The Heat Treatment Of Mono Diamond Fines

In general, a diamond fine that can achieve high removal rate may also cause scratches to the super smooth surface. Tomei solved this problem by carbonization of diamond's surface by heat treatment under vacuum (e.g. PM series fines). The carbonization consumes all sharp points on the diamond particle. Moreover, the superhard core is now enveloped by a more benign coating. In addition, due to the spherization of the diamond fines, the sizing can be more precise. Tomei's PM series fines are uniquely capable of policing read/write heads (e.g. GMR, TMR) fast without causing many scratches. Although dynamite fines are scratch free, but their removal rates are slow. Hence, nonodia-mond fines with carbonization still dominate the head polishing business.

The diamond fines may be formed in slurry. If organic solvent is used as the medium (e.g. for polishing heads), the diamond surface should by hydrophobic. On the other hand, if water suspension is needed (e.g. for texturing hard drives), the diamond surface must be hydrophilic. The hydrophobic surface can be made by terminating surface carbon atoms with unpolarized absorbents, such as hydrogen or fluorine. For example, by heating diamond under hydrogen at 600°C for 2 h. The nanodiamond's surface may

Figure 5.9. The carbonization of micron fines (left) and its effect on size distribution after heat treatment at 1250°C (right). Source: Ryuji Ohshima, Hiroshi Yamanaka, Satoru Hosomi, Carbon, TANSO, 2001, No. 197,88-91.

become hydrophilic by adding polarized radicals, such as ions of hydroxide, surfate, nitrate, and chlorate. For example, by soaking diamond in a solution of H2SO4 and HNO3 at 250°C for 1 h (Figs. 5.10-5.13).

The pulverized micron diamonds are relatively perfect crystals with little defects or impurities. This is in contrast to other forms of nanodiamond that may be synthesized by explosion (see later chapters).

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