**3.3. Wear of PS-type bamboo specimens**

temperature-increasing condition) were automatically recoded. The friction coefficients (*μI*

volume wear rate of the friction materials was evaluated and calculated as follows:

), *d*<sup>1</sup>

is the average thickness of specimen after test (mm), and *f*

*V*(*t*) = \_\_\_\_ <sup>1</sup>

**3. Experimental results and discussion**

mens presented better wear resistance than P<sup>I</sup>

**3.2. Wear track morphologies of gray iron rings**

higher than that of its inner layer.

be discussed in the following sections.

the specimen and the disk (*A* = 625 m2

**3.1. Wear tests of bamboo specimens**

5600) at a voltage of 25 kV.

92 Bamboo - Current and Future Prospects

*d*2

*μD*(*i*)), and specific wear rate (*V*(*i*)) were obtained after 5000 rotations of the disc, where *i* = 1, 2, …, 6, corresponding to the temperature of 100, 150, 200, 250, 300, and 350°C, respectively. The

> 2*πR* \_\_ *A n d*<sup>1</sup> − *d* \_\_\_\_\_2 *f m*

where *n* is the number of revolutions of the disk (*n* = 5000), *R* is the distance between the center of the rotating disk and friction material specimen (*R* = 0.15 m), *A* is the contact surface between

Worn morphologies of the specimens after tests were observed using the SEM (JEOL JSM-

**Figure 3** illustrates the wear volume of the three types of bamboo specimens versus the normal load at 0.42 and 0.84 m s−1. It can be seen from **Figure 3** that the wear volume was dependent upon the normal load, the sliding velocity, and the relative orientation of bamboo fibers with respect to the friction surface. The wear volume and its difference between different types of specimens increased with the increase of normal load. The wear rate at 0.42 m s−1 velocity was lower than that at 0.84 m s−1. The N-type specimens presented excellent wear

tal conditions, suggesting that the wear resistance of the outside layer of bamboo stem was

The materials transfer from the bamboo specimens to the iron surface occurred due to adhesion. In the initial stage, transferred material formed some patches on the gray iron ring surface, as shown in **Figure 4a**. As the sliding distance was increased, transferred material patches were extended along the sliding direction because of crushing action and further adhesion. When the interfacial contact reached a steady state, the adhesion transfer film was in a relatively steady state as shown in **Figure 4b**. The transferred material film did not cover the entire friction surface of the iron ring, as the material transferred to the ring surfaces could be detached. This transferring-detaching process resulted in the adhesive wear of bamboo. Features of adhesive wear were also found on worn surfaces of bamboo specimens and will

resistance. Although they had the same relative orientation of friction surface, PS

is the average thickness of specimen before test (mm),

*<sup>m</sup>* is mean value of the force.


(*i*),

(1)


**Figure 5** illustrates typical morphologies of worn surfaces of P<sup>S</sup> -type specimens. There were mainly three wear features: pits, microcracks, and grooves. Pits were produced because of adhesion between bamboo specimens and iron. Some materials from these pits were transferred onto the gray iron ring surface and the remainder became wear debris. Because of asperities of the gray iron ring surface, a tensile stress existed in the bamboo surface layer at the rear of the contacting asperity and a compressive stress at front. This stress distribution could easily lead to microcracking of the bamboo surface layer across the friction direction. Moreover, the adhesion force existing at the contacting interface strengthened this microcracking process. However, microcracks along the friction direction may be directly nucleated and propagated due to the tensile stress. As the normal load was raised, the influence of microploughing-microcutting on the wear of the bamboo specimens surface layer became stronger, and the microploughing-microcutting grooves on worn surfaces generated under 60–90 N load at 0.42 m s−1 velocity were shallow (**Figure 5a**–**d**). For this case, damage of cellulose fibers was not severe. However, when the normal load reached 120 N, particularly at 0.84 m s−1 velocity, the cellulose fiber walls had been cut, but leptodermous

**Figure 5.** SEM micrographs of worn surfaces of P<sup>S</sup> -type bamboo specimens. (a) 60 N, 0.42 m s−1; (b) 90 N, 0.42 m s−1; (c) details from (b); (d) 120 N, 0.42 m s−1; and (e) 120 N, 0.84 m s−1.

cell tissue between the cellulose fibers and the material inside vascular bundles were not completely removed (**Figure 5e**).

It can be considered from the surface morphology shown in **Figure 5** that adhesion, microcracking, and microploughing-microcutting were the main wear mechanisms of P<sup>S</sup> -type bamboo specimens. When the normal load and sliding velocity were low, the adhesive wear, microcracking, and microploughing were predominant, while, when the load and velocity were high, the predominant wear mechanism was microploughing-microcutting.

asperities of the ring surface cut into the P<sup>I</sup>

**Figure 6.** Worn surface micrographs of P<sup>I</sup>

microploughing-microcutting damage of P<sup>I</sup>

cutting. The degree of damage of P<sup>I</sup>

**3.5. Wear of N-type bamboo specimens**

type ones.


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the middle texture material of vascular bundles (see **Figure 6f**), or very deep grooves were produced (see **Figure 6g**). These topographies represented severe wear rupture of P<sup>I</sup>

specimens. Comparing the morphologies illustrated in **Figures 5** and **6**, it was found that the

bamboo specimens were also adhesion, specially, microcracking and microploughing–micro-

**Figure 7** illustrates typical morphologies of worn surfaces of the N-type bamboo specimens. The structure of the bamboo stem was basically revealed at low magnification, as shown in **Figure 7a**, **d**, **g**, and **h**. The larger dark zones are the ends of vascular bundles consisting of several vascular, and the remainder is matrix tissue. It can be seen that there existed a certain difference in wear behavior between vascular bundles consisting of sclerenchyma cells and matrix tissue consisting

specimens under identical experimental conditions. The main wear mechanisms of P<sup>I</sup>

in (a); (c) 60 N, 0.42 m s−1; (d) 90 N, 0.42 m s−1; (e) 60 N, 0.84 m s−1; (f) and (g) 120 N, 0.84 m s−1.

#### **3.4. Wear of PI -type bamboo specimens**

**Figure 6** illustrates typical morphologies of worn surfaces of P<sup>I</sup> -type bamboo specimens. It was seen that microcracks and microploughing grooves existed on the worn surfaces at low magnification, as shown in **Figure 6a** and **e**. **Figure 6c** shows some extent of the adhesive wear. At high magnification, besides some microcracks with random distribution illustrated in **Figure 6a**, some regular microcracking took place, as shown in **Figure 6c** and **e**. The regular microcracking under low load mainly displayed two states. One was local microcracking, causing some strips of bamboo material to be dug out from the surface layer, as shown in **Figure 6b**. Another was microcracking along the fiber direction (i.e., the rubbing direction) and then across fiber direction, as shown in **Figure 6e**. The tensile and compressive stresses of the bamboo surface layer under the contacting asperities of the ring surface and the adhesion force between both rubbing surfaces played important roles in the surface microcracking during the rubbing process. Generally, the former regular-microcracking occurred at the lower velocity (0.42 m s−1) and the latter took place at the higher velocity (0.84 m s−1). Compared with P<sup>S</sup> -type, P<sup>I</sup> -type specimens were severely ploughed, as shown in **Figure 6b** and **d**. When the load was increased to 120 N, particularly at high velocity (0.84 m s−1),

**Figure 6.** Worn surface micrographs of P<sup>I</sup> -type bamboo specimens. (a) 60 N, 0.42 m s−1; (b) details of ploughing grooves in (a); (c) 60 N, 0.42 m s−1; (d) 90 N, 0.42 m s−1; (e) 60 N, 0.84 m s−1; (f) and (g) 120 N, 0.84 m s−1.

asperities of the ring surface cut into the P<sup>I</sup> -type specimen surface layer and ploughed up the middle texture material of vascular bundles (see **Figure 6f**), or very deep grooves were produced (see **Figure 6g**). These topographies represented severe wear rupture of P<sup>I</sup> -type specimens. Comparing the morphologies illustrated in **Figures 5** and **6**, it was found that the microploughing-microcutting damage of P<sup>I</sup> -type specimens was larger than that of P<sup>S</sup> -type specimens under identical experimental conditions. The main wear mechanisms of P<sup>I</sup> -type bamboo specimens were also adhesion, specially, microcracking and microploughing–microcutting. The degree of damage of P<sup>I</sup> -type specimens was severe in comparison to that of P<sup>S</sup> type ones.

#### **3.5. Wear of N-type bamboo specimens**

cell tissue between the cellulose fibers and the material inside vascular bundles were not

It can be considered from the surface morphology shown in **Figure 5** that adhesion, microcracking, and microploughing-microcutting were the main wear mechanisms of P<sup>S</sup>

bamboo specimens. When the normal load and sliding velocity were low, the adhesive wear, microcracking, and microploughing were predominant, while, when the load and velocity

was seen that microcracks and microploughing grooves existed on the worn surfaces at low magnification, as shown in **Figure 6a** and **e**. **Figure 6c** shows some extent of the adhesive wear. At high magnification, besides some microcracks with random distribution illustrated in **Figure 6a**, some regular microcracking took place, as shown in **Figure 6c** and **e**. The regular microcracking under low load mainly displayed two states. One was local microcracking, causing some strips of bamboo material to be dug out from the surface layer, as shown in **Figure 6b**. Another was microcracking along the fiber direction (i.e., the rubbing direction) and then across fiber direction, as shown in **Figure 6e**. The tensile and compressive stresses of the bamboo surface layer under the contacting asperities of the ring surface and the adhesion force between both rubbing surfaces played important roles in the surface microcracking during the rubbing process. Generally, the former regular-microcracking occurred at the lower velocity (0.42 m s−1) and the latter took place at the higher velocity (0.84 m s−1).

and **d**. When the load was increased to 120 N, particularly at high velocity (0.84 m s−1),


were high, the predominant wear mechanism was microploughing-microcutting.

**-type bamboo specimens**


**Figure 6** illustrates typical morphologies of worn surfaces of P<sup>I</sup>




completely removed (**Figure 5e**).

94 Bamboo - Current and Future Prospects

**Figure 5.** SEM micrographs of worn surfaces of P<sup>S</sup>

details from (b); (d) 120 N, 0.42 m s−1; and (e) 120 N, 0.84 m s−1.

**3.4. Wear of PI**

Compared with P<sup>S</sup>

**Figure 7** illustrates typical morphologies of worn surfaces of the N-type bamboo specimens. The structure of the bamboo stem was basically revealed at low magnification, as shown in **Figure 7a**, **d**, **g**, and **h**. The larger dark zones are the ends of vascular bundles consisting of several vascular, and the remainder is matrix tissue. It can be seen that there existed a certain difference in wear behavior between vascular bundles consisting of sclerenchyma cells and matrix tissue consisting of leptodermous cells. The wear rupture of vascular bundles was severe as compared with that of the matrix tissue. This result was opposite to the abrasive wear behavior of N-type specimens in free-abrasive wear conditions, in which the ends of vascular bundles were protruded on the matrix of the abrading surface of N-type specimens, suggesting that vascular bundles of bamboo possessed higher wear resistance than matrix tissue [5]. It was seen to the naked eye that the ends of vascular bundles were darker than matrix tissue after sliding friction, particularly at high velocity or under high load (see **Figure 7d**, **e**, **g**, and **h**). High temperature caused by frictional heat would burn the contacting bamboo surface. The vascular bundles were mainly burned for the N-type. This phenomenon can be observed clearly after grinding a bamboo block (N-type mode) against an abrasive wheel. The burnt material may easily be removed by asperities of the ring surfaces. The interfacial temperature was dependent upon the normal load and, particularly, sliding velocity [29]. Therefore, topographies of the ends of vascular bundles of worn surfaces of N-type bamboo specimens varied with the load and velocity.

and **b**). As the load or velocity was increased, the area of the protruded part became smaller, comparing **Figure 7a**, **d**, **g**, and **h**. It was considered that the center part of a vascular bundle would have leptodermous cell texture because its tribological behavior was similar to that of

Bamboo Wear and Its Application in Friction Material http://dx.doi.org/10.5772/intechopen.69893 97

Worn micrograph of N-type bamboo specimens mainly presented adhesive wear and microcracking. No microploughing-microcutting feature was observed on worn surfaces except those of 120 N load and 0.84 m s−1 velocity (**Figure 7h**). Besides the microcracking of matrix

The mechanical and physical properties of bamboo fibers in this study are listed in **Table 2**. From **Table 2**, it can be seen that the elongation at fracture of alkaline-treated bamboo fibers was about 1.2%, the tensile strength was about 56.3 MPa, and the elastic modulus was about 18.6 GPa. However, for untreated bamboo fibers, the elongation at fracture was 1.7%, the tensile strength was 46.7 MPa, and the elastic modulus was 23.3 GPa. It indicated that the tensile strength and elastic modulus were increased by the modification of bamboo fibers. It is because the crystallinity of the alkaline-treated bamboo fiber was increased, which could improve the polarity of molecules and the adhesion strength between the high molecules. The slip produced by destruction of the binding force of molecules was relatively small, so the elongation at fracture was reduced, and the tensile strength and elastic modulus was increased after modification for bamboo fiber. The stress-strain curve of the alkaline-treated bamboo fibers is shown in **Figure 8**. At the initial stage, the stress was proportional to the strain, which is consistent with the Hooke's Force Law. There was no yield and necking phenomenon, and the stress and strain were very low before breaking. It indicated that the bamboo fiber was a brittle material. The brake pads were basically acted by compression pressure when they work. Hence, the lower tensile strength of

tissue, cracking between bundles and matrix occurred, as shown in **Figure 7f** and **h**.

matrix tissue. **Figure 7c** gives the details of the worn surface of matrix tissue.

bamboo fiber does not have impact on the braking performance of brake pad.

uneven stress that resulted from the bonding of pectin with lignin among the fibers.

Untreated bamboo fiber 1.7 46.7 18.6

**Table 2.** Test results of mechanical properties of bamboo fiber.

Alkaline-treated bamboo

fiber

It can be seen from **Figure 9a** that a major part of the fracture surfaces presented brittle failure. Some small molecule impurities on bamboo fiber surface were removed by the alkaline solution that caused the adhesion strength among fibers to be reduced considerably. The fracture of the untreated fiber (**Figure 9b**) showed that the uneven break presented because of the

The bamboo fibers connected with each other under pressure. The tangential resistance generated in relative sliding is called friction force. The tangential resistance is called cohesive force at the normal press of zero. **Figure 10** shows that the bamboo fibers were assembled, entangled, bonded,

1.2 56.3 23.3

**Elongation (%) Tensile strength (MPa) Elastic modulus (GPa)**

**3.6. Mechanical and physical properties of bamboo fibers**

**Figure 7b** shows that the part surrounded by vascular of cellulose fibers (i.e., the center area of a vascular bundle) protruded. This was more obvious at low load and velocity (see **Figure 7a**

**Figure 7.** Worn surface micrographs of N-type bamboo specimens. (a) 60 N, 0.42 m s−1; (b) details of the center area of a vascular bundle in (a); (c) details of leptodermous cell of the lower area of (a); (d) 120 N, 0.42 m s−1; (e) details of a vascular bundle of the center area of (d); (f) details of the center area of the vascular bundle of (e); (g) 60 N, 0.84 m s−1; (h) 120 N, 0.84 m s−1 (the friction direction of the ring was from left to right).

and **b**). As the load or velocity was increased, the area of the protruded part became smaller, comparing **Figure 7a**, **d**, **g**, and **h**. It was considered that the center part of a vascular bundle would have leptodermous cell texture because its tribological behavior was similar to that of matrix tissue. **Figure 7c** gives the details of the worn surface of matrix tissue.

Worn micrograph of N-type bamboo specimens mainly presented adhesive wear and microcracking. No microploughing-microcutting feature was observed on worn surfaces except those of 120 N load and 0.84 m s−1 velocity (**Figure 7h**). Besides the microcracking of matrix tissue, cracking between bundles and matrix occurred, as shown in **Figure 7f** and **h**.
