Spark Plasma Sintering of SiAlON Ceramics Synthesized via Various Cations Charge Stabilizers and Their Effect on Thermal and Mechanical Characteristics

Muhammad Zulqarnain Falak,Bilal Anjum Ahmed,Usman Ali Akbar,Hasan Aftab Saeed,Abbas Saeed Hakeem,Sajid Ullah Butt

doi:10.3390/cryst11111378

Abstract

Oxygen-rich SiAlON ceramics doped with various nanosized metal oxide (MO) stabilizers were synthesized with a view to examine their effect on thermal and mechanical characteristics. The nanosized starting powder precursors comprising Si3N4, AlN, Al2O3, and SiO2 along with oxides of Ba, Y, Mg, La, Nd, Eu, Dy, Er, and Yb as the MO charge stabilizer were employed in developing different SiAlON samples. Ultrasonic probe sonication was utilized to develop a homogenous mixture of initial powder precursors followed by spark plasma sintering (SPS) of the samples at the low temperature of 1500 °C coupled with 30 min of isothermal treatment. Sample compositions (according to general formula of alpha SiAlON: Mm/vv+Si12−(m+n)Alm+nOnN16−n) selected in the present study are represented by m value of 1.1 and n value of 1.6. The synthesized samples were evaluated for their physical behavior, microstructural and crystal structure evolution, and thermal and mechanical characteristics. More specifically, the sintered ceramics were examined by X-ray diffraction and electron microscopy to comprehend and relate the structural characteristics with the densification, thermal conductivity, hardness, and fracture toughness. The high reactivity of the nanopowders and the localized heating provided by SPS resulted in densified ceramics with relative densities in the range of 92–96%. Vickers hardness values were found to be in the range of 12.4–17.0 GPa and were seen to be profoundly influenced by the grain size of the alpha SiAlON (primary) phase. The fracture toughness of the samples was measured to be in the range of 4.1–6.2 MPa·m1/2. SiAlON samples synthesized using Er and Yb charge stabilizers were found to have the highest fracture toughness of 5.7 and 6.2 MPa·m1/2, primarily due to the relatively higher content of the elongated beta phase. While there was no obvious relationship between the thermal conductivity and the alpha SiAlON metal charge stabilizers, the values were seen to be influenced by the grain size of alpha phase where Dy-SiAlON had the lowest thermal conductivity of 5.79 W/m⋅K and Er-SiAlON showed the highest value of thermal conductivity (6.91 W/m⋅K). It was concluded that scientifically selected metal oxide charge stabilizers are beneficial in developing SiAlON ceramics with properties tailored according to specific applications.

Highlights

Ceramic materials are extensively employed in industrial applications due to their exceptional wear resistance, stability at elevated temperatures, hardness, and stable physical and chemical properties
It has been established that the formation of a eutectic liquid phase in SiAlONs acts as the synthesis aid to facilitate densification behavior of ceramic composites, such as the work reported on SiAlON/ZrB2 -SiC composites produced by spark plasma sintering (SPS) and hot pressing techniques [22,23]
Ceramics having values in between 3.17–3.92 g/cm3 were developed by employing a nanosized starting powder mixture coupled with the unique and efficient heating mechanism provided by spark plasma sintering

Summary

Introduction

Ceramic materials are extensively employed in industrial applications due to their exceptional wear resistance, stability at elevated temperatures, hardness, and stable physical and chemical properties These ceramics have a highly brittle nature and low fracture toughness, which imposes a severe limitation on their use in many engineering applications [1,2,3]. It is due to this reason that metal oxides and different types of additives have been used as synthesizing and densifying agents which help avoid a long duration of synthesis and high temperature complexities These additions modify the structure of silicon nitride and lead to the creation of SiAlONs [9,10,11,12,13]. It has been established that the formation of a eutectic liquid phase in SiAlONs acts as the synthesis aid to facilitate densification behavior of ceramic composites, such as the work reported on SiAlON/ZrB2 -SiC composites produced by SPS and hot pressing techniques [22,23]

Methods

Results

Conclusion