Amorphous embrittlement

Strength Fibrous minerals Ductility gaining tensile strength. Carbon fibers are more expensive fibrous minerals are least expensive but only slightly reinforcing. Reinforcement makes brittle resins tougher and embrittles tough resins. Fibrous minerals are not commonly used in amorphous resins. 

The combination of the above factors has rendered the nanocrystalline solution competitive, not only with amorphous Co-based alloys, but also with classical crystalline alloys and ferrites. The consequence is a steadily increasing level of applications in magnetic cores for ground fault interrupters, common mode chokes and high frequency transformers. Fig. 14 shows some typical examples. The worldwide production rate meanwhile approaches an estimated 1000 tons/year, and the trend is increasing. The only drawback of the nanocrystalline material appears to be the embrittlement that occurs upon crystallization, which requires final shape annealing and, thus, restricts application mainly to toroidally wound cores. 

Hopefully, future research will be able to reverse the embrittlement of the cellulose fibers. Perhaps the cut ends of the cellulose chain can be decrystallized, and with the amorphous regions thus restored, the ends of the chains can be bonded chemically but this has yet to be done. 

When molten sulfur is heated above 159°C, preferably to 200°-250°C, and then rapidly quenched to about — 20°C, a translucent elastic dark-brown material (plastic sulfur) is obtained (28). Plastic sulfur, which is a mixture of amorphous S8 rings and amorphous polymeric sulfur, is thermodynamically unstable (28). It undergoes embrittlement, especially above —10 °C, because of the rapid crystallization of octameric sulfur to orthorhombic sulfur (28). Also polymeric sulfur depolymerizes and crystallizes to orthorhombic sulfur slowly at ambient temperature and rapidly above 90°C (28). 

The entanglement molar mass Me for PE is 1.7 kg.mof. The ratio M c/Mg ranges thus between 23 and 59 versus 5 for low crystalline or amorphous polymers. This means that embrittlement occurs while the... 

The second condition to validate the scheme B is that embrittlement must correspond to a critical morphological state that is the only approach to explain its sudden character. The extensive and careful work of Kennedy et al. (//) on relationships between fracture behavior, molar mass and lamellar morphology, shows that this condition is fulfilled in the case of PE. Comparing various samples of different molar masses with different thermal histories, they found that the thickness of the amorphous layer (la) separating two adjacent lamellae is the key parameter (Fig. 6). As a matter of fact, there is a critical value lac of the order of 6-7 nm. For la > lac the samples are always ductile whatever their molar mass, whereas for U < laC the samples are consistently brittle. As a result, lac appears to be independent of the molar mass. Indeed, there is a specific molar mass, probably close to 70 kg.mof for PE below which crystallization is so fast that it is impossible to have la values higher than lac whatever the processing conditions. 

The morphological changes are induced by a chemicrystallization process. They lead to embrittlement when the thickness of the amorphous layer between adjacent lamellae becomes lower than a critical value of the order of 6 nm in PE. 

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 hardening and embrittlement of polyimides by ion implantation has been also studied (Pivin, 1994). Nanoindentation tests performed using a sharp diamond pyramid of apical angle 35° provided very quantitative depth profiles of microhardness in polyimides implanted with C, N, O, Ne or Si ions. In all cases the microhardness increased steeply when the amount of deposited energy reached the order of 20 eV atom". For energies of 200 eV atom" the polymer is transformed into an amorphous hydrocarbon and the microhardening factor saturates at a value of 13-20. However, the carbonized layer is poorly adherent, as is evidenced by reproducible discontinuities in the depth vs load curves, when the indenter tip reached the interface. 

Irradiation of PP in air leads to oxidative degradation, evidenced by discoloration and embrittlement. The extent of the degradation depends on crystallinity, MW, MWD, and chain mobility [Kadir et al., 1989 Kashiwabara and Seguchi, 1992 Williams, 1992]. Neat PP does not discolor on irradiation up to 100 kGy [Williams, 1992]. The antioxidants should be selected so as not to cause the discoloration. However, most commercial preparations containing phenolic antioxidants turn yellow on irradiation. Phenolic antioxidants produce stable phenoxyl radicals that convert into colored quinonoids. Other stabilizers and antioxidants are compounds that contain either phosphorous [Bentrude, 1965 de Paolo and Smith, 1968], sulfur [Jirackova and Pospisil, 1979], or hindered piperidine derivatives [Carlsson, et al., 1980 Felder et al., 1980 Allen et al., 1981]. A comprehensive list of stabilizers and their mode of action was given by Dexter [1992]. It is noteworthy that antioxidants and stabilizers are excluded from the crystalline regions [Winslow et al., 1966] thus they would provide protection only within the amorphous domains. 

PHYSICAL PROPERTIES bluish-black, amorphous powder or grayish-white lustrous metal soft, malleable, ductile solid hexagonal lattice below 865°C, body-centered cubic above 865°C may become embrittled by the absorption of nitrogen, oxygen, and carbon soluble in hot, very concentrated acids insoluble in water and cold acids MP (1857°C, 3375°F) BP (3577°C, 6471°F) DN (6.506 g/cm at 20°C) SG (6.51) CP (25.4 J/K-mol crystal at 25°C) VD (NA) VP (0 mmHg at 20 C) BHN (85). 

The best non-oxide fibers have good creep resistance but are susceptible to degradation by formation of an amorphous silica layer upon oxidation. This layer offers some resistance to further oxidation, but prolonged exposure to oxidizing environments results in oxidative embrittlement of the composite. 

In semi-crystalline polymers such as polyolefins, initial oxidation occurs in the amorphous tie molecules between crystallites and sometimes even in the inter-lamellar amorphous chains within crystallites. This allows a relatively low levels scission to cause a disproportionately large changes in mechanical properties due to brittle failure in the amorphous regions (Celina, 2013). In PP for instance, beginning stages of bulk embrittlement corresponded to only 0.01% of oxidation (Fayolle et al., 2004). 

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