Experimental

In order to verify experimentally the models presented in the preceding section, we prepared eight samples of MMCs, four with relatively smooth surfaces and four roughened by etching, and measured their roughness and contact angles. In fact by using etching, we tried to simulate the corrosive and erosive wear on the samples which are probable especially due to environmental conditions.

Fig. 7.16 Variation of contact angle versus graphite aligned fibers volume fraction in metal matrix for different average roughness, Ra and fractions of liquid-air in contact and a = 0.5 (C-B model)

7.3.1 Sample Preparation

The four samples of Al- and Cu-based alloys and their graphite composites were sectioned to 2.0 x 1.5 x 0.2 cm pieces. The diameter of graphite particles used to make MMCs is estimated to be between 10 and 15 im. We used Al and Cu because they are the standard material used as a matrix in the literatures due to their high conductivity and ductility for making MMCs. Furthermore, Al- and

Table 7.2 Chemical composition of the samples

Sample material

Composition

Copper base alloy

Cu (79.0-82.0%), Sn (2.5-3.5%), Pb (6.3-7.7%), Zn (7.0-10%),

P (0.02%), Al (0.05%), Si (0.005%)

Copper-graphite

Cu (81%), Ni (5%), Fe (4%), Al (9%), Mn (1%), 60% volume of

composite

graphite.

Al base alloy

Al (88%), Si (12%)

Al-graphite composite

Al (35%), Si (5%), 60% volume of graphite

Fig. 7.17 SEM images of polished samples

Copper graphite composite Copper base alloy

Aluminum graphite composite âjï Aluminum base alloy ^

Cu-based samples are easy to work with and inexpensive as well. Table 7.2 presents the chemical composition of the samples.

The samples were ground and polished to create a smooth surface before the etching process. The grinding involved successive steps with 400, 600, and 1,200 grit SiC paper. Polishing was done with a soft cloth impregnated with 1 i alumina. The Scanning electron microscope (SEM) images of the polished samples are as shown in Fig. 7.17.

After that four samples were etched. The reagents used in the selective dislocation etching are as follows [13]. For the aluminum base alloy and the aluminum-graphite composite, the etchant consisted of 40 ml of 37 wt% HCl, 12.5 ml of H2O, and 2.0 ml of 48 wt% of HF. The etching time was set for 20 s, for a two-cycle test. On the other hand, the etchants used for the copper-based alloy and the copper-graphite composite was 0.1 molar concentration of 37 wt% HCl. The etching time was 20 h for both the sample types. Since the Al-based samples are softer than Cu-based samples, these time periods for etching, make roughness of the same order of magnitude for all samples. All samples were washed, cleaned, and dried before conducting experiments.

Fig. 7.17 SEM images of polished samples

Copper graphite composite Copper base alloy

Table 7.3 Measured and calculated surafce roughness and contact angles Measured Calculated

Sample Etching Ra Measured Rfm Contact angle Contact angle time (im) contact (Wenzel, fSLm = 1) (Cassie-Baxter, _angle_fsLm = 0)_

Table 7.3 Measured and calculated surafce roughness and contact angles Measured Calculated

Sample Etching Ra Measured Rfm Contact angle Contact angle time (im) contact (Wenzel, fSLm = 1) (Cassie-Baxter, _angle_fsLm = 0)_

Al-graphite

0

0.2

59.1°

1

69°

92°

composite

Cu-graphite

0

0.2

86.7°

1

69°

92°

composite

Al base alloy

20 s

16

82.5°

1.89

73°

Cu base alloy

20 h

8

99.4°

1.28

29°

97°

Al-graphite

20 s

14

86.5°

1.72

56°

composite composite

composite

Fig. 7.18 Images of water droplet on non-etched samples

Copper base alloy Copper graphite compsite
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