Laminated Ceramic Structures within Alumina / YTZP System Obtained by Low Pressure Joining

J. Gurauskis a, A. J. Sánchez-Herencia b, C. Baudín c

Instituto de Cerámica y Vidrio (CSIC), C/Kelsen 5, 28049 Madrid, Spain

a gurauskis@icv.csic.es b ajsanchez@icv.csic.es c cbaudin@icv.csic.es

Keywords: Al2O3, ZrO2, processing, laminates, residual stresses.

Abstract. The production of multilayer ceramics by laminating stacked green ceramic tapes is

one of the most attractive methods to fabricate layered materials. In this work, a new

lamination technique was employed to obtain laminated ceramic structures in the alumina-

zirconia system with residual stress compression at the outer layers. This reinforcement

mechanism would lead to ceramics with changed material properties and R-curve behaviour.

The optimization of processing parameters for fabrication of defect free monolithic and

laminated structures is described. The residual stresses developed in the laminated structures

are discussed in terms of the results obtained from piezo-spectroscopic technique

measurements and finite element method calculations.

Introduction

Laminated structures have been investigated due to their capability for the reinforcement of

ceramics. One of the reinforcement mechanisms associated with laminated structure is the

residual stress compression at the outer layers. This reinforcement mechanism can lead to

higher fracture strength and toughness values of the ceramic material, and to activate rising

crack growth resistance (R-curve) behaviour [1-3].

One of the most versatile ceramic systems alumina - yttria stabilized zirconia (YTZP) was

chosen in this work. In this system, tapes with different compositions can be combined to give

rise to the desired residual compressive stresses in the outer layers for macro-structural scale

reinforcement [2-4]. In addition, the presence of YTZP might originate reinforcing

mechanisms, which operate at the microstructural level [5, 6].

The control of the residual stresses and, consequently, the reinforcing mechanism, is

achieved by a strict control of the composition and thickness of the layers that have to be

designed for an optimum behavior. In this sense colloidal processing technique based on tape

casting is adequate for the fabrication of designed layered structures [7-9].

The lamination of individual tapes into one structure involves the use of elevated pressures

and temperatures [7-10]. Such a process is time and energy consuming, furthermore various

authors [10, 11] observed the presence of varying mechanical properties for monolithic pieces

processed by this route. It has been suggested that this behaviour might be due to the

development of some degree of texture in the microstructure related to the implication of

elevated pressures and temperatures. Therefore, new lamination technique is proposed in

order to simplify this step. The quality of the interfaces between the different tapes and the

presence of any texture in the microstructure within sintered specimens obtained via new

lamination technique was evaluated by means of the fracture behaviour of the pieces during

the ball on three balls test.

Key Engineering Materials Vol. 333 (2007) pp. 219-222online at http://www.scientific.net© (2007) Trans Tech Publications, Switzerland

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No clear data is presented in literature that would indicate the reason of the presence of

curve-R behaviour within laminated structures with residual compressive stresses in the outer

layer and the influence of magnitude of residual stresses to this phenomenon. Therefore the

laminated structures with compressive residual stresses of different magnitude at the outer

layer with fixed thickness were fabricated and their values were evaluated by the piezo-

spectroscopic technique and finite element method approximation.

Processing of laminated structures

Two compositions within the system alumina - YTZP, A-5 (alumina-5vol.%YTZP) and A-40

(alumina-40vol.%YTZP) were selected for fabrication of ceramic tapes via tape casting

procedure of aqueous ceramic slurries. A new processing route for production of layered

structures by laminating stacked green ceramic tapes with help of adhesive layer and pressing

at room temperature was employed. Full details of tape pre-treatment, gluing agent

application and the pressing procedure are given elsewhere [12, 13].

As a first approach, the optimum pressure levels to obtain defect free sintered monolithic

materials were investigated. For this purpose, the quality of the interfaces between the tapes

and any formation of textures in sintered monolithic specimens A-5 and A-40 were evaluated

by means of the fracture behaviour of the pieces during the ball on three balls test [14]. This

analysis was chosen because any processing defect will be better revealed under a biaxial

stress distribution. Different fracture patterns were found as a function of processing pressure

and the characteristics of the tapes, giving the value of optimum pressure of 18 MPa for both

compositions. By applying this pressure dense pieces with maximum sizes of the defects of

the same level as those of the microstructural ones and without interface effects on

mechanical behaviour were obtained via this new processing route employed.

The established processing route was applied for the processing of designed laminated

structures, L1 and L2, using the tapes with different compositions, A-5 and A-40. Two

different distributions of tapes were selected in a way to obtain two different symmetrical

laminated structures with external A-5 layers. In one of them (L1) four A-5 layers were

alternated with three A-40 layers of the same thickness (≈430 µm). In the other (L2) three A-5

layers (≈430 µm) were alternated with two A40 layers of double thickness (≈860 µm).

The cross-sections of sintered pieces showed the presence of large transverse cracks

though the internal A-40 layers. Smooth and rounded grains at the surface of these cracks

suggested that crack formation took place before or during sintering process. Following this

observation, the colloidal processing parameters of the tapes, in particular the suspension

solid load, were modified to obtain tapes with similar green densities for both compositions.

The result was that the initial differential sintering was avoided and defect free laminated

structures L1 and L2 were obtained (Fig.1).

Functional Gradient Ceramics, and Thermal Barriers220

Figure 1. SEM micrographs at the polished cross-section showing the symmetrical laminate

structures.

a) Laminate L1.

b) Laminate L2.

Evaluation of residual stresses

The difference in thermal expansion between the compositions A-5 and A-40, resulted in the

development of residual stresses in layered structures L1 and L2. The magnitude and sign of

the expected residual stresses were evaluated using the simplified model of a symmetric plate

constituted by alternate layers of the same thickness having a uniform biaxial distribution of

stresses across each layer [4]. This model gave compressive stresses in the A-5 layers of -182

and -268 MPa for L1 and L2 structure, respectively.

The magnitude and the distribution of residual stresses in layered structure L1 were

assessed by the piezo-spectroscopic technique [15]. As expected, A-5 layers underwent

compression, whereas A-40 layers tension. The residual stress within the layers varied with a

parabolic trend, with the highest values at the interfaces and a reduction toward the centre of

the layers. The maximum values obtained showed a good agreement with the ones evaluated

using the simplified model of a symmetric plate. The observed distribution of residual stress

values was attributed to the free surface effect, which is recorded by low penetration of

fluorescence spectra (∼5-20 µm) [16].

In order to confirm the effect of the free surface on the distribution of residual stress within

the laminate structures L1 and L2, the simulation of residual stresses using finite element

method (FEM) was performed. The simulation results for laminated structure L1 near the free

surface (∼5 µm) showed the coincident results to the ones observed by the piezo-

spectroscopic technique. This fact confirms the correctness of residual stress determination

techniques employed and depending on details needed about the residual stresses present in

the structure it is possible to use one or other.

The profiles of residual stress distributions obtained by FEM at the outer layer in the

middle of sample (minimum effect of free surface) showed similar trends for both laminated

structures (L1 and L2). Showing the minimum values near the surface (L1 = -161 MPa, L2 = -

247 MPa) and the maximum values near the interface with A-40 tape (L1 = - 174 MPa, L2 = -

268 MPa). This distribution of residual stresses should be taken into account during

evaluation of the reinforcement mechanism based on residual compressive stresses at the

outer layer of laminated structures.

a A-5

A-40

b A-5

A-40

Key Engineering Materials Vol. 333 221

Summary

Dense multilayer ceramics with strong interfaces and free of defects can be obtained by

stacking water based tapes via new processing route and pressureless sintering. By controlling

green density of the tapes, development of stresses during initial stages of sintering as well as

the associated defects in laminated structures can be avoided. Determination of residual

stresses by the piezo-spectroscopic technique and by FEM approximation gives similar

results. Even the minimum effect of free surface affects the distribution of residual stresses.

Acknowledgments

Special thanks to Luca Ceseracciu from Departament de Ciencia dels Materials i Enginyeria

Metalurgica (Universitat Politécnica de Catalunya) for the help provided with simulation of

residual stresses using finite element method .

This work was supported by the projects CICYT MAT 2003-00836 and CAM GR

MAT07072004 (Spain).

Work supported in part by the European Community's Human Potential Programme

under contract HPRN-CT-2002-00203, [SICMAC].

Jonas Gurauskis acknowledges the financial support provided through the European

Community's Human Potential Programme under contract HPRN-CT-2002-00203,

[SICMAC].

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