Zirconia Toughened Alumina Composites Research Articles

X-Ray Diffraction Analysis of Mechanically Milled Alumina and Zirconia Powders

Ball milling is one of the top down approach used for reducing the particle size of bulk powder. Especially high energy ball milling is done to reduce the particle size to nanodomain. The Zirconia Toughened Alumina nanocomposite has diverged application in different engineering fields. The alumina and zirconia powders used for fabrication of Zirconia Toughened Alumina composite are subjected for ball milling. The effect of ball milling on the powders is reported on the work. The characterisation of the powder samples were done by X-ray diffraction. This was done to evaluate the effect of the starting material on the final product. Using the Scherer’s formula, Williamson-Hall analysis the change in crystallite size and strain were analysed.

Slow crack growth and hydrothermal aging stability of an alumina-toughened zirconia composite made from La2O3-doped 2Y-TZP

A very tough zirconia matrix is interesting to fabricate alumina-toughened zirconia (ATZ) and composites generally processed from 3Y-TZP do not exhibit very high toughness. The strategy of lowering the yttria content to increase toughness however is normally associated with an increased hydrothermal aging susceptibility. In this work, a 0.4mol% La2O3 doped 2Y-TZP matrix was investigated to realize a 20wt.% alumina toughened zirconia composite with a substantially high aging resistance. The higher transformation toughening in the composite shifted the V-KI towards higher KI values, while preserving the slope of the curve, resulting in a threshold KI0 of 4.0MPam1/2 and fracture toughness (KIC) of 7.1MPam1/2. These composites can offer a better compromise between aging and crack resistance than traditional 3Y-TZPs and plain ATZ composites without La2O3 doping.

Development and characterization of zirconia–alumina composites for orthopedic implants

In this study, three different grades of alumina toughened zirconia composites (ATZ) (from 80 to 90wt% of zirconia), and three grades of zirconia toughened alumina (ZTA) from 80 to 90wt% of alumina were developed. Two types of stabilized zirconia were used: 3mol% yttria stabilized zirconia used on the ATZ, and 2mol% yttria stabilized zirconia for the ZTA. The composite powders were obtained by spray-drying with a controlled granulometric distribution. Uniaxial and cold isostatic pressing were performed. High density ceramics with densification above 97% were obtained after a low temperature sintering (1400°C) leading to enhanced mechanical properties. Higher values of fracture toughness and flexural strength were achieved for the ATZ samples (up to 5MPam1/2 and 1394MPa respectively), while ZTA samples presented higher values of hardness (up to 1846HV). Aging tests showed that ZTA samples did not present monoclinic zirconia after 96h. MTT and alkaline phosphatase activity assays were performed on the sintered samples. This study shows that ATZ and ZTA composites with good mechanical properties, aging resistance and osteoblastic cytocompatibility can be attained.

Effect of Cr2O3 derived from Cr(NO3)3·9H2O precursor on the densification and mechanical properties of zirconia-toughened alumina (ZTA) composites

The effects of Cr2O3 derived from Cr(NO3)3·9H2O precursor on the densification, microstructure and mechanical properties of zirconia-toughened alumina (ZTA) ceramics were investigated. The Cr2O3 content in ZTA varied from 0 to 0.8wt%. Cr2O3 derived from Cr(NO3)3·9H2O can form solid solutions with Al2O3 and ZrO2 in ZTA more efficiently than Cr2O3 additive directly added to ZTA. The precursor-derived Cr2O3 can significantly promote the densification of ZTA and nearly fully dense ZTA composites were obtained by pressureless sintering at 1540°C for 2h. In contrast, the effect of directly added Cr2O3 additive on densification of ZTA was limited due to its low amount of dissolution in ZTA. ZTA with 0.5wt% Cr2O3 derived from Cr(NO3)3·9H2O had fine microstructure and its fracture toughness was as high as 4.67±0.03MPam1/2 which was about 30% higher than that of undoped ZTA and ZTA with 0.5wt% Cr2O3. ZTA with 0.5wt% precursor-derived Cr2O3 had the maximum Vickers hardness about 18.10±0.15GPa. Higher content of Cr2O3 (0.8wt% or higher) would have adverse effect on the densification due to the vaporization and condensation of Cr-containing species during pressureless sintering.

Shrinkage characteristics studies on conventional sintered zirconia toughened alumina using computed tomography imaging technique

Shrinkage is an important issue during sintering of ceramics as it affects the structural and mechanical properties. In the current work, the shrinkage characteristics of sintered zirconia toughened alumina (ZTA) compact produced by conventional powder forming process was investigated by using computed tomography imaging technique. The effect of process parameters such as weight percentage of zirconia added to toughened alumina, compaction pressure, and sintering temperature on ZTA was studied. The Box–Behnken technique in response surface methodology was used to develop the experimental design to analyze the shrinkage phenomena. The sintered samples were subjected to computed tomography scan and analyzed by using MIMICS Software wherein the three dimensional shrinkage of the ZTA composites was done. The mathematical regression model relating powder forming process parameters to the shrinkage was developed. It was observed that due to the effect of gravity, shrinkage is found to be 20% in the top for 1600°C sintered sample. From the study shrinkage of the sintered ZTA was influenced by the sintering temperature. Being a non-contact type digital measurement method the odds of error created by instrument or human was avoided. Shrinkage of three directional complex shapes can also be identified effectively with the help of solid modeling packages.

Effect of graphene and CNFs addition on the mechanical and electrical properties of dense alumina-toughened zirconia composites

Fully dense carbon/alumina-toughened zirconia composites were prepared by using a combination of aqueous colloidal powder processing and spark plasma sintering technique (SPS). Various carbon elements were introduced in alumina-toughened zirconia matrix (ZA) as filler; carbon nanofibers (CNFs) and graphene oxide (GO). The influence of the addition of different carbon forms on the microstructure and on the mechanical and electrical properties was investigated. In the case of the ZAGO composites, the SPS technique allowed, in one-step, the in situ reduction of the graphene oxide during the sintering process. The fracture toughness increases for ZAGO composites in comparison to the ZA material while the hardness decreases slightly with carbon elements addition. The electrical conductivity of the ZA composite drastically increased with the addition of graphene oxide, and it reached 10Ωcm at 2vol%.

Physical and mechanical properties of Ta2O5 doped zirconia-toughened alumina (ZTA) composites

The aim of this research is to investigate the characteristics and mechanical properties of Ta2O5 doped zirconia-toughened-alumina composites. Samples of partially stabilized-zirconia toughened alumina (ZTA) with 5% volumetric ZrO2 addition doped with Ta2O5 in amounts ranging between 0.12 and 0.36vol% were prepared by sol–gel technique. The powders of each composition were uniaxially pressed into discs and rectangular bars, and then sintered at 1500°C up to 1650°C for 1h in air. The phase constitution and microstructures of sintered ceramic composites were characterized using X-ray diffractometer and scanning electron microscope respectively. The mechanical properties of the ceramic composites were evaluated by the Vickers hardness, flexural strength and fracture toughness tests. It was found that increase in Ta2O5 concentration has a strong effect on the densification behavior and mechanical properties of the ZTA composites.

Investigation of the microstructure, mechanical properties and cell viability of zirconia-toughened alumina composites reinforced with carbon nanotubes

Investigation of the microstructure, mechanical properties and cell viability of zirconia-toughened alumina composites reinforced with carbon nanotubes

The Effect of Zirconia Particle Size on Mechanical Properties of Zirconia Toughened Alumina

【The purpose of this study was to investigate the microstructures and mechanical properties of zirconia toughened alumina (ZTA) ceramics prepared from two kinds of 3Y-TZP powders. ZTA composites were prepared by adding two kinds of 3Y-TZP powders, 3YEH (BET = $7m^2/g$ ) and 3YEM (BET = $16m^2/g$ ), to ${\alpha}$ -alumina in the range of 5-25 wt%. It was found that the microstructure photographs of the ZTA composites showed that the average grain size of alumina decreased as the content of zirconia increased. In our present study, specimens containing 3YEM zirconia exhibited smaller grain sizes compared to those of 3YEH zirconia. The Vickers hardness of the ZTA composites that were sintered at $1600^{\circ}C$ for 2 hrs was found to smoothly decrease with increasing zirconia content because of the low Young modulus in zirconia. The Vickers hardness of the ZTA containing 3YEH zirconia was greater than that of the 3YEM zirconia. In substance, the fracture toughness ( $K_{1c}$ ) of the ZTA composites increased as the content of zirconia increased. The fracture toughness ( $K_{1c}$ ) of ZTA containing 3YEM zirconia was greater than that of 3YEH zirconia.】

Structural evolution of ZTA composites during synthesis and processing

Abstract We have studied the structural properties of industrial Zirconia Toughened Alumina (ZTA) composites for different compositions of zirconia and yttria at the main stages of the process. Influences on phases stability, lattice parameters, homogeneous and non-homogeneous strains, particles and crystallites sizes, have been considered and discussed. The structural states are explained from a very delicate balance of three competing effects: yttria-doping, strain and size effect. The first and third one have been shown to stabilize the tetragonal phase relatively to the monoclinic phase, and to destabilize the tetragonal phase relatively to the cubic phase whereas the second one is doing the opposite. In addition to this competition, the most prominent local effects have also being considered to fully understand the microscopic situation: i.e. the micro-strains and the inhomogeneous distribution of yttria which both increases the metastable coexistence of phase. At high yttria content a core–shell structure of the grains is evidenced.

Effect of Zirconia Substitution on Structural and Mechanical Properties of ZTA Composites

Zirconia Toughened Alumina (ZTA) composites are attractive structural materials, which combine the high hardness and elastics modulus of the alumina matrix with additional toughening effects, due to the zirconia dispersion. In the present study, ZTA composites containing different amounts of zirconia (in the range 0-16 wt %) were prepared by slurry method and sintered at 1450oC and 1550oC for two hour. Microstructure of the composites was observed using scanning electron microscopy and phase content was detected by means of X- ray diffraction. Mechanical properties such as hardness, elastic modulus and flexural strength were measured. Density was increased with the increase of zirconia content for both sintering temperature, while grain size and elastic modulus was decreased with the same change of zirconia content. Hardness and flexural strength were carried out using the Vickers indentation and were discussed as a function of the ZrO2 content and sintering temperature. The toughening effect is attributed mainly to the zirconia t→m phase transformation. About 61% more flexural strength was found for 16 wt % ZTA composites when compared to the pure alumina.

Low temperature, spark plasma sintering behavior of zirconia added by a novel type of alumina nanofibers

A novel type of alumina nanofibers has been used for fabrication of the alumina toughened zirconia composites and the effect of different sintering techniques on the characteristics of the material was studied. ZrO2–10vol% Al2O3 composite was consolidated by spark plasma sintering (SPS) in a temperature range from 1000°C to 1200°C and by pressureless sintering in air at 1300°C. Dilatometry sintering of the said composite material was performed in order to assess its sinterability. Al2O3 used in fabrication of the composite is a novel type of alumina nanofibers and thus the effect of different sintering techniques on the formation of the material was investigated. Microstructural analysis has revealed that Al2O3 nanofibers partially retained their fiber states after being sintered at 1100°C under SPS conditions while some of the fibers turned into rounded inclusions. Specimens produced by the SPS routine at temperatures of 1100°C and 1200°C were consolidated 93% of theoretical density while the same material sintered with the conventional PM method in air had a densification degree of 74%.

X-ray diffraction evidence of a phase transformation in zirconia by the presence of graphite and carbon nanotubes in zirconia toughened alumina composites

We have demonstrated by X-ray diffraction a transformation from the t-ZrO2 phase into the m-ZrO2 phase by the presence of multiwall carbon nanotubes (MWCNTs) and graphite powder (GP) in zirconia toughened alumina composites sintered in air. The phase transformation does not occur in sintering under an argon atmosphere. The quantitave phase analysis was carried out by fitting the X-ray diffraction profile with the Rietveld method making use of the FullProf software. The obtained amounts of m-ZrO2 phase by the presence of GP and MWCNTs were 5% and 2.2%, respectively, at a sintering temperature of 1520°C for 1h.

Alumina toughened zirconia from yttria coated powders

Abstract Alumina toughened zirconia (ATZ) ceramics are attractive materials for biomedical implants and other engineering applications requiring high strength and abrasion resistance at ambient temperature. As the toughness of conventional ATZ composites is moderate it was tried to improve the mechanical properties by starting from a very tough and transformable 2.5Y-TZP powder derived from powder coating. 2.5Y-TZP and ATZ materials with 20–40 vol% of alumina were produced and tested with respect to mechanical properties, microstructure and phase composition. ATZ materials with a bending strength of up to 1900 MPa and a fracture toughness of 4.8–5.5 MPa √m were obtained. Transformability of the TZP distinctly declines with rising alumina content. Calculation of cooling and transformation stresses of the materials qualitatively confirms the measured toughness values. Alumina addition initially induces tensile residual cooling stress in the zirconia matrix which facilitates phase transformation. Formation of compressive stresses during transformation limits the transformability and toughness.

Wire-EDM of ZTA-TiC Composites with Variable Content of Electrically Conductive Phase

Electrical Discharge Machining (EDM) enables an economical production of high performance ceramics with high hardness and toughness in complex shapes and fine structures. Recent research work identified ZTA-TiC composites as one of the most promising ED machinable materials with high strength for injection molds, which significantly reduce abrasive wear. In order to improve the surface finish and processing time of ED-machined ceramics zirconia toughened alumina composites with addition of 20-30 vol.-% titanium carbide as electrically conductive phase were developed. Processing was performed by hot pressing at 50 MPa and 1525°C. Mechanical and electrical properties were investigated. The influence of TiC volume fraction on the surface quality after wire-EDM and the maximum possible sample feed rate was analyzed. The variation of TiC content moderately influences toughness and bending strength while electrical resistivity and indentation modulus strongly depend on the volume fraction of TiC. A competitive feed rate of 2.45 mm/min for samples with a height of 13 mm was obtained. Further improvement of the surface quality can be achieved by increasing the amount of the electrically conductive phase. Additional machining steps proved feasible to decrease significantly the surface roughness and corrugation depth without inducing any defects in bulk material.

Improving the surface hardness of zirconia toughened alumina (ZTA) composites by surface treatment with a boehmite sol

This paper reports a novel way of enhancing the hardness of a zirconia-toughened alumina (ZTA) composite with a zirconia content of 20vol% by surface treatments with a boehmite sol. More specifically, a ZTA composite was first prepared by heat-treating a mixture of alumina and zirconia powders containing Cr2O3 and SrAl11CrO19, as a reinforcement at 1400°C for 1h, and then infiltrating them with the boehmite sol, followed by heat-treatment at 1650°C for 1h to densify them. This treatment led to a significant increase in the surface hardness of the ZTA composite, which was attributed mainly to an increase in the volume fraction of an alumina phase with greater hardness, whereas the flexural strength and fracture toughness decreased slightly. The Vickers hardness, flexural strength and fracture toughness were 17.1±2.5GPa, 738±88MPa and 4.2±0.11MPam1/2, respectively.

Effects of Ca-, Mg- and Si-doping on microstructures of alumina–zirconia composites

The aim of this work was to obtain zirconia toughened alumina composites with different microstructures, using a simple process (powder mixing and natural sintering). Adjusting the amount of zirconia directly controls the size and localization of zirconia grains and the size of the alumina grains. Doping the composite with CaO, MgO and SiO2 allows further control of the microstructures. The influence of the thermal treatments is also investigated. The composites exhibit different structures (nano/nano-, micro/nano- and micro-composites) with zirconia and alumina grains as small as 100 and 200 nm, respectively, and with the proportion of intragranular zirconia grains varying between 0% and 90%. Zirconia plays a major role on grain size distributions as compared to CaO and MgO, whose role is almost negligible. The use of SiO2 leads to micro/nano composites with intragranular zirconia particles. The influence of these different additions is related to adjustments of the grain boundaries mobility.

Evaluation of mechanical reliability of zirconia-toughened alumina composites for dental implants

Zirconia-toughened alumina composites containing 0–30vol% of 3Y-TZP were fabricated by sintering at 1600°C for 2h in air. The effect of the 3Y-TZP content on the mechanical properties and microstructure of the alumina ceramics was investigated. The fracture toughness and biaxial flexural strength increased as the 3Y-TZP content increased. The Young's modulus decreased with 3Y-TZP content according to the rule of mixture, while the hardness showed the contrary tendency. The Weibull modulus of the Al2O3 with 20vol% 3Y-TZP composite is higher than that of alumina. The residual hoop compressive stress developed in ZTA ceramic composites probably accounts for the enhancement of strength and fracture toughness, as well as for the higher tendency of crack deflection. No monoclinic phase and strength degradation were found after low temperature degradation (LTD) testing. The excellent LTD resistance can be explained by the increased constraining force on zirconia embedded in alumina matrix.

Hard and tough carbon nanotube-reinforced zirconia-toughened alumina composites prepared by spark plasma sintering

It is demonstrated that 0.1 wt% of multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs) added to zirconia toughened alumina (ZTA) composites is enough to obtain high hardness and fracture toughness at indentation loads of 1, 5, and 10 kg. ZTA composites with 0.01 and 0.1 wt% of MWCNTs or SWCNTs were densified by spark plasma sintering (SPS) at 1520 °C resulting in a higher hardness and comparable fracture toughness to the ZTA matrix material. The observed toughening mechanisms include crack deflection, pullout of CNTs as well as bridged cracks leading to improved fracture toughness without evidence of transformation toughening of the ZrO 2 phase. Scanning electron microscopy showed that MWCNTs rupture by a sword-in-sheath mechanism in the tensile direction contributing to an additional increase in fracture toughness.

Pressureless sintering of gelcast ZTA–MgO–TiO2 systems as potential dental ceramics

The pressureless sintering behaviour of gelcast zirconia toughened alumina (ZTA) containing the total 5 wt-% additives of MgO and TiO2 was investigated. The effects of four combinations of MgO and TiO2 on the bulk properties, morphology and phase composition of the ZTA composites were evaluated in comparison with pure ZTA in the temperature range of 1150–1600°C. Four combinations of MgO and TiO2 initiated effective densification and enhancement at 1450°C, as indicated by the relative density and three-point bending strength measurements and fracture morphological observations by using scanning electron microscopy. X-ray diffraction analyses confirmed that the difference in the bending strength among four groups of ZTA composites was due to the MgO dependent formation of minor MgAl2O4 and TiO2 induced polymorphic transformation of tetragonal phase to monoclinic ZrO2. These results are useful to further develop dental ceramics, including glass infiltrated ZTA.