S110 Special Review "Launching into The Great New Millennium" Journal of the Ceramic Society of Japan 109 [7] S110-S120 (2001) Grain Boundary and Interface Structures in Ceramics Yuichi IKUHARA EngineeringResearch Institute, The University of Tokyo(PRESTO, JST), 2-11-16,Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan セ ラ ミ ッ ク ス の 粒 界 及 び 界 面 の 構 造 幾 原 雄- 東京大学工学部総合試験所 (科学技術振興事業団), 113-8656 東京都文京区弥生 2-11-16 This paper reviews grain boundary and interface structures in ceramics. Firstly, geometrical treatments are briefly described for both grain boundary and hetero-interface. For the grain boundary, grain boundary character, small angle grain boundary, coincidence site lattice (CSL) theory, structure unit, segregation and amorphous grain boundary are reviewed to show a couple of examples observed in ceramics. For the het ero-interface, O-lattice theory, lattheory,latry,latace,theory,latry, latttticecoherency O-lattice and interfacial dislocation are discussed and the results ob tained for metal/ceramics interfaces are introduced. Transmission electron microscopy (TEM) enables us to characterize the very narrow regions less than 1nm, and therefore is powerful technique for investigating grain boundary and interface structures. In this paper, recent experimental TEM studies are presented to demonstrate grain boundary and interface characterization at high spatial resolution. [Received November 27, 2000] Key-words: Grain boundary, Hetero-interface, Transmission electron microscopy, Analytical electron microscopy, Grain boundary character, Small angle grain boundary, Coincidence site lattice (CSL), Structure unit, Segregation, Amorphous, O-lattice, Lattice continuity, Interfacial dislocation, Lattice mismatch, Alumina, Silicon carbide, Siliconn nitride, Vanadi um, Magnesia 1. Introduction puritu-doped alumina which has high creep resistance.12) HE mechanical and electronic properties of ceramics are Covalent bonded ceramics such as Si3N4 and SiC are known strongly influenced by the atomic structure of grain as hard sintering materials, so sintering additives are usual boundaries.1)-3) On the other hand, grain boundary and in ly used for the sintering. In this case, amorphous film with terface structure itself are sensitive to the grain boundary the thickness of about 1 nm is frequently formed along the character. Therefore, it is important that we investigate the grain boundaries. The chemical composition and bonding relationship between grain boundary structure and its state in the film are considered to determine the high-tem character so that we can understand how grain boundary perature mechanical properties of such ceramics.13) Amor structure affects the intrinsic properties of ceramics. In this phous grain boundary is also briefly discussed in this paper. review paper, the importance of grain boundary character is Hetero-interfaces are always formed in ceramic compo briefly described, and typical examples are introduced for a sites and thin films. The hetero-interface structure is of wide small angle grain boundary4) and a coincidence site lattice interest not only from a fundamental point of view but also (CSL) grain boundary5) as in the case. Like the grain boun because of its importance in many modern materials which daries of metals, the grain boundaries of ceramics can be are composites consisting of two or more phases. The sig described as dislocation boundary or CSL boundary. nificance of the interface lies in the fact that many of the However, the atomic structures in ceramics are a little bit properties of structural or electronic composites depend complicated, compared with simple metals. Ceramics gener sensitively on it. The O-lattice theory can treat geometric ally consist of more than two kinds of elements , and often matching at hetero-interface.6),7) But, this involves the use have a hexagonal structure . Examples of such ceramics are of several complicated transformations between two lat Al2O3, Si3N4 and SiC. In these ceramics, there is little simple tices, and the theory can not predict the most stable orienta geometrical matching at grain boundaries except for the ro tion relationship (OR). Recently, the concept of the coinci tation around the special axis. CSL theory , therefore, can dence of reciprocal lattice points (CRLP) has been not fully describe grain boundaries in ceramics . For this proposed, and applied to predict stable ORs for many kinds purpose, O-lattice theory6),7) or an evaluation of lattice of hetero-interfaces.8),9) The contents of these theories are continuity8),9) is needed as shown in Section 3. The struc explained in detail in this paper. The adhesion between two ture of interfacial dislocation at boundaries is also influenced materials is basically made by atomic interactions across the by the intrinsic structure of ceramics because more than two interface and its strength is ultimately determined by the elements interact at grain boundaries. Several examples of strength of interfacial bonds between the atoms of two con this are shown for the grain boundary in Al2O3.10) stituent materials.2) Hence, the adhesion between two Sutton and Vitek11) proposed the idea of structure unit to materials is inherently related to the structure of, and describe grain boundary atomic structure. The concept is defects in, the interface between them. The strength and that a grain boundary generally consists of some structure fracture properties of a structural composite are in turn units, and it has been successfully applied for a couple of related to adhesion. Besides its practical importance, the ceramics. It therefore will be briefly covered in this paper. structure of a hetero-interfaces is important from a fun Doping impurities into ceramics is a useful way to control damental viewpoint because of the desire to understand how grain boundary properties. The impurities often segregate nature accommodates the mismatches across the contact along the grain boundary, changing intrinsic properties. A plane of two translationally periodic structures.1) Interfacial typical example of this is shown for a small amount of im dislocations play an important role here, and are described Special Review "Launching into The Great New Millennium" S111 Yuichi IKUHARA Journal of the Ceramic Society or Japan 109 [7] 2001 in detail for the case of metal/ceramics interface in this paper. Transmission electron microscopy (TEM), in particular, high-resolution electron microscopy (HREM) is one of the most useful techniques for studying the atomic structure of grain boundaries and has in recent years been used to inves tigate various kinds of grain boundaries in many kinds of ceramics.2),14) Analytical electron microscopy (AEM) ena bles us to evaluate an element distribution and a chemical bonding state with nanometer-order spatial resolution,6) and recent improvements in the field emission gun have made it possible to employ energy-dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) with spatial resolution better than 1nm.14),15)This paper includes many HREM and AEM results of the grain boundary and in terface structures in ceramics. 2. Grain boundary 2.1 Grain boundary character Grain boundary character can be described in terms of the relative orientation relationship between two crystals and the orientation of the boundary plane. Geometrically speak ing, there are nine degrees of freedom that we must con sider to exactly describe the grain boundary character.1) Consider a grain boundary with the plane normal to the vec tor P, in which one crystal is rotated by ƒÆ around the rotation axis n with respect to the other crystal. In this case, there are totally five macroscopic parameters because two degrees of freedom are, respectively, given for n and P. The remaining four are microscopic parameters, and are in troduced from atomic structure relaxation at grain bounda ry. That is to say, they are three degrees of freedom of rigid body displacement on grain boundary plane and one degree of freedom of displacement perpendicular to the boundary. A grain boundary can be classified into a small angle boundary or a large angle boundary depending on the degree of rotation angle ƒÆ. Although it varies a little by materials, the angle of a small angle grain boundary is generally limited 10-15•‹ which is close to the overlapping of Fig. 1. High resolution electron micrographs of (a) 10•‹ small an dislocation cores.16) On the other hand, among the angles of gle tilt grain boundary in Al2O3 and (b) Burgers circuit around the large angle grain boundaries, there are some specific angles dislocation in (a). at which two adjacent grains well-match geometrically. A grain boundary having such an angle is called a coincidence site lattice (CSL) grain boundary,5) and generally the energy is low and its structure is considered to be stable. boundary as indicated by the arrows. Figure 1(b) is a Burg CSL grain boundary is expected to be mechanically strong, ers circuit around the dislocation shown in (a). From this and therefore has been used to design grain boundary con circuit, the Burgers vector of the dislocation can be identi trolled materials.17)-19) fied as 1/3[110n] since the circuit is just a projected circuit We often use the terms tilt boundary and twist boundary along [0001] direction. As candidate Burgers vectors, b1= to describe grain boundary character. A tilt boundary has 1/3[1102], b2=1/3[1101] and b3=1/3[1100] can be consi the plane parallel to the rotation axis n, while a twist boun dered, in which b1 and b2 are perfect dislocations and b3 is a dary has the plane perpendicular to n. A grain boundary that partial dislocation. The sizes of these Burgers vectors are falls in between these two is called a mixed grain boundary, |b1|=0.908, |b2|=0.512 and |b3|=0.274nm, respectively, which comprises both tilt and twist components. Whether a and thus b3 has the smallest vector among them. If bl and b2 grain boundary becomes a tilt or twist grain boundary de dislocations are formed along the grain boundary, a large pends on the location of the grain boundary plane even if the twist component is introduced at the boundary.