High Temperature Nanoindentation - The State-of-the-art Laser Heating Technique

1. Introduction and Challenges
2. Key Features of the Laser Heater System and Technical Measurements
3. Capabilities and Specifications of the G200 Laser Heater System
4. An industry-leading solution for nanoindentation from Keysight Technologies
5. Summary
6. Related Information

High temperature Nanoindentation measurements have had a progressing trend over the last decade (>25%) by utilizing advanced techniques to perform stable indentation testing at elevated temperatures; meanwhile providing a very fast temperature stabilization with minimum thermal drift similar to those observed at ambient temperatures. Directly extracting the mechanical properties of material from the load-displacement curves using the nanoindentation instrument at elevated temperatures facilitates the measurement of temperature sensitive material properties, particularly at small volumes over the conventionally-scaled tension or compression tests. Additionally, minimum requirements of sample and test geometry eliminate extra sample preparations when compared to other methods, i.e. machining.

This makes nanoindentation a promising and unique tool to investigate the material behavior and properties at various temperatures which can be related to the actual service temperature of tested materials in industrial applications. The academic and research societies has also benefited from this unique capability to expand their knowledge of underlying science of materials behavior at elevated temperature, specifically by investigating the fundamental aspects of small-scale transient plasticity or specific properties, e.g. fracture toughness of newly developed multilayer thin film materials. However, as the temperature goes up, the associated thermal drift becomes a significant issue which can affect the accuracy of measurements. Therefore, the grand challenge is to provide a very stable and well controlled testing condition with minimum thermal drift, in order to precisely measure the mechanical response at elevated temperatures while avoiding more complexity in the process.

This backgrounder discusses the key features of laser heater system designed for the Nano Indenter G200 introduced by Keysight Technologies, with the capabilities of performing high temperature nanoindentation with high accuracy. Utilizing laser heater system to heat both sample and indenter tip enables engineers to obtain more reliable data in much faster and easier testing conditions.

Developing of the Keysight's G200 laser heater system over the past decade involved several setups with modification to the conventional sample holder, utilizing a laser as the heating element to precisely heat up the sample to the desired temperatures. Keysight's high-performance G200 Laser heater is capable of independently heating both indenter and sample which results in lower contact thermal drift during measurements caused by the heat transfer between the sample and indenter. Thermal stabilization and reducing other sources of thermal drifts are managed within current laser heater design by using insulating components, thermal expansion materials of low coefficient and water-cooling systems.

The Nano Indenter G200 from Keysight Technologies, Inc. is the world's most flexible, and user-friendly instrument for nanoscale mechanical testing. It is now equipped with a laser heater system to provide very precise and stable testing conditions for elevated nanoindentation. Advantages include the capability to measure various nanomechanical properties, e.g. hardness and modulus at precisely controlled temperatures up to 500°C (with 0.1°C accuracy) and to test a wide range of samples under highly dynamic temperature conditions. To ensure reliable data, the system minimizes drift associated with heating by using a heated tip and the laser as a heating source. Capability of precise control on heating and cooling rates is another advantage of this feature. Ultra-fast heating/cooling at ~20-25°C/sec is achievable. The system is also fully integrated with NanoSuite software to adopt various prevailing test methods, including dynamic measurements enabled by Continuous Stiffness Measurement (CSM) option. The G200 laser heater system also gives users the option to purge samples with various gases to avoid contamination and oxidation. New XP indenter head is uniquely designed for high temperature, long-term usage; and sapphire and boron nitride tips are available for reactive testing conditions.

Apart from elevated hardness and modulus measurements, scratch and wear tests can also be performed on various materials under operationally relevant conditions. Understanding nanoscale fundamental deformation mechanisms in time or displacement-rate measurements, i.e. creep and super plasticity and rapid thermomechanical cycling with high accuracy becomes possible. Thermal activation phenomena such as phase transformation and brittle to ductile fracture transitions can also be studied, as well as elevated fracture toughness measurements, for example, on multilayer metallic/ceramic thin films.

Here we provide the basic specifications and capabilities of the G200 laser heater system.

	G200 Laser Heater System
Sample Heating Method	Laser
Heatable Indenter Tip	Laser (optical fiber) (Various geometries for diamond, sapphire and conical cub boron nitride indenters are available)
Heating and Cooling Rates	Ultrafast 20-25 °C/sec
Thermal Equilibrium	High (Both sample and tip are heated to the same temperature - no thermal drift)
Accuracy	High                                 (Due to the fast and high thermal equilibrium and no thermal drift)
Sample Size	No restrictions (Conducting tapes can be used to transfer heat from bottom to top faster for large thicknesses)
Measurements	Real time temperature measurement on tip and sample; Hardness and Young modulus; Compatible with Dynamic Nanoindentation

As the chart shows, the G200 laser heater provides a closed loop control of the tip temperature and the sample temperature for elevated nanoindentation test.  This laser heater setup innovates the heating methodology by using specially designed heatable indenters to achieve highest accuracy in measurements. Ultra-fast control on testing temperature using G200 laser heater provides opportunities for scientists and engineers to design more accurate tests for evaluating time and displacement-rate sensitive material behaviors at various temperatures

To expand the applications of Nano Indenter G200, Keysight has developed a new G200 system solution that makes use of a stage with sample heated by a precise high-power diode laser. Figure 1 shows the laser heater product which includes the stage, controller and the software.

Figure 1: The Keysight G200 laser heater system

Advantages of laser heater include the ability to measure various nanomechanical properties at precisely controlled temperatures and to test a wide range of samples under highly dynamic temperature conditions. To improve the reliability of measurements, the system minimizes drift associated with heating by using a heated tip and the laser as a heating source. Figure 2 shows test results on fused silica sample conducted at 27 °C and 500 °C, respectively.

Figure 2: Nanoindentation on fused silica at elevated temperatures

Conventional heaters not only heat the substrate, but the instrument as well, resulting in excessive thermal drift. This common yet formidable problem greatly compromises the accuracy of nanoindentation measurements. Keysight’s new laser heater, however, implements the latest available diode laser technology to minimize the heat-generating area in the heater to precisely matches the substrate size. In addition to its adjustable laser spot size, the G200 uses function-optimized materials that reduce the thermal drift of the system to enable the dynamic nanoindentation measurement.

Keysight’s laser-heated indenter tip for the G200 prevents disturbance of the substrate temperature during measurement, which is critical for working with materials of poor thermal conductivity and mechanical properties that are strongly affected by temperature. The tip and the sample are kept at the same temperature as one another, allowing G200 users to perform highly precise high-temperature continuous stiffness measurement as well. Figure 3 shows an example of using laser heater system to measure the modulus values at elevated temperature on Titanium sample.

Figure 3: 10 sets of continuous stiffness measurement (CSM) on Titanium at 20°C, 200°C, 400°C, 450°C and 500 °C, respectively

Nanoscale mechanical properties evaluation at elevated temperatures is always accompanied by measurements uncertainties, due to increasing amount of temperature gradient at the contact surface and in the whole system frame. Keysight’s G200 laser heater overcomes these limitations and makes these measurements much faster with high accuracy. Using laser heating source to independently heat both indenter and sample while monitoring the temperatures provides the highest thermal stabilization and eliminates all sources of thermal drift.

Keysight Technologies is a leading technology company that helps its engineering, enterprise and service provider customers optimize networks and bring electronic products to market faster and at a lower cost. Keysight’s solutions go where the electronic signal goes, from design simulation, to prototype validation, to manufacturing test, to optimization in networks and cloud environments. Customers span the worldwide communications ecosystem, aerospace and defense, automotive, energy, semiconductor and general electronics end markets. Keysight generated revenues of $2.9B in fiscal year 2016. In April 2017, Keysight acquired Ixia, a leader in network test, visibility, and security. More information is available at www.keysight.com.