Image analysis and ceramics

Ceramics are always fabricated from powders or a mixture of powders. Bulk m aterials are obtained either via a liquid (slurry infiltration), solid (sin tering, electromelting) or gas (chemical vapour deposition or infiltration) process routes. The ceramics can be monolithic (alumina, silicon carbide, silicon nitride, ...). or polyphased (alumina-zirconia, silicon carbide-sil icon nitride ...). All these ceramics, whatever are their state - powder, g reen or sintered materials - can be characterized from their morphological point of view by using automatic image analysis. That is also the case for ceramic matrix composites (CMCs) or cermets (ceramic metal), which are gene rally obtained by sintering via a liquid phase route from a metal matrix an d a dispersed ceramic phase. The knowledge of the morphological characteristics of a ceramic is presentl y very important as these parameters control most often the mechanical stre ngth or electrical properties of the material. So, if is necessary to estab lish relationships between morphology and physical properties in order to h ave a return to the scientists in charge of the process to improve the cera mic material. Before any image analysis, it is necessary to observe the microstructure of the ceramics at the microscopic scale. It corresponds, in fact, to one of the most difficult problem to solve. After a classical polishing to reveal the ceramic grain boundaries, one can use generally either thermal etching (which can modify the size distribution if time and temperature are too hig h) or chemical etching (which do not reveal generally all the grain boundar ies). More recently plasma etching technique can be used, which offers nice results specially in the case of silicon nitrides. Observations are genera lly made either with an optical or a scanning electron microscope. The first step in image analysis is to segment the image in order to enhanc e (or defect) all the grain boundaries. The different methods of segmentati on which can be used in the case of porous ceramics, of ceramics with a low porosity and of bulk ceramic are presented and illustrated. At that stage, the ceramic can be considered as a two-phase system : 1) por es and ceramic; 2) matrix and refractory phase. The methods of parametrical characterization of the microstructure can be c lassified in two types : if the grain phase has not being segmented, classical stereological paramet ers can be used : the P(I) function will give access to the main morphologi cal size parameters; 2) if there is a correct segmentation of the ceramic grains, then an indivi dual analysis of the size and of the shape can be performed, like for the p owder analysis. These different cases are presented and some applications are given in the case of BaTiO3, UO2 and glass. For materials presenting an anisotropy (due to a hot-press process for exam ple) or for ceramics with whisker or fiber reinforcement, anisotropy can be obtained in using the rose of intercepts or of directions. These methods c an lead also to the size distribution of the neighbour number of a grain in R-2, which permits to inform on the grain growth. The sampling and homogeneity of ceramics can also be known in using the cov ariance or the regionalized variogram. That is the only way to reach such i nformation regarding the homogeneity at the morphological scale. Examples a re given in the case of UO2. Finally modelling can also be performed by automatic image analysis. Stereo logy is often sufficient to estimate 3D parameters, Nevertheless, the topol ogical parameters in R-3, the phase dispersion parameters, ... are not acce ssible from measurements in R-2. Probabilistic models are the elegant solut ion. They allow, for example, to accede to the number of particles per unit volume without any hypothesis and without using a serial sectionning Two e xamples are presented, in the case of MOX and WC-Co materials.