Effect of the Al2O3 content on the mechanical properties and microstructure of 3Y-TZP ceramics for dental applications

Alumina-reinforced zirconia composites containing 0 to 30 vol% of alumina were fabricated by sintering at 1550 °C for 2 h in air. The effect of the Al₂O₃ content on the mechanical properties and microstructure of 3Y-TZP ceramics was investigated. Al₂O₃ acted as an inhibitor of the grain growth of 3Y-TZP. As the alumina content increased, the fracture mode changed gradually from the transgranular mode to the intergranular mode and the Young’s modulus and hardness increased. The biaxial flexural strength also showed a slight increase with an increase in Al₂O₃ content, due to the grain size refinement of the ZrO₂ matrix, while the fracture toughness, which was investigated by the SEVNB method, showed a contrary tendency. The decrement of the fracture toughness can be explained by the increase in the critical transformation stress, the decrease in the volume fraction of the transformable t-ZrO₂ and the increase in the tensile residual stress.     

Ti3AlC2掺杂SPS法制备Ti2AlC/TiAl复合材料的研究

通过2TiC-Ti-1.2Al体系的原位热压反应制备了Ti₃AlC₂陶瓷,然后以59.2Ti-30.8Al-10Ti₃AlC₂(wt%)为反应体系,采用放电等离子烧结技术制备出Ti₂AlC/TiAl基复合材料。借助XRD、SEM分析了产物的相组成和微观结构,并测量了其室温力学性能。结果表明：原位热压烧结产物由Ti₃AlC₂和TiC相组成,Ti₃AlC₂呈典型的层状结构,TiC颗粒分布在其间。SPS法制备的Ti₂AlC/TiAl基复合材料主要由TiAl、Ti₃Al和Ti₂AlC相组成,Ti₂AlC增强相主要分布于基体晶界处,表现为晶界/晶内强化作用。力学性能测试表明：Ti₂AlC/TiAl基复合材料的密度、维氏硬度、断裂韧性和抗弯强度分别为3.85 g/cm₃、5.37 GPa、7.17 MPa?m_1/2和494.85 MPa。     

Effects of Al2O3 content on densification,microstructure, mechanical properties and oxidation resistance of TaC-Al2O3 composites

TaC-Al₂O₃ composites were prepared by hot pressing. Influence of Al₂O₃ content ranging from 10 to 40 vol. % on densification, phase composition, microstructure, mechanical properties and oxidation behavior of the TaC-Al₂O₃ composites was investigated. With 30 and 40 vol. % Al₂O₃ addition a closed porosity was achieved. The Al₂O₃ particles were uniformly distributed among TaC grains retarding grain growth and resulting in refined microstructures with grains below 2 μm in size. The most densified composite with 40 vol. % Al₂O₃ addition exhibited good mechanical properties with a Vickers’ hardness of 17.8 GPa, a flexural strength of 485 MPa and a fracture toughness of 5.4 MPa·m^1/2. After holding at 700°C for 3 h in air, the dense 30 and 40 vol. % Al₂O₃ compositions showed hardly noticeable and mainly surface oxidation, whereas less densified TaC-Al₂O₃ composites with 10 and 20 vol. % Al₂O₃ content and with open porosity were disintegrated to powders.     

Structure and Properties of Y2O3-Doped Al2O3-MWCNT Nanocomposites Prepared by Pressureless Sintering and Hot-Pressing

This study describes the combined effects of multi-walled carbon nanotubes (CNTs) additions and Y₂O₃ doping on the microstructures and mechanical properties of Al₂O₃-CNT nanocomposites fabricated by pressureless and hot-press sintering processes. A uniform dispersion of CNTs within the Al₂O₃ matrix was successfully attained via a combined approach using surfactant, sonication, and adequate period of incubation. Small amounts (1 wt.%) of Y₂O₃, as dopants, significantly affected the densification and properties of pressureless sintered monolithic Al₂O₃ and its nanocomposites at low CNT concentrations (<1 wt.%); however, they hardly showed any improvement at higher CNT contents. As opposed to the pressureless sintering, pressures applied during high temperature sintering in combination with the Y₂O₃ doping contributed in generating a homogenous microstructure and improved the densities (7 and 15%) and microhardness (11 and 12%) of Al₂O₃ reinforced with higher CNT contents (2 and 5 wt.%), respectively. Adding on, hot-pressed Y₂O₃-doped Al₂O₃ reinforced with 2 and 5 wt.% CNTs showed higher hardness (19 and 70%), flexural strength (10 and 5%), and fracture toughness (26 and 11%), respectively, compared to similar but CNT-free samples. These results showed that pressure-assisted sintering and Y₂O₃ are promising for the fabrication of CNT-reinforced Al₂O₃ nanocomposites, especially at higher CNT concentrations.     

Fractal analysis of crack paths in Al2O3-TiC-4%Co composites

1 Introduction Al2O3-TiC composite (denoted by AT) is an important material for structural components due to the high strength, hardness, as well as chemical stability and wear resistance. However, the low fracture toughness still cannot match the comman…     

Mechanical Properties and Thermal Shock Resistance of Al2O3-TiC-Co Composites

Ductile cobalt was introduced into Al₂O₃-TiC (AT) composites by using a chemical deposition method to improve toughness and resistance to thermal shock. The mixture of Co-coated Al₂O₃ and TiC powders was hot-pressed into an Al₂O₃-TiC-Co (ATC) composite. The flexure strength and fracture toughness of the ATC composites have been improved considerably, compared with AT and Al₂O₃. The fracture surface of ATC shows a large proportion of transgranular cracks with some intergranular type, unlike the intergranular fracture modes of AT and Al₂O₃. The thermal shock properties of the composites were evaluated by water quenching technique and compared with the traditional AT and Al₂O₃. The composites containing only 3.96 vol.% cobalt exhibited higher critical temperature difference and retained flexure strength. The SEM examination of the fracture surfaces of the ATC composites after single thermal cycle showed that voids increased in number and size, and most isolated voids coalesced with increasing temperature difference, which caused the density and strength to decrease. The ATC composite is less sensitive to repeated thermal shock than the AT composite.     

Simultaneous synthesis and consolidation of nanocrystalline Fe2Al5 and Fe2Al5-Al2O3 by pulsed current activated sintering and their mechanical properties

Nanopowders of Fe, Al and Fe₂O₃ are fabricated by high energy ball milling. Using the pulsed current activated sintering method, the densification of nanocrystalline Fe₂Al₅ and Al₂O₃ reinforced Fe₂Al₅ composites were simultaneously synthesized and consolidated within two minutes from mechanically activated powders. The advantage of this process is that it allows very quick densification to near theoretical density and prohibition of grain growth in nanostuctured materials. Nanocrystalline materials have received much attention as advanced engineering materials with improved physical and mechanical properties. As nanomaterials possess high strength, high hardness, excellent ductility and toughness, undoubtedly, more attention has been paid to the application of nanomaterials. Not only the hardness but also the fracture toughness of the Fe₂Al₅-Al₂O₃ composite was higher than that of monolithic Fe₂Al₅ due to the addition of the hard phase of Al₂O₃ and the crack deflection by Al₂O₃.     

Strengthening of Ti3AlC2 by incorporation of Al2O3

Ti₃AlC₂/Al₂O₃ composites were fabricated by the in-situ hot pressing/solid–liquid reaction process. The hardness, strength and toughness are enhanced by the incorporation of Al₂O₃. The strengthening and toughening mechanisms are discussed.     

Ti3AlC2-Al2O3-TiAl3 composite fabricated by reactive melt infiltration

Porous preforms were fabricated by cold-pressing process using powder mixture of TiC, TiO₂ and dextrin. After pyrolysis and sintering, Al melt was infiltrated into the porous preforms, leading to the formation of Ti₃AlC₂-Al₂O₃-TiAl₃ composite. Effects of cold-pressing pressure of preforms on microstructures and mechanical properties of the composites were studied. Synthesis mechanism and toughening mechanism of composite were also analyzed. The results shows that TiO₂ is reduced into Ti₂O₃ by carbon, the decomposition product of dextrin, which causes the spontaneous infiltration of Al melt into TiC/Ti₂O₃ preform. Then, Ti₃AlC₂-Al₂O₃-TiAl₃ composite is in-situ formed from the simultaneous reaction of Al melt with TiC and Ti₂O₃. With the increase of cold-pressing pressure from 10 MPa to 40 MPa, the pore size distribution of the preforms becomes increasingly uniform after pre-sintering, which results in the reduction of defects, and the decrease of property discrepancy of composites. Nano-laminated Ti₃AlC₂ grains and Al₂O₃ particles make the fracture toughness of TiAl₃ increase remarkably by various toughening mechanisms including stress-induced microcrack, crack deflection and crack bridging.     

Fabrication of Al matrix composite reinforced with submicrometer-sized Al<Subscript>2</Subscript>O<Subscript>3</Subscript> particles formed by combustion reaction between HEMM Al and V<Subscript>2</Subscript>O<Subscript>5</Subscript> composite particles during sintering

To fabricate an Al-V matrix composite reinforced with submicron-sized Al₂O₃ and Al_xV_y (Al₃V, Al₁₀V) phases, high energy mechanical milling (HEMM) and sintering were employed. By increasing the milling time, the size of mechanically milled powder was significantly reduced. In this study, the average powder size of 59 μm for Al, and 178 μm for V₂O₅ decreased with the formation of a new product, Al-Al₂O₃-Al_xV_y, with a size range from 1.3 μm to 2.6 μm formed by the in-situ combustion reaction during sintering of HEM milled Al and V₂O₅ composite powders. The in-situ reaction between Al and V₂O₅ during the HEMM and sintering transformed the Al₂O₃ and Al_xV_y (Al₃V, Al₁₀V) phases. Most of the reduced V reacted with excess the Al to form Al_xV_y (Al₃V, Al₁₀V) with very little V dissolved into Al matrix. By increasing the milling time and weight percentage of V₂O₅, the hardness of the Al-Al₂O₃-Al_xV_y composite sintered at 1173 K increased. The composite fabricated with the HEMM Al-20wt.%V₂O₅ composite powder and sintering at 1173 K for 2 h had the highest hardness.     

Pulsed electric current sintered Fe3Al bonded WC composites

WC based composites with 5, 10 and 20 vol.% Fe₃Al binder were consolidated via pulsed electric current sintering (PECS) in the solid state for 4 min at 1200 °C under a pressure of 90 MPa. Microstructural analysis revealed a homogeneous Fe₃Al binder distribution, ultrafine WC grains and dispersed Al₂O₃ particle clusters. The WC-5 vol.% Fe₃Al composite combines an excellent Vickers hardness of 25.6 GPa with very high Young’s modulus of 693 GPa, a fracture toughness of 7.6 MPa m^1/2 and flexural strength of 1000 MPa. With increasing Fe₃Al binder content, the hardness and stiffness decreased linearly to 19.9 and 539 GPa, respectively with increasing binder content up to 20 vol.%, while the fracture toughness and flexural strength were hardly influenced by the binder content. Compared to WC–Co cemented carbides processed under exactly the same conditions, the WC–Fe₃Al composites exhibit a substantially higher hardness and Young’s modulus.     

Effects of SiC interfacial coating on mechanical properties of carbon fiber needled felt reinforced sol-derived Al2O3 composites

3D carbon fiber needled felt and polycarbosilane-derived SiC coating were selected as reinforcement and interfacial coating, respectively, and the sol−impregnation−drying−heating (SIDH) route was used to fabricate C/Al₂O₃ composites. The effects of SiC interfacial coating on the mechanical properties, oxidation resistance and thermal shock resistance of C/Al₂O₃ composites were investigated. It is found that the fracture toughness of C/Al₂O₃ composites was remarkably superior to that of monolithic Al₂O₃ ceramics. The introduction of SiC interfacial coating obviously improved the strengths of C/Al₂O₃ composites although the fracture work diminished to some extent. Owing to the tight bonding between SiC coating and carbon fiber, the C/SiC/Al₂O₃ composites showed much better oxidation and thermal shock resistance over C/Al₂O₃ composites under static air.     

Mechanical property and cutting performance of yttrium-reinforced Al2O3/Ti(C,N) composite ceramic tool material

Effects of yttrium on the mechanical property and the cutting performance of Al₂O₃/Ti(C,N) composite ceramic tool material have been studied in detail. Results show that the addition of yttrium of a certain amount can noticeably improve the mechanical property of Al₂O₃/Ti(C,N) ceramic material. As a result, the flexural strength and the fracture toughness amount to 1010 MPa and 6.1 MPam^1/2, respectively. Cutting experiments indicate that the developed ceramic tool material not only has better wear resistance but also has higher fracture resistance when machining hardened #45 steel. The fracture resistance of the yttrium-reinforced Al₂O₃/Ti(C,N) ceramic tool material is about 20% higher than that of the corresponding ceramic tool material without any yttrium additives.     

Alloying effects on properties of Al2O3 and TiO2 in connection with oxidation resistance of TiAl

In the present work, a first-principles method is used to calculate the oxidation energies of Al₂O₃ and TiO₂ as well as the formation energy of oxygen vacancy in TiO₂ containing various alloying elements, in order to shed some light on the alloying effects on the oxidation resistance of γ-TiAl. Our calculations demonstrate that almost all alloying elements increase the oxidation energies of Al₂O₃ and TiO₂. The alloying elements with number of d electrons from 2 to 5 in the forth and fifth rows of the periodic table (e.g., Zr, Nb, Mo, Hf, Ta, W) increase significantly the oxidation energy difference between Al₂O₃ and TiO₂, i.e., reduce the relative stability of Al₂O₃ to TiO₂. On the other hand, these alloying elements increase the formation energy of oxygen vacancy in TiO₂. The effects of other alloying elements are less significant or opposite. Observing the experimental mass gains of TiAl alloys and unalloyed TiAl due to oxidation, we find that the elements reducing the relative stability of Al₂O₃ to TiO₂ and increasing the formation energy of oxygen vacancy enhance the oxidation resistance of TiAl whereas others do not. Such correlations are rationalized by analyzing the alloying effects on the internal oxidation of Al in the γ-TiAl matrix and the diffusion of oxygen in TiO₂ surface scale.     

Effect of Al2O3 content and swaging on microstructure and mechanical properties of Al2O3/W alloys

Al₂O₃-reinfored tungsten alloys were fabricated by powder metallurgy method and hot swaging technology. The investigation was made on the microstructure, relative density, nano-hardness and fracture toughness (K_IC) of the sintered and swaged Al₂O₃/W alloys. The swaging process and addition of Al₂O₃ are beneficial to comprehensive properties of the sintered and swaged alloys. After swaging, the Al₂O₃/W alloys can achieve the full density. According to the nano-indentation test and three-point bend test, the swaged W-0.25 wt% Al₂O₃ alloy possesses the highest hardness of 7.02 GPa, the greatest modulus of 435.09 GPa and the maximum fracture toughness of 21 MPa·m^1/2. The observation of fracture morphology shows that the recrystallization behavior and grain growth occur above 1400 °C in the swaged pure W alloy, which leads to recrystallization brittleness. At the same time, the microstructure of the swaged W-0.25 wt% Al₂O₃ alloy does not change apparently.     

The mechanical milling of Al/TiO2 composite powders

The processing of Al/TiO₂ composite powders produced by high-energy mechanical milling leads to production of a range of valuable, titanium-based materials. They include Ti(Al,O)/Al₂O₃ and Ti_xAl_y(O)/Al₂O₃ composite powders, bulk composites and Ti₃Al/TiAl alloy powders, and corresponding bulk materials. The strength of the Ti(Al,O)/Al₂O₃ and Ti_xAl_y(O)/Al₂O₃ composites is moderate, but their high-temperature oxidation resistance is exceptionally high, making the titanium-based composite powders favorable feedstock materials for protective coatings. The hardness of the Ti(Al,O)/Al₂O₃ and Ti₃Al(O)/Al₂O₃ composites is also very high (10–16 GPa). For more information, contact D.L. Zhang, University of Waikato, Waikato Centre for Advanced Materials, Department of Materials and Process Engineering, Private Bag 3105, Hamilton, New Zealand; 011-64-7-838-4783; fax 011-64-7-838-4835; e-mail d.zhang@waikato.ac.nz.     

Mechanical behavior of 3Al2O3·2SiO2 films under nanoindentation

A complete and systematic nanoindentation study was conducted on a mullite (3Al₂O₃·2SiO₂) coating ～1 μm thick, deposited by means of chemical vapor deposition on a silicon carbide (SiC) substrate. The investigation included using different indenter tip geometries (Berkovich, spherical and cube-corner), complemented with atomic force microscopy and three-dimensional focused ion beam tomography to characterize the indentation response, deformation and damage micromechanisms. The intrinsic mechanical properties of the 3Al₂O₃·2SiO₂ film and the interfacial toughness of the coated (3Al₂O₃·2SiO₂/SiC) system were critically evaluated to assess the influence of substrate and film residual stresses. Through appropriate implementation of specific indenter tip geometry, different length-scale mechanical properties in the materials studied were successfully determined: yield strength and fracture toughness for the film, together with energy of adhesion per unit area and interface fracture toughness for the coated system.     

First-principles study of the bonding characteristics of TiAl(111)/Al2O3(0001) interface

First principles calculations were carried out for α-Al₂O₃(0001) surface and γ-TiAl(111)/α-Al₂O₃(0001) interface to study the adhesion properties of the interface and to clarify the mechanisms that govern the adhesion of TiAl and its oxidation product Al₂O₃. Two type interface models, the TiAl(111) with Al- and O-terminated α-Al₂O₃(0001) surfaces denoted as T(A₁)-type and T(O_T)-type interface, respectively, were considered. The Universal Binding Energy Relation (UBER) was used to determine the separation between TiAl and Al₂O₃ and the work of adhesion of the γ-TiAl(111)/α-Al₂O₃(0001) interface. Optimization was then performed for all interfaces considered here using the separation obtained with UBER. The lowest work of adhesion is −1.05 J/m² for the T(A₁)-type interface and is −4.04 J/m² for the T(O_T)-type interface. There exists competition between O–Ti and O–Al (on the TiAl side) interactions; however O–Al bond is stronger than O–Ti bond because the main body of the Al valences is involved in the hybriding with O p electrons, while only part of the Ti d valence is involved in the O–Ti bonding. Thus the O–Al interaction dominates the adhesion between TiAl(111) and Al₂O₃(0001) surfaces, and it can be inferred that an Al-rich TiAl surface will favor the adhesion between TiAl/Al₂O₃.     

Interfacial Interaction and Wetting in the Ta2O5/Cu-Al System

Wettability and interfacial interaction of the Ta₂O₅/Cu-Al system were studied. Pure Cu does not wet the Ta₂O₅ substrate, and improved spreading is achieved when relatively a high fraction of the active element (~40 at.% Al) was added. The Al₂O₃ and AlTaO₄ phases were observed at the Ta₂O₅/Cu-Al interface. A thermodynamic evaluation allowed us to suggest that the lack of wetting bellow 40 at.% Al is due to the presence of a native oxide, which covers the drop. The conditions of the native oxide decomposition and the formation of the volatile Al₂O suboxide strongly depend on the vacuum level during sessile drop experiments and the composition of the Cu-Al alloy. In our case, Al contents greater than 40% provides thermodynamic conditions for the formation of Al₂O (as a result of Al reaction with Al₂O₃) and the drop spreading. It was suggested that the final contact angle in the Ta₂O₅/Cu-Al system (50°) is determined by Ta adsorption on the newly formed alumina interlayer.     

Improving the High-Temperature Oxidation Resistance of TiAl Alloy by Anodizing in Methanol/NaF Solution

An aluminum- and fluorine-enriched anodic film was fabricated on TiAl alloy by anodizing in NaF-contained methanol solution. The high-temperature oxidation resistance and protection mechanism of the anodized TiAl alloy were investigated. It showed that the formation of aluminum-enriched oxide scale originates from halogen effect and quite small anodization current density is essential to improving the high-temperature oxidation resistance. Upon oxidation at 850 °C for 100 h, no cracks and/or spallation were exhibited on the anodized TiAl alloy. At temperature higher than 300 °C, titanium fluoride will sublimate, accompanying with the transformation of aluminum fluoride into protective Al₂O₃ layer, as was indicated by XPS analysis. This freshly generated Al₂O₃ layer, together with the anodization formed one, can efficiently inhibit the outward diffusion of Al and inward diffusion of oxygen, thereby improving the high-temperature oxidation resistance of the TiAl.