Ion Implantation for Tool Steel Dies and Cutting Tools

Ion implantation puts nitrogen into the outermost layer of a finished tool without putting a coating on it. Charged ions are accelerated at high energy into the steel under high vacuum and come to rest 0.25 to 25 nm (2.5 to 250 Å) below the surface, so the treatment changes the composition of the surface itself. Nothing is added on top of the tool, which means there is no coating to spall and no edge buildup to work around. The table on this page is the measured life change for implanted tools, dies, molds and rollers, part by part and energy by energy.

What the process does to the steel

A beam of charged ions of the chosen element is formed and accelerated towards the tool surface, which is held under high vacuum. The ions penetrate the solid and stop within the top few nanometres. The source is explicit that this is not a coating process. There is no discrete layer sitting on the steel, only an alteration of the chemical composition near the surface, which is why the treatment does not change the tool dimensions and does not create an interface that can fail. The element most often implanted into tool steel is nitrogen, and the property it is used to improve is adhesive and sliding wear. Titanium and carbon have also been implanted into stamping and cutting tools, and both improved service life.

Tool life results for implanted tooling

Table 6 gathers the source results across machining, tapping, cutting, forming, molding and rolling. The largest multiplier in the table belongs to a tap. An HSS tap cutting 4140 steel gained 10 times the life of the untreated tap when nitrogen was implanted at 50 keV, and the same tap steel gained 3 times at 80 keV on the same workpiece. Dies and molds sit in the middle of the range, with a D2 forming die on 321 stainless steel at 2 times life, a D6 die forming titanium dioxide and rubber at 6 times, and a D2 mold forming polymers at 5 times. An H13 roller in rolling service gained 5 times its original life.

Source, Table 6 of Surface Engineering of Specialty Steels, in ASM Handbook Volume 5, Surface Engineering.

The row with no gain

One row in Table 6 reports a negligible effect, an M2 die forming 1020 steel at 100 keV. The contrast inside the same table is useful. The same M2 die steel forming a different workpiece at the same energy, and in one case with a spread of 2 to 12 times life, shows that the outcome tracks the wear mechanism at the tool surface rather than the tool steel alone. Implantation is aimed at adhesive and sliding wear. A die that fails by chipping, cracking or thermal fatigue is failing for a different reason, and an implanted surface layer does not answer that failure.

What the numbers assume

Each row comes from a different laboratory or production shop with its own tool geometry, surface finish, machine and cutting data, so the multipliers are not additive and are not a ranking of tool steels against each other. Two practical points follow from the mechanism. The process is a line of sight vacuum operation, so the faces that wear are the faces that have to be presented to the ion beam. The effect also lives in the outermost nanometres, so grinding or polishing after the treatment takes the implanted layer off with the metal. That places the treatment at the end of the tool route, on the finished geometry.

Source: ASM Handbook, Volume 5, Surface Engineering, ASM International, 1994.