Surface roughened zirconia: towards hydrothermal stability

Surface roughness is needed in several yttria-stabilized zirconia components used in restorative dentistry for osseointegration or adhesion purposes. This can be achieved by different treatments, which may also modify the microstructure of the surface. Among them, sandblasting and chemical etching are widely used, but their effect on hydrothermal aging of zirconia is not fully understood. In the present work, the zirconia long-term stability of rough surfaces prepared by these techniques is analyzed and a method is proposed for preventing hydrothermal aging while maintaining the original surface appearance and mechanical properties. The method involves pressure infiltration of a Cerium salt solution on the roughened surfaces followed by a thermal treatment. The solution, trapped by surface defects and small pores, is decomposed during thermal treatment into Cerium oxide, which is diffused at high temperature, obtaining Ce codoping in the near-surface region. In addition, the microstructural changes induced in the near-surface by sandblasting or chemical etching are removed by the thermal treatment together with surface defects. No color modification was observed and the final roughness parameters were in the range of existing implants of proved good osseointegration. The aging resistance of Ce co-doped materials was strongly enhanced, showing the absence of aging after artificial degradation, increasing in this way the surface mechanical integrity. The proposed treatment is easily applicable to the current manufacturing procedures of zirconia dental posts, abutments, crowns and dentures, representing a solution to hydrothermal aging in these and other biomedical applications.Peer ReviewedPostprint (author’s final draft

Hydrofluoric acid etching of dental zirconia. Part 2: effect on flexural strength and ageing behavior

Among the diverse treatments proposed to promote the osseointegration of zirconia dental implants, hydrofluoric acid (HF) etching appears to be a good candidate. However little is known on the effect of this process on the mechanical properties and long-term reliability. In this work, the surface integrity, the flexural strength and the ageing sensitivity of yttria-stabilized zirconia were assessed after etching in HF 40%. Results show that etching induces an increase of monoclinic phase content and a decrease in flexural strength. The strength decrease is limited to 15% for etching times below 60 min, whereas it reaches 29% after 120 min because of the formation of large etching pits. No substantial change in the ageing sensitivity was evidenced. Within the limits of this study, HF 40% etched zirconia appears to be reliable for long-term implantation provided that the etching duration does not exceed 60 mm. (C) 2015 Elsevier Ltd. All rights reserved.Peer ReviewedPostprint (author's final draft

Micro-mechanical testing of afvanced ceramics: tools, procedures and first results

Micro-mechanical testing has lately become a more accessible tool for understanding deformation, strengthening and failure mechanisms at small scales. It has been found that the often considered intrinsic or “intensive” properties of materials, i.e. not size dependent, start to exhibit an extrinsic behaviour if the volume of material tested is reduced down to the level of the micro- or nano-scale. This is true at least for metals, where diverse experimental approaches have shown that the ultimate strength strongly increases in enough small material volumes in the micro-nano range. In ceramics, the small scale testing approach has received much less attention probably because of the absence of dislocation-controlled deformation mechanisms. Even though, it is the only direct method for the study of the mechanical behaviour of ceramics in thin coatings, superficial layers induced by surface degradation processes as in wear, corrosion, etc. Besides, in ceramics with a grain size dependent transformation toughening mechanism, such as zirconia-based ceramics, a clear effect is expected when testing at the micro-scale. In this work the methodology of micro-mechanical testing is presented and is applied to yttria-stabilized zirconia. Advantages and limitations of the technique are discussed and details about the combination of FIB-machining and nanoindentation testing are illustrated. At the same time, first results of the strength in compression of zirconia micropillars are presented and the failure mechanism is discussed.Postprint (published version

Mechanical reliability of dental grade zirconia after laser patterning

The aim of this work is to test the mechanical properties of dental zirconia surfaces patterned with Nd:YAG laser interference (¿¿=¿532¿nm and 10¿ns pulse). The laser treatment produces an alteration of the topography, engraving a periodic striped pattern. Laser-material interaction results mainly in thermal effects producing microcracking, phase transformation and texturization. The role of such microstructural modifications and collateral damage on the integrity and mechanical performances has been assessed. Laser patterned discs of zirconia doped with 3% mol yttria (3Y-TZP) have been tested before and after a thermal treatment to anneal residual stresses and revert phase transformation. Both groups of samples behave in a similar manner, excluding residual stresses and phase transformation from the origin of properties modification. Result show that laser patterning induces a minor decrease in mechanical properties and surface integrity of 3Y-TZP surfaces. The biaxial strength decreases as a consequence of the damage induced by laser patterning. Fractographic observations identify preexisting defects enlarged by local laser interaction as the fracture origins. The Hardness and Young modulus of treated surfaces tested with nanoindentation also decrease slightly after laser treatment and this may be attributed to laser-induced microcracking.Peer ReviewedPreprin

Hydrothermal Ageing of Cracked 3Y-TZP

En la circona dopada con 3% molar de itria, el vapor de agua puede inducir la transformación de la fase tetragonal a monoclínica en la superficie. Dicha transformación se extiende hacia el interior acompañada por la aparición de microfisuras, lo que induce una pérdida de la integridad estructural. Este fenómeno de degradación a baja temperatura (LTD) es una clara desventaja para la utilización del material en atmósferas húmedas. Sin embargo, el efecto del agua sobre grietas de indentación no ha sido estudiado. Este trabajo estudia la influencia de LTD en la resistencia mecánica de dichas grietas. Los resultados muestran sorprendentemente un incremento de la resistencia mecánica en probetas indentadas sometidas a largos tiempos de degradación; al contrario del comportamiento en probetas sin indentar. Para explicar este comportamiento, se ha evaluado también la influencia de las tensiones residuales y el tratamiento de recocido adecuado para eliminarlas. Finalmente, se plantean los mecanismos que explican el aumento de la resistencia mecánica observado. // In zirconia stabilised with 3% molar of yttria, the water vapour can induce the transformation from tetragonal to monoclinic phase at the surface. This transformation propagates to the bulk with the apparition of microcracks; which induce a loss in the structural integrity. This low temperature degradation (LTD) is a clear disadvantage for the application of the material in humid environments. On the other hand, the effect of the water on indentation-induced cracks has not been previously studied. This work studies the change in mechanical strength of indentation cracks submitted to water vapour. The results show how, after long degradation times, there is surprisingly an increase in mechanical strength. This is the opposite behaviour as the one expected in normal specimens which are not indented. To explain the observed behaviour here, the influence of the residual tensions was evaluated included annealing treatments. Finally, the mechanisms explaining the observed increase in strength are discussed.Peer ReviewedPreprin

Surface grain size and texture after annealing ground zirconia

The surface microstructure induced after grinding and annealing 3 mol% Y2O3 stabilized tetragonal zirconia was investigated in a range of annealing temperatures between 1200 and 1600 °C. For annealing temperatures close to 1200 °C, a surface nano-grain size layer was formed, while annealing at 1600 °C induced a very high fraction of surface grains of size and yttrium content higher than the bulk. The use of TEM-EBSD observation along the surface of the specimen ground and annealed at 1200 °C also revealed the existence of a (0 1 0) preferential orientation.Peer ReviewedPostprint (author's final draft

Surface microstructural changes of spark plasma sintered zirconia after grinding and annealing

Spark plasma sintered zirconia (3Y-TZP) specimens have been produced of 140 nm 372 nm and 753 nm grain sizes by sintering at 1250 °C, 1450 °C and 1600 °C, respectively. The sintered zirconia specimens were grinded using a diamond grinding disc with an average diamond particle size of about 60 µm, under a pressure of 0.9 MPa. The influence of grinding and annealing on the grain size has been analysed. It was shown that thermal etching after a ruff grinding of specimens at 1100 °C for one hour induced an irregular surface layer of about a few hundred nanometres in thickness of recrystallized nano-grains, independently of the initial grain size. However, if the ground specimens were exposed to higher temperature, e.g. annealing at 1575 °C for one hour, the nano-grain layer was not observed. The resulted grain size was similar to that achieved by the same heat treatments on carefully polished specimens. Therefore, by appropriate grinding and thermal etching treatments, nanograined surface layer can be obtained which increases the resistance to low temperature degradation.Peer ReviewedPostprint (author's final draft

Enhanced reliability of yttria-stabilized zirconia for dental applications

An increasing number of dental applications based on yttria-stabilized zirconia (3Y-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 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 3Y-TZP and high resistance to hydrothermal aging. This optimal combination was obtained with the 1150 ºC pre-sintering temperature, 50 wt% solution and sintering at 1450 ºC, leading to a superficial CeO2 content of about 3 mol%.Postprint (author’s final draft

Weibull characterization of the flexural strength of hydrothermally degraded 3Y-TZP zirconia

The flexural strength of 3% mol yttria-stabilized tetragonal polycrystalline zirconia (3Y-TZP) with a ~0.30m grain size was studied under 4-point bending test. The material was evaluated both as received and after water vapour exposure for long times of 100hr and 200hr at 131ºC in autoclave. Results were systematically analyzed by Weibull statistics and fractography. It was found that the mean flexural strength and Weibull modulus change from 960MPa and m=10.9 in the as received condition to 916MPa (m=19.5) and 860MPa (m=19.8) for 100 and 200 hours ageing, respectively. This increase in m can be explained by a change in the origin of fracture from natural flaws in the original material to original defects in the degraded surface layer which extend under loading to a depth equal to its thickness. This layer can be clearly appreciated during fractographic analysis after long degradation times. In the present case the thickness of the degraded layer was on average equal to 8.5m after 100 hours of hydrothermal exposure.Peer ReviewedPostprint (published version

Influence of heat treatments on YSZ electrolyte for sofc manufactured by HVSFS

High-velocity suspension flame spraying (HVSFS) is a promising technology to manufacture yttria stabilized zirconia (YSZ) as solid electrolyte for solid oxide fuel cells (SOFC). In this paper isothermal ageing and thermal cycling have been carried out in order to evaluate their microstructural and mechanical stability. X-ray diffraction (XRD), scanning electron microscopy (SEM) and focused ion beam (FIB) techniques have used to analyze the microstructure of YSZ electrolyte before and after ageing at high temperature. Elastic modulus has been monitored by nanoindentation, while scratch tests have been performed in order to estimate the influence of ageing on the interface electrolyte/anode adhesion.Postprint (published version