A. Mestra

Enhanced reliability of yttria-stabilized zirconia for dental applications

An increasing number of dental applications based on yttria-stabilized zirconia (3 Y-TZP) have been developed in recent years as a result of the advances and versatility of dry-processing and soft machining at the pre-sintered state. Nonetheless, the long-term surface stability of these materials in humid environment is still a matter of concern and may limit its application. In this work, a simple method to prevent hydrothermal degradation on the zirconia surface is studied in detail. This method involves the infiltration of pre-sintered parts with optimized solutions containing Ce salts, leaving unchanged the other processing steps, allowing the diffusion of Ce during conventional sintering. Several pre-sintering conditions, solution concentrations and sintering temperatures were studied and characterized, obtaining working parameters for the production of zirconia parts with mechanical properties similar to standard 3 Y-TZP and high resistance to hydrothermal aging. This optimal combination was obtained with the 1150 °C pre-sintering temperature, 50 wt.% solution and sintering at 1450 °C, leading to a superficial CeO2 content of about 3 mol.%.

Stability of Nanocrystalline Spark Plasma Sintered 3Y-TZP

Spark plasma sintered 3Y-TZP has been investigated with respect to hydrothermal ageing and grinding. The sintering was performed between the temperatures of 1,100 and 1,600 °C for a soaking time of 5 minutes and the resulting materials were obtained with grain sizes between 65 to 800 nm and relative densities between 88.5 to 98.8%. Experiments on hydrothermal ageing in water vapour at 131 °C, 2 bars during 60 hours shows that phase stability is retained, elastic modulus and hardness of near surface region measured by nanoindentation does not change in fine grain (<200 nm) materials, in spite of porosity. In ground specimens, very small amount of transformation was found for all grain sizes studied.

Development of a novel zirconia dental post resistant to hydrothermal degradation

Tetragonal Zirconia Polycrystals stabilized with 3% mol. content of yttria (3Y-TZP) has excellent properties in terms of strength and fracture toughness. These properties are mostly imputable to the transformation toughening mechanism, by which the doped metastable tetragonal phase of zirconia transforms to monoclinic under applied stress ahead of a crack. This phenomenon is accompanied by a volume expansion of 5%, and increases the resistance to crack growth, thus leading to higher toughness and strength. An important drawback of this material is represented by the Low Temperature Degradation (LTD or aging), which consists in the progressive tetragonal-to-monoclinic phase transformation by the influence of water. This work focuses on the improvement of 3Y-TZP aging behavior in order to develop a novel dental post, by means of the addition of ceria from the surface. This was achieved through the impregnation of the pre-sintered samples with a solution containing Cerium, followed by sintering. Various pre-sintering temperatures were studied in terms of microstructure, mechanical properties and aging resistance. The novel zirconia dental posts developed in this work are much more resistant to LTD as compared to the base material with no loss in mechanical properties.

Effect of thermal treatments on the wear behaviour of duplex stainless steels

Duplex stainless steel (DSS) is a family of steels characterized by two-phase microstructure with similar percentages of ferrite (α) and austenite (γ).Their attractive combination of mechanical properties and corrosion resistance has increased its use in last decades in the marine and petrochemical industries. Nevertheless, an inappropriate heat treatment can induce the precipitation of secondary phases which affect directly their mechanical properties and corrosion resistance. There are few works dealing with the influence of heat treatments on wear behaviour of these steels in the literature. For instances, this paper aims to determine wear kinetic and sliding wear volume developed as a function of heat treatment conditions. Therefore, the samples were heat treated from 850 °C to 975 °C before sliding wear tests. These wear tests were carried out using ball on disk technique at constant sliding velocity and different sliding distances. Two methodologies were used to calculate the wear volume: weight loss and area measurement using a simplified contact model. Microstructural observations showed the presence of sigma phase for all studied conditions. The formation kinetics of this phase is faster at 875 °C and decrease at higher temperatures. Results related to wear showed that the hardness introduced due to the presence of sigma phase plays an important role on wear behaviour for this steel. It was observed also that wear rates decreased when increasing the percentage of sigma phase on the microstructure.

Sliding Wear Behavior of a Duplex Stainless Steel

Duplex stainless steels contain similar amounts of austenite  and ferrite α. This two-phase microstructure leads to an excellent combination of mechanical properties and corrosion resistance. However, there are few works dealing with the wear behaviour of these steels. This paper aims to determine the sliding wear mechanisms of a duplex stainless steel type 2205. In order to do it, three different sliding velocities (0.2, 0.7 and 1.2 m/s) and six sliding distances (500, 1000, 2000, 3000, 4000 and 5000 m) were selected. The results show that wear rate depends on both sliding velocity and sliding distance. The wear mechanisms detected were plowing, microcracking and microcutting (typical mechanisms of fatigue wear). These mechanisms evolve according to sliding velocity and sliding distance, highlighting a transition zone in which wear rate is reduced.

Surface mechanical properties of advanced zirconia after hydrothermal exposure

The surface mechanical properties of different types of advanced zirconia ceramics have been assessed after being subjected to hydrothermal degradation. Three different types of zirconia were considered: standard tetragonal polycrystalline zirconia doped with 3 % molar yttria (3Y-TZP) produced by conventional sintering; 3Y-TZP produced by spark-plasma sintering (3Y-TZP(SPS)) and a Ce-TZP/Al2O3 nanocomposite. Hydrothermal ageing was assessed by using X-ray diffraction. Berkovich nanoindentation was performed before and after the materials being exposed to 131 °C water vapour in autoclave, in order to assess changes in the surface hardness and Young modulus. It is shown that, while standard 3Y-TZP suffers a substantial decay of the surface mechanical properties with hydrothermal exposure, this is not the case for 3Y-TZP(SPS) and the Ce-TZP/Al2O3 materials. On the contrary, the later maintained their surface integrity during hydrothermal exposure. The results are explained in terms of microstructure and chemical composition.

Improvement of 3Y-TZP hydrothermal degradation resistance by surface modification with ceria without impairing mechanical properties

Surface modification with cerium oxide of tetragonal zirconia polycrystals stabilised with 3% molar yttria (3Y-TZP) has been investigated in order to avoid low temperature degradation. The surface modification was performed by annealing 3Y-TZP with surface coated CeO2 powder at temperatures of 1400 °C and 1500 °C for periods of time up to 10 hours. These heat-treatments diffused cerium up to about 10 μm depth. The bulk fracture toughness, Vickers hardness and Young modulus of the surface modified specimens showed no significant deviation as compared to the non-treated original material. Even the surface mechanical properties measured by penetration curves corresponding to nanoindentations of up to 2 μm depth, did not show significant differences after surface modification. All heat-treatments produced an increase in the surface grain size and a large increase in resistance to degradation.