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YAG-alumina platelets
Materials Science and Engineering A 380 (2004) 237–244
Fibrous monoliths of mullite-AlPO4 and alumina/YAG-alumina platelets Dong-Kyu Kim, Waltraud M. Kriven∗ Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA Received 30 May 2003; received in revised form 23 March 2004
Abstract Two-layer, fibrous monolithic composites consisting of mullite-aluminum phosphate (AlPO4 ) and 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet interphase components, were fabricated by a co-extrusion technique. The four powders were characterized for particle size, specific surface area, and SEM analysis. The mixing formulations for extruding the powders were developed using ethylene vinylacetate copolymer as a binder. The variation in the mixing torque, in a Brabender mixer, as a function of temperature was measured. The binder removal behavior of the mullite-AlPO4 fibrous monolithic composite was studied by thermogravimetric analysis (TGA). The AlPO4 and alumina platelet interphase layers formed a porous and less porous interphase region, respectively, after sintering. The sintered mullite-AlPO4 two-layer fibrous monolithic showed non-brittle fracture behavior. Its 3-point bend strength and work of fracture were 76 ± 5 MPa and 0.45 ± 0.02 kJ/m2 , respectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Fibrous monolithic composite; Mullite-aluminum phosphate(AlPO4 ); 50 vol.% Alumina:50 vol.% YAG in situ composite-alumina platelet; Sintering; Mechanical property
1. Introduction To overcome the brittleness of ceramics, several different approaches have been adopted. After the work of Clegg et al. [1], different kinds of laminated ceramic composites were made to increase toughness [2–6]. Fibrous monolithic composites are an alternative approach to the design of tough ceramics, without the incorporation of ceramic fibers into a composite. This concept was first introduced by Coblenz [7]. Fibrous monolithic composites are sintered (or hot–pressed) monolithic ceramics with a distinct fibrous texture, consisting of cells of a primary phase, separated by cell boundaries of a tailored secondary phase [8]. Halloran et al. [8–11] conducted extensive research on non-oxide, fibrous monolithic composites of SiC/C and Si3 N4 /BN. Mullite is an attractive structural material due to its excellent strength and creep resistance at high temperatures, low thermal expansion and conductivity, good thermal stability, and chemical inertness [12–14]. The reported bend strengths of mullite at room temperature lie between 250–400 MPa, and these strengths are maintained up to temperatures rang-
∗
Corresponding author. Tel.: +1 217 333 5258; fax: +1 217 333 2736. E-mail address: [email protected] (W.M. Kriven).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.03.083
ing from 1200 to 1400 ◦ C [12,15–18]. Mullite is known to be an excellent creep resistant material. Lessing et al. [19] reported the creep rate of mullite to be, approximately, an order of magnitude less than that of Al2 O3 . YAG (yttrium aluminum garnet, Y3 Al5 O12 ) and alumina are chemically compatible with each other and can constitute structurally sound composites [20–22]. YAG–alumina eutectic materials have been made by induction heating [23–25]. The alumina–YAG eutectic composite has a flexural strength of 360–500 MPa. This strength is maintained nearly constantly over the range from room temperature to 1700 ◦ C [25–28]. The creep resistance of the alumina–YAG eutectic composite is better than that of polycrystalline YAG and that of a-axis sapphire fiber [28]. An alumina–YAG eutectic composite has excellent high temperature strength and creep resistance, but its processing route is complicated and expensive. Therefore, as an alternative route to the benefits of a eutectic composite, an alternative, cheap, alumina–YAG in situ matrix composite was developed in our laboratory [29]. Aluminum phosphate (AlPO4 ) is chemically compatible with mullite and has a density corresponding to 61% of theoretical value. It has a bending strength of 1.5 ± 0.2 MPa after sintering at 1600 ◦ C for 10 h, resulting in AlPO4 acting as a chemically stable, porous, and weak interphase material [30]. Alumina platelets were aligned in random,
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Table 1 Mixing formulations for the oxide powders Powder
Binder (Elvax) 210
Mullite 50 vol.% Al2 O3 50 vol.% YAG
52 52
2.4 2.4
In situ composite AlPO4 Al2 O3 platelets
40 40
9.6 9.6
220 4.8 4.8 24 24
Plasticizer (DP)
Lubricant (SA)
40.8 40.8
– –
– –
14.4 14.4
9 9
3 3
250
Note: All ingredients are in vol%, elvax: ethylene vinylacetate copolymer (Dupont, 210, 220, and 250 are products names, indicating a molecular weight increase from 210, 220 to 250); DP: dibutyl phthalate (99%, Aldrich), SA: stearic acid (95%, Aldrich).
three-dimensional orientations, and induced constrained sintering of ceramic powders, finally functioning as a debonding interphase in the composite structures [31]. In this research, the processing of an oxide matrix–oxide interphase, 2-layer, fibrous monolithic composite was studied. Mullite and a 50 vol.% alumina:50 vol.% YAG in situ composite were used as possible high temperature, structural, matrix materials. Aluminum phosphate (AlPO4 ) and alumina platelets were investigated as interphase materials. A two-layer, mullite-AlPO4 and a 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet, fibrous monolithic material were fabricated by co-extrusion using ethylene vinylacetate (EVA) copolymer as a binder. The powders used for making composites were characterized. The microstructure and room temperature mechanical behavior of the two composites were investigated.
210, 220 to 250. The reason for using the mixture was to facilitate the binder removal process over a broad temperature range. Dibutyl phthalate (Aldrich, 99%) and stearic acid (Aldrich, 95%) were used as a plasticizer and lubricant, respectively. Ceramic powder, binder, plasticizer, and lubricant were mixed in a computer controlled, high shear mixer (Model 2100, C. W. Brabender, NJ). The mixing formulation is shown in Table 1. To make the fibrous monolithic texture, without intermixing between inner mullite matrix and AlPO4
2. Experimental procedure Commercial mullite (Kyoritsu, KM 101) and alumina platelet powders of 5–10 m diameter by 1 m thick (Elf Atochem, France) were used. AlPO4 and 50 vol.% alumina:50 vol.% YAG in situ composite powders were synthesized by the organic, steric entrapment method [32–37]. The particle size of the powder was measured in a centrifugal, automated, particle size distribution analyzer (Model CAPA-700, Horiba, Kyoto, Japan). The specific surface area of samples was measured from seven-point BET analyses using nitrogen gas adsorption (Model ASAP2400, Micrometrics, Norcross, GA). The bulk density of sintered alumina platelet pellets was measured by Archimedes’ method (ASTM C373). The morphology of powders and the microstructure of composites were investigated by scanning electron microscopy (SEM, Model S-4700, Hitachi, Osaka, Japan). The composite pellets for SEM were polished down to a 1 m using diamond paste finish, and then thermally etched at 1550 ◦ C for 30 min. A mixture of Elvax 210, 220, and 250 ethylene vinyl acetate copolymers (Dupont, Wilmington, DEL) was used as binder phases. The molecular weight increased from Elvax
Fig. 1. The stainless steel mouldings (a) and extrusion (b) dies.
D.-K. Kim, W.M. Kriven / Materials Science and Engineering A 380 (2004) 237–244
interphase layer during extrusion, the viscosity of the inner layer should be higher than that of interphase material, and so more powder was put into the matrix rod formulation to increase the viscosity. In the case of the mullite and 50 vol.% alumina:50 vol.% YAG in situ composite, for the inner matrix rod, the powder loading was 52 vol.%, and plasticizer and lubricant were not used to obtain a higher viscosity. 40 vol.% of powder loading was used for the AlPO4 and alumina platelet interphases. The Brabender mixing temperature was 150 ◦ C. The mixing torque of the formulation was measured by reading the shear force between two mixing blades. The inner rod was extruded at a rate of 50 mm/min into a cylindrical shape. The interphase formulation was warm pressed into a half tubular shape with the aid of a thickness controllable die at 150 ◦ C, under an applied pressure of about 34.5 MPa. Photographs of the molding and extrusion die are seen in Fig. 1. The center rod and half-tubular shape were arranged into a concentric, cylindrical die of 23 mm total diameter by 150 mm length, and extruded into the die with a 2.0 mm diameter orifice. This was referred to as the first extrusion and the extrudate was called a “monofilament rod” having a two-layer texture. The ratio of relative volumetric amounts of inner
Fig. 2. SEM micrographs of the matrix powders. (a) Mullite powder and (b) 50 vol.% alumina:50 vol.% YAG in situ composite (after 1 h attrition milling).
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mullite and interphase AlPO4 in the “green” monofilament rod was 1.11–1. 93. Monofilament rods are arranged in the cylindrical die and extruded again into a die with a 2 mm diameter orifice. This was referred to as the second extrusion and the extrudate was called a “multifilament rod”. The multifilament rods were cut into 47 mm lengths. Fifty-five multifilament rods were arranged into a rectangular die and warm pressed at 150 ◦ C, under a pressure of 34.5 MPa. The binder was then removed from the pressed pellet. The schedule for the binder removal was as follows: from 25 to 250 ◦ C at a ramp rate of 0.05 ◦ C/min, from 250 to 450 ◦ C at a ramp rate of 0.1 ◦ C/min, from 450 to 650 ◦ C at a ramp rate of 0.3 ◦ C/min, maintained at 650 ◦ C for 2 h, and subsequently cooled down to room temperature at a ramp rate of 0.5 ◦ C/min. The binder-free body was CIPed at 413.7 MPa and then sintered at conditions of 1600 ◦ C/10 h and 1650 ◦ C/10 h for the mullite-AlPO4 and 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet body, respectively. The microstructures of monofilament rods, multifilament rods, and green bodies were studied by optical microscopy (Model SMZ-2T, Nikon, Tokyo, Japan). The binder removal behavior of the mullite-AlPO4 , fibrous monolithic composite, was studied by thermogravimetry. (Model 7,
Fig. 3. SEM micrographs of the interphase powders (after 1 h attrition milling). (a) Alumina platelets and (b) aluminum phosphate (AlPO4 ).
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3. Results and discussion
Fig. 4. The variation of mixing torques with temperature for the mullite inner rod and AlPO4 interphase layer.
Perkin Elmer, Connecticut, USA). The temperature ranged from room temperature to 900 ◦ C, and the heating rate was 5 ◦ C/min. Flexural strengths were measured with a screw-driven testing machine (Model 4502, Instron Corp., Canton, MA) in 3-point bending. The samples were cut from sintered pellets where the length of the testing sample was aligned with the longitudinal direction of the filaments. The samples were ground with diamond paste and finally polished with 600 grit SiC paper, after which the four edges of each sample were bevelled. The flexural strengths were determined after testing 3–5 samples The supporting span was 30 mm, the cross head speed was 0.1 mm/min and sample size was 3 mm (H) × 4 mm (W) × 40 mm (L).
SEM micrographs of the mullite and the 50 vol.% alumina:50 vol.% YAG in situ composite matrix powders are seen in Fig. 2. The mullite powder had a granular morphology, with an average particle size and surface area of 0.8 m and 8.5 m2 /g, respectively. The in situ alumina-YAG composite powder showed an irregular shape with sharp edges and it had a specific surface area of 2.7 m2 /g after 1 h attrition milling. Alumina platelets had a hexagonal plate shape of 5–10 m diameter, and 1 m thickness with a specific surface area of 1.4 m2 /g (Fig. 3(a)). An alumina platelet pellet CIPped at 413.7 MPa and then sintered at 1600 ◦ C for 3 h, had a density of 80% of theoretical density and a 3-point bend strength of 205 ± 7 MPa. This bending strength was much higher than 1.5 ± 0.2 MPa of AlPO4 [30]. This means that alumina platelets can possibly function as a stronger debonding material, resulting in a higher strength and toughness composite material. AlPO4 powder had an irregular shape with sharp edges (Fig. 3(b)). The 1 h attrition-milled AlPO4 powder had a particle size and specific surface area of 0.9 m and 87 m2 /g, respectively. The variations of the mixing torque for the mullite inner matrix rod and AlPO4 interphase layer are plotted in Fig. 4. To prevent intermixing between the inner matrix layer and interphase layer during extrusion and, finally, to obtain a fibrous monolithic texture after extrusion, the viscosity of the inner layer should be higher than that of the interphase region. Fig. 4 shows that the inner layer had about a five-times higher mixing torque than did the aluminum phosphate, interphase layer. This means that the viscosity requirement to obtain fibrous monolithic composite texture was well satisfied for the formulations in Table 1. The mixing torque decreased with increasing temperature. The binder
Fig. 5. The binder burnout TGA profile from room temperature to 900 ◦ C, for the mullite-AlPO4 two-layer, fibrous monolithic composite.
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Fig. 6. Optical micrographs of the mullite-AlPO4 , two-layer, fibrous monolithic composite, after binder removal. The fibrous monoliths were laid up into a rectangular sample for subsequent sectioning into bend test specimens.
removal from the mullite-AlPO4 green pellet was studied by TGA. The results are presented in Fig. 5. The weight loss from room temperature to 900 ◦ C consisted roughly of 5 steps: 25–70, 70–160, 160–305, 305–430, 430–570, and 570–900 ◦ C. This indicates how the binder was removed gradually over a wide temperature range. Fig. 6(a) is an optical micrograph of binder-free rectangular pellets and Fig. 6(b) is the enlarged view of the surface. The sample surface was clean and had no distortion after binder removal. Fig. 7(a and b) are optical micrographs of green bodies of a monofilament rod and multifilament rod of the mullite-AlPO4 laminated composite. The interphase is distinguished from the matrix phase by a food coloring dye. The thickness of the AlPO4 interphase layer was 0.33 mm. A multifilament rod with diameter 2.1 mm had approximately 93 two-layer, sub-cells in it. The microstructure of a green body of the two-layer, mullite-AlPO4 fibrous monolithic composite is shown in the optical micrograph of Fig. 8. The
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Fig. 7. Optical micrographs of the mullite-AlPO4 fibrous monolithic composite filaments (M: matrix center rod (mullite), I: interphase layer (AlPO4 ). The thickness of green interphase layer is 0.33 mm). (a) Monofilament rod and (b) multifilament rod.
Fig. 8. Optical micrograph of the mullite-AlPO4 fibrous monolithic composite green body.
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Fig. 9. SEM micrographs of the sintered mullite-AlPO4 , two-layer, fibrous monolithic composite. (M: matrix center rod (mullite), I: porous interphase layer (AlPO4 )).
mullite matrix and AlPO4 interphase formed a homogeneous and uniform microstructure. Fig. 9(a and b) are SEM micrographs of the sintered mullite-AlPO4 , two-layer, fibrous monolithic composite. The microstructure is homogeneous, and the higher magnification image confirms that the AlPO4 interphase layer was porous. SEM micrograph of the sintered 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet; fibrous monolithic composite are presented in Fig. 10. The higher magnification micrograph of the interphase region, however, indicates that alumina platelets in the layer had sintered together. Because of the high CIPping pressure of 413.7 MPa, the platelets in the interphase region were thought to be compacted together without forming a porous pocket structure, which is necessary for debonding to occur. Because the platelets were compacted together after CIPping and the composite pellet was sintered at 1650 ◦ C for 15 h, the alumina platelets grains grew to about 2–3 times larger compared to their starting sizes.
Fig. 10. SEM micrographs of the sintered 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet interphase, forming a two-layer fibrous monolithic composite. (M: matrix center rod (50 vol.% alumina:50 vol.% YAG in situ composite), I: interphase layer (alumina platelets)).
The two composites were mechanically evaluated by 3-point bend testing and the load versus displacement curves are given in Fig. 11. The two-layer, mullite-AlPO4 fibrous monolithic composite exhibited apparent non-brittle fracture behavior with ∼0.1 mm of displacement, and had a strength and work of fracture of 76 ± 5 MPa and 0.45 ± 0.02 kJ/m2 , respectively. The graceful failure of the mullite-AlPO4 composite, is attributed to crack deflection along the porous AlPO4 interphase as can be seen in Fig. 12. This SEM micrograph clearly shows that crack deflected along the weak AlPO4 interphase. On the other hand, the sintered 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet, two-layer fibrous monolithic composite underwent brittle fracture, showing a displacement less than 0.02 mm, and had a room temperature, 3-point bend strength and work of fracture of 219 ± 7 MPa and 0.25 ± 0.03 kJ/m2 , respectively. The reason for the brittle
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Fig. 11. Stress–strain curves, for the bend tested, mullite-AlPO4 and 50 vol.% alumina:50 vol.% YAG in situ matrix–alumina platelet, fibrous monolithic composites.
fracture of this composite, can be attributed to the relatively dense microstructure of the alumina platelet interphase region because of the compaction of platelets due to high CIPping pressure and sintering the pellet at high temperature and for a long time at 1650 ◦ C for 10 h, as can be seen in Fig. 10. The reasons for the relatively lower composite strength of 219 ± 7 MPa are attributed to the grain growth of alumina platelets in the interphase region. Fig. 13 shows the fracture surfaces of two composites after 3-point bend testing. The mullite-AlPO4 fibrous monolithic composite indicated extensive fiber pull-out and a rough fracture surface. However the 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet, two-layer fibrous monolithic composite showed very little fiber pull-out and relatively smooth fracture surface.
Fig. 12. SEM micrograph of the fractured mullite-AlPO4 fibrous monolithic composite.
Fig. 13. SEM micrographs of the fracture surfaces of the two-layer fibrous monolithic composites. (a) Mullite-AlPO4 composite and (b) 50 vol.% alumina:50 vol.% YAG in situ composite-alumina platelet composite.
4. Conclusions Processing procedures for a mullite-AlPO4 and 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet, two-layer fibrous monolithic composite were successfully demonstrated. The synthesized 50 vol.% alumina:50 vol.% YAG in situ composite and AlPO4 powders had irregular shapes with sharp edges. The fact that the mixing torque value of the mullite inner matrix rod was about five-times higher than that of the outer aluminum phosphate interphase layer, indicated that the formulation systems using graded ethylene vinylacetate copolymer as a binder, were good to make homogeneous fibrous monolithic composite texture without intermixing of two materials. TGA analysis of mullite-AlPO4 , two-layer green pellet, indicated that a multi-step binder removal was successful. The 50 vol.% alumina:50 vol.% YAG in situ matrix–alumina platelet, fibrous monolithic composite, exhibited brittle fracture, because of significant densification in the alumina platelet interphase during CIPping and sintering. The porous AlPO4 interphase layer allowed the mullite-AlPO4 fibrous monolith composite to show some non-brittle fracture.
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Acknowledgements This work was supported by the US AFOSR under grant number: F49620-03-1-0082.
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Fibrous monoliths of mullite-AlPO4 and alumina/YAG-alumina platelets Dong-Kyu Kim, Waltraud M. Kriven∗ Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA Received 30 May 2003; received in revised form 23 March 2004
Abstract Two-layer, fibrous monolithic composites consisting of mullite-aluminum phosphate (AlPO4 ) and 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet interphase components, were fabricated by a co-extrusion technique. The four powders were characterized for particle size, specific surface area, and SEM analysis. The mixing formulations for extruding the powders were developed using ethylene vinylacetate copolymer as a binder. The variation in the mixing torque, in a Brabender mixer, as a function of temperature was measured. The binder removal behavior of the mullite-AlPO4 fibrous monolithic composite was studied by thermogravimetric analysis (TGA). The AlPO4 and alumina platelet interphase layers formed a porous and less porous interphase region, respectively, after sintering. The sintered mullite-AlPO4 two-layer fibrous monolithic showed non-brittle fracture behavior. Its 3-point bend strength and work of fracture were 76 ± 5 MPa and 0.45 ± 0.02 kJ/m2 , respectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Fibrous monolithic composite; Mullite-aluminum phosphate(AlPO4 ); 50 vol.% Alumina:50 vol.% YAG in situ composite-alumina platelet; Sintering; Mechanical property
1. Introduction To overcome the brittleness of ceramics, several different approaches have been adopted. After the work of Clegg et al. [1], different kinds of laminated ceramic composites were made to increase toughness [2–6]. Fibrous monolithic composites are an alternative approach to the design of tough ceramics, without the incorporation of ceramic fibers into a composite. This concept was first introduced by Coblenz [7]. Fibrous monolithic composites are sintered (or hot–pressed) monolithic ceramics with a distinct fibrous texture, consisting of cells of a primary phase, separated by cell boundaries of a tailored secondary phase [8]. Halloran et al. [8–11] conducted extensive research on non-oxide, fibrous monolithic composites of SiC/C and Si3 N4 /BN. Mullite is an attractive structural material due to its excellent strength and creep resistance at high temperatures, low thermal expansion and conductivity, good thermal stability, and chemical inertness [12–14]. The reported bend strengths of mullite at room temperature lie between 250–400 MPa, and these strengths are maintained up to temperatures rang-
∗
Corresponding author. Tel.: +1 217 333 5258; fax: +1 217 333 2736. E-mail address: [email protected] (W.M. Kriven).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.03.083
ing from 1200 to 1400 ◦ C [12,15–18]. Mullite is known to be an excellent creep resistant material. Lessing et al. [19] reported the creep rate of mullite to be, approximately, an order of magnitude less than that of Al2 O3 . YAG (yttrium aluminum garnet, Y3 Al5 O12 ) and alumina are chemically compatible with each other and can constitute structurally sound composites [20–22]. YAG–alumina eutectic materials have been made by induction heating [23–25]. The alumina–YAG eutectic composite has a flexural strength of 360–500 MPa. This strength is maintained nearly constantly over the range from room temperature to 1700 ◦ C [25–28]. The creep resistance of the alumina–YAG eutectic composite is better than that of polycrystalline YAG and that of a-axis sapphire fiber [28]. An alumina–YAG eutectic composite has excellent high temperature strength and creep resistance, but its processing route is complicated and expensive. Therefore, as an alternative route to the benefits of a eutectic composite, an alternative, cheap, alumina–YAG in situ matrix composite was developed in our laboratory [29]. Aluminum phosphate (AlPO4 ) is chemically compatible with mullite and has a density corresponding to 61% of theoretical value. It has a bending strength of 1.5 ± 0.2 MPa after sintering at 1600 ◦ C for 10 h, resulting in AlPO4 acting as a chemically stable, porous, and weak interphase material [30]. Alumina platelets were aligned in random,
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Table 1 Mixing formulations for the oxide powders Powder
Binder (Elvax) 210
Mullite 50 vol.% Al2 O3 50 vol.% YAG
52 52
2.4 2.4
In situ composite AlPO4 Al2 O3 platelets
40 40
9.6 9.6
220 4.8 4.8 24 24
Plasticizer (DP)
Lubricant (SA)
40.8 40.8
– –
– –
14.4 14.4
9 9
3 3
250
Note: All ingredients are in vol%, elvax: ethylene vinylacetate copolymer (Dupont, 210, 220, and 250 are products names, indicating a molecular weight increase from 210, 220 to 250); DP: dibutyl phthalate (99%, Aldrich), SA: stearic acid (95%, Aldrich).
three-dimensional orientations, and induced constrained sintering of ceramic powders, finally functioning as a debonding interphase in the composite structures [31]. In this research, the processing of an oxide matrix–oxide interphase, 2-layer, fibrous monolithic composite was studied. Mullite and a 50 vol.% alumina:50 vol.% YAG in situ composite were used as possible high temperature, structural, matrix materials. Aluminum phosphate (AlPO4 ) and alumina platelets were investigated as interphase materials. A two-layer, mullite-AlPO4 and a 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet, fibrous monolithic material were fabricated by co-extrusion using ethylene vinylacetate (EVA) copolymer as a binder. The powders used for making composites were characterized. The microstructure and room temperature mechanical behavior of the two composites were investigated.
210, 220 to 250. The reason for using the mixture was to facilitate the binder removal process over a broad temperature range. Dibutyl phthalate (Aldrich, 99%) and stearic acid (Aldrich, 95%) were used as a plasticizer and lubricant, respectively. Ceramic powder, binder, plasticizer, and lubricant were mixed in a computer controlled, high shear mixer (Model 2100, C. W. Brabender, NJ). The mixing formulation is shown in Table 1. To make the fibrous monolithic texture, without intermixing between inner mullite matrix and AlPO4
2. Experimental procedure Commercial mullite (Kyoritsu, KM 101) and alumina platelet powders of 5–10 m diameter by 1 m thick (Elf Atochem, France) were used. AlPO4 and 50 vol.% alumina:50 vol.% YAG in situ composite powders were synthesized by the organic, steric entrapment method [32–37]. The particle size of the powder was measured in a centrifugal, automated, particle size distribution analyzer (Model CAPA-700, Horiba, Kyoto, Japan). The specific surface area of samples was measured from seven-point BET analyses using nitrogen gas adsorption (Model ASAP2400, Micrometrics, Norcross, GA). The bulk density of sintered alumina platelet pellets was measured by Archimedes’ method (ASTM C373). The morphology of powders and the microstructure of composites were investigated by scanning electron microscopy (SEM, Model S-4700, Hitachi, Osaka, Japan). The composite pellets for SEM were polished down to a 1 m using diamond paste finish, and then thermally etched at 1550 ◦ C for 30 min. A mixture of Elvax 210, 220, and 250 ethylene vinyl acetate copolymers (Dupont, Wilmington, DEL) was used as binder phases. The molecular weight increased from Elvax
Fig. 1. The stainless steel mouldings (a) and extrusion (b) dies.
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interphase layer during extrusion, the viscosity of the inner layer should be higher than that of interphase material, and so more powder was put into the matrix rod formulation to increase the viscosity. In the case of the mullite and 50 vol.% alumina:50 vol.% YAG in situ composite, for the inner matrix rod, the powder loading was 52 vol.%, and plasticizer and lubricant were not used to obtain a higher viscosity. 40 vol.% of powder loading was used for the AlPO4 and alumina platelet interphases. The Brabender mixing temperature was 150 ◦ C. The mixing torque of the formulation was measured by reading the shear force between two mixing blades. The inner rod was extruded at a rate of 50 mm/min into a cylindrical shape. The interphase formulation was warm pressed into a half tubular shape with the aid of a thickness controllable die at 150 ◦ C, under an applied pressure of about 34.5 MPa. Photographs of the molding and extrusion die are seen in Fig. 1. The center rod and half-tubular shape were arranged into a concentric, cylindrical die of 23 mm total diameter by 150 mm length, and extruded into the die with a 2.0 mm diameter orifice. This was referred to as the first extrusion and the extrudate was called a “monofilament rod” having a two-layer texture. The ratio of relative volumetric amounts of inner
Fig. 2. SEM micrographs of the matrix powders. (a) Mullite powder and (b) 50 vol.% alumina:50 vol.% YAG in situ composite (after 1 h attrition milling).
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mullite and interphase AlPO4 in the “green” monofilament rod was 1.11–1. 93. Monofilament rods are arranged in the cylindrical die and extruded again into a die with a 2 mm diameter orifice. This was referred to as the second extrusion and the extrudate was called a “multifilament rod”. The multifilament rods were cut into 47 mm lengths. Fifty-five multifilament rods were arranged into a rectangular die and warm pressed at 150 ◦ C, under a pressure of 34.5 MPa. The binder was then removed from the pressed pellet. The schedule for the binder removal was as follows: from 25 to 250 ◦ C at a ramp rate of 0.05 ◦ C/min, from 250 to 450 ◦ C at a ramp rate of 0.1 ◦ C/min, from 450 to 650 ◦ C at a ramp rate of 0.3 ◦ C/min, maintained at 650 ◦ C for 2 h, and subsequently cooled down to room temperature at a ramp rate of 0.5 ◦ C/min. The binder-free body was CIPed at 413.7 MPa and then sintered at conditions of 1600 ◦ C/10 h and 1650 ◦ C/10 h for the mullite-AlPO4 and 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet body, respectively. The microstructures of monofilament rods, multifilament rods, and green bodies were studied by optical microscopy (Model SMZ-2T, Nikon, Tokyo, Japan). The binder removal behavior of the mullite-AlPO4 , fibrous monolithic composite, was studied by thermogravimetry. (Model 7,
Fig. 3. SEM micrographs of the interphase powders (after 1 h attrition milling). (a) Alumina platelets and (b) aluminum phosphate (AlPO4 ).
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3. Results and discussion
Fig. 4. The variation of mixing torques with temperature for the mullite inner rod and AlPO4 interphase layer.
Perkin Elmer, Connecticut, USA). The temperature ranged from room temperature to 900 ◦ C, and the heating rate was 5 ◦ C/min. Flexural strengths were measured with a screw-driven testing machine (Model 4502, Instron Corp., Canton, MA) in 3-point bending. The samples were cut from sintered pellets where the length of the testing sample was aligned with the longitudinal direction of the filaments. The samples were ground with diamond paste and finally polished with 600 grit SiC paper, after which the four edges of each sample were bevelled. The flexural strengths were determined after testing 3–5 samples The supporting span was 30 mm, the cross head speed was 0.1 mm/min and sample size was 3 mm (H) × 4 mm (W) × 40 mm (L).
SEM micrographs of the mullite and the 50 vol.% alumina:50 vol.% YAG in situ composite matrix powders are seen in Fig. 2. The mullite powder had a granular morphology, with an average particle size and surface area of 0.8 m and 8.5 m2 /g, respectively. The in situ alumina-YAG composite powder showed an irregular shape with sharp edges and it had a specific surface area of 2.7 m2 /g after 1 h attrition milling. Alumina platelets had a hexagonal plate shape of 5–10 m diameter, and 1 m thickness with a specific surface area of 1.4 m2 /g (Fig. 3(a)). An alumina platelet pellet CIPped at 413.7 MPa and then sintered at 1600 ◦ C for 3 h, had a density of 80% of theoretical density and a 3-point bend strength of 205 ± 7 MPa. This bending strength was much higher than 1.5 ± 0.2 MPa of AlPO4 [30]. This means that alumina platelets can possibly function as a stronger debonding material, resulting in a higher strength and toughness composite material. AlPO4 powder had an irregular shape with sharp edges (Fig. 3(b)). The 1 h attrition-milled AlPO4 powder had a particle size and specific surface area of 0.9 m and 87 m2 /g, respectively. The variations of the mixing torque for the mullite inner matrix rod and AlPO4 interphase layer are plotted in Fig. 4. To prevent intermixing between the inner matrix layer and interphase layer during extrusion and, finally, to obtain a fibrous monolithic texture after extrusion, the viscosity of the inner layer should be higher than that of the interphase region. Fig. 4 shows that the inner layer had about a five-times higher mixing torque than did the aluminum phosphate, interphase layer. This means that the viscosity requirement to obtain fibrous monolithic composite texture was well satisfied for the formulations in Table 1. The mixing torque decreased with increasing temperature. The binder
Fig. 5. The binder burnout TGA profile from room temperature to 900 ◦ C, for the mullite-AlPO4 two-layer, fibrous monolithic composite.
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Fig. 6. Optical micrographs of the mullite-AlPO4 , two-layer, fibrous monolithic composite, after binder removal. The fibrous monoliths were laid up into a rectangular sample for subsequent sectioning into bend test specimens.
removal from the mullite-AlPO4 green pellet was studied by TGA. The results are presented in Fig. 5. The weight loss from room temperature to 900 ◦ C consisted roughly of 5 steps: 25–70, 70–160, 160–305, 305–430, 430–570, and 570–900 ◦ C. This indicates how the binder was removed gradually over a wide temperature range. Fig. 6(a) is an optical micrograph of binder-free rectangular pellets and Fig. 6(b) is the enlarged view of the surface. The sample surface was clean and had no distortion after binder removal. Fig. 7(a and b) are optical micrographs of green bodies of a monofilament rod and multifilament rod of the mullite-AlPO4 laminated composite. The interphase is distinguished from the matrix phase by a food coloring dye. The thickness of the AlPO4 interphase layer was 0.33 mm. A multifilament rod with diameter 2.1 mm had approximately 93 two-layer, sub-cells in it. The microstructure of a green body of the two-layer, mullite-AlPO4 fibrous monolithic composite is shown in the optical micrograph of Fig. 8. The
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Fig. 7. Optical micrographs of the mullite-AlPO4 fibrous monolithic composite filaments (M: matrix center rod (mullite), I: interphase layer (AlPO4 ). The thickness of green interphase layer is 0.33 mm). (a) Monofilament rod and (b) multifilament rod.
Fig. 8. Optical micrograph of the mullite-AlPO4 fibrous monolithic composite green body.
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Fig. 9. SEM micrographs of the sintered mullite-AlPO4 , two-layer, fibrous monolithic composite. (M: matrix center rod (mullite), I: porous interphase layer (AlPO4 )).
mullite matrix and AlPO4 interphase formed a homogeneous and uniform microstructure. Fig. 9(a and b) are SEM micrographs of the sintered mullite-AlPO4 , two-layer, fibrous monolithic composite. The microstructure is homogeneous, and the higher magnification image confirms that the AlPO4 interphase layer was porous. SEM micrograph of the sintered 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet; fibrous monolithic composite are presented in Fig. 10. The higher magnification micrograph of the interphase region, however, indicates that alumina platelets in the layer had sintered together. Because of the high CIPping pressure of 413.7 MPa, the platelets in the interphase region were thought to be compacted together without forming a porous pocket structure, which is necessary for debonding to occur. Because the platelets were compacted together after CIPping and the composite pellet was sintered at 1650 ◦ C for 15 h, the alumina platelets grains grew to about 2–3 times larger compared to their starting sizes.
Fig. 10. SEM micrographs of the sintered 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet interphase, forming a two-layer fibrous monolithic composite. (M: matrix center rod (50 vol.% alumina:50 vol.% YAG in situ composite), I: interphase layer (alumina platelets)).
The two composites were mechanically evaluated by 3-point bend testing and the load versus displacement curves are given in Fig. 11. The two-layer, mullite-AlPO4 fibrous monolithic composite exhibited apparent non-brittle fracture behavior with ∼0.1 mm of displacement, and had a strength and work of fracture of 76 ± 5 MPa and 0.45 ± 0.02 kJ/m2 , respectively. The graceful failure of the mullite-AlPO4 composite, is attributed to crack deflection along the porous AlPO4 interphase as can be seen in Fig. 12. This SEM micrograph clearly shows that crack deflected along the weak AlPO4 interphase. On the other hand, the sintered 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet, two-layer fibrous monolithic composite underwent brittle fracture, showing a displacement less than 0.02 mm, and had a room temperature, 3-point bend strength and work of fracture of 219 ± 7 MPa and 0.25 ± 0.03 kJ/m2 , respectively. The reason for the brittle
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Fig. 11. Stress–strain curves, for the bend tested, mullite-AlPO4 and 50 vol.% alumina:50 vol.% YAG in situ matrix–alumina platelet, fibrous monolithic composites.
fracture of this composite, can be attributed to the relatively dense microstructure of the alumina platelet interphase region because of the compaction of platelets due to high CIPping pressure and sintering the pellet at high temperature and for a long time at 1650 ◦ C for 10 h, as can be seen in Fig. 10. The reasons for the relatively lower composite strength of 219 ± 7 MPa are attributed to the grain growth of alumina platelets in the interphase region. Fig. 13 shows the fracture surfaces of two composites after 3-point bend testing. The mullite-AlPO4 fibrous monolithic composite indicated extensive fiber pull-out and a rough fracture surface. However the 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet, two-layer fibrous monolithic composite showed very little fiber pull-out and relatively smooth fracture surface.
Fig. 12. SEM micrograph of the fractured mullite-AlPO4 fibrous monolithic composite.
Fig. 13. SEM micrographs of the fracture surfaces of the two-layer fibrous monolithic composites. (a) Mullite-AlPO4 composite and (b) 50 vol.% alumina:50 vol.% YAG in situ composite-alumina platelet composite.
4. Conclusions Processing procedures for a mullite-AlPO4 and 50 vol.% alumina:50 vol.% YAG in situ composite matrix–alumina platelet, two-layer fibrous monolithic composite were successfully demonstrated. The synthesized 50 vol.% alumina:50 vol.% YAG in situ composite and AlPO4 powders had irregular shapes with sharp edges. The fact that the mixing torque value of the mullite inner matrix rod was about five-times higher than that of the outer aluminum phosphate interphase layer, indicated that the formulation systems using graded ethylene vinylacetate copolymer as a binder, were good to make homogeneous fibrous monolithic composite texture without intermixing of two materials. TGA analysis of mullite-AlPO4 , two-layer green pellet, indicated that a multi-step binder removal was successful. The 50 vol.% alumina:50 vol.% YAG in situ matrix–alumina platelet, fibrous monolithic composite, exhibited brittle fracture, because of significant densification in the alumina platelet interphase during CIPping and sintering. The porous AlPO4 interphase layer allowed the mullite-AlPO4 fibrous monolith composite to show some non-brittle fracture.
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Acknowledgements This work was supported by the US AFOSR under grant number: F49620-03-1-0082.
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