Abstract
Nano-indentation, a depth sensing technique, is a useful and exciting tool to investigate the surface mechanical properties of a wide range of materials, particularly polymers. Knowledge of the influence of experimental conditions employed during nano-indentation on the resultant nano-mechanical response is very important for the successful design of engineering components with appropriate surface properties. In this work, nano-indentation experiments were carried out by selecting various values of frequency, amplitude, contact depth, strain rate, holding time, and peak load. The results showed a significant effect of amplitude, frequency, and strain rate on the hardness and modulus of the considered polymer, ultrahigh molecular weight polyethylene (UHMWPE). Load-displacement curves showed a shift towards the lower indentation depths along with an increase in peak load by increasing the indentation amplitude or strain rate. The results also revealed the strong dependence of hardness and modulus on the holding time. The experimental data of creep depth as a function of holding time was successfully fitted with a logarithmic creep model (R2 ≥ 0.98). In order to remove the creeping effect and the nose problem, recommended holding times were proposed for the investigated polymer as a function of different applied loads.
Highlights
Ultrahigh molecular weight polyethylene (UHMWPE) belongs to the polyethylene group of polymers with a repeating unit, [C2 H4 ]n, where n represents the degree of polymerization
A large decrease in hardness and modulus values occurred with increasing the contact depths, which we suggest, are again linked with the indentation size effects and will be explained
The results revealed a strong dependence of the nano-mechanical creep properties on the holding time, indicating a significant decrease in both hardness
Summary
Ultrahigh molecular weight polyethylene (UHMWPE) belongs to the polyethylene group of polymers with a repeating unit, [C2 H4 ]n , where n represents the degree of polymerization. The surface mechanical characteristics of polymers strongly depends on the applied contact conditions, for example the indenter loading rate (strain rate), the penetration depth (the strain), amplitude, frequency, holding time, etc. A detailed analysis of this creep phenomenon as a function of different holding times is required for UHMWPE to predict the appropriate value of holding time to guarantee the eradication of this undesired phenomenon and to obtain the accurate mechanical properties of the polymer By keeping these objectives in mind, the present study was focused on the investigation of the nano-mechanical behavior of UHMWPE as a function of various experimental variables, including frequency, amplitude, peak load, strain rate, and holding time. The creeping effect in UHMWPE was studied by varying the applied holding times and peak load in order to be able to predict the appropriate holding time for the considered polymer
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