JOURNAL OF MATERIALS SCIENCE 3211997) 1703-170‘)
`
`Titanium diboride copper-matrix composites
`
`,
`P. YIH, D. D. L.CHUNG
`Composite Materials Research Laboratory, State University of New York at Buffalo,
`Buffalo, NY 14260, USA
` a
`
`Copper-matrix titanium diboride platelet (3-5 um) composites containing15—60 vo|% TiB2,
`were fabricated by powder metallurgy, using copper-coated TiB2 (60 vol % TiBz) and various
`amounts of copper powder. The porosity was <0.5% when TiB, was <48 vo|%. Above
`48 vol % TiB,, the porosity increased abruptly with increasing TiB, content, reaching 6.7% at
`60 vol% TiB,. As a result, the hardness and compressive yield strength dropped
`precipitously with increasing TiB2 volume fraction beyond 48%. At 48 vol % TiB2, the thermal
`conductivity was 176 Wm“ °C“, the electrical resistivity was 3.42 x1O‘6Qcm, the
`coefficient of thermal expansion (CTE) was 10.2 x10"5‘ C“, the compressive yield strength
`was 659 MPa, and the Brinell hardness was 218. For composites made by conventional
`powder metallurgy, using a mixture of TiB, platelets (not coated) and copper powder, the
`porosity was <1.8°/o when TiB2 was at $42 vol %; above 42 vol% TiB2, the porosity
`increased abruptly and the hardness and compressive yield strength decreased abruptly.
`The electrical resistivity and thermal conductivity were also affected by the porosity, but less
`so than the mechanical properties. Composites made using copper-coated TiB, exhibited
`lower electrical resistivity, higher thermal conductivity, lower CTE, higher compressive yield
`strength, greater hardness, greater abrasive wear resistance, greater scratch resistance and
`lower porosity than the corresponding composites made from uncoated TiB2.
`
`
`1. Introduction
`Titanium diboride (TiB3l is well-known for its stiffness
`and hardness. Furthermore. in contrast to most cer-
`amics.
`it
`is electrically and thermally conductive.
`Metals. on the other hand. are electrically and ther-
`mally conductive. but most of them exhibit a low
`coefficient of thermal expansion (CTE). The combina-
`tion of low CTE and high thermal conductivity is
`particularly attractive for electronic packaging. such
`as heat sinks. housings. substrates, lids. etc. The com-
`bination of high electrical and thermal conductivity
`and hardness is particularly attractive for welding
`electrodes. motor brushes and sliding contacts. Owing
`to these attractive combinations of properties and the
`availability of TiB3 in discontinuous forms (such as
`platelets). TiB2 is an important reinforcement
`for
`composites. In particular. metal-matrix TiB3 com-
`posites are attractive because metals usually have high
`CTE and limited stiffness and hardness. The TiB3
`
`addition greatly increases the stiffness. hardness and
`wear resistance and decreases the CTE, while reducing
`the electrical and thermal conductivity much less than
`the addition of most other ceramic reinforcements
`
`[l~l0]. Metal matrices previously used for TiB3 com-
`posites include aluminium [1—7], Al22Fe3Tis [8]. in-
`termetallic compounds [9—12]. iron [3, 13-14], nickel
`[14]. copper [3. 15. 16]. bronze [3] and titanium [17].
`This work focuses on the use of copper as the matrix
`owing to its high electrical and thermal conductivities
`compared to most metals and the importance of these
`conductivities for numerous applications.
`
`0023-2461
`
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`
`Previous work on copper-matrix TiB3 composites
`includes TiB2 in the form of a sintered porous block
`(which is impregnated by molten copper to form the
`composites) [3],- and TiB3 in the form of discontinu-
`ous platelets (which are hot pressed with copper below
`the melting point of copper in order to form the
`composite) [10]. In other works.
`the TiB3 volume
`fraction is limited to 56.5% [3] and 15% and 60%
`[10]. The present work provides a systematic study of
`Cu/TiB; composites as function of the TiB2 volume
`fraction. which includes 15%, 30%. 35%. 42%. 48%.
`50% and 60%. Because the CTE decreases and the
`hardness increases with increasing TiB= volume frac-
`tion. while the thermal and electrical conductivities
`decrease with increasing TiBz volume fraction.
`the
`optimal TiBl volume fraction depends on the particu-
`lar combination of properties desired. As a result.
`a systematic study as a function of the TiB3 volume
`fraction is necessary in order to optimize the TiB3
`volume fraction for a particular application.
`The composite fabrication method of Viswanadham
`at u]. [10] gave composites of much lower porosity
`than that of loo er al. [3]. This work used the same
`method and the same TiB3 platelets as Viswanadham
`ct (if. [10]. As in the latter work. both the admixture
`method and the coated filler method of powder metal-
`lurgy were used. though the latter gave composites of
`lower porosity than the former. The admixture
`method refers to the method in which the reinforce-
`ment and matrix powder are mixed and then sintered
`together. In the coated liller method the reinforcement
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`is coated witn me matrix material and men smtered,
`
`such that mixing with the matrix powder is optional.
`
`2. Experimental procedure
`The TiB2 platelets described in Table I were supplied
`by Union Carbide Advanced Ceramics (Cleveland,
`OH). The copper powder used was supplied by GTE
`Products Corporation (Towanda, PA); the mean par-
`ticle size was 3.3 mm.
`
`Cu/TiB2 composites containing 15-60 vol % TiB1
`platelets were fabricated by hot-pressing, using the
`two methods, namely the coated filler method (using
`copper-coated TiB2 platelets, optionally mixed with
`copper powder to obtain the desired composition),
`and the admixture method (using a mixture of copper
`powder and TiB3 platelets).
`In the coated filler
`method, the surface of the TiB2 platelets was metal-
`lized by electroless plating with copper and sub-
`sequently electroplated with
`copper
`to obtain
`copper-coated TiB2 platelets containing 60 vol %
`TiB2. In the admixture method, mixtures of copper
`powder and TiB2 platelets were prepared at the same
`corresponding compositions by weight as the com-
`posites made by the coated filler method.
`Before composite fabrication,
`the copper-coated
`TiB2 platelets (or a mixture of copper-coated TiB2
`platelets and copper powder. for the coated filler
`method) and the mixture of TiB3 platelets and copper
`powder (for the admixture method) were reduced in
`purging hydrogen gas at 250 “C for 60 min. The com-
`posite fabrication involved cold compaction of the
`coated platelets (or the mixture) in a graphite die at
`155 MPa to form a cylindrical green compact (0.5 in
`or 12.7 mm diameter). The green compact was then
`heated and hot pressed in the same die in purging
`nitrogen gas at 950 °C and 116 MPa for 25 min. Dur-
`ing heating, the pressure was kept at 77 MPa until the
`temperature reached the hot-pressing temperature.
`Composite testing involved measurements of the
`density, hardness (Brinell). compressive yield strength,
`abrasive wear resistance. scratch resistance. volume
`
`electrical resistivity. coefiicient of thermal expansion
`(CTE) and thermal conductivity.
`The density of Cu TiB3 composites was measured
`by using the buoyancy (Archimedes) method (ASTM
`B328-92). The hardness measurement was performed
`using a Brinell Hardness Tester (Detroit Testing Ma-
`chine Co.. Model HB-2) at a load of 1000 kg. Com-
`pressive testing was conducted on a flat
`face of
`a cylindrical specimen (0.5 in or l2.7 mm diameter.
`0.5 in or 12.7 mm high), using an MTS hydraulic
`mechanical testing system.
`The abrasive wear test was conducted on a Tele-
`
`dyne Taber Model 503 standard abrasion tester. Fig. I
`shows the abrasive wear testing geometry. The cylin-
`drical samples. 0.5 in (12.7 mm) diameter. were posi-
`tioned in a disc-like sample holder. Two Crystalon (a
`clay composite impregnated with 180 grid SiC par-
`ticles) girding wheels were loaded by 1 kg weights in
`a perpendicular direction on the samples. which ro-
`tated with the sample holder in a horizontal plane.
`The rotating speed of the sample holder was constant
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`TA 3 L E l Properties of Til}, platelets
`
`Density
`4.50
`lgcm")
`
`
`3-5
`Particle size or diameter (um)
`~3
`Aspect ratio
`10-30
`Electrical resistivity (l0"’ Qcm)
`~l00
`Thermal conductivity (Wm" °C“)
`3.1
`CTE 00"" "C")
`350-570
`Elastic modulus (GPa)
`
`Poisson’s ratio 0.13-0.19
`
`Figure I Abrasive wear testing geometry.
`
`at 72 rev min“. The number of cycles used for the test
`was 600000. After the abrasive wear test, the weight
`loss of the sample was measured. The weight
`loss
`relates to the volume loss through the density. Because
`the weight loss depends on the wear conditions (such
`as load. rotating speed and the number of cycles), the
`relative wear under the same wear conditions was
`considered. Relative wear is defined as the volume loss
`ofa sample due to wear divided by that of a standard
`sample.
`In this work.
`the composite made by the
`admixture method and containing 50 vol % TiB2 was
`chosen as the standard sample.
`The scratch resistance test was conducted on
`a Teledyne Taber Model 502 shear/scratch tester un-
`der a load of 1 kg. After testing. the scratch width on
`the surface of the sample was measured by optical
`microscopy. This width relates to the scratch resist-
`ance of the composites. Moreover. the greater the
`width. the lower was the shear strength.
`For measurement of the volume electrical resistiv-
`ity. the four-probe method was used. Silver paint was
`used for electrical contacts. The CTE was determined
`by using a Perkin—Elmer TMA-7 thermal mechanical
`
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`temperature
`-l
`the
`analyser, with
`25—l00 ‘C at a rate of 3 "C min
`The thermal conductivity. K, was determined by the
`
`scanned
`
`from
`
`gtluzttion
`
`K =upCp,
`
`(1)
`
`where :1, p and CP are the thermal diffusivity, density
`and specific heat. respectively, of the sample. For ob-
`taining the thermal conductivity, the thermal diffusivity
`was measured by the laser flash method (neodymium
`glass laser. 10-15 J energy, 0.4 ms pulse“) [18], while
`the specific heat was measured by differential scanning
`calorimetry (Perkin—Elmer DSC-7).
`After fabrication. the composite was cut into pieces
`using a diamond saw for testing. For density and
`hardness tests. one sample was measured three times
`for each test, whereas for the compressive test. two
`samples were used. For abrasive testing, one sample
`was used and weighed three times after testing. In the
`scratch test, one sample was tested three times, where-
`as in the thermal diffusivity test, one sample was
`measured five times. For specific hcat
`testing, one
`sample was measured three times. and for CTE test-
`ing, one sample was measured ten times. Two samples
`were measured three times each for electrical resistiv-
`ity measurement.
`
`3. Results and discussion
`3.1. Microstructure
`
`Fig. 2 shows optical micrographs of polished sections
`of Cu/TiB3 platelet composites made by the two
`
`methods. At a low content of TiB2 platelets ( 15 vol °/o),
`dense C u/TiB 2 platelet composites were made by both
`the coated filler and the admixture methods and there
`
`was no apparent difference between the microstruc-
`tures of the composites made by the two methods (Fig.
`2a and bl. At a high content of TiB2 platelets
`(60 vol "/o),
`the composite made by the admixture
`method had a much higher porosity (Fig. 2d) than the
`composite made by the coated filler method (Fig. 2c).
`For all the composites made by the two methods, the
`TiB2 platelets were distributed uniformly in the cop-
`per matrix (Fig. 2).
`
`3.2. Porosity
`Fig. 3 shows that the porosity of Cu/TiB3 platelet
`composites made by the admixture method increased
`sharply with increasing TiB2 volume fraction when
`the TiB; volume fraction exceeded 42 %, but the por-
`osity of the composites made by the coated filler
`method remained low up to 50 vol % TiB2. The rea-
`son is that.
`in the coated filler method. by using
`copper-coated TiB2 platelets, even at a high TiB3
`platelet content. the matrix copper coating separated
`the TiB3 platelets from one another. thus making it
`possible to obtain a dense composite. This is sup-
`ported by Fig. 4, which shows optical micrographs of
`Cu/TiB3 composite containing 42 and 50 vol % TiB3
`and made by the two methods. Fig. 4a and b show that
`at a TiB3 content of 42 vol “/0. dense composites can
`still be made by the two methods. but at the higher
`TiB3 platelet content (50 vol °/o), many pores existed
`
`I5 vol "/0 TiB3 cottted tiller method: (ht
`Figure 2 Optical micrographs of the Cu/TiB3 platelet composites nittdc by the mo methods: (:1)
`15 ml ";.. TiB3 udinixturc method: (cl (:0 vol "/4, TiB3 coated lillcr method: (tit 00 ml "u TiB3 admixture method.
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`in the composite made by the admixture method
`(Fig. 4d), whereas there were no apparent pores
`in the'composite made by the coated filler method
`(Fig. 4c)
`
`3.3. Properties OT tne COmpOS|IeS
`3.3.1. Mechanical properties
`Fig. 5 shows that the hardness of the composites made
`by the coated filler method increased with increasing
`
`20
`
`
`
`Porosity(vo|%) 8
`
`.
`
`0
`
`1o
`
`20
`
`30
`
`40
`TiB,(vol%l
`
`so
`
`so
`
`70
`
`240
`
`220
`
`200
`
`180
`
`160
`
`140
`
`120
`
`100
`
`80
`
`60
`
`Hardness,HB
`
`10
`
`20
`
`30
`
`40
`TiBz (vo|%)
`
`50
`
`60
`
`70
`
`Figure 3 Variation of porosity with TiB3 platelet volume fraction in
`copper-matrix composites made by (CI the coated filler method and
`(Z) the admixture method. The vertical bar at each data point is an
`error bar.
`
`Figure 5 Variation of Brinell hardness with TiB3 platelet volume
`fraction in copper-matrix composites made by (O) the coated filler
`method and ID) the admixture method. The vertical bar at each
`data point is an error bar.
`
`Figure 4 Optical microgruphs of the Cu TiB; plutclct composites made by the two methods: Ia) 42 vol "/o TiB; coated filler method: (bl
`42 vol "/1. TiB:. admixture method: (cl 50 Vol % TiB3. coated tiller method: (d) 50 vol "/0 TiB3. admixture method
`
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`TiB2 content up to 48 vol % and reached the highest
`Brinell hardness value of 218.
`ln contrast, for the
`composites made by the admixture method, the hard-
`ncss level was lower than that of the composites made
`by the coated filler method at any TiB2 platelet con-
`tent exceeding 15 vol %, and-dropped markedly when
`the TiB2 content exceeded 42 vol %. Fig. 6 shows the
`compressive yield strength of the composites made by
`the two methods; the trend is similar to that of the
`hardness shown in Fig. 5.
`Because the applications of Cu/TiB2 platelet com-
`posites include electrical contacts and sliding contacts,
`the hardnessgabrasive wear resistance and scratch
`resistance are important properties. Table II lists the
`measured hardness, abrasive wear resistance (in terms
`of the relative wear) and scratchresistance (in terms of
`the scratch width) of selected Cu/TiB2 platelet com-
`
`posites.
`Table II shows that at a TiB2 content of 50 vol %,
`the composite made by the coated filler method had
`a much higher hardness, abrasive wear resistance and
`scratch resistance than those of the corresponding
`
`
`
`Compressiveyieldstrength(MPa)
`
`
`
`
`
`700
`
`600
`
`Ԥ
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`002§
`
`200
`
`100
`
`10
`
`20
`
`30
`
`40
`TiB, (vol %)
`
`50
`
`60
`
`70
`
`composite made by the admixture method Even at
`a lower TiB3 content (42 vol °/o), the composite made
`by the coated filler method was superior to the
`composite made by the admixture method at a
`higher TiBz content (50 vol %). The superiority of
`the composites made by the coated filler method in
`mechanical properties, especially at high TiB2 con-
`tents (>42 vol %), to the composites made by the
`admixtures method,
`is related to the difference in
`porosity (Fig. 3).
`
`3.3.2. Thermal and electrical properties
`Fig. 7 shows that the thermal conductivity of the
`composite made by the coated filler method was high-
`er than that of the corresponding composite made by
`the admixture method, when the TiBz content ex-
`ceeded 35 vol %. The thermal conductivity difference
`between the composites made by the two methods
`increased with increasing TiB2 content. Fig. 8 shows
`that the coeflicient of thermal expansion (CTE) was
`lower for the composites made by the coated filler
`method than the corresponding composites made by
`the admixture method when the TiBz content ex-
`ceeded 15 vol %. As shown in Fig. 9, the electrical
`resistivity of the composites made by the coated filler
`method was slightly lower than that of the corres-
`ponding composites made by the admixture method.
`when the TiB, content exceeded 35 vol %. At a high
`TiB2 content (> 50 vol %). the electrical resistivity of
`the composite made by the admixture method in-
`creased sharply, while the electrical resistivity of the
`composite made by the coated filler method increased
`to a much smaller extent.
`
`Porosity is an important factor which influences the
`thermal conductivity and electrical resistivity. How-
`ever, at low TiB2 contents (<35 vol %). although the
`porosity difference between the composites made by
`the two methods was small. there was still consider-
`able differences in thermal conductivity and electrical
`resistivity between the composites made by the two
`methods. Therefore, porosity alone cannot explain
`these differences. Another possible reason is
`that
`a cleaner or less-contaminated (contaminants such as
`oxides or impurities) interface results in a lower ther-
`mal barrier and lower contact electrical resistivity. and
`this can be provided by using the coated filler method
`rather than the admixture method.
`
`Because porosity has no effect on CTE [19]. the low
`CTE of the composites made by the coated filler
`method compared to that of the composites made by
`the admixture methodimay be due to the stronger
`
`Figure 6 Variation of compressive yield strength with TiB, platelet
`volume fraction in copper-matrix composites made by (O) the
`coated tiller method and (II) the admixture method. The vertical bar
`at each data point is an error bar.
`
`TA BL E II Measured hardness. abrasive wear resistance and scratch resistance of selected Cu/TiBz composites made by the coated filler
`method and the admixture method
`
`_ Composite fabrication method
`
`Coated filler method
`Coated filler method
`Admixture method
`
`
`42 ii
`50 ii
`so _+_l
`TiB1(\'ol°/ti)
`I85 i7
`208 :8
`143 :5
`Hardness (H5)
`74 ii
`42 fl
`100
`Relative wear ("vi
`0.47 i0.0|
`0.41 tom
`0.7(w_:0.()l
`Scratch width (mm)
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`10
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`30
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`40
`TiB,(vo|%)
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`so
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`so
`
`70
`
`Figure 9 Variation of electrical resistivity with TiB, platelet volume
`fraction in copper-matrix composites made by (O) the coated filler
`method and (El) the admixture method. The vertical bar at each
`data point is an error bar.
`
`mined by the net effect of the strains (which are asso-
`ciated with the internal stresses produced by the CTE
`mismatch between elastically accommodated rein-
`forcement and matrix) on the length of the composites
`in a given direction. Under an extreme condition of
`a composite with absolutely no bonding between the
`reinforcement and the matrix, because there is no
`possibility of an internal stress arising, the reinforce-
`ments dispersed in the matrix are akin to pores, and
`thus make no contribution to the low CTE reinforce-
`ment of the CTE of the composite. In contrast, at
`a given reinforcement content, a stronger bond be-
`tween the reinforcement and the matrix gives a lower
`CTE for the composite.
`
`3.4. Comparison with previous copper-
`matrix composites made by the coated
`filler method and other materials
`Table 111 lists the properties of copper-matrix com-
`posites made by the coated filler method in this work
`and in previous work. together with those of two
`alloys [20, 21]. Although Monel alloy has good mech-
`anical properties (with the highest compressive yield
`strength), it suffers from high electrical resistivity and
`low thermal conductivity. (In metals and alloys, a high
`electrical resistivity relates to a low thermal conduct-
`ivity). Therefore, Monel does not meet the require-
`ment for electronic packaging. Kovar alloy has been
`a common electronic packaging material due to its
`low CTE, but its poor electrical and thermal conduc-
`tivities limit its application in high-power and high-
`density microelectronic packaging technology. Com-
`pared to Monel and Kovar alloys, all copper-matrix
`
`400
`
`350
`
`300
`
`250
`
`200
`
`150
`
`100
`
`
`
`
`
`Thermalconductivity(Wm"“C“‘)
`
`50
`
`10
`
`20
`
`30
`
`40
`TiB, (vol %)
`
`50
`
`60
`
`70
`
`Figure 7 Variation of thermal conductivity with TiB3 platelet vol-
`ume fraction in copper-matrix composites made by (C) the coated
`filler method and (El) the admixture method. The Vertical bar at
`each data point is an error bar.
`
`15
`
`14
`
`13
`
`12
`
`11
`
`10
`
`
`
`
`
`
`
`Coefficientofthermalexpansion(10‘°"C“)
`
`8
`
`10
`
`20
`
`30
`
`40
`Tie, (vol =/.)
`
`50
`
`60
`
`70
`
`Figure 8 Variation of coefiicient of thermal expansion with TiB;
`platelet volume fraction in copper-matrix composites made by (O)
`the coated tiller method and (El) the admixture method. The vertical
`bar at each data point is an error bar.
`
`bond between the TiB3 platelet and the copper matrix
`in the composites made by the coated filler method. In
`a metal-matrix composite (with it regid reinforcement
`and a soft metal matrix), the overall CTE is deter-
`
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`TABLE 111 Properties of copper-matrix composites made by the coated filler method and of alloys. all tested identically
`Material
`Filler
`Density
`Hardness
`Compressive
`Electrical
`Thermal
`content
`tgcm")
`(HB)
`yield strength
`resistivity
`conductivity
`(vol”/o)
`(MPa)
`(l0“‘Qcm")
`(Wm" C"‘)
`
`'
`
`CTE
`(10“"’C")
`
`7.3 :0.2
`145 :2
`3.9 :0.1
`647 :18
`193 :8
`9.69 :0.0l
`70° :1
`Cu/Mo“
`10.2 :0.l
`176 :3
`3.4 :0.1
`659 :15
`218 :10
`' 6.78 :0.01
`48“ :1
`Cu/TiB;
`10.2 :0.1
`60 :2
`19.5 :0.7
`651 :18
`260 :12
`5.92 :0.01
`50‘ :1
`Cu/SiC..,"
`12.3 :0.1
`270 :8
`2.4 :0.1
`282 :11
`107 :5
`9.32 :0.01
`30 :1
`Cu/Mo“
`11.7 :0.1
`220 :4
`2.8 :0.l
`442 :17
`148 :5
`7.37 :0.0l
`35 :1
`Cu/T1131
`12.2 :0.1
`174 :3
`7.7 :0.3
`425 :11
`178 :7
`7.00 :0.0l
`33 :1
`Cu/SiC..."
`13.5
`—
`54.4
`730
`233
`—
`—
`Monel“
`5.3
`17
`50
`—
`—
`8.3
`—
`Kovar"
`
`" Mo particle composite from [21].
`“ SiC whisker composite from [20].
`* Volume fraction above which the porosity increased abruptly with increasing volume fraction.
`“ Ni-29 Cu—3 Al alloy.
`° Fe—27 Ni~7 Co alloy.
`
`electrical and thermal conductivities than Cu/SiC...
`composite. and higher hardness and compressive yield
`strength than Cu/Mo composite. Cu/SiC,, composite
`has higher hardness than the other two composites.
`These property variations for different composites at
`low reinforcement contents provide the possibility of
`choosing a suitable composite for a specific application.
`
`3.
`
`4.
`5.
`
`6.
`
`7.
`
`8'
`9.
`
`References
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`
`11.
`
`13.
`
`20.
`21.
`
`composites at any reinforcement content made by the
`coated filler method in this work and previous work
`have higher thermal and electrical conductivities. For
`the Cu/Mo composite at a high molybdenum content
`(70 vol %), its low CTE, relatively high electrical and
`thermal conductivities,
`together with its excellent
`mechanical properties. make it very attractive in ap-
`plications related to electronic packaging, sliding elec-
`trical contacts. motor brushes and resistance welding
`electrodes. At a high SiC whisker content Cu/SiC
`whisker (50 vol % SiC..) composite, because of the
`extraordinarily high SiC whisker content reached by
`using the coated filler method. the composite exhibits
`exceptionally high hardness (even higher than that of
`Monel) and compressive yield strength, compared to
`other metal-matrix composites. At the same time, it
`has higher thermal and electrical conductivities than
`Monel and Kovar. Also its CTE value is lower than
`that of Monel. These properties make Cu/SiC whisker
`(50 vol % SiC..) composite attractive for brushes or
`conductive applications where high hardness. wear res-
`istance. electrical and thermal conductivities and low
`CTE are required. For the Cu TiB3 composite contain-
`ing 48 vol % TiB3 platelets. its hardness is lower than
`that of the Cu/SiC... composite (50 vol % SiC..,) but
`higher
`than
`that
`of
`the Cu/Mo
`composite
`(70 vol °/o M0). the compressive yield strength is com-
`parable to those of Cu/SiC“. and Cu/Mo composites.
`the CTE is higher than that of Cu/Mo. but equal to that
`of Cu.rSiC.. and. most importantly. the electrical resis-
`tivity is lower and thermal conductivity higher than
`both Cu/SiC.,. and Cu/Mo composites. Considering the
`.
`.
`.
`.
`relatively low cost. chemical stability at elevated tem-
`P91311115 and €XCe“€m Wear PC5151’-"106 01 T132 1313191313
`Cu/TiB3 platelet composite at this reinforcement con-
`tent will be attractive in certain situations. such as in
`the applications of electronic packaging, sliding electri-
`cal contacts and motor brushes.
`At low reinforcement contents. Table 111 shows that
`Cu/Mo composite (containing 30 vol % Mo particles)
`has lower electrical resistivity and higher thermal con-
`ductivity than both Cu/SiC... (containing 33 vol % SiC
`whiskers) and Cu:TiB2 (containing 35 vol % TiB2
`platelets) composites. The Cu TiB2 composite has
`a lower CTE than the other two composites. higher
`
`*
`
`Rccewud 24 June
`and accepted 17 September 1996
`
`1709
`
`000007
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`000007