Mechanical properties of Al
2
O
3
/ZrO
2
composites
W.H. Tuan*, R.Z. Chen, T.C. Wang, C.H. Cheng, P.S. Kuo
Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 106, ROC Received 14 June 2001; received in revised form 28 January 2002; accepted 24 February 2002
Abstract
In the present study, both t-phase zirconia and m-phase zirconia particles are incorporated into an alumina matrix. Dense Al2O3/
(t-ZrO2+m-ZrO2) composites were prepared by sintering pressurelessly at 1600 C. The microstructure of the composites are
characterized, the elastic modulus, strength and toughness determined. Because the ZrO2inclusions are close to each other in the
Al2O3matrix, the yttrium ion originally in t-ZrO2particles can diffuse to nearby m-ZrO2particles during sintering, and the m-phase
zirconia is thus stabilized after sintering. The strength of the Al2O3/(t-ZrO2+m-ZrO2) composites after surface grinding can reach
values as high as 940 MPa, which is roughly three times that of Al2O3alone. The strengthening effect is contributed by
micro-structural refinement together with the surface compressive stresses induced by grinding. The toughness of alumina is also enhanced by adding both t-phase and m-phase zirconia, which can reach values as high as two times that of Al2O3alone. The toughening
effect is attributed mainly to the zirconia t–m phase transformation. # 2002 Elsevier Science Ltd. All rights reserved. Keywords:Al2O3; ZrO2; Composites; Strength; Toughness and toughening
1. Introduction
Zirconia has three crystallographic forms, namely: monoclinic (m), tetragonal (t) and cubic (c) phases.1The
transformation of pure zirconia from t-phase to m-phase occurs at a temperature around 950C, which is
accompanied by a volume expansion of 4%. This volume expansion generates both dilatational and shear stresses, and these stresses prohibit the opening of an advancing crack, so the toughness of zirconia at room temperature is high compared with other ceramics. In addition to the transformation toughening associated with the t–m transformation around advancing cracks, other mechanisms, such as crack deflection, crack brid-ging and the presence of microcracks, may also enhance the toughness. Nevertheless, the contribution to tough-ness from these mechanisms is smaller than that from the transformation toughening.2The phase
transforma-tion temperature from t to m can be suppressed by doping with suitable alloy elements, such as Y2O3,
CeO2, CaO, MgO, etc.3,4 Furthermore, the size of
zir-conia particles must be lower than a critical size, to
ensure the stable of t-phase at room temperature.5
Apart from size and composition control, the transfor-mation can also be manipulated by controlling external stresses,6 external environment,7 etc. The complexities
involved in the phase relationships give many possibi-lities to design new materials by combining various phases and microstructures.8
Zirconia particles are frequently employed as a toughening agent for other ceramics, and these zirconia-toughened ceramics (ZTCs) have received great atten-tion in the last two decades.24 Among these
cera-mics, many research groups have a very high interest in zirconia-toughened alumina (ZTA), in which either t-phase3,4 or m-phase9 zirconia particles were added
into alumina. Although the toughness of alumina is indeed enhanced by adopting this approach, the enhancement of toughness may, depending on flaw control or transformation control, be accompanied by a decrease in strength.9,10 Thus, optimizing the
mechanical properties of ZTCs is therefore a long-standing pursuit. In the present study, an alternative design for the composition of ZTA is proposed, where both t-phase and m-phase zirconia particles are added simultaneously into an alumina matrix. The mechanical properties of the Al2O3/(t-ZrO2+m-ZrO2) composites
are investigated. 0955-2219/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
P I I : S 0 9 5 5 - 2 2 1 9 ( 0 2 ) 0 0 0 4 3 - 2
www.elsevier.com/locate/jeurceramsoc
* Corresponding author. Tel.: 2365-9800; fax: +886-2-2363-4562.
2. Experimental procedures
An alumina (TM-DR, Taimei Chem. Co. Ltd., Tokyo, Japan) powder was ball milled together with two ZrO2 powders (TZ0.5, ZrO2+0 mol% Y2O3,
d50=0.3 mm; TZ-3YP, ZrO2+3 mol% Y2O3, d50=0.24
mm, Hanwha Ceramics Co., Australia) in ethyl alcohol for 24 h, using zirconia balls as grinding media. The compositions investigated in the present study are shown in Table 1. The slurry of the powder mixtures was dried with a rotary evaporator, and the dried lumps were crushed and passed through a plastic sieve. Powder compacts with dimensions of 7650 mm were formed by uniaxially pressing at 44 MPa. The sintering was carried out in a box furnace at 1600 C for 1 h in air
with heating and cooling rates of 5 C/min. For
com-parison, the Al2O3, Al2O3/t-ZrO2 and Al2O3/m-ZrO2
specimens were also prepared with the same techniques. Some discs of 25.4 mm in diameter were prepared for the measurement of elastic modulus with an ultrasonic technique at 5 MHz (Pulser Receiver 5055PR and Oscilloscope 9354CM, LeCoroy Co., USA).
The sintered specimens were machined longitudinally with a 325 grit resin-bonded diamond wheel at a depth of 5 mm/pass. The final dimensions of the specimens were 3436 mm. The strength of the specimens was determined by four-point bending at ambient, room-temperature conditions. The upper and lower spans were 10 and 30 mm, respectively. The rate of loading was 0.5 mm/min. To determine the effect of surface grinding, the strength of some specimens before surface grinding was also determined. The fracture toughness
was determined by the single-edge-notched-beam
(SENB) technique. The notch was generated by cutting
with a diamond saw. The width of the notch was approximately 0.3 mm. No annealing treatment was applied to the notched specimen before the toughness measurement.
Phase identification was performed on sintered, frac-tured and surface ground surfaces by X-ray dif-fractometry (XRD) with CuKa radiation. The relative phase content of zirconia was estimated by using the method proposed by Evans et al.11 The final density of
the specimens was determined by the Archimedes method. The solubility between the materials used in the present study was low; the relative density of the sin-tered composites was estimated by using the theoretical density of 3.98 g/cm3for Al
2O3, 5.83 g/cm3for m-ZrO2
and 6.05 g/cm3for t-ZrO
2. Polished surfaces for
micro-structure observation were prepared by grinding and polishing with diamond paste to 6 mm and with silica suspension to 0.05 mm. The polished specimens were thermally etched at 1500C for 0.5 h to reveal the grain
boundaries of matrix grains. Microstructural character-ization used scanning electron microscopy (SEM). The size of Al2O3 grains and ZrO2 inclusions was
deter-mined by using the line intercept technique. More than 200 grains or inclusions were counted for each speci-men.
3. Results and discussion
XRD analysis shows that the initial ZrO2 powders
containing 0 mol% Y2O3and 3 mol% Y2O3are mainly
monoclinic and tetragonal phases, respectively; the powders are thus denoted below as m-ZrO2and t-ZrO2
powders.
Table 2 shows the dependence of relative density of Al2O3/t-ZrO2, Al2O3/m-ZrO2 and Al2O3/(t-ZrO2
+m-ZrO2) composites on total zirconia content. The density
values shown in the table are the average value of 8–10 specimens. The density of the composites decreases slightly with the increase of zirconia content, indicating that the presence of zirconia particles prohibits the densification of alumina matrix. Although the solubility of zirconia in alumina is as low as 2000 ppm, the presence of Zr+4solute can slow down the densification
of Al2O3.12 However, the relative density of the
speci-mens, except for the composites with high inclusion content such as 15% t-ZrO2, 7.5% m-ZrO2+7.5%
t-ZrO2, 15% t-ZrO2+15% m-ZrO2, is higher than 98%,
indicating that the composites can be prepared with straightforward powder mixing and pressureless sinter-ing.
Fig. 1 shows the microstructure of the Al2O3
/(t-ZrO2+m-ZrO2) composites; and the microstructures of
Al2O3, Al2O3/t-ZrO2and Al2O3/m-ZrO2specimens are
also shown for comparison. The zirconia inclusions are distributed uniformly within the composites. The ZrO2
Table 1
Composition of the specimens investigated in the present study. The nearest neighbor distance between ZrO2 particles in Al2O3matrix as calculated by Eq. (1) is also shown
Composition Total zirconia content (vol.%) Nearest neighbour distance (mm) A12O3 0 – +5% t-ZrO2 5 0.87 +7.5% t-ZrO2 7.5 0.90 +10% t-Zro2 10 0.73 +12.5% t-ZrO2 12.5 0.70 +15% t-ZrO2 15 0.67 +5% m-ZrO2 5 1.6 +7.5% m-ZrO2 7.5 1.4 +10% m-ZrO2 10 1.2 +12.5% m-ZrO2 12.5 1.1 +15% m-ZrO2 15 1.0 +5% t-ZrO25% m-ZrO2 10 1.1 +7.5% t-ZrO2+7.5% m-ZrO2 15 0.92 +10% t-ZrO2+10% m-ZrO2 20 0.87 +12.5% t-ZrO2+12.5% m-ZrO2 25 0.91 +15% t-ZrO2+15% m-ZrO2 30 0.83
particles, both t-phase and m-phase, are mainly located at the grain boundaries of alumina, so the micro-structure of alumina is thus refined due to the pinning effect exerted by the zirconia particles, as shown in Table 2. The size of alumina grains in the t-ZrO2
-con-taining composites is smaller than that in the Al2O3
/m-ZrO2composites, indicating that the presence of a small
amount of Y2O3, the stabilizing agent for ZrO2, can
further prohibit the grain growth of alumina. Though the ionic charge of yttrium is the same as that of Table 2
The relative density, size of Al2O3grains, size of ZrO2inclusions, the percentage of m-ZrO2over total ZrO2on the sintered and fracture surfaces of the Al2O3/ZrO2composites
Composition Relative density (%) Size of Al2O3 grains (mm) Size of ZrO2 inclusions (mm) m-ZrO2on sintered surface (%) m-ZrO2on fracture surface (%) Al2O3 99.7 10.2 – – – +5% t-ZrO2 99.5 2.1 0.27 0 4 +7.5% t-ZrO2 98.2 2.1 0.34 0 5 +10% t-ZrO2 99.4 2.0 0.32 0 6 +12.5% t-ZrO2 99.4 1.7 0.34 0 9 -i-15% t-Zr02 97.5 1.5 0.36 0 10 +5% m-ZrO2 99.3 3.6 0.50 13 25 +7.5% m-ZrO2 98.7 2.6 0.53 19 29 +10% m-ZrO2 99.1 2.4 0.54 22 38 +12.5% m-ZrO2 99.5 2.4 0.54 26 39 +15% m-ZrO2 99.4 2.5 0.55 50 22 +5% t-ZrO2+5% m-ZrO2 99.9 1.4 0.49 4 10 +7.5% t-ZrO2+7.5% m-ZrO2 97.3 1.6 0.49 4 12 +10% t-ZrO2+10% m-ZrO2 99.9 1.2 0.54 6 11 +12.5%t-ZrO2+ 12.5% m-ZrO2 99.7 1.2 0.63 8 13 +15% t-ZrO2+15% m-ZrO2 96.1 1.0 0.63 32 10
aluminum, the yttrium ion is much larger than the alu-minum ion (0.89 angstrom vs. 0.53 angstrom).13 Large
yttrium ions tend to segregate at the grain boundaries of alumina, thus reducing elastic strain energy.14Although
the solubility of yttrium in alumina is extremely low (< 10 ppm),15large yttrium ions can block the diffusion of
ions along grain boundaries, leading to reduced densifica-tion and grain growth rates.16Though the yttria content in
the composites is low, the amount is high enough to sup-press the coarsening of alumina matrix grains.
Table 2 also shows the size of zirconia particles of the Al2O3/ZrO2composites. The ZrO2inclusions grow to a
size that is roughly two times that of the starting particle size after sintering. The size of ZrO2 inclusions in the
sintered Al2O3/m-ZrO2composites is larger than that in
the sintered Al2O3/t-ZrO2composites. The grain growth
of zirconia in alumina matrix is a process of coales-cence; namely, the coarsening of zirconia particles is accompanied by the grain growth of alumina matrix.17
The size of Al2O3grains in Al2O3/m-ZrO2composites is
larger than that in Al2O3/t-ZrO2 composites, so the
ZrO2 inclusions in Al2O3/m-ZrO2 composites are thus
larger than those in Al2O3/t-ZrO2composites. For the
Al2O3/(t-ZrO2+m-ZrO2) composites, no attempt is
given to distinguish the phase of each ZrO2particle. The
value shown for the ZrO2 inclusions in the Al2O3
/(t-ZrO2+m-ZrO2) composites in Table 2 is the average
size for all ZrO2inclusions. The size of ZrO2inclusions
in the Al2O3/(t-ZrO2+m-ZrO2) system is between the
other two systems. Some fine ZrO2 particles in the
Al2O3/m-ZrO2composite are trapped into Al2O3matrix
grains, Fig. 3(c), perhaps due to the relatively greater grain growth of the alumina matrix.
Table 2 shows the amount of m-ZrO2on the surface
of the sintered composites. The amount of m-ZrO2on
the surface of Al2O3/m-ZrO2composites is the highest,
on the Al2O3/t-ZrO2 composites the lowest, on the
Al2O3/(t-ZrO2+m-ZrO2) composites in the
inter-mediate. The presence of Y2O3 lowers the
transforma-tion temperature from t to m down to a temperature below room temperature,2so less m-phase is detected in
the t-ZrO2containing systems. Though less constraint is
imposed on zirconia particles near the surface region, there is hardly any m-phase detected on the sintered Al2O3/t-ZrO2composite. Even though m-ZrO2particles
are used as the starting material for the Al2O3/m-ZrO2
composites, only part of the ZrO2particles transform to
m-phase, indicating that after sintering some ZrO2
par-ticles remain at its high-temperature phase as metastable t-phase. The elastic modulus of pure alumina is high, 396 GPa, as determined by the ultrasonic technique. The rigid Al2O3matrix constraints the fine ZrO2
inclu-sions, thus suppressing the extent of phase transforma-tion. Furthermore, the size of ZrO2 particles in the
Al2O3/m-ZrO2 composites is larger than that of ZrO2
particles in the other two systems. Many ZrO2particles
can thus be larger than the critical size for the trans-formtion,5 so more m-phase is thus detected in the
Al2O3/m-ZrO2composites.
The amount of m-phase is also very low, 4%, on the surface of the sintered Al2O3/(t-ZrO2+m-ZrO2)
composites, as shown in Table 2. For a composite con-taining monosized inclusions, the nearest neighbor dis-tance, l, between inclusions depends on the size of inclusion, d, and its volume fraction, F, as18,19
l ¼
6 1=2 d
F1=2 ð1Þ
The calculated values for the distance between nearest neighboring ZrO2particles in Al2O3matrix are shown
in Table 1. The diffusion coefficient of yttrium ion in alumina is not available from the literature. However, the distance between ZrO2particles is so small that the
transportation of yttrium ions from t-ZrO2 to nearby
m-ZrO2particles is thus possible. The m-ZrO2particles
are stabilized after the adsorption of Y2O3 from the
nearby t-ZrO2 particles. Therefore, the amount of
m-Fig. 2. Elastic modulus of composites as function of total zirconia content. The straight line predicted by the rule of mixtures is shown for comparison.
Fig. 3. Flexural strength of composites as a function of total zirconia content.
phase in the sintered Al2O3/(t-ZrO2+m-ZrO2)
compo-sites is lower than that in the Al2O3/m-ZrO2composite,
even though the same amount of m-ZrO2 was used in
the starting compositions.
Fig. 2 shows the elastic modulus of the composites as a function of total zirconia content. The values calcu-lated from the rule of mixtures are also shown in the figure for comparison. The elastic modulus of zirconia, 200 GPa,20is lower than that of alumina; thus the
elas-tic modulus decreases with the increase of zirconia con-tent. As-sintered specimens, without surface grinding, were used for the elastic modulus measurement. The ultrasonic wave penetrates through the specimens, unlike the XRD analysis, which detects only the region near the surface. The elastic modulus measurement can thus provide more information for the interior of the composites. The presence of porosity and microcracks can reduce the elastic modulus. The densities of the composites with low zirconia content ( < 10 vol.%) are almost equal (Fig. 2); however, the elastic modulus of the Al2O3/m-ZrO2composites is slightly lower than that
of the Al2O3/t-ZrO2composites and of the values
pre-dicted by the rule of mixtures, suggesting the possibility of the presence of a small amount of microcracks in the Al2O3/m-ZrO2 composites. The density of Al2O3/
(15%t-ZrO2+15%m-ZrO2) composite is the lowest of
the composites, whereas it has the largest zirconia inclusions, so some microcracks may be present in the composite. Thus the elastic modulus of this composite is the lowest.
Fig. 3 shows the strength of the composites as a function of total zirconia content. The presence of either or both t-ZrO2and m-ZrO2 refines the microstructure
of alumina matrix, as shown in Table 2. The strength-ening effect is partly attributed to the refinement of microstructure. The strength of the Al2O3/(t-ZrO2
+m-ZrO2) system is the highest among the three systems,
reaching 940 MPa. The low density of Al2O3/(7.5%
t-ZrO2+7.5% m-ZrO2) and Al2O3/(15% t-ZrO2+15%
m-ZrO2) composites, Table 2, underlines their low
strength. The size of matrix grains in the Al2O3
/(t-ZrO2+m-ZrO2) composites is reduced to 1/5 that of
Al2O3alone, Table 2. The strength of ceramics is
inver-sely proportional to the square root of the grain size;21
however, the strength of Al2O3/(t-ZrO2+m-ZrO2)
composites is nearly three times that of Al2O3 alone.
The microstructural refinement alone is not sufficient to account for such strength enhancement. The strength of the Al2O3, Al2O3/5% t-ZrO2, Al2O3/5% m-ZrO2 and
Al2O3/(5% t-ZrO2+5% m-ZrO2) specimens before and
after surface grinding is shown in Table 3. The strength of Al2O3 specimens increases by 20% after the surface
grinding treatment. The strength of a brittle solid depends on the size of its critical flaws and the surface grinding process can alter the size of critical flaws and introduce compressive stresses into surface layer.22 The
population of flaws tends to be higher near surface region because contamination is easily introduced into the surface region during various processing steps. The strength is thus enhanced because the surface region is removed after grinding. In addition to the surface modification, residual compressive stress is also intro-duced into the surface layer by grinding, and the resi-dual compressive stress can also contribute to increased strength.
The strength of Al2O3/5% t-ZrO2, Al2O3/5% m-ZrO2
and Al2O3/(5% t-ZrO2+5% m-ZrO2) composites
increases by 60, 40 and 120% after surface grinding, respectively. There are approximately 3% ZrO2
parti-cles transformed from t to m phase in the surface region of the machined Al2O3/5% t-ZrO2composite as shown
in Table 3. The expansion of ZrO2particles during t–m
transformation can further introduce compressive stres-ses into the surface layer, so the strength of Al2O3
/t-ZrO2composites is therefore enhanced.
The critical transformation stress from t to m-phase increases with the increase of Y2O3 content.23 The
stresses, shear and tensile stresses, applied by the dia-mond wheel during grinding seems too small to trigger a significant amount of phase transformation of ZrO2
particles in Al2O3/t-ZrO2 (3 mol% Y2O3) composite
(Table 3). The effective Y2O3content in ZrO2particles
in the Al2O3/(t-ZrO2+m-ZrO2) composites is lower
than 3 mol%, so the ZrO2 particles are thus easier to
transform. Therefore, 16% of the ZrO2particles
trans-form to m-phase. Consequently, the strength of the machined Al2O3/(5% t-ZrO2+5% m-ZrO2) composite is
twice that of the composite before grinding. The Table 3
The strength of the Al2O3, Al2O3/5% t-ZrO2, Al2O3/5% m-ZrO2and Al2O3/(5% t-ZrO2+5% m-ZrO2) specimens before and after surface grinding. The percentage of the phase transformation on the surface before and after grinding is also shown
Strength/MPa Extent of phase transformation/%
As-sintered After surface grinding
As-sintered After surface grinding
Al2O3 269 18 323 30 – –
Al2O3/5% t-ZrO2 310 25 502 31 0 3
Al2O3/5% m-ZrO2 303 23 421 33 13 34
amount of m-phase zirconia is high, 34%, on the sur-face of the machined Al2O3/5% m-ZrO2composite. The
amount of transformation may be too high to produce some interconnected microcracks after phase transfor-mation, and the strength increase is thus limited by the excess transformation.
Fig. 4 shows the dependence of toughness of the composites on total zirconia content. The toughness of the Al2O3/m-ZrO2 composites is the highest among
three systems, reaching 11.8 MPam0.5. The amount of
m-phase on the fracture surface of the Al2O3/t-ZrO2
composites is very low, as shown in Table 2. For Al2O3/
m-ZrO2composites, more m-phase can be detected on
the fracture surface, indicating more phase transforma-tion participating in the fracture process. The amount of m-phase on the fracture surface of Al2O3/(t-ZrO2+
m-ZrO2) composites is also low, suggesting that m-ZrO2
particles are stabilized, or metastable, due to the supply of Y2O3from nearby t-ZrO2particles. Fig. 5 shows the
toughness as a function of percentage of phase trans-formation. Hannink et al.2suggested that the toughness
could increase linearly with the amount of transform-able zirconia, provided the transformation toughening
dominates during fracture. Such a linear relationship is indeed exhibited in the systems investigated in the pre-sent study. Fig. 5 demonstrates that the toughness enhancement for all the composites investigated in the present study can be attributed mostly to a transforma-tion toughening effect. The contributransforma-tion from other toughening mechanisms, such as microcracking, crack deflection, is small. The toughness of the composites thus depends strongly on the extent of phase transfor-mation. The toughness of the Al2O3/m-ZrO2
compo-sites, where no stabilizing agent is added to the ZrO2, is
thus the highest among three systems.
Fig. 6 presents all the toughness and strength data for the composites, showing that the strength of composites increases with the increase of toughness. However, the strength of Al2O3/m-ZrO2 composites is significantly
lower than that of Al2O3/t-ZrO2 and of Al2O3
/(t-ZrO2+m-ZrO2) composites in terms of toughness. A
small amount of microcracks may exist in the Al2O3
/m-ZrO2 composites, as demonstrated by the elastic
mod-ulus analysis (Fig. 2). The strength thus suffered due to the presence of microcracks.
Though the toughness of Al2O3/(t-ZrO2+m-ZrO2)
composites ranges between those of Al2O3/t-ZrO2and
Al2O3/m-ZrO2 composites, its strength is the highest
among all three systems. For example, the strength and toughness of Al2O3/(5% t-ZrO2+5% m-ZrO2)
compo-sites is 943 MPa and 7.2 MPam0.5, respectively. The
total zirconia content for the composite is only 10%; the strength and toughness are respectively, three and two times that of alumina alone. There was 3 mol% Y2O3in
the t-ZrO2particles in the beginning, and Y2O3can
dif-fuse from t-ZrO2 particles to m-ZrO2 particles during
the sintering of Al2O3/(t-ZrO2+m-ZrO2) composites.
The final effective Y2O3 content in the ZrO2 particles
may be in the range of 1–2 mol%. These ZrO2particles
transform easier under external stress, so a residual compressive stress is thus introduced into the surface region during grinding, and the strength is thus enhanced significantly. Many zirconia powders are Fig. 4. Toughness of composites as a function of total zirconia
con-tent.
Fig. 5. Toughness of composites as a function of the percentage of
available on the market; however, these are mainly 0 or 3 mol% Y2O3powders. The present study demonstrates
that the amount of Y2O3 dopant can be easily
manipulated by mixing various amounts of t-phase and m-phase powders together. The approach adopted in the present study provides an alternative to design Al2O3/ZrO2 composites with improved mechanical
properties.
4. Conclusions
The present study demonstrates that adding both t-ZrO2and m-ZrO2particles can significantly enhance the
mechanical properties of alumina. The presence of Y2O3, originally in the t-ZrO2 particles, can affect the
microstructural evolution of Al2O3 matrix and the
phase transformation of ZrO2. The m-ZrO2 phase is
stabilized due to the adsorption of yttria from nearby t-ZrO2phase. Fewer zirconia inclusions are transformed
from t to m in the Al2O3/(t-ZrO2+m-ZrO2) composites
than in the Al2O3/m-ZrO2 composites. A compressive
surface layer is formed on the machined surface due to the volume expansion accompanied by the t–m trans-formation. The strength can thus be enhanced due to the microstructural refinement and the presence of the surface stresses. The toughness enhancement is propor-tional to the amount of transformable zirconia, indicat-ing that the toughenindicat-ing effect is mainly contributed by a transformation toughening effect.
Acknowledgements
The National Science Council, R.O.C. supported the present study through contract number NSC89–2216-E002–049.
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