HVOF Coatings Research Articles

The Role of Oxide Tribofilms on Friction and Wear of Different Thermally Sprayed WC-CoCr

The tribolgical response of five different thermal sprayed WC-CoCr coatings sprayed with HVOF (JP-5000), HVAF (UniqueCoat M3) and Warm Spray (NIMS, Japan) has been evaluated during dry sliding in air and in nitrogen gas. Tests were conducted at room temperature and at 200 °C where both friction and wear were evaluated. Test results indicate that the HVAF and Warm Spray coatings were most dense, while HVOF coatings showed higher degree of porosity and discontinuities in the cross sections. Results show that the sliding atmosphere affects both friction and wear: Sliding in nitrogen reduced predominantly specific wear rate as well as friction coefficient. Higher friction was linked to the formation of tribofilms on the surfaces, aided by higher oxygen content in the sliding atmosphere and temperature.

Characterization of WC-10Ni HVOF Coating for Carbon Steel Blade

High Velocity Oxy-Fuel, HVOF is a depositing methods of a material layer over a base metal or substrate with characteristics of high flame velocity and moderate temperature. Where, tungsten carbide, WC cermet HVOF coatings is widely used to protect machine components from wear and corrosion. The main purpose of this present paper is to characterize the WC-10Ni coating deposited by HVOF thermal spray onto a carbon steel blade. The morphology and chemical composition of the coating were characterized by Scanning Electron Microstructure (SEM), electron dispersive spectrometer (EDS), and X-ray diffraction (XRD). The hardness test was carried out by using Vickers micro-hardness tester with loads of 490.3 mN (HV0.05). From XRD results, no sharp nickel peak was identified and has been replaced by a hump which indicate the amorphous Ni. The major crystalline phases were compounds WC, W2C and metallic phase of W. The WC-10Ni coating shows high hardness with low porosity distribution.

Determination of selected properties and fracture toughness of HVOF coatings

Abstract The article is aimed at evaluation of chosen properties of thermally sprayed coatings based on tungsten carbide. It was evaluated two types of coatings – WC-FeCrAl and WC-WB-Co. The coatings were deposited on basic material steel AISI 316L. The aim of experimental works was to determine the quality and resistance of coatings in corrosion environment depending on number of thermal cycles. The quality of coatings was evaluated by determination of their adhesion depending on thermal cycles, determination of microhardness and indentation fracture toughness. Coatings were exposed in tribo-corrosion conditions with the presence of 1% NaCl. The results of experimental showed a higher quality of coating WC-WB-Co.

Wear and corrosion properties of HVOF coatings from Superduplex alloy modified with addition of boron

Fe-based protective coatings are promising alternatives to replace large or expensive mechanical structures in aggressive environments, such as, in petrochemical industries. In this work, we have evaluated the corrosion and abrasive wear resistance of coatings produced by the HVOF thermal spray process using the commercial SAF2507 Superduplex alloy modified by the addition of 3.0 wt% boron. Three coatings were obtained from different metal powders produced by different routes (spray forming and high energy ball milling). The coatings had a partially amorphous structure containing a dispersion of nanocrystalline borides, such as Fe3B and (Fe,Cr)2B. The coatings showed significant improvement in abrasive wear resistance and preserved the high corrosion resistance of the commercial SAF2507 Superduplex alloy.

Effect of roughness on the wear behavior of HVOF coatings dry sliding against a friction material

WC-CoCr coatings on cast iron discs were prepared with different surface roughness and submitted to dry sliding wear tests against a commercial friction material using a pin-on-disc configuration. The effect of roughness on the friction and wear of the tribological system was evaluated. The pin wear rate increases with the coating roughness and becomes very high for an arithmetic average roughness, Ra, in excess of 1 μm. In contrast, the friction coefficient was found to increase as roughness decreased. The discs wear was anyway negligible. The experimental results were explained considering the characteristics of the friction layer that forms during sliding at the interface of the mating bodies in dependence of the initial roughness of the coatings. It is shown that the friction layer plays an important role in determining the relative contribution of the abrasive and adhesive interactions and, thereby, the resulting friction and wear behavior of the tribological system.

Fabrication and properties of NiCr/CNTs nanocomposite coatings prepared by High Velocity Oxy-Fuel Spraying

Abstract Nickel-Chromium (NiCr)/CNTs composite coatings were deposited onto mild steel substrates by High Velocity Oxy–Fuel Spraying (HVOF). The composite powder feedstock was prepared by: (1) mixing by ball milling NiCr powder with 0, 0.5, 1 wt% CNTs for 48 h: (2) growing carbon nanotubes on NiCr powder using chemical vapor deposition (CVD) technique at 650 °C for 60 min under ethanol atmosphere. The microstructure of NiCr-CNTs powder was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). Chemical compositions of the different powders were studied using X-ray Diffractometry (XRD) and Energy Dispersive X-ray spectroscopy (EDX) techniques. Porosity and Vickers hardness of the coatings were also measured. Sliding wear tests were carried out using a pin-on-disk apparatus. Dense coatings which contained mainly Nickel phase could be achieved. Thickness of the coatings was about 300–350 μm with a similar % porosity of 0.49–1.01 vol%. Vickers hardness tests show that the hardness of Nickel-Chromium/CVDCNTs was 20% higher than that of a pure Nickel-Chromium coating leading to a better wear resistance. Polarization test showed that carbon nanotubes could improve the corrosion resistance of the coating. The main objectives of this study are to prepare the NiCr/CNTs composite feedstock powders and also to evaluate the properties of HVOF coatings from those powders.

Superalloys under the Effect of Hot Corrosion and Role of HVOF Coatings - A Review

Superalloys under the Effect of Hot Corrosion and Role of HVOF Coatings - A Review

Possibilities of Utilization Thermally Sprayed Coatings in the Field of Sink Rollers Renovation

The paper deals with failure analysis of sink rollers used in hot-dip galvanizing of steel sheets. It presents the possibility of renovation the surface of the roller using thermally sprayed coatings. The paper contains results of research focused on quality evaluation of three coatings applied using HVOF technology - WC-Co, WC-NiMoCrFeCo and Co-MoCrSi. There was also investigated impact of sealing agent - boron nitride used for sealing the coating on corrosion rate of the coating. HVOF coatings were subjected to metallographic study with aim to study the structure, thickness, mechanical anchoring and hardness of coatings. There was also realised EDX analysis of chemical composition of coatings. Adhesion of coating was investigated in dependence of thermal cyclic loading. Main part of paper is focused on determining wear resistance of coatings by pin-on-disc in 1M NaCl solution and corrosion rate by Tafel analysis. Experiment confirmed that sealing coating WC-Co with boron nitride improves corrosive resistance of the coating. The boron nitride seals the pores in the structure of coating and thus prevents the formation of corrosion and subsequent depreciation of coating in various corrosive environments. Based on reached results, coatings for renovation rollers surfaces can be recommended according to preferred priority. If corrosion resistance of rollers plays key role, WC-Co coating sealed with boron nitride can be recommended together with WC-NiMoCrFeCo coating and the Co-MoCrSi. If the main priority is wear resistance of roller, unsealed WC-Co coating or alloy Co-MoCrSi coating can be used.

Wear behaviour of WC–Co HVOF coatings at different temperatures in air and argon

The wear behaviour of tungsten carbide–cobalt (WC–Co) high-velocity oxy-fuel coatings within the range of room temperature (RT) to 800°C in air and RT to 650°C in argon was studied using a ball-on-disc tribometer. The worn track morphologies, compositions and oxide phases after the tests were analysed by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction analysis and Raman spectroscopy. The correlation between the oxidation and wear of WC–Co coatings was investigated. Results show that oxides, as well as the oxidation of coatings, have an important role during the tribological process of WC–Co coatings. The volume loss of WC–Co coatings in air is low at RT to 600°C, which indicates that oxidation inhibits wear. Friction promotes the formation of CoWO4. CoWO4 can reduce friction and wear, and enables the coating to maintain good tribological properties even at 600°C. Above 600°C, the coating oxidises vigorously and its property deteriorates rapidly, which cause serious wear. In argon, the volume loss of WC–Co coating is significantly higher than that in air at RT to 600°C. Wear decreases as temperature rises because of the gradual formation of oxides. Consequently, WC–Co coatings should not be used as wear-resistant coatings in O-deficient environments at RT or at elevated temperatures.

Fatigue Performance of AISI 4340 Steel Ni-Cr-B-Si-Fe HVOF Thermal Spray Coated

Coatings are deposited on high strength materials to improve the surface degradation resistance of several elements. As a consequence, the fatigue strength of coated materials is significantly influenced by internal residual stresses. Despite the fact that hard chromium electroplated is used to guarantee combinations of adhesion, hardness, corrosion and wear resistance, the method decreases the fatigue resistance of aeronautical components due to the high tensile residual stresses and microcracks density contained in the coating.In order to avoid this problem thermally sprayed HVOF coatings and shot peening process are widely used to improve surface properties of several alloys. The aim of the present study is to analyze the influence of Ni-Cr-B-Si-Fe coating applied by HVOF process on the axial fatigue strength of AISI 4340 steel. The shot peening effect on the fatigue results of coated AISI 4340 steel was also evaluated. The fractured fatigue specimens were investigated using a scanning electron microscope in order to obtain information about the crack initiation points.

Quality Evaluation of HVOF Coatings on the Basis of WC-Co in Tribocorrosive Conditions

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.

Optimizing NiCr and FeCr HVOF Coating Structures for High Temperature Corrosion Protection Applications

In order to achieve a desired dense structure for coatings employed in high temperature corrosion conditions, thermal spray process optimization with diagnostic tools can be applied. In this study, NiCr (51Ni-46Cr-2Si-1Fe) and FeCr (Fe-19Cr-9W-7Nb-4Mo-5B-2C-2Si-1Mn) powders were sprayed with HVOFGF (gas-fueled) and HVOFLF (liquid-fueled) systems, and the spray processes were monitored with diagnostic tools, including SprayWatch for measuring the temperature and velocity of the spray stream, and in situ coating property (ICP measurement) for measuring the stress state. Various spray parameters were applied to attain the best coating characteristics for high temperature applications. Selected coatings were exposed to high temperature corrosion conditions both in laboratory and actual power plant. The coatings were analyzed by microscopic means and mechanical testing. The application of process-structure-properties-performance methodology with the process monitoring, analysis of the coating characteristics, and results of corrosion performance are presented in this paper.

Development of HVOF Coating of Hydroxy Apatite on Titanium Alloy with Carbon Nano Tube Intermediate Layer

Hydroxyapatite (HA) is considered as convenient material for the coating of titanium alloy in implant aplication. With the purpose to improve the morphology structure of HAP coating, the present research concerns in studying the effect of carbon nanotube (CNT) intermediate layer on HAP coated of titanium alloy. From SEM characterization, it can be observed that both of HAP coated sample, with and without CNT intermediate layer have porous nodular structure a characteristic of thermal spray coating without any micro-crack. However finer and more irregular structure observed on the sample with CNT intermediate layer, which is confirmed from XRD characterization, that lower degree of crystallinity occur on sample with CNT intermediate layer. After the coating, both of the samples were post annealed at 600 and 700°C for one hour. It was observed that micro-crack observed on the sample without CNT intermediate layer after post annealed at 700°C, while the rest of both samples observed without micro-crack.

Influence of Nickel-Coated Nanostructured WC-Co Powders on Microstructural and Tribological Properties of HVOF Coatings

In this research, a novel nickel-coated nanostructured WC-12Co powder (Ni/nc-WC) was developed and used as feedstock material for high velocity oxygen fuel process. The Ni/nc-WC powders with average WC grain size of ~15 nm were produced by mechanical milling and electroless plating processes. The microstructural and tribological characteristics of Ni/nc-WC coating were investigated and compared with those of microcrystalline WC-12Co (mc-WC) and nanostructured WC-12Co (nc-WC) coatings. X-ray diffractometry, high-resolution field emission scanning electron microscopy, and transmission electron microscopy were used to evaluate the microstructure of the powders and coatings. A ball-on-disk technique was used to probe the wear behavior of the coatings. The Ni/nc-WC coating showed negligible decarburization of ~5.4%, while mc-WC and nc-WC coatings suffered from higher decarburization levels of 16.3 and 36.8%. The wear rate of Ni/nc-WC coating was 2.5 × 10−4 mg/m indicating ~ 75 and 82% increase in wear resistance compared with mc-WC and nc-WC coatings. The wear track analysis of mc-WC and nc-WC coatings showed evidences of delamination mechanism. Besides, a severe carbide pullout mechanism was operative in wear of nc-WC coating. As for Ni/nc-WC coating, individual carbide pullout following the elimination of Ni(Co) matrix was the predominant wear mechanism.

Analysis of HVOF Coating on Molds Used for Refractory Fireclay Shapes

Molds used for pressing refractory fireclay shapes are exposed to very strong abrasive wear, which is given by the combination of applied pressure of more than 60MPa and processed materials like alumina (Al2O3) and silica (SiO2). Typical lifespan of molds is in several thousand cycles, our aim was to improve the lifespan 10 fold at minimum. To increase the lifespan of the critical parts of the molds, it was decided to use HVOF coating technology based on WC. This article evaluates the quality of the coating on the pins for pressing tools based on the technology used for deposition. An analysis was made on two sets of HVOF coated pins from different suppliers marked as a sample "A and B". Pins were analysed on tactile CMM with scanning system and samples from these pins were analysed on a multisensor CMM.

Thermal Spray Coatings for High-Temperature Corrosion Protection in Biomass Co-Fired Boilers

There are over 1000 biomass boilers and about 500 plants using waste as fuel in Europe, and the numbers are increasing. Many of them encounter serious problems with high-temperature corrosion due to detrimental elements such as chlorides, alkali metals, and heavy metals. By HVOF spraying, it is possible to produce very dense and well-adhered coatings, which can be applied for corrosion protection of heat exchanger surfaces in biomass and waste-to-energy power plant boilers. Four HVOF coatings and one arc sprayed coating were exposed to actual biomass co-fired boiler conditions in superheater area with a probe measurement installation for 5900 h at 550 and 750 °C. The coating materials were Ni-Cr, IN625, Fe-Cr-W-Nb-Mo, and Ni-Cr-Ti. CJS and DJ Hybrid spray guns were used for HVOF spraying to compare the corrosion resistance of Ni-Cr coating structures. Reference materials were ferritic steel T92 and nickel super alloy A263. The circulating fluidized bed boiler burnt a mixture of wood, peat and coal. The coatings showed excellent corrosion resistance at 550 °C compared to the ferritic steel. At higher temperature, NiCr sprayed with CJS had the best corrosion resistance. IN625 was consumed almost completely during the exposure at 750 °C.

Comparison of the Mechanical and Electrochemical Properties of WC-25Co Coatings Obtained by High Velocity Oxy-Fuel and Cold Gas Spraying

Cold gas spray (CGS) coatings were previously produced by spraying WC-25Co cermet powders onto Al7075-T6 and low-carbon steel substrates. Unlike conventional flame spray techniques (e.g., high-velocity oxy-fuel; HVOF), no melting of the powder occurs; the particles are deformed and bond together after being sprayed by a supersonic jet of compressed gas, thereby building up several layers and forming a coating. WC-Co cermets are used in wear-resistant parts, because of their combination of mechanical, physical, and chemical properties. XRD tests were previously run on the initial powder and the coatings to determine possible phase changes during spraying. The bonding strength of the coatings was measured by adhesion tests. Here, WC-25Co coatings were also deposited on the same substrates by HVOF spraying. The wear resistance and fracture toughness of the coatings obtained previously by CGS and the HVOF coatings obtained here were studied. Their corrosion resistance was determined by electrochemical measurements. It was possible to achieve thick, dense, and hard CGS coatings on Al7075-T6 and low-carbon steel substrates, with better or the same mechanical and electrochemical properties as those of the HVOF coatings; making the former a highly competitive method for producing WC-25Co coatings.

Performance testing of iron based thermally sprayed HVOF coatings in a biomass-fired fluidised bed boiler

Managing high temperature corrosion problems in biomass firing boilers has been challenging especially due to high amounts of chemically active compounds, in particular alkali chlorides. Thermally sprayed coatings with high chromium content can offer a solution for protecting low alloyed substrate materials in locations prone to high temperature corrosion. Two thermally sprayed (HVOF — high velocity oxy-fuel) iron based coatings (Fe–27Cr–11Ni–4Mo and Fe–19Cr–9W–7Nb–4Mo) were exposed to biomass boiler conditions for two years. The fluidised bed boiler for district heating used mainly wood-based fuels mixed with small amounts of peat. The coated tubes were located at the hot economiser of the boiler, where the estimated material temperature was about 200°C maximum. After the exposure the coatings and the carbon steel St35.8 substrate material were analysed with SEM–EDX. It was detected that corrosion due to elements such as chlorine, potassium, zinc, lead and copper had caused severe material wastage in the biomass boiler with relatively low heat exchanger surface temperatures. The low alloyed boiler tubes had suffered severely with a corrosion rate as high as 2mm/year, whereas dense thermal spray coatings offered excellent protection during the exposure.

Influence of HVOF Coating on the Fatigue Strength of 15-5 PH Stainless Steel

Structural component failures due to cyclic loading are associated to surface damage of materials and its interaction with environment. Fatigue failure occurs with stresses below the yield strength of each material and is a result of crack initiation and propagation. In aeronautical components is an important parameter to be considered in project, as well as the corrosion and wear resistance. Thermally sprayed HVOF coatings have been considered to replace galvanic chromium deposits with comparable performance for wear and corrosion resistance. The aim of present study is to study the influence of WC-13Co-4Cr applied by HVOF, on the axial fatigue strength of 15-5 PH stainless steel. The shot peening treatment was used to restore fatigue performance.

Mechanical Property of HVOF Inconel 718 Coating for Aeronautic Repair

The module of elasticity is one of the most important mechanical properties defining the strength of a material which is a prerequisite to design a component from its early stage of conception to its field of application. When a material is to be thermally sprayed, mechanical properties of the deposited layers differ from the bulk material, mainly due to the anisotropy of the highly textured coating microstructure. The mechanical response of the deposited layers significantly influences the overall performance of the coated component. It is, therefore, of importance to evaluate the effective module of elasticity of the coating. Conventional experimental methods such as microindentation, nanoindentation and four-point bending tests have been investigated and their results vary significantly, mainly due to inhomogeneous characteristics of the coating microstructure. Synchrotron radiation coupled with a tensile test rig has been proposed as an alternative method to determine the coating anisotropic elastic behavior dependence on crystallographic orientations. The investigation was performed on Inconel 718 (IN718) HVOF coatings sprayed on IN718 substrates. Combining these experimental techniques yield a deeper understanding of the nature of the HVOF coating Young’s modulus and thus a tool for Design Practice for repair applications.