Hybrid PP–EPR–GF composites. Part II: fracture mechanisms

Abstract

The fracture mechanism of hybrid iPP–EPR–GF composites has been studied by notched Charpy, three- and four-point bending fracture tests. The results of impact tests illustrate that increasing both temperature and EPR/GF ratio increases the impact energy of iPP–EPR–GF. Indeed, with increasing temperature, a brittle, ductile, transition temperature (BDTT) occurs. The results of a three-point bending test show that fracture toughness (K_IC ) can be improved by addition of both GF and EPR. Since the trend in the fracture toughness values is close to what would be expected by the rule of blends, it can be concluded that the use of both GF and EPR has no significant synergistic effect on toughening. The results of a four-point bending test show that craze-like damage appears in front of the pre-crack and its propagation is dictated by the GF and EPR content. Looking in more detail at the damaged zone by means of cross-polarised light microscopy, evidence of birefringence can be revealed. Briefly, the dominant mechanism of fracture in the iPP–EPR–GF system studied in this work can be related to a craze-like type damage, which includes both highly localised dilatational shear bands, due to cavitation of the EPR particles, and some crazing induced by the stress concentration associated with GF.     

Rheological characterisation of hydroxyapatite filled polyethylene composites. Part 1 – Shear and extensional behaviour

Abstract

The shear and extensional properties of injection moulding grade hydroxyapatite–polyethylene composites developed for orthopaedic applications have been studied. The composite was prepared without processing aids owing to concerns over the potential biological responses to such additives. The composite containing 20 vol.-% hydroxyapatite filler showed typical pseudoplastic behaviour. However, that containing 40 vol.-% hydroxyapatite filler tended to exhibit yield. The Maron–Pierce equation was found to be useful in predicting the viscosities of the composite systems. The activation energy of the composite and the unfilled polymer were equal, indicating that the 20 vol.-% system exhibits the same flow mechanism as the unfilled polymer. A qualitative assessment of extensional properties was made following Cogswell's method. The extensional stress of the unfilled polymer decreases with increasing temperature whereas the composites behave in a complex manner. For all the systems the Trouton ratios tend to increase with apparent shear rates. The Trouton ratio also indicates that at higher temperatures the flow of these composites is dominated by extensional properties.     

TiN – AlN-Based Hot-Pressed Composites. Part 1. Structure and Properties

Results of a study into the structure and mechanical properties of a hot-pressed TiN – AlN ceramic with the concentration of AlN in it varying from 0 to 100% are given. Composite materials with a strength of 750 MPa, a crack resistance of 4.4 MPa · m^1/2, and a hardness of 13 – 15 GPa measured over a wide range of the indenter load (50 – 100 N) are prepared. These materials exhibit an extremely high resistance to fracture under localized load. The TiN – AlN-based materials are promising for use in tribo-engineering applications — such as ceramic bearings and wear-resistant friction couples.     

Rheological characterisation of hydroxyapatite filled polyethylene composites. Part 2 – Isothermal compressibility and wall slip

Abstract

Rheological characterisation of hydroxyapatite–high density polyethylene (HA–HDPE) composites has been performed in terms of isothermal compressibility and wall slip. Addition of HA to the polymer melt decreases the compressibility of the melt. The unfilled HDPE was found to exhibit wall slip at shear stresses as low as 0·10 MPa. The flow curves of the composites showed three distinct regions: a gradient at low shear rates; a plateau region; and a gradient at higher shear rate. An increase in rheometer pressure seems to suppress the slip in composites. The 40 vol.-% HA–HDPE composite exhibited two critical shear stresses, one corresponding to wall slip, which occurs in the lower shear rate region of the flow curve, and the other corresponding to a plateau, which is identified with the stick–slip behaviour of unfilled HDPE reported in the literature. The plateau shear stress increased with filler volume fraction and this effect is attributed to the decreased compressibility of the melt. A good correlation with a negative correlation coefficient was found to exist between compressibility and shear stress in the plateau region. The slip observed in unfilled HDPE and at low shear rates in the 40 vol.-% HA–HDPE systems has been explained in terms of a low molecular weight polymer layer formed at the melt/wall interface. The large interfacial slip observed in the plateau region is attributed to complete disentanglement of adsorbed chains from free chains at the melt/wall interface at and beyond the plateau region.     

Deformation and damage processes during tensile creep of ceramic-fibre-reinforced ceramic–matrix composites

The tensile creep and creep fracture properties in air at 1300°C are compared for SiC_f/SiC and SiC_f/Al₂O₃ composites, each reinforced with 0.38 volume fractions of interwoven silicon carbide (Nicalon™) fibre bundles aligned parallel and normal to the stress direction. The differing behaviour patterns displayed by these 0/90° woven composites are analysed to identify the processes controlling creep strain accumulation and crack development.     

Severe wear mechanisms in Al2O3–AlON ceramic composites

Severe wear mechanisms in Al₂O₃-AlON ceramic composites during their friction against a bearing steel were investigated and analysed by different techniques, mainly transmission electron microscopy (TEM). It was shown that ceramic damages correspond not to a classical intergranular cracking but to a breakdown of alumina–alumina grain boundaries leading to their pull off. Consequently, a new model of the severe wear of ceramics, based on a dielectric approach is proposed. Moreover, the existence of AlON located at such boundaries induces a delaying effect of this damage and seems to participate in the forming and the stability in the contact of a third body essentially constituted by iron oxides.     

New hydroxyapatite–hydrotalcite composites I. synthesis

The synthesis and characterization of original materials, composed by hydrotalcite and hydroxyapatite, is discussed. All the syntheses were carried out in presence of microwave irradiation during the crystallization step. The interactions between the two compounds depend on the synthesis procedure. If hydroxyapatite is incorporated to hydrotalcite, the first compound is encapsulated by hydrotalcite. Instead, if hydroxyapatite is first prepared, the resulting solid is essentially a hydrotalcite with interlayered hydroxyapatite. When the composite material is synthesized by a simultaneous coprecipitation, the small clusters of hydroxyapatite and hydrotalcite are homogeneously dispersed. Consequently, the specific surface area and the particle size vary.     

Palygorskite–TiO2 nanocomposites: Part 1. Synthesis and characterization

In this paper we describe the synthesis and characterization of small-sized TiO₂ particles supported on palygorskite (Pal) with Pal to TiO₂ mass ratios of 10:90, 20:80, 30:70, 40:60 and 50:50. The above Pal–TiO₂ nanocomposites were prepared by deposition of anatase form of TiO₂ on the Pal surfaces using a sol-gel method with titanium isopropoxide as a precursor under hydrothermal treatment at 180 °C. Phase composition, particle morphology and physical properties of these samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), attenuated total reflection using Fourier transform infrared spectroscopy (ATR-FTIR) and N₂ surface area analysis by BET. In order to investigate the absorption properties of the catalysts, UV–vis reflection spectra were measured. Preparation of Pal–TiO₂ nanocomposites led to good dispersion of TiO₂ on Pal surfaces. By increasing the amount of TiO₂, the deposited 3–10 nm TiO₂ particles were found to be aggregated on the surfaces of the Pal particles. However, by decreasing the amount of TiO₂, the Pal particles were found to be aggregated. After treating with TiO₂, Pal samples largely showed interparticle mesopores of about 5.8 nm. It was observed that the commercial titania P25 showed no absorption in visible light region. In contrast, the prepared Pal–TiO₂ samples showed gray color and absorption in visible light region.     

Heat conduction mechanisms in hot pressed ZrB2 and ZrB2–SiC composites

Thermal diffusivity and conductivity of hot pressed ZrB₂ with different amounts of B₄C (0–5 wt%) and ZrB₂–SiC composites (10–30 vol% SiC) were investigated experimentally over a wide range of temperature (25–1500 °C). Both thermal diffusivity and thermal conductivity were found to decrease with increase in temperature for all the hot pressed ZrB₂ and ZrB₂–SiC composites. At around 200 °C, thermal conductivity of ZrB₂–SiC composites was found to be composition independent. Thermal conductivity of ZrB₂–SiC composites was also correlated with theoretical predictions of the Maxwell–Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB₂ and ZrB₂–SiC composites at room temperature were confirmed by Wiedemann–Franz analysis by using measured electrical conductivity of these materials at room temperature. It was found that electronic thermal conductivity dominated for all monolithic ZrB₂ whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB₂–SiC composites.     

Oxidation mechanisms of SiC-fiber–reinforced Si eutectic alloy matrix composites at elevated temperatures

SiC-fiber–reinforced binary Si eutectic alloy composites have been developed for aerospace applications using the melt infiltration method. In this study, the oxidation mechanisms of various binary Si eutectic alloys were evaluated at elevated temperatures. We suggest that the oxidation resistance of eutectic alloys could be predicted using the Gibbs energy change for the oxidation reaction. Based on these calculations, eutectic alloys of Si-16at%Ti, Si-17at%Cr, Si-22at%Co, Si-38at%Co, and Si-27at%Fe were prepared. These alloys produced uniform SiO₂ layers and showed the same oxidation resistance as Si at 1000°C under humid conditions. Therefore, SiC composites using Si alloys with excellent oxidation resistance can be predicted using thermodynamic calculations.     

Mechanical properties and deformation mechanisms of B4C–TiB2 eutectic composites

Samples of B₄C–TiB₂ eutectic are laser processed to produce composites with varying microstructural scales. The eutectic materials exhibit both load dependent and load independent hardness regimes with a transition occurring between 4 and 5 N indentation load. The load-independent hardness of eutectics with a microstructural scale smaller than 1 μm is about 31 GPa, and the indentation fracture toughness (5–10 N indenter load) of the eutectics is 2.47–4.76 MPa m^1/2. Indentation-induced cracks are deflected by TiB₂ lamellae, and indentation-induced spallation is reduced in the B₄C–TiB₂ eutectic compared to monolithic B₄C. Indentation-induced amorphization in monolithic B₄C and the B₄C phase of the eutectic is detected using Raman spectroscopy. Sub-surface damage is observed using TEM, including microcracking and amorphization damage in B₄C and B₄C–TiB₂ eutectics. Dislocations are observed in the TiB₂ phase of eutectics with an interlamellar spacing of 1.9 μm.     

On the deformation mechanisms of β-polypropylene: 1. Effect of necking on β-phase PP crystals

PP samples containing β-phase crystals were prepared by doping with a β-nucleating agent. The β-PP specimens were tensile stretched at room temperature under various crosshead rates and the yielding behaviour was analyzed on the basis of Eyring's theory. After tensile testing, the β-phase was examined with WAXS and D.S.C. over the necking region and the deformation mechanisms were discussed. The β-phase was found to be mechanically stable at room temperature up to yielding point. However, once the samples were stretched to necking, the β-phase began to transform to the -phase; this phase transformation continued over the necking region. The phase transformation implied that local melting and crystallization occurred during cold drawing and gave powerful support to Flory's model of the deformation mechanism of crystalline plastics.     

Blends containing core–shell impact modifiers. Part 5 – Ductile–brittle transition temperatures in impact and slow bending

Abstract

Dart drop, Charpy impact and single edge notched bending fracture tests were conducted over a range of temperatures on blends of polycarbonate (PC), poly(methyl methacrylate) (PMMA), poly(styrene-co-acrylonitrile) (PSAN), and poly(vinyl chloride) (PVC) with two core–shell impact modifiers, which were based, respectively, on polybutadiene and poly(butyl acrylate-co-styrene). The T _g values of the two rubbers were ?75 and ?8°C, respectively. The transition from fully brittle to partially or fully ductile behaviour occurred at various temperatures T _BD depending upon a number of factors, which included the T _g of the rubber phase, the choice of thermoplastic matrix, and the type of test. In disc specimens subjected to dart drop impact, ductility was observed at lower temperatures than in sharply-notched specimens tested in slow three-point bending. It is concluded that the T _g of the rubber phase generally marks a lower limit for ductility in rubbertoughened blends, but that other factors can shift the brittle–ductile transition to higher temperatures.     

Blends containing core–shell impact modifiers. Part 3 – Effects of temperature on tensile impact behaviour

Abstract

The performances of two contrasting core–shell impact modifiers, in blends with polycarbonate (PC), poly (methyl methacrylate) (PMMA), and poly (styrene-co-acrylonitrile) (PSAN), have been evaluated using tensile impact tests at temperatures between -80 and +50°C. In both modifiers, each individual particle has a 10 nm thick outer shell of PMMA, which is grafted to the rubber phase. In the case of modifier PB, the core of the particle is a 200 nm diameter homogeneous sphere of polybutadiene, with a T _g of -86°C. Modifier PBA has a 260 nm core of PMMA, surrounded by a 20 nm inner shell of poly (butyl acrylate-co-styrene), which has a T _g of -17°C. Tensile impact tests show that the T _g of the rubber does not necessarily control the brittle–ductile transition temperature T _BD. Both the PC–PB and PC–PBA blends exhibit some ductility at -80°C, although neither blend is as tough as plain PC at any temperature. The blend of PB with PMMA shows a modest increase in toughness above -40°C and there is a similar but rather larger increase in the toughness of the PMMA–PBA above -20°C. In PSAN blends, the PBA modifier is the more effective toughening agent ahove 0°C. It is concluded that these differences originate from differences in the balance between shear yielding and crazing in the matrix polymer, and in the ability of cavitated rubber particles to prevent crazes from turning into cracks. In PMMA and PSAN blends, the PBA modifier is the more effective toughening agent at 23°C because of its rigid core, which enables stable rubber fibrils both to form and to contribute to local strain hardening, thereby stabilising the yield zone.     

Size effect of carbon fiber-reinforced silicon carbide composites (C/C-SiC): Part 1 – bending load and statistical effects

This study analyzed the influence of the sample volume, number of tested specimen, and testing method on the flexural strength of fabric-reinforced ceramic matrix composites. For this purpose, seven different batches of C/C-SiC were prepared with four different sample thicknesses to determine the flexural strengths and Weibull moduli by three- and four-point flexural tests. The result showed that C/C-SiC exhibits a size effect of strength under bending load because a decrease of measured flexural strength with increased specimen size was observed. This size effect was discussed regarding the Weibull weakest link approach and the concept of quasi-brittle materials.The determined Weibull moduli were comparable for the same load condition but dissimilar for the identical material if the load condition were changed from three- to four-point bending. Hence, the Weibull modulus was found to be not an inherent material constant for C/C-SiC and the Weibull weakest link approach seems not appropriate.     

Compatibilisation of polypropylene–polystyrene blends: Part 1 – Effect of mixing intensity on morphology andrheological properties

Abstract

Propylene copolymer blends have been prepared using a proprietary, in reactor, grafting polymerisation technique, to give two phase materials composed of 70% polypropylene, 20% polystyrene, and 10%polypropylene- graft-polystyrene (PP-g-PS). The present study was carried out to determine the efficiency of PP-g-PS relative to block-copoly(styrene/ethylene–butylene/ styrene) (SEBS) as a compatibiliser for polypropylene–polystyrene blends by measuring rheological properties and examining the morphological structure. Mixing experiments were conducted on a twin screw mixing element evaluator and on a commercial ZSK-30 extruder. Dynamic mechanical analysis was used to characterise the viscoelastic properties of the extruded polymer pellets and transmission electron microscopy was used to examine the morphology of the polymer throughout its processing history, i.e. from carcasses at different stages of the mixing operations to injection moulded specimens. Both the graft copolymer, PP-g-PS, and the black terpolymer, SEBS, were found to enhance the compatibilisation of the blends, but the graft copolymer was found to be more effective. The results also show that the intensity of mixing affects the molecular and morphological structures of the graft and graft–rubber blends, as indicated by the intrinsic viscosity values of the extracted isotactic polypropylene fraction of the blends. This is supported by morphological examination of the graft blend, which showed a phase inversion from co-continuous to dispersed phase morphology during extrusion.     

Tensile creep properties and damage mechanisms of 2D-woven C/HfC–SiC composites in high temperature

2D-C/HfC–SiC composites were prepared by a combination of precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI). Creep tests were performed at 1100°C in air under different stress conditions. Unlike most, C/SiC and SiC/SiC ceramic matrix composites only underwent primary and secondary creep stages, and the C/HfC–SiC composites underwent tertiary creep stage in the creep process. The reason was that the mechanical properties of C/HfC–SiC materials prepared by PIP + CVI methods were different from those prepared by traditional methods. The microscopic morphological analysis of the sample fracture showed that the oxidation products SiO₂ and Hf–Si–O glass phases of the HfC–SiC matrix played a crack filling role in the sample during creep. In turn, it provided effective protection to the internal fibers of the sample. The creep failure of C/HfC–SiC composites in a high-temperature oxidizing atmosphere was caused by the oxidation of the fibers. The total creep process was dominated by the oxidation of carbon fibers. It is noteworthy that there was the generation of Hf_xSi_yO_z nanowires in the samples after high-temperature creep. The analysis of the experimental data showed that the creep stress had a linear negative correlation with the creep life.     

Interfacial instabilities in multimaterial co-injection mouldings Part 1 – Background and initial experiments

Abstract

Interfacial adhesion between the skin and core is vital for successful co-injection moulding. This is the first paper in a series, which introduces and describes an in mould method of mixing that is applicable regardless of the compatibility of the materials. It works by inducing turbulent mixing at the interface between the skin and core materials. It makes use of the change that occurs from laminar to turbulent flow at high injection speeds in co-injection moulding. This novel approach takes advantage of the moulding parameters already available within the co-injection system to offer an expanded range of material combinations for multimaterial moulding. Comparisons are made between multimaterial mouldings made with miscible polymers, immiscible polymers with no compatibiliser, and immiscible polymers bonded by compatibilisers.     

New hydroxyapatite–hydrotalcite composites II. microwave irradiation effect on structure and texture

Acid-basic materials are often used to catalyse organic reactions. Hydroxyapatite is acidic and hydrotalcite presents basic properties. The association of both compounds in a single material should present a rather unique catalytic behavior. Three preparations of hydroxyapatite impregnated with hydrotalcite are presented. The effect of microwave irradiation, at different preparation levels, is discussed. A homogeneous distribution of hydrotalcite on hydroxyapatite surface is obtained when hydrotalcite is precipitated over a previously microwave irradiated hydroxyapatite. Instead, if the hydrotalcite mixture is incorporated to the hydroxyapatite precursor gel and the resulting mixture microwave irradiated, hydrotalcite is preferentially deposited in the hydroxyapatite interparticle spaces. When both hydroxyapatite and hydrotalcite solutions are irradiated, mixed and irradiated again, the composite behaves as the addition of the two components.     

Drying behavior of dense refractory ceramic castables. Part 1 – General aspects and experimental techniques used to assess water removal

Despite the continuous evolution on the performance of refractory ceramic products, monolithic materials still require special attention during their processing steps as various phase transformations may take place during the curing, drying and firing stages. Drying is usually the longest and the most critical process observed during the first heating cycle of refractory linings, as the enhanced particle packing and reduced permeability of the resulting microstructure may lead to recurrent explosive spalling and mechanical damage associated with dewatering and the development of high steam pressure at the inner regions of such dense materials. In this context, this review article mainly addresses (i) the theoretical aspects related to the drying process of dense refractories, (ii) the influence of the phase transformations derived from the binder additives, and (iii) the usual and advanced experimental techniques to assess the water removal from consolidated castable pieces. Many studies have pointed out that due to the complex nature of this phenomenon (i.e., considering combined thermal stresses and pore pressure, heterogeneous microstructure, evolving pore structure with temperature, etc.), the mechanisms behind the water withdrawal and castables’ explosive spalling are lacking further understanding and, consequently, it has been difficult to save time and energy during the first heating of industrial equipment lined with ceramic materials. On the other hand, different methods are used for refractory spalling assessment and many efforts have been carried out in applying in situ imaging techniques (such as NMR and neutron tomography) to follow the moisture evolution during such thermal treatments. These novel techniques, also addressed in this review, might be of particular importance to provide more accurate data for the validation of many state-of-the-art numerical models, which can be used to predict the steam pressure developed in refractory systems and help in the design of proper heating schedules for such products.