Elsevier

Wear

Volume 301, Issues 1–2, April–May 2013, Pages 747-752
Wear

Tribological and thermal characteristics of reduced phosphorus plain ZDDP oil in the presence of PTFE/FeF3/TiF3 under optimized extreme loading condition and a break in period using two different rotational speeds

https://doi.org/10.1016/j.wear.2012.11.034Get rights and content

Abstract

There is a recent concern about the effects of P-containing anti-wear additives in regard to emissions reduction in automobiles and trucks. Research described here is aimed at providing good friction and wear performance while reducing suspected catalyst poisoning effects from compounds like zinc dialkyl dithiophosphate (ZDDP). The effects of adding submicron particles of titanium fluoride or iron fluoride along with polytetrafluoroethylene to ZDDP were investigated with bearing steel using a sliding ball-on-ring apparatus that operates under boundary lubrication. An optimized load of 336 N (2.6 GPa Hertzian contact pressure) and 60 μl of oil quantity were used to run several tests using 0.05 P% plain ZDDP with and without additives (1% catalysts and 2% PTFE) under two different speeds (3.14 m/s for the first 943 m (5000 revolutions) then 2.2 m/s until failure or 18,856 m (100,000 revolutions) whichever comes first) and steady state speed of 2.2 m/s. Special attention was paid to the effects of running-in and temperature. Therefore, a break in period of 3 min was chosen when the tribotest temperature reached 100 °C. Results showed that 0.8% FeF3+0.2% TiF3 and 2% PTFE additives when combined with 0.05 P% (phosphorus) ZDDP plain oil performed better than that when ZDDP is used alone. Thermally treated surfaces showed better coating and support our findings. A good transfer layer under extreme pressure was responsible for improving the antiwear resistance of additives and for lowering the wear volume and wear debris as it is evident by SEM and TEM images.

Introduction

Increasing demand for environmentally friendly oil drives many researchers around the world to understand the behavior of oil additives under different loading and boundary conditions. Zinc dialkyl dithiophosphates (ZDDP) have been used as additives in engine oil for the past decades owing to their antiwear resistance and low price [1]. Reducing phosphorus by using less ZDDP can improve the life of catalytic converter in a combustion engine. The P-contaminants once deposited on the converter are not effectively removed at high temperature [2]. Thermo gravimetric (TGA) measurements indicate a better ZDDP decomposition temperature in the presence of catalysts and PTFE [3], [4]. Improvement in lubricant formulation by imposing limits on ZDDP phosphorus and replacing it by fluorinated catalysts and PTFE has been investigated under extreme boundary conditions by several researchers [5], [6], [7].

DOE analysis of variance (ANOVA) from previous testing indicates very significant wear volume equation (mm3) for plain ZDDP oil with catalysts and PTFE [8]:F(x,y,z)=0.146+6.54x-0.015y-3.7zx1.7xy+0.1yzwhere F(x, y, z) is the wear volume that is a function of all additives combinations, x is the percentage of phosphorus in ZDDP oil, y is the percentage of PTFE, and z is the percentage of catalyst (FeF3+TiF3). Minimization of the wear volume revealed that approximately 2% PTFE+1% catalyst is adequate to reduce ZDDP phosphorus to 0.05 P% and thus minimizing the wear volume.

In this study thermal and tribological experiments were conducted using the above catalysts and PTFE additives with 0.05 P% plain ZDDP oil to investigate the antiwear performance. A break in period, complete stop for 3 min before resuming the test, was part of the testing protocol when the extreme boundary condition temperature reached 100 °C, then the test will resume until 18,856 m (100,000 revolutions) or failure. 3 min break in period was a result of extensive testing [8], and it simulates the condition of heavy traffic in large cities where complete engine shut down is usually involved. Design of Experiment (DOE) was used to optimize the contact load and oil quantity under extreme conditions and different rotational speeds. High resolution SEM and TEM have been employed to chemically and structurally characterize the additives and wear debris that are incorporated in the tribofilms under thermal and tribological conditions.

Section snippets

Experimental details

A modified ball on cylinder Plint T53 SLIM wear tester shown in Fig. 1 was used for all tests. A surface finish of 0.1 Ra was selected and examined using a profilometer (Mahr Perthometer M1). Details about the machine, severity of the boundary test, establishment of a consistent surface finish, were provided elsewhere by Nehme et al. [7].

A secondary ZDDP (7.2 wt% P) containing a mixture of neutral and basic form was used to prepare all plain oil solutions. Finely dispersed submicron powder

Thermal results

Fig. 2(a–d) presents two typical dispersions of finely milled powder (FeF3+TiF3) and unmilled powder catalysts. The submicron powder dispersion is homogenous and well distributed when compared to other dispersions. The effects and significance of PTFE and FeF3+TiF3 catalysts on the surface tension of 0.1 μm Ra bearing steel coupons revealed significant results in Table 2. The water drop contact angle improved when TiF3 and FeF3 weight percentage concentration increased to 1% in 0.05 P% plain ZDDP

Conclusion

Friction and wear losses improved greatly in the presence of 1% (FeF3+TiF3)+2% PTFE with 0.05 P% ZDDP and break in period of 3 min under two different speeds and high contact load (2.6 GPa Hertzian contact pressure). The improvement in wear performance with the addition FeF3/TiF3/PTFE is demonstrated with several analytical instruments: (1) amorphous film containing P compound provided by Transmission Electron Microscopy (TEM) were able to protect the surface under the higher loading condition and

Acknowledgment

The author thanks University of Balamand and its affiliates and the University of Texas at Arlington for providing support.

References (9)

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