Vickers Hardness of Β-Sialon Prepared by a Combination of Combustion Synthesis and Spark Plasma Sintering

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Journal of the Ceramic Society of Japan 118 [3] 250-252 2010 Technical report

Vickers hardness of β-SiAlON prepared by a combination of combustion synthesis and spark plasma sintering

Xuemei YI, Kotaro WATANABE and Tomohiro AKIYAMA*,†

Graduate school of engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo 060-8628 *Center for Advanced Research of Energy Conversion Materials, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo 060-8628

This paper describes the Vickers hardness (Hv) of dense β-Si6–zAlzOzN8–z (z = 1, 2, and 3) prepared by the combination of combustion synthesis (CS) and spark plasma sintering (SPS). The Vickers hardness was measured by a Vickers microhardness tester in the applied load ranging from 0.981 N (0.1 kg) to 19.614 N (2.0 kg) at room temperature. A significant indentation size effect (ISE) is observed for each sample at low indentation test load. The ISE shows more clearly on the β-SiAlON with z = 3, while it reaches a saturation value when the applied indentation load F ≥ 4.903 N for z = 1 and 2. In conclusion, the Vickers hardness is 17.6 GPa for z = 1, 16.2 GPa for z = 2, and 14.7 GPa for z = 3 under the load of 19.614 N. ©2010 The Ceramic Society of Japan. All rights reserved.

Key-words : SiAlON, Sintering, Mechanical properties, Hardness

[Received May 25, 2009; Accepted January 15, 2010]

edge, there are very few reports on the mechanical properties of 1. Introduction CS-SPSed β-SiAlON. Therefore, the purpose of this study was β-SiAlON, commonly described as β-Si6–zAlzOzN8–z (z takes 0 to measure the Vickers hardness of CS-SPSed dense β-SiAlON, to ~4.2), is the solid solution of β-Si3N4 in which Si–N has been and to observe the ISE on these products. The relationship substituted with an equivalent amount of Al–O.1),2) Ever since it obtained between this property and z value will give valuable was discovered in the early 1970s, β-SiAlON has been actively information for the design of β-SiAlON. developed for many area applications on account of their excel- 2. Experimental procedure lent mechanical and thermal properties, superior chemical stabil- ity, and a conspicuous thermal-shock resistance as engineering 2.1 Sample preparation ceramics.3) Due to the outstanding properties, it can be used to The synthesis method has been described in detail else- produce mechanical bearing to replace conventional bearing steel where.14) Here we only repeated the preparation method briefly. in some special industries, such as avigation technology, high- Commercially available powders of Si (98% purity, 1–2 μm in temperature and corrosion environment. size), Al (99.9% purity, 3 μm in size), and SiO2 (99.9% purity, To produce β-SiAlON ceramics, many methods such as hot 0.8 μm in size) were used as starting materials. β-SiAlON pow- press (HP),4) pressureless sintering,5) hot isostatic pressing ders (CSed product, unknown purity, 0.5 μm in size) were added (HIP),2) carbothermal reduction-nitridation (CRN),6) and com- as the diluent. The chemical reaction for the synthesis of β- bustion synthesis (CS)7)–10) have been previously used. However, SiAlON from the abovementioned starting materials can be there is still considerable interest in developing new methods to shown as follows: prepare dense β-SiAlON ceramics. Spark plasma sintering (SPS) ()615− ..zzSi++ Al 05 z SiO22 +−() 405 . zN → is a newly developed sintering technique which has been proved (1) − a rapid, effective densification technique to synthesize many β Si68−−zzz Al O N z kinds of materials,11)–13) because it has such many merits as quick where, z takes values of 1, 2, and 3. The mass percent of the heating and cooling rate, uniform heating and short sintering diluent was determined according to our preliminary experiment. time. The CSed powder was first subjected to planetary ball milling As is known, to densify a product, oxide sintering aids such as for 60 min, then was compacted into a carbon die of 10 mm in Y3O2 are commonly used via a transient liquid phase, however, inner diameter and sintered using a SPS system under vacuum the glassy phase can deteriorate the thermodynamics properties of lower than 4 Pa at a compressive stress of 50 MPa. The result- 3) 14) at elevated temperature. In our previous study, dense β-Si6– ing compacts were heated from room temperature to 600°C in zAlzOzN8–z (z = 1, 2, and 3) has been successfully synthesized by 5 min, and then were heated to 1600°C at a rate of 30°C/min. the combination of CS and SPS without any sintering aid. Vickers The compacts were maintained at this temperature for 10 min hardness as a principal parameter for the mechanical character- before the power was turned off. ization of materials has been commonly used as a technique to The phases of the CS-SPSed products were analyzed using an measure the mechanical properties of materials, but the micro- X-ray diffraction (XRD) (Cu Kα-radiation). The morphology hardness commonly decreases with applied load, which is known was examined by scanning electron microscopy (SEM). as the indentation size effect (ISE).15),16) To the author’s knowl- 2.2 Vickers hardness experiment † Corresponding author: T. Akiyama; E-mail: [email protected]. The Vickers hardness of polished CS-SPSed compacts was ac.jp measured using a Vickers microhardness tester with a diamond

Table 1. Characteristics of CS-SPSed β-SiAlONs Aimed z Lattice constant a (Å) Lattice constant c (Å) Calculated z Relative density value (–) [data from JCPDS] [data from JCPDS] value (–)* (%) 1.0 7.633 [7.635] 2.926 [2.934] 0.99 98.8 2.0 7.661 [7.666] 2.912 [2.960] 1.96 99.5 3.0 7.689 [7.678] 2.976 [2.977] 2.91 99.5 * Ekström proposed equations: a = 0.7603 + 0.00296z (nm); c = 0.2907 + 0.00255z (nm).

indenter of regular pyramid with an opposite angle of 136°. The experiments were performed under the loads between 0.981 N (0.1 kg) to 19.614 N (2.0 kg) at room temperature. The dwell time for each load was 20 s. An average of at least 5 readings at different locations of the specimen surfaces was taken for each specimen. The Vickers hardness (Hv, GPa) was calculated according to Eq. (2) F Hv =18. 19 (2) d2 Where, F is the applied test load (N), d is the average diagonal distance of the indented impressions (mm). 3. Results and discussion The characteristics of CS-SPSed β-SiAlON products are sum- marized in Table 1. The z value after SPS was calculated follow- ing Ekström proposed equations.2) The lattice constants a and c were calculated from the XRD data and compared with the corresponding JCPDS, while a value of z was calculated by using the lattice parameter a. Evidently, the calculated z values were in good agreement with the expected z values in the raw mixtures. The relative densities of the CS-SPSed β-SiAlON with z = 1 to 3 were about 99% of the corresponding theoretical density. Figure 1 shows the SEM images of β-SiAlON products with different z values after SPS process. The products looks dense solid, and the grain size increases clearly with the increase in z value. Figure 2 shows the measured Hv as a function of indentation load for the β-SiAlON (z = 1, 2, and 3). The variation of Hv with applied indentation test load for all samples shows that Hv decreases with the increase of applied load at low load region. It reaches a saturation value at higher loads when F ≥ 4.903 N for z = 1 and 2. However, for z = 3 the ISE shows more clearly than that of z = 1 and 2. Also, as the figure shows, Hv decreases with the increase in z value. At the max applying load of 19.614 N the Vickers hardness value of 17.6 GPa was obtained when z = 1, and with the increase of z value, it became lower values of 16.2 and 14.7 GPa when z = 2 and 3. The decrease of Vickers hardness with the increase of z value Fig. 1. SEM images of CS-SPSed β-Si Al O N (a) z = 1 (b) z = 2 can be mainly attributed to the increase of grain size shown in 6–z z z 8–z (c) z = 3. Fig. 1. In addition, the expansion of crystal lattice with the increase of z value shown in Table 1 can also give function on the decrease of Vickers hardness. When bond Si–N is substituted by Al–O, there are also Al–N and Si–O bonds formed simulta- and ionic bond. According to the electronegativation difference neously. As we know, the bond energy between different ele- of elements,17) the covalent bond ratio of Si–N bond shows ments gives contribution to hardness and strength of materials. higher than Al–O, Al–N and Si–O bonds, consequently, bond Bond energy gets higher with the decrease of bond length, and energy should get weaker with the increase in z value. covalent bond has higher bond energy than ionic bond. There- Figure 3 shows the Vickers hardness of our CS-SPSed β- fore, by more Al–O replacing of Si–N, the lattice constants, a, SiAlON comparing with β-SiAlON, β-Si3N4, and other ceramics and c, get larger, correspondingly, the bond energy gets weaker prepared by different methods. The hardness of HIPed β- with the increase in z value. In addition, as we know, the chem- SiAlON2) shows lower than that of the CS-SPSed β-SiAlON in ical bond of most ceramics is mixed bond between covalent bond lower z values, while it shows higher in higher z values. This

251 JCS-Japan Yi et al.: Vickers hardness of β-SiAlON prepared by a combination of combustion synthesis and spark plasma sintering

test load. The Vickers hardness reaches a saturation value when the applied indentation load F ≥ 4.903 N for z = 1 and 2. How- ever, the ISE shows more clearly on the β-SiAlON with z = 3 than those with lower z values. The results also show that the Vickers hardness decreases with the increase of z value. This should mainly attribute to the increase in grain size, and to the expansion of crystal caused by substitution of Si–N by Al–O. The Vickers hardness ranges between 14.7 and 17.6 GPa due to different z values under the indentation load of 19.614 N.

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