In Part 5, we discussed casting, powder- metallurgy, and roll bonding processes for manufacturing bearing- material systems in single and multilayer types. Now we cover electroplating processes, and we discuss tin alloys, lead alloys, overlays, and copper alloys.
Electroplating processes
Plated overlays. Lead alloy surface layers (overlays) whose thickness must be no more than about 0.002 in. (Table 3, classes 4 to 12) are most commonly produced by electroplating the lead alloy on finish-machined bimetallic bearings. Special plating racks ensure uniform distribution of plating current over the bearing surface. Often, with close current control, important dimensions can be maintained so precisely that no machining of the electrodeposited alloy surface is needed. One manufacturer has developed a process in which the lead alloy electroplating is applied continuously to precision-rolled bimetal strip. Here, all forming and machining follows electroplating.
Electroplated lead babbitts comprise both binary lead-tin and ternary lead-tincopper compositions, all of which are commercially codeposited from fluorate electrolytes. To prevent bond and plate defects, extreme care is needed in preparing the basis metal.
Besides cleaning and etching, preplating basis-metal preparation usually includes deposition of one or more thin metallic interlayers. A thin layer of nickel is most frequently used over copper-lead alloys and bronzes to prevent tin diffusion from the plated surface layer into the copper basis metal. Copper is most often used over aluminum alloys to ensure full adhesion of the plated lead alloy layer, and nickel sometimes is plated over the copper to prevent tin diffusion from the lead alloy layer into the copper layer.
Binary lead-indium alloy overlays are also used with copper-lead and leaded bronze intermediate layers. These alloys are produced by electroplating separate layers of pure lead and pure indium and subsequently diffusing indium into the lead in a low-temperature heat treatment. Here, no diffusion barrier is needed between overlay and intermediate alloy layer.
Plated silver intermediate layers. Pure silver and silver-lead alloy bearing liners are applied to steel shells by electrodeposition from cyanide plating baths. Usually, final machining comes after plating, leaving a substantially thick layer, (typically 0.010 to 0.015 in.), of bonded silver liner material. Although as-plated thickness tolerances are not critical, special racking and masking techniques restrict plating to surfaces where it is required and to eliminate local high current densities. If the plated layer structure is to be uniform and the bond strength of the liner uniformly high, the steel basis metal must be prepared carefully, and plating-bath compositions and cleanness must be properly controlled. The principles of silver plating of bearing liners are the same as for decorative silver plating. However, unusually thick deposits (normally more than 0.020 in.) and extremely high quality requirements for bond and plated-metal soundness have spawned several unique operating and control practices.
Tin alloys
Tin-base bearing materials (babbitts) are alloys of tin, antimony, and copper that contain limited amounts of zinc, aluminum, arsenic, bismuth, and iron. Zinc in these bearing metals generally is not favored. Arsenic increases resistance to deformation at all temperatures; zinc has a similar effect at 100 F, but causes little or no change at room temperature. Zinc has a marked effect on microstructures of some of these alloys. Small quantities of aluminum — even less than 1% — modify microstructures. Bismuth is objectionable because, in combination with tin, it forms a eutectic that melts at 279 F. Above this eutectic temperature, alloy strength decreases appreciably.
Bulk mechanical properties of ASTM grades 1 to 3 are available in “Friction and Wear of Sliding Bearing Materials,” ASM HANDBOOK: Friction, Lubrication and Wear Technology. These properties have some value for initial materials screening comparisons among alloys. However, they are not reliable predictors of performance of thin layers bonded to strong backings; which is how most tinbase babbitts are used in modern bearing practice. Layer thickness effects in Figure 4 and temperature effects in Figure 6 are more important practical considerations than mechanical-property differences among various alloy compositions.
Compared with most other bearing materials, tin alloys have low fatigue resistance but strength is enough to warrant use at low loads. They are easy to bond and handle and have excellent antiseizure qualities.
The alloys vary in microstructure in accordance with composition. Alloys containing about 0.5 to 8% Cu and less than about 8% Sb are characterized by a solid-solution matrix in which are distributed needles of a copper-rich constituent and fine, rounded particles of precipitated SbSn. The proportion of the copper-rich constituent increases with copper content. Alloys containing about 0.5 to 8% Cu and more than about 8% Sb exhibit primary cuboids of SbSn, and needles of the copper-rich constituent in the solid-solution matrix. In alloys with about 8% Sb and 5 to 8% Cu, rapid cooling suppresses cuboid formation. This is especially so of alloys with lower copper percentages.
Lead alloys
Lead-base bearing materials (lead babbitts) are alloys of lead, tin, antimony, and in many cases arsenic. Many such alloys have been used for centuries as type metals, and were probably first used as bearing materials because of properties they were known to possess in other uses. The advantage of arsenic was recognized about 1938.
Comments in the previous section about significance of bulk mechanical properties of tin-base babbitt alloys apply equally to those of lead-base alloys.
For many years, lead-base bearing alloys were considered just low-cost substitutes for tin alloys. However, the two groups of alloys do not differ greatly in antiseizure characteristics, and when lead-base alloys are used with steel backs and in thicknesses below 0.03 in., fatigue resistance is equal to, if not better than, that of tin alloys. Bearings of any of these alloys are serviceable longest when they are less than 0.005 in. thick, Figure 4.
Without arsenic, microstructures of these alloys comprise cuboid primary crystals of SbSn, or of antimony embedded in a ternary mixture of Pb-Sb-SbSn in which lead forms the matrix. The number of these cuboids per unit volume of alloy increases with increasing antimony content. If antimony content exceeds about 15%, the total amount of hard constituents increases so much that the alloys become too brittle to use as bearing materials.
Arsenic added to lead babbitts improves mechanical properties, particularly at high temperature. All lead babbitts are subject to softening or loss of strength during prolonged exposure to temperatures at which they serve as bearings in internal-combustion engines, 200 to 300 F. Arsenic minimizes such softening. With suitable casting conditions, arsenical lead babbitts develop remarkably fine, uniform structures. They also have better fatigue strength than arsenic- free alloys.
Pouring temperature and cooling rate markedly influence lead-alloy microstructures and properties, especially when used as heavy liners for railway journals. High pouring temperatures and low cooling rates, such as those from overly hot mandrels, promote segregation and formation of a coarse structure. A coarse structure may cause brittleness along with low compressive strength and low hardness.
Overlays
We have already discussed the fatigue- life improvement you can get by decreasing babbitt-layer thickness. Economically as well as mechanically it is hard to get consistent, thin, uniform babbitt layers bonded to bimetal shells by casting. Therefore, electroplating a thin precision babbitt layer on an accurately machined bimetal shell was developed. Special plating racks let the plated babbitt layer thickness be regulated so accurately that machining usually is not needed.
Electroplated tin alloys were found to be generally inferior to lead alloys, and only lead alloys are in commercial use as electroplated bearing overlays. The four most common compositions are SAE 191, 192, 193, and 194. Tin in Alloys 191, 192, and 193, and indium in Alloy 194 confer corrosion resistance. Tin also increases wear resistance. Copper and indium both enhance fatigue resistance.
When a tin-containing overlay is plated directly onto a copper-lead or bronze surface, the tin tends to migrate to the copper interface, forming a brittle copper-tin intermetallic compound. This decreases the corrosion resistance of the overlay and causes embrittlement along the bond line. To avoid this, a continuous barrier layer, preferably nickel about 0.00005 in. thick, is plated onto the copper alloy surface just before overlay plating. Besides providing better surface behavior, overlays improve fatigue performance of some intermediate layers by preventing cracking in this layer. Plated overlays generally range from 0.0005 to 0.002 in. thick, with fatigue life increasing markedly with decreasing overlay thickness. To take full advantage of improved fatigue life gains with thin overlays, you must minimize assembly imperfections such as misalignment, and maintain close tolerances on machined shafts and bearing bores. In adverse wear conditions, premature removal of the overlay will not necessarily impair bearing operation, because the exposed intermediate bearing alloy layer should keep working satisfactorily.
Copper alloys
Copper-base bearing alloys comprise a large family of materials with a wide range of properties. They include commercial bronze, copper-lead alloys, and leaded and unleaded tin bronzes. They are used alone in single-metal bearings, as bearing backs with babbitt surface layers, as bimetal layers bonded to steel backs, and as intermediate layers in steel-backed trimetal bearings, Tables 1, 2, and 3.
Pure copper is a soft, weak metal. The principal alloying element used to harden and strengthen it is tin, with which it forms a solid solution. Lead is present in cast copper-base bearing alloys as a nearly pure, discrete phase, because it has nearly no solid solubility in the matrix. The lead phase, which is exposed on the running surface of a bearing, constitutes a site vulnerable to corrosion in some operating conditions.
The antifriction behavior of copperbase bearing alloys improves with increasing lead content, though at the same time strength is degraded because of more interruption of the continuity of the copper alloy matrix by the soft, weak lead. Thus, by judicious control of tin content, lead content, and microstructure, a large family of alloys has evolved to suit a variety of bearing applications.
Commercial bronze. Poor antifriction properties but fairly good load capacity characterizes lead-free copper alloys. You can readily press-form wrought commercial bronze strip (SAE 795) with 10% zinc into cylindrical bushings and thrust washers. Cold working can increase strength of this inexpensive material.
Unleaded tin bronze. Unleaded copper- tin alloys are called phosphor bronzes because they are deoxidized with phosphorus. They serve principally in cast form as shapes for specific applications, or as rods or tubes from which solid bearings are machined. They have excellent strength and wear resistance, both of which improve with increasing tin content, but poor surface properties. They are used on bridge turntables and trunnions in contact with high-strength steel, and in other slow-moving applications.
Low-lead tin bronzes. Small amounts of lead can improve the inherently poor machinability of tin bronzes. Such additions do not significantly improve surface properties, however, and applications for these alloys are essentially the same as those for unleaded tin bronzes.
Medium-lead tin bronzes. The only wrought strip material in this alloy group is SAE 791, which is press-formed into solid bushings and thrust washers. C83600 is used in cast form as bearing backs in bimetal bearings. SAE 793 is a low-tin, medium-lead alloy that is cast or sintered on a steel back and used as a surface layer for medium-load bimetal bushings. SAE 792 is higher in tin and slightly higher in lead, cast or sintered on a steel back and used for heavy-duty applications such as wrist-pin bushings and heavy-duty thrust surfaces.
High-lead tin bronzes. These contain medium to high amounts of tin, and high lead contents to markedly improve antifriction characteristics. SAE 794, widely used in bushings for rotating loads, has the same bronze matrix composition as SAE 793 (4.5% Sn), but three times as much free lead. It is cast or sintered on a steel back and used for somewhat higher speeds and lower loads than Alloy 793. The bronze matrix of SAE 794 is much stronger than that of a plain 75-25 copper- lead alloy. Alloy 794 can serve as the intermediate layer with a plated overlay, in heavy-duty trimetal applications such as main and connecting-rod bearings in diesel truck engines. This construction provides the highest load capacity available in copper alloy trimetals.
Copper-lead alloys. These are used extensively in automotive, aircraft, and general engineering applications. They are cast or sintered to a steel backing strip from which parts are blanked and formed into full or half-round shapes.
High-lead alloy SAE 48 can be used bare on steel or cast iron journals. Its tin content is restricted to maintain a soft copper matrix, which together with the high lead content improves the alloy’s antifriction and antiseizure properties. Bare bimetal copper-lead bearings are used infrequently today because the lead phase, present as nearly pure lead, is susceptible to attack by corrosive products that can form in the crankcase lubricant between extended oil-change intervals. Therefore, most copper-base alloys with lead contents above 20%, including both SAE 48 and 49, are now used with plated overlays in trimetal bearings for automotive and truck engines.
SAE 485 is a special sintered and infiltrated composite material, produced by methods described in the preceding section, “Powder Metallurgy Processes.” By these methods you can combine a strong, continuous copper alloy matrix with a high lead content; and alloy the lead-rich constituent with enough tin to resist corrosion. SAE 485 is used mostly in bushing and bearing applications involving problems in alignment, shaft surface finish, or unusual dirt contamination.
Mechanical properties of copperbase bearing alloys. Table 4 shows the ranges of mechanical strength properties of copper-base bearing alloys, according to alloy families just discussed. Indentation hardness tests provide the best indications of behavior with compressive loads, and are the only standard strength tests applicable to all alloy forms. Conventional tensile and compression testing can be done only on solid alloy bodies, which represent a small part of total copper- base bearing alloy use.
Test information of this kind helps in materials selection, as a supplement to information generated in dynamic rig tests and in real service. Except in certain solid-alloy bearings and bushing applications, alloy strength and hardness values are rarely stated as absolute specification requirements.
*Material in this series is condensed from the chapter “Friction and Wear of Sliding Bearing Materials,” by George R. Kingsbury, ASM HANDBOOK, Friction, Lubrication and Wear Technology, ASM International, Materials Park, Ohio, 1992, pages 741-757. For ordering information about the entire book, contact ASM International, Materials Park, OH 44073-0002, ph. (216)-338-4634.
George R. Kingsbury, P.E., recently retired as Senior Engineer from Glacier Vandervell Inc., a major producer of metal plain bearings, is principal of his own metallurgical engineering consulting practice in Lyndhurst (Cleveland), Ohio. He is well known in the bearing materials field as an author, lecturer, inventor, and consultant.