E. Jiménez-Piqué

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.

Scale dependence of the Young's modulus measured by nanoindentation in columnar YSZ EB-PVD thermal barriers coatings

Electron beam-physical vapour deposited (EB-PVD) yttria-stabilized zirconia (YSZ) thermal barrier coatings have been investigated by nanoindentation. It is shown that the mechanical properties, and particularly the Youngs modulus, of these coatings are closely related to their columnar microstructure. The transition from intracolumnar to intercolumnar deformation is characterized by a difference between the mechanical properties of the bulk YSZ and those of the coatings. The use of different indenter shape showed that the contact area was the parameter characteristic of this scale dependence. Deformation mechanisms studied by AFM have shown a good correlation with the evolution of the mechanical properties during the deformation.

Tomography of indentation cracks in feldspathic dental porcelain on zirconia

OBJECTIVES The objective of this work is to study the crack produced by spherical and sharp indentation on veneering feldspathic dental porcelain in order to understand the morphology of the cracks in the surface and beneath the indentation using a tomographic technique. The geometry of cracks produced under contact loading are directly related to the structural integrity and reliability of dental prosthesis. METHODS Monotonic Hertzian contact loading and nanoindentation tests were performed on feldspathic porcelain (VITA-VM9) coatings. Residual imprints and the cracks produced by the indentations were characterized by 3-dimensional reconstruction using focused ion beam tomography. RESULTS Under nanoindentation, the propagating crack deflects due to the interaction with the leucite particles resulting in a crack with a complex morphology. Under spherical contact loading, multiple ring cracks were observed at the surface, with a conical shape beneath the residual imprint. SIGNIFICANCE These results will help to improve the mechanical performance of these materials by detecting potential causes of failure for the long term structural integrity and reliability of the prosthesis.

A parametric study of laser interference surface patterning of dental zirconia: Effects of laser parameters on topography and surface quality.

OBJECTIVE The aim of this work is to generate micrometric linear patterns with different topography on dental grade zirconia by means of UV laser interference and to assess the quality of the produced surface, both in term of the geometry produced and of the surface damage induced in the material. METHODS The third harmonic of a Q-switched Nd:YAG laser (355nm, pulse duration of 10ns and repetition rate of 1Hz) was employed to pattern the surface of 3Y-TZP with micrometric-spaced lines. The resulting topography was characterized with White Light Interferometry and Scanning electron microscopy: pattern depth (H), amplitude roughness parameters (Sa, filtered-Sa), Fourier spatial analysis and collateral damages were related to laser fluence and number of pulses employed. RESULTS With our experimental setup, line-patterning of zirconia surfaces can be achieved with periodicities comprised within 5 and 15μm. Tuning laser parameters allows varying independently pattern depth, overall roughness and surface finish. Increasing both fluence and number of pulses allows producing deeper patterns (maximum achievable depth of 1μm). However, increasing the number of pulses has a detrimental effect on the quality of the produced lines. Surface damage (intergranular cracking, open porosity and nano-droplets formation) can be generated, depending on laser parameters. SIGNIFICANCE This work provides a parametric analysis of surface patterning by laser interference on 3Y-TZP. Best conditions in terms of quality of the produced pattern and minimum material damage are obtained for low number of pulses with high laser fluence. With the employed method we can produce zirconia materials with controlled topography that are expected to enhance biological response and mechanical performance of dental components.

Instrumented indentation on alumina–alumina/zirconia multilayered composites with residual stresses

The instrumented indentation response of polycrystalline alumina and alumina-based ceramic multilayers with well-known residual stresses is investigated. Youngs modulus and hardness were evaluated as a function of penetration depth with Berkovich and spherical indenters by continuous stiffness measurements. A size effect was observed due to the length scale of the microstructure. Values of hardness and Youngs modulus were in good agreement with previous works. No significant differences in the P–h curves were observed for materials with and without residual stress, making it difficult to employ the various methodologies in the literature for estimation of residual stress or crack length evaluation. Crack length had, therefore, to be measured by direct observation. Thus, cube corner indentations did not produce well-developed radial cracks but chipping of the material, whereas indentations with Berkovich indenters produced radial cracks suitable for apparent fracture toughness estimation and, consequently, residual stress.

Corrosion damage in WC–Co cemented carbides: residual strength assessment and 3D FIB-FESEM tomography characterisation

Abstract The effect of corrosion damage on cemented carbides was investigated. The study included residual strength assessment and detailed fractographic inspection of corroded specimens as well as detailed 3D FIB-FESEM tomography characterisation. Experimental results point out a strong strength decrease associated with localised corrosion damage, i.e. corrosion pits acting as stress raisers, concentrated in the binder phase. These pits exhibit a variable and partial interconnectivity, as a function of depth from the surface, and are the result of heterogeneous dissolution of the metallic phase, specifically at the corrosion front. However, as corrosion advances the ratio between pit depth and thickness of damaged layer decreases. Thus, stress concentration effect ascribed to corrosion pits gets geometrically lessened, damage becomes effectively homogenised and relatively changes in residual strength as exposure time gets longer are found to be less pronounced.

Contact fatigue of veneer feldspathic porcelain on dental zirconia

OBJECTIVES To understand the resistance to cyclic and static contact loading of feldspathic porcelain on dental zirconia (3Y-TZP), in order to understand the partial failure of porcelain (chipping or cracking). METHODS Hertzian contact techniques were used to evaluate the appearance of damage as a ring crack in terms of applied load and number of cycles in air and simulated saliva. RESULTS Static contact loading showed the presence of stress corrosion cracking in the porcelain; the environmental crack growth in air was determined from the results of time to damage under static load. There was also a contribution of fatigue effects due to the interactions of the crack with the microstructure. From the obtained results, a time to failure was estimated depending on the material counterpart. Cracking can occur on porcelain coatings if the contact counterpart is teeth or porcelain in a time interval of a few years, consistent with clinical studies. SIGNIFICANCE Contact loading, particularly against teeth or other ceramic materials can be a significant cause of failure and chipping of feldspathic porcelain on zirconia, especially if the patient suffers from bruxism. Protection, by e.g. a guard, against repetitive contact against the porcelain can increase the lifetime of the veneer.

Resistance to Contact Deformation and Damage of Hard Ceramics

In this chapter, Hertzian (spherical) indentation as applied to hard ceramics, with focus on the damage produced under indentation and its consequences for strength are discussed. The typical experimental setup and the related contact mechanics relationships are explained. The different types of damage (ring cracking, cone cracking, and quasiplastic damage) and the influence of the different material parameters on the induced damage are described. Cyclic fatigue and influence of indenter materials are also discussed, and several examples reported in the literature on Al 2 O 3 , SiC, and Si 3 N 4 are described.

Micro-Structural Characterization and Stress-Corrosion Cracking Behavior of a FGM Glass-Based Coating on Ti6Al4V for Biomedical Applications

Microstructural and stress-corrosion cracking characterization of two glass-based coatings on Ti6Al4V with different SiO2 content (61% and 64% of SiO2) have been investigated in this work. These coatings belong to the SiO2-CaO-MgO-Na2O-K2O-P2O5 system and were obtained using a simple enamelling technique. They will be used as the first layer of the bioactive FGM coating. Microstructural characterization performed in the coatings by SEM shows the separation of the sintered glass particles after acid etching. The XRD integration method shows that the percentage of the crystalline phase (2.4CaO•0.6Na2O•P2O5) due to the partial devitrification is between 3-16 % vol. Mechanical characterization was made using Vickers and Hertzian indentation. Both coatings were sensitive to Vickers indentation subcritical crack growth with longer crack lengths for the smaller SiO2 content. This coating was also more sensitive to stress-corrosion “ring” cracking by Hertzian indentation. These two results are related with the larger residual stresses due to the thermal expansion mismatch.

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 (λ = 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.