Fractography of biaxial tested Si3N4-specimens

doi:10.1016/j.jeurceramsoc.2013.12.038

Journal of the European Ceramic Society

Volume 34, Issue 14, November 2014, Pages 3283-3289

https://doi.org/10.1016/j.jeurceramsoc.2013.12.038 Get rights and content

Abstract

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.

Introduction

At low temperatures brittle fracture is the most significant damage mechanism in ceramics. Fractographic analyses are used to investigate failure in service or to find fracture initiating flaws in test specimens in order to provide a guide to material, process or design improvement. In general, the strength of ceramics is measured with uniaxial bending tests. In the past many authors have described the performing of a fractographic analyses including practical examples (e.g. 1, 2, 3, 4, 5, 6, 7).

However, in most cases these examples were given for specimens, which were tested in uniaxial bending experiments on bars and rods. The Ball-on-Three Balls Test is a new possibility for biaxial testing and can be applied on specimens, which have different shapes (like discs or plates) and sizes ranging from approximately 1 mm up to some 10 mm in diameter (details concerning the B3B-test can be found in 8, 9, 10, 11, 12). Since this test is relatively new, only little fractographic work on B3B-specimens has been published yet (e.g. [13]). In this paper we present the results of an extensive investigation of silicon nitride specimens fractured using the B3B-test.

Section snippets

Identification of fracture origins

The failure of polycrystalline ceramics with no or low porosity is caused by the extension of a single crack. Failure starts from a defect which acts as (or develops into) a sharp crack, is called the fracture origin. One definition for the fracture origin, given in,⁵ states that the “fracture origin is both a location and a flaw”, and defines the fracture origin as “the source from which brittle fracture begins”. With increasing crack propagation the speed of the crack accelerates and can

Investigated material

The investigated Si₃N₄-specimens were disc shaped (diameter 20 mm, thickness approximately 2 mm) and manufactured by FCT Ingenieurkeramik, Germany. The surfaces of batches of the 260 specimens were machined in eight different ways; afterwards approximately half of each batch was annealed (temperature 1000 °C, exposure time 10 h¹³). Fig. 2 summarizes the surface conditions and the results of the strength tests of the different batches.

The size of the fracture origins (calculated with Eq. (3) for the

Fractographic investigation and discussion

During the strength experiments, particularly the annealed specimens (which have a higher strength) broke in many fragments and splints. Commonly the number of fragments increases with the stored elastic energy, see Fig. 3. Due to the high fracture forces and the contact of the fragments with balls and jig secondary damage occurred. Because sometimes small fragments or shards get lost during the experiment, it was not possible to identify typical fracture origins in many cases. On the tensile

Conclusions

The B3B-test applies a biaxial stress field to the specimen; the highest tensile stress appears in a small area on the surface of the specimens and decreases very rapidly towards the periphery and into the volume of the disc. Therefore the result of the test is very sensitive to surface defects which appear in the centre of the disc.

The fracture origins which were identified in this investigation were therefore in most cases surface defects (type 1 and type 2) or near surface located flaws

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Fractography of biaxial tested Si3N4-specimens

Abstract

Introduction

Section snippets

Identification of fracture origins

Investigated material

Fractographic investigation and discussion

Conclusions

J Eur Ceram Soc

J Eur Ceram Soc

J Eur Ceram Soc

J Eur Ceram Soc

J Eur Ceram Soc

J Eur Ceram Soc

Cer Int

Wear

Ceramic Fracture Features, Observations, Mechanisms, and Uses

ASTM STP 827

Quantitative fractography: an assessment

Ceram Trans

Fractography and fracture mechanics property assessment of advanced structural ceramics

Int Mater Rev

Fractography of brittle materials (Measurement Good Practice Guide No. 14, HMSO)

Fractography of biaxial tested Si₃N₄-specimens