Conventional and high resolution TEM investigation of the microstructure of compositionally graded TiAlSiN thin films

Microstructure and growth mechanism of graded TiAlSiN thin films have been investigated using conventional and high-resolution transmission electron microscopy (TEM). The films, having a total thickness of approximately 2 μm, were deposited by arc plasma PVD technique on WC–Co substrates. The film region close to the substrate was engineered as Ti-rich, and with the growing film thickness the Al+Si content increased, although the Al/Si ratio remained constant. Selected area electron diffraction and local chemical analysis (EDX microanalysis) were used to determine the exact concentration of Ti, Al and Si in the coating, TEM and HRTEM of cross-sectional cuts through the coating revealed distinct microstructures in different film regions. The Ti-rich zone close to the substrate exhibited crystalline structure with pronounced columnar growth. The addition of Al+Si leads to a crystallite refinement in the central part of the coating. Chemically modulated nanolayers with a period of 5 nm were superposed on the weak columnar structure in this region. Further increasing of the Al+Si concentration resulted in the formation of nanocomposites consisting of equiaxial, crystalline nanograins surrounded by a disordered, amorphous SiNx matrix.