A. Karimi

A study of high velocity oxy-fuel thermally sprayed tungsten carbide based coatings. Part 1: Microstructures

Abstract The microstructures of two tungsten carbide–cobalt (WC–Co) coatings, deposited using high velocity oxy-fuel (HVOF) thermal spraying method in different conditions, are studied. They are compared with that of the WC–Co powder grains injected in the flame, in an attempt to understand the transformations that occur during deposition. For this purpose, various imaging and analytical techniques in electron microscopy are used, in addition to global characterization methods such as X-ray diffraction and fluorescence. These methods reveal that the coatings are made of distinct islands, elongated along the substrate direction, which exhibit a nano-crystalline matrix containing tungsten, cobalt and carbon. The fraction of WC grains in the coating is smaller than that in the powder and fluctuates throughout the coating. A net loss in carbon is evidenced in the coatings as compared to the powder grains. New phases, W 2 C and W, appear in specific locations in the microstructure in relation with the local composition of the matrix. Very little metallic cobalt is retained. The extent of the transformation is related to the spraying conditions. Some processes that account for the change in microstructure and composition during spraying are proposed.

Microstructure and hydroabrasive wear behaviour of high velocity oxy-fuel thermally sprayed WCCo(Cr) coatings

Sand erosion tests were performed on WC-Co and WC-CoCr coatings deposited by the high velocity oxy-fuel spraying method. Several analytical techniques, including X-ray diffraction, Auger electron spectroscopy and energy-dispersive spectroscopy in a transmission electron microscope were used to characterize the microstructures formed during powder processing and spraying. It was found that a substantial fraction of WC decomposed into W2C or reacted with the cobalt matrix to form ternary carbides such as Co3W3C and other mixed compounds. In both cases the binder phase had a nanocrystalline structure of size 4-8 nm containing tungsten, cobalt, carbon and chromium elements. The addition of chromium inhibits to a large extent the decomposition of WC and avoids the formation of metallic tungsten. In addition, chromium improved the erosion resistance by several times compared with the WC-Co coating. Scanning electron microscopy showed that the CoCr matrix binds carbides better than the cobalt matrix, thereby inhibiting carbide loss at the spray particle boundaries. The hydroabrasive wear behaviour of coatings and the mechanisms for material removal are discussed with respect to the microstructures formed during spraying.

Fracture mechanisms in nanoscale layered hard thin films

Abstract Depth sensing nanoindentation and nanoscratch testing were combined with atomic force microscopy and electron microscopy observations to study mechanical properties and fracture behavior of a number of TiAlN(Si, C,…) hard thin films. Various nanoscale multilayer thin films were deposited onto the cemented carbide substrates using cathodic arc PVD. Failure modes were activated either by a cycle of indentation or by microscratching of the samples to provide an estimation of the fracture toughness and interfacial fracture energies. Load–displacement curves were characterized by small discontinuities due to the formation of nano and microscale cracks through the film thickness. Under sufficiently high load the occurrence of large-scale cracks consists of the successive lateral cracks at the contact site, or corner Palmqvist type radial cracks around the contact area depending on the film structures. Such indentation behaviors can be attributed to small modulus mismatch between the film and the substrate, good adhesion of the film, and in particular high toughness of both substrate and films in spite of great differences in their respective hardness. Various modes of failure and the sequences of fracture events were determined using stepwise or continuously increasing load scratch tests. Some films were found to be more sensitive to tensile stress behind the indenter which generates through thickness vertical microcracks on the scratch track. Other films appeared to be more susceptible to compressive stress ahead of the indenter leading to local delamination at the interface between the layers and irregular microcracks under the contact area.

Mechanical properties and failure modes of TiAl(Si)N single and multilayer thin films

Abstract Mechanical properties and failure modes of TiAlN and TiAlSiN single layer and multilayer thin films were investigated by nanoindentation measurements. It was found that alloying of silicon into TiAlN single layer led to a significant increase of hardness attributed to the reduction of grain size described by the Hall Petch effect. The mechanical properties of TiAlN/TiAlSiN multilayers were found to be very close to the average of the two constituent materials. This allows to suggest a negligible role of the interface affecting the hardness and modulus of TiAlSiN thin films. In contrast, the formation of cracks is significantly delayed and fracture resistance improved in multilayers as compared to the single layer films. The critical load for crack propagation was found to be higher in multilayer films due to a more efficient sharing of the work of indentation between the two layers with different elastic modulus. The morphology of cracks observed using atomic force microscope mainly consists of lateral microcracks parallel to the contact edge for shallow indentations. The radial corner cracks occur only for deeper indentation when the indenter reaches to the interface between the film and the substrate.

Slurry erosion behaviour of thermally sprayed WC-M coatings

Abstract Coatings of tungsten carbide-metal matrix cermets deposited using high velocity oxyfuel spraying were subjected to sand erosion tests. The experimental conditions were as following; flow velocity varying between 20–145 m s −1 , abrasive particle size distributed over 20–150 μm with a mean at 120 μm, and abrasive concentration 0.3 wt.% in tap water. Cermets of cobalt binder phase showed higher strength than those of nickel matrix, and addition of chromium improved the erosion resistance several times. Electron microscopy observations and microanalysis techniques were used to characterize the microstructure of coatings and the mechanisms of erosion. It was found that during powder processing and thermal spraying a substantial fraction of WC was melted and reacted with metal matrix to form ternary carbides or mixed W-C-M compounds which exhibit a nanocrystalline character (M is Co, CoCr, and Ni). Another part of WC led to decarburization and resulted in the appearance of W 2 C or metallic W within the coatings. The molten appearance of some single impact sites in conjunction with the results of extraction replicas from the eroded surface, indicated a local temperature rise within the surface layers during tangential erosion. The erosion results are discussed with respect to the microstructural modifications occurred during powder processing and thermal spraying.

Microstructural and analytical study of thermally sprayed WC-Co coatings in connection with their wear resistance

The microstructure of High Velocity Oxy-Fuel (HVOF) thermally sprayed WC-Co coatings was quantitatively evaluated in an attempt to describe the transformations that take place during thermal spraying. Slurry erosion tests were performed to measure the weight loss as a function of flow velocity and incident angle. It was found that an optimum amount of transformation is required to ensure a maximum of erosion resistance.

Hydroabrasive wear behaviour of high velocity oxyfuel thermally sprayed WC-M coatings

Abstract Hydroabrasive wear tests were conducted on the coatings of tungsten carbide-metal matrix cermets of type WC-M, where M≡Co, CoCr and Ni. The coatings were deposited using the high velocity oxyfuel (HVOF) spraying process. Cermets of the cobalt binder phase showed 50%–200% higher strength than those of the nickel matrix depending on the erosion conditions, and the addition of chromium improved the erosion resistance several times. Analytical techniques including X-ray diffraction and energy-dispersive spectroscopy (EDS) as well as transmission electron microscopy were used to characterize microstructures formed during powder processing and spraying. Owing to overheating of powder particles within the spray torch, the HVOF cermets develop complex microstructures. The hard phase consists basically of tungsten monocarbide (WC) as well as ditungsten carbide (W2C), while the binder phase obtains a nanocrystalline structure of the size between 4 and 8 nm, containing ternary carbides and mixed compounds. The erosion results are discussed with respect to the microstructural features developed during spraying.

Unified cluster expansion method applied to the configurational thermodynamics of cubic Ti1-xAlxN

We study the thermodynamics of cubic Ti1-xAlxN using a unified cluster expansion approach for the alloy problem. The purely configurational part of the alloy Hamiltonian is expanded in terms of concentration- and volume-dependent effective cluster interactions. By separate expansions of the chemical fixed lattice, and local lattice relaxation terms of the ordering energies, we demonstrate how the screened generalized perturbation method can be fruitfully combined with a concentration-dependent Connolly-Williams cluster expansion method. Utilizing the obtained Hamiltonian in Monte Carlo simulations we access the free energy of Ti1-xAlxN alloys and construct the isostructural phase diagram. The results show striking similarities with the previously obtained mean-field results: The metastable c-TiAlN is subject to coherent spinodal decomposition over a larger part of the concentration range, e.g., from x >= 0.33 at 2000 K.

On the behaviour of indentation fracture in TiAlSiN hard thin films

Abstract Structural properties of nanocomposite TiAlSiN thin films prepared as monolithic single layer, multilayers and gradient films have been characterized by X-ray diffraction, nano-indentation and atomic force microscopy. The films were deposited onto WC–Co substrates using cathodic arc-plasma. The Al content was varied within the range of Al/Ti=0.5–1.5 and the Si content did not exceed 3.5 at.%. The highest hardness (38–39 GPa) was obtained for samples with Al/Ti=1.0, associated with more effective combination of solid solution hardening and Hall–Petch effect arising from structure refinement by Al and Si addition. In contrast, the films with Al/Ti=1.5 showed the lowest hardness values (30–33 GPa) due to the formation of softer wurtzite-like hcp-AlN phase. Formation of layered films slightly improved hardness with respect to monolithic single layer films of comparable chemical composition. Meanwhile, gradient films (Al/Ti=1.0) showed hardness and modulus values very similar to those obtained for single layer films. Unlike mechanical properties, the formation and propagation of indentation induced cracks appeared more sensitive to film structures (columnar, equiaxed nanograins, …) as well as to growth morphology (monolithic, layered, gradient). The primary cracks appear for all films beneath the indenter at the contact site. The morphology of these cracks consists of a network of short, discontinuous, irregular cracks in columnar films, which gives rise to straight, well-organized repeated crack in fine grain structures. Such modification of crack patterns is attributed to the role of grain boundary sliding which is more pronounced in coarse columnar films than in nanograined materials.

Influence of nitrogen on the growth mechanism of decorated C:N nanotubes.

Nanotubes like feather boas were synthesised by the decomposition of methane in a nitrogen atmosphere using a bias-enhanced hot-filament chemical vapour deposition technique. This new type of slightly nitrogenated carbon (C:N) nanotubes exhibits a vermicular shape. Graphitic thin foils growing perpendicular to the tube axis typically cover the surface of each C:N tube. High-resolution transmission electron microscopy reveals the graphitic structure to be characterised by strong disorder at different length scales. The role of small amounts of nitrogen, which affects the structural formation of the growing C:N tubes, is discussed.