Tribochemistry of Frictional Ignition in High-Pressure Oxygen

Abstract

Frictional heating of metals at sliding contacts in high-pressure oxygen environments can result in catastrophic metal fires. This phenomenon, known as frictional ignition, presents an ongoing challenge in the development of oxygen-compatible components for next-generation reusable rocket engines. Early NASA investigations on frictional ignition indicated that ignition-resistant materials developed a robust oxide tribolayer. Breakdown of this tribolayer was hypothesized to drive the onset of frictional ignition. While this early work ranked the relative ignition resistance of several engineering alloys, it lacked a physical explanation for the mechanisms of tribolayer breakdown and frictional ignition, limiting its utility in predicting ignition conditions and designing ignition-resistant components. This thesis addresses this knowledge gap through a combination of experiments and modeling, which reveal the fundamental mechanisms governing frictional ignition of metals. These insights are then developed into a quantitative framework to predict ignition conditions and identify safe operating limits for sliding systems in high-pressure oxygen environments. The present work considers frictional ignition in the context of thermal ignition theory. Using high-speed dry sliding experiments and finite element modeling, we establish two conditions that must be satisfied for frictional ignition. The first condition is tribolayer breakdown, which we confirm through in situ measurements of the friction coefficient and complementary thermochemical modeling. Three distinct tribolayer breakdown mechanisms are identified – oxide melting, substrate metal melting, and solid-state mechanical failure. The dominant breakdown mechanism is found to depend on alloy chemistry and operating conditions. The second condition for frictional ignition is satisfaction of the thermal ignition criterion for thermal runaway, i.e., when the oxidative heating rate exceeds the rate of heat loss. Numerical modeling shows that the tribolayer acts as a diffusion barrier that slows oxidative heating. Thermal runaway is only possible in the absence of this tribolayer once the temperature at the sliding surface exceeds a critical threshold. The critical temperature provides a conservative estimate for evaluating ignition risk, since below this temperature, ignition cannot occur. This framework is used to explain the ignition behaviors of all materials with available frictional ignition data, highlighting the exceptional ignition resistance of the Nibase alloy MA754. Experiments show that this behavior derives from the unique mechanical properties, microstructure, and growth kinetics of the MA754 tribolayer. The above framework is extended to assess the ignition behaviors of dry dissimilar metal sliding systems. Frictional ignition experiments revealed that tribolayers may form through oxidation of the parent metal or via material transfer between counter-surfaces, potentially resulting in tribolayers with disparate chemistry from the base alloy. The dynamics of material transfer depend on the contact geometry, operating conditions, and material-pair combination. We find that favorable material transfer may result in the formation of thick, lubricating, and protective oxide tribolayers on both surfaces that suppress ignition. We develop expressions to establish safe operating bounds for dissimilar contacts with different geometries – symmetric contacts and a pin-on-disk geometry. These expressions highlight the effects of material-pair combination and contact geometry on ignition conditions. This thesis concludes by providing materials selection and component design strategies for ignition-resistant tribosystems. The first strategy is to select or design materials with high thermal conductivity, low enthalpy of oxidation, and favorable oxidational wear behaviors, i.e., rapid formation of a refractory, delamination-resistant tribolayer that lubricates the rubbing surface and protects against oxidation. The second strategy is to enhance cooling by optimizing component design (e.g., contact geometry) or modifying operating conditions (e.g., working fluid). Collectively, this thesis provides a roadmap for designing ignition-resistant sliding components capable of withstanding extreme oxygen pressures, with direct application to the development of safer oxygen-compatible hardware for next-generation reusable rocket engines.