What is Phosphor Bronze？
Phosphor Bronze has come a long way from just being an alloy of copper and tin; it is now a broader class of copper alloys that are still finding new uses to this day. The diversification of bronze can make material selection difficult, however, so this article will help alleviate some confusion by exploring one kind of bronze, the phosphor bronzes. By examining the physical, chemical, and mechanical properties of phosphor bronze, this article aims to help interested designers Phosphor Bronzedetermine if this copper alloy will be applicable to their projects. We will first examine the composition of phosphor bronze, and then discuss its strengths, drawbacks, mechanical properties, and finally common applications of such a useful metal.
Physical Properties of Phosphor Bronze
It may be helpful to review our article on the types of bronzes, so as to understand the commonalities between copper alloys and their popular applications.
Below, in Figure 1, is a qualitative chart depicting the composition of phosphor bronze:
Pie chart representation of the material composition of phosphor bronze.
Figure 1: Qualitative breakdown of phosphor bronze.
Notice how the included zinc, iron, lead, and phosphorus percentages are infinitesimal.
From examining Figure 1, it may be difficult to understand why phosphor bronze is named as such when it is mainly an alloy of copper and tin. This is because it only takes a small percentage of phosphorus, around 0.03-0.035% by weight, to imbue these alloys with unique properties such as improved flow, wear resistance, and stiffness. They are also known as tin bronzes, but always have some amount of phosphorus present to be truly considered a phosphor bronze. Their density is around 8.8 g/cm3, and most forms of these alloys respond to hot working, cold working, and heat treatment. They are exceptionally elastic in nature, resistant to fatigue and corrosion, and are readily formable, castable, and solderable. The addition of lead will make phosphor bronzes stronger and more machinable, but unleaded phosphor bronzes are also strong and resistant by themselves. They are most often used for corrosion-resistant applications, as well as in other applications that need a springy-yet-durable material.
Resistances and weaknesses
Most phosphor bronze alloys are tarnish and corrosion resistant, making them useful in electrical conduit and other caustic environments. They are also resistant to fatigue, meaning they retain their strength over many cycles of loading. The addition of tin will increase this resistance along with its strength, and the addition of lead will make a phosphor bronze that is colloquially known as the “Machinist’s friend” (alloy COLPHOS 90/C54400), as it is nearly indistinguishable from free-cutting brass alloys. Most alloys form an attractive patina in the presence of oxygen and are therefore also useful for decorative applications. Phosphor bronze’s main disadvantages can be its lower electrical conductivity, as phosphorus will reduce the ability of the metal to conduct current, and its price tag. The dust and fumes of phosphor bronze are also toxic when inhaled, so be wary of both your wallet and your lungs.
Mechanical Properties For Phosphor Bronze
Table 1, below, shows some mechanical properties relevant to phosphor bronze’s uses and strengths. This section will briefly explain each property and show how they make this copper alloy unique.
Table 1: Summary of mechanical properties for phosphor bronze – note that this table is only a general summary and not a de-facto list of properties for all phosphor bronzes.
Note: This table is only a general summary and not a de-facto list of properties for all phosphor bronzes.
Mechanical Properties
Metric
English
Tensile yield strength
380-450 MPa
55100-65260 psi
Modulus of Elasticity
110 GPa
16000 ksi
Electrical conductivity (relative to pure copper)
15%
Hardness (Rockwell B)
75-85
Machinability
20-100%
Tensile yield strength is a stress value that determines the point after which the material will plastically deform. It is a common measure of a material’s non-yielding strength, as stress below this point will not bend or stretch the alloy (though, there are always exceptions). It is a useful measure for fabricators when machining, as well as in applications where the material must keep its form without jeopardizing the structure’s integrity. Phosphor bronze has an impressive yield strength, matching some aluminum alloys and surpassing even other bronzes, and this is thanks to its high percentage of tin and phosphorus. When combined with its good spring characteristics, this creates an alloy that is strong and will stay strong.
Modulus of elasticity, or Young’s modulus, describes the ability of a material to elastically deform- that is, how stiff a material is. Counterintuitively, a higher Youngs modulus suggests a more elastic material, but this is not a measure of “stretchiness”; it is a measure of internal material strength and how likely the material is to return to its original shape in the presence of an increasing force. A high Youngs modulus, therefore, means a generally strong material as it will not plastically deform under increasing loading conditions. Phosphor bronze has a decently high elastic modulus (almost half that of most steels), which suggests it is not as strong as other alloys but can hold its own under moderate conditions. This causes phosphor bronze to be both useful for shaping operations and more difficult to machine, but more on this in a bit.
The electrical conductivity of phosphor bronze is about 15% that of pure copper – so why use it in electrical applications when copper is better able to carry electricity? The answer lies in phosphor bronze’s good balance of fatigue strength, corrosion resistance, and electrical conductivity. While not a standout alloy in any one of these categories, its ability to be both resistant, strong, and conductive means electrical components made from phosphor bronze will last longer than copper analogs. Therefore, phosphor bronze has been used for a long time as a material in electrical switches, fasteners, connectors, and more, as it can bare both mechanical and electrical loads better than typical copper counterparts.
Material hardness is always relative to some standard hardness machine, and the various hardness scales provide a comparative list of materials within the same general class/material. Table 1 shows hardness values from the Rockwell B scale, which is often used to describe the hardness of copper alloys and other metals. Materials with lower hardness values are generally softer, meaning they are more easily scratched, dented, and locally deformed on their surface, while higher numbers suggest a material that is difficult to scratch (think borosilicate glass, ceramic, etc.). For reference, the Rockwell B hardness of copper is around 50, so it is clear from Table 1 that phosphor bronze is more scratch-resistant than its pure form. It is also not as strong as other, more harden-able alloys such as steel, which can be good or bad depending upon the application. For example, if etching and inlay is part of your application, phosphor bronze may be a good balance between strength and workability, but this is dependent upon the specific alloy type and strengthening procedures.
Like hardness, machinability is a comparative measure of a material’s ability to be machined and is always in reference to some standard machining material (for the bronzes, this material is UNS C36000 – free cutting brass). This standard material is given a machinability score of 100%, meaning it is easy to machine and causes little problems in the shop. Alloys compared to this reference with a score lower than 100% are more difficult to machine, and this is generally the case with phosphor bronzes; however, phosphor bronze can be specially made to address these concerns (along with most other alloys, depending upon your source). Phosphor bronze can be bought with increased lead levels that will remove its machining difficulty, as it becomes more self-lubricating. So, while most phosphor bronze alloys are somewhat difficult to mill, this issue can be addressed with the right alloy selection.
Applications of Phosphor Bronze
Phosphor bronze has been used for many years in electrical components, but it is still finding new uses thanks to technological advances and the demand for more specialized materials. Below is a list of phosphor bronze applications but know that it has many uses outside of this list, and new applications are still being developed.
Some common applications include:
Springs
Sleeve bushings & bearings
Welding rods
Jewelry
Guitar strings
Dental bridges
and more.
Phosphor bronzes meets many designer specifications and is a proven engineering material. If it sounds attractive to your application, be sure to contact your supplier, as they will have the newest alloys available.
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