ICMCTF2016 Session B4-1: Properties and Characterization of Hard Coatings and Surfaces

Due to their high hardness and strong oxidation resistance, transition metal nitrides, such as CrN, are common materials for hard coatings applications [1]. A new approach to enhance their performance relies on optimizing their thermal conductivity in order to lower the substrate temperature during operation and, hence, reduce the thermal gradient between the coating and the substrate [2].

A previous work revealed that CrN_x (x≥ 0.05) showed good mechanical properties even at low at% nitrogen content due to a reduced grain size and the presence of point defects in the Cr structure [3]. As the hardness is stable with increasing N concentration, it seems relevant to establish a correlation between nitrogen content and thermal conductivity.

In this study, CrN_x thin films were deposited by reactive DC magnetron sputtering at 200^oC in N₂/Ar gas mixtures with the N₂/Ar flow ratio varying from 0.02 to 0.3, while the total pressure was fixed at 5 mTorr. The resulting films are characterized by θ-2θ X-ray diffraction, together with X-ray photoelectron spectroscopy, SEM and TEM. As analyses revealed that the substrate bias V_s and Cr-target power P_dc influence the composition and the nanostructure of the CrN_x films, we focused our study on the effect of both V_s and P_dc on film hardness and thermal properties. A correlation between nitrogen content, hardness and thermal properties is then established.

[1] Hones et al., J. Phys. D: Appl. Phys. 36 (2003) 1023

[2] Böttger et al., Thin Solid Films 549 (2013) 232

[3] Greczynski et al., IEEE Trans. Plasma Sci. 38 11 (2010) 3046

Plasma enhanced magnetron sputtering (PEMS) is an advanced version of the traditional magnetron sputtering technique by introducing an extra global plasma generated by an electron source, e.g. hot filaments, to enhance ionization in the entire vacuum chamber. Deep oscillation magnetron sputtering (DOMS) is one version of high power impulse magnetron sputtering (HiPIMS) technique that uses large voltage oscillation packets to achieve high power pulses for magnetron sputtering. DOMS has shown interesting capabilities in generating virtually arc-free high power pulsed depositions for reactive sputtering. Both the DOMS and PEMS techniques aim at increasing the degree of ionization and plasma density, which can be usefully utilized to improve the structure and properties of the coatings.

In the first part of the presentation, the fundamental and technical aspects of the two technologies will be introduced. Plasma diagnostics indicate the two technologies show different dominating ion species and ion energies. The PEMS process generates a large amount of low energy gas ions, while the DOMS process introduces ionized metal target ions with a distribution of ion energies. A combination of the two technologies was found to be a powerful tool for the deposition of dense, low stress, thick multifunctional tribological coatings for high performance tribological applications that demand multifunctional properties, e.g., high wear and erosion resistance coupled to low coefficient of friction, good toughness, and increased adhesion to the substrate. An increased coating thickness will also provide for greater protection of the substrate in harsh environments.

In the second part of the presentation, recent development and applications of thick CrAlN (up to 25 μm), TiSiCN (up to 30 μm), and DLC (up to 10 μm) coatings by DOMS and PEMS for wear and solid particle erosion protection, and automotive engine friction reduction will be reported. It was found that the PEMS assistance affected the target hysteresis behavior and the reactive sputtering process stability. The low energy ions and high deposition temperature associated with the PEMS assistance effectively reduced the residual stress of thick coatings and improved the coating adhesion. By controlling the peak target power in DOMS and/or the filament discharge current in PEMS, the substrate ion currents are tens of times higher than in conventional magnetron sputtering. These thick coatings exhibited extremely dense microstructure and improved mechanical and tribological properties as compared to the coatings grown by conventional magnetron sputtering.

In the present work, coatings in the material systems B-C-W and B-C-Ti have been synthesized by pulse magnetron sputtering, to exploit their potential as coatings for hard, wear resistant applications. These layers offer interesting properties, because of the possible amounts of carbides and borides that can form in the non-equilibrium deposition processes. Especially the amount of tungsten in the layers promises high wear resistance and corrosion protection at elevated temperatures.

The B‑C‑W and B‑C‑Ti thin films have been deposited by pulsed magnetron co-sputtering in bipolar mode, using a boron carbide (B₄C) target and a metal target (Ti or W) with a maximum power input of 18.75 W/cm². The films have been deposited onto various steel substrates at 300 °C and at a pressure of about 0.5 Pa. The amount of metal in the layers has been varied by adjusting the sputtering pulse length of the respective target material. The influence of bias voltage on the coating properties and the influence of an adhesion promoting metal rich layer between the substrate and the top layer have been investigated.

The coating adhesion has been examined by scratch tests. The hardness and the Young's modulus of the layers have been determined by nanoindentation. Their composition has been analysed by using the optical glow discharge emission spectroscopy (GD OES) and the ion beam analysis.

Whereas the majority of coatings exhibits hardness around 20 GPa or lower, maximum hardness of 28 GPa and Young’s modulus of 420 GPa are observed in a certain compositional range, in which coatings also distinguish by good thermal stability.

Ti-Al-N coatings are widely used as hard coatings on cutting tools. One of the proven effective techniques to grow these coatings is cathodic arc evaporation. In cathodic arc evaporation the cathode material is transformed into plasma at a localized spot on the cathode surface. The dynamics of such cathode spot are affected by multiple factors including cathode material, surface roughness, applied magnetic field, surrounding gas, and the grain size etc.

In this study we explore the surface evolution of powder metallurgically prepared Ti_0.5Al_0.5–cathodes of different grain sizes and their influence on the TiAlN coatings synthesis. The results show that when circular (63 mm diameter) cathodes with 10 and 80 µm grain size are compared after being arced in a N₂ ambience (using otherwise identical deposition conditions of a Sulzer/Metaplas MZR-323deposition system), the depth of the affected zone in both cases is about 6 µm. It also contain the same intermetallic phases, independent of grain size. However, when the cathodes are arced in an Ar ambience, this depth is increased to 20 µm for the 10 µm grain size cathode and to 200 µm for the 80 µm grain size cathode. Also here, the affected layer display the same phases for both cathodes. Using these cathodes, TiAlN coatings were deposited on WCo substrates, which were positioned in such a fashion that they cover an angular range of 0^o–50^o from the surface normal of the cathodes. Coatings grown by the 80 µm grain size cathode show higher macroparticle density and lower coating thickness than the coatings grown by the 10 µm grain size cathode.

The composition of the coatings deposited from the 10 µm grain size cathode were stoichiometric for substrates placed within the angular range of 0^o–45^o, i.e. Ti_0.5Al_0.5N₁,while beyond 45^o it becomes N-enriched. In the case of coatings grown from the 80 µm grain size cathode more aluminum was found in the coating over the entire angular range. Here N-abundance was detected beyond 30^o.

Mo₂BC has been shown to exhibit an unusual combination of very high stiffness (460 GPa) and moderate ductility based on density functional theory data [1]. The ab initio calculated positive Cauchy pressure of Mo₂BC [1] and the bulk to shear modulus ratio (B/G) over 1.75 [1] predict an intrinsic ductile behavior [2, 3]. This has been verified via growing Mo₂BC thin films and subsequent characterization, where a very good agreement was observed between quantum mechanically generated results and measured values [1]. Recently, utilizing high-power impulse magnetron sputtering (HiPIMS), crystalline Mo₂BC was deposited at 380 °C [4], which is significantly lower than a first reported required temperature of 900 °C by direct current magnetron sputtering (DCMS) [1]. Plasma characterizations have been made to reveal the underlying physical mechanisms responsible for this low temperature growth [4]. The reduced synthesis temperature greatly expands the range of technologically interesting substrate materials for application. Subsequently, electronic structure and mechanical properties of X₂BC phases with different transition metals were studied using ab initio calculations to investigate other promising candidates [5]. Further attempts have been made to synthesis and characterize a new ternary phase from X₂BC family.

[1] J. Emmerlich, D. Music, M. Braun, P. Fayek, F. Munnik, J.M. Schneider, Journal of Physics D-Applied Physics 42 (2009) 185406.

[2] S.F. Pugh, Philosophical Magazine 45 (1954) 823.

[3] D.G. Pettifor, Materials Science and Technology 8 (1992) 345.

[4] H. Bolvardi, J. Emmerlich, S. Mráz, M. Arndt, H. Rudigier, J.M. Schneider, Thin Solid Films 542 (2013) 5.

[5] H. Bolvardi, J. Emmerlich, M. to Baben, D. Music, J. von Appen, R. Dronskowski, J.M. Schneider, Journal of Physics: Condensed Matter 25 (2013) 045501.

Molybdenum nitride coating has low coefficient of friction in comparison to other nitride of transition metals, yet its wear resistance decreases dramatically at elevated temperature. In the present study, MoBCN coatings were synthesized by ion beam enhanced magnetron sputtering deposition from a Mo/B₄C composite target. The hardness of MoBCN coating increased with the increase of B content, and the wear resistance could be improved. The wear tests at elevated temperature demonstrated that its wear rate could be reduced to 1/20 of the MoN coating. The fiction coefficient of MoBCN coating was much lower than that of MoN coating. After annealed in atmosphere, the thickness of the oxide layer of MoBCN coating was only about 1/6 of MoN coating, and large amount of oxygen deficient phases such as Mo₈O₂₃ and Mo₄O₁₁ were found. In contrast, the only oxide phase in the annealed MoN coating was MoO₃. This revealed that the introduction of B decreased the oxidation rate of MoN. Since the formation MoO₃ phase with low strength and high coefficient of friction was suspended, the coefficient of friction as well as the wear loss of MoBCN coating were reduced. Therefore, doping of B is an appropriate way to solve the problem of low wear resistance of MoN coating at elevated temperatures.

Annealing heat treatment of TiN coatings have been the subject of many investigations which have resulted in the stress annihilation, and reduction of internal stresses and hardness of coatings. In our previous studies we have observed that by using high voltage pulse bias it is possible to change preferred orientation, grain size and tribological properties of TiN coatings. In this study, we aim to investigate the role of preferred orientation on temperature dependent structural stability of TiN coatings that are produced by using DC and pulse bias. For this purpose, TiN films were prepared by means of the cathodic arc method on high-speed steel substrates, using DC and pulse bias voltages. Vacuum annealing heat treatment was performed at different temperatures on the as-deposited coatings, to annihilate the stress. XRD structural investigations and nano-indentation hardness measurements were performed on all as-deposited and heat treated samples. According to the results, changes in the preferred orientation from (111) to (220) that is induced by pulse bias application also resulted in less sensitivity to heat treatment cycles.

In present study, Tantalum nitride, TaN, coatings with crystalline, amorphous, and multilayer microstructure features were fabricated and investigated. The microstructure for TaN coatings was modulated through the adjustments on Ar/N₂ gas flow ratio andRF input power. With Ar/N₂ gas flow ratio control from 18/2 to 12/8, the TaN films possessed a crystalline to an amorphous feature, respectively. The grain size for crystalline TaN single layer coatings decreased from 36.8 to 30.1 nm with input powers, leading to an elevated mechanical properties. The smaller grain size with more grain boundaries of TaN film fabricated under high Ar/N₂ ratio and input power slowed down the corrosion rate_. For amorphous TaN coatings deposited at Ar/N₂ = 12/8, there was no significant phase transformation when RF input power changed from 75 to 150W. With combination of appropriate parameter regulation for crystalline and amorphous TaN layers, the TaN multilayer coatings were then manufactured for optimization of protective behavior. Hardness, elastic modulus, and tribological characteristics of the TaN multilayer film were discussed in terms of layer modulation.