ICMCTF2014 Session B5-2: Hard and Multifunctional Nanostructured Coatings

Based on a combination of thermodynamics and first-principles quantum molecular dynamics (QMD) calculations, we shall discuss the prospects of a variety of one monolayer thick (1 ML) interfacial XY-layers (XY = SiNx, BN, AlN and SiC) for achieving super- and ultrahardness in fcc-TiN/XY heterostructures and nanocomposites. In the past we have shown that superhardness of ≥ 65 GPa can be achieved in long-term stable quasi-binary nc-TiN/Si3N4 nanocomposites, and ultrahardness of 80 to ≥ 100 GPa in quasi-ternary nc-TiN/Si3N4/TiSi2 systems only when the oxygen impurities are sufficiently low, in the range of few 100 ppm. The present work deals with clean systems essentially free of impurities. We present new results regarding the dynamic stability of different SiNx interfacial layers in the TiN/SiNx system, which is immiscible and forms a semi-coherent interface. The possible hardness enhancement in the TiN/BN system, which is from thermodynamical point of view also immiscible, is limited due to the strongly incoherent nature of the TiN/BN interface and, as shown by the QMD calculation, by the inherent dynamic instability of the interfacial BN layer which transforms into a highly disordered h-BN-like phase already at 0 K. The hardness enhancement reported for the nc-TiN/BN nanocomposites is therefore more probably due to the refinement of the TiN crystallite size to about 10 nm (the "strongest size") and not to a semicoherent interfacial layer strengthened by valence charge transfer like in the TiN/SiNx system. QMD calculations show that the AlN interfacial layer in the TiN/1 ML AlN/TiN is dynamically stable within the whole temperature range of 0 to 1400 K considered. However, thermodynamic considerations suggest that, although the heterostructures with 1-2 ML AlN have been shown to possess a relatively high stability, the formation of nc-TiN/AlN nanocomposites with thermal stability comparable to the nc-TiN/Si3N4 system seems to be questionable because of the relatively low positive mixing enthalpy (i.e. low de-mixing energy) of the TiAlN solid solution. The interfacial SiC layer in the fcc-TiN/1 ML SiC/TiN heterostructures is, according to the QMD calculations, stable only up to about 600 K above which it transforms into strongly distorted 3C-SiC phase. Moreover, the formation of nc-TiN/SiC nanocomposites is unlikely because, from the thermodynamical point of view, the solid solution TiN(1-x)Cx should form, particularly at a higher temperature. These combined thermodynamical, DFT and QMD calculations allow to identify the TmN/XY systems which are promising candidates for new superhard nanocomposites and rule out those which are less or not suitable.

TiSiN hard coatings are well established in commercial tribological applications due to their excellent oxidation, and extremely high hardness. The aim of this investigation was to compare the properties of TiSiN films deposited by DC reactive magnetron sputtering and high power impulse magnetron sputtering (HIPIMS). HIPIMS could remedy the drawbacks of standard DC process such as: lower film density, high number of voids and defects, and issues related to the nanocomposite structure formation. Further, the effect of V incorporation in the TiSiN films was also considered. The idea was to take advantage of the lubricious properties of V rich oxides to decrease the friction coefficient of TiSiN films. The structure, mechanical properties, oxidation resistance and tribological behaviour of DC and HIPIMS deposited films were characterized by nanoidentation, X-ray diffraction, scanning electron microscopy, thermo gravimetric analysis (TGA) and pin-on-disc tests. In the range of Si contents studied (up to 12 at.%), all the coatings presented an fcc NaCl-type structure characteristic of TiN phase. Improvement of the mechanical properties was achieved with HIPIMS deposition. Hardness and Young modulus of coatings are not changed with increasing V content. V incorporation successfully reduced the friction coefficient and consequently the wear volume loss of films. Scanning electron microscopy and Raman spectroscopy showed that V₂O₅ phase is the responsible for this performance. HIPIMS improved the wear performance of deposited coatings although no significant changes were observed in the friction behaviour.

The primary motive for most new PVD nanostructured coating development has been to enhance the tribological properties of engineering components. However, there are many applications in which there is a simultaneous requirement to accommodate abrasion, erosion, corrosive environment, and provide other functional properties such as lubricity, antibacterial properties, sensing or actuating capability – or to adapt to fluctuating conditions. As well as the potential to improve both wear and corrosion properties of engineering components, nanostructured PVD coatings (where combinations of ceramic, metallic, crystalline, and amorphous phases can be generated) have unrivalled design flexibility to develop multifunctional capabilities. Such coatings can exhibit unusual combinations of high hardness, long elastic strain to failure, strong resistance to crack formation and/or propagation – and often exhibit surprisingly good corrosion resistance, especially in terms of pitting.

In this research, the tribological and corrosion behaviour of some novel Al-based multi-element metal-alloy PVD coatings were studied. The coatings were deposited on austenitic stainless steel and low alloy steel, materials which are in widespread use, yet tend to exhibit poor tribological properties and poor corrosion properties respectively. These coatings have the potential to provide alternatives to electroplated cadmium, IVD-aluminium, electroplated hard chromium, electroless nickel and other ‘traditional’ coatings used in aerospace, automotive and other engineering applications – where environmental legislation is restricting the use of many of the toxic materials and processes required to deposit and functionalise them. We demonstrate that metallic PVD nanostructured/amorphous coatings with multifunctional and adaptive properties could be developed to suit a range of technically challenging environments.

The characteristics of the coating-substrate systems were studied using a reciprocating-sliding ball-on-plate test and a microabrasion wear test. SEM, and EDX were conducted to check the composition and measure the thickness of the coatings. XRD and TEM investigations provide more detailed explanations for the unusual properties of these coatings. Hardnesses and elastic modulii were also determined. The reactive addition of nitrogen to the multi-element metal-alloy films is shown to have a significant influence on the wear behaviour of the system. Open circuit potential scans and potentiodynamic polarisation measurements were used to investigate the pitting potential, passivation and sacrificial corrosion behaviour of the coatings in a 3.5 wt. % NaCl environment.

This presentation provides a review of adaptive surface materials and thin film technologies that were recently explored to impart adaptive behavior for mechanical contacts. Two main functions are considered for the surface adaptation which are most practically important for a multitude of mechanical contacts operating at high loads, speeds, and temperatures: i) adaptive tribological behavior with an emphasis on reduced friction and wear through sliding surface chameleon self-adaptation; ii) adaptive heat transport regulation at contact interfaces with an emphasis on surface engineering (texture, morphology, inclusions) to control heat flow at interfaces and mitigate thermal spikes. The focus is placed toward adaptive behavior at high temperatures in air, as this is one of the most challenging environments due to accelerated oxidation in addition to structural evolution processes. Examples include: multilayered oxide and nitride coatings with inclusions of noble metals for controlled surface diffusion and regulation of surface friction and thermal conductivity; transition metal nitride coatings with inclusions of silver for the formation of lubricating ternary oxides with a low shear strength; hard oxide coatings with vanadium additions to induce low melting point oxides for liquid lubrication and heat spike mitigation; age-hardened nitride coatings with thermal conductivity adaptation through structural evolutions; intrinsically nano-laminated complex carbides with low shear planes for friction reduction; and nano-laminated transition metal dichalcogenides with anisotropic thermal properties for heat redirection. While some of these concepts are experimentally verified and used in real-life applications, many are yet in an early design and development stages. The conclusions outline challenges and possible directions toward future developments in nanostructured coatings with adaptive behavior.

Ceramic Me-B-C thin films are interesting because of their multifunctional properties, such as high hardness, chemical inertness, conductivity and temperature resistance. The microstructure and thereby the properties are strongly dependent on elemental composition and type of transition metal Me. A basic understanding of how these parameters affect the film structure makes it possible to tailor the properties. We have investigated the influence of composition on Me-B-C (Me= Cr, Nb, Mo) thin films deposited by DC magnetron sputtering, at temperatures 300 - 500 °C, using MeB₂/C or Me/B₄C/C target combinations. The microstructure has been characterized using XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy) and TEM (transmission electron microscopy).

Ternary aluminum nitride alloys are commonly used as hard coatings for wear resistant applications. Coating materials such as TiAlN are known for high hardness even after being exposed to high temperatures. The good mechanical properties at high temperatures are an effect of spinodal decomposition of the unstable cubic (c) TiAlN phase leading to formation of nanostructured domains of TiN and AlN. For ZrAlN coatings, the larger miscibility gap between ZrN and AlN hinders the formation of a c-ZrAlN phase for AlN-contents larger than ~35 at.%. High Al-content ZrAlN coatings instead exhibits a hexagonal (h) structure in which nano-sized ZrN and AlN domains form during annealing at temperatures above 900 °C resulting in age hardening of the coating [1].

In this work, we study the decomposition paths of h-ZrAlN coatings and its effect on the coating’s mechanical properties. h-Zr_1-xAl_xN coatings with 0.46>x>0.71 were grown on WC-Co substrates by cathodic arc evaporation. For AlN contents between 0.51 and 0.71 the coatings have a single phase hexagonal structure with a strong 002 preferred growth orientation. Simultaneous in-situ small angle and wide angle scattering during annealing show that decomposition of the h-ZrAlN phase begins at temperatures around 750 °C and that the temperature at which decomposition is initiated is independent on chemical composition of the coating. The wide angle scattering results reveal that the c-ZrN formed is aligned with the 111 direction parallel to the h-(Zr)AlN 002 direction. From the small angle scattering results, the coatings are observed to form a layered structure during annealing. The layer period depends on chemical composition of the coating and increases from 1.7 nm for x=0.51 to 2.5 nm for x=0.71 while the periodicity is not changing with annealing temperature or time. The hardness of the coatings ranges between 22 and 26 GPa where the highest hardness is found for the highest Al-content coating (x=0.71). After 2 h annealing at 1100 °C, the hardness has increased from 22 to 26 GPa for the coating with x=0.51 while for the coating with x=0.71 the hardness of the annealed coating is stable at 26 GPa. The increased or retained hardness of the annealed coatings is assigned to the nanoscale layering formed during decomposition of the h-ZrAlN phase.

[1] L. Rogström, M.P. Johansson, N. Ghafoor, L. Hultman, M. Odén, J. Vac. Sci. Technol. A 30 (2012) 031504.

Hard nanocrystalline coatings prepared by magnetron sputtering possess complex gradients of residual stresses, phases, and microstructures. Hardness and elastic modulus characterization performed by nano-indentation provides volume-averaged quantities which do not reveal the mechanical response and function of individual coating components, regions, and/or intrinsic gradients. The presented approach is a step towards the characterization of thickness- and direction-dependent coating mechanical properties at the micro- and sub-micro-meter scale performed on compact coatings as well as on isolated coating segments. For the detailed studies we have selected TiAlN and CrN/AlN multilayer coatings as model systems. Coating cantilevers with the sizes in the range of 0.1-4 µm prepared using focused ion-beam milling are tested using in-situ micro- and pico-indentation in scanning and transmission electron microscopes, respectively. The results reveal the integral elastic and fracture properties of the coatings as well as the mechanical response of selected coating regions. The approach allows the identification of features, which provide an increase in the toughness and elastic moduli or deteriorate the coating mechanical behaviour.