Cocrmo Alloy Chemical Composition

The chemical composition of CoCrMo alloy is strictly governed by international standards, primarily ASTM F75 (cast) and ASTM F1537 (wrought), to ensure biocompatibility and mechanical integrity. The alloy consists of a Cobalt balance, 27.0%–30.0% Chromium for corrosion resistance, and 5.0%–7.0% Molybdenum for strength. Here is the typical chemical composition breakdown by weight:

• Cobalt (Co): Balance (~60–65%)
• Chromium (Cr): 27.0 – 30.0%
• Molybdenum (Mo): 5.0 – 7.0%
• Nickel (Ni): < 0.50% (Strictly limited to prevent allergic reactions)
• Carbon (C): < 0.35% (Adjusted to balance ductility and wear resistance)
• Iron (Fe): < 0.75%
• Silicon (Si): < 1.00%
• Manganese (Mn): < 1.00%

While the base elements provide the alloy’s structural phase stability, trace elements are critical. For instance, Carbon levels are often manipulated (low-carbon <0.05% vs. high-carbon >0.20%) to tailor the material for specific medical load-bearing applications, while Nickel is capped to ensure biological safety.

The Role Of The Main Elements In CoCrMo Alloy

The superior mechanical properties of CoCrMo are largely due to the precise ratio of its three main components-cobalt, chromium, and molybdenum. By figuring out the specific contribution of each element, you can understand why this particular chemical component has become the industry standard for medical implants.

• Cobalt (balance/matrix): As the matrix of the alloy, cobalt accounts for the majority of the weight (about 60-65%). It provides the structural phase stability of the foundation, which is essential for the alloy not to deform when subjected to high stresses.
• Chromium (27.0-30.0%): This high proportion is decisive for the lifetime of the material in the human environment. Chromium is responsible for forming a dense passive oxide film on the surface, thereby providing excellent corrosion resistance. If it is lower than the range of 27-30%, the material will become very fragile in the physiological environment and easily degrade.
• Molybdenum (5.0-7.0%): Molybdenum is added primarily for mechanical strengthening. It can significantly improve the strength and hardness of the alloy, and cooperate with the cobalt matrix to prevent fatigue failure of the material under cyclic loading.

Strict Control Of Trace Elements For Biocompatibility

In the chemical composition control of CoCrMo alloy, what is excluded is as important as what is added. International standards such as ASTM F75 and ASTM F1537 impose very strict upper limits on impurities to ensure biosafety.

• Nickel (< 0.50%): Nickel is arguably the impurity we most need to guard against. As noted in the composition table, the nickel content must be strictly controlled below 0.50%. This is not only a standard requirement, but also a clinical iron law, because nickel allergy is quite common in the patient population, and we must eliminate allergic reactions from the source.
• Iron, silicon and manganese: The content of these elements is also limited to very low levels (iron <0.75%, silicon and manganese <1.00%) in order to maintain the purity of the alloy structure and avoid excessive impurities that will damage its original excellent corrosion resistance.

Optimizing Performance Through Carbon Content

About the chemical composition of CoCrMo alloy, the most interesting point is the manipulation of carbon. Although the general standard allows for carbon content of up to 0.35 percent, in actual engineering applications, we will customize it for specific applications:

• Low carbon (< 0.05%): When ductility is a priority due to design requirements, we choose low carbon content. This makes the alloy slightly flexible and has better fracture toughness.
• High carbon (> 0.20%): Higher carbon content promotes the formation of carbides. These carbides can significantly improve wear resistance, which is an indispensable property for load-bearing surfaces such as artificial hip joints and knee joints.

As long as these chemical parameters defined by ASTM standards are strictly followed, manufacturers can ensure that CoCrMo alloys achieve the perfect balance between strength, corrosion resistance and biological safety.

Author：David
“I am a Senior Materials Engineer specializing in biomedical alloys. With over a decade of experience in medical implant manufacturing, I focus on interpreting international standards like ASTM F75 to help engineers understand material properties, chemical compositions, and biocompatibility requirements.”