Browsing by Author "Janakiraman, Narayanan"

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    Deformation and fracture mechanical properties of precursor-derived Si-C-N ceramics
    (2007) Janakiraman, Narayanan; Aldinger, Fritz (Prof. Dr.)
    This thesis deals with the investigation of the deformation and fracture mechanical properties of precursor-derived (PDC) Si-C-N ceramics. The materials were synthesized from a liquid poly(ureamethylvinyl)silazane precursor. In order to access the intrinsic mechanical behavior of the materials, fully dense defect-free PDC specimens devoid of process induced extrinsic features were fabricated using a special casting and crosslinking process and controlled thermolysis procedures. The investigations were performed on a range of Si-C-N-(H) PDC with progressively varying material structures from hydrogenated amorphous to phase-separated nanocrystalline microstructures, from which the influence of material structure on the mechanical behavior was analyzed. The crack-tip fracture toughness (KI0) of the materials were evaluated using the novel crack opening displacement (COD) method. The estimated KI0 values ranged from 0.6 to 1.2 MPa m1/2. The variation in KI0 was well correlated with the structural evolution in the materials, effected by the progressive stripping of the one-fold coordinated hydrogen in the amorphous materials leading to increased network connectivity, and the nucleation and growth of turbostratic graphite (TG) and nanocrystalline SiC in the phase-separated materials. The net change in the resistance to fracture in these materials was effected by the change in the average fracture surface energy and crack deflection toughening. Crack deflection observed even in the amorphous materials revealed the presence of structural and compositional inhomogeneities. To further understand the cause and effect of crack deflection in terms of crack tip damage mechanisms, roughness analysis of fracture surfaces was carried out using the fractal approach. The analysis revealed self-affine scaling up to a correlation length of around 50 nm and a self-affine roughness exponent (ζ) of 0.8 ± 0.1 in all the materials, the latter in agreement with the universal roughness exponent conjectured in literature. However, no correlation was observed between the observed roughness exponents and the fracture toughness of the corresponding materials. Examination of the crack opening near the crack tip by high resolution AFM imaging revealed no persistent damage cavities along the crack, concluding that the fracture in the investigated Si-C-N ceramics proceeded in a brittle manner in the resolvable length scales, at crack velocities employed in the present experiments. The deformation behavior of the present Si-C-N ceramics under contact loading conditions was investigated using spherical as well as sharp (Vickers and Berkovich) indentation experiments. The elastic moduli and nanoindentation hardness evaluated from the analysis of the nanoindentation load-displacement curves correlated well with the evolution of material resistance to elastic and plastic deformation, commensurate with the structural and microstructural evolution in the materials. Analysis of the elastic and plastic deformation work quantities derived from the load-unload cycle in the Berkovich nanoindentation enabled the discrimination of the different plastic deformation characteristics of the amorphous and phase segregated materials. The equi-proportional variation of elastic and plastic deformation in the amorphous Si-C-N materials identical to vitreous silica indicated the anomalous character of plastic deformation in these materials that induced appreciable strain hardening under progressive densification. This was manifested in the load-dependant increase in hardness. The contrasting variation of plastic deformation work in the phase-separated materials indicated the emergence of an additional plastic deformation mechanism in these materials, that proceeded by a shear deformation promoted by the TG-phase. The anomalous densification behavior in amorphous Si-C-N materials also led to a load dependence of the strain rate sensitivity (m), also observed in vitreous silica, and controlled the evolution of indentation size effect (ISE). The magnitude and direction of ISE was determined by the relative dominance of the two concurrent effects, namely strain hardening and indentation creep deformation. The evolution of strain rate sensitivity in the range of investigated materials showed good agreement with the cluster model, which relates the increase in the number density of isolated regions in the microstructure to a corresponding increase in m. The non-densifying shear mode of plastic deformation in phase-separated materials led to a decrease in the strain hardening capability, increase in m and increased vulnerability to ISE.