It is now well established that Cr2O3 is one of the hardest oxides both on the mineralogical scale (8.5 Mohs) and on the microhardness scale (up to 29.5 GPa). Much of the recent research work has focussed on Al2O3 due to its chemical and thermodynamic stability which makes it a viable option as a barrier layer in tribological and microelectronic applications.
Mechanical Properties of Chromium Oxide
Comparatively little work has been dedicated to chromium oxide and its mechanical properties. The major drawback of oxide films compared to the generally softer transition metal nitrides, which are now standard in the wear protective coating industry, is their lower toughness. However, Cr2O3 has now found application as a protective coating on read-write heads in digital magnetic recording units.
This article summarises some results obtained in a recent study of various Cr2O3 thin films prepared by reactive RF sputtering. The target material was metallic chromium (99.5 % purity) and all depositions were performed at a total pressure of 10-1 Pa in mixed Ar and O2. The ratio between the inert gas (Ar) and reactive gas (O2) was varied in the range 5 - 30 % in order to obtain different oxygen concentrations in the films produced. Nanoindentation studies combined with scanning force microscopy (SFM) were thus used to investigate mechanical properties.
Figure 1. Berkovich indentation data for a Cr2O3 thin film of thickness 1μm. Example (a) shows the result of a 50 mN maximum load monocycle, whereas (b) shows the cracking and delamination effects which occur above a critical applied load of approximately 80 mN. At higher loads the measured mechanical properties are almost entirely those of the Si substrate.
The results presented in Fig. 1 are for a Cr2O3 thin film deposited with a substrate temperature of 90°C and a deposition rate of 0.73 Å/s. The film in this case has an amorphous structure, in contrast to films produced at higher substrate temperatures which contain small crystallites (5 - 15 nm diameter).
At low loads (< 20 mN) the coating only properties are measured giving values of H = 21 GPa and E = 205 GPa. At higher applied loads, for example 50 mN (see Fig. 1 (a)), the values are already lower as the Si substrate begins to influence the measurement (Si has a hardness of ~ 9 GPa). The corresponding SFM image shows pile-up around the indentation site which is no doubt due to delamination of the coating during unloading of the indenter.
An indentation at a relatively high applied load of 200 mN (Fig. 1 (b)) displays a flat in the loading curve in the depth range 550 - 650 nm. Only subsequent SFM imaging allows this phenomenon to be fully investigated, confirming that subsurface median and lateral cracking has taken place around the indentation site. The measured mechanical properties are almost entirely those of the Si substrate. The considerable extra information provided by the SFM is again demonstrated for the case of nanoindentation on thin films and coatings.