Making light of lightweighting
Hailin Sun and Kevin Cooke from Teer Coatings Ltd and Colin Sharples from Kyocera-Unimerco consider the future of self-solid-lubricating coatings for dry drilling applications.
There is an increasing demand for the use of light alloys, such as aluminium, to reduce the weight of vehicles in the automotive and aviation industries.
In the modern structural design of vehicles, light-weighting could dramatically increase efficiency in terms of fuel consumption, not only accelerating commercial growth, but also contributing to environmental protection through the associated reductions in CO2 and other damaging emissions.
Due to the soft and sticky qualities of aluminium alloys, during faster drilling processes, various kinds of liquid lubricants are used in every aspect of mass-production manufacturing to achieve desired process results. As a typical example, a medium sized commercial aircraft aluminium structure needs to have thousands of high-quality holes of various diameters drilled into each wing. To extend the life time of drilling tools and get the high-quality hole surfaces, significant amounts of lubricant have to be used in mechanical machining, much of which has to be carried out in situ during assembly, and this results in a massive emission of wasted lubricant. At the same time, the mist and the vapour from conventional external liquid lubricants in the open factory environment has raised possible health and safety issues. A wet work floor can cause injury through slips and falls, and any inhalation of airborne particle from such drilling applications is less than desirable.
The pressure to reduce the use of conventional liquid lubricants as far as possible in environmentally sustainable manufacturing processes is higher than ever, and this is also a route to signiﬁcantly reduce the environmental burden. It would be a great achievement to totally remove such hazardous liquid lubricants without reducing the efficiency of the drilling process, while maintaining drilling speed and feed rate, and retaining the high surface quality of the resulting holes. The automotive and aerospace industries have, for some years, investigated the possibility of removing lubricants from the majority of their operations.
For these reasons, dry machining has become a challenging issue to be solved, and this is now a demand in many industries.
Meeting the challenge
Is it possible to achieve this? Manufacturing and research scientists have already started to work together. Some new manufacturing systems, such as solid lubricant coatings and high pressure air-cooling channels on drills, have been invented to enable dry machining without any use of external lubricant. One of the key technological issues lies in the tribological control of the interface between the tool and the workpiece materials.
The design of drills is complex. It demands the combination of a sharp cutting edge, highly wear-resistant surfaces, and the desired flute characteristics that ensure chips can move away quickly, especially for cutting soft and sticky materials. The failure of tools in drilling sticky materials is mainly caused by workpiece chips stacked on the drilling surface, which cause the drills to break or damage the quality of the holes.
One idea is to have drills coated with hard and low-friction surface layers. For example, because of the nature of the material being drilled in the aerospace industry, the introduction of a special tool coating is proposed, rather than making multiple changes to the design of the tool. The coating will aid in the drilling process where the material sticks to the cutting tool edges, which causes pick-up and eventual damage to the hole, limiting the useful life of the drill. This problem is exacerbated by the higher temperatures encountered in dry drilling. The coating will aid in the smoother removal of swarf and chips via the cutting tool flutes, preventing the sticking effect that aircraft-grade aluminium (particularly 2,000 series alloy) has on the tool. Previous research has shown that dry drilling is possible, but at least an order of magnitude improvement in tool life is now needed.
Some of the relevant modern surface engineering coating treatments include physical vapour deposition (PVD) and plasma enhanced chemical vapour deposition (PECVD), which are both processes carried out in a vacuum at temperatures of 150–900°C. These techniques entered industrial production in the 1990s. A thin, highly adherent coating with different properties (especially higher hardness, wear resistance and lower friction) are deposited on the tool or components’ surface to improve the service life and increase the efficiency of drilling. For example, in a typical PVD process, the solid initial coating materials are either evaporated by high-energy arc evaporation or atom-by-atom by sputtering bombardment with ions. At the same time, some reactive gases (such as nitrogen or a gas containing carbon) can also be introduced. These constituents form a desired compound that can deposit on the substrate surfaces. The properties of the coating (such as hardness, structure, chemical and temperature resistance and adhesion) can be controlled to a desired level.
One typical cutting tool coating is titanium nitride, which has been widely used for protection of dies and tools from severe wear during the machining process. It has been used for drilling against stainless steel or ductile metallic alloy counter parts. Due to the higher friction of this protective coating, lubricants are indispensable to reduce the friction coeﬃcient and for the cutting process to be free from metallic sticking. As such, the engineering conditions for dry drilling require some ideal tribological coatings, instead of coatings with higher hardness and wear resistance alone. The ideal tribological coating for dry-cutting conditions exhibits no adhesive wear modes over a relatively wide range of cutting parameters with a given sliding velocity. It is very important to realise dry machining without requiring a decrease in the production rate. Tribological features produced by the coatings need to be as smooth as original surface and well preserved, even after initial cutting.
The cutting edge
Recently, new coatings based on graphite and diamond-like carbon (DLC) have been developed by many research scientists and some coating companies, and they have been widely used for tooling and automotive industries in the manufacture of key components such as piston pins and precision dies. These coatings combine very low friction with high hardness, very low wear rates and high-load bearing capacity. Coatings on the drills act as solid lubricants, providing protection for both the coated surface and any uncoated opposing surface. The development of these coatings has been published widely, including many details of the combination of tribological properties and structure of the coating, which are related to the cutting properties.
Solid lubricating coatings, such as Graphit-iC, DLC and duplex coatings, deposited on drills by PVD and PECVD technology, have good wear resistance, reasonable temperature tolerance and very low friction coefficients against the particular metals of interest. This gives them the potential to be employed to fulfil all the essential criteria, such as machining quality and tool longevity. The self-lubrication concept can be introduced into the drilling process on components. Many drilling results have confirmed a promising future for this approach. In different industrial sectors, the range of cutting materials currently designed for many applications is very wide, and the chemical composition and the microstructure of the materials are different. Even though many coatings have achieved some good results on certain materials, they still need to be modified or improved for particular applications to achieve reliable drilling results, especially for the aviation industry.
Due to the future restrictions on the use of external lubricants for drilling components in the automotive and aviation industries, dry drilling, especially for aluminium alloy structural components, provides one solution widely regarded as the way forward to tackle the health and safety issues in these industries. Solid lubricant coatings, such as those based on graphite, DLC and duplex coatings, combined with hard, tough and wear-resistant sub-layers, are being deposited on drills by PVD and PECVD technology, and have good wear resistance, reasonable temperature tolerance and very low-friction coefficients against the lightweight alloys of interest that is needed.
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