Now showing 1 - 8 of 8
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    Coarsening induced phase transformation of hafnia in polymer-derived Si-Hf-C-N-O ceramics
    (01-09-2011)
    Sujith, Ravindran
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    Kousaalya, Adhimoolam Bakthavachalam
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    Coarsening induced reverse phase transformation of hafnia in polymer-derived Si-Hf-C-N-O ceramics was studied experimentally. X-ray diffraction and high resolution transmission electron microscopy were used to obtain information regarding nanocrystalline structure. Hafnia crystallites precipitated out from the amorphous ceramic matrix on heat treatment were observed to remain as tetragonal hafnia below a critical crystallite radius of 4 nm. Prolonged heat treatment has resulted in coarsening, as explained by Lifshitz-Slyozov-Wagner model, resulting in reverse phase transformation from tetragonal to monoclinic hafnia. © 2011 The American Ceramic Society.
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    Synthesis and phase stability of precursor derived HfO 2/Si-C-N-O nanocomposites
    (01-03-2012)
    Sujith, Ravindran
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    Kousaalya, Adhimoolam Bakthavachalam
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    Hafnium alkoxide modified polysilazane was synthesized by the drop-wise addition of hafnium tetra(n-butoxide) to polysilazane. The solid state thermolysis (SST) temperature and the ceramic yield for both the polysilazane and modified polysilazane were determined by performing thermogravimetry. Fourier transform infrared spectroscopy was performed to understand the polymer to ceramic conversion as well as the bonding characteristics of the ceramics. The modified polymer after crosslinking was subjected to SST at 800 °C at a constant heating rate of 5 °C/min for a dwell time of 2 h in atmospheric ambience. From the X-ray diffractograms, the as-thermolysed ceramics were observed to remain X-ray amorphous and on heat-treatment resulted in the crystallization of tetragonal hafnia. However, on heat-treatment at 1500 °C, reverse phase transformation from tetragonal to monoclinic hafnia was observed. Raman spectroscopy and transmission electron microscopy were employed to further understand the phase evolution. The thermal stability and the influence of amorphous matrix on the coarsening of HfO 2 were also evaluated. © 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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    In situ age hardening and grain refinement in as-sprayed Al-Sc binary alloy deposits
    (25-02-2018)
    Raghukiran, Nadimpalli
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    Sujith, Ravindran
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    Agrawal, Harshal
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    Shabadi, Rajashekhara
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    In situ age hardening phenomenon observed in as-sprayed Al-Sc binary alloys is reported. This, along with substantial grain refinement in the as-sprayed alloys exhibited hardness in the range of 605 MPa (Al-0.3 wt% Sc) to 923 MPa (Al-0.8 wt% Sc) that are comparable to the optimum hardness values typically obtained by age hardening in similar as-cast alloys. Transmission electron microscopy of the spray deposits revealed densely spaced Al3Sc precipitates of about 10 nm suggesting in-situ age hardening of these alloys during spray forming. This behavior is attributed to the inherent heating cycle associated with the spray forming process and the rapid kinetics of Al3Sc precipitation.
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    Small-scale deformation of pulsed electric current sintered silicon oxycarbide polymer derived ceramics
    (01-11-2013)
    Sujith, Ravindran
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    Srinivasan, Nedunchezhian
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    The deformation behavior of pulsed electric current sintered silicon oxycarbide ceramics produced by the solid-state thermolysis of polyhydridomethylsiloxane at small-scale is investigated. The ceramics remained X-ray amorphous after sintering at 1300 °C in vacuum and a high density of ≈2.35 g cm-3 is achieved. The elastic constants of these ceramics are determined using non-destructive ultrasonic testing method. The elasto-plastic deformation under contact loading is determined using depth sensing nanoindentation technique. An indentation hardness of ≈11 GPa and reduced elastic modulus of ≈105 GPa is observed. The load-displacement curves display significant elastic recovery with an elastic work ratio of ≈0.71. The evolution of Hertzian cone cracks upon microindentation indicates an anomalous deformation behavior. Small-scale deformation behavior of pulsed electric current sintered silicon oxycarbide ceramics is investigated. The ceramics remained X-ray amorphous after sintering at 1300 °C in vacuum and a high density of ≈2.35 g cm-3 is achieved. An indentation hardness of ≈11 GPa and reduced elastic modulus of ≈105 GPa is observed. The load-displacement curves display significant elastic recovery with an elastic work ratio of ≈0.71. The evolution of Hertzian cone cracks upon microindentation indicates an anomalous deformation behavior. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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    Room temperature strain rate sensitivity in precursor derived HfO 2/Si-C-N(O) ceramic nanocomposites
    (01-01-2014)
    Sujith, Ravindran
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    Investigation on the room temperature strain rate sensitivity using depth sensing nanoindentation is carried out on precursor derived HfO 2/Si-C-N(O) ceramic nanocomposite sintered using pulsed electric current sintering. Using constant load method the strain rate sensitivity values are estimated. Lower strain rate sensitivity of ∼ 3.7 × 10 -3 is observed and the limited strain rate sensitivity of these ceramic nanocomposites is explained in terms of cluster model. It is concluded that presence of amorphous Si-C-N(O) clusters are responsible for the limited flowability in these ceramics. © 2014 Author(s).
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    Indentation response of pulsed electric current sintered polymer derived HfO2/Si-C-N(O) nanocomposites
    (01-12-2013)
    Sujith, Ravindran
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    The influence of pulsed electric current sintering on the structural evolution and indentation response of hafnia incorporated silicon carbonitride polymer derived ceramics was studied. Despite the absence of any sintering additives, appreciable sintering at 1400 C with a density of ∼2.7 g/cc was observed. Sintering beyond 1300 C resulted in the phase separation and nano-crystallization of amorphous Si-Hf-C-N-O ceramic. The crystallite coarsening was insignificant and thereby the tetragonal phase of hafnia was retained in the ceramic nanocomposite. A relatively high indentation hardness and elastic modulus of ∼18 GPa and ∼313 GPa were determined by depth sensing nanoindentation technique. © 2013 Elsevier Ltd and Techna Group S.r.l.
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    Crack Evolution and Estimation of Fracture Toughness of HfO2/SiCN(O) Polymer Derived Ceramic Nanocomposites
    (01-09-2015)
    Sujith, Ravindran
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    Zimmermann, Andre
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    In this study the crack evolution and fracture toughness of HfO2/SiCN(O) ceramic nanocomposites were studied. These ceramics crack radially under indentation in contrast to Hertzian crack in SiOC ceramics. This difference was attributed to the variance in Poisson's ratio and elastic modulus to hardness ratio. Fracture toughness by crack opening displacement and radial crack length measurement was 0.8-1.2 MPa.m1/2 and 2.5-3 MPa.m1/2, respectively. The open network forming structure resulted in the over estimation of fracture toughness value by RCL.
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    Processing and characterization of polymer precursor derived silicon oxycarbide ceramic foams and compacts
    (01-12-2013)
    Nedunchezhian, Srinivasan
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    Sujith, Ravindran
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    This work focused on the fabrication of silicon oxycarbide ceramic (SiOC) foams as well as dense compacts using poly(hydridomethylsiloxane) (PHMS) as a polymer precursor. The room-temperature cross-linking of PHMS was achieved by the addition of 1,4-diazabicyclo [2.2.2] octane (DABCO) with the release of hydrogen gas as a by-product. This resulted in self-blowing of the polymer precursor at room temperature and thereby offered the possibility of producing SiOC foams without the need of any external blowing agents. We also reported the fabrication of crack-free silicon oxycarbide compacts by cold compaction and pyrolysis route using polyvinyl alcohol (PVA) as a processing additive. Cylindrical-shaped pellets were pyrolysed at 1300 °C in argon atmosphere with a ceramic yield of approximately 85%. Increased resistance to phase separation and crystallization up to 1400 °C was attributed to the presence of large volume fraction of free carbon in the material which was confirmed by Raman spectroscopy.