Water droplet erosion (WDE) is a well-known phenomenon. This type of erosion is due to the impingement of water droplets of several hundred microns to a few millimeters size at velocities of hundreds of meters per second on the edges and surfaces of components. The solution to this problem is in high demand especially for the moving blades of gas turbines’ compressors and those operating at the low-pressure (LP) end of steam turbines. Thermal sprayed tungsten carbide based coatings have been the focus of many studies and are industrially accepted for a multitude of wear and erosion resistance applications. The present work studies the microstructural, phase analysis and mechanical properties and their effects on water droplet erosion resistance of such coatings deposited with high velocity oxygen fuel (HVOF) and high velocity air fuel (HVAF) processes. The feed nano-agglomerated tungsten carbide-cobalt powders are in either sintered or non-sintered conditions. The WDE tests were performed using 0.4 mm water droplets at 300 m/s impact velocity. The study shows promising results for this cermet (better than the Ti6Al4V bulk material) as WDE resistant coatings when deposited using HVOF or HVAF processes under optimum conditions.

This paper compares the coating characteristics of two HVOF processes: air-cooled converging-nozzle Diamond Jet (DJ) spraying and Hybrid 2600 air/water-cooled converging/diverging technology. WC-Co coatings were deposited on steel substrates using different combinations of spray parameters, gas flow ratios, and cooling gas types. The coatings were then examined and tested to determine the extent to which microstructure, hardness, surface roughness, wear resistance, and deposition efficiency can be controlled. In addition to investigating process relationships, the paper also addresses the issues of practicality and cost.

The interaction between thermal spray parameters and coating characteristics of atmospheric plasma sprayed WC-12Co was investigated by means of statistically designed experiments. A central composite design was employed to study the influence of the process parameters’ ratio Ar/H 2 , plasma gas volume flow, powder feed rate and stand-off distance on the deposition efficiency, tensile strength, porosity, roughness, particle temperature, particle velocity and the percentage of molten carbides. The process parameters’ volume flow and powder feed rate showed a considerable impact on the microstructure and morphology of the coatings, which could be traced back to a change in the melting degree of the powders as well as a change in particle temperature and velocity. On the basis of the statistical experiments, mathematical expressions were found to describe the coating characteristics. The quality of the mathematic model was scrutinized in a subsequent evaluation stage.

The development of new hardmetal coating applications such as fatigue-loaded parts, structural components and tools for metal forming is connected with improvement of their performance and reliability. For modelling purposes the knowledge of thermophysical, mechanical and other material data is required. However, this information is still missing today. In the present work the thermophysical data of a WC-17Co coating sprayed with a liquid-fuelled HVOF-process from a commercial agglomerated and sintered feedstock powder from room temperature up to 700 °C was determined as an example. The dependence of the heat conductivity on temperature was obtained through measurement of the coefficient of thermal expansion, the specific heat capacity and the thermal diffusivity. Heat conductivities ranging from 29.2 W/(mK) at 50°C to 35.4 W/(mK) at 700 °C were determined. All measurements were performed twice (as-sprayed and after the first thermal cycle) in order to take into account the structural and compositional changes. Extensive XRD and FESEM studies were performed in order to characterize the phase compositions and microstructures in the as-sprayed and heat-treated states. Bulk samples obtained by spark plasma sintering from the feedstock powder were studied for comparison.

Tungsten carbide – cobalt powders (WC-17wt.%Co) were plasma sprayed by a water-stabilized system WSP. A matrix of experiments with variable feeding distances and spray distances was carried out. Thinner coatings were carried out on carbon steel substrates and thicker coatings on stainless steel substrates to compare fast cooling conditions – the former with slower cooling conditions. Basic characterization of coatings was done by XRD, SEM and light microscopy plus image analysis. Microhardness was measured on polished cross sections. The main focus of investigation was on resistances against wear in dry as well as wet conditions. The appropriate tests were performed with set-ups based on ASTM G65 and G75, respectively. The influence of spray parameters onto coating wear performance was observed. The results of mechanical tests are discussed in connection with changes of phase composition and with the character of the coating’s microstructure. The results show that for obtaining of the best possible WC-17Co coating with WSP process, from the viewpoint of wear resistance, the desired parameters combination is long feeding distance combined with short spray distance.

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).

Air plasma sprayed tungsten carbide-cobalt coatings are being used at Kelly Air Force Base for a fretting application for convergent seals in aircraft engines. Experimental and analytical studies were conducted to investigate the plasma spraying of two powders for this application. Statistical processing schemes were accomplished in conjunction with analytical modeling of the air plasma spray (APS) process. Classical and statistically designed experiments (SDE) chosen to be conducted were determined by analytical modeling. The coatings were characterized for composition, hardness, porosity, surface roughness, deposition efficiency, and microstructure. Attributes of the coatings are correlated with the changes in operating parameters. Wear screening of the coatings from the experiments was conducted using an abrasion tester based on ASTM Standard Test B611-85.

In this study, microstructural characterization is conducted on WC-17Co coatings produced via High Velocity Oxygen Fuel (HVOF), High Velocity Air Fuel (HVAF), and Cold Spraying (CS). All coatings prepared were observed to be of good quality and with relatively low porosity content. SEM study showed important microstructural features and grain morphologies of each coating. While composition of feedstock material was approximately similar, elemental composition using EDS showed higher Co content and lower WC in the CS deposited coating. XRD experiment identified formation of more complex oxides and tungsten phases in coatings deposited technologies involving melting of powders such as HVOF and HVAF. These phases consisted mainly of cobalt oxides and brittle phases such as W 3 Co 3 C or W 2 C caused by decarburization of the tungsten carbide particles. Hardness of all coating samples were examined and CS deposited coating exhibited considerably lower hardness compared to the other two coating samples instead of having significantly lower porosity content. It could be contributed to dissociation and physical loss of hard carbide phase during high velocity impact of particles in CS process. It is in good agreement with detection of higher amount of cobalt in CS deposited coating material. It is strongly believed that results obtained from this study can be used for future investigation in thermo-mechanical properties of WC-Co coatings.

The Under Secretary of Defense mandated all DoD facilities reduce or eliminate Hexavalent Chrome from all processing environments. The USAF Landing Gear Systems Engineering located at Hill AFB Utah is converting all line of sight EHC plating to High Velocity Oxygen Fuel (HVOF) applied tungsten carbide-cobalt (WC/Co) and/or tungsten carbide-cobalt chrome (WC/Co/Cr) coatings. The original HVOF coating thickness of 0.015 inch has been reduced to 0.010 inch due to spallation concerns. This has resulted in the condemnation of many expensive assets due to this coating thickness reduction. ES3 is developing an HVOF applied spall resistant coating for the USAF that may be applied up to 0.030 inch for High Strength Steels (HSS) above 200 KSI stress levels. Material test data and recommendations for use are detailed in this paper.

An important problem for the petrochemical industry is the behaviour of materials in aggressive environments, when hydrogen sulphide, carbon dioxide and sand, which contribute to corrosion erosion of the surface, are present. Generally, the use of hard materials such as thermal sprayed tungsten carbide and chromium carbide reduces this problem. Cemented carbides are quite suitable for this purpose: they are composite materials of pure carbides with binder metal alloys of low melting point and high ductility; the selection of the binder metals depends mostly on its ability to wet the surface of the carbide particles to ensure secure coating adhesion. Among the cemented carbides, namely tungsten carbide cobalt-chromium based (WC/CoCr) is considered as the standard for application to ball valve bodies and seats in the petrochemical field, while chromium carbide nickel-chromium based (Cr 3 C 2 /NiCr) is suitable for particular applications. Inconel 625 is also used in this field and usually applied by welding. This paper addresses the characterization of corrosion behavior of HVOF coated samples of WC/CoCr, Cr 3 C 2 /NiCr and Inconel 625 in aggressive environments, and in particular ferric chloride test according to standard ASTM G48-92 and H 2 S/CO 2 test based on NACE standards has been carried out. According to the test results, WC/CoCr based coatings show the best behavior both in terms of corrosion, thus confirming to be very versatile and useful for the application in petrochemical field.

The appropriate selection of bulk materials and coatings of valve components, is an important factor for the economic success of oil and gas production activities in petrochemical field. Materials and coatings are important because particle erosion and surface wear is associated to corrosion by hydrogen sulphide during oil and gas flow. The wear of high pressure valves of gas system will lead to pollution, safety problem and cost increases. The most popular solution of these problems is the deposition of hard material like tungsten carbide or chromium carbide by thermal spray. Particularly these coatings are deposed by HVOF (High Velocity Oxygen Fuel) to obtain a very high hardness with excellent cohesion and adhesion. Tungsten carbide cobalt-chromium based coating, chromium carbide nickel-chromium coating as well as Inconel 625 are adopted actually in the specifications of the industrial petrochemical companies and their behavior and wear, erosion and corrosion properties are reported in literature. This paper addresses the study and surface analysis and characterization of alternative coatings such as NiAl and composite material WC / intermetallic compounds containing mainly Ni, Cr, Co and Mo. The best parameters to produce these coatings has been found by implementing a DOE and the obtained coatings have been systematically submitted to corrosion and functional tests based on the determination of the behaviour of the thermal spray coatings in an atmosphere of H 2 S and CO 2 [1] and to wear and erosion test according to ASTM G75-95; removed material weight and usured surface damages have been determined. Furthermore the coatings have been completely characterized before and after the tests from the point of view of the structure (porosity, coating cohesion and adhesion, hardness, wear) and of the surface properties by means of a prototype 3- dimensional stylus micro-topography surface analysis system. Their corrosion and functional behaviour have been finally compared with the behaviour of the above mentioned coatings applied at present as standard in the petrochemical sector. The results state that WC/intermetallic compound could be a good substitute of IN625 for certain kind of application where good antierosion behaviour is requested.

It is widely known that during high velocity oxy-fuel (HVOF) spraying of tungsten carbide – cobalt (WC-Co) coatings, decomposition occurs resulting in the formation of W2C and a relatively brittle amorphous binder phase (along with other carbides and even metallic tungsten). Decomposition has generally been seen to be deleterious to the wear resistance of these coatings and, as such, there have been moves to reduce it. Since decomposition during spraying initiates with WC dissolution into the molten binder phase, strategies for its minimization have been based on reduction of particle temperatures and exposure times during spraying. Moves in spraying from gas-fuelled systems to liquid-fuelled systems have contributed towards these goals. This paper examines microstructural features and wear behaviour of WC-Co coatings deposited with both a liquid-fuelled and a gas-fuelled system. Contrary to expectation, it was found that the wear rate of the liquid-fuel sprayed coating was five to ten times higher than that of the gas-fuel sprayed coating. It was shown that whilst the degree of decomposition was limited during spraying with a liquid-fuelled system, the solid core of WC-Co suffers significant mechanical damage on impact as it is deposited, resulting in carbide fracture and size reduction and thus to the low observed wear resistance.

Tungsten carbide cobalt thermal spray coatings are used in the aircraft industry to reduce wear damage of lightweight metals such as titanium The performance and life of tungsten carbide (WC-Co) coated titanium materials depend on many factors. An important factor that has received increased attention in thermal spray research is the residual stresses in the coating and substrate. Residual stresses depend on the parameters of the application process. Parameters affecting residual stresses include the prespray treatment of the substrate material (grit blasting, shot peening) and the type of spray application process (HVOF, plasma arc) During the in-service life of a WC-Co coated material, residual stresses can change significantly. The goal of this work is to quantitatively evaluate the changes in residual stresses of the substrate and the WC-Co coating during various stages of processing. A destructive laboratory method, called the "Modified Layer Removal Method," was used to evaluate the through-thickness residual stresses of the WC-Co coating and the titanium substrate material. Residual stresses are determined for three conditions: (1) shot peened, (2) shot peened and grit blasted, and (3) shot-peened, grit blasted and thermal spray coated. The changes in the residual stresses are shown at selected stages during the processing history of the coated materials. Differences between residual stress levels at selected stages are identified and discussed. The effect of coating thickness and HVOF application process on the residual stress in the coating is also examined.

Tungsten carbide-cobalt coatings are extensively used to protect surfaces from wear in many types of applications, such as compressor piston rods, pump plungers, shaft sleeves on centrifugal pumps and fans, and midspans of compressor blades in gas turbines. The wear behavior in any application is strongly influenced by the basic physical and mechanical properties of such coatings. Fracture toughness as a mechanical property indicates the resistance to fracture in the presence of a sharp crack, and thus provides a measure of the intrinsic strength of the cemented carbides coatings. In this study, Vickers indentation tests have been used to quantify the in-plane fracture behavior of various WC-based coatings deposited by the High Velocity Oxy-Fuel (HVOF) spray process. The indentation cracks are analyzed in terms of standardized relations that utilize radial-median crack geometries. It is shown that the fracture properties of HVOF WC-Co coatings are anisotropic, and depend strongly on the microstructure and composition of the coatings. The crack propagation is determined by the porosity, binder mean free path, and the shape, size, and distribution of the reinforcing carbide particles. The erosion resistances of the coatings have also been discussed as a function of the fracture properties and mechanisms. It is shown, in this study, that the Vickers indentation method is a useful and convenient technique for determining the in-plane fracture toughness of HVOF sprayed WC-based coatings.

In order to address deficiencies in thermal spray coatings applied using air plasma spraying (APS) and high velocity oxygen fuel (HVOF), namely, adhesion, cohesion, porosity and line of sight limitations, novel hybrid coatings using post - thermal spray chemical vapor deposition via the pack cementation process were developed. Coatings based on tungsten carbide-cobalt chrome and chrome carbide-nickel chrome followed by boron or chrome diffusion were applied to multistage and single stage pump components for severe service applications in sand and alumina catalyst. Field testing established the effectiveness of using the dual coating approach.

Thermo-abrasive blasting is a technique, which combines conventional abrasive blasting and HVAF processes to prepare surfaces prior coating. Thermo-abrasive blasting has a number of advantages over conventional abrasive blasting as the result of a higher nozzle pressure and heat, which helps to remove impurities from the surface. However, practice showed that the short life of blasting nozzles due to thermal stresses and excessive wear is the biggest drawback of this method. Therefore, the correct nozzle geometry and suitable materials are critical for an efficient operation of thermo-abrasive blasting systems. In this study, computational fluid dynamics and finite element analyses were used to obtain the temperature distribution and to evaluate thermal stresses in nozzle materials. The materials investigated include tungsten carbide-cobalt (WC-6wt.% Co), hot pressed dense silicon carbide (SiC) and SiALON (Si 3 N 4 -Al 2 O 3 -AlN). The analysis and experiments showed that WC-CO nozzles produce the best overall results of thermal shock resistance and wear in thermo-abrasive blasting.

The Kinetic Metallization (KM) process allows the coating and dimensional restoration of interior diameter (ID) surfaces. It is a low temperature, low pressure, solid-state deposition process that is compliant with the recent MIL-STD-3021 cold spray materials deposition standard. The unique attribute of preheating powder particles to enhance ductility within the Kinetic Metallization process allows for high quality coating deposition onto the inner diameter of small-bore components using very short sonic nozzles. Inovati has developed a KM ID Spray Gun that can deposit a coating normal to the surface of the bore down to interior diameter sizes of 80-mm, with depth-to-diameter ratios exceeding 10-to-1. These ID deposition guns, when used with KM systems, can deposit the full range of coatings including structural aluminum alloys for rebuilding damaged forgings, superalloys for corrosion and/or oxidation protection, and hard-phase carbide coatings for wear resistance. This paper presents a case study for rebuilding of damaged 7075-Al forged landing gear outer cylinders requiring ID coating repairs of 0.020-0.030 inches in thickness. A separate study focuses on tungsten-carbide cobalt (WC-17Co) on steel liners and actuating cylinders for replacing hard chrome coatings and repairing worn cylinder bores.

Low-pressure cold spray has been used as an innovative method to deposit metal matrix composite (MMC) coatings: boron carbide-nickel (B4C-Ni) and tungsten carbide-cobalt-nickel (WC-Co-Ni) composites. The coatings were studied using scanning electron microscopy, X-ray diffraction with Rietveld refinement, and acoustic emission-coupled four-point flexural test. Indentation fracture toughness tests were performed on the WC-Co-Ni coatings, only. The results showed that the composites had reinforcing particle volume fractions of 45.8 ± 0.3 vol.% and 22.7 ± 0.1 vol.% for the WC-Co-Ni and B4C-Ni MMC coatings, respectively. Flexural tests were used to evaluate the fracture strain of the composites. In these tests, the WC-Co-Ni composite failed by brittle facture at approximately 0.5% nominal strain. The B4C-Ni composite showed flexural behaviour similar to that of an unreinforced Ni matrix. These results suggest that there was insufficient B4C within the coating to affect significantly the ductile failure mode of Ni matrix. Post bending fracture analysis showed the presence of straight, continuous cracks on the WC-Co-Ni surface and the indentation fracture toughness of WC-Co-Ni was found to be 1.2 ± 0.2 MPa·m0.5. Discontinuous, random cracks were observed on the B4C-Ni surface. The quantification of these properties is essential in evaluating the performance of the low-pressure cold sprayings to determine their potential applications.

In recent years, laser-based post-processing of thermally sprayed coatings has gained significant attention as an alternative post-processing route; to mitigate the microstructural defects such as pores, microcracks, and splat boundaries associated with thermally sprayed coatings. Optimisation of the parameters for the laser post-processing is of paramount importance to maintain the required properties of these coatings. The current thermo-mechanical model simulates the impact of laser heat treatment on thermally sprayed Tungsten Carbide Cobalt (WC-17Co) coating and AISI 316L as substrate. A sequentially coupled transient thermal and structural analysis is performed. Transient temperature field from thermal analysis due to laser source will become input loads for the subsequent stress-strain analysis with appropriate boundary conditions. Both the coating and substrate are given temperature-dependent material properties. A gaussian heat flux distribution is used to model the laser source. The finite element analysis results underline the importance of temperature gradients and the presence of thermally induced stress-strain fields responsible for promoting coating degradation. The obtained results also revealed that heat input and dimensional characteristics play a vital role in the annealing treatment's efficacy. Three separate test cases were considered wherein the hatch spacing was varied, keeping the other parameters (scan speed, laser power, and laser spot diameter) constant. The impact of hatch spacing on the temperature and residual stress distribution across the coating was assessed by this simulation. Residual compressive stress was observed in the coating for two out of the three test cases, which further improved the durability of the coating.

This paper describes an investigation of the influence of impingement angle of a solid/liquid jet upon the erosion corrosion behaviour of a tungsten carbide-cobalt-chromium thermal sprayed coating. The coating type investigated was a nominal WC-10%Co-4%Cr material, HVOF-sprayed onto a stainless steel substrate. This coating was subjected to a submerged impinging jet at 12 m/s of 3.5% NaCl solution containing various concentrations of suspended sand particles at 18°C. The angles of impingement employed were 30, 45, 60, 75 and 90 degrees and the measured total weight losses exhibited a significant influence of impingement angle with reduced material losses at more oblique angles of jet impact. The implementation of cathodic protection to the specimens enabled the pure mechanical component of the overall erosion-corrosion damage to be determined and this was also found to be dependent upon the angle of impingement of the liquid/solid stream. In contrast, the pure corrosion component (determined from in-situ electrochemical monitoring) exhibited no systematic trends with impingement angle. The findings are discussed in terms of the detailed erosion-corrosion mechanisms and implications for operational durability of cermet coatings