Compressive properties of open-cell ceramic foams

  ZHANG Jun-yan(张俊彦)^{1, 2}, FU Yi-ming(傅衣铭)¹, ZENG Xiao-ming(曾晓明)³

1. Department of Engineering Mechanics, Hunan University, Changsha 410082, China;

2. College of Civil Engineering and Mechanics, Xiangtan University, Xiangtan 411105, China;

3. China Machinery Interntional Engineering Desing and Research Institute, Changsha 410007, China

Received 10 April 2006; accepted 25 April 2006

Abstract: The compressive experiments of two kinds of ceramic foams were completed. The results show that the behavior of ceramic foams made by organic filling method is anisotropic. The stress-strain responses of ceramic foams made by sponge-replication show isotropy and strain rate dependence. The struts brittle breaking of net structure of this ceramic foam arises at the weakest defects of framework or at the part of framework, which causes the initiation and expanding of cracks. The compressive strength of ceramic foam is dependent on the strut size and relative density of foams.

Key words: ceramic foam; stress-strain curves; elastic properties; compressive experiment

1 Introduction

Ceramic foam is a kind of porous materials with porosity ranging from 70% to 90%, volume density from 0.3 to 0.6 g/cm³. It has 3D frameworks structure, and interconnected pores. Because of its many advantages, such as low density, high porosity, large specific surface area, low heat transfer rate, high temperature resistance, corrosion resistance and excellent acoustic property, it is applied in a variety of industries such as filtration, heat insulation, sound insulation, catalyze, and extends to electron, optics and biochemistry over the past decade. Recently, ceramic foams made of hydroxyapatite are employed in the biotechnology and biomedical industries, which can simulate bone and bio-implants. Many researches have been studied on candidates in ceramic materials for microelectromechnical systems applications, such as ceramic foams and ferroelectric thin films [1-4].

Open-cell ceramic foam manufacturing techniques can be classified into three general categories: sponge-replication, adding foaming agent and organic filling [5-9].

The sponge-replication was first developed in the early 1960 s. It uses a natural sponge or polyurethane foam as a form, which is infiltrated with ceramic slurry. The ceramic slurry is then fired to form ceramic foam.

Based on gas bubbles in preceramic melts, gas evolving constituents are added to the melt. The  generated bubbles causes the foam. Foaming uniformity and cell geometry can be adjusted by careful selection of surfactants and foaming agents.

The organic filling technique is based on a space holder concept. i.e. a polyurethane foam infiltrated with ceramic slurry is dried and indurated at room temperature.

Ceramic foams sample 1 used in this experiment was made by the organic filling technique. Samples 2 and 3 were made by the sponge-replication technique. Quasi-static uniaxial compressive experiments were performed, in order to get their mechanical properties under uniaxial compression.

2 Experimental

The porosity and density for two kinds of ceramic foams used in this test are 78%- 83% and 0.541-0.557 g/cm³ respectively [8]. The size of Sample 1 is 40 mm×40 mm×40 mm, those of samples 2 and 3 are d 60 mm×22 mm and 50 mm×50 mm×22mm, respectivey.

Uniaxial compression tests were performed using an Instron universal testing machine. Load and compression displacement were input into x-y function recorder.  The sensor and resistance strain gauge, and force-displacement curves were auto saved. At the same time, the sequence of whole compression events was recorded with a imaging system(CCD camera) in order to get morphological pictures of samples.

3 Results and discussion

Sample 1 was uniaxially compressed in y and z directions separately at the same strain rate 0.001 4 s^-1, The stress—strain curves of both directions are shown in Fig.1.

Fig.1 Stress—strain curves of ceramic foam 1 under compression in y and z directions

Fig.1 shows that the stress—strain curve of ceramic foam 1 is similar to that of the foamed material characterized by three distinct regions, i.e. elastic, collapse plateau and densification region, but the elastic region is very small. It can be seen from Fig.1 that the shapes of the two stress—strain curves in y and z directions are similar but not overlapping, which indicates ceramic foam 1 has anisotropic mechnical property. Because of its preparatory method, ceramic foam sample 1 is really a mixture of sponge and ceramic slurry. This compressive response reflects the mechanical behavior of the sponge mainly, and the sponge is anisotropic. Sponge made by pouring the polymer plus a hardener and a foaming agent into an open mould usually has cells that are elongated in the rise direction. Those made by spraying also rise and in these too, the cells are usually elongated in the spray direction [11]. The anisotropy of ceramic foam sample 1 arises in the way mentioned above, and it is called structural anisotropy. Fig.2 shows the morphological graphs of ceramic foam 1 in compressive process. It can be seen that ceramic foam 1 framework consisted of sponge, a kind of elastic material, is not breakdown. The struts of framework are bent under load. As load increased, force on framework would exceed yield strength of struts, resulting in plastic deformation.

Ceramic foams sample 3 were compressed in y and z two directions separately at the same strain rate 0.001 4 s^-1. The stress—strain curves of y and z directions are showed in Fig.3. Fig.3 shows that this ceramic foam is of isotropy. The stress strain curves of ceramic foam 3 compressed at different strain rates (0.001 4 s^-1, 0.000 3 s^-1) are shown in Fig.4. It can be seen that these ceramic foam properties depends on the strain rate, and the elastic modulus and elastic strength of the ceramic foam increase with the increase of strain rate. Moreover, there is no plastic deformation in the whole compression process, because this ceramic foam framework is ceramics, a kind of typical brittle material. The morphological graphs of ceramic foam sample 2 at different load stages are shown in Fig.5. It can be seen that the framework of ceramic foam 2 is fractured before ceramic foam is fully densified. In compression process the force on framework will exceed its fracture strength and result in brittle fracture. The deformation of framework is mainly result from its fracture partly, which causes the crack arised at the weakest part of framework or at the defects of framework. With the increasing force, cracks gradually extend until the specimen are eventual failure.

The strut strength of open-cell ceramic foam has been estimated from both mechanical tests and fractography. BREZNY et al [12] measured the strut strength of open-cell ceramic foams by threading a thin wire around a single horizontal strut in a block of foam that was clamped to the crosshead of a testing machine and giving three-point bending, with the sample ends assumed to be rigidly clamped. In practice, the degree of constraint at the ends depends on the stiffness of the adjacent struts: the failure stress in a rigidly constrained strut is half that in one constraint. The difficulties of estimate the size of the central void within the strut led to additional errors of roughly ±50%.

4 Conclusions

1) The behavior of ceramic foams made by organic filling methods is of anisotropy. The elastic-plastic deformation of strut contributes to that of ceramic foam mainly when loaded. The quasi-static uniaxial compressive curves consist of elastic, collapse plateau and densification regions and exhibit the characteristic of sponge.

2) The stress—strain responses of ceramic foams made by sponge-replication show isotropy and strain rate

Fig.2 Morphologies ceramic foam 1 in compressive process: (a) 0; (b) 80 s; (c) 160 s; (d) 240 s; (e): 320 s; (f) 400 s

Fig.3 Stress—strain responses of ceramic foam 3 compressed in y and z directions

dependence. The breakdown of net structure of this ceramic foam results from struts brittle breaking, which caused the birth of crack arised at the weakest part of

Fig.4 Stress—strain curves of ceramic foam 3 compressed at different strain rates

framework or at the part of framework with defects. The  compressive strength of foam depends on its struts size and relative density of foams.

Fig.5 Images of ceramic foams sample 2 at different load stages: (a) 0; (b) 39 s; (c) 78 s; (d) 117 s; (e) 156 s; (f) 195 s

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(Edited by ZHAO Jun)

Corresponding author: ZHANG Jun-yan; Tel: +86-732-8293794; Fax: +86-732-8293240; E-mail: zhangjy@xtu.edu.cn