Monocrystalline diamond

Microfabrication of the parallel channels was performed by mechanical surface cutting of metal tapes [31]. In the case of aluminum alloys, ground-in monocrystalline diamonds were used [45]. In the case of iron alloys, ceramic micro tools have to be used owing to the incompatibility of diamonds with that material. Such a microstructured platelet stack is provided with top and cover plates, diffusion bonded and connected to suitable fittings for the inlet and withdrawal ducts by electron beam welding (Figure 3.9). 

Two major classifications for diamond crystals are mono- or single crystalline and polycrystalline. The monocrystalline diamond particles tend to have more uniform surfaces and sharp edges. The abrasiveness of the monocrystalline diamond is mainly governed by its particle size. In the case of polycrystalline diamond, it is sometimes determined by the packing arrangement and the interaction of these single crystals that are related to friability [84]. Friability is defined as the readiness of a substance to crumble and form fine particles or fibers under the application of external pressures. 

The reconstruchon of the surface plays an important role especially for monocrystalline diamond films, but polycrystalline materials are affected too. The it-bonds aheady existing on such diamond faces largely fadhtate a graphitization, which manifests in a strong reduction of the friction coefficient at very high temperatures (when graphitization assumes a significant extent). 

The characteristics of monocrystalline diamond films are much more clearly defined. Still polycrystalline films are employed in most cases as the high price interferes with large scale application of the monocrystalline material. Even for thin layers there is no significant change to the essential characteristics of diamond. For this reason as well as to save further material, it is a common practice to employ coated substrates with a film the thickness of micrometers spread on their surface (Section 6.6.1). The endurance of such films against mechanical stress is essentially influenced by two factors Firstly, by delamination (peeUng off) of the film from the substrate, and secondly by normal, gradual wear. 

For the fabrication of ultraprecise parts, monocrystalline diamond tools are used ( diamond mming ). For example, modem optics fabrication processes like plastic injection molding or precision glass molding require ultraprecise steel... 

In addition to the material s extreme hardness, monocrystalline diamond exhibits extremely low coefficients of friction on most metals and non-metals (of the order of 0.05), resulting in minimal chip adhesion to the cutting edge. A thermal conductivity of up to 2,000 W/m.K and a thermal expansion coefficient of less than 2 ppm/K ensure minimum thermal deflections and deviations. 

Single crystal cBN is not of industrial importance for cutting applications as it offers few benefits over monocrystalline diamond and is intrinsically more difficult to synthesize. As such, all industrial cBN tools are composite materials prepared using powder metallurgical techniques, but sintered under similar conditions to those used for the synthesis of diamond and cBN from their softer allotropes (graphite in the case of diamond). 

As the contracting company (Warren Diamond Co.) is a member company, we can identify it as the primary beneficiary. The project s main goal is to optimize the lapping process, by finding a correlation between the process parameters (of which very important is the type of the slurry) and the final parameters of the ceramic parts (dimensional accuracy and surface roughness). Therefore, a part of the optimization process is concerned with a comparative study of the behavior of polycrystalline versus monocrystalline diamond slurry. The aspects that will be taken into account are the MRR and the grain size distribution. 

With the 1, 3, and 15 pm abrasive sizes, it can be considered that the polycrystalline diamond is effective in improving the surface finish whereas the monocrystalline diamond generates a better MRR. This is possible because of the different abrasive action of each type of diamond and its behavior determined by its physical properties. 

The monocrystalline diamond behaves differently from the polycrystalline one only because of larger grain size. This could be due to the breakage of larger polycrystalline grains (25 pm) into much smaller particles than those resulted after the breakage of monocrystalline diamond. One can see... 

The ideas mentioned here can explain why the MRR for the 25 pm polycrystalline grains is lower than that for the monocrystalline diamond. 

Grain size distribution for 25 j.m monocrystalline diamond (a) before lapping (b) after lapping. 

Comparison between the polycrystalline and monocrystalline diamonds suggests using the former for better surface finish and the latter for better MRR. 

For 0.25 (xm monocrystalline diamond, we removed 7.395 xm per 12 min of machining or 0.6163 pm per min. This rate is with a different charging method and is showing lesser efficiency than the one described earlier. 

Stefan, R.I. and Bairu, S.G. (2003) Monocrystalline diamond paste-based electrodes and their possible applications to the determination of Fe(II) in vitamins. A a/. Chem., 75, 5394-5398. 

The greatest amount of material removal is achieved with bonded diamond grinding disks. Progressively lower rates of removal are associated with polycrystal-hne diamond suspensions, monocrystalline diamond suspensions, diamond sprays, diamond pastes, SiC papers, and AI2O3 slurries (in that order). However, as the rate of material removal increases, so does the amount of damage caused to the sample smface. In the case of relatively soft or porous ceramics, it can be practical to use an... 

In general, industrial HPHT synthesis of diamond produces crystals with size ranging from tens to hundreds of micrometers. Up to now, ordinary mechanical milling of as-grown HPHT diamond (20-50 pm) yields a very small and, thus, commercially expensive fraction of NDs. However, a powder of commercial-type lb-synthetic monocrystalline diamond fabricated for industrial polishing purposes can be used to produce monocrystalline material by crushing, purification, and precision grading to achieve a particle size distribution below 50 nm. ... 

FIGURE 10.6 Spherical carbon onion with a monocrystalline diamond core 10 nm in diameter. The core shows the lattice fringes of diamond with a separation of 2.06 A. (Adapted from Banhart, R, Ajayan, P.M.,... 

Tool material PCD (polycrystalline diamond), MCD (monocrystalline diamond), diamond... 

Special care was taken in order to overcome problems that might be caused by impurities in the system. The electrode surface was firmly polished with a Struers DP-Suspension, P polishing suspension that contains monocrystalline diamond of a granular size of % pm. The electrode was then rinsed with pure ethanol and Milli-Q water, and in order to thoroughly remove all traces of contamination from the surface, it was cleaned ultrasonically for several minutes and washed extensively with ultrapure water. 

Surface parasitic phases. SEM analysis of the films shows that generally the crystalline quality of the film improves with increasing boron content up to lO cm [4]. However, they still contain a high concentration of defects and poor homogeneity. On monocrystalline diamond, only the 1332 cm diamond line is observed (Fig. 5.5). 

Fig. 5.5. Raman spectra of a monocrystalline diamond (type IFa) and lightly doped polycrystalline diamond film (B/C ratio in the gas phase = 7 ppm), after removal of the background due to the fluorescence (632.8 nm excitation line).

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