Mechanical and Tribological Properties of Epoxy Nanocomposites

Transcription

1 Chapter 7 Mechanical and Tribological Properties of Epoxy Nanocomposites 7.1 Introduction This chapter discusses the mechanical and tribological properties of silicon dioxide (SiO 2 ) and alumina (Al 2 O 3 ) filled epoxy nanocomposites. Implications of introducing SiO 2 and Al 2 O 3 nanoparticles on mechanical and dry sliding wear properties are discussed using micrographs of cast samples and through observations of wear affected surface of nanocomposites. Distribution of nanoparticles and their influence on properties are being emphasized for understanding the wear properties. The data on mechanical and tribological properties determined experimentally are compared with published literature. The main focus is to highlight the importance of nanofillers in the design of wear resistant epoxy based composites. A detailed study of hardness, ultimate tensile strength and modulus of elasticity of nanofilled epoxy nanocomposites was taken up as a part of this investigation. A discussion on density, hardness, tensile properties, friction and dry sliding wear behavior of nanocomposites and analysis of results by comparison with prevalent theoretical models and published results of experiments are also presented in this chapter. 7.2 Density Density is a material property which is of prime importance in several weight sensitive applications. Thus, in many applications, polymer composites are found to replace conventional metals and metal based materials primarily for their low densities. Density of a composite depends on the relative proportion of matrix and the reinforcing materials. There is always a difference between the measured and the theoretical density values of a composite due to the presence of voids and pores. These voids significantly affect some of the mechanical, thermal and electrical properties and even the service performance of composites. A higher void content usually implies lower fatigue resistance, greater susceptibility to water penetration and weathering [326]. The theoretical and measured densities of nanocomposites along with corresponding volume fraction of voids are presented in Table 7.1. The theoretical density was calculated for nanocomposites by weight additive principle, which states that:. w. w (7.1) c m m p p Where, ρ c is the density of the nanocomposite, w m and w p are the weight fractions of the constituents and ρ m and ρ p are the densities of matrix and particulates respectively. It may be noted from Table 7.1 that density values of the nanocomposite calculated 168

2 theoretically from weight fractions using equation 7.1 are not in agreement with the experimentally determined values. The theoretically calculated density values are higher as compared to corresponding experimentally measured values due to void formation, pores and poor interfacial adhesion between epoxy matrix and filler in the nanocomposites. Table 7.1. Theoretical, experimental densities and void fractions in nanocomposites Composition (wt.%) epoxy-sio 2 epoxy-al 2 O 3 Density (g/cm 3 ) Theoretical Experimental Void fraction (%) The knowledge of void fraction is therefore essential for estimation of the quality of the composites. From the measured densities of epoxy-sio 2 /Al 2 O 3 nanocomposites (Table 7.1), it is clear that pure epoxy matrix has an average density of 1.152g/cm 3. With the incorporation of SiO 2 /Al 2 O 3 nanofillers, an increase in the density of the nanocomposites is observed. This observed result is true in epoxy matrix with nanosized SiO 2 /Al 2 O 3 filler inclusions. It is evident from Table 7.1 that, the density values for epoxy-sio 2 nanocomposite increases with the nanofiller loading. However, the increase in the density is small in epoxy-sio 2 nanocomposites. With the addition of 20wt.% SiO 2, the density of epoxy matrix increases by 5% and the density of Al 2 O 3 filled epoxy increases by 9.2%. Higher densities of Al 2 O 3 filled epoxy nanocomposites are attributed to the higher density of Al 2 O 3 as compared to the density of SiO 2 nanoparticles. 7.3 Mechanical properties Hardness Hardness is considered as one of the most important factors that governs the wear resistance of any material. In the present work, hardness (Shore D) values of the nanofillers containing epoxy nanocomposites with different filler loadings have been obtained and are compared with those of a similar particulate filled polymer composites. Figure 7.1 shows the experimental results of hardness of pure epoxy, epoxy-sio 2 /Al 2 O 3 nanocomposites with different wt.% of filler loading. From the Figure 7.1, it can be observed that the incorporation of nanofillers into epoxy results in significant improvement in hardness of nanocomposites. Mechanical properties of 169

3 nanocomposites generally depend on factors such as filler content, particle size and shape, the degree of adhesion between the filler and polymer matrix and the degree of dispersion of filler within the matrix. The reason for improvement in the hardness is the inclusion of surface treated hard nanoparticles and uniform dispersion in the epoxy matrix. Figure 7.1. Shore D hardness of epoxy-sio 2 and epoxy-al 2 O 3 nanocomposites The major difference between nanometric (size < 100nm) and micro metric (size >100nm) particles is that nanoparticles have significantly large specific surface area, which greatly facilitates the transfer of load from polymer matrix to nanoparticles. As a result, hard nanoparticles in the epoxy effectively restrict indentation and increase the hardness of the nanocomposites [327]. Similarly, incorporation of Al 2 O 3 into epoxy increases the degree of dispersion, there by increases the hardness of nanocomposite. The SiO 2 and Al 2 O 3 nanoparticles have much greater surface hardness because of their ceramic like nature. Therefore, the contribution of SiO 2 /Al 2 O 3 to the hardness is greater as compared to pure epoxy matrix. From the observed data it is clear that the increment in hardness is more significant up to 15wt.% of filler loading. The increase in hardness for 15wt.% filled epoxy matrix may be partly attributed to the intrinsic hardness of nanofiller and the nanoparticles might be offering better resistance against epoxy segmental motion under indentation. However, further loading of filler above 15wt.%, increases hardness only marginally. This suggests that higher filler loading gives rise to poor dispersion and agglomeration in the pure epoxy matrix. As far as the comparison between the nanocomposites with SiO 2 and Al 2 O 3 loading is concerned, the epoxy-al 2 O 3 nanocomposite exhibit superior hardness values for the four filler loadings studied. Among all the nanocomposites investigated, maximum hardness value is recorded in case of epoxy nanocomposite filled with 15wt.% Al 2 O

4 7.3.2 Tensile properties The measured tensile properties of pure epoxy, epoxy-sio 2 /Al 2 O 3 nanocomposites with different wt.% of filler loading are listed in Table 7.2. It can be seen from the Table 7.2 that higher values of tensile strength and lower values of elongation at break are obtained for nanocomposites than those of pure epoxy matrix. Furthermore, the Young s moduli are evidently enhanced by the addition of nanofillers. As seen in Table 7.2, the tensile strength of these nanocomposites slightly increases with increase in filler loading irrespective of the type of filler. Similar observations have been reported by some investigators [328, 329]. The strength and toughness of a nanocomposite depends upon the shape, size of the filler and the amount which is compounded with the polymer matrix, the bonding between filler and the matrix, the toughness of the matrix, and sometimes the toughness of the filler. Fillers affect the tensile properties according to their packing characteristics, size and interfacial bonding. The maximum volumetric packing fraction of filler reflects the size distribution and shapes of the particles. The space between the particles is assumed to be filled with matrix and no voids/air bubbles are expected. Under these conditions, for a given composite, the matrix volume is less and it acts as individual segment or pocket to support tensile load. Properties of composites are also influenced by the individual properties of the filler, matrix and also the filler/matrix interface. The ability of the matrix is to transfer the load to the filler particles. This primary function depends on the adhesion and compatibility between the filler and matrix. The tensile strengths of the nanoparticles filled epoxy are higher than that of pure epoxy matrix. The increasing trend of adhesion up to 15wt.% and decreasing trend with respect to relatively higher filler loading points towards the phenomenon of dewetting occurring at the interface. In the present study, failure of all nanocomposites at strengths higher than that of pure epoxy may be attributed to the processing method. The compounding of ceramic fillers in epoxy matrix with mechanical mixing produces a highly viscous and foamy material. A higher content of ceramic filler leads to higher viscosity. The presence of voids in higher wt.% (15 and 20) of SiO 2 and Al 2 O 3 nanocomposites (Table 7.2) further confirms this fact. The other reason for voids could be air trapped during pouring of the highly viscous material onto the mold. Finally, the failure of all specimens at a strength range of 55 to 71MPa indicates crack initiation from similar types of defects. Therefore, it can be assumed that under tensile loading, cracks can initiate from these tiny voids and can result in specimen failure at relatively low strains [330]. The other reason is that the corner points of the irregularly shaped filler namely Al 2 O 3 particulate, may result in stress concentration in the epoxy matrix. These two factors are perhaps responsible for reduction in tensile strengths of the higher filler loaded epoxy nanocomposites. A comparison of the results reveals that the 10wt.% SiO 2 filled epoxy nanocomposite possess higher tensile strength confirming the effect of incorporation of silane treated SiO 2 filler which improves the filler-matrix interface in the composite. The tensile modulus shows a marked increase 171

5 with increasing SiO 2 and Al 2 O 3 content from 0 to 10wt.% in epoxy matrix. The improvement in the Young s modulus can be attributed to the uniform dispersion of nanosized filler particles that restricts the mobility of polymer chains under loading as well as good interfacial adhesion between the particles and the polymer matrix. For understanding the effect of silane treatment of nanoal 2 O 3 on the mechanical properties of epoxy based composites the interfacial bonding of the materials should be evaluated in advance. Unlike fiber reinforced composites where some direct measurements are available, the filler/matrix interaction in particulate composites has to be assessed in terms of indirect methods. Since stiffness of composite materials is a reflection of the capability of composite interface to transfer elastic deformation [331], tensile moduli of nanoal 2 O 3 filled epoxy composites are listed in Table 7.2 for different particle loading. The general trends demonstrated by these data show that the addition of the nanoparticles increases the stiffness of the matrix. With an increase in filler loading, the modulus increases mainly due to the contribution of the hard particles. Comparatively, the treated SiO 2 nanoparticles exhibit higher stiffening efficiency, especially at higher filler loading. The lower moduli of the nanocomposites with higher loading (15 and 20wt.%) of treated nanoal 2 O 3 are attributed to the agglomeration and poor bonding between the matrix and filler. Table 7.2. Mechanical properties of polymer nanocomposites Composition (wt.%) Tensile Tensile Elongation at strength modulus break (%) 2% (MPa) 1.5% (GPa) 1.5% epoxy-sio epoxy-al 2 O As reported in literature, the elongation at break of nanocomposites usually declines with increasing filler content. Low filler loadings cause a significant drop in fracture strain. It may be recalled that the composite is partly made up of filler and partly matrix. Due to the rigid nature of the fillers, most of the deformation comes from the polymer. The actual deformation is experienced only by the polymer matrix which is much larger than the measured deformation of the sample. Due to this, polymer failure reaches failure strain limit at lower level of total deformation. Hence, the total nanocomposite elongation to break decreases. However, it is interesting to 172

6 observe that nanocomposites of the present study show contrary results in that the elongation to break behavior is comparable to microparticles filled composites. It tends towards slightly higher values for filler content of less than or equal to 10wt.% (Table 7.2). This increase suggests that the nanoparticles are able to introduce additional mechanisms of failure and energy consumption without blocking matrix deformation. Particles may induce matrix yielding under certain conditions and may furthermore act as inhibitors to crack growth by pinning the cracks. Nevertheless, if the fillers exceed 10wt.%, the failure strain undergoes a drastic decrease for Al 2 O 3 - filled epoxy. Such a reduction is due to the large proportion of fillers dominating, leading to reduction in the matrix deformation by mechanical restraining. The decrease in tensile strength and the improvement in hardness with the incorporation of higher filler loading (15 and 20wt.%) can be explained as follows: under the action of a tensile force the filler matrix interface is vulnerable to debonding, depending on interfacial bond strength and this may lead to a break in the composite. But in case of hardness, a compression or pressing stress is in action. So the matrix phase and the solid filler phase would be compressed together and they touch each other more tightly. Thus the interface can transfer pressure more effectively although the interfacial bond may be poor. This results in enhancement of hardness Dry sliding performance of pure epoxy and its nanocomposites In the earlier section analysis of mechanical properties of the epoxy based nanocomposites and the interfacial interaction has been discussed. However, it does not mean that the tribological performance of the composites can be predicted because no direct correlation has been established. High performance polymer nanocomposites are emerging as a new class of materials for the demanding applications as electrical insulation for high voltage equipment. During operation of power equipment, the insulation is not only subjected to electrical stresses but also to thermal stresses due to heat generated by the flow of current, by short circuits and vibrational stresses arising from the mechanical movement of moving parts of electrical machines. Thus, the insulation system of electrical apparatus under goes multistress ageing. Therefore, reinforced polymer composites are being tried for such applications which can withstand mechanical loads by using functional fillers Effect of sliding distance on wear loss and specific wear rate Figures 7.2 and 7.3 illustrate the wear loss and specific wear rate of the epoxy nanocomposites as a function of sliding distance for different weight percentage of SiO 2 and Al 2 O 3 content at an applied load of 60N. It is observed that the wear loss and specific wear rate of pure epoxy and SiO 2 /Al 2 O 3 filled epoxy nanocomposites increases with increasing sliding distance. The high wear rate of pure epoxy due to the three dimensional cross-linking network is greatly decreased by adding nanosio 2 /Al 2 O 3 particles. However, addition of nanosio 2 particles to epoxy could reduce the wear loss and specific wear rate effectively when compared to nanoal 2 O 3 inclusion as shown in Figures 7.2 and 7.3 respectively. 173

7 As compared to the nanocomposites with 15 and 20wt.% loading of nanoparticles, low wear rates are noticed in case of nanocomposites filled with 10 or <10wt.% nanoparticles. The phenomenon is similar to what is reported [332] in spite of the fact that the specimen configurations and conditions for evaluation are different. Figure 7.2. Wear loss as a function of sliding distance for epoxy nanocomposites Figure 7.3. K s as a function of sliding distance for pure epoxy and its nanocomposites In general, the friction and wear properties do always describe the whole tribological system rather than a material property alone. Such systems always consist of a counterpart, the specimen material, a media in between (e.g. lubricant), the environment and, of course, the stress conditions over a certain time range. After the specimen has accomplished a running-in period which may take some hours, a steadystate is reached where the frictional coefficient and the frictional force remain approximately at a constant level. In this state also, the heat generation and heat flow 174

8 reach equilibrium. Depending on the conditions, the mechanisms involved in the wear process may change significantly, especially with rising temperature. It is known that differences in polymer morphology can have a strong impact on the material properties, like for example crystalline v/s non crystalline areas in polymers [333]. Therefore, the material properties would change, when the interface content is high enough i.e., addition of ceramic particles into polymer matrix. It is assumed that the higher the interface between the matrix and reinforcement, higher is the change in material properties. The interface volume is estimated by using the interface volume model (IVM) for the nanocomposites. The input parameters are the mass densities of the matrix material (1.17g/cm³) and the filler material (1.4g/cm³). Furthermore, the effective particle diameter is needed and can be estimated from the magnified TEM images. Since both particles form aggregates, the effective particle diameters are larger than the 21nm as shown in material data sheet. The results for the estimated interface volume are shown in Figure 7.4 for an effective particle diameter of 30nm. Figure 7.4. Interface content according to IVM for pure epoxy with SiO 2 particles and interface thicknesses i, for a particle diameter d = 30nm It can be seen that for high interface thicknesses of 10nm and 30nm, a distinct maximum of the interface volume is formed in the material. As a result, the estimated interface volume is in good agreement for 10wt.% SiO 2 filled epoxy nanocomposite. The wear resistance obtained for this material composite system is superior when compared to the pure epoxy and other nanofiller loaded epoxy nanocomposites and are in good agreement with the interface volume results. Low wear rates represent a high material resistance against wear. In the present work, epoxy containing 10wt.% SiO 2 had the smallest specific wear rate (Figure 7.3), while the wear rate of pure epoxy and increased to some extent at an excessive content of the SiO 2 particles (>10wt.%), possibly owing to the poor adhesion of the nanoparticles to the epoxy 175

9 matrix and the conglomeration of the nanoparticles at higher filler content. However, a further rise in filler loading leads obviously to a deterioration of the wear rate. A change in wear mechanism should be associated and this may be ascribed to the large amount of hard ceramic Al 2 O 3 particles causing a higher abrasive wear in the present case. Similar observations were also made in epoxy-al 2 O 3 nanocomposites. Some aspects of the reinforcing role that ceramic microparticles can play in the tribological behavior were studied by Durand [334], who found that large particles, e.g. carbides, protect the matrix better than small ones because they can shield the polymer if they are not pulled out. Small particles, on the other hand, were removed and then involved in an abrading wear process. Contrary to this, in the present study, the nanoparticles of size <25nm and <50nm SiO 2 and Al 2 O 3 were used and as a result of small particles, i.e., SiO 2 addition shows better wear resistance properties. Among the nanocomposites studied, the incorporation of 10wt.% SiO 2 particles was most effective in reducing wear loss and specific wear rate of pure epoxy Surface morphology of wear worn specimens Observations of worn surfaces by scanning electron micro scope (SEM) provides knowledge about, the role that particles play in the reduction of wear rate and the acting wear mechanisms. The worn surface of the nanocomposites was observed by SEM to establish a correlation between worn surface and the sliding wear loss. Figures 7.5 and 7.6 (arrow indicates the sliding direction) show morphology of worn surface of the pure epoxy and its nanocomposites worn under the applied load of 60N for 3,000m sliding distance at a velocity of 0.5m/s. Figure 7.5. Worn surface morphologies of pure epoxy a) lower magnification (X50) and b) higher magnification (X750) Severe plastic deformation can be clearly seen on the worn surface of pure epoxy. Further, because of mechanical action and the accumulated frictional heat, the molecular structure of epoxy is partially destroyed. It can be seen that some ploughed marks (Figure 7.5(a)) with a width of more than 50µm are formed on the worn 176

10 surface. The reason for the wide ploughed marks is that the pure epoxy transferred and adhered to the countersurface during sliding wear test. The surface of the ploughed marks was rough; some scale like debris is formed along the sliding direction (Marked as D in Figure 7.5(a)). Because the epoxy is brittle it can crack and break away from the surface of the sample to form brittle cleavage fracture surfaces (marked as C in Figure 7.5(b)) under shear force. Therefore, the wear loss increases drastically with increasing load (Figure 7.2(a-b)). It clearly indicates that the adhesive wear and ploughing are the main wear mechanisms in pure epoxy. Figure 7.6. Worn surface morphologies of 10wt.% SiO 2 filled epoxy a) lower magnification (X50) and b) higher magnification (X750) The micrographs of the worn surface of 10wt.% SiO 2 filled epoxy nanocomposite are as shown in Figure 7.6. It reveals a network of micro cracks, small and shallow ploughed grooves in the sliding direction due to incorporation of SiO 2 (Figure 7.6(a)). This kind of worn surface corresponds to excellent friction and dry sliding wear behavior as shown in Figures 7.2 and 7.3. Elastic and plastic deformations on the surface of epoxy become the main cause for wear under the influence of SiO 2 inclusion. The repeated deformation causes cracking on the surface or subsurface of the nanocomposites (Figure 7.6(b)). The wear rate is reduced by more than three times in terms of magnitude when 10wt.% of SiO 2 is added. The beneficial effect as a result of SiO 2 filler is mainly due to reduced ability of ploughing, tearing, and other non adhesive components of wear. At the beginning, the surface is cut by the asperities of the counterpart. Micro burrows are generated on the surface of the sample, and the transfer film is formed on the surface of the counterpart. The micrographs of the worn surface of 20wt.% SiO 2 filled epoxy nanocomposite are as shown in Figure 7.7. As the content of SiO 2 increases to 20wt.%, the damage to the epoxy matrix is more severe (Figure 7.7(a)), and it is likely that in this case many SiO 2 particles are enriched at some locations of the worn surface of the nanocomposite. This implies that the epoxy filled 20wt.% SiO 2 is 177

11 responsible for abrasion of the counter surface during the sliding process. Figure 7.7 (b) also shows a smooth surface with micro-cracks at the centre and the ploughed zone marked by extensive plastic flow and deformation (marked as P in Figure 7.7(b)). Some worn surfaces of Al 2 O 3 filled epoxy nanocomposites are shown in Figure 7.8 and Figure 7.9. The morphologies of worn surfaces for the nanocomposites at 10 and 20wt.% Al 2 O 3 look similar to the SiO 2 -filled epoxy nanocomposites and thus substantiate the results of evaluation. Higher loading of nanoparticles (20wt.%, Figure 7.9(a-b)) is not preferable and as it does not bring about improvements in wear resistance. Figure 7.7. Worn surface morphologies of 20wt.% SiO 2 filled epoxy a) lower magnification (X50) and b) higher magnification (X750) The wear performance of a composite material is indeed known to be weight dependent. On the other hand, the surface of the worn nanocomposites containing 10wt.% Al 2 O 3 (Figure 7.8(a-b)) appears to be completely different. Very fine scratches are visible (Figure 7.8(a)) resulting from the nanofiller particles which are pulled out of the matrix, and are further moved across the surface by grooving and rolling. The resulting wear mechanism should be described as a mild abrasive wear. Due to their size, the particles should be able to move into gaps and asperities on the counterpart surface and may then be exposed to rolling rather than sliding/grooving movement. Large matrix fragments were observed in these specimens. However, the wear resistance suffers if the filler loading exceeds 15wt.% of Al 2 O 3 since increase in concentration of particles cannot provide any wear reducing effect. Abrasive wear is accompanied by delamination and fatigue cracking of the matrix (Figure 7.9(a-b)). The Al 2 O 3 particles are detached from the matrix surface by the acting forces. During the processes that follow they may adhere to the counterpart on the epoxy matrix surface and thus unfold abilities to protect the regions below from severe wear. An additional mechanism may come into play due to the size of 178

12 nanoparticles. Since the Al 2 O 3 particles are bigger in size (around nm), they can protrude out of the matrix and act as stiff spacers towards the steel counter surface. The steel disc then slides partially on harder particle material than the matrix itself. Figure 7.8. Worn surface morphologies of 10wt.% Al 2 O 3 filled epoxy a) lower magnification (X50) and b) higher magnification (X750) Figure 7.9. Worn surface morphologies of 20wt.% Al 2 O 3 filled epoxy a) lower magnification (X50) and b) higher magnification (X750) Effect of applied normal load on wear loss and specific wear rate The variation in wear loss and specific wear rate of pure epoxy and its nanocomposites under different loads (15 to 60N) are as shown in Figure 7.10(a-b) respectively. The experimental results reveal that the wear loss increases linearly with load. The trend in the wear loss with load is the same for all samples. Another interesting feature observed is that the wear loss increases gradually with increase in load for SiO 2 filled epoxy nanocomposites, whereas in the pure epoxy, a higher gradient is seen. The wear loss decrease with increasing SiO 2 filler loading except in 179

13 case of 15 and 20wt.% filled epoxy samples and shows an upward trend indicating greater wear loss at higher load (Figure 7.10(a)). As shown in Figure 7.10(b), the specific wear rate of pure epoxy with the addition of SiO 2 filler is clearly lower than that of the pure epoxy. The wear loss and specific wear rate of nanocomposites increases with increasing load as shown in Figure 7.10(a-b). The specific wear rate of the composites appear to be strongly influenced by the stability of the transfer film on the counterface. The addition of lower wt.% of SiO 2 filler (10wt.%) remarkably enhance the tribological performance of epoxy. At higher loads (45 and 60N) little noise could be heard during the wear process. Figure a) Wear loss as a function of applied load of pure epoxy and its nanocomposites at 3,000 m, 0.5 m/s. b) K s as a function of applied load of pure epoxy and its nanocomposites at 3,000 m, 0.5 m/s The addition of SiO 2 to epoxy tends to arrest the stick-slip and induced plastic flow at the surface. This appears to stabilize the film which interacts with the 180

14 counterface film in a low adhesion regime, resulting in lesser wear. The dispersion state of nanoparticles in polymer matrix is of great importance for the mechanical properties of the composite. A homogeneous dispersion of the nanoparticles is believed to contribute better improvement. The ultimate tensile strength of pure epoxy and its nanocomposites increases with increasing SiO 2 filler loading. Hence it is obvious that the ultimate tensile strength of SiO 2 -filled epoxy nanocomposite is higher than that of pure epoxy. From Figure 7.11, it is observed that, the addition of SiO 2 to epoxy resin is expected to reinforce the matrix and increases its ultimate tensile strength. With increasing content of SiO 2 from 5 to 10wt.%, the ultimate tensile strength of the epoxy increases as a result of increased interfacial area between the matrix and the nanoparticles. However, further increase of SiO 2 content above 10wt.%, slightly decreases the ultimate tensile strength. On the other hand, the specific wear rate of SiO 2 filled epoxy composites is lower than that of pure epoxy. Figure Effects of nanosilica content on K s and tensile strength of pure epoxy and its nanocomposites It is obvious that the wear resistance of a material has a relation to its ultimate tensile strength and hardness (Table 7.2, Figure 7.1). Thus, the improvement in the wear resistance of epoxy nanocomposites can be attributed to the improved ultimate tensile strength and hardness by the SiO 2 particles. When the SiO 2 content is above 10wt.%, the specific wear rate increases and this may be attributed to the agglomeration of SiO 2 particles and a slight decrease in the hardness of epoxy nanocomposites Correlation of ultimate tensile strength with electrical properties The dielectric strength, arc resistance, comparative tracking index, tracking resistance and ultimate tensile strength as a function of percentage by weight of SiO 2 in the epoxy is shown in Figure 7.12(a-d). From the figure, it can be seen that the decrement 181

15 of the ultimate tensile strength is greater than that of the dielectric strength, arc resistance, tracking resistance and comparative tracking index. Furthermore, 11-12wt.% of SiO 2 filler loading in epoxy shows good electrical strength and ultimate tensile strength. It is also observed from Figure 7.12(a), that dielectric strength for the 14wt.% SiO 2 loading is reduced by 2.2%, whereas, the ultimate tensile strength of the epoxy-sio 2 nanocomposites improves by 12.5%. The arc resistance, tracking resistance and comparative tracking index for 11-12wt.% reduce by 1.6, 2.8 and 3%, whereas ultimate tensile strength improves by 19, 16 and 16% respectively as shown in Figure 7.12(b-d). Figure Effect of SiO 2 filler loading on ultimate tensile strength and electrical properties of epoxy nanocomposites It is known that, the ultimate tensile strength of particulate composite usually gets reduced with filler content as reported by several researcher s and it follows a power law in case of poor filler/matrix bonding [335, 336]. Theoretically, the ultimate tensile strength of the composite is expected to be lower than that of the pure polymer due to the fact that particles are unable to transfer the load during tensile loading [337]. But in this work results contrary to this are observed [338]. This indicates that the increase in ultimate tensile strength with addition of filler up to 10wt.% of SiO 2 is due to the interaction between filler and matrix. As a consequence, this interaction would result in better stress transfer between the filler particles and the matrix and 182

16 thus enhances the ultimate tensile strength of the composite. This result is in agreement with the findings reported by Wu and co-authors [337] Correlation of wear resistance and surface resistivity The wear resistance and surface resistivity of epoxy-sio 2 nanocomposite with variation in filler concentration are shown in Figure From the figure, both wear and surface resistivity of epoxy-sio 2 nanocomposite show increase by 65% and 75% up to nanosilica loading of 10wt.% and with further increase in filler loading, both tend to decrease by 73% and 4% respectively. Figure Effect of SiO 2 filler loading on wear and surface resistivity of epoxy nanocomposites As explained in section , the interface volume data holds good for wear and surface resistivity at 10wt.% of SiO 2 filled epoxy nanocomposite and it is highest. This result is in good agreement with the interface volume results. This shows that the surface resistivity results are influenced by surface quality and there exist synergism between the surface resistivity and wear resistance in relation to filler concentration. 7.4 Conclusions This study shows that a fair degree of success has been achieved in fabrication of pure epoxy, SiO 2 /Al 2 O 3 filled epoxy nanocomposites. The mechanical properties and dry sliding wear of the pure epoxy, SiO 2 /Al 2 O 3 filled epoxy nanocomposites have been examined by laboratory experiments and based on results the following conclusions can be drawn: i) The ultimate tensile strength and Young s modulus of epoxy containing 10wt.% nanosilica composite is comparable with those of the pure matrix and Al 2 O 3 filled epoxy nanocomposite, but increasing nanosilica/alumina contents lead to reduction. ii) Inclusion of nanosilica/alumina particles influences the properties of epoxy matrix. Mechanical properties determined by static tension method exert 183

17 remarkable influence on the wear behavior of the nanocomposites. The increased ultimate tensile strength of the composites with 10wt.% nanosilica corresponds to high wear resistance. iii) The wear loss of pure epoxy and epoxy-sio 2 /Al 2 O 3 nanocomposites increases with increase in sliding distance and applied load. iv) Epoxy-SiO 2 /Al 2 O 3 nanocomposites show signs of mild abrasive wear due to the hard ceramic particles. The wear mechanism of the composite changes from severe abrasive wear (for pure epoxy) to mild abrasive wear. v) The excellent overall properties of SiO 2 /Al 2 O 3 filled epoxy nanocomposites may open avenues for new applications of high performance polymers, leading to innovative product development in the automotive industry, electronics, abrasion resistant coatings, and many other applications. vi) Better correlation were obtained for ~12.5wt.% SiO 2 filled epoxy nanocomposites in respect of wear, surface resistivity, dielectric strength, arc resistance, tracking index and tracking resistance. Electrical insulation has the dual requirement of providing insulation and dissipating heat in an efficient way, hence thermal conductivity and distortion under thermo-mechanical loads are important. These characteristics of the nanocomposites are discussed under Chapter