Islam Shyha

Mechanical, Thermal, and Electrical Properties of Graphene-Epoxy Nanocomposites—A Review

Monolithic epoxy, because of its brittleness, cannot prevent crack propagation and is vulnerable to fracture. However, it is well established that when reinforced—especially by nano-fillers, such as metallic oxides, clays, carbon nanotubes, and other carbonaceous materials—its ability to withstand crack propagation is propitiously improved. Among various nano-fillers, graphene has recently been employed as reinforcement in epoxy to enhance the fracture related properties of the produced epoxy–graphene nanocomposites. In this review, mechanical, thermal, and electrical properties of graphene reinforced epoxy nanocomposites will be correlated with the topographical features, morphology, weight fraction, dispersion state, and surface functionalization of graphene. The factors in which contrasting results were reported in the literature are highlighted, such as the influence of graphene on the mechanical properties of epoxy nanocomposites. Furthermore, the challenges to achieving the desired performance of polymer nanocomposites are also suggested throughout the article.

Use of morphological features of carbonaceous materials for improved mechanical properties of epoxy nanocomposites

The influence of reinforcement morphology on damage tolerance and fracture toughness of epoxy based nanocomposites has been studied. Two different forms of carbonaceous reinforcements were used: multi-layered graphene (MLG) and nanostructured graphite (NSG). The maximum increase in Youngs modulus was observed from 609.6 MPa to 766 MPa (25.7% increase) in the case of 0.1 wt% NSG. The NSG showed a maximum increase in hardness up to 7.9% while MLG showed up to 18.3%. The MLG and NSG increased the storage modulus and Tg while loss modulus and tan δ decreased with MLG and NSG. SEM images of the fractured surfaces of tensile specimens showed that the fracture mode was significantly altered by MLG and NSG.

The degradation of mechanical properties in polymer nano-composites exposed to liquid media – a review

The advancement of polymer nano-composites has been motivated by the need for materials with a specific combination of mechanical properties beyond those achieved from only one material. Integration of reinforcement into polymers at the nanoscale can provide a significant increase in numerous physical and mechanical properties of polymer nano-composites. However, in applications where contact with liquid media is unavoidable, the mechanical properties of polymer nano-composites suffer degradation which is a commonly observed phenomenon. Non aggressive liquid such as water is capable of lowering the mechanical properties of polymer nano-composites by acting as plasticizers while moderate and severe aggressive liquid when combined with residual stresses can cause unexpected brittle failure known as ESC. To date, only a few studies are reported discussing the ability of nano-fillers to resist degradation of mechanical properties in polymer nano-composites when exposed to liquid media. In this review, various factors responsible for mechanical property degradation caused by liquid media in polymer nano-composites and their remedies are studied.

The degradation of mechanical properties due to stress concentration caused by retained acetone in epoxy nanocomposites

Multi-layered graphene (MLG)–epoxy nanocomposites of three different types were produced using the solution casting technique with MLG dispersed in three different mediums; acetone (MA), an epoxy (ME), and a hardener (MH). In the case of MLG dispersed in the hardener (MH), the maximum increases in tensile and flexural properties, fracture toughness, and microhardness were observed at 0.3 wt% of MLG. The Youngs modulus increased from 610 MPa to 758 MPa (24% increase) and the tensile strength increased from 46 MPa to 60 MPa (31% increase). The fracture toughness (K1C) increased from 0.8 MPa m1/2 to 1.1 MPa m1/2 (29% increase) and the Charpy impact toughness increased from 0.85 kJ m−2 to 1.61 kJ m−2 (89% increase). An increase in the storage modulus and glass transition temperature (Tg) was also observed which is attributed to the high stiffness and restriction of polymer chains. Also, if the acetone is not completely removed, the products would have porosity which acts as a stress concentrator and significantly degrades the mechanical properties of the nanocomposites.

Modeling and experimentation of multi-layered nanostructured graphene-epoxy nanocomposites for enhanced thermal and mechanical properties

The influence of multi-layered nanostructured graphene as reinforcement on thermal and mechanical properties of epoxy-based nanocomposites has been studied. The maximum improvement in mechanical properties was observed at 0.1 wt%. The Young’s and flexural moduli increased from 610 MPa to 766 MPa (26% increase) and 598.3 MPa to 732.8 MPa (23% increase), respectively. The tensile and flexural strengths increased from 46 MPa to 65 MPa (43% increase) and 74 MPa to 111 MPa (49% increase), respectively. The mode-1 fracture toughness (K1C) and critical strain energy release rate (G1C) increased from 0.85 MPa.m1/2 to 1.2 MPa.m1/2 (41% increase) and from 631 J/m2 to 685 J/m2 (9% increase), respectively. The increase in fracture toughness is attributed to the obstruction of cracks by graphene layers. The reinforcing effect of nanostructured graphene was also manifested in dynamic mechanical properties. The storage modulus and alpha-relaxation temperature values significantly increased indicating the fine integration of NSG in epoxy chains. The thermal properties of nanocomposites were simulated which showed that graphene is very efficient in significantly increasing the scattering and dissipation of thermal flux.

The Degradation of Mechanical Properties in Halloysite Nanoclay-Polyester Nanocomposites Exposed in Seawater Environment

Polyester based polymers are extensively used in aggressive marine environments; however, inadequate data is available on the effects of the seawater on the polyester based nanocomposites mechanical properties. This paper reports the effect of seawater absorption on the mechanical properties degradation of halloysite nanoclay-polyester nanocomposites. Results confirmed that the addition of halloysite nanoclay into polyester matrix was found to increase seawater uptake and reduce mechanical properties compared to monolithic polyester. The maximum decreases in microhardness, tensile and flexural properties, and impact toughness were observed in case of 1 wt% nanoclay. The microhardness decreased from 107 HV to 41.7 HV 61% decrease. Young’s modulus decreased from 0.6 GPa to 0.4 GPa 33% decrease. The flexural modulus decreased from 0.6 GPa to 0.34 GPa 43% decrease. The impact toughness dropped from 0.71 kJ/m2 to 0.48 kJ/m2 32% decrease. Interestingly, the fracture toughness KIC increased with the addition of halloysite nanoclay due to the plasticization effect of the resin matrix. SEM images revealed the significant reduction in mechanical properties in case of 1 wt% reinforcement which is attributed to the degradation of the nanoclay-matrix interface influenced by seawater absorption and agglomeration of halloysite nanoclay.

The degradation of mechanical properties in halloysite nanoclay–polyester nanocomposites exposed to diluted methanol

The degradation of mechanical properties in halloysite nanoclay–polyester nanocomposites was studied after an exposure of 24 h in diluted methanol system by clamping test specimens across steel templates. The glass transition temperature (Tg) and storage modulus increased steadily with the increase of halloysite nanoclays before and after diluted methanol exposure. The addition of nano-fillers was found to reduce liquid uptake by 0.6% in case of 1 wt% reinforcement compared to monolithic polyester. The mechanical properties of polyester-based nanocomposites were found to decrease as a result of diluted methanol absorption. After diluted methanol exposure, the maximum microhardness, tensile, flexural and impact toughness values were observed at 1 wt% of halloysite nanoclay. The microhardness increased from 203 to 294 HV (45% increase). The Young’s modulus increased from 0.49 to 0.83 GPa (70% increase) and the tensile strength increased from 23 to 27 MPa (17.4% increase). The impact toughness increased from 0.19 to 0.54 kJ/m2 in diluted methanol system (184% increase). Surprisingly, the fracture toughness of all types of nanocomposites was found to increase after exposing to diluted methanol due to plasticization effect. Scanning electron microscope images of the fractured surfaces of tensile specimens revealed that the methanol increased the ductility of the matrix and reduced the mechanical properties of the nanocomposites.

Electro-Discharge Machining of Metal Matrix Composite Materials

Abstract Following a review on the classification and applications of metal matrix composites (MMCs) and the use of unconventional machining processes to cut MMCs, the paper presents experimental results for electro-discharge machining of an aluminium matrix reinforced with silicon carbide particle composite. A modified fractional factorial experimental design based on Taguchi orthogonal array L9 was employed. This involved three control variables at three levels each, namely discharge current, spark gap and electrode type (varying size and geometry). Process responses included material removal rate (MRR), tool wear rate (TWR) and average surface roughness of the eroded surface. Maximum MRR/TWR ratio of 21.36 was achieved when 3 A and 10 μm discharge current and sparking gap respectively were used with 5 mm diameter solid electrode (with a corresponding Ra of 1.8 μm).

Machinability analysis when drilling Kovar shape memory alloys

Abstract The use of shape memory alloys is noticeably increasing in numerous industrial sectors including aerospace, automotive and marine. This is mainly owing to their intrinsic properties such as expansion-control ability when subjected to thermal loads, large recoverable strain as well as superior mechanical and corrosion resistance properties. Following to producing semi-final parts of shape memory alloys such as bars or sheets, several machining processes are typically employed to generate required features such as holes and slots and to meet dimensional and surface finish requirements. This study aims to investigate the machinability of a shape memory alloy known as Kovar when mechanical drilling using high-speed steel (HSS) twin-lipped twist drills. The influence of cutting speed, feed rate and backup/support conditions is evaluated against process key responses involving cutting temperature, exit burr height and micro-hardness. Additionally, limited qualitative assessment of the machine surface topography and cutting edge was undertaken. Cutting speed was found to be statistically significant affecting the cutting temperature at the 90% confidence level. Dramatic reduction (up to 85%) in exit burr size was obtained when drilling backed workpiece materials compared with the un-backed counterparts.

Biodegradation of Halloysite Nanotubes-Polyester Nanocomposites Exposed to Short Term Seawater Immersion

Halloysite nanotubes (HNTs)-polyester nanocomposites with four different concentrations were produced using solution casting technique and the biodegradation effect of short-term seawater exposure (120 h) was studied. Monolithic polyester was observed to have the highest seawater absorption with 1.37%. At 0.3 wt % HNTs reinforcement, the seawater absorption dropped significantly to the lowest value of 0.77% due to increase of liquid diffusion path. For samples tested in dry conditions, the Tg, storage modulus, tensile properties and flexural properties were improved. The highest improvement of Tg was from 79.3 to 82.4 °C (increase 3.1 °C) in the case of 0.3 wt % HNTs. This can be associated with the exfoliated HNTs particles, which restrict the mobility of polymer chains and thus raised the Tg. After seawater exposure, the Tg, storage modulus, tensile properties and flexural properties of polyester and its nanocomposites were decreased. The Young’s modulus of 0.3 wt % HNTs-polyester dropped 20% while monolithic polyester dropped up to 24% compared to their values in dry condition. Apart from that, 29% flexural modulus reduction was observed, which was 18% higher than monolithic polyester. In contrast, fracture toughness and surface roughness increased due to plasticization effect. The presence of various microbial communities caused gradual biodegradation on the microstructure of the polyester matrix as also evidently shown by SEM images.