Thermal spray WC based powders are now frequently used as chrome replacement alternatives for a wide range of industrial and aeronautical applications. In numerous cases, the carbide materials outperform the hard chrome in many property evaluations. However, their usage on highly stressed parts, especially in fatigue loading, can be limited by spalling resistance of the coating. While HVOF is being used on many flight critical parts, stringent applications like the landing gear components of carrier-based aircraft are still under investigation. This work, on WC-17%Co, relates coating bend test performance and fatigue/cyclic step loading behavior to the processing history using different HVOF systems. Initially, twelve (12) different coatings were monitored using a DPV- 2000 for temperature/velocity profiles. The mechanical properties were then assessed using an instrumented four-point bend test as well as uniaxial cyclic loading. After mechanical testing, the coating microstructures were characterized using X-Ray diffraction and electron microscopy in order to investigate the phase content and nature. In particular, the cracks generated during the bend test were measured using SEM on sample cross sections to understand characteristics such as spacing and crack penetration to the substrate. The interactions of processing parameters with the cracking/spalling resistance of the various coating deposits will be discussed and a potential criteria for the control of cracking phenomena will be presented.

Nanostructured coatings of Al 2 O 3 and ZrO 2 (1.5% Y 2 O 5 ) are produced by suspension plasma spraying, introducing the liquid feedstock internally into the central part of three converging plasma jets of a Mettech Axial III torch (Northwest Mettech Corp.). Spraying nanosized ceramic powders in a liquid carrier can yield thinner coatings with more refined microstructures than conventional plasma spraying. In-flight particle states are measured for a number of plasma conditions of varying torch current, gas flow rates and compositions (Ar, H 2 , N 2 ), and related to the resulting microstructure and phase composition in the coatings, as determined by EDS, SEM and XRD. Results show that particle velocities up to 600 m/sec can be reached, yielding high impact velocities and cooling rates. Some comparison is made to previous work performed using external injection of a suspension droplet stream into a conventional dc plasma flame. At optimized conditions, dense γ-Al 2 O 3 deposits with an average crystallite size below 30 nm are obtained at deposition efficiency above 80%, which was not previously attainable. Eutectic alumina-zirconia composite coatings with a finely layered lamellar structure are also realized. The laminates have potential advantages as thermal barrier coatings with beneficial properties arising from nano-composite components.

For solid fuel cell application, thin electrolyte is needed for novel materials that allow reducing their operating temperature. The influence of what called ‘the thermal management of the coating and the substrate’ is discussed. In particular, influence of substrate temperature and the nature of the substrate. It was found that adequate control of the coating and substrate temperature, together with an equivalent substrate/coating thermal expansion coefficients (CTE) are the key-factor to successfully obtain an SDC nanosized thin and dense coating free of cracks.

Plasma spraying of ceramic nano-powders suspended in a liquid carrier medium is an emerging technology, which allows the formation of thinner coatings with microstructures more refined than conventional plasma spraying. An external injection system, where the suspension enters the plasma jet radially, is installed on a F4-Sulzer Metco dc torch for the production of nanostructured Al 2 O 3 and ZrO 2 coatings. The effect of injection parameters, such as initial droplet diameter, droplet velocity and suspension flow rate is studied. The suspension droplets are continuously generated through an exchangeable micron-sized nozzle with a superimposed pulse of variable ultrasonic frequency. The heat transfer from the plasma to the liquid feed is optimized at high droplet velocity, moderate counter-current injection angle and flow rates not exceeding a threshold value, which depends on the plasma enthalpy and the latent heat of the suspension medium. A significant effect of initial droplet size (220 – 500 µm) or solid concentration (5 – 15 %) is not observed. In-flight particle states are measured for different plasma conditions, and are related to the resulting microstructures by SEM and XRD. High particle temperatures give rise to a refinement in crystallite size, while the particle velocities govern the deposition efficiencies and porosity levels. The results show that the particles follow closely the gas flow in the free stream, as well as in the stagnation boundary layer close to the substrate by virtue of their limited inertia. The prominent difference in microstructure between highly porous alumina and very dense zirconia coatings is explained in terms of particle impact velocities, which are simulated for typical operating conditions as a function of particle size and free-stream gas velocity.

The high cooling rate (>105 - 106 K/s) achieved in thermal spraying makes it possible to deposit coatings that exhibit a high content of amorphous phase and can be, afterwards, heated above the crystallization temperature to form nano-structured coatings. In this study, the relationship between the characteristics (temperature and velocity) of the sprayed particles at impact and degree of amorphization of the as-sprayed coatings was investigated for both plasma and wire-arc spraying techniques A Fe-Cr-based alloy that exhibits a high glass forming ability and is commercially available in the form of powder and wire with close chemical composition was used. The amorphous phase content was higher in plasma sprayed coatings than in wire arc sprayed ones and both types of coatings presented an increase in microhardness after heat treatment.

A new WC-WB-Co feedstock material was sprayed with two HVOF torches. During spraying, the in-flight temperature and velocity history were monitored using the DPV2000 tool. Coatings sprayed with both guns had a relatively high microhardness and good abrasion properties, even with a higher level of porosity for some coatings. This was explained by the nature of the matrix which was composed of an amorphous/nanocrystalline structure and W-Co-B phases. It is suggested that the matrix is harder than conventional binders such as cobalt, for instance, and exhibits a better cohesion with the WC hard phase.

WC thermal spray based powders are now frequently used as chrome replacement alternatives for a wide range of industrial and aeronautical applications. In numerous cases, the carbide materials outperform the hard chrome in many property evaluations However, its usage on highly stressed parts, especially in fatigue loading, can be limited by spalling resistance of the coating. While HVOF is being used on many flight critical parts, stringent applications like the carrier based landing gear components are still under investigation. This work, on WC-17%Co, relates the processing history of different HVOF processes used at a variety of industrial sources for hard chrome replacement to the coating microstructure and mechanical properties. The thermal history of the WC particles was monitored using a DPV-2000. The mechanical properties of the coatings were assessed following an instrumented four-point bend test as well as uniaxial cyclic loading. The coating microstructures were characterized using X-Ray diffraction and electron microscopy in order to investigate the phase content and nature. In particular, the cracks generated during the bend test were measured using SEM on samples cross sections to measure characteristics such as spacing and crack penetration to the substrate. The goal of the investigation was to better understand the interaction of processing parameters with the cracking/spalling resistance of the varied coating deposits. Abstract only; no full-text paper available.

The crack propagation characteristics obtained by observing the microstructure of bent coatings at different stages of applied strain were conducted. At the same time, acoustic emission (AE) features of released events were recorded. Generated cracks were found to emit weak acoustic emission events at the initiation (creation) of the crack at the coating surface, and then become strong releasing a uniform acoustic emission energy per event, when propagating transversally in the coating. However, cracks propagating at the interface with the substrate could generate both weak and strong events. It was also found that increasing the thickness of the coating leads to a decrease in the number of events and an increase of their released acoustic emission energy.

Amorphous metallic coatings are of high interest because of their good wear and corrosion resistance as well as their high hardness and toughness relative to the crystalline alloys with the same composition. Thermal spray that makes it possible to reach quenching rates in the order of 106-107 K/s, has the ability to deposit coatings with a high content of amorphous phase. However, very few studies dealt with the understanding of the spraying factors that affect the formation of the amorphous phase under thermal spray conditions. In this work, the relationship between temperature and velocity of the spray particles and coating characteristics is investigated. Special attention is given to the degree of amorphisation of the as-sprayed coatings. The latter were produced both by plasma and wire-arc spraying in order to get a larger range of particle parameters at impact and different particle heating history in the gas flow before impingement onto the substrate. A commercial iron-based alloy available both in powder and wire forms was used. Microstructural analyses show that the as-sprayed coatings are partially amorphous and that the proportion of the amorphous phases depends on the sizes of the sprayed particles as they control the heating and acceleration of particles in the gas flow and their cooling rate on the substrate.

Rietveld analysis was used to measure the phase content of twenty-four coatings sprayed with a multimodal WC-12Co feedstock using three HVOF spray guns. Different correlations were investigated involving, on the one hand, the thermal spray history (particle temperature and velocity) and the mechanical properties of the deposits (microhardness and abrasion resistance), and, on the other, the phase structure of the tested coatings. The results show an inherent effect of the spraying temperature, regardless of the HVOF process used, on the generation of sub-carbide species, mainly W 2 C, W and an amorphous-nanocrystalline phase. In particular, it was found that decarburization of the WC to produce the amorphous/nanocrystalline and tungsten phases is beneficial in enhancing the mechanical properties as long as there is no large amount of tungsten present in the coating. A linear relationship relates this phase with the presence of the amorphous-nanocrystalline phase, independently of the spraying process used in this study. It was also shown that metallic tungsten precipitated in a nanocrystalline structure only with the Diamond Jet HVOF process.

The abrasion and erosion resistance of six different coatings were evaluated in relation to their microstructure. The coatings were produced from six different powders: four containing WC and two containing CrC. Microstructural analysis highlights the relationship between the starting powder morphology and chemistry and the spray conditions in the development of the final coating microstructure. The wear performance of the coatings was evaluated according to the ASTM G-65 standard for the abrasion resistance and a slurry containing 0.66% of 180 μm alumina particles flowing at 20 m/s for wet erosion resistance. The results show that for all tested coatings the abrasion wear resistance is mostly governed by the hardness distribution. For the chrome carbide, coatings having the lowest hardness are the lest abrasion resistant. For the WC containing coatings, carbide debonding and pullout is the main wear mechanisms. The most resistant material being the WC-6Co-8Cr. All the coatings performed better than the D2 tool steel reference sample. The erosion wear resistance is controlled by the local hardness, the matrix properties and the droplet debonding. The most wear-resistant materials are the WC-Co-Cr cermets. The least wear-resistant materials are the clad CrC-20(NiCr) and the WC-Ni cermets.

Four-point bend test using acoustic emission (AE) was used to compare coating properties under mechanical solicitation, mainly the toughness and the spalling behaviour. Coatings are made from the same material; the 2005NS (WC-17Co), sprayed with an HVOF gun at various spray parameters. Coatings deposited on thin rectangular substrates were tested with the coating bent in tension and in compression. AE features like the event number, energy per event and cumulative energy were used to assess the damage in the coatings. The results are analyzed in relation with the coating microstructure.

HVOF-sprayed tungsten carbide coatings are materials of choice to replace electrolytic hard chrome in corrosion and wear environments. This paper evaluates different WC-10Co-4Cr powders for HVOF spraying using propylene as combustion gas. Five commercially available powders are applied using various process parameters. The results show that the choice of powder has the greatest influence on layer quality. Paper includes a German-language abstract.

This paper evaluates the effect of adding graphite and amorphous carbon to WC-10Co-4Cr powders during HVOF spraying. The carbon-based materials are added in order to protect the WC phase from decarburization during the spraying process. The powders were made by ball milling and spray drying and the resulting layers are characterized by means of SEM, X-ray diffraction, microhardness, and wear testing. Paper includes a German-language abstract.