Surface modification of diamond and its effect on the mechanical properties of diamond/epoxy composites

2.2.1 Modification in aqueous alcohol solvent

Ten percent silane coupling agent aqueous alcohol solution was prepared by ultrasonic dispersion for 10 min followed by pre-hydrolysis for 2 h. The weight ratio of KH550, distilled water and anhydrous ethanol was 1:1:8. Diamond was added into the abovementioned solution in the weight ratio of 1:4. The system was stirred and heated up to 80°C using heating magnetic stirrer. After stirring for about 4 h, the suspension was filtrated and the modified diamond was obtained after a repeated wash using anhydrous ethanol. These diamond particles were dried for 10 h at 100°C in a vacuum oven, and this helped to obtain modified diamond treated in aqueous alcohol solvent.

2.2.2 Modification in toluene solvent

KH550 was added to the toluene in a weight ratio of 1:9, which was dispersed ultrasonically for 10 min. Diamond particles were added into the KH550 toluene solution in the same weight ratio of diamond to solution of 1:4. After allowing the reaction to occur for about 4 h at a temperature of 100°C, the suspension was filtrated and the diamond was washed repeatedly with acetone. Finally, the diamond particles were dried for 10 h at 100°C in a vacuum oven. This process helped to obtain modified diamond treated in toulene solvent.

3.1.1 The surface groups of diamond particle

Figure 1 shows the FTIR spectra of diamond particles. The peaks at 3430 cm^-1and 1645 cm^-1 in Figure 1A are that of stretching vibration and deformation vibration of -OH, and the peak of 1394 cm^-1could be attributed to C-H vibration [26]. In Figure 1B and 1C, the peaks at 3430 cm^-1 and 1645 cm^-1 are narrowed and weakened. These are likely the absorption bands of rest of the -OH of diamond [27] and of -NH₂ of KH550. The peaks at 871 cm^-1 of Figure 1B and 885 cm^-1of Figure 1C belong to -Si-OH [17]. New peaks of 1468 cm^-1(-CH₂-)and 1040 cm^-1(C-O-Si/Si-O-Si) [17], [28] indicate that KH550 may be chemically grafted into the diamond surface both in aqueous alcohol and in toluene solvent. However, in comparison to Figure 1B, the peaks at 1468 cm^-1 and 1040 cm^-1 in Figure 1C are more intense, signifying the presence of more amount of KH550 on the diamond surface when treated in toluene solvent. Based on this, the following modification mechanism is proposed to happen during the process.

Figure 1:

FTIR spectra of (A) pristine diamond, (B) KH550 modified diamond in aqueous alcohol, (C) KH550 modified diamond in toluene.

3.1.2 Modification mechanism

In this paper, the chemical kinetics of the modification mechanism in aqueous alcohol solvent is hypothesized, as per the details shown in Figure 2. As shown in Figure 2, because of the presence of aqueous solvent, KH550 molecules will hydrolyze. However, if the experimental conditions are not controlled, the unimolecular hydrolysis products will easily self-polymerize into bigger molecules instead of reacting with -OH on the diamond surface. Also, the reaction of bigger molecules with the hydroxyl of the diamond surface is unfavorable, which couples further with the fact that the bigger molecules will hinder the reaction between unimolecular silanol and diamond. Thus, the number of molecules of KH550 reacting with hydroxyl on the diamond surface decreases, thereby making the amount of KH550 molecules less on the surface of diamond.

Figure 2:

The action mechanism of KH550 on the diamond surface in aqueous alcohol solvent.

Figure 3 shows schematically the plausible reaction mechanism of KH550 on the diamond surface in toluene solvent. In toluene solvent, the alkoxy groups of KH550 directly react with the hydroxyl on the diamond surface. During subsequent filtration process, the -Si-O-C₂H₅groups may hydrolyze to -Si-OH because of the steam in air and polymerize with each other subsequently. This would be expected to provide better adhesion, which has been detailed in the next sections.

Figure 3:

The modification mechanism of diamond with KH550 in toluene solvent.

3.1.3 The surface topography of diamond particles

Surface characterization of diamond under three different experimental conditions was performed using scanning electron microscopy (SEM) examination, which is shown in Figure 4. From Figure 4B and 4C, the presence of KH550 molecules on diamond surface is evident. However, the amount of KH550 on the surface of diamond modified in aqueous alcohol solvent is comparatively lower. Figure 4C shows that a silane layer was formed on diamond surface when treated with toluene solvent, which could offer better adhesion between diamond and resin.

Figure 4:

FE-SEM photographs of (A) pristine diamond, (B) KH550 modified diamond in aqueous alcohol, (C) KH550 modified diamond in toluene.

3.1.4 Chemical analysis of diamond particles using XPS

X-ray photoelectron spectra (XPS) analysis was considered reliable to unravel the understanding of the characteristics of bonding between diamond and KH550. Figure 5 and Table 1 show the XPS spectra of the pristine diamond and a comparison with the spectra of diamond obtained after modification by KH550. The contents of N (9.48%) and Si (13.87%) on diamond surface modified in toluene solvent were observed to be more than N (2.21%) and Si (3.11%) on the diamond treated in aqueous alcohol solvent. This is in agreement with the results of FTIR spectra and SEM investigation. Figure 6 displays the XPS spectra of Si 2p on the surface of the modified diamond. The binding energy of C1s (284.8 eV) is used as reference. The peaks at 101.53 eV, 102.3 eV and 103.15 eV are the binding energy of C-Si-O-Si, C-Si-O-H and C-Si-O-C, respectively [29]. The presence of C-O-Si-C indicates that the chemical reaction between KH550 and the hydroxyl group of diamond would proceed both in aqueous alcohol and toluene solvent. It can also be seen that much of the Si element on the surface of diamond modified in aqueous alcohol solvent is in the form of C-Si-O-Si. It means that most of the unimolecular hydrolysis products of silane coupling agent polymerized before reacting with the hydroxyl group of diamond. Moreover, through the integration of area enclosed by the peak of C-Si-O-C and the baseline, the area in Figure 6A and 6B is 1794 and 2406, respectively. It indicates that more C-Si-O-C was formed during the reaction between KH550 and -OH on diamond surface in toluene solvent.

Figure 5:

XPS spectra of (A) pristine diamond, (B) KH550 modified diamond in aqueous alcohol, (C) KH550 modified diamond in toluene.

Figure 6:

Si 2p XPS spectrum of diamond (A) KH550 modified diamond in aqueous alcohol, (B) KH550 modified diamond in toluene.

3.2.1 Mechanical properties of composite

After proceeding with the chemical investigation, mechanical analysis was performed to draw a comparison in the mechanical properties of three types of diamonds. Figure 7 shows the influence of the chemical treatment on the mechanical properties of diamond/epoxy composites under three test conditions. It can be seen that the bend strength of diamond/epoxy composite increases from 97.3 MPa to 103.6 MPa and 111.2 MPa, respectively, after diamond particles were treated with KH550 in aqueous alcohol and toluene solvent. This is because of the fact that during the curing process, the amine groups on the surface of modified diamond produce chemical cross-linking with the epoxy groups of resin, which increases the interfacial bonding strength. Therefore, the bend strength of the diamond/epoxy composite improves by 6.5% and 14.3% in respective conditions. Furthermore, as shown above, adhesion of more number of KH550 molecules in toluene solvent also contributes to higher bending strength. However, the hardness (Rockwell HB) of different samples (52.58, 53.13 and 55.17) showed a very small variation. It is anticipated that the acting force of the indenter was mainly applied on resin, which might be the reason that the hardness obtained here may not provide a good assessment.

Figure 7:

The bend strength and hardness of diamond/epoxy composites (i) 1#: sample with pristine diamond; (ii) 2#: sample with KH550 modified diamond in aqueous alcohol; and (iii) 3#: sample with KH550 modified diamond in toluene.

3.2.2 Interfacial conditions

Figure 8 is a SEM micrograph of the fracture surface of diamond/resin composite. Figure 8A shows the presence of several cracks (white arrows) at the interface of pristine diamond and resin. The surface of pristine diamond is hydrophilic, while the resin is hydrophobic. Therefore, the poor compatibility results in the appearance of cracks. However, post-treatment with KH550, the surface property of diamond was observed to change, which results in improved wettability between diamond and resin and eventually a better interfacial adhesion. After the diamond specimen was modified in aqueous alcohol solvent, a relatively smaller amount of KH550 molecule was chemically grafted into the surface of diamond (Figure 8B). Finally, when the modification was conducted in toluene solvent, more KH550 gets chemically grafted to the surface of diamond. This indicated that more -NH₂ groups will react with epoxy groups through chemical cross-linking, which accounts for better interfacial state between diamond and epoxy resin, and no cracks were observed as a result (Figure 8C).

Figure 8:

SEM micrograph of fracture surface of diamond/resin composite (A) sample with pristine diamond, (B) sample with KH550 modified diamond in aqueous alcohol, (C) sample with KH550 modified diamond in toluene; D represents diamond particles. The white arrows in (A) and (B) indicate racks.