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Instrumented Indentation Testing

The “Instrumented Indentation Testing” feature in the October 2003 issue of SN is thought provoking since the IIT draft it references (Work Item WK382) has been in development by Task Group E28.06.11 for more than six years without a single ballot. However, the author’s views are presented as if they were based on a full consensus of the ASTM members or the technical community. The appropriate (i.e., precisely descriptive and non-confusing) title of the IIT practice should be “Standard Practice for Depth-Sensing Pyramidal Nanoindentation of Thin Films and Coatings,” in order to be compatible with the scope and the two figures of the article. The article contains too many errors to address in a brief response, so I will discuss just a couple of the most egregious.

Mr. Tobolski has no technical basis to state that other groups developing indentation test methods must wait (perhaps indefinitely) until the completion of his draft practice or to reference his draft practice even if there is no relevance to his “one size fits all” draft practice. His comparison of the IIT to hardness tests provides no argument for a need for a common IIT practice because, although hardness tests have the word “hardness” in common, each hardness test type has its own ASTM test method (e.g., E 10 for Brinell, E18 for Rockwell, E 92 for Vickers, etc.). There is no need for a common hardness practice to set basic parameters for how to properly run a hardness test or for how to write a test method for each test, and such a document does not exist. The ASTM Form and Style Manual (Blue Book) provides clear definition and distinction between a standard practice (that does not produce a test result) and a standard test method (that produces a test result), and that a standard practice is not a downgraded test method. Also, any test method included in the draft IIT practice must have its own “Precision Statement” from a complete round robin study performed by at least six laboratories.

The author’s claim that “Almost any property that can be measured by uniaxial tension or compression testing can be measured or estimated using IIT” ignores the physical and technical limitations of the indenter geometry (linear versus non-linear), the indentation force, and the volume and type of test material. For example, comments regarding obtaining the yield strength and strain-hardening characteristics from pyramidal indenters have no technical basis. A linear/pyramidal indenter (see figure 1 of the IIT article) can only produce a single value of strain regardless of its indentation depth (because of the fixed angle specific to the shape of pyramidal indenter). Moreover, bulk/macro behavior cannot be obtained from indenting a fraction of a single grain of a metallic material. Hence, one cannot obtain a stress-strain curve from a single point on the curve. Readers are encouraged to compare the IIT draft with the “ABI Test Methods” draft (Work Item WK381) being developed by task group E28.06.14 and should contact ASTM for copies.

I am not aware of any publications authored by Mr. Tobolski on the IIT subject. There is also no round robin study to prove or to support the article’s claim that “The most universally accepted property determined from IIT is elastic modulus,” and it is insufficient to merely refer to the schematic in figure 2. Moreover, performance tests of the nanoindentation IIT equipment using test blocks will not be possible since reference blocks with certified elastic modulus values do not exist.

Fahmy Haggag
Advanced Technology Corporation
Oak Ridge, Tenn.
Chairman, E28.06.14 on
Automated Ball Indentation
Test Methods (WK381)

Author’s Response

My article on IIT testing was intended to provide the members of ASTM an overview of a new standard that is being developed within E28. It contained a brief description of IIT, where it’s used, and why a new standard is needed. I believe that the article accomplished what I was asked to write about.

Mr. Haggag has chosen to attack the article and the standard by using inaccurate conclusions and misleading statements. For example, he states that some of the properties I suggest IIT can measure cannot be done with the pointed indenter shown in figure 1. That’s correct; current methods use a spherical indenter for those evaluations. In a short article it is not possible to discuss every level of detail.

The practice does allow for the use of several indenter types including Vickers, Berkovitch, cube corner, and spherical. Therefore, test methods using spherical indenters over a wide test force range will all fit within the practice.

I have personally given Mr. Haggag several draft copies of our practice for his review so he has access to the sections of the document that address spherical indenters and force ranges. In the interest of the readers of SN it would have been better if he actually had read and understood the intention of the standard before writing such a negative letter.

The task group, E28.06.11, is comprised of three of the world’s leading producers of IIT instruments and a scientist from NIST, who works with IIT equipment. Mr. Haggag’s letter seems to be an attempt to discredit the work of the task group. Currently my company does not offer any commercial IIT equipment, therefore my role within the task group is one of mediation. I played a similar role in the successful development of the new International Organization for Standardization IIT standard, ISO 14577. As a group we are dedicated to producing a truly generic standard that can be used by all manufacturers as described in my article and not favor any commercial concern.

Mr. Haggag’s recently balloted standard is written exclusively around his company’s product. Section 6.1 of his test method states that the instrument used “shall conform to the requirements of U.S. patent...etc.” Mr. Haggag holds the patent.

E28.06.11 will continue to develop a world class ASTM practice that will be beneficial to anyone desiring to perform IIT tests. We will forward it for ballot as soon as it is complete.

Ed Tobolski
Wilson Instruments
Canton, Mass.

Copyright 2003, ASTM