Papers by Keyword: HVOF Coating

Abstract: In the present study, D-optimal based mixture design of experiments (DoE) is used to find the optimum powder mixture for High Velocity Oxygen Fuel (HVOF) coating. Twenty five experimental trials are performed by varying the powder mixtures based on mixture design. Cr₃C₂, Ni, Cr, B, Si powder mixtures are deposited on Inconel 718 substrate by HVOF process.The responses porosity and hardness are determined by the cross-sectioning the specimens.The effect of powder mixtures on coated samples are characterized by X-ray diffraction, optical microscope, Vickers hardness tester and Scanning electron microscope.The optimal powder mixture is determined to obtain minimum porosity and maximum hardness and the confirmatory experiment confirms with the predicted results. From the analysis it is observed that Cr₃C₂, Ni and Cr are the major factor which influences the porosity and hardness followed by B and Si respectively.

Abstract: Article deals with analysis of production and characteristics of ceramic coatings applied using HVOF technology. It presents the results of research focused on quality evaluation of HVOF coatings on the basis of WC-Co and impact of Boron Nitride protective layer. HVOF coatings were exposed to hard tribocorrosive conditions. For determining of corrosive resistance of coatings 1M solution of NaCl, potentiodynamic tests and accelerated salt spray test were used. Experiment confirmed that using of Boron Nitride protective coating clearly improves corrosive resistance of the coating on the basis of WC-Co even in hard combined tribocorrosive conditions. Protective layer seals the pores in the structure of coating and thus prevents the formation of corrosion and subsequent depreciation of coating in various corrosive environments. Using of protective layer thus improves lifetime and reliability of spraying and the part itself. Improved lifetime positively influences also the economics of the operation of energetic equipment in which these coating are used.

Abstract: The present investigation was carried out in order to determine residual stresses inNi-based self-fluxing alloy based high velocity oxy-fuel (HVOF) coatings with different wearresistant additives. The main components of the alloy were Ni, Cr, Si and B. To increase the wearresistance of self-fluxing NiCrSiB alloy based coatings, different ceramics and hard metals(WC-Co, TiC-NiMo and Cr2C3-Ni) were added. Residual stresses were measured by the holedrillingmethod and the X-ray method. The incremental hole-drilling technique combined with theintegral method was applied. This combined method allows to analyse non-uniform throughthicknessresidual stresses. The results obtained with the incremental hole-drilling method indicatethat through-thickness residual stress distribution is non-uniform. The determined residual stresseswere tensile on the surface of the coating and became compressive toward the interface.The values of the modulus of elasticity and microhardness of the coatings were obtained byinstrumented indentation. The microstructures of the studied coatings were investigated with theSEM technique.

Abstract: Erosion behavior of the high velocity oxy fuel (HVOF) deposited Cr₃C₂-NiCr and WC-Co coatings on boiler tube steels was evaluated. The solid particle erosion study was conducted, using an air jet erosion test rig at a velocity of 26 m/s and impingement angles of 30⁰ and 90⁰, on uncoated as well as HVOF spray coated boiler tube steels at 250°C. The coatings are significantly harder than the substrate steel and less porous. Scanning electron microscopy (SEM) technique was used to analyze the eroded surface. Mass loss of the coatings was found higher than the boiler tube steel.

Abstract: A hybrid surface treatment was performed to improve the surface properties and the durability improvement of turbo shaft material Inconel718. A micron sized (µ) WC-metal powder (86% WC 10% Co 4% Cr) was coated onto a substrate surface using HVOF thermal spraying, and the coating was heat treated by a CO2 laser. With the HVOF coating of the powder onto the substrate, the surface hardness of substrate increased approximately 300% from 399 Hv to 1260±30 Hv, and further increase of approximately 40% from1260±30 Hv to 1820±100 Hv by laser heating the coating for 0.6 s. Porosity of coating decreased more than five times from 2.2±0.3% to 0.4±0.1% by laser heating. According to the reciprocating sliding test, friction and wear behavior of coating improved by coating for both sliding surface temperature of 25°C and 450°C. Therefore, the HVOF coating and laser heat treatment of coating are recommended for the durability improvement of turbo shaft materials.

Abstract: Abstract. Micron-sized WC-metal (WC-0.6%C-21%Cr-6%Ni) powder was coated onto the substrate of magnetic bearing shaft material Inconel718 (substrate or In718) using JK3500 HVOF thermal sprayer for the improvement of the surface properties of the substrate. The optimal coating process for the highest surface hardness was obtained using the Taguchi experimental program. The coating was laser heat-treated (LH) by CO2(g) laser for further improvement of the properties. During the thermal spraying, a small portion of metal carbides of powder decomposed to W2C, metals and free carbon. The free carbon reacted with excessively sprayed oxygen, and formed carbon oxide gases, forming porous coating. By laser-heating, porosity decreased and the porous strips at the interface of coating and substrate (coat/sub) compacted. At the interface, the precipitated graphite concentration decreased and the metal elements diffused from both the coating and substrate increased, enhancing the functions as buffer zone and increasing adhesion of coating. The surface hardness of substrate increased by coating and further increased by laser-heating from 410±30 Hv to 983±101 Hv and 1425±94 Hv respectively. Porosity of coating decreased by laser-heating from 2.6±0.4% to 0.35±0.06%, and coating thickness shrank from 280㎛ to 200㎛. Friction coefficients of substrate decreased from 0.52±0.02 to 0.36±0.04 by coating, because the free carbon formed by decomposition of WC to W2C functioned as a solid lubricant. By increasing sliding surface temperature from 25°C to 450°C, the friction coefficients of substrate and coating were decreased from 0.52±0.02 to 0.31±0.02 and from 0.36±0.04 to 0.23±0.04 respectively, because of easy formation of free carbon and metal oxides which functioned as solid lubricants. Wear depth of surface was decreased by coating and by LH coating from 55µm to 32 µm and to 12 µm respectively. HVOF coating of WC-metal powder on In718 surface and laser heat-treatment of the coating are highly recommended for the improvement of the properties of magnetic bearing shaft.

Abstract: HVOF coating finds application in industry to protect the surface from the harsh environments such as high temperature, corrosion, and abrasion. In the present study, HVOF coating consisting of Inconel 625 powders blended with WC particles and sprayed on to 304 steel is carried out. The mechanical properties, such as elastic modules and fracture toughness, of the resulting coating are determined using the indentation tests. The influence of the mass fraction of WC on the fracture toughness and elastic modulus of the coating are also examined. It is found that addition of WC particles in Inconel 625 powder enhances the fracture toughness of the resulting surface. This is attributed to increased elastic modules and hardness.

Abstract: HVOF-sprayed coatings (WC–17%Co) and hard chromium coatings corrosion resistances have been compared through electrochemical polarization test in 3.5% NaCl solution. WC–17%Co alloy coatings were deposited on mild steel substrates by High Velocity Oxy-Fuel (HVOF) spray process. The layers of standard and crack free hard chromium coatings were prepared by using Direct Current (DC) and Pulse Current (PC) electroplating process on the mild steel substrates. Hard chromium coatings was characterized as a reference material, to verify whether HVOF-sprayed coatings are suitable as a hard chromium coatings replacement. The microstructure of the coatings was examined by OM, SEM and XRD. Standard hard chromium coatings passivate in NaCl environment, but crack free hard chromium coatings were prepared by using Pulse Current (PC) electroplating do not passivate. The lowest corrosion current densities (Icorr) were recorded for crack free hard chromium coatings. Comparative electrochemical test results showed that, the Standard hard chromium coating has the highest Icorr and were significantly damaged after the electrochemical tests. It is seem to be that WC–17%Co alloy coatings can be substituted for standard hard chromium coatings but crack free chromium coatings bring new challenge for HVOF-sprayed coatings!

Abstract: A feedstock of Ti(Al,O)-Al2O3 composite powders was produced by high energy milling of a mixture of Al and TiO2 powders followed by a thermal reaction process. The feedstock was then thermally sprayed using a high velocity oxygen-fuel (HVOF) technique on H13 steel substrates to produce Ti(Al,O)-Al2O3 composite coatings. The performance of the coatings was assessed in terms of thermal fatigue behaviour and reaction with molten aluminium (soldering). The composite powders and coatings were characterised using scanning electron microscopy (SEM) and X-ray diffractometry (XRD). This paper reports the experimental observations and discusses characteristics and potential applications of the composite coatings.