Many commonly used metals and alloys, such as iron, titanium, and nickel, form passive films when exposed to oxidising environments. These passive films, which are often considered to be stable oxides, are exploited for their ability to significantly reduce corrosion and oxidation.
Testing of Passivation Layers
Electrochemical testing of these materials and their various alloys is now common practice for assessing the corrosion resistance of a particular metal in a given environment. However, the mechanical properties of these passive films and the material on which the film has formed have not been thoroughly explored. This is partly due to the small scale of the films, which are generally oxide layers with thicknesses on the nanometer scale. With the silicon family of materials, passivation layers and their performance in service are a topic of increasing importance in todays’ microelectronics and micromachining industries. This application note focuses on assessment of the scratch resistance of SiO2, SiN and SiC passivation layers. These types of coatings are ideal examples owing to their distinct and reproducible critical failure points.
The results presented in Fig. 1 summarise the four distinct failure points encountered when a SiN/SiC passivation layer is scratched with a progressive load range of 0-15N and a diamond tip of radius 20μm.
The thickness of the coating is approximately 400 nm. The first critical force value of 3.26N corresponds to damage along the sides of the scratch path. At 9.77N first cracking occurs, after which continuous flaking occurs at 10.60N. Shortly afterwards, the coating is seen to completely delaminate at a critical force of 11.74 N, causing substantial debris to be strewn around the scratch path (see Fig. 1 (d)).
Figure 1. Progressive load scratch data for an SiN/SiC composite layer which exhibits four distinct and reproducible critical failure points, namely (a) first damage, (b) first cracking, (c) continuous cracking and (d) full delamination.
In addition to scratch testing, passivation layers may also be characterised by indentation testing with a conducting tip or by potentiostatic control of the sample. The tip is loaded and the electrical resistance measured between the tip and the sample. For many systems, particularly metals, a sudden drop in resistance can be attributed to the breaking of the oxide film and the resultant contact between the two conductors. Future work is hoped in adapting potentiostatic control to scratch testing.