Features of Fracture Surface and Grain Boundary Structure of Boride/Nitride Materials
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ABSTRACT Nanostructured bulks and films of TiB 2 , TiN, AIN, Ti(B,N), and (Ti,AI)N prepared by high pressure sintering and nonreactive magnetron sputtering accordingly. Fracture surfaces and structure of grain boundaries have been investigated by high-resolution scanning and transmission electron microscopy as well as by atomic force microscopy. The revealing of transgranular fracture, homogeneous and inhomogeneous deformation, and the availability of dislocations and amorphous phases in structure are discussed. INTRODUCTION Unusual properties of nanostructured (nanocomposite, nanophase) materials (NM), which are normally characterized by a grain size in the range 3-100 rnm, catalyzed the numerous investigations in this field. However, the mechanism of the fracture and deformation as well as grain boundary structure are still not clear understood and need further consideration (see, for example, reviews [1, 2]). In addition, only metals, alloys, intermetallics, and oxides are the subjects of the majority of these investigations. High-melting point compounds such as borides, carbides, and nitrides have been studied in limited scale. In elaboration of our previous works [3-5], the present report is devoted to high-resolution electron microscopy (HREM) and atomic force microscopy (AFM) study of boride/nitride NM.
EXPERIMENT Bulks and films of Ti (B,N) and other compounds have been prepared both by high pressure sintering of ultrafine powders and by nonreactive magnetron sputtering respectively. The details and features of this experimental technique have been published elsewhere [6, 7]. Fracture surfaces of bulks and films have been studied by high-resolution scanning electron microscopy (SEM), using a Hitachi S-4000 field emission gun, and AFM [8, 9]. The structure of films was examined in a JEM-3010 transmission electron micriscope operating at 300 kV [10]. Deformation was realized by cleavage fracture and under a Vickers indentor. RESULTS AND DISCUSSION Fracture surface of consolidated bulks In addition to previous our observations [3], Figs. 1 (a) and (b) show SEM micrographs of fracture surfaces of TiN bulks. As is easy to see, there are intergranular fracture and moreover the feature of crack propagation is also clearly visible. This observation of intergranular fracture seems to be important for understanding and explanation of fracture toughness data for onephase and two-phase NM (see, for example, [11]). 583 Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
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(b) Figure 1: SEM images of fracture surface (a) of nanostructured TiN bulk including that with crack propagated through crystallite boundaries (b).
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Fracture surface of films Some preliminary results have been published elsewhere [5, 8]. These results were obtained by the cross-section SEM observation of the indentation impressions. Fig. 2 shows two types of deformation. The formation of shear bands and localization of deformation is observed in the case of boride film with the hexagonal structure (Fig. 2, a). On the othe
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