Elsevier

Surface and Coatings Technology

Volume 201, Issue 7, 20 December 2006, Pages 4389-4394
Surface and Coatings Technology

PVD-coated tools and native ester – an advanced system for environmentally friendly machining

https://doi.org/10.1016/j.surfcoat.2006.08.089Get rights and content

Abstract

This paper presents results from the turning of stainless steel and the drilling of the nickel-based alloy, Inconel 718, using carbide tools coated with different PVD coatings. Newly developed coating systems like finecrystalline TiAlN resp. AlTiN, nanocomposites based on TiN/a-AlN and crystalline γ-Al2O3 have been analysed during these cutting tasks. Instead of an emulsion, synthetic esters are used which are readily biodegradable. The phenomena and the causes of tool wear have been studied. In the cutting experiments, the tested hard material coating systems demonstrated significantly enhanced wear behaviour in comparison to a conventional coating based on TiAlN. In combination with adapted coating systems, synthetic esters may represent an efficient alternative to conventional water-based lubricoolants, even in difficult machining operations. They are not only able to contribute to environmental protection and to improve the working environment, but also enhance tool performance.

Introduction

The insight that lubricoolants generate high costs and represent a hazard to the health and to the environment formed the impetus to develop dry machining methods and minimum lubrication technology [1]. As experience has shown, however, not all processes may be carried out dry or using minimum lubrication. In these cases, the use of oils as lubricoolants represents an alternative to emulsions. With respect to achieving a production process that is of less cause for concern related to the health and to the environment, rapidly biodegradable ester oils produced from vegetable raw materials which are free of additives should represent the lubricoolant of choice.

The loss of tribological functions associated with the lack of additives that the lubricating oil undergoes must be compensated for by surface modification of the cutting tools [2]. Tailored PVD wear-protection coatings possess potential to provide such compensation.

Section snippets

Experiments

Turning and drilling experiments with differently coated carbide tools have been performed to investigate the system “PVD-coated tool/native ester oil”. Turning of the austenitic steel material, X5CrNi18-10 (AISI 304), and drilling of the nickel-based alloy, Inconel 718, were carried out. These materials were chosen, as their physical and mechanical properties lead to great demands being placed on the wear resistance of cutting tools. Furthermore, they are generally machined only when

Environmentally friendly lubricants

Besides the base oil and water, emulsions also contain a range of additives, such as emulsifiers, corrosion inhibitors, biocides and fungicides. These are harmful to the environment and hazardous to the health of those in contact with them [1]. In comparison to emulsions, lubricoolants which are immiscible with water demonstrate a significantly lower hazard to the health and to the environment. They thus represent an environmentally friendly alternative to the use of emulsions. In many

Test results for turning austenitic steel and discussion

The dominant type of wear which occurs when turning austenitic steel materials using uncoated cemented carbides is the formation of crater wear. In the case of the uncoated WC–Co cemented carbide shown at Fig. 3a, the breakthrough of the crater to the minor cutting edge after a cutting time of only 40 s led, under the given conditions, to the failure of the insert. The reason for the crater wear in uncoated cemented carbides is the diffusion of iron from the chip into the cobalt binder phase.

Test results for drilling Inconel 718 and discussion

Drilling is a highly challenging process that places great demands on the tool and on the lubricoolant. The following are characteristics of the drilling process: the cutting speed which drops to zero from the outer diameter to the centre; the difficult removal of the chips, which occurs via the flutes in the opposite direction to the feed motion of the drilling tool; friction and adhesion processes between the lands and the wall of the hole, as well as between chip, tool and the wall of the

Conclusions

When machining austenitic steels and the nickel-based alloy, Inconel 718, emulsions are usually used as a lubricoolant. A more environmentally friendly alternative is the use of readily biodegradable synthetic esters made of renewable raw materials. On the downside, the use of ester oils instead of emulsions causes the cooling and lubricating effects to change in proportion in the cutting process. Due to the high water content in emulsions, which is usually in the region of > 90%, it is the

Acknowledgments

The work presented in this paper has been supported within the framework of the Collaborative Research Centre (Sonderforschungsbereich – SFB 442) entitled “Environmentally Friendly Tribological Systems” of the German Research Foundation (Deutsche Forschungsgemeinschaft – DFG).

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