Layers carbides

Another approach is to coat the cutting tool material with a carbide former, such as titanium or siUcon or their respective carbides by CVD and deposit diamond on top of it. The carbide layer may serve as an iaterface between diamond and the cemented carbide, thus promoting good bonding. Yet another method to obtain adherent diamond coatings is laser-iaduced microwave CVD. By ablating the surface of the substrate with a laser (typically, ArF excimer laser) and coating this surface with diamond by microwave CVD, it is possible to improve the adhesion between the tool and the substrate. Partial success has been achieved ia this direction by many of these techniques. 

Testing of linear variation of hardness allows us to check the effectiveness of complex technological processes in the ceramics industry. As an example, we consider the results obtained by van Naderveen (1980) for sintered carbide layers superposed on cylindrical items of soft steel in cold... 

Pure boron carbide (layers with index 2, bulk tensile stress). 

Micrograph of a B4C grain in (a) the pure boron carbide layer, and (b) the B4C-30wt%SiC layer, of the three-layered laminate. 

From these results it has become apparent that both polymerized and monolayer aminosilane coatings can be converted to a silicon carbide layer. A further assessment and fine-tuning of carbide layer thickness can be effectuated from controlled variation of the modification parameters. This is subject to further studies. 

The chlorides of the transition metals can also be employed to obtain transition metal carbides (equation 4). This reaction proceeds in the gas phase at temperatures above approximately 600 °C. Because transition metal halogenides are rather volatile, this reaction is usually used for chemical vapor deposition of carbide layers on solid substrates or to produce carbide powders with very fine (submicron) grain sizes ( nanopowders ). In the latter case, the nucleation of the... 

Subsequent preliminary comparative studies of the behavior of an SiC based layer on Ta, Mo, Ti and steel substrates showed that better mechanical stability was obtained with a coating deposited on tantalum. This element was consequently considered to make PFCVD deposit/interlayer/steel stacks. Tantalum can be produced by physical vapor deposition (PVD), at variable thickness, with reproducible morphology. Note that preparation by chemical vapor deposition with or without plasma assistance (CVD or PECVD) is possible at low temperature but would require an optimization study in order to be compatible with the deposition conditions of the silicon carbide layer, the aim being to increase the mechanical stability. 

Diffraction from overlayers has been reported in a very few cases only, including H2O and ethanol (but not CCI4) on NaF(lOO), a carbide layer on W(llO), and oxygen on tungsten carbide. 

The two-phase boride/carbide layer described in item 1 extends downward by reaction as the boron-rich liquid below it is drawn into the porous boron carbide by capillary action. Boron and carbon rapidly diffuse through this thin B-rich liquid layer, causing the two-phase layer to thicken as the directed reaction proceeds. 

L.M. Sorokin, N.S. Savkina, V.B. Shuman, A.A. Lebedev, G.N. Mosina, J. Hutchinson, Features of the structure of a porous silicon carbide layer obtained by electrochemical etching of a 6H-SiC substrate, Tech. Phys. Lett., 28, 935-938 (2002). 

Joffreau et al. and Lindlbauerl conducted systematic studies of diarriond growtli on carbide-fonrung refractory metals and observed that diamond nucleation occurred only after the formation of a thin carbide layer. Lux and Haubnerf subsequently postulated a model to elucidate the... 

Figure 9. Schematic diagram showing the proposed nucleation mechanism diamond nuclei form on a carbide interlayer on a carbide-forming refractory metal substrateJ Initially, carburization consumes all available C to form a carbide surface layer. A minimum C surface concentration required for diamond nucleation cannot be reached on the substrate surface. With increasing carbide layer thickness, the C transport rate stows and the C surface concentration increases. When the C surface concentration reaches a critical level for diamond nucleation, or a surface C cluster attains a critical size, a diamond nucleus forms. (Reproduced with permission.)...

In diamond growth experiments on Mo and Si substrates using MW PACVD by Meilunas et al., M02C and SiC layers of approximately 1.5 pm and 10 nm in thickness were observed with SEM within 1 min and after 5 min, respectively. The growth rate of SiC was much less thanthat of M02C. Diamond nanocrystallites were observed after 1 min, and no further carbide layer growth was detected once the surface was covered with diamond. 

The formation of SiC is kinetically limited by Si diffusion to the surface through the growing carbide layer (note that Lux and Haubner suggested that it is limited by the C diffusion through the carbide layer) thus, even if there... 

---