ABSTRACT
An overview of experimental data and theoretical models of fracture processes in single-phase and composite nanocrystalline ceramics is presented. The key experimentally detected facts in this area are discussed. Special attention is paid to the theoretical models describing toughness enhancement in nanocrystalline ceramics. Also, we consider theoretical models of generation of cracks at grain boundaries (GBs) and their triple junctions in deformed nanocrystalline ceramics. 9.1 INTRODUCTIONThe outstanding mechanical properties of single-phase and composite nanocrystalline ceramics (often called nanoceramics) represent the subject of intensive research efforts motivated by a wide range of their applications.1-6Commonly, nanoceramics have such technologically attractive characteristics as superior strength, superior hardness, and good fatigue resistance.1-6 At the same time, in most cases, superstrong nanoceramics at ambient temperatures show both low fracture toughness and poor ductility/machinability,3, 7, 8 which are undesired from an applied viewpoint. In particular, low fracture toughness of nanocrystalline ceramics, as with their conventional microcrystalline ceramics, is treated as the key factor limiting their practical utility.1 Mechanical Properties of Nanocrystalline Materials Edited by James C. M. Li Copyright © 2011 Pan Stanford Publishing Pte. Ltd. www.panstanford.com
However, recently, certain progress has been reached in enhancement of fracture toughness of ceramic nanocomposites at comparatively low homologous temperatures (for details, see, e.g., reviews,1-5 book,6 and original papers9-17). Also, several research groups reported on enhanced ductility or even superplasticity of nanoceramics at comparatively low homologous temperatures.18-22 This experimental data serves as a basis for the technologically motivated hopes to develop new, superstrong nanocrystalline ceramics with good fracture toughness and machinability. To do so, of crucial interest are the specific structural features and generic phenomena responsible for optimization of mechanical characteristics (strength, ductility/machinability, and fracture toughness) of nanoceramics. In particular, it is very important to understand the fundamental fracture mechanisms operating in these materials and reveal the sensitivity of fracture processes to their structural and material parameters. The main aim of this chapter is to review experimental studies and theoretical models of fracture of nanocrystalline ceramics. 9.2 SPECIFIC STRUCTURAL FEATURES OF NANOCRYSTALLINE
CERAMICSThe fracture processes in nanoceramics strongly depend on their structural features and phase content. This section briefly describes the specific structural features of single-phase and composite nanoceramics. Also, we discuss the peculiarities of plastic deformation processes affecting fracture of nanoceramics.First, let us consider the structural features of single-phase nanocrystalline ceramics, compositionally homogeneous solids consisting of nanoscale grains (nanocrystallites) divided by GBs (Fig. 9.1). Their grains are characterized by the grain size d < 100 nm and have a crystalline structure. In most cases, nanocrystalline ceramics consist of approximately equiaxed grains with a narrow grain size distribution (Fig. 9.1a). At the same time, there are other examples of grain geometry in nanocrystalline ceramics.20 Besides, in recent years, nanocrystalline ceramics with a bimodal structure have been produced during superplastic forming.20 The bimodal ceramic structure consists of both nanoscopic and microscopic rodlike grains (Fig. 9.1b). (Formation of such rodlike grains during superplastic deformation can effectively occur either by the Li mechanism of grain rotation resulting in coalescence of neighboring grains23 or through stress-driven collective GB migration.24)
Figure 9.1 Scanning electron microscopy photographs of Si3N4 nanoceramics. (a) The microstructure of as-received nanoceramics and (b) nanoceramics after deformation at 1,500°C with a strain of 0.45 under an initial strain rate of 3·10-5 s-1. Reprinted from Xu et al.20; copyright 2006, with permission from Elsevier.The crystal lattices of grains are misoriented relative to each other. Neighboring grains are divided by GBs, planes, or faceted layers that carry a geometric mismatch between adjacent misoriented crystalline grains. Typical thickness of conventional (nonamorphous) GBs is around 1 nm. Amorphous GBs have a thickness ranging from 1 to several nanometers. The atomic structure and properties of GBs are different from the structure and properties of grains. In particular, the arrangement of atoms in GBs is disordered compared with that in grain interiors. GBs join at triple junctions that are tubular regions with diameters around 1-2 nm when they adjoin conventional GBs. Triple junctions of conventional GBs are recognized as line defects, whose structure and properties are commonly different from those of GBs that they adjoin. Triple junctions of amorphous GBs are commonly
amorphous and have a typical diameter ranging from 1 to several nanometers. With the nanoscale range of grain-sizes, both GBs and their triple junctions occupy large volume fractions in nanocrystalline ceramic materials and thereby strongly influence the mechanical properties of these materials.
