CoCrMo Powder: Composition, Biocompatibility, and the LPBF Window

CoCrMo is roughly 64% cobalt, 28% chromium, and 6% molybdenum. The number that most decides whether a medical implant wears well or embrittles is smaller than all three: carbon, capped at 0.35 wt%. In cobalt-chromium additive manufacturing, that one trace limit is where powder chemistry and the LPBF process window intersect.

The Narrow Band Behind ASTM F75

The cast CoCrMo used in orthopedic and dental implants is specified in ASTM F75 (UNS R30075) and its ISO equivalent, ISO 5832-4. The standard fixes chromium at 27.0–30.0 wt% and molybdenum at 5.0–7.0 wt%, with cobalt as the balance. The trace limits are equally binding: nickel ≤0.50%, iron ≤0.75%, silicon ≤1.0%, manganese ≤1.0%, and carbon ≤0.35%. In practice CoCrMo is a solution-strengthened alloy, not a carbide-strengthened one — carbide volume is only a few percent — yet that small carbon budget is what nucleates M₂₃C₆ (Cr₂₃C₆) carbides at cell boundaries when solidification is slow.

Chromium Passivates, Molybdenum Resists, Nickel Stays Low

Biocompatibility is a surface-chemistry problem. Chromium forms a spontaneous Cr₂O₃ passive film that isolates the bulk metal from body fluid; molybdenum strengthens that film against pitting in the chloride-rich environment. Nickel is held to a maximum of 0.50% — far below the 12–14% in 316L stainless steel — which is why CoCrMo is the standard choice for nickel-sensitive patients. The boundary is worth stating precisely: orthopedic CoCrMo still contains up to ~0.5% nickel; only dental-grade CoCr is labeled nickel-free (≤0.1%).

Why LPBF Changes the Phase Balance

Cast CoCrMo solidifies as the FCC γ phase and partially transforms to the HCP ε phase on cooling. LPBF medical implant production suppresses that transformation: cooling rates on the order of 10⁵–10⁶ K/s freeze in a predominantly FCC γ structure with a fine cellular morphology. The window is process-controlled — too little energy density leaves lack-of-fusion porosity, too much builds residual stress from steep thermal gradients — and carbon decides how much M₂₃C₆ carbide precipitates, which in turn sets the 550–800 HV hardness and wear resistance against ductility.

Medical Means Tighter Than Industrial

The same Co-28Cr-6Mo chemistry that clears an industrial print can still fail an implant. Particle size distribution, oxygen content, and the carbon trace that drives carbide formation all sit inside a narrower window for medical powder — which is why the governing stack is ASTM F75 and ISO 5832-4 for chemistry, ISO 13485 for the quality system, and the EU MDR for the finished device. Implant-grade CoCrMo powder is defined by how tightly that window is held — by Jiangsu VMP.

Figure 1: ASTM F75 (UNS R30075) composition — the three major elements (left) and the trace elements that govern biocompatibility and carbide formation (right). 

By Jiangsu VMP Advanced Materials Co., Ltd. | VMP — Professional Metal Powder Solutions

For international inquiries: Ling Chen, lc@cnvilory.com