Preceramic Paper Derived Alumina/Zirconia Ceramics

Preceramic paper offers a novel approach for manufacturing of lightweight ceramic structures applying versatile paper shaping technologies. Substitution of bioorganic pulp fibers in alumina loaded preceramic paper by inorganic short zirconia fibers was investigated. A retention of more than 90 wt% was achieved by a combination of cationic and anionic retention aids. Powder packing density in the paper sheet decreased with increasing amount of non‐deformable zirconia fibers used to substitute highly deformable pulp fibers. Applying post‐pressing, however, resulted in a pronounced improvement of packing density while retaining excellent flexibility and strength of the preceramic paper preform. Thus, preceramic paper containing a zirconia fiber volume fraction of 19% sintered at 1 600 °C attained a rupture strength measured by ball‐on‐three‐balls loading of 120 ± 17 MPa (porosity 44%) which after pressure consolidation of the paper increased to 180 ± 49 MPa (porosity 28%).     

氧化铝/氧化锆前驱体纤维纺丝液的制备技术研究

以铝粉、盐酸、醋酸锆和氧氯化锆为原料,聚乙烯醇(PVA)为纺丝助剂,采用溶胶-凝胶法制备了氧化铝/氧化锆前驱体纤维纺丝液,借助高速离心甩丝机、纳米粒度仪、旋转流变仪等仪器设备,研究了原料配比、浓缩温度、含水率、PVA添加量和种类对纺丝液稳定性、流变性和纺丝性的影响机制。结果表明,按m(氧氯化锆)∶m(醋酸锆)=(0.5~0.9)∶1配制成锆溶液,按照n(铝粉)∶n(盐酸)∶n(蒸馏水)=2∶1∶20配制成铝溶胶,两者按m(氧化锆)∶m(氧化铝)=1∶1均匀混合,加入5%PVA(PVA占氧化铝、氧化锆质量总和),在75℃下减压蒸馏4~8h,室温下陈化24~48h,可得到粘度在1500~2500mPa·s,含水率在35%~45%,粒度在20nm左右,流变性好的氧化铝/氧化锆前驱体纤维纺丝液。采用高速离心甩丝机对纺丝液进行成纤实验,获得直径在4~10μm的氧化铝/氧化锆前驱体纤维,说明该纺丝液具有良好的成纤性能。     

Ionic Conductivity and Mechanical Properties of Post-HIPed Cubic Zirconia/Alumina Composites

8 mol.% yttria-doped cubic zirconia (8Y-CSZ)/AI₂O₃ composites containing 0-30 vol.% Al₂O₃ particles were fabricated by sintering, followed by hot isostatic pressing (post-HIPing). All composites were densified to at least 99·5% of the theoretical density by post-HIPing. The bending strength of composites sintered at 1500°C in air was independent of A1₂O₃ content, but a significant improvement in the bending strength was achieved by the post-HIPing technique. The bending strength and the fracture toughness of the HIPed composites increased with increasing A1₂O₃ content. Ionic conductivity of the composites was evaluated and the total, lattice, and grain boundary conductivities slightly decreased with increasing A1₂O₃ content. The HIPed composites containing up to 20 vol.% A1₂0₃ appear to be suitable candidate materials as electrolyte for solid oxygen fuel cell.     

Three‐Dimensional Printing of Complex‐Shaped Alumina/Glass Composites

Alumina/glass composites were fabricated by three‐dimensional printing (3DP?) and pressureless infiltration of lanthanum‐alumino‐silicate glass into sintered porous alumina preforms. The preforms were printed using an alumina/dextrin powder blend as a precursor material. They were sintered at 1600 °C for 2 h prior to glass infiltration at 1100 °C for 2 h. The influence of layer thickness and sample orientation within the building chamber of the 3D‐printer on microstructure, porosity, and mechanical properties of the preforms and final composites was investigated. The increase of the layer thickness from 90 to 150 µm resulted in an increase of the total porosity from ～19 to ～39 vol% and thus, in a decrease of the mechanical properties of the sintered preforms. Bending strength and elastic modulus of sintered preforms were found to attain significantly higher values for samples orientated along the Y‐axis of the 3D‐printer compared to those orientated along the X‐ or the Z‐axis, respectively. Fabricated Al₂O₃/glass composites exhibit improved fracture toughness, bending strength, Young's modulus, and Vickers hardness up to 3.6 MPa m^1/2, 175 MPa, 228 GPa, and 12 GPa, respectively. Prototypes were fabricated on the basis of computer tomography data and computer aided design data to show geometric capability of the process.     

Dispersion and Rheological Properties of Alumina/Zirconia Slurries with Methyl Isobutyl Ketone/Ethanol Solvents

The dispersion and rheological behavior of alumina, zirconia, and alumina/zirconia mixed slurries were investigated using various solvent ratios of methyl isobutyl ketone (MIBK)/ethanol (EtOH), by measuring sedimentation bulk density, particle size distribution, and viscosity. Well-dispersed suspensions were obtained in MIBK-rich solvents with additional dispersant and in EtOH-rich without dispersant. The shear viscosity of the slurries was dependent on both the Al₂O₃/ZrO₂ ratio and MIBK/EtOH ratio. At a constant solvent ratio, however, similar rheological behavior was shown regardless of the relative amounts of the two solids. At low shear rate, a Newtonian plateau was absent in the Al₂O₃/ZrO₂ slurries. With increasing shear rate (>600 s^–1), Al₂O₃ slurries exhibited a Newtonian plateau while ZrO₂ demonstrated continuous shear thinning.     

Comparison of Diamond‐Like‐Carbon and Alumina‐Oxide articulating with Polyethylene in Total Hip Arthroplasty

To reduce wear in joint bearings of total hip arthroplasty (THA) is the most important issue for improving long term results and implant survival. Due to low wear rates and excellent tribological features in simulator tests Diamond‐Like‐Carbon coating (DLC) of femoral balls is still discussed as an alternative articulation in THA. This clinical prospective study compares survivorship of DLC‐coated femoral heads and of Aluminia‐Oxide‐ (Al₂O₃) heads articulating with Polyethylene (PE). Over a period of two years 101 THA with DLC‐coated heads and PE cups (DLC‐group) and another 101 THA consisting of Al₂O₃ heads (Al₂O₃‐group) and PE cups as well were implanted. Both articulations were based on the same type of cementless hip joint prosthesis. All hips were implanted by one surgeon in consecutive series consisting of 51 Al₂O₃ and 101 DLC‐articulations and further 50 Al₂O₃. All perioperative and follow‐up data was processed with SPSS®. Survival of THA in both groups was evaluated according to Kaplan‐Meier survivorship analysis with an intervall of 90 months (range:78‐101). Qualitative surface analysis was performed in nineteen retrieved DLC‐heads which were revised for aseptic loosening using field scanning electron microscopy (FE‐SEM, XL 30 SFEG Philips, Eindoven NL). 178 patients (88.2 %) were evaluated for follow‐up. Fourteen patients died meantimes (nine DLC, five Al₂O₃) with the implant components not revised. Ten patients (five DLC, five Al₂O₃) were lost to follow‐up. Both groups were comparable regarding patient age, weight and indications for THA with a normal distribution. Survivorship analysis for aseptic loosening 8.5 years following implantation resulted in a significant difference between both groups with a 54 % survival for DLC/PE compared to 88 % for Al₂O₃/PE bearings (p <0.001). No correlation to variables as age, gender or bodyweight could be detected. Surfaces of nineteen retrieved DLC‐heads showed numerous smallest pits of the diamond‐carbon layers in different quantity. SEM showed delamination of the carbon layer which caused excessive debris of polyethylene and in some cases even of the metallic substrate of the heads. Despite modern manufacturing technology and excellent experimental results for its tribochemical characteristics and wear, even “new” DLC‐coating of femoral heads is to be considered critically due to very high rates of clinical failure.     

Controlled Crack Propagation Experiments with a Novel Alumina‐Based Refractory

Innovative, carbon‐reduced and carbon‐free refractory materials are currently under development within the framework of the DFG priority program “FIRE”. Among various novel material solutions an alumina‐based refractory with titania and zirconia additives (AZT) has gained special interest for application in high temperature processes under thermal shock conditions. The resistance against fracture of the AZT material and, for comparison, of a pure alumina refractory was examined by controlled crack propagation experiments. Wedge splitting and compact tension tests with in situ crack growth observation, partially on microstructural level, have been performed for both materials. Based on the measured room temperature values of dissipated energy, refractory stiffness and fracture stress, the Hasselman thermal shock parameter R″″ was determined. The results allow to predict that AZT is less prone to scale thermal shock damage than pure alumina. The microstructural observations reveal that growth and opening displacement of the main crack is accompanied in AZT by pronounced microcracking, branching and bridging processes. First efforts are also directed towards a mechanical quantification of this fracture behavior in terms of an R‐curve representation (fracture resistance as a function of apparent crack length). The specific problems of R‐curve evaluation that exist in AZT due to nonlinear deformation behavior are addressed and the influence of the observed crack growth mechanisms is discussed.     

核壳结构的氧化锆包裹氧化铝纳米复合粉体的制备研究

采用简单的一步醇盐水解包裹工艺在市售纳米氧化铝颗粒表面均匀地包裹了一层厚度为4～5nm的纳米晶氧化锆, 形成核壳结构的氧化物纳米复合粉体. 通过简单的锆源及酸浓度的控制, 可以实现包裹的氧化锆层厚度的可控调节. 借助XRD、TEM、FE-SEM及 EDS等测试手段对合成的材料进行了形貌与结构表征, 结果表明这种纳米氧化锆包裹氧化铝的核壳复合结构材料, 可以有效地抑制氧化铝在高温煅烧下的晶粒过分长大, 实现在较低煅烧温度下即具有均匀致密的表面形貌.     

Surface Modification of an Alumina‐Based Bioceramic for Cement Application

Biograde zirconia toughened alumina (ZTA) has found wide application in load bearing endoprosthetic implants due to high strength, fracture toughness, and wear resistance. In order to enhance bonding to acrylic bone cement (BC) for implants, fixation modification of ZTA with a thin layer of porous anodic alumina (PAA) was investigated. An Al‐layer of approximately 500 nm was sputtered on the ZTA substrate which subsequently was electrochemically oxidized by anodic polarization in H₂C₂O₄ or H₃PO₄ solution. PAA layers with a total porosity ranging from 11 to 30%, mean pore spacing of 90–200 nm and pore diameters of 30–110 nm were prepared. Compared to unmodified ZTA/BC interface (≈ 30 MPa), the PAA modified specimens (ZTA/PAA/BC) achieved a significantly higher interface bonding strength (≈ 60 MPa) measured by four point bending on composite beam specimens. While crack propagation in the unmodified ZTA/BC specimen was found to proceed along the interface, fracture analysis on the ZTA/PAA/BC specimens showed a mixed mode fracture with part of the fracture propagation localized along the PAA/BC interface and part through BC. Thus, pore structure controlled mechanical interlocking is expected to offer a high potential for applying PAA surface modification to improve biomaterial to BC bonding.     

Al2O3掺杂对YSZ固体电解质烧结及电性能的影响

研究了用常规共沉淀法掺杂Al2O3对YSZ固体电解质的烧结及电性能的影响．结果表明：适量的Al2O3能提高YSZ材料的烧结性能，促使其致密化，但过量的Al2O3对材料的致密化不利；同时，材料的晶界电导随Al2O3含量的增大表现出先增大后减小的变化趋势，这与Al2O3对YSZ晶界两方面的不同影响有关，Al2O3偏析于晶界一方面能清除晶界上对氧离子电导不利的SiO2，但另一方面也会降低晶界空间电荷层中的自由氧离子空穴的浓度．     

氧化锆增韧氧化铝在酸碱介质中的摩擦学特性

研究了氧化铝（F99.7)对氧化锆增韧氧化铝（ZTA）摩擦副在盐酸，氢氧化钠介质中的摩擦学特性，用扫描电子显微镜（SEM）观察了磨损表面形貌，结果表明，该摩擦副在盐酸中的摩擦因数比在据氧化钠中的小；但线磨损总量则相反，在盐酸中的线磨损总量要比在氢氧化钠中的大，磨损产生的原因可能是由于基体材料和介质之间发生摩擦化学反应而引起的。