Antonella Tucci

Use of soda-lime scrap-glass as a fluxing agent in a porcelain stoneware tile mix

Abstract The study was directed towards determining the feasibility of using soda-lime scrap-glass as a fluxing agent in a porcelain stoneware tile mix. Both laboratory and industrial scale tests were carried out. Starting from a reference mix composition, different amounts (5–20 wt.%) of sodium feldspar were replaced with the same amounts of soda-lime scrap-glass. The soda-lime scrap-glass was added in the form of powder, prepared by wet grinding in a centrifugal ball mill. The rheological study of the resulting slips showed that increasing the amount of glass in the mix composition increased the viscosity but decreased the yield stress. Both the laboratory experiments and results of industrial trials showed that the only mix in which the soda-lime glass acted as a good fluxing agent, i.e. lowered the firing temperature, was the mix in which the scrap-glass replaced 10 wt.% of the sodium feldspar. This mix also showed better mechanical characteristics, attributed to enhanced microstructural homogeneity.

Fractal analysis of cracks in alumina–zirconia composites

Abstract The crack paths, induced by Vickers indentation in alumina–zirconia composites, were analyzed using fractal geometry. The fractal dimension n S was calculated for each crack. This parameter refers to a corresponding three-dimensional fracture surface and indicates how its geometry varies by changing the magnification. An interesting correlation between K IC and n S was found: it suggests that the samples with high percentages of alumina and also the pure zirconia are characterized by an intergranular fracture mode, while the composites with high zirconia content present a transgranular fracture mode. This result is confirmed by analyzing the energies of fracture calculated using both the classical and fractal approaches. The results obtained in this research not only made it possible to understand the fracture behavior of the analyzed composites, but also confirmed the good potential of fractal analysis to explain complex mechanisms such as those involved in the fracture of brittle materials.

Microstructural changes produced by abrading Y-TZP in presintered and sintered conditions

OBJECTIVE To evaluate the effect of abrading before and after sintering using alumina-based abrasives on the surface of yttria-tetragonal zirconia polycrystals. Particular attention was paid to the amount of surface stress-assisted phase transformation (tetragonal→monoclinic) and the presence of microcracks. METHODS Pre-sintered zirconia ceramic specimens (ZirCAD; Ivoclar Vivadent) were first surface-ground flat with #600-800-1000-grit SiC paper. They were then surface-treated with different grain size abrasives before and after the sintering step. Samples that underwent no surface treatment were used as controls. For each condition, eight specimens were prepared. The physical/mechanical characteristics of zirconia material were determined by measuring density, porosity, grain size, hardness, and fracture toughness. The effects of surface treatments were assessed by surface roughness measurements, quantitative X-ray diffraction analysis, and scanning electron microscopy. RESULTS With increased dimensions of the abrasive particles, the abraded surfaces of zirconia specimens exhibited a widespread system of microcracks and an increased monoclinic zirconia quantity. These structural changes likely affect the aging of the material during its clinical service.

Effects of temperature and sliding velocity on the dry tribological characteristics of high purity, high density polycrystalline aluminium oxides

Abstract The effects of temperature (up to 600 °C) and sliding velocity on the tribological properties, i.e. volumetric wear rates and friction coefficients, of high density (greater than 97% theoretical density) and high purity alumina (99.7% Al 2 O 3 ) were investigated using a pin-on-disc tribometer. Increases in volumetric wear rates with temperatures were noticed, while the friction coefficient values decrease. The formation of smooth agglomerated compacted layers of water debris is thought to be responsible for the decrease in the friction coefficient. The wear of alumina couples is a complex process which may involve brittle fracture and plastic deformation which are active in different degrees depending on the operating conditions and material properties, e.g. microstructure and composition. The compressive stresses, exerted on the wear debris at the interface between pin and disc during sliding, caused a reduction in the crystallite size, as revealed by the peak broadening of X-ray diffraction patterns.

Second phases and material transfer in alumina ceramics sliding systems

Abstract To explore the effects linked to the presence of second phases in the wear mechanisms and performances of alumina, some low-purity alumina ceramics with an alumina content ranging from 79.0 to 95% were investigated. The tribological behaviour of these materials was evaluated using a pin-on-disc apparatus, coupling them with a reference material, a high-purity alumina, 99.6% alumina content. Extensive SEM observations at normal incidence and on the cross sections of the worn surfaces of specimens showed fairly close relationships among chemical–mineralogical composition, material transfer and friction and wear responses. Particularly interesting are some behaviours directly related to the presence of the transferred layers: (i) adhesion and sticking phenomena, due to the softened low-melting layers at the interfaces and (ii) an increasing wear rate of the high-purity alumina pins as the alumina content of the coupled discs decreases.

Adhesion mechanisms at the interface between Y-TZP and veneering ceramic with and without modifier ☆

OBJECTIVE This study investigated the mechanism of action at the interface between a commercially available Y-TZP and its veneering ceramic after final firing. Particular attention was paid, from a microstructural point of view, to evaluating the effects of different surface treatments carried out on the zirconia. METHODS In total, 32 specimens of presintered zirconia Y-TZP (LavaFrame, 3M ESPE, Germany) were cut with a low-speed diamond blade. The specimens were divided in two major groups, for testing after fracture or after mirror finishing, and were sintered following the manufacturers instructions. Each major group was then randomly divided into four subgroups, according to using or not using the dedicated framework modifier, with or without a preliminary silica coating (CoJet, 3M ESPE). A suitable veneering ceramic was used for each group (Lava Ceram Overlay Porcelain, 3M ESPE). A detailed microstructural study of the interfaces of the zirconia-veneering ceramic was performed using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer to evaluate chemical variation at the interfaces. RESULTS When the framework modifier was not applied on the Y-TZP surface, microdetachments, porosities, and openings in the ceramic layer were observed at the interlayers. A degree of diffusion of different elements through the interfaces from both the zirconia and veneering layers was detected. CONCLUSIONS Application of the framework modifier can increase the wettability of the zirconia surfaces, allowing a continuous contact with the veneering layer. The micro-analysis performed showed the presence of a reaction area at the interface between the different materials. CLINICAL SIGNIFICANCE the increase of the wettability of the zirconia surface could improve the adhesion at interface with the veneering ceramic and reduce the clinical failure as chipping or delamination.

Quantitative Evaluation by Fractal Analysis of Indentation Crack Paths in Si3N4–SiCw Composites

Abstract The crack paths, generated by Vickers indentation in Si 3 N 4 –SiC w composites, were analysed from a quantitative point of view, in order to distinguish between the possible toughening mechanisms involved. It was found that the roughness parameter R L and the fractal dimension D describe the geometrical features of the cracks and evidence the anisotropy of the microstructure, due to the whisker and Si 3 N 4 β -grain orientation. The relationships found between fracture toughness and these geometrical parameters emphasise the importance of the crack deflection mechanism in improving toughness and provide information on the role played by microstructure in determining the mechanical properties of a composite.

Surface Abrasion of Glazed Ceramic Tiles: A New Investigation Technique

In the present study, the resistance to surface abrasion of two commercial glazed ceramic tiles was determined according to the test method specified in the Standard ISO 10545-7 The worn areas of all the specimens were analysed using a non-destructive technique, an optical profilometer. These results, coupled with a morphological analyses carried out with a scanning electron microscope, allowed to understand the mechanisms acted during the material removal.

Surface Deterioration and Scratch Resistance of Polished Porcelain Stoneware Tiles

Three stoneware porcelain commercial products, in as fired and polished conditions, were examined to study the effect of polishing on the surface mechanical properties. The polishing procedure used was that of a current industrial machining process. The surface morphology of the samples was thoroughly analysed and the scratch resistance measured using an automatic scratch tester. The correlation between the surface morphology and the decrease of the surface mechanical characteristics, such as Vickers hardness and scratch resistance, are discussed.

The Effect of Spodumene and Zirconium Based Materials on the Rheology of Slips for Porcelain Stoneware Tile

The aim of the work was the study of the effects of new additives like zirconium oxide, silicate and spodumene on the rheology of a porcelain stoneware slip, in an attempt to understand the mechanisms of slurry stabilisation. A complete rheological characterisation was performed in function of the different additives employed. Introduction In industrial production, porcelain stoneware mix is obtained by wet milling process so that the rheological properties of the slips are very important, especially in regard to the milling, mixing, pumping and spray drying processes [1,2]. Many parameters affect the rheological properties of ceramic slips like the chemical and mineralogical properties of the raw materials, density of the slip, types and contents of deflocculants and additives, pH and ions soluble in water, and particle size distribution. A better understanding of the rheology of porcelain stoneware tile slips is important in terms of being able to control the manufacturing process [3]. The addition of zircon oxide in the mix for porcelain stoneware tile, represents one possible way to further improve the mechanical properties of this material which is characterised by very high density (water absorption lower than 0.5%), while the zircon silicate improves the whiteness of the final product which is particularly appreciated. Spodumene is a fluxing agent which reduces the firing temperature and can be successful used to stabilise the suspension and to reduce the high firing temperature necessary to obtain the characteristic compact structure of porcelain stoneware. The presence of new additives in the slip can change the rheological properties of the slip. So that when a new additive is added in a slip, it is important firstly to study as the rheological parameters are changed. In general the ceramic slurries are non-Newtonian systems and its rheological properties change by changing the shear conditions, so that it is important to evaluated the viscosity at different shear conditions. These systems are rheologically complex as they change during the time, in particular they exhibit a time-dependent behaviour which can be reversible (tixotropy) or irreversible (viscoelasticity) [4]. An other aspect of the time dependent behaviour is the presence of yield stress as these systems need an initial stress to move. These phenomena of time-dependency are all due to the presence of a colloidal structure. It is also clear that the changes in the composition of a such complex system as a ceramic slurry needs a very carefully study of the rheological parameters to avoid a drastically change in all the manufacturing process. The rheological study was also performed by a rotational rheometer which guarantees to evaluate the flow curve at different shear conditions and in different modes: in control rate mode to characterise the shear behaviour and the time behaviour and in control stress mode, to determine the yield stress. Material and Methods Four different formulations were considered: a standard mix (sand 28wt%, fluxes 37.5wt% and kaolinitic clays 34.5wt%), labelled as “Std”, and three modified mixes, containing i) zirconia TZKey Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 1531-1534 doi:10.4028/www.scientific.net/KEM.264-268.1531