ICMCTF2015 Session B4-4: Properties and Characterization of Hard Coatings and Surfaces

High-performance, multifunctional protective coatings for machining applications have to combine for example high hardness, high thermal stability and excellent tribological properties. Especially ceramic-like coatings are well known for their outstanding properties combinations. Whereas many research activities are conducted on TiN, ZrN and CrN based coatings, only little is known for Mo–N based materials. Especially the cubic phase of molybdenum nitride (Mo₂N) exhibits excellent mechanical properties but is extremely sensitive to the nitrogen content and the N₂-partial pressure used during deposition. We therefore studied in detail the structural evolution of Mo–N coatings as a function of the nitrogen partial pressure used during deposition. Based on ab initio calculations and experimental studies within the binary Cr–N and Mo–N systems, ternary Mo–Cr–N thin films along the quasi-binary Mo₂N–CrN tie-line were developed. Whereas all available literature reports deal with the Cr-rich side Mo/(Mo+Cr)<0.5 our studies concentrate on the Mo-rich side with Mo/(Mo+Cr)>0.5.

Erosion damage by high-velocity impinging particles is commonly observed such machine elements as blades of jet engine or power generation turbine. Conventionally, these materials are made of ferrous materials and recently it becomes popular to use Ti alloys due to specific strength at elevated temperature. Aerodynamic integrity is compromised once blade shape is changed by erosion and to prevent such deterioration in engine efficiency, so application of hard coating to those machine elements is common practice. Commonly used coating is the combination of ductile and brittle material such as Ti/TiN. Detailed mechanism, however, not clarified yet particularly between property of the coating and erosion behavior for nitride coatings and in this study, erosion behavior of (Ti_1-xAl_x)N coating, depending on x, is investigated

(Ti_1-xAl_x)N coatings with different Al contents (x) were deposited by newly developed arc cathode. This new cathode is characterized by optimized magnetic field design and makes it possible to deposit thick (up to 20um) coating with low level of residual stress. Ti or TiAl alloy target was discharged in nitrogen atmosphere of 4 Pa at 400°C. Coatings deposited on SUS630 substrates were subjected to the MSE test [1] for erosion resistance evaluation. 1 mass% suspension containing alumina particle with average grain size of 50 um was used for MSE test. Coatings deposited on WC-Co were subjected to compositional, structural and mechanical property analysis using EDX, X-ray diffraction and nano-indentation technique.

Preliminary study on effect of crystallographic orientation on erosion behavior was conducted using TiN with nearly random orientation. EBSP image of the surface before and after the erosion test indicate that coating grains rather than (111) texture are preferentially removed by the impact of incident particles.

Regarding effect of Al content on erosion resistance, TiN showed a largest erosion rate and it decreased with Al content is increased. To investigate this behavior hardness and Young’s modulus of the coating was measured. Hardness increased as the Al content increased almost linearly while Young’s modulus stayed fairly constant over the range of Al ratio. Correlation between erosion rate and these mechanical properties is investigated. The erosion rate sharply drops as the H/E ratio range up to 0.06. Best erosion resistance was obtained at the highest Al ratio of 0.6 where H/E ratio is also highest.

[1] Y, Iwai, et al., Wear, 251/1-12 (2001) 861-867

Weight reduction of aeronautic devices raises composite machining issues. The challenge lies in designing cutting tools able to resist to the specific machining conditions of these materials.

One of the possible solutions is to use diamond as a wear resistant coating. However, bare cutting tools, made of cobalt-bound tungsten carbide, can not be coated because cobalt promotes graphite formation instead of diamond. We propose to interpose an interlayer between the diamond coating and the substrate. Its function is not only to limit cobalt diffusion toward the surface, but also to control carbon diffusion phenomenon during the process in order to enhance diamond nucleation. Tantalum nitride (TaN), which can be carburized during CVD diamond deposition, appears as a good candidate for this purpose.

TaN exhibits two crystallographic structures. An accurate control of cathodic deposition conditions allows us to isolate both phases. The efficiency of the stable hexagonal phase of TaN as a cobalt diffusion barrier has already been shown but its influence on diamond nucleation is limited. In this study, we gauge the potentiality of the metastable face-centered-cubic phase of TaN. Its original response to carbon diffusion during the CVD process opens up new horizons for diamond nucleation and adhesion.

The objective of this study was to evaluate the fracture toughness of Ti_1-xZr_xN hard coatings with different compositions using the internal energy induced cracking (IEIC) method [1], from which the optimum composition for fracture toughness could be attained. Ti_1-xZr_xN remained single phase structure in the entire compositional range when deposited at temperatures below 500 °C. Three compositions of Ti_1-xZr_xN, x=0.25, 0.55 and 0.85, were deposited using unbalance magnetron sputtering. The IEIC method involved the residual stress measured by the laser curvature method, Young’s modulus obtained from nanoindentation and the film thickness from SEM cross-sectional image. The elastic stored energy (G_s) was calculated from the residual stress and film thickness before specimen fracture, from which the fracture toughness could be derived. The resultant fracture toughness of Ti_1-xZr_xN varied with Zr fraction, ranging from 26.5 to 48.7 J/m², and reaching a maximum for Ti_0.15Zr_0.85N. Adding Zr atoms into TiN could effectively increase the fracture toughness, which was possibly due to the atomic size difference of Zr and Ti. The increase of fracture toughness for Ti_0.15Zr_0.85N was higher than that for Ti_0.75Zr_0.25N. This asymmetrical behavior could be attributed to the difference in lattice constants between Ti-rich and Zr-rich compounds, where the capability of increasing elastic stored energy may be higher for a smaller Ti atom incorporate into a larger ZrN lattice. If the cracks penetrate into the substrate, the contribution of substrate cracking should be considered.

[1] An-Ni Wang, Ge-Ping Yu, Jia-Hong Huang, Surf. Coat. Technol. 239(2014)20.

Oxide and nitride coatings grown by CVD or PVD techniques have found successful applications as tool coatings in the field of metal machining. An important parameter for this success is the ability to control and adjust its stress distribution during growth, which not only affects the coating hardness but also the overall toughness of the cutting tool. The present study focuses on residual stress depth profiling of coatings with respect to their process condition, microstructure and mechanical properties. Several PVD and CVD coated cutting tool case studies including the effect of pre- and post-deposition surface treatments, i.e., grinding, polishing, brushing, blasting, etc. will be emphasized. The coatings were synthesized by both industrial CVD and PVD techniques with a controlled through-thickness stress profiles ranging from a tensile to a compressive stress state, even gradients. The stress depth profiles were evaluated by grazing incidence XRD at different information depths using Cu-Ka and Cr- Ka radiation. Results show strong stress profiles both in the PVD coatings as a result of deposition process parameters, and the CVD coatings as a result of ex-situ post treatment, e.g., wet blasting. In both cases the evaluated stress levels in the surface region is about -4 GPa, i.e., highly compressive, gradually changing to lower compressive stress levels with increased depth into the coatings. Here, the compressive stress in PVD coatings mainly correlates with the bias level whereas the blasting pressure is the strongest contributor to the different compressive stress levels in the CVD coatings. The validity of the results are discussed and related to complementary studies on coating structure and properties by, e.g., electron microscopy in scanning and transmission mode and nanoindentations.

Elemental composition control in ternary and quaternary hard coatings synthesized via cathodic arc deposition (CA) is extremely challenging. To address these challenges, the current work tailored the individual elemental compositions of nanostructured ternary Ti_1-xAl_xN coatings using a novel mosaic target configuration consisting of multiple tiles of Ti and Al via steered CA. The quantity, size and arrangement of each individual tile was found to be a critical factor in controlling the concentration of constituent elements within the deposited Ti_1-xAl_xN coating. Ti_1-xAl_xN coatings deposited by mosaic steered CA and conventional steered CA using a compound TiAl alloy target with 50:50 ratio deposited under similar conditions were compared. The results for Ti_1-xAl_xN coatings 2.5 µm thick and deposited under two different N₂ partial pressures will be presented. The coating roughness, microstructure, composition, mechanical and tribological properties were investigated using optical profilometry, SEM, XRD, EDS, nanoindentation and pin-on-disc tribology, respectively. Ti_1-xAl_xN coatings deposited by each method as a function of N₂ partial pressure showed comparable coating composition, structure and properties for the respected conditions. However, the results reveal that surface roughness of the mosaic Ti-Al steered CA deposited coating (Ra - 0.07 µm) was twice that of the traditional Ti_1-xAl_xN coating deposited from solid TiAl alloy o (Ra - 0.035 µm). All coatings showed dense columnar microstructures with cubic NaCl structure with (111) preferred orientation. Coating hardness (H) was found to be approximately 29 GPa for all coatings, whereas the Elastic modulus (E) for mosaic Ti-Al steered CA deposited coating was slightly higher ~390 GPa when compared to solid TiAl CA deposited coating (350 GPa). A systematic comparative study has been carried out for Ti_1-xAl_xN coatings deposited by steered CA deposition technique using two different target configurations (i) traditional compound TiAl alloy and (ii) mosaic Ti-Al target, to tailor coating composition and properties.

The gray iron cast surfaces are submitted to a high index of corrosion in the exposition with different environmental substances due mainly to the formation of iron oxides such as goethite, hematite, maghemite, and lepidocrocite, causing a deterioration in the mechanical properties of these materials.

This work describes the optical reflectance technique that has been developed to the study of this corrosion products. Specially, this technique will allow the identification of the presence of iron oxides in situ, is a fast and cheap tool and environmentally friendly procedure that could be an alternative to the conventional surfaces analysis method.

The diffuse reflectance spectrum in the visible and infrared regions were recorded and studied to assess the spectral dependence of these oxides, predict their relationship and thereby identify it. The agreement with the experimental results show that the diffuse reflectance spectroscopy could be a helpful method of quantifying of iron oxides on surfaces.

Hydrogen containing diamond-like carbon (a-C:H) coatings tend to exhibit an ultra-low Coefficient of Friction (CoF) in dry sliding; however, the CoF of such coatings can rise dramatically with increasing humidity. In contrast, the CoF of hydrogen-free diamond-like (a-C) coatings typically shows an opposite trend. Furthermore, silicon-doped a-C:H (a-C:H:Si) coatings may exhibit better humidity adaptability compared with either a-C:H or a-C coatings.

In this study, a-C:H:Si coatings with different silicon doping contents were deposited by PECVD with different experimental parameters and compared to a-C:H and a-C coatings. The mechanical properties of these coatings, e.g. their hardness to elastic modulus ratio (H/E), and substrate interfacial adhesion were investigated by Nano-indentation and scratch testing. The microstructure, stress state and chemical composition were also examined by SEM, TEM and Raman Spectroscopy. Pin-on-disc sliding wear tests were performed in an environmentally controlled chamber to evaluate coating friction and wear performance under atmospheric conditions with different levels of humidity.