Zirconia Toughened Alumina Ceramics Research Articles

Effect of MgO Addition on the Mechanical and Dynamic Properties of Zirconia Toughened Alumina (ZTA) Ceramics.

Zirconia toughened alumina (ZTA) is a promising advanced ceramic material for a wide range of applications that are subjected to dynamic loading. Therefore, the investigation of dynamic compressive strength, fracture toughness and hardness is essential for ZTA ceramics. However, the relationship between these mechanical properties in ZTA has not yet been established. An example of this relationship is demonstrated using ZTA samples added with MgO prepared through conventional sintering. The microstructure and mechanical properties of ZTA composites were characterized. The hardness of ZTA composites increased for ≤0.7 wt.% MgO due to the pinning effect of MgO and decrease of the porosity in the microstructure. Oppositely, the fracture toughness of ZTA composites continuously decreased due to the size reduction of Al2O3 grains. This is the main reason of deteriorate of dynamic compressive strength more than 0.2 wt.% of MgO addition. Therefore, the SHPB test shows the improvement of the dynamic compressive strength only up to a tiny amount (0.2 wt.% of MgO addition) into ZTA ceramics.

Thermophysical Properties of Zirconia Toughened Alumina Ceramics with Boron Nitride Nanotubes Addition

The demand for high thermal conductivity substrates with electrically insulating materials are increasing with the emerging markets in power electronics and mobile telecommunication device packages. Effective heat transfer in those packages is important to provide high performance and reliability of the product. This paper mainly presents the thermophysical properties of zirconia toughened alumina ceramics with the addition of small amount of boron nitride nanotubes (BNNTs). The effects of the boron nanotubes addition on the sintering behavior, the microstructure and the thermal properties of the yttria-stabilized zirconia toughened alumina (YZTA), nanocomposite ceramics are investigated. The addition of 0.3 wt% boron nitride nanotubes into the YZTA matrix enhanced the thermal diffusivity as well as a mechanical strength. Above all, the addition of boron nitride nanotubes greatly decreased the coefficient of thermal expansion (CTE) of the composites in which the CTE of pure alumina increases with increasing temperatures. Moreover, the BNNTs added YZTA composites revealed a drastic decrease in CTE at high temperature range, 400–800 °C. This enhanced thermal stability of YZTA–BNNT composites may have a potential application to the high temperature structural ceramics and high power semiconductor packaging substrate.

Liquid Phase Assisted Superplastic Deformation of TiO2-Doped ZTA Ceramics.

In this study, the compressive deformation of zirconia toughened alumina (ZTA) ceramics doped with different amounts of TiO2 dopants were investigated in the temperature range of 1300–1400 °C to evaluate the stress exponent (n value) and apparent deformation activation energy (Q value). With 0–8 wt.% TiO2 dopants, the n values and Q values of the TiO2-doped ZTA ceramics were calculated as 2–3 and 605–749 kJ/mol, respectively. Moreover, three grain boundary features were observed in these deformed materials, named the clean grain boundary, thin liquid phase grain boundary, and thick liquid phase grain boundary. Based on the deformation behavior and microstructure evolution, it was found that the lower apparent activation energy and higher strain rate of TiO2-doped ZTA ceramics are intensively related to the grain boundary feature.

A critical note on nanoscale plasticity in 20 ZTA ceramics

The present work reports the very first observations on initiation of nanoscale plastic events in 20 ZTA (Zirconia Toughened Alumina) ceramics. The nanomechanical properties as well as the intrinsic contact deformation resistance of the present ZTA ceramic are studied here as a function of low loads (i.e., 10–1000 mN). Here we report for the very first time, the detailed mechanisms on the genesis of ‘micro pop-in’ events that characterize the nanoscale plasticity initiation in the 20 ZTA ceramics. These new results along with field emission scanning electron microscopy (FESEM) based evidences confirm that the combined contributions from the maximum shear stress generated underneath the nanoindenter, the formations of shear bands and localized microcracking play significant roles in the initiation of nanoscale plastic events in the 20 ZTA ceramics.

Synthesis and characterization of zirconia toughened alumina ceramics prepared by co-precipitation method

Abstract Undoped and 3 mol% yttrium doped ZrO 2 –Al 2 O 3 composite powders with partially stabilized ZrO 2 (PSZ) content varying from 0 to 30 wt% were prepared by a co-precipitation route using inorganic precursors Al(NO 3 ) 3 , ZrOCl 2 and Y(NO 3 ) 3 . The precipitates were characterized by DTA and subsequently calcined at 1200 °C for 4 h to achieve fine grained composite powders. The calcined powders were characterized by FTIR and XRD. In order to enhance the sinterability, the calcined powders were wet milled in a high energy ball mill. Powders were uniaxially pressed to form pellets and sintered at 1600 °C for 5 h to achieve greater than 96% relative density. Microstructural analysis of the sintered compacts revealed the uniform distribution of the zirconia particles among the alumina matrix. It was also observed that the faceted intergranular zirconia grains were present at the grain boundaries and junctions in the alumina matrix. Vickers indentation was carried out at 1 kgf load for hardness and 2 kgf load for estimating the critical stress concentration factor (K c ). Microscopic studies of the indented samples showed that cracks were propagating around the grain boundaries. Highest K c ∼8.40 ± 0.4 MPa√m and hardness ∼16.31 ± 0.58 GPa was obtained for the 30 wt% PSZ-Al 2 O 3 composite. The sintered density and critical stress intensity factor (K c ) achieved were compararble to that achieved earlier by hot press and SPS.

Improvement of dielectric performance of solid/gas composite insulation with YSZ/ZTA coatings

N2 has been introduced to gas-insulated switchgears (GIS) as an environmentally friendly insulation medium for SF6 gas. Considering the low breakdown strength of N2, it’s essential to apply a dielectric coating on part of the electrode in GIS. In the current work, yttria-stabilized zirconia (YSZ) and zirconia-toughened alumina (ZTA) ceramics are tactfully applied as dielectric coating materials and successfully coated on the surface of rod electrode via high efficiency supersonic atmospheric plasma spraying method. It is demonstrated that both YSZ and ZTA have excellent thermal stability. In the measured frequency range 10−2–106 Hz, the dielectric constant (ε′) decreases with the increase in frequency. At measured temperatures (0–140 °C), ε′ increases from 15.45 to 16.31 for YSZ ceramic at 1 kHz, while it varies between 30.23 and 39.34 for ZTA ceramic. As compared to that of bare electrode, the 50% BDV (U50) of solid/gas composite insulation system is significantly improved when using YSZ and ZTA as coating materials. Furthermore, the U50 increases with the increase of coating thickness. For the electrode coated with ZTA ceramic (500 μm), the U50 value reaches to 86 kV, which is enhanced about 21.13% in comparison with that of bare electrode. With respect to YSZ ceramic coating (500 μm), a higher U50 value of 88 kV is obtained. This is mainly due to that YSZ has a lower permittivity, which can generate a more uniform electric field distribution between the rod and plane electrodes, bringing about a higher breakdown voltage.

Effect of ZTA volume fractions on the microstructure and properties of ZTAp/high manganese steel composites

The non-pressure casting infiltration technique was used to fabricate ZTA (zirconia toughened alumina) reinforced high manganese steel composites in order to study the influence of ZTA volume fractions on the microstructure and properties of it. The grain size of matrix in recombination region reduces from 154.4 μm to 63.2 μm when the volume fraction of ZTA particles increases from 0% to 32.8%. The Rockwell hardness and yield strength of the composites were found to be strongly dependent on the volume fraction of ZTA particles. The compressive strength of composites is closely related to the distribution of ceramic particles and the expansion of cracks around the particles. The interfacial layer of composites can absorb the stress from the ZTA ceramics and the metal matrix during the thermal expansion and contraction. The composites with 24.3 vol% ZTA showed the highest the impact abrasive wear and its wear loss is 2/3 times than that of high manganese.

Role of addition of La2O3 on the phase contents, microstructure and mechanical properties of ZTA-MgO ceramics

Abstract Addition of MgO into Zirconia Toughened Alumina (ZTA) enhanced the Vickers hardness (1710 HV) but reduced the fracture toughness (3.38 MPa.m1/2). The reason is attribute to the formation of MgAl2O4 phase in the ZTA-MgO system. Hence, the La2O3 was added to ZTA-MgO compounds to acquire both high hardness and fracture toughness. The mixtures were prepared with addition of La2O3 (0.1 wt.% to 10.0 wt.%) to ZTA/1.0 wt% MgO. The mixtures were pressed to a rectangular bar of (54*12*5) mm3 at 1600 oC for 4 hrs. The characterisation of phase contents, microstructure, density, porosity and mechanical properties were done through XRD, SEM, Archimedes principle and Vickers indentation technique, respectively. The optimum La2O3 addition was between (0.7 wt. % - 1.0 wt.%) for better fracture toughness of ZTA ceramics with a reasonable high hardness. The addition of La2O3 has a significant effect on the formation of finer Al2O3 grains, creating strong interface between the Al2O3 grains and LaAl11O8 grains result in denser ZTA-MgO ceramics of hardness of 2028 HV. Also, the presence of rodlike LaAl11O8 and/or LMA grains in the ZTA-MgO ceramics improved the fracture toughness up to 6.67 MPa.m1/2.

Tougher and harder zirconia toughened alumina (ZTA) composites through in situ microstructural formation of LaMgAl11O19

Advanced ceramic materials are highly demanded for various modern industrial and practical engineering applications. Nonetheless, the requirement of attainment hard and simultaneously tough materials is a crucial matter for industrial sectors. To accomplish this target, the strategy of designing a novel microstructure of Zirconia Toughened Alumina (ZTA) ceramics was utilised through in situ formation of LaMgAl11O19 (LMA) based on the stoichiometry ratio of constitute oxides inside ZTA composites during sintering process. ZTA Samples were conventionally sintered at 1600 °C for 2 h to ensure fully formation of LMA while their characteristics were analysed using SEM-EDX and XRD. The LMA elongated grains significantly enhanced ZTAs' Indentation Fracture Resistance (KIFR) due to crack deflection and crack bridging mechanisms combined with transgranular and intergranular fracture modes. Additionally, higher hardness was obtained due to high LMA density, creation of low porous ZTA and strong interface bonding between LMA and ZTA grains. The KIFR was stable while the hardness was steadily declined due to more porosity accompanied by LMA in situ formation. Consequently, sustainable high hardness and enhanced KIFR of ZTA ceramics were achieved. The optimum outcomes were superior KIFR (7.8 MPa.m^1/2) and hardness (1660 HV) than the pure ZTA (5.9 MPa.m^1/2, 1568 HV). The investigation provides a new hint for future design and fabrication of economic and high performance ceramics through in situ formation of LMA according to the stoichiometric ratio of the reinforcing oxides.

Ceramic prosthesis surfaces induce an inflammatory cell response and fibrotic tissue changes.

Early evidence has emerged suggesting that ceramic-on-ceramic articulations induce a different tissue reaction to ceramic-on-polyethylene and metal-on-metal bearings. Therefore, the aim of this study was to investigate the tissue reaction and cellular response to ceramic total hip arthroplasty (THA) materials in vitro, as well as the tissue reaction in capsular tissue after revision surgery of ceramic-on-ceramic THAs. We investigated tissue collected at revision surgery from nine ceramic-on-ceramic articulations. we compared our findings with tissue obtained from five metal-on-metal THA revisions, four ceramic-on-polyethylene THAs, and four primary osteoarthritis synovial membranes. The latter were analyzed to assess the amount of tissue fibrosis that might have been present at the time of implantation to enable evaluation, in relation to implantation time, of any subsequent response in the tissues. There was a significant increase in tissue fibrosis with implantation time for all implant types tested. Interestingly, the tissue fibrosis in ceramic-on-ceramic THAs was significantly increased compared with metal-on-metal and ceramic-on-polyethylene. Additionally, we found ceramic wear particles in the periprosthetic tissue of ceramic implants. Fibroblasts responded with expression of cytokines when cultured on alumina-toughened zirconia (ATZ) and zirconia-toughened alumina (ZTA) ceramic surfaces. This response was more pronounced on ATZ ceramics compared with ZTA ceramics. The same inflammatory response was observed with peripheral blood mononuclear cells (PBMCs) cultured on ZTA and ATZ. Our findings therefore, corroborate the previous findings that ceramic-on-ceramic periprosthetic revision tissue is fibrous and offer an explanation for this observation. We detected a long-term inflammatory response of PBMCs and an inflammatory response of fibroblasts to ATZ and ZTA ceramic. These findings partially explain the fibrotic tissue change in periprosthetic tissue of ceramic-on-ceramic bearings. Cite this article: Bone Joint J 2018;100-B:882-90.

Scanning electron microscopical observation of an osteoblast/osteoclast co-culture on micropatterned orthopaedic ceramics

Category: Basic Sciences/Biologics Introduction/Purpose: In biomaterial engineering, the surface of an implant can influence cell differentiation, adhesion and affinity towards the implant. On contact with an implant, bone marrow-derived mesenchymal stromal cells (BMSCs) demonstrate differentiation towards bone forming osteoblasts, which can improve osteointegration. The process of micropatterning has been shown to improve osteointegration in polymers, but there are few reports surrounding ceramics. Several implants in forefoot surgery like the Moje metatarsal phalangeal joint replacement were made from smoother ceramic zirconia toughened alumina (ZTA). The aim was to establish whether the pits were specifically bioactive towards osteogenesis or were generally bioactive and would also stimulate osteoclastogenesis that could potentially lead to osteolysis. Methods: We established a co-culture of bone marrow-derived mesenchymal stromal cells (BMSCs) with osteoclast progenitor cells and to observe the response to micropatterned ZTA ceramics with 30 µm diameter pits compared to smooth controls. Micropatterned masks were fabricated using a standard photolithography and electroplating technique. Micropatterned ceramic substrates were produced by embossing of visco-plastic green ceramic tapes at room temperature followed by sintering. After informed consent was obtained from healthy patients undergoing routine total hip arthroplasty, bone marrow was aspirated from the femoral medullary canal. At day 3, non-adherent cells were removed within the supernatant and cultured separately until a confluent BMSC layer was identified. The co-culture was maintained up to specific time points (days 3, 7, 14, 21 and 28) with thrice weekly media exchange. At each time point, duplicate samples were fixed and prepared for analysis by scanning electron microscopy (SEM) with immuofluorescence and histochemical staining, Results: SEM analysis of the patterned materials demonstrated successful micropatterning of the ZTA ceramic with approximately 30 µm diameter, 1.7 µm depth pits due to sintering shrinkage. At the longest time-point (day 28), osteoclast-like cells were visible across the planar surfaces and had strong interactions with the ceramic grains. Podosomes were notable on some of the osteoclast membranes. On the pitted surfaces, we observed significantly less osteoclast-like cells. Also, pit bridging by macrophage-like cells was regularly noted. Nodular clusters of osteoblast-like cells were noted statistically more often on the micropatterned ceramics. Conclusion: These results demonstrated specific bioactivity of micropatterned ZTA ceramics towards osteogenesis, with more bone nodule formation and less osteoclastogenesis compared to planar controls. In addition, we found that that macrophage and osteoclast-like cells did not interact with the pits and formed fewer full-size osteoclast-like cells on the pitted surfaces. This may have a role when designing ceramic orthopaedic foot implants. We are now using this co-culture for research on other micropatterned biomaterials used in foot and ankle surgery.

Microstructure and Hardness of Zirconia Toughened Alumina (ZTA) Ceramics by Hot Isostatic Press Sintering

Microstructure and Hardness of Zirconia Toughened Alumina (ZTA) Ceramics by Hot Isostatic Press Sintering

Indentation size effect and energy balance issues in nanomechanical behavior of ZTA ceramics

It is well known that physical, structural, mechanical as well as other functional properties change drastically at the nanoscale, thereby giving rise to size effects in all materials, especially ceramics. Therefore, it is extremely relevant today to understand the basic scientific issues involved in the development of size effects in materials, particularly ceramics which are characteristically brittle in nature. Hence, to be able to design better contact resistant ceramics; it is of significant importance to understand the genesis of indentation size effect (ISE) in the nanomechanical response of structural ceramics like zirconia toughened alumina (ZTA). Here we report the first-ever systematic study on ISE in nanoindentation behavior of 10, 20 and 40 volume% (vol%) ZTA. The nanoindentation experiments were conducted at an ultra low load range of 1–1000 mN. As the experimental data showed the presence of strong ISE, the efficacies of existing models in explaining the same; were critically examined. Among existing models, the strain gradient plasticity model provided the real physical reason for the genesis of ISE in ZTA and hence, explained the data the best. Similarly, existing models were used to predict the variations in experimentally measured ratios of plastic to total energy spent in the nanoindentation process. The results showed that the Malzbender model predicted experimental data the best. This observation implied the best efficacy of the internally expanded cavity concept in explaining the nanoindentation response of the present ZTA ceramics. In addition, the other possible mechanisms of ISE in ZTA were discussed. Finally, the linkage of microstructural parameters to ISE in ZTA was explored.

Study of hole characteristics in Laser Trepan Drilling of ZTA

Zirconia Toughened Alumina ceramic is widely used for aerospace components, combustion chambers, heat exchangers, bearings and pumps mainly due to its improved mechanical and thermal properties. To make holes in thick section Zirconia Toughened Alumina ceramics is a major challenge due to its unfavorable machining characteristics. Recent researches have explored that laser machining can overcome the machining limitations of advanced materials having improved mechanical properties. In present research, authors have analyzed the effect of Laser Trepan Drilling on hole characteristics of 6.0 mm thick Zirconia Toughened Alumina. Effect of significant process parameters on hole characteristics such as hole circularity at top and bottom, hole taper, and spatter size have been studied. The optimum ranges of these parameters have been suggested on the basis of empirical modeling and optimization.

Dielectric relaxation in alumina based ceramics studied by thermally stimulated depolarization current method

Thermally stimulated depolarization currents (TSDC) method was used to investigate the dielectric relaxation processes in alumina based ceramics. TSDC of 99% purity alumina ceramics (A99) and zirconia toughened alumina ceramics (ZTA) with varied polarization electric fields and polarization temperatures were studied. Measurement results show that the TSDC was induced by dipole relaxation. The dipole in A99 ceramics consists of a substitutional Mg2+ ion and a trapped hole on one of the nearest oxygen ions. And the dipole in ZTA ceramics is defect association of YZr'−VO••−YZr'. The orientation of defect dipoles was due to the hopping of oxygen vacancies.

Fabrication of dense zirconia-toughened alumina ceramics through a stereolithography-based additive manufacturing

We report a novel approach to fabricate dense zirconia-toughened alumina (ZTA) ceramics with excellent properties via an additive manufacturing process based on stereolithography. The XRD patterns show the ZTA sample consists of α-Al2O3 and t-ZrO2 with the dominance of α-Al2O3. The zirconia grain with the average size of 0.35µm is small enough to trigger the toughening behavior of zirconia in the ZTA. The prepared ceramics showed a density, Vickers hardness, bending strength, and fracture toughness of 4.26g/cm3, 17.76GPa, 530.25MPa, and 5.72MPam1/2, respectively. These properties are comparable to those for ceramics obtained through conventional ceramic processing.

Selective etching of injection molded zirconia-toughened alumina: Towards osseointegrated and antibacterial ceramic implants.

Zirconia-toughened alumina (ZTA) is the current gold standard for the fabrication of orthopedic ceramic components. In the present work, we propose an innovative strategy to provide both osseointegrative and antibacterial properties to ZTA surfaces: we demonstrate that injection molding allows a flexible design of surface micro-topography and can be combined with selective etching, a novel process that induces nano-roughness and surface interconnected porosity without the need for coating, avoiding reliability issues. These surface modifications have the potential to improve osseointegration. Furthermore, our results show that the porosity can be used for drug delivery and suggest that the etched surface could reduce bacterial adhesion.

Effects of in vitro shocks and hydrothermal degradation on wear of ceramic hip joints: Towards better experimental simulation of in vivo ageing

In an ageing world population, increasing the lifetime of orthopedic devices is of vital importance for reducing surgical risks for patients and reducing medical costs. Ceramic materials are gaining interest because of their hardness and chemical inertia with respect to metal prosthesis. Wear of the prosthetic couple (femoral head and cup) is one of the main rationales for using ceramic hip implants, because they induce much lower wear debris. The present study investigates the combined effects of shocks due to microseparation and hydrothermal ageing on the wear of Zirconia Toughened Alumina (ZTA) bearings, which have been under strong development these past ten years. Wear stripe patterns are formed on femoral heads tested on a shock device. 3D profilometry is used to evaluate the volume of worn material released from the head that averages at 0.46mm3/year during simulation mimicking about 15 years of ageing. This volume is larger than formerly estimated with hip walking simulators with microseparation, an effect that we tentatively attribute to a larger force (9000N) applied here than on hip simulators where the force was not specified. The present experiment should allow refining the effect of force on the wear of ceramic femoral head, hence to simulate different patient conditions (weight, activity, accidents,…). Micro Raman spectroscopy reveals significant zirconia phase transformation within wear stripes. ZTA components show minor phase transformation during hydrothermal degradation in an autoclave when compared to changes induced by shocks. Finally, five clinically retrieved bearings are characterized for in vivo comparison, and also reveal phase transformation within wear track areas. The association of phase transformation enhancement with wear areas on in vivo samples indicates that wear is induced by shocks, rather than by hydrothermal ageing. Shock simulations are therefore essential for in vitro experimentation aiming at reproducing wear of ceramic implants and in vivo environment. On a material׳s point of view, it is shown that the ZTA ceramics can exhibit a beneficial phase transformation under stress, which may limit wear stripes, without suffering from significant ageing.

The capability of hibonite elongated grains to influence physical, microstructural, and mechanical properties of zirconia toughened alumina–CeO2–MgO ceramics

This work investigated the capability of hibonite (CaAl12O19) phase on structure and microstructure of zirconia toughened alumina–CeO2–MgO ceramics. Three different additives (CeO2, MgO and CaCO3) were introduced into zirconia toughened alumina ceramics prepared by solid state reaction. X-ray diffraction and FESEM analyses were employed to observe the role of secondary phases especially hibonite and its influence on the microstructural features and other properties. Among the secondary phases present, EDX analysis revealed that hibonite phase contributed to elongated grains. Vickers indentations hinted a strong difference in the efficiency of compositional adjustment among the composites. The excellent Vickers hardness and fracture toughness results obtained for 3wt.% CaCO3 additions showed the appearance of 5.9% hibonite with the value of 1485HV and 7.10MPa·√m, respectively.

Effect of SrCO3 addition on the dynamic compressive strength of ZTA

Ceramic parts usually experience dynamic load in armor applications. Therefore, studying the dynamic behaviors of ceramics is important. Limited data are available on the dynamic behaviors of ceramics; thus, it is helpful to predict the dynamic strength of ceramics on the basis of their mechanical properties. In this paper, the addition of SrCO3 into zirconia-toughened alumina (ZTA) was demonstrated to improve the fracture toughness of ZTA due to the formation of the SrAl12O19 (SA6) phase. The porosity of ZTA was found to be increased by the addition of SrCO3. These newly formed pores served as the nucleation sites of cracks under dynamic load; these cracks eventually coalesced to form damaged zones in the samples. Although the K IC values of the samples were improved, the dynamic strength was not enhanced because of the increase in porosity; in fact, the dynamic strength of ZTA ceramics decreased with the addition of SrCO3.