Mullite/Alumina Particulate Composites by Infiltration Processing: III, Mechanical Properties

The effect of incorporating mullite into alumina by an infiltration process on the mechanical properties was investigated. Data for Young's modulus, strength, and fracture toughness for various composite compositions were compared with those for the unreinforced matrix (alumina). Measurements of Young's modulus by a resonance technique showed that the addition of mullite decreased Young's modulus. Up to 14 vol%, these changes were close to those expected, but above this mullite content, the decrease was more dramatic and indicated specimen damage during processing. The addition of mullite led, in some cases, to increases of more than 60% in both the strength (biaxial flexure) and indentation fracture toughness. These increases have been attributed to the method of introducing mullite and the resulting residual compressive surface stresses. The strength of the indented composite bodies deviated from the ideal behavior, indicating the probability of R -curve behavior in these materials.     

Fracture Origin and Strength in Advanced Pressureless-Sintered Alumina

Advanced raw materials and shaping approaches enable the production of pressureless-sintered alumina parts where, in bending, the average maximum stress at the fracture origin is as high as 800 MPa. In individual specimens that fracture at lower stresses (450–600 MPa), failure often originates at volume flaws, as known for hot-pressed alumina with a similar strength. Also, transgranular and intergranular fracture modes along the crack path are the same as those observed in hot-pressed alumina. If the size and the frequency of volume flaws are reduced, fracture initiates at smaller defects in the ground surfaces and bodies with a bending strength of >800 MPa are produced without hot pressing. The grain-size dependence of grinding-induced surface damage contributes to a grain-size effect for the strength.     

Behaviour of cordierite materials under mechanical and thermal biaxial stress

Abstract

A commercial cordierite powder (< 0.17 wt-% impurities) was selected for a study of material behaviour under mechanical and thermal stresses. Disks were slip cast, sintered for 2 h at 1450°C, and indented (Vickers, 44.1 N) at the centre of the surface to be subjected to mechanical and thermal shock tests. The sintered bodies (84 wt-% cordierite, 10 wt-% mullite, 6 wt-% glass) reached 95% of theoretical density. The microstructure consisted of homogeneous, mainly equiaxed grains (mean size ≈0.5 μm) and a few elongated grains (aspect ratio ≈1.9). A glass phase was identified at triple points, and intergranular pores (< 10 μm) and a few isolated larger pores (up to 40 μm) were observed. The fracture strength σ_F was measured by biaxial flexure, employing a ball on discontinuous ring configuration with displacement con1 trol (0.05 mm min ^-1). In each thermal shock test, the indented specimen was heated to a selected temperature and the disk centre was then suddenly cooled using a high velocity air jet at room temperature. The initial temperature was increased by increments of 10°C until crack propagation was detected and the value of the thermal shock resistance Δ T_C was evaluated. The values obtained were compared with cordierite disks without indents and with alumina materials. The fracture features of the specimens broken in both mechanical and thermal shock tests (crack patterns and fracture surfaces) were characterised, taking into account the developed microstructures (grains, phases, pores) and the fracture origin at the controlled size defect introduced by indentation.     

Reliability Analysis of Structural Ceramics Subjected to Biaxial Flexure

Sintered alumina and silicon nitride were tested in uniaxial (four-point and three-point bend) and biaxial (uniformpressure-on-disk) flexure tests in inert conditions. Fracture origins were identified to be surface flaws in alumina and subsurface pores in silicon nitride. Batdorf's statistical fracture theory and two different fracture criteria, the critical normal stress criterion and a noncoplanar strain energy release rate criterion, were used to examine size and stress-state effects on fracture strengths of the two ceramics. Size effects assessed in four-point and three-point bend tests were in good agreement with the theoretical predictions for both ceramics. Measured biaxial strengths of alumina were in good agreement with the prediction when a noncoplanar strain energy release rate criterion and random surface flaw orientations were assumed. On the other hand, biaxial fracture strength of the silicon nitride was consistent with a prediction based on preferred flaw orientation (i.e., normal to the principal stress in the disks) and the normal stress fracture criterion. Orientation distributions of the fracture planes assessed from the fracture patterns of the disks supported the assumptions of random flaw orientations (alumina) and the preferred flaw orientations (silicon nitride), respectively, for the two ceramics. The preferred flaw orientation in silicon nitride is suggested to originate at subsurface pores as a result of crack nucleation in the plane of maximum tensile stress concentration, i.e., a diametral plane normal to the maximum principal stress.     

Specimen design,manufacturing and testing procedures for flat carbon fiber reinforced plastic laminates under biaxial loading

The optimization of a specimen design allowing the investigation of the biaxial strength of composite laminates over the full range of failure strain was the primary objective of this work. Multiaxial strength criteria are often found unreliable mainly as a result of the inherent complexity of biaxial tests and, in many cases, as a result of inefficient specimen designs. As a result of a development program combining numerical simulations and experimental measurements, a flat cruciform-shaped specimen has been developed for carbon fiber reinforced plastic (CFRP) laminates. The design fulfills the basic criteria for such a specimen, namely allowing for a uniform biaxial stress/strain state to exist in the gauge area and for testing the virgin material up to failure in both the tension-tension and tension-compression quadrants of the strain/stress space. The fabrication of the specimen is described and a three-step testing procedure for generating biaxial strength data is proposed. Typical results obtained from specimens of the proposed configuration tested in accordance with this procedure are presented. Results compare well with those obtained from tubular specimens, thus confirming the effectiveness of the proposed design. Experimental data obtained for the AS4/3501-6 carbon/epoxy composite system are finally compared against strength predictions of recognized failure theories.     

Sodalite zeolite as an alternative all-ceramic infiltrating material for alumina and zirconia toughened alumina frameworks

The purpose of this study was to determine the effects of synthesized sodalite zeolite infiltration achieved by a direct in-situ hydrothermal reaction followed by sintering process on the flexural strength and hardness of alumina and zirconia-toughened alumina (ZTA) frameworks. Ceramic core materials were prepared as disk-shaped specimens with 16 mm diameter and 1.2±0.2 mm thickness. The case-study group was synthesized sodalite zeolite-infiltrated alumina (IA-SOD) and synthesized sodalite zeolite-infiltrated ZTA (IZ-SOD); and the control group was glass-infiltrated alumina (IA-glass) and glass-infiltrated ZTA (IZ-glass). The biaxial flexural strength (piston-on-three-balls test) and Vickers microhardness were compared among groups (n=10 specimens in each group). Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were used to investigate the structural characteristics of specimens at the fracture and cross-sectional surfaces. For both IA-SOD and IZ-SOD, the biaxial flexural strength exceeded the required value of 100–150 MPa as specified by ISO 6872(2015), indicating their potential as all-ceramic core materials. The flexural strengths and Vickers microhardness of IZ-SOD were respectively 324.7 MPa and 1162 VHN, while these values were measured 233.6 MPa and 1013 VHN for IA-SOD. The mechanical properties and microstructure of core materials have been advocated as crucial to the clinical performance of all-ceramic dental restorations. This investigation provides data regarding the flexural strength, hardness and microstructure of partially sintered alumina and ZTA frameworks with synthesized sodalite zeolite infiltration.     

Standardized Weibull statistics of ceramic strength

In order to estimate Weibull parameters in the Weibull statistical fracture theory as truly material properties independent of specimen geometry and loading mode, first the Weibull statistical fracture theory is transformed into the ordinary Weibull distribution function under certain approximation. Then the standardized format of ordinary Weibull distribution is introduced to enable Weibull modulus as the single parameter for estimation via the maximum likelihood method. The method of using standardized Weibull distribution for strength data synchronization and Weibull modulus estimation is validated by analyzing extensive strength data sets measured from uniaxial flexure, biaxial flexure and their combination, and from smooth and notched specimens. The technical path to estimate the scale parameter and threshold strength as material properties in the Weibull statistical fracture theory and effect of sample size on the estimation accuracy are also discussed.     

Effect of Polyaxial Stress States on Failure Strength of Alumina Ceramics

The effect of polyaxial stress fields on the brittle fracture strength of polycrystalline alumina was investigated through the use of thin-walled cylinders. Combinations of internal pressure, external pressure, and axial loads produced stress states of tension-compression, tension-tension, and compression-compression. The failure envelope was generated for these stress states. The results indicated that biaxial tensile stresses reduced the strength of the material; however, the tensile strength increased at least 50% when a compressive stress existed normal to the tensile direction. Compression strengths as high as 640,000 psi were measured for a biaxial compressive stress state.     

Fractography of biaxial tested Si3N4-specimens

The strength of ceramic materials is limited by flaws which are distributed in the volume or on the surface of the material. Commonly, fractographic investigations are performed after the strength tests to interpret the strength values.The relatively new Ball-on-Three Balls (B3B)-bending test applies a biaxial stress state (which is more searching for cracks than a uniaxial stress state) on the specimen. To identify typical fracture initiating flaws and to get a better understanding of the fracture behaviour of B3B-specimens a systematic fractographic investigation was performed on 260 silicon nitride specimens divided into batches with different surface qualities. It could be shown that in most cases (at least those in which origins could be clearly identified) surface or near surface located defects were responsible for failure. On specimens with poor surface qualities, surface defects were introduced through machining. On specimens with a better surface quality, volume defects, which were exposed on the surface by polishing, could be identified as fracture origins. In only a few cases defects in the bulk were fracture origins.     

A general formulation for strength prediction of advanced ceramics by ball-on-three-balls (B3B)-test with different multiaxial failure criteria

The determination of biaxial strength of ceramics plays a large role in the design of ceramic components. The ball-on-three-balls (B3B)-test is one of the most useful methods for measuring the biaxial strength of ceramics. The strength measured by B3B-test, with any specimen, is dependent on the size of the specimen and loading conditions (type and position of loading); therefore, the strength value, measured with a set of specimens, has to be adjusted by effective volume and/or surface. The standardized strength value obtained from this adjustment can be then used for the design process. Consequently, there is a need for calculating the effective volume/surface of the B3B-test specimens. In this article, general fitting functions are provided for effective volume/surface of B3B-test specimens with different multiaxial criteria, these can be used for all ceramic materials and for various test configurations. Verification of numerical effective volume and effective surface values with experimental measurements show that B3B specimens fail due to surface flaws according to normal stress criterion (NSC).     

Anomalous Defects and Dynamic Failure of Armor Ceramics

The ballistic performance of state-of-the-art silicon carbide armor material can exhibit a fairly wide variability in certain test configurations, which, it is proposed, may be due to the presence of large (>0.1 mm), rare defects, termed, herein, "anomalous" defects. SiC rubble resulting from ballistic tests was examined, as were quasi-static test samples. Ballistic fragment fracture surfaces revealed large carbonaceous defects that seemed to affect fracture path and mode. Low-strength biaxial flexure samples demonstrated similar defects (>0.1 mm) as failure origins. Carbonaceous defects similar in appearance but smaller in size were also found at the fracture origins of SiC bend bars. Frequently, alumina inclusions were found within the carbonaceous discontinuities. These alumina inclusions may cause the graphitic regions to form during sintering. The random distribution of such large, rare carbonaceous discontinuities from sample-to-sample, as well as batch-to-batch variability, may explain high ballistic variability for SiC armor ceramics.     

Equibiaxial Stress Effect on Fracture Strength of Indented Soda-Lime Glass in Water Environment: An Examination for Dependence on Crack-Tip Blunting

Equibiaxial stress effects were observed in constant stressing rate tests of indented soda-lime glass in a water environment. To discuss whether the equibiaxial stress effects on fracture strenght in a water environment were caused by the blunting of crack tips by chemical reaction between the glass and moisture, the specimens with controlled surface flaws soaked in hot water to make the crack tip blunt were fractured under both uniaxial and equibiaxial tensile stresses under both vacuum and air environments. The biaxial strengthening observed under restricted subcritical crack growth indicates that equibiaxial stress effects in a water environment are caused by the rounding of the initial crack tip.     

Effect of Surface Finish on Structural Ceramic Failure

The relation between fracture strength and surface finish of brittle nonmetallic materials was examined and related to surface-crack theory. Specimens used in an experimental illustration were made of a commercially available 96%-pure alumina with an average grain size of 3.8 μm and a porosity of about 6%. Several groups of specimens, each having a different surface finish, were used in a biaxial ball-and-ring     

Flexural strength of a conventionally processed and additively manufactured debased 94% alumina

Mechanical strength of a 94 wt% debased alumina was measured using ASTM-C1161 specimens fabricated via conventional and lithography-based ceramic manufacturing (LCM) methods. The effects of build orientation and a 1500°C wet hydrogen fire added to the LCM firing sequence on strength were evaluated. A Weibull fit to the conventional flexural specimen data yielded 20 and 356 MPa for the modulus and characteristic strength, respectively. Weibull fits of the data from the LCM specimens yielded moduli between 7.5 and 11.3 and characteristics strengths between 333 and 339 MPa. A Weibull fit to data from LCM specimens subjected to the wet hydrogen fire yielded 14.2 and 376 MPa for the modulus and characteristic strength, respectively. The 95% confidence intervals for all Weibull parameters are reported. Average Archimedes bulk densities of LCM and conventional specimens were 3.732 and 3.730 g/cm³, respectively. Process dependent differences in surface morphology were observed in scanning electron microscope (SEM) images of specimen surfaces. SEM images of LCM specimen cross-sections showed alumina grain texture dependent on build direction, but no evidence of porosity concentrated in planes between printed layers. Fracture surfaces of LCM and conventionally processed specimens revealed hackle lines and mirror regions indicative of fracture initiation at the sample surface rather than the interior.     

Indentation crack profiles of cordierite materials under mechanical and thermal biaxial stresses

The indentation crack profiles under mechanical and thermal stresses of cordierite materials obtained from a commercial powder were studied. Disks were prepared by slip casting and sintering, and they were indented (Vickers; 44.1 N; 15 s) at the center of one of their surfaces to be subjected to flexion in the mechanical test or to a sudden temperature change in the thermal shock test. Indented specimens were fractured in biaxial flexure (BF) or thermal shock (TS) tests. Some indented disks were thermally treated following two different schedules that involve exposures at high temperatures without (T₁) and with a sudden cooling (T₂). These specimens were fractured in biaxial flexure to observe the fracture surfaces. In all specimens (BF, TS, T₁ and T₂), the evolution of the indentation crack profiles was fractographically analyzed by SEM. In order to study the crack shape and the profile evolution, both the length and the depth of the indentation cracks were measured.     

Effects of Combustor Rig Exposure on a Porous-Matrix Oxide Composite

The present study explores the effects of exposure in a laboratory combustor on microstructural stability and property retention of an all-oxide fiber-reinforced ceramic composite. The material consists of a porous mullite–alumina matrix and Nextel 720 fibers in an eight-harness satin weave. To assess the effects of matrix strength, two matrix conditions are used, distinguished from one another by the amount of alumina added through precursor impregnation and pyrolysis (1.8% and 4.8%). In both cases, the dominant damage mode upon exposure involves interply delamination along the panel midplane. However, significant reductions in the rate and extent of cracking are obtained in the material with higher alumina content: a result of the higher delamination resistance. Mechanical tests performed on exposed specimens reveal a slight (10–20%) reduction in tensile strength along the fiber direction and a comparable increase in shear strength. These trends suggest some sintering of the matrix upon exposure. Examinations of fracture surfaces provide additional supporting evidence. Implications for long-term performance and strategies for imparting improvement in microstructural stability and delamination resistance are discussed.     

Mechanical Failure Characteristics of Ceramic Multilayer Capacitors

The results of both uniaxial and biaxial flexure as well as toughness testing on actual commercial ceramic capacitor samples are reported. Necessary procedural adjustments are outlined for miniaturizing the applied-moment double-cantilever-beam test to accommodate these small samples. Strength and toughness testing showed that the metal electrodes lowered the fracture toughness and that the metal-ceramic interface was a preferred fracture path for crack propagation parallel to the electrodes. However, toughness did not clearly depend on crack propagation parallel or perpendicular to the electrodes, nor with flexural strengths. Flexural strengths were 0 to 40% lower, with the electrodes perpendicular to the tensile surface vs. parallel to the tensile surface. Fractures initiated from machining (or impact) flaws, or voids of various sizes, shapes, and locations, thus explaining the poor correlation between strength and toughness. Biaxial flexure tests of dielectric specimens of various sizes indicate that such a test could be scaled down and "calibrated" for actual testing of capacitors; e.g., a decrease in strength with increasing specimen thickness to support diameter ratio is indicated.     

Rising Fracture Toughness from the Bending Strength of Indented Alumina Beams

The analytical function of crack extension to a fractional power is used to represent the fracture resistance of a vitreous-bonded 96% alumina ceramic. A varying flaw size, controlled by Vickers indentation loading between 3 and 300 N, was placed on the prospective tensile surfaces of four-point bend specimens, previously polished and annealed. The lengths of surface cracks were measured by optical microscopy. Straight lines were fitted to the logarithmic functions of observed bending strength versus indentation load in two series of experiments: (I) including the residual stress due to indentation and (II) having the residual stress annealed out at an elevated temperature. Within the precision of measurement these lines have the same slope, being about 32% less than the -1/3 slope which a fracture toughness independent of crack extension would indicate. Considering the criteria for crack extension and specimen failure, the fracture mechanics equations were solved for the conditions of the two series of experiments. Approximately the same values of fracture toughness, rising as a function of indentation flaw size, were obtained from both series of experiments.     

Biaxial and Uniaxial Data for Statistical Comparisons of a Ceramic's Strength

The uniaxial and equibiaxial tensile strengths of a brittle material were measured in bending. Equibiaxial tension was attained by concentric ring loading of disks and uniaxial tension by four-point line loading of plates. The two specimen designs give equal volumes, surface areas, and stress gradients. Ground surfaces and lapped surfaces were tested. The equibiaxial tensile strength of a dense alumina was lower than the uniaxial tensile strengths for both ground and lapped surfaces, 8.5 and 8.1%, respectively. The Batdorf theory of flaw statistics, in which biaxial tensile strengths can be predicted from the statistical distribution of uniaxial tensile strength measurements, agreed with the data.