Penetration Depth Analysis

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Figure 21: Dependence of the penetration depth on dwell times for different indentation peak loads Figure 22: Dependence of the penetration depth on dwell time for different indentation peak loads in ln-ln plot
5.4 HV vs. penetration depth
Figure 23 shows the dependence of the Vickers hardness on the indenter penetration depth for 100mN to 1000mN and 10 s to 60 s dwelling time. The highest hardness value is 15.6GPa. As the indenter penetration depth increases, the hardness value decreases. This phenomenon suggests that there exist an indentation size effect (ISE). The ISE, according to Nix and Gao’s model, is related to the concept of geometrically necessary dislocations (GNDs) (Nye 1953), an inverse relationship between the GND density
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Hardness value is relatively steady with increasing the indentation load. In addition, 20 s group subjected to the shortest dwelling period attains highest hardness result, when the dwelling period is extended to 60 s, the hardness value decreases from 14.5 GPa at 20 s dwelling to about 12 GPa. Figure 24: Vickers hardness value versus indentation load
5.6 Reduced modulus – load curve
Figure 25 shows the dependence of reduced modulus on indentation load for different dwelling periods. The contact modulus is obtained from linear fitting of the unloading curves. The contact modulus begin with relatively large deviation of 320 GPa, 235 GPa and 270 GPa for dwelling period of 20 s, 30 s and 60 s for small indentation load and then gradually converge at around 220 GPa from 700 mN of the indentation load. Figure 25: Load dependence of contact modulus for different dwell times
5.7 Reduced modulus – dwelling time curve
Figure 26 shows the dependence of the contact modulus on the dwelling time for the indentation of Bi57In26Sn17 sample at different indentation loads. This plot further proves the steadiness of the data at the indentation load larger than 700 mN, and according to Oliver-Pharr’s model, the contact moduli decrease with increasing dwelling time and finally converge to relatively constant
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There’s no bulge, break point, pop-in, and pop-up observed from the load-displacement curves of the 100mN to 1000mN peak load indentation tests for a fixed dwell time, which indicates a good consistency among the indentations. Also, the imprints showed good clarity. The load dependence of the diagonal length for different dwell times further confirmed that. The validity of the microindentation for Bi57In26Sn17 lead-free alloy data were ensured.
2. The Vickers hardness value for Bi57In26Sn17 lead-free alloy was found affected by the following factors: a). indentation depth b) dwell time.
3. The contact modulus for Bi57In26Sn17 lead-free alloy was dwell-time dependent for small dwell times, and became constant for large dwell times.
6.2 Suggestion
It was strongly recommended to prepare the Bi-In-Sn alloy sample with care, since the alloy has a low hardness value comparing to most of the metals and ceramics. Mechanical polishing is very likely to cause material surface

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