Nanoindentation

Nanoindentation provides a much more sensitive mechanical test for advanced materials such as nanometer thin films and coatings or nanoscale features on surfaces where more traditional methods of hardness testing or tensile testing would be less useful.

The testing instrument used for performing the nanoindentation tests is a Nanomechanical Test System (manufactured by Hysitron, Inc., USA). This Nanomechanical Test System is a high-resolution nanomechanical test instrument that performs nano-scale quasi-static indentation by applying a force to an indenter tip while measuring tip displacement into the specimen. During indentation, the applied load and tip displacement are continuously controlled and/or measured, creating a load-displacement curve for each indent.  From the load-displacement curve, nano-hardness and reduced elastic modulus values can be determined by applying the Oliver and Pharr method and a pre-calibrated indenter tip area function and a pre-determined machine compliance value. The instrument can also provide in-situ SPM (scanning probe microscopy) images of the specimen before and after indentation. Such nanometer resolution imaging function is accomplished quickly and easily by utilizing the same tip for imaging as for indentation. The in-situ SPM imaging capability is not only useful in observing surface features, but also critical in positioning the indenter probe over such features for indentation tests.

Typical Experimental Results

Applications

For more information please read our application notes:
In-situ and Small-Volume Fracture Toughness Measurement via Nanoindentation, PDF
Nanoindentation for Hardness and Elastic Modulus Measurements at Nanoscale, PDF

Nanohardness and Elastic Modulus Depth Profiles of Solvent Soaked Polycarbonate

Instrument: Multi-Technique and Full-Feature Nanoindentation System

Key Specifications

In-situ and Small-Volume Fracture Toughness Measurement via Nanoindentation

For hardness testing via nano/micro indentation, any cracking caused by sharp indenter tip and excessive load is undesirable and will lead to questionable data. However, such phenomenon and capability has been proven to be very useful in evaluating one of the critical mechanical properties of materials: fracture toughness. Fracture toughness is a measure of the materials ability to resist crack propagation and fracture under stress. Commonly used methods for evaluating fracture toughness of materials include bending, tension and impact tests of a specimen with a sharp crack or a defined notch. As regulated and recommended by many ASTM and international testing standards, these methods require the specimen with sufficient thickness and dimensions to ensure measurement validity. In many industrial and technical applications that involve small volume of materials, however, these requirements could not be practically met, for example in thin films, coatings, welds and miniaturized devices. The unmet needs by the conventional fracture toughness measurement methods have offered an excellent opportunity for the nanoindentation based techniques that are developed for mechanical characterization of small volume materials at nanoscale. Benefited from the established model and in-situ scanning probe microscopy (SPM) imaging capability, fracture toughness measurement via nanoindentation has become a preferred technique for in-situ and small-volume fracture behavior study of materials.

To measure fracture toughness of small volume of materials, a relative high load is chosen for a nanoindentation routine with the goal of creating cracks at the corners of the indent. Then the indented surface is imaged using the nanoindenter’s in-situ SPM imaging function to capture the fine features of corner cracks of the indent as shown in Figure 1. The fracture toughness of the material is calculated using the following equation:

Where K_c is the indentation fracture toughness; E is elastic Young’s modulus, H is the hardness, P is the peak load and c is the average crack length from the center of the indent to the tips of the cracks. The constant, α, is a value related to the tip geometry and its values are known for cube cornered, Berkovich and Vickers tips.

Very conveniently, the hardness values required for determining the fracture toughness can be easily measured by using a smaller load and performing the normal nanoindentation test without causing any cracks. The Young’s modulus is derived from the reduced elastic modulus determined through nanoindentation and the Poisson’s ratio of the material. The crack length is measured from the center of the indent for all formed cracks using imaging software. Table 1 presents indentation fracture toughness values of a few specimens made of different materials using nanoindentation method.

Nanoindentation for Hardness and Elastic Modulus Measurements at Nanoscale

Nanoscience and nanotechnology accelerate the proliferation of novel materials and devices possessing small sizes and low dimensions such as nanomaterials and ultra thin films. Mechanical testing and characterization of these materials have exposed challenges to the traditional hardness and tensile testing and measurement tools. Nanoindentation, also referred to as instrumented or depth-sensing indentation, is a proven technology for measuring nanomechanical properties of materials and miniaturized devices. Both hardness and elastic modulus of a material can be precisely determined through analysis of a load-displacement curve enerated by a single nanoindentation test using the well-established Oliver-Pharr method.

The nanoindenter equipped in Ebatco’s Nano Analytical and Testing Laboratory (NAT Lab) is a full-feature, multi-technique nanomechanical and nanotribological test system. It performs closed-loop controlled nanoindentation tests with sub-nanometer and nanoNewton resolutions. Experiments can be conducted at room, elevated or reduced temperature, submerged in liquid, or under humidity control. The in-situ scanning probe microscopy (SPM) capability of the instrument enhances the nanoindentation function by enabling SPM imaging of the surface and positioning the indenter tip with nanometer precision over the feature to be studied. The materials that have been tested cover almost all man-made materials including alloys, ceramics, composites, glass, metals, polymers, steels. The applications of nanoindentation onto natural materials like wood, bamboo, geological samples, and biological samples such as bones, tooth, and arteries are increasing. Examples of material formats and devices that can be tested through nanoindentation include thin films, coatings, nanoparticles, nanowires, bulk material surfaces and interfaces, MEMS, and electronic and biomedical devices.

Figure 2. Nanohardness profile for a chemically treated titanium alloy (upper) and the tested surface with the indents imaged through in-situ SPM imaging (lower).