A simple literature review, or search on the Internet for “intermetallic compounds” would lead one to believe that this represents the state of the art in material science. While the study of many new compounds is on the leading edge of technology, the intermetallic nature of copper and tin in an alloy form has been studied for more than 5000 years. Without the aid of high tech tools, the Chinese developed the beta bronze alloy form of tin and copper some 1400 years ago. This was the first metal that could be intentionally heat treated to provide a wide range of mechanical properties. In more recent research, much attention has been paid to the formation of Cu6Sn5 and Cu3Sn intermetallic compound layers, and their effect on solder joints in electronic assembly. Unfortunately, little, or no attention has been paid to the identical reaction that occurs when bonding a tin based Babbitt to a copper alloy backing material typical of many fluid film bearings used in industry today.
My first direct exposure to the resultant phenomenon of the formation of these compounds came about 10 years ago. During the dis-assembly of a high speed gas compressor, the thrust pads were removed from the unit for inspection. In this particular bearing, the pads were designed with ASTM-B23 Grade 2 babbitt bonded to a copper alloy containing approximately 2% chrome for increased mechanical strength. In this application, the high sliding velocity present in the oil lubricated thrust bearing would have yielded unacceptably high bearing temperatures if conventional steel backing material had been used. The copper alloy backing material was used due to its high thermal conductivity to provide improved bearing performance. In this instance, following successful dimensional checks, and ultrasonic inspection of the babbitt bond, the pads were returned to the compressor deck to be re-installed in the machine. During the installation process, one of the pads was inadvertently dropped from a height of about three inches on to a steel workbench. As a result of this minor impact, the babbitt completely separated from the copper alloy backing material. This was indeed somewhat disturbing that the babbitt could fall off of an otherwise acceptable part that was ready for installation in a very expensive machine that operates in excess of 10,000 RPM.
Careful inspection of the subject thrust pad, the babbitt that had been bonded to the pad, and the remaining 5 pads from this bearing yielded more disturbing questions than answers. Each of the intact pads could pass an ultrasonic inspection of the babbitt bond, and yet the babbitt could be readily removed from the backing layer with a pocket knife. Metallurgical inspection of the babbitt and the backing material indicated that all of the material compositions were correct. Scanning Electron Microscope (SEM) scans of the parts indicated that the failure had occurred along an intermetallic layer that had formed along the babbitt-copper interface. Literature reviews undertaken at that time, as well as today, would show that the only industry that acknowledges the formation of these intermetallic compounds is the electronics industry. Apparently, solder joints have been plagued with this problem for generations.
Intermetallic compounds differ from simple metal alloys in a few basic, but important ways. A metal alloy consists of a base material to which certain percentages of other elements have been added. Most alloys are simply a disordered solid solution of individual components. The atoms in a conventional alloy link together with relatively weak metallic bonds. In an intermetallic compound, there is a discrete, narrow range of chemical composition. The atoms bond to one another with a combination of ionic and covalent bonds. As a result, the individual atoms begin to take up preferred positions within the crystal lattice of the intermetallic compound. This helps to explain the general tendencies of intermetallic compounds: higher strength, higher melting points, and poor ductility.
As stated earlier, there are two discreet intermetallic compounds formed as by-products of the combination of copper, tin, heat, stress, vibration, and time. Cu6Sn5 forms the instant molten tin comes in contact with the copper material. This compound grows rapidly while the tin is liquid, and slows considerably once the metals solidify. The presence of Cu6Sn5 is essential for good adhesion of the two materials. Cu3Sn does not form while the tin is molten. It forms between the copper and the Cu6Sn5 layer through solid state diffusion once the metals have hardened. Both of the intermetallic compound layers will grow when exposed to heat, stress, and vibration. The mechanical problems associated with this phenomenon are multi-fold. The intermetallic compounds are extremely brittle, and have different coefficients of thermal expansion then those of either the tin based babbitt, or the copper backing material. This alone makes the bond susceptible to cracking from mechanical or thermal stress. In addition, the depletion of the tin from the layer of babbitt adjacent to the intermetallic compound leaves behind an alloy of questionable mechanical properties.
In a testing program, it was established that this problem could be reproduced, discretely and repeatedly simply by heat cycling the subject parts. The babbitt bond on perfectly good parts could be rendered dangerously brittle, simply by cycling the parts between 250 and 80 degrees F for a couple of weeks, for a total of less than 300 cycles. The heat cycled parts all had the following properties, identical to those found in the field:
1. Without additional stress, the babbitt bond remained intact
2. The babbitt can be separated from the backing by inserting a sharp object at the bond line with minimal force
3. As the babbitt is separated from the backing, an audible “crackling” can be heard
4. Un-magnified inspection of the copper surface following removal of the babbitt shows a matte grey color
5. 100X magnification of the copper surface shows a combination of areas where the babbitt has been removed clean to the copper, as well as areas of remaining babbitt. The areas of remaining babbitt do not appear as a crystalline fracture surface, but more as smooth shiny, almost polished areas.
Now that the problem was identified, and shown to be repeatable, the question remained as to the appropriate solution. A similar problem had been solved in the early days of satellite construction. Apparently, solder joints would fail after some of the early satellites would have made numerous orbits around the earth. The formation of these intermetallic compounds was halted by applying a thin barrier plating to the copper prior to soldering. Testing was then undertaken, and electroless nickel was found to provide an excellent barrier between the tin and the copper, as well as providing a surface that would readily accept the bonding of the babbitt. The application of a nickel barrier layer to all copper and copper bearing alloys prior to babbitting, has prevented this phenomenon from recurrence. This step should be taken with all new design, as well as any re-babbitting operations with copper alloy backed fluid film bearings.
TRI Transmission and Bearing Corp.