ICMCTF2016 Session B5-1: Hard and Multifunctional Nano-Structured Coatings

The influence of surface nano architecture on the sputtering and erosion of tungsten and molybdenum is discussed. We present an experimental investigation of the effects of low energy (150 eV) Ar ions on surface sputtering in Mo and W nano-rods and nano-nodules at room temperature. Measurements of the sputtering rate from Mo and W surfaces with nano architecture indicate that the surface topology plays an important role in the mechanism of surface erosion and restructuring. Chemical vapor deposition (CVD) is utilized as a material processing route to fabricate nano-architectures on the surfaces of W and Mo substrates. First, Re dendrites form as needles with cross-sections that have hexagonal symmetry, and are subsequently employed as scaffolding for further deposition of W and Mo to create nano rod surface architecture. The sputtering of surface atoms in these samples shows a marked dependence on their surface architecture. The sputtering rate is shown to decrease at normal ion incidence in all nano- architecture surfaces as compared to planar surfaces. Moreover, and unlike an increase in sputtering of planar crystalline surfaces, the current measurements show a decrease in the net sputtering rate at oblique angles as compared to normal incidence. Energy deposition in the near surface layer shows that W is also amorphized at room temperature by low energy Ar ions to a depth of 5–10 nm. Long duration experiments in Ar plasmas show that sputtered atoms are re-deposited onto surface architechure, showing surface islands that increase in density and size with plasma exposure. These islands coalesce and re-build the surface achieving partial self-healing of the surface nano-architecture. A discussion of the theory of nano-patterning will be presented to explain the observed experimental features.

Nanostructured coatings are increasingly exposed to a challenging combination of wear factors such as mechanical impact, abrasion and oxidation. This contribution will show ways how physical vapor deposition (PVD) coatings can be optimized to withstand these challenges.

Taking advantage of the benefits of the process flexibility offered by the lateral (LARC^®) and central (CERC^®) cylindrical rotating arc cathodes technology, coating properties can be adjusted to achieve a good balance of both hardness and toughness. Based on structural and physical optimizations, several coating families were designed for use in the most challenging operations such as milling and turning of hard and new, difficult to machine materials. Two design approaches were mostly followed, a multi-zone strategy for metal cutting applications and a dedicated multilayer structure for thick coatings on molds and dies. Substrate pre-treatment as well as plasma etching were taken into consideration in order to optimize coating adhesion on the respective substrate material.

For milling and drilling of hardened or difficult to machine materials, such as new steel types or Ni-base alloys, titanium-based nanocomposite coatings such as the multi-zone coatings nACo⁴, based on the TiAlN/SiN_x nanocomposite, and TiXCo⁴, based on the TiN/SiN_x nanocomposite, were found to provide the best performance. Here, a combination of good high-temperature mechanical properties and abrasion resistance was found to be crucial. In turning, adding an oxynitride top layer to any of these coating systems could up to double the performance in some applications.

For applications in dry or wet high-performance milling, but also for mold and dies, a toughness optimization of chromium-based coatings by structural design on the nano- and microscale produces versatile yet performant coatings. In milling, significant advances were achieved using nACRo⁴ (based on the CrAlN/SiN_x nanocomposite) and ALL4 (a nanolayered AlCrTiN with an AlCrN base layer). Here, both an optimized coating internal stress and an adjusted nanostructure were the keys to achieve stable high performance. For thick coatings on hot-working dies, a multilayer strategy was applied, containing an AlTiCrN-based temperature resistant coating together with rather soft interlayers, thus making the coatings more compliant even at a total thickness of up to 10 μm.

Additional tests and properties of these structurally optimized coating generations are shown for coatings produced by the recently introduced π¹⁵¹¹ large-volume coating unit, featuring a combination of newly developed big LARC^® cathodes and flexible traditional arc cathodes.

The MAX phase Ti₄AlN₃ was confirmed and characterized in the late 90´s, with a slight deviation from stoichiometry, i.e. Ti₄AlN_3-δ (δ ≈ 0.1) [1], [2]. The synthesis route was optimized to achieve a fully dense single-phase bulk material while reducing the aluminium loss during annealing. For technical purposes there is an interest to obtain MAX phases as thin films, reducing the synthesis temperature, and suppress the formation of metastable phases. Here we report a route to form Ti₄AlN₃ by solid state reaction from arc-deposited N-deficient (Ti,Al)N_y (y<1) coatings.

Reactive cathodic arc-deposition was used to grow substoichiometric solid solution cubic (c)-(Ti_0.52Al_0.48)N_0.45 thin films at a temperature of 450 °C. The films were then heated in an Ar-environment to 1400 °C and the formation of Ti₄AlN₃ via solid state high temperature reactions was established. Characterization of the reaction path was performed using a combination of X-ray diffractometry, differential scanning calorimetry, scanning- and transmission electron microscopy, and atom probe tomography. The solid state reaction of c-(Ti_0.52Al_0.48)N_0.45 started with the formation of w-AlN and Al₃Ti at 1000 °C followed by Ti₂AlN formation at 1100 °C. At 1300 °C two MAX phases, Ti₂AlN and Ti₄AlN₃, coexist and finally at 1400 °C only Ti₄AlN₃, w-AlN and c-TiN were observed. Highly nitrogen-deficient c-TiAlN solid solutions have a strong driving force for precipitation of metallic Al or Al-Ti mixtures, resulting in an Al-depleted matrix. It has been suggested that such situation favors the formation of hexagonal MAX phases [3]. In our case, this is consistent with the formation of intermetallic Al₃Ti and Ti₂AlN. Additionally, we note that in the subsequent reaction step, the presence of Ti₂AlN (and its stability up to 1300 °C) is necessary for obtaining Ti₄AlN₃, possibly via deintercalation of aluminium layers.

[1] M. W. Barsoum, L. Farber, I. Levin, A. Procopio, T. El-Raghy, and A. Berner, J. Am. Ceram. Soc., vol. 82, no. 9, pp. 2545–2547, 1999.

[2] M. W. Barsoum, T. El-Raghy, and A. Procopio, vol. 31, no. February, pp. 0–5, 2000.

[3] B. Alling, A. Karimi, L. Hultman, and I. A. Abrikosov, Appl. Phys. Lett., vol. 92, no. 7, p. 071903, 2008.

The control of friction as well as its adaption is essential for forming operations. Thin hard coatings have a significant influence on the performance of production processes and the service life of tools, especially for Sheet-Bulk Metal Forming processes with high contact normal stresses and issues concerning the filling of filigree functional elements.

To handle these challenges, the coating system CrAlCN is generated by means of bipolar-pulsed reactive magnetron sputtering, using Design of Experiments. A Central Composite Design is selected investigating the cathode power, bias voltage, as well as the reactive gas flow composition (nitrogen and acetylene). The aim is to evaluate the correlations and the interaction of the investigated process parameters on the morphology as well as the tribo-mechanical behavior of the CrAlCN coating and to develop models to obtain the desired coating properties.

As substrate material the high speed steel AISI M3:2 (material number 1.3344) with a hardness of 62 ± 1 HRC, which is typical of forming tools, is used for the deposition process. To investigate the mechanical and tribological properties, the coated specimens are analyzed by nanoindentation, a ball‑on‑disc‑tribometer with different counter parts (100Cr6, WCCo and Al₂O₃), a scratch-tester, and by utilizing a scanning electron microscope, including energy dispersive X-ray analyses. To determine the elemental composition of the CrAlCN coatings, Glow Discharge Optical Emission Spectroscopy (GDOES) as well as XRD for phase analyses are used.