Papers by Keyword: HVOF

Abstract: Thermally sprayed tungsten carbide-based coatings deposited by High Velocity Oxygen Fuel (HVOF) has been widely developed for its high corrosion and wear resistance. The finishing of the coated surface by grinding process is essential because of the high surface roughness of carbide coatings. Residual stresses existing in most hard coatings have significant influence on the adhesion, mechanical properties and tribological performance. The present study investigated the effect of grinding process on the residual stress of WC-10Co-4Cr coating by HVOF. The residual stress of coating was measured by the X-ray diffraction (XRD) technique using sin²ψ method. In addition, the effect of diffraction peak positioning method on the accuracy of residual stress of WC-10Co-4Cr coating was determined in the XRD technique. Four peak positioning methods and their effects on the accuracy of X-ray diffraction (XRD) residual stress measurement are considered. The results showed that the compressive residual stress of the WC-10Co-4Cr coating increase after grinding process. The parabolic method was selected for the peak positioning to measure the precise residual stress of HVOF-deposited WC-10Co-4Cr coating.

Abstract: This study aimed to understand the effect of a different binder on fundamental properties and corrosion behavior of WC based HVOF coatings. Three kinds of coatings, each with a different binder, were prepared using commercial feedstock powders, i.e. WC-12Co, WC-10Co4Cr and WC-12Ni. Micro Vickers hardness tester, XRD and SEM were used. The corrosion performance was evaluated using a salt spray tester. It was found that the WC-10Co4Cr coating showed the highest corrosion resistance and superior hardness comparable to the WC-12Co coating.

Abstract: The present research paper covers statistical analysis of the major tribological factors affecting the dry sliding wear behavior of Tungsten Carbide (WC) and Chromium Carbide (Cr-C) coatings on plain carbon P/M steel substrate using High Velocity Oxy Flame (HVOF) Coating Technique. Experiments were conducted on pin-on-disc wear testing equipment. The various parameters related to wear such as wear rate, Coefficientof Friction of the coated Disc, Hardness and Surface Roughness were measured.In comparison, WC and Cr-C coating, WC coating has comparably good effect.

Abstract: Thermal barrier coatings (TBC’s) protect superalloy components from excessively high temperatures in gas turbines. TBC’s comprise of a ceramic top coat, a metallic bond coat and a thermally grown oxide (TGO). The creep behaviour of the MCrAlY bond coat, which is sensitive to the composition and the method of deposition, has a significant effect on the life of the TBC. High velocity oxy-fuel (HVOF) thermal spraying is a popular deposition method for MCrAlY bond coats however the creep properties of HVOF MCrAlY coatings are not well documented. The creep behaviour of a HVOF thermally sprayed CoNiCrAlY coating has been determined by small punch creep (SPC) testing. Tests were conducted between an equivalent uniaxial stress range of 37-80 MPa at 750 °C on two different SPC rigs and between 30-49 MPa at 850 °C on a single SPC rig. The measured steady-state creep deformation rates at 750 °C were consistent across the two rigs. A comparison with previous work demonstrated that the creep behaviour of HVOF CoNiCrAlY coatings is not sensitive to the manufacturing variability associated with HVOF thermal spraying. The CoNiCrAlY coating exhibited typical SPC deformation at 750 °C. At 850 °C the CoNiCrAlY coating showed significantly different creep behaviour which could be attributed to the onset of superplasticity.

Abstract: Thermally sprayed MCrAlY bond coats are important elements of thermal barrier coating (TBC) systems which are applied to the surface of gas turbine components to protect them in high temperature environments. Knowledge of their mechanical properties is essential in preventing TBC failure which can have catastrophic consequences. However, limited data on modulus, strength and ductility are available for such coatings. In this work, the ductile to brittle transition behaviour of a CoNiCrAlY coating has been investigated via the small punch tensile test (SPTT). Displacement controlled tests were carried out on free standing coatings at room temperature (RT) and between 400-750 °C at a rate of 1 μms^-1. At low temperatures there was evidence of elastic-brittle behaviour and at high temperatures there was clear evidence of yielding and plastic deformation. The ductile to brittle transition temperature was found to be between 500-750 °C. The yield stress ranged from 1000-1500 MPa below 600 °C to less than 500 MPa above 650 °C. The elastic modulus was found to be approximately 200-230 GPa at 500 °C and 55 GPa above 700 °C. At room temperature the fracture surface showed flat, smooth features indicating brittle failure whereas at 700 °C there was evidence of ductile tearing.

Abstract: Deposition of NiCr-CrC(20NiCr) metal matrix composite (MMCs) coating have been applied on the substrate of boiler tubes material with High Velocity Oxygen Fuel (HVOF) thermal spray method and constant parameter. Variation of particle size and composition on MMCs was conducted to determined the optimum conditions for boiler applications. Microvickers hardness, metallography and thermal shock resistance testing were investigated. The best performance for boiler tubes application is MMC NiCr-CrC(20NiCr) with 270 mesh of NiCr particles size and 60:40 of composition as evidence by the highest of hardness value (410 Hv) and slightly of discoloration after thermal shock resistance with two variation cooling medium. While at MMC NiCr-CrC(20NiCr) with 70:30 variation composition, coating hardness value will decrease in line with the smaller of particle size of NiCr.

Abstract: In order to protect parts against wear, the carbon steel used are commonly coated by cermet coatings to increase the wear resistance. In this paper, the coatings consist of tungsten carbide 17wt% cobalt (WC-17Co), tungsten carbide 9wt% nickel (WC-9Ni), electrodeposited nickel (electro Ni) and electrodeposited nickel-silicon carbide (electro Ni-SiC) coatings. All coatings are deposited onto AISI 1018 carbon steel by using two different methods which are high velocity oxygen fuel (HVOF) and co-electrodeposition method. Abrasive wear test were observed under two‑body dry abrasion conditions with pin-on-disc test arrangement. Based on the volume loss after the wear test, the wear rates were calculated by using Archard’s law. The wear tracks of the coatings were investigated by using scanning electron microscope (SEM) and atomic force microscopy (AFM). The hardness of each coating was measured by using Vickers microhardness. The results showed that HVOF coatings have lower wear rate compared to the electrodeposited coatings. WC-9Ni has the lowest wear rate which is 4.06×10^-3 mm³/Nm much lower compared to electro Ni-SiC of 16.36×10^-3 mm³/Nm. This result was expected as the hardness of WC-9Ni is 1625.37 HV higher than electrodeposited coatings which approximately 380.51 HV. In conclusion, the methods of coating deposited affect the wear resistance as well as the hardness of the coatings.

Abstract: Wear and corrosion are problems prevalent in the industrialized world. One way to overcome this matter is the process of coating with a thermal spray coating. The coating process is categorized into two processes based on the heat source, i.e., combustion and electric. One example of combustion process is the high-velocity oxygen fuel (HVOF). The HVOF system which is operated manually has been studied in previous works. Computerized HVOF system is developed and manufactured in this study to improve reproducibility of coating and safety of operator. ATMega16 microcontroller was connected with several sensors (oxygen mass flow controller, nitrogen mass flow controller, propane mass flow controller, and pressure sensor), and Visual Studio.Net 2013 was used to create a graphic user interface (GUI). Based on test results obtained, it was found that the GUI successfully communicated using serial communications and could access the input/output (I/O) required by the microcontroller. Combinaton of gas mass flow controller and pressure sensors result in precise control of oxy-fuel combustion process.

Abstract: In present study HVOF spraying process had been employed for depositing 93(WC-Cr₃C₂)-7Ni, 75Cr₃C₂-25NiCr, 83WC-17CO and 86WC-10CO-4Cr coatings on ASME SA213 T22. All the coatings were found dense and uniform having thickness between 200-250µm. All the coatings on ASME SA213 T22 used in present studies have provided resistance to corrosion in coal fired boiler environment in superheater zone when exposed for 10 cycles at 900°C. Each cycle consists of 100 hours heating followed by 1 hour cooling at ambient conditions. X-ray diffraction (XRD), Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) techniques were used to analyse corrosion products. The results showed that among coated specimens 93(WC-Cr₃C₂)-7Ni and 75Cr₃C₂-25NiCr coatings has shown maximum and minimum resistance to corrosion respectively. The better corrosion resistance of 93(WC-Cr3C2)-7Ni coated steel alloys may be attributed to the formation of thin band of oxides of nickel, chromium; and carbides of tungsten.

Abstract: The influence of high temperature exposure on the mechanical and wear properties of selected HVOF sprayed CrC and No/Co – based alloy coatings were tested. Comparison of as-sprayed and 600°C/116 h annealed coatings’ microhardness, cohesive strength, abrasive and adhesive wear resistance showed that the Co-based Stellite 6 coating’s wear properties were deteriorated by heat exlosure despite of the increase of its hardness and cohesive strength. The heat exposure was found to be beneficial for both mechanical and wear properties of NiCrBSi self-fluxing coating. On the contrary, mechanical properties of Cr₃C₂-NiCr coating slightly decreased, while its wear resistance slightly increased as a result of high temperature exposure.