Nuria Cinca

WC-Co cemented carbides, commonly known as hardmetals, are composite materials constituted by hard ceramic particles embedded in a ductile metal matrix. Due to their unique microstructural assemblage, these materials exhibit excellent combinations of hardness, strength, and toughness, consolidating them as a first choice for tools, structural and wear components. During recent decades, extensive research and technological advancements have driven the development of alternative cemented carbide grades, where traditionally used WC or Co are partially or entirely replaced. Within this context, hardmetals containing a third γ-phase (mixed cubic carbides) represent an interesting alternative. However, accurate evaluation of their fracture toughness remains a significant issue, especially as conventional methods using either indentation or precracking approaches are limited by either restricted implementation of fracture mechanics analysis or testing challenges. Within this context, this study proposes, implements, and validates the use of a novel laser-micronotching methodology to evaluate the fracture toughness of a γ-phase containing cemented carbide grade. For comparison purposes, the investigation also includes assessment of such a property by means of two other well-established testing methodologies. Moreover, similar experimental work was conducted in a plain WC-Co system with similar microstructural features. It is shown that machining of a through-thickness micronotch by means of ultra-short pulsed laser ablation is a reliable and efficient method for fracture toughness evaluation of γ-phase containing hardmetals. The main reason behind this is its capability for providing a precise and reproducible micronotch, with minimal thermal damage, that finally acts as a real through-thickness crack for which a stress-intensity factor is well-defined under flexural testing. Furthermore, toughness values obtained are in satisfactory agreement with those determined using precracked specimens with machined large notches and/or indentation techniques.

Cemented carbide alloys are well known powder metallurgically processed materials used for a wide range of tooling and components that require a good balance of hardness and fracture toughness, together with wear resistance. After 100 years of the first patent, research and development within this area continues to fulfil more demanding applications and adapt to new requirements. The last decade especially has witnessed important advances. In that sense, Co-free compositions are being studied due to the health issues that its use implies and its criticality in the supply chain. Secondly, the steps towards near-net-shape components by means of additive manufacturing technologies to avoid waste of powder and the technological advance of fast sintering processes are promising. Finally, new microstructural and mechanical characterization methods at micro and nanoscale provide helpful insights for a better understanding of these materials under performance.

The electrochemical corrosion performance of WC-12 wt% Co in coating and bulk forms has been evaluated in a 3.56 wt% NaCl solution. The coatings were deposited by means of thermal spray techniques, i.e., cold gas spraying (CGS) and high-velocity air fuel (HVAF) spraying, while bulks with different WC sizes were manufactured by conventional pressing and sintering. Microstructural characterizations and phase composition determinations were carried out using scanning electron microscopy and X-ray diffraction. Differences in WC grain size and morphology, carbide dissolution, and cobalt binder phase transformation are discussed according to the inherent characteristics of each processing method. Together with surface roughness (polished/as-sprayed), these features have been observed to directly affect the electrochemical corrosion performance. Electrochemical measurements (open circuit potential, polarization resistance, electrochemical impedance spectroscopy, and polarization curves) showed that the as-sprayed CGS coating presented an electrochemical behavior similar to those of the bulk materials. This was attributed to the higher metallic character of this coating in comparison to that of the HVAF coating. The polished HVAF coating showed anodic activity lower than those of the bulk samples, most likely due to the presence of cobalt–tungsten carbide phases and eventually the lower amount of Co available for dissolution. Finally, the as-sprayed HVAF coating showed very high resistivity due to the presence of surface oxides generated during the deposition process.

Reinforced Inconel625 coatings have been successfully deposited by means of cold gas spray (CGS). Alumina has been simultaneously sprayed achieving a homogeneous distribution along the deposit with good cohesion between particles. The aim of this study was to investigate whether ceramic reinforcement could improve the mechanical and tribological properties of Inconel625 cold-sprayed coatings, while keeping the oxidation and corrosion resistance capacity. Furthermore, alumina particles were found to improve the spraying feasibility, by avoiding WC nozzle clogging during the process. A proper optimization of the spraying conditions was carried out in order to obtain the lowest possible porosity and best embedment of the alumina. Then, the mechanical and tribological properties as well as corrosion and oxidation behavior were characterized. Alumina acts as a reinforcement, improving the effects of abrasive and sliding wear. Regarding the oxidation and corrosion behavior, the coatings exhibit reasonably good oxidation resistance at temperatures up to 900 °C. The electrochemical corrosion performance in NaCl solution showed potentially lower noble corrosion values and corrosion current densities than bulk Inconel.

(2020). Editorial. Surface Engineering. Ahead of Print.

This work describes the influence of standoff distance (SoD), and gas temperature on the morphology and corrosion resistance of Al-10%Al₂O₃ coatings deposited by cold gas spray (CGS) on carbon steel. The results showed that the standoff distance had little effect on the thickness and microstructure of the coating. However, a 100 °C decrease of the spraying temperature reduced the coating thickness by 300 µm. The use of electrochemical analyses and SEM images showed that all the coatings studied were able to protect the substrate during at least 1300 h of immersion, due to the dense microstructure obtained by CGS.

The medium and high temperature tribological behavior of different iron aluminide thermal spray coatings was investigated. Several powders synthesized through different routes (ball milling, self-decomposition, and self-propagating high-temperature sintering (SHS)) were evaluated. High heterogeneity of conventional High Velocity Oxygen Fuel (HVOF) coatings plays a vital role in their sliding performance, but as long as their integrity is preserved under high temperature oxidizing conditions, the wear rates are found to be acceptable, as it occurs in the case of ball milled Fe-40Al (at.%) powder. The friction phenomenon and wear mechanisms were analyzed in detail through the wear track morphology, contact surface, and friction coefficients. The occurrence of brittle phases in the sprayed coatings, which are also present when tested at high temperatures, appeared to be crucial in accelerating the coating failure.

In this review, the beginnings and evolution of the cold gas spray (CGS) technique are described, followed by the main fundamental aspects of the technique together with a description of the several spraying systems up to date. Sequentially, the main spray parameters and their influence on the properties of the coatings are reported. Afterwards, the most important methodologies for preparing the feedstock powders to be sprayed, the effect of the powder composition, microstructure, particle size and shape on the properties of the coatings are discussed. The nature of the spray gun and nozzle, and the substrate pre-treatments were also discussed. With regard to microstructure and properties, the chemical and physical characterization of the coatings and the performance in protecting the substrates against corrosion together with some mechanical properties are presented and compared. The lacking systematic studies about the great part of investigated systems is the main drawback to compare the published results. Closing this review, the main applications, and the potentialities of the CGS coatings are evidenced.