Nanocomposites creep

Galgali and his colleagues [46] have also shown that the typical rheological response in nanocomposites arises from frictional interactions between the silicate layers and not from the immobilization of confined polymer chains between the silicate layers. They have also shown a dramatic decrease in the creep compliance for the PP-based nanocomposite with 9 wt% MMT. They showed a dramatic three orders of magnitude drop in the zero shear viscosity beyond the apparent yield stress, suggesting that the solid-like behavior in the quiescent state is a result of the percolated structure of the layered silicate. 

Another notable example of a reduction in creep rate through the addition of second-phase particles concerns nanocomposites . In alumina-SiC, systems, several investigations have reported significant reductions in creep rate compared with monolithic alumina [48, 49], Figure 4.9 shows the results of Ohji and co-workers [48], At 1200°C the creep rate of an AI203-17vol%SiC nanocomposite was less than that of alumina for a given stress by a factor of 250, and the time to rupture at 50 MPa was increased from 120 h to 1120 h. The SiC inhibits creep primarily because it is difficult to remove or deposit... 

Log-log plot of tensile creep strain rate against applied stress for alumina and an alumina-17vol%SiC nanocomposite tested in tension at 1200°C. 

It was found that dramatic improvements in toughness, strength, creep strength and thermoresistance could be achieved by incorporating nano-SiC dispersion in a microcrystalline matrix. Subsequently, similar improvements were found in other nanocomposites [1],... 

Rendtel, A., Htibner, H., Hermann, M., Schubert, C., Silicon nitride/silicon carbide nanocomposite materials II, Hot strength, creep and oxidation resistance, J. Am. Ceram. Soc., 81(5), 1998, 1109-1120. 

In order to obtain a competitive product, the PHB performance can be greatly enhanced with the addition of nanometer-size inorganic fillers. This kind of materials are called nanocomposites and have an interesting characteristic The mechanical properties [42], the barrier properties [43], the thermal properties [44], and some others such as the flammability [45], water adsorption [46], and creep resistance [47] can be greatly enhanced with the addition of a small amount of filler (usually less than 10 wt%) [6,42-48]. 

FIGURE 5-2 Tensile creep rate of Al203-SiC nanocomposite containing 5 volume percent of 0.15 pm (0.006 mils)... 

Ohji, T., A. Nakahira, T. Hirano, and K. Niihara. 1995. Tensile creep behavior of alumina/silicon carbide nanocomposite. Journal of the American Ceramic Society 77(12) 3259-3262. ... 

I. Gofman, B. Zhang, W. Zang, Y. Zhang, G. Song, C. Chen, et al.. Specific features of creep and tribological behavior of polyimide-carbon nanotubes nanocomposite films Effect of the nanotubes functionalisation. Journal of Polymer Research, 20 (10), 1-9, 2013. 

Cyclic, thermomechanical tensile tests were performed for the nanocomposites with POSS/polyol ratio = 2.63 (see Fig. 15b). The sample was firstly heated to 80°C (T > Tg) and deformed (1) by ramping to a load of 0.3N. The sample was cooled under this load (2) to 10°C, to fix the temporary, elongated shape. After unloading (3) the sample was heated (4) to 80°C to recover the permanent shape. The first cycle showed about 5% creep occurring between the elongation and fixing step over... 

Shen and co-workers [91] used nano-indentation to study the effects of clay concentration on the mechanical properties, such as hardness, elastic modulus and creep behaviour of exfoliated PA 6,6-clay nanocomposites. Results were compared with those obtained by DMA and tensile tests. 

Fig. 9.30 Comparison of the compression creep property of nanocomposites with those of existing silicon-nitride ceramics (additive in weight percentage unless specified, molecular formula simplified for clarity. For instance, 6YO" in figure legend stands for 6 wt%Y203 ) [38]. With kind permission of John Wiley and Sons...

The SiaN SiC nanocomposite illustrated in Fig. 9.31 indicates the well-known fact that monoUthic ceramics are weaker than nanocomposite ceramics. In this figure, the creep strain of monolithic Si3N4 is substantially greater than that of the nanocomposite. 

Also in this case, Y2O3 was added to the nanocomposite. The creep tests were performed by four-point bending at temperatures of 1200 and 1450 °C within a stress range of 50-150 MPa. The creep rate was calculated from the slope of the c versus t curve (Fig. 9.31) and steady-state creep was evaluated using Fq. (9.5), i.e., the Norton equation. An alternative explanation for the observed increase in creep resistance in the nanocomposite is that the SiC nanoparticles hinder the grain growth... 

Fig. 9.32 Tensile creep curves of the monolith and nanocomposite at 1200 °C and 50 MPa. Slight accelerated creep and steady-state creep were present in the monolith, while they were little observed in the nanocomposite [23]. With kind permission of John Wiley and Sons...

The curve of the monolith consists of primary, steady-state and very small tertiary creep. The specimen lifetime was 150 h and 4 % of creep strain was obtained at fracture. A large number of microcracks were also identified by optical microscopy. Compared to the monolith, the nanocomposite exhibited excellent creep resistance. At 1200 °C and 50 MPa, its creep hfe was 1120 h, which is 10 times longer than that of the monolith. The creep strain at fracture was 0.5 %, which is eight times smaller than that of the monohth. Furthermore, the superior creep resistance of the nanocomposite was also obtained by flexure creep tests. Similar to tensile-creep curves, the strain of the nanocomposite tended to decrease over time, while the monolith exhibited steady-state creep and sometimes accelerated creep. 

The strain rate, as a function of applied stress, is shown in Fig. 9.33 for the steady-state creep rates of both the monolith and the nanocomposite ceramics. It may be observed that the creep rate of the nanocomposite is about three orders of magnitude lower than that of the monohth under tension, and three to four orders of magnitude lower under flexure. One of the most characteristic changes in microstmctures during creep is the rotation of the intergranular sihcon-carbide particles, accompanied by GBS and small cavity formation around the particles. This may be seen in Fig. 9.34a. 

Table 7.7 Creep data of polyurethane/C20A nanocomposites at 26 wt% hard-segment content.

J = Ji+Ji+ h- Js can be considered as zero for a crosslinked or highly crystalline polymers. Table 7.7 lists creep-related data of PU/C20A nanocomposites. The instantaneous compliance decreases with the addition of clay, and the nanocomposites observes the nearly same equilibrium compliance except 5wt% C20A, which is similar to the equilibrium stress during stress relaxation. It is also similarly found that both the creep rate and retardant time increases with the addition of clay, which should also be the result of enhanced phase microseparation. 

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