Chemical and Morphological Characterization of Magnetron Sputtered at Different Bias Voltages Cr-Al-C Coatings

Abstract

MAX phases (M = transition metal, A = A-group element, and X = C/N) are of special interest because they possess a unique combination of the advantages of both metals and ceramics. Most attention is attracted to the ternary carbide Cr2AlC because of its excellent high-temperature oxidation, as well as hot corrosion resistance. Despite lots of publications, up to now the influence of bias voltage on the chemical bonding structure, surface morphology, and mechanical properties of the film is still not well understood. In the current study, Cr-Al-C films were deposited on silicon wafers (100) and Inconel 718 super alloy by dc magnetron sputtering with different substrate bias voltages and investigated using Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), and nanoindentation. Transmission Electron Microscopy (TEM) was used to analyze the correlation between the growth of the films and the coating microstructure. The XPS results confirm the presence of Cr2AlC MAX phase due to a negative shift of 0.6-0.9 eV of the Al2p to pure aluminum carbide peak. The XRD results reveal the presence of Cr2AlC MAX Phase and carbide phases, as well as intermetallic AlCr2. The film thickness decreases from 8.95 to 6.98 µm with increasing bias voltage. The coatings deposited at 90 V exhibit the lowest roughness (33 nm) and granular size (76 nm) combined with the highest hardness (15.9 GPa). The ratio of Al carbide to carbide-like carbon state changes from 0.12 to 0.22 and correlates with the mechanical properties of the coatings. TEM confirms the columnar structure, with a nanocrystalline substructure, of the films.

Keywords: AFM; MAX phase; TEM; XPS; chemical bonding; surface morpholog.

Figure 1

Cross-section SEM images of the Cr-Al-C coatings deposited at various bias voltages (a) 60 V; (b) 90 V; and (c) 120 V.

Figure 2

X-ray diffraction (XRD) patterns of the deposited Cr-Al-C coatings.

Figure 3

(a) Atomic composition at the surface; (b) O/(Al_Ox + Cr_Ox) and Al/Cr atomic ratios for Cr-Al-C coating at 90 V as a function of Ar⁺ etching time obtained two different ways.

Figure 4

Cr2p, Al2p, and C1s XP-spectra obtained for Cr-Al-C films at (a) 120 V; (b) 90 V; and (c) 60 V after Ar⁺ ion etching for 120 min.

Figure 5

(a) Atomic concentration of carbide-like carbon in relation to all elements; atomic ratio of chromium carbide (574.3 eV) and aluminum carbide (72.5–72.8 eV) states to carbide like carbon (282.8 eV); and Ar⁺ etching time obtained for Cr-Al-C at 90 V; (b) Cr_carbide/C_carbide and Al_carbide/C_carbide ratios obtained for all samples after 120 min of Ar⁺ ion etching vs. bias voltage.

Figure 6

Atomic force microscopy (AFM) topography images of Cr-Al-C films on a Si (100) wafer at (a) 60 V; (b) 90 V; (c) and 120 V; (d) A lateral force map at 90 V.

Figure 7

(a) Transmission electron microscopy (TEM) cross-sectional bright-field (BF) image of the Cr₂AlC coating, deposited at 90 V; (b) Selective area electron diffraction (SAED) obtained in the place marked by the circle.

Figure 8

(a) TEM cross-sectional BF image of the Cr₂AlC coating interface, deposited at 90 V (1; substrate part of interface, 2; coating part of interface); (b) Selective area electron diffraction (SAED) obtained in the place marked by the circle.

Figure 9

(a) TEM cross-sectional BF image of the single Cr₂AlC crystal on the top layer of the coating, deposited at 90 V; (b) Selective area electron diffraction (SAED) obtained in the place marked by the circle.