Plasma carburizing

Plasma Carburizing. Plasma carburizing generates carbon atoms at the surface by ionization of a carbon-containing gas, eg, methane. The process is similar to that described for ion nitriding. Because the process is carried out in partial vacuum, there is less chance of oxidation. 

Modern techniques are currently available using carburizing and nitriding systems under vacuum. In these processes of vacuum carburizing and plasma carburizing, the components are heated under vacuum to around 950 °C. Methane is leaked into the chamber to a pressure of between 3 and 30 mbar to add carbon to the system. In the absence of a plasma, the methane will only decompose to the extent of about 3%, probably on the surface of the components according to a sequence such as that shown in Equation (11.19) ... 

M. Tsujikawa, S. Noguchi, N. Yamauchi, N. Ueda and T. Sone, Effect of molybdenum on hardness of low-temperature plasma carburized austenitic stainless steel. Surf. Coat. 

Y. Sun and T. Bell Effect of layer thickness on the rolling-sliding wear behavior of law- temperature plasma-carburized austenitic stainless steel. Tribology Letters (2002) 13,1, 29-34 Y. Sxm, Kinetics of low temperature plasma carburizing of austenitic stainless steels, J. Mater. Proc. Tech. 168 (2005) 189-194. 

Y. Sun, X.Y. Li and T. Bell, Low temperature plasma carburizing of austenitic stainless steels for improved wear and corrosion resistance. Surf. Eng. 15 (1999) 49-54. 

Carbides may also be prepared, either by dhect carburizing, as in the case of steel, in which a surface carbide film dissolves into the subsuate steel, or by refractoty metal carbide formation as in die cases when one of the refractory metal halides is mixed with methane in the plasma gas. 

In general, several possible chemical reactions can occur in a CVD process, some of which are thermal decomposition (or pyrolysis), reduction, hydrolysis, oxidation, carburization, nitridization and polymerization. All of these can be activated by numerous methods such as thermal, plasma assisted, laser, photoassisted, rapid thermal processing assisted, and focussed ion or electron beams. Correspondingly, the CVD processes are termed, thermal CVD, plasma assisted CVD, laser CVD and so on. Among these, thermal and plasma assisted CVD techniques are widely used, although polymer CVD by other techniques has been reported. ... 

Elowever, as stated in Section 10.2,2 [222], a secondary plasma could be generated in the center of the substrate between the main plasma ball and the substrate holder as well as the outer rim of the substrate holder. The substrate holder has a complicated shape and was precoated with diamond film, and the 3-inch Si(lOO) substrate had been carburized prior to the BEN treatment. As a result, the diamond nucleation occurred both in the center and the peripheral of the substrate. Thus, the position of the secondary plasma depends on the geometry of the reactor chamber. 

As discussed in Ch. 6, diamond nucleation enhancement may be achieved by, for example, predeposition of a thin DLC layer in combustion flame or DC glow-discharge plasmas, or, by carburization/biasing to develop a complete SiC layer on the substrate surface in MW PACVD. 

These reactions may be stimulated to provide 80% decomposition by using a plasma process to excite the methane molecule. In this case, the molecular breakdown may occur in the plasma to produce charged species. Hydrocarbons other than methane may be used as the feedstock. The usual operating sequence involves flushing and evacuation, heating to temperature under the inert atmosphere, carburizing for a predetermined time followed by a diffusion anneal in a carbon-free atmosphere. This cycle is designed to provide the optimum surface carbon content and carburized depth. - ... 

Zirconium carbide powder is prepared by the reaction of Zr02 with carbon at 1800-2400 C in hydrogen by the carburization of zirconium sponge by the auxiliary bath technique, or by plasma-CVD. Zirconium carbide coatings are deposited by CVD, evaporation or sputtering (see Chs. 14 and 15). 

It is nowadays widely accepted that hard, wear and corrosion resistant surface layers can be produced on Austenitic stainless steel by means low temperature nitriding and/or carburizing in a number of different media (salt bath, gas or plasma), each medium having its own strengths and weaknesses (Bell, 2002). In order to retain the corrosion resistance of austenitic stainless steel, these processes are typacally conducted at temperatures below 450 °C and 500 °C, for nitriding and carburizing respectively. The result is a layer of precipitation free austenite, supersaturated with nitrogen and/or carbon, which is usually referred to as S-phase or expanded austenite (Sun et al, 1999 Li, 2001 Li, et al., 2002 Christiansen, 2006). 

T. Bell and Y. Sun, Low temperature plasma nitriding and carburizing of austenitic stainless steels. Heat Treatment of Metals 29 (3) (2002) 57-64 T. Bell, Bodycote-AGA Seminar, Lidingo, 2005. 

Thus, process availability might negate the selection of plasma cartmriz-ing over conventional methods, despite the reduced carburizing times and more uniform case depths associated with plasma methods. A similar situation exists for gas nitriding and plasma (ion) nitriding. 

Using carbon ions, this technique has been used to carburize a substrate surface prior to deposition of a hard coating. The process is similar to ionitriding, where the reaction in-depth depends on thermal diffusion. In plasma source ion implantation (PSII), the plasma is formed in a separate plasma source and a pulsed negative bias attracts the ions from the plasma to bombard and heat the surface. 

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