1 23
`
`Comparison of the Thermal Expansion
`Behavior of Several Intermetallic Silicide
`Alloys Between 293 and 1523 K
`
`S. V. Raj
`
`Journal of Materials Engineering and
`Performance
`
`ISSN 1059-9495
`Volume 24
`Number 3
`
`J. of Materi Eng and Perform (2015)
`24:1199-1205
`DOI 10.1007/s11665-015-1390-8
`
` 1
`
`UTC 2023
`General Electric v. United Technologies
`IPR2016-01289
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`1 23
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`JMEPEG (2015) 24:1199–1205
`DOI: 10.1007/s11665-015-1390-8
`
`ÓASM International
`1059-9495/$19.00
`
`Comparison of the Thermal Expansion Behavior
`of Several Intermetallic Silicide Alloys Between 293
`and 1523 K
`
`S.V. Raj
`
`(Submitted November 12, 2014; in revised form December 19, 2014; published online January 21, 2015)
`
`Thermal expansion measurements were conducted on hot-pressed CrSi2, TiSi2, WSi2 and a two-phase
`Cr-Mo-Si intermetallic alloy between 303 and 1523 K during three heat-cool cycles. The corrected thermal
`expansion, (DL/L0)thermal, varied with the absolute temperature, T, as
`
`ð Þ þ Dð Þ2þ C T 293Þthermal¼ A T 293ð Þ3þ B T 293
`ð
`
`
`
`
`
`where, A, B, C, and D are regression constants. Excellent reproducibility was observed for most of
`the materials after the first heat-up cycle. In some cases, the data from first heat-up cycle deviated
`from those determined in the subsequent cycles. This deviation was attributed to the presence of
`residual stresses developed during processing, which are relieved after the first heat-up cycle.
`
`DL=L0
`
`Keywords CTE, disilicides, intermetallic alloys, thermal expan-
`sion
`
`1. Introduction
`
`Intermetallic silicides have found applications as ohmic
`contacts in the semiconductor industry (Ref 1–5), thermoelec-
`tric materials (Ref 6), heating elements (Ref 7, 8), protective
`coatings (Ref 9), and they have been proposed for structural
`applications (Ref 10). A knowledge of the thermal expansion
`behavior of these silicides is important in these applications in
`order to reduce or eliminate thermal stresses between the
`intermetallic silicide and the substrate. Although a compilation
`of the thermal expansion data on many intermetallic silicides is
`available (Ref 11), the data are fairly old and it is unclear
`whether they were influenced by impurities and processing
`methods. Verkhorobin and Matyushenko (Ref 12) reported
`limited data on the coefficients of thermal expansion (CTE) of
`CrSi2, MoSi2, NbSi2, TaSi2, and VSi2 determined from x-ray
`data. A more complete thermal expansion data on transition
`metal silicides also determined by x-ray diffraction (XRD) have
`been reported by Engstro¨m and Lo¨nnberg (Ref 13). In contrast,
`there are only limited thermal expansion data on hot-pressed
`silicides generated by dilatometric measurements.
`The present investigation was undertaken to determine and
`compare the thermal expansion behavior of hot-pressed CrSi2,
`
`TiSi2, WSi2, and a two-phase Cr-30(at.%)Mo-30%Si* inter-
`metallic alloy between 303 and 1523 K by dilatometric
`measurements. It is noted that there is no previous CTE data
`for the Cr-30Mo-30%Si alloy.
`
`2. Experimental Procedures
`
`Commercially produced powders (325 mesh) of CrSi2,
`TiSi2, WSi2, and a Cr-30%Mo-30%Si alloy were hot-pressed into
`25.4 mm long and 9.5 mm in diameter cylindrical specimens.
`The Cr-30%Mo-30%Si alloy was procured from ATI Powder
`Metals, Pittsburgh, PA as gas atomized powder. Table 1 gives the
`total purity and major impurity content of the powders. The
`powders were hot-pressed using conditions given in Table 2.
`Three CrSi2 specimens were fabricated in separate hot-press runs
`(582, 619, and 620) in order to evaluate the effect of batch-to-
`batch variabilities on the thermal expansion data. The two faces
`of hot-pressed specimens were machined to ensure that they were
`flat and parallel. The thermal expansion measurements were
`conducted using a NETZSCH Dilatometer Model DIL 402C
`equipped with a high purity alumina as a calibration standard.
`Measurements were made over three heat-cool cycles to (a)
`minimize the effects of compositional, microstructural and
`processing variables on the data, (b) determine the extent of
`scatter in the data, and (c) to evaluate a statistical average of the
`coefficients for the regression curve. The specimen was placed in
`a sample holder and aligned with a single push-rod with an
`applied constant load of 0.2 N. The specimens were heated from
`303 to 1523 K at 10 K/min in the first cycle and cooled to 373 K
`at 10 K/min. in the first cool-down cycle. Subsequent cycles
`consisted of heating and cooling between 373 and 1523 K at
`10 K/min. All measurements were conducted in a He atmosphere
`
`S.V. Raj, NASA Glenn Research Center, MS 106-5, 21000 Brookpark
`Road, Cleveland, OH 44135. Contact e-mail: sai.v.raj@nasa.gov.
`
`*Unless specifically stated, all compositions are reported in at.% in this
`paper.
`
`Journal of Materials Engineering and Performance
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`Volume 24(3) March 2015—1199
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`

`

`Table 1 Chemical compositions of the silicide powders in wt%
`
`Powder
`
`CrSi2 (Alfa Aesar)
`Cr-Mo-Si (ATI)
`TiSi2 (Cerac)
`WSi2 (Materion)
`
`Al
`
`0.05
`…
`…
`0.005
`
`Cr
`
`46.5
`35.2
`…
`0.001
`
`Fe
`
`0.38
`0.038
`…
`0.005
`
`Mo
`
`…
`50
`…
`…
`
`Ni
`
`0.13
`0.015
`…
`…
`
`Si
`
`52.4
`14.7
`…
`22.9
`
`Ti
`
`0.33
`…
`…
`…
`
`W
`
`…
`0.06
`…
`76.6
`
`Total purity
`
`98.9
`99.9
`99.5
`99.5
`
`Table 2 Processing data for hot-pressing the silicide powders
`
`Silicide powder
`
`Run #
`
`CrSi2
`CrSi2
`CrSi2
`Cr-Mo-Si
`TiSi2
`WSi2
`
`582
`619
`620
`621
`641
`622
`
`T, K
`
`1183
`1523
`1523
`1723
`1523
`1573
`
`P, MPa
`
`Time, h
`
`Environment
`
`68.9
`89.6
`89.6
`89.6
`89.6
`89.6
`
`2.0
`0.25
`2.0
`4.0
`0.67
`2.0
`
`Ar
`Ar
`Ar
`Ar
`Ar
`Ar
`
`Fig. 1
`runs
`
`(a-f) Low and high magnification optical micrographs of the transverse cross-sections of hot-pressed CrSi2 specimens fabricated in three
`
`1200—Volume 24(3) March 2015
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`Fig. 2
`
`(a-b) Low and high magnification optical micrographs of the transverse cross-sections of a hot-pressed Cr-30%Mo-30%Si specimen
`
`Fig. 3
`
`(a-b) Low and high magnification optical micrographs of the transverse cross-sections of a hot-pressed TiSi2 specimen
`
`Fig. 4
`
`(a-b) Low and high magnification optical micrographs of the transverse cross-sections of a hot-pressed WSi2 specimen
`
`flowing at 60 cm3/min. The length changes were recorded by a
`computerized data acquisition system. The experimental strain,
`DL/L0, where DL is the differential change in length, L L0, L is
`the instantaneous length, and L0 is the original length of the
`specimen at room temperature, were measured.
`
`3. Results and Discussion
`
`3.1 Microstructures
`
`Microstructural observations of the hot-pressed specimens
`revealed that most of them were well consolidated although the
`
`extent of homogeneity varied from specimen to specimen.
`Figure 1(a-f) show optical micrographs of the transverse cross-
`sections of three hot-pressed batches of CrSi2. While the
`microstructures for specimens from hot-pressing runs 619 and
`620 show a great degree of homogeneity and a distribution of fine
`grain boundary porosity, the microstructure for run 582 shows the
`boundaries of the powder particles and extensive porosity. It is
`noted that run 582 was hot pressed at 1183 K while runs 619 and
`620 were hot pressed at 1523 K. The optical microstructures of
`the hot-pressed Cr-30Mo-30Si alloy were homogeneous and
`fully consolidated with little porosity (Fig. 2a-b). In contrast, the
`hot-pressed microstructures of the TiSi2 (Fig. 3a-b) and WSi2
`(Fig. 4a-b) showed a greater amount of grain boundary porosity.
`
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`3.2 Thermal Expansion of CrSi2
`
`Figure 5(a-c) show the variation of the thermal expansion,
`(DL/L0), with increasing absolute temperature, T, for the hot-
`pressed three specimens from runs 582, 619, and 620. On close
`examination of the results, it is evident that DL/L0 for the first
`
`Fig. 5 Temperature dependence of the thermal expansion behavior
`of CrSi2 during three heat up-cool down cycles between 303 and
`1523 K. The data are shown for three hot-pressed specimens from
`runs (a) 582; (b) 619; and (c) 620
`
`heat-up run is higher than the values for subsequent cool-down
`and heat-up runs for all three specimens with the difference
`being more pronounced for run 582. This discrepancy in the
`values of DL/L0 between the first and other cool-down and heat-
`up cycles has been attributed to the relieving of residual stresses
`developed in the specimens during hot-pressing of the powders
`during the first heat-up cycle (Ref 14). The measurements from
`the first cool-down to the third cool-down cycles were fairly
`reproducible for runs 619 and 620 but a little more scattered for
`run 582 presumably due to the extensive porosity and
`heterogeneous microstructure (Fig. 1a and b). Thus, neglecting
`the data from the first heat-up cycle, the data from the first cool-
`down to the third cool-down cycles were fitted with Eq. 1:
`ð
`
`Þthermal¼ A T 293ð
`
`Þ3þ B T 293ð Þ2þ C T 293
`
`
`ð Þ þ D
`DL=L0
`ðEq 1Þ
`where (DL/L0)thermal is the magnitude of the DL/L0 without
`any residual processing strains, A, B, C, and D are regression
`constants. Table 3 compiles the values of the regression con-
`2,
`stants and the corresponding coefficients of determination, Rd
`for the three CrSi2 specimens. Figure 6 compares the regres-
`sion plots for the three specimens, where it is seen that the
`curves are reasonably close thereby suggesting that batch-to-
`batch variability is relatively small. Unfortunately, a sparsity
`of published data on polycrystalline CrSi2 did not permit a
`meaningful comparison with the present results.
`
`3.3 Thermal Expansion of Cr-30%Mo-30%Si
`
`Figure 7 shows the variation of DL/L0 with increasing T for
`hot-pressed Cr-30%Mo-30%Si. Unlike the observations on
`CrSi2,
`the curves almost overlap for all
`thermal cycles
`including the first heat-up cycle. This observation suggests
`that
`the amount of residual strains developed during hot-
`pressing of this specimen was negligible. The constants given
`in Eq. 1 were determined by regression analysis of all the data
`including those measured during the first heat-up cycle. The
`values of these constants are shown in Table 4. Since the Cr-
`30%Mo-30%Si alloy was prepared by substituting Mo for Cr
`and Si in Cr3Si to improve its oxidation resistance (Ref 15),
`published data for Cr3Si are shown in Fig. 7 for comparison
`(Ref 11). The two sets of data increasingly deviate from each
`other with increasing temperature, where the deviation is
`relatively small. It is unlikely that this observed deviation in the
`thermal expansion can be attributed to the addition of Mo since
`the microstructure of the Cr-30%Mo-30%Si alloy consists of
`(Cr,Mo)3Si and (Cr,Mo)5Si3 (Ref 15). Therefore, this deviation
`is attributed to experimental scatter.
`
`3.4 Thermal Expansion of TiSi2
`
`The cyclic thermal expansion characteristics of hot-pressed
`TiSi2 are similar to the Cr-30%Mo-30%Si alloy (Fig. 8). The
`curves from the first heat-up to the third cool-down cycle are in
`excellent agreement with negligible scatter in the data. Once
`
`Table 3 Values of the regression constants describing the thermal expansion of CrSi2 between 293 and 1523 K
`
`Hot press run no.
`
`Cycle description
`
`582
`619
`620
`
`1st cool-down to 3rd cool-down
`1st cool-down to 3rd cool-down
`1st cool-down to 3rd cool-down
`
`A, %K23
`2.0 9 1010
`4.2 9 1010
`4.2 9 1010
`
`B, %K22
`1.2 9 1007
`1.4 9 1007
`1.8 9 1007
`
`C, %K
`1.0 9 1003
`1.1 9 1003
`1.1 9 1003
`
`D, %
`1.8 9 1001
`1.4 9 1001
`9.3 9 1002
`
`2
`Rd
`
`0.993
`0.999
`0.999
`
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`

`the residual stresses were
`it can be concluded that
`again,
`negligible during hot-pressing of the specimens. Table 5 gives
`the values the regression coefficients from the first heat-up to
`the third cool-down cycle. As shown in Fig. 8, the regression
`line describes the experimental data very well. Comparison of
`the present results with literature data on TiSi2 (Ref 11) reveals
`that the two sets of data are in reasonable agreement although
`they increasingly deviate with increasing temperature.
`
`cients for WSi2 are shown in Table 6, where the data from the
`first heat-up cycle were not included in the regression analyses.
`Figure 9 compares the present experimental results with the
`recommended values
`for polycrystalline WSi2 based on
`calculations from ‘‘selected axial thermal expansion values’’
`(Ref 11). Although the latter results are slightly larger than the
`current experimental regression curve, the two curves are in
`reasonable agreement with the limits of experimental scatter.
`
`3.5 Thermal Expansion of WSi2
`
`the
`to the thermal expansion behavior of
`In contrast
`Cr-30%Mo-30%Si alloy (Fig. 7) and TiSi2 (Fig. 8), the behav-
`ior of WSi2 is similar to CrSi2 in that the magnitudes of DL/L0
`are larger in the first heat-up cycle than those in the subsequent
`thermal cycles (Fig. 9). However, the difference is smaller than
`the observations on CrSi2 (Fig. 5a-c). The regression coeffi-
`
`Fig. 6 Comparison of the regression equations for the three CrSi2
`specimens between 303 and 1523 K
`
`3.6 Comparison of the Thermal Expansion Behavior
`of Different Silicides
`
`Metal silicides are used in the semiconductor industry as
`contact materials (Ref 1–5) as well as being considered for
`other potential applications, such as oxidation-resistant coatings
`(Ref 9) and matrix constituents in SiC fiber-reinforced com-
`posite materials (Ref 10, 16–19). Therefore, it is useful to
`compare the present
`results with thermal expansion data
`reported in the literature for MoSi2 (Ref 11), polycrystalline
`Si (Ref 20), SiC (Ref 11), and Si3N4 (Ref 11) (Fig. 10). An
`examination of Fig. 10 reveals that the magnitudes of DL/L0 for
`the metal silicides are higher than those for Si, SiC and Si3N4
`with the deviation from the latter set of data increasing with
`increasing temperature. The thermal expansion values are
`similar for both Si and SiC up to 1000 K but larger than those
`for Si3N4. While using metal silicide contacts on Si or SiC-
`based semiconductors are unlikely to debond from the substrate
`if they operate close to room temperature, Fig. 10 suggests that
`debonding is increasing likely at higher operating temperatures.
`Similarly, SiC fiber-reinforced silicide matrix composites are
`likely to develop large thermal strains due to the significant
`differences in the magnitudes of DL/L0 between the silicides
`and SiC. Nevertheless, it has been successfully demonstrated
`that mixing adequate amounts of Si3N4 with MoSi2 to form 0/
`90 MoSi2-Si3N4/SiC(f) laminated composites lasted over 1000
`
`Fig. 7 Temperature dependence of the thermal expansion behavior
`of Cr-30%Mo-30%Si during three heat up-cool down cycles between
`303 and 1523 K. The current results are compared with data for
`Cr3Si (Ref 11)
`
`Fig. 8 Temperature dependence of the thermal expansion behavior
`of TiSi2 during three heat up-cool down cycles between 303 and
`1523 K. The data reported by Touloukian et al. (Ref 11) are shown
`in the figure
`
`Table 4 Values of the regression constants describing the thermal expansion of Cr-30%Mo-30%Si between 293 and 1523 K
`
`Hot press run no.
`
`Cycle description
`
`621
`
`1st heat-up to 3rd cool-down
`
`A, %K23
`1.3 9 1010
`
`B, %K22
`1.7 9 1008
`
`C, %K
`9.2 9 104
`
`D, %
`2.5 9 102
`
`2
`Rd
`
`0.9993
`
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`

`Table 5 Values of the regression constants describing the thermal expansion of TiSi2 between 293 and 1523 K
`
`Hot press run no.
`
`Cycle description
`
`641
`
`1st heat-up to 3rd cool-down
`
`A, %K23
`1.8 9 1010
`
`B, %K22
`1.7 9 1007
`
`C, %K
`1.2 9 1003
`
`D, %
`2.0 9 1002
`
`2
`Rd
`
`0.999
`
`Fig. 9 Temperature dependence of the thermal expansion behavior
`of WSi2 during three heat up-cool down cycles between 303 and
`1523 K. The present data are compared with those compiled by Tou-
`loukian et al. (Ref 11)
`
`Fig. 10 Comparison of the temperature dependence of the average
`thermal expansion behavior of hot-pressed CrSi2, Cr-30%Mo-30%Si,
`MoSi2 (Ref 11), Si (Ref 20), SiC (Ref 11), Si3N4 (Ref 11), TiSi2 and
`WSi2 between 293 and 2000 K
`
`Table 6 Values of the regression constants describing the thermal expansion of WSi2 between 293 and 1523 K
`
`Hot press run no.
`
`Cycle description
`
`622
`
`1st cool-down to 3rd cool-down
`
`A, %K23
`1.2 9 1010
`
`B, %K22
`1.2 9 1007
`
`C, %K
`8.3 9 1004
`
`D, %
`8.0 9 1002
`
`2
`Rd
`
`0.999
`
`thermal cycles at 773 K (Ref 18). This concept will be further
`developed and investigated in a future paper.
`
`4. Summary and Conclusions
`
`Thermal expansion measurements were conducted on hot-
`pressed CrSi2, TiSi2, WSi2, and a two-phase Cr-Mo-Si
`intermetallic alloy between 303 and 1523 K during three
`heat-cool cycles. The optical microstructures of the cross-
`sections of the hot-pressed specimens revealed that they were
`well consolidated except in the case of one CrSi2 specimen,
`where there was a large amount of grain boundary porosity.
`Batch-to-batch comparisons with measurements made on two
`other CrSi2 specimens revealed that the values of DL/L0 for this
`specimen were slightly lower. Unlike for the other materials
`investigated in the present research, the first heat-up cycle data
`for the CrSi2 and WSi2 deviated from those determined in the
`subsequent cycles due to the presence of residual stresses
`developed during processing. Once these stresses were relieved
`after the first heat-up cycle, the data generated in the second
`and third heat-up and cool-down cycles were in excellent
`agreement. The corrected thermal expansion, (DL/L0)thermal,
`varied with the absolute temperature, T, as
`ð
`
`Þthermal¼ A T 293ð Þ3þ B T 293
`
`ð Þ2þ C T 293
`
`DL=L0
`where, A, B, C and D are regression constants.
`
`ð Þ þ D
`
`
`
`The present data were compared with those reported for
`MoSi2 (Ref 11), Si (Ref 20), SiC (Ref 11) and Si3N4 (Ref 11). It
`is demonstrated that the thermal expansion for the silicides was
`larger than those for Si, SiC, and Si3N4 with the deviation
`between the former and latter set of data increasing with
`increasing temperature. It is concluded that the thermal strains
`would increase with increasing temperature if silicides are used
`as contact materials on Si or SiC, or in silicide-based SiC fiber
`reinforced composites.
`
`Acknowledgments
`
`The author thanks the late Ms. Anna Palczer for conducting the
`thermal expansion measurements. This research was supported by
`a generous Grant from NASAÕs ARMD Seedling Fund Program,
`and this is gratefully acknowledged.
`
`References
`
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`

`

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`Volume 24(3) March 2015—1205
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`Author's personal copy
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`9
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