It has been found that uniaxial pressing (UP) may be as suitable as cold isostatic pressing (CIP) for forming bearing rings. Favourable conditions for debinding the forming additives by heating in air were found to be a temperature of 600°C and an intensive air flow depending of the charge geometry. Materials with an additive content > 5 wt% sintered above 99% theoretical density regardless of the kind of powder. Pressureless sintering at aid contents < 5 wt% resulted in residual porosity and unecceptable performance.
Additive contents of 3-4 wt% yttria and 1,5-2 wt% alumina proved to be suitable for silicon nitride bearing components by gas pressure sintering (GPS) or, for extreme high demands, by Sinter-HIP.
Material requirements were specified in two forms - 'basic' and 'preferred'. The majority of inspected market available materials and project materials met the basic requirements. Of the new developed materials many could be classified as "preffered" application as bearing components.
The developed finish technology based on a SiC tool achieved the surface quality and accuracy of the critical components. Further development of the machining process in production of high precision rollers for cylindrical roller bearings lead to good success. All machined rings met the dimension tolerances and running accuracies according to P5 bearing specification which is higher than standard. Surface qualities of up to Ra< 0,03-0,05µm can be realised in the serial production of high precision silicon nitride rollers that are used in high speed spindle roller bearings.
None of the silicon nitride materials was corroded in Ammonia, but corrosion resistance in strong acids or bases is clearly less than that of Silicon Carbide. A SiC 2nd-Phase additive did not improve the corrosion resistance significantly. A sinter-HIP Silicon Nitride, (Baysinid, Additives: 3A1,5Y) shows that a highly densified Silicon Nitride sintered from a high purity powder with low amount of additives can exhibit a good corrosion resistance against strong acids like HNO3 and HCL.
Under dry running conditions few to no wear occurred at loads of < 1 GPa and it could be shown that unlubricated operation is possible with ceramic rolling bearings with very low wear of the bearing raceways. Testing in hot water resulted in strong wear of the bearing already at relatively low specific loads. In NaOH bearings reached running times of 1.000h at 3.000 rpm at a fairly high specific load of 1,75 GPa. The results show that long term media lubricated operation at appropriate loads is possible with all ceramic rolling bearings even in fluids with extreme low viscosity.
The proposed project is directed towards developing high performance, low cost silicon nitride bearings for use in aggressive environments and proving their use in pumps on chemical process plant. The bearings will be fabricated from ultrafine silicon nitride powder which can be processed using more economic methods than are currently needed for such materials.
The major research tasks are:
A. Development of a silicon nitride powder which has high sinter activity.
B. Development of low cost processing methods (forming and sintering) suited to such powder and to produce blanks of both rolling elements and rings.
C. Further development of the powder into a ready-to-use form to facilitate technology transfer to SME's and less technically advanced areas.
D. Development of high quality, high accuracy but low cost machining methods for both rolling elements and rings to a defined specification.
E. Fabrication of all-ceramic bearings and laboratory testing in a modified magnetic coupling unit.
F. Field testing of pumps containing these magnetic couplings on a chemical process plant.
In this direct application alone savings of 5% pa (ca. 500,000 ECU pa) can be made in the maintenance costs of a typical chemical plant.
Funding SchemeCSC - Cost-sharing contracts
3439 DT Nieuwegein