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.     

Thermal residual stresses in co-continuous composites

The thermal residual stresses in two types of co-continuous composites copper/aluminum oxide (Cu/Al₂O₃) and aluminum/aluminum oxide (Al/Al₂O₃) were measured by neutron diffraction experiments. These stresses were generated during the cooling after high processing temperature. The coefficient of thermal expansion (CTE) mismatch of metal and ceramic phases led to significant amount of thermal stresses. In both the composites, the metallic phase was found to be under tension and aluminum-oxide phase under compression. Even though the magnitude of compressive stress in both the composites was similar; the two metal-phases had very different magnitude of tensile stresses. The difference in volume fraction, CTE, elastic stiffness and plastic flow properties led to this difference. The hydrostatic stresses were found to be predominant in both the phases. Finite element simulations were used to predict the stress distributions inside each phase and at the interfaces. A representative unit cell approach was considered to represent the composite. Concept of effective ΔT was utilized to simulate the thermal stress distribution inside the two phases in the unit cell. This model utilized the neutron diffraction measurements to predict the stress distribution inside each phase and at the interface. The simulations showed that significant amount of tensile stresses develop at the metal–ceramic interfaces.     

Investigation of residual stress effects in an alloy reinforced ceramic/metal composite

This study aims at investigating the thermal expansion behavior and internal residual strains in metal reinforced ceramic matrix composites (CMCs). A variety of Al₂O₃/A356 CMCs composites with an interpenetrating network structure and varying metal content, ranging from 10 to 40 vol.%, were produced using the pressure infiltration technique of Squeeze casting. Values of coefficients of thermal expansion (CTEs) were found to vary significantly with temperature, indicating an influence of the flow characteristics of the metal. Comparisons are made with well known methods for predicting CTEs values of metal/ceramic composites. The overall strain was found to increase with temperature and scaled proportionally with the metal content of the composite. Comparisons were also made with non-infiltrated porous ceramic preforms and a pure metallic sample. The uniform heating and cooling curves for the composite samples were found to exhibit hysterisis. Residual stress analysis and failure simulation were performed based on thermomechanics and the finite element method (FEM). This analysis is often utilized for the analysis of stress distribution or deformation of a structure. High angle X-ray and CTEs mismatch equation analysis were utilized to analyze the residual stresses at the ceramic/metal interface of the Al₂O₃/A356 composites. The relationship of residual stresses and the contact area of the ceramic/metal interface are also discussed.     

Microstructural characterization of alumina–zirconia layered ceramics using positron annihilation spectroscopy

Positron annihilation spectroscopy (PAS), indentation, nanoindentation experiments and scanning electron microscopy (SEM) observations were performed on Al₂O₃–ZrO₂ laminates samples to assess the effect of residual stresses on their mechanical and microstructural properties. Layered samples were implemented by slip-casting, constituted by two thin Al₂O₃ external layers and an intermediate thick one, consisting of a mixture of Al₂O₃ and monoclinic ZrO₂ in the range 0–30 vol.%. In these systems residual tensile stresses fields were generated inside the external layers during cooling from the sintering temperature, by the expansion of the adjacent ZrO₂-containing layer. SEM observations showed the microstructural effects due to the level of tension related to the zirconia content. A correlation between the PAS parameters and the microstructural changes caused by the presence of residual stresses was found. Nanoindentation measurements were used to trace the sign and magnitude of the residual stress gradient across the interface between the layers.     

Piezo-spectroscopy at low temperatures: residual stresses in Al2O3–ZrO2(Y2O3) eutectics measured from 77 to 350 K

High-resolution, piezo-spectroscopic studies were performed in alumina/zirconia eutectic composites with different Y₂O₃ contents and microstructures at different temperatures using the ruby R-line luminescence. Measurements at 77 K allowed the precise determination of the average stress and its distribution in the alumina phase. A normal distribution function was obtained in most of the cases. In some composites the highest stresses are relaxed by micro cracking, giving an asymmetrical distribution function. In the composites with stabilized zirconia the residual stresses originate from the differences in the thermal expansion coefficients of the component phases. A linear dependence of the thermostresses with temperature was obtained in the 77–350 K temperature range. The effective elastic modulus for thermostresses was 118±2.7 GPa. Using the coefficients of thermal expansion of the component phases and making an extrapolation of the low temperature values the stress-free temperature of 1270±35 K was determined.     

Thermal residual stresses and their toughening effect in Al2O3 platelet reinforced glass

Fluorescence spectroscopy has been used to measure the thermal residual stresses in Al₂O₃-platelet/borosilicate glass composites. Tensile residual stresses were found in the platelets, implying the presence of compressive residual stresses in the glass matrix. Measurements of stresses in the bulk of the composite could be obtained using fluorescence from platelets below the specimen surface. The measured stresses lay between the predictions of models for spherical particles and thin platelets, but were closer to the former for the range of platelet contents investigated (5–30 vol.%). Estimates of the increase in toughness associated with the compressive residual stresses in the matrix suggest that this mechanism makes a significant contribution to the toughening effect of the Al₂O₃ platelets.     

High-temperature deformation of Al2O3/Y-TZP particulate composites

Al₂O₃/Y-TZP particulate composites with compositions of between 20 and 80 vol.% Y-TZP were produced by tapecasting, lamination, and sintering. The processing methods employed resulted in fine grain sizes with only small variations between the composites produced. The resulting particulate composites were tested in compression at a temperature of 1350 °C over strain rates from 10⁻⁵ to 3.16×10⁻⁴ s⁻¹. Microstructural changes during testing were minor. Stress exponents were measured to the range from approximately two to three, which are consistent with published data on similar materials from tensile experiments. Models of composite creep behavior are compared to the experimental data over the full range of compositions studied. A constrained isostrain model is found to provide better predictive capabilities than either an unconstrained model, an isostress model, or a rheological model. Furthermore, the constrained isostrain model provides the most reasonable predictions for creep rates of 100% Al₂O₃ and 100% Y-TZP materials.     

Residual stresses in HVOF-sprayed ceramic coatings

In this paper, the residual stress state of thermally sprayed ceramic coatings was examined by combining different experimental and analytical techniques, in order to provide a thorough characterisation of through-thickness stress profiles and a cross-verification of results. HVOF-sprayed ceramics, manufactured using commercial and nanostructured Al₂O₃ powders and commercial Cr₂O₃ powders, and atmospheric plasma-sprayed (APS) ceramics, manufactured using commercial Al₂O₃ and Cr₂O₃ powders, were investigated.The near-surface stress was measured by X-ray diffraction. The through-thickness profile and the intrinsic quenching stress were analytically computed by the Tsui-Clyne iterative model, using the X-ray measurement result as input, and results were validated by the substrate chemical removal method. Further verification was achieved by applying the in-situ curvature technique to the deposition of HVOF-sprayed Al₂O₃ coating.HVOF-sprayed Al₂O₃ coatings deposited using both conventional and nanostructured powders feature a similar, almost equibiaxial tensile stress on the top surface (116.5 MPa and 136.5 MPa, respectively) and a moderate through-thickness gradient (about 12 MPa and 20 MPa, respectively). Their intrinsic quenching stresses were analytically estimated to be 184 MPa and 205 MPa, respectively. APS Al₂O₃ possesses higher top surface stress (220 MPa) and quenching stress (311 MPa). However, it shows a less pronounced stress gradient (≈ 3 MPa) than HVOF-sprayed Al₂O₃-based coatings, because cracks, pores and weak lamella boundaries in the APS coating can accommodate the deformations induced by the bending moments arising both during coating deposition and during cooling.The model-derived quenching stress of the conventional HVOF Al₂O₃ coating was validated by the in-situ curvature measurement technique.Cr₂O₃-based coatings are significantly different. They display a lower residual stress in the near-surface region: 20 MPa in the APS coating, 27.5 MPa in the HVOF one. The HVOF coating also exhibits a very large stress gradient of ≈ 77 MPa. Machining and sliding processes (like polishing and dry sliding tribological testing) change their surface residual stresses to compressive ones.     

Effect of thermal-cooling cycle treatment on thermal expansion behavior of particulate reinforced aluminum matrix composites

Two micron SiC particles with angular and spherical shape and the sub-micron Al2O3 particles with spherical shape were introduced to reinforce 6061 aluminium by squeeze casting technology.Microstructures and effect of thermal-cooling cycle treatment(TCCT) on the thermal expansion behaviors of three composites were investigated.The results show that the composites are free of porosity and SiC/Al2O3 particles are distributed uniformly.Inflections at about 300 °C are observed in coefficient of thermal expansion(CTE) versus temperature curves of two SiCp/Al composites,and this characteristic is not affected by TCCT.The TCCT has significant effect on thermal expansion behavior of SiCp/Al composites and CTE of them after 3 cycles is lower than that of 1 or 5 cycles.However,no inflection is observed in Al2O3p/Al composite,while TCCT has effect on CTE of Al2O3p/Al composite.These results should be due to different relaxation behavior of internal stress in three composites.     

Aging behavior of Al2O3 short fiber reinforced Al-Cu alloy composites

Al2O3 short fiber reinforced AI-Cu composites containing 1%, 3%, 5% and 7% Cu were fabricated by a squeeze casting technique. The as-cast Al2O3/Al-Cu composites were solution treated at 535 ℃ and then aged at 170, 190 and 210 ℃, respectively. Age hardening behavior of the Al2O3/Al-Cu composites was analyzed by measuring the hardness of the samples at different aging temperatures and aging time. Microstructures of the composites were observed by transmission electron microscope（TEM）. The results indicate that the hardness of the Al2O3/Al-Cu composites containing 7% Cu is much higher than that containing 1%-5% Cu because of the large amount of CuAl2 precipitant in the Al2O3/Al-Cu composite. With the increase of Cu content from 1% to 7%, the time needed for the appearance of peak hardness shortened, indicating that the addition of Cu can accelerate the kinetic of CuAl2 precipitation in the Al2O3/Al-Cu composites. The Al2O3/Al-Cu composite containing 7% Cu shows the highest increment of hardness by aging treatment. Therefore, in order to get a higher peak hardness, the Al2O3/Al-Cu composites need more Cu addition as compared with the un-reinforced Al-Cu alloys.     

20 Hz synchrotron X-ray diffraction analysis in laser-pulsed WC-Co hard metal reveals oscillatory stresses and reversible composite plastification

The lifetime of WC-Co inserts used in cutting processes, such as milling, is limited by millisecond temperature and mechanical pulses, which occur as a result of interrupted tool-workpiece contact, thermal fatigue and wear. In the current work, synchrotron X-ray diffraction (XRD) was used in conjunction with a pulsed laser heating set-up to characterise the time-dependent development of stresses and microstructure in locally irradiated WC-Co inserts coated by chemical vapour deposition with 6.5 and 3.5 μm thick TiCN and α-Al₂O₃ films, respectively. Diffraction data from the WC phase were used to evaluate the time and temperature-dependent evolution of in-plane stresses, thermal strains and integral breadths of WC diffraction peaks in experiments with a single and five successive laser shocks applied within 2.2 and 20 s, respectively, using a laser spot diameter of ~5.8 mm and an X-ray beam size of 1 × 1 mm². The laser heating induces the formation of compressive stresses in the inserts' substrates. Above a temperature of ~750 °C, at the onset of WC-Co composite plastification, compressive stresses relax and then vanish in WC at the maximal applied temperature of ~1300 °C, followed by the build-up of tensile stresses. The applied cyclic heating up and cooling down led to the repetitive formation of compressive and tensile stresses, with temperature dependencies oscillating with the number of applied laser pulses. The observed relatively high tensile stress level of ~1100 MPa in WC was a consequence of the stabilising function of the coating, which hindered the initiation of surface hot cracks and stress relaxation. The stress evolution was coupled with changes in XRD peak broadening, which however strongly depended on the particular hkl reflections and showed oscillatory behaviour within a single temperature cycle. In summary, the unique diffraction set-up revealed stress levels and provides insight into the WC-Co composite plastification mechanism governing the stress build-up and relaxation in locally thermo-shocked WC-Co inserts at millisecond time resolution.     

Influence of internal stresses in superplastic joining of zirconia toughened alumina

Joints between various compositions of 3 mol% Y₂O₃-stabilized ZrO₂/Al₂O₃ composites have been produced by superplastic flow at 1350 °C and strain rates of 1×10⁻⁵ s⁻¹. The joints are pore free and Vickers indentation induced cracks indicate that the joints are strong. The cracks have been used to measure residual stresses, which can be modified by using an interlayer between the materials to be joined. The results agree well with those obtained by finite-element analysis. Ruby fluorescence has been used to measure the hydrostatic stress.     

Quantifying residual stresses in resistance spot welding of 6061-T6 aluminum alloy sheets via neutron diffraction measurements

Residual strains of resistance spot welded joints of 6061-T6 aluminum alloy sheets were measured in three different directions denoted as in-plane longitudinal (σ₁₁), in-plane transversal (σ₂₂), and normal (σ₃₃). The welding process parameters were established to meet or exceed MIL-W-6858D specifications (i.e., approximately 5.7 mm weld nugget and minimum shearing force of 3.8 kN per weld confirmed via quasi-static tensile testing). Electron backscatter diffraction (EBSD) and optical microscopy (OM) were performed to determine grain size and orientation. The residual stress measurements were taken at a series of points along the weld centerline at depths corresponding to the weld mid-plane and at both 1 mm below the top surface of the plate and 1 mm above bottom surface. The residual stresses were captured on the fusion zone (FZ), heat affected zone (HAZ) and base metal (BM) of the resistance spot welded joint. Neutron diffraction results show residual stresses in the weld are approximately 40% lower than yield strength of the parent material. The maximum variation in residual stresses occurs, as expected, in the vertical position of the specimen because of the orientation of electrode clamping forces that produce a non-uniform solidification pattern. Despite the high anisotropy of the welding nugget and surrounding area, a significant result is that σ₃₃ measured stress values are negligible in both the horizontal and vertical directions of the specimen. Consequently, microstructure–property relationships characterized here can indeed inform continuum material models for application in multiscale models.     

Effect of Surface Roughness and Structure Features on Tribological Properties of Electrodeposited Nanocrystalline Ni and Ni/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Coatings

Metal matrix composite coatings obtained by electrodeposition are one of the ways of improving the surfaces of materials to enhance their durability and properties required in different applications. This paper presents an analysis of the surface topography, microstructure and properties (residual stresses, microhardness, wear resistance) of Ni/Al₂O₃ nanocomposite coatings electrodeposited on steel substrates from modified Watt’s-type baths containing various concentrations of Al₂O₃ nanoparticles and a saccharin additive. The residual stresses measured in the Ni/Al₂O₃ coatings decreased with an increasing amount of the co-deposited ceramics. It was established that the addition of Al₂O₃ powder significantly improved the coatings’ microhardness. The wear mechanism changed from adhesive-abrasive to abrasive with a rising amount of Al₂O₃ particles and coating microhardness. Nanocomposite coatings also exhibited a lower coefficient of friction than that of a pure Ni-electrodeposited coating. The friction was found to depend on the surface roughness, and the smoother surfaces gave lower friction coefficients.     

In situ measurement of lattice strains in mixed ceramic cutting tools under thermal and mechanical loads using synchrotron radiation

To completely understand wear mechanisms of mixed ceramic cutting tools (Al₂O₃–TiC), residual stress states and the superposition of external loads during hard turning should be investigated. This can be done via X-ray diffraction using high-energy synchrotron radiation to determine lattice strains in the material. For this reason, in first model tests, strain states in mixed ceramics were determined during the application of external loads. An experimental setup was developed to measure lattice strains in the different phases of the ceramic material in situ during thermal, mechanical and thermo-mechanical loading for first reference. The accuracy of the setup was sufficient to clearly determine shifts in lattice parameters in the different phases due to external loads. By applying a thermal load on the mixed ceramic material the two main phases showed different elastic lattice strains. Thus, a slightly lower coefficient of thermal expansion in the Al₂O₃-phase than in the Ti(O,C)-phase could be determined. This indicated the development of compressive stresses in the Al₂O₃-phase and tensile stresses in the Ti(O,C)-phase at room temperature. By applying external bending stresses to the mixed ceramic material, for both phases equal lattice strains could be determined. From these strains stresses could be calculated for both phases which were in the same order of magnitude as external stresses. With further in situ investigations of strain and stress states in the different phases of mixed ceramics during friction and turning experiments a more comprehensive characterization of wear mechanisms is possible.     

Evolution of the fine structure of Pd–5.3 at % Ιn–0.5 at % Ru solid solution after hydrogenation

Fine-structure parameters, such as the effective coherent domain size D_hkl and microstrain ε_hkl, of Ρd–5.3 at % Ιn–0.5 at % Ru alloy foil, which was subjected to electrolytic hydrogenation and subsequent prolonged (for 55000 h) relaxation, have been determined using X-ray diffraction data. The hydrogenation and subsequent relaxation of the alloy have been found to result in the refinement of coherent domains CD(hkl) and increase in the microstrains ε_hkl.     

复合钎料钎焊Al_2O_3接头剪切应力场的数值模拟

对Al2O3颗粒增强复合钎料钎焊Al2O3/Al2O3接头的残余应力场进行了数值模拟,分析了Al2O3陶瓷颗粒的加入对陶瓷接头残余剪切应力的影响.模拟发现,陶瓷接头的最大应力位于陶瓷-钎缝界面,陶瓷颗粒的加入对钎焊接头应力起到了缓解作用,其缓解程度随陶瓷颗粒体积分数的增加而增大;在陶瓷颗粒百分比一定的情况下,钎缝厚度的...     

Combustion synthesis of FeAl−Al2O3 composites with TiB2 and TiC additions via metallothermic reduction of Fe2O3 and TiO2

Combustion synthesis involving metallothermic reduction of Fe₂O₃ and TiO₂ was conducted in the mode of self-propagating high-temperature synthesis (SHS) to fabricate FeAl-based composites with dual ceramic phases, TiB₂/Al₂O₃ and TiC/Al₂O₃. The reactant mixture included thermite reagents of 0.6Fe₂O₃+0.6TiO₂+2Al, and elemental Fe, Al, boron, and carbon powders. The formation of xFeAl−0.6TiB₂−Al₂O₃ composites with x=2.0−3.6 and yFeAl−0.6TiC−Al₂O₃ composites with y=1.8−2.75 was studied. The increase of FeAl causes a decrease in the reaction exothermicity, thus resulting in the existence of flammability limits of x=3.6 and y=2.75 for the SHS reactions. Based on combustion wave kinetics, the activation energies of E_a=97.1 and 101.1 kJ/mol are deduced for the metallothermic SHS reactions. XRD analyses confirm in situ formation of FeAl/TiB₂/Al₂O₃ and FeAl/TiC/Al₂O₃ composites. SEM micrographs exhibit that FeAl is formed with a dense polycrystalline structure, and the ceramic phases, TiB₂, TiC, and Al₂O₃, are micro-sized discrete particles. The synthesized FeAl−TiB₂−Al₂O₃ and FeAl−TiC−Al₂O₃ composites exhibit the hardness ranging from 12.8 to 16.6 GPa and fracture toughness from 7.93 to 9.84 MPa·m^1/2.     

Lanthanum hexaaluminate—a new material for atmospheric plasma spraying of advanced thermal barrier coatings

One of the main application fields of the thermal spraying process is thermal barrier coatings (TBCs). Today, partially stabilized zirconia (YSZ or MSZ) is mainly used as a TBC material. At temperatures above 1000 °C, zirconia layers age distinctively, including phenomena shrinkage and microcrack formation. Therefore, there is a considerable interest in TBCs for higher temperature applications. In this paper, lanthanum hexaaluminate, a newly developed TBC material with long-term stability up to 1400 °C, is presented. It ages significantly more slowly at these high temperatures than commercial zirconia-based TBCs. Its composition favors the formation of platelets, which prevent a densification of the coating by postsintering. It consists of La₂O₃, Al₂O₃, and MgO. Its crystal structure corresponds to a magnetoplumbite phase. Lanthanum hexaaluminate powders were produced using two different fabrication routes, one based on salts and the other one based on oxides. To optimize the granulate, various raw materials and additives were tested. The slurry was spray dried in a laboratory spray drier and calcined at 1650 °C. Using these two powders, coatings were produced by atmospheric plasma spraying (APS). The residual stresses of the coatings were measured by the hole drilling method, and the deposition process was optimized with respect to the residual stresses in the TBC. The coatings were extensively analyzed regarding phase composition, thermal expansion, and long-term stability, as well as microstructural properties.     

Microstructure and mechanical properties of 7075 Al alloy based composites with Al2O3 nanoparticles

A new method was used to fabricate 7075 Al alloy based composites with Al₂O₃ nanoparticles to improve the distribution of particles. In this study, nano-sized particles were fed into the molten alloy through the flow of argon gas, then the Al₂O₃/7075 composites were prepared by solid-liquid mixed casting. The results indicated that the composite samples showed fine microstructure and achieved a homogeneous distribution of particles. Also, it was found that relative to the as-cast 7075 alloy, the Al₂O₃/7075 composites exhibited higher mechanical properties, which is due to the effect of uniform distributed Al₂O₃ nanoparticles reinforcement.