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.