The Oxidational Wear of Diesel Engine Materials

Abstract

The valves and valve seats in the exhaust of the diesel engine operate in hostile conditions at temperatures of up to 700°C. A material combination commonly used is that of 21-4N (valve) and Brico 65 (valve seat). The present work isolates the fundamental wear mechanisms of this material couple, under various conditions of load, speed and elevated temperatures. Sliding experiments were carried out on a wear machine of pin-on-disc geometry, in which friction and wear were continuously monitored during each test. The wear debris, and the worn pin (Brico 65) and disc (21-4N) specimens were examined using numerous physical methods of analyses (X-ray Diffraction, Scanning Electron Microscopy etc.). Three wear mechanisms were identified, namely, (1) Oxidation and (2) Transfer followed by Abrasion. At medium temperatures and heavy loads the mechanisms of transfer during 'running-in', followed by abrasion during the equilibrium wear regime were prevalent. The abrasive wear rates appear to be dependent on the amount of silicon which is transferred from the pin material to the disc. For other conditions of sliding the predominant wear mechanism is oxidation. Heat flow analysis shows that there is significant correlation between the bulk surface temperature, the contact temperature of the sliding members and the wear rate. Activation energy (Q_) values deduced from the wear and heat flow results suggest that in the case of tribo-oxidation Q is much lower in comparison to static oxidation.