Abstract
In the field of hard protective coatings, nano-crystalline Ti-B-N films are of great importance due to the adjustable microstructure and mechanical properties through their B content. Here, we systematically study this influence of B on Ti-B-N during reactive as well as non-reactive DC magnetron sputtering. The different deposition routes allow for an additional, very effective key parameter to modify bond characteristics and microstructure. Plasma analysis by mass spectroscopy reveals that for comparable amounts of Ti + , Ti 2+ , Ar + , and Ar 2+ ions, the count of N + ions is about 2 orders of magnitude lower during non-reactive sputtering. But for the latter, the N + /N 2 + ratio is close to 1, whereas during reactive sputtering this ratio is only 0.1. This may explain why during reactive deposition of Ti-B-N, the B N bonds dominate (as suggested by X-ray photoelectron spectroscopy), whereas the B B and Ti B bonds dominate for non-reactively prepared Ti-B-N. Chemically, reactively versus non-reactively sputtered Ti-B-N coatings follow the TiN-BN versus TiN-TiB 2 tie line, respectively. Detailed X-ray diffraction and transmission electron microscopy studies reveal, that up to 10 at.% B can be dissolved in the fcc-TiN lattice when prepared by non-reactive sputtering, whereas already for a B content of 4 at.% a BN-rich boundary phase forms when reactively sputtered. Thus, we could not only observe a higher hardness (35 GPa instead of 25 GPa) as well as a higher indentation modulus (480 GPa instead of 260 GPa), but also a higher fracture energy (0.016 instead of 0.009 J/m during cube-corner indentations) for Ti-B-N coatings with 10 at.% B, when prepared non-reactively. • Major differences in plasma species between reactive and non-reactive sputtering • The elemental composition also varies with the deposition route. • Significant differences in mechanical properties at higher boron contents
Highlights
Physical vapor deposited (PVD) TiN is an extremely successful coating, not just for decorative purposes or diffusion barrier abilities, and to protect cutting tools against heavy mechanical and corrosive loads, since the 1960s [1]
When plotting the chemical composition of individual Ti-B-N coat ings in the corresponding equilateral concentration triangle, hard and superhard (> 40 GPa [22]) coatings are centered along the TiN-TiB2 or TiN-TiB tie lines [10,13,14,15,16,17,23,24,25,26]
X-ray photoelectron spectroscopy (XPS) of all Ti-B-N coatings was conducted with a custom-built SPECS XPS-spectrometer equipped with a monochromatic Al-Kα X-ray source and a hemispherical WAL-150 analyzer (Acceptance angle 60◦)
Summary
Physical vapor deposited (PVD) TiN is an extremely successful coating, not just for decorative purposes (due to its shiny yellow color) or diffusion barrier abilities (e.g., for microelectronics), and to protect cutting tools against heavy mechanical and corrosive loads, since the 1960s [1]. Boron is a special alloying element (due to its semi-metallic character), as it may occupy the metal [10] as well as non-metal sub lattice [10,11] and forms BN-rich boundary phases [12] This leads to many diverse studies, reporting about a significant hardness increase of TiN with the addition of B (up to 40 GPa) [13,14,15,16,17,18,19], and about a hardness decrease [20,21]. A decrease in hardness (upon adding B to TiN) is often related to the formation of weaker B–N bonds, typically present in pronounced amorphous grain boundary phases of the coating microstructures [21] These B–N dominated grain boundary phases are able to reduce the coefficient of friction [20]. Reactive depositions are realized through co-sputtering of a Ti and a ceramic TiB2 target in a mixed Ar + N2 glow discharge
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