How hot will your materials get in service?
Material properties can vary greatly with changes in temperature. Thus, when developing or characterising materials or coatings which are to be used in high temperature applications, test conditions should mimic in-service conditions as closely as possible. This is the benefit of high temperature nano-mechanical testing.
The NanoTest Vantage high temperature stage allows nanoindentation, nano-scratch & wear and nano-impact & fatigue to be performed at temperatures of up to 850 °C.
Benefits of the NanoTest high temperature stage:
- Different high temperature modules for 500 ºC or 850 ºC (Vantage) and 1000 ºC (Xtreme) to allow you to pick the hardware that best suits your experimental requirements
- True depth sensing indentation for hardness and modulus determination at elevated temperature.
- Nano-scratch and high strain rate nano-impact experiements can also be performed at elevated temperature to assess the tribological behaviour of your materials at operating temperatures
- Horizontal loading and a localised heating approach minimise heat transfer to the instrument allowing more stable measurements
- Separate tip and sample heaters allow isothermal contact ensuring minimal thermal drift and more repeatable experiments
- Stability allows long duration nano-scale creep tests to determine accurate creep behaviour unaffected by instrumentation
High Temperature Stage
The horizontal loading design of the NanoTest systems is critical for accurate and reliable testing at elevated temperatures. The configuration is shown in Figure 1.
- Horizontal Loading – No heat flow into the loading head or depth measurement sensors
- Isothermal contact – The NanoTest Vantage hot stage controller uses separate active heating of both probe and sample to ensure no heat flow occurs during the indentation process (UK patent).
- Highly localised heating – ensures instrument stability
- Time-dependent measurements – As no significant thermal drift occurs during elevated temperature measurements it becomes possible to perform longer duration tests such as indentation creep tests.
- Non-ambient gases – The NanoTest Vantage has a choice of a temperature controlled environmental chamber or a purging chamber that provides a choice of ambient atmospheres and vastly reduces oxidation of samples.
Assessing in service performance: Hardness, Modulus and Creep to 850 °C
Properties measured at room temperature are not always a good indication of elevated temperature performance. In applications where temperature is a factor, mechanical properties at relevant temperatures are vital to predicting material behaviour.
Figure 2 shows the Hardness/Modulus ratio, which strongly influences wear in a variety of tribological situations, as a function of temperature for two PVD coatings. The TiAlN coating is much more stable with temperature than the TiCN coating.
The results show why TiAlN outperforms TiCN in high speed turning where frictional heating is an important factor despite having a lower hardness value at room temperature.
Creep behaviour also changes as a function of temperature. Most materials show a change in creep behaviour around 0.3 Tm. This can have a marked effect on the performance and lifetime of components.
Figure 3 shows that the creep strain on Ti6Al4V is notably higher at 650 ºC than at 25 ºC and that in high speed cutting operations, wear resistance and lifetime of coated cutting tools are strongly correlated with their high temperature mechanical properties.
High Temperature Wear
In addition to hardness, Young’s modulus and creep behaviour, tribological behaviour may alter significantly at elevated temperature. The ability to directly assess this behaviour at relevant temperatures is an important tool in determining a materials’ suitability for it’s final application.
Figure 4 shows how sample behaviour changes as temperature increases when the high temperature stage was used in conjunction with the nano-scratch and wear module in testing the sliding wear of a PVD AlTiN coating at high load. The coating fails totally during the third scan at 25 ºC but shows greater ductility at 500 ºC and the final wear depth is lower.
Optimising wear resistance of high speed cutting tools at high temperature
The nano-scratch and wear module is not the only complimentary nano-mechanical testing technique available at elevated temperature. The high strain rate nano-impact and fatigue module can also give useful information about material behavior.
Figure 5 shows the results of high strain rate nano-impact experiments performed on a TiAlN coating at elevated temperature. These results are consistent with the higher plasticity shown at 500 ºC in nanoindentation. Nevertheless, TiAlN still fractures at 500 ºC which is supported by the increased fracturing and unstable wear compared to AlTiN in interrupted cutting applications that generate significant heat.
For more information see also BD Beake et al, Int Heat Treat Surf Eng 5 (2011)17 and BD Beake et al Surf Coat Technol 201 (2007) 4585]