ICMCTF2014 Session B4-4: Properties and Characterization of Hard Coatings and Surfaces

Thin films of transition metal nitrides are a field of great interest for academia and industry alike. TiAlN films exhibit high hardness, thermal stability and wear resistance, making them suitable for metal cutting applications. One major mechanism for maximizing hardness of such films is based on self-organized compositional modulations on the nanometer scale through spinodal decomposition and phase separation of immiscible components. All group IV transition metal nitrides are expected to phase separate when combined with AlN, owing to their immiscibility due to volumetric, electronic, and/or magnetic mismatch. Calculations have predicted that the mixing enthalpy of HfAlN is approximately twice that of TiAlN [1], yielding a stronger driving force for phase separation in HfAlN than in TiAlN. Previously, laser assisted atom probe tomography (ATP) methods were developed for TiAlN thin films [2,3]. In this study, Hf_1-xAl_xN (x = 0.17, 0.29, and 0.48) thin films deposited by magnetron sputtering on MgO(001) substrates have been analyzed using ATP. We observe the very onset of compositional modulations in samples with x > 0.2, but with less discrimination than initially seen by scanning transmission electron microscopy (STEM) [4]. Differences in the results from tomography and microscopy evaluation are discussed with respect to the choice of field evaporation and reconstruction parameters, and the contrast-forming mechanisms (atomic number contrast vs. strain-field contrast), respectively.

[1] B. Alling, A. Karimi, I. A. Abrikosov, Surf. Coat. Technol. 203 (2008) 883.

[2] R. Rachbauer, E. Stergar, S. Massl, M. Moser, P.H. Mayrhofer, Scr. Mater. 61 (2009) 725.

[3] L.J.S. Johnson, M. Thuvander, K. Stiller, M. Odén, L. Hultman, Thin Solid Films 520 (2012) 4362.

[4] B. Howe, J. Bareño, M. Sardela, J.G. Wen, J.E. Greene, L. Hultman, A.A. Voevodin, I. Petrov, Surf. Coat. Technol. 202 (2007) 809.

Hard materials like CrN, TiN, CrAlN, and TiAlN are well established as protective coatings for tools used in various machining and forming applications. Even though the thermal stability and thermo-mechanical properties of such coatings have already been investigated in numerous studies only little information is available concerning the influence of transfer-material-diffusion during e.g. machining applications. Therefore, thin layers of Fe, Cr, and C ─ which are common transfer-elements during machining ─ were deposited on arc evaporated CrN, TiN, Al_0.7Cr_0.3N as well as on Ti_0.5Al_0.5N coatings. The concentration-depth profiles after vacuum annealed for 30 min at 600, 800, and 1000 °C were evaluated by cross-sectional EDX line-scans as well as by secondary ion mass spectroscopy. Additional to the determination of the diffusion coefficients at high temperatures, low temperature annealing treatments at 600 °C for 10, 100, and 1000 min were carried out to investigate in more detail the in-depth diffusion behaviour of Fe, Cr, and C. These results serve as a basis for a better understanding of the complex interaction between coated tool and work-piece as well as the influence of impurities on the thermal and mechanical behaviour of ceramic-like protective coatings.

The high strength of nanocrystalline metals suggests that these materials are promising for wear-resistant coatings, where protective films must be able to survive the repetitive application of high contact stresses. However, nanocrystalline metals also demonstrate novel grain boundary-dominated deformation mechanisms and a tendency for mechanically-driven structural evolution. This talk first addresses the tribological response of nanocrystalline Ni-W alloys across a range of grain sizes from 3 to 100 nm, with a focus on understanding how the extreme conditions produced during wear can lead to dynamic nanostructures and properties. Both experiments and simulations reveal evidence of grain growth and relaxation of grain boundary structure during tribological contact, which are actually beneficial to wear resistance. We next isolate the importance of atomic grain boundary structure on mechanical properties using small-scale uniaxial testing and molecular dynamics simulations. An ordered boundary structure is found to increase strength at the expense of ductility. Finally, we synthesize these results and identify strategies for metallurgical design of optimal nanocrystalline coatings.

Were deposited ZrO_xNy/ZrO₂ thin films on stainless steel by two different methods ultrasonic spray pyrolysis-nitruration and DC sputtering magnetron unbalance technique. Using the first technique is initially deposited ZrO₂ and subsequently nitrided in anhydrous ammonia atmosphere at 750 ºC atmospheric pressure [1]. By DC sputtering the film is deposited in atmosphere of air/argon, with a flow ratio Φair/ΦAr of 3.0. The ZrO_xN_y/ZrO₂ films were obtained from a 4 in. ×1/4 in. Zr (99.9%) target (CERAC, Inc.). The parameter set used during the deposition process was: base pressure at 1.0×10⁻³ Pa, deposition time 30 min, target–substrate distance at 5 cm and argon (99.999%) flow at 9.0 sccm at room temperature (287 K) [2]. Structural analysis was carried out through X-ray diffraction (XRD); morphological analysis was done through scanning electron microscopy (SEM) and chemical analysis was determined using X-ray photoelectron spectroscopy (XPS). From spray pyrolysis-nitriding grows ZrO_xNy rhombohedral polycrystalline film, whereas DC sputtering the oxynitride films were grown with cubic crystalline structures Zr₂ON₂ and preferentially oriented along the (222) plane. Chemical analysis determined that in the last 5.0 nm, the Zr coatings present the following spectral lines: Zr3d_3/2 (184.6 eV) and 3d_5/2 (181.7 eV), O1s (531.3 eV), and N1s (398.5 eV). SEM analysis presented the homogeneity of the films. Zirconium oxynitride films enhance the stainless steel’s resistance to corrosion by the two techniques. The protective efficacy has been evaluated using electrochemical techniques based on linear polarizations (LP). The results indicate that the layer provides good resistance to corrosion in chloride-containing media [3,4].

Shot peening and plasma processes are widely used to improve the surface properties of metals and alloys. This work aims at broadening the applicability of interstitial free (IF) steels by combining both processes. In addition, we evaluate the influence of prior shot peening on the plasma nitriding process, as shot peening introduces non-equilibrium defects on the surface, which enhance the diffusions rates of nitrogen [1, 2]. The plasma nitriding was carried out for 4h at temperatures of 450°C, 475°C and 500°C on a Ti-stabilized IF steel, with and without pre-shot peening. The microstructure and structural phases were characterized by means of backscattered scanning electron microscopy (SEM) and x-ray diffraction (XRD); while mechanical properties were investigated using micro- and nanoindentation as well as dry sliding wear tests.

SEM showed that the morphology and distribution of the precipitates formed in the surface layers strongly depend on the pre-shot peening and plasma process temperature. Long nitride needles were only observed in the samples without pre-shot peening. For all samples the XRD patterns showed the presence of ε-Fe_2-3N, γ’-Fe₄N and the Fe-α matrix ferrite peak. Ultramicrohardness exhibits a two fold increase in hardness values near the surface relative to the substrate, and a hardened region, which extends up to a depth of 500µm in the IF steel treated at 500°C. For all test temperatures, a slight increase in hardness was observed for the shot peened samples compared to the ones without prior shot peening. Further investigation of the sample cross-sections by nanoindentation showed that shot peening caused a hardening effect up to a depth of 20 µm on the untreated substrate. It was also shown that for the plasma processed samples at 500°C, up to a depth of 70 µm, the shot peened samples had a higher average nanohardness than the samples which were not pre-treated. This suggests that an enhanced nitrogen diffusion due to the pre-mechanical treatment might have increased the nitrogen concentration in this region. The dry sliding wear tests showed an improvement of wear resistance of the IF steel after the different treatments, which was compared by the worn volume and measured using 3D profilometry. Characteristic undulation on the coefficient of friction versus sliding distance curves indicated a tribochemical wear process [3], which is confirmed by EDS compositional maps of the wear tracks.

[1] Hashemi et al., Material and Design. 3287-3292 (2011) 32.

[2] Shen et al., Surf. Coat. Technol . 3222-3227 (2010) 204.

[3] Zhang et al. Tribology International. 214-223 (2013) 61.

The influences of increasing copper content in the NbN-Cu coatings on mechanical and tribological properties were studied. The coatings were deposited on M2 tool steel by using reactive magnetron sputtering method. Copper content of the coatings was varied from 1 to 24at.%. The microstructure, microhardness and wear properties of the coatings were examined. The addition of Cu into NbN coatings has modified the grain size and morphology. Mechanically, the microhardness values of the coatings other than NbN-1at% Cu were significantly reduced by Cu doping. The hardness of the films reached maximum level, 38GPa, at 1%at copper content and decreased with a further increase of the Cu content. Introduction of 1at % Cu into niobium nitride showed a beneficial effect on abrasive character of the coating. Wear of counter material increased at these coatings while others showed exact opposite behavior.