A transmission electron microscope study has been made of a silicon nitride component with 6 w/o yttrium oxide as a sintering aid hot isostatically pressed (HIP) with an encapsulation glass of borosilicate. The TEM study concentrated on the interface region between ceramic and glass. Two different types of hexagonal boron nitride were formed near the interface. One, with a textured structure, seemed to nucleate heterogeneously on the surfaces of silicon oxynitride grains. The (001) planes of the crystals extended outwards, giving a thickness of approximately 0.5 microns. The other type formed as hexagonally shaped grains separate from the first type and appeared to have grown as several segments in different directions around a nucleus. In each segment BN layers are parallel to each other and perpendicular to their common [001]BM direction. This second type of BN crystal was also detected a little further from the surface within the silicon nitride. The volume fraction of additive glassy phase tended to be lower in this surface region than in the bulk. Possible mechanisms of prevention of encapsulation glass penetration into the porous ceramic component during HIP were discussed

Interaction between ceramic compacts and the encapsulation glass during the HIP process has been studied in a model system of silicon nitride and borosilicate glass. Attention has been focused on what happens when the pressure is first applied in the HIP-cycle, i.e. between about 1200 and 1500°C. At this stage the pore system of the ceramic green body is still rather unaffected by sintering. The model system was characterised to evaluate a possible viscous flow of glass into the green body. Two glass compositions, one with high and one with low viscosity, were used, measurements being made of their viscosity and their contact angle on the nitride. Applying Darcy's law it was predicted that the encapsulation glass with the lowest viscosity should penetrate about 1200 microns into the still open pore structure at 1450°C, but this was not observed experimentally. In the calculations no chemical reactions were assumed to take place. However, increases in hardness of heat-treated mixture of glass and silicon nitride powder indicates that nitrogen dissolves in the glass. It is known that nitrogen increases the viscosity of the glass and this would result in a more limited glass intrusion. After HIP the surface region of the dense ceramic exhibited a phase composition gradient of silicon oxynitride, down to approximately 100-200 microns into the bulk

The objective of this work was to prepare for transmission electron microscopy (TEM) a layered structure of materials with fragile microstructure. The samples consisted of two layers of different materials, silicon nitride and borosilicate glass, loosely bonded together. The low strength of the sample resulted in fragmentation during more conventional preparation. However, it was possible to prepare the fragments by mounting them in a titanium specimen carrier with aluminium strips as support. After grinding and polishing, a technique of low-angle ion milling was used to obtain electron beam transparent areas at the nitride/glass interface.

Chemical interactions in the system of silicon nitride with borosilicate glass have been studied as part of an evaluation of glass encapsulated HIP. Theoretical calculations have been performed to predict the thermodynamically stable phases under conditions reflecting different stages in a HIP-cycle. Experimental studies were made on heat treated mixtures of the silicon nitride and the silicate glass. These samples were evaluated with X-ray diffraction. At temperatures commonly used for densification, the system reacted to BN and Si2N2O in agreement with the theoretical calculations. At typical temperatures for pressure application no chemical reactions could be detected but the theoretical calculations showed that BN and, for larger amount of silicon nitride, also silicon oxynitride were stable. Minor amounts of the phases may have formed or non-equilibrium conditions could be explanations for the absence of the expected phases

Hot isostatic pressing (HIP) is one of the industrial techniques for production of structural ceramics. A high quality of the as-HIPed surface is highly desirable, as it may make it possible to minimise or eliminate costly finishing machining. When a porous ceramic is HIPed, an encapsulation of glass can be applied on the component as a gas tight envelope. Silicon nitride ceramics work well with a borosilicate encapsulation glass but oxides, such as alumina and zirconia, react severely. A protective barrier between the oxides and the borosilicate glass has been developed earlier. With such a system of protective layers it was possible to demonstrate HIP of some oxides, with good mechanical as well as biocompatibility properties as a result. However, there is a difference between the bulk and the surface properties of these materials. A model system of silicon nitride and borosilicate glass was chosen for a more thorough study on interactions between the encapsulation glass and the porous ceramic green body during HIPing. Mechanical, chemical and compositional gradient from the surface of the ceramic into the bulk were studied. Theoretical calculations on chemical reactions in the system Si3N4 - B2O3 were done accompanied by practical experiments. Special attention has been given the part of the HIP-cycle when the pressure is applied and the glass is pushed against the still porous ceramic component. Possible viscous flow of the glass into the ceramic was analysed. TEM-studies of the densified silicon nitride revealed two different types of hexagonal boron nitride formed in the surface region and their role in the encapsulation is discussed. The increased knowledge can beused in work for improved surface quality and for development of intermediate protective layers.

Ultra-micro indentation using both pointed and spherical tipped indenters has been used to characterize mechanical properties of silicon nitride densified by glass encapsulated hot isostatic pressing (HIP). Young's modulus and hardness have been studied as a function of distance to the interface between silicon nitride and the encapsulation glass. The Young's modulus values are 10 to 20% lower in the close vicinity of the silicon nitride surface compared to bulk values. At distances of 75 to 150 microns from the glass-silicon nitride interface, bulk values are measured. The differences in hardness values between the region close to the surface and the bulk is less pronounced. A possible explanation for these gradients is formation of new phases at the surface of the silicon nitride. Routines for the calibration of both the pointed and spherical tipped indenters are presented

In the manufacture of an object of a powdered material by isostatic pressing of a body (10) preformed from the powdered material with a gaseous pressure medium, the preformed body is provided with a casing (13) of glass which is made gas-impenetrable by heating before carrying out the isostatic...