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Instrumented Indentation Testing: A Draft ASTM Practice

A task group of ASTM Subcommittee E28.06 on Indentation Hardness Testing, part of Committee E28 on Mechanical Testing, is working on a draft practice that will define the basic parameters of instrumented indentation testing. The draft practice is intended to be the first of several planned standards that will address the needs of industry for standards designed to cover this relatively new material testing technique.

What Is IIT?

Basically, instrumented indentation testing is a mechanical testing process designed to determine the properties of materials by putting an indentation in the surface of a material sample. Surface indentation has long been the domain of hardness testing. Rockwell, Vickers, Knoop, and Brinell are traditional indentation hardness tests that are well defined by various ASTM standards: E 18, Test Methods for Rockwell Hardness and Rockwell Superficial Hardness of Metallic Materials; E 10, Test Method for Brinell Hardness of Metallic Materials; E 384, Test Method for Microindentation Hardness of Materials; E 92, Test Method for Vickers Hardness of Metallic Materials, etc. However, while hardness values are frequently determined using IIT, the technique has gone beyond normal hardness tests by allowing tests at much lower test forces and indentation depths (into the nano ranges) and by determining other properties of materials in addition to hardness from the indentation process.

The mechanics of IIT are simple in principle. Like a hardness test, an indenter with a well-defined geometry is forced into the surface of a sample using a known test force (see Figure 1 and Table 1). Unlike a hardness test, where only the indent size or depth is measured after the total force has been removed, IIT utilizes high-resolution instrumentation to continuously monitor and control the displacement of the indenter as it penetrates into and is withdrawn from the sample. The data points generated during the indentation process are stored together in the memory of a computer. A visual load/unload curve (see Figure 2) is normally generated from the data. All of these data are then analyzed by any number of specifically developed algorithms to determine the desired material property of the sample.

Why Is IIT Testing Needed?

IIT was developed to determine the properties of materials that, due to their configuration, could not be tested using other conventional methods. After approximately 20 years of development, IIT has become the primary technique to examine the properties of thin films and coatings as well as surfaces that have been laser heat-treated or ion implanted. Ceramics and other super-hard coatings and materials are also frequently tested using IIT. Hardness values can be determined from IIT test data, however, a major objective for doing IIT is to determine additional mechanical properties of the subject materials.

The most universally accepted property determined from IIT is elastic modulus (E/(1-v2) where E = Young’s modulus and v = Poisson’s ratio). The material’s modulus can be easily determined from the slope of the unload curve (see Figure 2, line 3). Other properties such as yield stress and strain-hardening characteristics of metals can also be observed, however, the analyses for those properties have not been developed to the point that they are widely accepted. Almost any property that can be measured by uniaxial tension or compression testing can be measured or estimated using IIT. In addition, coating separation can be detected by observing discontinuities in the load curve, and fracture toughness may be estimated by optically measuring the cracks created by the indenter.

Why Is a New Standard Needed at This Time?

Worldwide, there are several manufacturers of instruments that have combined to sell hundreds of testers that can perform IIT tests. In addition, there are a number of instruments that have been built by various universities and other organizations to experiment with the test data that they generate. The numbers of instruments are growing and the amount of data being generated is significant. The traditional hardness test methods have some similarity to IIT (for example, a Vickers indenter using ASTM E 384 can be used by both), as does a standard tensile test. However, no existing standard can cover the range of requirements needed to do a proper IIT test. Some of these requirements are as follows:

• Extremely large ranges of test forces — as low as 1 nN up to 3,000 kg;
• Extremely small displacement measurements – down to 0.1 nm;
• Zero point determination;
• Indenter shape correction; and
• Instrument frame compliance determination.

No existing standard can address these items satisfactorily. In addition, there is no common format for reporting the test results.

Given the current use of IIT, and the variety of instruments available on the market, there is a major need for a standard to establish a level of performance for everyone to achieve. The draft practice is intended to provide the common ground among all of the instrument manufacturers and to give users confidence that their instruments have the capability to perform a proper IIT test. By defining the accuracy of the basic parameters of the test, i.e., test force, displacement, indenter shape, etc., the practice will allow manufacturers to produce instruments that can achieve a performance level that will provide the user knowledge that the tests they are conducting have a level of accuracy consistent with the technology currently available. The scope of the practice is to define the basic test parameters and the methods needed to verify and monitor the performance of the instrument.

Why a Practice Rather than a Test Method?

The IIT process can clearly be divided into two separate important sections — the test procedure that generates the test data and the analysis that is used to produce a result. Although there is more than one way to perform the indentation process, there is currently agreement among the manufacturers on how to do the test and record the raw data. Also, there is little disagreement on how to address the critical items such as zero point, indenter shape and frame compliance. There is however, a significant concern that the methods used to perform the analysis that provides the material property results, except for modulus, is not at the same level of acceptance. Indeed, there is serious concern within the community that some of the algorithms used to determine yield strength, for example, are not ready for wide acceptance. Therefore, faced with this situation and not wanting to risk delays dealing with numerous negative ballots, the task group felt that it was more beneficial to industry to define the items that can be outlined properly now in the form of a practice and allow the test methods to be generated later as they gain wider acceptance by the users. In addition, it’s expected that having a practice in place will provide a sound structure that will enable the analysis methods to be defined more quickly.

There is one exception. While a practice normally only defines the items necessary to perform the test correctly, a standard for IIT requires more. Indentation tests require that some performance testing using test blocks be done to verify that the instrument is operating correctly. This is an excellent system test and is relatively easy to perform. To accomplish that function, some sort of ability to generate a test result is required. Therefore, the practice will include a test method that will use all of the IIT functions to determine modulus and hardness of reference specimens. This test method within the practice will close the loop by providing a way to verify that the instrument is working properly during day-to-day operations.

Another critical item that the practice will address is the test report. Without a common indentation procedure, it is possible to generate widely different results doing what may appear to be the same test. With a proper test report it will be much easier for the user to understand results and reproduce them on a variety of similar instruments with a reasonable level of confidence.

The draft practice is only the first step. After it is completed, the foundation will exist for other groups to write a variety of different test methods that refer back to the practice for the basic process. While IIT has a way to go before everyone will be using it, there are a significant number of tests being done today that cannot be done any other way. The goal of Task Group E28.06.11 is to provide a test practice that will advance the process to the point where meaningful results can be obtained by anyone who uses the technique. The draft practice should provide that starting point. //

Copyright 2003, ASTM

Ed Tobolski has been involved with hardness testing for over 28 years, all with Wilson Instruments. He was engineering manager for Wilson for five years before becoming the president in 1990. Since the acquisition of Wilson by Instron Corporation in 1993, he has been the general manager of hardness products. Tobolski is chairman of ASTM Task Group E28.06.11 on Instrumented Indentation Testing.