Thermal stability of nanolamellar fcc-Ti1-xAlxN grown by chemical vapor deposition

In recent years, nanolamellar aluminum-rich face-centered cubic (fcc) Ti1-xAlxN coatings with x as high as 0.8–0.9 grown by thermal chemical vapor deposition (CVD) have been investigated extensively. However, detailed information about their microstructure characteristics, local chemical composition and phase stability at elevated temperatures is still missing. Thus, within the present work, the temperature-induced microstructural changes of a nanolamellar fcc-Ti0.2Al0.8N coating, synthesized by thermal CVD at ∼790 °C, were studied up to temperatures of 1300 °C. In situ high-temperature X-ray powder diffraction and differential scanning calorimetry were employed to follow the phase evolution at elevated temperatures. Scanning electron microscopy and electron backscatter diffraction, carried out ex situ for six different microstructural states after isothermal annealing, revealed the distribution of individual phases and morphology of different phase regions. Complementary atom probe tomography as well as transmission electron microscopy experiments were performed on the as-deposited, an intermediate and the final decomposed and transformed state. The results provided 3D elemental information as well as detailed morphology, phase and orientation information of the respective samples. In the as-deposited state, the coating was characterized by columnar, relatively large fcc grains exhibiting a nanolamellar microstructure. Decomposition of initially supersaturated fcc-Ti1-xAlxN was detected at temperatures of ∼900–1000 °C. Transformation of metastable Al-rich fcc regions into the thermodynamically stable wurtzitic modification started at ∼1000 °C and persisted up to ∼1175 °C. In this temperature range, intact nanolamellar fcc areas coexisted with fully decomposed and transformed regions, leading to a constant reduction of the fcc fraction with increasing temperature. At temperatures above ∼1175 °C, the coating was fully decomposed and transformed into fcc-TiN and w-AlN. The obtained findings provide a detailed description of the decomposition and transformation behavior of nanolamellar CVD fcc-Ti1-xAlxN coatings, which significantly contributes to the fundamental understanding of this complex coating system.