Zirconia Toughened Alumina Research Articles

Effect of Cr<sub>2</sub>O<sub>3</sub>-TiO<sub>2</sub> Addition on the Physical Properties of Zirconia Toughened Alumina

Zirconia toughened alumina (ZTA) has good mechanical properties and widely used in cutting insert application. Although the ZTA is well known of its good mechanical properties, its still can be improved in order to produce better properties. It can be done by adding additives as reinforcement such as magnesium oxide (MgO), cerium oxide (CeO), titania (TiO2) and chromia (Cr2O3). The effect of TiO2-Cr2O3 addition on the physical properties of ZTA were investigated in this study. The composition of TiO2 was varied from 0 wt% to 3.5 wt% while Cr2O3 was fixed at 5 wt%. The starting powder materials were mixed by wet mixing for 30 minutes in acetone. Then the powder mixtures were hydraulically pressed at 260 MPa. The green pellets were sintered at 1600°C for 1 hour using an electrical furnace in presureless condition. The results were characterized by XRD, density, and also Vickers hardness. The ZTA-Cr2O3-TiO2 ceramic composite achieved the highest density of 4.1 g/cm3 and Vickers hardness of 1919 HV prior to the addition of 2.0 wt% TiO2. Therefore, it can be decisively concluded that the addition of Cr2O3-TiO2 does affected the properties of ZTA.

Synthesis and characterisation of nano powders for production of zirconia toughened alumina bioceramic implant materials.

Zirconia toughened alumina (ZTA) is a very important bio ceramic implant material with excellent biocompatibility as an inert bone substitute material for orthopedic applications and also for dental implants. In this study, alumina (Al2O3) and yttria stabilised zirconia (YSZ) nano powders in soft agglomerate form were synthesised by co-precipitation method. Aluminium nitrate {Al(NO3)3•9H2O}, zirconium oxynitrate {ZrO(NO3)2•xH2O} and yttrium nitrate hexahydrate {Y(NO3)36H2O} were dissolved in distilled water. NaOH solution was added as precipitant. The solution was turned into gel by mixing on hot plate magnetic stirrer at ambient temperature. In this way, Al2O3 and YSZ nano powders were synthesised together to produce the ZTA bioceramic implant material. The crystal structure, morphology and other physical properties of synthesised nano powders were investigated and characterised by scanning electron microscope (SEM) and X-ray diffraction (XRD). The composition ratio of ZrO2/Al2O3 and co-precipitation conditions had significant effect on the synthesised powder morphology and particle size and distribution in final product. The powders have a different morphology like formless and spherical and have the particle size distribution in the range of 100-900 nm. The final products have a very high relative sinter density about 91%, a small particle size of < 2 µm and homogeneity of particle and porosity distribution in microstructure.

Synthesis and Characterization of Stabilized Zirconia Toughened Alumina (ZTA) Doped Niobium Pentoxide (Nb2O5)

The main target of this research is to study the mechanical properties of the Nb2O5 doped with alumina-zirconia composites. Pellets form samples of ZTA (5YSZ) doped niobium pentoxide in varied amount of 0 wt%, 0.3 wt%, 0.5 wt% and 0.7 wt% were prepared by solid-state conventional method with respective 4:1 ratio or alumina and 5YSZ. The effect of different dopant addition on the morphology, phase transformation and hardness was studied. Each mixture of the compositions were calcined at 1000°C followed by pressing at 220 MPa and then they were sintered at 1650°C with heating rate 10°C min -1 and holding for 2 hours in air. Phase transformation, phase quantification and crystal structure analysis was revealed by using X-ray diffraction data with the help of X-Pert HighScore Plus software using Rietveld Refinement Method while microstructure was observed by scanning electron microscopy. The hardness of the composite is determined by using Vickers hardness test. It found that with 0.5 wt% niobium pentoxide addition would give a great effect on the microstructure distribution, densification and mechanical properties of the ZTA-doped sample.

Investigation of Ablation Behaviour in Micro-EDM of Nonconductive Ceramic Composites ATZ and Si3N4-TiN

Micro-EDM is a well-known process for precision micro machining of difficult to cut materials. It is especially beneficial for machining of complex micro structures such as deep bores, grooves or undercuts in highly wear resistant materials.In the last years, research has successfully applied the assisting electrode method as a suitable process for machining non-conducting ceramics by micro-EDM. There, the EDM process is initiated after applying an electrically conductive starting layer onto the surface of the ceramic. Due to the great variety of ceramics like oxide ceramics, non-oxide ceramics or ceramic composites, different mechanisms for material removal can be observed depending on the raw material and its composition. Particularly for production oriented reasons it is necessary to determine the specific ablation behaviour of each ceramic with regard to the used process parameters. In this study two different ceramic composites, ATZ (Alumina Toughened Zirconia, ZrO2-Al2O3) and Si3N4-TiN (Titanium Nitride toughened Silicon Nitride), were investigated considering their ablation behaviour. Experiments were carried out to determine the maximum achievable aspect ratios of micro bores, the resulting surface quality and the possibility for machining complex 3D-structures by EDM-milling.

Role of Microstructural Features in Toughness Improvement of Zirconia Toughened Alumina

Ceramics constitute an integral part of highly efficient armours due to their low density, high hardness, strength and stiffness. However, they lack toughness and multi-hit capability. Therefore, zirconia toughened alumina is investigated. The hardness is evaluated using Vickers, Knoop and instrumented indentations, while the fracture toughness is evaluated using the indentation technique and Charpy tests. The strength is evaluated using ring-on-ring, four point bend and drop weight tests. The Young’s modulus is evaluated using the unloading instrumented indentation curves. Microstructure, porosity and density are characterised using ultrasonic scanning, Archimedes principle, optical and scanning electron microscopy. Results show an indentation size effect on all mechanical properties. A substantial improvement in toughness is achieved through retardation of crack initiation by tetragonal-to-monoclinic phase transformation in zirconia particles, crack deviation thanks to appropriate grain structure, as well as energy absorption by densification due to remaining porosity. This improved toughness is expected to promote multi-hit capability.

Chemically driven tetragonal-to-monoclinic polymorphic transformation in retrieved ZTA femoral heads from dual mobility hip implants

Two short-term (two and nine months) retrieved zirconia-toughened alumina (ZTA) femoral heads and nine pristine femoral heads from the same manufacturer have been investigated with respect to their surface stability by means of confocal Raman spectroscopy. Quantitative estimations of monoclinic volume fraction have been carried out in both non-wear and main wear zones of the retrieved heads, which invariantly showed high volume fractions of monoclinic polymorph. In-depth (sub-surface) profiles, non-destructively collected in the main wear zones with the Raman probe in confocal configuration, indeed confirmed that polymorphic transformation was extended down to 100μm below the bearing surface of the femoral heads. Acceleration of tetragonal-to-monoclinic transformation rate leads to unexpectedly high fractions of monoclinic phase within very short-term in-vivo exposures. Phase transformation in-vivo is much more marked than what one could actually predict according to simply simulating a hydrothermal environment in-vitro and could not be simply ascribed to the mechanical stress fields generated during normal service at the bearing surface. Instead, the chemical consequences of metal contamination on the ZTA femoral head surface are shown to play the most detrimental role in phase destabilization.

Synthesis of zirconia toughened alumina (ZTA) fibers for high performance materials

Polycrystalline zirconia toughened alumina (ZTA) fibers were prepared from aqueous solutions of aluminum hydroxide chloride 2.5-hydrate and zirconium oxychloride octahydrate. Fiber processing was accomplished via dry spinning. Poly(vinylpyrrolidone) (PVP) was used as spinning aid. Polycrystalline ZTA fibers were obtained by calcination of the green fibers followed by sintering at defined temperatures in air. Ceramic fibers were 10μm in diameter and had an average tensile strength of 1010 (±363)MPa with peak values reaching 1535MPa. Differential scanning calorimetry/thermogravimetric analysis coupled with mass spectrometry (DSC/TGA-MS) showed an exothermic peak at 1156°C assigned to the crystallization of α-Al2O3 and tetragonal ZrO2 and an overall ceramic yield of 41.7% at 1400°C. X-ray diffraction (XRD) analysis showed that tetragonal ZrO2 can be obtained at 1350°C directly from the amorphous precursor whereas pure-phase α-Al2O3 is formed stepwise via transition alumina phases.

Bi-layered zirconia/fluor-apatite bridges supported by ceramic dental implants: a prospective case series after thirty months of observation.

The aim of this study was to determine the success and survival rate of all-ceramic bi-layered implant-supported three-unit fixed dental prostheses (IS-FDPs) 3years after implant placement. Thirteen patients (seven males, six females; age: 41-78years) received two one-piece ceramic implants (alumina-toughened zirconia) each in the region of the premolars or the first molar and were finally restored with adhesively cemented bi-layered zirconia-based IS-FDPs (3 in the maxilla, 10 in the mandible) composed of CAD/CAM-fabricated zirconia frameworks pressed-over with fluor-apatite glass-ceramic ingots. At prosthetic delivery and the follow-ups after 1, 2 and 3years, the restorations were evaluated using modified United States Public Health Service (USPHS) criteria. Restorations with minor veneer chippings, a small-area occlusal roughness, slightly soundable restoration margins, minimal contour deficiencies and tolerable color deviations were regarded as success. In case of more distinct defects that could, however, be repaired to a clinically acceptable level, IS-FDPs were regarded as surviving. Kaplan-Meier plots were used for the success/survival analyses. To verify an impact on subjective patients' perceptions, satisfaction was evaluated by visual analog scales (VAS). All patients were seen 3years after implant installation. No IS-FDP had to be replaced, resulting in 100% survival after a mean observation period of 29.5months (median: 30.7). At the 3-year follow-up, 7/13 IS-FDPs showed a veneer chipping, 13/13 an occlusal roughness and 12/13 minimal deficiencies of contour/color. Since six restorations showed a major chipping and/or a major occlusal roughness, the Kaplan-Meier success rate was 53.8%. However, patients' significantly improved perceptions of function, esthetics, sense, and speech at prosthetic delivery remained stable over time. Bi-layered zirconia/fluor-apatite IS-FDPs entirely survived the observation period but showed a high frequency of technical complications. Nevertheless, the treatment highly satisfied patients' expectations.

Machinability evaluation and desirability function optimization of turning parameters for Cr2O3 doped zirconia toughened alumina (Cr-ZTA) cutting insert in high speed machining of steel

In present study, mechanical properties, microstructure and machining parameter optimization of Cr2O3 doped zirconia toughened alumina (ZTA) ceramic insert have been investigated for application in high speed turning of AISI 4340 steel with achieving maximum tool life. The yttria stabilized zirconia (YSZ) in α-Al2O3 matrix with varying percentage of co-doped chromia (Cr2O3) is prepared to study the phase transformation behaviour. The samples are uniaxially pressed in the form of cutting inserts and subsequently sintered at 1600°C to evaluate the mechanical properties. Hardness and fracture toughness reaches the highest value i.e. 17.40GPa and 7.20MPam1/2 respectively at 0.6% Cr2O3 doped ZTA due to more metastable tetragonal ZrO2 phase present in the alumina matrix. After 50min of machining, the flank wear and surface roughness are found well below the tool rejection criteria. The cutting force also does not affect detrimentally on the job–tool interface. Turning experiments have been adopted as per central composite design (CCD) of response surface methodology (RSM) with varying 3 levels of cutting speed (140m/min, 280m/min, 420m/min), feed rate (0.12mm/rev, 0.18mm/rev, 0.24mm/rev) and depth of cut (0.50mm, 1.00mm, 1.50mm). The effect of each input parameter on output responses is investigated using analysis of variance (ANOVA) and modelled using regression analysis. The influence of cutting speed, feed rate and depth of cut is observed maximum for determination of flank wear, cutting force and surface roughness respectively. Cutting speed of 420m/min with feed rate of 0.12mm/rev and depth of cut of 0.5mm has been shown as optimized condition with 83.32% desirability for minimum tool failure and maximum tool life.

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.

Osseointegration of Zirconia in the Presence of Multinucleated Giant Cells.

Current strategies to reduce medical device-associated infections propose zirconia as a potential implant material which may limit bacterial adhesion. Because multinucleated giant cells (MNGCs) have been detected on these implant surfaces, concerns have been raised regarding tissue integration. The present study examined the presence of MNGCs and their subsequent effect upon tissue integration. Surface-modified implants made of yttria-stabilized (TZP) and alumina-toughened zirconia (ATZ) were compared with commercially pure titanium (Ti). Seven miniature pigs received three implants on either side of the maxilla. After healing periods of 4 and 8 weeks, the tissue response at the implant surfaces was characterized according to three specific parameters: bone-to-implant contact (BIC), MNGC-to-implant contact (MIC), and the peri-implant bone density (BD). Despite being present on all tested implant surfaces, MNGCs were not associated with an inflammatory cell infiltrate or with fibrous encapsulation. MNGCs were less numerous on the Ti implants (range: 3.9-5.2%) compared with the ceramic implants (range: 17.6-30.3%, p < .0001). Even though the values of newly formed bone and pristine bone in direct contact with the implant surfaces were high at 4 weeks (tBIC: Ti = 82.3%, TZP = 64.3%, ATZ = 70%), a negative correlation was observed between the presence of MNGCs and newly formed bone at the implant surface (p < .001). Interestingly, the newly formed peri-implant bone density, defined as the percentage of new bone area inside the screw threads (nBD), was not diminished by the presence of MNGCs. Differences in the presence of MNGCs and the BIC parameters between Ti and the ceramic implants appear to be a local cellular phenomenon which is restricted to the implant-bone marrow interface and do not affect the peri-implant bone formation. Factors triggering MNGC differentiation and their persistence in response to biomaterial surface need to be investigated in future studies.

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%.

Microstructure and Properties of Zirconia Toughened Alumina Fabricated by Powder Injection Moulding

Zirconia toughened alumina (ZTA) ceramic has been fabricated by the powder injection moulding process. The ZTA ceramic, composed of 80 wt% alumina and 20 wt% zirconia, was mixed with a water-soluble, multi-component binder system. The binder ingredients were polyethylene glycol (PEG), polyvinyl butyral (PVB) and stearic acid (SA). Powder injection moulding was performed with powder loadings in the range of 48-52 vol%, using a laboratory-scale injection moulding machine. Water leaching was used for partial binder removal prior to thermal debinding and then sintering at 1650 °C for 2 hours. Microstructural examination of the ZTA ceramic revealed that zirconia inhibited alumina grain growth and, therefore, improved the mechanical and physical properties of the specimens. It was found that powder loading had an influence on density, hardness and strength of the specimens. A flexural strength of 334 MPa and hardness value of 2093 kg/mm2 was obtained from specimens injection moulded with 52 vol% powder loading feedstock. The highest sintered density achieved was 97% of the theoretical value.

Clinical and Patient-reported Outcomes of a Zirconia Oral Implant

The objective of this study was to determine the clinical, radiographic, and patient-reported outcomes of a 1-piece alumina-toughened zirconia implant restored with single crowns (SCs) or 3-unit fixed dental prostheses (FDPs) after 3 y of observation. Forty patients received 53 implants, placed in a 1-stage operation with immediate temporization. Finally, 50 implants were restored with 24 SCs and 13 FDPs. To evaluate peri-implant bone loss, standardized radiographs were taken at implant insertion, at final restoration delivery, and after 1 and 3 y. Additionally, several soft tissue parameters and patient-reported outcome measures were evaluated. Linear mixed models with random intercept for each patient and patients as clusters were used to compare subgroups. Three patients did not receive a SC due to early implant loss, and 1 patient died. As a result, 36 patients with 49 implants were followed-up for 3 y, giving a cumulative survival rate of 94.2%. The average marginal bone loss amounted to 0.79 mm (SCs, 0.47 mm; FDPs, 1.07 mm; P < 0.001). After the delivery of the final prosthetic restoration, further bone loss was not statistically significant (0.09 mm; P = 0.700). Probing depth, clinical attachment level, and modified bleeding index increased significantly at the implant sites, whereas gingival recession decreased significantly. Compared with the pretreatment questionnaires, the patient-reported outcome measures showed a permanently improved perception of function, aesthetics, sense, speech and self-esteem. The survival rate of the investigated ceramic implant system seems to be comparable to reported survival rates of titanium implants when immediately restored. The recorded parameters suggest its potential for clinical utilization.

Oxide ceramics with unidirectional pore channels by electrophoretic deposition

Disc-shaped alumina, alumina-toughened zirconia, and mullite green bodies, few millimetres in thickness, with parallel aligned pore channels were produced by an electrophoretic deposition method that takes advantage of the gas generation by electrolysis in aqueous suspensions. Appropriate amounts of an anionic polyelectrolyte, chosen on the basis of zeta potential measurements, and ammonia solution were used as additives. Adding ammonia promoted the gas evolution at a given polyelectrolyte amount. The electrolyte content influenced the porosity values and the diameters of the pore channels. Volume fractions of the pore channels in the sintered bodies up to 40% were determined by computed tomography in combination with 3D image analysis. The maxima of the pore diameter distributions of the sintered samples were in the range between 200 and 300μm.

Processing and Properties of Zirconia-Toughened Alumina Prepared by Gelcasting.

Zirconia-toughened alumina (ZTA) using yttria-stabilised zirconia is a good option for ceramic-ceramic bearing couples for hip joint replacement. Gelcasting is a colloidal processing technique capable of producing complex products with a range of dimensions and materials by a relatively low-cost production process. Using gelcasting, ZTA samples were prepared, optimising the stages of fabrication, including slurry preparation with varying solid loadings, moulding and de-moulding, drying and sintering. Density, hardness, fracture toughness, flexural strength and grain size were observed relative to slurry solid loadings between 58 and 62 vol. %, as well as sintering temperatures of 1550 °C and 1650 °C. Optimal conditions found were plastic mould, 4000 g/mol PEG with 30 vol. % concentration, 61% solid loading and Ts = 1550 °C. ZTA samples of high density (maximum 99.1%), high hardness (maximum 1902 HV), high fracture toughness (maximum 5.43 MPa m1/2) and high flexural strength (maximum 618 MPa) were successfully prepared by gelcasting and pressureless sintering.

A Novel Technique for the Connection of Ceramic and Titanium Implant Components Using Glass Solder Bonding.

Both titanium and ceramic materials provide specific advantages in dental implant technology. However, some problems, like hypersensitivity reactions, corrosion and mechanical failure, have been reported. Therefore, the combining of both materials to take advantage of their pros, while eliminating their respective cons, would be desirable. Hence, we introduced a new technique to bond titanium and ceramic materials by means of a silica-based glass ceramic solder. Cylindrical compound samples (Ø10 mm × 56 mm) made of alumina toughened zirconia (ATZ), as well as titanium grade 5, were bonded by glass solder on their end faces. As a control, a two-component adhesive glue was utilized. The samples were investigated without further treatment, after 30 and 90 days of storage in distilled water at room temperature, and after aging. All samples were subjected to quasi-static four-point-bending tests. We found that the glass solder bonding provided significantly higher bending strength than adhesive glue bonding. In contrast to the glued samples, the bending strength of the soldered samples remained unaltered by the storage and aging treatments. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analyses confirmed the presence of a stable solder-ceramic interface. Therefore, the glass solder technique represents a promising method for optimizing dental and orthopedic implant bondings.

Electrophoretic Method for Fabricating Porous Ceramics – Application to Different Oxide Materials

Electrophoretic deposition (EPD) with controlled gas evolution by electrolysis was used for producing ceramic green bodies with unidirectional pore channels. The method was applied to alumina, alumina-toughened zirconia, and mullite. It requires stable aqueous suspensions with sufficiently high particle charge for the electrophoresis and appropriate amounts of electrolyte for the gas bubble formation. An anionic polyelectrolyte and ammonia were shown to be a suitable additive combination for all three oxide powders. The amount of polyelectrolyte was chosen for each of the powders on the basis of electroacoustic measurements of the zeta potential. The addition of ammonia solution was necessary in order to obtain the desired porous structures.

Microstructure and Mechanical Properties of ZTA Ceramic-Lined Composite Pipe Prepared by Centrifugal-SHS

A type of zirconia-toughened alumina (ZTA) ceramic-lined composite steel pipe was fabricated by using self-propagation high-temperature synthesis and centrifugal casting technique. The microstructure and phase constituents of ZTA ceramic layer were analyzed by means of optical microscope, scanning electron microscope and X-ray diffractometer (XRD). The fracture toughness of ceramic layers and mechanical shock of Al2O3 and ZTA ceramic-lined composite pipes were measured using Vickers indentation microfracture method and the repetitive impacting method. The results show that the phase constituents of ZTA ceramic layer were Al2O3, FeAl2O4 and t-ZrO2 phases, and no m-ZrO2 phase was detected by XRD. Addition of ZrO2 reduced the width of Al2O3 dendrites and led to formation of a microstructure with fine ZrO2 particles distributed at boundaries of Al2O3 dendrites. The fracture toughness was increased from 0.56 MPa m1/2 of Al2O3 ceramic layer to 5.74 MPa m1/2 of ZTA ceramic layer. The mechanical shock resistance at the center of the ceramic-lined composite pipe was increased from twice to 19 times after addition of ZrO2 into the Al2O3 matrix.

Thermal Expansion Reduction in Alumina‐Toughened Zirconia by Incorporation of Zirconium Tungstate and Aluminum Tungstate

A series of composites containing yttria‐stabilized zirconia, Al2O3, and the thermomiotic (negative thermal expansion) materials ZrW2O8 and Al2W3O12 were prepared by an in situ solid‐state reaction during sintering. Microstructural analysis of the composites indicated several sub‐micron sized components. 27Al MAS NMR was used to characterize the aluminum‐containing phase distribution. Measurement of the coefficients of thermal expansion showed that the addition of thermomiotic components reduced the thermal expansivity of the alumina‐toughened zirconia matrix, but the reduction was considerably smaller than what would be predicted by the rule of mixtures. The Turner model was much more accurate in predicting the coefficients of thermal expansion of the composites due to the large differences in stiffness between the components.