Structure and chemistry of interfaces in Si3N4 ceramics studied by transmission electron microscopy

Interfaces in liquid-phase sintered Si3N4 materials were investigated employing both analytical and high-resolution transmission electron microscopy techniques (AEM, HREM). Owing to the sintering process that involves the addition of metal oxides to the Si3N4-starting powder, densified materials are composed of Si3N4-grains and amorphous as well as partly crystalline secondary phases. Most Si3N4 ceramics contain continuous, thin interlayers of residual glass along grain boundaries. HREM studies in conjunction with electron energy-loss spectroscopy (EELS) focused on the characterization of these grain-boundary films. A comparison between the diffuse dark field, the Fresnel fringe and the high-resolution lattice imaging technique revealed the latter technique to be most accurate in order to determine intergranular him width quantitatively. Depending on the interface composition, HREM imaging revealed conspicuous differences in film thickness. However, with a given batch composition each as sintered material is characterized by a constant film width within +/-0.1nm. Crystallization of secondary phases can alter the interface chemistry and thus change intergranular film thickness. The latter result is in line with the variation of interfacial film widths before and after oxidation. Owing to cation outward diffusion, a thinning of the interlayer was observed. In contrast, with increasing impurity-cation concentration at the interface, first a thinning and furtheron a widening of the intergranular film was observed. Si3N4 materials with only SiO2 present at interfaces showed a him thickness of 1.0 nm, independent of glass volume fraction. Apart from cations, the influence of anion segregation at Si3N4-grain boundaries was studied. Fluorine was found to markedly affect the mechanical response of the material. Owing to a lowered cohesive interface strength, as a consequence of F-segregation at grain boundaries, intergranular fracture was predominantly monitored. Moreover, in comparison to undoped specimens, anion-doped samples revealed markedly higher creep rates, which are related to a decrease in the apparent grain-boundary viscosity, as elaborated from internal friction measurements. In addition to the characterization of internal interfaces at room temperature, microstructures at high service temperatures were studied by rapid cooling Si3N4 materials from high temperatures. Quenching a MgO-doped Si3N4 from 1350 degrees C produced no observable variation in the grain-boundary film thickness. A substantial increase and a non-uniform width of the amorphous films was, however, observed when rapidly cooling from temperatures of about 1420 degrees C. Specimens quenched from higher temperatures or longer residence times above the entectic temperature again revealed an equilibrium thickness that was slightly wider compared to the film widths observed in the material slowly cooled down to room temperature. By studying a MgO-doped Si3N4 that was exposed for about 14,000h to 1100 degrees C, the question was addressed as to whether internal grain-boundary films are actually stable and can sustain a long-term exposure to high testing temperatures. It is shown that, apart from other microstructural changes induced during high-temperature exposure, the grain-boundary structure was fundamentally altered and depleted grain boundaries were formed. Moreover, the characteristics of interfaces, observed in Si3N4/SiC microstructures obtained by the pyrolysis and subsequent crystallization of organometallic precursors, are briefly discussed. Here, similar to the long-term exposure experiment, complex interface structures without the presence of continuous grain-boundary films were observed. In brief, the content of this overview is as follows: 1 Introduction 2 Experimental procedures 3 General comments, experimental results and discussion 3.1 Overall Si3N4 microstructure 3.2 Grain-boundary film imaging techniques 3.2.1 Diffuse dark field imaging 3.2.2 Fresnel fringe imaging 3.2.3 High-resolution lattice imaging 3.3 Statistical analysis of film thickness measurements 3.4 Undoped and SiO2-doped Si3N4 3.5 Si3N4 With the addition of sintering aids 3.5.1 Volume fraction of sintering aids 3.5.2 Cation segregation at the interface 3.5.3 Crystallization of secondary-phase glass pockets 3.6 Model systems 3.6.1 Ca2+ segregation at the interface 3.6.2 F- segregation at the interface 3.7 High-temperature microstructure 3.8 Dynamic microstructure 3.9 Interfaces in polymer-derived Si-C-N 4 Conclusions