Depth sensing indentation and methodology for mechanical...

Details Depth sensing indentation and methodology for mechanical... Depth sensing indentation and methodology for mechanical...

1998-02-19

2001-06-19

Raevis, Robert (Department: 2856)

Measuring and testing

Hardness

By penetrator or indentor

Reexamination Certificate

active

06247355

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to measuring mechanical properties of materials, and more particularly to indentation testing with the purpose of measuring such properties as hardness, yield strength, strain hardening exponent, and Young's modulus.
BACKGROUND OF THE INVENTION
The testing of mechanical properties of materials is a well-studied art. Standard tests exist for measuring mechanical properties such as Young's modulus, strain hardening exponent, yield strength, hardness, and the like, and many materials have been carefully characterized in terms of mechanical properties. One set of techniques for determining mechanical properties in materials involve tests in the macro regime in which, for example, a sample of material is stretched and its overall mechanical response inferred in terms of stress and deformation. These and other techniques have served, and continue to serve, an important role in determining critical mechanical properties so that careful processing and selection of materials for use in a variety of industrial settings can be made.
The above-described techniques, however, typically require large samples of material and generally are destructive of those samples. There is an increasing need in the field of materials science for analysis of small-scale samples in a manner that can be essentially non-destructive. The increasing rate of miniaturization in the semiconductor field, increasing interest in thin coatings for optical, electronic, magnetic, and mechanical devices, and increasing use of functionally-graded materials have led to a need for in situ testing of mechanical properties in small-scale structures. Additionally, there is interest in probing properties of individual phases, grain boundaries, and interfaces between phases and properties of novel materials such as nanocrystalline materials, or laminated or fibrous composites.
Indentation testing has developed as a viable technique for determining certain properties in a variety of materials at a very small scale, essentially non-destructively. Indentation testing typically involves placing a sample to be tested on a stage and applying a load to a surface of the sample via an indenter so as to slightly deform or penetrate the surface, followed by removal of the load. Several techniques can be employed to derive certain properties of the material from characteristics of the interaction of the indenter with the material. One set of techniques involves measuring an area of indentation during or after indentation, for example, optically, refractively, via surface profilometry, etc. U.S. Pat. No. 4,627,096 (Grattoni, et al.), U.S. Pat. No. 4,945,490 (Biddle, Jr. et al.), U.S. Pat. No. 5,284,049 (Fukumoto), U.S. Pat. No. 5,355,721 (Las Navas Garcia), U.S. Pat. No. 5,483,621 (Mazzoleni), U.S. Pat. No. 5,486,924 (Lacey), U.S. Pat. No. 4,852,397 (Haggag), U.S. Pat. No. 5,490,416 (Adler), U.S. Pat. No. 3,822,946 (Rynkowski), and others follow this procedure. For example, the measured area of indentation can be used to determine a simple “flow” or hardness value for the material, which is defined as the load applied divided by the projected area of the indentation. Or, the dimension of any cracks formed in the sample surface can be measured to determine the toughness of the material. Alternatively, the depth of penetration of the indenter as a function of applied load can be measured, and calculations performed to estimate roughly some mechanical properties. As discussed below, these techniques, in the prior art, have disadvantages.
Various shapes of indenters, for example spherical, cone-shaped, and pyramidal geometries can be used in indentation testing. Sharp indenters (e.g., cone-shaped and pyramidal) can be used in conventional tests to apply a load to a sample surface to form an imprint, or until the surface cracks, followed by measurement of the area of imprint or determination of the crack length to measure hardness or toughness, respectively. One piece of indentation testing equipment utilizing a sharp indenter at ultra low loads is sold by Nano Instruments, Inc. as the Nanoindenter™ indentation tester. The Nanoindenter™ is a relatively complex, self-contained unit including an indenter system, a sharp indenter, a light optical microscope, a moveable x-y table, and a computer. Analysis of load/depth curves with loads of less than one Newton and displacement of less than one &mgr;m using a three-sided pyramidal indenter is most typically carried out.
Blunt indenters, for example those having a surface contacting the sample surface that is spherical, are advantageous for use in indentation testing under certain circumstances for several reasons. First, less sample-destructive analyses often can be carried out. However, with blunt (spherical) indenters, sensitivity problems are maximized since displacement of the sensor into the sample surface, at a particular applied load, is less than displacement with a sharp indenter. This is especially problematic in measuring very soft materials. Spherical indenters have, therefore, found most use in techniques in which load is applied to a sample surface and the diameter of the indentation formed thereby is measured using, for example, optical means.
U.S. Pat. No. 4,820,051 (Yanagisawa, et al.) discloses self-contained apparatus for measurement of the hardness of materials. A load is applied to a sample via a sharp indenter (having a tip with a radius of curvature between 0.01 and 0.1 &mgr;m), and the displacement of the indenter relative to the sample is determined. An optical sensing mechanism determines the penetration depth of the indenter. Yanagisawa, et al. measure load/displacement values only during application of the load, with a self-contained unit, and measure only hardness of the material. Yanagisawa, et al. measure penetration and, with knowledge of the indenter geometry and assuming that no pile-up or sinking-in of the material at the contact perimeter occurs (which is known to be a factor that must be taken into account for accurate measurement), appear to calculate what the area of the indentation would be without sinking-in or pile-up, to measure hardness. Measurements are made in a load range of less than one Newton.
Gattoni, et al. (U.S. Pat. No. 4,627,096) recognize that sinking-in during indentation testing should be taken into account when measuring hardness of a sample (see, e.g., FIGS. 1 and 4). Therefore, Gattoni, et al. illuminate the sample carrying the impression and optical processing equipment is used to determine the contact area between the indenter and the sample.
U.S. Pat. No. 4,699,000 (Lashmore, et al.) describes self-contained apparatus and methods for determining hardness. Displacement of the indenter into the sample as a function of time, using sharp indenter geometries, is made and a load versus displacement curve is thereby derived. Lashmore, et al., measure penetration and, with knowledge of the indenter geometry and assuming no pile-up or sinking-in, calculate the area of the indentation (column 5, lines 5-43) to measure hardness using a self-contained unit. The displacement sensor of Lashmore, et al. is quite removed spatially from the indenter (FIGS. 2, 3). Lashmore, et al. state that modulus, yield strength, impact, hardness, creep and fatigue also can be determined. No indication, however, is given as to how to go about determining these properties or whether, using the described techniques, accurate determination of these properties can be made.
U.S. Pat. No. 5,133,210 (Lesko, et al.) exploits thermal expansion in applying a load to a sample surface via a spherical indenter. Various measurements of load versus penetration (displacement) are made and theories are presented as to how various mechanical properties can be derived. However, Lesko, et al. do not take into account sinking-in or pile-up of material at the contact perimeter. Additionally, it appears from FIG. 5 of Lesko and theoretical analysis (Col. 5, lines 35-39 and Col. 4, lines 50-53) of Lesko, et al., that the

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