Advanced Structural Ceramics (Part 2)

264 Pages·179.189 MB·English

by  Basu B., Balani K.

11
Chapter
S - Phase S i AlON Ceramics:
Microstructure and Properties
In this chapter, the microstructure and properties of the newly developed nearly
monophasic S - SiAlON ceramics, based on the composition of Ba Si Al O N
2 12 − x x 2 + x 16 − x
( x = 2 ± 0.2) are discussed. It is shown that the sintering mechanism is based on
liquid - phase sintering with formation of a Ba – Al silicate liquid ( < 5%) at intergranu-
lar pockets. The formation mechanism of the elongated - platelet morphology of
S - phase SiAlON will also be discussed. Another focus of this chapter is to analyze
and interpret the mechanical properties of hot - pressed Ba - doped S - SiAlON ceram-
ics. Crack defl ection by elongated S - phase grains in combination with crack bridging
by β - Si N needles has contributed to the observed high toughness.
3 4
11.1 INTRODUCTION
In view of moderately good toughness and high - temperature strength, silicon nitride
(Si N ) – based ceramics have been investigated as the potential candidate for various
3 4
engineering applications, such as cutting tool inserts, valve seals, sealing rings, and
cylinder liners, as well as for a variety of structural components in high - effi ciency
engines and other mechanical systems.1 Among silicon nitride – based ceramics,
SiAlON ceramics, in particular, have been developed for structural applications,
because of their easier processability and high oxidation resistance. 1 – 8 In the ceram-
ics literature, the solid solutions of α - Si N and β - Si N with Al and O are widely
3 4 3 4
known as α - SiAlON and β - SiAlON, respectively. Of these two variants, α - SiAlON
ceramics are characterized by a nearly single - phase microstructure with better
thermal shock resistance, chemical stability, and high hardness over a large tempera-
ture range. This, coupled with lower density (3.2 gm/cm 3 ), make these materials a
potential candidate for bearing applications. However, the lower strength and frac-
ture toughness remain bottlenecks for their successful application. Among many
approaches attempted so far, “ single - phase i n situ toughened α - SiAlON ” has been
reported to exhibit an optimized combination of hardness and fracture toughness.9 – 14
Advanced Structural Ceramics, First Edition. Bikramjit Basu, Kantesh Balani.
© 2011 The American Ceramic Society. Published 2011 by John Wiley & Sons, Inc.
215
216 Chapter 11 S-Phase SiAlON Ceramics: Microstructure and Properties
Interestingly, a 2007 paper reported good cytocompatibility of rare - earth oxide -
doped SiAlON ceramics, indicating potential biomedical application of SiAlON
materials.7 It needs to be mentioned here that an optimal combination of hardness,
toughness, elastic modulus, and strength is desired for load - bearing implants.
Many of the engineering applications of β ′ / α ′ ceramics remain in the lower -
temperature regime (generally less than 1000 ° C) and a greater market penetration
is restricted largely by economic factors. From this perspective a continuing quest
exists for novel SiAlON ceramics that could be sintered at lower temperatures and
might be more tolerant of less - expensive starting powders. In M – Si – Al – O – N
systems (where M is a cation in Group II of the periodic table), many phases have
comparatively high oxygen- to - nitrogen ratios, with reduced covalency and intrinsic
properties, and the phases with high nitrogen content are diffi cult to obtain with
controlled phase content.1 5,16 A relatively new SiAlON phase is the “ S phase, ” having
the composition M Al Si N O with x ≈ 2. 17,18 The Ba - containing S - phase
2 x 12 −x 16 −x 2 +x
ceramic is reported in this chapter with an aim to illustrate the main features of
microstructural evolution, because it can be readily sintered to the nearly phase- pure
state and it is less susceptible to evaporative loss of the stabilizing cation. A major
focus is to discuss the relationship between microstructure and mechanical proper-
ties of S - SiAlON ceramics.
11.2 MATERIALS PROCESSING AND
PROPERTY MEASUREMENTS
To sinter S - SiAlON ceramics, high purity ( > 99%) commercial powders were used:
BaCO (Aldrich), Si N (Ube grade SNE10), AlN (Starck grade C), and Al O
3 3 4 2 3
(Alcoa). The precursor powders with a targeted S - phase composition of BaAlSi O N
5 2 7
were ball - milled in Si N media for 24 hours and the dried powder mixture was hot
3 4
pressed in BN - coated graphite dies in the temperature range 1600 – 1750 ° C for 2
hours in nitrogen environment.
The density, measured by the Archimedean method, was > 97% of the theoretical
density for all sintering temperatures; the microstructure was examined using x - ray
diffraction ( XRD ) and a scanning electron microscope (SEM) and transmission
electron microscope (TEM) equipped with energy - dispersive x - ray analysis spec-
trometers (SEM - EDS and TEM - EDS).
For all the hot - pressed ceramics the elastic modulus was measured using an
ultrasonic pulse - echo technique. The hardness and indentation fracture toughness of
S - SiAlON materials were determined with a Vickers hardness tester. It should be
noted here that the use of microhardness measurements would cause a large variation
in the observed hardness property of S - SiAlON materials (depending on the location
of indents). In fact, Krell observed that the use of microloads needs to be avoided
for determining hardness properties of various structural ceramics with complex
microstructures.1 9
The indentation fracture toughness K (MPa · m 1/2 ) was estimated using the
IC
formula proposed by Niihara et al.2 0 for median cracks (l / a ≥ 1.5):
11.3 Microstructural Development 217
K =0.0667(l+a)−3/2×P×(E/H)2/5. (11.1a)
IC
H owever, for the Palmqvist type of crack (0.25 ≤ l / a ≤ 2.5), a different formulation
was used2 1 :
K =(0.0181/ l)(P/a)(E/H)2/5, (11.1b)
IC
where P is indentation load (N), 2a is the average indent diagonal length (μ m), 2c
is the crack length (from one crack tip to another), l is the difference of c and a
( μ m), E is the elastic modulus (GPa), and H is the hardness (GPa).
11.3 MICROSTRUCTURAL DEVELOPMENT
The properties of the hot - pressed S - SiAlON ceramics are presented in Table 11.1 .
Note that the highest sinter density (3.65 g/cm 3 ) was measured after hot pressing at
1750 ° C for 2 hours. While theoretical density (∼ 3.6 g/cm 3 ) is achieved at relatively
lower hot - pressing conditions (1600 ° C, 2 hours), the XRD traces (Fig. 11.1 ) reveal
Table 11.1. Summary of the Indentation Data — Crack Length (l ), Indent Diagonal Length
(2 a ) — for the Hot - Pressed S - SiAlON Ceramics4 1
Load 100 N 200 N 300 N
Material l ( μ m) a ( μ m) l / a l ( μ m) a ( μ m) l /a l ( μ m) a ( μ m) l / a
S1600 – 61.0 – 275.0 81.0 3.4 376.0 93.6 4.0
S1700 135.0 55.5 2.4 201.0 83.5 2.4 206.0 94.5 2.2
S1750 84.0 57.0 1.5 137.0 81.0 1.7 191.0 95.5 2.0
Ba-S Phase
β-Si N
3 4
1750 °C
1700 °C
Figure 11.1 XRD spectra
1600 °C showing S - phase crystallization
over a range of hot - pressing
30 35 40 temperature. The sintering time
2θ at each temperature is 2 hours. 40
218 Chapter 11 S-Phase SiAlON Ceramics: Microstructure and Properties
β′
β′
Glass
Ba-S phase
5 μm
Figure 11.2 Backscattered electron (SEM) image showing minor phases (β ′ and residual glass) in
dark and light contrast relative to the major S - phase in an S - SiAlON ceramic, hot pressed at 1700 ° C
for 2 hours. 40
that the solution – reprecipitation reaction for S - phase recrystallization remains
incomplete. XRD spectra also show a trace amount of β ′ - SiAlON and the partial
crystallization of residual liquid to BaAl SiO . The acicular β ′ - phase and the residual
2 6
silicate glass phase in an S - SiAlON ceramic (hot pressed at 1700 ° C for 2 hours) can
be clearly distinguished in backscattered electron (BSE) SEM images (Fig. 11.2 ) in
darker and lighter contrast, respectively. The S - phase has an intermediate Ba content,
which results in the atomic number contrast. Careful phase analysis (semiquantita-
tive) using SEM - EDS reveals that the hot - pressed microstructure contains (4 – 5%)
residual glass and (6 – 8%) α / β - Si N , besides the characteristic S - phase.
3 4
To investigate the important features of the microstructural evolution, an
S - SiAlON ceramic, hot pressed at 1700 ° C, was selected for detailed TEM analysis.
As shown in Figures 11.3 and 11.4 , bright fi eld TEM images display a microstructure
dominated by a contiguous array of elongated platelet crystals of S - phase with a
minor intercrystalline liquid residue at triple - junction channels. The presence of fi ne
β ′ crystals within the S - phase are suggestive that they are formed early via sintering
reaction and, subsequently, act as heterogeneous nucleants for the S - phase. The
aspect ratios (ARs) of S - phase grains lie in the range 3.1 – 4.3, with an average value
of 3.4. The width of S - phase grains (minor axis) varies in the range 0.6 – 1.4 μ m,
while the length (major axis) varies in the range 1 – 5 μ m. The average width and AR
of β - Si N needles were estimated to be 0.57 μ m and 7, respectively. In Figure 11.2 ,
3 4
a typical acicular β - Si N phase, with an AR of 7 – 8, is shown.
3 4
The analysis of the selected area diffraction patterns (SADPs) reveals that the
S - phase crystals exhibit a preferred growth in the [001] orthorhombic axis with
primary facets parallel to (010) and (100) planes (Fig. 11.3 ). TEM - EDS analysis
11.3 Microstructural Development 219
Si
Al Ba
O Ba
N Ba
1 2 3 4 5
O
002
1 μm
020
Figure 11.3 B right fi eld TEM image illustrating the elongated platelet morphology of S - phase in an
S - SiAlON ceramic, hot pressed at 1700 ° C for 2 hours. A selected area diffraction pattern ( SADP ) and
an energy dispersive X - ray (EDX) spectrum taken from the S - phase are also shown in insets.4 0
confi rms an average composition for S - phase as Ba Si Al O N ( x = 2.0 ± 0.2),
2 12 – x x 2 + x 16 – x
but hot pressings with larger variations in x in the initial powder mixture are pos-
sible. The presence of the residual glass phase can be clearly observed in Figure
1 1.4a . TEM- E DS analysis of the glass, as shown in Figure 1 1.4b , reveals that the
glass phase is Ba - r ich aluminosilicate. Based on our analysis, the possible composi-
tion range of the triple - pocket glass is shown in Figure 11.5 .
Many of the S- phase crystals are characterized by stacking - faults, imaged with
characteristic fringe contrast, which have a weak crystallographic preference (Fig.
11.4 ). It is possible that the stacking - faults occur during the growth process and such
defects terminate either on partial dislocations or on the crystal surface. Some rep-
resentative bright fi eld TEM images revealing the defect structure in the S - SiAlON
ceramic are provided in Figures 11.6 and 11.7 . The network of partial dislocations,
appearing in fringes, is observed in Figure 11.6 . In many of the investigated
S - SiAlON grains, closely spaced antiphase boundaries (APBs) can be observed
(Fig. 11.7 ).
220 Chapter 11 S-Phase SiAlON Ceramics: Microstructure and Properties
glass
glass
S
β′
S
APB
β′
200 nm
(a)
ops
20
Ba
15
O
10 Al Ba
5
Ba
Ba BaBa
0
1 2 3 4 5 6
Energy (keV)
(b)
Figure 11.4 (a) Bright fi eld TEM image of S - phase crystals with residual intercrystalline glass,
included β ′ and antiphase boundaries ( APB s) in an S - SiAlON ceramic hot pressed at 1700 ° C for 2
hours. (b) TEM - EDS analysis of the glass phase.4 0
11.4 MECHANICAL PROPERTIES
The elastic modulus, as provided in Table 11.1 , shows a value in the range 210 –
230 GPa, with the highest modulus of 228 GPa measured with the ceramic hot
pressed at 1700 ° C. Some representative SEM images of the indented SiAlON sur-
faces are shown in Figure 11.8 . A high - magnifi cation SEM image (Fig. 11.8 a) did
11.4 Mechanical Properties 221
[Ba Si Al O N ]
2 12–x x 2+x 16–x
Ba N BaO
3 2
x=0
Si3N4 x=2 SiO2
Si N O
2 2
z ~ 1
α′ β′
Glass-forming
compositions
AIN Al O
2 3
Figure 11.5 P hase equilibria of interest showing the possible composition range of the residual
liquid phase, formed at the triple pocket during hot pressing of the investigated S - SiAlON ceramic. 40
200 nm
Figure 11.6 B right fi eld TEM image of S - phase revealing the presence of a network of partial
dislocations in an S - SiAlON ceramic hot pressed at 1700 ° C for 2 hours.4 0
222 Chapter 11 S-Phase SiAlON Ceramics: Microstructure and Properties
100 nm
(a)
Figure 11.7 Bright fi eld TEM
images revealing the details of the
defect structure within an S - phase grain
in an S - SiAlON ceramic hot pressed at
1700° C for 2 hours. The defect structure
is characterized by APBs (formed due
to faults in the framework structure),
infrequently observed to terminate at the
partial dislocation (a) and also by the
100 nm presence of APBs extending between
(b) the facets of an “ S ” crystal (b). 40
11.4 Mechanical Properties 223
50 μm
(a)
Figure 11.8 S EM topography
images of the Vickers indents and
indentation- induced radial crack pattern
100 μm
in the Ba - S - SiAlON ceramic hot
pressed at 1750 ° C and indented at
(b) varying loads: 100 N (a) and 300 N (b). 41
not reveal any signifi cant cracking at the indent edges. Figure 11.8 b also illustrates
well - developed radial – median crack morphology, and multiple cracking from a
single indent corner is generally not observed.
As seen in Table 11.2 , the apparent hardness values lie in the range 8.6 – 16 GPa
at varying loads of 50 – 300 N. A hardness - versus - load curve, plotted in Figure 11.9 a,
indicates that the apparent hardness modestly increases with indent load; such an
observation can be attributed to behavior called the reverse indentation size effect
(RISE).1 8,22 In contrast to the normal indentation size effect ( ISE ), the indented mate-
rial undergoes relaxation, which involves crack formation, dislocation activity, and/
or elastic deformation of the tip of the indenter. 18,22 Therefore, it can be argued that,
750), 0 N K IC1/2Pa · m ±5 0.3 ±1 0.6 ±6 0.2
C (S1 at 30 M 5. 11. 11.
0), and 1750 ° Indentation (GPa) H v ± 16.0 2.0 ± 15.5 0.2 ± 15.3 1.8
0
1700 ° C (S17 n at 200 N K IC1/2(MPa · m ) ± 5.9 0.03 ± 8.5 0.3 ± 12.0 0.8
0 ° C (S1600), Indentatio (GPa) H v ± 13.7 0.9 ± 13.2 0.8 ± 14.1 1.0
0
6
ot Pressed at 1 on at 100 N K IC1/2(MPa · m ) – ± 7.4 0.2 ± 11.6 0.6
S - SiAlON, H Indentati (GPa) H v ± 12.6 0.8 ± 15.1 1.3 ± 14 0.7
of the Basic Mechanical Properties of 41ent Load of 100, 200, and 300 N Indentation at 50 N E (GPa) H (GPa) K vIC1/2(MPa · m ) ±± 215 8.6 0.1 2.4 0.2 ±± 228 12.7 0.4 3.9 0.3 ±± 212 12.5 0.4 3.7 0.1
y nd
Table 11.2. SummarWhen Indented with I Load Density 3(gm/cm ) Material S1600 3.62 S1700 3.63 S1750 3.65
224