Zirconia Toughened Alumina Ceramics Research Articles

Preparation of high-performance zirconia toughened alumina doped with Cr2O3 and SrO for artificial joints by spray granulation

Zirconia-toughened alumina (ZTA) ceramics have attracted much attention in the medical field as joint replacement materials due to their good mechanical properties and biocompatibility. However, the flexural strength of conventional ZTA is still slightly insufficient in traditional preparation techniques. In this study, by improving the slurry system, the effects of the solid content of the slurry, additives, and the pH of the slurry on the slurry system were investigated. ZTA powders with a uniform particle size distribution, high degree of sphericity and high flowability were prepared by spray granulation. The mechanical properties, microstructure and biocompatibility of the ZTA pressing were investigated after degreasing and sintering processes. The results showed that the hardness of the prepared ZTA ceramics was 1658.24 ± 28.04 HV, the fracture toughness was 4.016 ± 0.027 MPa·m1/2, and the biaxial bending strength was 501.43 ± 21.44 MPa, and the three-point bending strength reached 635 MPa. The ZTA ceramics also possessed good biocompatibility and a low haemolysis rate, which aligns with the requirements of clinical biomedical implant materials.

Study of Cu-doped zirconia toughened alumina ceramics (Cu-ZTA) subject to lubrication behaviors for arthroplasty

Zirconia-toughened alumina (ZTA) is widely applied in total hip arthroplasty (THA). However, problems with insufficient fracture toughness and lubrication remain, leading to wear, squeaking, and fracturing. In this study, the Cu doping of ZTA (Cu-ZTA) is proposed to solve these problems. To further promote the use of Cu-ZTA, the role of Cu doping on its tribological properties must be studied. In this study, the microstructure, mechanical properties, and stability capacity in a physiological environment, as well as the tribological behaviors of Cu-ZTA in phosphate-buffered saline (PBS) and bovine serum albumin (BSA), were systematically studied with varying amounts of Cu. The results demonstrated that holes and soft Cu-rich phases appeared, and grains grew after Cu doping, causing a decrease in hardness. The fracture toughness increased after Cu doping because the Cu-rich particles formed “bridges” during crack propagation and release energy. The incorporation of Cu improved the stability of ZTA in physiological environments, which enhanced the long-term stability of Cu-ZTA. The protein adsorption results indicated that a higher hydrophobicity and roughness contributed to a greater protein adsorption on the Cu-ZTA surfaces after Cu incorporation. In the BSA solution, the 5.0 wt% Cu-ZTA ceramics achieved a lower coefficient of friction (COF) and wear rate compared to ZTA, and a higher protein adsorption and formation of the denatured protein layer catalyzed by Cu ions in the wear track during friction, thus clearly improving the tribological properties of the Cu-ZTA.

Grain size distribution analysis of MgO and VC inhibited zirconia toughened alumina ceramics using digital image processing

Abnormal grain growth (AGG) of zirconia toughened alumina (ZTA) during sintering decreases phase transformability at critical temperature, leading material failure. The first-ever grain size distribution analysis using image processing in materials science identifies the impact of carbide and oxide inhibitors on tetragonality and grain refinement of ZTA ceramics. Incorporating insoluble vanadium carbide and soluble magnesium oxide inhibitor in ZTA restricts AGG, reducing average grain size of 0.5VC_ZTA by ∼ 86.84% compared to 0.5 MgO_ZTA due to Zener pinning effect of refractory carbide. The inhabitation of dehydroxylation process helps to decrease alumina crystallite size by ∼33.37% in case of VC, thereby increasing the density. Automated analysis using digital image processing makes the process less affected by user bias. A perceptible improvement in micro-mechanical properties through grain refinement of pressure-less sintered 0.5VC_ZTA confirms the structure property correlation of the matrix. This type of analysis has a beneficial aspect towards the fabrication of advanced ceramics with improved properties.

Zirconia Toughened Alumina Ceramics via Forming Intragranular Structure.

The distribution of second phase particles in the microstructure of composite ceramics affects the mechanical properties, and the intragranular structures often result in better properties compared to the intergranular structures. However, it is difficult to obtain composite ceramics with intragranular structure by conventional route. To produce composite ceramics with an intragranular structure in a simpler route. In this work, starting powders with different phase compositions were obtained by the co-precipitation method, and zirconia toughened alumina (ZTA) composite ceramics were prepared with these starting powders by spark plasma sintering (SPS). The results show that it is easier to fabricate ZTA composite ceramics with an intragranular structure by using composite powders containing amorphous or transition phase Al2O3 as starting materials. The phase composition of the powder prepared by the co-precipitation method after calcination at 1100 °C is θ-Al2O3 and t-ZrO2, and the average grain size after sintering at 1500 °C is 1.04 ± 0.28 µm, and the maximum Vickers hardness and fracture toughness of the specimens reach 19.37 ± 0.43 GPa and 6.18 ± 0.06 MPa·m1/2, respectively. The ZrO2 particles were the core of crystallization and grow together with the Al2O3 matrix, forming the intragranular structure of ZTA ceramics. This work may provide a new idea for preparing composite ceramics with intragranular structure.

Microstructure, mechanical properties, and bone cell interactions of ZTA composites reinforced with BN

Due to their remarkable mechanical properties and biocompatibility, zirconia toughened alumina (ZTA) based composites are preferred materials for dental and orthopedic uses. This study focuses on the production and characterization of hexagonal boron nitride (BN) containing ZTA ceramics. To achieve this, we formulated composite systems with a composition of 70Al2O3-(30-x)YSZ-(x)BN (in vol%, where x ranges from 1 to 5). These mixtures were densified via spark plasma sintering (SPS) at a temperature of 1350 °C under a pressure of 40 MPa, with a dwell time of 5 min. The densification, resulting microstructure, and mechanical properties, including Vickers hardness and indentation fracture toughness, were thoroughly examined. All ZTA-BN composites achieved sintering densities exceeding 98.5% of their theoretical values. Incorporating BNs into ZTA composites increased fracture toughness by ∼55% while maintaining Vickers hardness values. Notably, crack deflection mechanism played a dominant role in toughening, particularly for the composite containing 3 vol% BNs. Furthermore, in vitro biocompatibility assessment of the YZTAB3 sample demonstrated that the composite had no cytotoxic effects towards human bone cells, indicating its potential for biomedical applications.

Effect of TiO2 reinforcement on low temperature (1300 °C) sinterability of zirconia toughened alumina ceramics by correlating its structural and physico-mechanical properties

The primary goal of this research is to investigate the sinterability of zirconia toughened alumina (ZTA) at low temperature (1300 °C) utilizing two different sintering techniques: hot press and cold press. The effect of titanium oxide (TiO2) content variation (0–5 wt%) in ZTA is compared for each sintering method. The solubility limits of TiO2 in ZTA are identified to be 2 wt% and 3 wt% for hot press and cold press sintering respectively which have served exceptional physical and mechanical properties. In the case of hot press, outstanding relative density (99%) is achieved due to excellent compaction, including high microhardness of ∼2092 HV0.5, nanohardness of ∼25.05 GPa, elastic modulus of ∼371.59 GPa and indentation fracture toughness of ∼5.43 MPa·√m for 2 wt% TiO2 inclusion. Whereas in the case of cold press, 3 wt% TiO2 addition has revealed a considerable relative density (96%), alongside microhardness of ∼1975 HV0.5, nanohardness of ∼20.92 GPa, elastic modulus of ∼280.39 GPa and an indentation fracture toughness of ∼4.84 MPa·√m. However, further addition of TiO2 leads to the formation of secondary phases i.e., tialite and zirconium titanate which deteriorate the mechanical and physical properties due to the abnormal grain growth. Therefore, it has been postulated that the small additions of TiO2 have enormous capability of lowering the sintering temperature of ZTA-MgO ceramic without compromising the physical and mechanical characteristics.

The Effects of Compressive Residual Stress on Properties of Kyanite-Coated Zirconia Toughened Alumina Ceramics.

In this study, the prestressed coating reinforcement method was employed to create kyanite-coated zirconia toughened alumina (ZTA) prestressed ceramics. Due to the mismatch of the coefficient of thermal expansion (CTE) between the coating and substrate, compressive residual stress was introduced in the coating. The effects of compressive residual stress on the mechanical properties of ZTA have been demonstrated. Results show that the flexural strength of the kyanite-coated ZTA ceramics improved by 40% at room temperature compared to ZTA ceramics. In addition, the temperature dependence of mechanical properties has also been discussed. And the results show that the reinforcement gradually diminished with increasing temperature and eventually disappeared at 1000 °C. The modulus of elasticity of the material also exhibits a decreasing trend. Furthermore, the introduction of the prestressing coating enhanced the thermal shock resistance, but the strengthening effect diminished as the temperature increased and completely disappeared at 800 °C.

Zirconia-Toughened Alumina (ZTA) Nanoceramics with a Gradient Microstructure: A Comparative Study of ZTA Ceramics with Fibrous and Granular Morphology.

ZrO2-toughened Al2O3 (ZTA) ceramic composites with a porosity gradient and with improved mechanical properties have a wide range of possible applications. We fabricated nanofibrous and nanogranular Y-ZTA and Ce-ZTA composites with a gradient microstructure by creating a temperature gradient during SPS sintering, with the use of asymmetric graphite tool arrangement (ASY). In this study, we examined the morphology effect of the starting materials on the sintering process and on the final microstructure, as well as the mechanical properties of the composites. A large temperature difference was established for both the granular and fibrous samples fabricated in the ASY configuration, which resulted in gradient porosity along the ceramics bodies: the upper part of the ceramics showed a highly porous fine microstructure, while the opposite side was highly densified. The final microstructure of the composites can be tailored by varying the morphology of the starting ceramics or the graphite configuration. A highly porous skeleton-like structure was formed by sintering fibres in the ASY configuration, whereas the granular precursors resulted in a much less porous composite. The microstructure affected the mechanical properties of the composite. Improved hardness and more than 50% higher compression strength were obtained for the granular Ce-ZTA samples as compared to the fibrous sample. Gradient porosity with fibrous or granular morphology promotes the penetration of bioactive nanosized hydroxyapatite (HAp) into the pore structure. Fibrous ZTA absorbs HAp more effectively due to its higher porosity as well as its bimodal pore structure.

Mechanical properties and wear behaviors of Cu-doped zirconia-toughened alumina ceramics

Cu-doped, ZrO2-toughened alumina (ZTA) ceramics with varying amounts of Cu (0, 1.5, 3.0, and 5.0 wt%), denoted as Cu/ZTA, were prepared using a fast hot pressing sintering furnace. The microstructure, mechanical properties, tribological properties, and wear mechanism of Cu/ZTA ceramics were subsequently studied. The results showed that Cu doping can cause the formation of holes in the ZTA ceramics. The formation of holes originated from the O2 produced by CuO decomposition at high temperatures and the phase transformation of ZrO2. Cu/ZTA ceramics with 5.0 wt% Cu exhibited significantly improved toughness and tribological properties but diminished hardness. The worn surface analysis of the Cu/ZTA ceramics revealed that their wear mechanism primarily involved abrasive wear and material transfer. The ZrO2 and Cu particles were first pulled out from the wear track to form wear debris during friction. The 5.0 wt% Cu/ZTA ceramic exhibited decreased hardness and high toughness, which made it easier for the abrasive debris to adhere to the surface and holes. Additionally, the wear debris was crushed into the holes formed by Cu doping. The results indicated that a reduction in the number of wear particles at the friction interface could impede the wear of the abrasive particles. Overall, the Cu/ZTA ceramic with 5.0 wt% Cu was found to be a promising structure material with excellent tribological properties.

Material extrusion-based additive manufacturing of zirconia toughened alumina: Machinability, mechanical properties and biocompatibility

Material extrusion-based additive manufacturing (MEAM) has gained significant attention due to its versatile flexibility for fabricating a broad range of materials. In this study, fabrication of dense zirconia-toughened alumina (ZTA) ceramics with micro/nano powders of excellent mechanical and biological properties was investigated. The ZTA paste rheology was engineered to achieve appropriate rheological behavior. A novel machining scheme on MEAM produced ZTA green parts using CNC milling and high precision micro-milling was studied to investigate their effect on surface integrity and geometric accuracy. The MEAM produced ZTA ceramics show the highest relative density, Vickers hardness, bending strength, and fracture toughness of 99 %, 17.7 GPa, 422.5 MPa, and 6.2 MPa*m1/2, respectively. Biocompatibility of fabricated ZTA ceramics with different size of powders was investigated including cell adhesion, cell toxicity and cell proliferation, showing a good potential in biomedical usage. This framework provides a unique pathway for MEAM of ZTA ceramics to combine design and manufacturing freedom, multifunctionality, stability, and economical simultaneously.

Seamless joining of ZTA ceramics without the application of pressure

Seamless joining of zirconia-toughened alumina (ZTA) ceramics without the application of any external pressure was demonstrated in this work through a reaction-bonded aluminum oxide (RBAO) method. Reliable joining of ZTA ceramics in a green-state was successfully accomplished in ambient air at 1600 °C for 2 h with the use of an Al/3Y–ZrO 2 /Al 2 O 3 powder mixture as joining interlayer. The joining mechanism was discussed based on the RBAO process where the effective joining principally relies on the conversion of all Al particles to alumina. Thermogravimetry and differential thermal analysis confirmed the oxidation behavior of the milled Al particles. The joining interface exhibited excellent compositional and structural homogeneity without defects in commensurate with parent bodies. The mechanical properties of the joints were nearly equivalent to bare ZTA ceramics. This proposed simple and cost-effective joining technique can be used for the joining of other alumina-based advanced ceramics to realize large and complex-structures.

Improved ageing-resistance and fracture toughness of zirconia-toughened alumina bioceramics via composition and microstructure design

Low-temperature degradation (LTD) is the main obstacle that hinders the application of zirconia-toughened alumina (ZTA) ceramics as artificial hip joints. In this study, a new type of ZTA bioceramic with LTD resistance, high fracture toughness, and superior wear resistance was prepared by the in-situ formation of plate-like crystals and co-stabilised tetragonal phase zirconia (t-ZrO2). This new design is realised by synthesising nanocrystalline PrAlO3 through a one-step solution combustion method and introducing it into a ZTA matrix to form praseodymium hexaaluminate (Pr0.833Al11.833O19, PHA) plate-like crystals by solid-state sintering. PHA plays a key role in toughening and Y–Pr co-doped t-ZrO2 slows down the low-temperature degradation of ZTA bioceramics. After hydrothermal ageing, the combination strategies have a positive influence on the flexural strength, fracture toughness, and Vickers hardness of the ceramics. This study provides a novel direction for ensuring long-term safety of bioceramics.

Microstructure, mechanical properties, friction and wear performance, and cytotoxicity of additively manufactured zirconia-toughened alumina for dental applications

Zirconia-toughened alumina (ZTA) ceramic has potential dental restoration and repair applications due to its high mechanical strength, toughness, wear resistance, chemical durability, and biocompatibility. Nevertheless, there is a great deal of crack initiation caused by stress accumulation and also material wastage after computer-aided design/computer-aided manufacturing of ZTA ceramics in conventional clinical processing. In this study, we prepared a ZTA green compact using Stereolithography (SLA) technology followed by sintering at high temperatures ranging between 1450 °C and 1650 °C. The microstructures, mechanical properties, friction and wear performance, and cytotoxicity of the ZTA ceramic were investigated. The results indicate that all ZTA samples were composed of α-Al2O3, m-ZrO2, and t-ZrO2 phases before and after sintering. With increasing sintering temperatures from 1450 °C to 1650 °C, the density, mechanical properties, hardness, and wear performance of the ZTA samples were gradually improved. The ZTA sample sintered at 1650 °C with a relative density of 96.65% showed the best mechanical performance, with a flexural strength of 572.0 MPa, a Vickers hardness of 16.2 GPa, and a fracture toughness of 7.4 MPa m1/2. The sintered ZTA sample exhibited a good cell viability toward MC3T3-E1 cells.

Preparation and Characterization of Fibrous Alumina and Zirconia Toughened Alumina Ceramics with Gradient Porosity

This paper investigated a synthesis process for highly porous Al2O3, Y-ZTA, and Ce-ZTA ceramic nanocomposites with gradient microstructure and improved mechanical properties. Ceramic nanofibres were synthesized as the starting material. The gradient microstructure was developed during spark plasma sintering using an asymmetric graphite arrangement that generated significant temperature differences (80-100 °C) between the opposite sides of the samples. Structural and mechanical properties of the fibrous ceramic composites were investigated. The effect of the temperature gradient on properties was also discussed. While the asymmetric configuration resulted in a gradient porosity, reference samples fabricated in standard graphite configuration were uniformly porous. The gradient structure and the ZrO2 addition led to improved hardness and compression strength of the sintered samples. However, the opposite sides of the samples exhibited considerable variations in both microstructure and in terms of properties. The upper part of the Ce-ZTA ceramic showed a highly porous structure with 18.2 GPa hardness, while the opposite side was highly densified with 23.0 GPa hardness. Compressive strength was 46.1 MPa and 52.1 MPa for Y-ZTA and Ce-ZTA sintered at 1300 °C, respectively, despite their high porosity. The research provided a promising approach to prepare highly porous ZTA composites with high strength for a wide range of applications.

Influence of Cu doping on the biocompatibility of zirconia-toughened alumina ceramics for artificial joints

Zirconia-toughened alumina (ZTA) is considered the gold standard material for total hip arthroplasty (THA). However, it still lacks sufficient lubrication and fracture toughness, and it has the risk of wearing, squeaking, and fracturing. Cu doping is considered to address these issues. To further promote the application of Cu doping, its effect on biocompatibility needs to be studied. In this study, Cu-doped ZTA (Cu-ZTA) ceramics with different Cu contents were prepared using a fast hot pressed sintering (FHPS) equipment and their biocompatibility was systematically studied. The results demonstrate that, depending on the Cu content, Cu doping reduces the stability of the tetragonal zirconia, resulting in a tetragonal-to-monoclinic phase transition of zirconia, thereby creating pores on the surface of the ZTA. The biocompatibility of the Cu-ZTA with different Cu contents was analyzed based on osteoblasts and macrophages. The results suggested that Cu doping increased the adhesion of osteoblasts and slightly improved their viability. In contrast, Cu doping influenced the morphology of macrophages, causing the shrinkage and extension of their pseudopodia. Cu doping also induced the pro-inflammatory polarization of macrophages and stimulated the secretion of pro-inflammatory cytokines (TNF-α and IL-6). Therefore, Cu doping can improve the cytocompatibility of osteoblasts with ZTA ceramic surfaces, however, the enhanced effect of Cu-doped ZTA ceramics on the early inflammatory response requires special attention and further study.

High temperature tribological performance of molybdenum reinforced zirconia toughened alumina composites prepared by pressure-less sintering

Molybdenum (Mo) reinforced zirconia-toughened alumina (ZTA) composites are a vital class of self-lubricating ceramics with potential applications as seals, dies and cutting tools. However, their tribological behaviours under elevated temperatures remain largely unexplored. The present study investigates the tribological performances of pressure-less sintered ZTA ceramics containing various weight percentages (0, 5, 10 and 15 wt %) of Mo at elevated temperatures (500 °C and 750 °C). Formation of self-lubricating Mo oxides at tribo-interfaces was observed when the temperature increases from 500 °C to 750 °C. The least friction coefficient (0.34) and specific wear rate of (1.057 × 10−4 mm3/N-m) were observed at 750 °C for 15 wt % Mo – ZTA and 10 wt % Mo – ZTA composites respectively. Phase analysis, and morphological and elemental studies on the wear tracks confirmed the presence of molybdenum-oxide-rich thin lubricating tribo-films on the worn surface, and suggest adhesion as the dominant wear mechanism.

Simultaneous effect of CaF2 and TiC on tribological properties of ZTA ceramics for high temperature application

Liquid lubrication poses risk to environment; it can vaporize under high temperature or vacuum. Solid lubricant has the potential to replace liquid lubricants due to better stability at severe working environment. This work has been focused to improve tribological properties of zirconia toughened alumina (ZTA) by adding an optimized 4 wt% of CaF2 as a solid lubricant. Since CaF2 is softer material it degrades the mechanical properties of ZTA. To make a balance between the mechanical and the tribological properties, a comparatively harder material TiC is added simultaneously by 10 wt%. All the ceramic composites have been consolidated by hot press sintering at 1500 °C with 30 mins holding time under uni-axial pressing of 50 MPa. Four samples: ZTA, ZTA_CaF2, ZTA_TiC and ZTA_TiC_CaF2 were prepared through the same route. The relative density of the sample (ZTA_CaF2_TiC) achieved was 96.4%. The hardness and fracture toughness has been measured as ∼15GPa and ∼5 MPa m1/2 respectively for the hybrid ceramic composite which is better than ZTA_CaF2 ceramic composite. The tribological characteristics have been evaluated using high temperature tribo-meter with a ball on disc arrangement at 600 °C in vacuum condition. The COF of the ZTA_CaF2 has also been improved by ̴ 39 % compared to the main composite due to the self-lubricating film formation on wear track. A considerable improvement as ∼24.5% in coefficient of friction (COF) and ∼40% in specific wear rate has been found in hybrid ceramic composite as compared to the ZTA ceramic. Thus, it can be used for developing self-lubricating components used in high temperature application.

Is Surface Metastability of Today’s Ceramic Bearings a Clinical Issue?

Recent studies on zirconia-toughened alumina (ZTA) evidenced that in vivo aged implants display a much higher monoclinic zirconia content than expected from in vitro simulations by autoclaving. At the moment, there is no agreement on the source of this discrepancy: Some research groups ascribe it to the effect of mechanical impact shocks, which are generally not implemented in standard in vitro aging or hip walking simulators. Others invoke the effect of metal transfer, which should trigger an autocatalytic reaction in the body fluid environment, accelerating the kinetics of tetragonal-to-monoclinic transformation in vivo. Extrapolations of the aging kinetics from high (autoclave) to in vivo temperature are also often disputed. Last, Raman spectroscopy is by far the preferred method to quantify the amount of monoclinically transformed zirconia. There are, however, many sources of errors that may negatively affect Raman results, meaning that the final interpretation might be flawed. In this work, we applied Raman spectroscopy to determine the monoclinic content in as-received and in vitro aged ZTA hip joint implants, and in one long-term retrieval study. We calculated the monoclinic content with the most used equations in the literature and compared it with the results of X-ray diffraction obtained on a similar probe depth. Our results show, contrary to many previous studies, that the long-term surface stability of ZTA ceramics is preserved. This suggests that the Raman technique does not offer consistent and unique results for the analysis of surface degradation. Moreover, we discuss here that tetragonal-to-monoclinic transformation is also necessary to limit contact damage and wear stripe extension. Thus, the surface metastability of zirconia-containing ceramics may be a non-issue.

Interfacial characterization and erosive wear performance of zirconia toughened alumina ceramics particles reinforced high chromium white cast irons composites

To investigate the erosive wear-corrosion behavior of ZTAP ceramic reinforced high chromium cast iron matrix composites in two types of sand at various rotational speeds, the ZTAP/Fe composites were fabricated. The interface characteristics, erosion wear-corrosion and erosive wear mechanism have been investigated in a slurry condition at various erosion rotational speeds. The chemical composition of the matrix material was determined using a spark spectrometer. SEM, TEM, 3D laser scanning microscope and AFM were used to analyze the microstructure characteristics and worn surface morphology of the composites. Furthermore, the wear loss rate and relative erosive wear resistance of ZTAP/Fe composites and Cr15 standard specimen measurements were conducted. The results showed that the interface between the ZTAP ceramics and the Cr15 matrix exhibits a continuous transition layer without visible defects and close contacts. In the case of SiO2 and SiC abrasive sand, the wear rate of ZTAP/Fe composites and Cr15 standard specimens increases with erosion time. The wear rate of ZTAP/Fe against SiO2 sand is lower than that of against SiC sand. ZTAP/Fe composites was found to exhibit a better corrosion resistance property than that of Cr15 standard specimens.

Joining ZTA ceramics by using whiskers reinforced borosilicate glasses for high-temperature applications

We put forward a novel strategy for joining zirconia-toughened alumina (ZTA) ceramics using whiskers reinforced borosilicate glasses for high-temperature (HT) applications. Herein, the robust joining of ZTA ceramics by using B2O3-Al2O3-SiO2 (BAS) glasses as an interlayer was achieved in air at 1400 °C. The Al18B4O33 whiskers were generated in the joining seam during the joining process, thus strongly improving the HT strength. The flexural strength of the joint after the thermal cycle test at 1200 °C could still reach 84 % of that at room temperature (RT). The joining mechanism was mainly addressed from the perspective of the in-situ growth of whiskers.