Machinable α-SiAION/BN composites

Article abstract of DOI:10.1111/j.1551-2916.2006.01022.X

Dense machinable α-SiAlON/BN composites were fabricated by hot-pressing using turbostratic boron nitride (tBN) obtained from nitridation of melamine diborate. The tBN was added to the starting powders, or introduced as a coating that formed in situ on α-Si₃N₄ carrier powders during nitridation, and was subsequently converted to hexagonal boron nitride (hBN) during hot pressing by solution reprecipitation. These composites maintain high strength at 1000°C and their strength/hardness are much higher than similar composites prepared using commercial hBN powder, which yielded a coarser microstructure. Good machinability was achieved despite a flat R curve. The American ceramic society.

Full text of DOI:10.1111/j.1551-2916.2006.01022.X

J. Am. Ceram. Soc., 89 [7] 2147–2153 (2006)
DOI: 10.1111/j.1551-2916.2006.01022.x
r 2006 The American Ceramic Society
ournal
J
Machinable a-SiAlON/BN Composites
w
Roman Shuba and I-Wei Chen
Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272
Dense machinable a-SiAlON/BN composites were fabricated
by hot-pressing using turbostratic boron nitride (tBN) obtained
from nitridation of melamine diborate. The tBN was added to
the starting powders, or introduced as a coating that formed in
situ on a-Si N carrier powders during nitridation, and was sub-
sequently converted to hexagonal boron nitride (hBN) during
hot pressing by solution reprecipitation. These composites main-
tain high strength at 10001C and their strength/hardness are
much higher than similar composites prepared using commercial
hBN powder, which yielded a coarser microstructure. Good
machinability was achieved despite a flat R curve.
composition shifts the phase assemblage from single-phase a-
SiAlON to two-phase a-SiAlON/b-Si N .
The objective of this work is to prepare nanosized BN pre-
3
4
1
3–15
cursor using a stoichiometric melamine borate salt process,
and to use this precursor to fabricate a-SiAlON/BN composites.
The size effect of BN particles on microstructure development
and mechanical properties of the composites will also be
examined.
3
4
II. Experimental Procedure
The nominal composition of a-SiAlON/BN composites was
1.2
Y0.5Si9.3Al2.7O N
14.8 (m 5 1.5, n 5 1.2, or Y1512) with x
vol% BN (x 5 0–20) designated as a-SiAlON/xBN or simply
xBN composite (see Table I). As a sintering aid, 2 wt% of
La , which does not enter the crystal lattice of a-SiAlON, was
added. The starting powders for a-SiAlON were a-Si (SN-
E10, UBE Industries, Ube, Japan), AlN (Type F, Tokuyama
Soda Co., Burlingame, CA), Y (99.995%, APS 25–50 nm,
Alfa-Johnson Matthew Co., Ward Hill, MA), Al (AKP50,
Sumitomo Chemical America, NY), and La (99.995%, APS
80 nm, Inframat Advanced Materials, Farmington, CT). Source
chemicals for BN were H BO (99.5%, Acros Organics, Morris
Planes, NJ) and melamine C (99%, Acros Organics,
,
I. Introduction
ONOLITHIC Si N
3 4
/SiAlON ceramics have excellent mechan-
ical properties but poor machinability, which increases
the fabrication costs. As a remedy, several b-Si N /BN ‘‘nano-
O
2 3
M
3 4
N
3
4
composites’’ have been reported that combine machinability and
O
2 3
1
–4
excellent mechanical properties.
To achieve this goal, it is
2 3
O
imperative to maintain a highly homogeneous dispersion of fine
BN particles. Such dispersion proves difficult for conventional
powder mixing; in addition, the presence of highly refractory
BN sometimes retards sintering. To circumvent these problems,
in situ reaction (forming BN phase in situ during synthesis) is
commonly used in fabricating ‘‘nanocomposites.’’ Such work
has not been reported for a-SiAlON, which is the subject of the
present study.
2 3
O
3
3
3 6 6
N H
Morris Planes, NJ). Commercial hexagonal BN (99.5%, Alfa-
Johnson Matthew Co.) with a plate-like morphology (3–5 mm
diameter) was also used to fabricate reference composites for
5
–9
3 4
comparison. The oxygen content in a-Si N (1.26 wt%) and
In the in situ forming approach for b-Si N /BN ‘‘nanocom-
AlN (0.82 wt%) was taken into account in the formulation. Seed
crystals of Y1512 in the amount of 1% were also added in all
cases to control the microstructure. The procedure for obtaining
3
4
posites,’’ a BN precursor, typically in the form of a mixture of
boric oxide/acid and organic reducing agent (urea, melamine), is
first introduced into the starting powders. After pyrolysis tur-
bostratic BN (tBN) forms, intimately mixed with Si N and ox-
1
6–17
seeds reported elsewhere
using molten KOH, instead of a HNO
was followed with the exception of
1HF mixture, as a
3
4
3
ide powders. During subsequent sintering or hot pressing, tBN is
converted into the thermodynamically stable hexagonal BN
washing agent. This was done to prevent fluoride contamina-
tion that is harmful to the high-temperature properties.
Preparation of composites is schematically depicted in Fig. 1.
Melamine (72 g) was dissolved in boiling distilled water (2.5 L)
(hBN), which is responsible for improved machinability. This
approach was taken by Kusunose et al. and Oku et al., for ex-
In their work, reaction of boric acid and urea in
1–4,10
ample.
and a solution of H BO (90 g, dissolved in 200 mL of water)
3 3
hydrogen gas at 11001C, followed by annealing in nitrogen gas
at 15001C, was used to prepare tBN. The process, however, re-
quires excess urea and a small gas overpressure in order to pre-
vent the loss of urea (melting point 5 1331C), making it difficult
to control stoichiometry. These requirements were also reported
was added to it during stirring. The mixture solution was
cooled to room temperature to obtain melamine diborate
(C
N
H
ꢀ 2H
BO ) salt precipitate. This salt was washed with
3
6
6
3 3
water to remove unreacted boric acid and dried in an oven at
1
1
by Yamamoto, who described a method to obtain tBN by
heating urea and boric acid in a ratio of more than 2:1 in a
pressure vessel at 9501C. The poor stoichiometry control result-
Table I. Summary of SiAlON/BN Composites Described in
the Text
12
ed in a BN product that was typically oxygen rich ; moreover,
formation of hard agglomerates in a urea-based route requires
prolonged milling, contributing to partial oxidation of nitride
constituents. Such powders are unsuitable for fabricating a-Si-
AlON composites because, in our experience, an oxygen-rich
Precursor pyrolysis
temperature (1C) and
gas composition
%
BN
Source of BN
2
0
Si N coated
3
900 (NH3)
1500 (NH )11700 (N )
4
with BN
Si N coated
2
0
3
4
3
2
T. Ohji—contributing editor
with BN
tBN
tBN
0
2
20
, 5, 10, 20
0
900 (NH3)
1500 (NH )11700 (N )
No pyrolysis
3
2
Manuscript No. 21079. Received October 16, 2005; approved February 19, 2006.
This research was supported by the U.S. Air Force Office of Scientific Research, Grant
No. F49620-01-1-0150 and National Science Foundation, Grant No. DMR05-20020.
hBN
BN, boron nitride; tBN, turbostratic boron nitride; hBN, hexagonal boron
nitride.
w
Author to whom correspondence should be addressed. e-mail: roman@shuba.name
2
147

2
148
Journal of the American Ceramic Society—Shuba and Chen
Vol. 89, No. 7
with a span of 12.5 mm at a displacement rate of 0.2 mm/min.
Estimate for the bending modulus was obtained from the same
test using the linear portion of the load–displacement curve.
Here the compliance of the crosshead and fixture was measured
using the same setup but without the sample, and it was sub-
tracted from the total compliance of the actual test with the
sample. The procedure was calibrated using a monolithic a-Si-
AlON sample assuming its Young’s modulus was 300 GPa.
Hardness testing was performed with a Shimadzu Vickers Hard-
ness Tester (HSV-20), loaded at 10 kg for 10 s. Machinability of
composites was evaluated by drilling a hole in a 4-mm-thick
plate of ceramic with a spear-shaped carbide-tipped 1/8 inch
drill bit. Roughness of the drilled surface was measured using a
Dektak IIA (Veeco Inc., Woodbury, NY) profile measurement
system on 1-mm line scans in the drilling direction.
III. Results
(
1) Formation of BN
Melamine diborate precipitates had needle-like crystal morphol-
ogy with a length up to 100 mm and a width of 1–5 mm
Fig. 1. Flowchart of composite fabrication, where tBN stands for
turbostratic boron nitride and hBN for hexagonal boron nitride.
701C for 24 h. It was next pyrolyzed in an alumina tube by
heating, under flowing ammonia gas, at 11C/min to 9001C,
then held for 20 h, resulting in tBN. (As ammonia massively
dissociated into nitrogen and hydrogen during pyrolysis, pre-
cautions were taken against the ignition of hydrogen). To obtain
a tBN product that might be more stable, the pyrolysis temper-
ature was also raised to 15001C, held for 20 h, followed by an-
nealing at 17001C in a graphite furnace for 2 h in nitrogen.
To fabricate a-SiAlON/hBN composites, the tBN powders
2 3 3 4
were mixed with other constituents (Y O , a-Si N , AlN, a-Si-
AlON seeds) and ball milled for 24 h. The mixture was dried and
sifted through a 325-mesh sieve (45 mm openings). Hot pressing
was performed in a graphite furnace in nitrogen atmosphere
(
1 atm pressure) at a uniaxial pressure of 30 MPa. The heating
rate was 151C/min, followed by holding at 19001C for 1 h and
cooling in the furnace (approximately 501C/min down to
1
5001C). During initial heating, the chamber was evacuated
and held for 1 h at 10001C to remove absorbed organic residues
from the powder surface, before the pressure was applied. For
comparison, the same composites were fabricated using com-
mercial hBN powders.
In an attempt to obtain a more uniform dispersion of BN and
Si
as schematically described by the route on the right in Fig. 1.
Melamine diborate precipitates were first mixed with a-Si for
h (ball milling with 10 mm Si balls in an anhydrous is-
3 4 3 4
N , we also directly formed tBN onto Si N carrier particles
3 4
N
2
3 4
N
opropanol liquid medium in a polyethylene jar). This was fol-
lowed by drying in a rotary evaporator under vacuum, followed
by holding in an oven at 701C for 24 h, and then pyrolyzing
3 4
under the same conditions as above. This Si N /BN mixture was
also used to fabricate a-SiAlON/hBN composite after mixing
with other constituents and hot pressing in the same way as de-
scribed above.
Phase composition was characterized by X-ray diffraction
(
XRD) analysis using a diffractometer with CuKa radiation. IR
spectra were acquired with a Perkin-Elmer FTIR System (Mod-
el 2000, Wellesley, MA). Microstructure characterization of sin-
tered samples was performed on fractured or polished sections
using a scanning electron microscope (SEM) JEOL-6400 and
JEOL-6300 (Peabody, MA). The R curves of the hot-pressed
materials were recorded by in situ observation of crack propa-
gation in bars (30 mm ꢁ 2 mm ꢁ 3 mm) cut from the hot-pressed
samples and loaded in four-point bending with an outer span of
Fig. 2. Scanning electron micrographs of (a) as-precipitated C
2
3
N
6
H
6
ꢀ
1
8
2
0 mm and inner span of 10 mm. The flexural strength was
H
3
BO salt, used as a precursor for the preparation of turbostratic
3
measured on bars (30 mm ꢁ 2 mm ꢁ 1.5 mm) with edges cham-
fered at 451 and tensile surfaces polished down to 1 mm with
diamond paste. Samples were loaded in three-point bending
boron nitride (tBN), (b) tBN annealed at 17001C, the bulk retaining the
shape of (a), and (c), (b) at higher magnification showing individual 100-
nm-sized tBN platelets.

July 2006
Machinable a-SiAlON/BN Composites
2149
Fig. 3. X-ray diffraction patterns of tBN prepared by firing to different
temperatures: 9001C (20 h, NH ), 15001C (20 h, NH ), and 17001C
, after pyrolysis at 15001C for 20 h in NH ).
3
3
(
2 h in N
2
3
(
2
Fig. 2(a)). During pyrolysis, water vapor and CO as reaction
products were produced starting at B6001C. After pyrolysis at
001C the product was a white fluffy white powder that retained
9
the shape of the precipitate except for some delamination within
the needles. The mass yield was 100%–105% relative to the
theoretical value expected for pyrolysis in NH , meaning that
3
the conversion was essentially complete. To proceed, the batches
with a yield closest to the theoretical value (in the range 100%–
Fig. 5. Scanning electron micrographs of (a) as-received a-Si
der used as carrier particles for boron nitride (BN), and (b) BN-coated
Si powder (82 wt% Si 118 wt% BN) after pyrolysis at 15001C for
20 h in NH followed by annealing at 17001C for 2 h in N
3 4
N pow-
102%) were selected for subsequent experiments. Pyrolysis at
15001C followed by annealing at 17001C retained the overall
N
3 4
3 4
N
morphology of the precipitate crystals (Fig. 2(b)), but at higher
magnification they revealed fine platelet-like particles with a di-
ameter up to B100 nm and a thickness of B10 nm (Fig. 2(c)).
At this stage the XRD pattern corresponded to that of tBN with
a higher degree of crystallinity than achieved at 9001C (Fig. 3).
FT-IR spectra (Fig. 4) presented further evidence for the for-
mation of BN starting at 9001C, with features at 1360 and 810
3
2
.
Fig. 5(a)), after pyrolysis at either 9001C (not shown) or 17001C
(Fig. 5(b)) the powders maintained the same appearance. Nei-
ther did XRD reveal anything other than a- and b-Si N
3
4
3 4
(Fig. 6(a); a similar amount of b-Si N was also present in the
as-received a-Si
precursor on Si
3
N
4
powders). Apparently, dispersion of BN
carrier particles made it difficult to form
ꢂ1
cm that can be attributed to B–N stretching vibrations and N–
N
3 4
1
2,19
B–N bending vibrations, respectively.
The impurity traces
tBN with a sufficiently high crystallinity or large size to pro-
duce resolvable XRD reflections. It is also noted that hBN, the
(
marked with arrows in Fig. 4) in the 9001C product were very
similar to those observed by Hagio et al., while the product at
1
5
15001 and 17001C revealed only pure BN.
When melamine diborate precipitates were directly mixed
with a-Si N carrier powders (0.5 mm mean particle size, see
3 4
Fig. 6. X-ray diffraction patterns of (a) boron nitride (BN)-coated
Si
but not BN; and various a-SiAlON/20BN composites prepared from (b)
commercial hexagonal boron nitride (hBN), (c) Si /turbostratic boron
nitride (tBN) shown in (a), and (d) tBN. Both Si /tBN and tBN were
products of pyrolysis at 9001C.
3 4 3 4
N powder same as in Fig. 5(b), showing peaks of a- and b-Si N
Fig. 4. FT-IR spectra of boron nitride (BN) prepared by pyrolysis to
different temperatures same as in Fig. 3, compared with those of refer-
ence commercial hexagonal boron nitride (hBN) (top). Features attrib-
uted to impurities marked by arrows. (See text).
3 4
N
3 4
N

2
150
Journal of the American Ceramic Society—Shuba and Chen
Vol. 89, No. 7
Fig. 7. Densification curves of a-SiAlON (filled triangles) and
a-SiAlON/20 boron nitride (BN) composites. Sources for BN were
commercial hexagonal boron nitride (hBN), turbostratic boron
Fig. 8. X-ray diffraction peak height ratio of a-SiAlON (210)/(102) in
monolithic a-SiAlON and a-SiAlON/20BN composites same as in
Fig. 7. Degree of texture increases with BN particle size.
3 4
nitride (tBN), and Si N /BN, in the order of decreasing size.
sized hBN grains with agglomerates less than B2 mm in diam-
eter were seen, as shown in Figs. 9(b)–(c), respectively. These
hBN grains had no apparent preferred orientation even though
some a-SiAlON texture was still evident. Meanwhile, compared
with that of the a-SiAlON monolith (Fig. 9(d)), the size and
aspect ratio of elongated a-SiAlON grains were reduced, as
shown in Figs. 9(b)–(c). The homogeneous distribution of sub-
micron-sized hBN grains was also confirmed by backscattered-
electron (BE) SEM images as, for example, for the composite
that used tBN pyrolyzed at 9001C (Fig. 10). This micrograph
further established that the amount of residual grain boundary
phase, which appears white in Fig. 10 because of a higher Y
content, was quite small despite BN addition. Lastly, the grain
size of hBN was essentially independent of the BN amount in
the range from 5 to 40 vol% (not shown).
thermodynamically stable phase, was not observed in any
pyrolysis product, with or without Si
N .
3 4
(
2) Microstructure of a-SiAlON/BN Composites
Densification curves of a-SiAlON/20BN composites, regardless
of the source of BN, all contained a pronounced low-tempera-
ture densification step that was absent in a-SiAlON without ad-
ditives (Fig. 7). This step was the least evident when using
commercial hBN but was the most pronounced when using
3 4
Si N /tBN. There was also a rapid densification stage (between
T and T in Fig. 7), which has been attributed to nitride dis-
1
2
2
0,21
solution.
constant but the finishing temperature (T
Here the starting temperature (T
1
) was relatively
2
) systematically de-
creased with increasing magnitude of the low-temperature step.
The extra densification step was most likely related to BN dis-
3 4
solution, which was easier for smaller BN sizes, as in Si N /tBN
(
3) Mechanical Properties of a-SiAlON/BN Composites
where BN was well dispersed. XRD patterns of hot-pressed
composites all showed hBN peaks (Figs. 6(b)–(d)). Apparently,
although ‘‘dry’’ a-Si N and BN had no reaction with each other
The mechanical properties of these composites followed system-
atic trends. For conciseness, we will focus on comparing samples
made from commercial hBN powders and those from 9001C
pyrolyzed tBN powders, for which we have most extensive data.
3
4
3 4
up to 17001C (see Fig. 6(a) for 17001C pyrolyzed Si N /BN),
once oxide sintering liquid was available at the initial stage of
hot pressing, BN powders could dissolve and re-precipitate as
hBN.
Analysis of XRD patterns and SEM micrographs indicated a
further correlation between the size of the initial BN and the size
of hBN in the composites, as well as the texture of both hBN
and a-SiAlON. As the X-ray scattering cross-section for BN was
much lower than that of a-SiAlON, the rather strong XRD in-
tensity of the hBN (002) peak at B271 in the composites (Figs.
6
(b)–(d)) was entirely a result of the hBN texture. It follows that
the strongest hBN texture occurred when using hBN (Fig. 6(b)),
and the weakest texture was obtained when using Si /tBN
Fig. 6(c)). Meanwhile, the texture of a-SiAlON, compared with
3 4
N
(
that of monolithic a-SiAlON, as illustrated by the peak height
ratio of I(210)/I(102) shown in Fig. 8, was enhanced by BN ad-
dition when the hBN peak was strong.
The microstructures of a-SiAlON/20BN using different BN
powders are compared in Fig. 9. When hBN powders (diameter
B3–5 mm) were used (Fig. 9(a)), the length of the final inter-
granular BN grains, which have a flaky appearance, was several
micrometers. In support of the XRD evidence of strong texture,
these hBN grains were preferentially aligned in the plane per-
pendicular to the hot-pressing direction. These particles experi-
enced some agglomeration, forming aggregates of up to B10
mm in size. In contrast, when tBN (pyrolized at 9001C) or Si N /
Fig. 9. Scanning electron micrographs of a-SiAlO/20BN composites
prepared from (a) commercial hexagonal boron nitride (hBN), (b) tur-
bostratic boron nitride (tBN) (pyrolyzed at 9001C), and (c) Si N /BN
3
4
3
4
tBN (final annealing at 17001C) was used, primarily submicron-
(annealed at 17001C), compared with (d) a-SiAlON monolith.

July 2006
Machinable a-SiAlON/BN Composites
2151
Table II. Mean Roughness (R ) and Maximum Roughness
a
(
Rmax) of the Drilled Surface in the Microcomposite (from hBN)
and Nanocomposite (from Si N /tBN, Pyrolized at 9001C)
3
4
Roughness
Microcomposite
Nanocomposite
R (mm)
Rmax (mm)
0.61
6.09
0.35
2.78
a
tBN, turbostratic boron nitride; hBN, hexagonal boron nitride.
accompanied by
(
a similar decrease of elastic modulus
Fig. 14). The fracture resistance of the composites was consid-
erably lower than that of a-SiAlON and exhibited a rather flat R
curve (Fig. 15).
IV. Discussion
The melamine–diborate process proved to be more suitable than
the boric acid/urea process for preparing tBN, showing no sign
of precursor melting during pyrolysis. Indeed, even after an-
nealing at 17001C, the shape of salt crystals was preserved by
Fig. 10. Backscattered-electron scanning electron microscope (BE
SEM) image of polished surface of a-SiAlON/20BN composite pre-
pared from turbostratic boron nitride (tBN) (pyrolyzed at 9001C). Dark
areas: BN platelets (average diameter o2 mm). White areas: grain
boundary phase.
BN bundles (Fig. 2(b)), in agreement with similar reports in the
14,15
literature.
The product tBN was of good purity, with a mass
yield close to 100% and low impurity content according to IR
spectra. Although the variable interlayer distances of tBN make
it impossible to determine the crystallite size from XRD, SEM
micrographs showed that each BN bundle was a collection of
flake-like fine crystals B100 nm in diameter (Fig. 2(c)). These
bundles are likely to break up into finer crystallites during mill-
The 5BN composites were not machinable, and the 10BN com-
posites chipped during drilling. The 20BN composites could be
machined easily (Fig. 11) regardless of the source of BN, al-
though some edge chipping was occasionally observed in those
made from commercial hBN powders. This correlates with the
increasing number of intergranular BN particles in composites
with higher BN contents (not shown). The surface roughness of
ing in composite fabrication. For the Si N /BN powders, the
3 4
BN size is probably even smaller as XRD could not detect tBN
22
after pyrolysis (Fig. 6(a)), in agreement with Hojo et al. It was
reported in other studies that under similar annealing condi-
tions, but using urea and boric acid, carrier particles were coated
3 4
the ‘‘nanocomposite’’ (from Si N /tBN precursor) was only half
of that of the microcomposite from commercial hBN (see Table
II). The drill groove was continuous and apparent in the nano-
composite and less so in the microcomposite because of the
track-breakdown caused by the surface damage (Fig. 12). These
observations are in good accord with those reported in Kusu-
2
nose et al.
The hardness of the composites decreased with increasing BN
content; however, composites with a finer microstructure were
harder, with the 20BN composites made from tBN being almost
twice as hard as those made from commercial hBN (Fig. 13). A
size effect was also manifested in the room-temperature flexural
strength of the 20BN composites, being 811730 MPa for the
one made from tBN, 806740 MPa for the one made from
Si N /tBN (both pyrolyzed at 9001C), and 535715 MPa for the
3
4
one made from commercial hBN. Comparing these values with
the strength of B940 MPa of a-SiAlON monolith without BN,
it becomes apparent that a homogeneous addition of fine BN
particles to a-SiAlON is not detrimental to the strength despite
its softening effect on elastic (bending) modulus (Fig. 13). In-
deed, the strength retention of the composites with fine BN was
the same as that of monolithic a-SiAlON up to 10001C, above
which the composite strength decreased rapidly, which was
Fig. 12. Scanning electron micrographs of drilled hole surfaces in (a)
microcomposite and (b) nanocomposite (prepared from Si N /turbos-
3 4
Fig. 11. Hole drilled in a-SiAlON/20BN composite (prepared from
tratic boron nitride (tBN) pyrolized at 9001C). Arrow points to a chip in
microcomposite.
Si
3
4
N /turbostratic boron nitride (tBN)), demonstrating machinability.

2
152
Journal of the American Ceramic Society—Shuba and Chen
Vol. 89, No. 7
Fig. 13. Hardness and elastic (bending) modulus of a-SiAlON/xBN,
prepared using turbostratic boron nitride (tBN) pyrolyzed at 9001C
Fig. 15. R Curves of a-SiAlON and a-SiAlON/20BN composite,
prepared from turbostratic boron nitride (tBN; pyrolyzed at 9001C).
(open symbols) or hexagonal boron nitride (hBN) (solid symbols).
1
,10,19,23,24
with hBN films of a thickness of several nanometers.
composites has low elastic modulus, low hardness, and an an-
isotropic thermal expansion coefficient. (For hBN, E 5 43 MPa,
This thickness is below the resolution of the SEM we used, and
it also would not have resulted in any resolvable XRD peak of
BN. However, although the melamine-diborate process can lead
to a homogeneous mixture of very fine BN particles and other
powders, BN nevertheless dissolves and then re-precipitates dur-
ing liquid phase sintering. Therefore, the final size of hBN in the
composites is considerably coarser, of the order of 1 mm. At this
point, it is not clear whether BN from any precursor processing
routes can avoid solution/reprecipitation, yet undergo turbos-
tratic to hexagonal phase transformation, during liquid phase
sintering.
ꢂ6 ꢂ1
ꢂ6 ꢂ1
K perpen-
dicular to c-axis, hardness 5 1.5 on the Mohs scale and easy
a 5 0.57 ꢁ 10
K
parallel to c-axis, ꢂ0.46 ꢁ 10
2
8
cleavage along the basal plane.) During cooling hBN flakes
cleave easily perpendicular to the c-axis, forming microcracking,
2
7
as shown by Zhang et al. Microcracks can also form at the
interface between BN and the matrix. Size effect of BN particles
is related to the stress concentration around an elastically com-
pliant inclusion or microcrack. Thus, composite fracture is con-
trolled by the largest flaw size, which is probably correlated to
the size of the hBN platelet. The size effect on hardness may be
due to a similar reason, in that hBN platelet acts as an elastically
and plastically soft inclusion, causing a shear-stress concentra-
tion that induces plastic deformation nearby. This stress con-
centration also depends on the size of the platelet, and hence
larger hBN platelets have a larger softening effect on the com-
posite hardness.
Although BN coarsens during liquid phase sintering, its pres-
that inhibits the
1
,25,26
ence apparently provides Zener pinning
growth of a-SiAlON grains. As the pinning effect is stronger the
smaller the size of BN, it explains the different microstructures
in Fig. 9. In addition, as the hot-pressing texture typically in-
creases with the size of elongated grains, an associated BN size
effect on composite texture also results (Fig. 8). Similar texture
On the other hand, the weakening effect of second-phase
platelets on the elastic modulus of a composite is size independ-
ent; instead, it scales with the product of aspect ratio and the
observations on b-Si
N
3 4
3 4
grains in b-Si N /BN composites were
1
reported by Kusunose et al. Grain growth inhibition by BN
6
29,30
was also observed by Zhang and Ohji on SiC/BN and b-Si N /
SiC/BN composites, and by Kusunose et al. on b-Si N –BN
volume fraction of the platelets or microcracks.
The lack of
3
4
2
7
1
a size effect is consistent with our results in that the composite
moduli are independent of the source of BN, which have dif-
ferent diameters (Fig. 13). However, the modest modulus re-
duction, about 20% in 20BN composites at room temperature,
is smaller than expected for those hBN platelets studied here,
which have an aspect ratio considerably greater than one. This is
probably because, in bending, tensile and compressive stresses
on the two sides of the neutral axis are balanced, but only the
tensile side experiences a weakening effect of the microcracks,
whereas on the compressive side the shut cracks are still load
bearing. Therefore, only 50% of the hBN platelets contribute to
elastic weakening, and hence only a modest modulus reduction
results.
3
4
composites.
The mechanical properties of our composites manifest an-
other BN size effect: the smaller the BN particles, the higher
the strength and hardness. Note that hBN platelet in the final
Lastly, our observation of good machinability, high strength,
and lower, flatter R curve of the composites strongly suggests
that these properties are not directly correlated to each other. As
already mentioned, strength is controlled by the largest flaw size.
Therefore, high strength can still be achieved even if the com-
posite contains many weak inclusions, provided the inclusions
are small enough. It is also well known that the toughening ef-
fect (including rising R curve) due to second-phase bridging and
3
1
pullout decreases with the particle size. This, along with Zener
pinning that reduces the grain size of the matrix, explains the
low toughness of the composites that contain very small hBN
platelets. On the other hand, improved machinability is attrib-
uted to the weak interface afforded by hBN platelets, allowing
grain detachment and quasi-plastic deformation under machining
Fig. 14. Temperature dependence of strength and elastic (bending)
modulus of a-SiAlON (solid symbols) and a-SiAlON/20BN composite
(open symbols), prepared using turbostratic boron nitride (tBN; pyro-
lyzed at 9001C), with similar strength and modulus retention at 10001C.

July 2006
Machinable a-SiAlON/BN Composites
2153
8
Y. Uemura, S. Nisimura, T. Sato, and M. Tansho, ‘‘BN–Si
N
3 4
Composite
stresses. We may therefore expect that the size of the machining
chips formed is commensurate with the spacing between hBN
platelets, and that a small chip size in turn prevents the forma-
tion of critical machining flaws that can cause premature frac-
ture. With hBN platelets of the same size, an increase in hBN
content should decrease the chip size and lead to better mach-
inability. This is consistent with the microstructure observed in
our study, in which the hBN size appeared relatively constant
regardless of the BN volume fraction.
6
Synthesized by Direct Nitridation of SiB Alloy,’’ Key Eng. Mater., 206 [2] 1137–
4
0 (2002).
9
W. S. Coblenz and D. Lewis III, ‘‘In Situ Reaction of B
2 3
O with AlN and/or
3 4
Si N to Form BN-Toughened Composites,’’ J. Am. Ceram. Soc., 71 [12] 1080–5
(
1988).
10
T. Oku, T. Kusunose, T. Hirata, N. Sato, R. Hatakeyama, K. Niihara, and K.
Suganuma, ‘‘Formation and Structure of Ag, Ge and SiC Nanoparticles Encap-
sulated in Boron Nitride and Carbon Nanocapsules,’’ Diamond Relat. Mater., 9
[3–6] 911–5 (2000).
2
11
O. Yamamoto, ‘‘Crystalline Turbostratic Boron Nitride Powder and Method
for Producing Same’’; U.S. Patent No. 6,306,358, October 23, 2001.
M. Hubacek, T. Sato, and T. Ishii, ‘‘A Coexistence of Boron Nitride and Boric
12
Acid,’’ J. Solid State Chem., 109, 384–90 (1994).
K. Koeda and C. Ito, ‘‘Process for Producing Boron Nitride’’; U.S. Patent No.
V. Conclusions
13
4
,562,050, December 31, 1985.
F. Fauzi, M. Tani, and M. Suzue. Boron Nitride and Process for Preparing the
Nanosized tBN powder can be prepared by pyrolysis of mela-
powders
14
mine diborate salt in ammonia gas. Similarly, a-Si
N
3 4
Same U.S. Patent No. 6,319,602, November 20, 2001.
15
T. Hagio, K. Kobayashi, and T. Sato, ‘‘Formation of Hexagonal BN by
Thermal Decomposition of Melamine Diborate,’’ J. Ceram. Soc. Jpn., 102 [11]
coated with BN can be obtained when carrier a-Si N powders
3
4
are mixed with melamine diborate before pyrolysis.
1
051–4 (1994).
Using nanosized tBN or BN-coated Si
N
3 4
powders, a-Si-
16
M. Zenotchkine, R. Shuba, J. S. Kim, and I. W. Chen, ‘‘Synthesis of a-Si-
AlON Seed Crystals,’’ J. Am. Ceram. Soc., 84 [7] 1651–3 (2001).
AlON/BN composites with homogeneously distributed micron-
sized hBN platelets have been obtained. In contrast, composites
produced from commercial coarse-grained hBN powders con-
tained much larger hBN platelets with an overall coarser micro-
structure.
The size and distribution of the hBN platelets were found to
be the determining factors for the mechanical properties of the
composites. Coarse hBN platelets cause significant strength and
hardness degradation, whereas composites with fine hBN plate-
lets have strength close to that of monolithic a-SiAlON. Com-
posites with 20 vol% of fine hBN are machinable despite a flat R
curve.
17
M. Zenotchkine, R. Shuba, and I-W. Chen, ‘‘Liquid-Phase Growth of Small
Crystals for Seeding Alpha-SiAlON Ceramics,’’ J. Am. Ceram. Soc., 87 [6] 1040–6
2004).
(
18
M. Zenotchkine, R. Shuba, J. S. Kim, and I-W. Chen, ‘‘R-Curve Behavior
of In Situ Toughened—SiAlON Ceramics,’’ J. Am. Ceram. Soc., 84 [4] 884–6
(2001).
D.-F. Lii, J.-L. Huang, L.-J. Tsui, and S.-M. Lee, ‘‘Formation of BN Films on
Carbon Fibers by Dip-Coating,’’ Surf. Coat. Technol., 150 [2–3] 269–76 (2000).
M. Menon and I. W. Chen, ‘‘Reaction Densification of a-SiAlON: I. Wetting
Behavior and Acid–Base Reactions,’’ J. Am. Ceram. Soc., 78 [3] 545–52 (1995).
M. Menon and I. W. Chen, ‘‘Reaction Densification of a -SiAlON II. Dens-
ification Behavior,’’ J. Am. Ceram. Soc., 78 [3] 553–9 (1995).
19
20
21
0
22
J. Hojo, K. Eto, M. Uehara, and N. Enomoto, ‘‘Spectroscopic Evaluation of
Nanocomposite Formation from Amorphous Complex Compound in Si –BN
System,’’ Scripta Mater., 44 [8–9] 2169–72 (2001).
3 4
N
23
J. Li and L. Gao, ‘‘Preparation of h-BN Nano-Film Coated a-Si
3 4
N Com-
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