Theoretical and experimental mechanical properties and thermal conductivity of W-Al-B thin films deposited by magnetron sputtering

This work compares experimentally measue properties of W-Al-B thin films with mechanical properties, density, and thermal conductivity values calculated using DFT methods. Theoretical modelling was conducted to simulate two WB 2 stable structures alloyed with varying amounts of aluminium: α-WB 2 (P6/mmm) and ω-WB 2 (P6 3 /mmc), as well as α-AlB 2 (P6/mmm). Using the HiPIMS-DC magnetron sputtering technique, films with α-WB 2 structure and varying aluminium contents were deposited at 400 °C. When layers are composed with x = 1.4% aluminium (where x = at%Al / (at%Al + at%W)), their microstructure changes from amorphous to crystalline columnar. A back transformation to an amorphous microstructure occurs when the amount of aluminium exceeds x = 7.3%. An original method was used for the film density studies, which combined mass measurements and microscopic observation. These measurements were then used to determine the layers' thermal conductivity using the thermoreflectance method. The measured conductivity of the deposited ceramic films range from 3 to 6 W/(mK). Moreover, the obtained films are very hard, e.g. H = 36.1 ± 1.7 GPa for x = 1.4% Al, but exhibit a much lower Young's modulus than the theoretical values. The relatively high H/E⁎ ratio > 0.1 for films with low aluminium content indicates anmore elastic character. Ab-initio calculations showed that, based on the criteria of Cauchy pressure (C 12 -C 44 ) and Pugh's ratio (B/G), the α-WB 2 structure may have a ductile nature in contrast to the other structures. However, the deposited films are rather brittle in nature, resulting from an excess of boron. The fracture toughness measurements show higher K IC values for low aluminium content. They are 3.8 MPa√m for WB 2 , 2.8 MPa√m for x = 1.4%, and 3 MPa√m for x = 7.3% aluminium. • Stiffness tensors of thermally and mechanically stable W 1-x Al x B 2 compounds were determined • Increase of thermal conductivity and density decrease with growing aluminium content • High H/E ⁎ >0.1 of magnetron sputtered hard (H=34.1±0.7 GPa) W 0.927 Al 0.073 B 2.8 films • 1.4% of Al addition causes the change of microstructure to crystalline at 400 °C • Brittle/ductile character of DFT modelled α-W-Al-B 2 vs. brittle behaviour of films