This paper presents the design, construction and performance of a surface- referenced nanoindentation instrument termed a precision nanoindentation platform (PNP). The PNP is a symmetrically designed instrument with a centrally located indenter tip attached to a force cell for measuring the forces between the tip and specimen. Penetration of the indenter tip into the specimen surface is measured using two proximity sensors placed symmetrically about the indenter. Each proximity sensor is attached to a piezoelectric actuator that is servo controlled to maintain that sensor and surface reference frame to which it is attached at a constant height relative to the specimen surface. As the indenter tip penetrates the specimen surface, the movement of the tip relative to the two the surface reference frames is measured using capacitance gauges and the average of these displacements is used as a measure of penetration depth. The current indenter is capable of applying indentation forces of up to 150 mN with a noise floor below 2 µN rms for a sampling rate of 1 kHz, and measuring displacement with 0.4 nm rms noise for the same sampling rate. The proximity sensors are capable of maintaining surface height variations of less than 1.0 nm with penetration depths of up to 10 µm. Long-term stability tests indicate a total uncertainty in indentation depth less than 10 nm for periods as long as 12 hours. To demonstrate instrument accuracy, repeated indention cycles were performed on a fused silica specimen using incrementally increasing indention force. From this test, an average value of 72 GPa ± 1.5 GPa for Youngs modulus is obtained from the elastic unloading curves for 10 measurements ranging in maximum force from 5 mN to 50 mN. To demonstrate longer-term stability, a PMMA specimen was subjected to a fixed 5 mN indentation force for 4 hours; two distinct creep-like mechanisms were observed.
Citation: Review of Scientific Instruments
Pub Type: Journals
nanoindentation, instrumented indentation, surface referencing