Subsolidus phase relationships in the system Dy2O3-Si3N4-AlN-Al2O3

Journal of rhe European Ceramic Society 16 (19%) 1277-1282 0 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0955-2219/96/$15.00

Pil:S0955-2219(96)00053-2

Subsolidus Phase Relationships Dy,O,-S&N,--A 1N-A&O3

in the System

W. Y. Sun,” D. S. Y;an,’ L. Gao,” H. Mandaf’ & D. P. Thompsonb “The State Key Lab on High. Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Academia Sinica, Shanghai, IPeople’s Republic of China hWolfson Laboratory, Materials Division, Department of Mechanical, Materials & Manufacturing Engineering, University of Newcastle upon Tyne, UK (Received 20 November 1995;; revised version received 20 February 1996; accepted 28 February 1996)

In the systems Ln-Al-O-N (where Ln = Ce + Sm) two N-containing compounds exist - Ln,AlO,N and Ln2A11201gN2 (magneto-plumbite, MP compound) - and no garnet phase occurs; instead, the aluminate with 1 : 1 becomes stable.’ The Ln,AlO,N and MP compounds do not occur in those systems containing rare earth oxides with cations smaller than that of Gd.2,4 Therefore, in the five-component systems with low Z-value rare earth elements, the phase relationships become a little bit different from those in Y-Si-A1-O-N.5-7 In the oxygen-rich region where two five-component phases8,9 - U-phase (Ln3Si3,A13+,0,2+,N2-.J and W-phase (Ln4Si,A1503,,N) - exist, the former is stable for rare earth cations between La and Dy and the latter stable only in La, Ce and Nd systems. Recent work on Ln-melilite’“~” indicates that the formation of melilite solid solution (Ln2Si3_,A1,03+.UN4_.~) occurs easily for melilite containing light rare earth elements and that the top solubility limit of Al and 0 decreases with increasing Z-value (x = 1 for Nd and x = 0.6 for Er). In recent years, work has focused on the Sm(or Nd)-Si-Al-O-N system.‘2%‘3 The results indicate Nd has the same behaviour as Sm: below the subsolidus temperatures Nd(Sm)AlO, coexists with p-sialon (P-Si,N, solid solution, Si,,Al,O,N,, p’) and all AlN polytypoids and a-sialon can only be compatible with melilite solid solution. Dysprosium is one of the central elements in the rare earth series; in this region, phase relationships in Ln-Si-Al-O-N systems are changing from those observed for the low-Z elements (La, Nd, Sm) to those observed for the high-Z elements (Er, Yb... and Y). Studies on the Dy-Si-Al-O-N system will therefore give an indication of whether these changes occur simultaneously or whether there is overlap in this region between the different phase relationships observed for the high-Z and low-Z

Abstract

Subsolidus phase relationships in the system Dy-SiAl-O-N have been determined. Forty-two compatibility tetrahedra were established in the region Dy,O,Si,N,AIN-AI,O,. Within this region, DyAG(Dy,AljO,,) and M’-phase (Dy,Si,_,Al.~O,+,N,~,) are the only two important compounds which have tie lines joined to p-sialon and AIN polytypoid phases. a-Sialon coexists with borh M’ phase and DyAG (only with oxygen-rich a’). Copyright 0 1996 Elsevier Science Ltd

1 Introduction The importance of using rare earth oxides for the densification of silicon nitride ceramics has been recognised in recent years. Not only are they very effective along with alumina for densification, either singly or in combination with yttria, but they can also be accommodated in the a-S&N, lattice forming a-sialon (c&N4 solid solution with the composition Ln.uSi,2_(m+n~Alm+nO~N~~~, also called (Y’),thus providing an opportunity for decreasing the transient liquid phase content after sintering and hence reducing the amount of residual grain boundary glass. Therefore, phase relationships in Ln-Si-Al-O-N systems are of particular interest. It is known that elements in the lanthanide (Ln) series are similar to yttrium in compound formation and phase relationships. Previous work on Ln-Si-O-N subsystems,’ indicates that the phase relationships are nearly the same as in the Y-Si-O-N system, but in Ln-Al-O-N systems’ the phase relationships vary with the atomic number of the rare earth element. The phase relationships in systems with high Z-value of the rare earth element are similar to those in Y-Al-O-N.3 1277

1278

W. Y. Sun et al.

regimes. To elucidate the variation of phase relationships in the Ln-Si-Al-O-N system with rare earth element and to study the Dy-Si-Al-O-N system itself, the aim of the current work is to determine the subsolidus phase relationships in the region defined by the four end-members S&N,, AlN, A1,03 and Dy,03 (Fig. 1). This region includes the phases cu-sialon, p-sialon and AlN polytypoids, and covers the range of compositions used for the design and processing of commercial multi-phase sialon ceramics.

cooling rate (- 50°C min-I), whilst those at Newcastle were fired in graphite-resistant furnace which could be cooled down to room temperature very quickly (- 100°C min’). A nitrogen atmosphere was used in all cases. The specimens with compositions near liquid-phase region were heattreated after firing. All specimens were examined by X-ray diffractometry or using a Hagg-Guinier focusing camera. Only specimens showing less than 3% weight loss after firing were used for data analysis.

2 Experimental Procedure

3 Results and Discussion

The present paper is the result of two work programmes carried out separately at Shanghai Institute of Ceramics and the University of Newcastle upon Tyne. The symbol * is used to represent the experiments carried out at Newcastle. The starting powders used were c&&N4 (Starck Hl and Starck LClO*), AlN (supplied by Zhuzhou Institute of Hard Metal Alloys, containing 1.2% oxygen and Starck Grade B*), AllO3 (99.99%, produced by decomposing ammonium alum and Alcoa Al7*), Dy,O, (99.9%, Yaolung Chemical Works, China and 99.9%, Aldrich Chemical Co. Ltd*). The oxygen contents of the nitride powders were taken into account in calculating compositions. The starting powders were weighed out and ground in absolute alcohol or isopropanol* using an agate pestle and mortar. The mixed powders were dried, pressed into pellets 10 mm in diameter and then isostatically pressed under a pressure of 200 MPa. The specimens prepared at Shanghai were fired in a graphite-resistant furnace with a rather slow

Twenty-three compositions in the region bounded by S&N,, AlN, Al,O, and Dy,O, were studied to establish compatibility relationships. The binary tie lines established were based on the results listed in Table 1. Based on the much more extensive knowledge of phase relationships in the Y-Si-Al-O-N and Sm(Nd)-Si-Al-O-N systems and other information in the literature listed above, 42 compatibility tetrahedra were established in this region (Table 2). In the present work a-sialon is considered as a point composition and its detailed solid solution range has not been determined. As indicated in this table, DyAG(Dy3A150,,) is a stable phase and is compatible with all the AlN polytypoid phases, p-sialon (from PI0 to &,,) and a-sialon (oxygen-rich), forming 12 AlN-polytypoids-containing compatibility tetrahedra (Fig. 2) and one a’+‘-containing compatibility tetrahedron, a’-&-DyAG-12H (Fig. 3). M’ was also found to have tie lines with p-sialon from

DYN

Dy203

Si02

Fig. 1. Representation of Dy-SiAlON system showing phases occurring in the region bounded by S&N,, DyzO,, A&O, and AlN, and Si-AI-O-N behaviour diagram at 1700°C (cY-sialon plane = the plane with a-sialon composition Dy,Si,,_(,+,~l,+,O,N,,).

SO.52

77.70 73.43

70.00

69.39

63*7S

53.22 49.88

49.69

38.43

35.38

16.05

13.30

11.28

11.14

7.80 7.64 6.28

6.28

5.57 3.34

1.67

0.84

1*

2

3*

4

S

6

7 8

9

10*

11*

12

13

14

1s

16 17 18

19

20 21

22

23

88.42 88.24 66.53 50.02

6.01 8.42

1.80

9.14

30.00

40.00

160&l

1550-l

1700-l 1700-l

165&l

80.10

12.85

0.77

1700-l 1700-l 1700-l 16061

1700-l

88.86

170&l

170&l

180&l

1800-l

1750-l

1700- 1 1750-l

1800-l

1750-l

180&l

175&l

1800-l

l750-1

37.80

SO.56

SO.78

17.79

18.58

27.72

13.80 19.70

13.81 13x74

2356

14.61

14.62

13.83

Firing (“C-h)

88.59 36.83 73.35

3-61 23.13 19.60

13.85

4.87

1.87

17.49

11.03

11.71

23.13 21.18

15.16

11.26

644

5.86

2.55

2.92

32.40 0.77

37.05 -

1.29

1.29

29.34

31.97

10.88

9.85 9.24

6.82

5.53

0.00

6.09

5.14

2.74

Composition (wt’%) AlN Al&‘,

(s); M’ (ms); (w); (J’ (w) 1.4) (m); (m); M’ (w) l)(s) 1.4) (s) Dy,OJ(s); J’ (x = 0.6) (s) AlN (m) DyAP; (s); J’ (x= 1) (m) AlN (m)

DyAG DyAP J’ (x = DyAP J(x= J’ (x =

DyAG (s); 21R (w)

J 6) J’ x = 0.6) (s)

M’ (s); DyAG (s); 21R (m)

M’ (x = 0.3) (s); J’(x= l)(ms) M’ (x = 0.75) (s); J’ (x = 1.4) (w)

12H (ms); c-x’(m); p’ (w)

a’(m); 21R (m)

CI’(m); p’ (w); K(m)

p’ (s); 15R (VW)

p’ (s); 15R (VW)

cr’(m); p’ (m); 21R (s)

p’ (s); LY’ (m); 21R (m)

P (s); M’ (s)

P (~1;M’ 64 P’ W ; a’ (ml

P(s);M’ (~1

Phuses presen to

studied in the system Si,N4-AIN-Alz03-Dy203

P (ms); M’ (m); DYAG W P (ms); M’ (m)

1550-24 1550-5” 135&24

1_550-5/’

(~1;

DYAG 6)

(s); a’ (m); /?’ (w); K (w?) (s); (Y’(m); /r (w); K (w?) 21R (m) 21R (m); M’(m) 12H (m); (Y’(m); DyAG (m); p (w) 12H (m); (Y’(m); DyAG (m); /?’ (w)

DyAG DyAG a’ (m); (Y’(m);

P20

ho (s); DYAG 6)

Pm (~1;DYAG 6)

Pm (s); DYAG (4

P (s); M’ (m); DYAG (~1 p’ (s); cr’ (mw); DyAG (m); M’ (w) P’ (s); (Y’(w); M’ (m)

1350-24

135&24 1350-24 1350-24 1350-24 1350-24 1550-s 1350-24 1550-5 1350-24 15S&5

; i3j ;’ y;: s, ’ m

DYAG W

Phases present”

1350-24 1so-5A

(“C-h)

Heat treatment

DyAp = LnAlO,; M’ = Dy,Si,~,Al,O,+,~N,, /3 = /3-Si,N,; /3’, &a. pzo... = p-sialon (/3to.pzo = p’ with z = 0.8, 1.5, Si,zA1,O,N,& ‘cx = cy- Si,N,; (Y’= cY-sialon (Ln,Si,z_(,+,, Al,+,O,N,,,); (x = 0 + 0.75); J’ = Dy,Si,~,Al,0,+,N2_, (x = 0 + 2); K = Dy,Si,O,N,; s = strong; m = medium; mw = medium weak; ms = medium strong; VW= very weak.

Si,N,

No.

Table 1. Compositions

W. Y. Sun et al.

1280

Dy203

DYN

\ \ Jss

\

YAM

\ \

‘K

DyA103

%\

SiO2

Fig. 2. DyAG(Dy,Al,O,,) is compatible with A&O,, all polytypoid phases and AlN forming 12 compatibility tetrahedra: DyAG-&-ISR-Al,O,,DyAG-I5R-Al,O,, DyAG-15R-12H, DyAG-12H-21R, DyAG-21R-27R, DyAG-27R-2H”, DyAG-2H6AlN, DyAG-2HS-AlN-27R, DyAG-27R-AlN-21R,DyAG-21R-AlN-Alz03, DyAG-21R-Alz0342H and DyAG-12H-Al@-15R.

/?&Si,N,) to &, and (Y’,forming two &p-containing compatibility tetrahedra: c&-&-&-M’ (Fig. 4) and a’-&-M’-DyAG. Dy,Si,O,N, (J-phase) also forms a continuous solid solution with Dy,A1,09 in a way similar to Y4Si20702. In the Y-Si-Al-O-N system, the variation of unit cell dimensions (monoclinic) of J’-phase (solid solution of J-phase) with composition is not linear and the compositions near the centre of the range have simple X-ray patterns which index on a tetragonal unit cell related to the monoclinic cell.’ A similar variation of unit cell dimensions also occurs for Dy-J’-phase. The X-ray patterns of the compositions near x = 1.4 (Dy,Si,-,Al,O,+,N2-.~) can index on both tetragonal and monoclinic unit cells. The variation of the unit cell dimensions of Dy-J’ phase will be further determined and published elsewhere. The compound DyAlO, was

found to be compatible only with M’ (melilite solid solution) and %-rich terminals of AlN polytypoids (from AlN to 21R). The tie line between p and LnA103 that occurs in the Nd(Sm)-Si-Al-O-N systems does not exist in the Dy-Si-Al+N system. In the region studied, DyAG, DyAlO,, M’phase and J’-phase were the main Dy-containing stable phases observed. In the S&N,-rich compositions, DyAG phase could only be observed in the specimens after post-sintering heat treatment at a temperature around 1350°C and M’-phase was also observed to increase in post-sintering heat treatment around 1550°C. With the crystallization of DyAG and Ml-phase during heat-treatment, the cu-sialon, which was not expected to occur in these compositions, transformed into IT-sialon. Therefore, in establishing subsolidus phase relationships, judgement of the correct equilibrium

1281

Subsolidus phase relationships in the system Dy,O,-Si,N,AlN-A&O, Dy203

DYN \ \ \ \

_---

AlN Fig. 3. DyAG(Dy3Al,0,,)

2H6 27R21R12H15R

A1203:AlN

SiO2

A1203

is compatible with p-sialon (~10-/360)and a-sialon (oxygen-rich) forming a’-&-12H-DyAG tetrahedron.

phases under subsolidus temperature conditions can sometimes be a difficult problem. Using phase assemblages observed at the same temperature for all conditions studied might lead to mistaken results, since different compositions may need different temperatures to fully crystallize the liquid phase. In the present work, all starting compositions were designed to fall on supposed tie lines or into supposed compatibility triangles. For example, compositions Nos 2 and 3 were on the lines between M’ (Ln,Si,AlO,N,) and /3-sialon (& and &) and Nos 1, 5, 6 and 7 were designed on the composition lines between DyAG and p-sialon (PO, PI0 and & and /330, respectively). If the expected phases were obtained after heat-treatment under a particular set of conditions, then

compatibility

equilibrium was believed to have been achieved. Based on this philosophy, compositions marked with the symbol A in Table 1, i.e. compositions Nos 1, 3, and 4, were believed to have achieved equilibrium and were used to establish the subsolidus phase relationships. In the compositions 3,5 and 6, the expected phases are /Y with DyAG (Nos 5 and 6) or Ml-phase (No. 3). These phase assemblages could only be achieved in post-sintering heat-treatment. The occurrence of a-sialon as a transitional phase can be attributed to the kinetic priority in the formation of Dy-cr-sialon over the formation of DyAG and Ml-phase at high temperatures. In heat treatment the DyAG and Ml-phase is crystallized out at the exhaustion of Dy content in the liquid phase, thus,making cr-sialon transform Dy203

DYN

SiO2

AlN Fig. 4. M’ (melilite solid solution) is compatible

N k203 with p-sialon (PO-&,J and a-sialon forming a’-P-M’ compatibility

tetrahedron.

1282

W. Y. Sun

Table 2. Subsolidus

compatibility tetrahedra Si,N,AlN-A1,Os-Dyz03

AI,O,-&,,-I SR-DyAG Al@-lSR’-12I-I-DyAG Al@-2 1R’-AIN-DyAG I2H-12H’-2 1R-2 1R’-DyAG 27R-27R’-2HS-2H*-DyAG 21 R’-27R’-AlN-DyAG /36o-&5-1SR-DyAG &-&-l2H-DyAG 2 1R-&-j+’ 27R-&-&-a’ 2H’--_P,-/3-0’ AlN-&&-a’ PlO-12H-a’-DyAG 2 1R-27R-at-M’ 2H’-AlN-c&M’ 2H’-27R-M’-DyAP AlN-2H’-YAP-DyAG 27R-21 R-YAP-DyAG 2 1R-o’-M’-DyAG J’ (middle)-DyAM-DyAP-AlN J-J’ (middle)-M-M’-AlN

in the systems

Al@-1 5R-1 SRI-DyAG Al@-12H’-2 1R’-DyAG 15R-15R’-12H-IZI-I-DyAG 2 1R-2 1R’-27R-27R’-DyAG 2H’-2H*-AIN-DyAG 27R’-2H*-AlN-DyAG &-15R-12H-DyAG /3,0-12H-21R-cu’ &-2 1R-27R-(Y’ &27R-2HS-(r’ &2H’-AlN-a’ &&-L&M’ 12H-2 1R-o’-Dy AG 27R-2H*-a’-M AlN-2HS-M’-DyAP 27R-2 1R-M’-DyAP 2H’-27R-DyAP-DyAG 2 1R-M’-DyAG-DyAP cu’-&-DyAG-M’ Dy@-J’ (whole)-AlN J’ (middle)-M’-DyAP-AIN

DyAM = Dy,A1209; DyAP=DyAlO,; DyAG = Dy,A1,0i2; 15R, 12H, 21R, 27R, 2H” are Si-rich terminals of AlN polytypoids; 15R’, 12H’, 21R’, 27R’. 2H’ are Al-rich terminals of AlN polytypoids, for others see Table 1.

into /3-sialon to achieve the subsolidus phase equilibrium. Our current work on (Y’+ p’ phase transformation in the Ln-Si-Al-O-N systems during heat-treatment shows that, besides the liquid phase which assists the transformation,‘4 there still exist other effects, especially for Ln-a-sialons with low Z-values. Of the 42 compatibility tetrahedra established in the present work, the following five-which involve c&sialon and AlN polytypoids (which could be used as reinforcement phase for (Y’,p’ or &-p’ because of its fibre-like morphology) and the promising grain boundary phases DyAG and M-are the most useful for the design of multiphase sialon ceramics. They are a-sialon-&&-M’; a-sialon-&-M’-DyAG; a-sialon-P,,-DyAG-12H; cY-sialon-12H-2 1R-DyAG; and fir,,&-I 2H-DyAG.

4 Conclusions Subsolidus phase relationships in the system Dy,O,-S&N,-AlN-A&O3 have been determined. Forty-two compatibility tetrahedra have been established in this region. DyAG coexists with p-sialon, approximately from PI0 (z = O-8) to & (z = 4), IY’(oxygen-rich composition) and all AlN polytypoids. The coexistence of M’ with /3-sialon is restricted to the range &/3,0. a-Sialon is also

et al. compatible with M’. The compatibility tetrahedra a-sialon-&-p,,,-M’, cY-sialon-/?,,-M’-DyAG, a+-siaion-/3,,-DyAG- I2H, cY-sialon-12H-2I R-Dy AG and &,,-&-12H-DyAG are the most useful for the design and processing of multiphase sialon ceramic materials.

Acknowledgements This work was supported by NNSF (National Natural Science Foundation, China, contract number 59482001) and the Royal Society, UK.

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10. Cheng, Y.-B. & Thompson, D. P., Aluminium-containing nitrogen melilite phases. J. Am. Cerum. Sot., 77 (1994) 4%8. 11. Wang, P. L., Yu, H. Y., Sun, W. Y., Nygren, M. & Ekstrom, T., Study on the solid solubility of Al in the melilite systems R2Si,_.~l,0s+,N,,. J. Eur. Ceram. Sot., 15 (1995) 689-96.

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