Unexpectedly large intra-grain crystal rotations in CVD-grown (Ti,Al)N coatings and their relation to internal nanolamella structures

The crystal growth direction within a single grain is typically assumed to be uniform. However, this study presents the first observation of spontaneous intra-grain crystal rotations in (Ti,Al)N grains with an internal nanolamella structure. The rotation occurs spontaneously during grain growth in a chemical vapor deposition process. The crystal rotations exhibit a highly symmetrical pattern linked to the (Ti,Al)N grain morphology: a pyramid comprising three {001}-faceted domains. The crystal lattices systematically rotate within each domain toward the pyramid tip, with the largest rotations concentrated at the pyramid edges. Within a single (Ti,Al)N grain, the crystal orientation can shift up to 15° from base to tip, with a typical gradient of 1°/µm. The rotation gradient reaches a peak of 3 – 4°/µm in the horizontal direction near the pyramid edges. This phenomenon is attributed to the high density of grown-in dislocations, arising from interface mismatches between the (Ti,Al)N grain domains and hexagonal AlN nano-precipitates that form at the domain boundaries. Analysis of three (Ti,Al)N samples with varying nanolamella structures reveals that the presence of more nanolamellae corresponds to larger crystal rotations. These findings provide useful insights into controlling crystal rotation and its potential influence on material properties, opening new avenues for materials design and optimization.