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Silicon nitride bearings
temperature environment and the availability of data stored in the microcomputer relating to all the roller characteristics. These data can be used as a check on the manufacturing processes and tolerance trends can be identified so that corrective action can be taken to minimize the number of rejected rollerS. Another important feature is that the system can easily be modified to accommodate rollers of different sizes. Further information is contained in a 72-page report 'Advanced high speed roller bearing inspection techniques' by J.L. Jacoby and J.R. Bohn. It can be obtained, price £1.19 (microfiche) or £4.10 (reproduced paper-copy), quoting T78 5724 from TechAlert, TRC, Orpington, Kent, BR5 3RF, UK*
Glassy state lubricant behaviour Rheological properties &selected lubricants and their behaviour in a simulated elastohydrodynamic contact are being investigated at the Georgia Institute of Technology. A report for NASA on this work gives further insights into lubricant viscoelastic and viscoplastic transitions in typical EHD contact conditions. Temperature effects and factors limiting traction transmitted in these contacts and the effect of surface roughness on surface contact temperatures are also discussed. Rheological studies covered: high pressure low-shear rate viscosity measurements to 10 7 Pas; viscoelastic transition measurements by light scattering techniques at pressures from 0.1 to 0.7 GPa; measurements by dielectric relaxation at atmospheric pressure (but some to 0.55 GPa) and frequencies from 0.2 to 500 kHz; measurements by dilatometry up to 1.75 GPa pressure; and lubricant shear-stress/shear-strain behaviour in the amorphous glassy state at pressures to 0.7 GPa. The simulator studies consisted of contact surface temperature measurements for both thick and thin film elastohydrodynamic contacts with combined sliding and rolling (slide/roll ratios of -2 to +2) and for sliding contact where film thickness was equal to or less than surface roughness.
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Rough surface temperature measurements were also taken with a severely starved sliding elastohydrodynamic contact. Conclusions drawn from the research include the fact that many lubricants probably undergo viscoelastic transitions in typical EHD contact conditions and that a lubricant has a maximum shear stress it can support which in turn determines the traction in the contact except in the case of very low slide/roll ratio. Surface temperature data were used to produce a predictive equation for surface temperature as a function of pressure, sliding speed, and surface roughness. Further information is contained in a 126-page report 'Surface temperature and glassy state investigations in tribology' by W.O. Winer and D.M. Sanborn. It can be obtained, price £1.19 (microfiche)or £8.00 (reproduced paper-copy), by quoting T78 627.4, from TechAlert, TRC, Orpington, Kent, BR5 3RF, UK*
Silicon nitride bearings Until recently, fabrication of a fully dense, defect-free, silicon nitride bearing consisted of machining the part from a billet. Because of the extreme hardness of silicon nitride this is a tedious and expensive process, and consequently other methods of producing fully dense, high quality bearing parts to near finished shapes have had to be considered. In the US, fully dense parts formed by two methods from silicon nitride powder were used in the development of a process for finishing silicon nitride ball bearings to shape and size. The first method of fabricating the parts used a conventional hot pressing technique in which silicon nitride powder mixed with a suitable binder is pressed into a spherical shape at high temperatures (about 1700°C) in graphite dies. The second, a pressureless sintering technique, is a more recent development in which silicon nitride powder is blended with a suitable binder and then cold pressed to shape either by uniaxial pressure using metal dies or by isostatic pressure using polyurethane moulds. The cold pressed parts are then sintered at high temperature withont application of pressure. A description of the lbrming
TRIBOLOGY international February 1979
techniques and subsequent machining operations is contained in a US Navy report. The physical property and microstructural evaluations used, together with the non-destructive testing techniques and rolling four-ball fatigue life tests employed are also described. The sample batches of MgO- and Y2 Oz-b onded hot-press silicon nitride rough spheres were obtained from two different suppliers, and the pressureless sintered rough spheres from a third. Although there were marked differences in geometry between the two groups of hot-pressed rough spheres, they were capable of being machined ultrasonically and lapped to produce finished balls 17.5 mm diameter having an out-of-roundness ~<0.62/Jan and a surface roughness ~< 0.03/~m R a. Differences in the material removal rates during lapping were attributed to differences in the microstructures of the materials. Mg-O-bonded, hot-pressed Noralide NC 132 gave consistently superior wear and fatigue life performance in rolling contact. Although Y2 0 3 bonded, hot-pressed Ceralloy 147Y wore excessively under high stress conditions, it had a fatigue spalling life superior to bearing steel. The third group, comprising rough spheres produced by isostatic pressing followed by sintering, possessed very good geometry. However, the inhomogeneity of the material prevented the finished balls from rounding to better than 0.62 #m. These balls also wore excessively and spalled after very short service lives. The poor performance of this material is attributed to its relatively high porosity (2% by volume). Although there were differences in wear and spalling rates between the three groups of balls, extensive rolling contact fatigue life tests have demonstrated the potential of silicon nitride powder for reliable long life. All four non-destructive flaw detection techniques evaluated, ie ultrasonics, acoustic microscopy, krypton exposure and X-ray diffraction, were found to be effective in revealing defects, such as inclusions and porosity, detrimental to bearing fatigue life in silicon nitride. Further infornlation is contained in a 67-page report 'Final report on devel-
Glassy state lubricant behaviour Rheological properties &selected lubricants and their behaviour in a simulated elastohydrodynamic contact are being investigated at the Georgia Institute of Technology. A report for NASA on this work gives further insights into lubricant viscoelastic and viscoplastic transitions in typical EHD contact conditions. Temperature effects and factors limiting traction transmitted in these contacts and the effect of surface roughness on surface contact temperatures are also discussed. Rheological studies covered: high pressure low-shear rate viscosity measurements to 10 7 Pas; viscoelastic transition measurements by light scattering techniques at pressures from 0.1 to 0.7 GPa; measurements by dielectric relaxation at atmospheric pressure (but some to 0.55 GPa) and frequencies from 0.2 to 500 kHz; measurements by dilatometry up to 1.75 GPa pressure; and lubricant shear-stress/shear-strain behaviour in the amorphous glassy state at pressures to 0.7 GPa. The simulator studies consisted of contact surface temperature measurements for both thick and thin film elastohydrodynamic contacts with combined sliding and rolling (slide/roll ratios of -2 to +2) and for sliding contact where film thickness was equal to or less than surface roughness.
44
Rough surface temperature measurements were also taken with a severely starved sliding elastohydrodynamic contact. Conclusions drawn from the research include the fact that many lubricants probably undergo viscoelastic transitions in typical EHD contact conditions and that a lubricant has a maximum shear stress it can support which in turn determines the traction in the contact except in the case of very low slide/roll ratio. Surface temperature data were used to produce a predictive equation for surface temperature as a function of pressure, sliding speed, and surface roughness. Further information is contained in a 126-page report 'Surface temperature and glassy state investigations in tribology' by W.O. Winer and D.M. Sanborn. It can be obtained, price £1.19 (microfiche)or £8.00 (reproduced paper-copy), by quoting T78 627.4, from TechAlert, TRC, Orpington, Kent, BR5 3RF, UK*
Silicon nitride bearings Until recently, fabrication of a fully dense, defect-free, silicon nitride bearing consisted of machining the part from a billet. Because of the extreme hardness of silicon nitride this is a tedious and expensive process, and consequently other methods of producing fully dense, high quality bearing parts to near finished shapes have had to be considered. In the US, fully dense parts formed by two methods from silicon nitride powder were used in the development of a process for finishing silicon nitride ball bearings to shape and size. The first method of fabricating the parts used a conventional hot pressing technique in which silicon nitride powder mixed with a suitable binder is pressed into a spherical shape at high temperatures (about 1700°C) in graphite dies. The second, a pressureless sintering technique, is a more recent development in which silicon nitride powder is blended with a suitable binder and then cold pressed to shape either by uniaxial pressure using metal dies or by isostatic pressure using polyurethane moulds. The cold pressed parts are then sintered at high temperature withont application of pressure. A description of the lbrming
TRIBOLOGY international February 1979
techniques and subsequent machining operations is contained in a US Navy report. The physical property and microstructural evaluations used, together with the non-destructive testing techniques and rolling four-ball fatigue life tests employed are also described. The sample batches of MgO- and Y2 Oz-b onded hot-press silicon nitride rough spheres were obtained from two different suppliers, and the pressureless sintered rough spheres from a third. Although there were marked differences in geometry between the two groups of hot-pressed rough spheres, they were capable of being machined ultrasonically and lapped to produce finished balls 17.5 mm diameter having an out-of-roundness ~<0.62/Jan and a surface roughness ~< 0.03/~m R a. Differences in the material removal rates during lapping were attributed to differences in the microstructures of the materials. Mg-O-bonded, hot-pressed Noralide NC 132 gave consistently superior wear and fatigue life performance in rolling contact. Although Y2 0 3 bonded, hot-pressed Ceralloy 147Y wore excessively under high stress conditions, it had a fatigue spalling life superior to bearing steel. The third group, comprising rough spheres produced by isostatic pressing followed by sintering, possessed very good geometry. However, the inhomogeneity of the material prevented the finished balls from rounding to better than 0.62 #m. These balls also wore excessively and spalled after very short service lives. The poor performance of this material is attributed to its relatively high porosity (2% by volume). Although there were differences in wear and spalling rates between the three groups of balls, extensive rolling contact fatigue life tests have demonstrated the potential of silicon nitride powder for reliable long life. All four non-destructive flaw detection techniques evaluated, ie ultrasonics, acoustic microscopy, krypton exposure and X-ray diffraction, were found to be effective in revealing defects, such as inclusions and porosity, detrimental to bearing fatigue life in silicon nitride. Further infornlation is contained in a 67-page report 'Final report on devel-