Electro-Deposition of Nano Composites

8.1.2.3. Electro-Deposition of Nano Composites

As explained in the section 3.6, the nano-phase materials are also of considerable interest be- cause of their enhanced mechanical properties and abrasion-resistance relative to those of the bulk materials. One reason for the different properties of these materials is that an increasing fraction of the atoms occupy sites at interfaces, and hence the atomic packing at the interfacial positions is much better to resist any external mechanical force. It has been estimated that only about 3% of the atoms in a material are at the boundaries when the grain size is 100 nm, but this increases to 25-50% when the grain size approaches 5 nm. This is also evident in the small nano-sized magnetite grains, where some spins are disorganized, as the particle size is smaller (see the section 5.5.2 for spin canting in the nano particles of magnetite).

In metals, there is typically an increase in the yield strength with decreased grain size as de- scribed by the well-known Hall-Petch relation, which describes the yield strength as a linear function of the inverse square root of the grain size, as explained in the section 4.4.1, for the fracture toughness of sintered nano-particles of SiC. This behaviour is due to the influence of the grain boundaries on the ‘dislocation motion’. As the grain size approaches the nano scale, there is a large increases in strength to be obtained. In the nano regime, the crystallite size becomes smaller than the characteristic length scales associated with the generation of the dislocations and glide, which are the typical processes that determine mechanical behaviour in metals.

The deviations from the Hall-Petch relationship have been observed in nano-crystalline materi- als, even softening at the smallest grain sizes. This softening has been observed in nano-crystalline electro-deposits of Ni with grain sizes below approximately 12 nm. The thick-films of metals with nano-crystalline or amorphous structures for mechanical applications can be produced by co-depositng

a ‘metalloid element’, such as phosphorous boron with nickel and other iron group elements. Another approach is to take advantage of the high super-saturation that is achievable during the very high peak current densities possible in pulse plating. This is the electro-chemical version of ‘splat cooling’ of molten metal to make metallic glass for various magnetic applications.

The electro-chemical deposition is a very attractive processing route for the synthesis of com- posite materials. The low processing temperatures minimize the problem of chemical interaction and thermally induced stresses that are often serious problems in the conventional sintering, vapour-phase , or liquid metal processes, which are used to fabricate various composite materials. A simple approach is to suspend particulate material in the plating electrolyte and co-deposit this with the metallic matrix. This can be accomplished both by electrode-less deposition and by electro-plating. The commercial applications of this approach include co-deposition of alumina, silicon carbide , or diamond with a metal such as nickel. The challenge in this work is to prevent the ‘agglomeration’ of the particles prior to co-deposition.

OTHER METHODS AND OTHER NANO MATERIALS

279 The electro-chemical scheme for growing composites is electro-chemical infiltration. This ap-

proach of electro-chemical infiltration has been used to fill the 5 nm pores of a silica ‘xerogel’ film with nickel. In this manner at room temperature, 3-dimensional nanoscale networks of metal and ce- ramic were synthesized. There will be certainly an increased emphasis on the electro-deposition of nano-phase materials for magnetic applications, as well as for optical and electrical applications. An- other big area of application should be the co-deposition of very dissimilar materials. The low process- ing temperatures of electro-deposition allow the co-deposition of materials that would tolerate each other at high temperatures used for the traditional thermal processing.