Wear leads to high material and energy losses in various industries. The manufacturing of novel nano-carbide WC/Co powder feedstock materials promises a further increase in the performance of thermally sprayed wear protection coatings. A novel experimental powder and a commercial ultra-fine carbide WC/CoCr reference are thermally sprayed onto a 1.0038 substrate by High Velocity Air Fuel (HVAF) spraying. The specimens are metallographically prepared and analyzed by means of light microscopy (LM) and scanning electron microscopy (SEM). Vickers Hardness testing is conducted by microindentation and the porosities are determined by optical image analysis. X-ray diffractometry (XRD) analysis are used to investigate the phase retention. Fine nanocrystalline WC-structures are preserved in the dense coatings. A significant effect of powder type on the porosity of the coating was found. No systematic relationships could be identified between the coating structure and the parameter settings. It was possible to influence decarburization via both the powder type and the selected parameters. The resulting experimental coatings exhibit high hardness values in the range of the commercial ultrafine carbide WC reference. The novel nano-structured coating can contribute to reduced wear and therefore improve the efficient utilization of critical raw materials like tungsten.

Surface quality lifetime and wear resistance of protective coatings can be improved by decreasing carbide grain size from submicron to nanoscale. In this study, experimental WC-CoCr powders were manufactured via novel powder manufacturing approach using water-soluble raw materials. Produced powders were sprayed with the High-Velocity Air-Fuel (HVAF) spray process to control the particle temperature and to avoid in-flight decomposition of the nanocarbides. As a result, dense and wear resistant coatings with nanosized carbides were produced. Reference coatings were sprayed using commercial sub-micron WC-CoCr powder to compare the properties of the experimental coatings to the current state-of-the-art. Phase composition and microstructural characterization of the coatings were carried out with X-ray diffraction and electron microscopy, respectively. Mechanical properties were studied by using microhardness tester, as well as rubber wheel abrasion and cavitation erosion wear tests. The wear surfaces were characterized after the abrasion and cavitation erosion tests to understand the effect of nano-carbides on degradation mechanisms. Coatings with the nanosized carbides in the structure showed excellent mechanical properties in wear testing, and even outperformed reference coatings in cavitation erosion test. Based on the obtained results, these novel nano-carbide coatings are promising alternatives for demanding applications in which better surface quality lifetime is vital.

Thermal sprayed WC-Co coatings are widely used for various industrial applications due to their high hardness and corresponding wear resistance. Recently, it has been reported by many researchers, that the use of agglomerated and sintered micron powders with submicron or nanosized carbides can provide the deposition of WC-Co coatings with enhanced or even superior mechanical and tribological characteristics. This can only be achieved, as long as optimized coating conditions adapted to the specific thermo-kinetic behavior of such powders are considered. However, the porosity in the coating morphology represents an inherent problem when using powders with conventional agglomerate size (10-50 µm) and high internal porosity. Consequently, a minimum coating thickness is often necessary to provide suitable wear properties, which reduces the shape or dimensional accuracy when applying such coatings to complex surfaces. In addition, a reduction in surface roughness of the coating cannot be accomplished by fine carbides, since large agglomerates are employed. In this study we used two different fine WC-12Co powders in the HVOF process to manufacture nanostructured coatings with high hardness, moderate toughness, low surface roughness and low porosity. The first powder is a fine agglomerated and sintered powder with particle size of -10+2 µm and carbides in the ultrafine range (400 nm) The second one consists in two loose mixtures of fine Co (Fisher grain size FSS = 3.5 µm) with (a) WC (FSS = 3.0 µm) and (b) WC (FSS = 0.8 µm). Statistical design of experiments (DoE) were utilized to determine main effects of spray conditions on coating properties. Mechanical properties, microstructure and the phase development has been correlated to the in-flight particle behavior. Phase analyses were performed by XRD using synchrotron radiation.

Tungsten-based cermets are well-known engineering materials finding applications in aerospace, nuclear equipment, and many other fields. Plasma spraying is an interesting industrial process to manufacture those refractory materials. Original plasma sprayed hard coatings for wear protection composed of a stainless steel matrix and inclusions of tungsten carbide (WC) nanoparticles were developed. To built-up the coatings, two precursors were injected separately in the plasma jet : a stainless steel micrometric powder was classically injected into the plasma jet using a carrier gas whereas WC nanoparticles were injected with a liquid carrier, like in the so-called process suspension plasma spraying. One of the challenges is to maintain the WC phase stoichiometry in the deposit, without decomposing the carbide into brittle W 2 C, W 3 C, and metallic tungsten, phenomenon usually occurring with thermal spraying techniques. Another issue is to succeed in including homogeneously the carbide nanoparticles in a sufficiently dense stainless steel matrix. Coatings with different WC contents were deposited on stainless steel substrates and investigated with respect to their microstructure by optical and scanning electron microscopy, porosity level using the Archimedean method, phase composition by X-ray diffraction and Vickers micro-hardness. Results have shown that coatings consisting of a stainless steel matrix containing inclusions of carbide nanoparticles can be produced by plasma spraying. The phase composition analysis indicated that nanoparticles are largely composed of the WC phase and contain a small amount of WC1-x phases. A slight increase of the porosity level was measured for coatings containing nanoparticles, compared to the pure matrix, probably due to the cooling effect of the WC carrier liquid on the in-flight characteristics of the stainless steel particles. Micro-hardness measurements gave similar values for with or without nano-sized particles, showing that the amount of WC included in the samples was insufficient to improve the hardness property.

Thermal sprayed coatings produced from ultrafine, near-nano and nano grained powders provide improved properties as compared to conventional (micron size) powders. These ultrafine, near-nano and nano grained materials show significant potential for applications in the aerospace, energy, oil & gas and a great many other industries. A study was conducted to investigate the influence of grain size on the microstructures formed and mechanical properties of conventional, ultrafine, near-nano and nano size WC materials. Powders and coatings as well as consolidated forms of tungsten-carbide-10% cobalt- 4% chromium (WC-10Co-4Cr) and tungsten-carbide- 12% cobalt (WC-12Co) materials are examined. Thermal spray coatings are produced of carbides of several different grain sizes using high velocity oxygen-fuel (HVOF) thermal spray processing. Spark Plasma Sintering (SPS) is performed to provide consolidated forms of WC-10Co-4Cr materials. An examination of the thermal sprayed coatings is conducted using microstructural analysis and mechanical property testing. A brief examination of the wear and bend performance of a near-nano, and nano-enhanced material will be compared to a conventional material (micron sized).

The objective of this work is to demonstrate the ability of the Pulsed Gas Dynamic Spraying process (also known as Shockwave Induced Spraying) to produce nanostructured metal matrix composite. Nanocrystalline and microcrystalline (conventional) Al5356+20%B 4 C composite feedstock powders were used. The influence of the coating process as well as the nature of the feedstock material on the microstructure and mechanical properties of the coatings were studied. The new spraying process provides an opportunity to produce hard and dense coatings with good cohesion between deformed particles and good adhesion to the substrate. No phase degradation, low compressive residual stresses and high dry sliding wear resistance were observed which seem to be an advantage compared to the traditional thermal spray coatings.

Suspension plasma spraying (SPS) is able to process a stabilized suspension of nanometer-sized feedstock particles to form thin (from 20 to 100 µm) coatings with unique microstructures. The void network architecture of these ceramic coatings is a challenge to be characterized and quantified using commonly used techniques due to small sizes involved. Nevertheless, the discrimination of these pore architectures in terms of size and shape distribution, anisotropy, specific surface area, etc., is critical for the understanding of processing, microstructure, and properties relationships. USAXS (Ultra-Small Angle X-Rays Scattering) appeared as a suitable measurement technique allowing discriminating the void size distribution over a large range (up to four orders of magnitude). Results indicate that as-sprayed SPS coatings exhibit unusual porous architecture: 1) average void size is about the same than the feedstock one; i.e., nanometer sizes with multimodal void size distribution; 2) about 80% of the voids exhibit characteristic dimensions smaller than 30 nm; 3) the total void content varies between 13 to 20% depending upon considered operating parameters. In-situ annealing measurements were performed as they proved to deliver more relevant results compared to ex-situ measurements: even at temperatures as low as 800°C, the microstructure transforms - while the total void content does not change significantly. Indeed, it has been demonstrated that the smallest voids (equivalent diameters smaller than 50 nm) coalescence was the predominant mechanism and that it was more sensitive to temperature than time.

Thermal spray coatings are comprised of millions of heated particles that are driven at high velocity to impact against a substrate; thereby building up to form a consolidated coating. Thus, investigating single solidified droplets contributes to fundamental understanding of coating evolution and their properties. In this study, Scanning Electron Microscopy (SEM) studied the splat morphology of flame sprayed ethylene methacrylic acid (EMAA) with respect to the stand-off distance when deposited onto glass and mild steel substrates. A splat shape transition from a “splash” to a “disc shape” was observed. The morphology of EMAA droplets can be described as a ‘splash splat’ when sprayed onto mild steel at room temperature, whereas a 35 cm stand-off distance produced a disk-shaped splat when the polymer was deposited onto a glass substrate.

Alumina-titania plasma spray coatings are widely used for their tribological performances. The combination of these two ceramics in a particular mix percentage permits to manufacture coatings with better wear resistance in comparison to those made of pure alumina. Suspension plasma spraying permit to manufacture sub-micrometer structure coatings very fine structure thanks to precursors which have an initial size of 10 to 300 nm. The use of a liquid feedstock, aqueous or alcoholic, allows the use of nanometer particles directly without the need to agglomerate them to obtain conventional nanostructured micrometer-sized powders. This study aims at studying Al 2 O 3 and Al 2 O 3 -TiO 2 coatings made from aqueous and alcoholic suspensions produced by suspension plasma spraying. Microstructures and phase evolutions are considered. Manufactured coatings present different architectures depending of operating parameters and feedstock particle sizes; the lower the particle diameter, the thinner the microstructure. Phases composition are discussed and compared to conventional micrometer-sized structure Al 2 O 3 and Al 2 O 3 -TiO 2 coatings.

Suspension plasma spraying is gaining greater interest for emerging applications such as new thermal barrier coatings, next generation environmental barrier coatings and ceramic membranes as in solid oxide fuel cells. Mettech developed an axial injection plasma process coupled with an automatic suspension feed system, and demonstrated its capability to overcome the complexities of the process and deliver quality coatings. This paper aims at determining the durability and stability of the gun, suspension feeder and their components. A 120-hour duration test was performed, and the plasma torch and suspension feed parameters and performances were recorded. The test results indicate that the equipment and process are stable and reliable, and ready for industrial applications.

Yttrium oxide (Y 2 O 3 ) coatings have been prepared with high power axial injection plasma spraying using fine powder slurries. It is clarified that the coatings have high hardness, low porosity and high erosion resistance against CF4 contained plasma in the previous study. This suggests that the plasma spraying of Y 2 O 3 with slurry injection techniques is applicable to fabricating equipments for semiconductor devices, such as dry etching. Surface morphologies of the slurry coatings with splats are almost similar to conventional plasma-sprayed Y 2 O 3 coatings, identified from microstructural analysis by field emission SEM in this study. However, no lamellar structure has been seen from cross sectional analysis, which is apparently different from the conventional coatings. It has also been found that crystal structure of the slurry Y 2 O 3 coatings mainly composed of metastable phase of monoclinic structure, whereas the powders and the conventional plasma spray coatings have stable phase of cubic structure. Mechanism of coating formation by plasma spraying with fine powder slurries will be discussed based on the findings.

Thermal sprayed coatings produced from ultrafine- and nano- and near-nano grained powders of tungsten carbide- 10 wt.% cobalt-4 wt.% chromium (WC-10Co-4Cr) are reported to provide improved properties as compared to conventional powders. These materials show great potential for applications in the aerospace, oil & gas, power, and many other industries. A study is proposed to investigate the influence of WC grain size on HVOF coating properties. Thermal spray coatings will be produced from powders consisting of grains of WC from micron- to near-nano in size in a Co-Cr matrix. The Hall-Petch relationship cites the strengthening of materials by reducing the average crystallite (grain) size. An examination of consolidated forms will be performed using the same powders used in thermal spray in the spark plasma sintering (SPS) consolidation. The mechanical properties of thermal spray coatings have been reported to relate to those of bulk materials. Improvements observed in the HVOF spray coatings will be compared to those of bulk samples.