A new nanoindentation creep technique using constant contact pressure
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A new nanoindentation creep technique using constant contact pressure Olena Prach1,a), Christian Minnert1, Kurt E. Johanns1, Karsten Durst1 1
Physical Metallurgy, Materials Science, Technische Universität Darmstadt, Darmstadt 64287, Germany Address all correspondence to this author. e-mail: [email protected]
a)
Received: 7 February 2019; accepted: 9 May 2019
A new constant contact pressure (CCP) indentation creep method is presented, which is based on keeping the mean contact pressure as defined through Sneddon’s hardness constant, until a steady-state strain rate is achieved. This is in contrast to the conventional constant load–hold (CLH) creep experiments, where the load is held constant and relaxation in both hardness and strain rate occurs at the same time. Besides controlling the mean contact pressure, the dynamic stiffness is furthermore used to assess the indentation depth, thereby minimizing thermal drift influence and pile-up or sink-in effects during long-term experiments. The CCP method has been tested on strain rate sensitive ultrafine-grained (UFG) CuZn30 and UFG CuZn5 as well as on fused silica, comparing the results with those of strain rate jump tests as well as the CLH nanoindentation creep tests. With the CCP method, strain rates from 5 × 10−4 s−1 down to 5 × 10−6 s−1 can be achieved, keeping the mean contact pressure constant over a long period of time, in contrast to the CLH method. The CCP technique thus offers the possibility of performing long-term creep experiments while retaining the contact stress underneath the tip constant.
Introduction Instrumented indentation testing is a versatile tool commonly applied for measuring mechanical properties, such as hardness, Young’s modulus, and fracture toughness on a localized scale [1, 2, 3, 4, 5]. In recent years, significant advances have been made, which have allowed the measurement of thermally activated processes using indentation testing [6, 7, 8, 9, 10, 11, 12, 13]. Nanoindentation creep experiments play a significant role in determining the thermally activated properties on the nanoscale. The loading conditions for indentation creep tests reported in literature are fundamentally different from those for a conventional creep test, where deformation is observed under constant stress [6, 14, 15]. The pioneers in the nanoindentation creep testing are Mulhearn and Tabor [9]. Later, the constant loadingrate test, which was proposed by Mayo and Nix [16], enables an accurate determination of the strain rate sensitivity (SRS) m of the material. In the indentation load relaxation experiments [13], the position of the indenter is fixed after reaching a predetermined penetration depth and a decrease in the indentation load is monitored as a function of time. Nevertheless, it is rather difficult to keep the penetration depth constant, as a result of the continuous load and displacement change, which makes the
ª Materials Research Society 2019
analysis rather difficult [17]. The constant load test [12] is rather similar to the conventional uniaxial
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