Field assisted sintering of electro-conductive ZrO2-based composites

doi:10.1016/j.jeurceramsoc.2006.04.142

Journal of the European Ceramic Society

Volume 27, Issues 2–3, 2007, Pages 979-985

https://doi.org/10.1016/j.jeurceramsoc.2006.04.142 Get rights and content

Abstract

In order to reveal the fundamentals of the field assisted sintering technique (FAST), also known as spark plasma sintering (SPS), the evolution of the current density and temperature distribution in the punch-die-sample set-up during FAST of ZrO₂–TiN powder mixtures was modeled by finite element calculations supported by in situ measured electrical and thermal input data. The thermal and electrical properties of partially sintered composite powder compacts were estimated using theoretical mixture rules, allowing to calculate the current density and temperature distribution inside the tool and the specimen during the FAST sintering process. The electrical properties of the sintering composite powder compact, and hence the thermal distribution in the sinter set-up, changed drastically during densification once percolation occurred. Based on the calculated thermal distribution inside the composite powder compact, an optimal tool-powder compact design was determined in order to process electrically conductive ZrO₂–TiN composites from electrical insulating powder compacts within minutes with high reproducibility.

Introduction

The field assisted sintering technique (FAST), also known as spark plasma sintering (SPS) or pulsed electric current sintering (PECS), belongs to a class of sintering techniques that employ a pulsed DC current to intensify sintering.¹ Some general advantages of field assisted sintering, compared to traditional hot pressing or hot isostatic pressing, are technological advantages such as short processing time, the use of high heating rates hereby minimizing grain growth often leading to improved mechanical,² physical³ or optical⁴ properties and elimination of the need of sintering aids.

Lately, a lot of research is focused on a more fundamental understanding of the sintering process. Recent studies focused on the effect of (a) the electrical current and (b) the DC pulse pattern on the solid state reactivity of Mo and Si,5, 6 on the effect of contact resistances on the temperature distribution during the FAST process in case of monolithic fully densified materials7, 8 and on the enhanced sintering kinetics and diffusion mechanisms during the sintering process.⁹

Furthermore, high heating rates, especially in combination with short dwell times, can cause temperature gradients and subsequently sintering inhomogeneity leading to non-uniform microstructural and mechanical properties of the sintered parts.10, 11, 12 Therefore, the temperature field within the sintering powder compact during FAST sintering should be understood and controlled as well as possible. Up till now, most researchers controlled the temperature during a FAST sintering cycle by focusing a pyrometer on the outer die wall surface, leading to an underestimation of the sintering temperature. In order to correlate this temperature with the temperature of the sintering powder compact inside the die, the temperature distributions within the whole tool-specimen system should be known. The most practical way to find this out is theoretical modelling.

Previous research performed at our institute pointed out the importance of both the contact resistances induced by the graphite papers and of the electrical properties of the specimen. When a fully dense TiN compact was placed inside the FAST equipment, the radial temperature gradient at high temperatures (>1500 °C) was much higher compared to the gradient in a fully dense 3Y–ZrO₂ compact.⁸ Based on these observations, the thermal cycle controlling pyrometer was positioned more strategically i.e. focussing on the bottom of a borehole inside the upper punch of a graphite tool set-up (Fig. 1). In this way, the temperature at the specimen's centre differed less than 5 °C from the temperature measured by the central pyrometer, independent on the sample's electrical properties. Furthermore, it was suggested that the radial temperature gradient in electrically conductive samples can be reduced by reducing the radiation heat losses from the outer die wall surface. Therefore, the die can be surrounded by porous carbon felt insulation.

In the present work, the developed finite element method and ANSYS code were used to predict the thermal gradients inside a sintering ZrO₂–TiN (60/40) composite during the FAST process. The simulated data were in very good agreement with the experimentally measured temperature and resistance values. Interrupted sintering cycles were performed in order to correlate the shrinkage of the powder compact with its changing thermal and electrical properties.

Section snippets

Experimental procedures

ZrO₂–TiN (60/40) powder compacts were prepared by ball milling 3Y–ZrO₂ (Daiichi HSY-3U, d₅₀ = 20 nm), TiN (H.C. Starck, grade C, d₅₀ = 1.4 μm) and a small amount (0.75 wt.% with respect to the Y–ZrO₂ content) of Al₂O₃ (Taimicron, TM-DAR, d₅₀ = 100 nm) in a multidirectional Turbula T2C mixer.

Experiments were performed on a FCT FAST device (Type HP D 25/1, FCT Systeme, Rauenstein, Germany). More details about the used FAST device can be found elsewhere.⁸ During the presented experiments, a pulse-pause

Interrupted sintering cycles using a ZrO₂–TiN (60/40) powder compact

Typical microstructures of FAST sintered ZrO₂–TiN (60/40) composite powder compacts obtained at 1050 °C (a), 1200 °C (b), 1400 °C (c), 1500 °C for 1 min (d) and 1500 °C for 6 min (e) are shown in Fig. 3. Table 1 describes the evolution of the density of the sintering compacts as a function of temperature and applied pressure. The major part of the densification takes place between 1050 and 1400 °C. From 1200 °C on (Fig. 3(b)), the TiN particles start to form an interconnected particle network. At that

Conclusions

When ZrO₂ based, electrically conductive, ceramic composites are to be processed using the field assisted sintering technique, one should take into account the changing electrical and thermal properties of the sintering powder compact. Once a percolating path is formed inside the sintering powder compact, the temperature and current profile change very quickly leading to a large radial temperature gradient inside the specimen during the final stage of the sintering process.

In this work, the

Acknowledgements

This work is financially supported by the GROWTH program of the Commission of the European Communities under project contract No. G5RD-CT2002-00732. Prof. Laptev thanks the K.U. Leuven Research Council for his 8-month research fellowship (No. F/02/096).

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Field assisted sintering of electro-conductive ZrO2-based composites

Abstract

Introduction

Section snippets

Experimental procedures

Interrupted sintering cycles using a ZrO2–TiN (60/40) powder compact

Conclusions

Acknowledgements

Mater. Sci. Eng. A

J. Eur. Ceram. Soc.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Acta Mater.

Int. J. Refrac. Met. Hard Mater.

Int. J. Heat Mass Transfer

Formation of tough interlocking microstructures in silicon nitride ceramics by dynamic ripening

Nature

Field assisted sintering of electro-conductive ZrO₂-based composites

Interrupted sintering cycles using a ZrO₂–TiN (60/40) powder compact