Ravi Kumar N V

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.

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.

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.

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.

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).

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.

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.

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.

Different polymorphs of niobium pentoxide (Nb2O5) were synthesized using niobium ethoxide as a precursor by varying the pyrolyzing temperature. The room temperature X-ray diffractograms revealed the irreversible phase evolution from amorphous to pseudohexagonal (823 K) to orthorhombic (1023 K) and to monoclinic crystal structure (1223 K). While phase evolution was also confirmed by thermogravimetry and dilatometry, Raman spectroscopy clearly suggested complete elimination of free carbon in the pyrolysed ceramics. The sintering conditions were optimized to produce a highly dense (>95%) thermodynamically stable monoclinic Nb2O5. The electrical properties of stable monoclinic Nb2O5 sample were thoroughly studied. The monoclinic Nb2O5 was found to have a dielectric constant of around 28 with a dielectric loss of 0.008 at room temperature and at 100 kHz. At low measurement frequencies, an anomalous increase in the effective dielectric permittivity with increasing temperature was observed. Large values of the ε’ are associated with polarization due to the accumulation of free electrons at the grain boundaries. An analysis of the dispersion curves of Nb2O5 revealed that two relaxation processes are responsible for the observed anomalies, and the temperature dependencies of their parameters (dielectric strength, relaxation time and spectrum broadening parameter) were determined. The low-frequency process (relaxation time τ0 ~ 0.45 s), which makes the largest contribution to the dielectric constant, was apparently due to the inhomogeneous conductivity of ceramics. It was revealed that the DC conductivity of ceramics has thermoactivation character with activation energy of about 660 meV and was determined by the oxygen vacancies.

Since the last five decades, polymer-derived ceramics (PDCs) are in use and envisaged for a variety of applications. The transition of a precursor to an inorganic ceramic by pyrolysis and heat-treatment results in either amorphous or nanocrystalline composites with the evolution of phases strongly controlled by the processing conditions. Understanding the deformation behaviour under ambient conditions and at elevated temperatures is key to designing these materials for long-term use. However, quantitative reliable estimation of mechanical properties is quite challenging due to its unique structure which in turn is strongly governed by the precursor chemistry. The mechanical behaviour of PDCs in the form of fibres, bulk and foams are different and they are discussed separately. Both experimental and simulation-based studies are considered in this review. Recently, additive manufacturing processes have been used for the fabrication of PDCs, the mechanical properties of which are also included in this review.