Oxide tribolayer breakdown on sliding metal contacts drives thermal ignition

Frictional heating at sliding metallic contacts in high-pressure oxygen environments can cause frictional ignition and metal fires. Alloys that resist frictional ignition tend to grow a thick, protective oxide tribolayer during sliding. Ignition occurs when this tribolayer breaks down, exposing the hot underlying metal to oxygen. This paper establishes a quantitative link between frictional ignition and tribolayer breakdown using thermal ignition theory. The tribolayer breakdown mechanisms that drive frictional ignition of several engineering alloys are determined experimentally. For the alloy Haynes 214, the dominant tribolayer breakdown mechanism transitions from melting of the underlying metal to mechanical failure as oxygen pressure decreases, demonstrating how operating conditions affect ignition behavior. Thermal ignition theory is used to determine a critical interfacial temperature above which an alloy ignites in the absence of an oxide tribolayer. For the alloys of present interest, this critical temperature is lower than both the interfacial temperature at ignition (when the tribolayer breaks down) and the alloy melting point. In the temperature range between the critical and ignition temperatures, the sliding contact is metastable, meaning ignition occurs if the tribolayer is disturbed. Ignition resistance can be improved by raising the critical temperature towards the alloy melting point through changes in component design, operating conditions, and material properties. Additionally, tailoring alloy chemistry to promote the growth of a breakdown-resistant tribolayer allows safe operation in the metastable regime.