ICMCTF2019 Session B1-2-MoA: PVD Coatings and Technologies II

A patented & proprietary high density/high rate magnetron deposition source developed in-house from the ground up. The development includes a newly designed high-density plasma generator and an improved magnetron deposition source far more superior to any conventional magnetron technology in the market today. The data presented will include deposition rates and X-SEM images of different thin films such as DLC , High Rate Metal Nitrides ( MeN) and High Rate Metal Oxide (MeO), Highly Ionized Metal (Me).

Cathodic arc deposition, also known as arc ion plating or arc evaporation, is a PVD technique utilizing arc sources to deposit non-reactive and reactive coatings, such as carbide, nitride, carbonitrides, diamond like carbon, etc. Compared to traditional magnetron sputtering, denser coatings can be obtained by arc sources due to a higher degree of ionization and thus higher potential energy of the vapour flux towards the substrate. Also the ability of generating metal ions enables adhesion enhancement in some particular processes, like hard coating on cutting tools. Usually, target evaporation by arcs rather than momentum transfer in sputtering leads to a higher deposition rate in arc deposition, and provides a higher degree of heat on the substrate, which is beneficial for some processes. Unlike in Reactive Magnetron Sputtering, reactive arc has a very wide window of process conditions for a successful reactive process. However the deposition conditions all affect the arc parameters and the levels of macroparticle defect on the coating. Different solutions for filtering of the macroparticles have been devised, such as venetian blinds and Magnetic Guidance Filters. It is desired however that the basic level of macroparticles is reduced at its origin.

This work aims to obtain low defect coatings without using macroparticle filters and to obtain high target utilization by controlling the arc travelling. A 125mm circular arc source with optimized design will be employed for nitride deposition. Reliable arc triggering has been realized without mechanical trigger. The arc source will be powered by an arc generator with the capability to vary the arc current during deposition. A small but effective and easily adjustable controller will be used for varying the magnetic field. The effects of varying the arc current and the magnetic field on the defect generation and the target erosion will be investigated. SEM/EDX will be used to characterize the coating defects.

The Industrial Internet of Things (IIoT), which involves multi-level internet-connectivity of machines and systems, and additive manufacturing (AM) of complex 3D printed items hold great promise to revolutionize the manufacturing industry. In this paper we will discuss the impact of IIOT and AM on the machining sector and anticipated challenges and opportunities for cutting tools and hard coatings. We will examine how sensors that provide real-time internet-connected feedback on the conditions of the cutting edge will allow confident increase in cutting speeds and other machining parameters. This will lead to expansion of the safe zone in wear maps and call for improvements in tool materials and coatings. We will then review the structure and surface properties of AM 3D printed metal parts. Economic, large scale finish machining of rough 3D printed metals will challenge current finish cutting routines and accelerate suitable changes in design and structure of cutting tools. The IIOT sensors will affect, primarily, machining of bulk materials. AM and 3D printed metals will speed up progress in finish cutting. We will discuss both cases with appropriate examples of tools and coatings for machining high temperature alloys, gray cast iron, and carbon composites.

Transition metal nitride films TMN_x with a high stoichiometry x = N/TM > 1 are advanced films with new unique properties; here TM are the transition metals such as Ti, Zr, Mo, Ta, Nb, W, etc. We report on their properties and formation by the reactive magnetron sputtering using dual hybrid magnetron. The principle of formation of overstoichiometric TMN_x>1 films is explained. Three TMN_x>1 coatings were investigated in detail: ZrN_x>1, Ti(Al,V)N_x>1 and TiN₂ dinitride films [1]. It was found that (1) the overstoichiometric ZrN_x>1 films are two-phase films with c-ZrN and o-Zr₃N₄ structure, (2) the overstoichiometric Ti(Al,V)N_x>1 and TiN_x>1 films are one-phase films with c-TiN structure, (3) the one-phase overstoichiometric TMN_x>1 nitride films can form the TMN₂ dinitride films such as TiN₂ dinitride, (4) the film stoichiometry x is a strong parameter which enables to control its mechanical properties and electric conductivity; for example, the electrical resistivity of the ZrN_x film varies with increasing x from well electrically conducting films with x ≤ 1 through the semi-conducting films with x ranging from 1 to ≤ 1.26 to non-conductive with x ≥ 1.3, and (5) the high base pressure p₀ ≥ 0.001 Pa in the deposition chamber after its evacuation strongly influences the structure and phase composition of sputtered nitride films.

Reference

[1] J.Musil, M.Jaroš, Š.Kos, R.Čerstvý, S.Haviar: Hard TiN₂ dinitride films prepared by magnetron sputtering, J. Vac. Sci. Technol. A 36(4) 2018, 040602-1 to -3.

The flexible coating units which are able to work with ARC, sputtering and PACVD technologies are very suitable for the small and medium size enterprise. While the ARC brings the highest performance for cutting tools in cca 85% of the applications, the sputtering achieves very smooth surfaces for better chip evacuation and DLC coatings, made by PACVD, avoids build up edges at cutting sticky materials. The combination of these 3 technologies in one coating unit enables the use the advantages of all of them.

We would like to introduce the new LACS® technology (Lateral ARC and Central Sputtering by rotating cathodes) . The new feature of this hybrid technology is the combination of arc evaporated non-alloyed metallic (Ti,Al,Cr,W, etc) and magnetron sputtered ceramic (TiB2, B4C) targets, resp. cathodes.

It allows to deposit different high-performance coatings, like AlCrN/BN, AlTiN/BN, TiWN, TiCNWCC or TiB2. On the other hand, the supplementary electron injection provided by ARC discharge can improve the microstructure and performance of the plasma-enhanced magnetron sputtered coatings.

On the example of BN-containing coatings we would like to illustrate how is it possible to optimize mechanical parameters of the coating by tuning of the process parameters. We will also present industrial applications of the optimized coating.

The deposition of different coatings on shaped surfaces, such as on cutting tools (drills) faces problems especially on edges [1-2]. Normally the radii of such cutting edges are dramatically increased, if the coating thickness is increased. Also defects, damaging or resputtering of material on edges may occur. A multilayered coating based on AlCrSiN instead showed an interesting effect of edge sharpening during the deposition process. With this approach it would be possible to overcome the problem of edge rounding in PVD coating technology. The pictures below show different edge radii varying with the negative bias voltage and leading to a smaller edge radius as compared to the uncoated tools or tools without additional bias voltage [3].

Therefore the deposition process and important deposition parameters were investigated and the geometry of such edges was measured. Accompanying nano indentation hardness measurements give an overview of mechanical properties around the surface also in dependence of position and chemical composition of the coating.

REFERENCES:

[1] H. A. Jehn, “PVD coating of 3D parts studied with model samples”, Surface and Coating Technologies, Vol. 94-95, pp 232-236, 1997.

[2] J. Bohlmark, “Evaluation of arc-evaporated coatings on rounded surfaces and sharp edges”, Materials Science Forum, Vol. 681, pp 145-150, 2011.

[3]T. Krülle, FhI-IWS Dresden, Annual Report 2016, “Sharp edges thanks to coatings”, pp 106-107.

Hafnium nitride have excellent properties such as high melting point, high hardness, low resistivity, which makes them potential in many technological fields. HfN have been investigated for diffusion barriers in semiconductor devices. HfSi₂ has been used for high temperature oxidation resistant coatings. However, there are no detailed studies for growth of epitaxial HfN/HfSi₂ films on Si substrate.

HfN films were grown on Si(100) substrates by reactive DC magnetron sputtering with Ar/N₂ gas mixture using a Hf target. The deposition was carried out by varying N₂ flow ratio and the power at 5 mTorr and 850℃.

XRD results with cross-sectional TEM/STEM show that low N₂ flow ratio favors the formation of the HfSi­ interlayer between HfN and Si, while no HfSi­₂ diffraction peaks can be seen for high N₂ flow ratio of 12.5% . Also, increasing the applied power for sputtering results in the increase of the HfN peak intensity ratio of (200)/(111). Furthermore, it is shown that the orthorhombic HfSi₂ interlayer is in epitaxy with Si (100) and those (100) oriented HfN grains are found in epitaxy with both HfSi₂ and Si as well. The epitaxial relationship is HfN (100) [011] // HfSi₂ (020) [100] // Si (100) [011].

The piezo-related properties of ZnSnN₂ (ZTN) will be presented. Natural Sn₃N₄ and Zn thickness gradients were fabricated using combinatorial magnetron sputtering to form Zn–Sn₃N₄ composition spreads to enhance the relative variation of the cation ratios and to promote the formation of orthorhombic ZTN. Sn₃N₄ and the single crystallinity of orthorhombic (Pna2₁) ZTN nanocolumn arrays growing along the [001] direction were confirmed by locked-coupled XRD and TEM. The diffusion and variation of the atomic binding state of constituent elements were studied using SIMS depth profiling and XPS. The band gap of ZTN was estimated to be approximately 2.0 eV from a UV–vis measurement. The piezotronic and piezophototronic effects of ZTN were ascertained and illustrated by the Schottky barrier height variations. Excellent piezophotocatalysis was also observed, which was attributed to the reduced recombination of the photogenerated e⁻–h⁺ pairs. In addition, ^•O₂^- radicals were predominate in the photodecomposition process.

Advanced thin film applications e.g. in thin film transistors, electrochromic glasses or hard coatings frequently require sputtering targets consisting of multiple chemical elements with large differences in atomic weight. An understanding of sputtering, gas-phase transport and deposition characteristics of thin film systems deposited from such multi-element targets becomes increasingly important. For the previously investigated system Mo_0.70Al_0.20Ti_0.10 (m_Mo=95.95 u, m_Al=26.98 u, m_Ti=47.87 u) it was shown that not only (i) element-specific differences in sputter yield and initial emission angle, but also the (ii) transport of sputtered particles through the gas, accompanying (iii) collisions with background gas atoms with concomitant energy loss of sputtered particles, and (iv) the interaction of transmitted energetic particles with the surface of the growing thin film, such as preferential re-sputtering, will have an impact on the film morphology as well as the chemical composition of the films [1]. For the system NiW, which is used for the deposition of electrochromic thin films, the difference in atomic weight of the target constituents is significant (m_Ni= 58.69 u, m_W=183.84 u), making it an ideal model system to study the mechanisms mentioned above. Since it was concluded in [1] that the initial emission angle and the corresponding energy of sputtered atoms have a substantial impact on differences in morphology and chemical composition of the films, a 180 ° turnable magnetron with a mass-energy analyzer mounted opposite the target at a distance of 80 mm was used to d.c. sputter a Ni_0.80W_0.20 target at Ar pressures ranging from 0.01 to 1 Pa. This setup allowed measuring both, flux and energy of sputtered Ni and W, but also of Ar reflected from the target and ionized in the plasma, at any given angle between 0 and 90° with respect to the target normal. Subsequent thin film deposition with a spherical-shell substrate holder covering the examined angles at deposition positions from 0 to 80 ° in 10 ° steps allowed correlating the mass-energy measurements with thin film growth experiments. Film thickness measurements, structure and composition analysis yielded insights into the relation between emission angle and energy of transmitted particles and angular dependent structure and composition evolution of the deposited films.

The paper deals with the technique of forming coatings with nanolayer structure, including nanolayers (formed due to rotation of a turntable in a chamber) and subnanolayers (formed due to planetary rotation of rigging elements). The SEM and TEM methods were used to study the coating structure. In particular, the Ti-TiN-(Ti,Al,Si)N, Zr-ZrN-(Zr,Cr,Al,Si)N and ZrNb-ZrNbN-(Zr,Nb,Cr,Al,Si)N coatings were considered. The thickness of binary nanolayers in the coatings under study was 20-70 nm, while the thickness of subnanolayers was 2-20 nm. The study also considered the mechanism of coating failure in scratch testing, depending on thickness of nanolayers and subnanolayers. The investigation found significant differences in the value of the critical failure force L_C2 and the pattern of failure, depending on the thickness of coating nanolayers. The change in the value of adhesion component of the friction coefficient for samples with the coatings under study within the temperature range of 20-1000°C was studied. Cutting tests were conducted for carbide tools with the coatings under study in turning steel C45 at f = 0.25 mm/rev, a_p = 1.0 mm, v_c = 200-350 m/min. The patterns of wear and failure for coated tools were studied, as well as oxidation and diffusion processes. As a result of the conducted studies, a range of values for optimal thickness of nanolayers was determined to make it possible to increase the cutting speed by 25-30% while maintaining the tool life period and the high quality of the machined surface.