Observation of Soot Particles in Engine Lubricants
by Scanning Transmission X-Ray Microscopy
G. R. Morrison1, W. D. Shi1, A. D. H. Clague2, J. T. Gauntlett2, P. J. Shuff2
1
Department of Physics, King’s College London, Strand, London, WC2R 2LS, UK
Shell Research & Technology Centre, Thornton, PO Box 1, Chester, CH1 3SH, UK
2
Abstract. The agglomeration of soot in lubricating oils has been followed at
sub-micron resolution by STXM. A Peltier stage, utilised as a compact heater,
facilitates observation of this dynamic process at 80ºC. Sub-micron spherical
soot aggregates appear to form by accretion of smaller particles. Thereafter,
growth of the spheres proceeds by coalescence of the smaller aggregates in a
process that would minimise the specific surface area of the aggregates.
1 Introduction
Soot that is formed in the combustion process in an internal combustion engine, can
degrade the performance of engine lubricants by changing their rheological properties.
Primary soot particles are spheroids about 30 nm in diameter [1], which can combine
in suspension in the lubricant to form larger particles or extended networks.
The later stages of agglomeration can be observed by visible light microscopy.
Extended soot networks appear to form as the engine cools after switching off, leading
to gel formation within the oil, increasing the viscosity considerably. The gels can be
broken up by gentle agitation over several hours but re-form when the soot/oil mixture
reaches temperatures typical of the sump after the engine is stopped (50 - 100ºC).
Figures 1 and 2 show visible light micrographs of the same sheared lubricant before
and during heating.
The surface chemistry of the soot particles and the chemistry of the other species
in solution can have a significant effect on the agglomeration process. However, it is
difficult to relate changes in the oil formulation to processes on the sub-micron scale
without direct microscopic observation.
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Fig. 1. Optical micrograph of sheared lubricant
containing soot.
Fig. 2. Same sample as Figure 1 during
heating.
The high vacuum environment of the transmission electron microscope precludes
direct observation of the agglomeration process. X-ray microscopy is therefore
essential to studies of the early stages of agglomerate development (in the sub-micron
range) because these studies can be conducted in situ.
2 STXM Experiments at Daresbury
The scanning transmission X-ray microscope (STXM) on beamline 5U2 at the SRS,
Daresbury is able to observe the early stages of agglomeration in a lubricant. Similar
experiments to study gel formation in hydrated silica have been reported[2], but the
observation of soot agglomerates in lubricant is technically more challenging. As both
soot and lubricant are composed mainly of carbon, the main contrast mechanism is
photoelectric absorption. Discrimination of soot agglomerates therefore relies on the
different mass thicknesses of carbon under the X-Ray beam. In addition, the
agglomerates of interest lie close to the resolution limit of the STXM, so the image
contrast transfer function is low.
2.1 The in situ Heater
An in situ heating device has also been developed (Figure 3) so that the same area of
gel can be observed during agglomeration. The heater block consists of a ceramic
Peltier stage supported on, but thermally insulated from, a steel plate. The Peltier
effect is based on semiconductors pumping heat from one face to another. Usually
these devices are used as compact cooling stages. However, there is nothing in
principle to prevent their use as heating stages, simply by reversing the dc current
supply.
Observation of Soot Particles in Engine Lubricants
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Oil droplet
Ceramic
Peltier
stage
12 mm
Superglue
12 mm
Sample
mounted
here
3.2 mm
Thermocouple
Steel holder
Fig. 3. diagram of the heated sample stage. The silicon nitride windows are supported on a
brass or copper washer to aid handling. This is secured to the hot face of the Peltier stage by a
slow setting relatively weak glue such as 3M Spray Mount.
The oil film is held between windows of 100 nanometre thick silicon nitride
supported by silicon frames[3]. The windows were spaced by 1 micron thick bars of
acrylate photoresist, which were arranged perpendicular to each other to create a
nominal path length of 2 microns However, the surface tension of the oil film between
the windows tended to draw them in towards each other.
In one experiment, a series of circular species can be seen to grow and coalesce
within the oil film at 80ºC (Figures 4–6). In Figure 8 their lateral dimensions exceed
the distance between the windows, indicating that they are forming pillars. It is
possible that these species have nucleated at the windows due to some interaction
between the soot and the window material. The growth process is one that would
minimise the specific surface area of the aggregates.
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Figs. 4–6. STXM Images at 80ºC of agglomerated soot spheres coalescing with time.
Each image is 25.6 x 25.6 microns. Acquisition of each image took approximately
22 minutes and there are 48 minutes between the start of each scan.
References
1 J.-P. Donnet, R. C. Bansal and M.-J. Wang, Carbon Black, 2nd Edition, (Marcel
Dekker, New York, 1993).
2 W. D. Shi, G. R. Morrison, M. T. Browne, T. P. M. Beelen, H. F. van Garderen
and E. Pantos, Nucl. Inst. Meth. in Phys. Res. B97, 202 (1995).
3 P. A. F. Anastasi and R. E. Burge in X-ray Microscopy III, (Springer, Berlin,
1992) p. 341.