Papers by Keyword: HVOF

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Authors: M. Heydarzadeh Sohi, Shahin Khameneh Asl, Kazuyuki Hokamoto, M. Rezvani
Abstract: Five types of tungsten carbide based powders with different chemical compositions (WC-12Co, WC-17Co, WC-10Ni, WC-10Co-4Cr and WC- 20Cr-7Ni) were deposited onto ST37 mild steel substrate using high velocity oxy fuel (HVOF) spray technique. The feedstock powders and sprayed coatings were studied by using X-ray diffraction (XRD), and differential thermal analyzing (DTA). The results were shown during HVOF thermal spraying, WC-M powders become partially melted before being sprayed on the surface of the substrate with supersonic speed. In these types of coatings, the crystallographic structures are normally non equilibrium, because the cooling rates of the deposited splats are very high due to the cold substrate acting as a thermal sink. These partially melted powders are then rapidly solidified to an amorphous phase. XRD analysis showed that the amorphous phase was existed in all of the as sprayed coatings. The amorphous phase in WC-12Co, WC-17Co and WC-10Ni coatings was transformed to crystalline phases by heat treatment at high temperature. Heat treatment of these coatings at high temperature also resulted in partially dissolution of WC particles and formation of new crystalline phases. In cobalt base coatings, the new phases were eta carbide phases like Co6W6C and Co3W3C but in WC-10Ni coating a NiW intermetallic phase was formed. Heat treatment of WC-10Co-4Cr and WC-20Cr-7Ni coatings did not change the amorphous phases in these coatings. Differential thermal analysis of cobalt containing coatings revealed an exothermic reaction at approximately 880°C. This exothermic reaction may be related to the transformation of the amorphous phase to eta phases. On the contrary, DTA analysis of feedstock powders of these coatings showed an endothermic reaction at approximately 1000°C. DTA analyses of nickel containing cermets also showed similar results. Differential thermal analysis of chromium containing cermets did not show any noticeable exothermic or endothermic reactions.
Authors: W.T. Hsiao, W.H. Liao, M.S. Leu, Cherng Yuh Su
Abstract: The image of thermal spray splats is difficult to collect due to its high velocity of droplets. Especial in High Velocity Oxy-Fuel (HVOF) process, the process present higher velocity of flame jet correlated to other thermal spray process. The system presents at this article describes a useful splats catching method to obtaining splats during thermal spray deposited. Capabilities and advantages of using this instrument are declared at this theme. The final result presented the instrument caught the single spot of HVOF sprayed splats at sub-micro second. Splats of spot were dispersed well on the glass substrate at the obtaining system, and presented various information of droplets impact at different location on the substrate.
Authors: A. Mateen, T.I. Khan
Abstract: Abrasive wear mechanism in HVOF sprayed near-nano and microstructured WC-17wt.%Co coatings was studied. The crack propagation mechanism shows that it propagates only through cobalt matrix in the case of near-nanostructured coatings while both trans-granular and intergranular crack propagation was observed in the microstructured coatings. SEM images show a relatively ductile failure mechanism in the near-nanostructured coating, while a brittle failure mechanism dominated in the microstructured coating. AFM images show that the worn surface is smooth in near-nanostructured coatings with an average roughness value of 19 nm, while the roughness on the microstructured coating worn surface was recorded at 83 nm. Further, the wear debris formed on the near-nanostructured coated surface was mostly spherical and finer, whereas this was irregular and coarser in the microstructured counterpart. The wear behaviour suggested a lower wear rate in the near-nanostructured coatings than the microstructured coating. This was further confirmed by abrasive wear tests results.
Authors: Priit Kulu, Riho Tarbe, Arkadi Žikin, Heikki Sarjas, Andrei Surženkov
Abstract: The aim of the current study was to elaborate and compare abrasive wear resistance of thick coatings deposited with different hardfacing technologies. To produce metal matrix composite (MMC) coatings commercial iron and nickel based powders with recycled hardmetal content of 40 vol. % were studied. For deposition technologies plasma transferred arc (PTA) hardfacing, high-velocity oxy-fuel (HVOF) spraying were used. The microstructure of produced thick coatings was examined, including the distribution of hard phase and homogeneity of metal matrix. Micromechanical properties, including hardness and elastic modulus of features were measured by universal hardness measurements. Furthermore, behavior of coatings subjected to abrasive rubber-wheel wear (ARWW) and impact wear (AIW) tests were studied. Wear resistance of experimental PTA hardfacings at low velocity ARWW and AIW tests notably exceeds that of HVOF sprayed coatings. Wear mechanism dominating at abrasive wear in most cases is the removal of metal matrix due to lower hardness. Assignment of hardmetal waste as initial material can significantly decrease the cost of production, improve the mechanical characteristics of coatings and consequently increase their wear resistance. Results indicate, that the choice of matrix for the same reinforcements can also be as an important factor for combating abrasive wear. Fe-based thick coating, produced by PTA is more wear resistant compared to the Ni-based ones.
Authors: G.R. Heath, P. Heimgartner, G. Irons, Robert D. Miller, Stefan Gustafsson
Authors: Daniele Mari, L.M. Berger, S. Stahr
Abstract: Thermally sprayed hardmetal coatings can be used to improve the wear or fatigue resistance of mechanical parts. Depending on the deposition conditions, their microstructure and phase composition are out of equilibrium at different levels due to the extreme heating/cooling rates. In the present study, the changes that occur with temperature variation are monitored by mechanical spectroscopy. Requirements to specimen of mechanical spectroscopy created the need to prepare WC-17%Co coatings of 1.2 mm thickness by high velocity oxy-fuel (HVOF) spraying. The coatings, separated from the substrate by spark erosion, were tested in a forced torsion pendulum between room temperature and 1570 K at a temperature scanning rate of 1K/min. The mechanical loss spectrum shows different features. At 800 K, a maximum M1 is observed in coincidence with a sudden increase of the elastic modulus. The change of the elastic modulus is due to a densification of the material possibly related to cobalt recrystallization. A relaxation peak located at about 1100 K is typically found in WC-Co hardmetals. It is attributed to the movement of dislocations in the cobalt phase. A sharp peak is observed at 1510 K on heating and at 1410 K on cooling. Such peak is due to the reversible transition from W3Co3C at high temperature to W6Co6C at low temperature as proven by X-ray diffraction. The reversibility of such transformation was observed for the first time.
Authors: Nadimul Haque Faisal, John A. Steel, Rehan Ahmed, R.L. Reuben, G. Heaton, Bryan Allcock
Abstract: This research aims to characterise and quantify the acoustic emission (AE) generated during the high velocity oxy-fuel (HVOF) thermal spraying process, recorded using piezoelectric AE sensors. The HVOF process is very complex involving high temperature turbulent flow through a nozzle with entrained particles, the projection of these particles, and their interaction with the target surface. Process parameters such as gun speed, oxy-fuel pressure and powder specification affect various characteristics of the coating, including thermal residual stresses; the lamellar microstructure and the topology and geometry of pores, all formed when the fused powder hits the surface, forming “splats”. It is widely acknowledged in the thermal spray industry that existing quality control techniques and testing techniques need to be improved. New techniques which help to understand the effects of coating process parameters on the characteristics of the coating are therefore of value, and it was anticipated that recording the AE produced when the fused particles contact the surface would aid this understanding. As a first stage, we demonstrated here that AE associated with particle impact can, in fact, be discerned in the face of the considerable airborne and structure-borne noise. In order to do this, a new test method using a masking sheet with slits of varying size was developed. Thermal spraying was carried out for a range of spray gun speeds and process parameters. The AE was measured using a broad band AE sensor positioned on the back of the sample as the spot was traversed across it. The results show that the amplitude and energy of the AE signals is related to the spray gun speed, powder used and the oxy-fuel pressure. Using a simple geometrical model for particle impact, the measured AE was found to vary with the energy and number of particles impacting on the sample in a predictable way.
Authors: Ping Zhang, Jing Hua Jiang, Ai Bin Ma, Ze Hua Wang, Yu Ping Wu, Ping Hua Lin
Abstract: With the aim to obtain the high cavitation-erosion-resistance coating for the surface safeguard of fluid machinery, two kinds of hard coatings (WC-Cr-Co and Cr3C2-NiCr) were prepared on 1Cr18Ni9Ti substrate by high velocity oxy-fuel spraying (HVOF), which microstructure and performance were investigated respectively by optical microscope, X-ray diffraction, Vickers hardness, scanning electron microscope (SEM), and vibratory cavitation apparatus. The results indicated that these two coatings had the higher hardness and the finer structure than ZG06Cr13Ni5Mo, which is used actual in water conservancy. The results of cavitation erosion continued for 39 hours presented that the mass loss of WC-Cr-Co and Cr3C2 -NiCr coatings increased ceaselessly with time, and the cavitation erosion speeds varied with time. Compared with ZG06Cr13Ni5Mo, their cavitation erosion resistance performances were better due to their higher hardness and finer structure, and WC-Cr-Co coating was much better than Cr3C2-NiCr coating. Obviously, the higher hardness and finer structure could strengthen the cavitation erosion resistance of coating materials.
Authors: Sang Yong Lee, Han Shin Choi, Chang Hee Lee, Yong Jin Kim
Abstract: This study was conducted to observe the influence of oxygen to fuel ratio[O2/H2 ratio] of HVOF(High Velocity Oxygen Fuel) process on the amorphous formation and coating properties of NiTiZrSiSn bulk amorphous alloy. The bulk amorphous feedstock was prepared by inert gas atomization. Amorphous contents were followed by differential scanning calorimetry(DSC). Microstructural characterization was done by scanning electron microscope(SEM) and x-ray diffraction(XRD). Mechanical properties were obtained by Vickers microhardness testing and pin on disk wear testing. A maximum 72% amorphous phase could be obtained with an optimum O2/H2 ratio. High amorphous content resulted in a low friction coefficient and weight loss than high oxide content coating and fully crystallized coating.
Authors: K. Meekhanthong, S. Wirojanupatump
Abstract: Thermal spray coatings have become one of the most potential for hard chrome replacement, particularly for wear and corrosion applications. In this study four types of hard coating materials were selected and thermally sprayed by suitable processes as the following designated codes (material/spray technique): Cr-Fe/HVOF, Cr-Ni/HVOF, WC-Ni/SF and Cr-Fe/AS. All of starting materials were characterized in order to create a correlation between spray materials and coating characteristics. SEM was employed for morphology and microstructure investigation. Particle size analysis was investigated by SEM and laser particle size analyzer. Coating characterization included surface roughness measurment by profilometer, porosity evaluation by image analysis and hardness test by Vicker microhardness tester. Microstructure of coatings was also revealed by SEM. Wear performance was evaluated by pin on disc test and dry sand rubber wheel abrasion test. Corrosion resistance was tested by potentiodynamic method. The results showed that particle size and morphology of starting powders were various depending on manufacturing method. Chemical compositions of starting materials showed great effect on coating properties and performance. Coatings deposited by HVOF and spray & fuse (SF) method showed typically dense and homogeneous structure than arc sprayed coating. WC-Ni/SF coating showed lowest abrasive wear rate compared to other thermally sprayed coatings. All Ni base coatings had significantly lower corrosion rates while Cr-Ni/HVOF coating had lowest corrosion rate. Although Cr-Ni/HVOF and WC-Ni/SF could be applicable for both wear and corrosion applications, for heat sensitive parts, Cr-Ni/HVOF coating could be a better alternative to spray and fuse.
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