Indentation and Scratch Testing of Adhesion Properties

One of the most viable and already used sources of renewable energy are concentrating solar power systems (CSP) that convert solar thermal energy into electrical power^1-4. Mirrors are used by parabolic trough solar collector (PTSC) technology to redirect and focus sunlight.

This sunlight heats up the transport fluid in the tube, which is subsequently used for producing electricity production (Figure 1). Over the last several decades, the PTSC technology has been effectively used around the world and it is now become the most advanced of all CSP designs in terms of technical and scientific research.

Although existing systems are designed to operate at a temperature of 400 °C, new systems are now being made with operating temperature of up to 600 °C. The new generation of systems should boost the performance by as much as 5 to 10% so that the technology continues to remain competitive in the green technology market. However, new degradation processes can occur that can reduce the system efficiency⁵ at these elevated temperatures. CSP systems can provide a viable solution if they have a lifetime of about 20 to 25 years and therefore, understanding the degradation processes is very important to further develop these systems.

Figure 1. Schematic of the parabolic trough solar collector⁴ and the collector tube.

In order to ensure durability and efficiency of the PTSC technology, the absorber coating which is deposited on stainless steel tubes (where the working liquid circulates) represents an important factor. This coating warrants several key requirements which can be met only by using a multilayer structure^6-8.

Table 1 shows the composition of such multilayer absorber coating. While thorough investigation of the thermal stability and optical performance of the absorber coatings has already been made, very limited knowledge is available regarding the mechanical properties of these multilayer absorber coatings. Deterioration of optical properties can be indicated by changes in the elastic modulus and hardness of the coating. Such deterioration is attributed to degradation through thermal cycling day-night, which, in turn, can cause adhesive failure of the coating.

This article focuses on the characterization of adhesion and mechanical properties of standard CSP absorber coatings. The study was part of the FP7 NECSO project whose aim was to establish a standardized approach to evaluate the lifetime of the CSP absorber coatings through accelerated aging tests. Nanoscratch testing was performed to characterize the adhesion of the absorber coating, while low load nanoindentation was applied to characterize hardness and elastic modulus. All tests were performed on samples before the aging tests and after the accelerated aging tests.

Figure 2. The collector tube with absorber coating.

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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.

For more information on this source, please visit Anton Paar GmbH.