Automakers and lubricant formulators alike recognize that soot is a bad actor, promoting engine wear and untimely engine failure. Researchers hope a fundamental understanding of how soot particles and agglomerates behave will ultimately lead to better antiwear protection and longer engine life.
At the 5th World Tribology Congress in Turin, Italy, in September, Professor Fabrice Dassenoy with the Ecole Centrale de Lyons Laboratory of Tribology and Systems Dynamics in Ecully,
France, presented results of a collaborative research program between his team and global additive company Infineum. Funded by Infineum, Dassenoy and his colleagues are studying the hardness and deformation of soot particles and agglomerates.
Soot, Dassenoy explained, refers to impure carbon particles resulting from the incomplete combustion of hydrocarbons. Carbon makes up more than 90 percent of the particles weight. Individual soot particles are spherical, 20 to 30 micrometers in size, and are often found as aggregates fused together during the combustion process.
In the automotive industry, Dassenoy continued, soot is generated mainly during the combustion of diesel fuel in engines. Although a majority escapes through the exhaust, some soot gets past the piston rings with blow-by gases and through thin-layer adsorption.
Excessive soot levels in the oil can quickly overwhelm the dispersant additives in lower quality engine oils, Dassenoy said. Agglomerates lead to reduced lubrication due to higher viscosity that impedes oil flow through the engine, as well as the oil filter.
High soot levels impact the performance of anti-wear lubricant additives and lead to increased wear and premature engine failure.
Several wear mechanisms have been proposed in the literature, Dassenoynoted. These include:
Preferential adsorption by soot of ZDDP (zinc dialkyldithiophosphate) decomposition products;
Reduction and removal of antiwear film formation on metal surfaces, resulting in metal-to-metal contact and soot abrasion;
Reduction in the surface coverage rate by ZDDP because of the competition for metal surface sites between ZDDP and soot;
Pumpability problems due to soot agglomeration.
Dassenoy and his team, however, are focusing on the hardness of soot particles as the origin of abrasive wear.
The objective of the study was to observe the behavior of soot particles and agglomerates during mechanical stress – that is, under compression and sliding. The fundamental understanding of how the soot particles and aggregates behave in a tribological contact is of first importance for establishing the link between the soot particles and their wear mechanisms, he said.
The researchers used a high-resolution transmission electron microscope equipped with a nanoindenter to manipulate the soot particles at the sub-nanometer scale, and to characterize their mechanical properties in real time when an external load is applied. The nanoindentation sensor was equipped with a truncated diamond tip.
The transmission electron microscope nanoindenter permitted imaging the compression process in real time, recording movies as well as acquiring data from both compression tests and sliding tests. Soot particles for the tests were extracted from the crankcase of an Infineum heavy-duty diesel dynamometer engine test, and were purified.
The compression tests of agglomerates of soot particles featured 100 nanometer displacement controlled compression of the tip; a loading rate of five nanometers per second; and maximum applied force of 60 micronewtons.
The researchers found that the agglomerates resist pressure and do not break down into primary soot particles.
In compression tests of single soot particles (featuring 40-nm displacement-controlled compression of the tip, loading rate of two nm/sec and maximum applied force of 25 μN), Dassenoy reported that the particle is slightly deformed, but does not undergo major structural damage. Soot particles are able to withstand very high elastic stress.
In sliding tests of agglomerates, Dassenoy found the whole agglomerate is able to roll in the contact. No particle detaches from the agglomerate. There is high cohesion between the particles, he said, and sliding is only observed on very few occasions.
With a single soot particle in the sliding test, both rolling and sliding were observed. There was no major deformation or crushing of the particle, Dassenoy said, and the sliding tests again showed the exceptional resistance of the soot particles to mechanical stress.
Dassenoys movies showing the nano-size soot particles and agglomerates deforming slightly and recovering their original shapes, and showing no structural damage or crushing under pressure, then rolling like lumpy bearings in the slide tests, captivated the World Tribology Congress audience.
A first key conclusion, Dassenoy noted, is that his team was able to successfully conduct compression and sliding tests on 20- to 30-micron soot particles using the high-resolution transmission electron microscope equipped with an in situ nanoindenter. This allowed them to study the response of agglomerates of soot particles as well as individual soot particles during mechanical stress.
Both agglomerates and single particles are resistant to load. Agglomerates do not break; there are strong cohesion forces between the soot particles, Dassenoy said. In addition, the study showed partially reversible compaction of the particles within the aggregates and elastic behavior of the particles.
Finally, he noted, the study showed the ability of both agglomerates and single nanoparticles to roll in the contact zone. The next steps, said Dassenoy, will be research under more real-world conditions and with tribofilms.
Lube Report Asia occasionally includes articles originally published in sister publications of LNG Publishing Co. This article was originally published in December 2013, in Issue 54 of LubesnGreases Europe – Middle East – Africa, under the headline Soot is Tough Stuff.