Formation of Mo2C in molybdenum tubing inside a graphite furnace filled with pure nitrogen at high temperature

Pure Mo tubing (0.381 mm in thickness) were annealed at 1700 °C for 4 h in a graphite furnace filled with pure N2 (99.995%) to evaluate the feasibility of employing the Mo tubing for high temperature thermocouple sheath. It was found that the Mo tubes were shattered after several cycles of annealing treatment. EBSD measurements revealed that the fractured Mo tubing was transformed almost completely into Mo2C 99.4% in volume fraction. The Mo2C phase had a “zebra” eutectic structure, consisting of 83.25% of orthorhombic Mo2C (Orth.) and 16.15% of hexagonal Mo2C (Hex.). This indicated that the carbon from the heating elements in the furnace somehow reacted with the Mo tubing to form Mo2C without direct physical contact between them in N2 at 1700 °C. Thermodynamic analysis supported the hypothesis that N2 gas served as an intermedium transporting the C from the heating elements to the Mo tubes by first reacting with the C to produce a cyanogen (C2N2) gas which subsequently reacted with Mo to form Mo2C at 1700 °C in the graphite furnace. The O2 effect on the Mo2C formation was also analyzed, suggesting it was negligible because of the scarcity of oxygen in the pure N2 gas in the furnace. No Mo nitride could be formed in the Mo tube since N2 energetically prefers to react to C first at 1700 °C in the furnace.