Transmission Electron Microscope Investigation of the
Interface between Titanium and Zirconia
Kun-Fung Lin and Chien-Cheng Lin
*
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan 300
The interfaces between 3-mol%-yttria-partially-stabilized
zirconia and commercially pure titanium after reaction at
1750°C were analyzed with a scanning electron microscope
and an analytical transmission microscope. Zirconia was
reduced to oxygen-deficient zirconia (ZrO
2−x
) with an O/Zr
ratio as low as 1.53, causing the evolution of oxygen. Part of
the oxygen could accumulate at grain boundaries, the re-
mainder being dissolved in titanium as ␣-Ti(O). An or-
dered titanium suboxide (Ti
3
O) could be formed from a
solid solution of ␣-Ti(O) during cooling. A fine crystalline
ZrO
2−x
phase (O/Zr ≈ 2) was also found along with ␣-Zr
near the interface on the zirconia side. The ␣-Zr was
twinned with one of the twin planes being indexed as
{101
2}. The yttria stabilizer was excluded from zirconia as
the reaction was progressing, existing as oxygen-deficient
yttria. Extensive dissolution of zirconia in titanium gave
rise to the formation of ␣-Ti(Zr,O) solid solution. On cool-
ing, lamellae of Ti
2
ZrO precipitated from ␣-Ti(Zr,O) with
an orientation relationship of {11
0}
Ti
2
ZrO
//{100}
␣-Ti
and
〈111
〉
Ti
2
ZrO
//〈011〉
␣-Ti
.
I. Introduction
T
ITANIUM alloys have excellent properties such as high spe-
cific strength and good corrosion resistance. However, they
are extremely reactive to ceramics at high temperatures, result-
ing in chemical reactions.
1
Interstitial elements (e.g., C, N, O,
H) from ceramics have a great tendency to enter into titanium
alloys during casting and cause the deterioration of mechanical
properties, such as ductility, hardness, and toughness.
2–6
Titanium is often melted in a water-cooled copper crucible
by consumable electrode vacuum arc melting instead of
vacuum induction melting (VIM) because ceramic crucibles,
used in the VIM process, can react with the titanium melt.
However, there are many disadvantages with the arc melting
process, for example, the high cost of the equipment, difficulty
in temperature control, scrape recycle, alloy modification, and
long cycle time. Vacuum induction skull melting of titanium
castings requires chemical milling of the surfaces in order to
remove the reaction products. The VIM process could be prof-
itable in industry if a suitable crucible material were available.
Therefore, how to control the interfacial reactions between ti-
tanium and ceramics is of great concern.
Extensive investigations have been done on the reactions
between Ti and ZrO
2
, mostly by using scanning electron mi-
croscopy (SEM) and XRD analyses. The formation of oxygen-
deficient zirconia gives rise to the blackening of zirconia after
reaction.
2,7
Economos and Kingery
8
displayed the penetration
of titanium along grain boundaries without the formation of
any new interfacial phase except the oxygen-deficient zirconia.
However, Weber et al.
9
indicated that limited metals were dis-
solved in oxides, giving rise to the black color of the oxides.
They also reported that a crucible of ZrO
2
plus 15 at.% Ti had
the best performance for titanium melting, compared with TiC,
ThO
2
, CeS, TiO
1.97
, 15.5 mol% MgO–ZrO
2
, and pure ZrO
2
.
Ruh
2
reported that up to 10 mol% of each of Zr and O could be
retained in a solid solution of Ti at temperatures ranging from
1200° to 2000°C, while the solubility of Ti in ZrO
2
was up to
4 at.%. Some studies
10,11
stated that the dissolution of oxygen
due to the interfacial reactions could increase the hardness of
titanium, while the microhardness profile was primarily an in-
dicator of variation in oxygen concentration. Furthermore, oxy-
gen was preferably transferred from oxides to titanium, leaving
behind metallic components as lower oxides in the mold.
Lyon
12
observed that Y
2
O
3
was reduced to Y
2
O
2.94
by molten
titanium, and a few flowerlike Y
2
O
3
particles precipitated in
the Ti matrix.
In a previous study on the phase transformation of nonstoi-
chiometric zirconia, Ruh and Garrett
13
stated that a striated
intergranular ␣-Zr was precipitated during cubic–tetragonal
transformation. Upon subsequent annealing below the transfor-
mation temperature, this precipitate assumed a low-energy
spherical shape. Ruh
2
also reported that metallic ZrTi(O) could
form during the interfacial reactions between titanium and
zirconia.
Some previous studies discussed the effect of titanium ad-
ditive on the stabilization and mechanical properties of zirco-
nia.
14,15
Weber et al.
14
stated that the addition of titanium
(11.9–22.3 at.%) could improve the strength and thermal shock
resistance of zirconia. The improved properties were attributed
by Ruh et al.
7
to better sintering and densification. Lin et al.
15
reported that the enhancement in mechanical properties was
due to the partial stabilization of ZrO
2
in the system of (5–50
mol%) Ti/ZrO
2
as well as the refinement of grain size. The
dissolution of TiO, formed by internal oxidation, could stabi-
lize zirconia, while a second phase in the grain boundaries
could cause a decrease in the grain size of ZrO
2
. At tempera-
tures above 1200°C only about 4 at.% Ti was soluble in ZrO
2
,
even though titanium could dissolve more than 20 wt% ZrO
2
with the formation of a solid solution of ␣-Ti(Zr,O). Since the
titanium additive in these studies exceeded the solubility limit
of titanium in zirconia, the precipitation of (Ti,Zr)
3
O from the
solid solution of ␣-Ti(Zr,O) during cooling could be predicted
by a pseudobinary diagram of Ti–ZrO
2
.
16
However, Weber et
al.
14
and Lin et al.
15
reported neither (Ti,Zr)
3
O nor ␣-Ti(Zr,O).
The difficulties in preparing cross-sectional specimens have
discouraged many investigators from analyzing the microstruc-
tures of the interface between zirconia and titanium using trans-
mission electron microscopy. However, the microstructure
must be characterized before the reaction mechanisms are ex-
plored. In the present study, we will investigate the microstruc-
ture of the interface between Ti and ZrO
2
using analytical
transmission electron microscopy (TEM) as well as scanning
electron microscopy (SEM).
R. A. Cutler—contributing editor
Manuscript No. 190229. Received April 27, 1998; approved May 20, 1999.
Supported by the National Science Council, Taiwan, under Grant No. NSC87-
2216-E009-014.
*
Member, American Ceramic Society.
J. Am. Ceram. Soc., 82 [11] 3179–85 (1999)
J
ournal
3179
Fig. 1. (a) SEM micrograph of the cross section between titanium and 3-mol%-yttria-stabilized zirconia after reaction at 1750°C for 7 min. (b)
A larger magnification of the marked region in (a) showing the featherlike phase at the grain boundaries of titanium.
Fig. 2. (a) TEM micrograph showing parallel lamellae Ti
2
ZrO in the matrix of ␣-Ti. (b) SADP in the zone axis of [111]
Ti
2
ZrO
or [011]
␣-Ti
. (c)
SADP in the zone axis of [221
]
Ti
2
ZrO
or [012]
␣-Ti
. (d) EDS of Ti
2
ZrO. The Miller indices with a subscript ␣ are for ␣-titanium, while those without
any subscript are for Ti
2
ZrO.
3180 Journal of the American Ceramic Society—Lin and Lin Vol. 82, No. 11
II. Experimental Procedure
Several yttria-partially-stabilized ZrO
2
plates were first
made by tape casting from a slurry of poly(vinyl alcohol),
water, and 3 mol% Y
2
O
3
–ZrO
2
powder (1 m average; First
Rare Earth Inc., Japan), and then sintered in air at 1500°C/2 h.
After sintering, a zirconia plate (∼2 mm thick) was vertically
placed into each 5 mol% CaO–ZrO
2
crucible (ZR-5, Nikkato
Co., Osaka, Japan), and then tightly packed with commercially
titanium powder (−200 mesh, with impurities of 460 ppm Zr,
350 ppm N, 260 ppm O, <1 ppm Na, <10 ppm Ca; T-2045,
Cerac Inc., Milwaukee, WI). The zirconia crucible was then
loaded into an argon atmosphere furnace with tungsten mesh
heating elements (Model 4156, Centorr Inc., Nashua, NH),
whose chamber was evacuated to 10
−4
torr and then refilled
with argon to 1 atm. This cycle of evacuation and purging
was repeated at least twice. It took 30 min to raise the tem-
perature from 100° to 1600°C and 5 min from 1600° to
1750°C, and then it was held at 1750°C for 7 min. The zirconia
plate became immersed in the titanium melt above the melting
temperature of titanium (1670°C), causing an extensive inter-
facial reaction between the ZrO
2
plate and the titanium melt.
During cooling, the temperature was lowered to 1600°C at a
cooling rate of 30°C/min in the furnace, to 1000°C at 50°C/
min. The specimen was then continuously cooled down to
room temperature at 10°C/min. The interface of zirconia and
titanium was observed by using an analytical transmission elec-
tron microscope (Model JEM 2010, JEOL Ltd., Tokyo, Japan)
as well as a scanning electron microscope (Model JXA 6400P,
JEOL Ltd., Tokyo, Japan). Cross-sectional TEM specimens
perpendicular to the interface of zirconia and titanium were
prepared by standard procedures of cutting, grinding, polish-
ing, and ion milling. Quantitative composition analyses were
carried out based on the principle of Cliffs-Lorimer with an
energy-dispersive spectrometer (EDS; Model ISIS 300, Oxford
Instrument Inc., London, U.K.) attached to the transmission
electron microscope.
The oxygen partial pressure of the argon protective atmo-
sphere in the tungsten mesh furnace was quite low (∼10
−5
torr).
In addition, the zirconia plate was tightly packed in titanium
powders that could scavenge the residual oxygen to further
reduce the oxygen partial pressure. In the regions far away
from the interface, the oxygen content of titanium was negli-
gible and the zirconia was slightly reduced to ZrO
1.9
after
firing. Thus, the effect of the residual oxygen content in the
furnace on the interfacial reaction was very limited. In other
words, the compositional and microstructural variations ob-
served in the present study were mainly caused by the interfa-
cial reactions between titanium and zirconia.
III. Results and Discussion
Figure 1(a) is an SEM micrograph of the cross section nor-
mal to the interface of Ti/3Y-ZrO
2
after reaction at 1750°C
for 7 min. Zirconia, at the left of the micrograph, was believed
to be oxygen deficient and dissolved some titanium. At
the right portion of this micrograph, a featherlike phase existed
in the grain boundaries of titanium, being identified as one
of the Ti–Zr–O compounds. Figure 1(b) displays a larger
magnification of the featherlike phase. There were many
pores at both sides of the Ti/ZrO
2
. The larger pores in ZrO
2
were attributed to the Kirkendall effect, since Zr diffused to the
right more rapidly than Ti diffused toward the left. The smaller
pores at the grain boundaries of Ti were oxygen bubbles
formed by the reduction of ZrO
2
. While liquid titanium could
dissolve a significant amount of oxygen, excess oxygen was
likely to accumulate as bubbles along the grain boundaries
of ␣-Ti.
Figure 2(a) displays the lamellar structure of the featherlike
phase. From the selected area diffraction patterns (SADPs)
(Figs. 2(b) and (c)) and the energy dispersive spectrum (EDS)
(Fig. 2(d)), the lamellar phase was identified to be orthorhom-
bic Ti
2
ZrO that precipitated in ␣-Ti with an orientation rela-
tionship of {11
0}
Ti
2
ZrO
//{100}
␣-Ti
and 〈111〉
Ti
2
ZrO
//〈011〉
␣-Ti
.
Figure 3 presents a high-resolution TEM micrograph of
alternative layers of Ti
2
ZrO and ␣-Ti. One likely scenario is
that the lamellae of Ti
2
ZrO precipitated and grew preferably
along the (100) plane of ␣-Ti. ZrO
2
is very soluble in Ti,
allowing a solid solution of ␣-Ti(Zr,O) to form.
16
It is likely
that Ti
2
ZrO precipitated as ␣-Ti(Zr,O) was cooled down to
room temperature. On the transformation of ␣-Ti(Zr,O) →
Ti
2
ZrO, the crystal structure changed from hexagonal to or-
thorhombic. The morphology and location of the lamellar
Ti
2
ZrO are similar to those reported previously.
16
Figures 4(a) and (b) show micrographs of ␣-titanium and
oxygen-deficient zirconia, respectively, which were formed by
the reduction of zirconia by titanium at high temperatures.
They were identified to be hexagonal and cubic, respectively,
in structure by the inset SADPs. The EDS in Fig. 4(c) indicates
that titanium dissolved a small amount of Zr and O, being
designated as ␣-Ti(Zr,O). Meanwhile, oxygen-deficient zirco-
nia (ZrO
1.53
) dissolved 1.8 at.% Ti and retained some yttrium
(5.83 at.%) as indicated by Fig. 4(d). These results indicate that
zirconia was reduced to oxygen-deficient zirconia by titanium,
which was also featured by its dark gray color.
2,17,18
As men-
tioned in the previous section, the O/Zr ratio was compar-
atively large (∼1.9) in the regions far away from the interface.
This implies that the reaction between titanium and zirconia
significantly lowered the O/Zr ratio (∼1.53) of zirconia in the
region near the interface.
␣-Zr was found together with c-ZrO
2−x
near the interface in
the zirconia side. It had a twinned structure with kinks as
shown in Fig. 5(a). The corresponding SADP in Fig. 5(b) con-
firms that the twin plane of ␣-Zr was (101
2). This is consistent
with the results found in the Zr–20% Ti alloy by Banerjee and
Krishnan.
19
The phase diagram of Zr–O displays a two-phase
region of ␣-Zr(O) and c-ZrO
2−x
in a wide region (32.5 at.%
O–63.5 at.% O) at 1750°C, and the solubility of ␣-Zr in cubic
ZrO
2−x
decreased with decreasing temperature.
20
Therefore,
the ␣-Zr(O) has a tendency to precipitate from the supersatu-
rated solid solution of ZrO
2−x
during cooling, and zirconia with
a higher O/Zr ratio could be obtained by the exclusion of Zr.
Fig. 3. HRTEM micrograph of the lamellae of Ti
2
ZrO and ␣-Ti.
November 1999 Transmission Electron Microscope Investigation of the Interface between Titanium and Zirconia 3181
Fig. 4. (a) Bright-field image of ␣-Ti. (b) Bright-field image of c-ZrO
2−x
. (c) EDS of ␣-Ti. (d) EDS of c-ZrO
2−x
.
Fig. 5. (a) Bright-field image of microtwinned ␣-Zr(O). (b) SADP, Z ⳱ [2201]. The Miller indices with a subscript T are for the twins.
3182 Journal of the American Ceramic Society—Lin and Lin Vol. 82, No. 11
Fig. 7. (a) Bright-field image of YO
0.55
adjacent to ␣-Ti. The SADPs of YO
0.55
viewed in (b) Z ⳱ [011] and (c) Z ⳱ [001]. (d) EDS.
Fig. 6. (a) Bright-field image displaying nanocrystalline c-ZrO
2−x
. The ring pattern (b) and EDS (c) of these fine c-ZrO
2−x
.
November 1999 Transmission Electron Microscope Investigation of the Interface between Titanium and Zirconia 3183
![Fig. 8. (a) Bright-field image of Ti3O. (b) SADP,Z 4 [1100]. (c) EDS.](/figures/fig-8-a-bright-field-image-of-ti3o-b-sadp-z-4-1100-c-eds-31fhbcx7.webp)
![Fig. 2. (a) TEM micrograph showing parallel lamellae Ti2ZrO in the matrix ofa-Ti. (b) SADP in the zone axis of [111]Ti2ZrO or [011]a-Ti. (c) SADP in the zone axis of [221]Ti2ZrO or [012]a-Ti. (d) EDS of Ti2ZrO. The Miller indices with a subscripta are fora-titanium, while those without any subscript are for Ti2ZrO.](/figures/fig-2-a-tem-micrograph-showing-parallel-lamellae-ti2zro-in-3ckf1nhr.webp)
![Fig. 5. (a) Bright-field image of microtwinneda-Zr(O). (b) SADP,Z 4 [2201]. The Miller indices with a subscript T are for the twins.](/figures/fig-5-a-bright-field-image-of-microtwinneda-zr-o-b-sadp-z-4-1ln9e4zs.webp)