Material selection for engine internal components, e.g., bearings, is becoming increas ingly more complex and demanding as the operating environments become more aggres sive with the introduction of new technologies for the reduction o f C 02 emissions. Historically, engine bearings contained lead, which has excellent fundamental bearing properties such as compatibility (run satisfactorily under conditions o f marginal lubrica tion), conformability (deform and accept small scale geometrical inaccuracies of the crankshaft), and embeddability (tolerance to dirt and other foreign materials) while being readily alloyed to achieve good wear and fatigue resistance. However, facing new chal lenges, many original equipment manufacturers have started development programs to replace lead-containing with lead-free engine components in order to comply with new end-of-life vehicle directives or anticipated future directives. For more than 15 years, MAHLE has been successfully supplying the light, medium, and heavy duty market, with premium electroplated leaded composite bearings, which are designed to improve wear resistance. Some o f this market now demands a switch to lead-free materials, while main taining or exceeding its aforementioned requirements on bearing material properties. Composites of hard particles in a softer metal matrix are in this context ideally suited bearing materials as they can be tailored to obtain the optimal mix between soft and hard properties for the individual application. Typical hard particles that are commonly used comprise o f metal oxides, nitrides or carbides. In addition to higher load carrying capa bilities and longer service life, new engine technology trends demand that bearings also must operate under mixed or boundary lubrication conditions without having any adverse effect on the performance and integrity of the engine system. Boundary lubrication is commonly observed upon starting the engine before the elastohydrodynamic oil film is fully established. In this state, load is carried by surface asperities rather than by the lubricant. So far, the incorporation and even distribution of the hard particles into an electroplated lead-free matrix was not achievable using conventional direct current elec troplating techniques. MAHLE, therefore, has developed a patented pulse plating tech nique in order to incorporate hard particles into the overlay metal matrix. The refined and modified crystal structure of the resulting lead-free overlay, with incorporated hard particles, yields a premium electroplated bearing with superior wear and fatigue resist ance. Corresponding rig and engine test results have been completed to support the mate rial development.
The move to lead-free bearing materials is well known, and upcoming legislation, such as the restriction of hazardous substances, is increasing the drive to extend this trend toward heavy-duty diesel truck and off-highway applications. During the development of lead-free systems, new electroplated overlays and bronze-based substrates have been developed by various suppliers, but little attention has been given to the interlayer or diffusion barrier between the overlay and substrate materials. This interlayer is particularly necessary for tin-based solutions as it prevents the rapid diffusion of overlay species into the bronze substrate. The present development focuses on improving this often overlooked element in the system and provides a further robustness that could even be adapted to lead-based systems where increased performance is required. The incorporation of hexagonal boron nitride (hBN) as a solid lubricant in the nickel interlayer changes dramatically the interlayer properties and provides more typical bearing-like behavior for seizure resistance and scuff performance compared to nickel alone. The paper details the findings of respective rig tests as well as an actual engine test supporting the change in material characteristics and the associated improvement in seizure resistance.
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