Production and characterization of ZrO<Subscript>2</Subscript> ceramics and composites to be used for hip prosthesis

Tetragonal ZrO₂ polycrystalline (TZP) ceramics with varying yttria and ceria content (2–3 mol%) and distribution (coated or co-precipitated), and varying second phase content Al₂O₃ were prepared and investigated by means of microstructural analysis, mechanical properties, and hydrothermal stability, and ZrO₂-based composites with 35–60 vol% of electrical conductive TiN particles were developed. The effects of stabilizer content and means of addition, powder preparation, sintering conditions, and grain size have been systematically investigated. Fully dense Y-TZP ceramics, stabilized with 2–3 mol% Y₂O₃, 2 wt% Al₂O₃ can be achieved by hot pressing at 1,450 °C for 1 h. The hydrothermal stability increased with increasing overall yttria content. The jet-milled TiN powder was used to investigate the ZrO₂–TiN composites as function of the TiN content. The experimental work revealed that fully dense ZrO₂–TiN composites, stabilized with 1.75 mol% Y₂O₃, 0.75 wt% Al₂O₃, and a jet-milled TiN content ranging from 35 to 60 vol% could be achieved by hot pressing at 1,550 °C for 1 h. Transformation toughening was found as the primary toughening mechanism. The decreasing hardness and strength could be attributed to an increasing TiN grain size with increasing TiN content, whereas the decreasing toughness might be due to the decreasing contribution of transformation toughening from the tetragonal to monoclinic ZrO₂ phase transformation. The E modulus increases linearly with increasing TiN content, whereas the hydrothermal stability increases with addition of TiN content.     

Y<Subscript>2</Subscript>O<Subscript>3</Subscript> and Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> co-stabilized ZrO<Subscript>2</Subscript>-WC composites

Y₂O₃ + Nd₂O₃ co-stabilized ZrO₂-based composites with 40 vol% WC were fully densified by pulsed electric current sintering (PECS) at 1350 °C and 1450 °C. The influence of the PECS temperature and Nd₂O₃ co-stabilizer content on the densification, hardness, fracture toughness and bending strength of the composites was investigated. The best combination of properties was obtained for a 1 mol% Y₂O₃ and 0.75 mol% Nd₂O₃ co-stabilized composite densified for 2 min at 1450 °C under a pressure of 62 MPa, resulting in a hardness of 15.5 ± 0.2 GPa, an excellent toughness of 9.6 ± 0.4 MPa.m^0.5 and an impressive 3-point bending strength of 2.04 ± 0.08 GPa. The hydrothermal stability of the 1 mol% Y₂O₃ + 1 mol% Nd₂O₃ co-stabilized ZrO₂-WC (60/40) composites was compared with that of the equivalent 2 mol% Y₂O₃ stabilized ceramic. The double stabilized composite did not degrade in 1.5 MPa steam at 200 °C after 4000 min, whereas the yttria stabilized composite degraded after less than 2000 min. Moreover, the (1Y,1Nd) ZrO₂-WC composites have a substantially higher toughness (~9 MPa.m^0.5) than their 2Y stabilized equivalents (~7 MPa.m^0.5).     

Hot isostatic pressing of tetragonal ZrO2 solid-solution powders prepared from acetylacetonates in the system ZrO2-Y2O3-Al2O3

In compositions having ZrO₂/Y₂O₃=(74.25–71.25)/(0.75–3.75) (mol% ratio) with 25 mol% Al₂O₃, metastable t-ZrO₂ solid solutions crystallize at 780° to 860°C from amorphous materials prepared by the simultaneous hydrolysis of zirconium, yttrium and aluminium acetylacetonates. Hot isostatic pressing has been performed for 1 h at 1130 and 1230°C under 196 MPa using their powders. Two kinds of material are fabricated: (i) perfect ZrO₂ solid-solution ceramics and (ii) composites of ZrO₂ solid solution and -Al₂O₃. Their mechanical properties are examined, in connection with microstructures and t/m ZrO₂ ratios. Composites with a homogeneous dispersed -Al₂O₃ derived from solid-solution ceramics result in a remarkable increase of strength.     

Formation and hot isostatic pressing of ZrO2 solid solution in the system ZrO2-Al2O3

In the system of ZrO₂-Al₂O₃, cubic ZrO₂ solid solutions containing up to 40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminium alkoxides. At higher temperatures, they transform into tetragonal solid solutions. Metastable ZrO₂ solid solution powders containing 25 mol% Al₂O₃ have been sintered at 1000–1150 °C under 196 M Pausing the hot isostatic pressing technique. The solid solution ceramics consisting of homogeneous microstructure with an average grain size of 50 nm exhibited a very high fracture toughness of 23 MN m ^–1.5. They have been characterized by X-ray diffraction and electron probe surface analyses.     

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.     

Influence of CaTiO3 impurity in SHS-TiN powder on the mechanical properties of ZrO2–TiN composites

Yttria-stabilised tetragonal polycrystalline ZrO₂-based composites with 40 vol.% TiN were hot pressed at 1450 °C for 1 h using a jet-milled thermally synthesized and a self-propagating high-temperature synthesis (SHS) TiN powder. The ZrO₂ phase of the SHS-TiN powder-based composites was found to be substantially coarser than for the jet-milled TiN powder-based ceramics and prone to spontaneous transformation to m-ZrO₂ and microcracking, due to the CaTiO₃ impurity in the SHS-TiN starting powder. In order to prove this, a set of experiments was performed to investigate the effect of the addition of CaO and TiO₂ on an yttria-stabilised tetragonal ZrO₂ polycrystalline (Y-TZP). The addition of 0.2 mol% of CaO to a Y-TZP ceramic was found to destabilise the t-ZrO₂ phase, whereas the addition of 1 mol% TiO₂ results in significant grain growth and the formation of less transformable t-ZrO₂. The CaTiO₃ impurity could be removed from the SHS-TiN powder by hot hydrochloric acid leaching, allowing to obtain a similar microstructure and mechanical properties as with conventional TiN powder.     

Effect of zirconia addition on crystallinity,hardness, and microstructure of gel-derived barium aluminosilicate,BaAl2Si2O8

The effect of zirconia (ZrO₂) additions, in amounts equivalent to 5, 10, 20 and 40 mol%, to barium aluminosilicate, BaAl₂Si₂O₈, was studied by examination of the phase assemblage of the mixtures after crystallization heat treatments at ∼ 1050°C or more using X-ray diffraction (XRD). In all cases, BaAl₂Si₂O₈ gel crystallized into the hexacelsian polymorph. XRD results also indicated solid solubility of 10mol% or more ZrO₂ in hexacelsian material. Precipitation of an additional phase, tetragonal ZrO₂, occurred in the BaAl₂Si₂O₈ material containing 20 mol% ZrO₂. Also, 95% and 99% dense celsian ceramics were fabricated at 1450 and 1580°C sintering temperatures, respectively, using cold isostatically pressed pellets produced from powder mixtures containing 20 and 40 mol% ZrO₂. These pellets also contained 20 wt% gel-derived lithia-doped celsian “seed” powder to promote hexacelsian to celsian transformation during sintering. Indentation hardness values for the 99% dense celsian ceramic without and with 20 mol% ZrO₂ were 8.04 and 10.80 GPa, respectively. Scanning electron microscopy was used to examine the microstructures of these samples.     

Pressureless-sintering behavior of nanocrystalline ZrO2–Y2O3–Al2O3 system

ZrO₂–Y₂O₃–Al₂O₃ nanocrystalline powders have been synthesized using chemical coprecipitation method. Nano-powders were compacted uniaxially and densified in a muffle furnace. Densification studies showed that a fully dense pellet of ZrO₂(3Y) and a 99% relative density for 5 mol% Al₂O₃ doped ZrO₂(3Y) were obtained after sintering at 1200 °C. The presence of Al₂O₃ inhibits grain growth and suppresses the densification process. Full densification and the maximum microhardness of 17.8 GPa were achieved for the ZrO₂(3Y)/5 mol% Al₂O₃ composites sintered at 1250 °C.     

Microstructure and thermal shock resistance of Al2O3 fiber/ZrO2 and SiC fiber/ZrO2 composites fabricated by hot pressing

Al₂O₃ chopped fiber/ZrO₂ and SiC continuous fiber/ZrO₂ composites were fabricated by hot pressing at 1550°C and 15 MPa in vacuum. The mechanical properties of thermally shocked composites were measured at room temperature by four-point bending. The addition of Al₂O₃ fibers into ZrO₂ matrix degraded the fracture strength, but improved significantly the thermal shock resistance. In addition, the mechanical properties of SiC fiber/ZrO₂ composites were much lower than those of monolithic ZrO₂ because of the presence of microcracks on the surface. The SiC fiber/ZrO₂ composites showed an excellent thermal shock resistance.     

Design and fabrication of porous ZrO2/(ZrO2 + Ni) sandwich ceramics with low thermal conductivity and high strength

3Y–ZrO₂/(3Y–ZrO₂ + Ni) sandwich ceramics were fabricated through cold isostatic pressing and pressureless sintering. Porous 3Y–ZrO₂ ceramics with large connecting open pores and permeability were used as interlayers for insulation, whereas outer metal–ceramic layers were used as bearing loads. Microstructures and properties of the porous ZrO₂ and ZrO₂/(ZrO₂ + Ni) sandwich ceramics were investigated in detail. The ZrO₂/(ZrO₂ + Ni) sandwich ceramics exhibited better mechanical properties than the monolithic porous ZrO₂ ceramics at the same low thermal conductivity (approximately 0.85 W/m K). The mechanical properties of the sandwich ceramics were influenced by metal toughening and sintering-induced residual thermal stress.     

Characterisation and mechanical testing of hydrothermally treated HA/ZrO2 composites

Hydrothermal treatment is traditionally employed to improve the sinterability of powder compacts by reducing porosity and increasing apparent density. The effect of hydrothermal treatment on green powder compacts has been assessed in order to better understand how treatment may affect the sinterability of the bodies. Laboratory synthesised nano sized hydroxyapatite (HA) and a commercial zirconia (ZrO₂) powder have been ball milled together to create composite mixtures containing 0–5 wt% ZrO₂ loadings. Disc shaped bodies have been formed using uniaxial and subsequent isostatic pressure. The resultant coherent samples were subjected to hydrothermal treatment at either 120 or 250°C for 10 h in order to assess the effect of this processing technique on the physical, mechanical and microstructural properties of the green composites. ZrO₂ loadings up to 3 wt% increased apparent density from 90 to 92%, whereas increased loading to 5 wt% increased flexural strength, from 6 to 9 MPa. Increasing the hydrothermal treatment temperature increased open porosity, from ~44 to ~48% and reduced biaxial flexural strengths of the treated bodies compared to those of their room temperature isostatically pressed counterparts (~10 to ~6 MPa).     

Formation and sintering of 75 mol% alumina/25 mol% zirconia (2–3.5 mol% yttria) composite powder prepared by the hydrazine method

Al₂O₃/Y₂O₃-doped ZrO₂ composite powders with 25 mol% ZrO₂ have been prepared by the hydrazine method. As-prepared powders are the mixtures of AlO (OH) gel solid solutions and amorphous ZrO₂. The formation process leading to -Al₂O₃/t-ZrO₂ composite powders is investigated. Hot isostatic pressing has been performed for 1 h at 1500 °C under 196 MPa. Dense ZrO₂-toughened Al₂O₃ (ZTA) ceramics with homogeneous-dispersed ZrO₂ particles show excellent mechanical properties. The toughening mechanism is discussed.     

Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/TiN composites produced from TiO<Subscript>2</Subscript>-Modified Si<Subscript>3</Subscript>N<Subscript>4</Subscript> powders

Si₃N₄/TiN composites have been produced by hot pressing at temperatures from 1600 to 1800°C in a nitrogen atmosphere, using silicon nitride powders prepared by self-propagating high-temperature synthesis and surface-modified with titanium dioxide nanoparticles. We examined the effect of TiO₂ content on the microstructure, phase composition, and mechanical strength of the ceramics. It is shown that titanium nitride can be formed by the reaction Si₃N₄ + TiO₂ → TiN + NO + N₂O + 3Si. The Si₃N₄/TiN composites containing 5–20% TiN have a low density, high porosity, and a bending strength of 60 MPa or lower. In Si₃N₄/TiN ceramics produced using calcium aluminates as sintering aids, the silicon nitride grains are densely packed, which ensures an increase in strength to 650 MPa.     

Pressureless sintering of dense hydroxyapatite–zirconia composites

Hydroxyapatite (HA)–TZP (2.5 mol% Y₂O₃) containing 2, 5, 7.5 and 10 wt% TZP were prepared using calcium nitrate, diammonium hydrogen orthophosphate, zirconium oxychloride and yttrium nitrate. The composite powder was prepared by a reverse strike precipitation method at a pH of 10.5. The precipitates after aging and washing were calcined at 850°C to yield fine crystallites of HA and TZP. TEM study of the calcined powder revealed that while HA particles had both spherical and cuboidal morphology (∼50–100 nm) the TZP particles were only of spherical nature (∼50 nm). X-ray analysis showed that the calcined powder of all the four composition had only HA and t-ZrO₂. Uniaxially compacted samples were sintered in air in the temperature range 1,150–1,250°C. High sintered density (>95% of theoretical) was obtained for composites containing 2 and 5 wt% TZP, while it was 92% for 7.5 wt% and 90% for 10 wt% TZP compositions. X-ray analysis of sintered samples shows that with 2 wt% TZP, the retained phases were only HA and t-ZrO₂. However, for 5, 7.5 and 10 wt% TZP addition both TCP and CaZrO₃ were also observed along with HA and t-ZrO₂. Bending strength was measured by three point bending as well by diametral compression test. While in three point bending, the highest strength was 72 MPa, it was 35.5 MPa for diametral compression. The strength shows a decreasing trend at higher ZrO₂ content. SEM pictures show near uniform distribution of ZrO₂ in HA matrix. The reduction in sintered density at higher ZrO₂ content could be related to difference in the sintering behaviour of HA and ZrO₂.     

Preparation and characterization of an Al2O3–ZrO2 nanocomposite,Part I: Powder synthesis and transformation behavior during fracture

An alumina–zirconia composite containing 5 mol% zirconia was prepared by following a wet interaction process in aqueous medium via the sol–gel route. The formed hydrogel which was aged for proper growth and orientation at room temperature, was then dried at a low temperature to minimize agglomeration. The sol–gel derived precursor powder was properly characterized through determination of surface area, particle size and thermal analysis. The sintering behavior was studied by compacting the nano-powder through cold isostatic pressing where maximum densification of 98.4% was achieved at 1550 °C. During Vickers indentation using a 5 kg load, cracks were propagating around the grain boundaries and fractured the ZrO₂ particles, which was associated with t-ZrO₂ to m-ZrO₂ transformation. Sintering was studied in the absence of a mineraliser.     

Re−Ni2B2C superconducting borocarbides: “In situ” growth of thin films, transport and point contact spectroscopy

High quality c-axis oriented films of the intriguing intermetallic superconducting compound YNi₂B₂C have been obtained “in situ” by magnetron sputtering on MgO substrates held at about 800°C. The films showed maximum T_c=15.3 K, †T_c≈0.1 K, room temperature resistivity ρ≈50μΩ·cm, critical current J_c≈10⁵ A/cm² and B_c2≈6 T. Superconducting films were also obtained on Al₂O₃ and LaAlO₃ single-crystal substrates. From the ρ(T) dependence a value of the Debye temperature Θ _D =330±20 K can be deduced. At low temperatures the resistivity follows a quadratic power law possibly indicative of a high value of the electron-phonon interaction parameter λ. In order to clarify the role of λ in these compounds, point contact spectroscopy measurements have been performed on YNi₂B₂C and HoNi₂B₂C bulk samples prepared by inductive melting using a Low Temperature Scanning Tunneling Microscope (LTSTM). In the point contact regime clear evidence of a superconducting gap have been found in both compounds, corresponding to a moderate strong coupling behaviour (2†/KT_c≈3.8).     

Coprecipitation-derived mullite and mullite-zirconia composites

Precursor powders of mullite-zirconia (0–40 wt% ZrO₂) were prepared by a hydroxide coprecipitation method and their behaviour during calcination between room temperature and 1500 °C was studied using thermal analysis, X-ray diffraction and electron microscopy. The only crystalline phases present in the precalcined powders were bayerite and gibbsite, and these were stable up to 250 °C. Powders containing ZrO₂ were initially amorphous, but on calcination between 250 and 850 °C produced different crystalline phases at temperatures which depended on the amount of zirconia present. Thus in the case of mullite-40 wt% ZrO₂, zirconia crystallized at about 850 °C and was stable up to 1200 °C, when it reacted with free silica to form zircon (ZrSiO₄). Mullite formed above 1250 °C at the expense of zircon and remained stable at higher temperatures. The oxide powders were very homogeneous, and on sintering produced ceramics with a fine-grained uniform microstructure. The powders were very reactive and could be sintered conventionally to near-theoretical density at 1600–1700 °C without sintering aids. The fracture strength of mullite was about 275 MPa, and this could be improved to 350 MPa by hot isostatic pressing the presintered bodies. Addition of zirconia enhanced the sintering kinetics as well as the fracture strength of mullite.     

Effect of gadolinium ions on the structure and magnetic properties of zinc-borate glasses and glass ceramics

X-ray diffraction (XRD), electron paramagnetic resonance (EPR), and magnetic susceptibility measurements have been employed to investigate the samples from the (Gd₂O₃) _x ·(B₂O₃)_(60−x)·(ZnO)₄₀ (0 ≤ x ≤ 20 mol%) system. The XRD pattern for the prepared samples shows that the vitreous phase is present only for x ≤ 15 mol%. For the samples containing 20 mol% Gd₂O₃ the presence of a unique crystalline phase, GdBO₃, embedded in a vitreous matrix was evidenced. In this case the XRD patterns show the presence of nanometer sized crystals (64 nm) in a glassy matrix. The EPR spectra of the studied samples exhibit three important features with effective g-values of ≈6, 2.8, 2.0 and a weaker feature at g ≈ 4.8. For low Gd₂O₃ contents (x < 3 mol%), the EPR spectra have the typical ‘‘U’’-type shape. For higher contents of Gd₂O₃ (x ≥ 3 mol%), the spectral features are broadened and finally are dominated by a single broad absorption line located at g ≈ 2.0. This broad EPR line is associated to the Gd³⁺ ions present predominantly as clustered species. Magnetic susceptibility data show that for x > 1 mol% the Gd³⁺ ions are present not only as isolated species but also as species coupled through antiferromagnetic exchange interactions.     

Preparation and mechanical properties of self-reinforced in situ Si3N4 composite with La2O3 and Y2O3 additives

Self-reinforced in situ Si₃N₄ composite material was prepared with high amount of La₂O₃ and Y₂O₃ additives by two-step hot pressing, and the optimum amount of additives was determined. The volume fraction of boundary glass phase was calculated based on the equilibrium of equivalent number in chemical reaction. For material with 15 mol% additives, flexural strength and fracture toughness at room temperature were 960 MPa and 12.3 MPa m^1/2, respectively. At temperature of 1350°C, flexural strength was maintained to 720 MPa and fracture toughness was significantly increased to 23.9 MPa m^1/2 because of the high refractory of oxynitride glass containing compositions of La and Y. Self-reinforced mechanism was mainly responsible for crack deflection along the elongated β-Si₃N₄ grains.     

Compositional analysis and electrical properties of Sb,Mn-doped barium strontium titanate PTCR ceramics with TiO<Subscript>2</Subscript> and SiO<Subscript>2</Subscript> sintering additives

The secondary phase compositions in Sb-doped (Ba, Sr)TiO₃ ceramics containing SiO₂ and excess TiO₂ sintering additives have been examined by XRD, BEI and EPMA techniques. It is shown that alongside the primary (Ba_0.797Sr_0.2Sb_0.003)TiO₃ phase, (Ba_1.95Sr_0.05)₂(Ti_1.2Si_1.8)O₈ and Sb-doped (Ba_0.99Sr_0.01)₆Ti₁₇O₄₀ phases form at the intergranular regions. Substitution of up to 0.04 mol% MnO₂ enhances the PTCR effect, giving a ratio ρ_max/ρ_min ∼ 7 × 10⁵ for the optimum 0.04% MnO₂ composition. At this level Mn cannot be detected in the microstructure by EPMA, however in insulating samples containing 0.08 mol% of MnO₂, it was detected in the (Ba_0.99Sr_0.01)₆Ti₁₇O₄₀ intergranular phase.