ICMCTF2012 Session E4-2/G4-2: Coatings for Machining Advanced Materials and for use in Advanced Manufacturing Methods

High temperature loads in cutting processes can cause high tool wear and damages in the subsurface zone of the workpiece. Especially the interaction between the different cutting parameters affects the thermal loads in the cutting zone. Hence, the knowledge of temperatures in cutting processes is an important fact and is under the main focus of current investigations. Therefore, this paper deals with an in-process monitoring system for the resulting temperatures in a turning process. In contrast to the investigations performed hitherto, this research deals with a new tool sensor system for temperature measurement. This sensor system is realized by a PVD coating of a Nickel and a Nickel-Chromium layer on the rake face of cutting inserts. On the junction points of this layer system, three thermocouples are formed. The development of the coating system and the resulting measurement is shown. Additionally, the results are discussed in comparison to thermal imaging system and conventional thermocouples.

A very new development in cutting tools has been made for machining aluminum. Hybrid PVD coatings consisting of a superhard TiSiN base layer and a low friction CrC_x/a-C:H top layer are used on cutting tools where the combination of low friction and resistance to abrasion is required. For example when machining aluminum silicon, CrC_x/ a-C:H resists the adhesion and galling of aluminum to the cutting tool by reducing friction and TiSiN strongly resists abrasive wear.

Coatings deposited by sputtering Cr targets in an Ar + CH₄ glow discharge have been previously studied. A nanocomposite microstructure was formed containing CrC_x crystallites in a Cr containing amorphous diamond-like C:H matrix [1].

In this research, TiSiN films with a CrC_x/C top layer have been produced in a hybrid cathodic arc/sputter physical vapor deposition system. Applied by cathodic arc plasma deposition, X-ray diffraction shows TiN changes from a (111) preferred orientation to (200) texture structure with silicon additions. When CrC_x/C is subsequently deposited by sputtering, CrC XRD peaks are not found. The material is amorphous. SEM and TEM reveal the complex microstructure of the hybrid coating.

Measured by nano-indentation hardness tests, Si is shown to increase the hardness of TiSiN. When machining abrasive 390 aluminum, the wear resistance of taps coated with TiSiN base layer and a low friction CrC_x/C top layer is likewise improved with silicon additions to TiN. The CrC_x/C top layer is shown to have frictional and wear characteristics similar to DLC coatings.

The knowledge of tool wear mechanisms in milling is pivotal for explaining the coating failure and adjusting appropriately the cutting conditions. In the described investigations, coated cemented carbide inserts were applied in up and down milling hardened and stainless steel for monitoring the tool wear at various repetitive cutting loads and durations. The variable stress, strain and strain rate fields developed in the tool during cutting affect the film-substrate deformations and in this way the resulting coating loads and fatigue failure.

For investigating the influence of cyclic impact loads magnitude and duration on the film fatigue failure of coated specimens, an impact tester was employed which facilitates its force signal modulation. By this device, repetitive impact loads with variable duration and time courses were exercised on coated cutting inserts. These loads approximately simulated the developed ones in milling at various kinematics and process parameters. The attained tool life up to the film fatigue failure was associated to a critical force and an entry impact duration. The latter factors converge sufficiently to the tool life in all investigated milling kinematics and material cases.

Keywords: PVD coatings, milling, film fatigue, fatigue critical force, entry impact duration

The present paper investigates the cutting performance of PVD coatings during dry drilling of sustainable ADI material produced by the continuous casting-heat treatment process [1]. The cutting tools used were PVD coated carbide tools. SEM and EDS analyses were performed to investigate the surface characteristics of PVD coatings. Cutting performance of different coatings was evaluated by measuring cutting forces, chip types and under-surface chip morphology. The phenomena and the causes of the tool wear were studied too. The surface alteration at the machined subsurface was confirmed from the hardness variation. It was shown that the major cause of improved cutting performance and wear behavior of PVD coated tool is the enhanced tribological adaptability of multilayer (Ti,Al,Cr)N coating. The adaptability of multilayer (Ti,Al,Cr)N coating can be explained by the formation of oxide layers on the cutting tool's surface, which further increase tool wear resistance and lubrication at cutting zone. These oxide layers reduce the severity of friction at the tool-chip interface by reducing the strength of adhesion bonds at this interface.

In the field of diesel injection, pressures of up to p = 2500 bar are currently implemented. As a result, stresses of the materials require special steels with good levels of purity, so that the alloying with lead or sulfur to improve machinability is no longer possible. Those steels are machined with geometrically defined cutting edge in an annealed condition, which represents a particular challenge in terms of chip breakage. On the other hand, steels are also machined in the heat treatment condition needed for the application to avoid additional heat treatment steps and costs. One example is quenched and tempered steel which is drilled with a material hardness of 40 HRc.

In the area of fuel injection, the injection pressures are not comparable high, but because of the corrosive attack by the fuel stainless steels are used mostly. Due to the high content of alloying elements stainless steels have a specific property profile. Compared to engineering steels the heat capacity is higher and thermal conductivity is lower.

Thus the tool coating is of utmost importance for the process design. Particularly when machining stainless steels and high strength steels, the coating acts as a thermal barrier. A smooth surface is necessary for drilling processes to remove chips out of the hole safely. Reproducible and long tool lifes are also important for a trouble-free production. However, the short time of modern production development cycles allows no extensive testing for each machining task to determine the optimal coating. Therefore, the development of hard coatings with a broad application is also important.

The results of this investigation show examples for the design of modern precision machining processes for the high-volume production of components for diesel and gasoline injection technology.

CGI (compacted graphite cast iron) has been known for since long as ductile iron from 1940’s and recently becomes more popular to be used for blocks of a diesel engine. CGI has almost twice tensile strength than conventional gray cast iron and more heat resistant than aluminum alloys. CGI, on the other hand, is known to be a hard-to-machine material due to its high tensile strength and usually CVD coated thick oxide coating (more than 10 microns) is a choice for machining. Wet turning test of CGI with commercial thin TiAlN indicated that main wear is concentrated on the flank face and once the coating is worn our and WC-Co is exposed, intensive adhesive wear is observed. So our idea is to apply thick PVD coating to the level of more than 10 microns to prevent the worn out of the coating.

Conventional PVD coating process is usually limited thickness less than 10 um due to high residual stress involved. We have developed a new magnetically modified cathodic arc evaporation source thatenables to reduce the intrinsic stress substantially, making it possible to deposit thick PVD nitride coatings more than 10 microns. (Ti1-xAlx)N coatings with different compositions (x=0.25 to 0.6) were deposited by conventional and the new arc cathode. Cutting tests were performed against CGI, using WC-Co cutting inserts. Cutting conditions were 250m/min, DOC=0.25mm, Feed=0.34mm/rev. The effect of Al concentration on the life time of cutting tool is investigated in detail. As the criteria for tool life was set to 300 um of flank wear, TiN failed only cutting length of ca. 2.9km. The tool life was increased as the Al content was increased and reached maximum value of 15.7 and 17.5 km for 10 um (Ti0.5Al0.5)N and (Ti0.4Al0.6)N coatings deposited by the new cathode. Whereas commercially available (Ti0.5Al0.5)N coating failed at 10km. This clearly shows the advantage of thick PVD coating deposited by the new cathode. The effect of coating thickness on the tool life will also be reported.

While protective PVD tool coatings have, over the past years, become more and more resistant to the extreme environments associated with modern machining processes, a new challenge has occurred, to create measureable and homogeneous wear of such films in tribological lab-scale testing. Common pin-on-disk tribotests at elevated temperatures can often lead to either erratic wear mechanisms, that do not correlate well with cutting tests, or even to no measureable wear at all. In order to establish a valid set of high-temperature wear test conditions, we used a state-of-the-art high-temperature pin-on-disk tester and optimized conditions such as normal load, sliding speed and –distance. For this study, two families of coatings covering a range of physical properties such as hardness, modulus and adhesion strength were deposited using an industrial rotating cathodes arc PVD process.

The first set of coatings comprised a series of nanostructured Al-Cr-based oxynitrides deposited on cemented carbide, where nitrogen was subsequently substituted with up to 100 at.% of oxygen. These coatings are known to withstand extremely high temperatures in dry milling and turning of high-strength materials while still maintaining low mechanical wear, and differentiation of their wear resistance by laboratory tribotest had, in the past, proven difficult.

The second set of coatings investigated were thick nanolayered AlTiCrN PVD coatings deposited on cold-working steel, designed for use in challenging forming processes of high-strength steel. Here, the practical benefits of the coatings are rather related to a good fracture toughness and lubricious properties, while temperature resistance is limited by the substrate base material.

High-temperature pre-tests against alumina balls were run in order to identify the optimum parameters in ambient air at 600°C (AlCrON series) and 400°C (tool steel samples). The key parameter turned out to be the applied normal load, but a long enough total sliding distance was needed, too. The wear under these conditions was found by profilometry to be very low for the best coatings (≤10^-16 m³N^-1m^-1) and governed by an abrasive mechanism, where some micro-scale cohesive fracture was also observed, which was confirmed by FIB cuts. A straightforward correlation of the wear coefficient to coating hardness was not found. EDX mapping revealed that oxidation played a limited role for nitride coatings, while oxynitrides remained inert at 600°C thanks to their superior oxidation resistance. Therefore, supplementary experiments were conducted on selected AlCrON samples, using an increased temperature of 800°C, which allowed for a proper wear ranking.

Drilling is a cutting application with very high demands on the wear resistance of PVD-coatings: They have to withstand a high temperature influence at the cutting edges and along the margin, as well as the high friction and squeezing area along the main cutting edge and at the tip.

To solve these demands, a new (Al,Ti)N-based coating was developed. (Al,Ti)N-coatings are applied in a wide range of cutting applications today. An optimized coating architecture and a defined grain size result in a tailored hardness and fracture toughness of the coating, just as high oxidation and thermal shock resistance. With these optimizations the new coating offers a high performance as well as a broad band solution for drilling of different steels and cast iron even with a high variety of cutting parameters.

For the realization of the new coating, the chemical inertness of the surface was as important against chip adhesion as a smooth and polished surface in the flute. Therefore, the post-treatment is an important factor for the wear behaviour and the performance of drills . A detailed investigation of different post-treatment methods gives an overview and shows their influence by drilling test results. To understand the effectiveness and influencing parameters of post-treatment methods, a systematic investigation of selected processes was done. Their impact to the surface roughness and overall quality, the coating thickness as well as the coating stress and the micro hardness will be shown and are combined with the results of cutting tests and torque measurements. This leads to an essential input for the understanding of the correlations between post-treatment and machining results.