About Phosphor Bronze - Strength, Properties and Uses
Bronze has come a long way from being an alloy of copper and tin, it is now a much broader class of copper alloys that are still finding new uses today. However, the diversity of bronzes can make material selection difficult, so this article will help alleviate some of the confusion by exploring one type of bronze, phosphor bronze. By examining the physical, chemical, and mechanical properties of phosphor bronze, this article aims to help interested designers determine if this copper alloy is a good choice for their project. We will first examine the composition of phosphor bronze, then discuss its advantages, disadvantages, mechanical properties, and finally discuss common applications for this useful metal.
Physical Properties of Phosphor Bronze
It may be helpful to review our article on types of bronzes in order to understand the commonalities between copper alloys and their popular applications.
Below, in Figure 1, is a qualitative chart that describes the composition of phosphor bronze:
Pie chart representation of the material composition of phosphor bronze.
Figure 1: Qualitative breakdown of phosphor bronze.
Note that the percentages of zinc, iron, lead, and phosphorus included are infinitesimal.
By looking at Figure 1, it may be difficult to understand why phosphor bronze is so named when it is primarily composed of copper and tin as an alloy. That's because only a small amount of phosphorus, about 0.03-0.035% by weight, is needed to give these alloys their unique properties, such as improved flow, wear resistance, and stiffness. They are also known as tin bronzes, but a certain amount of phosphorus is always present to truly be considered phosphor bronzes. With a density of about 8.8 g/cm3, most forms of these alloys respond to hot working, cold working, and heat treatment. They are extremely resilient, resistant to fatigue and corrosion, and can be easily formed, poured, and welded. The addition of lead makes phosphor bronzes stronger and easier to machine, but lead-free phosphor bronzes are also strong and resistant on their own. They are most often used in corrosion-resistant applications, as well as other applications that require a resilient but durable material.
Resistance and Weaknesses
Most phosphor bronze alloys resist discoloration and corrosion, making them useful in electrical conduit and other corrosive environments. They are also fatigue-resistant, meaning they maintain their strength over many load cycles. The addition of tin will increase this resistance as well as its strength, while the addition of lead will make phosphor bronze, colloquially known as "machinist's friend" (alloy COLPHOS 90 / C54400) because it is virtually indistinguishable from free-cutting brass alloys. Most alloys develop an attractive patina in the presence of oxygen, so it can also be used in decorative applications. The main disadvantages of phosphor bronze are probably its lower electrical conductivity, as phosphorus reduces the metal's ability to conduct electrical current, and its price tag. Phosphor bronze dust and fumes are also toxic when inhaled, so be careful with your wallet and your lungs.
Mechanical Properties
Table 1 below shows some of the mechanical properties that are relevant to the uses and strength of phosphor bronze. This section will briefly explain each property and show how they make this copper alloy unique.
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%
Table 1: Summary of Phosphor Bronze Mechanical Properties - Please note that this table is a general summary and not a listing of the actual properties of all phosphor bronzes.
Tensile yield strength is a stress value that determines the point at which a material deforms plastically. It is a common measure of a material's non-yield strength, as stresses below this point will not bend or stretch the alloy (although there are always exceptions). It is a useful measure for manufacturers when machining and in applications where the material must hold its shape without compromising structural integrity. Phosphor bronze has an impressive yield strength that rivals some aluminum alloys and even exceeds other bronzes, thanks to its high proportion of tin and phosphorus. When combined with its good spring properties, this creates an alloy that is strong and holds its own.
The modulus of elasticity, or Young's modulus, describes a material's ability to deform elastically, i.e., the stiffness of the material. Counterintuitively, a higher Young's modulus indicates a more elastic material, but this is not a measure of "elasticity"; it is a measure of the internal material's strength, and how likely it is to return to its original shape under increased force. Therefore, a high Young's modulus means that the material is generally strong because it does not deform plastically under increased load conditions. Phosphor bronze has a fairly high modulus of elasticity (almost half that of most steels), which indicates that it is not as strong as other alloys, but can hold its own under moderate conditions. This results in phosphor bronze being both useful for forming operations and more difficult to machine, but more on that later.
Phosphor bronze has an electrical conductivity of about 15% that of pure copper - so why use it in electrical applications when copper can carry electricity much better? The answer lies in phosphor bronze's good balance of fatigue strength, corrosion resistance, and electrical conductivity. While not a standout alloy in any of the above categories, its ability to be durable, strong, and conductive means that electrical components made from phosphor bronze will last longer than their copper analogs. As a result, phosphor bronze has long been used as a material for electrical switches, fasteners, connectors, etc., because it can withstand mechanical and electrical loads better than its typical copper counterpart.
Material hardness is always relative to some standard durometer, and the various hardness scales provide a comparative list of materials within the same general category/material. Table 1 shows hardness values on the Rockwell B scale, which is commonly 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 the surface, while higher numbers indicate materials that are difficult to scratch (think borosilicate glass, ceramics, etc.). For reference, copper has a Rockwell B hardness of about 50, so it's clear from Table 1 that phosphor bronze is more scratch-resistant than its pure form. It's also not as strong as other, more hardenable alloys like steel, which can be better or worse depending on the application. For example, if etching and inlaying are part of the application, phosphor bronze may be a good balance between strength and machinability, but this depends on the specific alloy type and strengthening procedure.
Like hardness, machinability is a comparative measure of a material's ability to be machined, and is always referenced to some standard machining material (for bronzes, this material is UNS C36000 – Free Cutting Brass). This standard material has a machinability score of 100%, meaning it is easy to machine and poses few problems in a shop. Alloys with scores below 100% are more difficult to machine compared to this reference, and this is often the case with phosphor bronze; however, phosphor bronze can be specially manufactured to address these issues (as well as most other alloys, depending on your source). Phosphor bronze can be purchased with a higher lead content, which will eliminate 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 by choosing the right alloy.
Applications of Phosphor Bronze
Phosphor bronze has been used in electrical components for many years, but it is still finding new uses due to technological advances and the need 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
Bushings, bushings and bearings
Welding rods
Jewelry
Guitar strings
Dental bridges
And more.
