T. Nishida

Mapping of residual stresses around an indentation in β-Si3N4 using Raman spectroscopy

The stress dependence of the Raman bands of silicon nitride (β-Si3N4) have been investigated and applied to indentation experiments. Seven high-frequency bands have been found to have linear and negative stress dependencies. On the other hand, low frequency bands (namely 183, 205 and 226 cm-1 bands) showed small positive correlations with the stress. The piezospectroscopic (PS) coefficients of all the observed Raman bands have been determined. As an application, one of the PS coefficients has been used to determine the stress distribution around a triangular indentation.

Viscous behavior of interfaces in fluorine-doped si3n4/sic composites

Abstract The influence of fluorine addition on the grain/phase boundary structures and their viscous behavior at high temperature were systematically investigated in Si 3 N 4 /SiC composites. As a reference, a simple system densified by hot isostatic pressing (HIP) and containing only SiO 2 at the boundaries was selected for this basic investigation. In addition, increasing amounts of F dopant were incorporated into the composite bodies by adding Teflon during the mixing procedure of the raw powders and then pre-firing the mixture under high vacuum at 1200°C. Analytical transmission electron microscopy showed that fluorine remained localized at the grain boundary films and triple points, constituting an amount up to a few percent by weight of the intergranular glassy-SiO 2 phase. Detailed structural characterizations of both grain and phase boundaries were performed by using high-resolution electron microscopy (HREM) and atomic force microscopy (AFM). The high-temperature mechanical behavior of the undoped and F-doped SiO 2 phases was characterized by both measurements of torsional creep rate and variation of internal friction at temperatures up to 1600°C. F-doped materials showed creep rates several orders of magnitude higher compared to the undoped sample and damping temperature curves markedly shifted to lower temperature values. According to the above set of microstructural and mechanical data, the inherent viscosity of the SiO 2 intergranular phase could be quantitatively evaluated and the viscous-sliding mechanism under stress modeled.

Effect of size and morphology of particulate SiC dispersions on fracture behaviour of Si3N4 without sintering aids

The relation between microstructural characteristics and fracture behaviour of Si3N4/SiC-particle composites were evaluated for a series of materials containing a 25 vol% dispersion, with mean size in the range 7–106μm. All the composites were fabricated by hot isostatic pressing without external addition of sintering aids via glass encapsulation. Quantitative image analysis techniques were employed to assess the microstructural parameters, dealing with morphology and distribution of the SiC particles. A fracture mechanics analysis based on the determination of fracture strength, toughness, work of fracture and rising R-curve behaviour provided the basis for discussion of the effectiveness of the SiC dispersions.The results of mechanical tests are compared with those obtained on the monolithic material fabricated by the same process. The microfracture mechanisms in composites are discussed by relating microstructural data, obtained by image analysis, to toughness data.

Transmission electron microscopy characterization of a fluorine-doped Si3N4

The authors report on the microstructure of a HIPed and fluorine-doped Si 3 N 4 ceramic with no further sintering aid addition. Emphasis is placed on the structural and chemical characterization of the vitreous secondary phase performed by transmission and analytical electron microscopy.

Thermomechanically induced embrittlement in hot isostatically pressed Si3N4/SiC composites

The macroscopic fracture properties of an Si3N4/SiC-platelet composite fabricated by hot isostatic pressing (HIP) without sintering aids were measured by the chevron-notch technique in bending and related to micromechanisms of fracture by means of a quantitative profilometric analysis of the fracture surfaces. Compositional and processing parameters were varied systematically in order to maximize both the fracture toughness and the work of fracture of the composite. Data were compared with those of monolithic Si3N4 fabricated by the same process. Cooling-rate from the HI Ping temperature was indicated as a critical parameter especially when cooling was performed under high pressure. A marked embrittlement of the composite body was found by cooling at around 650 °C h−1 and it could not be completely recovered by successive annealing even up to temperatures above 1700 °C. The highest fracture toughness and work of fracture in the composite (obtained at a cooling rate of about 100 °C h−1), were measured as 4.6 MPa m1/2 and 58.6 J m−2, respectively. In agreement with fractal analysis results, they were estimated to be about 60%–70% of the maximum values, respectively, obtainable in the present composite system, provided that a complete debonding at the platelet/matrix interface can occur.

A quantitative evaluation of microstructure in Si3N4/SiC platelet and participate composites

A microstructural evaluation of Si3N4 containing 15–40 vol% SiC platelets or particles is presented. All the composites were fully densified by hot isostatic pressing without external addition of sintering aids. Size, morphology, surface roughness and crystal structures of the SiC phases before and after sintering were compared in order to discuss the structural stability of the reinforcements up to 2050 °C in Si3N4 matrix. Morphology and phase characteristics of the grain boundary are also discussed. In addition, homogeneity and isotropy of the composite bodies were quantitatively examined by image analysis techniques and it was recognized that, for a similar degree of dispersion, the characteristic of three-dimensional randomness could be preserved only at Vf<30% in the composites containing high aspect ratio platelets.

Phenomenology of fracture and fracture mechanisms under slow-crack-growth/creep regimes in high-purity Si3N4 and its SiC-platelet composite

Abstract The mechanical behaviors and the microfracture mechanisms of a high-purity dense Si 3 N 4 material and its 25 vol.% SiC-platelet composite have been investigated in the range of temperature 1400–1520°C by means of extensive mechanical testing coupled to transmission (TEM) and scanning (SEM) electron microscopy. Creep fracture was not observed even after several hundred hours exposure at high temperature and the slow (subcritical) crack growth (SCG) from a single, most critical pre-existing flaw was the dominant failure mechanism. Phenomenological crack-growth laws were discussed and compared with a theoretical model for diffusive crack propagation. Marked lifetime elongations and increased flaw crack tolerance were found in the composite material due to shielding mechanisms operated by the SiC platelets in the wake of the intergranular SCG crack. These positive effects, diminishing substantially with increasing the temperature, were well explained by an algorithm incorporating into the mechanical driving force acting on the crack tip, the closure-field contribution due to the rising R -curve behavior of the material.

Microscopy investigation on fracture mechanisms in hot-isostatically pressed Si3N4/SiC-platelet composites

Fracture mechanisms in hot-isostatically pressed (HIP) Si3N4/SiC-platelet composites have been investigated by transmission electron (TEM) and scanning electron (SEM) microscopy followed by profilometric analyses. Two composites containing 25 vol% platelets were compared. They were fabricated from the same raw materials and by the same procedure except for the cooling rate from the sintering temperature. The study consists of experimental observations as well as measurements of fractographic parameters which dictates the level of toughening, such as the percentage of intergranular fracture, lengths and angles associated with the debonding process at the matrix/platelet interface. The presence of microcracking in the neighbourhood of the main crack, a higher fraction of intergranular fracture, as well as substantial debonding at the nitride/carbide interface up to high orientation angles were found in the composite cooled at low rates (∼ 100°Ch−1) which, despite the unchanged microstructure, was substantially tougher than that cooled at ∼ 650°Ch−1. These trends were not observed in the composite subjected to fast cooling. The stronger interfacial bonding found after fast cooling under high pressure was attributed to an apparent compressive stress remaining stored at the grain boundary, rather than to a weakening of the platelets or the matrix grains. Calculations based on the mechanics analysis of crack/interface interactions and on quantitative profilometric data, indicated a difference of about one order of magnitude in the apparent interface fracture energy of the two composites.

Estimate of polytype fractions and dislocation density in SiC before and after sintering in Si3N4 matrix

A quantitative characterization of polytype fractions and dislocation morphology and density is presented for two α-SiC powders. The tools were X-ray diffraction (XRD) and etch-pit analysis carried out before and after hot-isostatic-press (HIP) sintering in an Si3N4 matrix at 2050 °C under 180 MPa. Results are compared with data from transmission electron microscopy and electron diffraction previously obtained on the same powders. To avoid overlapping of the major XRD peaks with that of the Si3N4 matrix and to make possible the observation of the Si plane during etch-pit analysis in the powders after sintering, a chemical etching procedure to separate nitride and carbide phases without damage was developed. The morphology and density of pits and dislocations were analysed to get quantitative information about the crystal structures of the SiC crystallites and their modifications after the HIP cycle. The polytype fractions were found to be unchanged after sintering. It was also determined that polytypes 6H, 4H and 15R generally share part of the surface in a single crystallite rather than existing as single crystallites themselves, the 15R polytype generally being a hosted structure by a 6H or 4H matrix. A high density of dislocations (1013–1014cm−2) was found in both the SiC powders after HIP sintering compared with the raw materials.

Interface structure of a chlorine-doped Si3N4 studied by high-resolution transmission electron microscopy

Abstracts are not published in this journal