A.C. Ferro

Wetting behaviour in the al-si/sic system: interface reactions and solubility effects

The sessile drop technique was used to study the wetting behavior of Al-Si alloys on SiC sintered ceramic substrates under vacuum in the 700--1,100 C temperature interval. Al-Si alloys with Si concentrations up to 50% were tested. An expected non-wetting/wetting transition was observed at 900--1,000 C due to the presence of an alumina film surrounding the molten alloy. At higher temperatures wetting was observed and the Si concentration of the alloy has a marked effect on the measured contact angles, {theta}. At 1,100 C {theta} decreases from 55 to 25{degree} when instead of pure Al an Al12.3%Si or an Al16.6%Si alloy is used. The suppression of the formation of a continuous Al{sub 4}C{sub 3} layer at the interface and a process of dissolution and reconstruction of the SiC surface, due to the increased Si concentration of the Al-Si alloys, are the key factors to explain the observed behavior.

Topology and transformations in cellular structures

Abstract Two- and three-dimensional structures respectively composed of polygons and trivalent polyhedra, connected to give the minimum edge valency of vertices, are analysed from the topological and thermodynamic point of view. Both structures are classified into three categories, namely: saturated, unsaturated and periodic structures, and their topological properties are reviewed. The unit topological operations through which structures of a given dimension transform into one another are the basic steps in cell growth, which results from the excess free energy of faces, edges and vertices. It is possible to define forces acting on these elements which are responsible for the transformation of the structures. Using this thermodynamic approach it is shown that the unit operations occur in such a way that growth takes place. A general formalism for predicting the kinetics of growth is developed.

Microstructural Characterization of Concrete Prepared with Recycled Aggregates

Several authors have reported the workability, mechanical properties, and durability of concrete produced with construction waste replacing the natural aggregate. However, a systematic microstructural characterization of recycled aggregate concrete has not been reported. This work studies the use of fine recycled aggregate to replace fine natural aggregate in the production of concrete and reports the resulting microstructures. The used raw materials were natural aggregate, recycled aggregate obtained from a standard concrete, and Portland cement. The substitution extent was 0, 10, 50, and 100 vol%; hydration was stopped at 9, 24, and 96 h and 28 days. Microscopy was focused on the cement/aggregate interfacial transition zone, enlightening the effect of incorporating recycled aggregate on the formation and morphology of the different concrete hydration products. The results show that concretes with recycled aggregates exhibit typical microstructural features of the transition zone in normal strength concrete. Although overall porosity increases with increasing replacement, the interfacial bond is apparently stronger when recycled aggregates are used. An addition of 10 vol% results in a decrease in porosity at the interface with a corresponding increase of the material hardness. This provides an opportunity for development of increased strength Portland cement concretes using controlled amounts of concrete waste.

Trivalent polyhedra: Properties, representation and enumeration

Abstract Polyhedra with three edges meeting at each vertex (trivalent polyhedra) are common in a variety of natural and man-made structures. The basic topological properties of the individual polyhedra are reviewed and their representation by planar graphs, including dual graphs, is described. Both straight ( n∗ ⩾ 3 ) and curved ( n∗ = 2 ) polyhedra are considered; n ∗ is the smaller polygonality of the faces of a polyhedron. The analogy between polyhedra and planar networks is discussed. The concept of a cluster of faces in a polyhedron is introduced and used to distinguish between convex and non-convex straight polyhedra, according to the valency (number of dangling edges) of the clusters they contain. Methods for generating all non-isomorphic trivalent convex polyhedra and curved polyhedra with a given number of faces are developed, including a method based on the clusters of inner faces. These methods are applied to generate all distinct convex polyhedra with F ⩽ 11 and all curved polyhedra with F ⩽ 7. Each polyhedron is indicated by its face content set, i.e. by the numbers of faces of each polygonality. The polyhedra that have an enantiomorphic pair are identified and the maximum symmetry that each polyhedron can admit is indicated. A large number of face content sets compatible with the basic topological relations do not correspond to any convex polyhedron. The reasons for this are briefly discussed.

Tribological behaviour of unveneered and veneered lithium disilicate dental material.

The friction and wear behaviour of a lithium disilicate dental ceramic against natural dental enamel is studied, including the effect of the presence of a fluorapatite veneering upon the tribological properties of the material. The tribological behaviour was assessed using reciprocating pin-on-plate test configuration, at pH 3 and pH 7. The surface energy of the plates was determined, as well as the zeta potential of fluorapatite, lithium disilicate and enamel particles in artificial saliva. It was found that the friction and wear behaviour of the tested enamel/plate material tribocouples is less severe in unveneered plates. Initial surface roughness of the plate does not affect wear results. However the topography of the resulting wear track affects the corresponding wear loss: a smoother final wear track is associated with lower wear. The surface topography of the wear track, and thus the tribological performance of the tested materials, is very sensitive to the pH of the sliding solution. This is because the dissolution trend, wettability and surface charge of the used materials are pH dependent. Overall friction and wear are higher under basic pH conditions, especially when plates are veneered. A wear model is proposed that correlates the effect of the described parameters with the observed tribological behaviour at pH 7. Attained results show that fluorapatite coating of lithium disilicate dental crowns affects tooth/crown wear behaviour, resulting in increased wear of both the artificial crown and the opposing natural teeth. Coating should therefore be avoided in occlusal crown surfaces.

Microstructure of concrete prepared with construction recycled aggregates

Conservation of natural resources, shortage of waste land and the high cost associated to treatment prior to disposal are driving growing interest in the recycling of construction and demolition waste materials (CDW). A challenging application for recycled CDW is the replacement of natural aggregates in the production of structural concrete. In the past few years several studies have examined the viability of this substitution. Although recycled aggregates are mostly heterogeneous, less dense and more porous than natural aggregates, satisfactory results have been attained by several authors regarding concrete workability, mechanical properties and durability. However, a systematic microstructural characterization of recycled aggregate concrete is still unaccomplished. In this context, the use of fine recycled concrete aggregates to replace natural fine aggregates in the production of structural concrete was tested, and attained microstructures are reported. The recycled aggregates were obtained from a standard concrete, produced and crushed under laboratorial conditions, thus allowing full control of concrete’s composition and setting. The used raw materials were natural aggregate (sand), recycled aggregates and Type 1 Portland cement. The substitution extent in the mixtures was 0, 10, 50 and 100 wt%; hydration was stopped at the ages of 9 h, 24 h, 96 h and 28 days. Microscopy study of the different mixtures enlightened the effect of the incorporation of recycled aggregates upon the formation and morphology of the different concrete hydration products. In this work, FEG-SEM (coupled with EDS microanalysis) was used on polished cross sections and fracture surfaces, to study the new mixtures. Focus was placed on the interfacial transition zone (ITZ) between cement matrix and aggregate. The natural aggregate-cement interfacial zone exhibits typical microstructural features of the ITZ of a normal strength concrete. After 28 days a large amount of well-crystallised C-S-H (and a small amount of poorly crystalline fibres) is present at the interface, together with CH deposits. Grains of unreacted clinker components (C 2 S, C 3 S, Ca 2 (Al,Fe), C 3 A) were also identified; ettringite crystals are barely present, even at the earlier hydration times. The ITZ is highly porous. Independently of the setting time, fracture takes place preferably along the surface between paste and aggregate, attesting the relatively loose nature of the interface. The structure of ITZ with recycled aggregates (Figure 1) is consistent with that observed in the reference natural concrete. Also, calculation based on EDS results rendered a lime to silica ratio (C/S) of 1.46 0.15, consistent with the typical 1.2-2.3 range. However, there are representative microstructural features that may contribute to variation of mechanical properties. Ettringite and plate-like CH hydrates are much more abundant, even at higher setting times. Overall porosity in the ITZ increases with the aggregate substitution extent; however maximum pore size decreases from approximately 30 µm for 0% substitution to 16 µm for 100 % substitution, as shown by image analysis results. In fresh natural concrete a water film forms around the aggregates, which is gradually replaced by the growing amount of hydration products. In recycled aggregates, active silica in residual cementitious materials reacts with the fresh cement hydration products. The secondary reaction products gradually fill the region, partially covering the recycled aggregates pore structure and creating additional interfacial bonding effects (Figure 2). In good agreement, it was observed that in substituted concretes fracture preferably takes place throughout the paste rather than throughout the contact surface. Concretes prepared with recycled aggregates exhibit typical microstructural features of the ITZ in normal strength concrete. Although porosity at the ITZ is affected by the extent of aggregate replacement, the interfacial bond is apparently stronger when recycled aggregates are used. This envisages an opportunity window for the development of increased strength Portland cement concretes.

Characterization of titanium–hydroxyapatite biocomposites processed by dip coating

Ti orthopaedic implants are commonly coated with hydroxyapatite (HA) to achieve increased biocompatibility and osseointegration with natural bone. In this work the dip-coating technique was used to apply HA films on Ti foil. A gel was used as the support vehicle for commercial HA particles. The experimental parameters like surface roughness of the metallic substrate and immersion time were studied. All coated substrates were heat treated and sintered under vacuum atmosphere. The produced coatings were characterized by field-emission gun scanning electron microscopy coupled with energy-dispersive spectroscopy, X-ray diffraction, Raman spectroscopy, microhardness, scratch test and profilometry. Additionally, the apatite-forming ability of the produced material was tested by exposure to a simulated body fluid. Higher substrate surface roughness and longer immersion time produce thicker, denser films, with higher surface roughness. Lower film porosity is accompanied by higher hardness values. However, thicker coating promotes differential shrinkage and crack formation during sintering. Both coating thickness and coating roughness increase with coating time. HA films∼30–40 μm thick with 45–50% HA theoretical density produced on Ti substrates with surface roughness of Rz∼1.0–1.7 μm, display an attractive combination of high hardness and resistance to spallation. Attained results are encouraging regarding the possibility of straightforward production of biocompatible and bioactive prosthetic coatings for orthopaedic applications using commercial HA.

Processing and characterization of alumina/LAS bioceramics for dental applications

Alumina allows to recreate the functionality and aesthetics of natural teeth. However, its low fracture toughness rises concern regarding use in dental restoration. Structural reliability is addressed here by formulating a material containing alumina and a glass–ceramic from LAS system. The presence of LAS in the mixtures result in formation of glass phase during sintering, promoting densification at lower temperature and enhanced surface finishing. A composite microstructure with increased toughness can thus be produced. Powder mixtures containing 0, 20, 50, 80 and 100 wt%-LAS were prepared by planetary milling and uniaxial pressing and sintered. The compositions were investigated regarding their processability, mechanical performance and biological behaviour. Aesthetics was evaluated by comparison with a commercial dental matching guide. Variations on hardness and fracture toughness with starting LAS fraction were assessed by indentation. Interaction with biological medium was evaluated by immersion in a simulated body fluid. Resulting microstructures were characterised by FEG–SEM, EDS and XRD.

A Study on CuO-Al2O3 Infiltration by Aluminium

This paper reports preliminary studies regarding a new fabrication process for aluminium alloy matrix particulate reinforced composites, which uses ceramic preforms with alumina and tailored amounts of reactive copper oxide, CuO. An Al2O3-CuO mixture with 75 mol% CuO was selected, aiming at a 10-40vol% reinforcement phase fraction in the final composite, after aluminium infiltration. Molten aluminium infiltration progress was studied as a function of ceramic’s composition, doping, and infiltration time. The resulting microstructures were investigated by OM, SEM, FESEM and EDS in order to establish the liquid aluminium infiltration profile at the metal/ceramic interface. Infiltration experiments showed that the 3CuO (s) + 2Al (l) → 3Cu (l) + Al2O3 (s) redox reaction is triggered at the experimental conditions used, but the infiltration process is slow and does not go to completion. The use of NaOH as a doping agent promotes effective infiltration of molten aluminium upon the ceramic green mixture.

Stabilisation of CuO Aqueous Suspensions

Obtaining ceramic bodies with enhanced mechanical properties via colloidal processing requires efficient dispersion of the ceramic powders. In this work, the dispersive effect of three low molecular weight quaternary ammonium hydroxides with different alkyl groups upon stabilisation of CuO aqueous suspensions is studied and compared with that of Tiron, a compound based on the benzene molecule. The purpose is to illustrate the effect of molecular structure, size and charge location upon dispersing effectiveness. To access these parameters, rheological and electrophoretic measurements using both bare and surface charge modified CuO were made. Tiron® revealed to be the most efficient dispersant for CuO in water, rendering viscosity values below 1 Pa⋅s and the highest variation in zeta potential amplitude.