ICMCTF2015 Session B4-1: Properties and Characterization of Hard Coatings and Surfaces

High film hardness is a necessary but not sufficient criterion to achieve good wear resistance, Instead a favorable hardness to toughness ratio is desirable, which is a long-standing materials development challenge. Here we present two different approaches to address the issue: (1) alloying to introduce microstructural variation such that plastic deformation and crack growth behavior is affected, and (2) multilayer architecture, where the epitaxially stabilized metastable phases are exploited to enhance both hardness and fracture resistance by stress-induced transformation toughening. The mechanical behavior of the films was evaluated by nanoindentation and extractions of transmission electron microscopy samples under the indent such that plastic deformation and crack patterns could be recorded. Reactive arc evaporation and reactive dual magnetron sputtering were used to grow the films.

In the first approach the microstructure of the films was varied from columnar to nanocomposite by Si additions to ZrN. The deformation and crack growth behavior is altered by these microstructural modifications, i.e. homogeneous dislocation glide dominates in the columnar structure resulting in a higher resistance to plastic deformation compared to the relatively softer nanocomposite in which grain boundary mediated heterogeneous plastic flow dominates. The columnar structure also offers higher fracture resistance due to additional energy dissipation mechanisms such as crack deflections, which is not seen in the nanocomposite structure.

In the second approach multilayer architecturing was used to tune both the hardness and fracture resistance of the films using ZrN/Zr_0.63Al_0.37N as a model system. Zr_0.63Al_0.37N is an immiscible material system that chemical segregates into nm-sized ZrN- and AlN-rich domainsduring high temperature growth. The crystal structure of the AlN-rich domains varies from cubic (c) to wurtzite (w) structure as a function of Zr_0.63Al_0.37N layer thickness. Maximum fracture resistance is achieved through a stress- induced transformation toughening mechanism, where the epitaxially stabilized metastable c-AlN-rich domains transform in to a more volume consuming w-AlN phase when subjected to the external stress fields caused by indentation. Hardness maximum is achieved when the w-AlN-rich domains are semi-coherent with the c-ZrN-rich domains. This crystallographic relationship with incompatible slip systems causes coherency strains that collectively generate a formidable resistance to dislocation glide.

Hard and wear resistant cubic (c)-(Ti,Al)N based coatings have many applications, such as protection of the underlying material and improved wear resistance. In the cutting tool industry, the improved wear resistance increases the lifetime of the coated tools. The mechanical properties of TiAlN deteriorates at high temperatures due to formation of the hexagonal (h) AlN phase, while by alloying of Cr in (Ti,Al)N coatings the detrimental effect of h-AlN on the mechanical properties can be reduced [1]. Further, the oxidation resistance of CrAlN coatings is improved compared to that of TiAlN [2], thus, a TiCrAlN coating could be expected to have improved high temperature properties under both oxidizing and non-oxidizing conditions. In this study, in-situ high-energy synchrotron x-ray diffraction studies during annealing in a vacuum or air atmosphere have been performed to study the phase evolution in (Ti,Al,Cr)N coatings. The results show that both the phase separation of the c-TiCrAlN phase as well as the growth of h-AlN depends on Al-content where a higher Al-content results in a faster growth of h-AlN in a vacuum atmosphere. Further, the effect of Al-content on the oxidation resistance of c-TiCrAlN during annealing in an air atmosphere was studied. The results reveal that in (Ti_xCr_yAl_z)N coatings with 38 < z < 60 at. % the oxidation behavior changes with Al-content. For coatings with z=0.48, the oxidation of the coating continues throughout the 3 h long annealing at a temperature of 1100 °C. In coatings containing higher Al-content and lower Ti-content, the amount of nitride phases does not decrease significantly after ~50 min of annealing suggesting that the oxide layer formed acts as a barrier for further oxygen diffusion into the remaining nitride coating.

[1] R. Forsen, et al., J. Vac. Sci. Technol. A 30 (6), 061506-061508 (2012).

[2] A.E. Reiter, et al., Surf. Coat.Technol. 200 2114 (2005).

We demonstrate that in case of a stable adhesion promoting interlayer, DLC coated articulating spinal-disk implants show no detectable wear up to 101 million articulations on a spine simulator - corresponding to about 100 years of articulation in vivo[1,2]. From the tribological point of view, this would allow to have a new generation of livelong articulating implants which do not generate wear particles and therefore will not trigger the known adverse effects to particles such as allergic reactions, pseudotumors and bone resorption. However, long term adhesion of the coating in-vivo is the main issue to be addressed. At each interface good mechanical adhesion is obtained by covalent chemical bonds resulting in a few atomic rows of a reactively formed interface material. The structure and composition of these interface materials, as well as an adhesion promoting interlayer, critically depend on the deposition conditions. It will be shown that even small amounts of oxygen contamination during deposition can change the composition and therefore also the corrosion properties of interfaces and interlayers. Delayed in-vivo delamination of the coating can then occur by slow crack growth due to stress-corrosion-cracking with delayed crack initiation, fatigue or by crevice corrosion. The adhesion tests such as scratch and Rockwell tests will only determine the fracture strength of an interface and are not addressing any corrosion related phenomena and may therefore generate false lifetime expectations in vivo. Especially critical is delamination by crevice corrosion since this process will not be accelerated by increased loads or by accelerated simulator testing [1]. Using biocompatible Ta as an adhesion promoting interlayer, its mechanical properties critically depend on the substrate surface and the feed gas used on deposition. On the one hand, by promoting a ductile body-centered cubic α-Ta phase with a TaN seed layer [3] the contact damage can be significantly reduced due to the plastic deformation of the interlayer material. On the other hand, residual oxygen or surface oxides will promote the brittle tetragonal β-Ta, resulting in increased contact damage upon local loading and also in coating failure during simulator testing.

[1] R. Hauert, et al. Surf. Coat. Technol. 233 (2013) 119.

[2] K. Thorwarth, et al. Int. J. Mol. Sci. 15 (2014) 10527.

[3] D. Bernoulli et al. Thin Solid Films 548 (2013) 157–161

By alloying small amounts of Fe to (Al_0.7Cr_0.3)₂O₃ cathodic arc evaporated thin films a significant increase in hexagonal phase fractions can be attained. However, the reasons for the enhanced occurrence of hexagonal crystallitesis not yet satisfactorily determined. Therefore, the focus of the present work is set on incorporated macroparticles in (Al_0.7Cr_0.3Fe_0.05)₂O₃ coatings and a potential correlation with the growth of distinct V-shaped hexagonal crystallites.

Detailed transmission electron microscopy studies give strong indication for film growth of an α-phased solid solution (Al,Cr,Fe)₂O₃—at least partially—triggered by smaller spherical droplets. Local analyses of the chemical composition of these particles reveal an enrichment of Cr and—in particular—Fe. Contrarily, flat-shaped Al-rich particles indicate re-nucleation of cubic film growth, independent on the phase constitution on which the droplet is incorporated. Additional studies on other—predominant cubic phased—coatings demonstrate that respective effect is also present, albeit less frequent.

According to present results we suggest, that the addition of Fe to powder metallurgically produced Al_0.7Cr_0.3 targets for arc evaporation processes in oxygen atmosphere significantly contributes to enhanced hexagonal crystal growth due to partial epitaxy on small Fe-rich particles.

CrN, TiN, CrAlN, and TiAlN coatings were deposited on AISI 304 stainless steel by cathodic arc evaporation technique. Microstructural and mechanical properties of the resulting coatings were examined. Solid particle erosion tests were carried out at various impingement angles. The erosion scars caused by alumina particle impingement were examined using scanning electron microscopy. The erosion rates of the coatings were estimated using the mass loss. The results indicated that ternary nitride coatings containing Al (CrAlN and TiAlN) exhibited higher erosion resistance as compared to the binary counterparts without Al (CrN and TiN). TiN, CrAlN and TiAlN coatings exhibited lower erosion rates while CrN coating showed higher erosion rate under the same test conditions. The erosion mechanism at 90˚ for all coatings was characterized by brittle fracture with the formation of craters in random positions. The erosion scars observed at 30˚ and 45˚ were typical of ductile fracture.

Fracture toughness is a significant mechanical property in material applications. Recently, we proposed an energy-based method, internal energy induced cracking (IEIC), for measuring fracture toughness on hard coatings. This method was successfully applied on TiN, ZrN and TiZrN hard coatings. The results showed that the fracture toughness of random-textured TiN hard coating was 16.7 J/m² , within the extension of reported values. For ZrN hard coating, the results revealed that the fracture toughness varies with different texture. In addition, the results of TiZrN hard coating indicated that composition could influence the fracture toughness. However, there is little information available on the variation of fracture toughness of multiphase coatings with different phase contents.

The major objective of this study was to investigate the effect of phase content on the fracture toughness. ZrN_xO_y hard coating, an adjustable thin film material, was chosen as a model system. Based on Griffith criterion, the release of elastic energy due to crack propagation is equal to the energy that creates two new surfaces, suggesting that fracture toughness is strongly related to the binding energy. ZrN_xO_y coating is composed of soft phase ZrO₂ and hard phase ZrN. Since the binding energy of ZrN, ZrO₂ and that of interface between two phases are different, fracture toughness may vary with changing phase ratio. ZrN_xO_y coatings on Si substrate with different phase contents of ZrN and ZrO₂ were deposited by adjusting the N₂/O₂ ratio of the reactive gases. The fracture toughness was measured following IEIC method. The residual stress, was assessed by laser curvature and XRD cos²αsin²φ methods, and the film thickness was measured by cross-sectional SEM image, from which the stored energy (G_s) could be calculated. The difference in stored energy before and after cracking was defined as the fracture toughness. Moreover, the critical thickness of coating could be predicted from the fracture toughness.

CrAlN/Si₃N₄ nanocomposite coatings with grains approximately 10 nm in size and surrounded by a grain boundary silicon nitride phase show a greatly improved wear resistance compared with conventional CrAlN coatings. This is generally attributed to the improvements in hardness, though there has been no clear understanding of how these coatings deform. In this work, micropillar compression was used to make direct measurements of the coating yield stress at both room and elevated temperatures. It is demonstrated, for the first time, that plastic flow at the theoretical yield stress can be obtained when the grain-size is sufficiently small, with shear yield stresses of ~ G/25 at room temperature, which extrapolates to ~ G/20 at 0 K. This suggests that the deformation occur by dislocation nucleation in the individual grains rather than by the formation of slip bands as is more often seen. The strain rate sensitivity of the coating flow stress was measured using nanoindentation at different strain rates, and it is shown that the deformation is determined by the CrAlN, rather than the grain boundary phase, suggesting that boundary effects should not influence the flow behaviour. A simple model explaining the deformation mechanism and its implications are also discussed.

Various metal nitride coatings for protecting the cutting tools have been widely developed and applied in the many applications of the tool and die. Despite of its excellent properties of the CrZrN coatings such as high hardness, very low surface roughness and friction coefficients under dry conditions, the utilization of the CrZrN deposited on WC substrate was much limited due to the poor adhesion strength between the coating and the substrate. Recent work reported that the H/E ratio of the interlayer between coating and substrate influences strongly tribological property of the coating. In this work, the CrZrN coatings with various interlayers with different H/E ratios were synthesized using unbalanced magnetron sputtering system on WC-6 wt.% Co substrate. The Cr-N interlayers were deposited with various N₂ partial pressures in the range from 0.6×10^-1 to 2.8×10^-1 Pa. With increasing N₂ partial pressure, the crystalline phase of Cr-N interlayer varied from Cr₂N to Cr₂N+CrN, and then CrN, and the hardness and elastic modulus of the Cr-N interlayers varied, showing the maximum hardness and elastic modulus of 28 GPa and 357 GPa (CrN interlayer at 2.8×10^-1 Pa of N₂ particle pressure). Wear and scratch test showed that the CrZrN coating with CrN interlayer exhibited the lowest friction coefficient of 0.28 and the highest adhesion strength of Lc3=46.1N. These improved tribological properties and adhesion strength could be attributed to the ratio of hardness to elastic modulus (H/E) of CrN interlayer between the CrZrN coating and WC. In view of the coating structure, there exists a gradual decrease in the H/E ratio from the CrZrN coating, to the CrN interlayer, and the substrate (0.090, 0.068, 0.045 respectively) and this would induce a smooth transition of the stress under loading conditions. Tribological and adhesion properties could be improved significantly by structuring the coating with an optimal gradient of the H/E ratio.

Acknowledgement This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Commerce, Industry and Energy, Republic of Korea.