Spark plasma sintering of a commercially available granulated zirconia powder: I. Sintering path and hypotheses about the mechanism(s) controlling densification

doi:10.1016/j.actamat.2007.01.048

Acta Materialia

Volume 55, Issue 10, June 2007, Pages 3493-3504

https://doi.org/10.1016/j.actamat.2007.01.048 Get rights and content

Abstract

Spark plasma sintering (SPS) of a commercially available granulated zirconia powder has been investigated. The “relative density/grain size” trajectory, or “sintering path”, has been established for a constant heating rate (50 °C/min) and a constant applied pressure (100 MPa). In addition, an attempt has been made to identify the mechanism(s) that could be invoked for the control of densification during the SPS experiments.

Introduction

Stabilized zirconia is an interesting material for a multitude of applications. Among these, refractory parts (stabilized zirconia exhibits a relative inertness to medium temperatures and to many hostile environments), solid oxide fuel cells (stabilized zirconia is an ionic conducting material) and very specific technical components (stabilized zirconia exhibits a low wear rate and a low coefficient of friction when it is in contact with high molecular weight polyethylene, as is the case for hip and knee prostheses) are potential issues.

Ionic conductivity of stabilized zirconia is directly related to the concentration of oxygen vacancies. It is therefore reasonable to think that a dense zirconia material with a small grain size, and so with a corresponding high level of grain boundaries that are potential sources of vacancies, would exhibit better ionic conductivity.

To limit grain growth whilst still obtaining a sufficient densification, spark plasma sintering (SPS) with other materials, such as TiN [1], Al₂O₃ [2], Si₃N₄ [3], 8 mol.% yttria-stabilized ZrO₂ [4] and β-SiC [5], has been shown to be effective. For this reason, we decided to investigate SPS as a possible densification method for a commercially available stabilized zirconia granulated powder. The microstructures obtained should be considered as potential material solutions for solid oxide fuel cells.

First, the “relative density/grain size” trajectory, or “sintering path”, has been drawn for a soak temperature in the range 950–1200 °C, a soak time in the range 5–180 min, a fixed heating rate of 50 °C min⁻¹ and a fixed applied macroscopic compaction pressure of 100 MPa.

Secondly, investigations have been undertaken to help formulate hypotheses concerning the mechanism(s) controlling densification during the SPS experiments.

Section snippets

Characterization of the raw powder

The commercially available TZ3Y raw powder (Tosoh Europe B.V., Amsterdam, The Netherlands) was selected as the starting material. It is a chemically co-precipitated powder (hydrolysis manufacturing process). Theoretically, the manufacturer ensures that no binder is incorporated during the formulation.

Apart from Y₂O₃, which is present as an intentional zirconia tetragonal phase stabilizer (5.49 wt.%, Table 1), the main impurities determined by inductively coupled plasma spectrometry (Varian Vista

SPS experiments

All the tests were conducted on the as-received granulated raw powder in vacuum, on equipment (SPS-2080, SPS Syntex Inc., Kanagawa, JP) located at the Plateforme Nationale de Frittage Flash (PNF2, CNRS-CIRIMAT, Université de Toulouse, France).

A graphite die, having an internal diameter of 8 mm and a wall thickness of 8 mm, was filled with the raw powder (0.5 g) and mounted on the SPS equipment (graphite punches). The initial sample height is around 3.2 mm for a starting relative green density of

Results

At this point, it is important to mention that the sintered samples exhibit a color modification as a function of the soak temperature maintained during the SPS experiments. At 1000 °C and below, the samples are white. When the temperature is 1050 °C or above, the samples turn grey/black. The higher the soak time, the more pronounced the black color. Preliminary investigations using SEM/TEM have shown that no carbon segregation/contamination (second phase, grain boundaries enrichment) is observed

Discussion

To be able to make a suggestion concerning which mechanism(s) controls the densification of the TZ3Y raw powder during SPS, it is critical to determine the values of the Q_d, p and n parameters in relation (8).

According to Fig. 6, the grain size may be assumed to be constant when the temperature is in the range 950–1050 °C and the soak time is in the range 5–180 min, and also when the temperature is 1125 °C and the soak time in the range 5–60 min. For these SPS conditions, the relation (8)

Conclusion

It has been shown that, depending on the experimental parameters other than the heating rate (fixed to 50 °C min⁻¹) and the applied macroscopic compaction pressure (fixed to 100 MPa), three kinds of microstructures are obtainable when using the SPS technology to densify a stabilized granulated zirconia raw powder. Porous materials (open porosity, relative density between 61 and 90%) with a nanometer grain size (around 80 nm) are obtained when the temperature is below 1050 °C whatever the soak time,

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Spark plasma sintering of a commercially available granulated zirconia powder: I. Sintering path and hypotheses about the mechanism(s) controlling densification

Abstract

Introduction

Section snippets

Characterization of the raw powder

SPS experiments

Results

Discussion

Conclusion

Acta Metall

J Eur Ceram Soc

Acta Mater

Solid State Ionics

Acta Mater

J Am Ceram Soc

J Am Ceram Soc

J Am Ceram Soc

J Mater Sci Lett

J Am Ceram Soc

Ultramicroscopy

Trans ASM