ICMCTF2011 Session B5-1: Hard and Multifunctional Nano-Structured Coatings

It is known that nanostructured multi-component nitrides like Ti-Al-Y-N may have a unique combination of properties: high hardness, wear resistance, thermal stability and oxidation resistance, which may allow using them as protection coatings on tools and machine parts, working under extreme conditions. Deposition of coatings from filtered vacuum arc plasma leads to additional quality improvement due to formation of more uniform structure and reduction in the surface roughness. In this paper the effect of the amplitude of the pulse substrate bias and percentage of yttrium on structure and properties of Ti-Al-Y-N vacuum-arc coatings, deposited from filtered vacuum arc plasma, were studied.

The coatings were produced in vacuum-arc system with a straight magnetic-electric filter by sputtering of as-cast TiAlY cathodes with the following deposition on the steel substrates at a nitrogen pressure of 0.1 Pa and pulse substrate bias amplitude of 0.5-2.5 kV. The deposition rate reached 14-16 μm/h, the coating thickness was of 6-8 μm. Scanning electron microscopy and atomic force microscopy (AFM) were used for investigation of the structure and surface morphology of the coatings. The elemental composition of the coatings was controlled using X-ray fluorescence analysis. Hardness was measured by nanoindentation. Oxidation resistance tests were performed using the thermal analyzer.

Elemental percentage of Ti, Al, Y in the coatings and cathodes were in a good mutual accordance. At the same time, the coatings were of uniform thickness and homogeneous composition. SEM of the surface samples showed the high quality filtering of the plasma flow, because only few defects on the films surface were observed. Samples cross section showed that the coating Ti-Al-N had a columnar structure like traditional vacuum-arc nitride coatings deposited at a constant substrate bias. Yttrium additions to the cathodes caused the changes in the coatings structure: with increasing of Y content in cathodes from 0 to 2.5 wt.%, columnar film structure became considerably less expressed. In this case, significant surface morphology changes were observed. In the AFM 3D images the surface of the films deposited at pulse bias of 1.5 kV amplitude was characterized by cellular-like microrelief with the cell size of about several hundred nanometers with hardness H=35-40 GPa.

According to the results of the thermogravimetric analysis, all produced Ti-Al-Y-N coatings possess sufficiently high oxidation resistance. The most oxidation-resistant coatings were deposited at pulse bias amplitude of 1.0-1.5 kV and contained of 1.0-2.5 wt. % of Y.

This work has been supported by the U.S. National Science Foundation (DMR-0806521, DMR-0922910) and the Regional Council of Burgundy, France.

In the field of superhard nanocomposites the research has focus on introducing new multifunctional materials as well as on the understandings of the structural complexity of the nanocomposites which boost their functionality, such as hardness, thermal stability, and corrosion resistance. Here we present an experimental study on Zr_1-xSi_xN_y (0 ≤ x ≤1; 1 ≤ y ≤ 1.22) alloys synthesized by reactive magnetron sputter deposition onto single-crystal MgO(001) and Al₂O₃(0001) substrates at 800°C that allows for phase separation during growth and hence make possible to design the structure of the internal interfaces by controlling the composition. For this high temperature, epitaxial growth allows incorporation of as much as 10 at.% of Si in ZrN, which enhances oxidation resistance and thermal stability of the films up to 1000°C, while retaining its superhard nature. The structure of Zr_0.8Si_0.2N films constitutes 3-5 nm thick lamellas that extend in the growth direction with a strong (002) texture. It is revealed by lattice resolved z-contrast transmission electron microscopy imaging that the lamellas consist of ZrN and amorphous Si₃N₄ separated by a possible crystalline SiN_x: Zr tissue phase.

Nanoscale multilayers made of borides, nitrides, and carbides, exhibiting desired mechanical properties, have been developed and applied to various kinds of industries for the past decades. In this study, we worked with W/ZrB nanoscale multilayers coatings. Tungsten, as a high atomic-number and refractory material, has been widely applied in applications such as wear- or corrosion-resistant components in high-temperature and high-vacuum environments due to its excellent mechanical, thermal and electrical properties. The refractory compound ZrB also has a high hardness, high melting point, high electrical conductivity and excellent corrosion resistance. Because W and ZrB have different crystal structures, we expect little mutual intermixing of both nanolayers at elevated temperatures and hence preserve the high-temperature tribological performance. Several different types of transition metal nitrides, borides, and carbides multilayered coatings, such as W/NbN, Mo/NbN, AlN/VN, AlN/TiN, TiN/TiB₂, TiB₂/TiC, TiN/SiNx, have been explored. For the W/ZrB₂ multilayers addressed in this work, studying the influences of various growth parameters will help us determine their microstructure and the related mechanical properties.

The M_n+1AX_n phases (n = 1 – 3, or ‘MAX phases’) are a group of ternary carbides and nitrides (X) of transition metals (M) interleaved with a group 12-16 element (A) [1]. Although a relatively little researched member of the MAX-phase family, Cr₂GeC exhibits a number of traits that render it interesting from a fundamental-research point of view, such as its reported thermal expansion coefficient which is the highest of the known MAX phases, and its reported anomalously high density of states at the Fermi level. Because of these intriguing observations and the limited amount of studies available, we are interested in Cr₂GeC. For synthesis of this phase, thin-film growth is a potentially important approach because of the relative ease with which numerous MAX phases can be epitaxially grown, often as single crystals. Here, we report growth of epitaxial phase-pure Cr₂GeC onto Al₂O₃(0001) both directly and using a TiN(111) seed layer. The use of a seed layer results in fewer defects as evidenced by the rocking curve full width at half maximum. For optimized composition, phase-pure epitaxial Cr₂GeC can be grown at temperatures above 700^oC. This enables determination of mechanical and electrical properties. Increased Ge or C content results in film containing additional impurity phases such as Cr₅Ge₃C_x and pure Ge segregated on the surface. A reduction in temperature yields polycrystalline growth of Cr₂GeC in the temperature range 500 – 650^oC.

[1] See review: P. Eklund et al Thin Solid Films 518 1851 2010

Hardness and high-temperature stability of hard coatings are indirectly controlled by their microstructure induced by the deposition process, as the initial microstructure strongly affects the kinetics of microstructure changes at elevated temperatures. Thus, the development of the microstructure at elevated temperatures is an important topic for Ti_1-xAl_xN coatings which can be used in machining applications. In this study, the development of microstructure in Ti_1-xAl_xN coatings at elevated temperatures was investigated using in-situ synchrotron high temperature glancing angle X-ray diffraction (HT-GAXRD) experiments and related to the original microstructure of the coatings, which was modified via the titanium to aluminium ratio and the bias voltage.

The Ti:Al ratio in the coatings varied between 60:40 and 33:67; the bias voltage ranged from -40 to -120 V. The in-situ synchrotron HT-GAXRD experiments were done at 450°C, 650°C and 850°C. After each annealing step, the coatings were cooled to 100°C and an additional GAXRD measurement was performed in order to seek for the spinodal decomposition of metastable fcc-(Ti,Al)N.

From the analysis of the GAXRD pattern, the phase composition as well as the macroscopic lattice strain and the stress-free lattice parameter of the fcc-(Ti,Al)N phase were determined for all investigated temperatures. At 850°C AlN segregated from fcc-(Ti,Al)N and formed c-AlN and w-AlN. It was found that for the same coating composition the ratio between the individual phases present in the coating after annealing at 850°C was different in the coatings deposited at -40 V, -80 V and ‑120 V. This effect of the initial microstructure, which was adjusted by the bias voltage during CAE deposition, on the phase evolution during annealing will be related to the results from GAXRD and transmission electron microscopy.

Using vacuum-arc source with HF discharge, superhard nanocomposite films of 1.05 to 1.2 µm thickness were fabricated. Their thickness depended on the bias potential on the substrates and N pressure in a chamber. It was found that the film hardness was ≥ 42 GPa and elastic modulus was E = 490 ± 10 GPa, nanograin size being 5 to 35nm. It was demonstrated that depending on the substrate bias potential, ratio of α-H₅S₄, nc-HfSi₂ and nc-TiN or (Ti,Hf)N phases changed. Using SIMS, RBS, EDS with SEM analysis, the films stoichiometry and concentration profiles over depth were measured. Analyzing the film cross-section, we found good quality of the films, very good adhesion to the steel 3 substrates, pores and columnar structures were not observed.

XRD and TEM with micro-diffraction were applied to determine sizes of nanograins of the above mentioned phases. 200^oC to 600^oC annealing in vacuum demonstrated that a hardness increased almost by a factor of 15 to 17% (48 to 52 GPa), i.e. an effect of self-hardening due to a process of spinodal phase segregation along grain interfaces was observed. Annealing at higher than 1100^oC temperature resulted in formation of an oxide layer of 120 to 140 nm thickness at the film surface.

The work was funded by the NAS project of Ukraine – Nanosystems, Nanocomposites, and Nanotechniques.