M. Guedes

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.

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.

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.

Effect of Quaternary Ammonium Hydroxides on the Stabilisation of Al2O3 Aqueous Suspensions

The effect of three low molecular weight quaternary ammonium hydroxides with different alkyl groups upon stabilisation of alumina 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 and charge location upon dispersing effectiveness. Rheological and electrophoretic measurements using both bare and surface charge modified alumina were carried out. Tiron ® revealed to be the most efficient dispersant for alumina in water.

Microstructural Changes in Copper–Graphite–Alumina Nanocomposites Produced by Mechanical Alloying

Microstructural features of nanostructured copper-matrix composites produced via high-energy milling were studied. Copper-graphite-alumina batches were planetary ball milled up to 16 h; copper-graphite batches were also prepared under the same conditions to evaluate the effect of contamination from the milling media. The microstructure of the produced materials was characterized by field emission gun scanning electron microscopy/energy-dispersive spectroscopy and related to Raman, X-ray diffraction, and particle size analysis results. Results showed that alumina was present in all milled powders. However, size reduction was effective at shorter times in the copper-graphite-alumina system. In both cases the produced powders were nanostructured, containing graphite and alumina nanoparticles homogeneously distributed in the copper matrix, especially for longer milling times and in the presence of added alumina. Copper crystallite size was significantly affected above 4 h milling; nanographite size decreased and incipient amorphization occurred. A minimum size of 15 nm was obtained for the copper crystallite copper-alumina-graphite composite powders, corresponding to 16 h of milling. Contamination from the media became more significant above 8 h. Results suggest that efficient dispersion and bonding of graphite and alumina nanoparticles in the copper matrix is achieved, envisioning high conductivity, high strength, and thermal stability.

Effect of SiCp Addition on Aqueous Tape Casting of LAS Glass-Ceramic Powders

The processing ability by aqueous tape casting of a commercial LAS glass ceramic reinforced with 20-vol% SiC platelets was studied. The formulations included the inorganic materials, an acrylic/styrene aqueous emulsion binder and a thickening agent. The suspensions presented shear-thinning behaviour, appropriate for tape casting, enabling the production of tapes with uniform and smooth surfaces. Laminates were prepared by cutting circular monolayers from the green tapes, stacking and thermocompressing at 50oC under 65MPa. Dilatometric and differential thermal analysis were used to study the sinterability of the powder mixtures and to optimise the sintering cycles. Sintering the laminates at 815oC, under uniaxial pressure of 11MPa rendered practically full dense glass-ceramic matrix composites.