Thin film degradation – sometimes desired and sometimes not
There are many processes all around us, both naturally occurring and artificially designed, where thin films or coatings degrade or breakdown. A typical example would be etching or corrosion, which in, for example, pipeline infrastructure, is an unwanted process, but which in manufacturing of electronic components is highly desired. Another area where film degradation is highly desired, is in soil removal by detergent. In both cases, it is important to understand the material degradation so that it can be optimized and controlled. Either to be able to prevent the unwanted degradation, or to increase the speed of the wanted one. To be able to control the process, it must be mapped out and understood. It can therefore be relevant to monitor both the degradation rate as well as the extent of the degradation. QCM-D, which essentially is a balance for small masses, can measure and quantify this film degradation. Both in terms of amount as well as the kinetics.
Measuring and quantifying thin film degradation
When a thin film degrades, there will be mass lost from the surface. There will also be a decrease in thickness of the initial surface-bound layer. These are two parameters that QCM-D measures in real-time at nanoscale resolution.
Example: Evaluating cleaning efficiency
As an example, let’s have a look at film degradation when cleaning a surface. We have a fat stain that we would like to remove from the surface using a surfactant. As outlined in Figure 1, we follow the steps below.
We start the measurement with a thin layer of fat deposited on the surface. The layer represents the soiling that we would like to remove. The background solution is water. We can see that no mass is removed in this step, i.e. that the soil does not dissolve in water
Next, we introduce our detergent and flow it over the fat stain.
The detergent will flow over the surface and begin to interact with the soil. Here we can see that the thickness of the soil is increasing. This is expected, since the detergent will penetrate the soil and make it swell.
In the next step, the soil begins to break up and be removed from the surface. We see a decrease in the thickness.
After the cleaning step, we add a rinse step in which we rinse with water. Again, we see that the thickness decreases as the rinse step removes soil from the surface.
In the end, 60% of the material was removed from the surface.
Figure 1. Monitoring a cleaning process with QCM-D technology.
Evaluating thin film degradation under different conditions
Monitoring the mass and thickness as a function of time, one can easily characterize and evaluate the degradation behavior and amount of material removed. It is also possible to compare the behavior under different conditions, such as by varying chemical concentration, temperature, and pH.
Example 2: Comparing the performance of two different detergents
Here we extend the first example, where we evaluated the detergent behavior and soil removal efficiency. Now, we use the same experimental setup and compare the cleaning performance of detergent A and B. The soil thickness results, plotted as a function of time in Figure 2, reveals that detergent B acts faster than detergent A, but has a lower overall cleaning efficiency. We also note that detergent A causes more initial swelling (increased thickness) than does detergent B.
Figure 2. Soil thickness as a function of time to evaluate the cleaning effectiveness of two different detergents. Water is used as a negative control
Thin film degradation in other areas
In addition to these two examples, other types of degradation that could be characterized by measuring the mass loss include, for example, enzymatic reactions, dissolution, light-induced degradation as well as temperature-induced degradation.
Biocompatibility, antibacterial qualities, and drug delivery can be achieved by for example polymer brushes, polyelectrolyte multilayers or hydrogels. When tailoring the interfacial properties of these thin films, the layer conformation, such as crosslinking and degree of hydration is important.
The ability to take up and release water is central for many materials, such as hydrogels, whose function depend on the ability to hydrate and dehydrate. Hydration and swelling are also central when dealing with hygroscopic materials. QCM-D can be used to characterize such swelling phenomenon.