Fractography of zirconia-specimens made using additive manufacturing (LCM) technology

A relatively new method to manufacture complex ceramic prototypes and components is additive manufacturing (AM). With the LCM (Lithography-based Ceramic Manufacturing)-technology the green body is manufactured layer-by-layer using selective curing of light-sensitive ceramic slurry by a mask exposure process. After curing by blue light the component is removed from the building platform and the green body is sintered to a ceramic component.The aim of this work is to investigate the influence of processing and layer architecture on the mechanical properties of an Yttria-stabilized zirconia ceramic. Strength tests were performed by uniaxial bending tests and by biaxial Ball-on-three Balls (B3B) tests. To identify typical fracture initiating flaws a systematic fractographic investigation was performed on different batches of Ball-on-three Balls-test and bending test specimens, respectively.Through additional investigations it was found that hardness and fracture toughness were independent on the layer architecture. But an extensive fractographic analysis showed that the strength was limited by flaws, which were introduced by processing and handling. If these flaws can be avoided by optimisation of the process the strength should be equal to that of conventional processed ceramics.     

Influence of surface defects on the biaxial strength of a silicon nitride ceramic - Increase of strength by crack healing

Failure of brittle materials starts in general from defects which exist in the volume or on the surface of the specimens. Surface flaws, which are more dangerous than volume flaws, can be introduced by machining. They decrease the strength of specimens and components.For this investigation silicon nitride specimens were produced using different machining conditions. About half of them were strength tested by use of the biaxial ball-on-three balls (B3B) test. It has been shown that better (more gentle) machining increases the strength but may also cause an increased scatter of strength data.The remaining specimens were heat treated (annealed) at 1000 °C in air and afterwards also strength tested using the B3B test. Compared to the non heat treated specimens a significant increase in strength could be proven, which was - depending on the machining conditions - between almost 300 MPa and more than 500 MPa. The scatter of strength data was largely decreased.The improvement was caused by the formation of a thin (0.5-2 μm) glassy layer which filled surface cracks and surface related pores during annealing.     

Fractographic Montage for a Si3N4–SiC Nanocomposite

A silicon nitride–silicon carbide nanocomposite has been prepared by an in situ method that utilizes C+SiO₂ carbo-thermal reduction during the sintering process. The materials consist of a silicon nitride matrix, with an average grain size of 140 nm, and inter- and intragranular SiC particles with sizes of approximately 250 and 45 nm, respectively. The four-point bending strength and its distribution were investigated. The fracture origins were identified and characterized using fractographic methods, and a fractographic montage of the Weibull plot and fracture origins was constructed. The fracture origins were subsurface and volume located processing defects with sizes from 5 to 460 μm, mainly in the form of clusters of pores, together with clusters of large SiC grains.     

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.     

Surface Integrity Effects on the Fracture Resistance of Electrical-Discharge-Machined WC–Co Cemented Carbides

The flexural strength evolution for two WC–16 vol% Co cemented carbides, with different mean carbide size, subjected to sequential and upgrading electrical-discharge machining (EDM) is studied. It is compared with the fracture behavior exhibited by a reference surface finish condition, attained through conventional mechanical grinding and polishing using diamond as abrasive. Considering that rupture is related to existing defects, either introduced during sample elaboration or induced by machining, a detailed fractographic examination by scanning electron microscopy is conducted to discern fracture origins. The experimental findings indicate that the flexural strength of WC–Co hardmetals may be strongly affected by EDM, depending on the correlation existing between natural defects, as given by particular microstructural parameters, and EDM-induced flaws. An analysis of the results using a linear–elastic fracture mechanics approach permits one to establish a clear connection between surface integrity and fracture resistance. Quantitative discrepancies between the estimated and the experimentally measured critical flaw sizes for all the EDM-related grades are rationalized through the existence of local residual tensile stresses of considerable magnitude at the shaped surface. Release of these stresses through final mechanical and annealing treatments is pointed out as a quite effective alternative for improving the fracture behavior of WC–Co cemented carbides shaped by EDM.     

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.     

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.     

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).     

Effect of Surface Flaw Size on Fracture Strength of Alumina Ceramics

Semielliptical surface flaws of different sizes were introduced into Al₂O₃ by Knoop microhardness indentation. The specimens were fractured by four-point bending and the profiles of the indentation flaws were determined by observing the fracture surfaces with a scanning electron microscope. The relation between the indentation flaw size and the fracture strength could be well explained by applying the fracture-mechanics analysis for semielliptical surface flaw in bending. The calculated values of the as-indented critical stress intensity factor, K_IC, were lower than previously reported presumably because of the influence of the residual stresses produced by the indenter.     

Oxidation-Induced Toughening and Strengthening of Y2O3/SiC Nanocomposites

Effects of oxidation on mechanical properties have been investigated for Y₂O₃/5 vol% SiC nanocomposite. The roomtemperature fracture strength and toughness substantially increased after oxidation around 900–1000°C for 5 h. On the other hand, little improvement was identified for specimens treated in an inert atmosphere under the same conditions. A TEM study of the oxidized specimen surfaces revealed formation of extensive residual strain contours around SiC nanoparticles. The improved strength and toughness could be caused by compressive surface stress, which was generated by volume expansion of the nanoparticles due to oxidation.     

Effect of Loading Rate and Surface Conditions on the Flexural Strength of Borosilicate Glass

This study evaluates the loading rate and surface condition dependence of the flexural strength of a borosilicate glass. The glass specimens are subjected to three different surface treatments before four-point bending tests to study the effect of surface flaws. Quasistatic (Material Test System 810) and dynamic (Kolsky bar) experiments are performed at loading rates ranging from 0.7 to 4 × 10⁶ MPa/s. The results show that the flexural strength of the borosilicate glass has a strong dependence on the loading rate. A chemically etched surface produces an enhanced flexural strength by about an order of magnitude. Scanning electron microscopy images on fracture surfaces indicate that the failure is governed by different types of flaws under different surface treatment conditions. Edge failure is also identified for samples possessing high flexural strength.     

Maximum likelihood estimation for the three-parameter Weibull cdf of strength in presence of concurrent flaw populations

For a correct strength characterization of brittle materials, not only the maximum stress at fracture, but also the geometry of the specimens has to be considered thus taking into account the variable stress state and the size effect. Additionally, fracture may occur due to different fracture modes, as for example surface or edge defects. The authors propose a maximum likelihood estimator to obtain the cumulative distribution functions of strength for surface and edge flaw populations separately, both being three-parameter Weibull cdfs referred to an elemental surface area or elemental edge length, respectively. The method has been applied to simulated 3-point bending test data. The estimated Weibull parameters have been used to compute the cdfs of strength for specimens with different size, providing also the confidence bounds calculated by means of the bootstrap method. Finally, fracture data of 4-point bending tests on silicon carbide have been evaluated with the proposed method.     

Mechanical Properties of Infiltrated Alumina-Y-TZP Composites

Small Al₂O₃ additions (∼ 4 vol%) made to Y-TZP using an infiltration technique increased the fracture toughness and strength by ∼15% and the amount of transgranular fracture. Ionic conductivity measurements showed decreased grain-boundary conductivity, confirming a change in the grain-boundary composition. The predominant failure origins for both the unmodified Y-TZP and the Al₂O₃—Y-TZP were surface flaws related to agglomerates in the original powder. Finishing reduced the severity of these flaws and substantially increased the strength of both materials (>50%). The infiltration approach introduced a new flaw population in some specimens; however, this problem was overcome by a simple processing modification.     

Fractographic analysis of silicate glasses by computer vision

ASTM C1678 describes the state-of-the-art's fractographic techniques to estimate the fracture strength of glasses and ceramics through empirical, strength vs. fracture mirror length relationships. However, the methodology is subjective and only applicable to a few loading scenarios and relatively pristine fracture surfaces. This work presents a semi-automated, alternative approach to objectively estimate the strength of silicate glasses for ampler loading and geometric scenarios. The proposed method relies on a baseline set of fracture surface profilometry-scans gathered on samples of known strengths. A computer vision-based algorithm compares relevant, topological features extracted from the baseline set to the features on the fracture surfaces investigated. An empirical relationship based on over 2,100 fractured silicate specimens is used to compute the strength of the trial sample. The proposed scheme could accurately estimate the strength of specimens beyond the capacity of ASTM C1678, such as in chemically strengthened glasses and fracture surfaces displaying significant damage.     

Threshold angle and valid fracture of the sectored flexural specimen

The sectored flexural specimen was developed over a decade ago to measure the strength of ceramic and glass tubes and cylinders in which flaws on a tube's or cylinder's outer surface are limiters of axial tensile failure stress. Using the specimen's geometry, the associated axial tensile failure stress can be analytically calculated from the failure force measured from simple uniaxial bending, and multiple specimens (and test data) can be harvested from a single tube or cylinder. The sector angles of specimens in previous studies were somewhat arbitrarily chosen and usually produced validly occurring fractures and data; however, if the angle used was too small (relative to the tube's or cylinder's geometry), then undesirable application-irrelevant edge-located failures resulted. To avoid such failures in specimen design, a threshold sector angle was identified to guide the selection of a minimum sector angle (and consequential cross section) for any arbitrary sector flexural specimen harvested from a tube or cylinder. If the sector angle of the specimen is larger than the threshold value, then fracture will not occur at a specimen's edge and the measured axial failure stress will be limited by surface-located flaws on the tube's or cylinder's outer surface.     

Controlled Surface Flaw-Initiated Fracture in Reaction-Densified Silicon Carbide

Controlled surface flaws were produced in commercial reaction-densified SiC by Knoop microhardness indentation. The flaws themselves could not be observed easily, thus an etching technique was used to delineate their semielliptical shape, thereby enabling calculation of the critical stress- intensity factor K _IC at room temperature. Room-temperature fracture was insensitive to annealing environment (air or vacuum), flaw "healing" being observed at ≫1000°C. The variation in fracture stress of indented specimens with temperature showed 3 distinct regions of behavior which were interpreted in terms of residual stress relief, flaw healing, and Si-SiC bond weakening.     

Fractography of silicon nitride based ceramics to guide process improvements

Fractography is an important tool to understand and identify the cause of the failure in materials. This understanding can be used to make changes in raw materials selection and processing to increase the strength of brittle materials. This study reports the fracture behavior of hot-pressed silicon nitride based ceramics, with focus on dominant flaw identification with respect to material and process parameters. Silicon nitride is an important material for structural applications which require high strength and wear resistance, such as bearings, nozzles, and cutting tools. Silicon nitride with a target base composition of Si_6-zAl_zO_zN_8-z (z = 0.5), along with varying boron dopant levels, was explored in this work. Detailed fractographic analyses revealed that the majority of fracture origins were internal flaws due to the foreign impurities introduced at various stages of processing. All materials were found to have reasonably high strength (800−1100 MPa). Strength was inversely proportional to the square root of the flaw size, however no correlation was found between measured flexural strength and fracture origin types. Mirror constants calculated from fracture mirror measurements ranged between 5.8 and 9.8 MPa.m^1/2.     

Effect of Cleaning and Abrasion-Induced Damage on the Weibull Strength Distribution of Sapphire Fiber

Fractographic analysis revealed the presence of concurrent flaw populations in sapphire fibers which were tensile tested in the as-received condition (sized and unsized) and after various cleaning procedures. The following flaw populations were identified: surface flaws attributed to handling and abrasion damage (type A), volume or internal flaws attributed to shrinkage voids which form during the manufacturing process (type B), localized fiber surface reaction flaws introduced during the flame-cleaning procedure (type C), and self-abrasion surface flaws intentionally introduced on unsized fibers (type D). The strength distribution associated with each flaw type was characterized using a censored data Weibull analysis for both the least-squares and maximum-likelihood estimation methods. The strength distribution for type C (flame-cleaning) flaws exhibited an approximately 20% degradation in strength compared to the distribution for type A flaws. The strength distribution for type D (self-abrasion) flaws exhibited an approximately 35% degradation in strength compared to the strength distribution for type A flaws. This result underscores the need for fiber sizings to prevent damage during shipping and handling. However, higher purity sizings and/or improved procedures for sizing removal are required to mitigate cleaning-induced fiber strength degradation during composite fabrication.     

Rotational flexural strength of cylindrical brittle specimens

Conventional static flexural strength testing of brittle cylindrical rods only subjects a small fraction of the entire specimen's area or volume to the maximum tensile stress. Thus, a nonconservative measured strength likely results since most flaws on the surface or in the bulk are not subjected to a sufficiently high tensile stress that can cause fracture. To mitigate this, a rotational flexural tester and corresponding test method were developed whereby rotation and monotonically increasing three-point flexure were superimposed to investigate fracture response of solid glass cylinders. This combination of rotation and flexure subjects more area and volume of a cylindrical test specimen to tensile stress than a standard static (nonrotating) flexural test. As anticipated, failure stresses were lower for the rotational flexural test. Expressions for effective area and volume are provided for rotating solid rods and tubes subjected to three-point, four-point, uniform, and uniformly distributed load bending configurations.