Abstract
The topic of the present experiments are transition metal diboride coatings of composition (Ti0.49W0.51)B2 and (Ti0.44W0.30Cr0.26)B2. The coatings were deposited on steel substrates using dc magnetron sputtering. We investigated how annealing in argon at elevated temperatures modifies microstructure. The as-deposited films are amorphous. Annealing between 700 and 1100 °C results in the formation of nano-crystalline precipitates with average grain diameters of about 10–50 nm. A TiC phase (Fm-3m; a ≈ 4.3 Å) is observed as the dominating precipitate phase. In addition, small amounts (10%–20%) of a Cr23C6 phase (Fm-3m; a ≈ 10.6 Å) are observed. In contrast to literature data on the same coatings deposited on silicon substrates, the formation of boride precipitate phases is strongly suppressed here. From investigations with X-ray diffractometry, electron microscopy and secondary ion mass spectrometry we conclude that the nanostructure of the coatings is formed by reactive phase formation of the boride coating with the carbon containing steel substrate.
Highlights
Titanium diboride (TiB2) ceramics exhibit various attractive properties which are interesting for technical applications: a high melting point (~3225 °C), high thermal conductivity (37–122 W·m−1·K−1 at 300 K), low electric resistivity (~0.13 μΩ·m), a relatively low density (4.5 g/cm3), excellent chemical stability, and a high hardness (20–30 GPa) [1,2,3,4,5,6,7]
Materials based on titanium diboride are in use as cutting tools, wear resistant parts, protective coatings, diffusion barriers, and for a reinforcement of carbide ceramics [1,2,3,4,5,6,7,8,9,10]
No boride phases are found as expected from preliminary work, where coatings of the same composition and with the same sputter parameters were deposited on silicon substrates [27]
Summary
Titanium diboride (TiB2) ceramics exhibit various attractive properties which are interesting for technical applications: a high melting point (~3225 °C), high thermal conductivity (37–122 W·m−1·K−1 at 300 K), low electric resistivity (~0.13 μΩ·m), a relatively low density (4.5 g/cm3), excellent chemical stability, and a high hardness (20–30 GPa) [1,2,3,4,5,6,7]. In order to improve toughness and creep resistance of bulk titanium diboride ceramics, tungsten rich platelets were precipitated in-situ in a supersaturated (Ti,W,Cr,)B2 solid solution [21,22,23,24,25]. These precipitates lead to crack deflection and branching [26]. Concerning thin films, it was recently shown [27] that it is possible to deposit (Ti,W,Cr,)B2 coatings with a thickness of about 1 μm on silicon substrates by DC magnetron sputtering of compound targets. Annealing the coatings at temperatures up to 1300 °C results in the formation of nano-crystalline precipitations composed of (Ti,W,Cr)B2
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