Phosphor Bronze: Its Properties and Characteristics

Phosphor Bronze, also known as tin bronze, is alloys made up of copper, tin and phosphorous and known for its strength, low coefficient of friction and toughness. The phosphor bronze contains between 0.5 and 11% tin and 0.01 to 0.35 % phosphorous. The addition of tin increases the resistance to corrosion and strength of the alloy. The phosphorous increases the resistance to wear and stiffness of the alloy. The phosphor bronze has superb spring qualities, resistance to high fatigue and corrosion, excellent formability and solderability. They are basically used for electrical products, corrosion resistant bellows, diaphragms, and spring washers. The phosphor bronze has to grades UNS C50100 through C54200. Leaded phosphor bronze combines good strength and resistance to fatigue, corrosion and wears along with good machinability.

Copper alone is not suitable for use in connectors because it is not elastic enough and loses its strength when held at the operating temperature of most electrical switchgear. Therefore Copper, in the form of an alloy has to be strengthened without unnecessary loss of electrical conductivity.

To remove oxygen Phosphorus is added to molten copper. The product of this deoxidation- Phosphorus pentoxide, is readily removed from the melt during the refining process. Any excess phosphorus goes into solution in the copper, where it provides some strengthening when the copper is cast and worked to a sheet. But phosphorus in solution reduces electrical conductivity dramatically. As little as 0.25% phosphorus will reduce conductivity to only 30% IACS, (International Annealed Copper Standard for the pure metal).  During the hot rolling stage of sheet production, too much phosphorus also causes problems. It is, therefore, a challenge, to maintain the amount of phosphorus at an optimal value to remove oxygen but keep it low enough to enable the material to be worked down to a sheet and still retain a relatively high electrical conductivity.

To strengthen the alloys Tin is added to conventional phosphor bronze. Like phosphorus, tin goes into solution in the molten copper and remains in solution in the solidified alloy. The maximum tin concentration that can be retained in solid solution is about 16%.

The tin content in this alloy determines the corrosion resistance and also gives it strength. Tin in solution produces two conflicting effects. Due to solid-solution strengthening, it increases the strength of the copper significantly. But the more the content of tin in solid solution the lower is the electrical conductivity. Fortunately, the effect of tin on electrical conductivity is less severe than in the case of phosphorus, and 1% of tin only reduces the conductivity to 60% International Annealed Copper Standard (IACS). Again, there has to be a balance, which depends on the cost of the material and the properties required.  If the tin content is above 2.5% the alloy becomes too expensive to process; if the content of tin below 1.5% the alloy lacks strength and the ability to retain strength at its operating temperature.

Hence, conventional phosphor bronze is a compromise, with optimum additions of phosphorus and tin to give as high strength as possible coupled with a relatively high electrical conductivity.

Properties and Characteristics

• Strength and resilience – phosphor bronze is an alloy which is resistant to fatigue. It holds up exceptionally well to wear. The inclusion of tin is what gives Phosphor bronze its added strength.
• Corrosion resistance – phosphor bronze is typically used in applications where it will have exposure to corrosive chemicals and/or physical elements.
• Electrical conductivity – the use of Phosphor bronze in electrical components is perhaps its most well-known application.
• Excellent elasticity – the fine grain size of this alloy enables the material to have a greater degree of spring back.