Elsevier

Materials Science and Engineering: A

Volume 707, 7 November 2017, Pages 159-163
Materials Science and Engineering: A

Effects of artificial defect on the material residual strength of SiC ceramics after thermal-shock

https://doi.org/10.1016/j.msea.2017.09.043Get rights and content

Abstract

Thermal shock behaviors of SiC ceramic with artificial defects were studied. Artificial surface cracks were simulated by Vickers and Knoop indentations, and artificial inner cracks were obtained using novel method of introducing plastic fibers. The residual flexural strength and microstructure were analyzed. The flexural strength of artificial defect free SiC ceramic maintains minimum 87% of the original strength after thermal shock ΔT no more than 400 ℃, and the residual strength of those with artificial crack, no matter surface type or inner type, can still be improved by the joint effect of thermal treatment, thermal shock and the stress relaxation by artificial crack if ΔT no more than 400 ℃. It is believed that inner and surface artificial defects ease the crack propagation during the thermal shock process.

Introduction

Ceramic materials are widely used as high-temperature structural ceramic because they have characteristics such as high melting point, good mechanical stability at high temperature and chemical inertness. Among all ceramic materials, SiC ceramic attracts more attention because of its advanced mechanical performance like high strength, high hardness, low thermal expansion coefficient and high thermal conductivity, and so is widely used as seals, bearings, heat exchange tubes and so on [1], [2], [3], [4]; in such harsh industry environment of high temperature and high pressure, SiC ceramic maintains its excellent mechanical properties as it does in room temperature [5], [6], [7]. Used as these structural components, usually SiC ceramic also undergoes thermal shock, i.e. rapid change of temperature accompanied by intense stress change: the material fast switches between compressive and tensile stress statuses during the rapid temperature up and down cycle [8], [9].

Numerous studies on the thermal shock resistance of ceramic materials were reported in recent years [10], [11], [12], [13], [14], [15]. Parameter R = σc (1-ν)/αE is usually applied to evaluate the resistance to crack initiation in thermal shock. The fourth thermal shock parameter R’’’’ = (KIcc)2 (1+ν) defined by Hasselman [16], [17], [18] indicates the resistance to propagation of crack, in which σc is the flexural strength, E Young's modulus, α the thermal expansion coefficient, ν Poisson's ratio and KIc the fracture toughness. As a typical brittle material of low fracture toughness, SiC ceramic has low R’’’’ value [19], [20], [21], and easy crack extending is expected. Since the formation and development of microcracks closely relates to material failure of ceramics, it is critical task to understand the relationship of crack behavior and residual mechanical strength of SiC ceramic in thermal shock conditions. Because crack formation under thermal shock is a rapid and highly complex process, and the random original defects in ceramics are uncontrollable, artificial cracks by standard Vickers and Knoop indentations on material surface are commonly used for study [22], [23], [24]; however, there are few studies involve artificial inner cracks.

In this work, we intent to have an insight of the artificial defect behavior of SiC ceramic in thermal shock conditions. Thermal shock was simulated using water-quenching experiment. In addition to the common Vickers and Knoop indentations for surface cracks, well-controlled pre-fabricated inner defects were introduced to investigate the inner crack evolution of SiC ceramic. Three-point bending strength was tested for study of strength degradation by thermal shock. Artificial indentations and pre-fabricated inner defects were systematically analyzed in order to reveal the crack development of SiC ceramic.

Section snippets

Experimental procedure

Commercial silicon carbide (SiC) powder (SIKA SINTEX, purity >99%, D50~0.5 µm) was firstly mixed with 6 wt% carbon and boron carbide as sintering additives, 1 wt% phenolic resin as binder and ethyl alcohol as solvent. These raw materials were blended for 4 h using a ball mill to obtain uniform mixture. The mixture was then dried, sieved to fine powder and dry pressed to obtain an 8×5×45 mm3 green body, which was later cold-isostatical pressed at 200 MPa. Finally, the as-prepared green bodies were

Results and discussion

All SiC ceramic samples prepared in this work are with relative density higher than 98%, and so were considered qualified samples for evaluation and discussion. Fig. 2 shows the typical alkali-etched polished surface of a fully densified SiC ceramic sample. It shows size of grains ranging from 3 to 10 µm and residual pores (black phase) left by etched carbon black.

The value of flexural strength, Vickers hardness and fracture toughness of as-prepared artificial-defect free SiC ceramic were ~480 

Conclusions

In this work, embedded plastic fibers were used to obtain inner defects, and Vickers and Knoop indent were used to obtain surface cracks. The fracture strength before and after thermal shock experiments and the crack development were studied carefully.

The flexural strength of SiC ceramics retains >87% of the original strength when thermal shock temperature ∆T is no more than 400 °C. After 400 °C quenching, residual strength of SiC ceramic samples with either artificial surface or inner defect is

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