Reversible microhardness

In summary, it can be concluded that the microhardness technique is sensitive enough to detect strain-induced polymorphic transitions in polymers. The results in this chapter reveal that in materials characterized by a high and reversible deformation ability it is possible to observe reversible microhardness provided the strain-induced structural changes are reversible too. 

Balth Calleja F J, Boneva D, Krumova M and Fakirov S (1998) Microhardness under strain. 4. Reversible microhardness in polyblock thermoplastic elastomers with poly(butylene terephthalate) as hard segments, Macromol Chem P/it/s 199 2217-2220. 

Microhardness, therefore, appears to be an elastic-plastic rather elusive parameter (Marsh, 1964). Microhardness as a property is, in fact, a complex combination of other properties elastic modulus, yield strength and strain hardening capacity. One way to differentiate between the reversible and irreversible components of contact deformation is to measure the elastic recovery during unloading of the indenter (Stilwell Tabor, 1961). Extreme cases of depth recovery are best described by soft metals, where it is negligible, and fully elastic rubber, where it is complete. 

Since thermoplastic elastomers can undergo large (up to a couple of hundred per cent) deformations which are to a great extent (up to 50%) reversible, we wish to discuss here the microhardness behaviour of these systems at different deformation levels, both, while maintained under stress, and after removal of the stress. In this way one may expect to observe reversibility in the microhardness values in so far as the polymer structure is restored after removing the stress. 

For the PEE under investigation it was shown that beyond a given deformation range, which depends on the ratio of the hard/soft segments, but typically is above s = 20-25%, the observed overall deformation under stress is only partially reversible (usually about 50%) (Stribeck et ai, 1997). This means that the remaining deformation under stress is a consequence of a plastic deformation. In Fig. 6.11(1 ) the microhardness H is plotted as a function of the residual plastic deformation as discussed above. 

The results shown in Fig. 6.11 not only support the reversibility of the strain-induced polymorphic transition (Boyle Overton, 1974) but also allow one to speak about reversibility of microhardness. This is feasible in cases in which, as a result of some treatment, it is possible to regenerate the starting structure of the polymer. Reversibility of the microhardness further emphasizes that this mechanical property depends primarily on the structure of material. 

Let us consider now the results of investigations of the substructure in nonmetal-lic compounds. The density of dislocations of shocked single crystals in this case also increases by several orders of magnitude, disorientation of blocks increases by 2- 5° and microhardness by several times. It is interesting to note that an increase of the dislocation density in many shocked solids is accompained by retaining the dislocation configurations which existed prior to shock loading. Only in KBr a new dislocation picture was observed, and that because the reversible phase transition had completely rebuilt the dislocation structure. 

It is to be noted that is intimately related to the packing of the chains in the crystals (4). Since the crystal hardness reflects the response of the inter-molecular forces holding the chains within the lattice, it has been shown that the microhardness technique permits to distinguish between polymorphic modifications of the same polymer (20,21). Indeed, the study of the transition from the a to the form in iPP confirmed that changes in H were directly related to the different crystal hardness values of each phase (20). More recently, the microhardness technique has been successfully applied to follow the reversible strain-induced poljmiorphic a p transition occurring on PBT (21). 

Hardness is a measurement of material resistance to plastic deformation in most cases. It is a simple nondestructive technique to test material indentation resistance, scratch resistance, wear resistance, or machinability. Hardness testing can be conducted by various methods, and it has long been used in analyzing part mechanical properties. In reverse engineering, this test is also widely used to check the material heat treatment condition and strength, particularly for a noncritical part, to save costs. The hardness of a material is usually quantitatively represented by a hardness number in various scales. The most utilized scales are Brinell, Rockwell, and Vickers for bulk hardness measurements. Knoop, Vickers microhardness, and other microhardness scales are used for very small area hardness measurements. Rockwell superficial and Shore scleroscope tests are used for surface hardness measurements. Surface hardness can also be measured on a nanoscale today. 

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