KeywordsIndentation Depth Indentation Hardness Indentation Modulus Test Force Indentation Force
Nanoindentation is a method for testing the hardness and related mechanical properties of materials, facilitated by high-precision instrumentation in the nanometer scale, as well as analytical and computational algorithms for result evaluation.
Theory and Application
The origin of the indentation method goes back to Martens at the end of the nineteenth century (Martens 1898). Later, the force–indentation depth curve and a method for deriving hardness and elastic modulus were described (Ternovskij et al. 1973). In the early 1980s, the development of a high-resolution nanoindentation followed (Newey et al. 1982). In the 1990s, refinements to the instrumentation and methods for extracting mechanical properties from the test contributed to the establishing of nanoindentation as an important tool in materials research (Oliver and Pharr 1992 and Field and Swain 1993). Since then, further developments of the technique and its use in a range of materials have been performed. This is highlighted by several special focus series, the most notable being the Journal of Materials Research series in the years 1999, 2004, and 2009. The CIRP Annals in 2010 published a keynote paper on nanoindentation (Lucca et al. 2010). The standardization of the instrumented indentation test started in the 1990s in Germany (DIN 50359-1 to -3), establishing the basis for the development of the international standard ISO 14577-1 to -3, published in 2002. To address the peculiarities of indentation of thin films and coatings, the ISO 14577-4 was developed. In 2008, the ISO/TR 29381 was published, allowing for the evaluation of tensile properties of metallic materials by instrumented indentation.
Method and Instrumentation
Nano-range: h max ≤ 200 nm
Micro-range h max > 200 nm and F max < 2 N
Macro-range 2 N ≤ F max ≤ 30 kN
Typical indenter materials include diamond, tungsten carbide, and sapphire. Indenter geometries are pyramidal with square base (Vickers), pyramidal with triangular base (Berkovich and cube corner), and spherical.
Indenter Tip Calibration Procedures
A prerequisite for high accuracy in nanoindentation result evaluation is the precise knowledge of the indenter and its tip geometry, especially when results attained using different indenters or even devices need to be correlated (Oliver and Pharr 1992; Alcala et al. 1998; Herrmann et al. 2010). In nanometer scale, indenter deviations from the ideal geometry due to manufacturing imperfections or operation wear significantly affect the result accuracy (ISO 14577 2002; Bouzakis et al. 2001, 2002a, b). Therefore, analytical–empirical and FEM-based methods have been developed to overcome this problem.
Although diamond pyramids possess very high elasticity modulus and hardness, their elastic deformation during penetration can be considerably high, depending on the properties of the test material, causing significant changes in the contact surface (Bouzakis and Michailidis 2006).
Effect of Surface Roughness on the Nanoindentation Accuracy
A further crucial parameter affecting the accuracy of the nanoindentation results is the surface roughness, through the different contact area of the indenter with the test piece when indenting on roughness peak or valley. When indenting on relatively rough specimens, as experienced in most surfaces of technical materials, an indentation depth scatter may occur as a consequence of the surface topography.
Determination of Material Properties (Stress–Strain Law, Hardness, etc.)
An improved method for the determination of the Martens hardness is offered for homogeneous materials by calculating the slope of the increasing force/indentation depth curve, to avoid the determination of the zero point.
The analytical–empirical methods are fast and simple, offering adequate approximation of material mechanical properties. However, they have limited accuracy: (i) in small indentation depths, where the real indenter area function is required; (ii) in materials with high Young’s modulus, since the indenter is considered rigid; and (iii) when materials have graded properties, like thin films. Moreover, significant differences between Young’s modulus and E IT may occur, when either pileup or sink-in is present.
Further algorithms for determining stress–strain characteristics of materials are based on the representative stress and strain (Ahn and Kwon 2001), an inverse analysis by FEM (Dao et al. 2001), and on neural networks (Huber and Tyulyukovskiy 2004). FEM-supported methods offer advanced capabilities in determining all kinds of mechanical properties of materials, such as Young’s modulus, yield, and rupture strength.
Characteristic applications of nanoindentation in production engineering for estimating mechanical properties can be found in coatings on cutting and forming tools and on anti-wear and anti-corrosion surfaces/surfaces of manufactured products for determining their internal stresses.
- Bouzakis KD, Michailidis N, Hadjiyiannis S, Skordaris G, Erkens G (2002a) Continuous FEM simulation of the nanoindentation. Z Met Kd 93:862–869Google Scholar
- ISO 14577 (2002) Metallic materials – instrumented indentation test for hardness and materials parametersGoogle Scholar
- Martens A (1898) Handbuch der Materialienkunde für den Maschinenbau. Springer, BerlinGoogle Scholar
- Ternovskij AP, Alechin VP, Shorshorov MC, Khrusshchov MM, Skvorcov VN (1973) About micromechanical material tests by indentation. Zavodskaja Laboratorija 39:1242–1247Google Scholar