EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

It is well known that creep effects can be observed during nanoindentation experiments when the load is hold constant at maximum force. Many attempts have been made to correlate nanoindentation creep curves under constant load obtained with sharp or spherical indenters with the results of macroscopic creep tests. However, they failed because the depth change under load is accompanied by a pressure reduction. This is in contrast to uniaxial creep tests under constant stress, for instance according to standard ISO 899-1. Another problem in long-term nanoindentation tests is the thermal drift that has to be considered if a depth resolution in the nanometer range shall be achieved. Several groups developed therefore a correction technique based on the dynamic contact stiffness during the constant load segment, assuming that the modulus stays constant over time.
A new test methodology has been developed that allows measurements under constant pressure by using sharp or spherical tips and that considers thermal drift effects by measuring the drift just before the creep segment and later on. The necessary calculations are done live during the measurement and used to correct the force accordingly. The dynamic contact stiffness is used to determine the pressure (equal to hardness) at maximum load and to adjust the force at lower constant pressure. It is necessary to reduce the pressure because otherwise the force for keeping the pressure constant would quickly exceed the instrument maximum. For long-term tests, the thermal drift is considered in the pressure calculation during the measurement by measuring the dynamic stiffness and assuming a constant modulus.
Results are presented for several materials and pressure levels and compared with creep results from constant force experiments.