Mechanical Properties of Nanogranular Metallic Glasses

Metallic glasses possessing extremely high strength, exceptional wear resistance, and appreciable elastic strain limit, as opposed to their crystalline counterparts, have spurred the interest of the scientific community. However, the inert brittleness of metallic glasses is attributed to the formation of highly localized shear bands, which cause cataclysmic failure. Two approaches to enhance the ductility of nanometallic glass are the deformation of metallic glass at high temperature and the reduction of the size of metallic glass. Nanogranular metallic glasses consist of metallic glass nanogranules (~ 100 nm or less) that are assembled together by a network of interfaces with similar or different compositions from the constituent nanogranules [31]. Nanogranular metallic glasses exhibit unique properties owing to a large fraction of interfaces, as opposed to that of the respective conventional metallic glass counterparts with a uniform amorphous structure. The low-density interfacial regions between nanogranules, which are believed to be softer than the nanogranules, are excess “free” volume- enriched regions. Consequently, these interfacial regions pose as a preferential site for shear localization, thereby promoting plastic flow. The shear bands nucleate at certain interfaces and traverse through the interface until they encounter hard nanogranules, only to propagate further or split into two. The nanogranules may rotate and stretch simultaneously, which further contributes to plasticity of the nanogranular metallic glass. Such a scenario is similar to multiple shear band nucleation and propagation, which cause heterogeneous-to-homogeneous deformation mechanism transition, thereby imparting tensile ductility (refer to Figure 4.10). Nanogranular metallic glasses demonstrate size-dependent strength properties akin to that observed by polycrystalline materials, owing to similar structure in the nanocrystalline materials and nanograined polycrystalline metals. Homogenous deformation in the nanogranular metallic glasses predominates over inhomogeneous deformation below a critical

granular size. Consequently, the ductility and plasticity of the nanogranular metallic glasses ameliorate with size reduction. A schematic diagram of variation of yield strength with granular size is exemplified in Figure 4. Ю.

Interfacial and Mechanical Properties of Epoxy Nanocomposites

Nanocomposite may be defined as a multiphase solid material in which the reinforcements have one or more dimensions ranging in the scale of nanoregime [32]. In addition to low specific weight, these nanocomposites possess a unique combination of tribological, mechanical, and degradation resistance properties, thereby ensuring economic efficiency and safety [33]. Epoxy resins are a suitable thermosetting polymer matrix, as they already possess a few of the required properties mentioned previously. However, to enhance their tribological and mechanical properties, they are reinforced with fillers.

Effect of Nanoparticles on Mechanical Properties of Epoxy Nanocomposites

The fraction, shape, and distribution of nanoparticles in the nanocomposite strongly influence the deformation behavior of the nanocomposite. The dispersion homogeneity of nanoparticles considerably affects the mechanical properties of the epoxy nanocomposite.

Tensile Strength of Epoxy Nanocomposite

The strength value shows a remarkable improvement when nanoparticles (of any shape or size) are distributed in the nanocomposite up until an optimum composition, after which drop in the tensile behavior is seen [34]. The decrease in the strength values is attributed to weak interfacial region bond between the nanoparticles and the matrix, owing to the agglomeration of nanoparticles. The shape of nanoparticles is also equally vital in determining the deformation behavior of the epoxy nanocomposites. For instance, platelet-shaped nanoparticles impart higher strength values than rod-shaped nanoparticles, resulting in shortening interparticle distance and hence in dense particle network, aiding transfer of stress [35]. Maximum amelioration of strength is observed for spherical nanoparticles, owing to the maximum nanoparticle-matrix interaction. The enhancement in strength values is because the nanoparticles incorporated have significantly higher elastic modulus than the epoxy polymer matrix. Another factor that contributes to the enhancement in the rigidity of the specimen is the adsorption of epoxy polymer resin on the surface of the nanoparticles [35]. The variation in tensile strength with different sizes of the nanoparticles with varying content is illustrated in Figure 4.11.

Flexural Strength of Epoxy Nanocomposite

The flexural strength of nanocomposite also seems to increase with increasing nanoparticles [34]. Increasing number of nanoparticles increases the inherent stiffness of the nanocomposite by restricting chain mobility [35]. Highest enhancement

UTS of different TiO, particle sizes at different weight fraction. (From Al-Turaif, H.A., Prog. Org. Coat., 69, 241-246, 2010. With permission.)

FIGURE 4.11 UTS of different TiO, particle sizes at different weight fraction. (From Al-Turaif, H.A., Prog. Org. Coat., 69, 241-246, 2010. With permission.)

Flexural stress of different TiO, particle sizes at different weight fraction. (From Al-Turaif, H.A., Prog. Org. Coat., 69. 241-246, 2010 [36]. With permission.)

FIGURE 4.12 Flexural stress of different TiO, particle sizes at different weight fraction. (From Al-Turaif, H.A., Prog. Org. Coat., 69. 241-246, 2010 [36]. With permission.)

in strength is observed for spherical nanoparticles, as observed for the previous case. Flexural strength also increases with increasing particles size [34]. Figure 4.12 exemplifies the variation in the flexural strength of the nanocomposite with varying weight percentage of Ti02 and varying size. The flexural strength is found to increase with the decreasing particle size. The strength value also increases with Ti02 content, but then, it remains almost constant with further increment.

Mechanical Properties of Epoxy/Graphene Nanocomposite

Graphene being associated with remarkable tensile strength, appreciably high elastic modulus and lower density, can be considered a potential candidate for reinforcement in the nanocomposite. With increasing graphene content, fracture toughness and flexural modulus show a considerable improvement in the flexural and fracture toughness. The epoxy composite has been toughened by incorporating graphene nanoplatelets and amine-terminated poly (butadiene-co-acrylonitrile) graphene nanoplatelets [37]. The flexural modulus value is found to increase to 18.1% consistently with the increasing pristine graphene nanoplatelets to 5 wt%, which indicates an amelioration of the strength properties of the nanocomposite. However, amine-terminated poly (butadiene-co-acrylonitrile)-graphene nanoplatelets exhibit more pronounced enhancement to 22% increment in flexural modulus for the same composition. The increase in the modulus properties is attributed to the improved interfacial interaction, owing to the covalent bonding between the amino group and the matrix. The flexural strength decreases constantly with the increasing graphene nanoplatelets, attributed to weak interfacial adhesion [37]. The strength properties of the nanocomposite are chiefly governed by the dispersion of graphene in the epoxy matrix. Amelioration of the strength properties necessitates homogenous dispersion of the nanoparticles in the matrix, leading to improvement in the load transfer to the graphene filler. Improvement in the mechanical interlocking between the matrix and the reinforced graphene can be observed with well-dispersed nanoparticles. In addition, the amelioration of the tensile strength properties is also associated with the suppression of slipping of entrapped polymer molecules and favorable adhesion between the graphene nanoparticles and the epoxy matrix.

Toughening of Epoxy Nanocomposites: Nano and Hybrid Effects

Toughening Mechanism Associated with Binary Nanocomposites

Debonding of nanoparticles such as silica, followed by plastic matrix void growth, is the governing strengthening mechanism in case of binary nanocomposites, for instance, anhydride-cured silica/epoxy nanocomposite. In addition, several factors such as plastic void growth in the near vicinity of the crack tip, nanoparticles debonding, shear yielding, low glass transition temperature, and lightly cross-linked epoxy also govern the toughening mechanism of the binary nanocomposites. The distribution of the nanoparticles has an insignificant effect on the mechanical properties of the binary nanocomposite, attributed to the diminutive size of the nanoparticle reinforcement, as compared with the crack opening displacement at the crack instability point, crack deflection, and crack pinning. Another dominant strengthening mechanism in the binary nanocomposite is matrix plastic shear deformation. Table 4.3 provides a summary of the toughening mechanism of a binary nanocomposite. It is imperative for the nanocomposite to sustain plastic deformation ability, because plastic matrix void growth and matrix plastic shear deformation play a very vital role in influencing the deformation behavior of the nanocomposites [38]. The lower ductility of the matrix makes it incapable of deforming adequately when subjected to loading conditions. However, as the ductility of the matrix increases, the plastic void growth and matrix plastic shear strengthening become activated via debonding of the nanoparticles, thus resulting in high fracture toughness behavior. Similar to silica-like nanoparticles, rubber nanoparticle-strengthened epoxy composites undergo shear yielding and plastic void growth deformation mechanism.

Toughening Mechanism Associated with Ternary Nanocomposites with Silica Rubber Hybrids

Ternary hybrid systems exhibit higher strength values than individual binary silica nanocomposite or rubber nanocomposite. A combined effect of debonding of

TABLE 4.3

Toughness Mechanism of Silica Epoxy Nanocomposites

Materials

Particle Size (nm)

Nanofiller Content (vol%)

Epoxy

Silica/ Epoxy

% Increase in K,c (fracture toughness)

Toughening Mechanism

Cycloaliphatic epoxy

25

1-14

0.42

0.47-0.74

76

Nanoparticles-induced dimples (crack pinning)

Diglycidyl Ether Bisphenol A (DGEBA)/ Methylhexahydrophthalic/acid anhydride

20

2.5-13.5

0.59

1.03-1.42

140

Plastic void growth

Diglycidyl Ether Bisphenol F (DGEBF)/ diethyltoluenediamine/acid anhydride

25

1-15

  • 0.64
  • 0.56
  • 0.65-1.13
  • 0.64-1.26
  • 76
  • 125

Local plastic deformation

Diglycidyl Ether Bisphenol A (DGEBA)/ Piperidine (PIP)

20

1-11

0.95

1-2.11

122

Shear yielding + void growth + plastic deformation

DGEBA/PIP

20

2.5-30

1.11

1.70-2.52

127

Shear yielding + void growth

74

1.75-2.89

160

170

1.68-2.65

138

Source: Marouf, B.T. et al., Poly. Rev., 56, 70-112, 2016.

silica nanoparticles, rubber particles undergoing cavitation, and matrix plastic shear deformation influences the deformation behavior in the ternary nanocomposite [38]. The increment in interparticle distance on the introduction of carboxyl-terminated butadiene acrylonitride (a rubber modifier) provokes more extensive-intensive shear yielding and the expansion of matrix in the near vicinity of SiO, nanoparticle, along with an increment in the density of shear band amid the rubber particles. As a result, these interactions enhance the fracture toughness, as advocated by the expanded crack-tip plastic zones.

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