With the explosion of research interest in nanocrystalline materials in recent years, one sub-area that has received significant attention is the mechanical behavior of materials with grain sizes less than 100 nm. The great interest in the mechanical behavior of nanocrystalline materials originates from the unique mechanical properties first observed and/or predicted by the pioneers of this field, Gleiter and co-workers, in materials prepared by the gas condensation method (Gleiter, 1989). Among these early observations or predictions were:

• lower elastic moduli than for conventional grain size materials – by as much as 30%–50%;

• very high hardness and strength – hardness values for nanocrystalline pure metals (~10 nm grain size) that are 2–10 or more times higher than those of larger grained (>1 μm) metals;

• increased ductility – perhaps even superplastic behavior – at low homologous temperatures in even normally brittle ceramics or intermetallics with nanoscale grain sizes, believed to be caused by grain boundary, diffusional deformation mechanisms.

While some of these early observations and predictions have been verified by subsequent studies, in particular the high hardness and strength values, some have been found to be caused by high porosity in the early bulk samples (for example, elastic constant behavior) or to other artifacts introduced by the processing procedures.

In this chapter we will describe and discuss the experimental evidence for the mechanical behavior of nanocrystalline materials. This will include pure metals, alloys, intermetallic compounds, ceramics, and multiphase materials. The range of mechanical properties for which measurements have been made will be covered. While the models and theoretical explanations for the various phenomena believed responsible for mechanical properties of nanocrystalline materials will be emphasized in Chapter 5, some discussion of deformation mechanisms must necessarily accompany the description of experimental results.

Elastic properties of nanostructured materials

The early measurements of the elastic constants on nanocrystalline materials prepared by the inert-gas-condensation method gave values, for example for Young's modulus, E, which were significantly lower than values for conventional grain size materials (Suryanarayana, 1995). While various reasons were given for the lower values of E, it was suggested by Krstic and co-workers (1993) that the presence of extrinsic defects, e.g. pores and cracks, was responsible for the low values of E in nanocrystalline materials compacted from powders. This conclusion was based upon the observation that nanocrystalline NiP produced by electroplating with negligible porosity levels had an E value comparable to fully dense conventional grain size Ni (Wong et al., 1993). Krstic et al. (1993) and Boccaccini et al. (1993) developed theories to account for the decrease in E with porosity which agree with E vs.% porosity data on nanocrystalline Fe produced by inert-gas condensation and warm consolidation (Fougere et al., 1995).

Introduction

Corrosion, degradation, and weathering of advanced materials are problems for which mankind has yet to find a proper solution. From the world's ancient man-made and natural monoliths to today's most modern buildings, bridges, and transportation facilities, the longevities of useful structures are closely regulated by the environment where they are located. Having little control over these aggressive environments, we must carefully select materials that have properties best suited for the conditions to which they are exposed. Even after 100 years of industrial revolution and the development of advanced materials, testing and monitoring procedures, we wonder if our knowledge of today's structural materials and their interactions in the current environment is sufficient to produce long-lasting structures that will benefit society.

For example, the total cost of corrosion and corrosion-related issues in the USA alone is quite significant, amounting to 6% GDP or $500 billion per year (Koch et al., 2002). This cost is distributed over direct costs on materials and structures and indirect costs on loss of productivity. About 90% of the corrosion cost is within iron-based materials. The annual direct cost of corrosion of highway bridges is estimated to be $8.3 billion, of which $3.8 billion is for bridge replacement, and $4 billion is for maintenance. It is estimated that indirect costs to the user due to traffic delays and lost productivity are about $38 billion annually. For more details, see the 2001 report by CC Technologies Laboratories, Inc., FWHA (2001).

Nanoscience and nanotechnology has become an identifiable, if very large, diverse, and multidisciplinary field of research and emerging applications. It is one of the most visible and growing research areas in science and technology. Government research funding agencies throughout the world have recognized its potential importance with substantial special initiatives to support its growth and development. The realms of nanotechnology applications are being explored not only of chemistry, materials, and engineering, but the frontiers of medicine as well. This on-going research is what enables the continuing expansions in nanotechnology. Over the past two decades there has been a revolution in the material-science field that has sparked great interest and research in all areas of science and engineering. Nanotechnology and nanoscience are leading this revolution fueled by the industrial progress, the scientific ability to fabricate, model and manipulate objects (things) with a small numbers of atoms, and the almost daily discovery of new phenomena of the nanoscale.

Nanostructured materials include atomic clusters, layered (lamellar) films, filamentary structures, and bulk nanostructured materials. The common thread to these various material forms is the nanoscale dimensionality, i.e. at least one dimension less than 100 nm (more typically less than 50 nm), often less than 10 nm. While this dimension requirement may appear to be arbitrary, it is usually at these length scales that the “physics” often changes and leads to very different properties, often superior, than those of conventional materials.

Introduction

Knowledge of the thermal stability of nanocrystalline materials is important for both technological and scientific reasons. From a technological point of view, the thermal stability is important for consolidation of nanocrystalline particulates without coarsening the microstructure. That is, many methods, as described in Chapter 2, for synthesis of nanocrystalline materials result in particulate products which must be consolidated into bulk form. Since most consolidation processes involve both heat and pressure, the thermal stability of the nanoscale microstructure is always at risk. The goal of particulate consolidation is to attain essentially 100% theoretical density and good particulate bonding while preventing or minimizing grain growth of the nanocrystalline grains.

Understanding the scientific nature of stability, grain growth of nanocrystalline microstructures is a criterion for allowing strategies for minimizing grain growth to be developed. A basic scientific question with regard to nanocrystalline materials is whether their behavior involves “new physics” or is simply the expected grain-size-dependent behavior extrapolated to nanocrystalline grain sizes. Thermal stability is an important phenomenon to be addressed in this regard. The thermal stability in a broader sense involves not only the stability of the grain structure, that is the microstructure, but also the stability of the structure of the grain boundaries in nanocrystalline materials. A number of investigations on the thermal stability of nanocrystalline materials have been conducted. Grain growth in nanocrystalline materials has been reviewed by Suryanarayana (1995), Weissmuller (1996), and Malow and Koch (1996a,b).

Introduction

The very early application of nanomaterials is utilized in systems where nanopowders are used in their free form, without consolidation or blending. For a simple example, nanoscale titanium dioxide and zinc oxide powders are now commonly used by cosmetics manufacturers for facial base creams and sunscreen lotions for UV protection. Nanoscale iron oxide powder is now being used as a base material for rouge and lipstick. Paints with reflective properties are also being manufactured using nanoscale titanium dioxide particles. Figure 7.1 lists some of the common current and future applications of nanomaterials. The list is not complete, however; it is a current summary and the view of the present authors.

Nanostructured wear-resistant coatings for cutting tools and engineering components have been in use for several years. Nanostructured cemented carbide coatings are used on some Navy ships for their increased durability. Recently, more sophisticated uses of nanoscale materials have been realized. Nanostructured materials are in wide use in the area of information technology, integrated into complex products such as the hard disk drives that provide faster communication in today's world.

Many uses of nanoscale particles have already appeared in specialty markets, such as defense applications, and in markets for scientific and technical equipment. Producers of optical materials and electronics substrates such as silicon and gallium arsenide have embraced the use of nanosize particles for chemomechanical polishing of these substrates for chip manufacturing.

Introduction

Structural nanomaterials are finding applications in bulk materials, films, coatings, and composites. Applications vary from wear-resistance coatings to load-bearing structures. Nanophase or nanocrystalline materials are also being used in electronics, refractory, biological, and catalytic applications. Progress in a wide range of structural applications for nanomaterials crucially depends on the development of new fabrication and processing technologies, along with a fundamental understanding of the relationship between the structure and properties of the feedstock powders and consolidated parts. Among the most important issues discussed here are experimental data, and theoretical and computer models concerning mechanical properties in nanostructured materials, which, in general, are different from the conventional coarse-grained counterparts. The competition between conventional and unusual deformation modes is believed to cause the unique mechanical properties of nanomaterials, serving as a basis for their structural applications. Fabrication of nanomaterials with bimodal (nano- and sub-micro-particles) composites, that exhibit both very high strength and reasonable ductility, represents a promising strategy in the synthesis of nanomaterials with enhanced properties for various structural applications. High strain rate and low-temperature superplasticity of some nanocrystalline materials are the subjects of growing fundamental research efforts motivated by a range of new applications of these super strong and super plastic materials in net shaping technologies.

Structural materials and composites containing at least one phase that is less than 100 nm are often termed as structural nanomaterials/composites (Roy et al., 1986).

Introduction

The rapidly growing scientific and technological interest in structural nanocrystalline bulk materials and coatings arises from their outstanding mechanical properties opening a range of new applications; see, for example, reviews (Koch et al., 1999; Gleiter, 2000; Gutkin et al., 2001; Mohamed and Li, 2001; Padmanabhan, 2001; Veprek and Argon, 2002; Kumar et al., 2003a; Milligan, 2003; Ovid'ko, 2004; Valiev, 2004; Chokshi and Kottada, 2006; Han et al., 2005; Ovid'ko, 2005a, b; Wolf et al., 2005) and books (Roco et al., 2000; Chow et al., 2000; Farkas et al., 2001; Berndt et al., 2003; Komarneni et al., 2003; Gutkin and Ovid'ko, 2004a). These outstanding mechanical properties are caused by the interface and nanoscale effects associated with structural peculiarities of nanocrystalline materials where the volume fraction of the interfacial phase is extremely high, and grain size d does not exceed 100 nm. For instance, nanocrystalline bulk materials and coatings often exhibit extremely high strength, superhardness and good fatigue resistance desired for numerous applications; see Chapter 4 and the literature (Siegel and Fougere, 1995; Hahn and Padmanabhan, 1995; Koch et al., 1999; Gleiter, 2000; Gutkin et al., 2001; Mohamed and Li, 2001; Niederhofer et al., 2001; Padmanabhan, 2001; Veprek and Argon, 2002; Kumar et al., 2003a; Milligan, 2003; Patscheider, 2003; Valiev, 2004; Chokshi and Kottada, 2006; Han et al., 2005; Ovid'ko, 2005a, b; Wolf et al., 2005). At the same time, in most cases, nanocrystalline materials show low tensile ductility at room temperature, which essentially limits their practical utility.