Effects of sintering additives on mechanical properties and microstructure of Si3N4 ceramics by microwave sintering

Effects of sintering additives on mechanical properties and microstructure of Si3N4 ceramics by microwave sintering

Materials Science & Engineering A 684 (2017) 127–134 Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: w...

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Materials Science & Engineering A 684 (2017) 127–134

Contents lists available at ScienceDirect

Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea

Effects of sintering additives on mechanical properties and microstructure of Si3N4 ceramics by microwave sintering

MARK



Weiwei Xua,c, Zengbin Yina,c, , Juntang Yuana,c, Zhenhua Wanga,c, Yihang Fangb a

School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China Zhejiang Provincial Key Laboratory for Cutting Tools, PR China Collaborative Innovation Center of High-End Equipment Manufacturing Technology (Nanjing University of Science and Technology), Ministry of Industry and Information Technology, PR China b c

A R T I C L E I N F O

A BS T RAC T

Keywords: Si3N4 ceramic Microwave sintering Sintering additives Mechanical properties Microstructure

Si3N4 ceramics with excellent mechanical properties and microstructure were fabricated by microwave sintering. The effects of sintering additives on the densification, phase transformation and grain growth of Si3N4 ceramics were investigated. The results showed that Y2O3 can promote the phase transformation, MgO can decrease the sintering temperature, Al2O3 can increase the Vickers hardness of the samples and the combination of Y2O3 and MgO was more effective in enhancing the densification and phase transformation in the microwave sintering. The samples with 5 wt%Y2O3, 5 wt%MgO and 2 wt%Al2O3 sintering additives had the optimum mechanical properties when sintered at 1700 ℃ for 10 min. Its relative density, fracture toughness and Vickers hardness were 98.52 ± 0.13%, 6.44 ± 0.02 MPa m1/2 and 14.92 ± 0.20 GPa, respectively. Compared to other microwave sintered Si3N4, the hardness and fracture toughness were enhanced by 6.6–24.3% and 5.6– 11%, respectively, while the sintering holding time was reduced by 33.3–83.3%.

1. Introduction

of ceramic materials. In recent years, microwave sintering of materials have attracted the attention of many researchers due to its advantages over conventional sintering techniques in fabricating of silicon nitride ceramics, such as enhancing densification and phase transformation, reducing sintering temperature and holding time, improving mechanical properties and capability of producing unique microstructure. There have been several studies on preparation of silicon nitride ceramics which make comparisons between microwave sintering and conventional sintering using the same sintering schedule. Yoon Chang Kim et al. [9] made comparisons between microwave sintering and conventional pressureless sintering. It was observed that the relatively density of microwave sintered specimens reached more than 99% and phase transformation rate reached about 100% at the sintering temperature of 1600 ℃, while in the case of conventional pressureless sintering, such high density and phase transformation rate could not be obtained even at a sintering temperature of 1850 ℃. Jones et al. [10,11] sintered silicon nitride ceramic by 28 GHz microwave sintering at the sintering temperature from 1200 ℃ to 1750 ℃. It showed that the temperature of densification and phase transformation sintered by microwave energy would be 200 ℃ lower than the conventional pressureless sintering. Chockalingam et al. [12] investigated the mechanical properties of

Silicon nitride is considered as one of the promising ceramic materials which has excellent high temperature mechanical properties, wear resistance and thermal properties and has been widely used in high-temperature gas filter, heat insulators, cutting tools, etc. [1]. Nowadays, the main conventional sintering techniques [2–6] of fabricating silicon nitride ceramics are pressureless sintering, reaction bonding sintering, gas pressure sintering, hot pressing sintering and hot isostatic pressing sintering, etc. For these conventional sintering, high sintering temperature, slow heating rate and long holding time are usually needed in order to fully dense the materials. The long sintering period is not cost effective and limits the wide use of the ceramic materials. With the development of science technology, some new sintering techniques are developed which include spark plasma sintering [7], microwave sintering [8], etc. The new sintering techniques not only improve the mechanical properties of ceramics but also greatly shorten the sintering period, while spark plasma sintering assisted with mechanical pressure which is similar with hot pressing sintering can only fabricate products with simple shape, moreover its equipment is very expensive and single furnace has low production. Microwave sintering is a potential way of promoting industrialization production



Corresponding author. E-mail address: [email protected] (Z. Yin).

http://dx.doi.org/10.1016/j.msea.2016.12.031 Received 14 October 2016; Received in revised form 29 November 2016; Accepted 7 December 2016 Available online 09 December 2016 0921-5093/ © 2016 Elsevier B.V. All rights reserved.

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Table 1 The characteristics of the raw powders of silicon nitride ceramic materials. Powder

Grain size (μm)

Purity (%)

Manufacturer

α-Si3N4 Al2O3 MgO Y2O3

0.7 0.5 1 1

99.9 99.9 99.9 99.9

Shanghai Shanghai Shanghai Shanghai

Table 2 The composition ratios of three series silicon nitride ceramic materials (wt%). Symbol

Si3N4

Y2O3

MgO

Al2O3

SYM SMA SYA

90 90 90

5 / 5

5 5 /

/ 5 5 Fig. 2. The relative densities and hardness of samples with 5 wt%−13 wt% Y2O3 and 5 wt% Al2O3 as additives microwave sintered at 1700 ℃ for 10 min.

2.45 GHz microwave sintered Si3N4 ceramics. They reported that microwave sintered samples exhibited higher relative density and hardness compared with the conventional sintered samples which performed in Centorr Industry's tungsten heating element furnace, its hardness was enhanced by about 40% and its holding time was reduced by 75%. Hirota et al. [13] reported microwave sintering can promote the production of columnar grain and bimodal microstructure compared with the conventional pressure sintering. From these reports, it can be found that microwave sintering is a potential way of fabricating Si3N4 ceramics with high properties.

Si3N4 is difficult to densify by solid state sintering for its high degree of covalent bonding, thus liquid-phase sintering with suitable additives is a preferred method of promoting densification. In the conventional sintering, various metal oxides, lanthanide oxides and rare earth oxides were used as sintering additives [14–17], and kinds of conventional sintered silicon nitride ceramics with high relative densities and superior mechanical properties had been fabricated using the compositions of two or three sintering additives [18–20]. Especially

Fig. 1. (a) relative densities, (b) hardness and (c) fracture toughness of SYM, SMA and SYA sintered at different sintering temperatures.

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Fig. 3. the XRD analysis of (a) SYM, (b) SMA and (c) SYA at different sintering temperatures.

Al2O3 were selected as the sintering additives. The effects of different combinations and content of additives on the mechanical properties and microstructure of silicon nitride ceramic were investigated. The experiments were conducted with controlling the content of additives in a proper range by comparing the effects of each additive. The optimal contents of sintering additives were obtained by orthogonal experiments. Moreover, the effects of sintering temperature on the phase transformation and mechanical properties were also studied.

in the studies of rare-earth oxide additives, there have been many reports in recent years, such as Tatarko, Peter [21–24] and Kašiarová [25] have done a lot of research work on the influence of various rareearth oxide additives on the oxidation resistance, wear resistance, microstructure, mechanical properties and thermal shock resistance of hot-pressed Si3N4-based composites, it was confirmed that the materials sintered with the additives having smaller ionic radius of rare-earth element exhibited higher oxidation resistance, wear resistance, hardness, fracture toughness and aspect ratio of β-Si3N4 grains, while its thermal shock resistance increased with the increase of ionic radius of rare-earth element. For the microwave sintered Si3N4 ceramics, sintering additives were also very important for promoting the densification. While few reports were focused on the effect of sintering additives on the microstructure and mechanical properties of microwave sintered Si3N4 ceramics. From the existing reports [1,26,27], it could be found that there were still some problems needed to be solved in the microwave sintering of Si3N4 ceramics, such as using higher contents of sintering additives than the conventional sintering, low relative density and poor mechanical properties. The content of sintering additives including some expensive rare earth oxides in microwave sintering were up to 15–19 wt%. The relative densities of some microwave sintered samples were only 93– 96% and the mechanical properties were still at a low level. These problems have greatly limited the application of microwave sintered Si3N4 ceramics. In the present paper, we attempted to prepare monolithic Si3N4 ceramics with good properties by microwave sintering in a cost saving way. Three common sintering additives including Y2O3, MgO and

2. Experimental procedures 2.1. Preparation of Si3N4 ceramics The characteristics of the raw powders of Si3N4 ceramics are listed in Table 1 and the composition ratios of three series ceramic materials are given in Table 2. The raw powders were ball-milled with silicon nitride balls for 48 h at a speed of 250rmp using a planetary ball mill (Model QM-3SP2, China). The mixed powders were dried in a vacuum drying oven (Model DZF, China), and then sieved through a 100 mesh sieve. Green compacts with the size of 13 mm×13 mm×6.5 mm (length × width × height) were produced in a die at a uniaxial pressure of 175 MPa for 2 min and then sintered in a 2.45 GHz microwave furnace (Model NJZ1, Nanjing, China) under a nitrogen atmosphere. The sintering temperature was 1500–1750 ℃ with the holding time of 10 min. The average heating rate was about 40 °C/min. After that, the samples were naturally cooled to room temperature in the furnace. 129

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a

b

5μm

5μm

c

5μm Fig. 4. SEM micrographs of etched surfaces of SYM system respectively sintered at (a) 1600 ℃, (b)1650 ℃ and (c)1700 ℃.

b

a

5μm

5μm c

5μm Fig. 5. SEM micrographs of etched surfaces of SMA system respectively sintered at (a)1600 ℃, (b)1650 ℃ and (c)1700 ℃.

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a

b

5μm

5μm

c

5μm Fig. 6. SEM micrographs of etched surfaces of SYA system respectively sintered at (a)1600 ℃, (b)1650 ℃ and (c)1700 ℃.

duration of 15 s. The indentation fracture toughness (KIc ) was determined by Vickers indention method, using the equation [28]:

Table 3 Experiment scheme design of orthogonal experiment L9(34). Test number

Sintering temperature A

Y2O3/wt% B

MgO/wt% C

Al2O3/wt% D

1 2 3 4 5 6 7 8 9

1(1600 ℃) 1(1600 ℃) 1(1600 ℃) 2(1650 ℃) 2(1650 ℃) 2(1650 ℃) 3(1700 ℃) 3(1700 ℃) 3(1700 ℃)

1(3%) 2(5%) 3(7%) 1(3%) 2(5%) 3(7%) 1(3%) 2(5%) 3(7%)

1(3%) 2(5%) 3(7%) 2(5%) 3(7%) 1(3%) 3(7%) 1(3%) 2(5%)

1(0%) 2(2%) 3(4%) 3(4%) 1(0%) 2(2%) 2(2%) 3(4%) 1(0%)

⎛ c ⎞−3/2 KIC =0. 203∙HV ∙a1/2∙⎜ ⎟ ⎝a⎠

(1)

Where a is half of the diagonal of the indent and c is half of the crack length. For each experimental condition, five specimens were tested. The etched surfaces were observed by scanning electron microscopy (SEM, Quant 250FEG, USA). Phase identification was carried out by X-ray diffraction analysis (XRD, D8 Advance, Germany) with copper Kα radiation. 3. Results and discussion

Table 4 The relative densities, hardness and fracture toughness of all the samples investigated in orthogonal experiments. Test number

Relative density (%)

Hardness (GPa)

Fracture toughness (MPa m1/2)

1 2 3 4 5 6 7 8 9

96.47 ± 0.21 96.88 ± 0.17 93.20 ± 0.34 95.36 ± 0.31 98.47 ± 0.17 96.69 ± 0.23 94.85 ± 0.28 98.80 ± 0.08 98.20 ± 0.11

13.55 ± 0.52 13.98 ± 0.12 12.37 ± 0.09 13.74 ± 0.15 13.28 ± 0.13 13.28 ± 0.15 13.84 ± 0.29 14.71 ± 0.16 14.14 ± 0.02

6.89 ± 0.28 6.18 ± 0.18 5.71 ± 0.30 7.67 ± 0.22 4.61 ± 0.09 7.6 ± 0.49 5.12 ± 0.06 6.38 ± 0.15 5.68 ± 0.10

3.1. Effects of the sintering additives on the mechanical properties and microstructure Fig. 1 shows the relative densities, hardness and fracture toughness of three samples sintered at different temperatures. In the conventional sintering, the total amount of sintering additives was about 6–12 wt%. For comparison, 5 wt% content of each additive was used in the present experiments. As can be seen in Fig. 1, the relative density, hardness and fracture toughness showed a downward trend after the first rise with the increase of sintering temperature. From the comparison of date in Fig. 1, the relative density and hardness of SYM was higher than another two series, its density was 96.35 ± 0.10%, hardness was 13.62 ± 0.31 GPa and fracture toughness was 7.84 ± 0.16 MPa m1/2 sintered at 1650 ℃. SMA with the density of 95.27 ± 0.10%, hardness of 12.34 ± 0.08 GPa and fracture toughness of 7.34 ± 0.37 MPa m1/2 can be sintered at a low temperature (1600 ℃). However, SYA was difficult to densify with the same content sintering additives, and its relative density was much lower than another two series, while the high hardness of 13.09 ± 0.22 GPa was obtained sintered at 1700 ℃ with the low relative density of 91.43 ± 0.15%. It was concluded that MgO can accelerate the densification and

2.2. Characterization The relative densities of specimens were measured by Archimedes' method with the distilled water as medium. The Vickers hardness (HV ) was measured on the polished surface by a Vickers diamond pyramid indenter (Model HV50 China) with a load of 196 N and a loading 131

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b

a

5μm c

10μm Fig. 7. (a)the XRD analysis, (b)SEM micrograph of etched surface and (c)crack propagation profile of SYMA552 sintered at 1700 ℃ for 10 min.

the microwave sintering was working under a negative atmosphere pressure. So, more additives probably were required to provide sintering driving force for the densification. As Tiegs et al. [27] reported that the Si3N4 ceramics with density of 96% by microwave sintering can be achieved with rising the content of additives up to 12 wt%Y2O3 and 4 wt%Al2O3. To this issue, some experiments for the composition of Y2O3 and Al2O3 were also conducted. The content of Y2O3 were changed from 5 wt% to 13 wt%, while the content of Al2O3 was constant as shown in Fig. 2. It showed that the relative densities increased with the increase of sintering additives, while the hardness only had a small increase with the content of additives increasing from 10 wt% to 12 wt% and it would decrease with the further increase of additives. As a result, it was not desirable to increase densities only by increasing content of sintering additives. And high content of additives would cause the increase of glass phase content and decrease of β-Si3N4 content which was detrimental to the mechanical properties. Additionally, a greater additive amount would affect the grain growth and phase transformation [35]. Fig. 3 shows the XRD analysis of Si3N4 ceramics with different sintering additives sintered at different temperatures. It can be seen that Si3N4 existed in α and β phase and no sintering additives phase existed in XRD analysis, it might that sintering additives reacted with

Table 5 Microstructure characteristics of hot-pressed specimen (plane N) [37]. Composition

Relative density (%)

Average grain size (μm)

Aspect ratio

Si3N4+5 vol% Al2O3/ CRE2O3 Si3N4+10 vol% Al2O3/ CRE2O3 Si3N4+15 vol% Al2O3/ CRE2O3 Si3N4+20 vol% Al2O3/ CRE2O3

98.2 ± 0.3

2.5 ± 0.8

6.5 ± 1.1

98.5 ± 0.2

2.9 ± 0.9

9.5 ± 1.4

98.6 ± 0.1

3.6 ± 1.0

10.1 ± 1.4

98.0 ± 0.2

4.1 ± 1.0

11.2 ± 1.5

decrease the sintering temperature. On the other hand, Y2O3 can also increase the hardness except enhancing densification in some ways. Al2O3 and Y2O3 were the most commonly additives used in the conventional sintering of Si3N4 ceramics [29–33], while in the microwave sintering it was difficult to be densified with the same content of additives which used in the conventional sintering. The possible reason was that conventional sintering was working under a positive pressure atmosphere and especially hot pressing sintering can achieve complete densification with the applying of mechanical pressure [34], oppositely Table 6 Comparison of mechanical properties of microwave sintered silicon nitride. Additives

Sintering temperature (℃)

Holding Time (min)

Relative density (%)

Hardness (GPa)

6 wt%Y2O3+2 wt%Al2O3+ 5 wt%ZrO2+6 wt%Li2CO3[1] 6 wt% Y2O3+2 wt% Al2O3[11] 6 wt%Y2O3+4 wt%MgO+2.5 wt%ZrO2[12] 10 wt%LiYO2+5 wt%ZrO2[26]

1550

30

97

12

/

1850 1750 1600

60 15 15

98 95 93

14 14 13

6 5.8 6.1

132

Fracture toughness (MPa.m1/2)

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relative density, hardness and fracture toughness were 98.52 ± 0.13%, 14.92 ± 0.2 GPa and 6.44 ± 0.02 MPa m1/2, respectively. The XRD analysis of SYMA552 sintered at 1700 ℃ for 10 min is shown in Fig. 7(a), and no α-Si3N4 was identified. It was proved that the phase transformation of Si3N4 from α phase to β phase was fully completed. From the SEM micrograph of etched surface of SYMA552 sintered at 1700 ℃ for 10 min as shown in Fig. 7(b), columnar β-Si3N4 grains can be observed, and a well-controlled bimodal microstructure which resulted in high fracture toughness was formed. The length of the big grain size was about 4.21 µm and the width was about 1.05 µm, while the length of small grain size was about 2.63 µm, the width was about 0.41 µm. Compared to the Si3N4 with Al2O3 and a mixed concentrate of yttrium and rare earth oxides (CRE2O3) sintered by hot-pressing as list in Table 5, the grain size of sample sintered by microwave was bigger and had two different sizes with relatively small aspect ratio. It indicated that microwave sintering was effective to promote the formation of a well-controlled bimodal microstructure. In Fig. 7(c), crack bridging(indicated by dotted box) and crack deflection (indicated by arrows) which can effectively consume the fracture energy was observed. It is beneficial to improve the fracture toughness of microwave sintered Si3N4 ceramics. The sintering conditions and mechanical properties of other Si3N4 ceramics with different sintering additives by microwave sintering are listed in Table 6. Compared to these, the hardness and fracture toughness of the present Si3N4 ceramics were enhanced by 6.6– 24.3% and 5.6–11%, respectively, while the sintering holding time was reduced by 33.3–83.3%. Thus, there is great potential for the industrial production of Si3N4 ceramics by microwave sintering.

each other and existed as glassy phase. The phase transformation of Si3N4 from α to β occurred below 1500 ℃ in the microwave sintering, and it fully transformed at 1650℃for the sample with 5 wt%Y2O3+5 wt %MgO and basically completed at 1700℃for the sample with 5 wt% Y2O3+5 wt%Al2O3. While the phase transformation rate of the sample with 5 wt%MgO+5 wt%Al2O3 was still at a relatively low level even though the sintering temperature was up to 1700 ℃. It can be obtained that Y2O3 can promote the phase transformation and the combination of Y2O3 and MgO had more remarkable effect than another two combinations on phase transformation, which could promote the transformation at a lower temperature. It was worth noting that this phenomenon was quite different from the conventional reaction bonding sintering. Lee et al. [36] reported that Y2O3 and MgO can enhance densification, while it was harmful for phase transformation. From Fig. 1 and Fig. 3, it could be found that the mechanical properties of silicon nitride ceramic materials were not only related with its densification but also closely related with the phase transformation rate. In the system of SYM, the densification of samples increased with the rise of temperature from 1700 ℃ to 1750 ℃, while its hardness decreased which corresponded to the decrease of phase transformation rate. Generally, higher phase transformation rate result in higher hardness for each series of microwave sintered Si3N4 ceramics. The surfaces of SYM, SMA and SYA sintered at 1600 ℃, 1650 ℃ and 1700 ℃ for 10 min by microwave sintering were etched in molten NaOH for 2 min. The SEM micrographs of etched surfaces are shown in Fig. 4, Fig. 5 and Fig. 6. It can be indicated that the formation and growth of β-Si3N4 had great relationship with sintering temperature and sintering additives. For the three samples, it was evident that a part of elongated β-Si3N4 had been formed since 1600 ℃. And different additives leaded to different distribution of grain size. As can be seen from Fig. 4, the grain size of β-Si3N4 was homogeneous at 1650 ℃ for SYM, while when sintering temperature was up to 1700 ℃, a typical bimodal grain size distribution which a part of grain size of β-Si3N4 in both direction of width and length was bigger than another part was formed, the length and width of big grain were about 4.93 µm and 0.82 µm, respectively, the length and width of small grain were about 2.74 µm and 0.41 µm, respectively. For SMA, only a small number of elongated β-Si3N4 can be observed for its lower phase transformation rate (Fig. 5). For SYA, bimodal grain structure was also evident as shown in Fig. 6(c) which sintered at 1700 ℃, the length and width of big grain were about 3.24 µm and 0.94 µm, respectively, the length and width of small grain were about 2.30 µm and 0.41 µm, respectively, it was evidence that the grain size and aspect-ratio of β-Si3N4 were smaller than that of microwave sintered SYM. It can be concluded that different additives had different effects on the mechanical properties, phase transformation and the grain growth of β-Si3N4, and it was inadvisable to sinter Si3N4 ceramics by microwave sintering with the same kinds and content of additives used in the conventional sintering. Therefore, in order to fabricate fully dense Si3N4 ceramics by microwave sintering, it is important to choose the appropriate sintering additives and their optimal content.

4. Conclusions One kind of Si3N4 ceramics with preferable mechanical properties and excellent microstructure were fabricated by microwave sintering. The effects of kinds and content of additives on the mechanical properties, phase transformation and microstructure were studied. Several conclusions can be drawn as follows. (1) For the microwave sintered Si3N4 ceramics, both MgO and Y2O3 can promote the densification, and MgO can decrease the sintering temperature. Adding a moderate amount of Al2O3 can improve the mechanical properties, but the combination of Y2O3 and MgO was more effective in enhancing the densification and phase transformation. (2) In the microwave sintering, a bimodal microstructure with two kinds of aspect ratio which were about 4.01 and 6.41 was formed. Compared with conventional hot-pressed sintering, microwave sintering was effective to enhance the grain growth and promote the formation of a well-controlled bimodal microstructure. (3) The Si3N4 ceramics with the sintering additives of 5 wt% Y2O3, 5 wt%MgO and 2 wt%Al2O3 which were sintered at 1700 ℃ for 10 min had the optimum mechanical properties. Its relative density, hardness and fracture toughness were 98.52 ± 0.13%, 14.92 ± 0.20 GPa and 6.44 ± 0.02 MPa m1/2, respectively. Compared to other microwave sintered Si3N4, the hardness and fracture toughness of the present Si3N4 were enhanced by 6.6– 24.3% and 5.6–11%, respectively, while the sintering holding time was reduced by 33.3–83.3%.

3.2. Optimization selection of the content of sintering additives For getting the optimal contents of the sintering additives, the orthogonal experiment was designed as shown in Table 3. In the orthogonal experiments, four elements which concluded sintering temperature and the contents of Y2O3, MgO and Al2O3 were considered and each element had three levels. The relative densities, hardness and fracture toughness of all the samples investigated in orthogonal experiments were listed in Table 4. By calculating and comparing the sum of the index value of each element under different levels, the best condition (A3B2C2D2) was obtained. It indicated that the sample (SYMA552) with 5 wt%Y2O3, 5 wt%MgO and 2 wt%Al2O3 sintered at 1700 ℃ would get the optimal density and mechanical properties. The

Acknowledgements The work is supported by Open Research Program of Zhejiang Provincial Key Laboratory for Cutting Tools (ZD201602), National Natural Science Foundation of China (51505227), Key Laboratory of High-efficiency and Clean Mechanical Manufacture at Shandong University, Ministry of Education Natural Science Foundation of Jiangsu Province of China (BK20150783). 133

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