Characterization of magnetron sputtered sub-stoichiometric CrAlSiN x and CrAlSiO y N x coatings

Abstract

The influence of varying nitrogen and oxygen partial pressures on microstructure, mechanical and optical properties of magnetron sputtered CrAlSiN x and CrAlSiO y N x coatings has been studied. The partial pressure of nitrogen reactive gas was varied from 0.037 Pa to 0.15 Pa for CrAlSiN x films, and the N 2 /O 2 (85%:15%) partial pressure was varied from 0.046 Pa to 0.21 Pa for CrAlSiO y N x layers. Transmittance and reflectance of samples were measured and were modeled to obtain the spectral optical constants, n and k. Chemical state, composition, morphology and microstructure of films were analyzed by XPS, RBS, XRD, Raman Spectroscopy and SEM. Films' hardness was evaluated using nanoindentation method. XRD results revealed that the two samples CrAlSiN x with P N = 0.15 Pa and CrAlSiO y N x with P NO = 0.21 Pa are polycrystalline with cubic (fcc-B 1 ) structure. On contrary, all other films prepared with lower reactive gases partial pressures are amorphous. The chemical composition changed with the variation of reactive gases partial pressure, although the Cr: Al: Si composition ratio remained approximately constant, 1.25:1.5:1. All samples showed low hardness, mainly due to lower content of reactive gases and higher content of Si. However, the sample CrAlSiN x with P N = 0.15 Pa has the highest value of 11.1 GPa. Optical constants are seen to be very sensitive to reactive gases partial pressure. The refractive index and extinction coefficient were lower for coatings with higher reactive gases partial pressure. These coatings are good candidates for designing selective solar absorber stacks for different applications.

A study of solar thermal absorber stack based on CrAlSiNx/CrAlSiNxOy structure by ion beams

“…as also confirmed by X-ray diffraction (XRD), as already reported [11]. Remaining layers are amorphous and reveal a featureless morphology as shown by single layers studied previously [16]. The final design shows simultaneously high solar absorbance in average α= 95.2 % and low emissivity ε= 9.8 % (calculated for 400 ºC).…”

“…These changes were seen after the first vacuum annealing step (50 h) but no significant further changes were found with subsequent anneling steps until 650 h. As a result, the values of solar absorptance and emittance show a very small variation due to that shift. Small changes after annealing can be related to the oxidation of some Si atoms, or it can be related to the diffusion of layer elements (such as the diffusion of Cr CrAlSiN x O y layers are substoichiometric in terms of (N+O)/(Cr+Al+Si) atomic ratio [16], which means that some of Cr atoms are in metallic oxidation state [16], because Cr are less reactive than Al and Si, and can diffuse towards the surface. The substoichiometry of those layers are due to required optical properties of HA and LA layers.…”