Effect of temperature and wear process

The friction coefficient performance of synthetic borates and ZDDP tribofilms for a range of temperature and wear process as described in section 3.43 and section 3.44 are shown in Figure 5-3 (a) and (b). The results shown in Figure 5-3 (a) indicated that friction coefficient performance by tribofilms from ABE and ZDDP-containing oils are not too different from one another (± 3.3 %) at 19oC when the sliding distance was doubled (from 3 h to 6 h). However, the friction coefficient increased when the sliding time was double on tribofilms formed by oils containing KBE additives at 19o_{C bulk}

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(a)

(b)

Figure 5-3 Average tribological test results for a range of temperature and wear process with error bars for three repeated samples to show the variability of the test results; (a) friction coefficient, (b) plate wear rates

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 3 h 6 h 3r3 3 h 6 h 3r3 3 h 6 h 3r3 ABE KBE ZDDP C oe ff ic ien t of f rict ion (C OF)

Oil Types and Test Duration

0 2 4 6 8 10 12 14 16 3 h 6 h 3r3 3 h 6 h 3r3 3 h 6 h 3r3 ABE KBE ZDDP Pla te W ea r Ra te (x 10 -19 m 3/N -m)

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In addition, tribofilms formed by each of the additives in base oil gave significantly higher friction coefficient when the sliding process was changed from 3h to 3r3; ABE (20%), KBE (13.0 %) and ZDDP (20%). This behaviour could partly be attributed to the effect of PAO or partly due to the effect of certain abrasive iron oxide that adversely affects friction reduction. At 80o_{C bulk oil test temperature, the friction}

coefficient results was higher than at 19o_{C. In addition, there was no significant}

changes in friction coefficient when the sliding distance was doubled (from 3 h to 6 h) on tribofilms formed by oils containing KBE (± 2.0 %) and ZDDP (6.0 %); but was considerably higher for ABE (24.0 %). However, when the sliding process was changed from 3 hr to 3r3 at 80o_{C, there was no significant increase in the friction}

coefficient performance of tribofilms from oils containing ABE and KBE additives. On the other hand, ZDDP tribofilms gave considerable increase in friction coefficient performance at 80oC when the sliding process was changed to 3r3 compared to 3 h tests. A similar friction coefficient response increase was observed from ZDDP tribofilms at different temperatures [21, 89]. The friction coefficient behaviour of tribofilms formed by ABE and KBE increased at 100o_{C when the sliding process was}

doubled (from 3 h to 6 h), unlike that of ZDDP that remained unaffected by this increase. A similar situation occurred when the sliding process was changed to 3r3 at 100o_{C. This is an indication that friction coefficient of ZDDP tribofilms is not}

significantly affected by sliding process at the test temperature of 100oC.

At 135oC tribotests temperature, the friction coefficient performance of tribofilms from all the antiwear additives increased when the sliding distance was increased from 3 h to 6 h. However, when the sliding process was changed from 3 hr to 3r3, tribofilms formed by the borates gave reduction in friction coefficient; ABE (11.0 %) and KBE (23%). On the other hand, ZDDP tribofilm gave increased friction coefficient (57.0 %) at 135oC tribotest temperature. This is an indication that durability of borate tribofilms in terms of friction coefficient is better than ZDDP at 135oC tribological test temperature. Hence, the different friction coefficient results from the antiwear films subjected to different temperatures and sliding distance could be related to the chemistry of their formation. These appears to depend on temperature and the sliding process as previously observed in the literature for ZDDP tribofilm [21, 47, 48]. The wear coefficient results shown in Figure 5-3 (b) indicated that all the antiwear additives gave higher wear rates with increasing temperature. However, wear rates of tribofilms from borate tribofilm increased up to 100o_{C temperature before decreasing}

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at 135oC to wear rate values that is similar to that one obtained at 19oC. This could be related to the physical and chemical nature of their tribofilms.

This behaviour was quite different to that of ZDDP tribofilm which gave decreasing antiwear performance that was consistent with results from similar study by Morina et al. [21]. At 100o_{C tribotests temperature, the boron-containing tribofilms both gave}

higher antiwear performance irrespective of the sliding distance. This behaviour could be related to the chemistry of the reacted layers at this temperature. For the borate-containing tribofilms, there were no significant changes in their antiwear performance when the sliding distance was doubled; unlike the antiwear performance of ZDDP antiwear films that varies with temperature and sliding distance. However, the antiwear behaviour of borate-based tribofilms at 3 hr and 6 hr test durations appeared to be sensitive to temperature rather than sliding process.

Durability performance in terms of wear rates during the 3r3 test conditions as shown in Figure 5-3 (b) indicated that borate-based tribofilms are more durable than ZDDP at 19oC. As temperature increases to 135oC, ZDDP tribofilms gave better durability performance than ABE and KBE-based tribofilms within this test temperatures; except at 135oC, where KBE-based tribofilms gave comparable antiwear performance to ZDDP.

In addition, durability of the borate tribofilms at 100oC was very poor when compared to ZDDP. This could be related to changes in the physical and chemical structure of the borates at this temperature. A tribological study of boron-containing oil at 100oC had observed weaknesses in the wear resistance potential of the borates at this temperature [273]. The result from this study could be used to elucidate the influence of borates and ZDDP in gear and lubricating engine oils as described by Kim et al. [81] and Komvopoulos et al. [82] respectively. Based on the results from this study, ZDDP in fully formulated oils containing borates plays a major role in providing better antiwear protection and durability than the borate additives at 100o_{C. On the}

other hand, antiwear performance and durability at ambient temperature and boundary lubrication regime could have been enhanced by borates in lubricating oils of IC engines during cold-starting operations.

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