Lube-Tech
For example, degraded engine oil on piston rings or on the cylinder wall has many more negative implications than degraded oil in the crankcase. Good lubricant should be ‘forgiving’ and should not cause major failures even after severe abuse. ‘Forgiving oils’ should not vaporise off completely even after prolonged degradation. On the other hand, they should not become dry solid either, so they could dissolve in fresh lubricant, when it is finally supplied. It is nearly impossible to test these aspects in bulk oils, so the film degradation approach is used in tests such as Panel Coker, Pressurised Differential Scanning Calorimetry (PDSC), micro-oxidation and several others.
Often lubricant films continue degrading even after equipment shutdown, because it might take some time until cylinder walls, hydraulic reservoir, gears or other surfaces cool down. Film degradation is still rapid, but no new oil is supplied, which might lead to residue formation, corrosion and other major issues. In fact, degradation mechanisms in thin films are significantly different from those in bulk oils. Although oxidation, hydrolysis or saponification reactions seem the same chemically, in thin films all these mechanisms can easily combine, leading to a variety of organometallic and other unusual processes. In addition, previous experiments showed [2] that in 10 µm or thinner films, electrochemical reactions can take place, for example dissociation or oxygen reduction, despite low electrical conductivity of oils, see Figure 1.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.116 page 2
The importance of thin film degradation in lubricants was clearly understood several decades ago, particularly in the US, but film-based tests did not become widespread worldwide. Panel Coker and micro-oxidation tests [1] never became official methods of globally accredited standards, such as ISO, DIN or ASTM, most likely due to repeatability issues. Thermo-oxidative Engine Oil Simulation Test (TEOST-MHT) using ASTM D7097, PDSC-based ASTM methods D6186 and D5483 for oils and greases along with several other standards are the only options for film degradation tests. Unfortunately, none of them achieved broad recognition in specification sheets, mostly because the results were often confusing and dependent on the skills of operators.
Thin film degradation tests for lubricants Film uniformity is usually a predominant problem in these tests. Panel Coker and TEOST-MHT don’t directly specify its thickness. In micro-oxidation (e.g. K29200 version) or PDSC, especially its SFI version and the film thickness is specified, but the film itself does not stay uniform, see Figure 2.
Figure 2. Comparison of typical thickness profiles of oil sample films in micro-oxidation (left), PDSC (centre) and current tests (right)
Figure 1. Diversity of chemical processes during oil degradation in thin films and bulk volume
One way of obtaining a much more uniform film thickness is to polish the sides of the holder to reduce the possibility of draining the film downwards by capillary forces. Films as thick as 500 µm were already successfully tested [3]. Temperatures between 50°C and 250°C or even higher can be used as well as various degrees of humidity, if below 100°C. Oil sample is coated on a freshly-polished coupon and exposed to selected heat and humidity. Air-flow has to be carefully controlled, but forced-draft ovens appear quite suitable. After some degradation the coupon can be carefully weighed to determine volatile losses and placed back to degrade further. Afterwards a
LUBE MAGAZINE NO.145 JUNE 2018 31
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