Titanium/hard carbon

Generally, fillers in sealant formulation are used as additives to boost the viscosity of the sealant and get better gap-filling properties and to reduce the material cost of the sealants. In sealant formulation, fillers cannot affect reinforcement and improved strength. However, they can affect other properties such as water resistance and hardness, etc. The most common filler used in sealant is calcium carbonate, because of its many advantages such as abundant resource, low cost, and stabUity. Other fillers are clays, silica, titanium dioxide, carbon black, and iron oxide. O Table 13.9 lists commonly used fillers in sealant formulations and functions in sealant systems. 

Other materials can be ion implanted and are under investigation for commercial applications. These include a combination of titanium and carbon implantation, which produces an amorphous surface layer at low temperatures and carbide precipitation at high temperamres. Ion implantation of active species has been shown to increase the erosion and wear resistance of surfaces (Ti/C on steel, N on steel), the hardness of surfaces (Ni on Al), the oxidation resistance of surfaces (Pt on Ti), and the tribological properties of surfaces. Ion implantation of inert species has been shown to increase the hardness of TIN films. Ion implantation can cause a metal surface to become amorphous. 

If the normal carbonate is used, the basic carbonate or white lead, Pb(OH),. 2PbCO,. is precipitated. The basic carbonate was used extensively as a base in paints but is now less common, having been largely replaced by either titanium dioxide or zinc oxide. Paints made with white lead are not only poisonous but blacken in urban atmospheres due to the formation of lead sulphide and it is hardly surprising that their use is declining. 

Other substances— for example, the silicate ores, the carbonate ores, the titanium minerals— may be similarly grouped for pmposes of experi-piental study, but it is hardly necessary here to make a complete inventory of such groups. 

Fluorosilicones can be compounded by the addition of mineral fillers and pigments. Fillers for such compounds are most commonly silicas (silicon dioxide), because they are compatible with the elastomeric silicon-oxygen backbone and thermally very stable. They range in surface areas from 0.54 to 400 m2/g and average particle size from 100 to 6 nm. Because of these properties, they offer a great deal of flexibility in reinforcement. Thus, cured compounds can have Durometer A hardness from 40 to 80. Other fillers commonly used in fluorosilicones are calcium carbonate, titanium dioxide, and zinc oxide. 

Titanium carbide, TiC, is made by the action of carbon black on titanium dioxide at 2000 °C. It is the most important hard metallic material after tungsten carbide, and in fact is the hardest of all the metal carbides with a hardness rating of 9 on the Mohs scale - diamond is 10. In itself it is too brittle to be used pure but when mixed with the carbides of tungsten, tantalum and niobium it delivers great strength. 

The nitrides and carbides of titanium and zirconium and the carbide of hafnium are extremely hard substances, resembling metals both in appearance and in electrical conductivity. Their formulae approach AxBh but some departure from stoichiometry is possible. Each of these refractory substances has the sodium chloride structure, described alternately (p. 190) as cubic close-packed arrays of metal atoms with the small nonmetal atoms in the octahedral holes. Note, however, that the parent metals themselves do not have cubic close-packed structures. Thus, the older view of such nitrides and carbides as lattices of the parent metals that are expanded to accommodate nitrogen or carbon atoms in the holes (interstices) is not admissible. The nature of the bonding in such refractory nitrides and carbides appears to be linked to the nature of bonding in metals in general, an important and interesting topic, but best pursued in more advanced works. 

Fillers. Fillers are added to reduce cost, increase hardness, and color the compound. Generally they do not provide the dramatic improvement in properties seen with reinforcing agents, but they may have some reinforcing capability. Typical fillers are clays, calcium carbonate, and titanium dioxide. 

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