Metal Finishing Guide Book

2013

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article. Due to stringent quality standards in the coating industry, it is necessary to be able to test the hardness of coatings with accuracy and repeatability. The hardness of thin coatings on tool bits, the viscoelasticity of protective coatings on optical lenses, the low friction coatings in consumer products all require precision application of millinewtons of force and corresponding measurements of depth in nanometers. This has led to the development of nanoindentation. Nanoindentation Instrumented indentation testing, more commonly referred to as nanoindentation – or, in simpler terms, depth-sensing indentation employs high-resolution instrumentation to continuously control and monitor the loads and displacements of an indenter as it is driven into and withdrawn from a material. The analysis of the measured force-displacement curves described in ISO 14577 is based on work by Doerner and Nix and Oliver and Pharr 2, 3. Developed in the mid-1970s, nanoindentation is used to characterize a variety of mechanical properties of any material that can be measured in a uniaxial tension or compression test. While nanoindenation is most often used to measure hardness, it is also possible to calculate the modulus and creep using the data collected in this test. Methods using nanoindentation testers have also been devised for evaluating the yield stress and strain-hardening characteristic of metals, the storage and loss modulus in polymers, and the activation energy and stress exponent for creep. The fracture toughness of brittle materials can be estimated as well using optical measurement of the lengths of cracks that have formed at the corners of hardness impressions made with sharp indenters. Construction of Testing Equipment Equipment used to perform nanoindentation consists of three basic components as shown in Figure 1: (a) An indenter mounted onto a rigid column (b) An actuator for applying the force (c) And a sensor for measuring the indenter displacements Small forces are generated either electromagnetically with a coil and magnet assembly or electrostatically using a capacitor with fixed and moving plates or with piezoelectric actuators. Displacements may be measured by eddy current sensors, capacitive sensors, linear variable differential transducers or laser interferometers. A diamond is typically used to make indenters because it has high hardness and elastic modulus. This minimizes the contribution to the measured displacement as compared to those that are made of other lessstiff materials like sapphire or tungsten carbide in which case the elastic displacements of the indenter must be accounted for. Vickers geometry indenter, a four-sided pyramid, is most commonly used in higher load nanoindentation tests for its durability. The Berkovich geometry indenter is used for measurements of a few nanometers for two reasons; they are very sharp, thus they cause Figure 1: Schematic of typical nanoindentation tester with a force actuator and displacement sensor. 451

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