The stiff and fragile structure of thermosetting polymers, such as epoxy, accomplices the innate cracks to cause fracture and therefore the applications of monolithic epoxy are not ubiquitous. However, it is well established that when reinforced especially by nano-fillers, its ability to withstand crack propagation is propitiously improved. The crack is either deflected or bifurcated when interacting with strong nano-filler such as Multi-Layer Graphene (MLG). Due to the deflection and bifurcation of cracks, specific fracture patterns are observed. Although these fracture patterns seem aesthetically appealing, however, if delved deeper, they can further be used to estimate the influence of nano-filler on the mechanical properties. Here we show that, by a meticulous examination of topographical features of fractured patterns, various important aspects related to fillers can be approximated such as dispersion state, interfacial interactions, presence of agglomerates, and overall influence of the incorporation of filler on the mechanical properties of nanocomposites.
The polymer matrix composites (PMCs) have found extensive applications in aerospace, automobile, and construction owing to ease of processing and high strength to weight ratio which is an important property required for aerospace applications [
Non-contact techniques are getting increasingly popular to measure topographical features, especially for surfaces that may be subject to damage using contact techniques. The results obtained are very similar to those of stylus techniques and can use the same parameter definitions. Some non-contact techniques, such as diffraction measurements, can measure topographical features easily and quickly and can potentially be used on the machining tool. The non-contact methods have certain limitations. For example, in high slope surfaces, an insufficient intensity of light reaches the detector and the focus lens begins to follow inaccurately. In addition, when the contaminated surfaces are studied, the contamination is measured as part of topographical features as there is no external agency to remove the contaminations from the surface [
In current work, Multi-Layered Graphene (MLG)-epoxy nanocomposites of three different types were produced using solution casting technique with MLG dispersed in three different mediums: epoxy (ME), hardener (MH), and acetone (MA). The maximum improvement in mechanical properties was observed in case of MLG dispersed in hardner (MH) at 0.3 wt% MLG. The second highest improvement in mechanical properties was observed in case of MLG dispersed in epoxy (ME). However, in case of MLG dispersed in acetone (MA), least improvement in mechanical properties was observed. It can be attributed to the presence of retained acetone that causes stress concentration and concomitant degradation of mechanical properties [
Multi-layered graphene (MLG) of 12 nm average thickness and 4.5 μm average lateral size with specific surface area of 80 m2/g and purity 99.2% was purchased from Graphene Supermarket. MLG was washed extensively with acetone to remove any impurities and tip sonicated for 6 h to fragment any aggregates. Bisphenol A-epichlorohydrin based epoxy having density of ~1.3 g/cm3 and dimethylbenzylamine isophorone diamine based low viscosity fast curing hardener with ~1.1 g/cm3 density were used in current study. The epoxy matrix used consisted of EPOPHENTM EL5 bisphenol A based liquid epoxy and EPOPHENTM EHA57 diamine hardener, purchased from Polyfibre, UK. This epoxy system is a multi-purpose resin offering good all-round properties with the epoxy group content of 4.76 - 5.25 mol/kg. The viscosity of liquid epoxy and hardener are 12,000 - 15,000 cps and 45 cps at room temperature, respectively. To prepare monolithic epoxy samples, the mix proportions are 50 parts by weight of hardener to 100 parts by weight of liquid epoxy. The gelation time of the resin was 43 min at room temperature. Acetone of purity 99.8% was purchased from Sigma-Aldrich and was used as dispersion medium for MLG.
MLG of different weight fractions (0.1 wt%, 0.3 wt%, 0.5 wt% and 1.0 wt%) were taken and dispersed in three different mediums; (1) acetone (MA), (2) epoxy (ME), and (3) hardener (MH). The nano-filler was dispersed in the hardener using sonication that was carried out using tip sonicator of power 750 W and frequency 250 kHz (Vibra-cell model VC 750, USA). The operation mode was 70% power with 10 s vibration and 5 s break. Although the sonication was carried out at room temperature, however, temperature of the system rose due to high energy vibration produced by tip sonicator. The acetone was removed at 60˚C for 2 h. The epoxy and hardener were vacuum degassed separately for 1 h. Then, the resins were mixed in epoxy: hardener ratio of 2:1. Following thorough hand mixing for 10 min, vacuum degassing was again carried out for 15 min. The resin was poured into silicone molds (without any release agent) and cured at room temperature for 6 h followed by post-curing at 150˚C for overnight to ensure completion of the crosslinking.
Tensile, three-point bending, and fracture toughness tests were conducted using Instron Universal Testing Machine (Model 3382). The displacement rate was kept 0.5 mm/min for tensile and fracture toughness tests and 1 mm/min for three-point bending test. Five specimens were tested for each composition. The schematics of the specimens are shown in
The fractography specimens are shown in
forcements, the cracks are deflected resulting in parabolic and non-linear fracture patterns [
The topographical features of fracture surfaces of tensile specimen of ME are shown in
in
observation of topographical patterns can reveal some specific attributes of fracture patterns and distribution of MLG.
The topographical features are summarized in
Apart from Rz, Ra is another important parameter to consider. The Ra values of monolithic epoxy without and with acetone are 0.7 μm and 0.61 μm, respectively. With the addition of 0.1 wt% MLG, the Ra values increased to 1.3 μm, 1.51 μm, and 0.85 μm in 0.1 wt% ME, MH, and MA, respectively. However, on the contrary to Rz values, the Ra values decreased with the increasing weight fraction of MLG. At 1.0 wt% MLG, the Ra values were 0.11 μm, 0.33 μm, and 0.06 μm in MA, MH, and MA, respectively. The decrease in Ra with increasing Rz may seem contradicting however can be explained on the basis of observed fractured patterns and surface roughness charts shown in
The epoxy based nanocomposites were successfully produced with MLG as nano-filler. Three different systems were made with each system having five compositions. Fractography of samples from each composition was carried out and roughness parameters were analyzed. It was observed that the topographical features of fractured patterns of polymer nanocomposites can be used to approximate the dispersion state, interfacial interactions, and presence of agglomerates, and overall influence of the incorporation of fillers on the mechanical properties of produced nanocomposites. A high value of Ra (with low Rz value) can be an indicator of uniform dispersion of filler. On the other hand, a low value of Ra (with high Rz value) indicates the poor dispersion of filler and the presence of agglomerates. A similar trend was observed in Rq values as in Ra values. However, no specific trend was observed in surface waviness and may not be indicative of dispersion state.
The authors would like to thank the Department of Mechanical and Construction Engineering, Northumbria University, UK for the provision of research facilities, without which the analysis of relevant data was not possible.
Atif, R. and Inam, F. (2016) Fractography Analysis with Topo- graphical Features of Multi-Layer Graphene Reinforced Epoxy Nanocomposites. Graphene, 5, 166-177. http://dx.doi.org/10.4236/graphene.2016.54014