Mechanical and tribological properties of Nd- and Yb-SiAlON composites sintered by hot isostatic pressing

Single phase alpha-, beta- and two phases alpha/beta-sialon composites
were sintered with both Nd2O3 and Yb2O3 by hot isostatic pressing
(HIP). The mechanical and tribological properties of these composite
ceramics have been investigated with respect to their application in
direct fuel injection pumps for gasoline engines. Scanning electron
microscopy (SEM) was used to analyse the fracture surfaces of sialons
following sintering and their worn surfaces after the tribological
tests. Microstructural evolution and its change with composition as
well as the influence of the microstructural changes on the mechanical
and tribological properties have been reported. It has been found that
increasing alpha-sialon phase fraction of the Nd-sialons increases the
hardness due to the harder nature of this phase, whereas no significant
change in the indentation fracture toughness was observed. Increasing
Nd content in these sialons resulted in no significant change in both
of the hardness and fracture toughness values. On the other hand,
single phase Yb-alpha-sialons showed a slight decrease in the hardness
and a gradual increase in the fracture toughness with increasing Yb
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content. In a comparative way, it can be seen that Yb-doping gives
slightly higher hardness and toughness than Nd-doping. This has been
explained by the difference between the cationic radii of Nd3+ and Yb3+
and the corresponding ionic field strength. Increased ionic field
strength of the Yb3+ in Yb-sialons resulted in an improvement in their
mechanical properties. Tribological tests were performed using a
laboratory tribometer with ball-on-plate geometry in reciprocating
sliding contact under lubrication with isooctane. Based on the friction
and wear data, a relative ranking of the different wear pairs of Nd-
and Yb-sialon plates together with a commercial Si3N4 plate (used as a
reference material) against Si3N4 ball was established. Sliding pairs
of Yb-doped alpha-sialon ceramics showed higher friction coefficient
and lower wear resistance compared to Nd-doped beta- and
alpha/beta-sialon composites and the commercial Si3N4 ceramic. The
lowest friction and the highest wear resistance were measured for the
Nd-doped sialon with an alpha/beta-ratio of 88:12. SEM micrographs of
the worn surfaces revealed that the main wear mechanisms were
microabrasion and tribochemical reactions that resulted in the
formation of a thin tribolayer composed of compact wear debris in the
tribocontact. (c) 2006 Elsevier B.V. All rights reserved.