Thermal Stability of Nanostructured TiN-TiB 2 Thin Films
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Thermal Stability of Nanostructured TiN-TiB2 Thin Films Paul H. Mayrhofer and Christian Mitterer Department of Physical Metallurgy and Materials Testing, University of Leoben, A-8700 Leoben, Austria ABSTRACT Nanocrystalline hard films have attracted increasing interest for wear-resistant applications. Especially, films within the system Ti-B-N with its numerous different phases are ideal candidates for materials science based studies on the nanoscale. In physical vapor deposited TiB-N films the nanostructure arises during growth by segregation-driven renucleation resulting in 2-3 nm sized TiN and TiB2 crystals. These films exhibit a hardness of ~ 42 GPa in the asdeposited state which increases to ~ 52 GPa during thermal annealing in vacuum. Here we show, that as-deposited films have a remarkable fraction of disordered regions surrounding TiN and TiB2 nanocrystals. During thermal annealing, the structural rearrangement causes the formation of compact boundary areas, leading to a hardness increase. At temperatures higher than 900 °C, the occurring B-loss, grain-growth, and recrystallization cause the hardness to decrease. INTRODUCTION Thin transition metal nitride, boride and carbide films are the topic of many papers due to their physical, chemical and mechanical properties which make them applicable for several industrial fields [1-12]. Among these, boron containing thin films have attracted increasing interest due to their outstanding hardness and thermal stability [1-9,13-16]. The increasing industrial demand for advanced protective thin films with tailored properties requires the development of multi-elemental and multi-phase ceramic layers. In particular, films within the system Ti-B-N with its numerous different phases are challenging candidates for materials science based studies [2,5], also due to their possible nanocrystalline structure. The latter results from a segregationdriven renucleation process during growth of the multiple phases present [13,14]. The focus of this work is about the nanostructure of sputtered TiN-TiB2 films, its modification during annealing treatments up to 1400 °C and the effects of the nanostructural arrangement on the film hardness. Here we show, using x-ray diffraction and high resolution transmission electron microscopy, that sputtered TiN-TiB2 films are composed of randomly orientated 2-3 nm sized TiN and TiB2 crystals surrounded by a B-rich disordered region. During post-deposition annealing treatments in vacuum, the nanostructural arrangement modifies itself, causing a hardness increase starting from ~ 42 GPa in the as-deposited state to ~ 52 GPa after annealing at 900 °C. Annealing at higher temperatures causes B-loss [16], grain growth and recrystallization [17] of the film, resulting in a hardness decrease. The results for TiN-TiB2 films show a clear correlation between their nanostructure, modified by annealing, and their mechanical properties. EXPERIMENTAL TiN-TiB2 films [13], ~ 3 µm thick, were grown on polished austenitic stainless steel substrates at 300 °C by
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