1. Study of the Lubricant Additives Using Electrochemical Techniques
Xiaoyin Xu, Hugh Spikes and Nigel Brandon
Department of Mechanical Engineering
Imperial College
London

2. What is Electrochemistry?
Electrochemistry is the study of chemical reactions which occur due to electron transfer at an interface.

3. Electrochemical techniques can be used
to measure the effective concentration of solutes in solution.
to study redox processes at solid surfaces, and thus investigate reaction mechanisms

4. Aims of the research work
measure the effective concentration of additives in oils
study lubricant additive reactions on metals
(c) study the effect of applied potential on friction/wear

5. WE RE CE
Potentiostat CE WE
Electromagnetic vibrator RE
Insulator
Combination of potentiostat and HFRR to study the influence of applied potential on friction

6. Test conditions for HFR rig

7. ioct-ZDTP (isooctyl Zinc dialkyldithiophosphate)
ZDTP used in this study
R: (CH2)5CH(CH3)2
ioct-ZDTP (isooctyl Zinc dialkyldithiophosphate)

8. Cyclic voltammogram of isooct-ZDTP in 0.8M LiClO4 DEA
Study of Boundary Lubricating Additives using Electrochemical Techniques
-1.00E-09
2.00E-09
5.00E-09
8.00E-09
-3.0
-2.0
-1.0
0.0
1.0
2.0
potential vs Pt (V)
Current (A)
with 1% ZDDP
Cyclic voltammogram of isooct-ZDTP in 0.8M LiClO4 DEA
(scan rate: 100mv/s)
(3) Xu, X, Brandon, N., Spikes, H.,Presentation on 28th Leeds-Lyon symposium on Tribology, Sep. 2-7, 2001

9. 0.5
without ZDTP
0.4
wear scar of ball/mm
0.3
0.2
0.1
with 1% ZDTP
-2.5 -2
-1.5 -1
-0.5
0.5 1
1.5 2
potential vs Pt (V)
Effect of electrode potential on the ball’s wear scar in 0.8M LiClO4 DEA

10. Possible dimerisation of ZDTP at positive potential:
[(RO)2PS2]2Zn  Zn2+ + [(RO)2PS2]2 + 2e-

11. Similar work is taking place using hexadecane
Effect of electrode potential on wear scar of ball in hexadecane with 0.01M TDDATPh-FB as electrolytes with and without 1% wt.ZDTP

12. Electrode current at different potential during HFR test with 1% wt
Electrode current at different potential during HFR test with 1% wt. ZDDP in hexadecane at 60oC

13. Conclusions
High frequency reciprocating tests have shown that ZDDP is effective in reducing wear only under oxidising conditions.
At positive potential (> 0.8V), ZDTP is oxidised to disulphide [(RO)2PS2]2, which may adsorb on electrode surfaces to produce a protective layer.