Microhardness time dependence

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

Hardness measurements such as Rockwell or Vicker s indentation properties are time-dependent as a result of the viscoelastic flow and relaxation processes (236) (see Hardness). Microhardness measurements have been used to correlate with other properties such as Yoimg s modulus and compressive yield stress in polyethylenes (237) and glass-transition temperature of amorphous plastics (238). Scratch resistance in polyproplyene studies was found to have shear yielding as the main cause of plastic flow scratch pattern with tensile tear effects on the surface and shear-induced fracture on the subsurface (239). 

In order to examine the possible influence of the crystalline phase on the Tg value, the H of a. PET sample with a degree of crystallinity of 0.38 was studied. Figure 3.4 compares the dependence of microhardness upon temperature for amorphous and semicrystalline PET. In order to separate the ageing contribution from the pure temperature dependence of the amorphous material the microhardness data for the former were taken after long annealing times (f = 100 h). The Tg value... 

Figure 3.10. The dependence of microhardness upon storage time for PEN (1) at room temperature, (2) subsequently annealed at Ta = 105 °C up to //max, (3) subsequently annealed at Ta = 105 °C to the final value (after 60-100 min) (see text). (After Rueda et al., 1995.)...

The question of whether microhardness is a property related to the elastic modulus E or the yield stress T is a problem which has been commented on by Bowman Bevis (1977). These authors found an experimental relationship between microhardness and modulus and/or yield stress for injection-moulded semicrystalline plastics. According to the classical theory of plasticity the expected microindentation hardness value for a Vickers indenter is approximately equal to three times the yield stress (Tabor s relation). This assumption is only valid for an ideally plastic solid showing sufficiently large deformation with no elastic strains. PE, as we have seen, can be considered to be a two-phase material. Therefore, one might anticipate a certain variation of the H/ T 3 ratio depending on the proportion of the compliant to the stiff phase. 

A characteristic feature of the material under investigation is its very low microhardness - between 25 and 35 MPa depending on the crystalline modification present. These values are up to 5-6 times lower than those for semicrystalline homo-PBT regardless of the crystalline modification. Moreover, the obtained values for H of PEE (Table 6.3, Fig. 6.5) are about half the amorphous hardness, // , of PBT, being 54 MPa as reported by Giri ef a/. (1997). This means that there should be other factors responsible for the very low H values of the copolymer. 

Maitra et al. determined the hardness of PVA with 0.6 wt% oxidized NDs using nanoindentation and the Oliver-Pharr method addition of nanofiller enhanced the hardness 80% of the neat polymer [48]. Hardness and modulus of FG/epoxy composites increased steadily with the incorporation of up to 1.5 wt% nanofiller. An increased amount of agglomerates was obtained at a loading of 2 wt% amino FGs as observed by the dramatic drop in the modulus this behavior also affected the nanocomposite hardness, as depicted in Figure 10.20 [113], The microhardness of amino FGs/PI nanocomposites showed a dependence on nanofiller content, although the dependence diminishes at loadings > 1 wt%, where the effect starts to saturate [115]. Covalently bonded amino NDs/epoxy composites showed a 200 times higher hardness compared to the neat... 

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