Functionally graded SiAlON ceramics

https://doi.org/10.1016/j.jeurceramsoc.2003.10.019Get rights and content

Abstract

Functionally graded SiAlON ceramics were developed by three different methods to improve the mechanical properties of SiAlONs; powder bed, rapid cooling and laminating. β-SiAlON compacts were embedded in two different homogeneously mixed powder bed compositions, α-SiAlON (100 wt.%) and AlN: BN (50:50 wt.%). The affect of powder bed composition and pressure on the formation of α-SiAlON on the compact surface was investigated. Rapid cooling to retain transformed α-SiAlON was also another method under investigation. Lamination is another method for production of functionally graded materials. For this purpose laminar type of functionally graded SiAlON ceramics with two layers, which have different compositions, were produced. Transition zone obtained in different methods were examined by phase characterization technique, microstructural investigations and hardness measurements. The results showed that laminar type is the most effective method for FGM SiAlON ceramics production.

Introduction

Functionally graded materials have been investigated extensively since 1987. The aim of production of FGMs, is eliminating the macroscopic boundary in materials in which the material's mechanical, physical and chemical properties change continuously and which have no discontinuities within the material. Thus, these materials exhibit superior mechanical properties compared to monolithic and composite materials.1

Silicon nitride and its derivative SiAlON (solid solutions containing Al and O in addition to Si and N) ceramics constitute an important class of structural ceramics. They are suitable for many commercial applications requiring wear resistance, high toughness, chemical stability and heat resistance.2 Due to these superior properties, SiAlON ceramics have many structural applications especially as wear components. It is well known that mechanical and chemical properties of SiAlON ceramics are affected by chemical composition and microstructure and crystal structure. In general, α-SiAlONs are in the form of equiaxed grains with high hardness and good wear resistance but low fracture toughness and thermal conductivity, whereas β-SiAlONs have elongated grains with high fracture toughness and good thermal conductivity but relatively low hardness. To combine the advantages of both SiAlONs, α/β-SiAlON composites have long been developed. These materials show intermediate values between those of α and β-SiAlONs.3 However, α→β SiAlON transformation requires attention in compositional design and microstructure. There have been various studies in order to overcome α→β transformation problem with considerable success.4 Microstructural improvements followed these studies which lead α/β SiAlON composites with much improved properties.5 Further way to especially improve the mechanical properties for these materials is to functionally design in a gradient manner that their surfaces are rich in α-SiAlON. In this way, high hardness of α-SiAlON can be utilized at the surface and high toughness of β-SiAlON can be taken advantage of in the core.

Previously, Chen and co-workers developed graded insitu SiAlON ceramics by embedding β-SiAlON green compacts in an α-SiAlON powder.6 Their results showed that compositions, microstructures and properties of the graded SiAlON ceramic change gradually from the hard α-SiAlON with spherical morphology on the surface to the tough and strong β-SiAlON with elongated grains in the core. Recently, Kang and Jiang developed a technique for insitu formation of α-SiAlON layer on β-SiAlON surface.7 Their technique consists of packing a compact of β-SiAlON composition in α-SiAlON powder. They found that it is possible to control the thickness of the α-SiAlON rich layer by changing the pre-sintering conditions during heating to sintering temperature. In another study, Mandal and co-workers obtained a gradual change of α-SiAlON content from surface through core by rapid cooling method.8 This gradual change is explained as a function of α→β SiAlON transformation. Pre-lamination of green compacts is another considerable technique for graded material production. Preliminary results for this technique were reported by Shen and Nygren.9 However there has been no detailed information in this reference and also elsewhere.

In the present study, functionally graded SiAlON ceramics were developed by three different methods, namely powder bed, rapid cooling and lamination. Comparison of these methods especially with respect to graded layer thickness is discussed.

Section snippets

Experimental procedure

All SiAlON compositions were prepared by wet milling with isopropyl alcohol in a Si3N4 media. The slurries were dried into powders in a rotary evaporator, which then were sieved through 250 μm. After sieving, the powders were compacted into pellets at 25 MPa by uniaxial pressing followed by isostatic pressing at 300 MPa.

For the powder bed method, a composition rich in β-SiAlON (coded as B1) was selected as a compact composition to observe compact-powder bed interaction. Two different powder bed

Powder bed method

Change in the amount of β-SiAlON from the surface of the samples for green and pre-sintered compacts is shown in Fig. 3, Fig. 4, respectively, after sintering in α-SiAlON and AlN:BN powder beds. Both figures clearly illustrate that AlN:BN powder bed is more effective for the formation of α-SiAlON at the surface than α-SiAlON powder bed. Formation of α-SiAlON rich layer on the component surface is due to transfer of α-SiAlON forming ions from the powder bed. This transfer could be in various

Conclusions

In this study, SiAlON ceramics enriched at the surface by α-SiAlON in order to improve surface hardness without loosing bulk toughness were attempted to be developed. Of the three different methods applied, physical lamination is the most effective one as its thickness can be controlled physically as required. Powder bed and rapid cooling is also effective to form an α-SiAlON rich surface layer but its thickness is limited to few hundred microns.

Acknowledgments

This work has been supported by the Turkish Academy of Sciences, in the framework of the Young Scientist Award Programme (HM/TÜBA-GEBIP/2001-2-15).

References (9)

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