Plasma Silicon Nitriding and Iron Boriding

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Abstract
Plasma hardening of silicon and iron surfaces was investigated to determine if such processing can provide any advantage over conventional methods. In particular, can suitably hardened surfaces be achieved at lower temperatures or with shorter process times? On solid silicon a nitride case (∼0.0002 in.) forms when the sample is heated for 2 h at 1420 °C in a nitrogen plasma at about 5 Torr. The rate at which the nitrogen diffuses into the solid silicon is greatly reduced once the case has formed. Boriding experiments on iron showed a 0.003-in. case is formed on compressed 1080 iron powder when treated at 780 °C in a boron trichloride plasma for 4 h. For these results the principal hardened layer appeared slightly below the surface and was predominantly Fe2B, contrary to the results of Lyakhovich et al. [Metal Sci. and Heat Treat. Metals (USSR) 1969]. These results show no advantage for hardening of silicon by plasma nitriding. However, iron may be plasma borided in comparable times at a lower temperature than by conventional means.
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