Implantables like artificial joints and bone plates are in direct contact with human tissues and body fluids. Therefore, the requirements for their materials are extremely strict: they must have high corrosion resistance, high strength, excellent wear resistance, and good biocompatibility at the same time.
316L stainless steel and cobalt-chromium-molybdenum alloy (CoCrMo) are the preferred materials for medical implants.
Material Definition and Advantages
316L Stainless Steel
316L is an austenitic stainless steel. The “L” in its name stands for extra-low carbon (carbon content ≤ 0.03%). This feature significantly enhances the material’s resistance to intergranular corrosion. 316L also has the following advantages:
- Excellent corrosion resistance: Resists corrosive media such as body fluids and chlorides.
- Good biocompatibility.
- High strength and toughness.
- Easy to machine and form.
Cobalt-Chromium-Molybdenum Alloy (CoCrMo)
Cobalt-chromium-molybdenum alloy (CoCrMo) is a high-performance alloy mainly composed of cobalt. It typically contains approximately 27%-30% chromium and 5%-7% molybdenum. Its key characteristics include:
- Extremely high wear resistance: Suitable for friction pairs like joints.
- Superior strength and hardness: Outperforms stainless steel.
- Excellent corrosion resistance and biocompatibility.
- Good fatigue resistance, making it suitable for long-term dynamic load scenarios.
The table below briefly compares the key properties of the two materials:
| Performance Indicator | 316L Stainless Steel | CoCrMo Alloy |
| Tensile Strength (MPa) | 485 – 620 | 900 – 1500 |
| Yield Strength (MPa) | 170 – 310 | 450 – 1000 |
| Hardness (HV) | 140 – 200 | 300 – 450 |
| Corrosion Resistance | Excellent | Extremely Excellent |
| Biocompatibility | Good | Excellent |
| Main Applications | Bone Plates, Screws, Stents | Joint Prostheses, Dental Implants |
In-Depth Material Analysis: Why 316L VM and CoCrMo?
The 316L and CoCrMo used in medical implants are not ordinary industrial-grade materials. Instead, they are custom medical materials optimized through special processes. Among them, 316L VM (Vacuum Melted) and high-purity CoCrMo are the most representative.
316L VM
Ordinary 316L stainless steel already has basic corrosion resistance. However, medical implant-grade 316L must be upgraded through the Vacuum Melting (VM for short) process. Its core advantages lie in two aspects:
Ultra-low carbon control to eliminate intergranular corrosion: The carbon content is ≤ 0.03%. Moreover, the distribution of carbon can be precisely controlled in a vacuum environment. This prevents the formation of “intergranular precipitated carbides” in the material after heat treatment — such substances will damage the passive film formed by chromium, causing local corrosion of the implant in body fluids and triggering the dissolution of metal ions.
Extreme purity to reduce harmful impurities: The vacuum environment can effectively remove gaseous impurities such as hydrogen and oxygen generated during the melting process. At the same time, it filters out trace harmful elements like sulfur and phosphorus. If these impurities remain, they may form “stress concentration points” when the implant is under long-term stress, leading to fatigue fracture of the implant or local tissue irritation.
Currently, 316L VM is widely used in “static support” implants such as bone plates, bone screws, and internal fixation plates. With its reliable strength and cost advantage, it has become a common material for orthopedic trauma treatment.
CoCrMo
Implants for parts of the human body such as hip joints and knee joints need to withstand rotational friction between the ‘joint head and acetabulum’for a long time. Ordinary materials will soon generate particles due to wear, which triggers “osteolysis”— the human body’s immune response to wear particles erodes the surrounding bones. With its excellent wear resistance, CoCrMo has become the first choice for “dynamic friction pair” implants. Its core advantages include:
Extremely low friction coefficient: The intermetallic friction coefficient of CoCrMo alloy is only 0.15-0.2 (far lower than 316L’s 0.3-0.4). When matched with ceramic or high-molecular polyethylene liners, it can ensure the service life of artificial joints exceeds 15 years.
Balanced mechanical strength and toughness: Its tensile strength is more than 50% higher than that of 316L, while it also has good impact toughness. It can not only bear the body weight load when a person stands, but also cope with dynamic impacts such as walking and going up and down stairs, preventing the implant from brittle fracture.
No nickel sensitization risk: Some patients are sensitive to the nickel element in 316L. CoCrMo, however, contains no nickel, so it has better biocompatibility and is suitable for patients allergic to nickel.
Vacuum Melting and Cleanliness Control
High-quality materials need to be paired with precision processes to ensure the stable performance of medical implants. At the QD casting factory, Vacuum Induction Melting (VIM) and full-process cleanliness control are the “two standard features” in medical implant production.
Vacuum Induction Melting (VIM)
Vacuum melting is a “standard” process for medical-grade metal materials, with advantages including:
- Oxidation prevention: Melting in a vacuum environment avoids the reaction between metal and oxygen.
- Gas removal: Effectively reduces the content of gases such as hydrogen and oxygen, minimizing porosity defects.
- Precise composition control: Ensures consistent composition of materials in each batch, meeting medical standards.
Full-Process Cleanliness Control
From wax pattern preparation, shell sintering to melting and pouring, QD strictly controls cleanliness in all steps:
- Shell-making process: Medical-grade zircon sand and corundum powder are used as shell-making materials. The shell-making workshop meets the ISO Class 7 (10,000-class) cleanliness standard. The operators’clothing, tools and equipment all comply with medical production regulations.
- Pouring process: Molten metal is directly injected into the mold through a vacuum pouring system, preventing the molten metal from contacting impurities when exposed to the atmosphere. After pouring, the castings need to cool in a clean area to avoid adsorbing air pollutants during the cooling process.
- Post-treatment process: Medical-grade grinding wheels are used for casting grinding and polishing to prevent residual metal debris. Ultrapure water is used for final cleaning to ensure no oil stains or impurities on the surface.
Certifications and Standards
The production of medical implants must comply with international standards and regulatory requirements. Below are some key certifications and standards:
ISO 13485: Quality Management System for Medical Devices.
ASTM F138: Standard for 316L Stainless Steel for Medical Applications.
ASTM F75: Standard for Cast Cobalt-Chromium-Molybdenum Alloys.
QD factory has obtained ISO 13485 certification and strictly adheres to standards such as ASTM and ISO. Every batch of 316L and CoCrMo implant blanks must pass mechanical property, corrosion resistance, and composition analysis tests conducted by third-party testing institutions. This ensures that 100% of the products delivered to customers are qualified.
Conclusion
316L and CoCrMo each perform excellently in terms of corrosion resistance, strength, wear resistance and biocompatibility. Behind their outstanding performance is the support of high-end processes and management, such as vacuum melting, clean production and strict quality inspection.
QD has mature capabilities in melting, precision casting and post-processing of 316L and CoCrMo materials. We provide customers with safe, reliable and high-performance medical casting solutions.
FAQ
1. Why are 316L and CoCrMo commonly used in medical implants?
316L has excellent corrosion resistance and biocompatibility. CoCrMo, on the other hand, offers higher strength and wear resistance. Both can meet the extreme material performance requirements of implants.
2. Which is more suitable for joint replacement: 316L or CoCrMo?
Due to its extremely high wear resistance and strength, CoCrMo alloy is more commonly used in friction pair components such as artificial joints.
3. Will implants corrode in the human body?
Both 316L and CoCrMo have excellent corrosion resistance and can withstand the human body fluid environment. They hardly corrode under normal use.
4. Beyond orthopedic implants, in which other medical fields can 316L and CoCrMo be used?
316L can be used for dental implant abutments and surgical instruments (such as hemostats).
Due to its excellent biocompatibility, CoCrMo can also be used in fields with higher material performance requirements, such as cardiac stents (requiring ultra-thin wall processing) and spinal implants.
5. Why is vacuum-melted 316L VM more expensive than ordinary 316L?
Vacuum melting requires specialized equipment and a longer production cycle. Moreover, it can remove more impurities and precisely control composition, which improves the material’s corrosion resistance and mechanical stability by over 30%. The cost mainly lies in “process complexity” and “performance enhancement”.