1 Introduction
2 Experimental Materials
Additive abbrev. | Additive details |
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ZDDP1 | Mixed primary/secondary zinc |
ZDDP2 | Primary zinc |
ZDDP3 | Secondary zinc |
D1 | PIBSA polyamine dispersant (high MWt.) |
D2 | PIBSA polyamine dispersant (no information) |
D3 | PIBSA polyamine dispersant (high MWt./high functionality) |
D4 | PIBSA polyamine dispersant (high MWt./medium functionality) |
D5 | PIBSA polyamine dispersant (high MWt./medium functionality) |
D6 | PIBSA polyamine dispersant (medium MWt./low functionality) |
D7 | PIBSA polyamine dispersant (high MWt./high functionality) |
D8 | PIBSA polyamine dispersant (very high MWt./high functionality) |
3 Experimental Methods
3.1 HFRR Wear Tests
Ball and disc material | LESCALLOY® 52100 VAC-ARC® high performance bearing steel |
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Ball hardness (HV) | 880 ± 3 |
Disc hardness (HV) | 772 ± 57 |
Ball roughness, Ra (nm) | 5.0 ± 0.1 |
Disc roughness, Ra (nm) | 5.1 ± 0.2 |
Load (N) | 3.92 (0.4 kg) |
Maximum hertz contact pressure (GPa) | 1.03 |
Frequency (Hz) | 50 |
Stroke length (mm) | 1 |
Test temperature (°C) | 100 |
Test duration (min) | 60 |
3.2 Scanning White Light Interferometer (SWLI) Wear Measurement
3.3 Rheology Tests
3.4 Dynamic Light Scattering for Particle Size Measurements
3.5 Rolling/Sliding MTM-SLIM Tests for Tribofilm Formation and Removal
3.6 Reciprocating MTM to Study Initial ZDDP Film Formation
3.7 SEM-EDS Surface Analysis
3.8 TEM-EDS Tribofilm Analysis Using FIB Milling Technique
4 HFRR Wear Test Results
4.1 Effect of Nitrogen Concentration
4.2 Effect of Phosphorous Concentration
4.3 Effect of CB Concentration
5 Discussion and Ancillary Tests
5.1 Possible Origins of Wear Behaviour
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the CB particles must enter the rubbing contact;
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they need to adhere to and accumulate on the surface;
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the ZDDP must react with the rubbing surfaces;
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the CB particles must abrade the ZDDP tribofilm as fast as it forms.
5.2 Influence of Dispersant Concentration on Viscosity and Particle Size
5.3 Surface Adhesion via SEM-FEG
5.4 Influence of Dispersant on ZDDP Reaction Rate
5.5 Origins of the Influence of Dispersant on ZDDP/CB Wear
6 Conclusions
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HFRR wear tests show that when ZDDP is added to solutions of dispersant and CB, much more wear occurs than when ZDDP is left out. ZDDP thus has pro-wear properties in CB solutions, confirming previous studies.
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The rate of wear seen with ZDDP and CB increases with phosphorus concentration, suggesting that the ZDDP reaction with ferrous substrate is a rate-determining step in the wear process.
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Further HFRR tests show that both dispersant concentration and type strongly influence the wear induced by CB in ZDDP-containing oils. Most dispersants show a maximum of wear at ca 0.02 wt% N, which is around the concentration used in fully formulated engine oils. Much less wear is seen at very low and at high dispersant concentrations.
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A few dispersants do not show this high wear behaviour at intermediate concentrations and instead enable low wear over the whole concentration range studied. These appear to be succinimide dispersants combining high functionality and high MWt.
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The increase in wear with increasing dispersant concentration at very low dispersant concentrations is believed to originate from an increase in the ability of CB particles to enter the rubbing contact as they become better dispersed.
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Two possible origins of the lower wear seen at high dispersant concentrations than at intermediate concentrations for most dispersants have been identified. These are: (i) at high concentrations dispersants may interact with ZDDP molecules in solution or at surfaces to reduce the latters’ reactivity and thus slow ZDDP reaction with rubbed surfaces; and (ii) at high concentrations dispersants may protect the initial ZDDP tribofilm from abrasion by adsorbing effectively either of the tribofilm itself or on the CB particles.
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The rates of ZDDP film formation by CB-free ZDDP blends containing dispersants that give high wear at intermediate dispersant concentrations are similar to those formed by blends containing dispersants that give low wear. This suggests that mechanism (i) above is not prevalent. Preliminary studies of the very thin ZDDP tribofilms present on rubbed surfaces when ZDDP and CB are present indicate that dispersants that enable low wear at intermediate concentrations are able to protect iron sulphide films on the rubbed surfaces, in support mechanism (ii) above.
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It is thus proposed that when ZDDP and soot are both present in an oil, the ZDDP reacts with the rubbed surfaces to form a film of iron sulphide that may be immediately abraded by the soot particles, resulting in rapid corrosive-abrasive wear. Key to this is that the initial tribofilm formed by ZDDP is iron sulphide and this is normally subsequently overlain by a protective zinc phosphate film [28]. However soot prevents build-up of this zinc phosphate film, thus allowing iron sulphide formation and removal to continue unchecked.
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High concentration levels of dispersant, and in particular some highly functionalised dispersants, appear to be able to protect this iron sulphide film from abrasion and so prevent the very high levels of wear otherwise seen with oils that contain both ZDDP and soot.