Date of Award
Master of Science
Materials Science and Engineering
Iver E. Anderson
Concerns regarding the safety of lead have provided sufficient motivation to develop substitute materials for the surface layer on a thrust bearing type component known as a valve plate in axial piston hydraulic pumps that consists of 10% tin, 10% lead, and the remainder copper (in wt. %). A recently developed replacement material, a Cu-10Sn-3Bi (wt.%) P/M bronze, was found to be unsuitable as valve plate surface layer, requiring the development of a new alloy. A comparison of the Cu-10Sn-10Pb and Cu-10Sn-3Bi powder metal valve plates showed that the differences in wear behavior between the two alloys arose due to the soft phase bismuth in the alloy that is known to cause both solid and liquid metal embrittlement of copper alloys.
A lead-free alternative was developed by using infiltration of high-tin alloys into porous bronze compacts compacted at 350MPa and sintered for 2 hours at 650°C to replicate the dual phase structure of leaded bronze. The resulting engineered composite had a lower volume loss than samples of leaded and bismuth bronze under lubricated wear test conditions. It also possessed a similar coefficient of friction and abrasive/plowing mechanism of wear as the Cu-10Sn-10Pb alloys. The results of wear testing show that the alloy demonstrates wear resistant properties comparable to those of the very successful leaded-bronze alloys.
Furthermore, the microstructure of the composite alloy can be engineered by increasing the compaction pressure and sintering temperature of the bronze to balance strength and percentage of the soft phase to match the end use. Also critical to the alloys performance is the control of copper-tin intermetallic formation and growth. The amount of intermetallics could be controlled by minimizing time at temperature during infiltration. In addition, it was shown that growth of the copper-tin Cu3Sn intermetallic can mitigated by the addition of manganese.
Vetterick, Greg, "Lead-free, bronze-based surface layers for wear resistance in axial piston hydraulic pumps" (2010). Graduate Theses and Dissertations. 11631.