Research On Measurement Traceability For Mechanical Properties Of Nanomaterial

Nanoindentation, also known as instrumented indentation or depth sensing indentation, is a well-established technique in materials testing, with its origins in the field of metals, ceramics, polymeric and biological materials as a means of characterizing hardness and modulus. In contrast to traditional hardness testers, nanoindentation systems record the raw data about force and displacement when the transducer pushes the indenter into the material. Then the software can calculate the hardness and modulus about material's mechanical character. The instrument should be taken a series of calibration which include the electrostatic force constant and plate spacing, machine compliance, piezo scanner calibration, load scale factor(LSF), displacement scale factor(DSF) and tip shape (area function) calibration before having a real test. This paper has made a research on the traceability of load scale factor and displacement scale factor in the quasistatic nanoindentation mode under nanomechanical testing system, establishing a system of traceability for the data analysis and processing on the results of the test, and applying the result of the test to the calibration of the nanomechanical testing system, which has been well verified in Interlaboratory Comparison through test. In the end, this paper also explores the direct and indirect traceability of the geometry of the tip of the indenter.1. Load scale factor. The transducer's center plate is suspended within the transducer by springs, so the load scale factor could be calibrated by using the traceable masses which are hung from the center plate through the use of a special hook-shaped tip. When the traceable masses are hung from the center plate, the center plate will displace according to Hooke's law. The load scale factor (LSF) is the slope of the displacement voltage, which can be obtained from the"MicroScope Feedback"on the transducer controller, vs. the traceable masses.2. Displacement scale factor. The displacement scale factor (DSF) represents the correlation between the displacement voltages from the transducer to the actual position of center plate of the transducer. To calibrate the DSF, the physical displacement of the center plate is simultaneously recorded by the transducer and interferometer. A special tip with a mirror at the end is attached to center plate so that an interferometer with high resolution can be used to measure the indenter's displacement. The applied voltage to the bottom outer plate is adjusted to displace the indenter by 1μm-5μm.The slope of the recorded voltage vs. recorded displacement is the DSF. A set of multi-pass position laser interferometer system is established. Its higher resolution can meet the needs of nanometrology. At the same time, The Renishaw's interferometer is used for doing this research. At last, the calibrated value of LSF and DSF are used for testing the fused quartz sample with known modulus. The result is better than before.3. Tip area function. The important source of uncertainty in nanoindentation measurement is the geometry of the indenter tip. In this paper, two methods of calibrating the tip area function will be deduced. (1) Direct determination from co-ordinate measurements obtained using traceable Atomic Force Microscope (AFM). (2)Indirect determination can get from indenting into reference materials with known Young's modulus and Poisson ratio. The direct measurement of geometry of a Berkovich indenter is determined using a traceably calibrated AFM from National Institute of Metrology. Because the calibration of tip area function is complicated, so the group will apply another subject to do this research.In addition, in this paper, a particular description of instrumented indentation is given with regard to current instrument technology and analysis method. Some of the most commonly encountered sources of error and methods of accounting for them are also included.