New techniques to create nanophase materials have resulted in the development of new class of materials, which have an appreciable fraction of their atoms residing in defect environment. For instance, a nanophase material with an average grain size of 5nm has about 50% of the atoms within the first two nearest neighbour planes of a grain boundary in which significant displacements from normal lattice positions are exhibited. The basic idea being to produce new disordered solid which contains a high density of defect cores whose 50% or more of the atoms reside in the core of the defects.
The atomic arrangement formed in the core of an interface is known to be an arrangement of minimum energy in the potential field of two adjacent crystal lattices with different crystallographic orientation on either side of boundary core. Owing to the different orientation, the two adjacent crystal lattice match poorly. The poor matching results in an atomic density lower than that in the perfect crystal. The boundary core density is found to be reduced by 15 – 40% relative to density of perfect lattice. Such boundary conditions are not found in glasses or perfect crystals.
Reasons for Interest in Nanocrystalline Solids
• The atomic structure exhibited is in a different form from that of crystalline solids (long range order) and glassy (short range order) materials with the same chemical composition.
• They permit the alloying of conventionally immiscible components like the alloys of Fe-Ag, even if Fe and Ag are immiscible in the solid and liquid states
• It is possible to generate solids with novel properties
• They show considerable enhancement of solid solubility over the conventional alloy systems
• It is possible to produce new high strength steels by use of a dispersion phase such as carbide in a nanoscale dimension
• Several conventional difficulties encountered in fabrication of ceramic matrix related to particle agglomeration and infiltration can be overcome by use of nanopowders
• A high reduction in density of 30-40% can be achieved which is remarkable.
• An enhanced surface area because of the small size of the nanocrystal is obtained.
• Coatings with nanoparticles would generate novel surface with anti-wear, anti-corrosive, anti-static and anti-bacterial.
• It would be possible to develop a fast and on to target drug delivery system.
The nanocrystalline solids comprise of small crystals separated by grain boundaries. The main feature is a very high density of grain boundary core in nanocrystalline solids (60-70%) in metals and 80% in ceramics, compared to conventional materials. The atomic structure of grain boundary core averages about 1019 boundary cores per cc as suggested by studies on Fe (6 nm) and Cu (10 nm). Extended X-ray Absorption Fine Structure Measurement (EXAFS) have suggested that the amplitude of FT of the first shell nanocrystalline copper are reduced by an amount comparable to the volume fraction of grain boundary components present in copper. It is based on the fact that if a nanocrystalline material consists of an ordered polycrystalline component and grain boundary component formed by displacement of atoms from their ideal lattice sites, the oscillation component results only from the crystalline component because materials with a wide spacing would exhibit no oscillation. Hence, nanocrystalline materials are expected to show reduced oscillation as compared to conventional material proportional to the fraction of atoms located in the boundary cores because of the displacement of atoms from the lattice sites and the disturbance of interatomic spacing within the boundary cores. Although there is no conclusive evidence, it appears that large fractions of the atoms of a nanocrystalline material are displaced from the lattice site. The high density of defects in nanomaterials enhances their influence on the macroscopic properties. Despite a remarkable progress and availability of state of art tools such as nanoscope, AFM,STEM and EMS of sub nanometer resolution we are just at the beginning to understanding the essential features of structures of nanophase materials.
Because of the difference in the atomic structure of nanocrystalline solids and crystals, the structural dependent properties of nanocrystalline solids show a considerable deviation. Some of the factors affecting the properties are:
• Porosity: Porosity in nanophase materials and ceramics is smaller compared to the grain size of the material The porosity can be reduced by an appropriate consolidation process
• A decrease in the grain size lowers the ductile-brittle transition temperature (DBTT) and therefore nanocrystalline materials should exhibit a lower DBTT than their coarse grain counterparts. A decrease in grain size from 10 µm to 10 nm increases the creep rate by nine orders of magnitude. The grain boundary diffusivity is also higher which allows ceramics and intermetallics to be plastically deformed. The atomic diffusion process in nanocrystalline solids is orders of magnitude higher than that observed in conventional crystals and glasses which have a significant bearing on mechanical properties such as super-plasticity, creep and other properties of nanomaterials. Enhancement of elasticity and plasticity are of prime interest for technological applications. Tools such as AFM and STEM have made it easier to predict the mechanical properties by study of individual atoms. For example, nanoparticles can move past each other making stretching easier. By packing of crystalline particles along the grain boundary materials with super strength can be produced. Nanaocrystals of titanium nitride embedded in thin films of silicon nitride showed a hardness of 60 Gpa. Nanocrystalline materials as hard as diamond can be produced. Mechanical properties can drastically change by introducing a nanophase in a metal matrix. For instance, attempts have been made to develop a carbon strengthened steel in which the dispersed carbide phase is of nanoscale elements. This steel is reported to have a super strength, wear resistance and fracture toughness. Ceramic materials with significant toughness at high temperatures could be developed by hybridizing a microcomposite and a nanocomposite. It can be achieved by reinforcing a nanocomposite with plate like grains, whiskers or long fibers. The fracture strength of Al2O3, nanocomposite jumped from 350 MPa to 1500 MPa by dispersing 5 vol% of nano SiC(p) in crystalline grains of the matrix.
Improvements brought about in mechanical properties of nanocomposites are shown in Table 1.
Table 1. Mechanical strength of selected nanocomposites at elevated temperatures
The field of nanocomposites is gaining a lot of momentum as researchers strive to enhance composite properties by using nanoscale reinforcements instead of particulate filled composites. This nanograin size plays a dominant role in influencing its mechanical properties. A reduction in grain size of 80 – 200 nm results in a hardness increase from 1600 – 1950 (Vickers) in WC 10% C cemented carbides. The strength of copper single crystals is 82 MPa which rises to 290 MPa at a grain size of 11 nm. Copper with a grain size of 27 nm had a tensile ductility of 30% even though the tensile strength was as high as 202 Mpa. On rolling, copper with a grain size of 20 nm, an extensibility of 500 % was obtained.
Despite considerable progress being made, the knowledge of mechanical properties of nanocomposite solids is in a state of infancy, one of the reasons being the inability to obtain large and defect free samples. Central to all is the understanding of the microscopic deformation and fracture mechanism in nanocomposite solids. There is a lot more needed to understand the dislocation activity in nanocrystalline solids. It is likely that for the longer end of the nanoscale grain size (50 – 100 nm) dislocation activity dominates for test temperatures less than 0.5 Tm. As the grain size decreases, dislocation activity also decreases. There appears to be a lack of dislocation at grain sizes below 50 nm. This appears to be a new phenomenon, which controls the deformation behavior such as grain boundary sliding. This is a great need to explore the deformation mechanism in nanocrystalline solids and very little of it is understood.
To conclude, our knowledge of the relationship between microstructure and mechanical properties is still in the elementary stage as it needs to be further developed to obtain a precise understanding. There is no published information available to demonstrate the response of nanocrystalline solids to environmental impact. With the enhanced surface area, high density, high density of defects at the interphase, the corrosion behavior of nanomaterials is worth exploring. In recent years, coatings of nanoxide particles have been applied and excellent corrosion resistant properties have been registered. Experimental evidence on the corrosion behavior of nanomaterials would be necessary for evaluating the application of these alloys.