WEAR-RESISTANCE PROPERTIES OF CERAMIC ROCKER ARM PADS

Rocker arm pads made of silicon nitride ceramics have been adopted in automotive engines in recent years, especially in taxi engines which exhibit severe wear in the cam and follower system. In this research, measurements of wear depth, observations of worn surface morphologies and analyses of lubrication films were performed for cam noses and their mating rocker arm pads following a series of engine bench tests. The results confirmed the superior wear resistance of silicon nitride pads for LPG taxi engines and it was found that excessive calcium and phosphorus adsorptions on contact surfaces lubricated with diesel engine grade oil contained primary type zinc dialkyldithiophosphate and large amounts of calcium detergent. The excessive adsorption of some additives caused the micro-pits observed on the cam noses following every test conducted with that grade of oil. It is thought that the pits were formed by acid corrosion following mechanochemical reactions.