- Email: [email protected]
Lightweight (Ca1−x, Mgx)Zr4(PO4)6 ceramics
Materials
Chemistry
and Physics,
Lightweight Dean-MO Materials
32 (1992)
161
161-167
(Ca, --x, Mgx)Zr4(P0&
ceramics*
Liu
Research
Laboratoties,
and Jesse J. Brown
Industrial
Research
Institute,
Hsinchu,
Chutung,
31015
(Taiwan,
ROC)
Jr.
Center for Advanced 0256 (USA)
Ceramic
(Received
20, 1992; accepted
February
Technology
Materials,
301 Holden
March
Hall,
Wginia
Polytechnic
Institute
and State
University
Blacksburg,
VA 24061-
27, 1992)
Abstract (Cat-,, Mg,)Zr.,(PO,), (CMZP) ceramics (X=0, 0.1, 0.2 and 0.3) have been fabricated with relative densities of less than 0.35 by the polymer foam technique, and higher than 0.35 by the polymer powder technique. The polymer powder method forms an inhomogeneous pore structure having an average pore size of 30-80 microns compared with the polymer foam method, which yields large and uniform pore structures with pores 250-300 microns in diameter. Gas permeability of these two ceramics was examined and distinct behaviors were exhibited. The lightweight CMZP ceramics showed flexural strength comparable to that of lightweight ZrO,, but lower in compressive strength. The excellent resistance to thermal shock makes the lightweight CMZP ceramics a strong candidate for applications in severe environments. Lightweight
Introduction
The rising costs of petroleum products and other energy sources have brought attention to the importance of energy utilization, particularly energy savings and service efficiency. One method to improve energy utilization is to develop new materials having characteristics such as low thermal conductivity to reduce heat dissipation during operation, low coefficient of thermal expansion to minimize thermal-shock-induced fracture when subjected to cyclic thermal environments, and good thermal stability for high-temperature applications. Currently, lightweight ceramics are widely used as thermal insulation such as thermal protection systems for space shuttles and the filter systems such as the filtration system for molten metals and hot gas clean-up. These applications of lightweight ceramics require high thermal shock resistance, low thermal conductivity, and sufficient mechanical strength. In the present study, the development of a lightweight ceramic having both a low coefficient of thermal expansion and a low thermal conductivity has been initiated by exploring a new class *This works was accomplished Engineering University.
and Science, Virginia Blacksburg, VA.
0254-0584/92/$5.00
in the Department of Materials Polytechnic Institute and State
of ceramic materials known as [NZP], i.e. NaZr,(PO,),, which has the desired properties [l, 2,3]. The processing techniques for manufacturing lightweight ceramics have been studied. The pore structure, gas permeability, mechanical properties, and thermal shock resistance were characterized.
Literature
survey
Lightweight ceramics may have four basic structures: (1) tangle fiber networks, e.g. the thermal protection system of the space shuttle formed by sintering silica glass fibers with a high temperature binder; (2) particulate networks, e.g. porous glassceramics formed using sol-gel methods; (3) opencell structures; e.g. filters produced by coating open-cell polymer foams with a ceramic and (4) closed-cell structures, e.g. sintered hollow spheres. The most widely used fabrication technique for producing open-cell ceramics is by coating reticulated polymer substrates with a ceramic slurry. The resulting ceramics possessing the same network structure as the polymer, can be obtained by the pyrolysis of the polymer substrate through a very slow heating rate to a desired firing temperature. This technique provides lightweight ceramics with a variety of unique properties such as controllable
0 1992 - Elsevier
Sequoia.
All rights
resewed
162
pore size, controllable tortuosity, and complex ceramic shapes for different applications and are favorable for many applications as pointed out by Lange and Miller [4]. Green [5] obtained closedcell glasses by sintering hollow glass spheres at a sufficient temperature and found that the densification behavior of the glass spheres became complex involving uniform shrinkage of the spheres, local densification, sphere deformation, and coarsening of the cells as the temperature increased to 1000 “C. Fujiu et al. [6] fabricated porous silica having closed-cell structures by controlling gel viscosity and foaming agent concentration in a solgel method, but the porous silica densified as the temperature increased above 1000 “C, indicating that this fabrication technique is inappropriate for high-temperature applications. In this investigation, an additional (except the slurry-coating technique) method used to fabricate porous ceramics with primarily a closed-pore structure is to mix polymer and ceramic powders leaving voids or pores after sintering. A promising characteristic of the well-known [NZP] is that they possess an ultra-low coefficient of thermal expansion which is due to their low and opposite axial displacement in the lattice and has been extensively studied [7, 8, 91. The CMZP ceramics, which were synthesized by incorporating Mg+* ions with CaZr,(PO,), [lo], possess a framework structure resembling that of the [NZP] and exhibit ultra-low thermal expansion coefficients [ll, 121 (CTEs are well below 2-3~ lO-‘j ‘C-l), and thermal conductivities which are lower than that of conventional thermal barriers [13]. The relationship between strength and porosity has been extensively studied during the last two decades [14]. The most widely used relation for mechanical strengths is the exponential dependence which was first proposed by Ryshkewitch [VI: S = S, exp( - bp)
has an intermediate b value, and rhombohedral has the highest b value. This indicates that the more complex the pore shape, the higher the porosity dependence b. Experimental CMZP powder preparation
Single phase (Ca, _-x,MgX)Zr4(P0& with x = 0, 0.1, 0.2 and 0.3 powders were synthesized using stoichiometric amounts of CaCO, (99.5%), MgC03 (99.5%), Zr02 (99%), and NI&H2P04 (98%) as starting materials. The mixture was homogenized in acetone either by hand mixing in a mortar with a pestle or by ball milling and then air dried. The dry powder was calcined at different temperatures to drive off the volatiles at 250 “C for 20 h, 650 “C for 4 h, and 1250 “C for 24 h. Before each heat treatment the material was thoroughly ground to ensure material homogeneity. The resultant powders were ground by ball milling to a particle size of 3-5 microns. Preparation of lightweight CMZP ceramics
Two methods were employed for manufacturing lightweight CMZP ceramics, namely, polymer foam and polymer powder methods. The first technique is conducted using a reticulated polyurethane foam, which is a flexible porous cellular plastic, with a pore size of 25CL300 microns, Fig. 1. The foam was impregnated with a ceramic slurry consisting of deionized water, 10-25 wt% of CMZP powders, 10-25 wt% of cellulose ether binder (based on the weight of the CMZP powders), and O-l wt% ZnO as a sintering aid. The slurry-coated foam was air dried and then heat-treated at 1300 “C for 10 min to 24 h with a heating rate of 100 “C h-l.
(1)
where S is the strength of the porous material, So is the zero-porosity strength, and b is determined by the complexity of the pore structure and p the volume fraction of pores. For the parameter b, Gannon et al. [16] indicated that it usually is larger due to inhomogeneous pore distribution and variety of pore shapes, but is smaller for materials possessing uniform pore shape and uniform pore distribution. Kundsen [17] proposed a set of models to describe the relationship between strength and porosity based on the different packing arrangements of coalescing spheres. Kundsen found that cubic packing has the smallest b value, orthorombic
Fig. 1. Microstructure
of polymer foam.
163
The alternative method is conducted by mixing the CMZP powders with 5-22.5 wt% of poly(viny1 chloride) having particle sizes of 20-30 microns and O-l wt% ZnO. The mixture was pressed to form a 12.7 x 12.7 X 100 mm rectangular bar at a hydraulic pressure of 140 MPa. The bar was heated using the same firing conditions as those in the polymer foam method.
Characterization The pore structure of the obtained lightweight CMZP ceramics was characterized using scanning electron microscopy. The gas permeability of the ceramics was examined. To characterize the mechanical behaviors; flexural strength, compressive strength, and Young’s moduli of the lightweight CMZP ceramics were determined using an Instron testing machine (model 4202) with a crosshead speed of 0.5 mm min-l, and an ultrasonic technique respectively. Thermal shock resistances of the lightweight CMZP ceramics were also determined after air quenching at different temperatures.
Fig. 3. Defects
between
pores.
Results and discussion Pore structure of lightweight CMZP ceramic The microstructure of the lightweight CMZP ceramics is controlled by the processing techniques. Figure 2 illustrates the resulting ceramic, which was manufactured using the foam method. It has circular pores and a pore structure resembles that of the original polymer foam, but consists of both open and closed pores with an average pore size of 300 microns. Defects such as holes as well as cracks (Fig. 3), can be easily found in the struts between pores and are caused by the pyrolysis of the polymer substrates. However, these defects
Fig. 2. Microstructure
formed by foam method.
Fig. 4. Elimination
of the defects.
can be reduced by using the fine CMZP powders (about 40 nm) derived from a sol-gel technique [lo], as shown in Fig. 4, which is a cross section of a broken strut. This is due to the fact that fine powders have better sinterability at elevated temperatures in comparison to the coarser powders. The pore structure becomes more complex and the relative densities of the resulting solids becomes higher ( > 0.35) in the powder method. Figures 5a and 5b illustrate two of the ceramics with different relative density, 0.50 and 0.67 respectively. There is a combination of large pores having an average pore size of 200 microns and small pores (20-30 microns) which interconnect forming a continuous phase at lower relative density. However, as the relative density increases, i.e. 0.67, the pore structure becomes isolated with average sizes of 30-80 microns. This transition in pore structure is revealed in the measurement of gas permeability as illustrated in Fig 6, where transition
164
occurred at a relative density of 0.63 and is in good agreement with observations by scanning electron microscopy. Accordingly, this transition can be explained by the percolation theory proposed by McCullough [18]. At a low fraction of porosity the pores are isolated, as the porosity increases to a critical value, the amount of isolated pores is increased and they begin to connect to form an open channel which in turn rapidly increases the gas permeability. However, the permeability of the lightweight CMZP exhibits different behaviors in the foam method (Fig. 7), where the values of permeability are higher than those from the powder method, even at the same relative densities. This difference is because the foam method produces more and larger open pores. Furthermore, higher porosity dependence in the specimens prepared by the foam method is a result of the percentage of open pores decreasing rapidly as the relative density is increased. The lower permeabilities in the powder method indicated that the pore structure consists mainly of closed and small-sized pores. The closed pores do not affect the permeability, while the small pores usually increase the pressure drop within the specimens, resulting in decreased permeability.
(4
Mechanical properties
(b) Fig. 5. Pore structure of the lightweight ChEP ceramics formed, using the powder method with relative densities of (a) 0.50, (b) 0.67.
For porous solids, the cracks tend to propagate along the largest pores or the most serious defect such as microcrack. Hence, the strength of the porous solids usually does not reflect the average properties, but rather the largest defects in the microstructure. Figure 8 illustrates the relation of flexural strength and relative density of the lightweight CMZP ceramics having a variable Mg con400 0
Relative Fig. 6. A transition in permeability has changed.
Density indicates that the pore structure
Relative Fig. 7. Permeability relative density.
Density
of the lightweight
CMZP as a function
of
165
.g
10000;
if
3
2
a
1 Cl00
?
L
0
m 33
o. _ Sol-Gel PO
-x,Mgx)Zr4(P04)6
n 2 I”,
I
02
00
I
Relative Fig. 8. Effect the lightweight
of particle CMZP.
size and
de
0’6
04
1’0
Density relative
density
on MOR
of
tent. As shown, the flexural strength appears to be independent of the composition, indicative that the fracture mode is not affected by slightly altering the framework structure of CMZP [ll]. No obvious strength variation is observed between the fabrication methods, indicating that the flexural strength is insensitive to the pore structure but is directly affected by the connected area of the solid network. The flexural strength can be expressed as a porosity-dependent relationship using the method of least squares, thus Sf= 183.6 exp( - 9.Op) (MPa)
(2)
density (denoted by open squares). The flexural strength of the lightweight CMZP ceramics approaches that of the lightweight ZrO,,, and the former may possess higher strength than the latter in certain relative density ranges by reducing the particle size. Typical compressive stress-strain curves of the lightweight CMZP ceramics exhibit three regimes of behavior: linear elastic, collapse, and densification, Fig. 9. The lightweight CMZP ceramics made by the powder method fail rapidly because the stress concentrates near large and complex pores [19], as in Fig. 5(b), which leads to catastrophic failure once the compressive stress exceeds the crushing strength. In contrast, the lightweight CMZP ceramics made using the foam method fail stepwise in the collapse mode, where the compressive stress is shared by cell-edge bending and cell-wall stretching [20]. The dependence of the compressive strength on the relative density of the lightweight CMZP ceramics is illustrated in Fig. 10. A transition in the compressive strength occurs at a relative density of 0.35, which is due to the change in forming technique. This shows that the compressive strength is strongly affected by the pore structure, i.e., compressive strength demonstrates a low porosity dependence for materials having an uniform pore structure, but a high porosity dependence, for a complex structure [14]. The relations between the compressive strength and relative density can be expressed in terms of porosity using the method of least squares:
having a correlation coefficient of 0.92. Sf is the flexural strength and p represents the volume fraction of pores. The particle size of the powders also affects the flexnral strength of the lightweight CMZP by changing the microstructure which has already been discussed above. As indicated in Fig. 8 with a open square, the flexural strength of the lightweight CMZP ceramics made using the sol-gel-derived powders is about three- to five-fold higher than those made using the coarser powders at relative densities between 0.22 and 0.42. Similarly, the relationship between the flexural strength and porosity for the fine CMZP powder is expressed as &(fine powder)
120 exp( - 6.7~) (MPa)
(3) For comparison, the conventional leading thermal barrier material, i.e. lightweight ZrO,, was made using the powder method and the resulting ZrO, has relative densities of 0.2 to 0.83. Figure 8 also describes the behavior of flexural strength of the lightweight ZrO, in terms of the relative =
0
1
Ncmtlizd Fig. 9. Compressive ceramics.
Strain
stress-strain
curves
if the lightweight
CMZP
166 00000
‘Ooo ! (ca ;
0
101 00
0.2
04
Relative Fig. 10. Compressive strength lightweight CMZP and ZrOZ.
0.6
0.8
IO
density
for the
0
I d.0
I -x,Mgx)Zr4(!‘04)6
x-o.0
0'2
Relative
Density
verSUS relative
S, = 965 exp( - 11.0~) (MPa)
4000
5
Density
Fig. 11. Young’s moduli of the lightweight functions of porosity and composition.
for the powder method,
018
016
014
CMZP ceramics are
(Cao.g,Mgo. I )Zf4(PO4)6
(4)
for the foam method, S, = 12.4 exp( -4-e)
(MPa)
(5) which have correlation coefficients of 0.95 and 0.96, respectively, and S, represents the compressive strength. The exponential b value in eqn. (4) is larger than that in eqn. (5), indicating substantial inhomogeneity of spatial distribution and shape of pores present in the lightweight CMZP ceramics made by the powder method, which is consistent with the SEM results and also agrees with the experimental results of Gannon et al. [16] and Kundsen [17]. The lightweight CMZP ceramics exhibit lower values of compressive strength than those of ZrOz, which can be expressed as, s c(zrOz)= 2758.4 exp( - 9.Q) (MPa)
(6) This is close to that presented by Ryshkewitch [15] for ZrO, foam with a uniform pore size of about 200 microns. His strength-porosity relation is S C,(ZrCh) = 2896.3 exp( - 7.0~7)(MPa)
(7) Again, the difference in the exponential b value between eqns. (6) and (7) indicates that a more inhomogeneous porosity exists in ceramics made by the powder method. The Young’s moduli of the lightweight CMZP and ZrOl ceramics were determined using an ultrasonic technique. Figure 11 illustrates the results where each composition is well-fitted into a linear relationship between the Young’s moduli and relative density using the method of least
!s
2 g
1000
L : :a
0.54+lwt% Zno
400
0
Quench Fig. 12. Thermal-shock x=0.1.
800
Temperature resistance
1600
1200
Difference
of the lightweight
(‘C) CMZP with
squares. The Young’s moduli of the lightweight CMZP ceramics increases with an increase of the Mg content for the same relative densities. One tentative explanation is due to the reduction of lattice volume by replacing larger Ca2+ cations with smaller Mg*+ cations leading to a more rigid framework structure [ll]. Thus, a higher strength is required to separate adjacent atoms, which in turn results in higher values of Young’s moduli. Thermal shock resistance The thermal shock resistance
of lightweight CMZP ceramics, made by the polymer powder method, is illustrated in Fig. 12, where five to six specimens were measured through three-point bending at each temperature, In Fig. 12, the strength of the lightweight CMZP ceramics having different relative densities is nearly constant after
167
air quenching, even up to 1500 “C, which is attributed to their ultra-low thermal expansion characteristics that minimize the internal stresses resulting from thermal expansion anisotropy. Conclusion
Lightweight CMZP ceramics can be fabricated using the foam and powder methods. The former technique produces large pores (280 microns) composing mainly of an uniform open-pore structure. The latter yields small pores (30-80 microns) with a pore structure consisting of both open and closed pores. Their excellent thermal shock resistance and sufficient mechanical strength make them a strong candidate for applications in severe environments. Acknowledgements
The authors thank the U.S. Department of Energy under Contract DE-AC05-840R21400, the Martin Maritta Energy system and the Center for Advanced Ceramic Materials of the Virginia Center for Innovative Technology for supporting this research. References 1 S. Y. Limaye, D. K. Agrawal and H. A. McKinstry, J. Am. &rum. Sot., 70 (1987) 232.
2 G. E. Lenain, H. A. McKinstry, S. Y. Limaye and A. Woodward, Mater. Rex Bull., 19 (1984) 1451. 3 G. E. Lenain, H. A. McKinstry, J. Alamo and D. K. Agrawal, J. Mater. Sci., 22 (1987) 17. 4 F. F. Lange and K. T. Miller, Adv. Ceram. Mater., 2 (1987) 827. 5 D. J. Green, /. Am. Ceram. Sot., 68 (1988) 403. 6 T. Fujiu, G. L. Messing and W. Heubner, J. Am. Cerum. sot., 73 (1990) 85. 7 H. Y.-P. Hong, Mater. Rex Bull., 11 (1976) 173. 8 J. Alamo and R. Roy, J. Mater. Sci., 21 (1986) 444. 9 L. 0. Hagman and P. Kierkegaard, Acta Chem. Stand., 22 (1968) 1822. 10 D.-M. Liu, D. A. Hirschfeld and J. J. Brown, Development of Lightweight (Ca,_MgJZr,(PO& Ceramics, presented at the Amer. Ceram. Sot. 93th Annual Meeting, Cincinnati, OH, 1991. 11 S. M. van Aken, Thermal Expansion and l7zermal Conductivity of (Cat-, MgJZr,(PO& wherex=O.O-0.4, M.S. Thesis, Virginia Polytechnic Institute and State University (1990). 12 D. M. Liu, 7’he Development and Characterization of Lightweight (Ca,_, MgJZr,(POJe Ceramics, M.S. Thesis, Virginia Polytechnic Institute and State University (1991). 13 D. A. Hirshfeld, D. M. Liu and J. J. Brown, CMZP-A New High Temperature Thermal Barrier Material, presented at 4th International Symposium on Materials and Components for Engines, Gothenburg, Sweden 1991. 14 R. W. Rice, in R. K. MacCrone, (ed.) Treatise on Material Science and Technology, Vol. 11, Academic press, New York, 1977, pp. 200-381. 15 E. Ryshkewitch, /. Am. Ceram. Sot., 36 (1953) 65. 16 R. E. Gannon, G. M. Harris and T. Vasilos, Am. Ceram. Sot. Bull., 44 (1965) 460. 17 F. P. Kundsen, .I. Am. Ceram. Sot., 42 (1959) 376. 18 R. L. McCullough, Compos. Sci. Tech., 22 (1985) 3. 19 B. D. Agarwal, G. A. Panizza and L. J. Broutman, J. Am. Ceram. Sot., 54 (1971) 620. 20 M. F. Ashby, Metal. Trans. A, 124 (1983) 1755.
Chemistry
and Physics,
Lightweight Dean-MO Materials
32 (1992)
161
161-167
(Ca, --x, Mgx)Zr4(P0&
ceramics*
Liu
Research
Laboratoties,
and Jesse J. Brown
Industrial
Research
Institute,
Hsinchu,
Chutung,
31015
(Taiwan,
ROC)
Jr.
Center for Advanced 0256 (USA)
Ceramic
(Received
20, 1992; accepted
February
Technology
Materials,
301 Holden
March
Hall,
Wginia
Polytechnic
Institute
and State
University
Blacksburg,
VA 24061-
27, 1992)
Abstract (Cat-,, Mg,)Zr.,(PO,), (CMZP) ceramics (X=0, 0.1, 0.2 and 0.3) have been fabricated with relative densities of less than 0.35 by the polymer foam technique, and higher than 0.35 by the polymer powder technique. The polymer powder method forms an inhomogeneous pore structure having an average pore size of 30-80 microns compared with the polymer foam method, which yields large and uniform pore structures with pores 250-300 microns in diameter. Gas permeability of these two ceramics was examined and distinct behaviors were exhibited. The lightweight CMZP ceramics showed flexural strength comparable to that of lightweight ZrO,, but lower in compressive strength. The excellent resistance to thermal shock makes the lightweight CMZP ceramics a strong candidate for applications in severe environments. Lightweight
Introduction
The rising costs of petroleum products and other energy sources have brought attention to the importance of energy utilization, particularly energy savings and service efficiency. One method to improve energy utilization is to develop new materials having characteristics such as low thermal conductivity to reduce heat dissipation during operation, low coefficient of thermal expansion to minimize thermal-shock-induced fracture when subjected to cyclic thermal environments, and good thermal stability for high-temperature applications. Currently, lightweight ceramics are widely used as thermal insulation such as thermal protection systems for space shuttles and the filter systems such as the filtration system for molten metals and hot gas clean-up. These applications of lightweight ceramics require high thermal shock resistance, low thermal conductivity, and sufficient mechanical strength. In the present study, the development of a lightweight ceramic having both a low coefficient of thermal expansion and a low thermal conductivity has been initiated by exploring a new class *This works was accomplished Engineering University.
and Science, Virginia Blacksburg, VA.
0254-0584/92/$5.00
in the Department of Materials Polytechnic Institute and State
of ceramic materials known as [NZP], i.e. NaZr,(PO,),, which has the desired properties [l, 2,3]. The processing techniques for manufacturing lightweight ceramics have been studied. The pore structure, gas permeability, mechanical properties, and thermal shock resistance were characterized.
Literature
survey
Lightweight ceramics may have four basic structures: (1) tangle fiber networks, e.g. the thermal protection system of the space shuttle formed by sintering silica glass fibers with a high temperature binder; (2) particulate networks, e.g. porous glassceramics formed using sol-gel methods; (3) opencell structures; e.g. filters produced by coating open-cell polymer foams with a ceramic and (4) closed-cell structures, e.g. sintered hollow spheres. The most widely used fabrication technique for producing open-cell ceramics is by coating reticulated polymer substrates with a ceramic slurry. The resulting ceramics possessing the same network structure as the polymer, can be obtained by the pyrolysis of the polymer substrate through a very slow heating rate to a desired firing temperature. This technique provides lightweight ceramics with a variety of unique properties such as controllable
0 1992 - Elsevier
Sequoia.
All rights
resewed
162
pore size, controllable tortuosity, and complex ceramic shapes for different applications and are favorable for many applications as pointed out by Lange and Miller [4]. Green [5] obtained closedcell glasses by sintering hollow glass spheres at a sufficient temperature and found that the densification behavior of the glass spheres became complex involving uniform shrinkage of the spheres, local densification, sphere deformation, and coarsening of the cells as the temperature increased to 1000 “C. Fujiu et al. [6] fabricated porous silica having closed-cell structures by controlling gel viscosity and foaming agent concentration in a solgel method, but the porous silica densified as the temperature increased above 1000 “C, indicating that this fabrication technique is inappropriate for high-temperature applications. In this investigation, an additional (except the slurry-coating technique) method used to fabricate porous ceramics with primarily a closed-pore structure is to mix polymer and ceramic powders leaving voids or pores after sintering. A promising characteristic of the well-known [NZP] is that they possess an ultra-low coefficient of thermal expansion which is due to their low and opposite axial displacement in the lattice and has been extensively studied [7, 8, 91. The CMZP ceramics, which were synthesized by incorporating Mg+* ions with CaZr,(PO,), [lo], possess a framework structure resembling that of the [NZP] and exhibit ultra-low thermal expansion coefficients [ll, 121 (CTEs are well below 2-3~ lO-‘j ‘C-l), and thermal conductivities which are lower than that of conventional thermal barriers [13]. The relationship between strength and porosity has been extensively studied during the last two decades [14]. The most widely used relation for mechanical strengths is the exponential dependence which was first proposed by Ryshkewitch [VI: S = S, exp( - bp)
has an intermediate b value, and rhombohedral has the highest b value. This indicates that the more complex the pore shape, the higher the porosity dependence b. Experimental CMZP powder preparation
Single phase (Ca, _-x,MgX)Zr4(P0& with x = 0, 0.1, 0.2 and 0.3 powders were synthesized using stoichiometric amounts of CaCO, (99.5%), MgC03 (99.5%), Zr02 (99%), and NI&H2P04 (98%) as starting materials. The mixture was homogenized in acetone either by hand mixing in a mortar with a pestle or by ball milling and then air dried. The dry powder was calcined at different temperatures to drive off the volatiles at 250 “C for 20 h, 650 “C for 4 h, and 1250 “C for 24 h. Before each heat treatment the material was thoroughly ground to ensure material homogeneity. The resultant powders were ground by ball milling to a particle size of 3-5 microns. Preparation of lightweight CMZP ceramics
Two methods were employed for manufacturing lightweight CMZP ceramics, namely, polymer foam and polymer powder methods. The first technique is conducted using a reticulated polyurethane foam, which is a flexible porous cellular plastic, with a pore size of 25CL300 microns, Fig. 1. The foam was impregnated with a ceramic slurry consisting of deionized water, 10-25 wt% of CMZP powders, 10-25 wt% of cellulose ether binder (based on the weight of the CMZP powders), and O-l wt% ZnO as a sintering aid. The slurry-coated foam was air dried and then heat-treated at 1300 “C for 10 min to 24 h with a heating rate of 100 “C h-l.
(1)
where S is the strength of the porous material, So is the zero-porosity strength, and b is determined by the complexity of the pore structure and p the volume fraction of pores. For the parameter b, Gannon et al. [16] indicated that it usually is larger due to inhomogeneous pore distribution and variety of pore shapes, but is smaller for materials possessing uniform pore shape and uniform pore distribution. Kundsen [17] proposed a set of models to describe the relationship between strength and porosity based on the different packing arrangements of coalescing spheres. Kundsen found that cubic packing has the smallest b value, orthorombic
Fig. 1. Microstructure
of polymer foam.
163
The alternative method is conducted by mixing the CMZP powders with 5-22.5 wt% of poly(viny1 chloride) having particle sizes of 20-30 microns and O-l wt% ZnO. The mixture was pressed to form a 12.7 x 12.7 X 100 mm rectangular bar at a hydraulic pressure of 140 MPa. The bar was heated using the same firing conditions as those in the polymer foam method.
Characterization The pore structure of the obtained lightweight CMZP ceramics was characterized using scanning electron microscopy. The gas permeability of the ceramics was examined. To characterize the mechanical behaviors; flexural strength, compressive strength, and Young’s moduli of the lightweight CMZP ceramics were determined using an Instron testing machine (model 4202) with a crosshead speed of 0.5 mm min-l, and an ultrasonic technique respectively. Thermal shock resistances of the lightweight CMZP ceramics were also determined after air quenching at different temperatures.
Fig. 3. Defects
between
pores.
Results and discussion Pore structure of lightweight CMZP ceramic The microstructure of the lightweight CMZP ceramics is controlled by the processing techniques. Figure 2 illustrates the resulting ceramic, which was manufactured using the foam method. It has circular pores and a pore structure resembles that of the original polymer foam, but consists of both open and closed pores with an average pore size of 300 microns. Defects such as holes as well as cracks (Fig. 3), can be easily found in the struts between pores and are caused by the pyrolysis of the polymer substrates. However, these defects
Fig. 2. Microstructure
formed by foam method.
Fig. 4. Elimination
of the defects.
can be reduced by using the fine CMZP powders (about 40 nm) derived from a sol-gel technique [lo], as shown in Fig. 4, which is a cross section of a broken strut. This is due to the fact that fine powders have better sinterability at elevated temperatures in comparison to the coarser powders. The pore structure becomes more complex and the relative densities of the resulting solids becomes higher ( > 0.35) in the powder method. Figures 5a and 5b illustrate two of the ceramics with different relative density, 0.50 and 0.67 respectively. There is a combination of large pores having an average pore size of 200 microns and small pores (20-30 microns) which interconnect forming a continuous phase at lower relative density. However, as the relative density increases, i.e. 0.67, the pore structure becomes isolated with average sizes of 30-80 microns. This transition in pore structure is revealed in the measurement of gas permeability as illustrated in Fig 6, where transition
164
occurred at a relative density of 0.63 and is in good agreement with observations by scanning electron microscopy. Accordingly, this transition can be explained by the percolation theory proposed by McCullough [18]. At a low fraction of porosity the pores are isolated, as the porosity increases to a critical value, the amount of isolated pores is increased and they begin to connect to form an open channel which in turn rapidly increases the gas permeability. However, the permeability of the lightweight CMZP exhibits different behaviors in the foam method (Fig. 7), where the values of permeability are higher than those from the powder method, even at the same relative densities. This difference is because the foam method produces more and larger open pores. Furthermore, higher porosity dependence in the specimens prepared by the foam method is a result of the percentage of open pores decreasing rapidly as the relative density is increased. The lower permeabilities in the powder method indicated that the pore structure consists mainly of closed and small-sized pores. The closed pores do not affect the permeability, while the small pores usually increase the pressure drop within the specimens, resulting in decreased permeability.
(4
Mechanical properties
(b) Fig. 5. Pore structure of the lightweight ChEP ceramics formed, using the powder method with relative densities of (a) 0.50, (b) 0.67.
For porous solids, the cracks tend to propagate along the largest pores or the most serious defect such as microcrack. Hence, the strength of the porous solids usually does not reflect the average properties, but rather the largest defects in the microstructure. Figure 8 illustrates the relation of flexural strength and relative density of the lightweight CMZP ceramics having a variable Mg con400 0
Relative Fig. 6. A transition in permeability has changed.
Density indicates that the pore structure
Relative Fig. 7. Permeability relative density.
Density
of the lightweight
CMZP as a function
of
165
.g
10000;
if
3
2
a
1 Cl00
?
L
0
m 33
o. _ Sol-Gel PO
-x,Mgx)Zr4(P04)6
n 2 I”,
I
02
00
I
Relative Fig. 8. Effect the lightweight
of particle CMZP.
size and
de
0’6
04
1’0
Density relative
density
on MOR
of
tent. As shown, the flexural strength appears to be independent of the composition, indicative that the fracture mode is not affected by slightly altering the framework structure of CMZP [ll]. No obvious strength variation is observed between the fabrication methods, indicating that the flexural strength is insensitive to the pore structure but is directly affected by the connected area of the solid network. The flexural strength can be expressed as a porosity-dependent relationship using the method of least squares, thus Sf= 183.6 exp( - 9.Op) (MPa)
(2)
density (denoted by open squares). The flexural strength of the lightweight CMZP ceramics approaches that of the lightweight ZrO,,, and the former may possess higher strength than the latter in certain relative density ranges by reducing the particle size. Typical compressive stress-strain curves of the lightweight CMZP ceramics exhibit three regimes of behavior: linear elastic, collapse, and densification, Fig. 9. The lightweight CMZP ceramics made by the powder method fail rapidly because the stress concentrates near large and complex pores [19], as in Fig. 5(b), which leads to catastrophic failure once the compressive stress exceeds the crushing strength. In contrast, the lightweight CMZP ceramics made using the foam method fail stepwise in the collapse mode, where the compressive stress is shared by cell-edge bending and cell-wall stretching [20]. The dependence of the compressive strength on the relative density of the lightweight CMZP ceramics is illustrated in Fig. 10. A transition in the compressive strength occurs at a relative density of 0.35, which is due to the change in forming technique. This shows that the compressive strength is strongly affected by the pore structure, i.e., compressive strength demonstrates a low porosity dependence for materials having an uniform pore structure, but a high porosity dependence, for a complex structure [14]. The relations between the compressive strength and relative density can be expressed in terms of porosity using the method of least squares:
having a correlation coefficient of 0.92. Sf is the flexural strength and p represents the volume fraction of pores. The particle size of the powders also affects the flexnral strength of the lightweight CMZP by changing the microstructure which has already been discussed above. As indicated in Fig. 8 with a open square, the flexural strength of the lightweight CMZP ceramics made using the sol-gel-derived powders is about three- to five-fold higher than those made using the coarser powders at relative densities between 0.22 and 0.42. Similarly, the relationship between the flexural strength and porosity for the fine CMZP powder is expressed as &(fine powder)
120 exp( - 6.7~) (MPa)
(3) For comparison, the conventional leading thermal barrier material, i.e. lightweight ZrO,, was made using the powder method and the resulting ZrO, has relative densities of 0.2 to 0.83. Figure 8 also describes the behavior of flexural strength of the lightweight ZrO, in terms of the relative =
0
1
Ncmtlizd Fig. 9. Compressive ceramics.
Strain
stress-strain
curves
if the lightweight
CMZP
166 00000
‘Ooo ! (ca ;
0
101 00
0.2
04
Relative Fig. 10. Compressive strength lightweight CMZP and ZrOZ.
0.6
0.8
IO
density
for the
0
I d.0
I -x,Mgx)Zr4(!‘04)6
x-o.0
0'2
Relative
Density
verSUS relative
S, = 965 exp( - 11.0~) (MPa)
4000
5
Density
Fig. 11. Young’s moduli of the lightweight functions of porosity and composition.
for the powder method,
018
016
014
CMZP ceramics are
(Cao.g,Mgo. I )Zf4(PO4)6
(4)
for the foam method, S, = 12.4 exp( -4-e)
(MPa)
(5) which have correlation coefficients of 0.95 and 0.96, respectively, and S, represents the compressive strength. The exponential b value in eqn. (4) is larger than that in eqn. (5), indicating substantial inhomogeneity of spatial distribution and shape of pores present in the lightweight CMZP ceramics made by the powder method, which is consistent with the SEM results and also agrees with the experimental results of Gannon et al. [16] and Kundsen [17]. The lightweight CMZP ceramics exhibit lower values of compressive strength than those of ZrOz, which can be expressed as, s c(zrOz)= 2758.4 exp( - 9.Q) (MPa)
(6) This is close to that presented by Ryshkewitch [15] for ZrO, foam with a uniform pore size of about 200 microns. His strength-porosity relation is S C,(ZrCh) = 2896.3 exp( - 7.0~7)(MPa)
(7) Again, the difference in the exponential b value between eqns. (6) and (7) indicates that a more inhomogeneous porosity exists in ceramics made by the powder method. The Young’s moduli of the lightweight CMZP and ZrOl ceramics were determined using an ultrasonic technique. Figure 11 illustrates the results where each composition is well-fitted into a linear relationship between the Young’s moduli and relative density using the method of least
!s
2 g
1000
L : :a
0.54+lwt% Zno
400
0
Quench Fig. 12. Thermal-shock x=0.1.
800
Temperature resistance
1600
1200
Difference
of the lightweight
(‘C) CMZP with
squares. The Young’s moduli of the lightweight CMZP ceramics increases with an increase of the Mg content for the same relative densities. One tentative explanation is due to the reduction of lattice volume by replacing larger Ca2+ cations with smaller Mg*+ cations leading to a more rigid framework structure [ll]. Thus, a higher strength is required to separate adjacent atoms, which in turn results in higher values of Young’s moduli. Thermal shock resistance The thermal shock resistance
of lightweight CMZP ceramics, made by the polymer powder method, is illustrated in Fig. 12, where five to six specimens were measured through three-point bending at each temperature, In Fig. 12, the strength of the lightweight CMZP ceramics having different relative densities is nearly constant after
167
air quenching, even up to 1500 “C, which is attributed to their ultra-low thermal expansion characteristics that minimize the internal stresses resulting from thermal expansion anisotropy. Conclusion
Lightweight CMZP ceramics can be fabricated using the foam and powder methods. The former technique produces large pores (280 microns) composing mainly of an uniform open-pore structure. The latter yields small pores (30-80 microns) with a pore structure consisting of both open and closed pores. Their excellent thermal shock resistance and sufficient mechanical strength make them a strong candidate for applications in severe environments. Acknowledgements
The authors thank the U.S. Department of Energy under Contract DE-AC05-840R21400, the Martin Maritta Energy system and the Center for Advanced Ceramic Materials of the Virginia Center for Innovative Technology for supporting this research. References 1 S. Y. Limaye, D. K. Agrawal and H. A. McKinstry, J. Am. &rum. Sot., 70 (1987) 232.
2 G. E. Lenain, H. A. McKinstry, S. Y. Limaye and A. Woodward, Mater. Rex Bull., 19 (1984) 1451. 3 G. E. Lenain, H. A. McKinstry, J. Alamo and D. K. Agrawal, J. Mater. Sci., 22 (1987) 17. 4 F. F. Lange and K. T. Miller, Adv. Ceram. Mater., 2 (1987) 827. 5 D. J. Green, /. Am. Ceram. Sot., 68 (1988) 403. 6 T. Fujiu, G. L. Messing and W. Heubner, J. Am. Cerum. sot., 73 (1990) 85. 7 H. Y.-P. Hong, Mater. Rex Bull., 11 (1976) 173. 8 J. Alamo and R. Roy, J. Mater. Sci., 21 (1986) 444. 9 L. 0. Hagman and P. Kierkegaard, Acta Chem. Stand., 22 (1968) 1822. 10 D.-M. Liu, D. A. Hirschfeld and J. J. Brown, Development of Lightweight (Ca,_MgJZr,(PO& Ceramics, presented at the Amer. Ceram. Sot. 93th Annual Meeting, Cincinnati, OH, 1991. 11 S. M. van Aken, Thermal Expansion and l7zermal Conductivity of (Cat-, MgJZr,(PO& wherex=O.O-0.4, M.S. Thesis, Virginia Polytechnic Institute and State University (1990). 12 D. M. Liu, 7’he Development and Characterization of Lightweight (Ca,_, MgJZr,(POJe Ceramics, M.S. Thesis, Virginia Polytechnic Institute and State University (1991). 13 D. A. Hirshfeld, D. M. Liu and J. J. Brown, CMZP-A New High Temperature Thermal Barrier Material, presented at 4th International Symposium on Materials and Components for Engines, Gothenburg, Sweden 1991. 14 R. W. Rice, in R. K. MacCrone, (ed.) Treatise on Material Science and Technology, Vol. 11, Academic press, New York, 1977, pp. 200-381. 15 E. Ryshkewitch, /. Am. Ceram. Sot., 36 (1953) 65. 16 R. E. Gannon, G. M. Harris and T. Vasilos, Am. Ceram. Sot. Bull., 44 (1965) 460. 17 F. P. Kundsen, .I. Am. Ceram. Sot., 42 (1959) 376. 18 R. L. McCullough, Compos. Sci. Tech., 22 (1985) 3. 19 B. D. Agarwal, G. A. Panizza and L. J. Broutman, J. Am. Ceram. Sot., 54 (1971) 620. 20 M. F. Ashby, Metal. Trans. A, 124 (1983) 1755.