High temperature deformation of silicon nitride ceramics with different microstructures

High temperature deformation of silicon nitride ceramics with different microstructures

MATERIALS SCIENCE & ENGIWEERIHG A Materials Scienceand Engineering A206 (1996)45-48 ELSEVIER High temperature deformation of silicon nitride ceram...

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MATERIALS SCIENCE & ENGIWEERIHG

A

Materials Scienceand Engineering A206 (1996)45-48

ELSEVIER

High temperature deformation of silicon nitride ceramics with different microstructures Naoki

Kondo”, “National

Fumihiro

Wakai”, Masamichi Yamagiwab, Akira Yamakawab

Takao Nishiokab,

hdustrial Research Insritute of Nagoya, Hirate-cho, Kila-ku, Nagoya, Aichi bSwniton~o Electric Industries Ltd., Koyakita, Itami, Hyogo 664, Japan

4G2, Japan

Received 16 March 1995; in revisedform 19 May 1995

Abstract

The deformation of some silicon nitride ceramics which consist of elongated p-S&N, grains in a fine-grained matrix containing an amorphous grain boundary phase was studied by tensile testing at elevated temperature. Superplastic elongations larger than 250% could be observed, and characteristic microstructural development during deformation was studied. Keywords:

Silicon nitride; Ceramics; Microstructures; Deformation; Superplasticity

1. Introduction

The strategy for ceramics superplasticity has been microstructural control to achieve a fine (less than about 1 pm) and equiaxed grain size that is reasonably stable during deformation [ 1,2]. The initial microstructures of superplastic silicon-nitride-based ceramics so far reported have been formed on the basis of this strategy, for example, S&N,-SIC composites [3,4], sialon, [2,5-81, and silicon nitride, [9,10]. However, such a fine and equiaxed grain structure lowers the fracture toughness of silicon nitride to less than 4 MPa ml’* [ll]. Most of silicon nitride ceramics with improved fracture resistance consist of elongated j3-Si,N, grains in a fine-grained matrix containing amorphous or partially crystalline grain boundary phases, because the elongated grains can reinforce the matrix by the crack bridging process [ll]. The excellent fracture toughness and strength are advantages of the material as structural components in engines and turbines. The present authors have found that a silicon nitride with a strength of 1.6 GPa and fracture toughness of 6 MPa m’i2 could exhibit superplastic elongation up to 250% recently [12]. The material consisted mainly of very fine and elongated @S&N4 grains. Ceramics SuperpIasticity is not peculiar to a uniform microstructure of fine and equiaxed grains; ceramics 0921-5093/96/S15.00 0 1996- Elsevier ScienceS.A. All rights reserved

with a heterogeneous microstructure containing elongated grains can be superplastic. This finding suggests that some conventional silicon nitrides may exhibit superplasticity. The aim of this paper is to re-examine the deformation of various silicon nitrides at elevated temperatures. Some preliminary results indicate that superplastic forming is applicable to commercially available silicon nitrides.

2. Experimental procedures

Samples used in this study were designated SNl, SN2 and SN3. The sample SNl is a sintered silicon nitride with Y,03, Al,O, and MgO. The mechanical properties and superplasticity of which were reported elsewhere [12,13]. The samples SN2 and SN3 are conventional silicon nitride ceramics. The sample SN2 is a sintered silicon nitride doped with Y,O, and Al,O,. The sample SN3 is a sintered silicon nitride doped with Y,O, and MgO. The mechanical properties of the materials are summarized in Table 1. The chemical compositons of dopants in silicon nitride are given in Table 2. The sample SNl contains 10 vol.% CIphase, but the samples SN2 and SN3 contain no a phase from results of X-ray diffraction analysis.

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IV. Kondo et al. / Materials Science and Engineering A,706 (1996) 45-48

Table 1 Mechanical properties of materials Material

SNl SN2 SN3

Density (x 10” kg mm3)

3.24 3.2 3.2

Bending strength (MPa) Room temperature

1200 “C

1600 1220 860

1100 600 780

Fracture toughness (MPa m’/2)

Hardness (GPa)

6.0 5.7 6.0

17 (Vickers) 15 (Vickers) 15 (Knoop)

Table 2 Chemical compositions of dopants in silicon nitride (wt.%) Material

Y

Al

Mg

0

SNl SN2 SN3

3.1 3.1 3.9

1.7 1.7 -

0.54 2.1

3.9 3.5 4.1

Scanning electron microscopy (SEM) observations were conducted to examine the microstructures of assintered specimens and those of superplastically deformed specimens by using a 25 keV microscope (model JSM-5200, JEOL, Japan). SEM specimens were cut, ground and polished, and then either chemically etched using melted sodium hydroxide salt or plasma etched by CF,. Tensile test specimens were cut from the sintered plates, ground rotationally by diamond wheels and polished by diamond pastes. The specimen had a gauge length of 10 mm and a circular cross-section 3 mm in diameter. The uniaxial tensile tests were performed using a universal testing machine (model 1361, Instron, USA) equipped with SIC fixtures and a furnace with tungsten heating elements. The tests were conducted under a nitrogen atmosphere at 1600 “C, and under a constant cross-head speed with the initial strain rate of 2 x 10m5 s-‘. The tensile tests were started after holding the specimen at the testing temperature for 10 min to homogenize temperature in the specimen.

3. Results and discussion SEM micrographs of chemically etched surfaces of as-sintered materials are shown in Fig. 1. All materials consist chiefly of elongated P-S&N, grains. Voids in the structures are the result of dissolution of matrix phase by excess chemical etching which was conducted to observe three-dimensional microstructures of elongated grains. The average size of elongated grains was largest in SN3 and smallest in SNl. Fig. 2 shows micrographs of plasma-etched surfaces of the materials. Silicon nitride grains were etched away, so that the microstructures were delineated by amorphous grain boundary phase. Large elongated P-S&N, grains are embedded in

Fig. 1. SEM micrographs of chemically etched surfaces of as-received materials: (a) SNl, (b) SN2, and (c) SN3.

a fine-grained matrix. The sizes of matrix grains in SNI and SN3 were smaller than that in SN2. As a consequence, the material SN2 had rather uniform microstructure, while SNl and SN3 had microstructures with a distinctive bimodal distribution of grain size. The average grain sizes of the materials are summarized in Table 3. True stress-true strain curves are shown in Fig. 3. The samples SNl and SN2 could be superplastically deformed up to 250% and 280% respectively. However, the sample SN3 was broken at an elongation of only 40%. The flow stress of SN2 was about 7 MPa initially, and then increased continuously up to 35 MPa at the end of the test. This strain hardening has been reported by other researchers also [3,4,6]. Small peaks are ob-

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Nominal Strain (%) /J&y--

I

0

I

I

I

I

I

I

I

I

0.5

I

III1

I

1

True Strain Fig. 3. True stress-true strain curves at 1600 “C.

Fig. 2. SEM micrographs of plasma etched surfaces of as-received materials: (a) SNI, (b) SN2, and (c) SN3.

served at the beginning of deformation in true stresstrue strain curves of SNl and SN3. Fig. 4 shows the specimen of SN2 with elongation of 280% sectioned in direction parallel to the tensile axis. Elongated P-S&N, grains aligned with their long axis preferentially along the tensile axis. Similar microstructures were observed in the specimen of SNl [l l] and other silicon-nitride-based ceramics [4,6]. The rotation of elongated grains during superplastic deformation brought about this microstructure as described by a simple model [6]. Anisotropic grain growth also accompanied the deformation, so that the diameter changed from 0.6 pm to 0.7 pm and the aspect ratio from 6 to 7. The observation of grain growth clearly reveals the

matter transport through a solution-precipitation process, because the intergranular amorphous phase becomes liquid like at elevated temperatures. This fact suggests that the solution-precipitation creep can take place as an accommodation process of grain boundary sliding in superplasticity [14]. Some cavities were observed in the ruptured specimen as shown in Fig. 5. The cavities were isolated and elongated along the tensile axis. The elongated shape of cavities was reported also in superplastically deformed Y,O,-stabilized tetragonal ZrO, polycrystals [ 151. The origin of strain hardening is supposed to be related to the following microstructural changes during deformation: (1) reorientation of elongated B-S&N, grains [4,6], (2) devitrification and crystallization of amorphous phase (41, and (3) grain growth. However, the observed grain growth was too small to explain the extent of strain hardening. By comparing the microstructure of the deformed specimen (Fig. 4) with the initial microstructure (Fig. 2(b)) both the reorientaiton of grains and a decrease in the amount of intergranular amorphous phase can be noticed. Further quantitative analysis is necessary to determine which process is dominant in the strain hardening. The factors influencing mechanical properties of S&N, at elevated temperatures include (1) size, mor-

Table 3 Grain sizes of materials Material

SNl SN2 SN3

Elongated grains

Matrix grain Diameter (pm)

Diameter (pm)

Aspect ratio

0.5 0.6 0.9

6 6 6

0.2 0.4 0.2

Fig. 4. SEM micrograph of specimen SN2 deformed up to 280% (the tensile direction is horizontal).

IV. Kondo et al. / Materials Science and Engineering A206 (1996) 4.5-48

phase at elevated temperatures. Probably Y,O,-MgOSiO, glass has lower viscosity and lower surface energy than Y,O,-A&O,-SiO, glass [16]. The flow stress of SN3 decreases owing to the lower viscosity of the glass. The threshold stress for cavity formation decreases with decreasing surface energy of the intergranular liquid phase. Thus premature fracture occurred in the deformation of SN3, even though the initial flow stress was lowest among the materials. Fig. 5. SEM micrograph of cavity in ruptured specimen (SN2, ruptured at an elongation of 280%; the tensile direction is horizontal).

References phology and orientation of elongated P-S&N4 grains, (2) size and a:P ratio of matrix Si,N, grains, (3) chemical composition and amount of amorphous phase at grain boundaries, and (4) crystallization of amorphous phase. The superplastical property and also the characteristic nature of the stress-strain curves of each materials are influenced by the above factors. Premature fracture occurred in the deformation of SN3, while the samples SNl and SN2 could be superplastically elongated. The most plausible reason for this is the difference in size of elongated P-S&N, grains. The length and diameter of elongated grains in SN3 were largest among these materials. When large grains are included in the matrix, the stress concentration at the grain boundary will be the origin of fracture during tensile deformation. The second reason for premature fracture of SN3 is the difference in chemical composition of amorphous phase at grain boundaries. The samples SNl and SN2 contains mostly Y,O,-A&O,-SiO, glass, while the material SN3 contains Y,O,-MgOSiOZ glass as shown in Table 2. The difference in chemistry of the amorphous phase changes the viscosity and surface energy of the intergranular amorphous

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