Connections, interlamellar

Their crystal structures have been mentioned briefly in connection with intercalation in Section 14.2. All five compounds can be obtained in the layered hexagonal crystal form, and most are also found in rhombohedral or trigonal form. The compounds of the Group 6 metals, molybdenum and tungsten, as well as niobium diselenide, have a hexagonal form similar to that of molybdenum disulphide, in which the metal atoms in one layer are displaced sideways from those in the layers immediately above and below. This structure results in the widest interlamellar spacing, the easiest interlamellar shear, and the lowest friction. 

These results demonstrate that the embrittlement of the PE implants accompanies a microhardening of a surface layer and an increase in crystallinity. The two pieces of evidence are complementary and imply a reduction in the crack-blunting ability of the material, i.e. a diminution of the number of interlamellar tie molecules which connect adjacent lamellar stacks. In consequence the elastic properties of the material diminish and cause the material to microharden during wear. The increase in microhardness at the wear surface is partly because the amorphous component decreases in quantity and partly because its chemical nature changes as it undergoes simultaneous microhardening and loss of elasticity. 

The results of Figure 11 indicate that the polymers studied were subject to different microstructural deformation mechanisms. In this connection it must be borne in mind that the maximum nominal deformation of POM and PA66 was only 1% whereas PP and PTFE were deformed up to 1.8% and 3.3% respectively. Therefore it may be assumed that for POM and PA66 only an instantaneously reversible deformation of the amorphous matrix of the spherulitic microstructure occurred (18) whereas for PP and PTFE some irreversible effects, like interlamellar shearing or reorientation of the lamellae may have taken place. 

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