WCCo/Cenosphere coatings is dealt next for tribological response. Dry sliding wear behavior of MDN 321 steel, Cr3C2-NiCr/Cenosphere/MoS2/CaF2, Cr3C2- NiCr/Cenosphere/MoS2/CaSO4, NiCrAlY/WC-Co/Cenosphere/MoS2/CaF2 and NiCrAlY/WC-Co/Cenosphere/MoS2/CaSO4 is carried out using high temperature pin on disc tribometer as outlined in ASTM G99-05 standard. All the four coatingsdisplayed a lower coefficient of friction and wear rate in comparison with the substrate. Excellent wear resistance of the coatings is attributed to the solid lubricants effect. Based on the wear rate data, the relative wear resistance of the coatings under dry sliding conditions is arranged in the following sequence: (Cr3C2-NiCr/Cenosphere/MoS2/CaSO4) > (Cr3C2-NiCr/Cenosphere/MoS2/CaF2) > (NiCrAlY/WC-Co/Cenosphere/MoS2/CaSO4) > (NiCrAlY/WC-Co/Cenosphere/MoS2/CaF2) Higher wear resistance of Cr3C2-NiCr/Cenosphere/solid lubricant coatings is attributed to the high hardness of Cr3C2-NiCr which is incorporated in the coatings. Developed coatings in the present study exhibit higher temperature resistance to erosion, oxidation and wear as compared to MDN321 steel making them suitable for components subjected to elevated temperature service conditions.