ICMCTF2005 Session E5/G4: Tribological Studies of Coatings for Green Manufacturing and Dry Machining

Uniform coating of thick diamond-like carbon (DLC) was prepared on many kinds of large three-dimensional works with a developed machine for a hybrid process of plasma-based ion implantation and deposition (PBII&D). In this system, a pulsed RF power for plasma generation and a high-voltage pulse with fast rise time for ion implantation were supplied to the work through the single electrical feed-through with matching network for superimposing both pulses. As the work worked as an RF antenna, RF plasma was produced along on the whole surface of work and ions were implanted to the work surface from an afterglow plasma by the high-voltage pulse. The pulsed RF (13.56 MHz) power was 50 - 3000 W with 5 - 150 μms and the negative high-voltage with the voltage of -5 ~ -20 kV and the duration of 2 - 5 μms pulse was supplied at 50 - 150 μms later after turn off of RF pulse. The repetition rate of both pulses were 0.1 ~ 4 kHz.

The thick DLC was made from hydrocarbon gas such as CH₄, C₂H₂, and C₆H₅CH₃ using 4 steps of PBIID process. Ion implantation was produced a graded interface between the DLC film and the surface of material, resulting in enhancement in adhesion. A deposition method with ion implantation using suitable voltage was able to reduce the residual stress in the film and as a result, thick DLC film with over 40 μmm thickness was prepared successfully.

On the other hand, the film thickness profile was observed to be extremely uniform (uniformity: above 93%) on the surface of cylindrical stainless pipe and the hexagonal sample holder. But case of using aluminum trench with an aspect ratio of 1 - 2 profile was diffused about 1.3 - 1.9 times. The hardness of DLC film was 1100 - 1500 Hv and the deposition ratio was approximately 1 - 3 μmm / h as using C₆H₅CH₃ gas.

DLC film was formed on the surface of many kinds of works, e.g., inside of pipe, piston of car engine, mold of shoes, printing roles, plastic sheet, and large metal plate of 73 cm in diameter.

Dry machining is considered as a key issue in environmentally conscious manufacturing in order to reduce the amount of waste generated through the use of lubricants and cleaning agents. Self lubrication through in-situ formation of lubricious oxide tribofilms has been reported to show potential in the further development of dry machining.

Tribological reactions during machining using cutting tools with Cl-implanted TiC, TiN and TiCN protective coatings have been found to result in the formation of Ti base oxide (TiO_x) films which are readily plastically deformed, hence act as self-lubricant with resulting low friction and wear. The implanted Cl diffuses in the coating layer and the self-lubrication continues until the coating is depleted. The film formed in the vicinity of the cutting edge has been reported as a complex distribution of oxides, with the region closest to the edge consisting mainly of TiO_x, which plays the key role in the process.

In addition, the cutting speed limit was significantly increased beyond 400 m/min in Cl-implanted TiCN coated tools. Above 300-400 m/min, the flank wear increased exponentially with cutting speed in the case of unimplanted TiCN coated tools, while the flank wear increased only linearly with cutting speed for the Cl-implanted TiCN coated tools. This improvement in dry machinability is attributed to the in-situ formed tribofilms along the flank surface.

The formation of self lubricating oxide tribofilms during dry machining using cutting tools with Cl-implanted Ti-base coatings was studied using various analysis techniques such as laser microscopy, SEM, EDX, and XPS. This was in order to investigate the processes involved in the tribological reactions and oxide formation, as well as the resulting improvement in dry machining performance.

High speed cutting and dry machining are two currently discussed issues for reducing production costs in metal cutting industry. Physical vapor deposition (PVD) coated cutting tools play an important role for making these technologies possible. The current study discusses the drawbacks and opportunities of new chromium based coatings for dry machining applications.

In order to find improved coatings, both tribological and practical wear tests were performed. The characterization of the coatings includes fundamental properties such as thickness and hardness, as well as a detailed analysis of the coating composition. The adhesion quality was determined by scratch tests. In reciprocating sliding tests friction and wear has been studied in a ball-on-disc arrangement. Dry turning operations were performed in order to study the wear behavior during practical application. The wear properties of Cr_xAl_yN and Cr_xAl_ySi_zN coatings were compared to uncoated carbide tools. SEM analyses supplement the picture of the wear mechanisms in dry machining with chromium based cutting tools.

Due to high costs, infrastructure demands, and environmental concerns, there is motivation to move toward dry machining, i.e., machining without the use of metal removal fluids (MRFs). Aluminum, which has become the material of choice for light-duty engines and transmissions, is particularly difficult to machine dry because of its tendency to adhere to the tool as temperatures rise. Machining performance suffers when machining is done without MRFs. For example, tool life during drilling is reduced from >10,000 holes/drill with MRF to about 40 holes/drill without MRF (dry).

The challenge, then, is to reduce the heat build-up through improved tribological surfaces on the tool. In this study a variety of carbon-based coatings on drills were tested to determine their performance in both bench and machining tests. Coatings included metal-containing carbon, graphitic, hydrogenated and hydrogen-free diamond-like carbon, and diamond. The best coatings gave a >100-fold improvement in performance compared to an uncoated drill, approaching the performance possible with MRFs.

Nowadays economical and cost-effectiv machining has become the main driving force for tool and coating development. Hence, PVD coatings already lead to a tremendous increase in performance of modern cutting tools. The present paper deals with an innovative PVD coating concept which will lead to an further increase in process and machining reliability.

The concept consists of special thin film sensor for wear measurement which has been integrated into a multilayered coating system of cutting inserts. The whole layer system consist of an interface, a hard isolating layers of Al₂O₃ and a nano-sensor film of TiN sputtered in one run on the surface of standard inserts. Whereas interface and sensor layer are in the order of 100 -500 nm, the Al₂O₃ layer was between 2 -5 µm, depending on the roughness and shape of the insert. After micro structuring of the sensor film, a second insulating layer of Al₂O₃ or AlN was applied followed optional by a typical wear protection of TiAlN or TiN. The whole layer system will be applied in only two deposition processes. The novel coating system proved to be very flexible and can be applied to hard metal as well as to ceramic tools.

Based on the bipolar dual magnetron approach, CemeCon contributed the High Ionization Pulsing (H.I.P.) process that results in a significantly increased ionization and thus allows the deposition of virtually any ceramic hard coating, such as conductive and insulating coatings as well. The highly flexible H.I.P. concept allows the consecutive deposition of nitrides and oxides within the same coating cycle.

For micro patterning of the sensor structures a fast laser direct writing system was used, which allowed the whole patterning of all four flank sides of an insert in less than 40 seconds. The smart tools have been tested by cutting steel C45. It turned out, that the life cycle was twice the value for conventionally coated inserts.

Tool wear is fully recognized as an important factor in materials cutting

The well established methods for its control are based either on process optimisation, application of lubricant or use of tool coatings to provide wear resistance and low friction

Nevertheless, use of external electromotive force sources (e.g. magnetic field) during cutting of material may accomplishes this role

The mains aims of paper are to give a discussion of research results on the improving surface wearing of tools by magnetization and to present quantitative description of the physical mechanisms causing observed effects.

The demand for higher productivity and lower costs in the modern metal-cutting industry has resulted in many improvements, particularly in tool coating technology. Most of these developments have been in the field of hard coatings. However, it has recently been shown that MoST^TM, a solid lubricating coating, deposited on top of an advanced CrTiAlN multilayer hard coating, can significantly enhance the performance of high-speed steel drills. This paper presents the results of an extensive study of the performance of these coatings under a range of severe cutting conditions. It also includes an investigation of the effect of different gaseous coolants on the behaviour of the coated drills.

Tests were performed on a Haas vertical machining centre, using CrTiAlN-coated drills, with and without a top layer of MoST^TM, to drill through medium carbon steel (070M55) workpieces under wet, dry and gas-jet-cooled cutting conditions. Cutting forces were recorded during the tests using a dynamometer, while the tool wear was monitored and measured. The results showed that MoST^TM solid lubricant coatings improved the tool life under all testing conditions, even in exceptionally severe dry-cutting tests. It is proposed that the MoST^TM coating delays the onset of severe wear due to its high inherent wear resistance, its low-friction characteristics and possibly reduced temperatures at the cutting edge. It was also found that a nitrogen gas-jet gave improved performance compared to dry-air-jet cooling, implying that inhibition of oxidation reactions may also be of importance.