Contact Fatigue Damage in Ceramic and Metal Valve Tips

A valve tip is subjected to contact fatigue damage in the form of micro-pitting due to its cyclic rolling and sliding contact with a rocker arm. The fatigue damage generated in steel, partially stabilized zirconia (PSZ), and silicon aluminum oxy nitride (sialon) tips is evaluated. Each tip material was engine tested for up to 1000 hours. The resulting damage was examined using a scanning electron microscope and correlated with contact stresses predicted by the Smith-Liu equations. The steel tips developed cracks and subsequent micropits by delamination and experienced the most damage. The PSZ tips developed micropits by microfracturing and cracking at existing voids, but experienced less damage. The sialon tips experienced no contact fatigue damage.