High-temperature tribology of Hf doped c-Al0.64Ti0.36N cathodic arc PVD coatings deposited on M2 tool steel
Introduction
Products and parts for automotive, aeronautical, and various other industrial sectors are manufactured Fe-based alloys, non-ferrous alloys (Al-, Ti-, and Ni-based), among others with highly optimized tools. Cutting tools are crucial elements that enable component production for cars, ships, transportation systems, and airplanes [1]. The cutting speed of metals has been increased (high-speed machining) to improve machining efficiency and productivity through different initiatives around the world [[2], [3], [4], [5], [6]]. The machines, cutting tools, and machining conditions were considered in those investigations. For instance, improving the processing conditions of difficult-to-machine metals such as Inconel alloys are state-of-the-art challenges [[6], [7], [8]]. These high-speed machining concepts consider the use of cryogenic coolants and cutting under dry conditions (dry machining), which would be ideal for metal processing considering the environmental benefits and health impacts since little or no waste and non-toxic side-products are produced. In turn, dry-machining of metals at high speeds leads to elevated mechanical and thermal loads, accelerating wear and causing catastrophic damage to the surfaces of tools, thus impacting the productivity and quality of the final products. For this reason, the use of hard coatings (e.g., nitrides, carbides or carbonitrides, boron-nitrides, and more recently, oxynitrides) is widely practiced to improve the surface performance working under mild and harsh conditions at high temperature [9].
Dry-machining or gas-cooled machining has been proposed as a green manufacturing process [10,11] that improves both the machinability and lubricity at the tool-workpiece-chip interface [12]. Different gas-protective streams containing N2, CO2, Ar, or a mixture of these have been mentioned for such gas-cooled machining operations. Successful application examples such as dry machining of Inconel 715 using Ar-cooled ceramic tools [13] point out the potential of these approaches. Argon- and oxygen-rich atmospheres reduced notch wear, while nose-wear was commonly observed in such conditions. These investigations strongly focus on tool performance and tool life, and little or no evaluation of the surface material is considered. Machining conditions under high temperatures with elevated friction coefficients and oxygen presence lead to surface oxidation processes. Oxide formation and its microstructural evolution impact the surface resistance to friction, affecting the surface quality and texture of the working material. Additionally, tool wear processes may be accelerated using inert gases such as Ar due to the poor heat conductivity and limited lubrication characteristics of the gas [13,14]. In summary, further knowledge that relates the surface quality and the chemical composition to the microstructural evolution and the surface oxidation (and tribo-oxidation) at high temperatures of protective hard coatings is needed.
The AlxTi1-xN coating is a well-known hard nitride system that has been widely deposited on tool surfaces for high temperature and wear protection; this is due to its resistance to oxidation and the chemical stability of the nitride under sliding against a second body. The oxidation resistance of cubic fcc AlxTi1-xN nitrides is due to the Al-presence, which participates in the formation of a protective Al2O3 layer and reduces the fragile rutile phase formation at relatively high temperatures (>800 °C) [15]. Moreover, Al-contents >5 at. % promote the formation of TiO, Ti3O5 that change the diffusion rate of oxygen and decrease the oxygen penetration depth. Consequently, the phase stability and oxidation resistance of the Al-containing nitrides directly relate to the coating tribological resistance [16]. More recently, AlxTi1-xN oxidation resistance has been further enhanced by adding small amounts of rare earth metals such as hafnium, which facilitates forming a protective α-Al2O3 scale in the air around 800 °C [17]. Hf in AlxTi1-xN also increases the coating hardness, which has been related to wear resistance improvement [18]. Additionally, the nitride phase stability is considerably improved with Hf in the crystal lattice; this was observed in the investigation on the isostructural decomposition of the doped nitride Al0.57Ti0.38Hf0.05N, which shifted to higher temperatures of about 200 °C than Al0.59Ti0.41N [19]. The property enhancement of Hf-contained nitrides is explained by an increase of the micro-strain caused by the size of the Hf atoms inserted into the lattice, crystal size refinement, grain boundary hardening, and grain boundary sliding ability of the crystallites [18]. The investigation of Hf-doped nitrides at high temperatures has been oriented to understand the Hf effect on the crystalline structure, the mechanical properties, and the oxidation resistance. These results show the importance of the Al1-x-yTixHfyN coating systems for high-temperature applications. In the case of dry-machining or gas-cooled machining, it is needed to determine the tribological behavior of the Al1-x-yTixHfyN coating. Studies regarding the tribological behavior in Al1-x-yTixHfyN coating have only been carried out at room temperature, and for these reasons, it is important to analyze the Al1-x-yTixHfyN coating oxidation and wear mechanisms (tribo-oxidation) at high-temperature.
The Hf-doped coatings have been deposited using targets with fixed alloy composition(s) and certain Hf-content [20]. An alternative to vary the Hf-content in the coating formulations can be through pure Hf-targets combined with the corresponding alloy targets, such as Al/Ti. Nevertheless, relatively big target materials, i.e., Ø = 100 mm or bigger, would be needed for industrial-scale production of the Hf-doped coatings, and pure Hf-targets in these sizes are extremely expensive, making the process unsuitable for commercial purposes. The single documented Hf-doped AlTiN coating developed in semi-industrial equipment appears to be the work of Xu et al. [20]. They investigated arc-PVD Ti0.48Al0.5Hf0.02N coatings deposited in a PVD unit from Balzers Oerlikon and used 99.99% pure Ti0.48Al0.5Hf0.02 targets. A detailed investigation on the Hf effect in the nitride, the thermal stability, and oxidation of the coating was reported. However, no significant information indicating the industrial feasibility was mentioned. For these reasons, in the present investigation, an alternative option to add Hf into the c-AlxTi(1-x)N-based coatings was explored. The effect of this methodology on the microstructural and mechanical properties of the Hf-doped c-Al0.64Ti0.36N coating is studied. A pin-on-disk test allowed the evaluation of the oxidation and the crystalline phase evolution at high-temperature and their correlation with the tribological performance of the coating at 900 °C in Ar-jet varying cycles number. Additionally, the influence of the oxides generated at 900 °C on the mechanical performance of the Hf-doped c-Al0.64Ti0.36N coating and the failure mechanisms induced by Rockwell indentations were analyzed.
Section snippets
The coating deposition
The pure Al/Ti-nitride and the Hf-doped coatings were deposited on hardened mirror-polished (surface roughness - Sa = 97 nm) M2 steel samples of 1″ diameter and 5 mm thick in the PVD semi-industrial coater model DOMINO Mini from OERLIKON, equipped with four cathodic arc sources, a 3-axis rotation system of 330 mm Ø × 400 mm high effective coating volume, and 120 Kg loading capacity. A 100 mm Ø alloy target of Al/Ti: 66/34 at. % and 99.5% purity from Plansee were used. One and two 99.8%
Microstructure and chemical composition
Fig. 2 displays the 3D AFM images of the Al0.64Ti0.36N (Fig. 2a) and Al0.638Ti0.36Hf0.002N (Fig. 2b) coatings. The surface roughness (Sr) of the reference Al0.64Ti0.36N nitride was Sr = 175 ± 3 nm while the roughness of Al0.638Ti0.36Hf0.002N was Sr = 179 ± 2.7 nm. These results indicate that only neglectable variations in both coatings are produced since similar surface quality was obtained. The main characteristics of the Hf-doped c-Al0.638Ti0.36Hf0.002N coating are summarized in Fig. 3. The
Conclusions
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Polycrystalline c-Al0.638Ti0.36Hf0.002N arc PVD coatings were deposited in 2- and 3-axis planetary systems using a semi-industrial coater.
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The 5.07 μm thick c-Al0.638Ti0.36Hf0.002N and AlTiN displayed similar microstructures and crystalline states. Both nitrides exhibited mainly the cubic fcc + traces of the hcp phases due to the Al-content. However, the mean hardness = 36 GPa & E-module = 416 of the c-Al0.638Ti0.36Hf0.002N indicated that such low Hf-content is enough to reinforce 12.5% more the
CRediT authorship contribution statement
G.C. Mondragón-Rodríguez: Conceptualization, Methodology, Writing – original draft, Writing – review & editing, Supervision, Project administration. J.L. Hernández Mendoza: Methodology, Investigation, Formal analysis. A.E. Gómez-Ovalle: Methodology, Formal analysis. J.M. González Carmona: Methodology, Writing – review & editing. C. Ortega-Portilla: Methodology, Writing – review & editing. N. Camacho: Writing – original draft, Writing – review & editing. D.G. Espinosa-Arbeláez: Resources,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was carried out under the financial support of CONACYT (National Council of Science and Technology), the “Frontiers of Science” program project No: 2015-02-1077, & the financial support provided through the FORDECYT projects 297265 and 296384.
Thanks to the group of the Ceramics Department of the Metallurgical Research Institute of the UMSNH in Morelia Michoacán for providing access to the nanoindentation equipment.
Thanks to the National Lab – CENAPROT, FADMAT for providing the
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