Effect of oil quality on electrostatic charge generation and transport

Scuffing detection of TU3 cam–follower contacts by electrostatic charge condition monitoring

Unsteady streaming flow based TENG using hydrophobic film tube with different charge affinity

Mineral oils are commonly used for cooling and electrical insulation purposes in electrical power transformers. The physicochemical process appearing at the Pressboard/Oil interface, leads to the formation of an Electrical Double Layer (EDL). Therefore the interface is polarized by the EDL, the charge of one sign is generated within the solid medium and the opposite charge remains in the liquid medium. The process of flow electrification is due to the convection of the diffuse layer inside the liquid. The test loop was recently optimized with the addition of an energized wrapped copper electrode in the sensor. This modification allows studying the effect of an external electric field on the charge production and the distribution of the EDL : in the liquid via the streaming current (global charge production) and in the pressboard via the capacitive current. This capacitive current allows quantifying the accumulated charge within the pressboard plate (local charge production). Therefore, the dynamics of the accumulated charge refer to two EDL affected in the same way by the DC electric field. This paper proposes to show the influence of an external DC electric field on the accumulated charge within the pressboard as well as on the streaming current.

Behavior of the charge accumulation at the pressboard/oil interface under DC external electric field stress

International standards require lubricant formulators to develop additive packages that increase fuel economy,
reduce environmental impact and minimise wear over ever increasing service intervals However, additive
behaviour and interactions between additives is not well understood An absence of real-time technology has
hindered understanding of additive behaviour and interaction between additives in tribo-contacts The work
presented in this thesis assessed whether electrostatic charge monitoring, which is sensitive to changes in
surface chemistry, can offer insight into additive-surface behaviour and how this affects tribological
performance
Electrostatic sensors were deployed on tribological test apparatus used to simulate: engine valve-train,
manual transmission synchroniser and automatic transmission clutch tribo-contacts Additive performance in
these simulated contacts was assessed by electrostatic surface charge measurements and cross-correlated with
friction, wear and off-line surface chemistry analysis
The first study involved electrostatic monitoring of valve-train contacts, which was a continuation of
previous electrostatic monitoring work carried out to relate wear and electrostatic charge During a simulated
TU3 cam-follower wear test, charge signals underwent a sign inversion; this was due to a transition between
tribocharging of the lubricant under running-in and mild wear, and contact potential difference generated at
the onset and progression of adhesive wear It was found that charge signals differed between different oils,
which could not be explained by the wear performance alone; this indicated that lubricant chemistry
significantly affected charge generation
Dynamic charge peaks produced by simulated valve-train contacts lubricated with zinc
dialkyldithiophosphate (ZnDTP) additive were related to the stripping of the tribofilm The source of this
charge peak was an increase in negative charge, which correlated with a dominance of phosphate and
sulphate (anions) compared to zinc (cations), as the film was worn away When friction modifier (FM) and
dispersant additives were combined to lubricate a simulated wet clutch contact, x-ray photo-electron
spectroscopy (XPS) analysis and friction data indicated that the dispersant dominated the tribofilm
composition; evidenced by nitrogen levels and friction levels similar to the dispersant alone Electrostatic
charge data showed that competition for surface sites is an extremely dynamic process; as indicated by charge
levels which alternated between the levels of the FM and dispersant alone When a potassium borate additive
was added to a polysulphide additive containing oil during testing (seeding), the charge data showed a
transition from being predominantly positive to predominantly negative This correlated with the formation
of a borate rich (anions) layer on top of the sulphur film
Further novel tribological discoveries were found through investigation into these additive-surface charge
behaviours An underdeveloped ZnDTP tribofilm, which predominantly contained sulphur, was formed at
room temperature; the sulphur promoted tribochemical wear and resulted in a pro-wear affect for primary
ZnDTP The combination of FM and dispersant showed a clear antagonism resulting in increased wear
compared to the individual additives
XPS of brass and steel simulated synchroniser contacts lubricated with potassium borate and polysulphide
identified the affect of surface chemistry on film formation For the potassium borate additive: potassium
(cation) preferentially adsorbed to brass, and borate (anion) preferentially adsorbed to steel Seeding the
borate additive into the oil, and therefore the contact, produced the same film composition and structure as
the combination from the start This inferred that the polysulphide additive drove initial film formation
Seeding is an extremely powerful technique, but its use is almost absent in the literature
It has been shown that lubricant chemistry dominates charge levels even in a wearing contact Electrostatic
monitoring is sensitive to the type of additive-surface adsorption and interaction between additive and
additive or contaminant Also, cross-correlation of surface charge and tribofilm chemical composition
demonstrated that electrostatic sensors have the ability to detect tribofilm kinematics This is a significant
finding; no current real-time technique used to monitor tribofilm kinematics derives its measurements from
the tribofilm composition Although interpretation of electrostatic charge data is currently an intensive
process, in the long term, lubricant development could see a move towards charge informed formulation

The feasibility of using electrostatic charge condition monitoring for lubricant additive screening

Streaming electrification under dc and ac electric fields was investigated by using test pipes and oil circulation apparatus. We used two different test pipes, one made of pressboard, and the other of insulating paper. At first, the amount of static charges which flowed into the test pipes was measured without electric field. For the experiment using an insulating paper pipe, when the oil temperature increased, the measured current curve shows a peak. When the velocity increased, the current increased proportionally from the 2nd to 4th power of velocity. For the experiment using a pressboard pipe, the current increased proportionally from the 3rd to 5th power of temperature, and by the 1st power of velocity. Then, we applied ac and dc electric fields on the test pipes and the current from a relaxation tank to ground was measured. The ions in oil carry the charges, so an electric field affects ions in the oil. It makes the mathematical approach to streaming electrification difficult. We know that as the electric field intensity increased, the current due to selective adsorption of the test pipe was increased. The current vs. velocity curve under ac electric field had a peak point in velocity.

Jung-II Jeong

Papers

Streaming electrification of thin insulating pipes under electric field