Strain-rate-dependent compressive and compression-shear response of an alumina ceramic

This paper assessed the microstructure and properties of CeramTec ALOTEC 98 SB alumina ceramic through microscopic characterization and mechanical experiments. The rate-dependent strength and failure response of an alumina ceramic were studied under both uniaxial compression and compression-shear loading. Under quasi-static uniaxial compression at rates of 10^?5 to 10³ s^?1, the strength had an average of 3393 ± 306 MPa, and at dynamic strain rates of 10² to 10³ s^?1, the strength ranged from 3763 to 4645 MPa. The CeramTec ALOTEC 98 SB alumina ceramic was found to have greater mechanical properties than other commercial alumina ceramics from the literature (i.e., AD-995). To monitor the strain field and the failure process of the alumina ceramic during testing, an ultra-high-speed camera coupled with digital image correlation (DIC) was used to visualize crack initiation and propagation processes, and obtain quantitative stress-strain information. A new data processing method was then proposed in this study to calculate the shear components for the compression-shear tests. Validation of the proposed method was confirmed by the shear strain obtained from the DIC analysis with the ultra-high-speed camera. Using the results obtained by the proposed model and the DIC analysis, new observations and understandings of failure mechanisms are obtained. (1) In compression-shear tests, the shear failure happens before complete failure, and shear behavior plays an important role during the failure process. (2) The equivalent peak stress (strength) of compression-shear test is smaller than the uniaxial compression one. (3) The directional cracks have weak influence on the compressive stiffness, but have a strong influence on the shear response.     

Determination of the multiaxial failure criteria for alumina ceramics under tension–torsion test

Failure probability of ceramic components in multiaxial stress state can be predicted using the uniaxial test results (e.g. tension test, 4-point-bending test) when a suitable multiaxial criterion, which introduces the triaxiality of stress state, is known. In this article, tension–torsion tests were performed with alumina (Alsint 99.7) specimens from a standard manufacturer under two different load cases. Next experimental results were compared with the numerically calculated effective volume and effective surface values according to different multiaxial failure criteria. It was concluded that the specimens failed due to surface flaws and the normal stress criterion is the most appropriate criterion for the strength prediction of alumina ceramics under multiaxial stress state. Furthermore, it was shown that the Weibull modulus does not play a big role for the prediction of strength of alumina ceramics.     

Development and mechanical characterization of novel ceramic foams fabricated by gel-casting

Porous ceramic materials are of considerable interest for a variety of chemical and industrial applications in extremely harsh conditions, particularly at very high temperatures for long time periods. A combined gel-casting-fugitive phase process employing agar as a natural gelling agent and polyethylene spheres as pore formers was exploited to produce porous ceramic bodies. Alumina and alumina–zirconia powders were used to prepare samples having a porosity of about 65–70–75 vol%. The composite powder was produced by a surface modification route, i.e. by coating a well-dispersed alpha-alumina powder with a zirconium chloride aqueous solution. On thermal treatment, ultra-fine tetragonal zirconia grains were formed on the surface of the alumina particles. SEM observations and image analysis were used to characterize the microstructure of porous samples and uniaxial compressive tests were carried out to measure their mechanical behavior.     

Micromechanical Stress Concentrations in Two-Phase Brittle-Matrix Ceramic Composites

A quantitative investigation was conducted on the effect of micromechanical stress concentrations on the strength of two-phase brittle-matrix ceramic systems. The materials consisted of a continuous brittle matrix containing dispersions with elastic properties different from those of the matrix. A soda borosilicate glass was used as the matrix and the dispersions consisted of spherical alumina particles 60μ in diameter and spherical pores 60μ in diameter. Stress concentrations were varied by measuring the strength of the composite under uniaxial and biaxial tensile stress conditions. The experimental results showed that micromechanical stress concentrations strongly affect the macroscopic strength of the composite. Under biaxial tensile stress, additions of either alumina microspheres or spherical porosity to the glass matrix resulted in a decrease in strength equal to the maximum calculated stress concentration factor. Under uniaxial tensile stress conditions, however, the reduction in strength for the glass-alumina system was negligible. The glass-porosity system gave a reduction in uniaxial strength which was not equal to the maximum calculated stress concentration factor. Experimental results suggest that differences in strength of brittle multi-component systems under uniaxial and biaxial stress states can be attributed in part to micro-structural features. On the basis of the experimental work, a hypothesis is developed relating the relative size of the region in the glass matrix over which stress concentrations act to the size of the Griffith flaws responsible for failure. This hypothesis is extended to the effect of porosity on the strength of polycrystalline brittle ceramic materials.     

Hot Forging of Mullite-Based Ceramics Prepared from Kaolin–Alumina

Mullite-based ceramics prepared from a mixture of kaolin and alumina were plastically deformed in uniaxial compression tests at high strain rates (     = 10⁻²–10⁻³ s⁻¹) at 1450° and 1500°C. Deformation occurred with barreling in tests that used SiC platens coated with h-BN. This study shows that more-suitable lubricants and molds (platens) must be found, to reduce friction between the mullite-based ceramics and molds. However, this hot-working method can be used to give a fine, textured surface to mullite-based ceramic substrates for polycrystalline silicon thin-film solar cells.     

A novel application of alumina fiber mats as TBC protection for CFRP/epoxy laminates –Laboratory tests and numerical modeling

A novel method is proposed to create a TBC on the CFRP/epoxy laminate surface in a single technological process by formation of one additional layer made of a commercially available alumina fiber mat. Both materials: the CFRP/epoxy laminate made of four layers and the ceramic mat were joined during prepreg curing.The quick thermal heating tests of the TBC system were conducted for different times with a propane-butane torch. Changes of material strains due to temperature influence are determined experimentally in uniaxial tensile tests via the Digital Image Correlation (DIC) method. A numerical model corresponding to the experimental quick heating test is built in Abaqus standard finite element software to determine temperature distribution in the CFRP/epoxy laminate.The experimental and numerical investigations lead to a conclusion that the alumina fiber mat is an effective protection of the CFRP/epoxy laminate against temperature degradation and can be used in industrial practice.     

Estimating tensile creep rate of ceramics from flexure data

A new model of ceramic creep in four-point bending is proposed to determine the creep rate that corresponds to tensile creep at an elevated temperature. Based on the assumption that ceramics creep only in tension and there is no creep in compression, the tensile creep rate which is invariant with time in the secondary mode is calculated in a simple way. Since the initially applied maximum tensile stress does not correspond to the stress at the secondary creep range, the creep-induced stress at the time of measurement is calculated based on beam deflection. Then, the calculated tensile creep rates from fourpoint bending data are compared with observed tensile creep rates for both an alumina ceramic at 1000 °C and a silicon nitride ceramic at 1200 °C. This study shows the usefulness of flexural creep tests not only to verify the accuracy of tensile creep tests, but also to obtain the tensile creep data in a less expensive and easier way.     

Consolidation Behavior of High-Performance Ceramic Suspensions

Colloidal processing has been shown to produce low defect and uniform ceramic microstructures from submicrometer ceramic powders. These concepts were applied to colloidal pressing to determine critical design relationships for uniaxial consolidation of dense and uniform green bodies from colloidal suspensions. Carefully controlled constant rate of strain consolidation experiments were carried out using alumina in water. The compression index decreased from 0.143 for a poorly dispersed alumina system to 0.077 for a well-dispersed alumina suspension compression curve, indicating that the well-dispersed system is stiffer in consolidation. The compression curves showed that, as the degree of dispersion decreases, increased consolidation stresses are required to achieve a given particle packing density. The compression index increased with increasing strain rate for well-dispersed alumina suspensions. Permeability through the sample ranged from 3 × 10^–8 to 4 × 10⁻⁷ cm/s, decreasing with decreasing void ratio during consolidation. Well-dispersed samples gave lower permeabilities than did poorly dispersed samples over a given consolidation increment. Coefficients of consolidation were nonconstant over the experimental effective stress range, invalidating the general solution to the linear consolidation equation. An approximate incremental solution was applied which indicated rapid pressing cycles are possible by starting with a suspension having a high solids concentration. Application of this consolidation data to nonlinear consolidation models is recommended for more exact prediction of consolidation time.     

Effect of Compressive Stress on Fatigue of Alumina under Uniaxial Cyclic Loading

The effect of compressive stress on fatigue behavior of alumina was investigated under uniaxial cyclic loading. Experimental data for alumina tension specimens under uniaxial tension–unloading and tension–compression cyclic loadings were compared. This comparison suggests that compressive stress is effective in advancing the crack growth under tension–compression cycling.     

Cyclic Fatigue from Frictional Degradation at Bridging Grains in Alumina

Tension—tension cyclic loading tests have been conducted on a coarse-grained alumina ceramic that exhibits toughnesscurve behavior by grain-interlock bridging. Fatigue effects are observed in the regions of both short cracks, using indentation flaws, and long cracks, using compact-tension specimens. A true mechanical fatigue effect is demonstrated by running the tests below the static fatigue limit. A custom-made device for in situ observation of crack propagation in the scanning electron microscope enables us to identify bridge degradation as a cause of the fatigue process. "Wear" debris cumulates at the sliding intergranular frictional contact points, indicating a loss of traction at the junction. The basis of a fracture mechanics model describing the effect of this frictional degradation in reducing crack-tip shielding is outlined and fitted to the data. It is suggested that the bridge degradation fatigue mechanism may be widespread in polycrystalline ceramics with pronounced toughness curves.     

Observed and Theoretical Creep Rates for an Alumina Ceramic and a Silicon Nitride Ceramic in Flexure

Observed creep curvature rates are compared to theoretical rates for both an alumina ceramic at 1000°C and a silicon nitride ceramic at 1200°C in four-point flexure. The observed rates have been calculated from published rise-displacement rates, and the theoretical rates have been calculated from published power-law parameters for compressive and tensile creep, which differ appreciably for these ceramics. Although both compressive and tensile creep measurements are easier to analyze than flexural creep measurements, the latter are usually less expensive and easier to conduct. The present work shows the usefulness of flexural creep tests to verify the accuracy of compressive and tensile creep tests.     

Analysis of the nanoindentation load–displacement curves measured on high-purity fine-grained alumina

This paper conducted a preliminary examination on the effect of microstructural inhomogeneity on the reproducibility of the nanoindentation data. Nanoindentation tests were conducted on a high-purity, fine-grained alumina ceramic. It was found that the reproducibility of the nanoindentation data were somewhat poor. The nanoindentation data were then analyzed with the widely employed Oliver–Pharr method to yield the hardness, H, and the Young’s modulus, E. Large scatters were observed in the resultant H and E. These experimental findings revealed that microstructural inhomogeneity may play an important role in the material response to nanoindentation.     

Low-Pressure Injection Molding of Ceramic Springs

Injection molding has important advantages over other methods for the production of advanced ceramic parts with complex shapes. In this work, low-pressure injection molding was used to produce helical ceramic springs using two different kinds of molds. The ceramic powders used were submicrometer-sized alumina and partially stabilized zirconia. Sintered alumina and zirconia springs were obtained free of defects, with densities from 96% to 99% of the theoretical value. In preliminary mechanical tests, these ceramic springs supported axial deformations up to 10% before failure.     

Impact of characteristic length and loading rate upon dynamic constitutive behavior and fracture process in alumina ceramics

Understanding the impact performance of ceramic materials requires accurate corresponding relationship between mechanical response and fracture behavior. In this study, constitutive behaviors of alumina ceramics were successfully determined via split-Hopkinson pressure bar (SHPB) system coupled with high-speed camera to track the deformation and failure process. Failure strength of alumina demonstrated a strong dependency on strain rate beyond a critical value (namely transition strain rate). Inelastic deformation in the dynamic stress-strain curves implied that degradation of modulus does occur. The incorporating such degradation (damage evolution) in modulus enabled a more accurate evaluation of transition strain rate as a function of characteristic length of specimen. On-line observation revealed that longitudinal cracks dominated the failure process of alumina with negligible interfacial friction. However, interfacial friction became significant with the decreased characteristic length, thus the inclined cracks dominated fracture in alumina. It was found that the effect of interfacial friction can be minimized by lowering the impact velocity to maintain the uniaxial loading status in SHPB loads. Finally, it is suggested that an aspect ratio of 1.0 for the specimen should be suitable for alumina due to its insensitivity to interfacial friction within the achievable strain rate.     

Tubular ceramic membranes coated with ZnO and applied in the disinfection of water contaminated with Staphylococcus aureus

In this study, commercial ceramic membranes of mullite and of alumina were coated with nanostructured ZnO using 3-aminopropyltriethoxysilane (APTES) as the connecting molecule between ceramic and ZnO. The characterization of ceramic membranes containing ZnO was performed by SEM, EDS and ICP-OES. The membranes without and with ZnO coating were applied in microfiltration tests to evaluate their antimicrobial capacity in the disinfection of distilled water contaminated with Staphylococcus aureus. In the microfiltration tests, ceramic membranes without the ZnO coating showed low efficiency in water disinfection, 35 and 45%, for mullite and alumina membranes, respectively. After the coating with nanostructured ZnO, both membranes showed disinfection efficiencies greater than 99%. The results obtained justify the application of ZnO as a promising coating on both mullite and alumina membranes for application in microfiltration aiming the disinfection of water contaminated with Staphylococcus aureus.     

Recovery strain of ceramics during sintering

A recovery strain after compressive loading was observed for bulk ceramic specimens during sintering. For pure alumina and low temperature co-firing ceramic (LTCC), a transient expansion occurs only in the loading direction after rapid removal of the load. This effect increases with stress magnitude and loading time.     

Design guidelines for titania-silica-alumina ceramics with tuned anatase to rutile transformation

Titania-based ceramics with adjustable anatase-rutile fractions were obtained by milling of anatase, quartz and corundum precursors, uniaxial pressing and firing at 1100?°C. The influence of silica and alumina, combined with milling time and compaction pressure, was studied by design of experiments. The L9 orthogonal array with a three-level noise factor was employed. Firing of pure titania at 1100?°C yielded complete anatase to rutile transformation (ART), whereas stabilized samples show that an optimum amount of 9% silica and 33% alumina reduces phase transformation to only about 5?wt% rutile. An extended correlation matrix combined with analysis of variance (ANOVA) was applied to assess the combined effects of quartz, alumina, milling time and uniaxial compressing pressure on relative density, and anatase to rutile transformation. Results show absence of ART after milling, and controlled partial conversion of anatase to rutile after firing. Very good fitting was obtained by multivariate analysis on considering first and second order terms for dependence on silica contents and interactions between silica and each of the remaining factors, including milling time. This empirical dependence could be interpreted on a sound physicochemical basis, allowing the prediction of suitable compositions and processing conditions to obtain rutile-free samples by conventional ceramic processing, and to design ceramic samples with controlled fractions of anatase and rutile.     

Evolution of Mechanical Properties of Porous Alumina during Free Sintering and Hot Pressing

This comparative study addresses the influence of microstructural evolution on mechanical properties of porous alumina sintered with and without the application of uniaxial pressure. A complete set of data on Young's modulus, long-crack fracture toughness, and fracture strength for two alumina powders as a function of density was obtained. The evolution of fracture strength with increased density was modeled using a porosity-dependent crack-tip fracture toughness, linked to the contact area of grains and a porosity-dependent size of the largest defect. The defect shape factor was found independent of porosity. A uniaxial pressure of 13 MPa during densification had negligible effect on the relation of strength to porosity.     

Pressable Glass Ceramic as a Repair Material for Fractures in Metal-Ceramic Restorations

Porcelain repair of fractured metal-ceramic restorations is a common challenge in the dental field. The current study introduces a novel technique for intraoral repair of porcelain fracture utilizing pressable glass ceramic (GC) as the repair material. The shear bond strength (SBS) of GC to different components of the metal-ceramic restoration (metal, porcelain, and metal-porcelain combination) under different surface treatments was also evaluated. The SBS was tested under the following five surface treatments: control, sandblasted with silica, sandblasted with silica then glow-discharge-treated, sandblasted with alumina, and sandblasted with alumina then etched with hydrofluoric acid. A self-adhesive resin cement and silane were used for bonding. Surface roughness was evaluated before and after each treatment. One-way ANOVA and post-hock Tukey's tests were used for data analysis. The results showed that SBS of GC to all surface-treated substrata ranging from 37 to 55 MPa was significantly higher than that of untreated surfaces ranging from 10 to 26 MPa. For all bonded surfaces, blasting with silica showed higher bond strength than blasting with alumina. On the contrary, alumina groups showed higher surface roughness than silica groups. Pressable glass ceramic in conjunction with all tested surface treatments, especially silica blasting, can be used successfully for fractured porcelain repair.