Cobalt-chromium alloy

Cobalt with a quarter to a third chromium, plus molybdenum or tungsten. It is hard, stiff, extremely resistant to wear and to corrosion, and it keeps its strength hot — the combination that makes it simultaneously an implant material and a turbine material.

In joint replacement it is the bearing surface. An artificial hip's ball articulates against polyethylene tens of millions of times, and the wear debris rather than the mechanical loading is what eventually fails the joint, so the hardness of the counterface matters more than its strength.

Its stiffness is the trade against titanium. Being twice as stiff as titanium and close to steel, a cobalt-chromium stem shields the surrounding bone from load more effectively, and bone that is not loaded resorbs. So the modern hip is often both alloys: a titanium stem for the modulus, a cobalt-chromium head for the wear surface.

Investment cast, which is the traditional route and is how dental frameworks and many implant components are still made, or wrought and forged for higher strength. Increasingly made by additive manufacturing, especially for dental work, where a scan becomes a printed framework without a wax pattern.

Hard to machine — it work-hardens rapidly under a tool — and normally finished by grinding and polishing. An implant bearing surface is polished to a few nanometres of roughness, and that finish is as much a part of the device's performance as the alloy.

Hip and knee replacement bearing surfaces; dental crowns, bridges and partial denture frameworks; bone plates. Turbine blades and vanes in the hot section of gas turbines, and combustor components. Hardfacing overlays welded onto valve seats, cutting edges and anything that erodes. Cutting tools and saw tips.

Also, historically, the cutting tool material Stellite was developed as before cemented carbide displaced it.

Elwood Haynes developed the cobalt-chromium alloys from 1907, looking for a tarnish-proof cutlery metal, and named them Stellite for their brightness. Their wear resistance turned out to be more valuable than their appearance.

Medical use dates from the 1930s, with Vitallium, and cobalt-chromium was the first alloy used successfully for internal fixation and joint replacement. The metal-on-metal hip resurfacings of the 2000s are the counter-example worth recording: cobalt and chromium wear debris and ion release caused adverse local tissue reactions in a significant fraction of patients, and the devices were largely withdrawn — a reminder that a material's biocompatibility is a property of the *system* and not of the alloy alone.