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Processing of laminated hybrid ceramic composites
PII: S1359-8368(97)00009-7
ELSEVIER
Composites Part B 29B (1998) 189-194 © 1998 Published by Elsevier Science Limited Printed in Great Britain. All rights reserved 1359-8368/98/$19.00
Processing of laminated hybrid ceramic composites M. M. Sharma and M. F. A m a t e a u Applied Research Laboratory, The Pennsylvania State University, P.O. Box 30, North Atherton Street State College, Pennsylvania 16804-0300, USA The thermoelastic response of ceramic composite laminates was studied in order to optimize their design and fabrication. Laminated ceramic composites were fabricated with induced thermal residual stresses resulting from subsequent cooling from elevated processing temperatures. Samples were fabricated by tape casting laminae of different compositions and then bonding them together by hot pressing to form the finished laminate. These laminates were analyzed to better understand the process occurring during hot pressing that produced bonded and consolidated composites. Calculated laminate properties were found to be comparable to measured values. Microprobe elemental analysis was used to determine the interlaminar and intralaminar reactions occurring during fabrication. X-ray diffraction stress analysis was used to verify predicted residual stress calculations. Residual stresses and stress-driven reactions were found to be contributing factors in the resulting microstructural variations found within the composites. © 1998 Published by Elsevier Science Limited. All rights reserved. (Keywords: A. laminates; B. interface/interphase; B. residual/internal stress; processing)
INTRODUCTION Laminated hybrid ceramic composites have potential applications in cutting tools, ballistic armor, and hightemperature structural components. Multilayered hybrid ceramics and ceramic composites present a special challenge in their fabrication. This is specifically due to the possibility of thermoelastic and chemical interaction between layers and constituents over the temperature range required for their consolidation. Classical laminate plate theory has been successfully employed to improve the strength, toughness and thermal shock resistance in laminated ceramic composites by the addition of particulates or whiskers, and through incorporation of compressive residual stresses in the appropriate layers of the structure 1. This thermoelastic tailoring of the composite is accomplished by manipulating the lamina composition and layer structure within the composite. Depending on the desired application, a composite can be specifically tailored for toughness, strength, stiffness, wear resistance, oxidation resistance or chemical resistance. The accurate measurement of thermally induced residual stresses and their effects within the ceramic composite is also an essential part of fabricating composites for optimum mechanical performance. Few attempts, however, have been made to measure or estimate accurately the residual stresses in ceramic bodies. Most studies conducted incorporate destructive techniques, whereby the subsequent deflection or strain change is measured as a result of
material or layer removal 2-5. These mechanical techniques are then compared to a mathematical model or theoretical calculation for verification 6. Unfortunately, only limited information is obtained through these methods. This paper addresses the development of residual stresses and microstructural changes in the composites during the fabrication process. Successful fabrication of laminated ceramic composites requires accurate knowledge of the thermoelastic properties of the constituent lamina, and the interfacial diffusion and reactions of layers and constituents. The discontinuous reinforcements used in the design of these composites were SiC, Si3N 4, and TiC. Two types of hybrid laminated composites were investigated in this study. These are (1) TiC particulate reinforced A1203 and unreinforced A1203 lamina, and (2) Si3N4 particulate reinforced A1203 with various amounts of additional SiC or Si3N4 whisker reinforcement in adjacent layers. Both types of composites were processed by hot pressing assembled tape cast and powdered layers. The layers were specifically arranged to produce different states of residual stress on the surface of the specimen and at each interface.
SPECIMEN F A B R I C A T I O N Tape cast sheets were used for the outer layers, while dry bulk powders were used for the core of the laminates. Tape cast slurries were comprised of ceramic powder, organic polymer, and solvent. Individual tapes of 0.102-0.152 mm
189
Processing of laminated hybrid ceramic composites: M. M. Sharma and M. F. Amateau Table 1
Hybrid laminae designs
Design
Composition
Thickness/mm
Stress X/MPa
GX-03
26TIC p/A120 3 20S iC w/A120 3 10SiC w/A120 3
0.127 0.317 3.874
- 197.93 - 348.27 +70.34
GX-06
A120 3 26TIC p/A12° 3
0.635 3.490
+498.74 - 181.36
GX-08
A1203 26TIC p/A120 3
0.254 4.254
+668.72 - 79.85
GX-14
10SiCw/Gem-2* Gem-2
0.254 4.255
-215.17 +25.52
GX- 15
20SIC w/Gem-2 10SiCw/Gem-2
0.635 3.493
- 195.96 +71.25
GX- 16
10SiC w/Gem-2 Gem-2
0.635 3.493
- 116.59 +42.39
GX-20
26TiCv/A1203 A1203
0.254 4.254
-447.29 +53.41
HX- 18
Gem-4 30SiCw-Gem-2
0.635 3.493
+37.14 - 13.50
HX-20
20SIC w/Gem-4"~ 10SiC J G e m - 4
0.254 4.254
- 138.62 + 16.55
HX-21
20S i 3N 4w/Gem-4 10Si3N4JGem-4 Gem-4
0.127 0.317 3.873
- 340.69 -144.14 +46.21
HX-22
20SIC w/Gem-4 10SiC w/Gem-4 Gem-4
0.127 0.317 3.873
-270.34 - 108.27 +35.17
HX-23
20Si3N4w/Gem-4 10Si3N4w/Gem-4
0.254 4.254
-177.93 +21.38
0.0635 mm radius to minimize the stress concentrations. In a specimen designated as GX-06 (see Table 1), the remaining plate was machined into a rectangular geometry, 12.7 mm × 12.7 m m × 4.76 mm thick, to verify residual stress calculations. A Position-Sensitive Scintillation Detector (PSSD)-based D-1000-A Stress Analyzer was utilized to make the measurement. Both the Sine-Square-Psi and Single-Exposure (SET) techniques were used. Cu Kc~ radiation was chosen because of the shorter data collection time and the precision of the subsequent data. In order to calculate the material properties of the specimens, the final individual layer thickness was measured using the microscopic and digital analyzer on the LECO model M-400-G1 hardness tester. Thermoelastic properties of the finished laminates were then calculated using measured constituent laminae properties. For composites where such data were not available, the lamina properties were calculated from constituent data using the Halpin-Tsai method 7,s. The elastic modulus of eight samples was measured using a dynamic resonance technique. Equilibrium dilatometry (Orton Model 1600 dilatometer) was performed to ascertain the rate of expansion during heating. Eight samples were studied. Heating rates were 4°C min -~ from room temperature to 800°C and 3°C rain -1 until 1000°C. Specimens were held at 800°C and 1000°C for 30 minutes in an argon atmosphere in order to minimize the formation of oxides. The dwell time of 30 minutes at 800°C allowed the machine to achieve thermal equilibrium with the specimen temperature. The residual stress in the outer layer of one laiminate was measured using X-ray diffraction.
* Gem-2, a Greenleaf Corporation composition is 26TiCp-A1203. t Gern-4, a Greenleaf Corporation composition is 18Si3Nae = A1203.
LAMINATE DESIGN AND PROPERTIES thick and up to 76 mm wide were deposited continuously onto a carrier tape, fed at a rate of 46 mm s -1, and under a doctor blade to control thickness. After approximately 20 rain in the drying chamber the tape was sufficiently flexible to be wound onto a spool. Square sheets measuring 50.8 mm and 0.127 mm thick were cut from the highest quality tapes, stacked and laminated at 83.3°C for 15 min at 24.1 MPa. These laminates were then used as the outer layers surrounding the thick, dry powder cores. All layers were then densified together by hot pressing for 1 h at 1750°C, producing a laminate 50.8 mm × 50.8 mm × 476 mm. Specific specimen designs for each sample are given in Table 1.
Verification of final laminae thickness was required for property calculations. Generally, the experimental results were consistent with predicted values. Laminates reinforced with Si3N4 particulates deviated slightly more than those reinforced with TiC particulates. This discrepancy is believed to be due to the difficulty in distinguishing the individual layers from one another in the Si3N4-reinforced samples. Table 1 gives the laminate designs for all the hybrid laminates in this study. All laminates are symmetric (either three or five layers); hence, designs are given to the center-line of the laminate. Table 2 gives the calculated versus the measured values for elastic modulus (E) and the coefficient of thermal expansion between 800-1000°C (c~) for selected specimens.
TESTING AND CHARACTERIZATION RESULTS AND DISCUSSION In order to verify the material property calculations, specimens were machined into test bars measuring 4.76 mm × 6.35 mm × 50.8 mm for coefficient of thermal expansion characterization and also cut into 4.76 mm × 6.35 mm × 6.35 mm pieces for microstructure and microprobe elemental analysis. The test bars were ground and the edges chamfered to 0.762 ram. The comers were finished to
190
In order to gain a more detailed knowledge of the effect of thermoelastic tailoring, the material properties were predicted and then measured. The predicted and measured values of the elastic modulus were in close agreement for the laminates and can be seen in Table 2. Although not by a great deal, the S i 3 N 4 particulate reinforced laminates
Processing of laminated hybrid ceramic composites: M. M. Sharma and M. F. Amateau Table 2
Calculated vs. measured laminate properties Measured values
Calculated values Design
E/GPa
o~ X 10 -6 °C-I
E/GPa
o~ X 10 -6 °C 1
GX- 14 GX- 15 GX- 16 HX- 18 HX-20 HX-21 HX-22 HX-23
412.45 393.56 412.08 321.07 400.88 328.56 337.79 349.36
8.45 7.32 8.29 6.97 5.05 6.43 6.16 6.19
408.0 384.6 394.3 351.0 374.6 369.9 377.0 315.2
8.31 7.51 8.75 7.46 5.25 6.62 6.12 6.56
showed the greatest difference between predicted and measured properties. This slight difference is believed to be a result of less precise laminae thickness measurements used to calculate the Young's modulus. Among the samples reinforced with TiC particulates, values for the measured Young's moduli ranged between 384.6 and 408.0 GPa and were within 1.1-4.3% of the calculated values. Similarly, for those laminates reinforced with Si3N4 particulates, Young's moduli differed only by 6.6-10.4% of calculated values and ranged between 315.2 and 377.0GPa. The agreement between measured and rule-of-mixtures values indicates that the processing parameters are sufficient for complete densification and indicates good bonding between the reinforcement and matrix. These results provide confidence in the use of calculated properties when experimental values are not available. Values for the coefficient of thermal expansion were measured in the temperature range between 800°C and 1000°C and are presented in Table 2. Generally, the measured values were slightly higher than predicted coefficients of thermal expansion. Although the coefficient of thermal expansion is consistently underestimated, the differences are not significant. Variation in measured thicknesses of the laminae also made a noticeable impact on the predicted values. Even small deviations in thickness values altered the results of the rule-of-mixtures calculations of the coefficient of thermal expansion. Despite this fact, experimentally obtained values were in very good agreement with predicted values. Samples reinforced with TiC particulates exhibited coefficients of thermal expansion of between 7.51 × 10 -6 °C J and 8.75 × 10 -6 ° C -1 (Table 2). The coefficients of thermal expansion are within 1.7-5.3% of the rule-ofmixture calculations. The laminates reinforced with Si3N4 particulates displayed coefficients of thermal expansion between 6.25 × 10 -6 ° C - I and 7.46 X -6 o C - 1 (Table 2). These values are within 0.6-6.6% of the predicted calculations. The differences in predicted and measured values were so small that calculated properties are considered to be accurate.
TiCp-reinforced Al 20 s composites In one composite, GX-06 (see Table l for laminate design), the residual stress of the outer layer was measured to verify predicted values. An average value of 483.12 MPa was obtained for both the Sine-Square-Psi and SET
techniques. This value is extremely close to the predicted value of 498.74 MPa (see Table 1). Therefore, the predicted values for the residual stress are believed to be accurate. In order to further enhance our understanding of the behavior of the material, metallography and microprobe elemental analysis were carried out on selected specimens. From this evaluation, it is concluded that the microstructure at the interface was strongly influenced by thermal residual stresses, because inter- and intra-layer diffusion was observed. Figure 1 displays the interface between the SiCw-reinforced A1203 layer (left) and the TiCp-reinforced A1203 layer (right) in one sample (GX-03). For the most part, a clean straight interface was observed with no unusually large pores, or evidence of excessive diffusion. Figure 2 is a micrograph showing the interface between the TiCp-reinforced A1203 layer and the unreinforced A1203 layer in another sample (GX-20). Both diffusion and void formation occurred from the interface into the unreinforced A1203 layer. Although the layer design varied, this phenomenon was still observed in all samples containing the TiCp-reinforced A1203 and unreinforced A1203 layers. A noticeable void growth near the interface was found in three of the four samples studied. Upon comparing the laminate designs of the four different types of TiCpreinforced A1203 composites studied (GX-03, GX-06, GX-08, GX-20), it was interesting to note that the distribution of large diameter pores occurred only in layers which were in a state of large tensile residual stress. As a result, the tensile residual stress field is believed to attract and then coalesce the pores, thus promoting void growth near the interface. However, several samples with similar residual stress patterns (GX-03 for example) did not exhibit this pore growth behavior; hence an additional process may be involved in this phenomenon. Microprobe elemental analysis of these samples found titanium ( < 1.0wt%) to be diffusing from the TiCpreinforced A1203 layer into the adjacent layer, as seen in Figure 3. The concentration profile of titanium corresponds to the same area displaying the characteristic pore size gradient seen in Figure 2. Figure 3 shows that the concentration of titanium is largest at the interface and diminishes to a trace amount, 20-30 tzm away from the interface. Due to the fact that the diffusion of titanium and the variable pore distribution occur within the same 2 0 30/~m area of the composite, these phenomena are believed to be related. Chemically driven diffusion was not found in any of the samples containing TiC particulates (GX-08
191
Processing of laminated hybrid ceramic composites: M. M. Sharma and M. F. Amateau
Figure 1 SEM micrograph showing the interface betwen SiC whisker-reinforced A1203 (left) and TiC particulate-reinforced A1203 (right). This interface displays uniform pore diameters and distribution of pores
Figure 2 SEM micrograph of the interface between TiC particulate-reinforced AI203 (left) and unreinforced A1203 (right) layers. The anomalous pore behavior can be seen 20-30/~m into the A1203 layer and is attributed to stress and chemically driven diffusion
for example); therefore, it is not solely responsible for the observed microstructure, which is attributed to a combination of stress and chemically driven diffusion across the interface. Despite the variable microstructure, the T i C p A1203 composites were still successfully consolidated. The effect of residual stresses on the interface did not prove to be detrimental to the mechanical properties of these
192
composites, but is considered to be a factor in the resulting microstructural variation observed within the composite.
Si 3N4p-reinforced Ale03 composites No variable pore structures were found in the microstructure of Si3N4p-reinforced A 1 2 0 3 laminates with
Processing of laminated hybrid ceramic composites: M. M. Sharma and M. F. Amateau .40 T i
.30 + ~z251
.00
. . . .
0
I
5
. . . .
~
10
. . . .
I
15
. . . .
!
20
'
'
'
25
Diffusion distance of Ti (ran) into alurrina layer Figure 3 Diffusion profile of titanium into the adjacent AI203 layer. Titanium concentration is greatest at the interface and depletes uniformly to a trace amount, approximately 20-30 ~m from the interface Figure 5 SEM micrograph of the interface betwen 10SiC-reinforced Si3N4p-AI203 (left) and unreinforced Si3N4p-AI203 (right) layers. Nodules consisting mostly of Si3N4 are present along the interface and continue 20-30 Izm into the layer. A few nodules consisting of A1203 are also present. The unreinforced Si3N4p-AI203 layer does not contain any nodules
Figure 4 SEM micrograph of the interface betwen 20SiC-reinforced Si3N4p-AI203 (left) and 10SiC-reinforced Si3N4e-A1203 (right) layers. Nodules consisting mostly of A1203 are present along the interface and continue 20-30/zm into the layer
additional SiC and Si3N4 whisker reinforcement. However, various lens-shaped nodules, 10-40/~m in length, scattered along interfaces of the layers were observed. Figure 4 and Figure 5 show the elemental profile of the two different interfaces in an Si3N4p-reinforced A1203 composite (HX-22), and clearly display the unusual nodules. These nodules are largely present at the interface, and continue to exist 20-30/~m into the layers. In bulk powder layers which do not contain whisker reinforcement, nodules were not observed (Figure 5, right side). Microprobe elemental analysis established that these lens-shaped particles consist mainly of two different compositions. The larger nodules consist mostly of pure A1203 while the smaller, more numerous, nodules were found to be made up of approximately 6.3 wt%A1, 40.1 wt%N and 53.5 wt%Si. The A1203 nodules are attributed to clustering of agglomerated powder which occurred during tape casting due to inhomogeneous mixing of the starting
slurry. This conclusion is supported by the fact that nodules were not found in dry bulk powder cores. The lens-like shapes consisting mostly of Si and N are believed to have resulted from particulate clustering and diffusion of A1 during processing. Areas which contained clustered particulates or agglomerated powder after tape casting were shaped into nodules upon hot pressing. Evidence of this comes not only from the composition of the nodules, but also from the fact that the lenses were longest in the direction perpendicular to hot pressing, indicating clusters were forced into lens-like shapes (Figure 4 and Figure 5). Unlike the TiCp-reinforced A1203 laminates, this phenomenon occurred mostly in layers containing compressive residual stress. However, no correlation was made between the residual stress and the anomalous microstructural behavior in these composites. Despite the observed lens-shaped nodules, all samples were successfully fabricated.
CONCLUSIONS Although some variation in results occurred, measured properties of hybrid laminates compared favourably to predicted values. For the most part, observed anomalous behavior occurring at the interfaces of the composites was not found to affect the residual stress predictions of laminates. Furthermore, this behavior is not believed to have adversely affected the mechanical properties of the laminate. Inter- and intra-layer reactions caused by stress and chemically driven diffusion were responsible for the unusual pore behavior in the TiCp-reinforced A1203 laminates. Nodules observed in the Si3N4p-reinforced A1203 composites were attributed to whisker clustering and diffusion. The X-ray stress analysis confirmed the predicted
193
Processing of laminated hybrid ceramic composites: M. M. Sharma and M. F. Amateau outer layer residual stress in one TiCp-reinforced A1203 composite.
REFERENCES 1. 2. 3,
194
Amateau, M.F., Stutzman, B.A., Conway, J.C. and Halloran, J., Performance of laminated ceramic composite cutting tools. Ceram. Inter., 1995, 21, 317-323. Virkar, A.V., Jue, J.F., Hansen, J.J. and Cutler, R. A., Measurement of residual stresses in oxide-ZrO2 three-layer composites. J. Am. Ceram. Soc., 1988, 71(3), C148-C151. Lange, F.F., James, H.R. and Gren, D.J., Determination of residual
4. 5. 6. 7. 8.
surface stresses caused by grinding in polycrystalline AI20 3. J. Am. Ceram. Soc., 1983, 66(2), C16-C17. Ramsey, R.M., Chandler, H.W. and Page, T.F., The determination of residual stresses in thin coatings by a sample thinning method. Surf. and Coat. Tech., 1990, 43/44, 223-233. Randon, R. and Green, D.J., Residual stress determination using strain gage measurements. J. Am. Ceram. Soc., 1990, 73(9), 2628-2633. Bitler, J.W., A model of the thermoelastic response of A1203/M2 tool steel functionally gradient materials. Master's Thesis The Pennsylvania State University, 1995. Halpin, J.C. and Kardos, J.L., The Halpin-Tsi equations: A review. Polymer Eng. Sci., 1976, 16, 344-352. Halpin, J.C. and Tsai, S.W., Environmental Factors in Composite Materials, Laboratory Report, AFML-TRG7-473, June 1969.
ELSEVIER
Composites Part B 29B (1998) 189-194 © 1998 Published by Elsevier Science Limited Printed in Great Britain. All rights reserved 1359-8368/98/$19.00
Processing of laminated hybrid ceramic composites M. M. Sharma and M. F. A m a t e a u Applied Research Laboratory, The Pennsylvania State University, P.O. Box 30, North Atherton Street State College, Pennsylvania 16804-0300, USA The thermoelastic response of ceramic composite laminates was studied in order to optimize their design and fabrication. Laminated ceramic composites were fabricated with induced thermal residual stresses resulting from subsequent cooling from elevated processing temperatures. Samples were fabricated by tape casting laminae of different compositions and then bonding them together by hot pressing to form the finished laminate. These laminates were analyzed to better understand the process occurring during hot pressing that produced bonded and consolidated composites. Calculated laminate properties were found to be comparable to measured values. Microprobe elemental analysis was used to determine the interlaminar and intralaminar reactions occurring during fabrication. X-ray diffraction stress analysis was used to verify predicted residual stress calculations. Residual stresses and stress-driven reactions were found to be contributing factors in the resulting microstructural variations found within the composites. © 1998 Published by Elsevier Science Limited. All rights reserved. (Keywords: A. laminates; B. interface/interphase; B. residual/internal stress; processing)
INTRODUCTION Laminated hybrid ceramic composites have potential applications in cutting tools, ballistic armor, and hightemperature structural components. Multilayered hybrid ceramics and ceramic composites present a special challenge in their fabrication. This is specifically due to the possibility of thermoelastic and chemical interaction between layers and constituents over the temperature range required for their consolidation. Classical laminate plate theory has been successfully employed to improve the strength, toughness and thermal shock resistance in laminated ceramic composites by the addition of particulates or whiskers, and through incorporation of compressive residual stresses in the appropriate layers of the structure 1. This thermoelastic tailoring of the composite is accomplished by manipulating the lamina composition and layer structure within the composite. Depending on the desired application, a composite can be specifically tailored for toughness, strength, stiffness, wear resistance, oxidation resistance or chemical resistance. The accurate measurement of thermally induced residual stresses and their effects within the ceramic composite is also an essential part of fabricating composites for optimum mechanical performance. Few attempts, however, have been made to measure or estimate accurately the residual stresses in ceramic bodies. Most studies conducted incorporate destructive techniques, whereby the subsequent deflection or strain change is measured as a result of
material or layer removal 2-5. These mechanical techniques are then compared to a mathematical model or theoretical calculation for verification 6. Unfortunately, only limited information is obtained through these methods. This paper addresses the development of residual stresses and microstructural changes in the composites during the fabrication process. Successful fabrication of laminated ceramic composites requires accurate knowledge of the thermoelastic properties of the constituent lamina, and the interfacial diffusion and reactions of layers and constituents. The discontinuous reinforcements used in the design of these composites were SiC, Si3N 4, and TiC. Two types of hybrid laminated composites were investigated in this study. These are (1) TiC particulate reinforced A1203 and unreinforced A1203 lamina, and (2) Si3N4 particulate reinforced A1203 with various amounts of additional SiC or Si3N4 whisker reinforcement in adjacent layers. Both types of composites were processed by hot pressing assembled tape cast and powdered layers. The layers were specifically arranged to produce different states of residual stress on the surface of the specimen and at each interface.
SPECIMEN F A B R I C A T I O N Tape cast sheets were used for the outer layers, while dry bulk powders were used for the core of the laminates. Tape cast slurries were comprised of ceramic powder, organic polymer, and solvent. Individual tapes of 0.102-0.152 mm
189
Processing of laminated hybrid ceramic composites: M. M. Sharma and M. F. Amateau Table 1
Hybrid laminae designs
Design
Composition
Thickness/mm
Stress X/MPa
GX-03
26TIC p/A120 3 20S iC w/A120 3 10SiC w/A120 3
0.127 0.317 3.874
- 197.93 - 348.27 +70.34
GX-06
A120 3 26TIC p/A12° 3
0.635 3.490
+498.74 - 181.36
GX-08
A1203 26TIC p/A120 3
0.254 4.254
+668.72 - 79.85
GX-14
10SiCw/Gem-2* Gem-2
0.254 4.255
-215.17 +25.52
GX- 15
20SIC w/Gem-2 10SiCw/Gem-2
0.635 3.493
- 195.96 +71.25
GX- 16
10SiC w/Gem-2 Gem-2
0.635 3.493
- 116.59 +42.39
GX-20
26TiCv/A1203 A1203
0.254 4.254
-447.29 +53.41
HX- 18
Gem-4 30SiCw-Gem-2
0.635 3.493
+37.14 - 13.50
HX-20
20SIC w/Gem-4"~ 10SiC J G e m - 4
0.254 4.254
- 138.62 + 16.55
HX-21
20S i 3N 4w/Gem-4 10Si3N4JGem-4 Gem-4
0.127 0.317 3.873
- 340.69 -144.14 +46.21
HX-22
20SIC w/Gem-4 10SiC w/Gem-4 Gem-4
0.127 0.317 3.873
-270.34 - 108.27 +35.17
HX-23
20Si3N4w/Gem-4 10Si3N4w/Gem-4
0.254 4.254
-177.93 +21.38
0.0635 mm radius to minimize the stress concentrations. In a specimen designated as GX-06 (see Table 1), the remaining plate was machined into a rectangular geometry, 12.7 mm × 12.7 m m × 4.76 mm thick, to verify residual stress calculations. A Position-Sensitive Scintillation Detector (PSSD)-based D-1000-A Stress Analyzer was utilized to make the measurement. Both the Sine-Square-Psi and Single-Exposure (SET) techniques were used. Cu Kc~ radiation was chosen because of the shorter data collection time and the precision of the subsequent data. In order to calculate the material properties of the specimens, the final individual layer thickness was measured using the microscopic and digital analyzer on the LECO model M-400-G1 hardness tester. Thermoelastic properties of the finished laminates were then calculated using measured constituent laminae properties. For composites where such data were not available, the lamina properties were calculated from constituent data using the Halpin-Tsai method 7,s. The elastic modulus of eight samples was measured using a dynamic resonance technique. Equilibrium dilatometry (Orton Model 1600 dilatometer) was performed to ascertain the rate of expansion during heating. Eight samples were studied. Heating rates were 4°C min -~ from room temperature to 800°C and 3°C rain -1 until 1000°C. Specimens were held at 800°C and 1000°C for 30 minutes in an argon atmosphere in order to minimize the formation of oxides. The dwell time of 30 minutes at 800°C allowed the machine to achieve thermal equilibrium with the specimen temperature. The residual stress in the outer layer of one laiminate was measured using X-ray diffraction.
* Gem-2, a Greenleaf Corporation composition is 26TiCp-A1203. t Gern-4, a Greenleaf Corporation composition is 18Si3Nae = A1203.
LAMINATE DESIGN AND PROPERTIES thick and up to 76 mm wide were deposited continuously onto a carrier tape, fed at a rate of 46 mm s -1, and under a doctor blade to control thickness. After approximately 20 rain in the drying chamber the tape was sufficiently flexible to be wound onto a spool. Square sheets measuring 50.8 mm and 0.127 mm thick were cut from the highest quality tapes, stacked and laminated at 83.3°C for 15 min at 24.1 MPa. These laminates were then used as the outer layers surrounding the thick, dry powder cores. All layers were then densified together by hot pressing for 1 h at 1750°C, producing a laminate 50.8 mm × 50.8 mm × 476 mm. Specific specimen designs for each sample are given in Table 1.
Verification of final laminae thickness was required for property calculations. Generally, the experimental results were consistent with predicted values. Laminates reinforced with Si3N4 particulates deviated slightly more than those reinforced with TiC particulates. This discrepancy is believed to be due to the difficulty in distinguishing the individual layers from one another in the Si3N4-reinforced samples. Table 1 gives the laminate designs for all the hybrid laminates in this study. All laminates are symmetric (either three or five layers); hence, designs are given to the center-line of the laminate. Table 2 gives the calculated versus the measured values for elastic modulus (E) and the coefficient of thermal expansion between 800-1000°C (c~) for selected specimens.
TESTING AND CHARACTERIZATION RESULTS AND DISCUSSION In order to verify the material property calculations, specimens were machined into test bars measuring 4.76 mm × 6.35 mm × 50.8 mm for coefficient of thermal expansion characterization and also cut into 4.76 mm × 6.35 mm × 6.35 mm pieces for microstructure and microprobe elemental analysis. The test bars were ground and the edges chamfered to 0.762 ram. The comers were finished to
190
In order to gain a more detailed knowledge of the effect of thermoelastic tailoring, the material properties were predicted and then measured. The predicted and measured values of the elastic modulus were in close agreement for the laminates and can be seen in Table 2. Although not by a great deal, the S i 3 N 4 particulate reinforced laminates
Processing of laminated hybrid ceramic composites: M. M. Sharma and M. F. Amateau Table 2
Calculated vs. measured laminate properties Measured values
Calculated values Design
E/GPa
o~ X 10 -6 °C-I
E/GPa
o~ X 10 -6 °C 1
GX- 14 GX- 15 GX- 16 HX- 18 HX-20 HX-21 HX-22 HX-23
412.45 393.56 412.08 321.07 400.88 328.56 337.79 349.36
8.45 7.32 8.29 6.97 5.05 6.43 6.16 6.19
408.0 384.6 394.3 351.0 374.6 369.9 377.0 315.2
8.31 7.51 8.75 7.46 5.25 6.62 6.12 6.56
showed the greatest difference between predicted and measured properties. This slight difference is believed to be a result of less precise laminae thickness measurements used to calculate the Young's modulus. Among the samples reinforced with TiC particulates, values for the measured Young's moduli ranged between 384.6 and 408.0 GPa and were within 1.1-4.3% of the calculated values. Similarly, for those laminates reinforced with Si3N4 particulates, Young's moduli differed only by 6.6-10.4% of calculated values and ranged between 315.2 and 377.0GPa. The agreement between measured and rule-of-mixtures values indicates that the processing parameters are sufficient for complete densification and indicates good bonding between the reinforcement and matrix. These results provide confidence in the use of calculated properties when experimental values are not available. Values for the coefficient of thermal expansion were measured in the temperature range between 800°C and 1000°C and are presented in Table 2. Generally, the measured values were slightly higher than predicted coefficients of thermal expansion. Although the coefficient of thermal expansion is consistently underestimated, the differences are not significant. Variation in measured thicknesses of the laminae also made a noticeable impact on the predicted values. Even small deviations in thickness values altered the results of the rule-of-mixtures calculations of the coefficient of thermal expansion. Despite this fact, experimentally obtained values were in very good agreement with predicted values. Samples reinforced with TiC particulates exhibited coefficients of thermal expansion of between 7.51 × 10 -6 °C J and 8.75 × 10 -6 ° C -1 (Table 2). The coefficients of thermal expansion are within 1.7-5.3% of the rule-ofmixture calculations. The laminates reinforced with Si3N4 particulates displayed coefficients of thermal expansion between 6.25 × 10 -6 ° C - I and 7.46 X -6 o C - 1 (Table 2). These values are within 0.6-6.6% of the predicted calculations. The differences in predicted and measured values were so small that calculated properties are considered to be accurate.
TiCp-reinforced Al 20 s composites In one composite, GX-06 (see Table l for laminate design), the residual stress of the outer layer was measured to verify predicted values. An average value of 483.12 MPa was obtained for both the Sine-Square-Psi and SET
techniques. This value is extremely close to the predicted value of 498.74 MPa (see Table 1). Therefore, the predicted values for the residual stress are believed to be accurate. In order to further enhance our understanding of the behavior of the material, metallography and microprobe elemental analysis were carried out on selected specimens. From this evaluation, it is concluded that the microstructure at the interface was strongly influenced by thermal residual stresses, because inter- and intra-layer diffusion was observed. Figure 1 displays the interface between the SiCw-reinforced A1203 layer (left) and the TiCp-reinforced A1203 layer (right) in one sample (GX-03). For the most part, a clean straight interface was observed with no unusually large pores, or evidence of excessive diffusion. Figure 2 is a micrograph showing the interface between the TiCp-reinforced A1203 layer and the unreinforced A1203 layer in another sample (GX-20). Both diffusion and void formation occurred from the interface into the unreinforced A1203 layer. Although the layer design varied, this phenomenon was still observed in all samples containing the TiCp-reinforced A1203 and unreinforced A1203 layers. A noticeable void growth near the interface was found in three of the four samples studied. Upon comparing the laminate designs of the four different types of TiCpreinforced A1203 composites studied (GX-03, GX-06, GX-08, GX-20), it was interesting to note that the distribution of large diameter pores occurred only in layers which were in a state of large tensile residual stress. As a result, the tensile residual stress field is believed to attract and then coalesce the pores, thus promoting void growth near the interface. However, several samples with similar residual stress patterns (GX-03 for example) did not exhibit this pore growth behavior; hence an additional process may be involved in this phenomenon. Microprobe elemental analysis of these samples found titanium ( < 1.0wt%) to be diffusing from the TiCpreinforced A1203 layer into the adjacent layer, as seen in Figure 3. The concentration profile of titanium corresponds to the same area displaying the characteristic pore size gradient seen in Figure 2. Figure 3 shows that the concentration of titanium is largest at the interface and diminishes to a trace amount, 20-30 tzm away from the interface. Due to the fact that the diffusion of titanium and the variable pore distribution occur within the same 2 0 30/~m area of the composite, these phenomena are believed to be related. Chemically driven diffusion was not found in any of the samples containing TiC particulates (GX-08
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Processing of laminated hybrid ceramic composites: M. M. Sharma and M. F. Amateau
Figure 1 SEM micrograph showing the interface betwen SiC whisker-reinforced A1203 (left) and TiC particulate-reinforced A1203 (right). This interface displays uniform pore diameters and distribution of pores
Figure 2 SEM micrograph of the interface between TiC particulate-reinforced AI203 (left) and unreinforced A1203 (right) layers. The anomalous pore behavior can be seen 20-30/~m into the A1203 layer and is attributed to stress and chemically driven diffusion
for example); therefore, it is not solely responsible for the observed microstructure, which is attributed to a combination of stress and chemically driven diffusion across the interface. Despite the variable microstructure, the T i C p A1203 composites were still successfully consolidated. The effect of residual stresses on the interface did not prove to be detrimental to the mechanical properties of these
192
composites, but is considered to be a factor in the resulting microstructural variation observed within the composite.
Si 3N4p-reinforced Ale03 composites No variable pore structures were found in the microstructure of Si3N4p-reinforced A 1 2 0 3 laminates with
Processing of laminated hybrid ceramic composites: M. M. Sharma and M. F. Amateau .40 T i
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Diffusion distance of Ti (ran) into alurrina layer Figure 3 Diffusion profile of titanium into the adjacent AI203 layer. Titanium concentration is greatest at the interface and depletes uniformly to a trace amount, approximately 20-30 ~m from the interface Figure 5 SEM micrograph of the interface betwen 10SiC-reinforced Si3N4p-AI203 (left) and unreinforced Si3N4p-AI203 (right) layers. Nodules consisting mostly of Si3N4 are present along the interface and continue 20-30 Izm into the layer. A few nodules consisting of A1203 are also present. The unreinforced Si3N4p-AI203 layer does not contain any nodules
Figure 4 SEM micrograph of the interface betwen 20SiC-reinforced Si3N4p-AI203 (left) and 10SiC-reinforced Si3N4e-A1203 (right) layers. Nodules consisting mostly of A1203 are present along the interface and continue 20-30/zm into the layer
additional SiC and Si3N4 whisker reinforcement. However, various lens-shaped nodules, 10-40/~m in length, scattered along interfaces of the layers were observed. Figure 4 and Figure 5 show the elemental profile of the two different interfaces in an Si3N4p-reinforced A1203 composite (HX-22), and clearly display the unusual nodules. These nodules are largely present at the interface, and continue to exist 20-30/~m into the layers. In bulk powder layers which do not contain whisker reinforcement, nodules were not observed (Figure 5, right side). Microprobe elemental analysis established that these lens-shaped particles consist mainly of two different compositions. The larger nodules consist mostly of pure A1203 while the smaller, more numerous, nodules were found to be made up of approximately 6.3 wt%A1, 40.1 wt%N and 53.5 wt%Si. The A1203 nodules are attributed to clustering of agglomerated powder which occurred during tape casting due to inhomogeneous mixing of the starting
slurry. This conclusion is supported by the fact that nodules were not found in dry bulk powder cores. The lens-like shapes consisting mostly of Si and N are believed to have resulted from particulate clustering and diffusion of A1 during processing. Areas which contained clustered particulates or agglomerated powder after tape casting were shaped into nodules upon hot pressing. Evidence of this comes not only from the composition of the nodules, but also from the fact that the lenses were longest in the direction perpendicular to hot pressing, indicating clusters were forced into lens-like shapes (Figure 4 and Figure 5). Unlike the TiCp-reinforced A1203 laminates, this phenomenon occurred mostly in layers containing compressive residual stress. However, no correlation was made between the residual stress and the anomalous microstructural behavior in these composites. Despite the observed lens-shaped nodules, all samples were successfully fabricated.
CONCLUSIONS Although some variation in results occurred, measured properties of hybrid laminates compared favourably to predicted values. For the most part, observed anomalous behavior occurring at the interfaces of the composites was not found to affect the residual stress predictions of laminates. Furthermore, this behavior is not believed to have adversely affected the mechanical properties of the laminate. Inter- and intra-layer reactions caused by stress and chemically driven diffusion were responsible for the unusual pore behavior in the TiCp-reinforced A1203 laminates. Nodules observed in the Si3N4p-reinforced A1203 composites were attributed to whisker clustering and diffusion. The X-ray stress analysis confirmed the predicted
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Processing of laminated hybrid ceramic composites: M. M. Sharma and M. F. Amateau outer layer residual stress in one TiCp-reinforced A1203 composite.
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Amateau, M.F., Stutzman, B.A., Conway, J.C. and Halloran, J., Performance of laminated ceramic composite cutting tools. Ceram. Inter., 1995, 21, 317-323. Virkar, A.V., Jue, J.F., Hansen, J.J. and Cutler, R. A., Measurement of residual stresses in oxide-ZrO2 three-layer composites. J. Am. Ceram. Soc., 1988, 71(3), C148-C151. Lange, F.F., James, H.R. and Gren, D.J., Determination of residual
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surface stresses caused by grinding in polycrystalline AI20 3. J. Am. Ceram. Soc., 1983, 66(2), C16-C17. Ramsey, R.M., Chandler, H.W. and Page, T.F., The determination of residual stresses in thin coatings by a sample thinning method. Surf. and Coat. Tech., 1990, 43/44, 223-233. Randon, R. and Green, D.J., Residual stress determination using strain gage measurements. J. Am. Ceram. Soc., 1990, 73(9), 2628-2633. Bitler, J.W., A model of the thermoelastic response of A1203/M2 tool steel functionally gradient materials. Master's Thesis The Pennsylvania State University, 1995. Halpin, J.C. and Kardos, J.L., The Halpin-Tsi equations: A review. Polymer Eng. Sci., 1976, 16, 344-352. Halpin, J.C. and Tsai, S.W., Environmental Factors in Composite Materials, Laboratory Report, AFML-TRG7-473, June 1969.