"Multi-Technique Problem Solving in an Industrial
Environment"
Shell, Thornton Laboratories, Chester-
Wednesday 6th of January 1999
"Multi-technique Analytical Studies of
Automotive Anti-wear Films"
G Smith, D Park, S Smith, J Bell, S Garner, G Roper (Shell),
S Hibbert (CSMA), D Johnson (Liverpool J.M.), A Pidduck (DERA Malvern).
Dr Smith described the inside of a car engine and highlighted the
complex role of the engine oil. Formulating engine oil is still a combination of art and
science. Dr Smith identified the key components in side the engine and pointed out that
the fit inside the modern car engine is so close that the roughness is almost identical to
the separation between the moving components! These tolerances present a major challenge
to Shell in the development of more effective, but also more environmentally compliant
materials. The work of Dr Smith and his group at the Thornton Laboratories supports the
fuel/oil formulation teams at shell in terms of analytical measurements and the
development of a more scientific understanding of the formation and role of automotive
anti-wear films.
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Figure 1: SEM Images of the
Anti-Wear Films |
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Figure 2:
RIARS spectrum from the surface of an Anti-Wear Film |
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Figure 3:
XPS Depth profile from a ZDTP+Dispersant&Detergent generated Anti-Wear Film |
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The first major obstacle for the analysis of is the preparation of the
samples. Given the delicate nature of the films formed during the wear experiments,
ideally that the samples have to be taken from test engines and put straight into the
spectrometer. For experiments using the rolling pin test instruments, this is possible
since the surface analysis instrument has been modified to accommodate specimens of those
dimensions.
The complex nature of the films formed during the wear process, means
that a range of analytical techniques need to be used in order to study the films in
detail. For example, SEM measurements indicate that the films have a flaky structure,
which is further highlighted by AFM measurements which also identify a "squishy"
organic outer layer. EDX measurements indicate that there is some lateral inhomogeniety.
Techniques such as RAIRS and XPS have been used to probe the surface
chemistry in more detail, the former being used to show the presence of a phosphate film
and the latter to show the structure of the films as a function of depth.
XPS depth profiling shows a Zn rich outer layer on films generated by
older Shell formulations. The profiles also indicate the presence of a buried S based
intermediate layer. The XPS depth profiles generated from similar samples or even
different areas of the same sample show that the film thickness is variable, which is not
surprising given the patchy nature of the films. Films formed using oils with added
dispersant plus detergent are more homogeneous than those formed with the basic oil, with
a more complex multilayered structure.
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Figure 4: EPMA Model used to
calculate the film thickness |
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Figure 5: The Anti-Wear film
formed using ZDTP + dispersant & detergent |
Having obtained these detailed sputter profiles, the next step is
defining the depth scale in nm not time. Combining SEM/EPMA data with a complex modelling
programme, which predicts that the films are around 66nm thick. This information is then
put back into the sputter profiles in order to determine the thickness of the individual
layers that make up the anti-wear films.
Using this approach, Dr Smith was able to develop the following model
for the anti-wear film formed using ZDTP with a combined dispersant and detergent.
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