Low Friction Coating for High Temperature Bolted Joints in IC Engines

The IC engine still plays an important role in global markets, although electrified vehicles are highly demanded in some markets. Emission requirements for stoichiometric operation are challenging. This requires the bolted joints for turbo, EGR (Exhaust Gas Recirculation) and exhaust manifold to work under much higher temperature than before. How to avoid fastener breakage due to bolt bending caused by cyclic changes of the thermal conditions in engines is a big challenge. The temperatures of the components in the exhaust, EGR (Exhaust Gas Recirculation) and turbo systems change from ambient temperature to about 800 ~ 1000 °C when engines run at peak power with wide-open throttle. The temperature change induces catastrophic cyclic bending and axial strain to the fasteners. This research describes a method to reduce the cyclic bending displacement in the fasteners using a low friction washer. Mathematical modeling and FEA methods have been employed to specify the design space based on the engine operating conditions. A series of tribological bench tests were conducted to evaluate different coatings on the washer under room temperature and up to 600 °C. A sensitivity study has also been done to identify the factors that affect the coefficient of friction (CoF). A multi-layer coating has been found to be able to provide a low coefficient of friction under high temperature and high pressure. It meets the design requirements and has been validated by engine dyno tests on the exhaust system.