INCISO C DE LA LEY DE CONTRATACION

The average friction and wear performance of all the antiwear additives under the influence of dissolved and free water are shown in Figure 5-4 (a) and (b) respectively. In Figure 5-4 (a), friction reduction by ABE and KBE-containing tribofilms when the relative humidity (RH) was varied between 5 and 95% RH indicated that both borate tribofilms gave friction coefficient performance that was lower than tribofilms formed without free- or added-water. However, friction coefficient performance of ZDDP tribofilms was not significantly different to its friction response without added water. This frictional behaviour of ZDDP tribofilms with water contamination was similar to that observed in the works of Cen et al. [83].

The performance of borate tribofilms between 65 % and 95 % RH suggests an interaction with moisture to form certain borate compounds with low shear strength and lubricious tribolayer. The decreasing coefficient of friction trends are similar to that from boron permeated surfaces [145, 184]. The literatures studies in Chapter 2 had shown that friction reduction performance of borate additives is due to the formation of certain low shear strength boron compounds within the friction zone. Friction results shown in Figure 5-4 (b) indicated that changes in free water content in oils containing ABE was lower than KBE and ZDDP up to 1.5 wt. % before drastically increasing by about 28 %.

In addition, ZDDP friction response at different added water concentrations was observed to be comparable to KBE-based tribofilms; except at 2.5 wt. % water content. Literature studies in chapter 2, had attributed friction reduction performance to the presence of low shear strength magnetite film (Fe3O4) and the protective oxide

film formed by oxygen could reduce metal-to-metal contact under boundary lubrication [274, 275]. The relative antiwear performances of tribofilms from all the additives-containing oils and contaminated with free-water will be assessed and compared to the wear rates of tribofilms when there was no free-water added to the oil as shown in Figure 5-4 (b). The results indicated that tribofilms from all the additives gave increased antiwear performances up to 1.0 wt. % free-water in oil. However, when the free-water in the oil rose to 1.5 wt. %, the antiwear performance of ABE-containing tribofilms reduced, unlike KBE and ZDDP.

106 (a)

(b)

Figure 5-4 Average tribological test results due to; (a) dissolved water for 3 hrs at 19o_{C, and (b) free water for 6 hrs at 80}o_{C. Error bars indicates standard}

deviations of three repeated experiments

0 1 2 3 4 5 6 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0 20 40 60 80 100 Plat e W ear Rat es (x 10 - 19 m 3/N -m) Coe ff ici en t o f friction (C OF) Relative Humidity (%)

ABE KBE ZDDP ABE KBE ZDDP

0.2 0.7 1.2 1.7 2.2 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0 0.5 1 1.5 2 Plate We ar rates (x 10 -19 m 3/N -m) Coe ff ici en t o f friction (C OF)

Concentration of added distilled water in oil (wt. %)

ABE KBE ZDDP ABE KBE ZDDP

COF Wear rates

107

As the level of free-water contamination of oils containing the additives was increased to 2.0 wt. %, the antiwear performance of ZDDP tribofilms was reduced, unlike the enhancement provided by ABE and KBE. These results are consistence with similar work in the literature on the interaction of humidity with tribofilms from boron coated systems [145, 184, 185]. However, wear rates of ZDDP-based tribofilms at 0.5 wt. % was observed to increase with increasing water content that was consistence with increasing wear trends with respect to increasing free-water content of the oil in the work of Cen et al. [83]. This is an indication that tribofilms formed by oils containing KBE additives contaminated by 2.0 wt. % free water formed a more effective antiwear film ABE and ZDDP.

5.5 Summary

Tribological test results are summarized as follows:

 At 0.5 wt. % additive concentration, tribofilms formed by oils containing boron gave better tribological performance than ZDDP

 The tribofilms formed at 2.5 wt. % concentration by synthetic borate additives (ABE and KBE) gave comparable tribological performance to ZDDP, unlike BTE that gave catastrophic wear rates

 The tribofilms formed by ZDDP-containing oils had been shown in the literature to be influenced by bulk oil test temperature and wear process. In this study, borate tribofilms was also shown to be influenced not only by temperature, but also by the sliding process

 The durability of borate tribofilms in terms of friction coefficient performance was found to be comparable to ZDDP at 19, 80 and 100oC tribotest temperature, but were significantly better than ZDDP at 135oC

 The antiwear performance of tribofilms from ZDDP-containing oils was found to increase with increasing temperature. This is unlike tribofilms from oils containing boron additives that gave decreasing antiwear performance at 80 and 100oC. However, at 135oC tribotest temperature, tribofilms from oils containing ABE and ZDDP gave decreased antiwear performance

 The durability of tribofilms from oils containing ABE and KBE in terms of wear rates were observed to be better that ZDDP at 19oC. However, tribofilms

108

from oils containing ZDDP gave better durability than borate-containing oils at 80 and 100oC. In addition, enhanced durability in terms of wear rates was provided by tribofilms from oils containing KBE compared to ZDDP at 135oC  Tribological tests results indicates that in dry air, tribofilms formed by

ZDDP-based oils gave better antiwear performance than oils containing borate additives. However, lubricious boron compound was formed on the borate tribofilms that provided better friction reduction than ZDDP in moisture-rich environment

 As humidity of the surrounding environment increases, antiwear performances of tribofilms formed by the borates increases, but that of ZDDP decreases. This result confirmed the earlier results from the literature that dissolved water contamination did not enhance the antiwear functions of tribofilms formed by ZDDP, unlike those formed by oils containing synthetic borate additives  Tribological test results on the effects of free water contamination on the

antiwear functions of borate tribofilms indicated a positive synergy. This is unlike tribofilms formed by oils containing ZDDP additives that gave adverse antiwear performance when the free-water added into the oil exceeds about 1.5 wt. % concentration.

In order to fully understand the tribochemistry that took place at variable; additive concentration, temperature and sliding time/process, induced tribo-oxidation due to humidity and free-water in the oil, further surface characterization of the tribofilms are necessary. The results of physical and chemical characterization of tribofilms formed under variable test conditions are presented in subsequent chapters. Physical characterization of tribofilms is to provide answers on how changes in morphology, thickness and hardness of the tribofilms affect their tribological behaviour. On the other hand, chemical analysis of the borate tribofilms will elucidate on how the friction reducing and antiwear mechanisms of borate tribofilms are influenced by different extrinsic test conditions.

109