Stephen H. Roby

Soot Wear in Diesel Engines

Abstract In response to regulatory requirements, lubricant manufacturers are seeking oils that minimize soot thickening and the accompanying soot wear. Formulation technology is being developed by additive manufacturers to satisfy these requirements. For example, such work is in progress at Chevron Oronite Company LLC, using the Cummins M-11 exhaust gas recirculation (EGR) engine test as a surrogate for the anticipated soot wear test for PC-10. Simultaneously, the authors developed bench tests to screen candidate formulations and reduce costs. A ball-on-disc sliding wear test, using a PCS Instruments MTM® tribometer, has been investigated. Sliding conditions at high pressure are required for soot polishing wear. Conditions that correlate tribometer test results with M-11 engine results at high soot concentrations (∼9 per cent) have been found. Both ball wear and Stribeck curves were determined in these tests. The high-wear oil progresses from mixed lubrication conditions to boundary lubrication at higher sliding speeds than the low-wear oil. X-ray photoelectron spectroscopy experiments were also conducted on the ball wear scars, revealing differences in the chemical constitution of the tribofilms from the two oils.

Antiwear film formation by ZnDTP, detergent, and dispersant components of passenger car motor oils©

Lubricant suppliers are under increasing pressure to provide high quality lubricants with superior oxidation and wear performance. In recent years, governmental regulations have required exhaust gas catalysts to reduce emissions of hydrocarbon and nitrogen oxides, key components of smog. Exhaust gas catalysts are sensitive to the types and levels of additives present in engine oil. The next generation of motor oils may have significantly lower sulfur, phosphorus, and sulfated ash contents to ensure exhaust gas catalyst performance throughout the life of the engine. However, phosphorus is a key component in commercial engine oils, used for both antiwear and oxidation inhibition. Engine testing of new low-sulfur, low-phosphorus antiwear components is expensive and time consuming so bench testing of potential candidates is highly desirable as a first step evaluation. Electrical contact resistance (ECR) has been shown to be a convenient method to assess antiwear film formation in a ball-on-flat bench wear test. Correlation of the bench test to fired engines was demonstrated for lubricants varying only in the type of detergent. Previous papers have examined film formation by one and two component formulations of zinc dialkyidithiophosphate (ZnDTP) and detergents. In this study, the ECR technique is systematically extended to formulations including ZnDTP, detergent, and dispersant. Both type and level of components are considered and the implications for engine performance are discussed. Presented at the 57th Annual Meeting Houston, Texas May 19–23, 2002

Time-Dependent Film Formation from ZnDTPs and Nonphosphorus Antiwear Agents

Electrical contact resistance (ECR) studies, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and X-ray absorption near-edge structure spectroscopy (XANES) were carried out on specimens run with oils containing 0.05% phosphorus as either primary zinc dialkyldithio-phosphate (ZnDTP) or secondary ZnDTP in Group II base oil. A series of progressively longer ECR experiments were run on each ZnDTP. At the end of each run in the series, the ball was removed and preserved for surface analysis. The surface analyses were designed to observe chemical species deposited on the surface and within the deposited films. The observation of surface phenomena at different intervening times during the ECR experiment, allowing for characterization of the maturing antiwear film, was the distinct feature of these experiments. In general, short ECR experiments gave poorer films than long ECR experiments. Atomic concentrations versus depth were determined from AES. Quite strikingly, the antiwear films formed after only 10 min of the ECR experiment showed that both primary and secondary ZnDTPs form a thin film (∼70 Å) very rapidly. Those films are rich in Zn, P, and S. Auger and XANES analyses of the same specimens were not as revealing, most likely due to the small wear scar on the balls and the unfortunately relatively large beam cross section. ECR, XPS, and AES were then performed on oils containing nonphosphorus antiwear agents in American Petroleum Institute Group II base oil. Several nonphosphorus supplemental antiwear inhibitors were evaluated. These experiments showed separation in apparent performance among the various components.

Antiwear Film Formation by ZnDTP, Detergent, and Dispersant Components of Passenger Car Motor Oils—Part II: Effects of Low-Molecular-Weight Dispersant on Film Formation

Passenger car motor oils (PCMOs) are undergoing a rapid evolution. Studies have found that some exhaust emission catalysts may be deactivated by phosphorus, largely derived from zinc dialkyldithiophosphate (ZnDTP), the mainstay antiwear and antioxidant agent in PCMO formulations for the past 50 years. Consequently, future engine oils will contain significantly reduced phosphorus levels. Since ZnDTP is the dominant antiwear and antioxidant in current PCMOs, lower phosphorus content will impact engine oil formulation strategies. To better understand the effects of ZnDTP reduction on wear control, electrical contact resistance (ECR) studies have been carried out on blends containing ZnDTP, detergent, and low-molecular-weight (LMW) succinimide dispersant. In contrast to previous results obtained with high-molecular-weight (HMW) dispersant, the combination of ZnDTP and LMW dispersant gave an ECR trace closely resembling that of ZnDTP alone. Thus, the chemical structure of the succinimide dispersant can have a profound effect on ZnDTP antiwear film formation. ECR experiments on three-way combinations of ZnDTP + LMW succinimide dispersant + overbased phenate detergent provided a much better film than that from a similar formulation using an HMW succinimide dispersant. This study demonstrates that the ECR experiment is sensitive to the chemical structures of components controlling the function of modern PCMOs, making ECR a convenient tool to optimize the performance of the remaining ZnDTP in lower phosphorus PCMO formulations.

Film Formation of Non-Phosphorus Wear Inhibitors

The time-dependent film formation of two potential non-phosphorus supplemental wear inhibitors in the presence of secondary zinc dialkyldithiophosphate (ZnDTP) was studied by electrical contact resistance (ECR), auger, and X-ray photoelectron (XPS) spectroscopy. One weight percent of a molybdenum dithiocarbamate or an ashless dithiocarbamate was blended with a secondary ZnDTP, sufficient in quantity to yield 0.05 wt% phosphorus at blend level. A thorough surface examination by auger and XPS, coupled with the ECR results, detailed the deleterious effects that these supplemental antiwear additives had on ZnDTP antiwear film formation. Both carbamates interfered with antiwear film formation by secondary ZnDTP. It is speculated that MoDTC generated a competing molybdenum sulfide film that oxidized over time to form MoO 3 , which promotes wear in the ECR bench test based on literature insight. Ashless DTC also formed a competing antiwear film but not as good a film as from ZnDTP alone.