Abstract
The correlation between structural properties of Al-rich self-assembled nano-lamellar AlxTi1−xN coatings and process parameters used during their chemical vapor deposition (CVD) remains unexplored. For this article, two gradient AlxTi1−xN coatings were prepared by a stepwise increase in temperature and pressure in the ranges of 750–860 °C and 1.56 to 4.5 kPa during the depositions at a constant composition of the process gas mixture. The cross-sectional properties of the coatings were analyzed using X-ray nanodiffraction (CSnanoXRD) and electron microscopy. Experimental results indicate that the variation of the process parameters results in changes in microstructure, grain morphology, elastic strain, nanolamellae’s chemistry and bi-layer period. At temperatures of ~750–800 °C and pressures of 2.5–4.5 kPa, preferably cubic nanolamellar grains are formed, whose microstructure correlates with the build-up of tensile stresses, which become relaxed in coating regions filled with nanocrystalline grains. CSnanoXRD superlattice satellite reflections indicate the period of the cubic Al(Ti)N-Ti(Al)N bilayers, which changes from 6.7 to 9 nm due to the temperature increase from 750 to ~810 °C, while it remains nearly unaffected by the pressure variation. In summary, our study documents that CVD process parameters can be used to tune microstructural properties of self-assembled AlxTi1−xN nanolamellae as well as the coatings’ grain morphology.
Highlights
Protective hard AlTiN coatings represent one of the most investigated coating systems as well as a common industrial standard for applications on cutting tools for dry and high-speed metal machining
The novel LP-chemical vapor deposition (CVD) coatings exhibit a unique nano-lamellar microstructure based on alternating Al- and Ti-rich Alx Ti1−x N lamellae, which are formed as a result of self-assembly during deposition [9]
high-angle annular dark-field (HAADF) scanning TEM (STEM) micrographs in Figure 2a,c show the Z-contrast, where dark and bright regions correspond to Al-and Ti-rich regions, respectively
Summary
Protective hard AlTiN coatings represent one of the most investigated coating systems as well as a common industrial standard for applications on cutting tools for dry and high-speed metal machining. These coatings, introduced in 2013 [5], attract attention because they break the Al-solubility limit of x ≈ 0.67 in the face-centered cubic (FCC) rock salt (B1) phase commonly observed in PVD coatings [6,7]. This solubility limit has been explained by ab initio studies addressing the forming enthalpies of the FCC and hexagonal Alx Ti1−x N phases, which favor the formation of the latter one for x & 0.67 in monolithic coatings [8]. The combination of high Al content in the FCC phase, and the nano-lamellar microstructure leads to superior thermal stability, oxidation resistance and mechanical properties, in comparison to traditional PVD-based
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