Hp-Ht synthesis

Plasma-jet diamond techniques yield growth rates of about 980 p.m/h (163,164). However, the rate of diamond deposition is still one to two orders of magnitude lower than the HP—HT technology, which is about 10,000 p.m/h (165). Diamond deposition rates of ca 1 p.m/s have been reported usiag laser-assisted techniques (166). This rate is comparable to the HP—HT synthesis. 

The activated CVD diamond techniques can be mote attractive in cases where the huge capital investment (several hundred million dollars) requited for the HP—HT technology is not available or where the high level of technical knowledge requited for HP—HT synthesis is not available. In addition, most wear-resistant apphcations requite diamond coatings only of the order of a few micrometers thick. Such coatings can be deposited ditecdy on the finished product without the need for further finishing if CVD techniques are employed. 

High-Pressure High-Temperature (HP-HT) synthesis, requiring specialized equipment, and capable of producing high-quality single crystals of, e.g., GaN. 

Experimental HP-HT synthesis of solid solutions in the B6O-B4C system and the first conclusive synthesis of a new boron subnitride BeNi- are reported by Hubert et al. [548, 549]. AU the experiments have been performed... 

Low-pressure deposition of diamond is a commonly used industrial process [184]. Because HP-HT synthesis for c-BN and diamond works under similar conditions, it was assumed that the low-pressure synthesis of c-BN should be possible analogous to that of diamond. However, there are a lot of differences between c-BN and diamond that make a low-pressure deposition of c-BN rather difficult ... 

For the c-BN formation a stress threshold was observed in the deposited layers. The h-BN intermediate layer shows a preferred orientation, where the c-axis of the h-BN is parallel to the substrate. Both effects are explained by the compressive biaxial stress induced by the ion bombardment. The mechanism for the conversion of h-BN into c-BN is explained by rather high temperatures originated during thermal spikes (direct h-BN —> c-BN transformation). The stress caused by the bombardment with high energetic ions is considered to be a reason for the growth of the c-BN crystals [190, 191]. A stress within the layer of up to 10 GPa has been observed. This biaxial stress causes a hydrostatic pressure up to the values usual in HP-HT synthesis. 

Limited supply, increasing demand, and high cost have led to an intense search for an alternative, dependable source of diamond. This search led to the high pressure (ca 5 GPa (0.5 x 106 psi)), high temperature (ca 1500°C) (HP—HT) synthesis of diamond from graphite in the mid-1950s (153—155) in the presence of a catalyst—solvent material, eg, Ni or Fe, and the subsequent development of polycrystalline sintered diamond tools in the late 1960s (156). 

Figure 15 shows the variation of diamond deposition rates by various activated CVD techniques as well as the HP—HT technique (165). It can be seen that the highest growth rate of activated CVD diamond synthesis is stiU an order of magnitude lower than the HP—HT technique. However, CVD has the potential to become an alternative for diamond growth ia view of the significantly lower cost of activated CVD equipmeat and lower miming and maintenance costs. 

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