D.-H. Kuo, W.M. Kriten/ Materials Science and Engineering A210(1996)123-134 Material properties can be controlled by adjusting the After drying at 200-300C on a hot plate, the powders tape compositions, reinforcement orientation and stack- were calcined at 950C (LP)and 1200C(LA, and ing sequence. Tough laminated composites can be ob- YAG). The calcined powders were ball milled for 3 tained by introducing ductile interlayers, e.g. metallic days, dried and passed through a number 100 sieve layers [17, 18] or carbon fiber/ epoxy prepregs [19], or by inserting a weak interlayer, e. g. carbon [20], in between 2. 2. Chemical compatibility and microstructural ceramic substrates. Nevertheless . these laminates have characterization problems in high temperature oxidizing applications The fiber pushout test has been widely used to char Studies of chemical compatibility were carried out on acterize the nature of interfaces in fiber-reinforced ce- pressed pellets composed of Lp powder as one compo- ramic composites. This test can be a cost-saving nent and Al,O3, YAG or LAu powder as the other screening test on model systems when used to evaluate Studies were also carried out on an LP-coated Al2O3 the mechanical responses(i.e debonding and sliding)of fiber (Saphikon, Inc, Milford, NH)/ AL2O3 matrix fibers in a matrix. Theoretical models [21, 22] and a nodel system. These materials were fired at 1550C shear-lag approach [23] can then be applied to calculate and 1600C for 3-6 h and the phases were identified the interfacial properties, although the models need to using X-ray diffractometry (XRD, model D-Max, be modified to allow for the effect of the coating on the Rigaku/USA, Inc., Danvers, MA)and scanning elec- interface behavior during the pushout experiment tron microscopy (SEM, model DS-130, International In this paper, LaPO4 was investigated as a weak Scientific Instruments, Santa Clara, CA)equipped with interlayer in three laminates and two fiber model sys- energy dispersive spectroscopy (EDS). Microstructural tems. The laminates were fabricated by a tape casting characterization was performed by optical microscopy (doctor blade) process. Al,O3(A),Y3AlSO12(YAG) and SEM, using as-fabricated specimens for better con and LaAlyOIs(LAu) were combined with LaPO4(LP) trast. The coefficient of thermal expansion(CTE)for to make the following laminates: LP/A, LP/YAG and LAu was measured on a NETZSCH dilatometer P/LAu. The combinations of LP/A and LP/ YAG are (model 402 ES, Selb, Germany)for temperatures up to related to the developments of Al, O3 fiber- and YAG 1200C fiber-reinforced ceramic composites, LAu is a member magnetoplumbite/B-alumina group 23 Laminate fa which contains weak basal planes [11]. Thus it was hoped that the LP/LA, combination would also have a The procedure for making laminated composites by weak interface. Flexural testing and the indentation tape casting is summarized in Fig. I. The formulation method were used to measure the flexural strengths and followed the technique of Plucknett et al. [ 14-16. The to examine the interfacial bonding slurry formulation contained approximately 20 vol. The interfacial shear strengths were measured by oxide powders, approximately 60 vol. solvent(con fiber pushout tests on the systems Al2O3 fiber/LP/ AL2O3 sisting of a mixture of trichloroethylene and ethanol) matrix and YAG fiber/LP/Al, O3 matrix, and compared and a dispersant, binder and plasticizers. The slurry with calculations made by the shear-lag and linear formulation for tape casting of the different materials is given in Table 1. a slight change in the amount of solvent was made for the slurry viscosity when needed. Slurries were tape cast to 2. Experimental procedures yield laminae 100-200 um thick with a doctor blade opening of 250-350 um. Eighty-layer laminated com- sites were fabricated by alternatively stacking two 2.1. Powder preparation kinds of oxide laminae having dimensions of 25 mm x 51 mm. Thermocompression was performed by Powdered 99.8% Al6-SG(Alcoa Aluminum Co., holding for I h at 50-80C under a pressure of 10 Pittsburgh, PA)Al,O3 was used. The LP, LAu and MPa. The organic additives were removed by heating YAG powders were prepared by dissolving 99.9% to 500C at a rate of 3C h, followed by a 3 h La, O3 or Y,O, powders(Molycorp, Inc, White Plains, holding time. Subsequently, the bulk materials were NY in nitric acid. Ammonium phosphate, dibasic isostatically cold pressed at approximately 170 MPa for Fisher Scientific, Pittsburgh, PA)or aluminum nitrate 10 min, and then loaded into a graphite die with Al,O nonahydrate (J.T. Baker Chemical Co., Phillipsburg, YAG and LAu powders surrounding the pressed LP/ NJ)was then added to the solution. An organic resin A, LP/YAG and LP/LAu specimens respectively. Con- ormed by mixing ethylene glycol(Fisher Scientific)and solidation was performed by hot pressing, under an for citric acid monohydrate(EM Science, Gibbstown, NJ) argon atmosphere at 28 MPa, at temperatures of 1600 was added to control drying and to form fine powders C for 3 h in the case of LP/YAG and LP/LAul124 D.-H. Kuo, W.M. Kriven / Materials Science and Engineering A210 (1996) 123-134 Material properties can be controlled by adjusting the tape compositions, reinforcement orientation and stack￾ing sequence. Tough laminated composites can be ob￾tained by introducing ductile interlayers, e.g. metallic layers [17,18] or carbon fiber/epoxy prepregs [19], or by inserting a weak interlayer, e.g. carbon [20], in between ceramic substrates. Nevertheless, these laminates have problems in high temperature oxidizing applications. The fiber pushout test has been widely used to char￾acterize the nature of interfaces in fiber-reinforced ce￾ramic composites. This test can be a cost-saving screening test on model systems when used to evaluate the mechanical responses (i.e. debonding and sliding) of fibers in a matrix. Theoretical models [21,22] and a shear-lag approach [23] can then be applied to calculate the interfacial properties, although the models need to be modified to allow for the effect of the coating on the interface behavior during the pushout experiments. In this paper, LaPO4 was investigated as a weak interlayer in three laminates and two fiber model sys￾tems. The laminates were fabricated by a tape casting (doctor blade) process. AI203 (A), Y3A15012 (YAG) and LaAll ~O18 (LAI~) were combined with LaPO 4 (LP) to make the following laminates: LP/A, LP/YAG and LP/LA~. The combinations of LP/A and LP/YAG are related to the developments of A1203 fiber- and YAG fiber-reinforced ceramic composites. LAl~ is a member of the refractory magnetoplumbite/fl-alumina group which contains weak basal planes [11]. Thus it was hoped that the LP/LA~ combination would also have a weak interface. Flexural testing and the indentation method were used to measure the flexural strengths and to examine the interfacial bonding. The interfacial shear strengths were measured by fiber pushout tests on the systems A1203 fiber/LP/A1203 matrix and YAG fiber/LP/A1203 matrix, and compared with calculations made by the shear-lag and linear approaches. 2. Experimental procedures 2.1. Powder preparation Powdered 99.8% A16-SG (Alcoa Aluminum Co., Pittsburgh, PA) A1203 was used. The LP, LAll and YAG powders were prepared by dissolving 99.9% La203 or Y203 powders (Molycorp, Inc., White Plains, NY) in nitric acid. Ammonium phosphate, dibasic (Fisher Scientific, Pittsburgh, PA) or aluminum nitrate nonahydrate (J.T. Baker Chemical Co., Phillipsburg, N J) was then added to the solution. An organic resin formed by mixing ethylene glycol (Fisher Scientific) and citric acid monohydrate (EM Science, Gibbstown, N J) was added to control drying and to form fine powders. After drying at 200-300 °C on a hot plate, the powders were calcined at 950 °C (LP) and 1200 °C (LA~l and YAG). The calcined powders were ball milled for 3 days, dried and passed through a number 100 sieve. 2.2. Chemical compatibility and microstructural characterization Studies of chemical compatibility were carried out on pressed pellets composed of LP powder as one compo￾nent and A1203, YAG or LAI~ powder as the other. Studies were also carried out on an LP-coated A1203 fiber (Saphikon, Inc., Milford, NH)/A1203 matrix model system. These materials were fired at 1550 °C and 1600 °C for 3-6 h and the phases were identified using X-ray diffractometry (XRD, model D-Max, Rigaku/USA, Inc., Danvers, MA) and scanning elec￾tron microscopy (SEM, model DS-130, International Scientific Instruments, Santa Clara, CA) equipped with energy dispersive spectroscopy (EDS). Microstructural characterization was performed by optical microscopy and SEM, using as-fabricated specimens for better con￾trast. The coefficient of thermal expansion (CTE) for LAtl was measured on a NETZSCH dilatometer (model 402 ES, Selb, Germany) for temperatures up to 1200 °C. 2.3. Laminate fabrication The procedure for making laminated composites by tape casting is summarized in Fig. I. The formulation followed the technique of Plucknett et al. [14-16]. The slurry formulation contained approximately 20 vol.% oxide powders, approximately 60 vol.% solvent (con￾sisting of a mixture of trichloroethylene and ethanol) and a dispersant, binder and plasticizers. The slurry formulation for tape casting of the different materials is given in Table 1. A slight change in the amount of solvent was made for the purpose of adjusting the slurry viscosity when needed. Slurries were tape cast to yield laminae 100-200 /~m thick with a doctor blade opening of 250-350 pro. Eighty-layer laminated com￾posites were fabricated by alternatively stacking two kinds of oxide laminae having dimensions of 25 mm x 51 mm. Thermocompression was performed by holding for 1 h at 50-80 °C under a pressure of 10 MPa. The organic additives were removed by heating to 500 °C at a rate of 3 °C h-l, followed by a 3 h holding time. Subsequently, the bulk materials were isostatically cold pressed at approximately 170 MPa for 10 min, and then loaded into a graphite die with A1203, YAG and LAll powders surrounding the pressed LP/ A, LP/YAG and LP/LA~ specimens respectively. Con￾solidation was performed by hot pressing, under an argon atmosphere at 28 MPa, at temperatures of 1600 °C for 3 h in the case of LP/YAG and LP/LA~1