Silicon nitride/SiAION ceramics -A reviewnopr.niscair.res.in/bitstream/123456789/24334/1/IJEMS 8(1...Indian Journal of Engineering & Materials Sciences Vol. 8, February 2001, pp. 36-45

Indian Journal of Engineering & Materials Sciences Vol. 8, February 2001, pp. 36-45

Silicon nitride/SiAION ceramics -A review

C B Raju", S Vermab, M N Sahua

, P K Jaina & Shompa Choudary"

aRegional Research Laboratory (CSIR), Bhopal 462 026, India

bGrasim Industries Ltd, Engineering and Development Division, Birlagram, Nagda 453 433, India

Received 4 May 1999; accepted 20 January 2000

Advanced ceramic materials such as Si)N4 and SiAION are reviewed here with particular reference to their synthesis, properties, and applications in various sectors such as transport and defense. It is noticed that synthesis by the simultaneous carbothermic reduction and nitridation (CTR) is relatively easy and economical. The products thus obtained by CTR, have unusual combination of unique properties, viz., high strength at higher temperature, chemical inertness, wear resistance, etc. , and therefore they are very much useful in high-tech areas. In addition, some of the leading manufacturers of Si)NJSiAION are listed in this paper. Ceramic whisker growth mechanism and its importance in improving the mechanical properties of ceramic matrix composites (CMCs) are also discussed. This paper is mainly intended to cover important properties like structural, mechanical, oxidation/corrosion and wear resistance of Si)N4 and SiAION.

Silicon nitride and SiAION have been identified as one of the most promising engineering materials with high strength and toughness in addition to their wear and corrosion resistance properties since late nineteen sixties. SiAION is the oxynitride of silicon and aluminum. It has high temperature oxidation resistance also. It has a variety of potential applications for ceramic gas turbine components, high-speed cutting tool bits, etc. Many components can be made out of SiAION and used in the automobile and aerospace industries, e.g., pre­combustion chambers, glow plugs, shroud rings, turbine blades, ball bearings, heat engine parts (gas turbines, turbochargers, cam roller, rotor blades), nozzle guide vanes, large shroud rings, shaft bearings, pins for vehicle chassis, injector needle valves, guides, tappets, caps, pistons, water facet valves, etc. Only a few companies in Japan, USA and UK are producing SiAlON in limited quantities because of its high production costs. As a result, synthesis of silicon nitride and SiAIONs has received considerable interest in recent times. Whiskers of alumina, AIN, silicon nitride have much higher tensile strength than the conventional alumina 1-3 and the corresponding particulates. Japan contributes up to 60% of the world production of advanced ceramics and is followed by US (30%) and Europe (10%). A recent study by the Japan's Ministry of International Trade and Industry has indicated that the demand in 2000 AD for ceramics with high heat resistance will grow ten times from that of 1987 and the market for ceramic gas turbines will grow to 13 billion US dollars by the turn

of next century. Some details on the status of SiAION, world-wide leading manufacturers of SiAION/silicon nitride components its prices and utilisation are discussed separately in the market opportunity report for SiAlON/Silicon nitride ceramic powder4

-5

. Jack6, Wilson 7 and Alan Hendr/ have dealt some of these subjects to some extent.

It is intended to bring an updated information based on the various issues related to si licon nitride and SiAION with a view to widen the scope of raw materials including alumino-sil icate minerals and their secondary resources for the development of these advanced materials in the present paper.

Synthesis of SiAION Table 1 covers most of the leading manufacturers

of Si3NJSiAION in the world and their product specifications. ~-SiAION, nitrides and oxy-nitrides of si licon are traditionally prepared by thorough mixing of the reactant powders in a ball mill followed by heat treating the mixture at elevated temperature in either N2 or NH3 atmosphere. However, the requirement of high purity ceramic materials has led to the development of sol-gel process, chemical vapour deposition (CYD), plasma spray, aerosol, and glow discharge, etc. But these methods give small quantities of pure, highly uniform and costly ceramic powders. The preparation of nitrides and oxy-nitrides of silicon and aluminum from alumino-silicates by si multaneous carbo thermal reduction and nitridation is of interest because of the versat ility of the process and the avai lability of several inexpensive mineral

RAJU et al.: SILICON NITRIDElSiAlON CERAMICS 37

Table I - Leading producers of SiJ N4 & SiAION with grade/specifications*

Country

USA

Japan

France

U.K.

Germany

Company

Ceradyne Inc.

Dow Corning Corporation

General Electric Company

Advanced Refractory Technologies Inc.

Benchmark Structural Ceramic Corporation

Vesuvius-Mc Danel Co.

Denki Kagaku Kogyo KK

Nippon Denko KK

Nippon Heavy Chemicals Ltd

Onada Cement Company Ltd

Shin-Etsu Chemical Co. Ltd

Showa Denka KK

UBE Industries Ltd

Tateho Chemical Industries Co. Ltd

Nihon Cement Company Ltd

Atochem

Cookson Syalons Ltd

Morgan Matroc Ltd

H.C. Starck GmbH & Co.

Product/Grade

Silicon nitride powder

HPZ silicon nitride continuous fibre

Silicon nitride powder

Silicon nitride whiskers and powders

SiAION powders/SiAlON whiskers blends

SiAION powder

Silicon nitride powder

(Refractory grade, automobile purpose)

Silicon nitride powder

Silicon nitride powder

Silicon nitride powder

Silicon nitride powder

(Coarse/Refractory ultrafine with 99% alpha phase)

Silicon nitride powder

(Size 0.7 microns,a phase 90-95 %)

Silicon nitride powder

whiskers

Silicon nitride whiskers

!3-SiAION powders

Polysilazane precursors

SiAION Powders

Silicon nitride powders

Silicon nitride powders (Grade S- Coarse SI ; LCI2SX­Fine)

* Vivien Mitchell , Mitchel Market reports -Silicon nitride and the SiAIONs (Elsevier Science Publishers Ltd., England), 3rd edit ion , vol.l &2, 1993

resources. Literature surVey reveals that nitridation of alumino-silicates such as clay, montmorillonite, pyrophyllite, illite, sillimanite, is thermodynamically possible and practically confirmed resulting in the formation of SiAION, nitrides of aluminium and si licon. These materials show better sintering properties as they are obtained from natural alumino­silicates containing small amounts of Ca, Mg, Fe, etc. It is also proved that iron present in the raw materials catalyses the carbothermal reduction and nitridation process of alumino-silicates. Intercalation compounds are more favorable than the mixtures with a larger amount of carbon for possible nitridation of alumino­silicates. This view has been confirmed by Sugahara et al. 9 while converting montmorillonite-PAN to a mixture of ~- SiAION, AIN and SiC at llOO-1500°C. Ammonia gas has been shown as an effective agent for nitridation of borosilicate glass5, silica gelID, fumed silica", and also phosphate glasses l2 at temperatures less than 1 160°C.

SiAION from pure materials By the interaction of silica gel-Al(OHh in a

mixture containing AIISi ratio equal to I at high temperature « 1500°C) in N 2 atmosphere, very fine SiAlON powders have heen produced 13. u-SiAlON has been prepared by carbothermal reduction and nitridation of sol gel derived systems viz. , CaO­AI20 3-Si02 and Y203-AIz03-Si0214. Raise in temperature of nitridation from 1300 to 1450 °C leads to agglomeration of SiAION powder (0.1 to 2 Jlm). Nitridation of mixtures containing carbon, Si02 and Alz03.2H20 in N2 atmosphere at 1450-1500°C brings about the formation of ~-SiAION with z = 3.5 where the increased flow rate of N2 causes more loss of SiO and higher carbon content leading to the formation of AlN + 15R phase on higher side l5 .

Mixtures of u- or ~-Si3N4' AI20 3, AlN, Si02 and SiON are heated and hot pressed in N2 atmosphere at 1700°C to produce SiAlON ceramic body. Si02 and Alz03 gels are co-precipitated and subjected to

38 INDIAN J. ENG. MATER. SCI., FEBRUARY 2001

1700°C in NH3 atmosphere resulting in the formation of SiAION based ceramic body. In addition, mixtures of alkoxides or Si3N4 + AI-isopropoxides, or SbN4 + AI(OHh have also been subjected to heat treatment in N2 atmosphere.

It has been shown that hydrolization of Si3N4, while mixing it with AI(OHh solution, gives more amounts of O'-SiAION compared to that prepared from AI(OCJH7h In other words, the selection of raw materials greatly influences the phase composition, microstructure and, properties of the SiAION powder. Recently, the practice of adding dopants, e.g., Y 203, Nd20 J, CaO, La203, dysprosium (Dy) and SC20J has shown promising results in the mechanical and high temperature oxidation properties of SiAION ceramic bodies. The addition of CaO, MgO and Y 203 helps to promote the conversion process and thereby lowering the temperatures of carbothermic reduction and nitridation reactions.

Thermite process ~-SiAION powder, 15 R SiAION along with small

amounts of AIN and Ah03 are formed when mixtures of aluminum metal powder, amorphous silica, opalite and volcanic ash are subjected to electric shock ignition at 1400-1700°C in N2 atmospherel 6

• In the thermite process 17-22, chemical vapor deposition (CYD)17, physical vapor deposition (PYD), plasma spray, aerosel and reaction formed techniques IS are used to synthesize the new ceramic materials. High temperature strengthened structural ceramics produced by gas phase synthesis in the thermal plasma are pure ultra fine powder with desired

compositions and morphologies. ~-SiC crystalline powder have been synthesized by keeping reaction temperature more than 1150 K with round shape of particles having 0.1-0.5f..lm in size l9 .

Both CYD and PYD result in the deposition of materials on substrate with the help of chemical decomposition such as thermal reduction, hydrolysis, oxidation, carburisation, nitridation and physical depositions of atoms, ions or molecules, respectively. Important steps involved in CYD are: Production of volatile carrier compound; Transportation of the gas to the substrate site without decomposition; and, Chemical reactions in between substrate and coating substances. Important steps involved in PYD are: Transportation of the gas to the substrate site without decomposition; Chemical reactions in between substrate & coating substances; and, Ion plating.

Advanced ceramic composites of SiC-TiS2 having doubled fracture toughness from SiC20 were produced by CYD process and composites of SiC with nitrides or carbides of metals were also synthesized and examined for their higher strength and toughness. Morphology of dispersed phase produced by CYD is very much useful to control the mechanical properties of the ceramic composites21 . The CYD techniques can successfully be used to develop the tool bit materials22

, high temperature oxidation protection coatings including a number of advanced ceramic products for high temperature structural applications. Direct powder coatings on substrate at room temperature with high diffusion temperature treatment require high temperature sources such as combustion flame, plasma flame or a laser for proper deposition on the substrate. This process is mainly based on the extent of melting of the powder and the velocity with which the molten droplets hit the substrate for coating.

Carbothermic reduction and nitridation of alumino-silicates Carbothermic reducion and nitridation of Kaolin

has been studied by several workers to obtain SiAIONs of ~-composition23.34 [(Si + AI)/(N + 0) = 3:4] Si6-zAlzOzNs-z where z = 0-4. Polyacrylonitrile, carbon black, anthracite coal and graphite were normally used as the reducing agents in the solid mixtures. The product contained in addition to SiAION, other phases such as alumina, mullite, AIN and AI20 3 mixtures. It has been noticed that the sintering properties of SiAION produced from kaolinite are better than those obtained from silicon nitride, AIN and AI20 3 mixtures. This route is still open for research to further improve the properties of the end products, obtained from natural alumino­silicates.

It has been shown that a counter current vertical shaft kiln is a suitable reactor for production of ~-SiAlON. Higgins et al. have proposed the basis for the carbothermic reduction-cum-ni tridation process in the counter current vertical shaft kiln and investigated the parameters necessary for the manufacture of SiAlON powder in an industrial processJI-32. According to the authors, important stages in the formation of SiAION at 1400°C in N2 atmosphere are: (1) decomposition of kaolin into mullite and silica, (2) formation of SiC by the reaction of silica and carbon, and (3) simultaneous reduction and nitridation of mullite by its reaction with silicon carbide and carbon as shown by the following reactions. Iron present in

RAJU et al.: SILICON NITRIDFlSiAION CERAMICS 39

the system catalyses the formation of SiC21 .23 and thus indirectly the formation of SiAlON.

3(2Si02.AI203.2H20)-t(3AI203.2Si03)+4Si02+6H20

4Si02+12C -t 4 SiC + 8 CO

3Al:!03.2Si02+4SiC+3C+SN2 -t 2Sb Al3 0 3 N 5+7CO

Carbon content is shown to have considerable effect on the reaction of clay carbon mixture at 14S0°C in N2 gas atmosphere leading to the formation of crystalline phases containing ~, X, lSR, Si3N4 and AlN24. Carbon black of commercial grade has been used as a source of carbon in the reaction mixture.

N2 flow rate has considerable effect on the phase composition of the reaction products when the mixture of clay/carbon (3: 1) is heated at high temperatures28. According to the authors, when the flow rate of N2 is high, Reynolds number is high and the reaction products contain a mixture of ~-Si3N4'

Ah03 and AlN in place of ~-SiAlON. Some workers28 have shown the formation of SiO

in the carbothermal reduction and nitridation of clay carbon mixtures in the ratio 4: 1 at 13S0-1S00°C. The quantities of N2 gas flow further affects AVSi ratio causing the formation of ISR and AlN29. Illite-kaolin­carbon mixtures have been converted at <lS00°C in N2 atmosphere to a mixture of ~-SiAlON3o with small quantities of AlN, Si20N2 and Ah03. According to the authors, ~-SiAlON formed from kaolin has better sintering properties than that obtained from illite. Similarly, Higgins et a1. 31

.37 have shown a higher

conversion rate to ~-sialon in the case of kaolin­carbon mixture due to formation of liquid phases by the impurities present in the starting materials favouring faster reaction for nitridation.

The reaction products obtained by carbothermic reduction and nitridation of seeding halloysite clay (Hadong pink kaolin clay, Korea)-graphite mixtures at 1300-1400°C in N2+ H2 atmosphere contain a ~-SiAlON (z=2.S) along with minor amounts of a-Ah03, AlN and lSR34.

Sugahara et al. 35·36 have shown a faster rate of conversion in the montmorillonite- polyacrylonitrils mixture heated at llOO-IS00°C in N2 atmosphere leading to a mixture of ~-SiAlON, ~-SiC and AIN. The authors opine that closer contact between aluminosilicate and carbon is responsible for the faster rate of reaction in the montmorillonite-PAN

mixture. ~-SiAION powders with z value 0.8 to 2 have been synthesized by carbo-thermal reduction and nitridation of kaolinite, sillimanite, fire clay and pyrophyllite36.37. Carbon black has been used as a source of C and Fe203 as the catalyst. Fireclay alone gives ~-SiAlON which is similar to that of the product from kaolin with catalyst. The major phase is ~-SiAION with z= 2, 2.S, 0.8 in the reaction products from kaolin, sillimanite and pyrophillite respectively. Mossbauer spectrometry has been used for studying the role of Fe in the reduction as well as nitridation process and the results confirm the catalytic role of Fe similar to the views expressed by Lee and cuttler24. With increased flow rate of N2 gas, the uptake of nitrogen content in the system is increased with a result in the formation of more ~-SiAlON and smaller amounts of alumina. Mukherji and Bandopadhyay37.39 have shown that the amount of carbon content in the system and nature of mixing are found to have a very effective role in controlling the crystalline phase composition of reaction products. Higher carbon in the starting materials results in the formation of AlN, ~-SiAION and Al ON where as low carbon results in the formation of mullite.

Demerits of carbothermal reduction and nitridation Siddique et al. 4O have shown that carbothermal

reduction and nitridation of silica have certain disadvantages though it is found to be an alternative and practical route to the nitridation of silicon for producing silicon nitride. Iron impurities, which may be present in the raw material, have to be removed from the final product.

SiAION Whiskers The most important reinforcing material for

improving the mechanical properties of ceramics matrix composites (CMC's) and metal matrix composites (MMC's) are ceramic whiskers which offer high melting point, low densities, high temperature strength and thermal shock resistance making them suitable for high temperature engineering applications. A filamentary/fibrous solid material having small cross-sectional dimensions below that of 100 J-lm diameter is generally known as whiskers. These single crystal filament type materials can be produced by the solid, liquid or gaseous precursors.

Whiskers grow vapour-solid-liquid (VS) mechanism

either by the vapour-solid or mechanism. The vapour-solid is based on the material

40 INDIAN J. ENG. MATER. SCI., FEBRUARY 2001

impingement at the end or sides of a whisker where it diffuses. In the vapour-liquid-solid (VLS) mechanism for the whiskers, growth, rapid expansion of material particles takes place followed by gradual thickening.

In VLS mechanism41 , new crystals in the form of short fibers with rounded tips known as whiskers are generated. It is also evident that the presence of impurities in liquid form helps in the growth of whiskers in VLS mechanism. It is reported that the existence of impurities like Fe which act as a catalyst (or solution forming agent) is very useful for the growth of whiskers.

~-SiAION whiskers have been produced by vapour phase route (CVD) and nitridation of the SiOrC­Na3AIF6 system. Fused silica bodies and machine tools are given coating of SiAION to improve the efficiency of substrate bodies.

Structure of Silicon Nitride Ellen Y.Sun et aL. 42 have investigated the interfacial

micro-structures of ~-Silicon nitride(w)-SiAION-glass systems by varying the nitrogen content and the AI:Y ratio of the glass matrix. Transmission Electron Microscopy (TEM), High Resolution Electron Microscopy (HREM) and Analytical Electron Microscopy (AEM) studies show that the interfacial microstructure is a function of the glass composition. In glasses with low AI:Y ratios as well as with high AI:Y ratios and low nitrogen content, interfacial phases are absent, where as in a glass matrix with high AI :Y ratio and high N2 content, an epitaxial growth of an interfacial layer of thickness ranging 100-200 Ilm on the ~-silicon nitride(w) has been observed. ~-SiAION phase is identified as the interfacial layer exhibiting high debonding energy compared to silicon nitride(w) glass interfaces. High resolution electron microscopy (HREM) studies prove that the lattice mismatch strain in the SiAION layer is relieved by dislocation formation at the SiAION­silicon nitride(w) interface. The difference in the interfacial debonding energy is due to the local atomic structure and bonding between the glass-~-SiAION phases. From these observations, it is evident that the glass chemistry influences the interfacial debonding behaviour by altering the interfacial microstructure.

Xiaoqing Pan43 has reported the structure of interface formed by a silicon nitride grain and silica rich amorphous phase by quantitative high resolution transmission electron microscopy (QHRTEM). The contrast and periodicity of HRTEM image of {3-silicon nitride strongly depends on the specimen

thickness and objective lens focus values. The thinning rate between silicon nitride and glass phase during ion milling and interface roughness parallel to the electron beam are responsible for ingredients of the specimen thickness at the interfaces. Due to these reasons, the HRTEM micrographs of thin specimen regions near the interface at a certain defocus value show the artificial occurrence of an ordered structure, which seems to be different from that of silicon nitride.

The absence of ordered phase in the interfacial region of silicon nitride is also reported43. The interfacial structure is likely to have direct silicon nitride/glass bonding rather than an ordered transition phase between the silicon nitride and the glass phase. The mechanical properties of sintered body are affected by the character and the compaction behaviour of the dried silicon nitride granules sprayed from a well dispersed slurry. Dense hard granules cause numerous post defects in sintered body. When pH of slurry is decreased to 7.9, it results in slurry flocculation and reduces the granule density and diametrical compression strength of the granules. Sintered bodies fabricated with these weak granules contain fewer defects and show remarkable strength44

Hans Joachim Kleebe45 has statistically analyzed the intergranular film thickness in silicon nitride ceramics and evaluated forty to fifty data points for each boundary, showing a Gaussian-like-distribution of the film thickness around a median value, which corresponds well with the film width measured from single HREM micrographs. The accuracy achieved by this statistical method is better than ±A. The amorphous intergranular films present along with the grain boundary in the silicon nitride material strongly affect the high temperature properties of the bulk material. HREM studies show that the variation of film thickness at grain boundaries is in between 5 to 15 A.

Sintering time controls the growth of elongated grains and extent of densification leading to desired micro-structure. Powders of ~-silicon nitride containing 1 mole% of equimolar Y20 3-Nd20 3 are sintered at 2000°C for 2-8 h, in a flow of nitrogen gas at 30 Mpa. The sample obtained after 2h of sintering had a Weibull modulus of 53, due to its uniform size and spatial distribution of large grains. A fracture toughness of 10.3 Mpa m 112 has been observed with the material sintered for the 4 h duration. A lower fracture toughness with increased duration of sintering up to 8h of the material has been observed,

RAJU et al.: SILICON NITRIDFJSiAION CERAMICS 41

which is due to the extensive microcracking resulting from excessive large grain size. It shows that fracture toughness is inversely related to grain size and

. . • 46 smtenng time .

Properties of SiAION ~-SiAlON produced from kaolin37

-40 has a maximum density of 2.81 g/cm3 with only 1.45% open porosity at 1750°C. The material has 72% N2 content and retains its room temperature strength (MOR) up to 1350°C. The thermal expansion coefficient is 2.65 x 1O-6/oC which is comparable to that of SiAlON (z =2) synthesized from pure components by reaction sintering.

More than 75 % of the room temperature strength (MOR) was retained after 30 cycles of thermal shock from 1350°C to ambient temperature. The total creep strain of the sample (z=2) conducted at a load of 18 MN.m-2 and at 1300°C was 0.06% in 20 hand 2.72% at 1400°C in 30 h. The material showed very good oxidation resistance with a maximum weight gain of 4 mg/cm2 at 1350°C after 72 h.

The corrosion behaviour of the sintered SiAlON was studied with molten aluminium at 800°C and also with fluoride bath at 1000°C in an ambient atmosphere. The material ~-SiAlON did not show any sign of attack or corrosion to molten aluminium even after 72 h. But the material was completely dissolved in flouride bath at 1000°C after 7h showing its weak corrosion resistance to fluoride, which was probably due to the replacement of N2 by oxygen in Si3N4. Dense sintered SiAlON with external additives was synthesized in the systems Si3N4-AlN-AI203 Si02 and Y 20 3-AlN-Si02. The highest density of 3.21 g/cm3

was attained at 1750°C after 90 min of sintering with 5 wt% additive in the system Y20 r AlN-Si02. The ambient fracture toughness of ~-SiAlON produced for a composition with 5 wt% liquid phase had the highest value of 6.5MPa. ml12 which was not changed even at 1200°C.

Mechanical properties Silicon nitride has fracture toughness of nearly 4-6

Mpa.m"2. Fracture toughness can be increased to 8-14 Mpa.m l12 by using two techniques, viz., self reinforcement and in situ toughening using gas­pressure sintering (GPS) or hot-pressing (HP) method in different additive systems.

Ivon E.Reimanis et.al. 47 have synthesized single crystals of a-silicon nitride by CYD process, using

HSiCb, NH3 and H2 gases in a chamber at a pressure of 0.5 torr (70 Pa) and temperature in the range of 1300-1500°C. After the synthesis of coarse grained polycrystalline silicon nitride, larger isolated crystals of a-silicon nitride of several mm in diameter and length have been grown. The single crystals of a-silicon nitride have been shown to contain a variety of structural defects largely due to the- high residual stresses inside the crystals. Silicon rich dislocation loops and stacking faults are the two kinds of defects commonly observed.

The macroscopic crystal morphology depends on the crystal growth direction which determines the degree of microcracking, type and density of structural defects. Indentation cracking studies indicated the identation fracture toughness of 1.5-2.0 Mpa. m l12 at the room temperature of relatively isotropic nature. At high temperature, erratic nature of cracking cast doubt on the precise fracture toughness values measured by the indentation methods. However, the results suggested that there is a little change in fracture toughness of these crystals for temperature up to 1200°C.

Fatigue behaviour The effect of environment of cyclic and static

fatigue behaviour of hot pressed silicon nitride materials has been studied48. It has been found that static and cyclic fatigue are sensitive to the environment and display the same trends, i.e., the chemisorption process takes place at the crack tips which influences the crack growth for both types of fatigue. Silicon nitride materials are most sensitive to water in the environment, and change in pH value. NH3 is less reactive to silicon nitride material than to vitreous Si02. Also, cyclic loading favours a lower growth rate of crack48.

Yttrium has been added up to 4 wt% as the sintering aid to the silicon nitride powder for preparing cylindrical button head specimens49. These specimens are pressed cold isostatically and finally HIPed to full density. Creep tests at 1477-1673 K temperature under uniaxial tension have been performed and creep deformation has been measured. It has been observed that the formation of lenticular cavities exists at two grain junctions at all the temperatures ranging from 1477 to 1677 K and the extensive triple junction condition occurs at high temperatures ranging from 1644 to 1673 OK condition in the part of net creep process. The stress rupture data show the stratification of the Monkman Grant

42 INDIAN J. ENG. MATER. SCI., FEBRUARY 2001

lines with respect to temperature. As the rupture temperature or rupture time increases, failure strain also increases. Failure strain increases as the stress decreases. Toughness values of silicon nitride ceramics with microstructure containing elongated grains increase with crack extension.

Oxidation/Corrosion resistance The oxidation state of the material gets affected by

the mixing and forming steps. The surface chemistry of silicon nitride is strongly influenced by the processing environment and different techniques of formation such as isostatic process and slip casting.

Surface oxidation of silicon nitride, i.e., the oxidized species and the extent of oxidation depend on the dispersion media, and the route selected for the forming process, have been studied by X-ray Photon spectroscopy (XPS). The variation in tendency to oxidation in the silicon nitride green compacts obtained by different processing conditions shows that the slip casting process yields less oxidized species than with isostatic pressing, because of the use of TMAH (tetra methyl ammonium hydroxide) as a dispersing agent which stabilises the suspensions50. Due to the chemical character of TMAH and the resulting high pH value, the formation of amine species at the surface of silicon nitride during the homogenization steps is also favoured.

During casting procedure, TMAH controls the oxidation because of the formation of a partially oxidation layer around the particles, acting as a screen preventing progressive oxidation50.

Oxidation improves the high temperature strength and fracture toughness. Two materials of different y 20-j Ah03 ratio have been chosen by Chuan He et ae l

• First material has SbN4-13.9 wt%, Y 203-4.5 wt%, AI20 3 and second material has the composition of SbN4 -6.0 wt%, Y203-12.4 wt%, A120 3. Both of these materials are oxidized at 1200°C in air for 1000 h. The oxidation is improved significantly more for the first material than that of the second material. The oxidation affects of the two materials are different at room temperature. Due to oxidation, the microstructure of the material changes, and as a result mechanical properties are also affected. The room temperature Wei bull modulus of first material is increased by more than half. At l200°C, Wei bull modulus decreases slightly after oxidation. If annealing treatment is done prior to the oxidation, there would be no effect of oxidation on the high temperature strength of materials.

Silicon nitride prepared by CYD has been oxidized in the CO-C02 atmosphere between 1823-1923 K by Takayuki Narushima et a1. 52

, using thermo­gravimetric techniques. Silicon nitride plates of a-type and of 1 mm thickness have been deposited on graphite substrate using SiCI4, NH3 and H2 as source gases. It has been observed that two types of oxidation take place. First is the active oxidation where mass loss of silicon nitride is observed in a region of PC02/PCO < l. Second is the passive oxidation region where mass gain is observed at around PcoiPco = 10. In active oxidation region below PcoiPco = 10-4, carbon particles are formed on the silicon nitride surface as an oxidation product. The mass loss rates are independent of the Pc02/Pco. In the active oxidation region above Pc02/Pco = 10-4, the mass loss rate decreases with the increasing Pc02/Pco. It has been observed that the critical value of PC02/PCO from the active to passive oxidation is second order of magnitude larger than that of the calculated value predicted from Wagner model.

Darryl P.Butt53 revaluated the kinetics of thermal oxidation of silicon nitride powders in the temperature range 650-l200°C using isothermal and non­isothermal thermo-gravimetric analysis, High Resolution Transmission Electron Microscopy and X­ray Photon Spectroscopy. He used two types of silicon nitride powders, i.e., H.e. Strack M-l1 and Ube SN-E 10 of same particle size and surface areas. The isothermal kinetics was modeled equally well using the Ginstling-Brown-Shtein and Zurarler­Lesokhin-Tempel'man equations. Despite their same particle size and surface area, they exhibited different oxidation kinetics. The activation energy for the oxidation of the H.e. Stark M-ll silicon nitride was determined to be 400 kllmole between 1000 and l200°C and 230 kJ/mole for temperature between 700-1000°C whereas the activation energy for the oxidation of Ube SN-E1O silicon nitride powder was 540 kllmole between 1000-l200°C and 260 kJ/mole for temperature between 700 and 1000°e. At high temperature, the parabolic rate constants for oxidation of two powders were comparable to those reported for monolithic CYD silicon nitride. At low temperature, the oxidation kinetics of M-ll powder was nearly linear and whereas the kinetics of SN-E1O powder remained as dependent on power law.

Yury G. Gogatsi and George Grothwahi54, observed the phenomenon of the stress corrosion in oxygen­containing environments in the case of porous and dense ceramics with or without additives in a wide of

RAJU et al.: SILICON NITRIDElSiAION CERAMICS 43

range of temperature (700-14S0°C) and found that stress enhances the oxidation of silicon nitride ceramics. Due to the stress, there is an alteration of the amount and composition of oxidation products. In addition to this, there is formation of pits and cracks on stressed parts of the specimens and changes of the surface colour and oxide scale morphology. Both tensile and compressive stresses affect the oxidation process, where mass gain is exponentially dependent on stress. Also, the oxidation of silicon nitride based ceramics affects the material response to mechanical stresses such as deformation, cavitation and cracking. The effect of stress corrosion on mechanical properties has been discussed by the authors. It has been found that the main reasons for the susceptibility of silicon nitride ceramics to stress corrosion is stress­assisted chemical reaction at lower temperature and stress affected diffusion at higher temperatures.

Wear properties of nitrided ceramics SiALON and Si3N4 are potential wear resistant

ceramic materials55. Several workers in the past56-60

have explained to some extent the friction and wear behavior of nitride ceramic materials. The properties usually described for silicon nitride and SiAlON are its high strength, wear resistance, chemical inertness, high temperature oxidation resistance, excellent thermal shock properties, low co-efficient of friction

and high resistance to corrosive environment, etc 57. However, utilization of Si3N4 (the covalently

bonded solid) is limited due to the major difficulty in fabricating shapes with the desirable properties. The self-diffusivity of pure silicon nitride is very small and it can not be sintered to maximum density by firing alone. Another difficulty of using pure ceramics for engineering applications is their brittleness, which limits their use for impact applications or where ductile properties are needed. Earlier attempts have considered friction and wear properties of monolithic silicon based ceramics in pure form33. Dry sliding wear of hard material against a diamond composite has also been reported54. Sliding wear of other structural ceramics like SiC-SiC, Zr02, etc., has also been reported by some authors34-38. Sliding wear behaviour of Si3N4 and hot pressed alumina have been studied and the effect of soaking time on wear resistance of specimens is reported60

. The machining and fabrication of pure ceramic component has been a major problem57 whereas MMC's can be easily machined and fabricated to desired shape and size. In case of high percentage of ceramic component (ceramet), techniques like EDM (Electric Discharge Machining) can be used because of its electrical conductivity due to presence of metal. Use of TiC in CMCs has been reported for facilitating EDM 61.

Table 2 - Properties of SiAION compared to other ceramicss

Property SiAION* Si)N4** RBSN# SiC## AI2O)+ PSZ++ I 2 3 4 5 6

Room temperature Mod. of Rupture, MPa 945 896 241 483 380 610

Typical Weibull Modulus II 10-15 10-15 10 10 10-20

Room temperature Tensile Strength, Mpa 450 580 145 299 210 466

Room temperature 3500 3500 1000 2000 2750 1850 Compressive Strength, Mpa

Room temperature Young's Modulus X 105, Mpa 3.0 3.1 2.0 4.1 3.6 2.0

Room temperature Hardness Kglmm2 (YHN,O.5 kg load) 2000 2200 950 2500 1600 1500

Fracture Toughness (Kid Mpa .ml12 7.7 5 1.87 3.0 1.75 9.5

Poisson 's Ratio 0.23 0.27 0.27 0.24 0.27 0.30

Density, glcm) 3.25 3.20 2.50 3.10 3.98 5.78

Thermal Expansion Coeff. (O-IOOO°Cx lO-6 /K 3.04 3.20 3.20 4.3 9.0 10.6

Specific Heat, J/kg/K 620 710 710 1040 1040 543

Room temperature Thermal Conductivity, 21.3 25 8-12 83.6 8.40 2

w/mlk

Thermal Shock Resistance 900 600 500 350 200 500

*Low poly type SiAION + Alumina ## Sintered silicon carbide # Reaction bonded silicon nitride ** Hot pressed silicon nitride ++Partially stabilized zirkonia sSAE Paper 850521, Lukas cookson syalon Ltd

44 INDIAN J. ENG. MATER. SCI., FEBRUARY 2001

Applications of Silicon Nitride/SiAION Silicon nitride not only has properties of high

melting point, chemical inertness, high strength and toughness and good wear resistance, but also has interesting electrical properties. SiAION has both the properties of chemical inertness and hardness. Table 2 shows that SiAION has high modulus of rupture (945 Mpa), fracture toughness (7.7 Kd and low thermal expansion coefficient in comparison to any other ceramic materials such as silicon nitride, silicon carbide, alumina and partially stablised zirconia. Reaction bonded Si)Ni and SiAION is used for: (i) Jigs and fixtures for heat treatment, brazing, soldering and welding; (ii) Spouts, nozzles, riser tubes, thermocouple sheaths, melting crucibles and other components for handling molten aluminium, zinc, tin, and lead alloys and most copper and iron alloys; (iii) Radiant heating element formers, induction coil and powder cable carriers, TIG and MIG arc welding nozzles and shrouds, and other components where electrical resistance at high temperature is required; (iv) Mechanical case seals, bearing and shaft sleeves for pumps and flow meters, wear plates and nozzles, ring gauges and wire guide pulley inserts and other components for sliding and abrasive wear applications and for use as gauzing components; (v) The films of amorphous silicon nitride for utilization in integrated circuit manufacture; (vi) Indexible lathe inserts for cutting metals especially high nickel alloys; (vii) Gas shrouds and location devices for resistance welding

Table 3 - Properties of silicon nitride for gas turblnes*

Density, g/cm3 3.26

HV (Gpa) ) 5.40

Youngs Modulus. (Gpa)

Bending Strength (Mpa)

Klc (Mpa min)

* J Mater Sci, 29 (1994) 3243-3247

310.00

920.00

6.00

machines; (viii) Extrusion dies for brass, copper, titanium and other metals; (ix) Seals and bearings molten metal handling; (x) Drawing dies, tube drawing mandrel plugs, roll guide plates and other wear applications; (xi) Diesel engine components, e.g. turbocharger rotors and bearings, valve train parts, tappet shims, head plates and piston parts; and (xii) Radom applications in missiles and robot aircraft vehicles for transparency to radar frequencies and erosion resistance as SiAION and silicon nitride are considered interchangeable 10 many of these applications.

Gas turbines; specifications of silicon nitride and SiAION for the purpose of automobile and refractories are shown in the Tables 3 and 4.

Conclusions It has been shown that there is an increasing

demand of silicon nitride and SiAION materials for high temperature engineering applications such as heat engine parts, turbine blades, turbo-charger rotors, cutting tool bits, ball bearings, nozzle guide vanes. The demand is picking up fast covering a broad range of applications because of their highly desirable properties. In spite of this, there are limited number of companies in USA, UK, and Japan which are producing these ceramics in small scale but with high production costs. We are on the threshold of production and usage of these advanced ceramics which are rapidly finding use as a substitute material in place of metals and polymers for hi-tech engineering applications.

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Table 4 - Properties of silicon nitride and other materials used for refractories*

Materials Density Resistivity Micro Thermal (g/cm3) (mho) Hardness Expansion

Coeff. (K'I)

Si)N4 3.21 1.00x 10. 11 30 3.25xlO·6

TiN 5.44 3.34x I 0'7 20.5 8.00xlO,6

zrC 6.73 4.90xlO,7 29.5 7xlO,6

ZrN 7.09 1.80x 10'7 16.7 7.24xlO,6

TiN 4.93 6. lOx 10'7 31.7 7.95xlO'6

TiBr2 4.48 9.00xlO,7 33.7 4.60xlO,6

ZrBr2 6.12 I.30xlO,7 22.3 3.90xlO'6

* J Mater Sci, 29 (1994) 2541-2556

Young's Modulus

(GPa)

280-320

430-469

355

427

571

350

RAJU et al.: SILICON NITRIDEISiAION CERAMICS 45

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