Sebastião Ribeiro

Substitution of Y2O3 by a rare earth oxide mixture as sintering additive of Si3N4 ceramics

Abstract The objective of this work was to compare the sinterability, the microstructures and the mechanical properties of Si 3 N 4 ceramics with additive mixtures based on yttrium oxide, Y 2 O 3 , and a mixed concentrate of yttrium and rare earth oxides. The RE 2 O 3 samples were prepared by a two-step gas-pressure sintering process. No significant differences between the samples have been found, thus demonstrating the potential of this low cost alternative sinter additive.

Fracture toughness measurements of LPS-SiC: a comparison of the indentation technique and the SEVNB method

Many methods are currently used to measure the fracture toughness of ceramic materials. Methods based on crack-length measurements of cracks introduced into the sample surface by the Vickers indentor have the advantage that they are easy to use, but are very unreliable due to subcritical crack growth and the difficulty in determining the exact length of the cracks. Furthermore, depending on the crack shape there are many equations to calculate KIc. Other methods like the Chevron Notch or Single Edge Pre-cracked Beam (SEPB) are often difficult to execute or expensive. The simple and inexpensive Single-Edge-V-Notched Beam (SEVNB) on the other hand gives reliable values of fracture toughness of ceramic materials. In this method a saw cut is tapered to a sharp V-notch using a razor blade sprinkled with diamond paste. Thus, it is possible to introduce a sharp crack with a notch width of less than 20 micrometers, necessary to conduct valid tests. In this investigation, fracture toughness measurements on LPS-SiC materials carried out by the indentation technique and the SEVNB method have been compared.

Liquid phase sintering of silicon carbide with AlN/Y2O3, Al2O3/Y2O3 and SiO2/Y2O3 additions

In this work, the influence of the additive system on the liquid phase sintering of silicon carbide has been investigated. The additives employed were mixtures of AlN/Y2O3, Al2O3/Y2O3 and SiO2/Y2O3. The total additive content was fixed at 20 vol.-%, maintaining the Y2O3 content in each additive system at 35 vol.-%. Cold isostatically pressed samples were sintered at 1900, 2000 and 2100 °C under Ar atmosphere during 30 min. The most promising results have been obtained by samples with AlN/Y2O3 additions sintered at 2000 °C, exhibiting the smallest weight loss of about 6% and the highest flexural strengths of about 433 MPa. Samples with Al2O3/Y2O3 and SiO2/Y2O3 additions exhibited high weight loss, because of reactions of Al2O3 and SiO2 with the SiC matrix, forming gaseous species such as Al2O, SiO and CO, resulting in depletion of the liquid phase, and, consequently, in inferior final densities and mechanical properties. Concerning the SiO2/Y2O3 additive system, the reactions seem to be completed already at temperatures below 1900 °C, turning this additive mixture unsuitable. The microstructural analysis indicated only the presence of the b-SiC phase for all samples; no phase transformation of the b-SiC into a-SiC has been observed.

Study of the fracture behavior of mortar and concretes with crushed rock or pebble aggregates

The objective of this work was to compare the fracture energy of mortar and concretes produced with crushed rock and pebble aggregates using zero, 10, 20, 30 and 40% of aggregates mixed with standard mortar and applying the wedge splitting method to achieve stable crack propagation. The samples were cast in a special mold and cured for 28 days, after which they were subjected to crack propagation tests by the wedge splitting method to determine the fracture energies of the mortar and concrete. The concretes showed higher fracture energy than the mortar, and the concretes containing crushed rock showed higher resistance to crack propagation than all the compositions containing pebbles. The fracture energy varied from 38 to 55 J.m-2. A comparison of the number of aggregates that separated from the two concrete matrices with the highest fracture energies indicated that the concrete containing pebbles crumbled more easily and was therefore less resistant to crack propagation.

Properties of SiC Ceramics Sintered via Liquid Phase Using Al2O3 + Y2O3, Al2O3 + Yb2O3 and Al2O3 + Dy2O3 as Additives: a Comparative Study

Silicon carbide (SiC) ceramics show excellent performance at high temperatures. Due to the high covalence of Si-C bonds, these ceramics are produced successfully only via liquid phase sintering (LPS). In this work, SiC ceramics were sintered via LPS using eutectic mixtures of Al2O3+Y2O3, which served as a standard for comparison, Al2O3+Yb2O3 and Al2O3+Dy2O3. The oxides mixtures were used to form liquid phase during the SiC sintering. Mixtures of SiC and additives were ground, pressed at 300 MPa and sintered at 1950oC for 2 hours. All mixtures showed similar hardness, fracture toughness and flexural strength slightly different. Also the microstructure and crystalline phase were similar, showing that the ytterbiums and dysprosiums oxides can be also used as additive as well the most used oxide, yttrium oxide.

Si3N4 ceramics sintered with Y2O3/SiO2 and R2O3(ss)/SiO2: a comparative study of the processing and properties

A comparative study was made of Si3N4 sintered with two types of additives, namely, Y2O3/SiO2 and R2O3(ss)/SiO2, R2O3(ss) being a rare earth metal oxide in solid solution. The processing conditions for both types of Si3N4 were 14 vol% of additives and a sintering temperature of 1800 oC for 30, 60 and 240 min. To compare the efficiency of the additives in the materials sintering process, the density, flexural strength, fracture toughness and hardness were measured and the phase composition and microstructure determined. The results indicated that R2O3(ss)/SiO2 as a sintering aid improved the materials high temperature strength and slowed down grain growth when compared with the Y2O3/SiO2 additive.

Elastic Work and Fracture Energy of Concretes Made with Crushed Stones and Pebbles Aggregates

This work examines the elastic work and fracture energy of a mortar and two concretes using the wedge splitting method to attain stable crack propagation. A comparison was made of the fracture energy in mortar and concrete using two different aggregates: crushed stone and pebbles, both with sizes between 4.8 and 9.5 mm. The mortar was made of sand, Portland cement and water, using a cement: sand ratio of 1:2. The water content was 0.46 of the cement mass. The samples were molded and then cured for seven days at 25oC in 100% relative humidity. After curing, the samples were dried at 55oC for 48 hours. The concretes were produced using the same procedure, but with the addition of the aggregates. The amount of aggregate was 10 wt.% of the total weight of cement plus sand. After curing and drying, the samples were subjected to the wedge splitting procedure. The tests were carried out at a constant displacement rate of 0.030 mm/min. The following results were obtained: elastic work: 80.4 ± 0.6, 114 ± 9 and 110 ± 12 mJ, and fracture energy: 30.3 ± 0.6, 40 ± 1, and 40 ± 5 J.m-2, respectively, for the mortar and for the concretes containing crushed stone and pebbles. These results allow us to conclude that the aggregates improved the elastic work and the fracture energy of the concretes. However, the type of aggregate did not make any difference to the properties. These findings contradict what is generally known, i.e., “that concrete produced with pebbles is inferior to concrete made with crushed stone”, at least insofar as it concerns the energies associated with the fracture process.

Melting Temperature and Wetting Angle of AlN/Dy2O3 and AlN/Yb2O3 Mixtures on SiC Substrates

This work aims to evaluate the melting temperature and wetting behavior of AlN/Re2O3 (Re = Dy, Yb) mixtures when in contact with SiC substrates at high temperatures, in order to define whether these compounds can be further used to induce a more effective liquid phase sintering of SiC-based products. The prepared samples were placed on SiC plates and thermally treated up to 1900 °C under argon. The melting point and spreading evolution of different compositions of AlN/Re2O3 on SiC were determined by analyzing images captured as a function of the heating temperature. The contact angle and melting point were measured using the ImageJ software and according to DIN 51730, respectively. Based on the obtained wetting curves, all evaluated conditions resulted in the decrease of the contact angle values with temperature. The mixtures presenting improved wetting (θ ~ 1° and 3°) on SiC plates were the ones above the selected eutectic point.

Mechanical Characterization of Tricalcium Phosphate Ceramics Doped with Magnesium

In this work, the mechanical properties of magnesium doped tricalcium phosphate ceramics, considered a bioceramic for bone repair applications, were studied. Pure β-TCP and β-TCP doped with 2.25 mol% of Mg powders were synthesized through neutralization, freeze dried, uniaxially pressed and sintered at 1200°C/1h. The mechanical properties were evaluated through four points flexural and compression strength tests, fracture toughness and Youngs Modulus. After the flexural strength test, the fractures surface and their homogeneity were characterized by scanning electron microscopy. It was verified that Mg addition into β-TCP structure lead higher linear shrinkage, followed by higher residual stress, decreasing the mechanical properties compared to pure β-TCP ceramics. However, this behavior does not hinder the use of such bioceramics as bone substitutive materials, mainly in the sites that do not require high mechanical solicitations.

FEA and microstructure characterization of a one-piece Y-TZP abutment.

OBJECTIVES The most important drawback of dental implant/abutment assemblies is the need for a fixing screw. This study aimed to develop an esthetic one-piece Y-TZP abutment to suppress the use of the screw. METHODS Material characterization was performed using a bar-shaped specimen obtained by slip-casting to validate the method prior to prototype abutment fabrication by the same process. The mechanical behavior of the prototype abutment was verified and compared with a conventional abutment by finite element analysis (FEA). The abutment was evaluated by micro-CT analysis and its density was measured. RESULTS FEA showed stress concentration at the first thread pitch during installation and in the cervical region during oblique loading for both abutments. However, stress concentration was observed at the base of the screw head and stem in the conventional abutment. The relative density for the fabricated abutment was 95.68%. Micro-CT analysis revealed the presence of elongated cracks with sharp edges over the surface and porosity in the central region. SIGNIFICANCE In the light of these findings, the behavior of a one-piece abutment is expected to be better than that of the conventional model. New studies should be conducted to clarify the performance and longevity of this one-piece Y-TZP abutment.