Lube-Tech
energy of the material surfaces. However, in the direct contact of lubricant and surface, the interfacial tension dominates, which cannot be measured directly, and which only indirectly can be determined via the contact angle. If no equilibrium occurs at the interface, the measurement is made more difficult because the contact angle decreases as a function of time. Furthermore, in the field of precision mechanics, capillary forces are increasingly superimposing the wetting effects in narrow gaps, on rough surfaces and small drop volumes, and thus the transferability of measuring results into practical application is difficult.
Another point to consider is that under practical running conditions of lubricated sliding bearings, frictional heat is created, which results in an acceleration of wetting processes due to Marangoni convection effects2
. The oil actively creeps out of the
bearing (and lubrication zone respectively) onto colder surfaces.
In order to better assess long term wetting effects in real parts, a method may be used, that allows one to visualize even thin oil films on surfaces. With the reflection-contrast-equipment, oil droplets which, for example, have been placed directly on the parts to be lubricated, may be monitored. A camera placed over the test setup makes time lapse recordings to visualise the effects which occur. Additionally, by raising the temperature of the test specimen, wetting and creeping effects can be accelerated, too. Results obtained give important information on the influence of oil type, surface roughness, material, etc. In Figure 1 four different situations are given of two oils, a polyglycol with high surface tension of 36mN/m (upper line) and a silicone oil with low surface tension (21mN/m, below), where droplets with oil volumes of 2µl have been placed on a glass slide, the left side with a smooth surface and the right side with a rough surface. Only on the smooth surface, the polyglycol droplet stays with a stable contact angle, on the rough surface even the polyglycol begins to creep
Figure 1. Spreading behaviour of polyglycol (above) and silicone droplets (below) on glass with smooth (left) and rough surface (right)
A typical exposure for lubricated fine mechanic components is given in Figure 2. Here olephobic coatings are used to realise forlife lubrication by a purposeful design of the contacting surfaces. A silicone oil on a rough plastic surface remains there with a stable contact angle, if a certain oleophobic layer has been applied (lower part). On the nontreated area (upper part) the silicone oil wets completely.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.117 page 2
through the roughness. The silicone oil immediately starts to wet the surfaces, on the rough surface to a much higher extent.
Figure 2. Spreading behaviour of silicone oil on a rough plastic surface with (zone below) and without olephobic coating (upper zone)
LUBE MAGAZINE NO.146 AUGUST 2018
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