Influence of Residual Stresses on the Wear Behavior of Alumina/Alumina–Zirconia Laminated Composites

Symmetric structures of laminated ceramic composites were produced by superimposing alternating layers of Al₂O₃ and Al₂O₃/ZrO₂ composite prepared by tape casting. The composites were designed to have an alumina surface layer on either side. This configuration caused residual compressive stresses to be induced on the surface due to the different thermal expansion coefficients of the various layers, leading to an increase in the apparent surface toughness. The amount of residual stress was determined using the indentation technique. The tribological behavior of these laminated structures was evaluated using the pin-on-disk method for different loads and sliding speeds. Comparison with the results obtained from stress-free alumina showed that, within the range of these experimental conditions, the improvement in surface toughness leads to a reduced friction coefficient and increased wear resistance of the composites. Possible wear mechanisms are proposed.     

Residual Stresses in Alumina/Ceria-Stabilized Zirconia Composites

Measurements of the residual stresses in Al₂O₃/Ce-TZP (12 mol% CeO₂) sintered composites, containing 10, 20, and 40 vol% zirconia, obtained by neutron diffraction and by piezospectroscopy using optical fluorescence and Raman are compared. The techniques give essentially the same values for the spatial average of the hydrostatic residual stresses in the two phases despite the difference in the parameters measured in the two techniques. The measured stresses are also in accord with those predicted from a stochastic stress analysis for materials cooling from a stressfree temperature of ∼1180°C. Over the range of volume fraction investigated the hydrostatic stress in the alumina phase varies linearly with zirconia content, corresponding most closely to the upper Hashin bound.     

Diffusion Bonding of Zirconia/Alumina Composites

ZrO₂/Al₂O₃ composites with from 0% to 100% Al₂O₃ content were diffusion bonded at 12.5 MPa for 30 min in the temperature range 1450° to 1500°C. Under appropriate bonding conditions, a bonding strength greater than 1000 MPa was achievable between dissimilar materials with different thermal expansion coefficients.     

Alumina and Alumina/Zirconia Multilayer Composites Obtained by Slip Casting

The slip casting technique has been revealed as a powerful method to obtain multilayer composites close to theoretical density. From zeta potential and viscosity measurements of Al₂O₃ and Al₂O₃/ZrO₂ (4 vol% ZrO₂) suspensions, the corditions for the preparation of multilayer composites by slip casting have been determined. A microstructural analysis of the different layers by scanning electron microscopy is also reported.     

Influence of Grain Size on the Indentation-Fatigue Behavior of Alumina

The surface fracture behavior of a high-purity, high-density alumina, as a function of grain size (3, 5, and 9 μm), was investigated using an indentation-fatigue technique. Increasing the grain size reduced the threshold for crack nucleation, reduced the resistance to surface spalling, and increased the volume of materials lost per spalling event. These results are explained in terms of residual stresses and fatigue damage.     

反应烧结法制备ZTM/Al2O3陶瓷材料的研究

通过测定试样的烧结密度、烧成收缩率、原料细度、两种研磨方法和两种烧成制度下抗弯强度和韧性,对工业上利用ZrSiO_4与AL_2O_3原位反应,制备还原海绵铁高温炉膛用ZTN/AL_2O_3陶瓷材料进行了研究。结果表明,球磨与过长的研磨时间都不能提高超细原料的细度。AL_2O_3含量较高的试样,原位反应烧结所产生的体积膨胀较小,试样的性能越高。两步烧成制度有利于反应烧结试样性能的提高。     

Effect of Dynamic Compaction on the Retention of Tetragonal Zirconia and Mechanical Properties of Alumina/Zirconia Composites

Dynamic consolidation techniques were employed to investigate the retention of tetragonal zirconia and degree of consolidation in alumina/zirconia powder compacts. Heating the specimens prior to explosive shock compaction increased the tetragonal-phase retention significantly. Low shock pressures yielded no macrocracking, although final densities were low (60% to 70% of the theoretical density). Heat treatment following dynamic consolidation enhanced the retention of the tetragonal zirconia polymorph regardless of the shock pressure employed. Compact densities were increased to over 90% of theoretical at relatively low sintering temperatures (1300°C). Hardness, toughness, and Young's modulus of the compacts were comparable to those achieved in composites that were synthesized using more conventional techniques. Dynamic compaction offers an alternative method for the fabrication of zirconia-toughened alumina ceramics.     

Role of Grain Size in the Strength and R-Curve Properties of Alumina

An investigation of the interrelationships between strength, crack-resistance ( R -curve) characteristics, and grain size for alumina ceramics has been carried out. Results of identation-strength measurements on high-density aluminas with uniform grain structures in the size range 2 to 80 μm are presented. A theoretical fit to the data, obtained by adjusting parameters of a constitutive frictional-pullout relation in a grain-bridging model, allows determination of the critical microstructural parameters controlling the R -curve behavior of these aluminas. The primary role of grain size in the toughness characteristic is to determine the scale of grain pullout at the bridged interface. It is shown that the strength properties are a complex function of the bridged microstructure, governed at all but the finest grain sizes by the stabilizing effect of the R -curve. The analysis confirms the usual negative dependence of strength on grain size for natural flaws that are small relative to the grain size, but the dependence does not conform exactly to the −1/2 power predicted on the basis of classical "Griffith-Orowan" flaws. The analysis provides a self-consistent account of the well-documented transition from "Orowan" to "Petch" behavior.     

Development of Surface Compressive Stresses in Zirconia–Alumina Composites by an Ion-Exchange Process

A two-step ion-exchange technique was developed for introducing compressive stresses on the surface of ZrO₂–Al₂O₃ composites. In the first step, a thin layer (∼250 μm) of Na-β"-Al₂O₃ was formed on the surface of the composite by a vapor-phase process at ∼1400°C. In the second step, Na⁺ ions were replaced by K⁺ ions by a heat treatment at ∼385°C for 2 h in a molten KNO₃ bath. Replacement of sodium by potassium led to the creation of surface compressive stresses. The flexural strength and Weibull modulus of ZrO₂–Al₂O₃ composite were ∼915 MPa and 10, respectively, for the as-sintered samples. By contrast, the flexural strength and Weibull modulus were ∼1140 MPa and 26, respectively, for the ion-exchanged samples. A residual surface compressive stress of ∼480 MPa was measured by a strain-gauge technique in K⁺-ion-exchanged samples. The presence of surface compressive stresses also was confirmed using an indentation technique. The technique developed here can be used to introduce compressive stresses on components of virtually any shape.     

Sintering and Microstructure Modification of Mullite/Zirconia Composites Derived from Silica-Coated Alumina Powders

This paper addresses the densification and microstructure development during firing of mullite/zirconia composites made from silica-coated-alumina (SCA) microcomposite powders. Densification occurs in two stages: in the presence of a silica–alumina mixture and after conversion to mullite. The first stage of densification occurs through transient viscous phase sintering (TVS). This is best promoted by rapid heating, which delays the crystallization of silica to higher temperatures. A further sintering stage is observed following mullitization. The introduction of seeds promotes solid-state sintering, most probably due to refinement of the mullite matrix. For seed concentrations up to about 1% the sintering kinetics depend on seed concentration. This suggests that nucleation still remains the rate-controlling mullitization step. Above this concentration the reaction becomes growth controlled. Introduction of seeds also promotes direct mullitization without transient zircon formation that was observed in a previous study of the same process without seeding. Seeding also promotes the development of elongated grains by way of a solid-state recrystallization process.     

Room-Temperature Aging of Laminate Composites of Alumina/3-mol%-Yttria-Stablilized Tetragonal Zirconia Polycrystals

The stresses of laminate structures obtained by joining single layers of pure alumina (A), pure yttria-stabilized tetragonal zirconia, 3Y-TZP (Z), and an intimate mixture of alumina and zirconia (AZ) have been determined by fluorescence (in alumina) and Raman (in zirconia) piezospectroscopy. Three symmetrical stacking sequences were examined, namely, A/Z/A, A/AZ/A, and AZ/Z/AZ, with the aim of designing structures where the higher coefficient of thermal expansion (CTE) of zirconia could be used to induce compressive stress in the external layers (and ensuing tensile stress in the central layer). Two experimental sessions, 6 years apart, were conducted on the same samples, also taking care to record the spectra from the same locations; during the time elapsed between the two sessions, the samples were kept at room temperature and humidity. The stress values in alumina obtained during the more recent session were markedly different from those observed in the first session. Monoclinic zirconia ( m -zirconia) was absent in all samples in the first session, whereas up to 25 vol% zirconia could be observed during the second session. m -Zirconia could only be observed in AZ layers and not in Z layers, irrespective of the position in the stacking sequence. It was concluded that 3Y-TZP underwent spontaneous tetragonal-to-monoclinic ( t – m ) transformation, that is, "aging," when mixed with alumina at the grain-size level. Aging occurred only where pristine t -zirconia was subject to tensile stresses larger than ∼400 MPa.     

Activation Energy for the Sintering of Two-Phase Alumina/Zirconia Ceramics

In an earlier paper we reported measurements of the activation energy for sintering from constant-heating-rate experiments with alumina/5% zirconia. Here, results from a full range of compositions in this two-phase system are described. They show that the activation energy remains in the range 700 ± 100 kJ/mol when the composition changes from 5 to 95 vol% zirconia. In comparison, pure zirconia sinters with an activation energy of 615 ± 80 kJ/mol and pure alumina with the energy of 440 ± 45 kJ/mol. The addition of 2.8 mol% yttria to zirconia does not have a measurable effect on the activation energy. The grain size dependence of the sintering rate suggests boundary-diffusion-controlled sintering. These activation energies are phenomenologically correlated with the interfacial energies in alumina, zirconia, and two-phase alumina/zirconia, suggesting that the bonding at the interface influences diffusional transport.     

Evolution of the Grain Size Distribution during the Sintering of Alumina at 1350°C

The objectives of this study are to provide some rare unfolded grain size distribution data for the sintering of alumina and to test for time invariance of the normalized grain size distribution as required by normal grain growth. The results show that ln ς doubled during the sintering times studied. The changes in the grain size distribution may be due to an increase in the number of relatively large grains combined with a reduction in the number of grain annihilation events compared with that required for time invariance of the normalized grain size distribution.     

Compressive Fiber Fragmentation in Carbon/Polypropylene Composites: Effects of Residual Thermal Stresses and Transcrystallinity

The effects of fiber volume fraction and transcrystallinity in single fiber composites, on the phenomenon of compressive fiber fragmentation due to residual thermal stresses, are studied. A concentric cylinder model is used, jointly with experimental data, to predict the Weibull shape parameter of the compressive strength distribution of pitch-based high and medium modulus (HM and MM) carbon fibers, with isotactic polypropylene as the semi-crystalline embedding matrix. A severe effect of the fiber content on the thermal residual stress in the fiber and, thus, on the fiber break density, is predicted and experimentally confirmed. The effect of the presence of isothermally grown polypropylene transcrystalline interlayers (using pitch-based HM carbon fibers as a substrate) on the compressive stresses induced upon subsequent quenching is investigated, both experimentally and theoretically. Cooling rate results are also presented. The thermoelastic constants of the interlayer are predicted to have a severe effect on the residual stresses generated in the fiber, the interphase, and the matrix. There is therefore, a definite need for direct experimental measurements of these constants.     

Effects of Grain Size of Hot-Pressed Silicon Nitride on Contact Damage Morphology and Residual Strength

Effects of the grain size of hot-pressed Si₃N₄ on contact damage morphology and residual strength were studied using the elastic/plastic indentation method with a spherical indenter. The contact damage, initially formed with increasing indentation load, was Hertzian cracks in the Si₃N₄ consisting of fine grains (mean grain size: 0.2 μm). In the coarser-grained Si₃N₄ (mean grain size: 0.8 μm), there was a damage texture, consisting of grain-sized microcracks. The residual strength was degraded at a load slightly higher than the critical load for contact damage formation. The strength degradation was not caused by contact damage but the residual stress formed around the impressions.     

Reduction of Thermal-Gradient-Induced Stresses in Composites Using Mixed Fibers

The possibility of using fibers of varying thermal expansivities to reduce stresses generated from thermal gradients in service was explored. Laminates of aluminosilicate- and alumina-matrix composites were fabricated using a combination of Nextel 610™ and Nextel 720™ fibers. The composites were tested for mechanical behavior and thermomechanical behavior in comparison to control composite specimens which contained a single type of fiber. Mixed-fiber composites show strength and fracture behavior similar to those of single-fiber-type composites. Their thermomechanical behavior is consistent with 2D laminate models, making it possible to design composites using such models. Interlaminar shear stress is a key, but sole limitation and the use of gradation in fiber mixture is suggested.     

Correlation between Neutron Diffraction Measurements and Thermal Stresses in a Silicon Carbide/Alumina Composite

Sintering of Al₂O₃ powder and SiC fibers at high temperatures leads to a state of high internal stresses in the composite material at room temperature because of the difference in thermal contraction between both materials. These stresses are calcualted in this work using the scheme of the elastic elliposidal inclusion of Eshelby and regarding the fibers as prolate ellipsoids. The predicted strain in the fibers is used to calculate variations in the spacing of crystallographic planes and the results are compared against experimental measurements obtained with neutron diffraction by other authors. We show that in order to correlate the measurements with the actual stress state, it is necessary to account for the elastic anisotropy and the proper crystallographic structure of the fibers. We demonstrate that the interpretation also depends on the distribution of fiber orientations inside the matrix and that results can be substantially changed depending on the assumptions employed.     

Abnormal Grain Growth of Alumina: CaO Effect

The critical concentration of Ca required for the onset of abnormal grain growth in alumina was determined by controlled doping of Ca in ultrapure alumina (>99.999%), by sintering under clean contamination-free conditions, and by microstructural characterization. As in the case of Si, the excess concentration of Ca beyond its solubility limit was inversely related to the average grain size at the moment of first appearance of abnormal grains, which corresponds to the moment of sufficient enrichment of Ca in grain boundaries to form stable intergranular liquid films. However, the critical concentration of Ca was found to be in the range of only a few tens of ppm, which is lower than that of Si by almost 2 orders of magnitude. The equivalent silica concentration to form such a stable intergranular calcium aluminate glass film and its minimum thickness were estimated from the inverse relationship with the assumption that the glass composition is close to calcium hexaluminate.     

Influence of Abrasive Properties on Residual Stresses in Lapped Ferrite and Alumina

The effect of abrasive-particle properties on surface-finishing residual stresses, surface finish, and material removal rates during the lapping of ferrite and aluminium oxide is quantified experimentally. It is shown that lapping with softer abrasives and smaller particles results in lower compressive residual stresses near the surface and improved surface finish. These results demonstrate possible methods for controlling surface finishing residual stresses in ceramics while at the same time achieving dimensional accuracy.