ceramic composites by electrophoretic deposition

PII: S1359-8368(96)00022-4

ELSEVIER

Composites Part B 28B (1997) 49-56 © 1997ElsevierScienceLimited Printed in Great Britain. All rights reserved 1359-8368/97/$17.00

Functionally graded ceramic/ceramic and metal/ceramic composites by electrophoretic deposition Partho Sarkar, S o m e s w a r Datta and Patrick S. Nicholson Ceramic Research Group. Department of Materials Science and Engineering McMaster University, Hamiltion. Ontario, Canada (Received 27 December 1995; revised 11 April 1996) Constant current electrophoretic deposition (EPD) has been used to synthesise AI2Oa/YSZ, A1203/MoSi 2, A1203/Ni and YSZ/Ni functionally graded materials (FGM). EPD is a cheap and simple technique to fabricate complicated ceramic shapes. By this technique it is possible to synthesize step FGMs as well as continuous-profile FGMs. The profile can be controlled precisely by controlling the deposition current density, second component flow rate, suspension concentration, etc. The microstructures of the FGMs produced were characterized by optical and electron microcopy and micro-indentation was used to track the Vicker's hardness and fracture toughness variation across the composition profiles. © 1997 Elsevier Science Limited. All rights reserved (Keywords:electrophoreticdeposition;FGM; ceramiccomposites;YSZ; Ni; MoSi2; AI203)

1 INTRODUCTION Continuous variation of the physical properties across functionally graded materials (FGM) is achieved by variation of composition. F G M s find applications in thermal shielding, joining metal to ceramics, optical and electronic functions, and biomaterial inplants I. F G M s have been synthesized by chemical vapor deposition, plasma spraying, self-propagating high-temperature synthesis2, tape casting 3, slip casting 4 and electrophoretic deposition (EPD) 5. EPD is a combination of two processes, electrophoresis and deposition. Electrophoresis is the motion of charged particles in a suspension under the influence of an electric field gradient. It was discovered by the Indian scientist, G. M. Bose in the 1740s 6 in a liquid-siphon experiment. The Russian, Reuss (1807) 7, was the first to observe electric-field-induced motion of clay particles in water. The second process is deposition, i.e. the coagulation of particles into a dense mass.

2

EPD OF SUSPENDED PARTICLES

EPD needs a stable suspension with particles charged so as to respond to an applied electric field. Therefore only electrostatic and electrosteric stabilization are appropriate for suspensions to be electrophoretically

deposited. Since most of the literature on EPD considers electrostatic stabilization, the present discussion will be limited to this mechanism. Oxide particle surfaces in air are usually covered by amphoteric hydroxyl groups. These suffer reaction with the H+/or O H - of a suspension liquid, which ion depending on the suspension pH. The reactions are as follows: MOH(surface) + O H - = MO~surface) q- H20 + MOH(surface) + H + = MOH2(surface)

(1) (2)

According to these equations, there is an intermediate pH where the number of positively and negativelycharged surface groups are equal thus resulting in zero surface charge. Counter ions from the liquid form a charge cloud around this-charged particles masking their surface charge (Figure 1). This ionic atmosphere is called the 'diffuse-double-layer'/or 'lyosphere'. When the particle/ lyosphere system moves, fluid mechanics dictates some ions of the lyosphere 'shear off' and counter-ions are lost. A moving particle and its remnant double-layer will thus be charged with the same sign as the particle surface charge. The residual potential at the shear plane is called the shear or zeta (~)-potential and is experimentally accessible. The DLVO (Derjaguin-Landau-Verwey-Overbeek) theory describes colloidal suspension stability using attractive- and repulsive-energy concepts. When two

49

Functionally graded ceramic~ceramic and metal~ceramic composites." P. Sarkar et al. Positively Charged ,. -" -- --~ ~, Particle / - x

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Figure 1 Schematic of the 'Particle-Lyosphere System'

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Figure 2 A schematic of the interaction energy as a function of separation between two particles in suspension

particles approach, their diffuse-double-layers overlap and they experience a repulsive force. The origin of the repulsive force between two similarly charged particles is osmotic, not electrostatic. The force at any point between the particles consists of two components, i.e. an electrostatic component, which is always attractive and osmotic pressure component, always repulsive. In the double-layer, the repulsive osmotic pressure between counter-ions forces them away from the particle surfaces and from each other, thus increasing their configurational entropy. The bringing of two particles together therefore forces the counter-ions from their preferred equilibrium state, i.e. against the osmotic repulsion but favoured by electrostatic interaction. Since the former dominates, the net force is repulsive. A van der Waals attraction always exists between

50

(3)

where A is the Hamaker constant, D the separation of the particles and rl, r2 their radii. Since this attractive relationship follows a power law, it dominates at small separations. A primary minimum is therefore observed at the position of the particle-particle contact (D = 0; Figure 2). As separation increases, the double-layer repulsion starts to dominate and, as a consequence, the total-interaction-energy curve exhibits a peak the height of which, Eb, constitutes the energybarrier against coagulation. A secondary minimum develops at large distances when the attractive van der Waals energy term dominates. If particles overcome the primary energy barrier (Eb), they will adhere strongly and irreversibly due to the primary minimum. The secondary minimum can also give rise to coagulation but particle adhesion is weak and reversible. The attractive energy is insensitive to suspension pH and ionic concentration for a given suspension but the double-layer repulsion, is highly sensitive thereto, i.e. one can 'design' the total interaction energy curve to induce stability.

3

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EXAMINATION OF THE EPD MECHANISM

The first attempt to explain the phenomenon of EPD was made by Hamaker and Verweys. They proposed deposition is based on the accumulation of particles at the electrode. They pointed out that successful EPD needs a stable suspension. They observed that an EPD suspension, on standing, produced a strongly adhering sediment. They suggested therefore that the phenomena of EPD and sedimentation are identical in nature and the primary function of the applied electric field in EPD is to move the particles toward the electrode to accumulate. Koelmans and Overbeek 9'1° studied the behaviour of suspensions of polar organic media on electrophoretic deposition. They proposed an electrochemical mechanism of deposit formation. The basis of their mechanism is the DLVO theory, i.e. an increase of electrolyte concentration can induce coagulation of the system. They pointed out the Hamaker mechanisms exclusively considered the accumulation of particles at the electrode and ignored the parallel transportation of ions. As a result, they also ignored the concomitant increase of ionic concentration and electrode reaction at the depositing electrode. They calculated the electrolyte concentration at the depositing electrode and showed it is comparable with that required to coagulate the powder and form a deposit. Thus they proposed the deposit forms due to particle flocculation via the increased electrolyte concentration thereabout and the result reduced (-potential near the electrode. The literature identifies other mechanisms. Grillon

Functionally graded ceramic/ceramic and metal/ceramic composites." P. Sarkar et al. e t al. 11 suggested particles

undergo charge neutralization as they touch the depositing electrode/or deposit and become static. Shimbo e t al. 12 proposed secondary processes at the electrode produce hydroxides, which absorb on the particles and polymerize, holding them together in the deposit. Later Sluzky et a l J 3 supported this mechanism. Mizuguchi et al. 14 suggested combined

LYOSPHEREDISTOI~IONBY EPD

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LOCALLYOSPHERETHINNING

COAGULA~ON ~.O® O e

®

e®

Figure 3 Schematic of the deposition mechanism by lyosphere distortion and thinning

m

Figure 4 Experimental set up for the synthesis of ceramic/ceramic and ceramic/metal FGMs

Figure 5 SEM microstructures of a step profiled A1203/YSZ FGMs. The dark phase is A1203 and the light phase is YSZ

particle neutralization and Shimbo's polymerization mechanism. They envisaged the discharge of particles at the electrode, bringing them closer and so nitrocellulose chains present on the particle surfaces of their suspension could form bridges and cause deposition.

51

Functionally graded ceramic~ceramic and metal~ceramic composites."P. Sarkar et

Figure 6

SEM microstructures of a continuously profiled AI203/YSZ F G M s . The dark phase is A1203 and the light phase is YSZ

In view of the multi-explanations, the present authors examined the mechanism of deposition in detail. The depositing electrode (the cathode in the case considered), was separated from the suspension by a dialysis membrane 15. Liquid, (the same as the suspension liquid), was filled into the cavity between the cathode and the membrane to provide an ionic path. A dc field was applied and the particles moved and stopped at the membrane. The current continued to pass, however, thus ions must permeate the membrane to complete the electrical path during EPD with a particle-exclusion membrane. A dense deposit formed on the latter, so particle/electrode reactions are not involved in electrophoretic deposition and particle neutralization at the electrode is not the mechanism. Brown and Salt 16 investigated the EPD of ,-~50 metals and oxides in organic media, calculated the minimum

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Figure 7 Through-the-thickness variation o f YSZ composition, Vicker's hardness and fracture toughness for an AI203/YSZ F G M

52

al.

field-strength required for deposition and compared it with observed values. Most of the latter were smaller than the calculated values by a factor of ~5. Some were smaller by a factor of ,-400. Hamaker had also observed this problem and attempted to explain it by considering that the accumulation of particles at the depositing electrode provided the necessary force for deposition. Recently Giersig et al. 17 deposited a monolayer of gold particles by EPD at I 0-100 mV. This result indicates that the accumulation of particles does not explain the low deposition voltage. Contrary to the Koelmans and Overbeak explanation, stable suspensions provide a dense, homogeneous deposit which cannot involve flocculation and its associated lowdensity structure. Also, dialysis membrane deposition occurred at any location between the anode and cathode. This eliminates the explanation of deposition due to local increases of electrolyte concentration around the electrode. A mechanism involving hydroxide formation and polymerization is also unlikely since deposition occurs at the anode or cathode depending on the particle size and noble metals, carbon, etc., can be deposited. An alternate explanation based on extension of the DLVO theory is proposed by the present authors TM. Considering an overall-positively-charged oxide particle/lyosphere system ([(M-OHz)+-X-], for instance), moving towards the cathode in an EPD cell (Figure 3). Fluid dynamics and the applied field will distort the double-layer envelope, thinner ahead and wider behind the particle. Cations in the liquid also move to the cathode with the positively-charged particles. The counterions in the extended 'tails' will tend to react with these accompanying cations in high concentration around them.

Functionally graded ceramic~ceramic and metal~ceramic composites: P. Sarkar et al. As result of this chemical 'reaction', the double layer around the 'tail' of the particle will thin so the next incoming particle (which has a thin leading double layer) can approach close enough that LVDW attractive forces dominate and coagulation/deposition results. Figure 3 shows schematically the mechanism of coagulation/deposition via lyosphere distortion and thinning.

4

EXPERIMENTAL P R O C E D U R E

The starting materials were 3m/o Y203-stabilized zirconia (YSZ) (TZ-3Y, Tosoh Corp. Tokyo, Japan), AI203 (AKP-50, Sumitomo Co., Tokyo, Japan), MoSi2 (Cerac Inc.) and Ni (Cerac Inc.). Suspensions are prepared in ethanol with pH values controlled at ~3.5 to produce positively charged particles. The experimental set up for electrophoretically-depositing, continuousprofile functionally-graded-materials of the AI203/YSZ system is shown in Figure 4. Deposition was started with a YSZ suspension, and a stream of A1203 suspension was slowly injected into the bottom of the EPD bath by syringe pump. A1203 and YSZ mixed at the bottom of the bath. The mixing area was shielded from the depositing cathode by a plate, as shown in Figure 4. The profile of AI203 can be precisely controlled by the deposition current, rate of pumping of the A1203 suspension into the YSZ and the concentration of the suspension. After deposition, the deposit was dried, the green compact removed and sintered in air at 1525°C for 6h. The compositional gradient across the A1203/YSZ FGMs was characterized by through-the-thickness variation of composition, hardness, and indentation fracture toughness. Microstructures were examined by SEM and compositional changes by X-ray microanalysis for zirconium and aluminium. Microindentation was conducted with a Vicker's indenter using a 3 kgf load. The indentation fracture toughness was determined by the indentation-crack-length method 19. MoSi2/ AlzO 3, YSZ/Ni and AI203/Ni FGMs have also been synthesised. In YSZ/Ni F G M 0.4kgf indentation load was used for determining the Vicker's hardness.

m 5

RESULTS AND DISCUSSION

Figure 5 is a micrograph of an A1203/YSZ step FGM with six layers in 20v/o steps. The A1203 (dark phase) and YSZ (light phase) are homogeneously distributed in the microstructure. The pure A1203 has an average grain size ~6 #m which reduces to ~2 #m in the 80 v/o layer. The sharp interface between the layers with a roughness of the grain size scale is clear in the microstructure. Figure 6 is a montage of representative sections of a continuously-graded AI203/YSZ ceramic. The YSZ and AI203 are uniformly distributed at all locations. The grain size of the YSZ is ~0.5#m and the

Figure 8 SEM microstructures of vacuum sintered (~1800°C)A1203/ MoSi2 FGMs. Bottom micrograph is the pure AI203 and MoSi2 amount increasesin the upward direction

A1203 < 1.5#m. Grain growth of the A1203 was inhibited by the second-phase YSZ. Figure 7 shows the through-the-thickness variation of the YSZ volume fraction. The continuous change of YSZ (and complementary A1203) is evident and is consistent with the microstructural observations. Figure 7 also shows the variation of Vicker's hardness and indentation fracture

53

Functionally graded ceramic~ceramic and metal/ceramic composites: P. Sarkar et al.

Figure 9 SEM microstructures of hot pressed (~ 1600°C) A1203/MoSi 2 FGMs. The dark phase is A1203 and the light phase is MoSi2

toughness across the functionally graded A1203/YSZ cross-section. The composites exhibit increasing hardness and decreasing fracture toughness from the YSZrich surface to the A1203-rich surface. Figure 8 is a micrograph of a MoSi2/A1203 FGM. The bottom of the micrograph is pure A1203. This sample was sintered in vacuum at ,~1800°C for 3 h. The large grains of pure A1203 reduced in size as MoSi 2 was introduced. Figure 9 is a micrograph of another MoSi2/A1203 sample hot pressed at 1600°C. Here the dark phase is A1203 and the light phase MoSi2. One side of the sample is pure A1203 and other 60 v/o MoSi2.

54

High magnification micrographs from three different regions (as marked in the low magnification micrograph) are also shown in the Figure 9. The micrographs indicate the sample is ~100% dense. Figure 10 is a micrograph of an Ni/A1203 FGM. The dark phase is A1203, light phase Ni. This sample was vacuum sintered at ,,d420°C and the high magnification micrograph shows large Ni areas, evidence of melting of the Ni at the sintering temperature. Figure 11 shows the optical micrograph of an micro indented YSZ/Ni FGM across the concentration gradient in the sample. This sample was hot pressed at 1300°C/1 at

Functionally graded ceramic/ceramic and metal/ceramic composites: P. Sarkar et

al.

Figure 10 SEM microstructures of vacuum sintered (~1420°C) AI203/Ni FGMs. The dark phase is Al203 and the light phase is Ni

20 MPa and shows no Ni melting. The picture clearly shows the increase in microhardness with decrease in metal phase concentration (bright phase). The lower part of the lowest series of indent marks indicate some plastic deformation in the direction of the metal rich phase. The asymmetric nature of the indentations are possibly due to uneven polishing of the softer metal rich phase in comparison to the ceramic rich harder phase. The picture also explains the lack of variation in microhardness value of the top three indented position. Figure 12 shows the variation of Vicker's microhardness across the functionally graded YSZ/Ni cross-section. The change matches the continuous increase of harder, ceramic-phase, YSZ component in the cross-section profile in the material. The hardness of the material varies from that of pure nickel (1.35 GPa) to the almost-pure ceramic phase (13.5 GPa). This profile can be tailored by controlling the rate of deposition (i.e. depositing current density, concentration of the suspension) and rate of change of YSZ concentration in the bath (i.e. rate of pumping).

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Figure 12 Variation of Vicker's hardness across the functionallygraded YSZ/Ni cross-section

55

Functionally graded ceramic~ceramic and metal~ceramic composites: P. Sarkar et al. The microhardness varies parabolically within the functionally-graded YSZ/Ni matrix and same is true in the case of A1203/YSZ reported above.

3 4 5

6

SUMMARY

Suspension stability has been discussed in the light of DLVO theory. EPD mechanisms have been discussed and an alternative mechanism was proposed based on DLVO theory and lyosphere distortion/thinning. An electrophoretic deposition technique to synthesise functionally graded materials is described. A wide range of FGMs, e.g. AI302/YSZ, AI302/MoSi2, A1302/Ni and YSZ/Ni were synthesised by EPD and their microstructures are reported. These show EPD is a facile technique for fabricating FGMs.

6 7 8 9 10 11 12 13 14 15 16 17

REFERENCES 18 1 2

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Functionally Gradiented Materials, Mater. Process. Rep. 1992, 7, 1 Koizumi, M. Ceram. Eng. Sci. Proc. 1992, 13, 333

Both, P., Chartier, T. and Huttepain, M. J. Am. Ceram. Soc. 1986, 69, C191 Takebe, H., Teshima, T., Nakashima, M. and Morinage, K. J. Ceram. Soc. (Jpn.) 1992, 100, 387 Sarkar, P., Huang, X. and Nicholson, P. S. J. Am. Ceram. Soc. 1993, 76, 1055 Bose, G.M. Phil. Trans, Roy. Soc. 1745, 43, 419 Shyne, J.J. and Scheible, H.G. 'Modern Electroplating', (Ed. P.A. Lowenheim), John Wiley and Sons, New York, 1963, p. 714 Hamaker, H.C. and Verwey, E.J.W. Trans. Farad. Soc. 1940, 36, 180-185 Koelmans, H. and Overbeek, J. Th. G. Disc. Farad. Soc. 1954, 18, 52-63 Koelmans, H. Phlips Res. Rep. 1955, 10, 161-193 Grillon, F., Fayeulle, D. and Jeandin, M. J. Mater. Sci. Lett. 1992, 11, 272-275 Shimbo, M., Tanzawa, K., Miyakawa, M. and Emoto, T. J. Electrochem. Soc. 1985, 132, 393-98 Sluzky, E., Hesse, K. J. Electrochem. Soc. 1989, 136, 2724-2727 Mizuguchi, J., Sumi, K. and Muchi, T. J. Electrochem. Soc. 1983, 130, 1819-1825 Sarkar, P., Prakash, O. and Nicholson, P. S. Ceram. Eng. Sci. Proc. 1994, 15, 1019-1027 Brown, D.R. and Salt, F.W.J. Appl. Chem. 1963, 15, 40-48 Giersig, M. and Mulvaney, P. J. Phy. Chem. 1993, 97, 63346336 Sarkar, P. and Nicholson, P.S. to be published in J. Am. Ceram. Soc.

19

Anstis, G.R., Chantikul, P., Lawn, B.R. and Marshall, D.B. J. Am. Ceam, Soe. 1981, 64, 533