This can add hardness, corrosion resistance and other desirable properties without spoiling the toughness of the original metal. Carbon and nitrogen are the two most popular elements added during surface treatment. Titanium alloy implants are currently treated using a process known as chemical vapour deposition to add nitrogen to the metal surface. As the temperature changes the nitride layer expands at a different rate from the underlying metal, and this can cause it to crack or peel off. Once the metal has lost its protective coating, ions such as chloride in the body can corrode the surface and start cracks which can cause the insert to break under repeated loading.
Corrosion also releases dissolved metal into the bloodstream, which is undesirable.
By keeping the treated layer thin and avoiding a sharp junction with the underlying metal, the researchers hoped to come up with a durable surface that would not crack or peel. The researchers used three groups of techniques. Because it does not require the beam of ions to be separated into different atomic masses, it costs only 20 to 50 per cent as much as conventional ion implantation, yet gives many of the same benefits. Both titanium alloys and stainless steel were treated with nitrogen using this method. Processes in the second group, also used to add nitrogen, are based on the use of plasma.