ENERGY SCALING TRANSITIONS IN MACHINING OF SILICON BY DIAMOND

The existence of an energy scaling brittle-ductile transition in the m achining of ceramics and glasses is now well established. We have exam ined the surface layers of silicon crystals machined in the ductile re gime by two methods: (i) a dedicated highly stiff single point diamond turning facility (SPDT), using cut depths of the order of 100 nm (ii) high precision cup grinding using a nominal cut depth of 500 nm. Surf aces were profiled by AFM and subsurface damage was characterized by R utherford ion backscattering and cross section TEM. The main results t o date are: (a) The SPDT specimens possessed a surface quality corresp onding to that achieved by optical polishing, the R(a) similar or equa l to 0.6 nm and R(max) (P-V) similar or equal to 6 nm. (b) The R(a) va lues of the ground specimens ranged between 7 nm and 20 nm, with 64 < R(max) < 148 nm. There were significant differences between wheels bon ded by resin and by cast iron. (c) The mean depths of surface damage w ere in the range 200-400 nm in all cases. (d) In the SPDT crystals the subsurface damage consisted of dislocation loops intersecting the sur face. In any one region of the specimen these represented slip on a si ngle close packed system. (e) In ground slices the permanent damage co mprised sub-micron length cracks as well as a high density of dislocat ions on various systems. Local regions of amorphous silicon were also present. (f) In SPDT ductile machining diamond tool wear is effectivel y homogeneous and much less variable than in normal machining.