Material-Dependent Wear Mechanisms of Soot in Presence of ZDDP

Chanaka Kumara, Jun Qu*

Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN

INTRODUCTION: Soot is generated from incomplete combustion of fuel during the engine combustion processes, particularly in diesel and gasoline direct injection engines. These soot particles may be blended in the circulating engine lubricant oil or discharged into the environment. The soot content in the engine oil has been reported to strongly correlate with engine wear. The soot-induced wear mechanism, however, is lacking consensus. Several hypotheses have been proposed in the literature,¹ such as  soot competing with antiwear (AW) additives by adsorbing onto contact surfaces, soot adsorbing on the AW additives to deactivate their functionality, soot abrading the tribofilms, soot abrading the contact surfaces, soot promoting oil degradation,  and soot accumulating at the contact inlet resulting in oil starvation. Some studies observed antagonistic interaction between ZDDP and soot or carbon black^1,2 (CB, as soot surrogate) while others³ did not. Considering distinct wear behavior reported for different steels, we suspected alloy dependency in the soot-induced wear process and, thus, conducted this study. The wear behavior and mechanism were investigated for four different steel alloys in seven contact material pairs when lubricated by CB-containing oils with and without ZDDP.

METHODS: CB particles were characterized using Transmission Electron Microscopy (TEM). A toluene suspension of CB particles was drop cast onto a copper grid and analyzed using a Hitachi HF-3300 TEM. Boundary lubrication tests were conducted to investigate the tribological behavior of PAO 4 cSt base oil containing CB (R250R) or ZDDP or both. The CB and ZDDP concentrations were 1.0 wt% and 0.8 wt%, respectively. A PIB dispersant (1.0 wt %) was used to help suspend the CB in the oil. For fair comparison, PIB was added to all test lubricants. Tribological tests were conducted on a Plint TE-77 tribotester using ball-on-flat reciprocating sliding configuration. Two types of steel balls (10 mm diameter) were used: AISI 52100 bearing steel and M50 tool steel. Four flat materials were:  AISI 52100 bearing steel, A2 tool steel, M2 tool steel, and silicon nitride. Tests were carried out at 150 ^oC under 100 N for 1000 m of sliding. Friction force was captured in situ and wear volume was quantified using a Wyko NT9100 optical profiler. Morphology of the worn surfaces was analyzed by SEM/EDS.

RESULTS AND DISCUSSION:  Morphology of the CB particles is shown in Fig. 1a with particle sizes in the range of 50-100 nm. The wear results of the 52100 steel ball-M2 steel flat pair are shown in Fig. 1b. As expected, adding the ZDDP alone (w/o CB) into the PAO effectively protected both the ball and flat, and blending the CB alone (w/o ZDDP) into the PAO oil caused significant wear increase for both the contact surfaces. It was noticed that the ball wear was significantly higher than the flat wear, with a ratio (52100 steel ball wear / M2 steel flat wear) around 5:1 in the neat PAO base oil, PAO+ZDDP, or PAO+CB. However, when both the ZDDP and CB were added to the base oil, the wear ratio between the ball and flat surprisingly flipped over from 5:1 to 1:4, a 20X change. Basically, adding ZDDP into the PAO+CB oil protected the 52100 steel ball with a wear reduction of >70%, but increased the M2 steel flat wear by >5X. To our knowledge, no such observation has been previously reported in the literature. Tests were then conducted using the 52100 steel ball sliding against an A2 tool steel flat (different composition than M2 steel and less wear resistant) and similar wear behavior was observed: while the total ball and flat wear volumes were about the

same when lubricated by PAO+CB and PAO+CB+ZDDP, the ball/flat wear ratio was reduced from more than 2:1 (CB alone) to less than 1:3 (CB+ZDDP).   

Figure 1. (a, left) Morphology of the CB used in this study; (b, right) Wear volumes for 52100 steel ball against M2 tool steel flat in PAO+PIB containing ZDDP alone, CB alone, and both ZDDP and CB.

Results here could not be explained by the recent hardness hypothesis proposed by Spikes⁴, because the 52100 bearing steel, M2 tool steel, and A2 tool steel have similar hardness, 878, 876, and 821 HV. Instead, a combination of adhesive wear, abrasive wear, and catalyzed tribo-corrosive wear is proposed. In PAO+CB, the increased wear may be ascribed to the CB-induced abrasive wear and consequently adhesive wear due to significantly roughened contact surfaces, as revealed by worn surface characterization. In contrast, the wear scars generated by PAO+CB+ZDDP appeared much smoother and are covered by significant amounts of tribochemical reaction products with ZDDP based on surface chemical analysis. This implies a tribo-corrosion on the M2 or A2 steel flat. The CB could act as a catalyst to trigger the tribo-corrosion between ZDDP and the M2 or A2 steel surface, in a similar manner to the DLC coating-catalyzed tribo-corrosion reported earlier.⁵ But why did the 52100 steel ball have a wear reduction instead? Comparing the alloy compositions for the 52100 bearing steel with these two tool steels, the element standing out is Mo, which accounts for 4.5-5.5% in the M2 steel and 0.9-1.4 in the A2 steel, but less than 0.1% in the 52100 steel. Mo seemed to be necessary for the hypothesized tribo-corrosion. To further understand the wear mechanism, five additional material pairs were tested: 52100 steel ball against 52100 steel and silicon nitride flat and M50 tool steel ball against 52100 steel, M2 tool steel, and silicon nitride flat. Results showed consistent wear reduction for the 52100 steel and wear increase for M-series tool steel when ZDDP was introduced to the CB-containing oil, which supported the Mo-involved tribo-corrosion hypothesis. In summary, the wear behavior and mechanisms in the CB-containing oil highly depend on the presence of ZDDP as well as the compositions and mechanical properties of the two contact surfaces.

ACKNOWLEDGEMENTS: Authors thank Cummins for providing the carbon black, ExxonMobil for providing the PAO and PIB, and Shell for providing the ZDDP, respectively. Research was supported by the Vehicle Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy.

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