IATERIALS ENE S ENGINEERING ELSEVIER Materials Science and Engineering A281(2000)305-309 www.elsevier.com/locate/msea Short communication Interface structure and mechanical properties of Al2O3-20vol%SiCw ceramic matrix composite F. Ye*. T.C. Lei. Y. Zhou School of Materials Science and engin Harbin Institute of Technology, PO Box 433, Harbin 150001, PR China eceived 4 January 1999; received in revised form 13 October 1999 Abstract Al2O3-20vol%SiCw composites with as-received and acid-leached whiskers were fabricated by a hot-pressing technique. The microstructure, mechanical properties, fracture behavior and toughening mechanisms of the composites were investigated by scanning electron microscopy (SEM), transmission electron microscopy ( TEM), high-resolution electron microscopy (HREM). X-ray photo spectroscopy (XPS), energy dispersive analysis of X-ray(EDAX) techniques and three-point bending tests. The results show that whisker surface acid-leached treatment obviously reduces the oxygen content of Sic whisker and makes the whisker surface smooth, resulting in composites with higher fracture toughness. C 2000 Elsevier Science S.A. All rights reserved. Keywords: SiC whisker; Composites; Acid-leached treatment; Interface 1. Introducti leached whiskers were fabricated to assess the whisker/ matrix interface structure and its effect on the Considerable improvement in fracture toughness and mechanical properties of the composite strength can be achieved via the incorporation of strong, small diameter whiskers into ceramic matrixes The operation of various toughness mechanisms, such ocedure as crack deflection, whisker pull-out and bridging, to a 2. Experimental pre large extent depends on the nature of the whisker/ma- The materials used in this study were alumina rein- trix interface [1-6]. Several factors affect the whisker/ forced with 20vol% silicon carbide whiskers. Ultrafine matrix interface, including matrix chemistry, whisker alumina powders, having an average grain size of - 0.I surface chemistry, whisker morphology and thermal expansion mismatches between whisker and matrix um, were proved to be a-Al,O3 by XRD technique and the chemical composition is shown in Table 1. The Extensive research has been carried out and signifi- B-SiC whiskers were supplied by Takai Carbon, Japan, cant progress has been achieved in the field of SiC having a diameter of 0.5-1 um and a length of 30-60 whisker reinforced alumina composites [4-10]. The um. The whiskers were leached by using 5% HF acid toughening behavior of these composites has en for 10 h and then washed five times with distilled water reatly influenced by the nature of the whisker. The TH he composites with both the as-received and omposites with a smooth surface and (or)low oxygen leached whiskers were hot-pressed at 1650C for Ih content have high fracture toughness under a pressure of 25 MPa. The density of the samples In this study, Sic whiskers were leached with HF was measured by archimedes method in distilled water acid to reduce the chemical impurity on the surface of at 20C the whiskers. The composites with as-received and acid Flexural strength and fracture toughness of the com- osites were measured in air at room temperature using Corresponding author an Instron-1186 machine. Flexural strength measure 0921-5093/00/S- see front matter o 2000 Elsevier Science S.A. All rights reserved PI:S0921-5093099)00714-5
Materials Science and Engineering A281 (2000) 305–309 Short communication Interface structure and mechanical properties of Al2O3 –20vol%SiCw ceramic matrix composite F. Ye *, T.C. Lei, Y. Zhou School of Materials Science and Engineering, Harbin Institute of Technology, PO Box 433, Harbin 150001, PR China Received 4 January 1999; received in revised form 13 October 1999 Abstract Al2O3 –20vol%SiCw composites with as-received and acid-leached whiskers were fabricated by a hot-pressing technique. The microstructure, mechanical properties, fracture behavior and toughening mechanisms of the composites were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution electron microscopy (HREM), X-ray photo spectroscopy (XPS), energy dispersive analysis of X-ray (EDAX) techniques and three-point bending tests. The results show that whisker surface acid-leached treatment obviously reduces the oxygen content of SiC whisker and makes the whisker surface smooth, resulting in composites with higher fracture toughness. © 2000 Elsevier Science S.A. All rights reserved. Keywords: SiC whisker; Composites; Acid-leached treatment; Interface www.elsevier.com/locate/msea 1. Introduction Considerable improvement in fracture toughness and strength can be achieved via the incorporation of strong, small diameter whiskers into ceramic matrixes. The operation of various toughness mechanisms, such as crack deflection, whisker pull-out and bridging, to a large extent depends on the nature of the whisker/matrix interface [1–6]. Several factors affect the whisker/ matrix interface, including matrix chemistry, whisker surface chemistry, whisker morphology and thermal expansion mismatches between whisker and matrix. Extensive research has been carried out and signifi- cant progress has been achieved in the field of SiC whisker reinforced alumina composites [4–10]. The toughening behavior of these composites has been greatly influenced by the nature of the whisker. The composites with a smooth surface and (or) low oxygen content have high fracture toughness. In this study, SiC whiskers were leached with HF acid to reduce the chemical impurity on the surface of the whiskers. The composites with as-received and acidleached whiskers were fabricated to assess the whisker/ matrix interface structure and its effect on the mechanical properties of the composite. 2. Experimental procedure The materials used in this study were alumina reinforced with 20vol% silicon carbide whiskers. Ultrafine alumina powders, having an average grain size of 0.1 mm, were proved to be a-Al2O3 by XRD technique and the chemical composition is shown in Table 1. The b-SiC whiskers were supplied by Takai Carbon, Japan, having a diameter of 0.5–1 mm and a length of 30–60 mm. The whiskers were leached by using 5% HF acid for 10 h and then washed five times with distilled water. The composites with both the as-received and acidleached whiskers were hot-pressed at 1650°C for 1 h under a pressure of 25 MPa. The density of the samples was measured by Archimedes method in distilled water at 20°C. Flexural strength and fracture toughness of the composites were measured in air at room temperature using * Corresponding author. an Instron-1186 machine. Flexural strength measure- 0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0921-5093(99)00714-5
F. Ye et al. /Materials Science and Engineering 4281(2000)305-309 Table 1 Chemical composition of Al,O, powders(wt%) Material Al,O 3 Cao Mgo TiO, HCI Alo <0.0l <0.01 <001 <0.0l <0.2 ents were performed on bar specimens(3×4×36 mm)using a three-point bend fixture with a span of 30 mm. Fracture toughness measurements were performed on single-edge -notched bar (SENB) specimens(2 x 4X 5 mm)with a span of 16 mm and a half-thickness notch was made using a diamond wafering blade The surface morphologies of the whiskers and frac- 真 ture surfaces of the composites were examined using an HITACHI S-570 scanning electron microscope(SEM) An indentation loading of 15 kg was used to introduce cracks into specimens for observing the crack propaga- tion modes The surface chemistry of Sic whiskers prior to fabri cation was determined using X-ray photoelectron spec troscopy(XPS). The microstructure of the composites was characterized by TEM. The foils of the composites were prepared by dimpling and subsequent ion-beam BINDING ENERGY. ev thinning. Orientation of the foils was normal to the hot-pressing axis. In addition, a high-resolution phase contrast imaging technique was employed in JEM 2000EX-II electron microscopy to determine interface structure at atomic level resolution and the distribution of chemical elements near the interface was also mea sured by the EDAX technique 时四巴 w州 3. Results and discussion The XPS results of Sic whiskers are shown in Fig. 1 which indicate that the as-received Sic whisker has a high surface oxygen content and HF acid-leached treat ment virtually eliminated the surface oxygen content of BINDING ENERGY, a SiC whisker. Yang and Stevens [2]and Smith and Singh Fig. I. X-ray photoelectron analysis of the surface of as-received ane [3] also found the same results. The surface morpholo- acid-leached Sic whiskers: (a) as-received Sic whiskers; (b) acid- gies of the whiskers are show in Fig. 2, revealing that leached Sic whiskers the surface of the as-received whiskers are compara tively rough(Fig. 2a and b)and the acid-leached treat ment makes the surface of the whiskers rather smooth ( Fig. 2c and d). The change of whisker surface charac- teristics undoubtedly influences the fracture behavior of composit The values of flexural strength and the fracture toughness of the Al,O3-20vol%SiCw composites are demonstrated in Table 2. The incorporation of Sic whiskers can effectively improve the fracture toughness and flexural strength of Al,O3 ceramic. The composite reinforced with 20vol% as-received Sic whiskers exhib Iopm ited a fracture toughness of 6.87 MPa m and flexural Fig. 2. Morphologies of SiC whiskers:(a)as-received SiC whiskers; strength of 566 MPa. The composite reinforced with (b)acid-leached Sic whiskers
306 F. Ye et al. / Materials Science and Engineering A281 (2000) 305–309 Table 1 Chemical composition of Al2O3 powders (wt%) Material Al SiO2 Fe2O3 CaO MgO TiO2 HCl 2O3 Al2O3 99.66 B0.01 B0.01 B0.01 B0.01 B0.01 B0.2 ments were performed on bar specimens (3×4×36 mm) using a three-point bend fixture with a span of 30 mm. Fracture toughness measurements were performed on single-edge-notched bar (SENB) specimens (2×4× 25 mm) with a span of 16 mm and a half-thickness notch was made using a diamond wafering blade. The surface morphologies of the whiskers and fracture surfaces of the composites were examined using an HITACHI S-570 scanning electron microscope (SEM). An indentation loading of 15 kg was used to introduce cracks into specimens for observing the crack propagation modes. The surface chemistry of SiC whiskers prior to fabrication was determined using X-ray photoelectron spectroscopy (XPS). The microstructure of the composites was characterized by TEM. The foils of the composites were prepared by dimpling and subsequent ion-beam thinning. Orientation of the foils was normal to the hot-pressing axis. In addition, a high-resolution phasecontrast imaging technique was employed in JEM- 2000EX-II electron microscopy to determine interface structure at atomic level resolution and the distribution of chemical elements near the interface was also measured by the EDAX technique. 3. Results and discussion The XPS results of SiC whiskers are shown in Fig. 1, which indicate that the as-received SiC whisker has a high surface oxygen content and HF acid-leached treatment virtually eliminated the surface oxygen content of SiC whisker. Yang and Stevens [2]and Smith and Singh [3] also found the same results. The surface morphologies of the whiskers are show in Fig. 2, revealing that the surface of the as-received whiskers are comparatively rough (Fig. 2a and b) and the acid-leached treatment makes the surface of the whiskers rather smooth (Fig. 2c and d). The change of whisker surface characteristics undoubtedly influences the fracture behavior of the composites. The values of flexural strength and the fracture toughness of the Al2O3 –20vol%SiCw composites are demonstrated in Table 2. The incorporation of SiC whiskers can effectively improve the fracture toughness and flexural strength of Al2O3 ceramic. The composite reinforced with 20vol% as-received SiC whiskers exhibited a fracture toughness of 6.87 MPa m1/2 and flexural strength of 566 MPa. The composite reinforced with Fig. 1. X-ray photoelectron analysis of the surface of as-received and acid-leached SiC whiskers: (a) as-received SiC whiskers; (b) acidleached SiC whiskers. Fig. 2. Morphologies of SiC whiskers: (a) as-received SiC whiskers; (b) acid-leached SiC whiskers
F. Ye et al. Materials Science and Engineering 4281(2000)305-30 Table 2 the acid-leached whiskers(Fig. 4c and d), cracks were Room-temperature mechanical properties of the composites always observed to deflect along the whisker/matrix Material Flexural strength Fracture toughness interface and there was a large amount of pulling-out M IPa and bridging whiskers in the wake of the extending cracks. This results in a tortuous fracture path. In 14 contrast, the crack propagation paths observed in the Al,OrSiCw composites with as-received whiskers exhibited much Al,OrSiCw 508+ 8.78+0.50 less interaction between the crack and microstructure (acid-leached) and cracks tended to propagate directly through the whiskers. This reveals that the fracture behavior of Al2O3-SiCw composites is strongly dependent on the surface characteristics of sic whiskers The typical interface structures of the composites are shown in Fig. 5. Whiskers can be seen to be well bonded with the matrix in the Al,O, composites with as-received or acid-leached Sic whiskers when observed using TEM. In addition, little amorphous layer located at the triple grain boundaries of A2O3 could be found in the composite with as-received whiskers(Fig. 6) EDAX microanalyses revealed that the amorphous phase contains Al and Si. Braue [7] also found the similar phase in Al2O3-SiCw composites The typical HREM images of the Al,O3-SiCw com posites are shown in Fig. 7, indicating that in the composites reinforced with as-received Sic whiskers AlO3 and Sic are separated by a layer of amorphous phase with a thickness of nearly 10 nm(Fig. 7a). The lattice image taken from the composites with acid leached whiskers indicates that the whiskers are directly A题。 Fig 3. Fracture surfaces of Al,O320vol%SiCw composites: (a),(b Al2O3-20vol%SiCw composites with as-received Sic whiskers;(c) (d) Al2O3-20vol%SiCw composites with acid-leached Sic whiskers. 可觉 acid-leached whiskers demonstrates a further increase n fracture toughness and the fracture toughness of the composite with 20vol% acid-leached SiC whiskers in- creases to 8.78 MPa m /2 Msum The fracture surfaces of the composites tested in flexure at 20C are shown in Fig. 3. The fracture behavior of the two kinds of composites is quite differ nt. The composite with the aid-leached whiskers pro- duced substantial interactions between the propagating crack and microstructure. with evidence of whisker pull-out on the order 1-5 um(Fig. 3c and d). In contrast, the composites reinforced with the as-received SiC whiskers exhibited a relatively smooth fracture surface with little evidence of whisker pull-out(Fig. 3a and b) The degree of the crack/microstructure interactions an be demonstrated more clearly by examining the ! ig. 4. Scanning electron micrographs of indentation crack propaga- crack propagation produced by a Vickers indentation, posites with as-received Sic whiskers:(c),(d) AL, 0, composites with as shown in Fig 4. For the composites reinforced with acid-leached Sic whiskers
F. Ye et al. / Materials Science and Engineering A281 (2000) 305–309 307 Table 2 Room-temperature mechanical properties of the composites Material Fracture toughness Flexural strength (MPa m1/2 (MPa) ) Al2O3 235910 4.4290.14 Al2O3–SiCw 566941 6.8790.28 (as-received) Al2O3–SiCw 508930 8.7890.50 (acid-leached) the acid-leached whiskers (Fig. 4c and d), cracks were always observed to deflect along the whisker/matrix interface and there was a large amount of pulling-out and bridging whiskers in the wake of the extending cracks. This results in a tortuous fracture path. In contrast, the crack propagation paths observed in the composites with as-received whiskers exhibited much less interaction between the crack and microstructure, and cracks tended to propagate directly through the whiskers. This reveals that the fracture behavior of Al2O3 –SiCw composites is strongly dependent on the surface characteristics of SiC whiskers. The typical interface structures of the composites are shown in Fig. 5. Whiskers can be seen to be well bonded with the matrix in the Al2O3 composites with as-received or acid-leached SiC whiskers when observed using TEM. In addition, little amorphous layer located at the triple grain boundaries of A2O3 could be found in the composite with as-received whiskers (Fig. 6). EDAX microanalyses revealed that the amorphous phase contains Al and Si. Braue [7] also found the similar phase in Al2O3 –SiCw composites. The typical HREM images of the Al2O3 –SiCw composites are shown in Fig. 7, indicating that in the composites reinforced with as-received SiC whiskers, Al2O3 and SiC are separated by a layer of amorphous phase with a thickness of nearly 10 nm (Fig. 7a). The lattice image taken from the composites with acidleached whiskers indicates that the whiskers are directly Fig. 3. Fracture surfaces of Al2O3-20vol%SiCw composites: (a), (b) Al2O3 –20vol%SiCw composites with as-received SiC whiskers; (c), (d) Al2O3 –20vol%SiCw composites with acid-leached SiC whiskers. Fig. 4. Scanning electron micrographs of indentation crack propagation paths in Al2O3 –20vol%SiCw composites: (a), (b) Al2O3 composites with as-received SiC whiskers; (c), (d) Al2O3 composites with acid-leached SiC whiskers. acid-leached whiskers demonstrates a further increase in fracture toughness and the fracture toughness of the composite with 20vol% acid-leached SiC whiskers increases to 8.78 MPa m1/2 . The fracture surfaces of the composites tested in flexure at 20°C are shown in Fig. 3. The fracture behavior of the two kinds of composites is quite different. The composite with the aid-leached whiskers produced substantial interactions between the propagating crack and microstructure, with evidence of whisker pull-out on the order 1–5 mm (Fig. 3c and d). In contrast, the composites reinforced with the as-received SiC whiskers exhibited a relatively smooth fracture surface with little evidence of whisker pull-out (Fig. 3a and b). The degree of the crack/microstructure interactions can be demonstrated more clearly by examining the crack propagation produced by a Vickers indentation, as shown in Fig. 4. For the composites reinforced with
F. Ye et al. Materials Science and Engineering 4281(2000)305-30 bonding strength and hence inhibited the pull-out of 12O3 Many researchers [3, 4, Il] have reported that the amorphous layer at the interface between the whisker and ceramic matrix inhibited the toughening effects of Sic whiskers. As shown in Figs. I and 2, the acid p03 leached treatment can obviously reduce the oxygen content at the surfaces of whiskers and also make the whisker surfaces smooth. with the result that the whiskers in the composites with acid-leached Sic 0.5m Al2(3 whiskers are directly bonded with Al,O3 matrix grain without interfacial amorphous layer. The elimination of Ah2O Al203 interfacial amorphous phase and improvement of whisker surface roughness undoubtedly promote the whisker pull-out and crack bridging during fracturing, and hence further increase the mechanical properties of the composites. Therefore, the acid-leaching pre-treat SiCw ment of whiskers may be one of the effective ap- proaches to promote the toughening effects of whiskers Fig. 5. Transmission electron micrographs of SICw/AlO, interface: (a), (b)Al,O, composites with as-received Sic whiskers; (c),(d)Al,O composites with acid-leached Sic whiskers. 102 Sicw Ab 5nm Fig. 6. Transmission electron micrographs of Al2O3-20vol%SiCw omposite with as-received whisker (a) TEM micrograph of Al,o SiCw composite, showing amorphous phase; (b) diffraction pattern of he amorphous phase as shown in(a); and (c) EPMA results of the bonded to Al,O3 grains. It reveals that the interfacial hous layer in the Al ith as-re- ceived Sic whiskers is caused by the surface impurity of as-received whiskers, and the acid-leaching treatment reduces the surface oxygen content of SiC whiskers and 5nm hence eliminates the amorphous layer at the whisker/ matrix interface The presence of the amorphous phase at the whisker/ Fig. 7. HREM images of SiCw/Al,O, interface in Al,O,-SiCw composites:(a) as-received SiCw-Al2O, interface; (b) acid-leache ceramic matrix obviously increased the interfacial SiCw-AL2O
308 F. Ye et al. / Materials Science and Engineering A281 (2000) 305–309 Fig. 5. Transmission electron micrographs of SiCw/Al2O3 interface: (a), (b) Al2O3 composites with as-received SiC whiskers; (c), (d) Al2O3 composites with acid-leached SiC whiskers. bonding strength and hence inhibited the pull-out of whiskers. Many researchers [3,4,11] have reported that the amorphous layer at the interface between the whisker and ceramic matrix inhibited the toughening effects of SiC whiskers. As shown in Figs. 1 and 2, the acidleached treatment can obviously reduce the oxygen content at the surfaces of whiskers and also make the whisker surfaces smooth, with the result that the whiskers in the composites with acid-leached SiC whiskers are directly bonded with Al2O3 matrix grains without interfacial amorphous layer. The elimination of interfacial amorphous phase and improvement of whisker surface roughness undoubtedly promote the whisker pull-out and crack bridging during fracturing, and hence further increase the mechanical properties of the composites. Therefore, the acid-leaching pre-treatment of whiskers may be one of the effective approaches to promote the toughening effects of whiskers. Fig. 7. HREM images of SiCw/Al2O3 interface in Al2O3 –SiCw composites: (a) as-received SiCw–Al2O3 interface; (b) acid-leached SiCw–Al2O3 interface. Fig. 6. Transmission electron micrographs of Al2O3 –20vol%SiCw composite with as-received whisker. (a) TEM micrograph of Al2O3 – SiCw composite, showing amorphous phase; (b) diffraction pattern of the amorphous phase as shown in (a); and (c) EPMA results of the amorphous phase. bonded to Al2O3 grains. It reveals that the interfacial amorphous layer in the Al2O3 composite with as-received SiC whiskers is caused by the surface impurity of as-received whiskers, and the acid-leaching treatment reduces the surface oxygen content of SiC whiskers and hence eliminates the amorphous layer at the whisker/ matrix interface. The presence of the amorphous phase at the whisker/ ceramic matrix obviously increased the interfacial
F. Ye et al. Materials Science and Engineering 4281(2000)305-309 Conclusions References The acid-leaching pre-treatment of Sic whiskers in [J.J. Brennan, S.R. Nutt, J. Am. Ceram. Soc. 75(1992) 5vol% HF acid can reduce the oxygen content at the surfaces of whiskers and reduce the whisker surface [2 R. Stevens, J. Mater. Sci. 26(1991)726. roughness []s J.P. Singh, J Am Ceram Soc. 76(1993)497 2. In the Al,O3 composites with as-received Sic 4J W.L. Vaughn, M.K. Ferber, J. Am. Ceram Soc. 73 whiskers, SiC whiskers and Al,O3 grains are sepa (1990)394. 5 E. Yasuda, T. Akatsu,Y. Tanabe, J. Ceramic Soc. Jpn. Int Ed rated by an interfacial amorphous layer, which in 99(1991)51 hibits the pull-out of whiskers. For the composites [6 T. Akatsu, Y. Tanabe, S Matsuura, M. Yamada, H. Ishii, M. with acid-leached whiskers, SiC whiskers are directly Munakata, E. Yasuda, J. Ceramic Soc. Jpn. Int. Ed 99(1991) bonded with the matrix grains, which promotes the oulling-out effect of whiskers [7 w. Braue, R.W. Carpenter, D.J. Smith, J. Mater. Sci. 25(1990) 3. The composite reinforced with acid-leached [8G.C. Wei, P F. Becher, Am. Ceram. Soc. Bull. 64(1985) whiskers has higher fracture toughness than that with as-received whiskers. This is conceivably at- 9T N. Tiegs, P F. Becher, J. Am. Ceram. Soc. 5(1987) tributable to the enhanced whisker pull-out effects [10] S Lio, M. Watanabe, M. Matsubara, Y. Matsuo, J Am Ceram. due to the reduction of frictional resistance at the Soc.72(1989)1880. whisker/matrix interface (1N. Claussen, G. Petzow, Mater. Sci Res. 20(1986)649
F. Ye et al. / Materials Science and Engineering A281 (2000) 305–309 309 4. Conclusions 1. The acid-leaching pre-treatment of SiC whiskers in 5vol% HF acid can reduce the oxygen content at the surfaces of whiskers and reduce the whisker surface roughness. 2. In the Al2O3 composites with as-received SiC whiskers, SiC whiskers and Al2O3 grains are separated by an interfacial amorphous layer, which inhibits the pull-out of whiskers. For the composites with acid-leached whiskers, SiC whiskers are directly bonded with the matrix grains, which promotes the pulling-out effect of whiskers. 3. The composite reinforced with acid-leached whiskers has higher fracture toughness than that with as-received whiskers. This is conceivably attributable to the enhanced whisker pull-out effects due to the reduction of frictional resistance at the whisker/matrix interface. References [1] J.J. Brennan, S.R. Nutt, J. Am. Ceram. Soc. 75 (1992) 1205. [2] M. Yang, R. Stevens, J. Mater. Sci. 26 (1991) 726. [3] S.M. Smith, J.P. Singh, J. Am. Ceram. Soc. 76 (1993) 497. [4] J. Homeny, W.L. Vaughn, M.K. Ferber, J. Am. Ceram. Soc. 73 (1990) 394. [5] E. Yasuda, T. Akatsu, Y. Tanabe, J. Ceramic Soc. Jpn. Int. Ed. 99 (1991) 51. [6] T. Akatsu, Y. Tanabe, S. Matsuura, M. Yamada, H. Ishii, M. Munakata, E. Yasuda, J. Ceramic Soc. Jpn. Int. Ed. 99 (1991) 416. [7] W. Braue, R.W. Carpenter, D.J. Smith, J. Mater. Sci. 25 (1990) 2949. [8] G.C. Wei, P.F. Becher, Am. Ceram. Soc. Bull. 64 (1985) 295. [9] T.N. Tiegs, P.F. Becher, J. Am. Ceram. Soc. 5 (1987) C109. [10] S. Lio, M. Watanabe, M. Matsubara, Y. Matsuo, J. Am. Ceram. Soc. 72 (1989) 1880. [11] N. Claussen, G. Petzow, Mater. Sci. Res. 20 (1986) 649.