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Part 3 of the ISO 14577 ¡°Instrumented indentation test for hardness and materials parameters¡° (IIT) concerns the calibration of reference blocks. Besides the widely spread activities for supplying reference blocks in the nano and micro range it is needed to be taken into account the macro range too. The paper reports on test results on ceramic materials which are tailored for reference blocks. The specimens made of Si3N4, SiC, or Al2O3 ceramics are highly homogenous and exhibit an increased quality with a roughness Ra ¡Ü 0.005 ¦Ìm. They are successfully used for reference blocks of the traditional Vickers and Knoop hardness. In comparison, the capability of hardened steel which is used for traditional hardness techniques is studied.
Instrumented indentation test for hardness and materials parameter from millinewtons to kilonewtons
(2002)
The ISO/DIS 14577 Metallic materials - Instrumented indentation test for hardness and
materials parameters Part 1-3 (IIT) concerns test forces up to 30 kN. The paper reports on
IIT at test forces ranging from 0.002 N to 1000 N on non magnetizable steel X8 CrMnN 18-
18 (1.386) with well polished surface using Vickers indenter and four hardness machines of
different design (Nano Indenter XP, Fischerscope H 100, Zwick Z005 with Universal
hardness head and a laboratory four-column set up materials testing machine) according to
the standard. Using mostly identical test parameters the results of the different machines are
almost in good agreement. Estimated small differences are caused by the different
uncertainties of the used machines and by different methods of mathematical analysis of the
detected raw data.
To validate the European standard ENV 843-4 for hardness measurements on ceramics, three classes of ceramic materials, silicon nitride, silicon carbide, and aluminium oxide, involving 19 ceramics in total, were tested using the traditional techniques Vickers (HV1), Knoop (HK2), and superficial Rockwell (HR45N). The use of new ceramic reference blocks certified according to the standards ISO 4547 and ISO 6507-1 for metallic materials was studied. If the hardness response of the tested materials does not involve chipping and cracking the application of high hardness reference blocks for training users to obtain hardness values comparable with the certified HV1 and HK2 values improves the reproducibility from about 10% to 1 to 3%. The scatter between the laboratories is similar to the scatter within the laboratories. The measurement of the indentation geometry on typical commercial ceramic materials can be made only with higher scatter and reduced reproducibility compared with typical metallic materials, which is caused by the stochastic indentation response. For such materials involving chipping and cracking (for instance SiC), the application of reference blocks with well-shaped indentations does not provide improved comparability of results between the laboratories. The actual indentation response of the ceramic material tested must be considered before selecting the appropriate hardness technique and test force. There is no significant difference between the abilities of the hardness techniques HV1, HK2, and HR45N to discriminate sensitively between materials of closely similar properties.
The fundamental quantities in the depth sensing hardness test are force and displacement. The penetration depth used for the calculation of a certain hardness parameter (for instance HU, Eq. (1)) makes it necessary to determine the contact point. As shown in the paper, fitting of the forcedisplacement curve, F(h), by a second order polynomial and extrapolating to F=0 is not always the right way to achieve a sufficiently small uncertainty of depth. The feature of that procedure is analysed by a simple mathematical model. In dependence of the testing force, the scatter of the experimental data, and the required limit of depth uncertainty, a profound selection of the fitting range, the fitting function, and the extrapolation function should be done.