Instrumented Indentation

Douglas Smith

Over the past fifteen to twenty years, an experimental technique known as instrumented indentation testing IIT, (also depth-sensing indentation or nanoindentation) has gained widespread acceptance as a method of obtaining local mechanical properties, particularly for coatings, thin films and other treated surfaces (e.g., ion-implanted). With this technique, an indenter, often a diamond pyramid or spherically tipped cone, is pushed into a test surface and withdrawn, in much the same way as is done with Vickers, Knoop or Rockwell hardness testing. The important difference between IIT and conventional hardness testing is that IIT machines are equipped with transducers that continuously monitor the force on the indenter and the displacement of the indenter into the test specimen through the entire loading/unloading cycle. These force-displacement data for each indent are stored and analyzed to provide information about both the elastic and plastic properties of the specimen. No measurements of the residual indentation impression are required. There is now a wide range of IIT machines commercially available, operating in ranges from micronewtons and nanometers to kilonewtons and millimeters. The properties most often obtained using IIT are the hardness and elastic modulus of the specimen (see the Hardness and Elastic Modulus section for definitions), although information about yield strength, fracture toughness and creep behavior can also be obtained in some cases.

The fact that mechanical properties are obtained from force-displacement data, without the need to image a residual impression, makes IIT particularly useful for very low-force testing of small volumes of material. It is not uncommon for researchers to obtain useful data from indentations as shallow as 10 nm to 20 nm in depth. When used at this scale, mechanical properties are obtained from a very small volume of material. Common applications of the technique at this scale include the study of coatings, thin films and individual phases in heterogeneous or composite materials.

At present, properties obtained by IIT are not commonly used in product specifications, because there are no national or international standard test methods that apply to the test or to the analysis of the force-displacement data. There is the German DIN standard, and there is good progress being made toward an ASTM Standard Practice and Standard Test Methods (in Task Group E28.06.11) and an ISO (TC 164, SC 3) testing standard. There also are no NIST Standard Reference Materials (SRMs) or international Certified Reference Materials (CRMs) currently available for indirect machine verification; these are being developed in conjunction with the test methods. Several machine manufacturers do however sell test blocks for checking machine performance. In addition, NIST and the Bundesanstalt für Materialforschung und-prüfung (BAM) in Germany have begun a joint program to develop thin film SRMs for mechanical property testing.