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Joining of zirconia ceramics with a CaO-TiO2-SiO2 interlayer
Volume 7, number
JOINING
1I
MATERIALS
OF ZIRCONIA
S.L. SWARTZ,
CERAMICS
B.S. MAJUMDAR,
LETTERS
WITH A CaO-Ti02-Si02
A. SKIDMORE
February
1989
INTERLAYER
and B.C. MUTSUDDY
Batelle Columbus Division. 505 King Avenue, Columbus, OH 43201, USA Received
27 October
1988; in final form 11 January
1989
MgO partially stabilized zirconia ceramic billets were joined using an interlayer with the composition: 50 wt% Ti02, 35 wt% CaO. and 15 wt% SiO*. The zirconia/zirconia bond was formed by hot-forging at a temperature of 1420°C and a pressure of 4.5 MPa. Characterization of the joined region by electron microprobe analysis revealed that substantial diffusion of the interlayer constituents occurred through the grain boundaries of the zirconia ceramic.
Several heat engine applications require complex ceramic parts that can withstand extreme conditions of temperature and stress. Joining of simple ceramic shapes to form complex ceramic parts is being considered as an alternative to the difficult and expensive methods associated with fabrication of near-netshape ceramic components. There has been significant recent effort directed at the joining of alumina [ l-61 and silicon nitride ceramics [7-l 11. However, relatively little effort has been directed to the joining of zirconia ceramics; the only methods reported for the bonding of zirconia ceramics are based on brazing using metallic alloys [ 561. Although these brazed joints have relatively high strength at room temperature, their strength at elevated temperatures is limited because of the relatively low softening temperature of the alloy braze material. The use of a ceramic interlayer for the bonding of zirconia ceramic should allow for increased joint strengths at high temperature. The present investigation relates to the joining of MgO partially stabilized zirconia (Mg-PSZ) ceramics by hot-forging (i.e. hot-pressing without lateral dimensional constraints) with a ceramic CaOTiOz-Si02 (CTS) interlayer material. The combined application of pressure and temperature (hotforging) enhances the joining process by facilitating densitication of the interlayer. Hot-forging is preferred over hot-pressing because it is simpler and requires less tooling. The hot-forging conditions must be selected carefully so that the Mg-PSZ material is 0167-577x/89/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
not degraded (or de-stabilized) by the joining process. Hot-forging temperatures in the range of 1400 to 1450°C are attractive because temperatures in this range are often used for the post-sintering annealing treatment of Mg-PSZ ceramics. Interlayer compositions in the CaO-TiO,-Si02 system were previously used to join silicon nitride ceramics [ 111. Compositions in this system (see fig. 1) were considered as interlayer materials for MgPSZ joining because the CTS system has one compound (sphene, or CaTiSiO,) and several eutectic
Melting eutectic A) 9) C) D) E)
cao
Ca,Ti,O,
CaTiO,
temperatures compositions:
of
1365 C 1365C 1346C 1316 c l398C
Ti02
Fig. 1. Phase diagram of the system CaO-SiOz-TiOz, indicating positions of eutectic compositions with melting temperatures of less than 1400°C (points labelled A through E), and the position of the interlayer composition used in this investigation (derived from ref. [ 121).
B.V.
407
Volume 7, number
MATERIALS
11
compositions with melting temperatures below 1400” C [ 12 1. The CTS interlayer material should at least be partially molten at the suggested hot-forging temperature range of 1400 to 1450” C. This would be advantageous for the following reasons: (i) the molten interlayer will distribute itself more evenly during hot-forging; (2) the molten CTS interlayer may wet the Mg-PSZ surfaces better; and (3) the molten interlayer may result in enhanced diffusion of the CTS material into the Mg-PSZ. Results of joining experiments using a specific CTS interlayer composition (50 wt% TiOz, 35 wt% CaO, and 15 wt% SiO*) are reported in this communication. Relative to the phase diagram in fig. 1, this CTS interlayer composition is located on the tie-line between CaTiSiO, (sphene) and CaTiO, (perovskite ). The sphene component, with a reported melting temperature of 1382°C [ 121, should be molten at the hot-forging temperature. The CTS interlayer powder of the above composition was prepared from reagent-grade raw materials in the following manner: ( 1) milling of CaCO,, TiOz, and SiOz in a nalgene bottle with alcohol and zirconia grinding media, followed by pan-drying at 90°C; (2) calcination at 1000 ’ C for 4 h; and ( 3 ) re-milling and drying. A differential thermal analysis (DTA) pattern of the calcined powder is presented in fig. 2. An endothermic peak was observed at 1372’ C in the DTA pattern; this endotherm probably corresponds to the melting of CaTiSiO,, as expected from the composition. The parts to be joined were Mg-PSZ (Zirconia Zycron L) billets, approximately 14 mm in diameter and 7.5 mm thick. The surfaces of the two zirconia
I
1
I 1200
I ,300
Temperature. C
Fig. 2. Differential thermal analysis (DTA) pattern of CaO-TiO,SiO, interlayer powder.
408
LETTERS
February
1989
billets were roughened slightly with 240-g& Sic paper and ultrasonically cleaned in alcohol. The CaOTiOz-SiOz powder was sieved to less than 40 mesh and a slurry was prepared with 1 g of the sieved powder and a solution of 0.1 g of polyvinyl butyrol binder in 10 ml of methanol. A thin layer of the interlayer slurry was evenly applied to both roughened faces of the Mg-PSZ billets, and the two billets were pressed together to form a green bond with minimal handling strength. The thickness of the interlayer in the green state was about 100 pm. The sandwich was placed in an air-ambient hot-forging apparatus, separated from the alumina rams by zirconia felt below the bottom face and an alumina disc above the top face. A slight pressure (0.2 MPa) was applied at room temperature and the temperature of the furnace was increased to 1400°C in approximately 3 h. When the temperature reached 1400” C, a compressive pressure of 4.5 MPa was applied and maintained as the temperature was increased to 1420°C and held for 2 h. The furnace was then turned off and the sample cooled to room temperature under the 4.5 MPa pressure. The joined billet sandwich was sectioned and a face was mounted in epoxy and polished down to 1 urn diamond for electron microprobe analysis. The microprobe used was a JEOL 733 superprobe. Fig. 3a is a back-scattered electron (BSE) micrograph of the joined region. The interlayer is the dark band across the middle of the micrograph; the dark contrast of the interlayer is due to the lower atomic numbers (and thus lower electron densities) of the interlayer constituents (Ti, Ca, and Si) compared to Zr in the Mg-PSZ. The interlayer is approximately 10 urn thick, and it has excellent connectivity with the zirconia base material. Examination across the entire length of the joint revealed that the interlayer is relatively dense and uniform throughout. A highly diffused region, approximately 5 pm wide on either side of the interlayer, is also apparent in the BSE micrograph of fig. 3a; this region is of brighter contrast than the interlayer but is darker than the Mg-PSZ base material. Diffusion of the interlayer material further into the zirconia through the grain boundaries is also evident from the BSE micrograph. Examination of the entire surface revealed that this grain boundary diffusion was significant up to approximately 50 pm into the zirconia.
Volume 7. number
11
MATERIALS
LETTERS
Fig. 3. (a) Back-scattered electron (BSE) micrograph ofjoint region. EDS elemental to the same image of the BSE micrograph shown in (a). The bright areas correspond particular elements compared to other constituents.
Elemental maps of Ca, Ti, and Si are presented in figs. 3b, 3c, and 3d, corresponding to the same image of the BSE micrograph in fig. 3a; the bright areas correspond to regions of relatively high concentrations of the respective elements. The elemental maps confirm the diffusion of Ca and Ti into the zirconia, with significant concentrations of these elements in the highly diffused region and in the grain boundaries. Surprisingly, the Si map indicates a comparatively low concentration of Si in the interlayer relative to the Mg-PSZ base material. High concentrations of Si were detected in pores at distances of up to 60 urn from the interlayer. It is possible that the high Si concentration in the pores was due to Sic particles remaining from an initial polishing of the sample with
February
I989
maps of(b) Ti, (c) Ca, and (d) Si, corresponding to regions of relatively high concentrations of the
coarse Sic. However, this seems unlikely because the 240-grit SIC was far coarser than the 2 urn diameter pores and because the sample was further polished with alumina and diamond that would likely have removed the SIC. Thus, the evidence seems to indicate that the SiOz diffused rapidly from the interlayer into the zirconia and deposited in the pores. The mechanism by which this diffusion took place is unclear from the microprobe data. The results of this study suggest that hot-forging using a molten CaO-Ti02-SiO, interlayer has good potential for joining Mg-PSZ structural ceramics. Zirconia billets joined in this manner had excellent continuity with the interlayer, and the interlayer material diffused partially into the Mg-PSZ ceramic. The 409
Volume7, number11
MATERIALS LETTERS
interlayer is only 10 urn wide, compared to much wider interlayers in metallic alloy brazed joints, and should lead to improved joint strength at higher temperatures. Further studies of this joining technique and mechanical characterization of joined Mg-PSZ ceramics are now in progress. This research was funded by the U.S. Department of Energy under Subcontract No. 86XSB046C to Martin Marietta Energy Systems. The authors would like to thank Dr. M. Santella of Oak Ridge National Laboratory for his support of this program. The authors are also grateful to Mr. E. Ylo of Zircoa, Inc., for supplying the Mg-PSZ material used in the joining experiments.
References
[ 1] H.P.Kirchner,J.C.ConwayandA.E.Se&, J. Am. Ceram. Sot. 70 (1987) 104.
410
February1989
[2] W.A. Zdaniewski, J.C. Conway and H.P. Kirchner, J. Am. Ceram. Sot. 70 (1987) 110. [3] W. Zdaniewski, P.M. Shah and H.P. Kirchner, Advan. Ceram. Mater. 2 (1987) 204. [4] A.J. Moorhead, Advan. Ceram. Mater. 2 (1987) 159. [ 51A.J. Moorhead, T.N. Tiegs and R.J. Lauf, in: Proceedings of the 21st Automotive Technology Development Contractors’ Coordination Meeting (Society of Automotive Engineers, Inc., Warrendale, PA, 1984) p. 223. [6] A.J. Moorhead and P.F. Becher, J. Mater. Sci. 22 (1987) 3297. [7] P.F. Becher and S.A. Halen, in: Ceramics for high performance applications, Vol. 2, eds. J.J. Burke, E.N. Lenoe and R.N. Katz (Metals and Ceramics Information Center, Columbus, OH, 1978) p. 1076. [8] P.F. Becher and S.A. Halen, Ceram. Bull. 58 (1979) 582. [9] S.M. Johnson and D.J. Rowcliffe, J. Am. Ceram. Sot. 68 (1985) 468. [lo] M.L. Mecartney, R. Sinclair and R.E. Loehman, J. Am. Ceram. Sot. 68 (1985) 472. [ 111 N. Iwamoto, N. Umesaki, Y. Haibara and K. Sibuya, in: Ceramic materials and components for engines, eds. W. Bunk and H. Hausner (Deutsche Keramische Gesellschaft, Bad Honnef, 1986) p. 467. [ 121 R.C. DeVries, R. Roy and E.F. Osbom, J. Am. Ceram. Sac. 38 (1955) 162.
JOINING
1I
MATERIALS
OF ZIRCONIA
S.L. SWARTZ,
CERAMICS
B.S. MAJUMDAR,
LETTERS
WITH A CaO-Ti02-Si02
A. SKIDMORE
February
1989
INTERLAYER
and B.C. MUTSUDDY
Batelle Columbus Division. 505 King Avenue, Columbus, OH 43201, USA Received
27 October
1988; in final form 11 January
1989
MgO partially stabilized zirconia ceramic billets were joined using an interlayer with the composition: 50 wt% Ti02, 35 wt% CaO. and 15 wt% SiO*. The zirconia/zirconia bond was formed by hot-forging at a temperature of 1420°C and a pressure of 4.5 MPa. Characterization of the joined region by electron microprobe analysis revealed that substantial diffusion of the interlayer constituents occurred through the grain boundaries of the zirconia ceramic.
Several heat engine applications require complex ceramic parts that can withstand extreme conditions of temperature and stress. Joining of simple ceramic shapes to form complex ceramic parts is being considered as an alternative to the difficult and expensive methods associated with fabrication of near-netshape ceramic components. There has been significant recent effort directed at the joining of alumina [ l-61 and silicon nitride ceramics [7-l 11. However, relatively little effort has been directed to the joining of zirconia ceramics; the only methods reported for the bonding of zirconia ceramics are based on brazing using metallic alloys [ 561. Although these brazed joints have relatively high strength at room temperature, their strength at elevated temperatures is limited because of the relatively low softening temperature of the alloy braze material. The use of a ceramic interlayer for the bonding of zirconia ceramic should allow for increased joint strengths at high temperature. The present investigation relates to the joining of MgO partially stabilized zirconia (Mg-PSZ) ceramics by hot-forging (i.e. hot-pressing without lateral dimensional constraints) with a ceramic CaOTiOz-Si02 (CTS) interlayer material. The combined application of pressure and temperature (hotforging) enhances the joining process by facilitating densitication of the interlayer. Hot-forging is preferred over hot-pressing because it is simpler and requires less tooling. The hot-forging conditions must be selected carefully so that the Mg-PSZ material is 0167-577x/89/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
not degraded (or de-stabilized) by the joining process. Hot-forging temperatures in the range of 1400 to 1450°C are attractive because temperatures in this range are often used for the post-sintering annealing treatment of Mg-PSZ ceramics. Interlayer compositions in the CaO-TiO,-Si02 system were previously used to join silicon nitride ceramics [ 111. Compositions in this system (see fig. 1) were considered as interlayer materials for MgPSZ joining because the CTS system has one compound (sphene, or CaTiSiO,) and several eutectic
Melting eutectic A) 9) C) D) E)
cao
Ca,Ti,O,
CaTiO,
temperatures compositions:
of
1365 C 1365C 1346C 1316 c l398C
Ti02
Fig. 1. Phase diagram of the system CaO-SiOz-TiOz, indicating positions of eutectic compositions with melting temperatures of less than 1400°C (points labelled A through E), and the position of the interlayer composition used in this investigation (derived from ref. [ 121).
B.V.
407
Volume 7, number
MATERIALS
11
compositions with melting temperatures below 1400” C [ 12 1. The CTS interlayer material should at least be partially molten at the suggested hot-forging temperature range of 1400 to 1450” C. This would be advantageous for the following reasons: (i) the molten interlayer will distribute itself more evenly during hot-forging; (2) the molten CTS interlayer may wet the Mg-PSZ surfaces better; and (3) the molten interlayer may result in enhanced diffusion of the CTS material into the Mg-PSZ. Results of joining experiments using a specific CTS interlayer composition (50 wt% TiOz, 35 wt% CaO, and 15 wt% SiO*) are reported in this communication. Relative to the phase diagram in fig. 1, this CTS interlayer composition is located on the tie-line between CaTiSiO, (sphene) and CaTiO, (perovskite ). The sphene component, with a reported melting temperature of 1382°C [ 121, should be molten at the hot-forging temperature. The CTS interlayer powder of the above composition was prepared from reagent-grade raw materials in the following manner: ( 1) milling of CaCO,, TiOz, and SiOz in a nalgene bottle with alcohol and zirconia grinding media, followed by pan-drying at 90°C; (2) calcination at 1000 ’ C for 4 h; and ( 3 ) re-milling and drying. A differential thermal analysis (DTA) pattern of the calcined powder is presented in fig. 2. An endothermic peak was observed at 1372’ C in the DTA pattern; this endotherm probably corresponds to the melting of CaTiSiO,, as expected from the composition. The parts to be joined were Mg-PSZ (Zirconia Zycron L) billets, approximately 14 mm in diameter and 7.5 mm thick. The surfaces of the two zirconia
I
1
I 1200
I ,300
Temperature. C
Fig. 2. Differential thermal analysis (DTA) pattern of CaO-TiO,SiO, interlayer powder.
408
LETTERS
February
1989
billets were roughened slightly with 240-g& Sic paper and ultrasonically cleaned in alcohol. The CaOTiOz-SiOz powder was sieved to less than 40 mesh and a slurry was prepared with 1 g of the sieved powder and a solution of 0.1 g of polyvinyl butyrol binder in 10 ml of methanol. A thin layer of the interlayer slurry was evenly applied to both roughened faces of the Mg-PSZ billets, and the two billets were pressed together to form a green bond with minimal handling strength. The thickness of the interlayer in the green state was about 100 pm. The sandwich was placed in an air-ambient hot-forging apparatus, separated from the alumina rams by zirconia felt below the bottom face and an alumina disc above the top face. A slight pressure (0.2 MPa) was applied at room temperature and the temperature of the furnace was increased to 1400°C in approximately 3 h. When the temperature reached 1400” C, a compressive pressure of 4.5 MPa was applied and maintained as the temperature was increased to 1420°C and held for 2 h. The furnace was then turned off and the sample cooled to room temperature under the 4.5 MPa pressure. The joined billet sandwich was sectioned and a face was mounted in epoxy and polished down to 1 urn diamond for electron microprobe analysis. The microprobe used was a JEOL 733 superprobe. Fig. 3a is a back-scattered electron (BSE) micrograph of the joined region. The interlayer is the dark band across the middle of the micrograph; the dark contrast of the interlayer is due to the lower atomic numbers (and thus lower electron densities) of the interlayer constituents (Ti, Ca, and Si) compared to Zr in the Mg-PSZ. The interlayer is approximately 10 urn thick, and it has excellent connectivity with the zirconia base material. Examination across the entire length of the joint revealed that the interlayer is relatively dense and uniform throughout. A highly diffused region, approximately 5 pm wide on either side of the interlayer, is also apparent in the BSE micrograph of fig. 3a; this region is of brighter contrast than the interlayer but is darker than the Mg-PSZ base material. Diffusion of the interlayer material further into the zirconia through the grain boundaries is also evident from the BSE micrograph. Examination of the entire surface revealed that this grain boundary diffusion was significant up to approximately 50 pm into the zirconia.
Volume 7. number
11
MATERIALS
LETTERS
Fig. 3. (a) Back-scattered electron (BSE) micrograph ofjoint region. EDS elemental to the same image of the BSE micrograph shown in (a). The bright areas correspond particular elements compared to other constituents.
Elemental maps of Ca, Ti, and Si are presented in figs. 3b, 3c, and 3d, corresponding to the same image of the BSE micrograph in fig. 3a; the bright areas correspond to regions of relatively high concentrations of the respective elements. The elemental maps confirm the diffusion of Ca and Ti into the zirconia, with significant concentrations of these elements in the highly diffused region and in the grain boundaries. Surprisingly, the Si map indicates a comparatively low concentration of Si in the interlayer relative to the Mg-PSZ base material. High concentrations of Si were detected in pores at distances of up to 60 urn from the interlayer. It is possible that the high Si concentration in the pores was due to Sic particles remaining from an initial polishing of the sample with
February
I989
maps of(b) Ti, (c) Ca, and (d) Si, corresponding to regions of relatively high concentrations of the
coarse Sic. However, this seems unlikely because the 240-grit SIC was far coarser than the 2 urn diameter pores and because the sample was further polished with alumina and diamond that would likely have removed the SIC. Thus, the evidence seems to indicate that the SiOz diffused rapidly from the interlayer into the zirconia and deposited in the pores. The mechanism by which this diffusion took place is unclear from the microprobe data. The results of this study suggest that hot-forging using a molten CaO-Ti02-SiO, interlayer has good potential for joining Mg-PSZ structural ceramics. Zirconia billets joined in this manner had excellent continuity with the interlayer, and the interlayer material diffused partially into the Mg-PSZ ceramic. The 409
Volume7, number11
MATERIALS LETTERS
interlayer is only 10 urn wide, compared to much wider interlayers in metallic alloy brazed joints, and should lead to improved joint strength at higher temperatures. Further studies of this joining technique and mechanical characterization of joined Mg-PSZ ceramics are now in progress. This research was funded by the U.S. Department of Energy under Subcontract No. 86XSB046C to Martin Marietta Energy Systems. The authors would like to thank Dr. M. Santella of Oak Ridge National Laboratory for his support of this program. The authors are also grateful to Mr. E. Ylo of Zircoa, Inc., for supplying the Mg-PSZ material used in the joining experiments.
References
[ 1] H.P.Kirchner,J.C.ConwayandA.E.Se&, J. Am. Ceram. Sot. 70 (1987) 104.
410
February1989
[2] W.A. Zdaniewski, J.C. Conway and H.P. Kirchner, J. Am. Ceram. Sot. 70 (1987) 110. [3] W. Zdaniewski, P.M. Shah and H.P. Kirchner, Advan. Ceram. Mater. 2 (1987) 204. [4] A.J. Moorhead, Advan. Ceram. Mater. 2 (1987) 159. [ 51A.J. Moorhead, T.N. Tiegs and R.J. Lauf, in: Proceedings of the 21st Automotive Technology Development Contractors’ Coordination Meeting (Society of Automotive Engineers, Inc., Warrendale, PA, 1984) p. 223. [6] A.J. Moorhead and P.F. Becher, J. Mater. Sci. 22 (1987) 3297. [7] P.F. Becher and S.A. Halen, in: Ceramics for high performance applications, Vol. 2, eds. J.J. Burke, E.N. Lenoe and R.N. Katz (Metals and Ceramics Information Center, Columbus, OH, 1978) p. 1076. [8] P.F. Becher and S.A. Halen, Ceram. Bull. 58 (1979) 582. [9] S.M. Johnson and D.J. Rowcliffe, J. Am. Ceram. Sot. 68 (1985) 468. [lo] M.L. Mecartney, R. Sinclair and R.E. Loehman, J. Am. Ceram. Sot. 68 (1985) 472. [ 111 N. Iwamoto, N. Umesaki, Y. Haibara and K. Sibuya, in: Ceramic materials and components for engines, eds. W. Bunk and H. Hausner (Deutsche Keramische Gesellschaft, Bad Honnef, 1986) p. 467. [ 121 R.C. DeVries, R. Roy and E.F. Osbom, J. Am. Ceram. Sac. 38 (1955) 162.