S. C. Tjong

Microstructural and mechanical characteristics of in situ metal matrix composites

During the past decade, considerable research effort has been directed towards the development of in situ metal matrix composites (MMCs), in which the reinforcements are formed in situ by exothermal reactions between elements or between elements and compounds. Using this approach, MMCs with a wide range of matrix materials (including aluminum, titanium, copper, nickel and iron), and second-phase particles (including borides, carbides, nitrides, oxides and their mixtures) have been produced. Because of the formation of ultrafine and stable ceramic reinforcements, the in situ MMCs are found to exhibit excellent mechanical properties. In this review article, current development on the fabrication, microstructure and mechanical properties of the composites reinforced with in situ ceramic phases will be addressed. Particular attention is paid to the mechanisms responsible for the formation of in situ reinforcements, and for creep failure of the aluminum-based matrix composites.

Morphological behaviour and instrumented dart impact properties of β-crystalline-phase polypropylene

Abstract Specimens consisting of high-purity β-phase polypropylene were prepared by adding a bicomponent β-nucleator consisting of equal amounts of pimelic acid and calcium stearate. Scanning electron microscopy (SEM), differential scanning calorimetry (d.s.c.), dynamic mechanical analysis and instrumented drop weight impact tests were used to characterize the morphology, thermal behaviour and the impact properties of the β-phase polypropylene. SEM examinations show that the β-spherulites exhibit a sheaf-like structure with no clear boundaries between them. This distinct spherulitic morphology results in a substantial improvement in the falling weight impact resistance. Fractographic analysis reveals that microfibrils and voids were formed in the fracture induction area of the β-form specimen. The greater impact strength observed in the β-form material is due to the larger energy dissipation which is associated with the formation of microfibrils. Furthermore, d.s.c. analysis showed that there is no β → α phase conversion during the impact test.

In-situ Ti-TiB metal-matrix composite prepared by a reactive pressing process

During the past decade, considerable research efforts have been directed towards developing metal matrix composites (MMCs) by means of the reactive processing technique. In this study, four reactive systems, namely Ti-B, Ti-TiB{sub 2}, Ti-B{sub 4}C and Ti-BN, are selected to fabricate in-situ titanium matrix composites by means of the reactive pressing technique. The microstructure and compressive mechanical property of these composites are examined. The objective of this work is to assess which system can produce in-situ titanium matrix composites with a higher mechanical strength.

Tribological behaviour of SiC particle-reinforced copper matrix composites

Abstract Pure copper and its composites reinforced with SiC particles were prepared by hot isostatic pressing (HIP) process. The tribological behaviour of copper and composites was studied on a pin-on-disc tester. The pins were slid against a hardened steel disc under dry ambient conditions. In two-body abrasive wear measurements, the disc surface was bonded with a SiC abrasive paper of 240 grit size. The abrasive wear measurements showed that soft copper exhibits an extremely high wear loss. However, additions of SiC particles up to 20 vol.% appeared to improve the abrasive wear resistance of copper significantly under the applied loads of 15–55 N. Dry sliding wear tests also indicated that the composite with 20 vol.% SiC exhibits a lower wear loss compared to pure copper. This was due to the reinforcing SiC particles being effective to reduce the extent of wear deformation in the subsurface region during sliding.

Abrasive wear behavior of TiB2 particle-reinforced copper matrix composites

Abstract Pure copper and its composites reinforced with TiB 2 particles were prepared by a hot isostatic pressing process. The two-body abrasive wear behavior of these specimens was investigated using pin-on-disk method where the sample slid against a SiC abrasive of 240-grit size. Pure copper exhibits a high abrasive wear loss because of its softness. The addition of only 5 vol.% TiB 2 particle to copper leads to a dramatic improvement in its wear resistance. The wear resistance increases by further increasing volume fraction of reinforcing particles that resist the microcutting action of SiC abrasives.

Thermal decomposition characteristics of poly(propylene carbonate) using TG/IR and Py-GC/MS techniques

The thermal decomposition behaviour of poly(propylene carbonate)s (PPC)s synthesized with varying molecular weights was studied at various pyrolysis temperatures by the combination of pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetric analysis/infrared spectrometry (TG/IR) techniques. The pyrolysis products of PPCs with lower molecular weight of 26,900 and higher molecular weight of 144,600 at different pyrolysis temperatures were identified using Py-GC/MS. The dynamic decomposition was also explored with the TG/IR technique. The results showed that chain scission occurs at relatively lower temperature than for the unzipping reaction, and an increase in molecular weight can reduce the amount of the active terminal groups and restrict unzipping reaction to some extent. It was also observed that the backbone structure plays a great role in thermal decomposition behaviour of PPC. The same perfectly alternating structure leads to the same decomposition mechanism whereas unzipping needs a high activation energy and takes place at high decomposition temperature. The final pyrolysates are cyclic propylene carbonate, and 1,2 propanediol. Low molecular weight PPC undergoes a one-stage pyrolysis and high molecular weight PPC pyrolysis obeys two-step pyrolysis mechanism, viz. main chain random scission and unzipping. The thermal decomposition behaviour of PPC in the absence and presence of a metal complex catalyst was studied by TG/IR. It was further observed that the metal complex catalyst has little effect on the thermal decomposition of the PPC. The catalyst only slightly reduced the activation energy leading to the accelerated depolymerization reaction.

Brittle–tough transition in PP/EPDM blends: effects of interparticle distance and tensile deformation speed

Abstract The toughness of polypropylene (PP)/ethylene–propylene–diene monomer rubber (EPDM) blends containing various EPDM contents as a function of the tensile speed was studied. The toughness of the blends was determined from the tensile fracture energy of the side-edge notched samples. A sharp brittle–tough transition was observed in the fracture energy versus interparticle distance (ID) curves when the crosshead speed

An investigation on the processing of sisal fibre reinforced polypropylene composites

In this paper, a pre-impregnation technique has been introduced for the injection moulding of sisal fibre reinforced polypropylene (PP/SF) composites. The major advantasge of the pre-impregnation technique is that the PP/SF composites can be injection moulded with relatively lower barrel temperature, and therefore significant thermal degradation of the sisal fibres could be avoided. The resulting injection mouldings possessed lighter colour and no odor.

Dielectric behavior and dependence of percolation threshold on the conductivity of fillers in polymer-semiconductor composites

Polymer-semiconductor PVDF∕LNO (polyvinylidene fluoride∕Li doped NiO) composites were fabricated via simple blending and hot-molding technique. The dielectric behavior of such composites was studied over broad frequency. The results revealed the dependence of percolation threshold on the conductivity of LNO filler in the composites. And the conductivity of the LNO fillers played an important role on the dielectric properties and critical exponents of the PVDF∕LNO composites. High dielectric constants and low conductivities of the composites were observed near the percolation threshold. Finally, critical exponents were also used to explain the experimental results, and provided useful information for understanding the resultant dielectric properties.

Mechanical behavior and fracture toughness evaluation of maleic anhydride compatibilized short glass fiber/SEBS/polypropylene hybrid composites

Short glass fiber (SGF) reinforced polypropylene composites toughened with styrene-ethylene-butylene-styrene (SEBS) triblock copolymer were injection molded. The effect of maleic anhydride (MA) functional group on the mechanical performance of the hybrid composites was investigated. In this study, MA was either grafted to PP (PP-g-MA) or SEBS copolymer (SEBS-g-MA). The mPP blend was prepared by compounding 95% PP with 5% PP-g-MA. The matrix of hybrid composites consists of either SEBS/mPP or SEBS-g-MA/mPP. Tensile tests showed that SGF exhibits a beneficial effect for restoring the stiffness of the SEBS/mPP blend. Impact measurements on SEBS/mPP exhibited superior impact strength. However, the incorporation of SGF into SEBS/mPP and SEBS-g-MA/mPP blends reduced the impact strength considerably. SEM observations revealed that the SGF surfaces of both SGF/SEBS/mPP and SGF/SEBS-g-MA/mPP hybrids are coated with a thin layer of matrix material. This implied that the MA functional group of mPP improves the adhesion between SGF and PP, and between SGF and SEBS. The essential work of fracture (EWF) method revealed that a strong interfacial bonding between SGF and PP is detrimental to the fracture toughness of ternary SGF/SEBS/mPP and SGF/SEBS-g-MA/mPP hybrid composites. Finally, dynamic mechanical analysis showed that SGF increases the storage modulus but decreases the intensity of damping factor for the hybrids.