Nano goes to the extreme

Dr Ben Beake explains why testing at sub zero and elevated temperatures is essential to the development of more reliable advanced materials operating in extreme environments

The introduction of nanomechanical test methods, such as nanoindentation, has been vital to the introduction of many advanced materials over the past 20 years. Now reliable hardness and modulus are possible on the thinnest coatings – independent of substrate – and the technique has advanced to have its own ISO standard. However, despite these advances, a major drawback for many applications is that the nanoindentation instrumentation has been limited to operating at (or very close to) room temperature due to their extreme sensitivity to thermal effects, while the materials of interest operate at raised temperatures so information about them has to be inferred from results at ambient temperature which increases the scope for error.

Designing instrumentation capable of reliable depth-sensing measurements at small scale and at elevated temperature presents a considerable technical challenge. However, reliable, high temperature, nano-scale depth-sensing measurements are possible from -50°C, up to ~750°C with a commercial nanoindentation instrument, the NanoTest. The NanoTest is pendulum-based, with the sample mounted vertically so that indentation occurs horizontally. Horizontal loading is well suited to elevated temperature measurements as the displacement measuring electronics are away from the hot zone. Also key is the use of separate heating of both indenter probe and test sample resulting in minimal heat flow (and thermal drift) occurring on indentation at elevated temperature. Early adopters of the technology include General Motors, MIT, Cranfield University and Cambridge University. As it becomes possible to accurately measure coating properties and performance at high temperatures, product developments will speed up accordingly. No longer will the actual testing be an afterthought.

Many applications are in coatings, with environmental and energy conservation concerns driving many of the developments. In the automotive industry low-sulphur diesel fuels are required by the emission regulations. However, a side-effect of the sulphur removal process is that many naturally lubricous compounds are also removed. Additives are one solution but may not always be possible so manufacturers are turning to coatings for wear protection under tighter tolerances and higher contact pressures (e.g. DLC-coated injector pumps). The properties of such coatings at the engine operating temperature, such as hot hardness and stiffness, are critical to their performance and elevated temperature nanoindentation provides a convenient route to testing this.

Weight saving is another important driver. For example, the valves in IC engines are usually made of steel. Advanced applications require lower inertial mass and light non-ferrous alloys have been suggested for engine blocks. Despite the weight saving, these materials can exhibit poor tribological properties. Researchers at Birmingham University have begun to study how the mechanical and tribological properties of FeAl and TiAl intermetallics vary with temperature. Development of low friction and oxidation-resistant coatings for these lightweight materials is a natural progression. There are potential applications in gear development; gears could be made smaller and lighter if coatings could be developed to tolerate the resulting higher stresses.

At Cranfield University, John Nicholls and co-workers have been using elevated temperature nanoindentation to investigate the mechanical properties of advanced TBCs for turbine engines. EB-deposited YSZ coatings are not at all “nano” in terms of their thickness, but are rather heterogeneous and columnar. Professor Nicholls realised that the individual erosive events occurring during use were on the scale of the individual columns and so it was necessary to probe their properties by small scale testing rather than bulk measurements. The variation in the TBC hardness/modulus ratio (a useful dimensionless index often important in tribological situations) with temperature is shown in Figure 1.

Another limitation of nanoindentation is that the loading rate is too low to simulate the actual deformation rates occurring in a material’s operation or production. Realising this, Micro Materials developed a fast indentation technique, nano-impact, to probe properties at high strain rates and investigate surface fatigue and fracture due to repetitive contact (think of a woodpecker pecking at a tree!). This enables the simulation of the small repetitive stresses many modern materials suffer in real-life over time and allows more accurate prediction of their behaviour and the design of better materials.

NPL have recently used the technique to show differences in energy absorption occurring when fatigue-induced fracture occurs in thin DLC films on steel. This is expected to be useful for testing the next generation of DLC coatings in automotive engines and we have already helped a major automotive parts manufacturer to optimise their graded DLC coatings to reduce scuffing wear in fuel injector plungers. The nano-impact technique has confirmed the excellent resistance to fracture of advanced H-free DLC coatings (such as Teer Coatings’ Graphit-iC) under highly loaded contact compared to standard DLCs.

Several years of parallel development were necessary until we were confident to integrate the nano-impact and the elevated temperature test capability. Now complete, the integration enables a wide range of temperature and strain rate to be accessed (Figure 2) compared to conventional room temperature nanoindentation.

PVD coatings such as TiAlN and AlCrN are often used as cutting tools, but their good oxidation resistance and high hot hardness have many other applications under extremes of loading, loading rate and temperature. Collaboration with scientists at McMaster University in Canada, Polytech Tours in France and Balzers AG in Leichtenstein, we have begun to investigate the nanomechanical properties of these and other coatings at high strain rates and elevated temperatures (Figure 3). We have found, for example, that their resistance to impact can actually increase at elevated temperature. Results with the hot nano-impact technique show excellent correlation with tool life-time in cutting applications such as milling steels and titanium alloys – but with a typical test time of only 5 minutes, are much quicker.

The simulation of real application environments is vital to get the most realistic test performance. A good example of the critical role the environment has on component or product is humidity changes affecting the polymer protective coating on sensitive micro electronics. The humidity level can affect the polymer properties and therefore how effective the coating is. The more environmental controls available (Figure 4) to the end user the closer the test is to the real application and hence greater confidence in the results. This is essential for sectors with very demanding environments, such as aerospace, automotive and oil and gas.

As knowledge of the possibilities for high and low temperature nano-scale testing grows, instrument development is becoming increasingly market-led. This has resulted in the test envelope (Figure 2) being pushed ever outwards to map more extreme conditions. Potential applications are opened up as the test temperatures reachable increase and we are working continually to increase this limit so watch this space!