The polishing process of advanced ceramic balls using a novel eccentric lapping machine
Abstract The finishing process of advanced ceramic balls can be divided into two steps. The first step is lapping in which most of the stock from the ball is removed at a higher material removal rate. The second step is polishing in which the required ball surface roughness, roundness, and dimensional and geometric accuracy are achieved. In polishing, the abrasive particle size is 1 μm, and the load and speed are lower than lapping. A novel eccentric lapping machine is used for polishing hot isostatically pressed (HIPed) silicon nitride balls. In the initial polishing stage, the polishing load is demonstrated as being most influential in the reduction of surface roughness value Ra. However, in the later polishing stages, the erosive process played a major role in the further reduction of Ra, although the high roughness peaks cannot be removed by erosive process alone. Experimental results also show that, in order to achieve desired surface roughness value, the initial surface quality of the upper plate should be reasonably high, and deep scratches on the ball surface from previous lapping processes should be avoided. The best polishing results achieved were surface roughness values of Ra of 0.003 μm and r.m.s. (Rq) of 0.004 μm, ball roundness of 0.08–0.09 μm. This proves that the novel eccentric lapping machine is also suitable for polishing advanced ceramic balls.
On the finishing of Si3N4 balls for bearing applications
The conventional method of producing of Si3N4 balls for bearing applications by grinding and lapping using diamond abrasive at low speeds ( < a few hundred rpm) and higher loads (several tens of N/ball) is generally an expensive and time-consuming operation (several weeks). It also leads to the formation of scratches, microcracks, and pits on the finished balls resulting from large radial and circumferential cracks and dislodgement of grains. Since failure of ceramics initiates from such defects, the reliability of Si3N4 halls in service is of prime concern. This paper deals with an alternate technology for finishing Si3N4 balls for hybrid bearing applications using magnetic float polishing (MFP) process that overcomes some of these limitations. A methodology for finishing of HIP'ed Si3N4 balls from the as-received condition by MFP is presented. It involves the mechanical removal of material initially using harder abrasives with respect to the workmaterial (of different materials of progressively lower hardnesses and finer grain sizes) followed by final chemo-mechanical polishing (CMP) using preferably a softer abrasive for obtaining superior finish with minimal surface or subsurface defects, such as scratches, microcracks, or pits on the Si3N4 balls. High material removal rates (1 μm/min) with minimal subsurface damage is obtained with harder abrasives, such as B4C or SiC (relative to Si3N4) due to the use of a flexible support system, small polishing loads (≈ 1 N/ball), and fine abrasives but high polishing speeds (compared to conventional polishing) by rapid accumulation of minute amounts of material removed by microfracture. Final polishing of the Si3N4 balls using a softer abrasive, such as CeO2 (that chemo-mechanically react with the Si3N4 workmaterial) results in high quality Si3N4 balls of bearing quality with superior surface finish (Ra < 4nm, R1 < 0.04 μm) and damage-free surface. It is found that CMP is very effective for obtaining excellent surface finish (R4 ≈ 4nm andR1 ≈ 40nm) on Si3N4 ceramic material and CeO2 in particular is one of most suitable material for this application.
Recalculation of Mass Defects
THE well-known mass defect curve of the old nuclear scheme calculated with regard to α-particles and protons presented a difficulty with its minimum of binding energy for tin and an increasing portion between tin and lead. On the other hand, the mass defect values against protons give a rather smoothly decreasing curve.1 As has already been pointed out,2 the number of α-particles must be considerably reduced from the point of view of the new scheme, which does not admit any electrons in nuclei, but only neutrons and protons (presumably joined as α-particles). The curve of mass defect against protons and neutrons (perhaps with a single ‘central’ α-particle) is very similar to the old curve against protons, but decreases less rapidly. Clearly the new mass defect values relatively to α-particles, neutrons and protons must lie somewhere between the old values computed relative to protons and α-particles respectively (because the number of α-particles is decreased). We may emphasise that this new mass defect curve shows no increasing portion between tin and lead. For illustration we give two typical values: Old: 50Sn124 = 31α + 12&#38;epsiv;; mass defect = 0.158 (in mass units); 82Pb208 = 52α + 22&#38;epsiv;; mass defect = 0.035. New: 50Sn124= 25α + 24ω (ω = neutron); mass defect = 0.304; 82Pb208 = 41α + 44ω; mass defect = 0.366.