Influence of transition metal type and sputter deposition conditions on the hardness of transition metal carbide thin films

Abstract

Transition metal carbides (TMCs) are known for their ultrahigh melting temperatures and high hardness making them ideal for coating applications in extreme environments. The present work investigates the role of atomic mass and chamber pressure on the resulting film hardness for a series of Zr, Nb, and Ta TMCs that are synthesized via reactive gas sputter deposition. For all TMCs, the hardness varied between approximately 5 and 15 GPa for deposition pressures between 7 and 13 mTorr, which is lower than their bulk values as measured from consolidated powder billets. However, at 4 mTorr, ZrC and NbC hardness rose to approximately 20 and 27 GPa, respectively, which is closer to and at their bulk values, and TaC rose to a hardness near 47 GPa, which is greater than its bulk value and is within the superhard regime. These collective effects are explained by the changes in crystallite sizes, film texture, and film densification that occur due to atomic peening effects resulting from the differences in transition metal atomic masses and sputtering pressures, which alter the arrival energy during deposition.