Giuseppe Pezzotti

Transmission electron microscopy of microstructures in ceramic materials

Abstract Based on selected examples from the area of Si3N4 ceramics, the value of utilizing transmission electron microscopy (TEM) as a technique to study ceramic microstructures as well as a characterization tool for the development of new materials is demonstrated. In the field of ‘new ceramics’, one Si3N4-based composite is discussed, which was processed via pyrolysis of liquid precursors (polysilazanes). Moreover, it is shown that TEM in general can helpfully accompany ceramic processing techniques. This applies to the characterization of ceramic starting powders as well as to the study of densified materials. The investigation of Si3N4 powders, in particular the influence of the addition of sintering aids via organometallic precursors, which leads to a homogeneous distribution of sintering additives in the powder compact, in contrast to the use of metal oxide powders, is shown. The variation of micro-structures during the densification process of liquid assisted sintering is also demonstrated. The most common application of the TEM technique is to characterize dense ceramic components in the as-processed (as-sintered) state. Post-sintering heat treatment can initiate secondary phase crystallization. However, the very important aspect of microstructure integrity at elevated temperatures, e.g. the stability of microstructures under severe service conditions, is also addressed. Emphasis is placed on the fact that ceramic microstructures, which are typically thought to be rather stable, can undergo serious microstructural changes when temperature and stress is applied simultaneously, which strongly limits potential applications of these materials.

On the near-tip toughening by crack-face bridging in particulate and platelet-reinforced ceramics

The effectiveness on toughening of a near-tip crack-face bridging mechanism has been evaluated and discussed by both an experimental and theoretical viewpoint in the case of ceramic-ceramic particulate and platelet composites. According to the assumption of a strong bonding at the phase-boundaries, which, in ceramics, is often related to the characteristic of high refractoriness, a Barenblatt-like mechanism of elastic bridging was considered. From the experimental side, several brittle-matrix materials reinforced by SiC single-crystal grains of various size and morphology were fabricated and their microstructural parameters and fracture behaviors evaluated according to image analysis, fracture toughness and acoustic emission measurements. Theoretical and numerical computations of bridging-zone length and fracture toughness were performed with taking care to explicit the dependences of these parameters on the size and morphology of the reinforcement phase as well as the mismatches in elastic and fracture properties between matrix and reinforcement. Three model bridge-stress distribution functions were adopted and compared. They were: (1) the Dugdales or constant clamping; (2) the distributed bridging spring; and (3) the discrete-pin distribution. It is pointed out that, in the range of intrinsic elastic and fracture properties generally reported for ceramic phases, the maximum toughening effect achievable is limited by stereological and micromechanical reasons. The critical stress intensity factor obtainable by optimizing the reinforcement size and morphology in the composite body is hardly higher than a few times that of the matrix material, an order-of-magnitude increase requiring a mismatch in the inherent elastic and/or fracture properties of matrix and reinforcement which is not easily achievable among known ceramics.

Internal friction study of sialon ceramics

Abstract The behaviours of the internal friction and shear modulus for a Si3N4 ceramic sintered with large amounts of Al2O3, Y2O3, and AIN additives to form a β-sialon structure are reported. When subjected to annealing cycles at increasing temperatures, this material showed marked changes in both microstructure and stiffness. The internal friction parameter was found to be phenomenologically very sensitive to such changes. Crystallization processes occurring at the sialon grain boundaries were clearly represented by relaxation peaks in the internal friction curve as a function of temperature. X-ray diffractometry and analytical microscopy techniques such as electron diffraction and energy dispersive X-ray spectroscopy were also employed in parallel experiments for obtaining direct information on the microstructural evolution of the material. Based on the results of these characterizations and on analyses of activation energy from internal friction data, the physical origin of each relaxation peak is disc...

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.

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.

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.

Fractal character of fracture surfaces and boundary values of toughness in a simple ceramic-ceramic system

Eight ceramic-ceramic composites with a common matrix but reinforcement of different size and/or morphology have been characterized by fracture mechanics testing, and their fracture surfaces quantitatively analyzed by means of computer-aided scanning electron microscopy. The fracture surfaces were found to obey approximately a fractal rule over about four orders of magnitude in the range investigated between the submicrometre and the millimetre scales. Therefore, each fracture surface could be characterized in terms of a single numerical parameter, i.e. its fractal dimension ξ. A simple proportionality relationship between the measured toughness value (the work of fracture) and the fractal dimension of its fracture surface also could be established. Finally, some insight has been gained into predicting the boundary values of toughness for the present ceramic-ceramic system, based on fractal concepts.

Evidences for dilute solid solutions in the Si3N4-TiN system

In this study, the authors present some evidence proving that the Si[sub 3]N[sub 4] and a non-oxide phase, the TiN, may have certain degrees of mutual solubility. In other words, they show the existence of dilute alloys obtained through the solution of Ti atoms into the Si[sub 3]N[sub 4] lattice as well as of Si atoms into the TiN lattice. These proofs could be only obtained by investigating a pure Si[sub 3]N[sub 4] system which does not contain any externally added oxide additives. It should be emphasized that, since reports have already been published on composite materials containing both Si[sub 3]N[sub 4] and TiN phases, the alloys discussed in the present work have possibility been already fabricated though not yet recognized. Furthermore, they came across a preliminary result showing much higher solubility of TiN into [beta]-Si[sub 3]N[sub 4].

Interfacial Glass Structure Affecting Micromechanism Of Fracture in a Fluorine-Doped Si3N4-SiC Composite

In the field of structural ceramics, ceramic-ceramic composites such as Si3N4-SiC components have gained wide interest owing to their potential application at elevated temperatures. The incoporation of ceramic reinforcements, e.g., SiC whiskers or platelets, into a ceramic matrix can improve mechanical properties, however, densification is rendered difficult. Due to the high covalent bonding character of Si3N4, liquid-assisted sintering is required for complete densification even of monolithic materials [1,2]. The addition of metal oxides or transition metal oxides, which react with the SiC2 present on the Si3N4-particle surface to form a silica-rich liquid at high sintering temperatures, enables liquid-phase sintering. The remains of this liquid are commonly present at triple-grain junctions and along grain boundaries as a secondary glass. Post-densification heat treatment can partially crystallize these glass pockets [3]. One of the aims to further improve high-temperature performance of ceramic composites is to drastically reduce the amount of secondary phases. A high volume fraction of an amorphous phase strongly decreases the mechanical properties, because the glass softens at relatively low service temperatures, i.e., about 900°C. Therefore, materials were prepared without the addition of further sintering aids [4,5]. Hereby, liquid-assisted densification is achieved by the SiO2 present in the Si3N4 starting powder. Owing to the high melting temperature of pure silica glass, hot-isostatic pressing (HIPing) was utilized as the densification technique. These materials, formed by HIPing without the addition of sintering aids, could be fully densified and, moreover, showed superior creep behavior, when compared with commercial materials [61]. It should be emphasized that such Si3N4 materials with pure silica present at triple pockets and along grain boundaries, revealed a rather low fracture resistance, which is due to the predominantly transgranular mode of fracture [7,8].