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Abstract
In recent years, near-nano (submicron) and nanostructured materials have attracted increasingly more attention
from the materials community. Nanocrystalline materials are characterized by a microstructural length or grain size of
up to about 100 nm. Materials having grain size of
0.1 to 0.3 mm are classified as submicron materials.
Nanocrystalline materials exhibit various shapes or forms, and possess unique chemical, physical or mechanical
properties. When the grain size is below a critical value (
10–20 nm), more than 50 vol.% of atoms is associated with
grain boundaries or interfacial boundaries. In this respect, dislocation pile-ups cannot form, and the Hall–Petch
relationship for conventional coarse-grained materials is no longer valid. Therefore, grain boundaries play a major role
in the deformation of nanocrystalline materials. Nanocrystalline materials exhibit creep and super plasticity at lower
temperatures than conventional micro-grained counterparts. Similarly, plastic deformation of nanocrystalline coatings
is considered to be associated with grain boundary sliding assisted by grain boundary diffusion or rotation. In this
review paper, current developments in fabrication, microstructure, physical and mechanical properties of nanocrystalline
materials and coatings will be addressed. Particular attention is paid to the properties of transition metal nitride
nanocrystalline films formed by ion beam assisted deposition process.
Introduction
Nanomaterials are experiencing a rapid development in recent years due to their existing and/or
potential applications in a wide variety of technological areas such as electronics, catalysis, ceramics,
magnetic data storage, structural components etc. To meet the technological demands in these areas,
the size of the materials should be reduced to the nanometer scale. For example, the miniaturization of
functional electronic devices demands the placement or assembly of nanometer scale components into
well-defined structures. As the size reduces into the nanometer range, the materials exhibit peculiar
and interesting mechanical and physical properties, e.g. increased mechanical strength, enhanced
diffusivity, higher specific heat and electrical resistivity compared to conventional coarse grained
counterparts [1]. Nanomaterials can be classified into nanocrystalline materials and nanoparticles. The
former are polycrystalline bulk materials with grain sizes in the nanometer range (less than 100 nm),
while the latter refers to ultrafine dispersive particles with diameters below 100 nm. Nanoparticles are
generally considered as the building blocks of bulk nanocrystalline materials. Research in nanomaterials
is a multidisciplinary effort that involves interaction between researchers in the field of physics,
chemistry, mechanics andmaterials science, or even biology andmedicine. It has been stimulated by theinterest for basic scientific investigations and their technological applications. Nanomaterials and most
of the applications derived from them are still in an early stage of technical development. There are
several issues that remain to be addressed before nanomaterials will become potentially useful for
industrial sectors. These issues include synthesis of high purity materials with large yield economically
and environmentally, characterization of new structures and properties of nanophase materials,
fabrication of dense products fromnanoparticles with full density and less contamination, and retention
of the ultrafine grain size in service in order to preserve the mechanical properties associated with the
nanometer scale.
Novel fabrication technology of nanoparticles is versatile and includes a wide range of vapor,
liquid and solid state processing routes. Available techniques for the synthesis of nanoparticles via
vapor routes range from physical vapor deposition and chemical vapor deposition to aerosol spraying.
The liquid route involves sol–gel and wet chemical methods. The solid state route preparation takes
place via mechanical milling and mechanochemical synthesis. Each method has its own advantages
and shortcomings. Among these, mechanical milling and spray conversion processing are commonly
used to produce large quantities of nanopowders.
Nanoparticles synthesized from several routes may have different internal structures that would
affect the properties of materials consolidated from them. Processing nanoparticles into fully dense,
bulk products or coatings which retain the nanometer scale grain size is rather difficult to achieve in
practice. Due to their high specific surface areas, nanoparticles exhibit a high reactivity and strong
tendency towards agglomeration. Moreover, rapid grain growth is likely to occur during processing
at high temperatures. As unique properties of nanocrystaline materials derived from their fine
grain size, it is of crucial importance to retain the microstructure at a nanometer scale during
consolidation to form bulk materials. It is also noticed that pores are generated in bulk nanocrystalline
materials consolidated from nanoparticles prepared by inert-gas condensation. Such nanopores
can lead to a decrease in Young’s modulus of consolidated nanocrystalline materials [2].
Electrodeposited samples are believed to be free from porosity, but they contain certain impurities
and texture that may degrade their mechanical performances. Therefore, controlling these properties
during synthesis and subsequent consolidation procedures are the largest challenges facing
researchers.
The unique properties of nanocrystalline materials are derived from their large number of grain
boundaries compared to coarse-grained polycrystalline counterpartes. In nanocrystalline solids, a
large fraction of atoms (up to 49%) are boundary atoms. Thus the interface structure plays an
important role in determining the physical and mechanical properties of nanocrystalline materials.
Huang et al. [3] reported that nanocrystalline copper has a much higher resistivity and a larger
temperature dependence of the resistivity than bulk copper. They attributed this effect to the grainboundary
enhanced scattering of electrons. Nanocrystalline metals have been found to exhibit creep
and superplasticity with high strain rates at lower temperatures than their micro-grained counterparts.
High strain-rate superplasticity at lower temperatures is of practical interest because it can offer an
efficiently near-net-shape forming technique to industrial sectors. Despite recent advances in the
development of nanocrystalline materials, much work remains to be done to achieve a basic
understanding of their deformation and fracture behavior. Nanocrystalline metals generally exhibit
significantly higher yield strength and reduced tensile elongation relative to their microcrystalline
counterparts. The hardness and yield strength tend to increase with decreasing grain size down to a
critical value (ca. 20 nm). When the grain size is below 20 nm, strength appears to decrease with
further grain refinement. At this stage, dislocation sources inside the grains can hardly exists. This
implies that dislocation pile-ups cannot form and the Hall–Petch relationship for conventional coarsergrained
materials is no longer valid. Instead, inverse Hall–Petch effect, i.e. softening is obtained whenthe grain size is reduced. The softening behavior of nanocrystalline materials is a subject of
considerable debate. Several mechanisms have been proposed to explain the anomalous deformation
behavior of nanocrystalline materials with the grain size below the critical value. These include grainboundary
sliding, grain-boundary diffusion, the triple junction effect, presence of nanopores and
impurities, etc. Therefore, comprehensive understanding of the processing-structure–property relationships
is essential in the development of novel nanomaterials with unique properties for structural
engineering applications. As dense nanocrystalline materials with grain size smaller than 20 nm are
difficult to acquire, molecular dynamics (MD) modeling has been used to simulate the interfacial
structure and mechanical deformation mechanism of such materials. Computer simulations play a
critical role in advancing our understanding of atomic level and deformation structures that are not
accessible by experimental routes.
Nanocrystalline coatings with grain sizes in the nanometer range are also known to exhibit
superior hardness and strength. The search for nanostructured coatings is driven by the improvement in
coating technologies and the availability of various kinds of synthesized nanopowders. Such
nanopowders can be used as feedstock materials for thermal spray processes; these include plasma
spraying and high-velocity oxygen fuel (HVOF) spraying. Thermal spraying involves particle melting,
rapid cooling and consolidation in a single-step operation. Thermal-sprayed nanocrystalline coatings
with moderate hardness are found to possess better wear performances than their counterparts
fabricated from microcrystalline powders. HVOF is particularly suited to deposit dense nanocrystalline
ceramic coatings as opposed to plasma spraying because of its lower spraying temperature. Today,
HVOF allows tailoring nanocrystalline coatings with low porosity, higher bond strength and increased
wear properties.
In the past decade, favorable applications have been found for hard and wear-resistant ceramic
coatings in industrial sectors. Transition metal nitride coatings are of particular interest due to their
high hardness, thermal stability, attractive appearance and chemical inertness. Conventional nitride
coatings have been prepared by physical and chemical vapor deposition. Nanocrystalline coatings of
transition metal nitrides can be deposited on substrate materials by means of ion beam assisted
deposition. The process is based on simultaneous ion bombardment of the growing physical vapor
deposited film using an independent ion source. This technique permits the deposition of nanocrystalline
nitride films at lower temperatures with better coating-substrate adhesion.
The improvement of tool materials coated with transition metal nitrides has led to interest in
developing superhard coatings for wear protection under complex loads and aggressive environments.
Optimal microstructural design and materials selection permit chemical, physical and mechanical
characteristics can be tailored for specific applications. Superhard coatings having hardness values
above 40 GPa are obtainable in multilayer structures with the period of the superlattice (bilayer
thickness) within the nanometer regime. The enhancement of superhardness is attributed to a
difference in shear modulus between two layer materials and to the presence of sharp interfaces
between the layers. In another approach, nanocomposite coatings with superhardness 40 GPa can be
prepared by dispersing the transition metal nitride nanoparticles in an amorphous covalent nitride
matrix (1 nm). In terms of MD computer simulations, grain-boundary accommodation mechanisms
such as grain-boundary sliding and diffusion are considered to be the main factors causing the
softening of nanocrystalline materials with grain sizes below 10 nm. Blocking of grain-boundary
sliding of nanocrystalline grains embedded in a thin amorphous matrix is believed to be responsible for
superhardness of the nanocomposite coatings.
Because of the multidsiplinary nature of nanomaterials, it would be difficult to cover all areas of
interest. In this review article, we address and focus the discussions on the following subjects, namely
nanoparticles, nanocrystalline materials and coatings.