F is for F75

F75 is one of a family of alloys that are often termed cobalt-chromium alloys. It will come of little surprise that the two dominant constituents of these materials are the two elements cobalt and chromium. More precisely, in the case of F75 we have a composition of around 29 wt% chromium and 6 wt% molybdenum with the balance being cobalt. Small levels of impurities are typically present, including iron, nickel, manganese and perhaps most importantly around 0.2-0.3 wt% carbon.

It is likely that many readers of this blog will not have heard of cobalt-chromium alloys. Unlike the aluminium, copper and iron alloys we have met from A–E, they are much less common in everyday usage. They do however occupy a particular niche as orthopaedic and dental implant materials.

The first criteria for such applications for a metal is biocompatibility. The three common engineering solutions all rely on the ability to form a highly adherent passive oxide on their surface. In this way the human body and the implant never actually contact each other—the human body ‘sees’ only an inert oxide. This oxide is TiO₂ in the case of titanium alloys (we will meet them later, when we reach ‘T is for Titanium’). In the case of cobalt chromium alloys (and also austenitic stainless steels, see ‘S is for 316 stainless steel’) the passive oxide is Cr₂O₃.

A passive adherent oxide is the starting point for a useful orthopaedic material—what else is required? Typical requirement are high strength, fatigue resistance and excellent wear resistance. Cobalt-chromium alloys exhibit these properties to an exemplary level. This makes them ideal for hip replacements and knee joints. These very high strength alloys are made by forging or similar thermo-mechanical processes. Other alloys that can be made by the alternative processing route of casting are also ideal for bespoke one-offs such as replacement jaw bone, etc. These alloys can also be fabricated by additive manufacture where bespoke geometry for a given patient is required.

Have they really changed the world? Well a replacement hip or knee joint change the ‘world’ of the individual concerned. In these applications cobalt-chromium alloys can enable a journey from pain and disability to recovery and a fuller life.