Interface analysis and wear behavior of Ni particulate reinforced aluminum–silicon composites produced by PM

This study is concerned with the influence of Nickel, as reinforcement, in an aluminum–silicon (AlSi) alloy when regarding wear behavior. For these composites, the effect of Ni content, in the tribopair performance, was evaluated. For this purpose, the pin but also the counterface wear behavior was analyzed.Nickel particulate reinforced aluminum–silicon (AlSi) composites, with 5, 12.5 and 20 wt.% Ni were produced by a hot-pressing route. Microstructural characterization showed a uniform distribution of the Ni particulates in the AlSi matrix. EDS and XRD analyses revealed that the particle/matrix interface was formed by Al₃Ni intermetallic. Reciprocating pin-on-plate wear tests were performed with AlSi and AlSi–Ni pins against a gray cast iron (GCI) counterface. It was observed that the wear behavior of the AlSi–Ni/GCI tribopair is improved when compared with the AlSi/GCI system.     

Microstructures and fracture toughness of directionally solidified Mo–ZrC eutectic composites

Microstructures and fracture toughness of arc-melted and directionally solidified Mo–ZrC eutectic composites were investigated in this study. Two kinds of directionally solidified composites were prepared by spot-melting and floating zone-melting. Microstructure of the arc-melted composite (AMC) consists of equiaxed eutectic colonies, in which ZrC particles are dispersed. The spot-melted composite (SMC) exhibits spheroidal colony structure, which is rather inhomogeneous in size and morphology. ZrC fibers in the eutectic colonies are aligned almost parallel to the growth direction. Well aligned, homogeneous columnar structure with thin ZrC fibers evolves in the floating zone-melted composite (FZC). Texture measurement by X-ray diffractometry revealed that the growth direction of Mo solid solution (Mo_SS) in FZC is preferentially 〈100〉, while that of SMC is scattered. Fracture toughness K_Q evaluated by three point bending test using the single edge notched beam method is >13 MPa m^1/2 for AMC, 20 MPa m^1/2 for SMC and 9 MPa m^1/2 for FZC. Intergranular fracture along colony boundaries is often observed in AMC. In contrast, transgranular fracture is dominant in SMC and FZC, although significant gaps caused by intergranular fracture are occasionally observed in SEM micrographs of SMC. Fracture surface in FZC is wholly flat. Pull-out of ZrC occurs owing to Mo/ZrC interfacial debonding in intergranularly fractured regions of AMC and SMC.Coarse elongated colonies in SMC and FZC induce transgranular fracture instead of intergranular fracture. Intergranular fracture and interfacial debonding in AMC and SMC causes frequent crack deflection accompanied by ligament formation and crack branching, which is responsible for the high fracture toughness of the composites. Preferred 〈100〉 growth of Mo_SS phase in FZC leads to brittle {100} cleavage fracture associated with low fracture toughness.     

Influences of processing methods and chemical treatments on fracture toughness of halloysite–epoxy composites

This study presented experimental reports on effects of processing methods (mechanical mixing and ball milling homogenization) on the fracture toughness of as-received and phenylphosphonic acid (PPA) treated halloysite–epoxy composites. It was demonstrated that with halloysite added, the composites held higher fracture toughness value than that of neat epoxy. Large-sized particle clusters occurred during mechanical mixing process can significantly decrease by ball milling homogenization and much more uniform dispersion was obtained in the cured composites. With PPA treatment, the morphology of halloysite changed from nanotubes to nanoplatelets with a substantial increase in the total contact area between halloysite and epoxy, it further enhanced fracture toughness in PPA-treated composites. The fracture toughness mechanisms of each nanocomposite with different processing methods and chemical treatments have been illustrated.     

The effects of water aging on the interphase region and interlaminar fracture toughness in polymer–glass composites

Three different unidirectional polymer–glass composite systems involving phenolic and polyester resins were aged for 6 and 11 weeks in tap water and tested in the mode I double cantilever beam (DCB) test. The results showed a dramatic increase in water absorption and a decrease in fracture toughness for phenolic/glass systems. Fractographic analysis revealed interfacial debonding to be dominant failure mechanism, indicating a strong influence of water degradation on fracture toughness results. The interphase region of each system was investigated using the nano-indentation and the nano-scratch techniques before and after aging in water. The nano-indentation test produced a series of indents as small as 30 nm in depth, to detect water degradation of the material properties at the interphase region between the fibre and the matrix. The nano-hardness results indicated interdiffusion in water aged interphase regions. The nano-scratch test was used in conjuction with the nano-indentation test, in order to detect the width of the interphase regions before and after water degradation. It was shown, from the coefficient of friction and the scratch profile depth, that the interphase region width increased and the material properties degraded during water aging. Qualitative links between water degradation of the glass–polymer interphase on a nanometer level and interlaminar fracture toughness are discussed.     

Thermodynamic and kinetic analysis of interfacial reaction between CBN and titanium activated Ag–Cu alloy

Abstract

Thermodynamic and kinetic analysis was performed in order to study the interfacial reaction mechanism of cubic boron nitride (CBN) abrasive grains and Ti activated Ag–Cu filler alloy during high temperature brazing. Meanwhile, microstructure of the interfacial layer was experimentally detected using scanning electron microscope (SEM), energy dispersion spectrometer (EDS) and X-ray diffraction (XRD) in the present paper. The results indicate that according to the thermodynamic theory, the interfacial reaction has been proved feasible, and during brazing the special active element Ti concentrated to and reacted with the CBN abrasive to form TiB₂ and TiN, which joined hard the abrasive grain and steel substrate. Furthermore, the diffusion activation energy of the growing process in the interfacial reaction layer has discovered that the layer growth largely depends on the new formed TiN under conditions of 1153–1193 K and 5–30 min.     

Characterization of the correlation between the interfaces and failure behaviors for particle reinforced Mg–Li composites

The interfacial microstructure of SiC_p or YAl_2p reinforced Mg–14Li–3Al matrix composites was comparatively characterized by scanning electron microscopy and electron probe microanalysis. A nanoindentation combined with scanning electron microscopy technique was used to characterize the interfacial mechanical properties between the reinforcements and matrix. The interfacial strength and failure behaviors for the composites were analyzed from the load–penetration curves and corresponding images. In situ tensile tests were used to observe the fracture and deformation processes with the aid of scanning electron microscopy. The results show that both the chemical and mechanical compatibilities between the YAl₂ particles and LA143 matrix are better than those between the SiC particles and LA143 matrix. The interfacial breakage load for the SiC/LA143 composite is lower than that for the YAl₂/LA143 composite because of the worse chemical and mechanical compatibilities between the ceramic particles and metal matrix. Interfacial breakage is the main failure mechanism for the SiC/LA143 composite, while the particle breakage and matrix crack are the main failure mechanism for the YAl₂/LA143 composite. These may be related to the stronger interfacial bonding between the intermetallic particles and metal matrix.     

Effect of amino functionalized MWCNT on the crosslink density,fracture toughness of epoxy and mechanical properties of carbon–epoxy composites

Amino functionalized multiwalled carbon nanotubes (A-MWCNTs) reinforced two phase (A-MWNT–epoxy) and three phase (A-MWCNTs–carbon fiber–epoxy) nanocomposites were fabricated with 0.25 wt%, 0.5 wt% and 1.0 wt% loadings of A-MWCNTs. It is observed that, A-MWCNTs can improve the crosslink density of epoxy significantly. Fracture toughness of epoxy matrix is found to increase up to an optimum crosslink density improvement, indicating the role of crosslink density in imparting toughness to epoxy apart from the crack deflection contributions of A-MWCNTs. In addition to that, this study infers that, tensile, flexural properties of the three phase composites are strongly influenced by the fracture toughness changes of the matrices. This study, thus proposes additional mechanisms of toughness enhancements for two phase and mechanical properties enhancements for three phase composites imparted by A-MWCNTs.     

Performance of calcium deficient hydroxyapatite–polyglycolic acid composites: an in vitro study

The strategic incorporation of bioresorbable polymeric additives to calcium-deficient hydroxyapatite cement may provide short-term structural reinforcement and modify the modulus to closer match bone. The longer-term resorption properties may also be improved, creating pathways for bone in-growth. The aim of this study was to investigate the resorption process of a calcium phosphate cement system containing either in polyglycolic acid tri-methylene carbonate particles or polyglycolic acid fibres. This was achieved by in vitro aging in physiological conditions (phosphate buffered solution at 37°C) over 12 weeks. The unreinforced CPC exhibited an increase in compressive strength at 12 weeks, however catastrophic failure was observed above a critical loading. The fracture behaviour of cement was improved by the incorporation of PGA fibres; the cement retained its cohesive structure after critical loading. Gravimetric analysis and scanning electron microscopy showed a large proportion of the fibres had resorbed after 12 weeks allowing for the increased cement porosity, which could facilitate cell infiltration and faster integration of natural bone. Incorporating the particulate additives in the cement did not provide any mechanism for mechanical property augmentation or did not demonstrate any appreciable level of resorption after 12 weeks.     

Deformation,yield and fracture of unidirectional composites in transverse loading: 2. Influence of fibre–matrix adhesion

The influence of the adhesion between fibre and matrix on the transverse properties of unidirectional composites was studied using a combination of experimental and numerical analyses. The interface is modelled on a nano(metre)-scale and the aim is to investigate its local influence on the ultimate macroscopic transverse properties. Fibre-to-matrix stress transfer (i.e. fibre-to-matrix surface interaction) is simulated by introducing elastic interface springs. Since these elastic springs represent the chemical (covalent) bonds formed at the interface as a result of oxidative chemical surface treatment, the micromechanical model can be directly related to the effects of this treatment. For the verification of the numerical analyses, the influence of the interface is determined experimentally by transverse testing of carbon fibre reinforced composites, using fibres that were subjected to different levels of surface treatment. A direct relation between the oxygen concentration on the surface of the fibres, the interfacial bond strength and the resulting transverse strength was found. The interface strength required to obtain perfect bonding was found to be dependent on the fibre volume fraction and at increased fibre volume fractions a higher level of adhesion is required.     

Influence of N and Fe on α-Ti precipitation in the in situ TiC–titanium alloy composites

High strength with high ductility can be achieved in the titanium alloys by using metal precipitated ceramic particle as reinforcement. In this work, α + β or β-Ti alloy composites were prepared with α-Ti precipitated TiC particles. A series of Ti–Fe–C–N alloys were prepared and a constitutional diagram was constructed as a function of N and Fe contents. Two criteria were identified for the formation of α-Ti precipitation. One is the existence of Ti₂C phase and the other is the presence of α-Ti phase in the matrix. The mechanism of α-Ti formation from the Ti₂C phase is discussed.     

Macro and micromechanics analysis of short fiber composites stiffness: The case of old newspaper fibers–polypropylene composites

Stiffness is one of the most relevant characteristics of composite materials. Natural wood fibers have demonstrated their ability to increase the Young’s moduli of composite materials, and old newspapers are a potential source of reinforcing fibers for composite materials. There are some micromechanic models to predict the Young’s modulus of composite materials, and one of the input data is the intrinsic modulus of their fibers. This intrinsic modulus is a value which is difficult or impossible to measure in the case of wood fibers, due to their measures. This paper evaluates the stiffening abilities of old newspaper fibers and the possibility to back calculate the value of the intrinsic Young’s modulus by means of micromechanic models. Different percentages of old newspaper fibers were compounded with polypropylene (PP). Micromechanics of the fibers were obtained using Hirsch model, Cox–Krenchel’s model, Tsai–Pagano model and Halpin–Tsai equations. The most important results were the average intrinsic Young’s modulus of the fibers, the mean orientation angle and the mean modulus efficiency factor.     

Multiobjective optimization and analysis of copper–titanium diboride electrode in EDM of monel 400™ alloy

Electrical discharge machining (EDM) is one of the most accepted machining processes in the precision manufacturing industry. In EDM process, finding an alternative tool material is the demand in modern manufacturing industry. Therefore, an attempt had been made to fabricate copper–titanium diboride powder metallurgy electrode to test in EDM on monel 400? material. The experiments are planned using center composite second-order rotatable design and the model is developed by response surface methodology. The machining characteristics have analyzed using the developed model. In this study, four input parameters such as titanium diboride percentage, pulse current, pulse on time, and flushing pressure are selected to evaluate the material removal rate (MRR) and tool wear rate (TWR). The adequacy of the developed regression model has tested through analysis of variance test. The desirability-based multiobjective optimization is used to find the optimal process parameter which has given maximum MRR and minimum TWR. The optimum process parameters obtained were titanium diboride of 16%, pulse current of 6 A, flushing pressure of 1 Mpa, and pulse on time of 35?µs. The validity of the response surface model is further verified by conducting confirmation experiments.     

Magnesia supported Au and Ag catalysts for the preparation of few-layer graphene–metal nanocomposites: relationship between catalyst structure and the properties of graphene composites

Magnesia supported Au, Ag, and Au–Ag nanostructured catalysts were prepared, characterized, and used to synthesize few-layer graphene–metal nanoparticle (Gr–MeNP) composites. The catalysts have a mezoporous structure and a mixture of MgO and MgO·H₂O as support. The gold nanoparticles (AuNPs) are uniformly dispersed on the surface of the Au/MgO catalysts, and have a uniform round shape with a medium size of ~8 nm. On the other hand, the silver nanoparticles (AgNPs) present on the Ag/MgO catalyst have an irregular shape, larger diameters, and less uniform dispersion. The Au–Ag/MgO catalyst contains large Au–Ag bimetallic particles of ~20–30 nm surrounded by small (5 nm) AuNPs. Following the RF-CCVD process and the dissolution of the magnesia support, relative large, few-layer, wrinkled graphene sheets decorated with metal nanoparticles (MeNPs) are observed. Graphene–gold (Gr–Au) and graphene–silver (Gr–Ag) composites had 4–7 graphitic layers with a relatively large area and similar crystallinity for samples prepared in similar experimental conditions. Graphene–gold–silver composites (Gr–Au–Ag) presented graphitic rectangles with round, bent edges, higher crystallinity, and a higher number of layers (8–14). The MeNPs are encased in the graphitic layers of all the different samples. Their size, shape, and distribution depend on the nature of the catalyst. The AuNPs were uniformly distributed, had a size of about 15 nm, and a round shape similar to those from Au/MgO catalyst. In Gr–Ag, the AgNPs have a round shape, very different from that of the Ag/MgO catalyst, large size distribution and are not uniformly distributed on the surface. Agglomerations of AgNPs together with large areas of pristine few-layer graphene were observed. In Gr–Au–Ag composites, almost exclusively large bimetallic particles of about 25–30 nm, situated at the edge of graphene rectangles have been found.     

Organic–inorganic interaction between hydroxyapatite and gelatin with the aging of gelatin in aqueous phosphoric acid solution

Hydroxyapatite (HAp)/gelatin (GEL) nanocomposite was prepared by the solution-precipitation process using Ca(OH)(2) in water and aqueous solution of H(3)PO(4) in GEL. Before the co precipitation process the GEL powders were dissolved in the aqueous phosphoric acid solution for the phosphorylation of GEL molecules. The chemical variation of the phosphorylated GEL macromolecules was investigated by using attenuated total reflection (ATR) measurement. The crystal growth of HAp became bigger with the long-time aging of the GEL molecules in the phosphoric acid solution, and it resulted from the reduction of length scale of the GEL molecules. The degree of the organic-inorganic interaction was decreased because of the degradation of the GEL macromolecules.     

Combined atomistic–crystal plasticity analysis of the effect of beta phase precipitates on deformation and fracture of lamellar γ + α2 titanium aluminide

Atomistic simulations based on the use of interatomic potentials and a finite element method based on the crystal plasticity theory are combined to investigate the deformation and fracture behaviour of polycrystalline lamellar -TiAl + ₂-Ti₃Al material containing 10 vol % of body centred cubic beta phase precipitates. The effects of both stable beta phase precipitates, which deform by slip, and metastable beta phase precipitates, which deform by a combination of stress-induced martensitic transformation and slip, are studied. To model the cracking along the grain boundaries and the matrix–precipitate interfaces, the grain boundaries and interfaces are modelled using a cohesive zone approach. The grain boundary–interface potentials are determined by carrying out atomistic simulations of the grain boundary–interface normal separation (decohesion) and sliding.The results obtained suggest that incompatibilities in the plastic flow between the adjacent grains in the single-phase material give rise to a large build-up in tensile hydrostatic stress in the region surrounding certain three-grain junctions, which, in turn, leads to nucleation of the grain boundary cracks and ultimate failure. The stable beta phase precipitates located at the three-grain junctions in the two-phase material help accommodate the incompatibilities in plastic flow, doubling the strain to failure. The lattice expansion, which accompanies martensitic transformation in the metastable beta phase precipitates, further delays nucleation of the grain boundary–interface cracks giving rise to an additional increase in the fracture strain.     

Architecturally modified Al–DRA composites: the effect of size and shape of the DRA rods on fracture behavior

Architectural modification of aluminum matrix composites is considered as an efficient method to improve fracture toughness. Al–DRA (Al–Al/SiC/20_p) composites were fabricated via “powder extrusion–casting–ingot extrusion” route with structures similar to that of reinforced concrete, so that DRA rods were surrounded by unreinforced aluminum. The effects of variation in shape, size, and number of DRA rods on fracture behavior of Al–DRA composites were investigated. Composites containing DRA rods with hexagonal cross-section exhibited higher resistance to crack initiation and growth, in comparison to those containing circular rods. In the case of hexagonal rods, increasing the number of rods (reducing the rods’ cross-section surface) led to further enhancement of fracture toughness. Fracture surface observations of all samples revealed the existence of desirable cohesion between rods and the surrounding matrix. The remained sharp and unblunted corners of hexagonal DRA rods caused stress concentration and microcrack formation upon loading. Hence, plastic deformation constraint of aluminum ligament between rods was alleviated, which, in turn, led to further energy consumption during the fracture process.