1. Spark Plasma Sintering of Ca-α-Sialon Matrix Reinforced with Cubic Boron Nitride Particles
B. Ahmed1, A. Hakeem1, A. Ibrahim1, M. Ehsan1, T. Laoui1
1King Fahd University of Petroleum and Minerals, Dhahran, KSA
Abstract
Results
Calcium stabilized alpha-Sialon ceramics reinforced with cubic boron nitride (CBN) particles were synthesized using spark plasma sintering (SPS) technique. Single phase alpha-Sialon precursors, namely CaO (160nm), SiO2 (40nm), Si3N4 ( nm) and AlN of sizes 50 and 100nm were used to synthesize the desired Ca-α-Sialon matrix. In order to attain enhanced mechanical properties, the designed matrix was reinforced with cBN particle, sizes of 20 and 40µm, in the range of 10 to 30wt. %. With the aid of ultrasonic probe sonication, the powder mixture containing the precursors and the respective amount of reinforcement were homogenized in ethanol. Consolidation of the powder mixtures was performed using SPS at 1500oC with a holding time of 30 minutes resulting in almost full densification. Alpha-Sialon/cBN composites synthesized using 100nm AlN and 20µm cBN resulted in a marked improvement in mechanical properties. Vickers hardness value of 24 GPa was achieved for the alpha-Sialon matrix reinforced with 30wt% cBN. However, for the same composition of starting precursors and at similar SPS processing conditions, an unanticipated phase transformation from alpha-Sialon to beta-Sialon was observed, as the size of AlN powder precursor was reduced to about 50nm. To our knowledge, this phenomenon was not reported earlier in literature.
Table 2. Mechanical Properties of samples sintered at 1500 °C.
Sample ID
5-Ca-α-2
5-Ca-α-3
5-Ca-α-4
5-Ca-α-5
5-Ca-α-6
5-Ca-α-7
Fracture Toughness (MPa√m)
9.03
6.70
5.67
7.95
11.51
12.46
Vickers Hardness
Hv10 (GPa)
21.84
23.10
24.01
15.50
11.60
9.41
Figure1. XRD pattern of 5- sialon sintered at 1500C and holding time of 30 min. Sialon and CBN (20m) were matched with ICDD # and respectively.
Introduction
In the few past decades, difficulties in sintering of Si3N4 has led to the development of Sialons, solid solution of Si3N4. Sialons exist in two major phases; alpha and beta. Synthesis of these sialons require high temperatures (greater than 1750oC) along with a compromise on mechanical properties in comparison to original Si3N4 material. The present study focuses on developing fully densified Cubic Boron Nitride (CBN) reinforced alpha-sialon composites at sintering temperature of as low as 1500oC, thereby improving the mechanical properties of the said materials. The effect of alpha forming precursors on the structural stability of alpha phase and hence on mechanical properties of the composites was explored to an extent.
(a)
(b)
(c)
(d)
Figure 2. FESEM images (a) 5-Ca-α-2 in polished condition, showing 30%CBN (20um) particle distribution, (b) 5-Ca-α-2 (matrix region) depicting equi-axed alpha sialon morphology, (c) 5-Ca-α-7 showing CBN (40um)-beta sialon interface (d) 5-Ca-α-7 high magnification beta- sialon matrix.
Conclusions
Successfully synthesized alpha sialon/CBN composites at low sintering temperatures of 1500oC.
Vickers hardness value of 24 GPa was achieved for the alpha-Sialon matrix reinforced with 30wt% CBN(20um).
Unanticipated phase transformation from alpha-Sialon to beta-Sialon was observed, as the size of AlN powder precursor was reduced to about 50nm
Materials and Methods
Alpha sialon satisfying the formula of Ca0.8Si9.2Al2.8O1.2N14.8 was selected for the synthesis of the composites. Precursors employed for synthesis include CaO (160nm), SiO2 (40nm), Si3N4 ( nm) and AlN of sizes 50 and 100nm. Classification based upon size of AlN precursor as well as that of size and amount of reinforcement employed in the synthesis of sialon composites is summarized in the Table.1. With the aid of ultrasonic probe sonication, the powder mixture containing the precursors and the respective amount of reinforcement were homogenized in ethanol. The sintering was performed at 1500oC with a holding time of 30 minutes followed by cooling at 10oC/min.
Acknowledgments
The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. 13-NAN as part of the National Science, Technology and Innovation Plan (NSTIP).
Table 1. Classification of CBN particle size and weight percent.
References
Sample ID
CBN Particle Size
CBN Wt.%
AlN (nm)
5-Ca-α-2
20µm 10
100
5-Ca-α-3 20
5-Ca-α-4 30
5-Ca-α-5
40µm 50
5-Ca-α-6
5-Ca-α-7
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Xu, X. et al. Fabrication of β-sialon nanoceramics by high-energy mechanical milling and spark plasma sintering. Nanotechnology 16, 1569–1573 (2005).
Copyright 2016 by B. Ahmed, A. Hakeem, A. Ibrahim, M. Ehsan, T. Laoui. All rights reserved.