Oxide laminated composites with aluminum phosphate (AlPO4) and

alumina platelets as crack deflecting materials

Dong-Kyu Kim, Waltraud M. Kriven *

Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, 1304 W. Green Street, Urbana, IL 61801, USA

Received 5 February 2005; received in revised form 14 September 2005; accepted 15 September 2005

Available online 3 April 2006

Abstract

Oxide–oxide laminated composites with aluminum phosphate (AlPO4) and alumina platelets as crack deflecting interphases were developed by

the tape casting method. Dense bodies of Al2O3, mullite, 50 vol% Al2O3$50 vol% YAG in situ composite, and 3Y-TZP were sintered and

characterized. Tape casting formulations for different oxides with solid contents of 25.1 and 30 vol%, respectively, were developed. XRD

indicated compatibility between alumina, mullite, zirconia and AlPO4. Laminated, matrix-interphase composite systems consisting of Al2O3–

AlPO4, mullite-AlPO4, 50 vol% Al2O3$50 vol% YAG in situ composite-AlPO4, and 50 vol% Al2O3$50 vol% YAG in situ composite-alumina

platelets were made. The 50 vol% Al2O3$50 vol% YAG in situ matrix-alumina platelet composite showed ‘quasi-elastic’ load–displacement

behavior under the conditions of fabrication, and had a 3-point bending strength and work of fracture of 188G8 MPa and 0.65G0.02 kJ/m2,

respectively. The 3Y-TZP–AlPO4 laminated composite could not be made because of delamination due to thermal expansion mismatch.

q 2006 Elsevier Ltd. All rights reserved.

Keywords: Laminated composites; Aluminum phosphate; Alumina platelets; Tape casting; Bending strength

1. Introduction

To overcome the brittleness and increase the toughness of

ceramics, laminated composites have been made. Laminated

ceramic composites have been fabricated by tape casting [1,2],

slip casting [3,4], electrophoretic deposition [5,6], die

pressing [7], sequential centrifuging [8,9], rolling [10,11],

and co-extrusion [12]. Some laminated, ceramic composite

systems reported in the literature are Al2O3/ZrO2 [13,14],

Al2O3/LaPO4 [15–21], YPO4/Y3Al5O12 [22], TiO2/MgSiO3

[23], Al2O3/Al2TiO5 [24], Al2O3/MoSi2CMo2B5 [25], Al2O3/

Al2O3 platelets [26], Al2O3/fluoromica [27], Al2O3/mullite

[28], SiC [29], SiC/C [10,11], Si3N4 [30], and Si3N4/BN

[31,32], etc.

Crack deflection in Al2O3/ZrO2 laminated composites is

attributed to residual stress at the interface [33]. Sarkar et al. [5]

fabricated 80 alternating layers of alumina and zirconia of

w1.5 mm total thickness by electrophoretic deposition. The

thicknesses of the densified alumina and zirconia layers were 2

and 12 mm, respectively. Chartier et al. [13] made five different

1359-8368/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compositesb.2006.02.003

* Corresponding author. C1 217 333 5258; fax: C1 217 333 2736.

E-mail address: kriven@uiuc.edu (W.M. Kriven).

kinds of Al2O3/Al2O3–ZrO2 laminates, and measured their

mechanical properties in 3-point bending, obtaining 335–

560 MPa strengths and fracture toughness of 4.6–8.0 MPa m1/2.

Morgan et al. [9,34] suggested that the monazite (LaPO4)–

alumina interface was weak enough to produce interfacial

debonding when a crack approached the interface, and that this

weak interface wasmaintained after 200 h at 1600 8C.Mawdsley

et al. [16] fabricated alumina/monazite laminates consisting of

44–54 alternating layers of alumina and monazite after hot-

pressing at 1400 8C for 1–1.5 h under a pressure of 30 MPa. The

thicknesses of the alumina andmonazite layers after hot-pressing

of the laminateswere 150and125 mm, respectively.Their 4-point

bend strengths ranged between 172.9 and 252.5 MPa. The

laminated composites had works of fracture in the range of 0.08–

0.6 kJ/m2. Liu et al. [31] hot-pressed Si3N4/BN laminates at

1750 8C/1.5 h under a pressure of 30 MPa. Their average bend

strength and work of fracture were 430 MPa and 6.5 kJ/m2,

respectively. Clegg et al. [10] produced SiC/graphite laminated

composites by sintering at 2040 8C for 30 min in an argon

atmosphere. They reported that the composites had an average

bend strength of 633 MPa and works of fracture in the range of

4.6–6.7 kJ/m2.

In this study, oxide/oxide laminated composite systems

were fabricated by the tape casting method. Tape casting

formulations for different oxides were developed. Alumina,

mullite, zirconia, and a 50 vol% alumina$50 vol% YAG in situ

Composites: Part B 37 (2006) 509–514

www.elsevier.com/locate/compositesb

Fig. 1. Schematic flow chart for making oxide–oxide laminated composites by

tape casting.

Table 1

The tape casting formulations used for the different ceramic materials

Powder Solvent Dispersant

(PS)

Eth (60%) MEK (40%)

Mullite 25.1 57.6 1.3

Al2O3 25.1 57.6 1.3

50%Al2O3–

50%YAG in situ

composite

25.1 57.6 1.3

3Y-TZP 25.1 57.6 1.3!2

AlPO4 25.1 57.6 1.3

Al2O3 platelets 30 57.6 1.3

Note: All ingredients are in vol%, Eth, ethanol (Ethyl Alcol USP, AAPER ALCO

phosphate ester (Emphos PS-21A, Witco); PVB, polyvinylbutyral (Butvar B90,

dibutylphthalate (99%, Aldrich Chemical).

D.-K. Kim, W.M. Kriven / Composites: Part B 37 (2006) 509–514510

composite were fabricated as strong matrix phases. Aluminum

phosphate (AlPO4) and alumina platelets were investigated as

crack deflecting interphases. The microstructure and room

temperature mechanical properties of the laminated composites

were characterized and evaluated for each of the laminated

systems fabricated.

2. Experimental procedures

Commercial alumina (Alcoa, A16 SG), mullite (Kyoritsu,

KM 101), zirconia (Tosoh, 3Y-TZP), and alumina platelet

(Atochem, Pierre-Benite, France, 5–10 mm) powders were

used. Aluminum phosphate and a 50 vol% alumina$50 vol%

YAG in situ composite powder were synthesized by a steric

entrapment synthesis method [35–46]. To synthesize AlPO4,

aluminum nitrate nonahydrate [Al(NO3)3$9H2O, Aldrich

Chemical Inc., 98C% purity] and ammonium phosphate

dibasic compound [(NH4)2$HPO4, Fisher Scientific] were

used as Al and P sources, respectively. Appropriate amounts

of aluminum nitrate nonahydrate and yttrium nitrate hexahy-

drate [Y(NO3)3$6H2O, Aldrich Chemical Inc., 99.9% purity]

were mixed as Al3C and Y3C sources, to make a 50 vol%

alumina$50 vol% YAG in situ composite matrix phase. The

nitrates were first dissolved in distilled water. After 30 min of

mixing, 5 wt% PVA solution was added to the solution,

followed by another 50 min of mixing. The solution was then

heated at 200 8C and then 400 8C to remove the water. The

partially dehydrated cake was dried at 150 8C overnight,

ground in a mortar and pestle, and finally calcined.

The solvent for the tape cast laminates was a mixture of

60 wt% ethanol (AAPER ALCOL and Chemical, ethyl alcohol

USP) and 40 wt% methyl ethyl ketone (99.8% purity, Fisher

Scientific, Fair Lawn, NJ). Phosphate ester (Emphos PS-21A,

Binder

(PVG)

Plasticizer Extra

additions

Comments

PG DP

5.7 4.7 5.6 – –

3.7 5.6 6.7 – Delamina-

tion after

binder

removal

5.7 4.7 5.6 – –

5.7 4.7 5.6 30% sol-

vent/30%

solvent (1st

ball milling)

Agglomera-

tion and too

high vis-

cosity

5.7 4.7 5.6 30% solvent Too high-

viscosity

8.6 1.0 1.5 300% sol-

vent/2 h

evaporation

Difficult to

form and

processing

problem

L and chemical); MEK, methyl ethyl ketone (99.8%, Fisher Scientific); PS,

Solutia); PG, polyethyleneglycol (300NF, FCC Grade, Union Carbide); DP,

Fig. 2. SEM micrograph of the 5–10 mm alumina platelets having thickness of

w1 mm.

Fig. 3. X-ray diffraction profiles indicating the compatibility between the four

oxide matrix materials and AlPO4 (temperature/time represents the sintering

condition).

D.-K. Kim, W.M. Kriven / Composites: Part B 37 (2006) 509–514 511

Witco Chemicals, Houston, TX) was used as a dispersant. The

binder was polyvinyl butyral (Butvar B90, Solutia Chemicals,

St Louis, MS). Dibutyl phthalate (99% purity, Aldrich

Chemical Inc., Milwaukee, WI) and polyethylene glycol

(300NF, FCC grade, Union Carbide, Danbury, CT) were

used as plasticizers. A conventional tape casting machine with

double doctor blades was used. The first doctor blade openings

for the strong matrix materials and crack deflecting materials

were 600 and 75 mm, respectively. The second doctor blade

openings were 1200 and 150 mm, respectively. The speed of

casting was 1 cm/s. The procedures for making laminated

composites are shown in the flow chart of Fig. 1. De-airing was

carried out by rotating a ball-free suspension at a very slow

speed. The laminated composite was thermo-compressed into a

rectangular pellet at 34.5 MPa after being maintained for 1 h at

80 8C. The binder removal was achieved by increasing the

temperature from room temperature to 150 8C at a ramp rate of

1 8C/min, then from 150 to 600 8C at a ramp rate of 0.1 8C/min,

and finally by maintaining the sample at 600 8C for 2 h. Cold

isostatic pressing (CIP) was carried out at 413.7 MPa. The

sintering conditions differed depending on the particular

materials.

The bulk density of sintered pellets was measured by

Archimedes’ method (ASTM C373). To study the chemical

compatibility between oxide matrix materials and AlPO4, a

Rigaku X-ray diffractometer (Model D-Max automated

diffractometer, Rigaku/USA, Danvers, MA) was used. Two

powders were mixed by 24 h ball milling, sintered, and

analyzed for any co-existing phases by XRD. The microstruc-

tures of the laminated composites were studied by scanning

electron microscopy (SEM, Model S-530, Hitachi, Osaka,

Japan). A screw-driven universal testing machine (Model

4502, Instron Corp., Canton, MA) was used to measure flexural

strengths in 3-point bend testing. The cross-head speed was

0.1 mm/min, the supporting span was 30 mm, and the

specimen size was 3 mm (H)!4 mm (W)!40 mm (L). The

flexural strength and work of fracture data were determined by

testing 3–5 samples. The final surface polishing of specimens

for bend testing were conducted by 600 grit SiC polishing

paper. The work of fracture of each sample was obtained from

the calculation of the area under the load–displacement curve

from bend testing.

3. Results and discussion

Table 1 summarizes the tape cast mixing formulations for

the different oxides. The amount of powder was 25.1 vol%,

except for the alumina platelets, in which case 30 vol% of

powder was used. For the alumina matrix, a lower amount of

binder of 3.7 vol% and higher amounts of plasticizers, i.e.

5.6 vol% of polyethylene glycol and 6.7 vol% of dibutyl

pthalate, were used, because of delamination after binder

removal. For the 3Y-TZP matrix, 30 vol% excess solvent was

added, before and after the first ball milling, respectively, to

lower the viscosity. The viscosity of the AlPO4 formulation

was lowered by adding 30 vol% excess solvent before the first

ball milling. To prevent possible change of their shape by

breaking during mixing, alumina platelets were mixed with

polymers by stirring without balls. The alumina platelets,

300 vol% excess solvent, and dispersant were mixed by stirring

for 12 h. Another 12 h mixing was carried out after adding the

plasticizers and binder into solution. The excess solvent was

evaporated before tape casting.

The morphology of the alumina platelets is seen in the SEM

micrograph of Fig. 2. They had a hexagonal platelet shape, an

approximate thickness of 1 mm, and size of 5–10 mm. The XRD

results indicated compatibility between the oxide matrix

materials and AlPO4, and are schematically summarized in

Fig. 3. The mixtures of Al2O3, mullite, 50 vol% alumina$50 -

vol% YAG in situ composite, 3Y-TZP and AlPO4 were

sintered under the conditions of 1600 8C/3 h, 1600 8C/10 h,

1650 8C/10 h, and 1550 8C/1 h, respectively. The aluminum

phosphate was compatible with alumina, mullite, and zirconia.

However, AlPO4 was not compatible with the 50 vol%

alumina$50 vol% YAG in situ composite matrix. AlPO4

reacted with YAG in the composite, and formed yttrium

phosphate (YPO4).

Table 2

Mechanical properties of six oxide ceramics

Bending strength (MPa) Fracture toughness (MPa$m1/2) Creep properties

25 8C 1000 8C 1300 8C 25 8C 1000 8C 1300 8C

Al2O3 [47] 380 (100%) 350 (92%) 280 (74%) 3.3 (100%) 2.5 (76%) 2.4 (73%) c-Axis sapphire: best creep

resistant [48]

Mullite [49] 240 (100%) 240 (100%) 250 (104%) 2.3 (100%) 2.5 (109%) 2.8 (122%) Approximately an order

less creep rate than that of

Al2O3 [50]

ZrO2 [51] 790 (100%) 200 (25%) – 7 (100%) 2 (29%) 2 (29%) –

YAG [52] 230 (100%) 210 (91%) 200 (87%) 1.5 (100%) 1.3 (87%) 1.4 (93%) [110] and [111] YAG has

higher creep resistance to

c-axis sapphire [53]

Alumina YAG

eutectic composite

[54,55]

420 (100%) 420 (100%) 420 (100%) 4.3 (100%) 4.1 (95%) 3.9 (91%) The creep resistance is

better than that of poly-

crystalline YAG and that

of a-axis sapphire [56]

Fig. 4. The SEM micrograph of the as-fabricated Al2O3–AlPO4 laminated

composite.

Table 3

The physical and mechanical properties of the four matrix materials used in this

study

Sintering

condition

Density

(g/cm3)

3-Point bend

strength

(MPa)

Average

grain size

(mm)

Al2O3 1600 8C/3 h 3.40 (98%) 437G13 2.29

Mullite 1600 8C/10 h 3.13 (98%) 308G11 1.44

50 vol% Al2-O3$50 vol%YAG in situ

composite

1700 8C/5 h – 361G19 Al2O3:2.14,

YAG:2.37

3Y-TZP 1550 8C/1 h 6.03 (99%) 1073G46 0.52

D.-K. Kim, W.M. Kriven / Composites: Part B 37 (2006) 509–514512

The variations of the mechanical properties of the oxide

matrix materials as a function of temperature were gathered

from the literatures and the results are summarized in Table 2.

Alumina had a bending strength of 380 MPa at room

temperature, and held 74% of its room temperature strength

at 1300 8C. Mullite had a higher strength and work of fracture

at 1300 8C than at room temperature. The mullite had 250 and

2.8 MPa m1/2 values for bending strength and fracture

toughness, respectively, at 1300 8C. The 3Y-TZP had the

highest bending strength and fracture toughness at room

temperature, but the values decreased dramatically at 1000 8C.

The Al2O3$YAG eutectic composite had 420 and 4.3 MPa m1/2

of bending strength and fracture toughness, respectively, at

room temperature. The composite retained 100 and 91% of its

room temperature bending strength and fracture toughness,

respectively, at 1300 8C [54,55]. The mullite, YAG, and

Al2O3$YAG eutectic composites all possessed good reported

creep properties.

The oxide matrix materials were sintered at different

temperatures, and their physical and mechanical properties

were studied. Table 3 presents the results. The densities of the

sintered Al2O3, mullite, and 3Y-TZP were 98, 98, and 99% of

theoretical density, respectively. The 3-point bending strengths

of the alumina, mullite, 50 vol% alumina$50 vol% YAG in situ

composite matrix, and 3Y-TZP were 437G13, 308G11,

361G19, and 1073G46 MPa, respectively. The average

grain sizes after sintering of Al2O3, mullite, and 3Y-TZP

were 2.3, 1.4 and 0.5 mm, respectively. In the case of the

50 vol% alumina$50 vol%mullite in situ composite matrix, the

average grain sizes of the alumina and YAG phases were 2.1

and 2.4 mm, respectively.

A SEM micrograph of the Al2O3–AlPO4 laminated

composite is shown in Fig. 4. The alumina layer was dense,

the aluminum phosphate layer was porous, and interphase

between the two materials indicated no delamination. The

results of the 3-point bending tests for the laminated

composites with mullite, alumina, zirconia, and 50 vol%

alumina$50 vol% YAG in situ composite as matrix materials

and aluminum phosphate and alumina platelets as crack

deflecting phases, are summarized in Table 4. The Al2O3–

AlPO4 laminated composite showed non-brittle fracture and

had a bending strength and work of fracture of 161G15, and

0.47G0.05 kJ/m2, respectively. The 50 vol% alumina$50 -

vol% YAG in situ composite matrix-AlPO4 laminated

composite also showed brittle fracture and had a bend strength

and work of fracture of 181G10 MPa and 0.26G0.06 kJ/m2,

respectively. The reason for this behavior is attributed to the

Table 4

The strength and work of fracture of oxide laminated composites

Strengh

(MPa)

Work of

fracture

(kJ/m2)

Mullite (600 mm)–AlPO4 (75 mm) 157G16 0.46G0.03

Al2O3 (600 mm)–AlPO4 (75 mm) 161G15 0.47G0.05

50 vol% Al2O3$50 vol% YAG in situ

composite (600 mm)–AlPO4 (75 mm)

181G10 0.26G0.06

50 vol% Al2O3$50 vol% YAG in situ

composite (600 mm)–alumina platelets

(75 mm)

188G8 0.65G0.02

3Y-TZP (600 mm)–AlPO4 (75 mm) Delamination

Fig. 5. The load vs displacement curve for the 3-point bending test of 50 vol%

Al2O3$50 vol% YAG in situ matrix-AlPO4 laminated composite.

D.-K. Kim, W.M. Kriven / Composites: Part B 37 (2006) 509–514 513

reaction of the AlPO4 to form YPO4 at the interface, so that the

AlPO4 could no longer function as a weak, porous, crack-

deflecting interphase. The 50 vol% alumina$50 vol% YAG

in situ composite matrix-alumina platelet laminated composite

exhibited non-brittle fracture, and had a strength and a work of

fracture of 188G8 MPa and 0.65G0.02 kJ/m2, respectively.

The 3Y-TZP–AlPO4 laminated composite showed

Fig. 6. Crack deflection along alumina platelet interphases in the laminate

composed of 50 vol% Al2O3$50 vol% YAG in situ composite matrix and

alumina platelets (corresponding to the specimen in Fig. 5).

delamination after sintering. The thermal expansion coeffi-

cients of 3Y-TZP and AlPO4 are 10.2 and 2.3!10K6/8C,

respectively, [57]. The reason for delamination of the

composite is attributed to the large thermal expansion

coefficient mismatch. Fig. 5 shows the load vs displacement

curve from bend testing of the 50 vol% alumina$50 vol% YAG

in situ composite matrix-alumina platelet interphase, indicating

‘quasi-elastic’ load vs displacement behavior. The composite

underwent almost 0.35 mm of displacement. The SEM

micrograph of the 3-point, bend-tested, 50 vol% alumina$50 -

vol% YAG in situ composite matrix-alumina platelet,

laminated composite is shown in Fig. 6. The crack was

deflected along the alumina platelet interphase layer and

showed a complicated crack path.

4. Conclusions

Various oxide–oxide laminated composites were fabricated

having porous AlPO4 or alumina platelets as crack deflecting

interphases. Tape casting formulations for oxide materials with

powder loadings of 25.1 vol%, were developed. In the case of

tape casting of alumina platelets, a solid loading of 30 vol%

was used. The AlPO4 was chemically compatible with alumina,

mullite and zirconia during various high-temperature annealing

conditions between 1550 and 1600 8C. However, AlPO4

reacted with YAG in the 50 vol% alumina$50 vol% YAG

in situ composite matrix, forming YPO4. The 50 vol%

alumina$50 vol% YAG in situ composite matrix itself had an

average 361G19 MPa 3-point bending strength, in which the

grain sizes of the alumina and YAG were 2.1 and 2.4 mm,

respectively, after sintering at 1700 8C for 5 h. Alumina–

AlPO4, mullite-AlPO4, 50 vol% alumina$50 vol% YAG in situ

composite matrix-alumina platelet laminated composites

showed some graceful failure and had works of fracture of

0.46G0.03, 0.47G0.05, and 0.65G0.02 kJ/m2, respectively.

The 50 vol% alumina$50 vol% YAG in situ composite matrix-

AlPO4 laminated composite showed brittle fracture because of

reaction of AlPO4 to form YPO4 within the interphase. The 3Y-

TZP–AlPO4 laminated composite was delaminated because of

too large a mismatch in the thermal expansion coefficients.

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