Thermodynamic stability of iron nitrides at temperatures below 350 °C

Abstract

Although iron nitride coatings are applied for their good tribological properties, their thermodynamic stabilities are unknown in the temperature range in which they are used. We have investigated the \ensuremath{\varepsilon} and \ensuremath{\gamma}' nitride phases between 0 and 350 \ifmmode^\circ\else\textdegree\fi{}C with ion implantation and find evidence that while the \ensuremath{\varepsilon}-${\mathit{Fe}}_{2}$${\mathrm{N}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ with x\ensuremath{\ge}2/3 is stable, \ensuremath{\gamma}'-${\mathrm{Fe}}_{4}$N is not. At T\ensuremath{\le}150 \ifmmode^\circ\else\textdegree\fi{}C, ion irradiation converts \ensuremath{\gamma}-${\mathrm{Fe}}_{4}$N into \ensuremath{\varepsilon}-${\mathrm{Fe}}_{2}$${\mathrm{N}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$, with x=0.5. This \ensuremath{\varepsilon} nitride can then be transformed back into \ensuremath{\gamma}'-${\mathrm{Fe}}_{4}$N by annealing at temperatures of 150 \ifmmode^\circ\else\textdegree\fi{}C and higher. Ion irradiation assists this process as well.