Coating of CaTiO3 on titanium substrates by hydrothermal reactions using calcium-ethylene diamine tetra acetic acid chelate

Titanium plates were treated in [Ti(O₂)EDTA]^2-– -Ca(EDTA)^2- mixed solutions and/or Ca(EDTA)^2- solutions (where EDTA is ethylene diamine tetra acetic acid) at pH 9–13 and 150–250 °C for 0.5–12 h. The film, about 50 m thick, and consisting of mixtures of CaTiO₃ and TiO₂ was formed in 0.01 M [Ti(O₂)EDTA]^2- – 0.01 M Ca(EDTA)^2- mixed solution at pH 13 and 250 °C for 6 h. The film consisted of large icosahedral and hexagonal particles, of about 10 m diameter, and small aggregated particles, of about 1 m diameter. On the other hand, the film, about 20 m thick, consisted of hexagonal plate-like CaTiO₃ particles, of about 1 m diameter, was formed in 0.01 M Ca(EDTA)^2- solution at pH 13 and 250 °C for 6 h. The thickness of both films increased with time, where the film formation rate in 0.01 M [Ti(O₂)EDTA]^2- – 0.01 M Ca(EDTA)^2- mixed solution was much faster. The CaTiO₃ film formed on the surface of titanium promoted the precipitation of hydroxyapatite on the substrate by the hydrothermal reactions in Ca(EDTA)^2-–PO ₄ ^3- mixed solutions.     

Thermal shock fracture behaviour of ZrO2 based ceramics

Thermal shock fracture behaviour of alumina, mullite, silicon carbide, silicon nitride and various kinds of zirconia based ceramics, such as magnesia partially stabilized zirconia (Mg-PSZ), yttria and ceria doped tetragonal zirconia polycrystals (Y-TZP and Ce-TZP), Y-TZP/Al₂O₃ composites and yttria doped cubic stabilized zirconia (Y-CSZ), was evaluated by the quenching method using water, methyl alcohol and glycerin as quenching media. Thermal shock fracture of all materials seemed to proceed by the thermal stress due to convective heat transfer accompanied by boiling of the solvents under the present experimental conditions. Thermal shock resistance of zirconia based ceramics increased with increasing the fracture strength, but that of Y-TZP and Y-TZP/Al₂O₃ composites was anormalously lower than the predicted value.     

Dispersion stability of Y-TZP/Ce-TZP powder system and slip casting

The zeta potential and apparent viscosity measurements of 3Y-TZP, 12Ce-TZP and 3Y-TZP/12Ce-TZP suspensions, has allowed the slip casting conditions for the preparation of multilayer composites have been examined. The influence of heat treatment on the sintered density, microstructure and crystalline phase of multilayer composites was also studied. The isoelectric point of both 3Y-TZP and 12Ce-TZP suspensions was near pH 8 and that of 3Y-TZP/12Ce-TZP was at pH 8.6. The suspensions exhibited pseudoplastic flow, showing a decrease in viscosity with increasing shear stress. A small (0.3 wt%) addition of an organic deflocculant gave 3Y-TZP and 3Y-TZP/12Ce-TZP suspensions with 15 and 20 vol% solid contents an appropriate fluidity for slip casting, but an additional electrolyte was required to reduce viscosity in 12Ce-TZP. Dense (>98% of theoretical) multilayer composites with grain size of 0.3–2.2 m were obtained after sintering at 1500°C.     

The effect of ceria co-doping on chemical stability and fracture toughness of Y-TZP

The fracture toughness and ageing resistance of yttria, ceria-stabilized tetragonal zirconia polycrystals (Y, Ce-TZP) were evaluated as a function of grain size and ceria content. Very fine grained, fully dense materials could be produced by sinter forging at relatively low temperatures (1150–1200 °C). The ageing resistance in hot water (185 °C) of 2 mol% Y₂O₃-stabilized TZP is strongly enhanced by alloying with ceria. The ceria content necessary to avoid degradation completely, decreases with grain size. The toughness of fully dense Y, Ce-TZP is 7–9 MPa m^1/2 for grain sizes down to 0.2 m. No or very little transformation took place during fracturing and no clear variation with grain size was observed for the toughness at grain sizes up to 0.8 m. Reversible transformation and crack deflection may explain the observed toughness values.     

Fabrication and sinterability in Y2O3-CeO2-ZrO2

CeO₂-stabilized tetragonal zirconia polycrystal (Ce-TZP) containing 1 to 6 mol % YO_1.5 have been fabricated as fine powders by a coprecipitation technique. The microstructure of the as-sintered surface and fracture surface were examined by electron microscopy. CeO₂ dopants reduced the phase transformation temperature from amorphous to tetragonal and stabilized the tetragonal phase at low temperature. The addition of Y₂O₃ to Ce-TZP inhibited the grain growth. The sintered density reached 99% theoretical for short sintering times at 1440 and 1540° C, but decreased slightly to 97 to 98% theoretical for longer sintering times. The decrease in density is attributed to the morphological development of agglomerates, which induce large pores during sintering. The average grain size decreased significantly as the yttrium content increased from 1 to 3 mol %. Specimens aged in water at low temperatures exhibited no phase transformation. This implies fairly good thermal stability in the Y₂O₃-doped Ce-TZP system.     

Calcium phosphates formation on CaTiO<Subscript>3</Subscript> coated titanium

In this study, performance of calcium phosphate formation of CaTiO₃ coating film on Ti in Hanks’ balanced saline solution (HBSS) was investigated. CaTiO₃ thin films with a thickness of 50 nm were deposited on Ti using radiofrequency (RF) magnetron sputtering. The temperature of Ti substrate was adjusted to room temperature (RT) and 873 K. Thereafter, the specimens deposited at RT were annealed at 873 K in air for 7.2 ks. The films were characterized by grazing incident angle X-ray diffractometry (GI-XRD) and X-ray photoelectron spectroscopy (XPS). After immersion in HBSS for 60 d, on CaTiO₃ coated Ti, the formation of hydroxyapatite (HAP) was observed. Furthermore, HAP layer formed was thicker on the specimen on which CaTiO₃ film was deposited at RT and annealed than that prepared at 873 K. The major difference between both specimens was the chemical properties of the outermost surface. In summary, CaTiO₃ thin film deposited at RT and followed by annealing at 873 K for 7.2 ks in air enhances calcium phosphate formation ability on Ti.     

Mechanical properties of ultra-fine grained zirconia ceramics

The mechanical properties of tetragonal zirconia (TZP) materials doped with Y, Ce or Ti were studied as a function of temperature and grain size. Fine grained Y-TZP (grain size < 0.3 m) shows values for fracture toughness and strength at room temperature, which are comparable with the coarse grained transformation toughened materials, despite lacking transformation toughening. The morphology of the fracture surface points to crack deflection as the most important toughening mechanism. At 800 °C fracture toughness and strength are higher than in coarse grained Y-TZP materials. Doping Y-TZP with Ce or Ti results in a similar trend in mechanical properties, for fine grained material, as for the Y-TZP materials.     

Mechanical properties of (Y-TZP) — alumina-silicon carbide nanocomposites and the phase stability of Y-TZP particles in it

Al₂O₃-SiC-ZrO₂ composites were investigated to obtain a better understanding of the effect of SiC particles and the stress-induced transformation of Y-TZP on its mechanical properties. The Al₂O₃-SiC-ZrO₂ composites were fabricated by hot pressing using -Al₂O₃, SiC and ZrO₂ mixtures. Fracture toughness and strength of Al₂O₃ were greatly improved by incorporating SiC and ZrO₂ particles which were located mainly inside and between Al₂O₃ grains, respectively. The toughening and strengthening mechanism of these composites and the phase stability of the tetragonal ZrO₂ in the composites before and after high-temperature annealing were investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. It was observed that there is a critical volume fraction of zirconia, above which the phase stability of the tetragonal zirconia increases, despite the grain growth of the zirconia. It is considered that another phenomenon, the residual stresses, affect the phase stability of the tetragonal zirconia. To remove the residual stresses the composites were annealed at 1100 °C. After slow cooling, the tetragonal zirconia became very unstable, especially in samples with the highest fabrication temperature and increasing zirconia content. Even quenching from 1100 °C caused an increase in the monoclinic phase of these samples.     

Microwave assisted-in situ synthesis of porous titanium/calcium phosphate composites and their in vitro apatite-forming capability

Microwave irradiation has been proven to be an effective heating source in synthetic chemistry, and can accelerate the reaction rate, provide more uniform heating and help in developing better synthetic routes for the fabrication of bone-grafting implant materials. In this study, a new technique, which comprises microwave heating and powder metallurgy for in situ synthesis of Ti/CaP composites by using Ti powders, calcium carbonate (CaCO₃) powders and dicalcium phosphate dihydrate (CaHPO₄·2H₂O) powders, has been developed. Three different compositions of Ti:CaCO₃:CaHPO₄·2H₂O powdered mixture were employed to investigate the effect of the starting atomic ratio of the CaCO₃ to CaHPO₄·2H₂O on the phase, microstructural formation and compressive properties of the microwave synthesized composites. When the starting atomic ratio reaches 1.67, composites containing mainly alpha-titanium (α-Ti), hydroxyapatite (HA), beta-tricalcium phosphate (β-TCP) and calcium titanate (CaTiO₃) with porosity of 26%, pore size up to 152 μm, compressive strength of 212 MPa and compressive modulus of 12 GPa were formed. The in vitro apatite-forming capability of the composite was evaluated by immersing the composite into a simulated body fluid (SBF) for up to 14 days. The results showed that biodissolution occurred, followed by apatite precipitation after immersion in the SBF, suggesting that the composites are suitable for bone implant applications as apatite is an essential intermediate layer for bone cells attachment. The quantity and size of the apatite globules increased over the immersion time. After 14 days of immersion, the composite surface was fully covered by an apatite layer with a Ca/P atomic ratio approximately of 1.68, which is similar to the bone-like apatite appearing in human hard tissue. The results suggested that the microwave assisted-in situ synthesis technique can be used as an alternative to traditional powder metallurgy for the fabrication of Ti/CaP biocomposites.     

Hydrothermal-electrochemical CaTiO3 coatings as precursor of a biomimetic calcium phosphate layer

The hydrothermal-electrochemical method was used to coat Ti and Ti6Al4V with calcium titanate CaTiO₃. The film exhibited a plate-like structure with several isolated pinholes around 100 nm in diameter. These coated samples were treated by a biomimetic procedure, immersing them in a simulated body fluid (SBF) solution for 5, 10 and 28 days. Characterization was performed by means of X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive spectroscopy. The CaTiO₃-coated samples after the SBF treatment showed a significant increment in their calcium and phosphorous amounts as compared with SBF-treated samples with no previous CaTiO₃ coating; the latter exhibited only surface phosphate incorporation.     

Solid-state reaction,microstructure and phase relations in the ZrO2-rich region of the ZrO2-Yb2O3 system

Phase relations below 1700°C in the ZrO₂-rich region of the zirconia-ytterbia system have been established using thermal expansion, room-temperature X-ray diffraction, precision lattice parameter measurements and microscopic observations. The solubility limits of ytterbia in both monoclinic and tetragonal zirconia were determined. A eutectoid reaction, tetragonal zirconia solid solution monoclinic + cubic zirconia solid solutions at 400 ± 20°C and 2.4 mol % ytterbia was found. The left-hand boundary of the cubic zirconia solid-solution field was redetermined between room temperature and about 1700°C. Long-range ordering was present at 40 mol % ytterbia and the formation of an ordered phase, Zr₃Yb₄O₁₂, isostructural with M₇O₁₂-type compounds was found. Its thermal stability was established between room temperature and 1630 ± 10°C, in which it decomposes into cubic zirconia solid solution by an order-disorder reaction.     

Titanium local environment and electrical conductivity of TiO2-doped stabilized tetragonal zirconia

Solid state reactions in the ZrO₂-Y₂O₃-TiO₂system have been studied and the solid solubility limit of TiO₂ in yttria stabilized tetragonal zirconia Y-TZP has been stablished. Structural characterization was carried out by XRD studies, and the changes in local structures with increasing TiO₂ in yttria stabilized tetragonal zirconia was analyzed for the first time by EXAFS and XANES measurements. The XANES results indicate a displacement of Ti ions from the center of symmetry with increasing titania content leading to assume a non randomly substitution of Ti on Zr sites in the tetragonal zirconia lattice. From the Ti-O and Ti-Ti measured distances, it has been assumed that the found electrical conductivity decrease with increasing titania content was due to both a trapping of the oxygen ion vacancies at Ti ions (Ti_Zr Vö) and the formation of more complex associations like (Ti_Zr Vö Ti_Zr Vö), which giving rise to a reduction in the global concentration of moving oxygen vacancies.     

Mechanical properties and microstructures of co-precipitation derived tetragonal Y2O3-ZrO2-Al2O3 composites

Y-TZP Al₂O₃ specimens (2.5 mol% Y₂O₃-ZrO₂ and 5 to 30 wt% Al₂ 03) were prepared from coprecipitated powders and their mechanical properties were studied. The addition of alumina to Y-TZP improves the attainable density of the materials after sintering at 1500° C and reduces the degradation of their densities due to porosity formation when the materials are sintered above 1500° C. Near theoretical density could be achieved for most of the samples after HIPing at 1500° C for 1/2 h at 200 M Pa pressure. The fracture strength of the HIPed specimens was in the range 2.0 to 2.4 GPa and the stress intensity factor was in the range 3.5 to 6.0 MPa m^1/2. The mechanical strength of the materials was not degraded seriously after autoclaving in water at 175° C for 24 h. The surface layer of transformed monoclinic zirconia was less than 70 m thick even after autoclaving at 175° C for 5 days.     

Fracture toughness,strength and Vickers hardness of yttria-ceria-doped tetragonal zirconia/alumina composites fabricated by hot isostatic pressing

Yttria-ceria-doped tetragonal zirconia ((Y, Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing (HIP) at 1400–1600 °C and 147 MPa for 30 min in Ar gas using fine powders prepared by hydrolysis of ZrOCl₂ solution. The mechanical properties of these ceramic composites were evaluated. The fracture toughness and bending strength of the composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5-4 mol% CeO₂-ZrO₂, 2.5 mol% YO_1.5-4 mol% CeO₂-ZrO₂ and 2.5 mol% YO_1.5-5.5 mol% CeO₂-ZrO₂ fabricated by HIP at 1400 °C were 6–7 MPa m^1/2 and 1700–1800 MPa. Fracture toughness, strength and hardness of (Y, Ce)-TZP/Al₂O₃ composites were strongly dependent on HIP temperature. The fracture strength and hardness were increased, and grain growth of zirconia grains and phase transformation from the tetragonal to the monoclinic structure of (Y, Ce)-TZP during HIP in Ar at high temperature (1600 °C) were suppressed by the dispersion of Al₂O₃ into (Y, Ce)-TZP.     

Microstructure and mechanical properties of zirconia-toughened lithium disilicate glass–ceramic composites

The lithium disilicate glass–ceramics composites reinforced and toughened by tetragonal zirconia (3Y-TZP) were prepared by hot-pressing at 800 °C with varying zirconia content from 0 to 30 wt.%. In the case of the composites of small zirconia content (below 10 wt.%), zirconia acted as nucleation agent primarily, and the microstructure was refined continuously. The morphology of Li₂Si₂O₅ crystals transformed from rod-shaped to spherical structure, and the mechanical properties decreased inevitably. For the composites of large zirconia content (from 15 wt.% to 30 wt.%), however, zirconia restrained the phase separation of glass. The morphology of Li₂Si₂O₅ crystals transformed to rod-shaped structure again. The mechanical properties of the composite at zirconia content of 15 wt.% increased up to 340 MPa and 3.5 MPa m^1/2 which were much higher than those of zirconia-free glass–ceramics. The improved properties were attributed mainly to compressive stress reinforcement, phase transformation and bridging toughening mechanisms.     

Enhanced performance of fluorine substituted hydroxyapatite composites for hard tissue engineering

Dense fluorine (F) substituted hydroxyapatite composites with yttria-doped zirconia (Y-TZP) and/or alumina (Al₂O₃) were successfully fabricated without applying pressure at 1400 °C for 3 h. The suppression of decomposition via the formation of a fluor-hydroxyapatite (FHA) solid solution allowed the sintered body to reach full density. Such fully densified FHA-composites exhibited improved mechanical properties, such as strength, toughness, and hardness, having values of more than 2–4 times higher than those of pure HA or HA-composites. The proliferation behavior of osteoblast-like cells on the FHA-composites showed no cytotoxicity and comparable cell viability to that observed in pure HA for up to 10 days.     

Disintegration process of yttria-stabilized zirconia ceramics using hydrothermal conditions

Capability of the recycling of high strength and high fracture toughness yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) sintered body utilizing “low-temperature annealing degradation” phenomenon was investigated. Hydrothermal treatment was employed to induce the phase transformation from tetragonal to monoclinic zirconia and to disintegrate the Y-TZP sintered body. 3 mol% Y₂O₃–ZrO₂ specimens sintered at 1,550 °C and more were disintegrated without leaving the original appearances when the treatment temperature was between 200 °C and 400 °C. The size of the disintegrated fragments of Y-TZP sintered body was much affected by hydrothermal treatment conditions. Only with hydrothermal treatment and simple ball milling, the sintered body was pulverized into the primary particle level. This technique is expected to apply to a sustainable recycling system for the zirconia ceramics, which restrains an energy consumption compared to crushing zirconia using mechanical procedures.     

Hydrothermal stability of yttria- and ceria-doped tetragonal zirconia-alumina composites

Changes in the crystalline phase and microstructure resulting from hydrothermal ageing of Y-TZP, (Y,Ce)-TZP, Y-TZP-Al₂O₃ composites and (Y,Ce)-TZP-Al₂O₃ composites were investigated under hydrothermal conditions at 180 °C and 1 MPa. Although (Y,Ce)-TZP showed no tetragonal-to-monoclinic (tm) phase transformation during low-temperature ageing in air as compared with 3Y-TZP, the tetragonal phase of (Y,Ce)-TZP easily transformed to monoclinic phase by ageing under hydrothermal condition. This tm phase transformation invaded the inside of the body accompanied by microcracks. (Y,Ce)-TZP-Al₂O₃ composites were resistant to phase transformation during hydrothermal ageing.     

Synthesis and processing of hydroxyapatite ceramic tapes with controlled porosity

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) ceramic sheets with a wide range of porosities (up to 62%) have been prepared. The process is based on the reaction between dicalcium phosphate (CaHPO₄) and calcium carbonate (CaCO₃). When mixed with the appropriate Ca/P ratio, this proves to be a reliable new method for obtaining hydroxyapatite. Moreover, CaCO₃ serves as a gas-forming agent (due to the evolution of carbon dioxide and water during the reaction), which leads to the development of highly porous microstructures. Alternatively, CaHPO₄ and CaCO₃ can be reacted by calcining at 1000°C to produce pure hydroxyapatite powders. When processed in a similar way, a dense ceramic results. By mixing 50 vol% of CaCO₃ and CaHPO₄ with precalcined powders, hydroxyapatite with an intermediate porosity was obtained. Moreover, it should be possible to achieve porosity control by mixing different amounts of uncalcined and precalcined powders. All of these powders are colloidally processed using tape casting to produce thin sheets 150–200 m thick. This technique can be used to make laminates, with or without porosity gradients, up to several millimetres thick.Currently enrolled in the CICESE PhD programme.     

Microstructure and mechanical properties of yttria-stabilized tetragonal zirconia polycrystals containing dispersed TiC particles

The microstructure and mechanical properties of hot-pressed yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) ceramics containing up to 30 vol % TiC particles were studied. Adding TiC particles to Y-TZP improved the bending strength and fracture toughness. With 20 vol% TiC particles the maximum bending strength and fracture toughness reached 1073±30.4 MPa and 14.56±0.25 MPa m^1/2, respectively. The residual tensile stress induced by the thermal expansion difference between ZrO₂ and TiC must have inhibited the tetragonal-monoclinic transformation. The stress-induced phase transformation was therefore not the dominant toughening mechanism. High-densities of dislocations within TiC particles and microcracking were detected by TEM. The improved toughness of the materials is considered to be the result of crack deflection, crack bowing of TiC particles and microcracking toughening of ZrO₂.