Mat. Res. Soc. Symp. Proc. Vol. 702 © 2002 Materials Research Society
High Toughness Ceramic Laminates by Design of Residual Stresses
Nina A. Orlovskaya, Jakob Kuebler1, Vladimir I. Subotin2, Mykola Lugovy3
Department of Materials Engineering, Drexel University,
Philadelphia, PA, 19104, U.S.A.
1
Department of High Performance Ceramics, EMPA,
Duebendorf, CH-8600, Switzerland
2
Institute for Problems of Materials Science,
Kiev, 03142, Ukraine
3
Université Catholique de Louvain-la-Neuve,
Louvain-la-Neuve, 1348, Belgium
ABSTRACT
Multilayered ceramic composites are very promising materials for different engineering
applications. Laminates with strong interfaces can provide high apparent fracture toughness
and damage tolerance along with the high strength and reliability. The control over the
mechanical behavior of laminates can be obtained through design of residual stresses in
separate layers. Here we report a development of tough silicon nitride based layered
ceramics with controlled compressive and tensile stresses in separate layers. We design
laminates in a way to achieve high compressive residual stresses in thin (100-150 micron)
Si3N4 layers and low tensile residual stresses in thick (600-700 micron) Si3N4-TiN layers.
The residual stresses are controlled by the amount of TiN in layers with residual tensile
stresses and the layers thickness. The fracture toughness of pure
Si3N4(5wt%Y2O3+2wt%Al2O3) ceramics was measured to be of 5 MPa m1/2, while the
apparent fracture toughness of Si3N4/Si3N4-TiN laminates was in the range of 7-8 MPa m1/2
depending on the composition and thickness of the layers.
INTRODUCTION
Layered ceramics with a strong interface is a very promising material for crosscutting
industrial applications since it has high fracture toughness, strength, damage tolerance and
improved reliability [1]. The way to achieve the highest possible mechanical properties is to
control the level of residual stresses in separate layers. There is a possibility to increase the
fracture toughness of ceramics by creating a layer with compressive stresses on the surface
and in such a way, arrest the surface cracks and achieve higher failure stresses. The variable
layer composition, as well as the system's geometry, allows the designer to control the
magnitude of the residual stresses in such a way that compressive stresses in the outer layers
near the surface increase strength, flaw tolerance, fatigue strength, resistance to oxidation
and stress corrosion cracking. In the case of symmetrical laminates, this can be done by
choosing the layer compositions such that the coefficient of thermal expansion (CTE) in the
odd layers is smaller than the CTE of the even ones. The changes in compressive and tensile
stresses depend on the mismatch of CTE's, Young's moduli, as well as on the thickness ratio
of layers (even/odd).
U8.7.1
Silicon nitride is the most promising and well-developed ceramics for structural
application because of its outstanding mechanical properties as well as its superior wear
resistance. The addition of TiN to Si3N4 leads to an increase of Young's modulus, electrical
conductivity, and CTE of Si3N4 ceramics. By varying the amount of TiN in silicon nitride
ceramics, we can increase the CTE/Young's modulus mismatch and develop composites
with compressive and tensile stresses in alternative layers. The ratio of CTE’s in alternate
layers in the case of strong interface should create such state of the material when residual
compression stresses are formed in the thin layers with lower to zero titanium nitride
contents, while the thick layers with its higher contents display tensile stresses [2]. Such
approach should further improve the mechanical properties of laminates.
The goal of this research is to study the interrelation between structure, mechanical
properties, and fracture behavior of complex particulate-layered Si3N4 based composites and
propose an optimal design and processing parameters for their fabrication.
EXPERIMENTAL
The preparation and investigation of Si3N4 based layered composites were done. It
included next stages:
First step included the choice of composition of ceramics for preparation of
multilayered composites and was depended on the coefficient of thermal expansion (CTE, α,
1/K) and Young's modulus (E, GPa) of compounds. Young’s moduli and CTE’s of
composites were calculated by the rule of mixture.
Five compositions of silicon nitride based materials were used:
1) Si3N4 - 5wt% Y2O3 - 2wt% Al2O3; E = 310 GPa; α = 3e-006 1/K
2) TiN; E = 440 GPa; α = 7.2e-006 1/K
3) Si3N4 (5wt% Y2O3 - 2wt% Al2O3) - 20wt% TiN; E = 327 GPa; α = 3.55e-006 1/K
4) Si3N4 (5wt% Y2O3 - 2wt% Al2O3) - 30wt% TiN; E= 336.2GPa, α=3.8 x10-6 1/K
5) Si3N4 (5wt% Y2O3 - 2wt% Al2O3) - 50wt% TiN; E = 358 GPa; α = 4.55e-006 1/K
6) Si3N4 (5wt% Y2O3 - 2wt% Al2O3) - 70wt% TiN; E = 385 Gpa; α = 5.43e-006 1/K
Grinding of mixtures of certain compositions were done in the ball mill for 5 h. Grain
size of powders were measured by "Laser micron sizer".
Second step included addition of binder/plastisizer to batches and roll compaction of
powder mixers. Crude rubber (4wt%) was added to the mixture of powders as a plastisizer
through a 3 % solution in petrol. A roll mill with 40mm rolls was used for rolling. The
thickness of tapes was either 0.4-0.5 or 0.8-1.0 mm, the width - 60-65 mm.
Third step included hot pressing of multilayered composites. Samples of ceramics were
prepared by hot pressing of tapes stacked together. The hot pressing was performed for
silicon nitride based composites at T=1780-1820oC; t=20min; P=30Mpa. The graphite dies
were used for the hot pressing silicon nitride based laminates without protective atmosphere.
Fourth step was mechanical testing and microstructure investigation. Fracture toughness
was measured also by Single-Edge-V-Notched-Beam (SEVNB) method [3]. 4-point bending
strength of the machined specimens was determined using a cell with an inner span of 20
mm and an outer span of 40 mm. Young's modulus was determined at room temperature by
measuring the deflection of samples during 4-point bending tests. Optical and scanning
electron microscopy was used for a microstructure investigation.
U8.7.2
RESULTS
Residual stresses calculation
In each layer, the total deformation after sintering is the sum of an elastic component
and of a thermal component [4]. The residual stresses in the case of a perfectly rigid bonding
between the layers of a two-component material are:
E ' E ' f (α − α )∆T
σ R1 = 1 2 2' T 2 ' T 1
(1)
E1 f1 + E2 f 2
and
E ' E ' f (α − α )∆T
σ R1 = 1 2 1' T 1 ' T 2
(2)
E1 f1 + E2 f 2
( N + 1)l1
( N + 1)l2
and f 2 =
, l1 and l2 are the thickness of layers of the first and
2h
2h
second component, α T 1 and α T 2 are the thermal expansion factors of the first and second
components respectively, ∆T is the difference in temperature, h is the total thickness of the
specimen. Using the materials parameters (E and CTE) given above, residual stresses were
calculated. The initial temperature used to determine the residual stresses was assumed to be
1200oC instead of hot pressing temperature of 1800oC. We consider that above 1200oC
layers are sufficiently plastic to have a zero stress state because of plastic or liquid glassy
phases at the grain boundaries. Microphotographs of Si3N4 based laminates with different
compositions of layers are shown in Fig. 1.
where f1 =
A
BB
C
DD
Figure1. Microphotographs (backscattered image) of Si3N4 based laminates with different
composition of layers with tensile stresses. Dark layers are pure Si3N4, bright layers are
Si3N4 with TiN additive. A) Si3N4/ Si3N4-20wt%TiN; B) Si3N4/ Si3N4-50wt%TiN; C) Si3N4/
Si3N4-70wt%TiN; D) Si3N4/TiN.
U8.7.3
These laminates, as well as other laminates were used to calculate residual stresses. It
was not any calculation for laminate with Si3N4-70wt%TiN layers (Fig. 1C), because pure
Si3N4 layers with compressive stresses were much thicker than layers with tensile stresses,
and as a result tensile stresses are much higher than compressive ones for this particular
composite. The examples of calculated residual stresses are shown in Table 1.
Table 1. Examples of calculated residual stresses of Si3N4 based laminates
Composition
σcompressive,
Thickness of Layers, µm
MPa
Si3N4 with TiN
Si3N4
Si3N4/Si3N4 –20wt%TiN
Si3N4/2(Si3N4 –20wt%TiN)
Si3N4/Si3N4 –50wt%TiN
Si3N4/TiN
250
250
200
200
210
420
330
400
129
175.5
527
1622
σtensile,
MPa
164.5
104.4
323
799
Mechanical properties and microstructure of multilayered composites
Mechanical properties of selected silicon nitride based layered materials are shown in
the Tables 2. As one can see from the Table 2, while laminates with no CTE mismatch in
between layers (#1 Si3N4/Si3N4 laminates) show no improvement in mechanical properties
over virgin Si3N4 ceramics (σf = 550MPa; K1C = 5.4 MPa.m1/2), there is an improvement in
KIC for the laminates with high compressive stresses in thin Si3N4 layers (laminates #3,
Table 2). Laminates with a significant CTE’s mismatch between layers (#4, 5 and 6) has a
lot of defects, originated during a cooling of the materials. These defects are present since
the tensile stresses can be in excess of the critical tensile stress and material simply cannot
survive during the cooling from the hot pressing temperature. Defects in hot pressed
multilayered ceramics originated during cooling stage of processes and appeared due to
mismatch of CTE’s and E moduli.
Table 2. Mechanical properties of Si3N4 based layered composites with different layer
compositions
E, Gpa
K1C, MPa.m1/2
#
Composition
σf, MPa
1
Si3N4/Si3N4
307.7
500.8±117.5
5.8±0.22
2
Si3N4/Si3N4-20wt%TiN
312.9
356.2±76.4
7.41±1.79
3
Si3N4/2(Si3N4-20wt%TiN)
450.4±82.9
8.5±0.01
4
Si3N4/Si3N4-50wt%TiN
297.7
157.9±14.9
5
Si3N4/Si3N4-70wt%TiN
226.9
185±6.5
4.59±0.35
6
Si3N4/TiN
157.4
140.8±10.9
3.97±0.52
The results of the study show that laminated materials on silicon nitride bases with
different CTE/Young's modulus of adjacent layers have residual stress fields, which can
effect crack propagation, and that such structures can augment the fracture characteristics of
a brittle material. The strength and crack propagation behavior of multilayered ceramics are
not generally limited by bulk properties but rather by the local interaction of cracks with such
stress fields. The selection of a crack trajectory in such bimaterial layered structures is
determined by a mutual competition between the direction of ‘maximum’ mechanical driving
U8.7.4
force and the weakest microstructural path. The crack is subject to deviation while it
approaches the centerline of layers with compressive stresses. When the crack enters the
layer with compressive stresses, tensile stresses appear near the edges of the crack. These
tensile stresses are parallel to the free surfaces (crack moving direction). They appear
because compressive stresses, which are perpendicular to the free surfaces, have to become
zero. The tensile stresses are maximal at the centerline of layers. KII ≠ 0 in the direction
parallel to the crack propagation at the centerline of layers with compressive stresses, but KII
→ 0 in the direction perpendicular to the crack propagation. As shown in [5], when
compressive stresses are very high (~ 2 GPa), the crack not only deviates at the centerline,
but also bifurcates, preventing the catastrophic failure of a sample during bending test. It was
noticed that as the crack continues to extend, it seeks the midplane of the layer with high
compressive stresses. Compressive stresses, as well as thickness of layers, influence crack
deviation and bifurcation behavior.
Fracture surface of Si3N4/Si3N4-50wt% TiN is shown in Fig. 2. As we can see from the
figure, there is a high roughness of the surface, and the crack bifurcation of moving crack
occurred when it approaches Si3N4 layer with residual compressive stresses, while at Si3N450wt%TiN layers there are fracture steps which are perpendicular to the interfaces of
composite. The fracture steps appeared only in layers with tensile stresses.
Channel cracks, debonding cracks were observed in the laminates with a difference in
composition of layers starting from 70wt% TiN content and higher. Cracks which appeared
in pure TiN layers with tensile stresses (Fig.3) do not have a consistent crack opening
displacement, they seems to have formed by a linkage of series of pores that roughly outlines
the crack path. These pores have been formed as a result of the tensile stress that developed
during cooling of Si3N4/TiN laminates that have the biggest CTE’s mismatch among all
investigated composites. As we can see from data on tensile stresses in TiN layers (Table 1),
when the value of residual tensile stresses approach to the value of strength of laminates they
generate cracks in the layers. To reduce or eliminate cracking it is necessary to make
composites with more close characteristics of the layers, especially CTE’s and elastic
moduli. The extent of tunnel cracking was decreased in laminates with Si3N4-70wt%TiN
layers in comparison with composites where one of the layers was pure TiN and was fully
eliminated for composites with Si3N4-20wt%TiN layer’s composition. Absent of pre-existent
cracks resulted in increase in mechanical properties, fracture toughness in particular.
A)
B)
C)
D)
Figure 2. Fracture surface of Si3N4/Si3N4-50wt% TiN composite. A) and C) SEI image; B)
and D) backscattered image.
U8.7.5
A
B
C
Figure 4. Showing the cracking/porosity defects which occurs in TiN layers of hot pressed
Si3N4/TiN laminate. Note that the cracking/porosity is transverse to the layer direction,
relieving the stresses, which are generated during hot pressing/cooling composite. A)
General view; B) TiN layer; C) Interface between Si3N4/TiN layers in laminate composite.
White phase in Si3N4 layer is particles of TiN, which were introduced during polishing of
surface.
CONCLUSIONS
Processing of multilayered silicon nitride-based composites and investigation of their
microstructure and mechanical properties such as strength, fracture toughness, and Young’s
modulus was done. Processing of multilayered ceramics with a strong interface includes
milling of powders, rolling and hot pressing of ceramic laminates. Crack bifurcation during
fracture processes was observed at the fracture surfaces by SEM investigation. The
maximum fracture toughness equal 8.5 MPa.m1/2 was obtained by SEVNB method on
Si3N4/2(Si3N4-20wt%TiN) layered composite. This composition of laminates is considered
as the most promising for the further research.
ACKNOWLEDGMENTS
This research was supported by the European Commission INCO-Copernicus grant
ICA2-CT-2000-10020 'LAMINATES'.
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U8.7.6