Elsevier

Ceramics International

Volume 35, Issue 6, August 2009, Pages 2481-2491
Ceramics International

Preparation and mechanical characterization of dense and porous zirconia produced by gel casting with gelatin as a gelling agent

https://doi.org/10.1016/j.ceramint.2009.02.017Get rights and content

Abstract

A modified gel casting procedure based on a natural gelatin for food industry and commercial polyethylene spheres as pore formers was successfully exploited to produce dense and porous ceramic bodies made of yttria stabilized tetragonal zirconia polycrystal (Y-TZP). Vickers and Knoop microhardness, elastic modulus and fracture toughness measurements on dense samples obtained by experimental investigation closely matched results found in the literature for similar materials. However, after a careful analysis of obtained results, no indentation size effect and a lower scattering of experimental data from low load indentations were observed, in comparison with literature.

Mechanical testing of porous samples (with reproducible values of porosity of about 40%) evidenced a high scattering of compressive strength values, suggesting that the uneven distribution of cavities in the material or the presence of defects from the agglomeration of pore forming agents could have a more direct influence on the mechanical properties of such materials than the absolute value of porosity.

Introduction

The importance of porous ceramic materials is continuously increasing because of a variety of interesting applications, such as, molten metals filtration, high-temperature thermal insulation, support for catalytic reactions, filtration of particulates from diesel engine exhaust gases and of hot corrosive gases in various industrial processes, and scaffolds for bone substitution [1], [2].

The characteristics of the pores influence the properties of the components, thus two crucial steps are involved in the production of cellular materials: first, the selection and set-up of a reliable forming method allowing a strict control of pore size, volume, distribution and morphology; second, the proper characterization procedures of the actual porosity features as well as of the mechanical/functional performance [1], [2].

This paper mostly deals with the exploitation of a novel gel-casting process to develop dense and porous materials having controlled porosity characteristics.

Gel casting is a well-known, wet forming method based on the combination of ceramic processing and polymer chemistry. This process involves the dispersion of a ceramic powder into a monomer solution and casting of this suspension into a non-porous mould. Polymerization is then promoted and consequently ceramic particles are entrapped into the rigid and homogenous polymeric network [3]. After gel formation, gel-cast green materials can be easily de-molded and are then dried in controlled conditions.

The main advantages of this method are the high strength of the green bodies, allowing their machinability, the low amount of organic additives, avoiding a preliminary thermal treatment as in the injection molding process [4], and also its versatility which allowed to extend such process to a variety of ceramic materials.

Different monomers were exploited for gel casting, starting from acrylamide systems, now withdrawn due to their neurotoxicity [5], as well as alternative gelling agents, such as, for example, agar [6], agarose [6], [7], [8], carrageenan gums [6], egg white [9], gelatin [10], [11], [12], [13], [14] and polyvinyl alcohol [12].

Independently of the selected gelling agent, several problems related to porosity distribution control and mechanical strength tailoring as a function of process parameters still remain unsolved, and a comprehensive study is not yet present in literature.

In the present paper a natural gelatin, precisely a pig-derived product for food industry, was used. The natural gelling agents present the advantages of a polymerization just promoted by a temperature lowering, without the use of a catalyst and an initiator, as synthetic monomers do, and of a low environmental impact.

Hardness, elastic modulus, indentation size effect and fracture toughness of the bulk materials were evaluated by the use of a specifically developed mechanical characterization procedure, based on Vickers–Knoop microindentation testing and nanoindentation.

Gel-casting process was initially set-up for preparing dense components, and then modified to fabricate also porous ceramics, by combining it with foaming techniques, or replica methods, or even the addition of a sacrificial phase [1], [15].

In this paper, a fugitive phase, made of commercial polyethylene spheres in a size range selected by sieving, was added to the ceramic suspension before gelling. The shape, size and size distribution of these spheres as well as their volume fraction with respect to the ceramic content into the slurry can allow a strict control of many porosity features of the final components. The feasibility of the modified process was demonstrated by developing porous components with a pore volume percentage of about 40 vol.%.

Section snippets

Sample preparation and characterization of the green bodies

Commercial stabilized zirconia with 3 mol% of yttria (yttria tetragonal zirconia polycrystal, Y-TZP, grade TZ-3Y supplied by Tosoh Co., Japan) was used for preparation of ceramic suspensions having a solid loading of 60 wt% and dispersed into water at natural pH by ultrasonication for 10 min [13]. Particle size distribution was determined by means of a laser granulometer (Fristch analysette 22) in ethanol after 10 min of ultrasonication.

A pig skin-derived gelatin produced by Italgelatine (Italy)

Powders characterization

After 10 min of ultrasonication, the mean diameter of the zirconia powder was about 0.6 μm and the diameters corresponding to 10% and 90% of the particle size distributions were, respectively, 0.33 and 1 μm.

TEM observations revealed that powder grains were almost regular and agglomerated into clusters up to 1 μm in diameter (Fig. 4). Image analysis on 50 grains showed that mean crystallite size was about 24 nm and the standard deviation was 8.5 nm.

Dense sample characterization

Knoop microhardness was firstly measured onto two

Dense samples

Dense zirconia samples having up to 97% of the theoretical density, with submicronic grains, were produced by an improved gel-casting process based on gelatin.

It is shown that by the combined use of Vickers–Knoop microindentation and nanoindentation techniques, and a proper application of the models available in literature, reproducible data can be obtained on elastic modulus, hardness and fracture toughness of samples; indentation testing is then confirmed to be an effective micro-probe for

Conclusions

Dense and porous ceramic bodies based on yttria tetragonal zirconia polycrystal were successfully produced by a gel-casting process based on a natural gelatin for food industry and commercial polyethylene spheres used as pore formers. As already pointed out by other authors [11], de-airing step is of crucial importance, as well as the long time for gelation and above all for drying in controlled conditions. Dense samples reached 96–97% of theoretical density with a fine microstructure, thus

Acknowledgements

The authors wish to thank the Italian Inter-University National Consortium on Material Science and Technology (INSTM) to have supported this research in the framework of the Prisma Project 2005 “Development of new cellular materials by gel-casting technique: optimization of production process and functional simulation of the microstructure”.

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