Conventional High Velocity Oxy-fuel Research Articles

HVAF vs oxygenated HVAF spraying: Fundamental understanding to optimize Cr3C2-NiCr coatings for elevated temperature erosion resistant applications

High velocity sprayed Cr3C2-NiCr coatings are a viable solution to improve the life of power plant components and are often influenced by the choice of processing conditions. The new generation convertible high-velocity air-fuel (HVAF) process offers higher versatility to spray wide ranging particle sizes including the standard high-velocity oxy-fuel (HVOF) grade. However, the use of such oxygenated spraying termed as ‘HVAF(O)’ mode necessitates better understanding for reliable and repeatable coating. Single particle splat morphology and fracture mechanisms of HVAF and HVAF(O) sprayed Cr3C2-NiCr coatings were correlated with the microstructural features, mechanical properties and erosion performance with two different particle sizes. Among the coatings studied, HVAF(O) sprayed coarse Cr3C2-NiCr and HVAF sprayed fine Cr3C2-NiCr provided the least thermal degradation, and better mechanical properties which collectively ensured enhanced erosion performance. The overall coating performance can be attributed to the appropriate choice of process conditions and suitable particle size. In comparison with conventional HVOF process that generally induces a certain degree of thermal degradation within the hard carbides, oxygenated HVAF(O) sprayed Cr3C2-NiCr coating still retained most of the starting carbide phase with minimal thermal degradation, which resulted in superior erosion performance.

Amorphous Steel Coatings Deposited by Cold-Gas Spraying

Cold-gas spray (CGS) deposition of amorphous steel coatings starting from a commercial feedstock powder containing boron, tungsten, and silicon was investigated. Microstructural characterization, carried out by X-ray diffraction (XRD), transmission electron microscopy, and backscattered electron diffraction (EBSD) analysis, confirmed the amorphous nature of deposited coatings. The amorphization phenomenon is related to high-strain/strain-rate deformation with shear instability caused by very high particle kinetic energy, with a mechanism that resembles the severe plastic deformation process. The CGS coatings were heat-treated at temperatures ranging from 650 to 850 °C to induce partial recrystallization. The effect of nanocrystal nucleation and growth on the hardness of the coatings was investigated, and the hardness of heat-treated samples was found to increase with respect to as-sprayed coatings, outperforming conventional high-velocity oxy-fuel (HVOF) deposits. Hardness was found to decrease after prolonged (<90 min) or higher temperature (>750 °C) exposures.

Deposition and characterization of WC-Co hard-metal coatings by high velocity oxy-fuel process combined with dry-ice blasting

The residual stresses arising during high velocity oxy-fuel (HVOF) process usually restrict the building up of thick coatings. The potential of in-situ dry-ice blasting treatment during HVOF process to deposit thick WC-Co coatings with high quality due to its efficient cooling and mechanical peening was investigated in the present work. Characterization of WC–Co coatings deposited by HVOF combined with dry-ice blasting was carried out and compared with that of the coatings prepared using conventional HVOF. Several techniques, including scanning electron microscopy, X-ray diffraction and energy dispersive spectroscopy, were used to characterize the microstructures and phase distribution of the powders and coatings. In addition, mechanical properties such as hardness and sliding wear resistance were evaluated. The results demonstrate that HVOF-sprayed WC-Co coating with dry-ice blasting is characterized by compact construction, grain refinement as well as high hardness. Different from the prevention phenomenon of decarburization observed for plasma spraying of austenitic steel, WC decarburization still occurs for HVOF-sprayed WC-Co coating with dry-ice blasting. The in-situ dry-ice blasting treatment results in minute improvement in wear resistance of WC-Co coatings.

Corrosion Mechanism of Tungsten Carbide-Based Coatings in Different Aqueous Media

In cermet coatings with both ceramic and metal present, these can have very different corrosion potentials. Therefore, determination of the corrosion behaviour performance and its mechanism is crucial, especially in very acidic and alkaline environment. A conventional high velocity oxy-fuel (HVOF) tungsten carbide (WC)-Co coatings and a weld overlay tungsten carbide-nickel alloy cermet with carbide size of ~0.3-5 mm and ~50-140 mm respectively are used. Potentiodynamic tests are carried out in acidic and alkaline electrolytes: 0.5 M sulphuric acid, pH 0.45 and bentonite drilling fluid, pH 10.45. The behaviour of both coating types tested were pH dependent. The HVOF coating showed preferential dissolution of the binder in both electrolytes, with material loss being approximately twice as severe in the 0.5 M sulphuric acid compared to the alkaline drilling fluid. For the weld overlay coatings, with the larger carbide particles, the Ni3B phase in the binder was preferentially removed in the acidic electrolyte whereas the Ni rich dendritic phase underwent preferential dissolution in the alkaline environment.

Mechanical property and wear performance dependence on processing condition for cold-sprayed WC-(nanoWC-Co)

WC-(nanoWC-Co) with micro-WC reinforcing particles in nanoWC-Co cermet matrix was cold-sprayed by using WC-Co with bimodal sized WC particles (with both micro-sized WC particles and nano-sized WC particles). The effect of spray powder property and post-spray treatment on the mechanical properties and wear performance of cold-sprayed WC-(nanoWC-Co) coating were examined in comparison to the conventional high-velocity oxy-fuel (HVOF)-sprayed WC-Co coating. Results showed that both hardness and toughness were influenced by the spray powder properties and the post-spray heat treatment. Compared to the HVOF-sprayed WC-Co coating with micro-sized WC particles, WC-(nanoWC-Co) exhibited much higher hardness and toughness and thereby much higher wear resistance. The further toughening of WC-(nanoWC-Co) by post-spray heat treatment significantly doubled the wear performance. The excellent wear resistance of WC-(nanoWC-Co) is attributed to the simultaneous strengthening and toughening effects resulting from the microstructure design of bimodal WC particle size distribution composed of both micro-sized and nano-sized WC particles.

Microstructure and properties of cold sprayed multimodal WC–17Co deposits

In the present paper, three multimodal WC–17Co deposits with different content of nano-sized WC (10%, 30% and 50%, in wt.%) were prepared by cold spraying process. The effect of micro/nano ratio of WC particles on the microstructure, phase composition and properties such as microhardness, fracture toughness and wear performance of cold-sprayed multimodal WC–12Co coating was investigated. The experimental results demonstrated that the cold-sprayed multimodal WC–17Co deposits exhibited dense microstructure with some stripes composing of Co and nano WC, and their phase structures were similar to that of feedstock. The microhardness and fracture toughness of cold sprayed multimodal WC–17Co deposits respectively increased and decreased with increasing the content of nano-sized WC. The obtained multimodal WC–17Co deposits had both a high hardness and high fracture toughness and exhibited a greatly enhanced wear resistance in comparison to the conventional high-velocity oxy-fuel (HVOF) sprayed WC–12Co coating. The abrasive wear behavior of cold sprayed multimodal WC–17Co deposits was mainly dominated by cutting or gouging related to the preferential wear of Co and the fracture and the peel-off of WC particle.

Study on stainless steel 316L coatings sprayed by a novel high pressure HVOF

The High-Velocity Oxy-Fuel (HVOF) technology has been widely used to deposit cermet, metals, and alloy coatings. However, the high temperature and oxidative flame jet results in oxidation and decomposition of spray particles during the spraying process; giving significant influence on phase composition, microstructure, properties, and performance of the sprayed coatings. In this paper, a high pressure HVOF system with a combustion chamber pressuring up to 3.0MPa along with characteristics of lower flame temperature and higher flame velocity was used to deposit Stainless Steel 316L coatings. Particle velocity was measured by using the DPV-2000 system. It was found that higher combustion chamber pressure enables higher particle velocity. The influence of oxygen/fuel ratio and spray distance on deposition efficiency, coating microstructure, and microhardness of the Stainless Steel 316L was investigated. It shows that the oxygen/fuel ratio has a significant influence on particle's melting state. Therefore, dense Stainless Steel 316L coating composed of semi-molten or solid state particles can be obtained by adjusting the oxygen/fuel ratio at the combustion chamber pressure of 3.0 MPa. In the experiment, deposition efficiency up to 90% was achieved at the optimized spray conditions. As a typical conventional HVOF system of JP-5000, the comparison to the JP-5000 sprayed coating was also performed.

Characterization of cold-sprayed multimodal WC-12Co coating

In present work, the deposition behaviors of individual bimodal WC-12Co particles impacting on stainless steel and WC-12Co substrates were investigated. The microstructure and properties of the cold-sprayed WC-12Co coating were characterized. The results showed that the WC-12Co particles partially penetrated into the stainless steel substrate. Flattening of the particles occurred with splashes at its periphery when impacting on the WC-12Co substrate. A movement of small WC particles accompanied with a plastic flow of the Co binder occurred during the impacting and flattening process of the powders. The sprayed coating exhibited a characteristic microstructure with a wide range of WC particle sizes varying from less than 100nm up to a micron scale. The obtained multimodal coating exhibited both a high hardness and high fracture toughness, e.g. the microhardness is similar to that of a cold sprayed nano WC-12Co coating and the fracture toughness is comparable to that of a bulk material of micro WC-12Co. The cold sprayed coating exhibited a greatly enhanced wear resistance in comparison to the conventional high-velocity oxy-fuel (HVOF) sprayed WC-12Co coating.

タングステンの効率的利用のためのウォームスプレー法による WC-Co 皮膜の開発

Tungsten carbide (WC) based hard metal is known for its excellent combination of hardness and fracture toughness and has been widely used in a wide range of industrial applications such as various kinds of cutting tools and wear resistant components. Recently, it is becoming great concern to save the amounts of WC due to the increase of global demands and the national strategy of the producing countries of the raw materials. High velocity oxy-fuel (HVOF) has been widely used to deposit coatings of WC-Co. However, fracture toughness of HVOF sprayed WC-Co coatings is much lower than its sintered bodies, due to the formations of the detrimental phases such as W2C and η phase during deposition. In other words, if the toughness of the coatings can be improved, there is a great possibility to substitute a part of sintered bulk applications, and hence to lead to resource savings of tungsten. In the present paper, we investigated the possibility to fabricate degradation free WC-Co coatings by Warm Spray deposition which is a new spray technique and has the capability to control sprayed particle temperature. The microstructure and mechanical properties of those coatings were evaluated and compared with the ones by conventional HVOF deposition and the sintered bulk WC-Co. WC-Co coatings without detrimental phases were successfully deposited by Warm Sprayings but the fracture toughness was lower than HVOF coatings. The reasons of this poor fracture properties were discussed.

Sliding wear behaviour of conventional and nanostructured HVOF sprayed WC–Co coatings

WC–Co hardmetals have been used traditionally as wear resistant materials and a significant body of research has demonstrated that sintered hardmetals with WC particles in the nanoscale range offer improved properties over their conventional counterparts. This work examines the performance of such conventional and nanostructured materials in the form of coatings deposited by high velocity oxy-fuel (HVOF) thermal spraying. WC–Co coatings were deposited using both conventional (sintered and crushed) and nanostructured powder feedstocks. Characterisation of the coatings by a range of techniques showed that both coatings not only contained WC but also reaction products such as W 2C and W and an amorphous Co-rich binder phase containing W and C. The nanostructured coating exhibited higher levels of these decomposition products due to the enhanced kinetics of dissolution of the small WC particles into the molten binder during HVOF spraying. The performance of the coatings were examined under sliding wear conditions against an alumina ball and compared with the behaviour of a conventional sintered WC–Co hardmetal. It was found that the nanostructured coating exhibited the highest wear rate of the three materials examined. Microstructural examination showed that wear was dominated by the loss of ductility in the Co-rich binder phase (due to decomposition during spraying) leading to subsurface fracture and the generation of wear debris.

Fabrication of WC–Co coatings by cold spray deposition

Processing of cermet such as WC–Co is not easy by cold (gas dynamic) spraying although cold spray process can eliminate the degradation of the phase as compared to conventional high velocity oxy-fuel (HVOF) spraying process. In this study, WC–12∼17%Co powders with nano- and microstructures were deposited by cold spray process using nitrogen and helium gases. Microstructural characterization and phase analysis of feedstock powders and as-deposited coatings were carried out by SEM and X-ray diffraction (XRD). The results show that there is no detrimental phase transformation and/or decarburization of WC by cold spray deposition as expected. It is also observed that nano-sized WC in the feedstock powder is maintained in the cold sprayed coatings. It seems that nano-sized WC is advantageous over micro-sized WC for cold spray deposition because higher particle velocity can be obtained with the same gas velocity. It is demonstrated that it is possible to fabricate the nano-structured WC–Co coating with low porosity and very high hardness (∼2050 HV) by cold spray deposition with reasonable powder preheating.

Synthesis and Oxidation Behavior of Nanocrystalline MCrAlY Bond Coatings

Thermal barrier coating systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO2/Y2O3). In this work, the oxidation behavior of conventional and nanostructured high-velocity oxyfuel (HVOF) NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Freestanding bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at 1000 °C for different time periods to form the thermally grown oxide layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared with that of the conventional one. The observed behavior is a result of the formation of a continuous Al2O3 layer on the surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating.