UDK 666.3/.7:537.311.3
Original scientific article/Izvirni znanstveni članek
ISSN 1580-2949 MTAEC9, 44(1)31(2010)
PREPARATION OF Si3N4-TiN CERAMIC COMPOSITES
PRIPRAVA KERAMIČNIH KOMPOZITOV NA OSNOVI Si3N4-TiN
Aljoša Maglica, Kristoffer Krnel, Tomaž Kosmač
Engineering Ceramics Department, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
aljosa.maglica@ijs.si
Prejem rokopisa - received: 2009-07-24; sprejem za objavo - accepted for publication: 2009-09-04
In this work we report on the preparation of particulate ceramic composites based on a Si3N4 or SiAlON matrix phase. The composites were prepared with reaction sintering of Si3N4/TiO2 powder mixtures using Y2O3, Al2O3 and, in case of SiAlON, also AlN as sintering additives. The results of X-ray diffraction investigation confirmed that TiN was formed during sintering in a nitrogen atmospere with chemical reaction of Si3N4 and TiO2. A comparison of the materials sintered with addition of TiO2 in the starting-powder mixture with the matrix-phase ceramics (Si3N4 or SiAlON) showed that materials with addition of TiO2 have higher densities and better flexural strength. The electrical conductivity of the sintered composites with addition of TiO2 in the starting powder mixture were also investigated. Their electrical conductivity was found to be highly dependent on the amount of added titania and on the sintering conditions.
Key words: Si3N4, TiN, electrically conductive ceramics, ceramic heater
V delu poročamo o pripravi delčnih keramičnih kompozitov na osnovi matrične faze iz silicijevega nitrida ali SiAlON-a. Kompozite smo pripravili z reakcijskim sintranjem mešanice prahu Si3N4 in TiO2, kot dodatke za sintranje pa smo uporabili Y2O3 in Al2O3, v primeru SiAlON-ov pa še AlN. Rezultati rentgenske difrakcije so potrdili, da med sintranjem v dušikovi atmosferi pri reakciji TiO2 s Si3N4 nastane TiN. Ko smo primerjali kompozite, pripravljenje z dodatom TiO2 v začetni mešanici z materiali matrične faze (sintrani Si3N4 ali SiAlON), smo ugotovili, da dosežemo višjo gostoto in boljšo upogibno trdnost pri materialu z dodanim TiO2. Raziskali smo tudi električno prevodnost kompozitov z dodanim TiO2 v začetni mešanici in ugotovili, da je njihova električna prevodnost odvisna od dodane količine TiO2 in od pogojev sintranja. Ključne besede: Si3N4, TiN, električno prevodna keramika, keramični grelec
1 INTRODUCTION
Recently, much attention has been devoted to the production of particulate-reinforced silicon nitride and SiAlON materials, not only because of their improved fracture toughness, strength and mechanical reliability, but also because of their potential multi-functionality, especially their electrical conductivity, which can be obtained with incorporation of electrically conductive particles into the matrix phase1. The most commonly used electroconductive particles are WC, MoSi2, TiN, TiC, TiCN, TiB2 and ZrN2,3. Electro-conductive composite ceramics with good thermal conductivities are interesting for the production of various heating elements, such as ceramic glow plugs, igniters, ceramic heaters, etc4,5. Such materials have also received great attention due to their compatibility with electrical discharge machining6,7. The electrical conductivity of ceramic materials is strongly affected by the distribution of the electrically conductive second phase, and so many studies have been performed to increase the electrical conductivity of ceramic materials8,9.
Composites of silicon nitride and titanium nitride have been investigated in order to obtain a combination of high hardness, high strength, good fracture toughness, and low electrical resistivity10,11,12. Some attempts were made to use a silicon nitride matrix with titanium nitride as the conductive phase. There are three main processing
routes to fabricate the Si3N4-TiN composites13. The first is conventional liquid-phase sintering of Si3N4 and TiN powder mixtures with a small amount of sintering additives. The second one is the chemical vapour deposition (CVD) method using gas components of SiCl4-TiCl4-NH3-H2. The third method, where Si and Ti are used as the starting materials, is called an "in-situ synthesis" process. By contrast to the conventional powder processing route and the CVD process, the in-situ chemical reaction method enables the production of composites with the following advantages: low cost, desirable microstructures and enhanced sinterability.
The aim of the work was to prepare Si3N4/TiN and SiAlON/TiN ceramic composites with sintering in a nitrogen atmosphere by the in-situ chemical reaction of Si3N4 and TiO2. The microstructure, mechanical properties and electrical conductivity of the composites were investigated. The results indicated that the flexoral strength improved and that electrically conductive materials can be prepared if the amount of added TiO2 is sufficient.
w(a)/w((a + /3) - Si3N4) = 0.8
2 EXPERIMENTAL
The starting powders used were Si3N4 SILZOT HQ (SKW, DE, d50 = 1.7 pm; BET =3.2 m2/g, w (a) / w
(a+/3)-Si3N4) = 0.8, Al2O3 (Alcoa, USA, d50 = 0.5 pm; BET = 3-7 m2/g), Y2O3 grade fine (H. C. Starck, DE, d50 (max) = 0.9 pm; BET = 10.0-16.0 m2/g), AlN grade C (H. C. Starck, DE, d50 = 1.2 pm; BET = 4.1 m2/g) and TiO2 RC8 (Cinkarna Celje, SLO, rutil, d50 = 0.35 pm; BET = 6.5-8.5 m2/g). The nominal compositions for the preparation of Si3N4 and SiAlON with and without the addition of TiO2 are listed in Table 1. The powders were mixed with Si3N4 ball milling in a planetary mill for 2 h in isopropanol. After evaporation of isopropanol using a rotating evaporator, the dry powder mixture was cold pressed at 100 MPa into bars with dimensions of 3 mm x 5 mm x 42 mm and subsequently cold isostatically pressed at 790 MPa. The pressed samples were then sintered at 1800 °C for 2 h in nitrogen atmosphere. The sintered samples were examined using X-ray powder diffraction (D4 Endeavor, Bruker-AXS, Germany) and a scanning electron microscope (SEM, Jeol-5800, Japan). The density of the sintered samples were determined using Archimedes' method. The flexural strength was measured on an Instron-1362 testing machine (Instron, USA), using the four-point bending method with a lower-span length of 20 mm and an upper-span length of 10 mm, and a crosshead speed of 1 mm/min. The bodies for the strength tests had the following dimensions: 2.3 mm x 3.9 mm x 38.5 mm. The electrical resistivity of the sintered specimens with dimensions of 2.3 mm x 3.9 mm x 19.2 mm was measured on a Multimeter 3457A testing machine (HP, USA) using four-probe measurements at room temperature (25 °C) with a direct current.
Table 1: Compositions of the starting powder mixtures (mass fractions, w/%)
Tabela 1: Začetna sestava mešanic prahov (masni delež, w/%)
Compositions	Si3N4	Y2O3	Al2O3	AlN	TiO2
SN	92	5	3	0	0
SN/TiO2	82.8	4.5	2.7	0	10
SiAlON	83.9	2.2	5.5	8.4	0
SiAlON/TiO2	75.5	2	4.9	7.6	10
3 RESULTS AND DISCUSSION
3.1 Ceramic materials based on SisNs
The X-ray diffraction pattern of Si3N4 with and without the addition of TiOa in the starting mixture (denoted as SN and SN/TiOa) after sintering at 1800 °C for 2 h in flowing N2 is shown in Figure 1. In both samples we could observe the presence of the /3-Si3N4 and YAG (Yittrium Aluminium Garnet) phases, while in the sample SN/TiO2 we also observed TiN. The peaks of TiO2 could not be detected, and it could be concluded that the transformation of TiO2 into TiN was completed. During the sintering of such composites the following chemical reactions take place141516:
6TiO2 + 4Si3N4 ^ 6TiN + 12SiO (g) + 5N2 (g) (1)
6TiO2 + 2Si3N4 ^ 6TiN + 6SiO2 +N2 (g)	(2)
Figure 1: X-ray analysis of SN and SN/TiO2 after sintering at 1800 °C for 2 h in flowing N2
Slika 1: Rentgenska analiza materiala SN in SN/TiO2, sintranega pri 1800 °C 2 h v pretoku N2
The TiO2 and Si3N4 react in the temperature range from 1150 °C to 1350 °C (equations 1 and 2) in a nitrogen atmosphere. In both reactions gaseous species are formed, which can influence the density of the final
Figure 2: Microstructure of sintered silicon nitride ceramic at 1800 °C for 2 h in N2: a) SN and b) SN/TiO2
Slika 2: Mikrostruktura sintrane silicijeve nitridne keramike pri 1800 °C, 2 h v N2: a) SN in b)SN/TiO2
Figure 3: The SEM micrographs of fracture surface of sintered silicon nitride samples: a) SN and b) SN/TiO2
Slika 3: SEM-posnetki mikrostruktur prelomov sintranih vzorcev silicijevega nitrida: a) SN in b) SN/TiO2
sample, especially in the samples with larger amounts of titania.
The microstructures of the SN and SN/TiO2 composites are presented in Figure 2. In both SEM images (Figure 2a and 2b) we can see elongated yÖ-Si3N4 grains (dark region), a brighter transient liquid phase based on Y2O3 and Al2O3, and some black pores. The material with the addition of TiO2 in the starting-powder mixture (Figure 2 b) also indicates white TiN particles with size around 0.5-1.0 ^m. The bright TiN particles were analyzed with EDXS, and the results confirm the presence of Ti and N.
Figure 3 shows the fracture surfaces of sintered SN and SN/TiO2 samples. As shown in Figure 3 a, the matrix )Ö-Si3N4 grains are surrounded by the secondary bright phase, while in Figure 3 b, the white TiN particles are located in between the yÖ-Si3N4 grains. From the SEM micrographs we can conclude that these materials have intergranular fracture, which is characteristic for this kind of ceramic1718.
The materials SN and SN/TiO2 (Table 2) exhibited around 90 % relative density, and suitable flexural strengths. However, due to the cheaper submicron silicon nitride powder and the pressureless sintering process the relative density of materials could not be higher than 97 %. Sample SN/TiO2 had a higher flexural strength due to the presence of TiO2 in the starting-powder mixture, which contributed to the larger amount of transient liquid phase. The electrical conductivity of this composite was relatively low, because it had the mass fraction only 10 % of conductive phase, which is not enough to exceed the percolation threshold for particles of this size.
Table 2: Comparison of a relative density, a flexural strength and a electrical conductivity of sintered Si3N4 samples Tabela 2: Primerjava relativne gostote, upogibne trdnosti in električne prevodnosti sintranih vzorcev Si3N4
Material	Prel./%	afl./MPa	ael. /(^ m)-1
SN	89.8	380	ND*
SN/TiO2	90.0	410	7.1 • 10-8
* ND _ the measured electrical resistivity is higher from the range of measurement / izmerjena električna upornost je večja od območja merljivosti
3.2 Ceramic materials based on a SiAlON matrix phase
For the formation of the SiAlON matrix the Si3N4 powder with the addition of sintering additives such as AlN, yttria and alumina was chosen (Table 1), together with and without the addition of TiO2 in the starting-powder mixture. The phase analysis was conducted on the sintered specimens with and without the addition of the mass fraction of 10 % of TiO2 in the starting-powder mixture (denoted as SiAlON and SiAlON/TiO2) using XRD analysis. The results of the XRD analysis, presented in Figure 4, revealed that in the case of sample without TiO2 we could observe ^Ö-SiAlON and some signals from Y2O3, whereas in the SiAlON/TiO2 material again ^-SiAlON was formed together with TiN, implying that complete transfor-
Figure 4: XRD patterns of sintered SiAlON and SiAlON/TiO2 materials at 1800 °C for 2 h in nitrogen atmosphere Slika 4: Rentgenska difraktograma sintranega materiala SiAlON in SiAlON/TiO2 pri 1800 °C 2 h v dušikovi atmosferi
Figure 5: SEM microstructure of sintered SiAlON samples: a) SiAlON and b) SiAlON/TiO2 at 1800 °C for 2 h in N2. Slika 5: SEM-posnetki mikrostruktur sintranih vzorcev SiAlONa: a) SiAlON and b) SiAlON/TiO2 pri 1800 °C, 2 h v N2
mation of TiO2 occurred during the sintering process, in accordance with the following chemical reactions16,19-21.
2TiO2+ 3AlN + 5Si3N4 ^ 3Si5AlON7 + 2TiN +
2TiO, + 2AlN
+ 12O2 (g) 2TiN + Al2O3 + 12O2 (g)
(3)
(4)
Figure 6: The SEM micrographs of fracture surface of sintered SiAlON samples: a) SiAlON and b) SiAlON/TiO2. Slika 6: SEM-posnetki mikrostruktur prelomov sintranih vzorcev SiAlONa: a) SiAlON in b) SiAlON/TiO2
Table 3: Comparison of a relative density, a flexural strength and a electrical conductivity of sintered SiAlON samples Tabela 3: Primerjava relativne gostote, upogibne trdnosti in električne prevodnosti sintranih vzorcev SiAlONa
Material	Prel/%	fffl./MPa	ael/(Q m)-1
SiAlON	95	406	ND*
SiAlON/TiO2	96.5	610	1.1 • 10-8
From equation 3 it is clear that TiO2 reacts together with AlN and Si3N4 to form SiAlON, TiN and O2. The second chemical reaction (equation 4) leads to the formation of TiN, O2 and Al2O3. The formation of Al2O3 contributes to a larger amount of transient liquid phase and therefore could increase the densification of the material.
The microstructures of the sintered SiAlON and SiAlON/TiO2 ceramics (Figure 5) contained as bright intergranular phase in a darker /3-SiAlON matrix. In the SiAlON/TiO2 ceramic, additional submicron (white) TiN particles with size around 0.8-1.2 pm are homogenously distributed around the elongated /3-SiAlON grains. The
* ND ^ the measured electrical resistivity is higher from the range of measurement / izmerjena električna upornost je večja od območja merljivosti
presence of TiN was also confirmed by EDXS analysis, where signals of Ti and N were observed.
The fracture surfaces of these materials (Figure 6) show dark /3-SiAlON grains, a brighter transient liquid phase and uniform white TiN particles.
The sintered SiAlON with and without the addition of TiO2 in the starting-powder mixture reached a higher relative density, flexural strength and comparable electrical conductivity (Table 3) compared to the SN and SN/TiO2 samples. The SiAlON/TiO2 material had a
higher density compared to the SiAlON and consequently the flexural strength was higher by about 33 %. The electrical conductivity of this composite is relatively low, and is in the same range as the conductivity of the SN/TiO2 material. This could be explained by the grain growth of the TiN particles during sintering.
4	CONCLUSIONS
The results show that by sintering in nitrogen at atmospheric pressure we were able to sinter Si3N4 and SiAlON ceramic materials, with and without the addition of TiO2 in the starting-powder mixture, to a relatively high density and with suitable mechanical properties. The SN and SN/TiO2 materials exhibited 90 % of relative density, while the SiAlON material reached 96 % of relative density and consequently a higher flexural strength. The addition of AlN enhanced the densification of SiAlON under the same sintering conditions compared to the Si3N4. The composites with the addition of TiO2 had a higher flexural strength, due to the larger amount of transient liquid phase. The SN/TiO2 and SiAlON/TiO2 composites were electrically conductive. However, their electrical conductivity is somewhat low and is not high enough to fulfill the requirements for the production of ceramic heaters. The reason for this is the amount of secondary conductive phase and the micro-structure of the samples.
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