Composites Surface Finishing and Tribological Coatings

Composite materials based on elongated nanostructures can bring additional functionality and structural reinforcement, allowing the number of applications of such materials to be extended. A few approaches to the manufacture of titanate nanotube-polymer composites have been reported recently using polystyrene154 or polyurethane155 matrices. In both cases, the effective dispersion of TiNT was achieved by the hydrophobization of the surface using siloxanes or surface adsorption of hexamethylene diisocyanate, prior to mixing with polystyrene and polyurethane, respectively. TiNT-polystyrene composite films were characterised by an increased Young's modulus and a tensile strength observed even at low nanotube loading levels.154 Comprehensive tribological studies of TiNT-polyurethane composites have revealed significant improvements in wear resistance and a lower friction coefficient compared to those of the unloaded polymer.155

TiNT-polyaniline composites, obtained by the oxidation of aniline adsorbed onto the surface of nanotubes in the presence of triblock copolymers,156 are interesting due to their unusual combination of electroconductive and proton conductive properties of the polymer and nanotubes, respectively. Similar composites of TiNT with proton conductive Nafion™ ion-exchange polymer have also demonstrated advantages when compared to the pure polymer.48 Long titanate nanofibres in aqueous suspensions with157 or without158 addition of Pluronic F127 fluorocarbon surfactant may form a pulp, which can be cast into a free-standing (paper-like) membrane. These membranes have an open-pore structure (pore size of ca. 0.05 mm) and can be used as a filter or catalyst during the oxidation of organic wastes.

The incorporation of nanotubular objects into metal matrices can significantly improve the mechanical properties of composites (see the example of Damascus steel in Chapter 1). One established industrial method for the incorporation of ceramic micro-particles into a metal matrix is co-deposition of particles during electroplating. Our recent results have shown that the embedding of elongated titanate nanotubes into metal during the electroplating of nickel from an electrolyte containing suspended titanate nanotubes, can result in improvement in the wear resistance of the composite coating against steel of approximately 20% (see Figure 5.10).159

An important processing requirement for many surface engineering applications is to immobilise titanate nanotubes on the surface of the substrate via film formation or the application of a coating. Many approaches have been

Figure 5.10 Wear resistance of nickel/TiO2 composites prepared by co-electro-deposition of TiO2 nanostructures during nickel electroplating. Ni represents electroplated nickel; TiO2/Ni is TiO2 (P25) co-deposited from a 20gdm"3 suspension; and TiNT/Ni (A) and TiNT/Ni (B) are titanate

nanotubes co-deposited from 10 and 20 gdm (Data adapted from ref. 159).

suspensions, respectively.

used to produce TiNT films, including: the use of the doctor blade technique from a slurry mixture;6,25 the electrophoretic deposition at the anode from an aqueous electrolyte with the addition of polycations as counter-ions;127,160 the anodisation of titanium in the presence of fluoride ions;131 the in situ growth on the surface of titanium under alkaline hydrothermal synthesis;7,8 spin coating;161 the hot pressing of TiNT powder;162 Langmuir-Blodgett film deposition;163 and layer-by-layer assembly.127,128 All of these methods can produce composite films, characterised by controlled thickness, density and titanate composition, together with a degree of self-assembly.

It is likely that further developments in the synthesis and characterisation of novel composite films will be an active area for future research, particularly in problems related to the self-assembly of elongated nanostructures. This task is challenging, but a favourable outcome would allow significant improvements in the performance of devices such as solar cells.

5.6 Other Applications

A number of recent publications suggest the possible applications of elongated titanates and TiO2 in specific areas, which are not intrinsic to TiO2. This includes the use of high surface area elongated titanates as a low-cost adsorbent for chromatography;164,165 for dye removal from the wastes producing during fabric staining in the textile industry;3 or for the removal of radioactive ions from waste water.166

The high surface area and the acidic nature of the titanate nanotube surface also render these materials useful as a coating for the quartz crystal microbalance-type devices used for sensing various amines in the gas phase, which can be applied as a detector of chemical warfare agents.167

The high aspect ratio of elongated titanates can also be utilized in liquid suspensions, namely nanofluids, which are characterised by unusual electro-rheological168 and thermoconductive169 properties. Such fluids can be used in the active control of conventional and intelligent devices where viscosity or thermal conductivity is modulated by an applied electric field.

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