PhD
Thesis – Abstract
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Engineering
Ceramics for Tribological Systems
Manufacture,
Properties and Testing
by
Lars
Kahlman
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A
broad range of commercial and experimental grades of
ceramics and ceramic composites was tested in various model
tests and simplified component tests. The tests were done to
establish important microstructural features, friction and
wear mechanisms. Important tribological systems used in the
tests included face seals, abrasive-waterjet nozzles and
hybrid bearings. The importance of mechanical and physical
properties at the micro-level has been highlighted in the
work, as they play vital roles in the tribology of ceramics
and are less commonly investigated than the bulk properties.
Novel ceramic whisker composites were produced by
hot-isostatic-pressing. The composites were based on a
silicon nitride matrix reinforced with silicon carbide
whiskers. The matrix of silicon nitride showed high strength
and fracture toughness. But the composites showed no
improvements in the bulk mechanical properties when compared
with the matrix. This was mainly due to impurities in the
whisker batch, causing defects and lower fracture toughness.
A new type of silicon carbide material was made
dense using liquid-phase sintering, and its wear behaviour
was established and compared with conventionally solid-state
sintered silicon carbides. The presence of grain boundary
phases was both an advantage and a disadvantage regarding
wear rates. At high wear rates the grain boundary phase
limits the severity, probably due to crack deflection of the
micro-fractures that occur. However, in mild wear the
limited high-temperature properties of the grain boundary
phases can be a disadvantage.
Monolithic
ceramics and composites with similar mechanical and physical
properties to the bulk showed dramatically different wear
behaviour in the tests. A newly developed, more homogeneous
grade of whisker reinforced alumina showed superior wear
resistance to the other monolithic ceramics and composites
that were considered.
A
high-speed abrasive-waterjet test was developed in the work
for wear testing, and was used to investigate the resistance
of ceramics to thermal wear. The wear was caused by thermal
spalling, oxidation, creep and plastic deformation.
Abrasive-waterjet tests were done at low inclination angles,
and newly commercialized machines were used. The tests
highlighted the influence of thermal wear in the mixing
nozzle for the abrasive and the waterjet, and the thermal
mechanisms present while machining engineering ceramics. It
was previously not known that an abrasive waterjet creates a
thermal zone when machining ceramics at the wear front and
in the mixing nozzles. The temperatures reached in the wear
zone of the test specimens were at least 1280°
C for the garnet abrasive, and 2050°
C for the alumina abrasive.
In
scratch testing with diamond tips the presence of four
different wear regimes was established for ceramics. On the
tested materials a transition was seen from entirely ductile
deformation at low loads, towards catastrophic brittle
fractures at high loads. In scratch testing some materials,
such as a hot-isostatic-pressed silicon nitride composite,
showed much higher resistance to the initiation of brittle
fracture.
Ceramic
composites reinforced with long fibres were studied to rank
their sensitivity to particle impact. The test showed that
composites with silicon carbide spheres on the surface were
more resistant to the impact of abrasive particles. The
spheres were present from the manufacturing process
(chemical vapour infiltration).
A
study of thrust bearings was performed to compare all-steel
bearings with hybrid ones. The hybrid bearing was of the
same type as the all-steel one except the balls were of
silicon nitride. The influence of contaminants in the
bearings was investigated for fairly soft micron-sized (0.1
5 µm titania) and macro-sized (75– 100 µm quartz)
ceramic contaminants. The hybrid bearing showed greatly
superior performance when contaminants were present compared
to the traditional all-steel type of bearing. It was shown
that it is even possible to reduce friction and wear in a
hybrid bearing system by the introduction of micron-sized
titania particles. |
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Keywords: abrasive
waterjet, abrasive wear, ceramic composite, engineering
ceramic, erosion, face seal, hybrid bearing, lubrication,
thermal spalling, thermal wear, tribology
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