Composition of NMCC

3 Zr-ZrN-(Zr,Cr,Al, Nb)N

4 Zr-ZrN-(Nb,Zr,Ti, Al)N

6 Zr-ZrN-(Zr,Cr,Al)

N

study.

Tool life Tc (min) VB = 0.4 mm

122 Novel Nanomaterials - Synthesis and Applications

Sublayer thickness (nm)

1 Uncoated 8 — —— 18 2 TiN 18 — 2.8 31 30

5 Ti-TiN-(Ti,Al)N 28 65–90 5.0 >40 38

24 200 3.8 >40 34

31 45–60 3.3 >40 34

37 15–45 3.4 39 36

Table 2. The basic properties of NMCC and periods of tool life of the carbide tools under study with the NMCC under

Total thickness (μm)

Adhesion, LC2 (N)

Hardness, HV (GPa)

> Figure 10. An example of the failure of the upper layer of NMCC Zr-ZrN-(Zr,Cr,Al)N because of the tearing force related to the adhesion interaction between the outer boundary of the coating and the material being machined (steel C45) [28].

> An important distinctive feature of the development of longitudinal cracks in nanostructured coatings is the formation of bridges in the process of cracking due to the alternation of less plastic sublayers with more plastic ones in the coating structure. Such bridges inhibit the development of a crack by exerting a positive influence on coating crack resistance and, consequently, on the tool life of a cutting tool (Figure 11). This mechanism of inhibition of

cracking is fairly close to the mechanism of action of bridges from a particle of a more plastic phase embedded in the brittle phase described, in particular, by Kumar and Curtin [23]. It should be noted that the studies of the propagation of longitudinal cracks in monolithic coatings revealed no such bridges. The strength of the bridges depends on the composition of the coating layers. In particular, in layers of (Zr,Cr,Al)N (Figure 11a), the bridges show significantly higher strength and ductility than in (Zr,Nb,Ti,Al)N (Figure 11b), where the bridges show a tendency to failure.

No such bridges are observed in NMCC Ti-TiN-(Ti,Al)N, and that may be connected with the high hardness and brittleness of the layer (Ti,Al)N. The failure of NMCC Ti-TiN-(Ti,Al)N often occurs in accordance with a pronounced "brittle fracture" scenario with the formation of a network of longitudinal and transverse cracks (Figure 12).

In the case of insufficiently strong adhesion bond between the coating layers or cohesive bonds between its nano-sublayers, delaminations of the classical form are formed between the layers of the coating or between its nano-sublayers. In particular, Figures 13 and 14 show obvious delamination between the intermediate TiN layer and the wear-resistant (Ti,Al)N layer. In the structure of the coating presented in Figure 14, transverse cracks and delaminations also occur between nano-sublayers of the wear-resistant layer. In addition, it is possible to note a relatively positive role of delamination (1) as a factor of inhibition of transverse cracks (3). The transverse cracks (3) are decelerated at the boundary of the intermediate and wear-resistant layers, and they are not spreading in the intermediate TiN layer (Figure 14).

Figure 11. Deceleration of a longitudinal crack in NMCC Zr-ZrN-(Zr,Cr,Al)N (a) and Zr-ZrN-(Zr,Nb,Ti,Al)N (b) due the formation of bridges of more plastic nanolayers [17, 28].

The patterns of formation of longitudinal cracks and delaminations often appear to be complex. In particular, Figure 15 shows the mechanisms of cracking and delamination in NMCC Zr-ZrN-(Zr,Cr,Al)N. The area of this picture that is marked as AREA I contains a crack of a complex kind, combining delamination between the substrate and the adhesion layer Zr,

Figure 15. An example of formation of longitudinal cracks and delaminations in NMCC Zr-ZrN-(Zr,Cr,Al)N [28].

Figure 14. Interlayer delamination (1), delamination between nano-sublayers (2), and transverse cracks (3) in the struc-

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125

Figure 13. Interlayer delamination in the structure of NMCC Ti-TiN-(Ti,Al)N [28].

ture of NMCC Ti-TiN-(Ti,Al)N [28].

Figure 12. Failure of NMCC Ti-TiN-(Ti,Al)N with the formation of a network of longitudinal and transverse cracks [28].

Delamination and Longitudinal Cracking in Multilayered Composite Nanostructured Coatings and Their Influence… http://dx.doi.org/10.5772/intechopen.72257 125

Figure 13. Interlayer delamination in the structure of NMCC Ti-TiN-(Ti,Al)N [28].

cracking is fairly close to the mechanism of action of bridges from a particle of a more plastic phase embedded in the brittle phase described, in particular, by Kumar and Curtin [23]. It should be noted that the studies of the propagation of longitudinal cracks in monolithic coatings revealed no such bridges. The strength of the bridges depends on the composition of the coating layers. In particular, in layers of (Zr,Cr,Al)N (Figure 11a), the bridges show significantly higher strength and ductility than in (Zr,Nb,Ti,Al)N (Figure 11b), where the

No such bridges are observed in NMCC Ti-TiN-(Ti,Al)N, and that may be connected with the high hardness and brittleness of the layer (Ti,Al)N. The failure of NMCC Ti-TiN-(Ti,Al)N often occurs in accordance with a pronounced "brittle fracture" scenario with the formation of a

In the case of insufficiently strong adhesion bond between the coating layers or cohesive bonds between its nano-sublayers, delaminations of the classical form are formed between the layers of the coating or between its nano-sublayers. In particular, Figures 13 and 14 show obvious delamination between the intermediate TiN layer and the wear-resistant (Ti,Al)N layer. In the structure of the coating presented in Figure 14, transverse cracks and delaminations also occur between nano-sublayers of the wear-resistant layer. In addition, it is possible to note a relatively positive role of delamination (1) as a factor of inhibition of transverse cracks (3). The transverse cracks (3) are decelerated at the boundary of the intermediate and wear-resistant

Figure 11. Deceleration of a longitudinal crack in NMCC Zr-ZrN-(Zr,Cr,Al)N (a) and Zr-ZrN-(Zr,Nb,Ti,Al)N (b) due the

Figure 12. Failure of NMCC Ti-TiN-(Ti,Al)N with the formation of a network of longitudinal and transverse cracks [28].

layers, and they are not spreading in the intermediate TiN layer (Figure 14).

bridges show a tendency to failure.

124 Novel Nanomaterials - Synthesis and Applications

formation of bridges of more plastic nanolayers [17, 28].

network of longitudinal and transverse cracks (Figure 12).

Figure 14. Interlayer delamination (1), delamination between nano-sublayers (2), and transverse cracks (3) in the structure of NMCC Ti-TiN-(Ti,Al)N [28].

Figure 15. An example of formation of longitudinal cracks and delaminations in NMCC Zr-ZrN-(Zr,Cr,Al)N [28].

The patterns of formation of longitudinal cracks and delaminations often appear to be complex. In particular, Figure 15 shows the mechanisms of cracking and delamination in NMCC Zr-ZrN-(Zr,Cr,Al)N. The area of this picture that is marked as AREA I contains a crack of a complex kind, combining delamination between the substrate and the adhesion layer Zr, passing into a transverse crack that cuts the adhesive layer, and turning into a series of delaminations between nano-sublayers of the intermediate coating layer. The initial factor stimulating the formation of this crack is microroughness of the substrate, formed by high carbide grain. In contrast, the area indicated as AREA II contains an example of extended delamination, reaching a width of 200–300 nm. The formation of this delamination resulted in (1) chipping of microcomponents of the coating, (2) formation of bridges of more plastic nanolayers, and (3) the crack development boundary.

Let us consider separately the process of delamination and failure in coating Zr-ZrN-(Zr, Cr, Al, Nb)N with thickness of sublayers of about 200 nm (Figure 18a). Due to thick sublayers, this coating cannot be called "nanostructured." This coating is characterized by a large number of delaminations, arising especially in the fracture zone adjacent to a wear crater. No bond bridges are formed between sublayers (Figure 18 Area A), while delaminations are an impor-

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127

Let us individually consider delaminations formed in NMCC of heavy thickness (usually exceeding 8 μm) because of heavy internal compressive stresses. Such delaminations can be formed with equal probability in the coating both on the rake and flank face of a tool. An example of formation of delaminations in "thick" NMCC is presented in Figure 20. It is possible to observe four clear delaminations located at approximately equal distance (about 20 nano-sublayers) from each other. Meanwhile, the delamination closest to the substrate (area A on Figure 20) passes exclusively along the boundary between the sublayers. At the same time, delaminations B, C, and D are rather longitudinal cracks because they also are character-

Various internal defects in "thick" NMCC (in particular, microdroplets embedded in the structure of the coating) become particularly important and result in local fracture of the coating because of the formation of multiple delaminations that weaken the coating structure and ultimately result in the formation of a transverse crack (Figure 20). As a result of the distortion of the coating structure associated with the curvature of the nano-sublayers because of rounding

Figure 18. The process of delamination and failure in coating Zr-ZrN-(Zr,Cr,Al,Nb)N: general structure (a), destruction of

the coating in the area of the crater wear boundary (b), boundary region "coating-adherent" (c).

tant factor in failure of coating (Figure 18c).

ized by breaks in the structures of nano-sublayers (Figure 19).

Various defects in coatings (in particular, embedded microdrops and pores) can play an important role in the formation of longitudinal cracks and delamination. Figure 16 shows how a crack reaches a macro droplet and forms branches. Meanwhile, one of the branches of the crack passes through a macro droplet, while the second crack branch traverses it along the contour. This photomicrograph reveals a separation of the material being machined from the coating; this separation indicates a low adhesive bond between the materials. Meanwhile, no separation of the coating from the tool material occurs due to a strong adhesive bond between them.

Another example of the effect of a microdroplet embedded in the structure of the coating on the formation of delaminations is shown in Figure 17. Here, a crack is formed directly above a microdroplet, and several parallel delaminations exist in the area adjacent to a microdroplet. These occurrences may be related to internal stresses arising during the coating deposition.

Figure 16. An example of development of a longitudinal crack in NMCC Zr-ZrN-(Zr,Cr,Al)N [28].

Figure 17. An example of development of a longitudinal crack in NMCC Ti-TiN-(Ti,Al)N [28].

Let us consider separately the process of delamination and failure in coating Zr-ZrN-(Zr, Cr, Al, Nb)N with thickness of sublayers of about 200 nm (Figure 18a). Due to thick sublayers, this coating cannot be called "nanostructured." This coating is characterized by a large number of delaminations, arising especially in the fracture zone adjacent to a wear crater. No bond bridges are formed between sublayers (Figure 18 Area A), while delaminations are an important factor in failure of coating (Figure 18c).

passing into a transverse crack that cuts the adhesive layer, and turning into a series of delaminations between nano-sublayers of the intermediate coating layer. The initial factor stimulating the formation of this crack is microroughness of the substrate, formed by high carbide grain. In contrast, the area indicated as AREA II contains an example of extended delamination, reaching a width of 200–300 nm. The formation of this delamination resulted in (1) chipping of microcomponents of the coating, (2) formation of bridges of more plastic

Various defects in coatings (in particular, embedded microdrops and pores) can play an important role in the formation of longitudinal cracks and delamination. Figure 16 shows how a crack reaches a macro droplet and forms branches. Meanwhile, one of the branches of the crack passes through a macro droplet, while the second crack branch traverses it along the contour. This photomicrograph reveals a separation of the material being machined from the coating; this separation indicates a low adhesive bond between the materials. Meanwhile, no separation of

Another example of the effect of a microdroplet embedded in the structure of the coating on the formation of delaminations is shown in Figure 17. Here, a crack is formed directly above a microdroplet, and several parallel delaminations exist in the area adjacent to a microdroplet. These occurrences may be related to internal stresses arising during the coating deposition.

the coating from the tool material occurs due to a strong adhesive bond between them.

Figure 16. An example of development of a longitudinal crack in NMCC Zr-ZrN-(Zr,Cr,Al)N [28].

Figure 17. An example of development of a longitudinal crack in NMCC Ti-TiN-(Ti,Al)N [28].

nanolayers, and (3) the crack development boundary.

126 Novel Nanomaterials - Synthesis and Applications

Let us individually consider delaminations formed in NMCC of heavy thickness (usually exceeding 8 μm) because of heavy internal compressive stresses. Such delaminations can be formed with equal probability in the coating both on the rake and flank face of a tool. An example of formation of delaminations in "thick" NMCC is presented in Figure 20. It is possible to observe four clear delaminations located at approximately equal distance (about 20 nano-sublayers) from each other. Meanwhile, the delamination closest to the substrate (area A on Figure 20) passes exclusively along the boundary between the sublayers. At the same time, delaminations B, C, and D are rather longitudinal cracks because they also are characterized by breaks in the structures of nano-sublayers (Figure 19).

Various internal defects in "thick" NMCC (in particular, microdroplets embedded in the structure of the coating) become particularly important and result in local fracture of the coating because of the formation of multiple delaminations that weaken the coating structure and ultimately result in the formation of a transverse crack (Figure 20). As a result of the distortion of the coating structure associated with the curvature of the nano-sublayers because of rounding

Figure 18. The process of delamination and failure in coating Zr-ZrN-(Zr,Cr,Al,Nb)N: general structure (a), destruction of the coating in the area of the crater wear boundary (b), boundary region "coating-adherent" (c).

4. Conclusions

nation):

layered nanostructured coatings reveals the following:

ment of longitudinal cracks and delaminations.

material being machined.

leads to failure of coating structure.

hard and brittle compound (Ti,Al)N.

Acknowledgements

project 16.9575.2017/6.7).

nano-sublayers.

This study of the nature of the formation of longitudinal cracks and delaminations in multi-

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129

1. Two important mechanisms result in formation of transverse cracks and delaminations:

tool substrate (more typical for the flank face of a tool).

a. Tearing force associated with adhesion interaction between the outer boundary of the coating and the material being machined (typical for the rake face of the tool).

b. Tearing force associated with plastic microdeformations in the surface layer of the

2. The nature of the formation of longitudinal cracks and delaminations varies significantly for different coating compositions. In coatings with more plastic nanolayers, bridges can be formed, which inhibit the development of cracks. This can be clearly observed in NMCC Zr-ZrN-(Zr,Cr,Al)N and to a lesser extent in NMCC Zr-ZrN-(Zr,Nb,Ti,Al)N, while in coatings with more hard and brittle nanolayers (e.g., (Ti,Al)N), such bridges are not

3. Such coating defects as embedded microdrops and micropores can stimulate the develop-

4. The following factors reduce the probability of formation of longitudinal cracks (delami-

a. Reduction of adhesion interaction between the outer boundary of the coating and the

b. Increase of adhesion bonds between coating layers and cohesive bonds between the

c. Decrease in the level of plastic microdeformations of the tool substrate, in particular, through heat strengthening and/or diffusion saturation with alloying elements.

5. In general, coatings with thickness of sublayers of more than 100 nm do not form bond bridges which inhibit cracking. In such coatings, delamination develops more actively and

6. NMCC of relatively large thickness (larger than 8 μm) may experience delamination during the deposition as a result of significant internal stresses. The presence of such delamination can contribute to brittle fracture of coatings, particularly NMCC based on a

This research was financed by the Ministry of Education and Science of the Russian Federation in the framework of the state order in the sphere of scientific activity (Leading researchers,

formed, and coatings are destructed under the mechanism of brittle failure.

Figure 19. An example of the formation of delaminations in NMCC Ti-TiN-(Ti,Al)N (total thickness of the coating is 10.3 μm) [28].

Figure 20. Formation of a transverse crack in the structure of NMCC Ti-TiN-(Ti,Al)N as a result of weakening of the structure of NMCC by multiple delaminations formed under the influence of internal stresses [28].

of an embedded microdroplet, internal stresses arise, which in turn result in the formation of corresponding delaminations and longitudinal cracks. Because (Ti,Al)N is a very hard, yet brittle compound, the chipping of fragments of nano-sublayers and formation of a transverse crack occur in the coating structure weakened by delaminations.

## 4. Conclusions

This study of the nature of the formation of longitudinal cracks and delaminations in multilayered nanostructured coatings reveals the following:

	- a. Tearing force associated with adhesion interaction between the outer boundary of the coating and the material being machined (typical for the rake face of the tool).
	- b. Tearing force associated with plastic microdeformations in the surface layer of the tool substrate (more typical for the flank face of a tool).
	- a. Reduction of adhesion interaction between the outer boundary of the coating and the material being machined.
	- b. Increase of adhesion bonds between coating layers and cohesive bonds between the nano-sublayers.
	- c. Decrease in the level of plastic microdeformations of the tool substrate, in particular, through heat strengthening and/or diffusion saturation with alloying elements.

#### Acknowledgements

of an embedded microdroplet, internal stresses arise, which in turn result in the formation of corresponding delaminations and longitudinal cracks. Because (Ti,Al)N is a very hard, yet brittle compound, the chipping of fragments of nano-sublayers and formation of a transverse crack

Figure 20. Formation of a transverse crack in the structure of NMCC Ti-TiN-(Ti,Al)N as a result of weakening of the

structure of NMCC by multiple delaminations formed under the influence of internal stresses [28].

Figure 19. An example of the formation of delaminations in NMCC Ti-TiN-(Ti,Al)N (total thickness of the coating is

occur in the coating structure weakened by delaminations.

10.3 μm) [28].

128 Novel Nanomaterials - Synthesis and Applications

This research was financed by the Ministry of Education and Science of the Russian Federation in the framework of the state order in the sphere of scientific activity (Leading researchers, project 16.9575.2017/6.7).

#### Author details

Alexey Vereschaka<sup>1</sup> \*, Sergey Grigoriev<sup>1</sup> , Nikolay Sitnikov<sup>2</sup> , Gaik Oganyan<sup>1</sup> , Anatoliy Aksenenko<sup>1</sup> and Andre Batako<sup>3</sup>


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**Chapter 8**

**Provisional chapter**

**Mechanical Properties of GO Nanostructures Prepared**

**Mechanical Properties of GO Nanostructures Prepared** 

Recently, 3D graphene oxide (GO) has attracted much attention due to its high specific surface area, multifunction, and facile preparation. Here, porous GO foams with extraordinary mechanical properties were prepared by using freeze-drying technique. The structure and mechanical properties of the GO foams have been characterized by X-ray diffraction, Fourier transform infrared spectroscopy, atomic force microscopy, and electronic universal testing machine. The unique structure endows the GO foams excellent elasticity, which can recover to its original shape even after compression hundreds of times. The density of GO foams has a significantly positive impact on the elastic modulus. Furthermore, the compressive strength of GO foams decreased linearly with decreasing relative humidity. A honeycomb model was constructed to investigate the effects of wall thickness, length, and included angle on the elastic modulus of GO foams. The structural evolution during the compression was revealed by finite element simulation. **Keywords:** GO foams, mechanical properties, humidity sensitivity, Gibson honeycomb

Recently, graphene and graphene oxide (GO) have been widely studied for various applications, such as sensors, field-effect transistors, Li-ion batteries, and polymer composites, due to their remarkable physical and chemical properties. As is well known, preparation of

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.71515

**from Freeze-Drying Method**

**from Freeze-Drying Method**

Ping Zhang

Ping Zhang

**Abstract**

**1. Introduction**

Yanhuai Ding, Hui Chen, Zheng Li, Huming Ren, Xianqiong Tang, Jiuren Yin, Yong Jiang and

Yanhuai Ding, Hui Chen, Zheng Li, Huming Ren, Xianqiong Tang, Jiuren Yin, Yong Jiang and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71515

model, finite element simulation


#### **Mechanical Properties of GO Nanostructures Prepared from Freeze-Drying Method Mechanical Properties of GO Nanostructures Prepared from Freeze-Drying Method**

DOI: 10.5772/intechopen.71515

Yanhuai Ding, Hui Chen, Zheng Li, Huming Ren, Xianqiong Tang, Jiuren Yin, Yong Jiang and Ping Zhang Yanhuai Ding, Hui Chen, Zheng Li, Huming Ren, Xianqiong Tang, Jiuren Yin, Yong Jiang and Ping Zhang

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71515

#### **Abstract**

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Metals; 1969 (fifth reprint)

1992;7:1564-1583

Recently, 3D graphene oxide (GO) has attracted much attention due to its high specific surface area, multifunction, and facile preparation. Here, porous GO foams with extraordinary mechanical properties were prepared by using freeze-drying technique. The structure and mechanical properties of the GO foams have been characterized by X-ray diffraction, Fourier transform infrared spectroscopy, atomic force microscopy, and electronic universal testing machine. The unique structure endows the GO foams excellent elasticity, which can recover to its original shape even after compression hundreds of times. The density of GO foams has a significantly positive impact on the elastic modulus. Furthermore, the compressive strength of GO foams decreased linearly with decreasing relative humidity. A honeycomb model was constructed to investigate the effects of wall thickness, length, and included angle on the elastic modulus of GO foams. The structural evolution during the compression was revealed by finite element simulation.

**Keywords:** GO foams, mechanical properties, humidity sensitivity, Gibson honeycomb model, finite element simulation

#### **1. Introduction**

Recently, graphene and graphene oxide (GO) have been widely studied for various applications, such as sensors, field-effect transistors, Li-ion batteries, and polymer composites, due to their remarkable physical and chemical properties. As is well known, preparation of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

graphene monolayers by chemical reduction of GO sheets is considered as an ideal method to obtain a high yield of graphene. Actually, GO is a kind of functional nanomaterials used widely, which contains large number of hydrophilic groups such as hydroxyl, carboxyl, and carbonyl on both sides of the sheets. Since graphene was reckoned as a strong contender to carbon nanotubes, GO-based materials have attracted a great deal of attention due to their wide applications in gas sensors [1], Li-storage [2, 3], catalyst supports [4], and water purification [5]. GO nanosheets can be easily prepared by chemical exfoliation of graphite in bulk quantities. Due to its good dispersity and ease of post-functionalization, the researchers not only focus on the reduction of GO to graphene but also shift to explore various chemical and physical properties caused by chemical modification and structure tuning. Theoretical and experimental studies have demonstrated that GO exhibited size-dependent properties when its size down to the nanometer scale. GO sheets with different lateral size displayed different mechanical strength.

**2.2. Preparation of GO foams**

than 20 Pa.

respectively.

**2.3. Tests of GO foams**

**3. Results and discussion**

at elevated temperatures.

undergo compression without collapsing.

GO foams were fabricated by freeze-drying method to form GO dispersion. The concentration of GO was set at 6.8 gL−1. GO suspension was frozen into an ice cube in a refrigerator (−18°C) and then freeze dried with a condenser temperature of −20°C and inside pressure less

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The morphology, structure, and mechanical properties of GO foam were investigated by X-ray diffraction (XRD; Bruker D8 Advance), Fourier transform infrared spectroscopy (FTIR; Perkin-Elmer Spectrum 10), scanning electronic microscopy (SEM; JSM-6610LV), atomic force microscopy (AFM; Veeco Multimode Nanoscope 3D), and electronic universal testing machine (UTM; Instron 5943). The diameter and height of the specimens were 10 and 5 mm,

AFM image, XRD pattern, and FTIR spectra of as-prepared GO foam are shown in **Figure 1**. Exfoliation of graphite oxide to GO was achieved by ultrasonication of graphite oxide. The exfoliated GO nanosheets are flat with an average thickness of about 2 nm, which means that the single-layer GO sheet was obtained, as shown in **Figure 1a**. XRD pattern of GO contains an intense 001 peak at 2θ = 10° that corresponds to a d-spacing of approximately 0.8 nm. The disappearance of the native graphite peak at about 26° revealed the successful oxidation of the graphite powders. The increased interlayer spacing can weaken the van der Waals interactions between GO layers and make facile exfoliation possible. FTIR spectra of GO show that the spectral bands are corresponding to C-O stretching vibrations (1300–1000 cm−1), C=O stretching vibrations from carbonyl and carboxylic groups (1720–1706 cm−1), C=C stretching vibrations from unoxidized graphitic domains (1450–1680 cm−1), C-H bending vibration (1465–1340 cm−1), and O-H stretching vibrations (3430 cm−1) [17, 18]. The presence of these functional groups on the GO surface leads to strong interaction of GO with water. In addition, the presence of the functional groups makes GO thermally unstable, as it undergoes pyrolysis

Digital picture of a cylinder-like GO foam is presented in **Figure 2a**. The shape and size of the GO foam can be easily adjusted in the freeze-drying process. Foam-like structures with high porosity and flexibility have many important applications in actuators, catalytic supports, adsorption, and separation. The size of the foam can be easily adjusted by simply changing the initial concentration of the GO suspension. SEM image reveals that interconnected pore structure is formed where GO nanosheets traversed laterally and connected with other sheets. This honeycomb features endow GO foam extraordinary mechanical properties, which can

Though we view GO as potentially powerful and widely applicable, it is very difficult to recycle GO from the dispersion. To overcome this problem, various methods such as blending GO with polymer matrix, formation of GO sponge, and chemical reduced GO gels generated by hydrothermal method have been investigated. Recently, significant progress has been made in self-assembly of GO nanosheets into microporous or mesoporous networks such as GO foams. Several methods have been employed to prepare 3D GO foams such as selfassembly [6, 7], leavening process [8], electrochemical erosion [9], electrospinning method [10, 11], hydrothermal reaction [12], solvent evaporation method [13], and freeze-drying technique [14–16]. Among them, the freeze-drying technique has been intensively studied due to its low-cost, high efficiency, and high yield. The mechanical performance of GO foam is critical to its application under harsh environments; however, most researchers focused on the functionality of GO architectures. Here, 3D GO foams were prepared by freeze drying of GO aqueous dispersion. The relationship between structure and mechanical behavior of GO foams was investigated, and the effect of RH on the mechanical properties of GO foams was also studied. Simultaneously, the structural evolution of GO under uniaxial compression was simulated by finite element method.

## **2. Experimental details**

#### **2.1. Preparation of GO nanosheets**

We used a modified Hummers' method to produce chemical exfoliated GO nanosheets with a thickness of 2–5 nm and lateral dimensions of 1–10 μm. In a typical process, graphite powders were oxidized by reacting with a mixture of NaNO<sup>3</sup> and concentrated H<sup>2</sup> SO<sup>4</sup> in an ice bath, then KMnO4 was added to the dispersion slowly. After fully oxidizing, the graphite oxide was washed with dilute HCl and deionized water pH of the wash solution was near neutral. The obtained graphite oxide was dispersed in deionized water and exfoliated through ultrasonication for 0.5 h.

#### **2.2. Preparation of GO foams**

graphene monolayers by chemical reduction of GO sheets is considered as an ideal method to obtain a high yield of graphene. Actually, GO is a kind of functional nanomaterials used widely, which contains large number of hydrophilic groups such as hydroxyl, carboxyl, and carbonyl on both sides of the sheets. Since graphene was reckoned as a strong contender to carbon nanotubes, GO-based materials have attracted a great deal of attention due to their wide applications in gas sensors [1], Li-storage [2, 3], catalyst supports [4], and water purification [5]. GO nanosheets can be easily prepared by chemical exfoliation of graphite in bulk quantities. Due to its good dispersity and ease of post-functionalization, the researchers not only focus on the reduction of GO to graphene but also shift to explore various chemical and physical properties caused by chemical modification and structure tuning. Theoretical and experimental studies have demonstrated that GO exhibited size-dependent properties when its size down to the nanometer scale. GO sheets with different lateral size displayed different

Though we view GO as potentially powerful and widely applicable, it is very difficult to recycle GO from the dispersion. To overcome this problem, various methods such as blending GO with polymer matrix, formation of GO sponge, and chemical reduced GO gels generated by hydrothermal method have been investigated. Recently, significant progress has been made in self-assembly of GO nanosheets into microporous or mesoporous networks such as GO foams. Several methods have been employed to prepare 3D GO foams such as selfassembly [6, 7], leavening process [8], electrochemical erosion [9], electrospinning method [10, 11], hydrothermal reaction [12], solvent evaporation method [13], and freeze-drying technique [14–16]. Among them, the freeze-drying technique has been intensively studied due to its low-cost, high efficiency, and high yield. The mechanical performance of GO foam is critical to its application under harsh environments; however, most researchers focused on the functionality of GO architectures. Here, 3D GO foams were prepared by freeze drying of GO aqueous dispersion. The relationship between structure and mechanical behavior of GO foams was investigated, and the effect of RH on the mechanical properties of GO foams was also studied. Simultaneously, the structural evolution of GO under uniaxial compression was

We used a modified Hummers' method to produce chemical exfoliated GO nanosheets with a thickness of 2–5 nm and lateral dimensions of 1–10 μm. In a typical process, graphite powders

washed with dilute HCl and deionized water pH of the wash solution was near neutral. The obtained graphite oxide was dispersed in deionized water and exfoliated through ultrasoni-

was added to the dispersion slowly. After fully oxidizing, the graphite oxide was

and concentrated H<sup>2</sup>

SO<sup>4</sup>

in an ice bath,

mechanical strength.

134 Novel Nanomaterials - Synthesis and Applications

simulated by finite element method.

**2.1. Preparation of GO nanosheets**

were oxidized by reacting with a mixture of NaNO<sup>3</sup>

**2. Experimental details**

then KMnO4

cation for 0.5 h.

GO foams were fabricated by freeze-drying method to form GO dispersion. The concentration of GO was set at 6.8 gL−1. GO suspension was frozen into an ice cube in a refrigerator (−18°C) and then freeze dried with a condenser temperature of −20°C and inside pressure less than 20 Pa.

#### **2.3. Tests of GO foams**

The morphology, structure, and mechanical properties of GO foam were investigated by X-ray diffraction (XRD; Bruker D8 Advance), Fourier transform infrared spectroscopy (FTIR; Perkin-Elmer Spectrum 10), scanning electronic microscopy (SEM; JSM-6610LV), atomic force microscopy (AFM; Veeco Multimode Nanoscope 3D), and electronic universal testing machine (UTM; Instron 5943). The diameter and height of the specimens were 10 and 5 mm, respectively.

### **3. Results and discussion**

AFM image, XRD pattern, and FTIR spectra of as-prepared GO foam are shown in **Figure 1**. Exfoliation of graphite oxide to GO was achieved by ultrasonication of graphite oxide. The exfoliated GO nanosheets are flat with an average thickness of about 2 nm, which means that the single-layer GO sheet was obtained, as shown in **Figure 1a**. XRD pattern of GO contains an intense 001 peak at 2θ = 10° that corresponds to a d-spacing of approximately 0.8 nm. The disappearance of the native graphite peak at about 26° revealed the successful oxidation of the graphite powders. The increased interlayer spacing can weaken the van der Waals interactions between GO layers and make facile exfoliation possible. FTIR spectra of GO show that the spectral bands are corresponding to C-O stretching vibrations (1300–1000 cm−1), C=O stretching vibrations from carbonyl and carboxylic groups (1720–1706 cm−1), C=C stretching vibrations from unoxidized graphitic domains (1450–1680 cm−1), C-H bending vibration (1465–1340 cm−1), and O-H stretching vibrations (3430 cm−1) [17, 18]. The presence of these functional groups on the GO surface leads to strong interaction of GO with water. In addition, the presence of the functional groups makes GO thermally unstable, as it undergoes pyrolysis at elevated temperatures.

Digital picture of a cylinder-like GO foam is presented in **Figure 2a**. The shape and size of the GO foam can be easily adjusted in the freeze-drying process. Foam-like structures with high porosity and flexibility have many important applications in actuators, catalytic supports, adsorption, and separation. The size of the foam can be easily adjusted by simply changing the initial concentration of the GO suspension. SEM image reveals that interconnected pore structure is formed where GO nanosheets traversed laterally and connected with other sheets. This honeycomb features endow GO foam extraordinary mechanical properties, which can undergo compression without collapsing.

A compression load of 1.5 N was applied, and GO foams were pressed to 80% of the original length. After 300 cycles, the compression load was not changed, which can be ascribed to the well-ordered microstructures oriented along the compression direction. The load-displacement curves recorded during the compression cycling are shown in **Figure 3b**. All curves have a familiar form, in which displacement increases as applied load gradually increases. Once a maximum value of 1.5 N is achieved, the load decreases as the indenter is retracted. As we can see from the figure, no residual displacement is observed after the indenter was fully retracted. GO foams exhibited good elasticity, and it recovered to its original height even

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The humidity sensitivity of the GO foam was characterized. As depicted in **Figure 4**, the maximum load compressed GO foam to its 80% of the original length is reduced with the increasing of RH. Surface properties of GO sheets were affected greatly by the RH due to its

hydrophilicity, which caused a change in the mechanical behavior of GO foams.

**Figure 3.** Uniaxial compression data of GO foams. (a) Uniaxial compression test. (b) Compression cycling.

**Figure 4.** Load-displacement curves at different ambient humidity.

after 300 cycles.

**Figure 1.** AFM image, XRD pattern, and FTIR spectra of as-prepared GO. (a) AFM image; (b) XRD; (c) FTIR.

**Figure 2.** Microstructure of GO foams. The red line shows the honeycomb-like structure in GO foam.

GO foams were submitted to compression for 300 cycles, and each cycle involved an contact between sample and the indenter, a displacement to a prescribed value, and a retraction to the original position. The results of uniaxial compression experiments are shown in **Figure 3a**. A compression load of 1.5 N was applied, and GO foams were pressed to 80% of the original length. After 300 cycles, the compression load was not changed, which can be ascribed to the well-ordered microstructures oriented along the compression direction. The load-displacement curves recorded during the compression cycling are shown in **Figure 3b**. All curves have a familiar form, in which displacement increases as applied load gradually increases. Once a maximum value of 1.5 N is achieved, the load decreases as the indenter is retracted. As we can see from the figure, no residual displacement is observed after the indenter was fully retracted. GO foams exhibited good elasticity, and it recovered to its original height even after 300 cycles.

The humidity sensitivity of the GO foam was characterized. As depicted in **Figure 4**, the maximum load compressed GO foam to its 80% of the original length is reduced with the increasing of RH. Surface properties of GO sheets were affected greatly by the RH due to its hydrophilicity, which caused a change in the mechanical behavior of GO foams.

**Figure 3.** Uniaxial compression data of GO foams. (a) Uniaxial compression test. (b) Compression cycling.

**Figure 4.** Load-displacement curves at different ambient humidity.

GO foams were submitted to compression for 300 cycles, and each cycle involved an contact between sample and the indenter, a displacement to a prescribed value, and a retraction to the original position. The results of uniaxial compression experiments are shown in **Figure 3a**.

**Figure 2.** Microstructure of GO foams. The red line shows the honeycomb-like structure in GO foam.

**Figure 1.** AFM image, XRD pattern, and FTIR spectra of as-prepared GO. (a) AFM image; (b) XRD; (c) FTIR.

136 Novel Nanomaterials - Synthesis and Applications

*<sup>E</sup>* <sup>=</sup> *Es* (

*<sup>υ</sup>*<sup>12</sup> <sup>=</sup> (cos*<sup>θ</sup>* <sup>+</sup> \_*<sup>t</sup>*

[( \_\_*h*

lus decreased with an increase in structural parameter *θ*.

density 15.45 mg cm−3; (c) density 10.93 mg cm−3.

[( \_\_*h* \_*t l*) 3 (cos*<sup>θ</sup>* <sup>+</sup> \_*<sup>t</sup> l*)

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

*<sup>l</sup>*) cos*<sup>θ</sup>* <sup>⋅</sup> [<sup>1</sup> <sup>+</sup> (1.4 <sup>+</sup> 1.5 *<sup>υ</sup>s*) (

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

\_*t l*) 2 ] 

⎫ ⎪ ⎬ ⎪ ⎭

http://dx.doi.org/10.5772/intechopen.71515

(1)

139

\_*t l*) 2 ]

\_*t l*) 2 ]

Mechanical Properties of GO Nanostructures Prepared from Freeze-Drying Method

*<sup>l</sup>* <sup>+</sup> sin*θ*) sin2 *<sup>θ</sup>*] <sup>⋅</sup> [<sup>1</sup> <sup>+</sup> (2.4 <sup>+</sup> 1.5 *<sup>υ</sup><sup>s</sup>* <sup>+</sup> cot2 *<sup>θ</sup>*) (

*<sup>l</sup>* <sup>+</sup> sin*θ*) sin*θ*] <sup>⋅</sup> [<sup>1</sup> <sup>+</sup> (2.4 <sup>+</sup> 1.5 *<sup>υ</sup><sup>s</sup>* <sup>+</sup> cot2 *<sup>θ</sup>*) (

Where *t*, *l*, and *θ* are the wall thickness, length, and included angle, respectively. The effects of wall thickness, length, and included angle on the mechanical properties of GO foams are shown in **Figure 7**. With an increase of *h*/*l*, the elastic modulus of GO foams decreased. However, the elastic modulus decreased with decreasing *t*/*l*. Additionally, the elastic modu-

**Figure 6.** Effects of density and strain rate on the stress-strain behaviors of GO foams. (a) Loading rate 1 mm min−1; (b)

**Figure 5.** Relationship between the RH and mechanical properties of GO foams.

The relationship between the RH and mechanical properties of GO foams was shown in **Figure 5**. It is surprise that a linear relationship is observed between the maximum compression load and RH. GO foam exhibited very high sensitivity and good linearity with the RH in the range of 65–93%. Here, we give some discussions about the humidity sensing mechanism of the GO nanostructures. We believed that the large decrease in mechanical properties of GO foams was related to the adsorption of water molecules. As porous structure materials, GO foams have higher specific surface areas to absorb moisture easily. Thus, striking interaction between GO sheets was released due to the lubrication of water molecules. It may be the main reason for the decay of mechanical properties.

The pore size and porosity of GO foams were easily adjusted by changing the concentration of GO dispersions in the freeze-drying process. The effects of density and strain rate on the mechanical properties of GO foams are presented in **Figure 6**. The elastic modulus of GO foams with a density of 5.25, 10.93, and 15.45 mg cm−3 are 0.0635, 0.1715, and 0.3822 MPa, respectively. As expected, the elastic modulus was significantly affected by the density of the GO foam. GO foam shows a high resistance against the compression with increasing density. The yield stress displayed an increase as the strain rate increased due to the strain rate-dependent behavior of GO foams.

A honeycomb structure was constructed for the calculation of elastic modulus of GO foam. Two hypotheses were proposed: (1) GO foam suffers small deformation under uniaxial compression. (2) Honeycomb structure is remained during the compression. Then, the elastic modulus *E* and Poisson's ratio *v* could be expressed as:

Mechanical Properties of GO Nanostructures Prepared from Freeze-Drying Method http://dx.doi.org/10.5772/intechopen.71515 139

$$\begin{aligned} \text{map.} \land \text{vac.output} & \begin{aligned} \text{map.} \land \text{vac.output} & \begin{aligned} \text{map.} & \text{index.output} \\ \hline \end{aligned} \\\\ \begin{aligned} \text{if } E &= \frac{E\_\* \left(\frac{t}{l}\right)^3 \left(\cos \theta + \frac{t}{l}\right)}{\left[\left(\frac{h}{l} + \sin \theta\right) \sin^2 \theta\right] \cdot \left[1 + \left(2.4 + 1.5 \,\upsilon\_\* + \cot^2 \theta\right) \left(\frac{t}{l}\right)^2\right]} \\\\ \text{if } \upsilon\_{12} &= \frac{\left(\cos \theta + \frac{t}{l}\right) \cos \theta \cdot \left[1 + \left(1.4 + 1.5 \,\upsilon\_\*\right) \left(\frac{t}{l}\right)^2\right]}{\left[\left(\frac{h}{l} + \sin \theta\right) \sin \theta\right] \cdot \left[1 + \left(2.4 + 1.5 \,\upsilon\_\* + \cot^2 \theta\right) \left(\frac{t}{l}\right)^2\right]} \end{aligned} \end{aligned} \tag{1}$$

Where *t*, *l*, and *θ* are the wall thickness, length, and included angle, respectively. The effects of wall thickness, length, and included angle on the mechanical properties of GO foams are shown in **Figure 7**. With an increase of *h*/*l*, the elastic modulus of GO foams decreased. However, the elastic modulus decreased with decreasing *t*/*l*. Additionally, the elastic modulus decreased with an increase in structural parameter *θ*.

The relationship between the RH and mechanical properties of GO foams was shown in **Figure 5**. It is surprise that a linear relationship is observed between the maximum compression load and RH. GO foam exhibited very high sensitivity and good linearity with the RH in the range of 65–93%. Here, we give some discussions about the humidity sensing mechanism of the GO nanostructures. We believed that the large decrease in mechanical properties of GO foams was related to the adsorption of water molecules. As porous structure materials, GO foams have higher specific surface areas to absorb moisture easily. Thus, striking interaction between GO sheets was released due to the lubrication of water molecules. It may be the main reason for the decay of mechanical

**Figure 5.** Relationship between the RH and mechanical properties of GO foams.

138 Novel Nanomaterials - Synthesis and Applications

The pore size and porosity of GO foams were easily adjusted by changing the concentration of GO dispersions in the freeze-drying process. The effects of density and strain rate on the mechanical properties of GO foams are presented in **Figure 6**. The elastic modulus of GO foams with a density of 5.25, 10.93, and 15.45 mg cm−3 are 0.0635, 0.1715, and 0.3822 MPa, respectively. As expected, the elastic modulus was significantly affected by the density of the GO foam. GO foam shows a high resistance against the compression with increasing density. The yield stress displayed an increase as the strain rate increased due to the strain rate-depen-

A honeycomb structure was constructed for the calculation of elastic modulus of GO foam. Two hypotheses were proposed: (1) GO foam suffers small deformation under uniaxial compression. (2) Honeycomb structure is remained during the compression. Then, the elastic

properties.

dent behavior of GO foams.

modulus *E* and Poisson's ratio *v* could be expressed as:

**Figure 6.** Effects of density and strain rate on the stress-strain behaviors of GO foams. (a) Loading rate 1 mm min−1; (b) density 15.45 mg cm−3; (c) density 10.93 mg cm−3.

**Figure 7.** Effects of wall thickness (*t*), length (*l*), and included angle (*θ*) on the elastic modulus of GO foams. (a) *θ* = 15°; (b) *θ* = 30°; and (c) *θ* = 45°.

**4. Conclusions**

**Acknowledgements**

(No. 17A205) is greatly acknowledged.

represents the strain concentrates on the middle of the long side.

Honeycomb-like structured GO foams were prepared by freeze-drying method. The GO network structure endows GO foam extraordinary mechanical properties. GO foams can recover to its original height even after 300 compression cycles. GO foams exhibits a linear relationship between the maximum compression load and RH. SEM characterization reveals that the deformation of the honeycomb-like structure under compression is mainly observed at the

**Figure 8.** FE simulation and SEM image of honeycomb-like GO foams before and after compression. The insert in **Figure 8b**

Mechanical Properties of GO Nanostructures Prepared from Freeze-Drying Method

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141

The financial support from the National Natural Science Foundation of China (No. 21376199 and 51,002,128) and Scientific Research Foundation of Hunan Provincial Education Department

long side wall of the cell, which is consistent with the FE simulation results.

The change of honeycomb-like GO foams before and after compression was simulated by FE method as shown in **Figure 8**. The FE model was created in Commercial software package (ANSYS 15.0). The bottom was fixed and the compression force was applied on the top of the model. The honeycomb structure can be described as a typical linear elastic material according to the experimental data. Then, the effect of cell size on the mechanical properties of honeycomb structure was calculated by FE method. The compressive stress of GO foams under small deformation can be released by the interconnected GO nanosheets. The strain was mainly concentrated at the middle of the long side when honeycomb structure undergone large deformation as shown in the strain nephogram. Buckling is the type of failure that has been observed in GO foam after compression. From the SEM image, the FE simulation has been proven to be effective in this case. The deformation of the honeycomb-like structure is mainly observed at the long side wall of the cell, which is consistent with the simulation results.

Mechanical Properties of GO Nanostructures Prepared from Freeze-Drying Method http://dx.doi.org/10.5772/intechopen.71515 141

**Figure 8.** FE simulation and SEM image of honeycomb-like GO foams before and after compression. The insert in **Figure 8b** represents the strain concentrates on the middle of the long side.

#### **4. Conclusions**

The change of honeycomb-like GO foams before and after compression was simulated by FE method as shown in **Figure 8**. The FE model was created in Commercial software package (ANSYS 15.0). The bottom was fixed and the compression force was applied on the top of the model. The honeycomb structure can be described as a typical linear elastic material according to the experimental data. Then, the effect of cell size on the mechanical properties of honeycomb structure was calculated by FE method. The compressive stress of GO foams under small deformation can be released by the interconnected GO nanosheets. The strain was mainly concentrated at the middle of the long side when honeycomb structure undergone large deformation as shown in the strain nephogram. Buckling is the type of failure that has been observed in GO foam after compression. From the SEM image, the FE simulation has been proven to be effective in this case. The deformation of the honeycomb-like structure is mainly observed at the long side wall of the cell, which is consistent

**Figure 7.** Effects of wall thickness (*t*), length (*l*), and included angle (*θ*) on the elastic modulus of GO foams. (a) *θ* = 15°;

with the simulation results.

(b) *θ* = 30°; and (c) *θ* = 45°.

140 Novel Nanomaterials - Synthesis and Applications

Honeycomb-like structured GO foams were prepared by freeze-drying method. The GO network structure endows GO foam extraordinary mechanical properties. GO foams can recover to its original height even after 300 compression cycles. GO foams exhibits a linear relationship between the maximum compression load and RH. SEM characterization reveals that the deformation of the honeycomb-like structure under compression is mainly observed at the long side wall of the cell, which is consistent with the FE simulation results.

#### **Acknowledgements**

The financial support from the National Natural Science Foundation of China (No. 21376199 and 51,002,128) and Scientific Research Foundation of Hunan Provincial Education Department (No. 17A205) is greatly acknowledged.

## **Author details**

Yanhuai Ding, Hui Chen, Zheng Li, Huming Ren, Xianqiong Tang, Jiuren Yin, Yong Jiang and Ping Zhang\*

[12] Xu Y, Sheng K, Li C, Shi G. Self-assembled graphene hydrogel via a one-step hydrother-

[13] Ding Y-H, Zhang P, Ren H-M, Zhuo Q, Yang Z-M, Jiang Y. Preparation of graphene/

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[15] Mi X, Huang G, Xie W, Wang W, Liu Y, Gao JP. Preparation of graphene oxide aerogel

[16] Mohandes F, Salavati-Niasari M. Freeze-drying synthesis, characterization and in vitro bioactivity of chitosan/graphene oxide/hydroxyapatite nanocomposite. RSC Advances.

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\*Address all correspondence to: zhangp@xtu.edu.cn

Institute of Rheological Mechanics, Xiangtan University, Hunan, China

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to SO<sup>3</sup>

catalyzed by


**Section 2**

**Applications**

**Section 2**
