Fernando García Marro

Phase transformation and subsurface damage in 3Y-TZP after sandblasting

OBJECTIVE The goal of this work is to investigate t-m phase transformation, and subsurface damage in 3Y-TZP after sandblasting. METHODS Commercial grade 3Y-TZP powder was conventionally sintered and fully dense specimens were obtained. Specimens were sandblasted using different particle sizes (110 and 250μm) and pressures (2 and 4bar) for 10s. Phase transformation was measured on the surface and in the cross-section using X-ray diffraction and micro Raman spectroscopy, respectively. Subsurface damage was investigated on cross-sections using SEM and in shallow cross-sections machined by focused ion beam. RESULTS Sandblasting induced monoclinic volume fraction is in the range of 12-15% on the surface. In the cross-section, a non-homogeneous phase transformation gradient is found up to the depth of 12±1μm. The subsurface damage observed was plastic deformation in grains with the presence of martensite plates, and this effect is found to be larger in specimens sandblasted with large particles. SIGNIFICANCE The extent of subsurface tetragonal-monoclinic transformation and damage induced by sandblasting are reported for different sandblasting conditions. This knowledge is critical in order to understand the effect of sandblasting on mechanical properties of zirconia used to fabricate dental crowns and frameworks.

Effect of sandblasting and residual stress on strength of zirconia for restorative dentistry applications.

Sandblasting is a commonly used surface treatment technique for dental crowns to improve the adhesion of the mating parts of a restoration. The goal of this work is to study the effect of different sandblasting conditions on the mechanical properties of 3mol% yttria stabilized tetragonal zirconia (3Y-TZP), such as biaxial strength, surface elastic modulus, contact hardness and residual stresses induced by sandblasting. The specimens were sandblasted considering two different particle sizes (110, 250μm), two pressures (2 and 4bar) and two impact angles (30° and 90°). Biaxial strength of 3Y-TZP increases when sandblasted with 110µm particles while its decreases with 250µm particles for impact angle of 90°. Strength increases slightly when sandblasting with 30° impact angle regardless of the size of the particle. Elastic modulus and contact hardness were not affected by sandblasting with 110µm particles, and compressive residual stresses are produced down to a depth of ~10µm.

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.

Degradation resistance of 3Y-TZP ceramics sintered using spark plasma sintering

Commercially available tetragonal zirconia powder doped with 3 mol% of yttria has been sintered using spark plasma sintering (SPS) and has been investigated for its resistance to hydrothermal degradation. Samples were sintered at 1100, 1150, 1175 and 1600 °C at constant pressure of 100 MPa and soaking for 5 minutes, and the grain sizes obtained were 65, 90, 120 and 800 nm, respectively. Samples sintered conventionally with a grain size of 300 nm were also compared with samples sintered using SPS. Finely polished samples were subjected to artificial degradation at 131 °C for 60 hours in vapour in auto clave under a pressure of 2 bars. The XRD studies show no phase transformation in samples with low density and small grain size ( 300 nm). Results are discussed in terms of present theories of hydrothermal degradation.

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.