Abstract
(Ta,W) and (Ta,W):C films with ~5 at.% C were deposited by non-reactive magnetron sputtering. They crystallised in a bcc structure with a columnar microstructure. The solid solubility of C in (Ta,W) alloys is very low, which suggests that the (Ta,W):C films are supersaturated with respect to carbon. This was confirmed by diffraction and atom probe tomography (APT) showing that carbon is in the as-deposited (Ta,W):C films homogeneously distributed in the structure without carbide formation or carbon segregation. Annealing at 900 °C for 2 h showed no significant column coarsening but an increased defect density at the column boundaries in the (Ta,W):C films. The films were still supersaturated with respect to carbon but APT showed a partial segregation of carbon presumably to defect-rich column boundaries after annealing. The (Ta,W) films exhibited a hardness of ~12–13 GPa. Alloying with carbon increased the hardness to ~17 GPa. The hardness increased to ~19 GPa for the annealed (Ta,W):C films. This annealing-induced hardness increase was explained by C segregation to the more defect-rich column boundaries, which restricts dislocation movements. (Ta,W):C coatings may be a potential alternative to ceramic coatings, worth exploring further by small scale mechanical testing to investigate if these materials are ductile.
Highlights
The aim of this study is to investigate the influence of carbon on the microstructure and mechanical properties of magnetron sputtered Ta-W films using a combination of characterisation techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), atom probe tomography (APT) and nanoindentation
The results show that single-phase (Ta,W) films (55 at.% Ta) with a nanocrystalline columnar microstructure can be synthesised with magnetron sputtering
The XRD and TEM results show that the (Ta,W) film crystallises in a single-phase bcc structure and the high hardness of 12 ± 0.3 GPa for the (Ta,W) film is a result of the nanocrystalline microstructure
Summary
Microstructure design is an efficient tool to control the mechanical properties and obtain a combination of high hardness and ductility This can be achieved by taking advantage of several hardening mechanisms, such as grain refinement, solid solution hardening and precipitation hardening. Grain refinement and formation of a material with nanocrystalline microstructure can be a fruitful pathway to significantly increase the hardness in bcc alloys. This has been demonstrated for magnetron-sputtered films of W and Ta. For example, Zhang et al [9] deposited nanocrystalline Ta films with a grain size of 77 nm that had an order of magnitude higher hardness (11.6 GPa) than coarse bulk Ta (~1 GPa). Sputterdeposited nanocrystalline Ta-W films with a grain size of 36 nm exhibited a hardness of almost 15 GPa [10]
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