10-11-2012, 04:58 PM
Rolling contact silicon nitride bearing technology: a review of recent research
Rolling contact silicon.pdf (Size: 499.9 KB / Downloads: 67)
Abstract
The paper reviews some of the extensive research in the field of ceramic rolling element bearings that has been carried out over the
past decade or so. As a result of this work hot isostatically pressed silicon nitride (HIPed Si3N4), has emerged as an extremely promising
material for fabricating high performance all-ceramic or hybrid steel/ceramic rolling contact bearings. Compared with conventional steel
bearings silicon nitride bearings have been shown to offer significant benefits in terms of rolling contact fatigue life, and the lower density
of the material greatly reduces the dynamic loading at ball/raceway contacts in very high speed applications such as machine tool spindles
and gas turbine engines. Investigations have shown the particular benefits of using silicon nitride bearings in severe lubrication and wear
conditions such as extreme temperature, large temperature differential, high speed, ultra-high vacuum and in safety-critical applications
where, for example, they can respond to the requirements of short periods of oil-off operation in an aircraft engine. Other benefits which
have been demonstrated are corrosion resistance and tolerance of contaminated lubricants. As a result of their sustained development and
testing it is expected that silicon nitride bearings will continue to achieve wide acceptance in all types of applications. © 2000 Elsevier
Science S.A. All rights reserved.
Introduction
Rolling contact bearings are widely used in rotational machinery
to support shafts and separate rotating machine elements
from stationary ones. In machine tool applications
bearing precision is of vital importance to ensure accurate
products and in chemical machinery corrosion resistance
must be guaranteed. In aircraft and space vehicles high reliability
and relatively long life are required of all assemblies
and elements, including bearings. With the advent of
the space era very demanding bearing operating conditions
such as high vacuum (<10−6 Torr), extreme temperatures
(e.g. C230 to −150C), large temperature differentials, long
life (both wear and fatigue life, usually 10–15 years without
maintenance) and low frictional power are quite common
[1,2]. These ever increasingly stringent demands present
great challenges for those responsible for the development
and validation of new rolling contact bearing materials.
Material evaluation of candidate silicon nitrides
The quest for suitable silicon nitride formulations and
processing techniques is driven by the bearing manufacturers
who demand reliability and cost effectiveness from
any new candidate material. The potential of silicon nitride,
although well appreciated by the bearing industry as a
promising rolling contact bearing material, has not been
fully exploited, though there are a number of competitive
material suppliers who are involved in blank fabrication of
candidate silicon nitride bearing elements. At this stage of
the development of the technology it is inevitable that suppliers
make frequent changes to material additive composition
and process parameters, the advantages of which, in terms of
improved bearing performance, are not always clear to their
customers. From the standpoint of traditional steel bearing
technology, silicon nitride is still perceived as a brittle material
with low fracture toughness (typically 7–8MPa(m)1=2
at best) which depends markedly on the material composition
and process as illustrated in Table 2.
Non-destructive evaluation (NDE) of ceramic
bearing elements
Defects such as voids, micro-scratches or micro-cracks at
the surface or in the vicinity of the surface (i.e. subsurface)
of silicon nitride bearing elements are particularly critical
to their rolling contact lifetime since their RCF failures can,
as in the case of steel bearing elements, be described as
surface or subsurface microcrack initiated with propagation
due to fatigue contact stress [26]. In a Hertzian contact the
maximum shear stress occurs in the subsurface at a depth
which depends on the ellipticity of the contact as listed in
Table 3. The stress concentration behaviour around the sharp
tip of flaws or cracks is very strong because of high values
of material hardness, brittleness and elastic modulus.
Under the action of both normal and tangential forces high
tensile stress occurs at the flaw tips and this tends to open
the flaws up towards the free surface [27]. Defects such as
voids, pin-holes, organic or non-organic inclusions, badly
distributed second-phase sintering aids (usually oxides such
as Al2O3, MgO and Y2O3) and inherent powder defects can
all occur during the sintering phase of manufacture, but micro
cracks, micro-scratches and delaminations can also be
introduced at any stage during both machining and final polishing
processes. Whatever their cause, all such defects will
result in reduction and variability in silicon nitride rolling
element properties. Quality control at all stages of the manufacture
of the bearing elements is therefore of crucial importance.
Conclusions
This review has briefly summarised the extensive studies
that have been made, and are currently taking place, in the
field of silicon nitride rolling element bearing technology.
It is clear that many of the perceived advantages of ceramic
bearings that were envisaged in their early stages are now
being realised as a result of sustained research and development
over the last two decades. Improvements in bearing
element fabrication, machining, NDE, surface treatments,
coatings, full-scale bearing tests and field tests have accumulated
much useful data and are helping engineers build
a sound basis for the future application of ceramic bearings
not only in the highly demanding specialised situations for
which they were first intended, such as aerospace, but also
for much more widespread use where they can offer a performance
advantage over conventional all steel bearings.