Papers by Keyword: HVOF

Abstract: The high-velocity oxy-fuel technique (HVOF) was used to produce dense NiCrRe coating on boiler steel substrate with a minimal amount of oxide impurities and low porosity. Microstructure analysis, tribology and nano-hardness measurement have been realized with the aim to characterize the systems. The microstructure was studied using scanning electron microscopy and Energy-dispersive X-ray spectroscopy. Tribological characteristics have been studied under the dry sliding condition at applied loads of 5, 10 and 20 N using the ball-on-flat technique with SiC ball at room temperature. Nano-hardness was investigated in continuous stiffness measurement (CSM) mode, the indentation depth limit was 1500 nm. Microstructure analyses proved that the HVOF sprayed layer, with a thickness approximately 870 µm, contains a relatively low volume fraction of porosity with a chemical composition based on Nickel, Chromium, with white areas of Rhenium. The wear rate of the coating is significantly lower than the wear rate of 16Mo3 steel. The average values of indentation modulus and hardness were E_IT = 237.6 GPa and H_IT = 6.3 GPa, respectively.

Abstract: The purpose of this study was to achieve a low-porosity HVOF (High velocity Oxygen Fuel) spray cover and to determine the effect of spray parameters on the porosity of the coating. The spray parameters studied include: Powder feed rate (A); Spray distance (B); Oxygen/Propane ratio to porosity of WC-12Co coating on 16Mn steel. Taguchi method and ANOVA (Analysis of variance) technique were used to optimize and analyze the effect of spray parameters. The results showed that the optimum spray parameters with A = 26 g/min, B = 0.2m, C = 5 for porosity smallest. Factor A has the greatest influence on porosity, followed by B and C. The experimental result is then compared to optimal results and the error is only 4.2 %. Hence, one can say that the optimal results are reliable and Taguchi method - ANOVA technique proved to be an effective solution to the optimization problem. Parameters for a WC-12Co coating with improved porosity on 16Mn steel substrate have been identified.

Abstract: Nowadays, thermal spray coatings are used to enhance mechanical properties of the material. One of the technologies used to produce thermal spray coating is HVOF spray technology. This is the most advanced and modern technology which has been widely used in the industry due to its flexibility and ability to create coatings with better adhesion in comparison with other thermal spray methods. This article presents some empirical findings from applying the 67Ni18Cr5Si4B alloy powder coating onto C 45 steel shaft by HVOF spray technology. It also analyzes the influence of some technological parameters on the adhesion of the coating. As a result, the parameters of HVOF spray technology are obtained suitable for recovering worn axis-sized workpieces.

Abstract: Thermal spray is often applied on steam turbine blade leading edge to increase abrasion resistance. Stellite 1 is one of the commonly used material as it is known to wear protection against abrasion, oxidation, and corrosion at elevated temperature. This paper evaluated microstructure, hardness, and surface cracks of HVOF-sprayed Stellite-1 coating applied to the steam turbine blade. Optical microscopy reveals that all cracked and no-cracked coating have similarity in the microstructure. Typical 1.89% porosity was found on the specimen. The unbonded interface between the coating and substrate was also located to about 38% in length. All samples have 4.1% different in coating hardness with an average value of 718 HV. Coating thickness has a relation with the amount of porosity produced. Analysis of variance showed that both of the thickness and the hardness of coating are influencing the coating in making the penetrant indication. The microstructure showed a less dense coating with apparent porosity and unbonded interface when compared with other HVOF experiment. Blasting with excessive pressure and or the improper angle have made an alumina deposit at and below interface which may interfere with coating adhesion. Chromium Carbide and Silicon Oxide are formed near the porosity of coating.

Abstract: Porosity, hardness, and adhesion mainly affect the performance of thermal spray coating and significantly depend on spray parameters. Therefore, determining value of the spray parameters and their effects on the coating properties are always taken into consideration. This paper studies optimization as well as evaluates influences of HVOF spray parameters which include powder feed rate (M), rotational speed of the details per minute (N) and step movement of the nozzle per revolution (S) to the adhesion, porosity, micro-hardness of Cr₃C₂ - 20(80Ni20Cr) coating on 40Cr steel shaft substrate. Taguchi experimental design L9 combined with analysis of variance (ANOVA) was used to determine optimum spray parameters and percentage of effect of each spray parameter on properties of the coating. From obtained results, the optimal spray parameters are m = 35 g.min^-1, n = 130 rpm, S = 6 mm for the highest hardness coating of 658.2 HV; with m = 45g.min^-1, n = 130rpm, S = 3 mm for the smallest porosity of 1.27%; with m = 35g.min^-1, n = 130 rpm, S = 3 mm for the coating with a maximum adhesion of 44.07 MPa. The percent effects of the parameters m, n, S to adhesion, porosity and hardness were (2.8%, 33.6%, 63.6%), (0.1%, 1.3%, 98.6%), (32.7%, 43.3%, 24%), respectively. The percent effects of spray parameters on corresponding coating property allows adjustment of spray parameters to obtain the desired coating. Verified experiment results shows that the results are reliable. Taguchi method and ANOVA can find optimal parameters of the HVOF spray to acquire high-performance coating.

Abstract: Current development in power generation industry leads to search of new technologies and ways how to protect surface against aggressive corrosion environment. Suitable method leading to enhancement of surface properties is application of protective coatings on surface. This approach allows to use components composed from less quality materials. The current development in steam turbines leads to increasing operating temperatures for the purpose to enhance performance. This process results in elevation of operating temperature reaching the potential limits of commonly used materials. One of the key areas of protection in such environment is protection against hot temperature corrosion. Possible solution can be found in application of coatings based on alloys and cermets prepared by HVOF (High Velocity Oxygen Fuel) technology. This paper examines local mechanical and microstructural properties of Cr₃C₂-25%NiCr coating after exposition to extremely severe hot corrosion environment. Furthermore, the nanoindentation measurements of this coating were performed before and after the corrosion test. The test environment composition was based on mixture of salts 59% Na₂SO₄ with 34.5% KCl and 6.5% NaCl. Test temperature was 525°C and 575°C. Duration of the exposition to hot corrosion environment was 168 hours in autoclave. The results confirmed that the coating matrix is deteriorated during the exposition to such aggressive environment. The deterioration is localized at surface of coating and is characterized by creation of oxide layer which decreases the speed further speed of matrix dissolution and stabilizes the coating.

Abstract: In the present study, Cr₃C₂-NiCr and WC-Co coatings have been deposited using high velocity oxygen-fuel (HVOF) spray technology from five available powders with various bonding phase content or manufacturing process. The microstructure of coatings was observed by scanning electron microscopy (SEM). Microhardness and erosion performance of the coatings were studied. The influence of powder characteristics on the microstructure and erosion performance of coatings was also investigated. The results indicated that an independent bonding phase distributed in the feedstock powder can effectively improve the erosion resistance of Cr₃C₂-NiCr coatings although the microhardness of the coatings may be lower. Deformation of the free NiCr binder layer in the coating is probably to prevent nucleation and propagation of cracks, which may result in improving the erosion resistance of the coating. Nano WC-Co coatings reached lower erosion resistance than micro WC-Co coatings due to the higher porosity and lower microhardness of nano WC-Co coatings.

Abstract: Thermal sprayed nickel chrome powders are known as the best materials for combined wear and corrosion applications and represent a good alternative to hard chrome plating. The coating from these materials are resistant to wear abrasion, contact with hard surfaces, particle erosion, fretting and have excellent submerged saltwater corrosion resistance. The work in this paper is focused on determining the adhesion strength for nickel powders and structural characterization of the deposited layers using metallographic analysis. The adhesion strength testing was carried out in accordance with ASTM C633 procedure. During the experimental program, an investigation was also carried out for studying the effect of the HVOF process on the air quality while operating with the powders mentioned. Study showed 2 times higher adhesion strength of coating on structural steel substrates than stainless steel. Variations in regimes gave understanding about the most recommended thermal spraying parameters for Metco 5803 powder.

Abstract: The paper presents some preliminary results regarding the combining of metallic, Cr3C2-20(Ni 20Cr) and ceramic (TiO₂) HVOF deposition of layers for corrosion protection applications. The deposited layers morphology was investigated by SEM and optical microscopy. The corrosion protection efficiency of the deposited layers was evaluated in 0.3M NaCl solution. The maximum corrosion protection efficiency was determined for the combination of layers using the carbide as the first layer although the inverse deposition order showed a small decrease. Depending on the main application (corrosion versus wear) one or the other are recommended.

Abstract: In the present investigation, WC–10Co–4Cr coating was deposited by high velocity oxy-fuel (HVOF) process on CA6NM hydro turbine steel to improve its erosion resistance. The coating was characterized in term of crossectional microstructure, phase, microhardness and fracture toughness using a field emission scanning electron microscope (FESEM), X-ray diffractometer and microhardness tester respectively. Solid particle erosion resistance of the substrate and coating were evaluated by air jet erosion tester at two different impingement angles (30° and 90°). Coating microstructure has shown a homogeneous and well-bonded laminar morphology. The microhardness of the coating was observed more than three times higher than CA6NM substrate. This resulted in significant improvement in erosion resistance of coated CA6NM steel at both impingement angles.