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Item type:Publication, Access status: without files , Using polystyrene spheres to produce thin, porous interlayers in alumina laminatesPorous alumina layers were produced by colloidal processing of alumina with the addition of 15, 30, and 45 vol% polystyrene spheres (PS) as pore formers. Alumina laminates were designed with dense layers alternated with porous interlayers using 45 vol% of PS spheres and sintered at 1350°C. The layers’ thickness ranged from 2 to 15 μm, with a random distribution of pores. The higher volume fraction of pores tends to decrease the alumina average grain size, but does not influence the final size of bulk and surface pores. The obtained values of hardness and Young's modulus for the porous interlayer are ∼30% of the values obtained for the dense layer. Vickers indentations suggested that crack propagation can be opposed by the porous interlayers. However, values of mechanical strength, fracture toughness (KIC), and work of fracture presented no relevant difference compared to a monolithic reference. R-curves presented a slight increase and KIC a decrease due to crack propagation through the porous interlayers. Although no macrodeviations of the crack path were observed in the fractured surfaces, microdeviations were detected in the interlayer regions.Publicationtype: Journal Article58 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Dopant diffusion at the interface of TiO₂-MnO-doped alumina/alumina layers in sintered laminates(2021-05); ; Ceramic laminates were produced by thermal pressing of pure alumina (Al2O3) and TiO2-MnO-doped alumina (d-Al2O3) tapes and subsequently sintered at temperatures ranging from 1100 to 1250°C. The diffusion of dopants through the interface was investigated by optical (OM), scanning electron (SEM) microscopy, nanoindentation, and energy-dispersive X-ray spectroscopy (EDS) together with scanning transmission electron microscopy. Temperature-dependent dopant diffusion and porosity profiles were, respectively, observed investigating the microstructure. For d-Al2O3, the average grain size, hardness, and Young's modulus were higher when compared to pure alumina, while the dopants diffusion zone presented intermediate values of these properties. The increase in sintering temperature resulted in higher values of hardness and Young's modulus in both layers. A further investigation showed a gradual decrease in grain size, hardness, and Young's modulus from the doped to the pure alumina layer, corresponding to a decreasing gradient in the dopants concentration in the Al2O3 layer with increasing distance from the interface. High concentrations of Mn and Ti were observed at triple points and grain boundaries, as well as within alumina grains.Publicationtype: Journal Article4 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Colloidal processing and characterization of TiO2–MnO-doped alumina/alumina slurries and tapesAqueous slurries of alumina doped with manganese and titanium oxides were developed. The powders and tapes were investigated by scanning electron (SEM) microscopy, X-ray diffractometry (XRD), and thermal analyses (TG/DSC). The stability and rheology of colloidal suspensions of undoped and doped alumina suspensions were investigated. At neutral pH of 7, pure alumina and MnO–TiO2 dopant particles present positive and negative surface charges and are prone to heterocoagulation. Optimized colloidal processing through proper adjustment of parameters was achieved to reach particle stabilization. The surface charge of the oxide particles was negative, and the suspensions did not agglomerate at a pH of 7 and higher. Tapes were cast, dried, and burned out. The thermal analyses revealed the release of residual water and decomposition of short-chain organics at temperatures under 215°C and of long-chain additives over 300°C.Publicationtype: Journal ArticleScopus© Citations 1 8 27 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Microstructure and mechanical behavior of TiO₂-MnO-doped alumina/alumina laminatesTapes of TiO2-MnO-doped alumina (d-Al2O3) and pure alumina (Al2O3) were shaped via tape casting. Laminates with three different layer numbers and respective thicknesses were produced and sintered at 1200°C. The microstructure and mechanical behavior of laminates were investigated and compared to the respective monolithic references (d-Al2O3 and Al2O3). The use of dopants in alumina decreased the initial sintering temperature, leading to higher densification at 1200°C (~98% theoretical density (TD)) when compared to Al2O3 (~73% TD). The higher density was reflected in a higher Young's modulus and hardness for doped alumina. A region of diffusion of dopants in pure alumina layers was observed along the interface with doped layers. The mechanical strength of d-Al2O3 samples sintered at 1200°C was not statistically different from Al2O3 samples sintered at 1350°C. The strength of laminates composed of doped layers with undoped, porous interlayers did not change. Nevertheless, as the thickness of these porous interlayers increases, a loss of strength was observed. Monolithic references showed constant values of fracture toughness (KIC), ~2 MPa·m1/2, and linear crack path. On the other hand, KIC of laminates increases when the crack propagates from weak Al2O3 layers to dense d-Al2O3 layers.Publicationtype: Journal ArticleScopus© Citations 5 195 294 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , R-curve behavior and flexural strength of zirconia-toughened alumina and partially stabilized zirconia composite laminates(2018-08-15); ; ; ; A composite-laminate formed by thick layers ( 320 µm) of zirconia-toughened alumina (ZTA) with thin ( 50 µm) interlayers of zirconia partially stabilized (Y-PSZ) has been fabricated by tape casting and pressureless sintering. Fracture behavior and strength has been investigated and compared to a “monolithic” reference, e.g. a stack of zirconia-toughened alumina (ZTA) without interlayers. The fracture behavior has been analysed using stable crack growth in V-notched specimens loaded in 3-point bending. The ZTA+Y-PSZ composite laminate presented a rising crack resistance with maximum values between 6 and 14 MPa m1/2. In contrast, the “monolithic” ZTA laminate shows a plateau R-curve behavior at 2.7 MPa m1/2. Several toughening mechanisms were identified in the ZTA+Y-PSZ composite laminate, such as, crack arrest/slow down, micro cracking and bifurcation. These toughening mechanisms are most likely caused by high tensile residual stresses that were estimated theoretically.Publicationtype: Journal Article153 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Mechanical behaviour of zirconia-toughened alumina laminates with or without Y-PSZ intermediate layersZirconia-toughened alumina (ZTA) laminates with 5 vol% (95A) and 30 vol% (70A) of yttria-partially stabilized zirconia (Y-PSZ), as well as laminated composites with thick ZTA layers and thin Y-PSZ layers were developed and processed by means of tape casting followed by co-firing. The addition of 5 vol% Y-PSZ led to grain refinement, higher densification and an increase in mechanical properties compared to those of pure alumina. In contrast, the addition of 30 vol% Y-PSZ refined the alumina grains and formed clusters of zirconia. However, it promoted lower densification when compared to pure alumina and 95A. Nevertheless, mechanical strength increased in the 70A composites owing to the zirconia toughening mechanism. Laminated composites with intermediate Y-PSZ layers have shown thermal residual stresses after sintering as a result of the different coefficients of thermal expansion (CTE) of the components and higher performance in mechanical behaviour owing to compressive stresses in ZTA layers and to the zirconia toughening mechanism present in Y-PSZ thin layers and in ZTA with 30 vol% thick layers.Publicationtype: Journal Article185