Additive-Additive Interactions

Interactions of two or more additives may lead to either synergistic or antagonistic effects on the tribological performance of component in contact.

It was reported by Rounds [115] that ZDDP in combination with metallic dithiocarbamate oxidation inhibitors, primary alkyl amine friction modifiers, sulphur and chlorine containing EP agents showed a detrimental effect on the wear performance; whereas, addition of detergents, dispersants, oxidation inhibitors, VI improvers and EP agents to ZDDP showed little or no effect on the wear performance. The literature on the interactions of

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additives and the related tribological performance will be reviewed in this section.

3.2.7.1. ZDDP Interactions with MoDTC

In general, the performance of engine oil additives, such as; ZDDP may be influenced when used in conjunction with other additives. This effect could be synergistic or antagonistic and therefore a great deal of research has carried out to study the possible effects [116] from which a considerable amount was focused on the ZDDP interaction with MoDTC.

In the literature several reports showed that MoDTC in combination with ZDDP was more effective in friction reduction [117-119]. Muraki et al. [119]

showed that under rolling-sliding conditions, combination of ZDDP with MoDTC resulted in both lower friction and better wear performance suggesting a synergistic effect of ZDDP on frictional behaviour of MoDTC.

However, mechanism by which ZDDP could promote MoS2 formation was not provided. ZDDP tribofilm formation was shown to be responsible for more effective friction reduction by MoDTC. It was shown that friction started to drop as soon as the ZDDP tribofilm was formed [120].

Martin et al. [121] reported a synergistic effect on both fiction and wear when ZDDP was used together with MoDTC compared to individual ZDDP or MoDTC. They suggested that the wear is reduced due to the reaction of MoO3 and possible iron oxides with zinc polyphosphate leading to their elimination. This would also preserve the pure MoS2 from oxidation which in turn could improve the friction performance. In contrast, Morina et al. [35]

found that ZDDP showed an improved wear performance than ZDDP/MoDTC. On the other hand, Kasrai et al. [117] showed that almost the same wear was given by ZDDP and ZDDP/MODTC whereas the friction was reduced for ZDDP/MoDTC compared to MoDTC or ZDDP alone.

Sogawa et al. [122] studied the contribution of MoDTC and ZDDP in providing sulphur for MoS2 formation in a ZDDP/MoDTC-containing solution.

They realized that about 40% of the required sulphur for MoS2 formation is

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derived from ZDDP which clearly shows the interaction between ZDDP and MoDTC to form MoS2.

MoDTC has been shown to have a detrimental effect on the structure of ZDDP tribofilm. It was shown that a thick patchy pad-like tribofilm which was formed by ZDDP alone became much thinner and the patchy structure was vanished [123]. In another work, MoDTC/ZDDP tribofilm was found to be rougher than ZDDP alone which could elucidate any relation between lower observed friction and the transformation to elastohydrodynamic lubrication [92].

3.2.7.2. ZDDP Interaction with Detergents and Dispersants

Detergents and dispersants are key additives in oil formulation as they will keep the insoluble products in suspension which would later be removed by filters. Detergents have also been shown to offer anti-wear properties by forming carbonates in the wear scar [115, 124, 125]. Metallic detergents were seen to have an antagonistic effect on the wear performance of ZDDP [124, 126, 127].

ZDDP effectiveness was deteriorated due to the interaction with overbased metallic detergents. This behaviour was attributed to the competition between these two additives for surface sites. It was shown that Ca²⁺ ions took over the Zn²⁺ in the polyphosphate structure of tribofilms, leading to the formation of short chain polyphosphate[124]. This was in agreement with a previous work by Willermet et al. [125] who showed the formation of ortho- and pyro-phosphates with lower molecular weight than phosphates when Zn partially replaced with Ca.

In agreement with other works, Kasrai et al. [127, 128] reported ineffectiveness of ZDDP in combination with detergents. They investigated the effect of overbased calcium sulphonates in formation of sulphur and phosphorous species. Surface analysis of the tribofilms showed that the presence of detergent along with ZDDP resulted in the formation of calcium phosphate instead of long chain polyphosphates. The higher wear given was

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thought to be related to the higher hardness of calcium phosphate formed from detergent in the solution compared to the wear given ZDDP only.

Yin et al. [72] showed that calcium phenate detergents have an adverse effect on the ZDDP film formation even at low temperatures whereas calcium sulphunates interaction with the absorbed ZDDP occurred only at high detergent concentrations.

Rounds [115] reported that succinimide dispersants increased wear when added to ZDDP solutions in 4-ball wear tests. Similarly, Shiomi et al.

reported the same effect when used in valve train tests [129]. However, this adverse effect was diminished by borating the succinimide. Formation of a borate component in the anti-wear film was responsible for this improvement [130]. The antagonistic effect of dispersants on wear reduction was attributed to the reduction in the amount of ZDDP available for film formation by forming a complex. The degree of their impact on wear, however, depends on the strength of the complexes formed with various dispersants and with amines [125]. However, no evidence was found for such behaviour by adsorption studies [131]. Borated dispersants, for instance, could contribute to this behaviour by formation of a borate component in the anti-wear film [130]. Succinimide together with other additives increases the decomposition temperature of ZDDP. This will promote scuffing wear at lower oil temperatures when using a succinimide and other additives with ZDDP [130].

Smith et al. [132] showed that with a simple ZDDP mineral oil solution, the film is mainly composed of phosphate which is bonded to the ferrous substrate. When both detergent and dispersant are used in the ZDDP solution, the film thickness was increased but in a more patchy shape which has no clear underlying sulphide layer. This was attributed to the competition for surface sites between the wider range of surface-active additives present in the formulation. Dispersant only formed a thin layer of nitrogen-containing material on the outer surface [132] and did not mainly contribute in the film structure. The schematic structure for the film formed with ZDDP-containing lubricant and ZDDP/detergents/dispersant solution is shown in Figure 3-16.

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(a) (b)

Figure ‎3-16 Schematic structures for films generated by (a) ZDDP and (b) ZDDP+ Detergents+ Dispersant [132].

It was also found that the mechanical properties of the tribofilm formed from ZDDP varied with the addition of the detergent to the oil formulation. The indentation modulus was measured to be lower for the film formed from ZDDP/detergent compared to ZDDP tribofilm. Contribution of CaCO3 in the tribofilm was thought to be the main reason for such measured values [91].

3.2.7.3. Additive Interactions in Fully Formulated Oils

Engine oil formulations contain different types of additives including anti-wear, extreme pressure, antioxidants, dispersant, detergents, corrosion inhibitors, etc. The physical and/or chemical properties of the base oil, or the (ferrous) surface might be affected by individual additives present in the lubricant formulation. However, the mechanisms by which each additive contributes to the overall performance of the surface/lubricant is complex and, thus studies on the tribofilms formed from fully formulated oils are limited.

Using a fully formulated oil (FF) which is commercially available, wear of 52100 steel was reported to increase substantially compared to ZDDP alone. Figure 3-17 shows the time dependent wear scar widths (WSWs) for the films formed from FF oils compared to ZDDP alone. Using X-ray Absorption Near Edge Structure (XANES), it was shown that tribofilm formed

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from the fully formulated oil was rich in medium chain Ca phosphate. It was concluded that ZDDP does not play its role fully as an anti-wear agent (by forming Zn-phosphate) in FF oils, and only initiate the film formation. After the initial film formation, ZnS and Ca phosphate film grow. ZDDP-derived tribofilm was found to be mainly consisted of ZnS (78%) whereas the remaining 22% Zn was identified as Zn Phosphate. In addition. MoS2

formation was also evidenced in the tribofilm whereas no Mo-oxide was present in the film [133].

Figure ‎3-17 Wear scar width as function of rubbing time [133].