We can coat the vast majority of tools, but the benefit of coatings can be maximized by the observation of some simple rules.
Coating should be the very last step in tool manufacturing; rework or alteration of coated tools often negates benefits of the coating.
Optimum tool performance can be achieved by observing the following guidelines:
Identify areas to be coated and/or masked. Specify the substrate material and its temperature tolerance (draw or tempering temperature for tool steels).
PVD coating is a vacuum process, and its results critically depend on the observation of basic rules of the relevant technology. Virtual leaks, that is, the release of impurities, trapped air, and humidity from enclosed blind volumes and crevices must be avoided or at least minimized. For this, parts should be completely disassembled before coating, screws, plugs, inserts removed, internal crevices, blind holes exposed. Cleanliness of crevices and coolant ducts is critical.
Items for PVD coating should be received free of shop soil, lapping and cutting compounds, lubricants, coolants, and other chemicals. The use of rust preventives and dry lubricants, which leave a dry, waxy, inert layer on the surface, must be avoided. A 1:16 mixture of machine oil and mineral spirit often yields effective cleaning and rust prevention for days to weeks, depending on the particular environment, temperature, and atmospheric humidity. Alternatively, one can use a water-soluble or light oil based rust inhibitor.
As a rule, the best surface finish yields the highest performance. Surface roughness, burrs, and nicks on forming and cutting tools can diminish or even negate the benefits of the coating.
Grinding or polishing should not cause the contamination the para-surface region by mechanical alloying. Mechanical alloying is the process of rubbing impurities, such as fine abrasive particles, dust generated during sand or bead blasting or wear debris, into the surfaces. During the mechanical working of a surface, such as polishing or grinding, there is always a dynamic equilibrium between contamination of the para-surface region and the removal of impurities together with the host material by the abrasive action.
This equilibrium can be shifted towards impurity removal by the frequent dressing of grinding wheels and avoiding ‘spark out’. For manual polishing, use and change frequently bonded abrasives, such as a metallographic quality wet-and-dry SiC paper. Start with a coarse grit; proceed in small steps with finer and finer paper to desired finish. Do not rub the surface; apply only a light pressure to cut the surface instead of rubbing, and change the abrasive paper frequently. Use plenty of water, change it and rinse the container and the polished surfaces between steps to eliminate lingering larger abrasive particles which can score an already-polished surface.
Do not use rubberized or resinoid buffing wheels. Try to avoid sandblasting, bead-blasting, or vapor-honing surfaces to be coated.
Use cadmium-free braze for brazed tools and other items; there should be no crevices and pinholes in the joints.
Black-oxided surfaces or those covered with heat-treat scale cannot be coated. Moreover, the presence of black oxides or scales can cause major problems in the coating process even if those surfaces are not critical for coating.