Processing and properties of tape-cast alumina/zirconia laminates composites

In the present work, laminar ceramic structures formed by layers of alumina and partially stabilized zirconia were fabricated by water-based tape casting. Rheological, physical and mechanical properties of slurries and laminates were evaluated. The laminates consisted of stacked alumina and zirconia green tapes produced by thermopressing. Pyrolysis was carried out at 450 °C and sintering at 1500 °C. The alumina/zirconia laminates were studied for a better understanding of the formation behavior and crack propagation at the laminate interface. The flexural strength values of laminates depend on the stress state on their surface. The laminates with the highest amount of zirconia layers presented low strength values (6.7 MPa), while the laminates with more alumina layers had a higher strength level (57.7 MPa). This is because these laminates have alumina layers on the surface which are in a state of residual compressive stress.     

Laminated alumina/zirconia ceramic composites prepared by electrophoretic deposition

The electrophoretic deposition of alumina and zirconia powders from isopropanolic suspension in the presence of monochloroacetic acid was studied in the constant-current regime. The different levels of electric current during deposition from 250 μA to 48 mA were used. The green density of the deposit depends on the current density and then on the particle velocity during deposition, reaching values from 58% to 61% according to the electric current used. It was found that the lower the green density of the green deposit, the larger the pores. The low green density led to low final fired density and subsequently to the low Vickers hardness HV5 ranging from 2000 to 1650 depending on electric current used. Based on these findings microlaminates having various thickness ratios to achieve different residual stress levels were prepared consisting of alternating layers of alumina and zirconia.     

Nacre-like alumina composites reinforced by zirconia particles

Nacre-like alumina is a class of bio-inspired ceramic composite manufactured by field-assisted sintering of green bodies made primarily of alumina platelets with an anisotropic microstructure. Here we investigate the addition of zirconia particles to enhance the mechanical properties of the composite. The resulting structure is a nacre-like anisotropic structure which features deflection and reinforcement during crack propagation. Monoclinic zirconia has no impact on the mechanical properties of the composite while tetragonal zirconia improves its fracture resistance properties. Both types of zirconia seem to slow down grain growth during sintering. The addition of zirconia stabilised in the tetragonal phase is thus a good option to obtain a composite with a fine microstructure and higher mechanical properties than a standard nacre-like alumina, with a flexural strength of 626 ± 39 MPa and a crack initiation toughness of 6.1 ± 0.6 MPa.m^0.5.     

Stabilization of zirconia lamellae in rapidly solidified alumina–zirconia eutectic composites

In-situ fabrication of ceramic eutectic composites by rapid solidification of eutectic drops is a cheap and quick method compared to directional solidification or to multi-step fabrication methods of fiber reinforced/layered materials for high temperature use. Binary eutectic composites with a homogeneous periodic microstructure have been obtained by directional solidification of eutectic melts for many years, but typical solidification velocities used in directional solidification are limited to the range of cm/hour or, more recently, up to 15 mm/min. The present study aims to determine the effects of faster solidification rates on the structure of the alumina–zirconia binary composites obtained at higher growth rates by rapid solidification from eutectic melts in air or vacuum. A binary composite with zirconia stabilized in the high-temperature tetragonal form is presented. The stabilization of the tetragonal phase has not been observed before in bulk crystalline pellets of binary Al₂O₃–ZrO₂ eutectic composites.     

Toughening of zirconia/alumina composites by the addition of graphene platelets

A study on graphene platelet/zirconia-toughened alumina (GPL/ZTA) composites was carried out to evaluate the potential of the new structural materials. GPL–ZrO₂–Al₂O₃ powders were obtained by ball milling of graphene platelets and alumina powders using yttria stabilized ZrO₂ balls. Samples were sintered at different temperatures using spark plasma sintering. Fracture toughness was determined by the single-edge notched beam method. The results show that the GPLs are uniformly distributed in the ceramic matrix and have survived high temperature sintering processes. Several sintering experiments were carried out. It is found that at 1550 °C, GPL/ZTA composites were obtained with nearly full density, maximum hardness and fracture toughness. A 40% increase in fracture toughness in the ZTA composite has been achieved by adding graphene platelets. The toughening mechanisms, such as pull out, bridging and crack deflection, were observed and are discussed.     

Effect of the volume ratio of zirconia and alumina on the mechanical properties of fibrous zirconia/alumina bi-phase composites prepared by co-extrusion

Fibrous zirconia/alumina composites with different composition were fabricated by piston co-extrusion. After a 3rd extrusion step and sintering at 1600 °C, crack-free composites with a fibre width of 50 μm were obtained for all compositions. The effect of the volume ratio of secondary phase on the mechanical properties was investigated. The Young's modulus of the composites decreased linearly with increasing the zirconia content. The fracture toughness of the composites was improved by introducing fine second phase filaments into the matrix. The maximum fracture toughness of 6.2 MPa m^1/2 was attained in the 3rd co-extruded 47/53 vol% zirconia/alumina composite. The improvement in toughness was attributed to both “stress-induced” transformation of zirconia and a crack deflection mechanism due to thermal expansion mismatch between the two phases. Bending strength of the composites was almost the same as that of the monolithic alumina regardless of the composition.     

Synthesis of zirconia–alumina and alumina–zirconia core–shell particles via a heterocoagulation mechanism

This study describes the synthesis of core–shell particles, consisting of a ZrO₂ or Al₂O₃ submicron nucleus coated by a nanolayer of Al₂O₃ or ZrO₂, respectively. The oxide layers around the cores are deposited via a heterocoagulation route, based on the attraction of oppositely charged core and shell particles.TEM micrographs clearly show a homogeneous Al₂O₃ shell (originating from boehmite or γ-Al₂O₃ particles) around the ZrO₂ cores and in the other case, a ZrO₂ layer (originating from hydrothermally prepared ZrO₂) around the submicron Al₂O₃ cores. From PCS measurements, it can also be deduced that the cores are enwrapped by a shell and it is calculated that the thickness of the Al₂O₃ shell is about 30–35 nm and the ZrO₂ layer is approximately 80 nm. The coated powders are additionally characterized by XRD.     

Dry sliding wear of zirconia-toughened alumina with different metal oxide additives

Tribological behaviour of zirconia-toughened alumina (ZTA) with different metal oxide additives was studied. Solid oxide lubricants (MO_x; M = Cu, Ti, Mg, Zn, Mn) were added in small quantities (∼8–11 wt%) to provide a self-lubrication action and thereby to decrease the coefficient of dry friction. Dry mixed ceramic composites powders were compacted and pressureless sintered at different temperatures. Densification studies show that near full densities (>97%) were obtained for ZTA ceramic containing both TiO₂–CuO and TiO₂–MnO₂ at 1450 °C. Phase and qualitative compositional analysis was done using X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). Tribological behaviour of the various ZTA composites was tested by dry sliding the different specimens against SiC abrasive grit paper at a load of 50 N. The lowest specific wear rate of 9.2 × 10⁻⁵ mm³/N m and coefficient of friction of 0.35 were observed for the ZTA ceramic composite with TiO₂–MnO₂. The basic wear mechanisms were abrasion and grain pullout as corroborated from scanning electron microscopy (SEM) images.     

Processing of alumina-coated tetragonal zirconia materials and their response to sliding wear

Alumina-coated tetragonal zirconia stabilised with 3 mol% of Y₂O₃ (YTZP) specimens (30 mm × 30 mm × 6 mm) have been obtained by dipping of pre-sintered YTZP compacts in alumina suspensions and subsequent sintering. The coated specimens present hardness values and a wear resistance similar to those of reference dense alumina specimens and significantly higher than those of the YTZP substrates.     

Continuous alumina fiber-reinforced yttria-stabilized zirconia composites with high density and toughness

Continuous alumina fiber-reinforced yttria-stabilized zirconia (YSZ) composites with a LaPO₄ fiber coating were fabricated by slurry infiltration and spark plasma sintering (SPS). The LaPO₄ coating was deposited on the reinforcement alumina fabrics by a modified sol-gel method. The YSZ slurry with good dispersion and stability was prepared by optimizing the pH value, dispersant addition and ball milling time. The fabricated composite with a high density of ∼ 92 % has a good flexural strength of 277 ± 43 MPa, and a superior fracture toughness of 15.93 ± 0.75 MPa·m^1/2 exhibiting a non-brittle failure behavior. It was found that the LaPO₄ coating reduced the residual stress near the fiber/matrix interface to 131 ± 41 MPa, which was 369 ± 63 MPa in the composite without the fiber coating. The LaPO₄ coating renders a weak interphase to improve the composite toughness by activating several toughening mechanisms including crack deflection, fiber debonding and pullout, and delamination behavior.     

Fracture of composite alumina/yttria-stabilized zirconia joints

Four-point bend tests have been performed on samples consisting of yttria-stabilized zirconia containing 0–80% alumina joined by plastic deformation to the same or different composition. The fracture strength of joints between the same composition was equal to the strength of the monolithic material. Fracture of joints made between different compositions occurred at the position of maximum tensile residual stress, as determined by finite-element analysis, not at the interface. Measured strengths were in accord with fracture mechanics and the calculated residual stresses.     

Creep mechanisms of laminated alumina/zirconia-toughened alumina composites

High-temperature plastic deformation of laminar composites containing alternate layers of Al₂O₃ and a mixture of 60 vol.% Al₂O₃ + 40 vol.% 3 mol% Y₂O₃-stabilized tetragonal ZrO₂ (ZTA) produced by tape casting is investigated in isostrain compression testing at temperatures between 1400 and 1500 °C. The stress exponent n and the creep activation energy Q are close to 1 and 700 kJ/mol, respectively. Microstructual observations reveal the lack of differential features in the ZTA layers and a general creep damage of the Al₂O₃ layers, with little microcracking by cavity coalescence even up to strains of 30%. The layer interfaces maintain their initial structural integrity after testing. An isostrain composite creep model predicts correctly the overall mechanical behavior of the laminates, which is dictated by the alumina phase via diffusional creep controlled by oxygen grain boundary diffusion.     

In situ formation of zirconia–alumina–spinel–mullite ceramic composites

A study was made to prepare and characterize multicomposite containing zirconia–mullite–spinel [ZrO₂–Al₆Si₂O₁₃–xMgAl₂O₄ (0.25 ≤ x ≤ 4)]. This multicomposite was prepared by reaction sintering of relatively pure commercial grades of zircon, alumina and magnesium carbonate. Spinel and mullite could be in situ formed, while ZrO₂ evolved mainly in monoclinic polymorph. The sintering properties in terms of bulk density (BD) and apparent porosity (AP) were measured. The formed phases were identified by X-ray diffraction pattern (XRD). Cold crushing strength (CCS) of the fired and thermally shocked bodies was evaluated. The fractured surfaces of the fired compacts were scanned by SEM. The results showed great dependence on the firing temperature as well as zircon content, type and amount of glassy phase. XRD exhibited that all samples contain m-ZrO₂ and Al₂O₃ and with decreasing the amount of MgO and increasing zircon, the spinel phase decreases while mullite begins to form at high zircon contents. Batches containing mullite, ZrO₂ and Al₂O₃ gave higher mechanical properties. Also, all samples retained more than 90% of their original strength after being subjected to 20 cycles of thermal shock (Δt = 1000 °C).     

Effect of temperature on friction and wear behaviour of CuO–zirconia composites

Results of wear tests using an alumina ball sliding against 5 wt% copper oxide doped tetragonal zirconia polycrystalline (CuO-TZP) ceramics are reported as a function of temperature up to 700 °C. The specific wear rate and friction coefficient are strongly dependent on temperature. Below a critical temperature (T < 600 °C), CuO-TZP showed a high coefficient of friction as well as a high wear rate. This was ascribed to the formation of a rough surface, caused by brittle fracture and abrasive wear, based on observations by scanning electron microscopy (SEM), laser scanning microscopy (LSM) and X-ray photoelectron spectroscopy (XPS). However, above 600 °C a self-healing layer is formed at the interface and results in low friction and wear. The mechanism of layer formation and restoration is discussed and rationalized by onset of plastic deformation caused by a reduction reaction of CuO to Cu₂O at high temperatures.     

Electrical transport properties in zirconia/alumina functionally graded materials

Functionally graded materials (FGMs) are promising candidates for the fabrication of technological components, not only as structural devices, but also in electrochemical ones, such as solid oxide fuel cells (SOFC), or high-efficiency hybrid direct energy conversion systems. In the present work FGMs were prepared by the sequential slip casting technique, starting with an yttria tetragonal zirconia polycrystalline layer and increasing subsequently the amount of Al₂O₃ in the following layers. Electrochemical impedance spectroscopy (EIS) analysis was used to evaluate the electrical characteristics of these materials and to compare with those of the monolithic compacts. In general, it was observed that the FGM conductivity is ruled by the conductivity of the layer which contains the highest amount of alumina blocking particles. By EIS no electrical interfaces between adjoining layers were detected and, accordingly, no specific electric ohmic losses were observed. The conductivity of the FGMs is close to that calculated using the normalized thicknesses and the alumina volume fractions of the layers after measuring the conductivity of the monolithic materials with the same composition to what correspond to that of the final layer in the FGM. These results suggest that the gradient structure can be used to control the oxygen vacancy motion, and then applied in electrochemical devices.     

Effects of Ca-, Mg- and Si-doping on microstructures of alumina–zirconia composites

The aim of this work was to obtain zirconia toughened alumina composites with different microstructures, using a simple process (powder mixing and natural sintering). Adjusting the amount of zirconia directly controls the size and localization of zirconia grains and the size of the alumina grains. Doping the composite with CaO, MgO and SiO₂ allows further control of the microstructures. The influence of the thermal treatments is also investigated. The composites exhibit different structures (nano/nano-, micro/nano- and micro-composites) with zirconia and alumina grains as small as 100 and 200 nm, respectively, and with the proportion of intragranular zirconia grains varying between 0% and 90%. Zirconia plays a major role on grain size distributions as compared to CaO and MgO, whose role is almost negligible. The use of SiO₂ leads to micro/nano composites with intragranular zirconia particles. The influence of these different additions is related to adjustments of the grain boundaries mobility.     

Effect of zirconia on ablation mechanism of asbestos fiber/phenolic composites in oxyacetylene torch environment

Micron-size zirconium oxide (ZrO₂) was used to improve the thermal stability and ablation properties of asbestos fiber/phenolic composites and to reduce their final cost. ZrO₂/asbestos/phenolic composites were prepared in an autoclave by the curing cycle process. The densities of the composites were in the range of 1.64–1.82 g/cm³. The ablation properties of composites were determined by oxyacetylene torch environment and burn-through time, erosion rates and back surface temperature in the first required 20 s. To understand the ablation mechanism, the morphology and phase composition of the composites were studied by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Thermal stability of the produced materials was estimated by means of thermal gravimetric analysis, in air which consisted of dynamic scans at a heating rate of 10 °C/min from 30 to 1000 °C with bulk samples of about 23±2 mg. The thermal stability of the composites was enhanced by adding ZrO₂. The results showed that the linear and mass ablation rates of the composites after adding 14 wt% ZrO₂ decreased by 58% and 92%, respectively. The back surface temperature of a sample with 14% zirconia was 49% lower than that of pure composite. The SEM studies showed that, modified composites displayed much lower porosity than that of non-modified composite and the destruction of asbestos fibers was very low. On the other hand, it appeared that a thin melted layer of ZrO₂ covered the surfaces of zirconia-containing composites.     

Electroconductive composite of zirconia and hybrid graphene/alumina nanofibers

Novel type of hybrid nanofillers representing graphene encapsulated alumina nanofibres was selected as an additive to develop toughened electroconductive partially stabilized zirconia. The sinterability, mechanical and electrical properties of the produced nanocomposites were studied as function of the filler/graphene content. Composites containing just 0.6 vol.% of graphene corresponding to 3 vol.% of hybrid nanofibres exhibited high electroconductivity of 58 S/m without deterioration of mechanical properties. They also showed a slight toughening effect that is reflected by an increase in the indentation fracture toughness by 20% as compared to monolithic zirconia.     

Tape casting of alumina/zirconia suspensions containing graphene oxide

The introduction of carbon derivatives (nanotubes, graphene, etc.) as a second phase in ceramic matrices has limitations arising from their difficult processing. This paper studies the colloidal stability and the rheological behaviour of concentrated suspensions of alumina with 5 vol.% Y-TZP (AZ) and the effect of the addition of 2 vol.% of graphene oxide (AZGO) on the suspension stability, rheological behaviour and tape casting performance. The colloidal stability was studied using zeta potential measurements in terms of concentration of deflocculants and pH and homogenisation was optimised adjusting the sonication mode and time. The best results were obtained for pulsed mode. The optimum rheological properties were obtained for solid loadings of 53 vol.% and 40 vol.% for AZ and AZGO. Homogeneous, flexible tapes with thickness of ∼120 μm were obtained reaching densities of >60% of theoretical density in which secondary phases are well dispersed.     

Chemical treatment on alumina–zirconia composites inducing apatite formation with maintained mechanical properties

Alumina–zirconia composites with submicrometric grain size were surface modified with the purpose to induce bioactivity using several chemical treatments. Among them, a quick attack by phosphoric acid induced on Zirconia Toughned Alumina (80–20 wt%) the formation of apatite-like calcium phosphate phases after immersion in simulated body fluid, indicating bioactivity induction. Such a treatment does not reduce the strength, hardness and ageing properties of this ceramic material, making it a suitable method for biomedical applications. Surface properties, topography and microstructure of oxide ceramics are also discussed.