Fracture Behaviour of Pressureless Sintered

Nickel-Reinforced Alumina Composites

A.J. Sánchez-Herencia1,a, N. Hernández1,b and R. Moreno1,b 1 Instituto de Cerámica y Vidrio (CSIC) Camino de Valdelatas s/n Madrid 28049, Spain

aajsanchez@icv.csic.es, bgilmer@icv.csic.es, crmoreno@icv.csic.es

Keywords: Nickel-alumina composites, fracture toughness, colloidal processing, sintering.

Abstract. Nickel-reinforced alumina composites have been manufactured by aqueous slip casting

and pressureless sintered under flowing atmosphere of argon with 0,36 and 1% of oxygen in order

to force interfacial reactions leading to the formation of a nickel-aluminum spinel. Colloidal

stability of concentrated suspensions of alumina with 5, 10 and 15 vol% of nickel has been studied

in terms of zeta potential, rheometry and packing density. The processed composites show a high

dispersion of the nickel into the alumina matrix and green densities of 60-70 %th. The effect of

sintering temperature and atmosphere on the mechanical behaviour of the composites has been

investigated through Vickers indentation and fractographic SEM observations.

Introduction

The dispersion of metallic particles is one of the methods to face the

low toughness of ceramics. In this sense several variables have been

related to the reinforcement, including composition [1], shape[2],

size[3] and sintering conditions[4]. Colloidal processing of metals has

probed to be a valid route for processing metallic and metal ceramic

compacts [5,6]. Using this technique, that requires the optimization of

the processing parameters, the metallic phase may be homogeneously

dispersed inside the ceramic matrix.

The inclusion of metallic particles inside the ceramic matrix can

reinforce the materials due to crack deflection, microcrack toughening,

crack bridging and crack blunting by a ductile phase. If the nickel

particles join to the alumina matrix residual stresses due to the

difference in thermal expansion coefficients will developed. Fig. 1

shows the stresses behaviour at a nickel alumina interface. This stress

distribution will force the crack to divert away from the nickel particles.

The joining of metals to ceramics can be obtained by chemical or reactive joining: In the case of

the materials used in this work (alumina and nickel), the wetting angle is very high, indicating that

no chemical bond will be reached. It is necessary the formation of a spinel interface and

consequently, the partial oxidation of the metallic particles.

The processing of Nickel by aqueous colloidal method will generate an oxidation layer of some

nanometers but not enough to promote the reactive joining. In this sense, the sintering of samples

under a small partial pressure of oxygen has proved to be a mechanism to develop the interface7.

The objective of this work is to evaluate the influence of the sintering conditions (temperature and

atmosphere) on the mechanical properties of the alumina nickel compacts obtained by aqueous

colloidal processing techniques.

Experimental

Starting powders were metallic nickel (INCO T-110 Canada) with a mean particle size of 2.5 and

alumina (Condea HPA 0,5; USA) with a mean particle size of 0.3µm.

Ni

Fig. 1 diagram of the

stresses near the

Ni/Al2O3 interface

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Slurries with a solid content of 50vol% were prepared by mixing the two powders in water at a

fixed pH of 10 using TMAH and with 0.8% of Duramax D-3005 (Rohm and Hass, Germany) as

dispersant. Slurries were dispersed with an ultrasound horn for 2 minutes (IKA U400S, Germany)

and maintained under mechanical stirring for 1 hour. Composite compacts were obtained by slip

casting on plaster of Paris molds. Three different compositions were fabricated based on alumina

with 5, 10 and 15vol% of nickel that were named as A5Ni, A10Ni and A15Ni respectively.

The obtained compacts were sintered in a tube furnace under flowing Argon with 0, 0.36 and 1vol%

of oxygen. The sintering temperatures were 1450, 1500 and 1550°C for 2 hours with heating and

cooling rates of 5°C/min. Sintered samples were cross-sectioned and polished with diamond paste

down to 1µm.

Vickers indentations were performed on polished samples using a controlled displacement system

with a load cell (Microtest, Spain). The loading speed was 0.01 mm/s up to reach a maximum load

of 100N. Cracks length were measured using a scaled optical microscope. For fracture toughness

calculations, Anstis equation was used assuming that alumina matrix will generate a median crack.

SEM observations were performed on indented and no-indented samples without any etching in

order to maintain the microstructure unaltered

Results

Characterization of compacts. Figure 2 shows the cross section of the samples sintered at 1500°C

under a) pure argon and b) argon with 1 vol% of oxigen . In fig 1.a it can was observed that nickel

squeezed from the samples as the melting temperature has been exceed and nickel do not wet

alumina. In the case of samples sintered under flowing argon with 1vol% of oxygen, it can be

observed that the edges of the samples have been oxidized allowing the formation of a

spinel/alumina layer. X-Ray Diffraction confirms that this outer layer was composed of alumina and

spinel (gray area-blue in the sample). But under these conditions neither spinel not nickel oxide was

detected inside the samples (dark area-black in the sample) for any composition and temperature

studied. In these samples it was observed a light weight gain due to the nickel oxidation at this

layer. Samples sintered under oxygen-containing atmospheres reached close to theoretical density

while samples sintered in pure argon do not reach the theoretical density because relative amount of

nickel was lower after sintering.

Figure 3 (a and b) shows the SEM microstructure of A15Ni samples sintered at 1550°C. The sample

sintered under flowing pure argon (figure 3.a) that a high porosity is produced as the melted nickel

has flown out of the sample. This phenomenon occurs when the alumina matrix has already sintered

so the pores have an irregular shape coincident with the nickel grains. In the case of the sample

sintered under flowing argon with oxygen small porosity observed was due to the triple points at the

nickel-alumina junctions.

Fig. 2. A15Ni samples sintered at 1550°C under pure argon (a) and argon with 1 vol% of oxygen

a b

Fractography of Advanced Ceramics II325

Mechanical properties. Microstrucutral observations reveal great differences between samples

sintered in pure argon atmosphere or in the presence of small amounts of oxygen. These differences

are expected to influence the indentation experiments. As the nickel samples sintered in pure argon

were degraded because of the nickel migration out of the sample, the hardness values were poor (7

to 8 GPa) and the fracture toughness range between 5.5 and 6 MPa·m1/2

, higher than the values

recorded for the samples sintered under oxygen containing atmosphere. Figure 4.a shows the

indentation mark of this sample. It is observed that the edges show fracture instead of plastic

deformation. Also the crack path (figure 4.b) was very difficult to visualize. This behaviour is due

to the lack of bonding between the ceramic matrix and the metallic reinforcement which make the

matrix to behave as a porous material.

Fig.3. A15Ni samples sintered at 1550°C under pure argon (a) and argon with 1 vol% of oxygen

a b

Fig 4. SEM observation of the indentation mark (a) and crack path

(b) for a A15Ni sample sintered at 1550°C under pure argon.

a b

Key Engineering Materials Vol. 290 326

Samples sintered under argon with 0.36 and 1vol% of O2 show very similar hardness behaviour,

decreasing the value as the nickel content increases. Those values range from 17 to 12 (GPa). These

indicate that under those atmospheres the metallic reinforcement particles have effectively joined to

the ceramic matrix. In fig. 5 the values of KIC are plotted. As expected the fracture toughness

increase with the nickel content. It can be seen that materials sintered in 1vol% of oxygen present

higher toughness values than those sintered in 0.36%, indicating a better joining between the

alumina matrix and the nickel reinforcement. For both atmospheres the largest toughness is reached

at 1500°C.

Fig. 5. KIC vs. nickel content for the samples sintered with different temperatures and

atmospheres.

According to SEM observations, two reinforcement

mechanisms may exist in these materials, as illustrated

in the micrograph of an A15Ni material sintered at

1500°C with 1% of oxygen (Figure 6). The first

mechanism is associated to residual stresses originated

as a consequence of the strong bonding of nickel to

alumina, which have different thermal expansion

coefficients. These stresses make the crack to move in a

tortuous way instead of a straight way, surrounding the

nickel particles (row 1).

The other mechanism is the crack blunting by the

metallic particle. In this case the shape of the particle

does not allows the crack to surround the nickel that

behaves as a resistant ligament (row 2).

Acknowledgments

This work has been supported by CICYT-Spain under

contract MAT 2003-00836

References

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Vol 206/2 (2002) p. 981.

[4] S. Tang and A.T. Zehnder, Eng. Fract. Mech., Vol 69 (2002) p. 701

[5] N. Hernández, R. Moreno and A.J. Sánchez-Herencia, Acta Mater (In press).

[6] A.J. Sánchez-Herencia, C.A. Gutiérrez, A.J. Millán, M.I. Nieto and R. Moreno, Key Eng.

Mater., Vol 206/2 (2002) p. 227.

[7] K. P. Trumble and M. Ruhle, Acta Metall Mater, Vol. 39 (1991) p. 1915.

2,0

3,0

4,0

5,0

6,0

0 5 10 15 20Nickel vol%

KIC

(M

Pa·m

1/2

)

1450 1%

1500 1%

1550 1%

2,0

3,0

4,0

5,0

6,0

0 5 10 15 20Nickel vol%

KIC

(M

Pa·m

1/2

)

1450 0,36%

1500 0,36%

1550 0,36%

Fig. 6 Crack path in a A15Ni simple

sintered at 1550°C under 1%vol O2.

Arrows indicate the crack blunting (1)

and the surrounding of nickel (2).

2

1

Fractography of Advanced Ceramics II327