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European Journal of Natural History
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THE PHENOMENON OF REDUCTION IN STRENGTH OF HARD-COATED CUTTING TOOLS AND THE CONDITIONS FOR ITS OCCURRENCE

Liberman Ya.L. 1 Letnev K.Yu. 1 Shterenson V.A. 2
1 Ural Federal University
2 Russian State Vocational Pedagogical University
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The article presents a summary of well-known facts on reduction in strength of metal cutting tools with diamond like carbon (DLC) coatings.

The last decade saw an increase in the usage of hard coatings of the diamond-like carbon (DLC) type and ones similar to them. As a rule, they are applied by dispersing coating materials (for example, graphite) in a vacuum with a cathode and depositing them over the working surface of a tool [1–2]. They are fastened on the surface by means of adhesion, without infiltration into it.

Today, significant experience has already been gained from the usage of cutting tools with hard coatings, DLC coatings, in particular. As a whole, the experience is positive, but the accumulated data is uncoordinated and, as a rule, presents results for a specific case only. If there are negative results among them, they are usually deemed to be accidental, caused by errors during either the experiment itself or mathematical treatment of its data.

In view of the above, the authors conducted at the Department of Metal Cutting Machines and Tools of the Ural Federal University an analytic research the goal of which was to generalize currently known facts and figures on the efficiency of tools with DLC coatings, to study deeper negative examples of their usage, and to find out reasons behind those negative outcomes.

The research studied data on the wear and wear resistance of end milling and disk milling cutters, lathe tools, drills of 0,5–2,5 mm in diameter and knives of guillotine shears [3–9]. The methodology of the research was based on the analysis of reliability of the mathematical models which describe the dependence of tools wear on duration of their usage in various combinations of cutting modes and for various machined materials. Also, other sources of data were used, like the one from metallographic examination of the structure of tools before and after they have been used, and also the results of the microscopic examination and profilography of the surface of tool cutting parts.

Thus, the following facts were established:

1. DLC coatings indeed reduce the wear and increase the resistance of cutting tools by 3–4 times within the whole range of cutting modes recommended by «The Reference Book for Production and Mechanical Engineers» [10] for fine-turning and milling operations.

2. In rough-turning or milling operations, on the contrary, the wear rate of DLC-coated tools turns out to be 3-4 times higher than that of uncoated tools.

3. In those operations when holes are machined with drills of 4 mm or more in diameter using the modes recommended by the reference book noted above, the wear of tools also decreases, similar to fine-turning and milling.

4. In those operations when holes are machined with drills of small diameter (up to 2,5–3 mm), the DLC coating contributes to a substantial tool life reduction, causing their breakage.

5. In those operations when metals are machined at high speeds by means of turning, milling or drilling, the strength of DLC-coated tools turns out to be higher than that of uncoated tools.

6. In those operations when thin-sheet metals are cut with guillotine shears, holes are reamed, other types of machining at low speeds of cutting (except those mentioned in paragraph 2 of this list) are used, the strength of cutting tools increases by 2–5 times after having them DLC-coated.

The physics analysis of cutting with DLC-coated tools and its comparison with the conditions of machining noted above allow us to state that those situations when using the coating gives no result or affects negatively are nonrandom at all. There exists quite an evident phenomenon of the reduction in strength of tools which have been DLC-coated, and the reasons behind it are quite specific.

The first one of those is a high brittleness of the coating. In rough-milling or turning operations, the impact of considerable forces of cutting and the deformation of the undercoat (however small it is) cause breakage of the coating and formation of chips and fly grit in the area of cutting. The grit, in its turn, increases the wear rate mentioned in paragraph 2 of the list above.

The second reason is the increase of tool surface roughness and the «tool-detail» friction coefficient. Nowadays, it is commonly accepted that this coefficient decreases after the coating have been applied. This conclusion is made from those experiments when a surface, with or without the coating, is acted on with a small but very hard ball. By the trail size of such a ball, they assess the coefficient – a harder surface is obviously indicated by a smaller trail. Which leads to the conclusion noted above. But, in reality, it is true only for the coefficient of rolling friction, and not as much so for the one of sliding friction, and the «tool-detail» pair is characterized by the latter. The coefficient of such friction increases after having a hard coating of the DLC type applied to the surface. That is because graphite, while being dispersed with a cathode, settles unevenly on the undercoat and forms significant microirregularities whose depth exceeds by more than 50 times the depth of irregularities of a polished undercoat. This leads to the increase of the friction coefficient by more than 30 % [1–2]. But such an increase of the coefficient and, accordingly, the friction force results in a corresponding rise of cutting forces. And if stress in the tool body exceeds the strength limit of this tool, it breaks. That is the reason behind the breakage of drills of small diameters mentioned in paragraph 4 of the list above.

Thus, the cases of the decrease in strength of DLC-coated cutting tools are quite natural and caused both by some peculiarities of their usage and certain specifics of how the coating is applied. However, one can find ways of combating the phenomenon. For this, special measures should be taken to prevent the increase of surface roughness when applying the coating to fine-sized tools, such as drills of small diameters.

Such measures could include oxygen-based ion etching of the coating just after it has been applied (proposed by the authors of the article as early as in 1998 in paper [4]), laser-based pulse vaporization [11], magnetic separation of the dispersed graphite (proposed by the «CreepService Sarl» company from Switzerland [12–13] and used by the «New Plasma Technologies» company since 2011). Also, you cannot use DLC-coated tools for rough or high-speed cutting machining. Although the forces of cutting at an increased speed, as a rule, decrease, the temperature of cutting rises and the structure of the coating changes [1–2]. This explains the inefficiency of the coating noted in paragraph 5 of the list.

The work is submitted to the International Scientific Conference «Engineering and modern production» France (Paris), October 19–26, 2016, came to the editorial office оn 10.12.2016.