TY - THES

T1 - Influence of the bilayer period on the structure of AlN and the mechanical properties of CrN/AlN superlattice coatings

AU - Mayer, Bernhard

N1 - embargoed until null

PY - 2012

Y1 - 2012

N2 - In recent years the research on CrN/AlN multilayer films has shown improved mechanical properties and oxidation resistance. Particularly CrN/AlN superlattice coatings show increasing hardness for a certain bilayer period (Λ). To obtain this superlattice structure both layer materials (CrN and AlN) have to exhibit the same crystal structure and a similar lattice parameter. The hardening effect corresponds to the metastable cubic (c) AlN phase, which is stabilized by coherency strains. However, a systematic study on the influence of the individual layer thicknesses of CrN and AlN on structural and mechanical properties of CrN/AlN superlattice coatings is still missing. Therefore, CrN/AlN multilayer coatings were prepared by DC reactive magnetron sputtering with AlN layer thicknesses of 1, 2, and 3.3 nm and varying the CrN layer thicknesses from 0.9 to 10 nm. The thereby obtained bilayer period and structure of the CrN/AlN coatings was investigated by low-angle and high-angle X-ray diffraction (LAXRD and HAXRD) as well as high-resolution transmission electron microscopy (HRTEM). The hardness as a function of Λ was determined with an ultra-micro-indentation system and the residual stresses were obtained by the substrate-curvature method at room temperature. The results suggest that the minimum CrN layer thickness, necessary to stabilize the AlN layers in their metastable cubic structure, is in the range of the AlN layer thickness themselves. Hence, for the CrN/AlN coatings composed of 1, 2, and 3.3 nm thin AlN layers the CrN layers need to be at least 1, 2, and ~3 nm, respectively. For thinner CrN layers an X-ray amorphous structure or a multiphase arrangement of cubic, wurtzite-like, and amorphous phases is obtained. Exemplarily, the CrN/AlN superlattice coating composed of 1 nm thin AlN and 1.9 nm thin CrN layers (Λ = 2.9 nm) was investigated by cross-sectional HRTEM. These studies confirmed the superlattice structure–suggested by LAXRD and HAXRD–by the almost perfect hetero-epitaxial relationship between c-CrN and c-AlN. This CrN/AlN superlattice coating, as well as the one composed of 2 nm thin AlN and 3.5 nm thin CrN (Λ = 5.5 nm) exhibit a hardness maximum of ~31 GPa. If the AlN layer thickness is ~3.3 nm the hardness peak is obtained only with ~28.5 GPa for a bilayer period of Λ = 6.3 nm. The resulting hardness-peak as a function of the bilayer period becomes broader with increasing AlN layer thickness. A corresponding dependence on the bilayer period is also obtained for the compressive stresses. Based on the results it can be concluded that the arrangement of 1 nm thin AlN layers with 1.9 nm thin CrN layers or 2 nm thin AlN layers with 3.5 nm thin CrN layers will result in the formation of a superlattice CrN/AlN structure having a hardness of ~40 % above that of the layers they are formed.

AB - In recent years the research on CrN/AlN multilayer films has shown improved mechanical properties and oxidation resistance. Particularly CrN/AlN superlattice coatings show increasing hardness for a certain bilayer period (Λ). To obtain this superlattice structure both layer materials (CrN and AlN) have to exhibit the same crystal structure and a similar lattice parameter. The hardening effect corresponds to the metastable cubic (c) AlN phase, which is stabilized by coherency strains. However, a systematic study on the influence of the individual layer thicknesses of CrN and AlN on structural and mechanical properties of CrN/AlN superlattice coatings is still missing. Therefore, CrN/AlN multilayer coatings were prepared by DC reactive magnetron sputtering with AlN layer thicknesses of 1, 2, and 3.3 nm and varying the CrN layer thicknesses from 0.9 to 10 nm. The thereby obtained bilayer period and structure of the CrN/AlN coatings was investigated by low-angle and high-angle X-ray diffraction (LAXRD and HAXRD) as well as high-resolution transmission electron microscopy (HRTEM). The hardness as a function of Λ was determined with an ultra-micro-indentation system and the residual stresses were obtained by the substrate-curvature method at room temperature. The results suggest that the minimum CrN layer thickness, necessary to stabilize the AlN layers in their metastable cubic structure, is in the range of the AlN layer thickness themselves. Hence, for the CrN/AlN coatings composed of 1, 2, and 3.3 nm thin AlN layers the CrN layers need to be at least 1, 2, and ~3 nm, respectively. For thinner CrN layers an X-ray amorphous structure or a multiphase arrangement of cubic, wurtzite-like, and amorphous phases is obtained. Exemplarily, the CrN/AlN superlattice coating composed of 1 nm thin AlN and 1.9 nm thin CrN layers (Λ = 2.9 nm) was investigated by cross-sectional HRTEM. These studies confirmed the superlattice structure–suggested by LAXRD and HAXRD–by the almost perfect hetero-epitaxial relationship between c-CrN and c-AlN. This CrN/AlN superlattice coating, as well as the one composed of 2 nm thin AlN and 3.5 nm thin CrN (Λ = 5.5 nm) exhibit a hardness maximum of ~31 GPa. If the AlN layer thickness is ~3.3 nm the hardness peak is obtained only with ~28.5 GPa for a bilayer period of Λ = 6.3 nm. The resulting hardness-peak as a function of the bilayer period becomes broader with increasing AlN layer thickness. A corresponding dependence on the bilayer period is also obtained for the compressive stresses. Based on the results it can be concluded that the arrangement of 1 nm thin AlN layers with 1.9 nm thin CrN layers or 2 nm thin AlN layers with 3.5 nm thin CrN layers will result in the formation of a superlattice CrN/AlN structure having a hardness of ~40 % above that of the layers they are formed.

KW - CrN/AlN superlattice

KW - hardness effect

KW - stresses

KW - Mehrlagenschicht CrN/AlN mit Übergitterstruktur

KW - Härteeffekt

KW - Spannungen

M3 - Diploma Thesis

ER -