decoating (stripping)

Article stripping v1.0.doc JPr + CBü, 21.09.2009 1 / 7

DECOATING (STRIPPING) 1. Introduction Most money of a complete life-span of a tool is not gained with new tools but with reground and recoated ones. The carbide drill is a good example: a reground and recoated tool sells for 1/3 of the price of a new tool and is reground up to 10 times. This adds up to more than three times the sales price of a new tool. The second example are gear hobs: They are reground and recoated up to 15 times. It is important that the tool life achieved with a reground and recoated tool is the same as with a new tool.

Fig. 1: Service life of an uncoated, coated, reground, and reground recoated tool (left to right)

2. Why decoating? Any coating must be deposited on a solid surface, in order to achieve a good adhesion of the coating and a good tool life (comparable to a new tool’s one) as a result. The adhesion of a second coating on a previous one drops as the thickness of the preceding coating(s) increases. The reground tools may hence be recoated two or three times without decoating, and still achieve a good life time.

Fig. 2: Over coating: worn before regrinding, reground, edge-rounded, and recoated. However, the thickness of the coating on the non-reground surfaces will increase (Fig. 2). The adhesion of the new coating is too low, after the third over coating latest. This results in a massively lower tool life. In order to avoid this problem, the tools must be decoated (stripped). Most of the decoating processes rely on chemicals, which decompose the coating but attack most likely the underlying tool material. This may happen especially with cemented carbide (see Section 3).


Fig. 3: The less good but often practiced tool remanufacturing process. The worn cutting edge is reground first, then decoated, edge-rounded and recoated. This damage result in a bad adhesion of the coating normally and thus to a lower tool life. This is the reason why it is very important to carry out the individual steps of tool remanufacturing in the proper order. Regrinding exposes the base material. This is the best condition for a good adhesion. But the base material may be damaged, if the stripping takes place after the regrinding, thus the adhesion decreases. The following edge-rounding step (e.g. microblasting or brushing one) will improve the situation. Nevertheless, depending on the depth of the damage the surface may still be in a bad state (see Fig. 3) and the coating adhesion insufficient.

Fig. 4: The best tool remanufacturing procedure: 1st decoat, 2nd regrind, 3rd edge preparation, 4th recoat. For this reason, first should be decoated and reground afterwards, as displayed in Fig. 4. In order to unfold the full potential of the edge-rounding step, one has to decoat first and regrind afterwards, like it is possible in an in-house coating center (see Fig. 5).

Fig. 5: Possible sequences of tool remanufacturing process steps. [1]


3. Why is it so difficult? All but one industrially applicable, chemical decoating procedures make use of water-based stripping solutions. There are several advantages (simple handling, low costs, health and environmental protection) with this methode but some disadvantages, too:

a. HSS corrosion: It is well known that HSS may corrode when in contact with water and oxygen. The risk of corrosion may be considerable in a water-based stripping solution, depending on its composition. The tool surface activated by the stripping solution will corrode easily in air, even when the decoating is finished and the tool has been cleaned.

b. Chemical likeness of coating and tool material The chemical reactivity of the cobalt in the tool material is quite similar to that of titanium and/or chromium in the coating. This fact makes it a big challenge to dissolve the coating and at the same time preserve the cobalt-containing base material. This task is even more complicated when titanium carbide or chromium carbide are present in the cemented carbide. After a loss of 0.1 to 0.5 !m of base material, the surface may be e.g. polish-blasted so that the coating applied afterwards adheres well. Note that some stripping solutions do not only attack cobalt but as well the tungsten carbide of the cemented carbide, with very similar consequences to the adhesion of the coating applied afterwards.

c. Cemented carbide corrosion - „Cobalt Leaching“ A carbide grinder knows that the carbide may get damaged if it remains in contact with water. Cemented carbide is a composite material consisting of tungsten carbide grains and a cobalt-based matrix as the binder (Fig. 6). The binder dissolves slowly when in contact with water and thus removed from the tool surface. This is called “cobalt leaching”. The lack of binder results in decreased cohesion of the carbide grains which remain loose on the base material. The surface becomes dull or even black. The depth of cobalt leaching is decisive. A depth from 0.1 to 0.5 !m may be “repaired” by post-treatment. Deeper cobalt leaching results in a mechanically unstable surface and if a coating is applied it shows weak adhesion or none at all. Therefore, the perfect stripping solution dissolves the coating but does not damage the cemented carbide.

Fig. 6: Schematic of an intact (left) and “cobalt-leached” cemented carbide surface.

d. Handling, general safety and environmental compatibilty The chemicals used or generated within the decoating process should preferably be easy to handle and harmless to people and environment. Usage of nature-identical chemicals is an advantage for their easier disposal.

e. Time and costs The decoating plant should not be too expensive, neither its purchasing price nor its operational costs. The operational costs comprise energy and chemicals consumption, personnel costs and waste disposal costs. The stripping process should be a fast one to enable short tool remanufacturing times.

Tungsten carbide

Cobalt


4. What methods are there? There are three main groups of decoating methods: mechanical, physical, and chemical. Mechanical decoating methodes (e.g. microblasting) remove more base material than the wear-resistant coating. Apart from that, rough blasting media leave rough surfaces. A physical decoating method to be named is plasma etching. This method may only be applied on DLC coatings, but it is quite expensive. The chemical methods use a liquid stripping media for decoating. The old coating must be oxidized and solve in the liquid media. The oxidizer is usually an oxygen-rich chemical (e.g. hydrogen peroxide); halogen-containing oxidizers are less popular due to their lower environmental friendliness. The stripping liquid may be either water-based or water-free (solvent-based). Although solvent-based liquids yield better results in laboratory tests, they may hardly be used in industrial environment due to safety concerns (expensive reagents are required along with flammable and poisonous solvents which have to be recovered expensively). Water-based methods are thus the only ones, which come into question for large-scale decoating. Some of them require electric current as well (electrolysis), in order to dissolve the coating.

5. PLATIT decoating processes PLATIT started to develop its own stripping media in 2003, due to unsufficient decoating methods available on the market. The first stripping media realized was for HSS, named AlZiRo for its inventor. The powder, consisting of several components, is heated up to 60°C with water and hydrogen peroxide in order to strip titanium and aluminum based coatings without damaging the base material. The development of a stripping recipe for carbide proved difficult. Only after many experiments, calculations and many more experiments, a stripping solution came to being, consisting of a coating removal component, cobalt-specific protection components, catalyst, and direct current. This stripping solution is based on hydrogen peroxide. Chromium containing coatings have been in the market since 1985, but started gaining importance only after 2003. They were not supported by PLATIT in the beginning, because the highly toxic and carcinogenic chromium(VI) is produced while stripping. In order to satisfy the demand of the market for decoating of chromium containing coatings, PLATIT has been offering a corresponding stripping solution under the responsibility of the customer using it since 2006. The stripping solution for chromium containing coatings on cemented carbide leaves a brown stain on the tool. These are removed by putting the tool into a second water-based solution. Chromium based coatings on HSS are, on the contrary, decoated in one step in a water-based solution and direct current.

Coating Type Base Material Module Chemistry Duration Environmental Compatibility

Ti,Al based carbide ST-40 HM Electrochemical 6 – 28 hrs. ok Cr based carbide ST-40 HM-C Electrochemical 0.5 – 1 hrs. Cr(VI)

Cr,Ti based carbide no method Ti,Al based HSS ST-40 HSS Wet chemical 1 – 4 hrs. ok Cr based HSS ST-40 HSS Electrochemical 0.2 – 0.5 hrs. Cr(VI)

Cr,Ti based HSS ST-40 HSS Electrochemical 1 – 4 hrs. Cr(VI)


Fig. 7: Functional principle of the ST-40 HSS and ST-40 HM modules.

5.1 PLATIT stripping modules PLATIT has developed the corresponding modules for the individual stripping solutions in order to decoat efficiently with the minimum possible quantities of chemicals.

Fig. 8: The stripping modules of the ST-40 range. On the right: A basket with shank tools and a holder with a hob.

The ST-40 HM consists of a chiller, a circulation system, and a DC power source. The ST-40 HSS consists of a water cooler, a heater, a circulation system, and a DC power source for accelerated stripping. The ST-40 HM-C has a tank out of a material optimized for the process and a shower to connect on a demineralized water line. The ST-40 R is a rinsing module with a shower and it is used for rinsing of the stripping media on the tools after the decoating process. The ST-40 P is used to dip the stripped tools into a solution for temporary protection against corrosion / cobalt leaching before the tools are cleaned in a tool cleaning machine. The tool holders conduct electric current to the tools and prevent them from any mechanical damage.


6. Waste disposal The disposal of spent stripping solutions is subject to various regulations depending on the country (EU / USA / other countries). All PLATIT solutions are based, as far as possible, on natural / nature identical chemicals. Some spent solutions contain, however, compounds which are treated as “Hazardous Waste” (e.g. Cr(VI)), and may not be disposed into public sewage systems. Most tool regrinder companies produce “difficult” heavy-metal containing grinding slurries as well as oil emulsions, which have to be disposed by local chemical waste disposal companies as required by law. These companies are able to dispose the spent stripping solutions without any problems and relatively cost-effectively. All this is due the low concentrations, relatively low quantities and, above all, absence of any oil-containing components in the spent solutions.

7. Costs The stripping costs per tool depend heavily on the stripping solution and the number of tools stripped in one batch. The decoating of chromium-containing coatings from HSS or carbide tools, as well as Ti,Al-containing from HSS, make up to 30-40 % of the total costs in the recoating process. For Ti,Al-based coatings on carbide, however, it is up to 70 % of the total costs in the recoating process. This is because of the more compex process and the more expensive chemicals.

Fig. 9: Decoating costs for a dia. 6 mm drill and a dia. 10 mm end mill

8. Future prospects A good adhesion of a new coating on previously decoated tools, and consequently a good tool life of remanufactured tools depend on many factors:

- proper order of the individual steps of the tool remanufacturing process - decoating plant technology - chemicals used for the decoating


9. Conclusion No later than after two recoatings, the tool must be decoated (stripped) in order to guarantee a good tool life after remanufacturing. Depending on coating and tool material, various chemical or physical decoating processes are applied. However, all these attack the tool material as well. For this reason, the decoating must precede the regrinding in the remanufacturing process. This is made possible with in-house coating [1]. PLATIT developed proprietary water-based electrochemical processes. The decoating plant consists of five individual modules out of which the proper ones are used for the given coating / tool material combination and post-treatment. Most of the used chemicals are environmentally compatible. Chemical waste disposal companies dispose the spent solutions. The decoating costs are for all other coating / tool material combinations much lower than for Ti,Al-based coating on carbide.

10. Literature [1] PLATIT Compendium 2009/10

decoating (stripping)

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