Volume 09 - Metallography and Microstructures Part 13 potx

Copper-lead alloy liner Copper-lead-tin alloy liner High-leaded tin bronze liner Leaded tin bronze liner Lead-tin-copper overlay on copper-lead alloy liner Nickel bronze infiltrated

Fig 13 Same material, thickness, cold reduction, and annealing as Fig 12, then temper rolled (10% additional

reduction) Ferrite grains are slightly elongated; some compression is evident at the surface 3% nital 100×

Fig 14 Same material, thickness, cold reduction, annealing, and temper-rolling reduction as Fig 11, but

heated to 940 °C (1725 °F) and held 2 min The recrystallized ferrite grains are very large 3% nital 100×

Fig 15 Same material, thickness, cold reduction, annealing, and temper-rolling treatment as Fig 13, but

reheated to 940 °C (1725 °F) and held 2 min Structure: large recrystallized ferrite grains 3% nital 100×

Fig 16 3% Si flat-rolled electrical strip (M-22), cold rolled 70% to 0.6 mm (0.025 in.) thick Structure is ferrite

grains elongated in the direction of rolling See also Fig 17 10% nital 100×

Fig 17 Same material, thickness, and cold reduction as Fig 16, decarburization annealed in moist hydrogen at

815 °C (1500 °F) and annealed for grain growth in dry hydrogen at 870 °C (1600 °F) Compare with Fig 18 10% nital 100×

Fig 18 3% Si flat-rolled electrical strip (M-22), cold rolled to ~0.6-mm (0.023 in.) (70% reduction),

decarburization annealed in moist hydrogen at 815 °C (1500 °F), and annealed in dry hydrogen at 925 °C (1700 °F) for grain growth Note large ferrite grain size 10% nital 100×

Fig 19 3.25% Si cold-rolled electrical strip, 0.3 mm (0.011 in.) thick, annealed at 1175 °C (2150 °F), showing

domain structure of (110) [001] oriented material with no applied field The specimen was demagnetized at 60

Hz Domains reveal where the deviation of [001] direction is not more than 3 or 4° from parallel As-polished Magnification not reported

Fig 20 3% Si flat-rolled electrical strip, cold reduced 70% to approximately 0.6 mm (0.025 in.) thick,

decarburization annealed in moist hydrogen at 815 °C (1500 °F) and at 1040 °C (1900 °F) in dry hydrogen for grain growth The ferrite grain size is larger than in Fig 17 and 18 10% nital 100×

Fig 21 3% Si flat-rolled, oriented electrical hot band, as hot rolled Cross section from near the surface (right)

to a depth of approximately 0.5 mm (0.02 in.) Hot finishing temperature was approximately 900 °C (1650 °F) The structure is ferrite grains; note difference in grain shape from near surface to near center 10% nital 100×

Fig 22 3% Si flat-rolled oriented electrical strip, cold reduced 65% and annealed at 925 °C (1700 °F)

Structure: recrystallized ferrite grains The material is ready for final cold reduction and a grain-coarsening anneal See also Fig 23 and 24 for effects of further processing 10% nital 100×

Fig 23 Same material as Fig 22, but given a second cold reduction of 50% to final strip thickness Ferrite

grains are elongated, ready for the grain-coarsening anneal See also Fig 24 10% nital 100×

Fig 24 Same material and reduction as Fig 23 after a grain-coarsening box anneal at 1095 to 1205 °C (2000

to 2200 °F) in hydrogen The upper strip shows portions of two grains; the lower strip, one complete and two partial grains 10% nital 100×

Fig 25 3.25% Si flat-rolled, oriented electrical strip, continuously cold rolled to 0.3 mm (0.011 in.) thick,

annealed at 1175 °C (2150 °F) Macrograph shows secondary recrystallized ferrite grains with cube-on-edge orientation 5 mL HCl, 1 mL HF, and 8 mL H2O Actual size

Fig 26 3% Si steel, 0.35 mm (0.014 in.) thick, gradient annealed by heating to 760 °C (1400 °F) in 1 h, then

at 65 °C (100 °F) per hour to 1065 °C (1950 °F) at hot end (right) in dry hydrogen and held 3 h Secondary grain coarsening started at approximately 815 °C (1500 °F) Grain size is typical 20% HNO3, then 20% HCl (alternated) Actual size

Fig 27 3% Si Steel, hot rolled to 0.35 mm (0.014 in.) thick, annealed 24 h at 1150 °C (2100 °F) in dry

hydrogen Note fine, poorly oriented grains, which are parallel to the rolling direction, in a matrix of normal, well-oriented grains This can result from stringers of inclusions or from rolled-in scale 5% HF, then 10% HNO 3 Actual size

Fig 28 3% Si steel, 0.3 mm (0.012 in.) thick, annealed 7.5 h in dry hydrogen at 1175 °C (2100 °F) Specimen

shows abnormally small grain size, which indicates poor orientation This may be caused by the wrong percentage of cold reduction or by heating too rapidly above 1010 °C (1850 °F) 20% HNO3, then 20% HCI

(alternated) Actual size

Fig 29 Cubic etch pits in cube-on-face grain-oriented 3% Si steel The area seen is a single ferrite grain,

obtained by annealing at 1175 °C (2150 °F) or higher Fe2(SO4)3 1000×

Fig 30 Scanning electron micrograph of cubic etch pits in cube-on-face grain-oriented 3% Si steel The area

shown is a single grain of ferrite See also Fig 29 and 31 Fe2(SO4)3 1000×

Fig 31 Etch Pits in a 3% Si steel, showing two orientations The hexagonal pits have cube-on-corner

orientation; the others have cube-on-edge orientations Fe 2 (SO 4 ) 3 1000×

Fig 32 Thermal faceting and pitting of (100) planes of ferrite in 3% Si grain-oriented steel, cold rolled from 0.3

to 0.1 mm thick and annealed 3 h in dry hydrogen at 1205 °C (2200 °F) Thermal etch in hydrogen at 1205 °C (2200 °F).100×

Fig 33 Solenoid-quality type 430FR ferritic stainless steel Note that some of the ferrite grain boundaries were

not revealed Ralph's reagent 100×

Fig 34 Fe-30Ni cold-rolled strip, batch annealed 6 h at 950 °C (1740 °F) and furnace cooled in dry hydrogen

Grains are coarse because carbon content was low HCl, CuCl2, FeCl3, HNO3, methanol, and H2O 100×

Fig 35 Fig 36

Austenitic Fe-50.5Ni soft magnetic alloy, showing the effects of different etchants Fig 35: etched using

a flat etchant, glyceregia Fig 36: etched using a grain contrast etchant, Marble's reagent Both 100× See Fig 37, 38, 39 for the effects of deformation (cold rolling) and heat treatment on the structure of a similar Fe-50Ni alloy

Fig 37 Fe-50Ni cold-rolled 0.15-mm strip, annealed 2 h in H2 at 900 °C and furnace cooled Structure: primary recrystallized grains of austenite 60 mL ethanol, 15 mL HCl, and 5 g anhydrous FeCl 3 100×

Fig 38 Fe-50Ni cold-rolled 0.03-mm strip, annealed 4 h at 1175 °C (2150 °F) in dry H2 and furnace cooled Structure is nonoriented primary recrystallized grains See also Fig 39 Saturated (NH4)2S2O8 100×

Fig 39 Fe-50Ni cold-rolled strip, 0.03 mm (0.014 in.) thick, annealed same as Fig 38 The structure is

comprised of nonoriented secondary recrystallized grains Thermal etch in hydrogen at 1175 °C (2150 °F) 3×

Fig 40 4-79 Moly Permalloy (4Mo-79Ni-17Fe), cold-rolled strip 0.03 mm (0.014 in.) thick, annealed 4 h in dry

hydrogen at 1175 °C (2150 °F) and cooled at 320 °C (575 °F) per hour to room temperature The structure is coarse-grained austenite HCl, CuCl 2 , FeCl 3 , HNO 3 , methanol, and H 2 O 100×

Fig 41 4-79 Moly Permalloy magnetic test-ring specimen taken from 0.3-mm (0.014-in.) thick cold-rolled

strip, annealed 4.5 h at 1120 °C (2050 °F) in dry hydrogen The structure is austenite grains Marble's reagent 0.875×

Fig 42 Fe-27Co cold-rolled strip, annealed 2 h in dry H2 at 925 °C (1700 °F) and furnace cooled The microstructure is ferrite solid solution HCl, CuCl2, FeCl3, methanol, and H2O 100×

Fig 43 Fe-Co-1.9V alloy, annealed 2 h in wet hydrogen at 885 °C (1625 °F) A duplex ferritic grain structure

Fig 46 Fe80 B 18.3 P 1.7 amorphous metal ribbon, as-cast Complex magnetic domain structure resulting from the residual stress pattern produced by rapid solidification (melt spinning) The image was produced using dark- field illumination and the powder pattern technique, which is referred to as the Bitter method (see the section

of this article on observation of domains for a description of this technique as well as the Faraday effect, Fig 47 and 48, and Kerr effect for observing domain structures) 165× (J.D Livingston)

Fig 47 Gd0.94 Tb 0.75 Er 1.31 Al 0.5 Fe 4.5 O 12 garnet, crystal grown in flux containing PbO and B 2 O 3 at 1300 °C (2370

°F) Magnetic domain structure of 0.05-mm (0.002-in.) platelet cut parallel to the (111) plane Black and white areas represent domains with opposing magnetic vectors normal to the surface As-polished 100×

Fig 48 Same garnet as Fig 47 and produced the same way, but with an applied magnetic field of 150 Oe,

which reduced white domains of Fig 47 to cylinders (seen here as dots) 0.5 μm in diameter The observation technique for this figure and Fig 47 was the Faraday effect (transmitted polarized light) As-polished 100×

Fig 49 Nickel ferrite (Ni0.6Fe2.4O4), prepared from powder precipitated from a solution of NiSO4 and FeSO4, pressed and sintered 4 h at 1250 °C (2280 °F) in N2 The fine grain structure was obtained by adding lithium hydroxide to the solution 1:1:2 HF, HNO 3 , and H 2 O 1000×

Fig 50 Nickel ferrite, same composition as Fig 49, prepared from powder as described in Fig 49, but with

potassium hydroxide added to the solution instead of lithium hydroxide As a result, the grains are much coarser than those in Fig 49 1:1:2 HF, HNO3, and H2O 1000×

Fig 51 Macrostructure of Chromindur II (Fe-28Cr-10.5Co) cup-shaped telephone receiver magnets that were

deep drawn from 1-mm (0.04-in.) thick strip and solution annealed at (left) 980 °C (1795 °F) and (right) 955

°C (1750 °F) Glyceregia See Fig 52 for a higher magnification view of a deep-drawn and solution-annealed Chromindur II structure A transmission electron micrograph of a sparkeroded and electropolished iron- chromium-cobalt specimen is shown in Fig 53 Approximately 2.5×

Fig 52 Chromindur II, deep drawn and solution annealed A ferritic grain structure See also Fig 53

Glyceregia 200×

Fig 53 Thin-foil transmission electron micrograph of Chromindur II The specimen was spark eroded, then

electropolished by the window method A very fine, uniform spinodal structure (the large black particles are σ ase) 20% HClO 4 in methanol at -40 °C (-40 °F) 115,000× (S Jin)

Fig 54 Alnico 5 (Fe-8Al-14Ni-24Co-3Cu), cast with directional grain, annealed 30 min at 925 °C (1700 °F),

cooled in a magnetic field of 1000 Oe minimum, and aged 24 h at 550 °C (1020 °F) Note the pattern and boundaries of directional grains See also Fig 55 and 56 Marble's reagent 30×

Fig 55 Alnico 5, cast with random grains, annealed 30 min at 925 °C (1700 °F) in a magnetic field of 1000 Oe

min, and aged 24 h at 550 °C (1020 °F) Note pattern and boundaries of random equiaxed grains of the α hase matrix Marble's reagent 30×

Fig 56 Replica electron micrograph of an Alnico 5 casting, solution annealed above the Curie temperature,

cooled in a magnetic field, and aged Particles of α ark; vertical orientation here is parallel to magnetic field) in

an α' matrix As-polished 100,000×

Fig 57 Alnico 5, pressed from powder, sintered at 1315 °C (2400 °F), annealed at 1260 °C (2300 °F), cooled

in a magnetic field of 1000 Oe min and aged 24 h at 550 °C (1020 °F) Structure: small equiaxed grains of α

phase Marble's reagent 30×

Fig 58 Alnico 9 (Fe-35Co-15Ni-4Cu-5Ti-7Al), cast with directional grain, annealed 1 h at 1260 °C (2300 °F),

held isothermally 15 min in a magnetic field at 805 °C (1480 °F), then aged by heating to 550 °C (1020 °F) and holding 24 h Structure is pattern and boundaries of directional grains of the α -phase matrix The elongated, needlelike particles scattered through the structure are titanium sulfide Marble's reagent 100×

Microstructure of SmCo5 sintered permanent magnet, as-polished (Fig 59 and 60) and after etching Fig 59: differential interference contrast Fig 60: bright-field illumination Fig 61: bright-field illumination after etching with 3 parts glycerol, 1 part acetic acid, and 1 part HNO3 All 400×

Kerr effect (polarized light) micrographs of SmCo5 permanent magnet (adjacent domains) Fig 62: large Co17Sm2 grains (arrows) Fig 63: dark Co7Sm2 grains continuous with the surrounding Co5Sm As-polished Both 850× (J.D Livingston)

Fig 64 Fig 65

Microstructures of SmCo5 sintered permanent magnets Fig 64: a hyperstoichiometric composition containing

Co7Sm2 (arrows) Fig 65: hypostoichiometric composition containing Co17Sm2 (arrows) Fig 66: a stoichiometric composition with a samarium-rich phase Fig 67: a near-stoichiometric composition showing

near-Co5Sm grain boundaries photographed using differential interference contrast Fig 64, 65, 66: as-polished Fig 67: 10 mL acetic acid, 10 mL H2O, 10 mL HNO3, and 40 mL HCl All 720×

Transmission electron micrographs showing the cellular microstructure of Sm(Co,Cu,Fe)7 permanent magnet

in the peak aged condition (aged 30 min at 850 °C, or 1560 °F) Fig 68: a section normal to the magnetic

alignment direction (c-axis) Fig 69: a section including the alignment direction (c-axis vertical) Cell interiors

show the 17:2 structure; cell boundaries have the 5:1 structure with the phases fully coherent As-polished 160,000×

Fig 70 Anisotropic barium ferrite (BaO · 6Fe2 O 3 ) This hard ferrite was pressed from micronsize barium ferrite powder in a magnetic field and sintered at 1205 °C (2200 °F) to obtain desired magnetic properties See also Fig 71 Marble's reagent 100×

Fig 71 Replica electron micrograph of a fracture surface of isotropic barium ferrite pressed from micron-size

powder and sintered at 1205 °C (2200 °F) to obtain final magnetic properties Crystals are ~1 μm in size, which approximates the dimensions of a single domain As-polished 10,000×

Fig 72 Cunife (Cu-20Ni-20Fe), cold rolled to 80% reduction, then aged 4 h at 565 °C (1050 °F) Specimen is a

longitudinal section; grains are elongated in the direction of rolling Marble's reagent 50×

Electrical Contact Materials: Metallographic Techniques and Microstructures

Introduction

ELECTRICAL CONTACT material specimens are prepared in much the same way as those of most other metals Because electrical contacts are often small, sectioning is more difficult Some electrical contact materials are made of soft metals, some are composites containing metals that vary widely in hardness, and some are composed partly or completely

of noble metals, which are difficult to etch Therefore, certain problems are likely to be encountered in the preparation of specimens, and some techniques of specimen preparation will differ from those for other materials

Specimen Preparation

Mounting. Preparation of unmounted specimens is rarely attempted, because electrical contacts are usually small Bakelite is the mounting material recommended for most applications Lucite is useful as a mounting material, particularly for contacts to be examined after testing, because its transparency allows visibility of the surface adjacent to the cross section For fragile specimens, a cold-mounting material should be selected (see the article "Mounting of Specimens" in this Volume) For maximum edge retention, such as for plated specimens, the specimen should be plated with another metal of contrasting color before it is mounted

Grinding. Electrical contact materials are usually wet ground on a series of silicon carbide papers of successively finer grit sizes through 600 mesh All electrical contact materials are treated the same through this stage of preparation

Polishing. Rough polishing of electrical contact materials is often performed manually using 6- to 9-μm diamond or

5-μm alumina (Al2O3) abrasive on a nylon or silk cloth Vibratory (automatic) polishing can also be used Manual polishing with diamond abrasive usually requires the use of lapping oil, kerosene, or a similar vehicle If Al2O3 is used, it is mixed with water whether polishing is performed manually or by the vibratory method

Rapid rough polishing of a hard contact material (for example, tungsten or a refractory-metal composite such as silver or tungsten-copper) is handled using a medium-speed wheel covered with cotton cloth of very low nap impregnated with diamond paste without fluid extenders Depending on the finish obtained from the 600-grit paper in grinding and on the percentage of tungsten in the specimen, rapid rough polishing can begin with 15- or 9-μm diamond paste and can proceed in steps through 6- and 3-μm pastes Cloths containing the different grades of paste can be reused several times if each is kept in a plastic bag to avoid contamination by the others Specimens must be thoroughly rinsed between polishings to avoid contaminating the subsequent cloth The use of light to medium pressure on the specimen minimizes pullout of particles

tungsten-The most widely used technique for final polishing employs an aqueous slurry of 0.3- or 0.05-μm Al2O3 on a soft cloth of medium to long nap Final polishing is performed manually or by the vibratory method

The harder contact materials can be final polished manually with 1-μm diamond on a short-nap cloth This procedure can also be used for final polishing of the softer metals if a few fine scratches can be tolerated The procedure is particularly useful for final polishing of the silver-cadmium oxide materials, because it produces no flow of the silver matrix to obscure the cadmium oxide particles The advantages of this procedure are its speed in polishing and its avoidance of disturbed metal

A technique for polishing contact materials that contain hard and soft constituents, such as refractory metals with silver or copper, uses vibratory polishing to accomplish rough and final polishing in one operation Specimens are prepared conventionally through the 600-grit grinding, then are vibratory polished in an aqueous slurry of 0.3-μm Al2O3 on nylon cloth for 1 to 6 h With minimum effort, this procedure produces a distortion-free surface on multiconstituent materials The procedure is useful also for polishing specimens of pure tungsten and pure molybdenum, because these metals, when prepared by conventional procedures, usually require alternate etching and final polishing to reveal the true structure

Another procedure for final polishing of tungsten, molybdenum, and tungsten carbide uses an aqueous slurry of 3% potassium ferricyanide (K3Fe(CN)6), 0.5% sodium hydroxide (NaOH), and 5% 0.05-μm Al2O3 on a soft napped cloth

As-Polished Examination. Microscopic examination of electrical contact materials is often accomplished without etching the specimen In many contact materials, the constituents vary so widely in hardness that one metal will polish in

relief, revealing the distribution of one metal in the other Tungsten compacts infiltrated with silver are often examined without having been etched; other typical examples are copper-tungsten and copper-graphite mixtures as well as silver-graphite, silver-cadmium oxide, and silver-nickel combinations These materials often are examined in the as-polished (not etched) condition and are subsequently etched for further examination Numerous examples of as-polished specimens can be found in the series of representative micrographs at the end of this article

Chemical Etching. The solutions used for etching specimens of electrical contact materials are listed in Table 1 Although these etchants have proved successful, all are not used equally A specific metal or combination of metals often can be etched successfully with two or more different etchants Similarly, a specific etchant is frequently used for two or more different materials

Table 1 Etchants and etching procedures

1 20 mL NH 4 OH, 10-20 mL H 2 O 2 (30%), 10-20 mL H 2 O Swab at room temperature, 3-10 s; use fresh; more water, less

H 2 O 2 for copper alloys and conversely for silver alloys

2 2 g K 2 Cr 2 O 7 , 1.5 g NaCl, 8 mL H 2 SO 4 (conc), 100 mL H 2 O Swab at room temperature, 5-10 s; good for etching

hard-to-etch copper alloys

3 50 mL NH 4 OH, 10-30 mL H 2 O 2 (30%) Swab at room temperature for 3-10 s; use fresh

5 A: 100 mL saturated aqueous solution of K 2 Cr 2 O 7 , 2 mL saturated

aqueous solution of NaCl (sodium chloride), 10 mL H 2 SO 4

B: 1 part solution A, 10 parts H 2 O

C: 98 mL H 2 O, 3 g CrO 3 , 2 mL H 2 SO 4

Use solutions of A, B, then C; swab at room temperature for 15-20 s with each solution; rinse in water between solutions

6 20 g CrO 3 , 4.5 g NH 4 Cl (ammonium chloride), 18 mL HNO 3

(conc), 15 mL H 2 SO 4 (conc), H 2 O to make 1/2 L (Waterbury

8 20 mL HNO 3 (conc), 20 mL acetic acid (glacial), 20 mL glycerol Swab at 38-42 °C (100-108 °F) for 3-10 s

9 0.2% CrO 3 and 0.2% H 2 SO 4 , in H 2 O Swab for 1 min

13 20 mL KCN (10%), 20 mL (NH 4 ) 2 S 2 O 8 (10%) Use in a hood; immerse at room temperature for 10-30 s

14 A: 5% nital etch

B: 5% FeCl 3 in methanol

Immerse specimen alternately in A and B

15 10 mL HNO 3 , 20 mL HCl, 10 mL glycerol Swab at room temperature for 3-10 s

16 30 mL HCl, 10 mL H 2 O Electrolytic; up to 5 V dc; 1.5 A/cm2 (9.7 A/in.2); room

76, 77 in the section "Atlas of Microstructures for Electrical Contact Materials" in this article)

Two etchants can sometimes be used consecutively, as in preparing the specimen for Fig 20 This specimen of 30W was etched first in a K3Fe(CN)6 + NaOH solution, then in an NH4OH + H2O2 solution Dual etching often is used for composites of two or more different metals A typical specimen etched in this way is a 90Ag-10CdO contact clad with silver that was brazed to a brass (Fig 36) A section of the joint was etched first with a nitric acid plus potassium dichromate (HNO3 + K2Cr2O7) solution, then with a chromic acid plus sodium sulfate (CrO3 + Na2SO4) solution It is sometimes advantageous to use two etchants alternately through two or more cycles This practice revealed the structure

70Cu-of a gold-plated nickel-iron alloy (Fig 93, 94, 95, 96)

Unalloyed palladium and palladium alloys (50 to 95% Pd) are most often etched in a mixture of potassium cyanide (KCN) and ammonium persulfate [(NH4)2S2O8] (etchant 13 in Table 1) A mixture of nitric acid (HNO3), hydrochloric acid (HCl), and glycerol (etchant 15 in Table 1) has proved useful for distinguishing the rhodium plate from the gold underplate on a cross section of plated nickel-iron alloy (Fig 111) An etchant composed of HNO3 and acetic acid (etchant 12 in Table 1) is often used for gold-base alloys and sometimes for palladium when welded to nickel silver (Fig

99 and 104)

Electrolytic etching is not necessary or appropriate for etching most electrical contact materials Exceptions are platinum-base alloys, such as platinum-ruthenium and platinum-iridium alloys Electrolytic etching is also used for etching contact materials that contain substantial amounts of platinum, such as the 35Pd-10Pt-10Au-30Ag-14Cu-1Zn alloy shown in Fig 109 in the section "Atlas of Microstructures for Electrical Contact Materials" in this article

An electrolyte that has proved successful for etching platinum alloys is a mixture of HCl and water (etchant 16 in Table 1) Also suitable is a 5% solution of sodium cyanide (NaCN) in water A 5 to 7% aqueous solution of KCN has been used for electrolytic etching of gold and silver plates Cyanide-containing etchants must be used under a hood Electrolytic etching is usually carried out at room temperature, using up to 5 V dc at a current density of about 1.5 A/cm2 (9.7 A/in.2) for 1 to 3 min

Microstructures of Electrical Contact Materials

The electrical contact materials depicted in this article include copper-, silver-, tungsten-, tungsten-carbide-, and molybdenum-base materials, as well as precious metals other than silver In form, these materials encompass unalloyed metals; bimetals; alloys; mixtures of metals; mixtures of a metal with a metalloid, a metal oxide, or a metal carbide; and electroplated overlays Among the processes used to produce or prepare these materials are melting and casting, powder metallurgy, cold drawing, mechanical bonding, and electroplating

The various electrical contact materials are selected for diverse service requirements For a discussion of the selection

criteria and properties, see the article "Electrical Contact Materials" in Properties and Selection: Nonferrous Alloys and

Special-Purpose Materials, Volume 2 of ASM Handbook, formerly 10th Edition Metals Handbook Detailed information

on electrical contacts produced by powder metallurgy techniques can be found in the article "Powder Metallurgy

Electrical Contact Materials" in Powder Metal Technologies and Applications, Volume 7 of the ASM Handbook The

usefulness of an electrical contact material depends on its electrical characteristics and mechanical properties, which are closely related to composition and microstructure

Copper-Base Materials. Unalloyed copper aluminum bronzes, brasses, and other copper-base materials are widely used as contact materials because of their high electrical and thermal conductivities, low cost, and ease of fabrication Unalloyed copper, bronze, and brass contacts are prepared from wrought products or by powder metallurgy techniques Graphite or tungsten are added to powder metallurgy contacts to resist sticking or welding; tungsten also contributes to wear resistance These additions are visible in the microstructures of mixtures containing either element

Binary age-hardenable alloys of copper-chromium and copper-beryllium are more resistant to thermal softening than unalloyed copper (see the article "Beryllium-Copper and Beryllium-Nickel Alloys" in this Volume) Several ternary and quaternary alloys, including those that contain cobalt and beryllium, or cobalt, cadmium, and silicon, are age hardenable

In these alloys, the precipitate responsible for hardening is usually observable in the microstructure

Silver-Base Materials. Silver, which has the highest electrical and thermal conductivity of all metals, is a widely used contact material; it is also used as a plated, brazed, or mechanically bonded overlay on other contact materials notably, copper and copper-base materials

Several binary and ternary alloys provide special properties Silver-copper alloys combine good electrical characteristics with a higher hardness than that of unalloyed silver; silver-cadmium alloys provide improved arc-quenching characteristics The ternary alloys silver-copper-nickel and silver-cadmium-nickel offer improved erosion resistance as well as other special properties

Another class of silver-base compositions contains semirefractory constituents, such as cadmium oxide, magnesium oxide, and graphite, that are made by powder metallurgy techniques However, additions of cadmium oxide or magnesium oxide can also be made by preparing binary alloys of silver and cadmium or of silver and magnesium, then converting the cadmium or magnesium into an oxide by internal oxidation In general, the semirefractory constituents promote nonsticking qualities or provide increased resistance to wear Most of these constituents are observable in the microstructure

Silver-base contact materials for switchgear contain tungsten or molybdenum, or tungsten carbide These refractory-metal constituents, which are readily identifiable in the microstructure, do not form solid solutions with silver Contacts for switchgear are made by powder metallurgy

Tungsten-Base and Molybdenum-Base Materials. Tungsten-base materials are used in contacts for low-current applications that require exceptional resistance to arcing, welding, and sticking These contacts are made by powder metallurgy techniques and have relatively simple microstructures in which the individual grains of tungsten are clearly defined In the tungsten-base contact materials that contain silver or copper, these elements appear as a light-etching constituent surrounding tungsten grains Those materials that contain small amounts of nickel exhibit a nickel precipitate, particularly at grain boundaries

Unalloyed molybdenum and molybdenum-base materials are also prepared by powder metallurgy techniques The micrographs included in this section indicate that the microstructure of unalloyed molybdenum is analogous to that of unalloyed tungsten and that the microstructure of a molybdenum-silver material is similar to that of a tungsten-silver material

Precious Metals. Several of the unalloyed precious metals, precious-metal alloys, and electrodeposited overlays of precious metals for which micrographs are shown in this article are high-purity, single-phase materials Minor insoluble impurities, if present, usually deposit at grain boundaries and, upon etching, will define individual grains

Microstructures can be enhanced by metallography employing the differential-interference contrast technique This technique, using polarized light, is based on the interference of two sets of light waves produced in a birefringent quartz prism above the microscope objective These beams may be shifted in phase by surface details of the specimen Surface

details not detectable in bright field or dark field assume noticeable contrast as a result of the phase shift Additional information on differential-interference contrast is available in the articles "Optical Microscopy" and "Color Metallography" in this Volume

Some of the single-phase alloys and unalloyed precious metals depicted in this article exhibit well-defined grain structures as well as grain deformation and slip planes resulting from cold work (Fig 101, 102, 103 in the section "Atlas

of Microstructures for Electrical Contact Materials" in this article) Response to recrystallization after cold working can also be shown in the microstructure of a single-phase alloy (Fig 108) The constituents in two-phase alloys can be well-defined (Fig 107) Multiple plated coatings can usually be clearly distinguished by differences in color, texture, or both (Fig 111)

Sleeve Bearing Materials: Metallographic Techniques and Microstructures

Milton W Toaz, Senior Scientist, Imperial Clevite

Introduction

SLEEVE BEARINGS usually consist of one or more layers of a comparatively soft bearing alloy(s), or liner, bonded to a relatively thick steel backing Therefore, they require metallographic preparation techniques that differ somewhat from those used for singular metals and alloys

Acknowledgements

The author wishes to thank the following individuals for their assistance: K Summerton, Plant Metallurgist, Imperial Clevite; J Rigler, Senior Metallographer, Imperial Clevite Technology Center; W.A Yahraus, Manager, Product Analysis and Field Engineering, Imperial Clevite; and A Blazy, Retired Senior Metallographer, Imperial Clevite

4 in.) for heavy-wall liners

When sampling a piece of bearing material to produce a specimen, the thickness of the steel backing should be minimized

in proportion to that of the bearing alloy A thick steel layer increases the probability that a relief will be produced during polishing, because the softer bearing liner will be polished away more rapidly As a result, the bearing alloy and the steel will not be viewed in the same plane, and part of the structure shown will appear out of focus

Initial cutting can be performed with a handsaw, power saw, or cutoff wheel Cutting should begin at the bearing alloy and proceed through the steel, or it should begin with the bearing alloy/steel interface parallel to the plane of cutting and proceed through the sleeve bearing Cutting should never take place from the steel into the bearing alloy If the initial cutting severely distorts the bearing alloy, the disturbed metal should be removed by wet abrasive belt grinding The finishing cut should also proceed from the bearing alloy into the steel backing

Mounting. Specimens of sleeve bearing materials are mounted in much the same way as specimens of other metals For additional information, see the article "Mounting of Specimens" in this Volume It is often convenient to mount several small specimens together Each specimen should then be positioned so that the bearing liner faces the same direction

Thermosetting polymers are frequently used as mounting materials They are unsuitable, however, for examining a bearing alloy for internal voids, which would likely be collapsed by the time-temperature-pressure combination used Such examinations require a cold-mounting material, such as a self-curing acrylic or an epoxy liquid A short vacuum treatment applied immediately after pouring the mounting liquid over the specimen will eliminate as much of the trapped air as possible and will improve penetration of the cold-mounting liquid

Grinding is performed first on an abrasive belt flooded with water or on a low-speed disk grinder (approximately 500 rpm) using water-cooled 180-grit silicon carbide abrasive The mount is ground next with 240-, 320-, 400-, then 600-grit silicon carbide papers placed on a flat marble pedestal, a plate-glass surface, or a rotating wheel The grinding is preferably performed wet, with thorough washings of the mount between abrasives As an alternative, grinding may be performed successively with grades 1, 0, 00, and 000 alumina (Al2O3) paper using frequent applications of kerosene as a lubricant; the mount should be thoroughly rinsed with kerosene between grindings

Grinding should begin at the bearing alloy and proceed toward the steel backing, or it should begin with the bearing alloy/steel interface parallel to the direction of grinding and proceed along the specimen Frequent washings of the abrasive paper with water or kerosene minimize contamination of the bearing alloy by abrasive particles or other debris

Polishing usually begins on a 500- to 600-rpm wheel covered with nylon, to which a coating of 6- to 10-μm diamond paste has been applied Polishing should continue with firm pressure for approximately 5 min or until the scratches and disturbed metal from grinding have been removed The mount should then be thoroughly washed to remove all the diamond abrasive

Intermediate polishing is performed on a 300-rpm wheel using 0.3-μm Al2O3 abrasive and a medium-nap cloth The abrasive is mixed with distilled water before polishing to form a suspension that should be applied frequently to the cloth during polishing The mount should be polished for 1 min using light pressure Additional distilled water is then added to remove most of the Al2O3 abrasive After the mount is removed from the wheel, the polished surface should be thoroughly and alternately rinsed in cold and hot water Excess water should be wiped away with a damp cotton square while the surface is exposed to a dry air blast

The intermediate polish often yields a surface that is suitable for routine examination However, a final step using a rpm wheel with a medium-nap cloth will produce a perfect polish The cloth is moistened with distilled water, then dusted with magnesium oxide abrasive, which is worked into a paste The mount should be polished for several minutes using heavy pressure The wheel is then flooded with distilled water, and the pressure is reduced to complete the polish The mount should be thoroughly rinsed with cold, then hot, water and wiped with a moist cotton square while the surface is exposed to a dry air blast

300-Certain polishing techniques that are standard for most singular materials are not always suited to bimetal or trimetal sleeve bearing structures For sleeve bearing materials, a circular rotation of the mount should be avoided Instead, the mount should be moved laterally across the wheel with the specimen oriented such that polishing proceeds from the steel toward the bearing alloy (the opposite of the orientation used in grinding) This procedure lessens the tendency of the bearing alloy to polish away at a faster rate than the steel, producing a relief or step at the interface between the two layers

A relief, if produced, is visible under low magnification as a dark line at the interface between the steel backing and the bearing alloy This line could easily be misinterpreted as a bond defect However, examination at high magnification will reveal the true nature of the bond line Minimizing the ratio of steel backing to bearing alloy, keeping the specimen size small, and following the instructions on mount orientation during polishing will help minimize bond-line relief

Etching of sleeve bearing materials for metallographic examination is usually performed by immersion, although it is sometimes desirable to swab the surface with a cotton square saturated with the etchant just before the rinse Immediately following removal from the etching reagent, the mount should be thoroughly rinsed in cold, then hot, water, and excess moisture should be removed with a damp cotton square while the surface is exposed to a dry air blast Rinsing or wiping the surface of a mount with alcohol will promote staining, especially on aluminum bearing alloys The most common etchants and some of the sleeve bearing alloys to which they best apply are listed in Table 1

Table 1 Etchants for microscopic examination of sleeve bearing materials

Etchant Bearing material

Copper-lead alloy liner

Copper-lead-tin alloy liner

High-leaded tin bronze liner

Leaded tin bronze liner

Lead-tin-copper overlay on copper-lead alloy liner

Nickel bronze infiltrated with lead-base babbitt

Nickel-tin bronze infiltrated with lead-base babbitt

Silver electroplate on steel

Silver-lead alloy electroplate on steel

Tin-base babbitt overlay on copper-lead-tin alloy liner

Tin bronze infiltrated with lead-base babbitt

Tin bronze infiltrated with Teflon

H 2 O 2

Trimetal bearing: lead-tin-copper electroplated overlay, brass electroplated barrier, copper-lead alloy

Aluminum alloy clad to steel

Aluminum-silicon alloy clad to steel

High-tin aluminum alloy clad with unalloyed aluminum

Lead-tin-copper overlay on aluminum alloy liner

Low-tin aluminum alloy clad to steel 0.5% HF

Trimetal bearing: lead-tin-copper electroplated overlay, copper electroplated barrier, aluminum-silicon-cadmium alloy

5% nital High-tin aluminum alloy clad to nickel-plated steel

Lead-base babbitt liner

Tin-base babbitt liner

Steel backing of any bearing alloy

Keller's reagent Lead-tin-copper overlay on aluminum-cadmium alloy

(a) Equal parts of concentrated NH4OH and water with 2-4 drops of H2O2 (30%) per 10 mL of solution

Microstructures of Sleeve Bearing Materials

Sleeve bearing materials depicted in micrographs in this article are identified and nominal compositions of the materials are given in Table 2

Table 2 Chemical compositions of sleeve bearing alloys

6, 7, and 8 SAE 13 rem 5.0-7.0 9.0-11.0 0.7 0.25 0.10 0.005 0.005 0.05 0.02

3, 4, and 5 SAE 14 rem 9.0-11.0 14.0-16.0 0.7 0.6 0.10 0.005 0.005 0.05 0.02

9, 10, and 11 SAE 15 rem 0.9-1.7 13.5-15.5 0.7 0.8-1.2 0.10 0.005 0.005 0.02 0.02

40 SAE 16(b) rem 3.5-4.7 3.0-4.0 0.10 0.05 0.10 0.005 0.005 0.005 0.40

designation

Composition, % (a)

designation

Pb Sn Cu In Others (total)

Plated overlay

36, and 37 SAE 191 rem 8.0-12.0 0.5

42, 43, 44, 45, 46, 51, 52, 53, 54, 55, 56, and 57 SAE 192 rem 8.0-12.0 1.0-3.0 0.5

38, and 39 SAE 193 rem 16.0-20.0 2.0-3.0 0.5

83, and 84 SAE 194 rem 5.0-10.0 0.5

19, 20, 30, and 31 SAE 792 77.0 min 9.0-11.0 9.0-11.0 0.7 0.50 0.50 0.7 0.40

17, and 18 SAE 793 83.0 min 7.0-9.0 3.5-4.5 0.5(h) 0.50 0.50 0.7 0.30

88, and 89 AMS 4815 Unalloyed

90, and 91 99.3-99.7 0.3-0.7 0.30

92, and 93 97.5-98.5 1.7-2.3 0.30

(a) All values not given as ranges are maximum except as shown otherwise

(b) SAE 16 is cast into and on a porous sintered matrix (usually copper-nickel bonded to steel) The surface layer is 0.025 to 0.13-mm (0.001 to 0.005-in.) thick

(c) A corrosion-resistant overlay, such as SAE alloys 191 to 194, may be used with SAE alloys 48, 480, and 481 and is recommended for SAE

49

(d) SAE 781 contains a magnesium content specified at 0.05-0.15%

(e) A 0.15% maximum zinc content is permissible within this range

(f) A modification of this alloy contains 0.05-0.15% Cu, 0.2-0.4% Mn, and 0.05-0.15% Mg (see Fig 83 and 84)

(g) SAE 791 is similar to SAE CA544

(h) The maximum zinc content may be raised to 3.0% upon agreement between purchaser and supplier

(i) Compositions listed are those of a sintered grid prior to infiltration with babbitt or Teflon

Materials used as solid bearings (no backing) include wrought bronzes and aluminum alloys, microstructures of which are shown in this article These and other bearing materials are usually used in thinner sections as bearing liners, as described below

Bimetal Bearings. Bearing materials are often bonded to a backing of stronger material such as steel to form a bimetal bearing with increased load-carrying capacity The bonded layer of the bearing material can be thinner than 0.13 mm (0.005 in.) Babbitts, copper-lead alloys, and leaded tin bronzes are often bonded to steel backing by such processes as continuous gravity casting onto a steel strip as well as static gravity and centrifugal casting against the inside surface of a cylindrical shell

Aluminum alloys are generally clad to a steel backing by warm rolling To facilitate bonding, the steel is roughened by belt sanding or grit blasting and is sometimes electroplated with a thin layer of nickel before being clad with the aluminum alloy (the resulting bearing is still referred to as "bimetal")

Other aluminum bearing alloy liners are bonded using an unalloyed aluminum layer This layer may be formed as part of the bearing alloy fabrication process (as is the case with the powder rolled Al-8Pb-4Si-1Sn-1Cu alloy), or it may be established during fabrication of the bearing alloy strip prior to its cladding to steel (as is often true for SAE 783 alloy) Silver and silver alloy liners can also be deposited onto a steel backing A layer of unalloyed silver is deposited by electroplating, followed by a layer of alloyed silver

Sintered liners for sleeve bearings are made from prealloyed powders of copper-lead alloys or high-leaded tin bronzes

by spreading the powder uniformly on a continuously moving steel strip that passes through a sintering furnace with a reducing atmosphere The particles of powder become sintered together, forming an open grid bonded to the steel strip This bimetal is then rolled to compact the liner and resintered to improve the bond strength However, aluminum alloy powder for liners is usually roll compacted and sintered before being roll clad to the steel backing strip

Liners are also made by infiltration of a lower melting material into a layer of sintered copper or copper alloy powder The powder layer, usually a copper alloy, is not compacted after sintering; the open grid of the sintered powder layer is infiltrated with molten material having a lower melting temperature than that of the grid alloy This infiltrant is often lead

or a lead alloy, but it may be a nonmetallic material such as Teflon Detailed information on sintering, infiltration, and roll

compaction of metal powders can be found in Powder Metal Technologies and Applications, Volume 7 of the ASM

Handbook

Trimetal Bearings. The fatigue strength of babbitt or lead-tin alloy can be increased significantly by reducing the thickness of the material to 0.013 to 0.05 mm (0.0005 to 0.0020 in.) Such layers are typically produced by electrodeposition Under severe operating conditions, these layers may easily wear through; if the backing is steel, seizure can result To avoid seizure, an interlayer of strong bearing material is placed between the steel backing and the babbitt surface layer

During the operation of some trimetal bearings, the temperature may rise enough to cause the diffusion of tin from overlays of lead-tin alloys into the intermediate liners of copper alloy, with resulting deterioration of the bearings To prevent this diffusion, the liner material of such bearings is first electroplated with a thin barrier layer of brass or nickel (the resulting bearing is still referred to as "trimetal") Aluminum alloy liners are given a zinc immersion coating (zincate), then a thin electroplated layer of copper or nickel in preparation for electrodeposition of the overlay Thin overlays can also be cast in place by using an excess amount of the lower melting material when infiltrating sintered liners as described above

Sleeve Bearing Materials: Metallographic Techniques and Microstructures

Milton W Toaz, Senior Scientist, Imperial Clevite

Atlas of Microstructures for Sleeve Bearing Materials

Fig 1 Tin-base babbitt liner (SAE 12), continuously cast on steel backing strip (bottom) White second-phase

particles (see Fig 2); matrix of tin saturated with copper and antimony Nital 100×

Fig 2 Same as Fig 1, except at higher magnification, which reveals starlike arrays of needles of copper-rich

constituent and small, round particles of precipitated antimony-tin Nital 500×

Fig 3 Lead-base babbitt liner (SAE 14), continuously cast on steel backing strip (bottom) White particles of

antimony-tin in a dark matrix of lead-rich solid solution See also Fig 4 Nital 100×

Fig 4 Same as Fig 3, except at higher magnification, which reveals that the white antimony-tin compound is

in the form of cuboid-shaped primary crystals and small eutectic particles See also Fig 5 Nital 500×

Fig 5 Same as Fig 3, but centrifugally cast against inside wall of cylindrical steel shell (bottom) Primary

crystals of antimony-tin segregated away from bond between babbitt and steel backing Nital 50×

Fig 6 Lead-base babbitt liner (SAE 13), continuously cast on steel backing strip (bottom) Dark primary

crystals of lead in a light matrix of antimony-tin and lead See also Fig 7 Nital 100×

Fig 7 Same as Fig 6, except at higher magnification, which reveals that the configuration of the lead dendrites

and the structure of the eutectic-like matrix of antimony-tin and lead Compare with Fig 8 Nital 500×

Fig 8 Same as Fig 7, except annealed to increase formability, which changed the microstructure to white

crystals of eutectic antimony-tin in a dark matrix of lead-rich solid solution Nital 500×

Fig 9 Lead-base babbitt liner (SAE 15), continuously cast on steel backing strip (bottom) Antimony-arsenic

phase (white) in dark matrix of lead-rich solid solution See also Fig 10 Nital 100×

Fig 10 Same as Fig 9, except at higher magnification showing the white particles of antimony-arsenic phase

to be of two types: small eutectic particles and large primary crystals Compare with Fig 11 Nital 500×

Fig 11 Same as Fig 10, except that improper casting conditions have caused the primary antimony-arsenic

crystals to form undesirable starlike patterns of needles, causing a decrease in formability Nital 500×

Fig 12 Copper-lead alloy liner (SAE 49), gravity cast against inside wall of cylindrical steel shell (bottom)

Copper dendrites (light) in a lead matrix (dark) 4% picral, 2% nital 200×

Fig 13 Copper-lead alloy liner (SAE 49), gravity cast against inside wall of cylindrical steel shell (bottom)

Coarse copper dendrites (light) in a matrix of lead (dark) Compare with Fig 14 NH4OH + H2O2 100×

Fig 14 Same as Fig 13, except the copper-lead alloy was continuously cast on a steel backing strip (bottom),

which resulted in faster cooling and thus produced finer dendrites of copper NH 4 OH + H 2 O 2 100×

Fig 15 Copper-lead alloy liner (SAE 48), gravity cast against inner wall of cylindrical steel shell (bottom)

Coarse copper dendrites, blunted by addition of silver, in a continuous matrix of lead Compare with Fig 16

NH4OH + H2O2 100×

Fig 16 Same as Fig 15, except the copper-lead alloy liner was continuously cast on a steel backing strip

(bottom of micrograph), which resulted in a faster cooling rate and thus produced finer dendrites of copper

NH4OH + H2O2 100×

Fig 17 Leaded tin bronze liner (SAE 793), continuously cast on steel backing strip (bottom) Cored, fine

dendrites of copper solid solution; particles of lead Compare with Fig 18 NH4OH + H2O2 100×

Fig 18 Same as Fig 17, except the strip casting was cold rolled and annealed, which produced globular

particles of lead (black) and small equiaxed grains of solid solution of tin and zinc in copper NH 4 OH + H 2 O 2 100×

Fig 19 Leaded tin bronze liner (SAE 792), gravity cast against inner wall of cylindrical steel shell (bottom)

Coarse dendrites of copper solid solution; particles of lead Compare with Fig 20 NH 4 OH + H 2 O 2 100×

Fig 20 Same as Fig 19, except the liner was centrifugally cast instead of gravity cast Globular particles of

lead (black); cored, fragmented dendrites of solid solution of tin in copper NH 4 OH + H 2 O 2 100×

Fig 21 Leaded tin bronze (SAE 791) strip, cold rolled and annealed Globular particles of lead (black) and

small, equiaxed, recrystallized grains of solid solution of tin and zinc in copper See also Fig 22 NH4OH + H2O2 100×

Fig 22 Commercial bronze (SAE 795) strip, hot rolled Grains are larger than those in Fig 21 Absence of lead

increases fatigue resistance, but also increases surface sensitivity (susceptibility to seizure) NH4OH + H2O2 100×

Fig 23 High-leaded tin bronze liner (AMS 4825, 74Cu-16Pb-10Sn), gravity cast against inside surface of

cylindrical steel shell (bottom) Cored, coarse dendrites of solid solution of tin in copper; interdendritic particles

of lead (black) NH 4 OH + H 2 O 2 100×

Fig 24 High-leaded tin bronze liner (SAE 794), gravity cast against inside wall of cylindrical steel shell

(bottom) Same structure as in Fig 23except for more interdendritic particles of lead See also Fig 25 NH4OH + H 2 O 2 100×

Fig 25 Same as Fig 24, except the liner was continuously cast on steel backing strip (bottom), resulting in

faster cooling, which reduced the coarseness of the cored, columnar dendrites of copper solid solution See also Fig 26 NH4OH + H2O2 100×

Fig 26 Same as Fig 25, except the strip casting was subsequently cold rolled and annealed, producing a

structure consisting of small, equiaxed grains of copper solid solution and an irregular dispersion of lead particles NH 4 OH + H 2 O 2 100×

Fig 27 High leaded tin bronze liner (SAE 49), gravity cast against inside surface of cylindrical steel shell

(bottom) Cored dendrites of solid solution of tin in copper; interdendritic particles of lead Compare with Fig

28 NH4OH + H2O2 100×

Fig 28 Same as Fig 27, except the liner was continuously cast on the steel backing strip (bottom), resulting in

more rapid cooling, which reduced the coarseness of the cored, columnar dendrites of copper solid solution

NH4OH + H2O2 100×

Fig 29 Leaded tin bronze liner (SAE 793); prealloyed powder, sintered on a steel backing strip (bottom), cold

rolled, and resintered Copper solid solution; intergranular lead (black) NH4OH + H2O2 100×

Fig 30 Leaded tin bronze liner (SAE 792); prealloyed powder, processed same as Fig 29, resulting in the

same structure, but with more intergranular lead (black) See also Fig 31 NH 4 OH + H 2 O 2 100×

Fig 31 Same as Fig 30, except at higher magnification, which reveals the details of the structure of the

bonded particles of copper powder and the intergranular particles of lead NH 4 OH + H 2 O 2 500×

Fig 32 High-leaded tin bronze liner (SAE 794); prealloyed powder, processed same as Fig 29 and 30,

resulting in the same structure but with still more intergranular lead (black) NH 4 OH + H 2 O 2 100×

Fig 33 Copper-lead alloy liner (SAE 49); prealloyed powder, sintered on a steel backing strip (bottom), cold

rolled, and resintered Unalloyed copper and intergranular lead (black) NH 4 OH + H 2 O 2 100×