Nanoindentation instruments are developed by employing very low indentation depths and loads. This standard is important for almost all instruments since they experience the problem of thermal drift to some extent.
The term 'thermal drift’ refers to the change in the displacement signal, while the normal force on the indenter remains constant and the material does not display mechanical properties, which are time dependent.
Thermal Drift Effect
Owing to thermal drift, the application of indentation instruments is quite limited for performing long-term measurements, like creep or grid indentation. Normally, two main techniques are used to reduce the effect of thermal drift.
One is to introduce a stabilization period prior to the measurement, and the other is to perform the indentation within a matter of seconds However, in both these approaches, the effect of thermal drift is reduced to some extent but not removed entirely. Hence, it is still difficult to conduct creep or long-term measurements, which can provide ambiguous results.
Ultra Nanoindentation Tester
Back in 1997, Anton Paar introduced the Nano Hardness Tester (NHT), which reduces the issue of thermal drift by using passive surface referencing. Continuous research and development at Anton Paar has resulted in the development of an advanced nanoindentation instrument called the Ultra Nanoindentation Tester (UNHT).
This device was developed using innovative materials and patented active surface referencing and takes a step forward towards a thermal drift free high resolution instrumented indentation. The special surface referencing principle combined with the use of next-generation materials and electronics nearly eliminates the issue of thermal drift and frame compliance.
Anton Paar has Compact and Open platforms on which the UNHT measurement head can be mounted easily. The system can be completed using other modules, like scratch testers, NHT, or Atomic Force Microscope. The UNHT can be utilized in displacement controlled mode or force controlled mode.
Figure 1. The UNHT on Anton Paar Open Platform modular system: 1 - UNHT head, 2 - NHT head, 3 - AFM, 4 - optical video microscope
Principle of UNHT
The UNHT uses two individual vertical axes, wherein one axis is used for indentation and the other for active surface referencing, as shown in the figure given below.
Figure 2. Two-axis UNHT principle
Each axis contains load, displacement and actuator sensors. For both axes, the displacement is deployed through piezo actuators A1 and A2, and the load on the reference and indenter is achieved from the displacement of the springs S1 and S2, calculated with capacitive sensors C1 and C2. The 3-mm spherical reference is located 2.5 mm away from the indenter. Then, the displacement of the indenter is determined corresponding to the reference via the differential capacitive sensor C3.
The force used on the reference is controlled on a steady level. This ensures that the reference exactly tracks each displacement of the sample surface. Constant control of normal force on the reference and the indenter is made certain by accurate feedback loops. In the measurement head, components made of Ze-rodur were utilized.
Ze-rodur is a type of material that exhibits low coefficient of thermal expansion. With the help of the UNHT, the issues of thermal drift and frame compliance have been nearly eliminated and all measurements include only raw data without involving any hardware or software rectification of frame compliance or thermal drift. The UNHT offers force stability and zero frame compliance during the indenter technique, and provides superior thermal stability during long-term measurements.
Figure 3. Thermal stability of the displacement signal during a 300 s hold at maximum load
The above figure displays two displacements against time curves captured during a 300 second hold at two diverse loads. The normal value of thermal drift recorded during various tests was less than 0.5 nm/min and in most cases was 0.2 nm/min.
In addition, the system displays extremely low thermal drift even at high temperatures. A unique heated stage is offered by Anton Paar wherein a sample can be heated to approximately 450°C. Initial tests have validated that the UNHT has a thermal drift on fused silica as low as 4 nm over 120s.
In active surface referencing, elimination of frame compliance is a critical aspect. Commercially available instruments must be able to rectify the frame compliance, which adds an error component in the displacement signal owing to frame deformation.
Hence, it is important to calculate the frame compliance and correct the displacement signal. Using the technique suggested by ISO 14577, attempts were made to measure the frame compliance and this led to the conclusion that the compliance value was so small that it is not possible to differentiate it from the scatter of the data around the zero value.
The indentation data obtained from the UNHT can thus be employed without correcting the frame compliance.
The above results demonstrate that the Ultra Nanoindentation Tester is an ideal instrument for complex applications of instrumented indentation.
This instrument offers a number of benefits such as high load and depth resolution, and low thermal drift, thus making it suitable for creep or stress-strain measurements as well as for experiments on innovative materials, like thin films and gels.
This information has been sourced, reviewed and adapted from materials provided by Anton Paar.
For more information on this source, please visit Anton Paar.