High-velocity Oxy-fuel System Research Articles

Studies on wear and microstructure assessment of WC-Co reinforced iron based HVOF coating

This research work uses a High-Velocity Oxy Fuel (HVOF) system to create a cermet based WC-10Co reinforced with a durable Iron-based (FeNiCrMo) advanced composite coating to resist high-temperature sliding wear on an ASTM-SA213-T11 steel substrate. Metco 41C (70%) + WC-10Co (30%) were mechanically blended. Then, feedstock was utilized by the HVOF system to create coatings. The base alloy and the coatings were subjected to various metallurgical and mechanical characterizations. Dry sliding wear test of base alloy and coating, were evaluated using the pin-on-disc device. The test was performed by varying parameters of load 10 N and 20 N with variable temperature of 200 °C, 300 °C and ambient conditions. SEM/EDS and XRD techniques were used to examine the worn samples and to identify the formation of new phases during wear test. The wear characteristics pertaining to wear rate, volume loss, and COF were computed. The coating exhibits both intermetallic phases, such as Mo2C and Co3W3C, and challenging stages, such as Cr3C2, Ni3C, W2C, Fe2C, and SiC. There has been a resultant improvement in wear resistance and hardness of coating relative to the base alloy.

Sliding wear performance of thermally sprayed Ni-Cr based alloy deposited on SAE 1045 steel

ABSTRACTThe present work has been carried out in order to investigate the sliding wear performance of a Ni-Cr based alloy, containing boron, carbon, silicon and iron, deposited onto a SAE 1045 steel substrate. The coatings were thermally sprayed by means of a High Velocity Oxy-Fuel system and post heat-treated for one hour at 760°C in an argon atmosphere. The relative sliding performance was evaluated under unlubricated dry conditions at different applied normal loads and sliding velocities, in order to analyze the influence of these parameters on the wear response. Test results are discussed and interpreted on the basis of both microstructural evaluations and mechanical properties measurements. The surface morphology and topography of the wear scars have been studied through of a Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) techniques. The results have shown an improved wear resistance of the coated steel samples as compared to the uncoated ones.

Effects of Combustion Model and Chemical Kinetics in Numerical Modeling of Hydrogen-Fueled Dual-Stage HVOF System

The present work examines the effect of utilizing different combustion models and chemical kinetics in predicting the properties of gas and particle phases in a hydrogen-fueled, dual-stage high-velocity oxy-fuel (HVOF) thermal spray system. For this purpose, effects of two combustion models, eddy dissipation concept (EDC) and eddy dissipation model (EDM), on the temperature and velocity fields in the system are studied. The computations using EDC model are performed for detailed and reduced chemical kinetics and for a range of mixture from lean to rich. It is found that EDC with multi-step reaction mechanism predicts higher temperatures for the flow and particle in the warm spray system. In contrast to EDC, the EDM with one-step global reaction shows extra heat release outside the HVOF barrel for rich mixtures which leads to unphysical higher prediction of particle temperature. The simulations using EDC model with detailed and reduced chemical kinetics show some exothermic reactions in converging-divergent nozzle of the system. The heat release from these reactions has profound impacts on the flow and particle temperatures and affects the gas dynamic behavior of flow considerably. Finally, it is discussed that moving toward rich mixtures is more reliable way to control the particles temperature.

Effects of the surface texture in a compressor impeller shaft on its remanufacturing using HVOF

This study investigated the effects of surface texture in a compressor impeller shaft on its remanufacturing using high-velocity oxy-fuel (HVOF). Nine main types of surface texture were prepared. The HVOF system was used to remanufacture samples related to the appearance of these surface texture features, and the samples without texture were selected as the control group. The performance of remanufactured samples was evaluated by the scratch method. The effect of the surface texture factor level on first cracks that occurred (critical load LC1) was investigated using the analytic hierarchy process (AHP) method. The levels of various factors that influence the indicator weight, as well as the preferred combination, were determined. The representative cross-sectional morphologies and element distribution of the preferred coated samples were investigated using scanning electron microscopy (SEM) and elemental line scanning, respectively. Results indicate that the coating combined with the substrate mainly through a mechanical method, and adhesion strength was substantially improved via a suitable surface texture. It can be concluded that fabricating a suitable surface texture is an effective way to improve the coating adhesion strength of the shaft remanufactured using HVOF.

Effects of Purity and Phase Content of Feedstock Powder on Thermal Durability of Zirconia-Based Thermal Barrier Coatings

The thermal durability of thermal barrier coatings (TBCs) obtained using feedstock powders with different purity and phase content was investigated by cyclic thermal testing, including the effects on the sintering and phase transformation behaviors. Three kinds of 8 wt.% yttria-stabilized zirconia, namely regular purity (8YSZ), high purity (HP), and no monoclinic phase (nMP), were employed to prepare top coats by atmospheric plasma spraying on a NiCoCrAlY bond coat using a high-velocity oxy-fuel system. Use of 8YSZ, HP, and nMP for plasma spraying affected the microstructure and lifetime of the TBC in furnace cyclic testing (FCT) at 1100 °C and the sintering rate during annealing at 1400 °C for 50, 100, 200, and 400 h. In FCT, the TBC formed from nMP showed the longest durability, while that formed from HP showed lifetime performance similar to that obtained with regular-purity 8YSZ. The TBC obtained with nMP also exhibited the lowest monoclinic phase transition rate, followed by those obtained using HP and 8YSZ.

Microstructural characterization of HVOF/plasma thermal spray of micro/nano WC–12%Co powders

This research focuses on studying the powder reactions and coating microstructures produced by thermal spraying various particle sized (micro and nano-sized) tungsten carbide cobalt powder. High Velocity Oxy Fuel (HVOF) and Atmospheric Plasma thermal spray methods were used to perform a multi-layer coating applied onto carbon steel specimens (typical of those used in oil/gas industry). The thermally sprayed powders/coatings were analyzed using X-ray powder diffraction (XRD), Environmental scanning electron microscope (ESEM), and Energy Dispersive Spectrometry (EDS) to predict the metallic powder reactions. In addition, surface roughness measurement of the coating, hardness assessment, and evaluation of coating porosity and adhesion were conducted to determine the coating characterization. The comparative metallographic results of the various test coatings are discussed. Results show that higher phase transformation occurred in the plasma thermal spray and more cobalt evaporation occurred during the deposition of the nano-sized powder. The plasma spray of nano-sized powders yielded lower coating roughness (from 5.5μm to 4.6μm), whereas it increased the roughness when sprayed with the HVOF system (from 3.1μm to 4.3μm). Nano-sized powders improved porosity but not significantly (to 1.2%). However, the higher percentage of nano-sized particles in the starting powder increased the hardness in the coatings to 1367HV. Among the four produced coatings, the results concluded that the coating produced by HVOF using nanostructured WC–12Co powder was found to have the best coating microstructure due to its low porosity, high density, good adhesion, and fracture resistance.

Study on stainless steel 316L coatings sprayed by a novel high pressure HVOF

The High-Velocity Oxy-Fuel (HVOF) technology has been widely used to deposit cermet, metals, and alloy coatings. However, the high temperature and oxidative flame jet results in oxidation and decomposition of spray particles during the spraying process; giving significant influence on phase composition, microstructure, properties, and performance of the sprayed coatings. In this paper, a high pressure HVOF system with a combustion chamber pressuring up to 3.0MPa along with characteristics of lower flame temperature and higher flame velocity was used to deposit Stainless Steel 316L coatings. Particle velocity was measured by using the DPV-2000 system. It was found that higher combustion chamber pressure enables higher particle velocity. The influence of oxygen/fuel ratio and spray distance on deposition efficiency, coating microstructure, and microhardness of the Stainless Steel 316L was investigated. It shows that the oxygen/fuel ratio has a significant influence on particle's melting state. Therefore, dense Stainless Steel 316L coating composed of semi-molten or solid state particles can be obtained by adjusting the oxygen/fuel ratio at the combustion chamber pressure of 3.0 MPa. In the experiment, deposition efficiency up to 90% was achieved at the optimized spray conditions. As a typical conventional HVOF system of JP-5000, the comparison to the JP-5000 sprayed coating was also performed.

A study of liquid droplet disintegration for the development of nanostructured coatings

AbstractThermal spray coatings produced from a liquid feedstock are receiving an increasing level of interest due to the advanced, nanostructured coatings which are obtainable by these processes. In this article, a high‐velocity oxy‐fuel (HVOF) thermal spray system is computationally investigated to make a scientific assessment of the liquid droplet behavior on injection. An existing liquid‐fuelled HVOF thermal spray gun is simulated using the computational fluid dynamic approach. The steady‐state gas‐phase dynamics are initialized by the introduction of liquid kerosene and oxygen which react within the combustion chamber producing a realistic compressible, turbulent jet. Discrete‐phase water droplets are injected at the powder injection port. On injection, the water droplets breakup and vaporize, while being entrained through the acceleration barrel of the HVOF system. The results obtained give an insight to the mechanism which control the water droplet sizes and disintegration process, and serve as a fundamental reference for future development of liquid feedstock devices. © 2012 American Institute of Chemical Engineers AIChE J, 2012

Investigation of Properties and Wear Behavior of HVOF Sprayed TiC-Strengthened Fe Coatings

High-velocity oxyfuel (HVOF) sprayed carbide based coatings (such as Cr3C2/NiCr) are industrially well established for wear protection applications. Due to their high carbide content of typically 75 wt.% and more, they provide very high hardness and excellent wear resistance. Unfortunately, costs for matrix materials such as nickel underlie strong fluctuations and are normally well above the prices for iron. Therefore an alternative concept to conventional carbides is based on TiC-strengthened low-cost Fe-base materials, which are already used for sintering processes. Depending on the carbon content, the Fe-base material can additionally offer a temperable matrix for enhanced wear behavior. The sprayability of TiC-strengthened Fe-powders with a gaseous and a liquid fuel driven HVOF system was investigated in this study. The resulting coatings were analyzed with respect to microstructure, hardness, and phase composition and compared with galvanic hard chrome, NiCrBSi, and Cr3C2/NiCr (80/20) coatings as well as with sintered Fe/TiC reference materials. Furthermore, the Fe/TiC coatings were heat treated to proof the retained temperability of the Fe matrix after thermal spray process. Tribometer tests (pin-on-disk tests) were conducted to determine wear properties.

Comparison of fatigue performance of HVOF spray coated and conventional roll bonded aluminium bearing alloys

A comparative study on fatigue resistance of thin aluminium bearing linings (supported by harder backing steel layers) produced by high velocity oxyfuel (HVOF) spray coating and conventional roll bonding (RB) processes has shown that the former is superior to the latter with similar lining composition (Al–20%Sn–1%Cu–0·25%Mn) when compared on the basis of oscillating lining surface strains under a three point bend test condition. In terms of the integrity of the multilayered bearing system under oscillating stresses, HVOF lining appeared to show worse fatigue resistance, due to poor/brittle bond between the lining and the backing steel layer resulting in the detachment of the lining from the backing layer. The newly developed RB alloy with reduced Sn content (Al–6·5Sn–2·5Si–1Ni–1Cu–0·25Mn) and scattered intermetallics showed higher fatigue resistance than the HVOF and previous RB systems. This was linked to delayed initiation of short cracks leading to a longer overall lifetime compared to all other systems.

High-temperature evaluation of solid lubricant coatings in a foil thrust bearing

PS304 and Korolon™ 1350A foil bearing coatings were evaluated in startup/shutdown testing of compliant thrust pads against thrust runner disks. Tests were conducted at 540 °C, with pads loaded at a constant 3.44 kPa pressure while disk speed cycled between 0 and 19 m/s. As opposed to the case of uncoated Inconel X750 pads against uncoated Inconel 718 disks, PS304 disk coatings allowed hydrodynamic conditions to be more readily attained, as indicated by reduced speeds at which pad liftoff and touchdown occurred and lower coefficients of friction that existed at full speed. During low-speed portions of the cycle where direct sliding occurred, the PS304 disk coatings also lowered friction. PS304 disk coatings that were high-velocity oxyfuel (HVOF)-deposited attained these improved tribological performances following fewer required run-in cycles than those that were plasma-sprayed, as a result of lower attainable levels of as-finished roughness. HVOF deposition required reduced-size feed powder, with which PS304 disk coatings could be deposited using conventional propylene-fueled in addition to hydrogen-fueled HVOF systems. HVOF-deposited PS304 additionally possessed higher as-deposited strength than plasma-sprayed PS304. If instead the thrust pad was coated, with K1350A, even better hydrodynamic performance was attained with substantial further reductions in liftoff and touchdown speeds and coefficients of friction at full speed. When coupled with a PS304-coated disk the K1350A pad coating immediately provided this better hydrodynamic performance, with very few run-in cycles required before attainment. However, one drawback of K1350A pad coatings appeared to be an increased level of friction during low-speed sliding conditions where direct sliding contact with the disk occurred.

Aluminium depletion in NiCrAlY bond coatings by hot corrosion as a function of projection system

Three different projection system are used to prepare NiCrAlY bond coats over metallic substrates: atmospheric plasma spray (APS), high velocity oxyfuel (HVOF) and high frequency pulse detonation (HFPD). These coatings were tested in hot corrosion experiments with sprayed Na 2SO 4 at 1000 °C for 20 and 100 h experiments in air. Coatings surface composition after thermal treatment was characterised by XRD and SEM. Cross section of coatings were analysed by SEM-EDX. A relationship between microstructural characteristics of initial coatings and final performance in hot corrosion was found in terms of porosity percentage: plasma sprayed coatings present higher percentage of porosity compared to HVOF and HFPD projection systems for the same composition and Al is heavily consumed in interparticle oxidation. This Al depletion in turn involves intrinsic chemical failure and surface layer is comprised by a porous spinel of mixed oxides. On the other hand, high energy projection systems produce dense coatings allowing the Al migration to external alumina layer, particularly in the case of HVOF coating.

Down-Selection and Optimization of Thermal-Sprayed Coatings for Aluminum Mould Tool Protection and Upgrade

This article details the down-selection procedure for thermally sprayed coatings for aluminum injection mould tooling. A down-selection metric was used to rank a wide range of coatings. A range of high-velocity oxyfuel (HVOF) and atmospheric plasma spray (APS) systems was used to identify the optimal coating-process-system combinations. Three coatings were identified as suitable for further study; two CrC NiCr materials and one Fe Ni Cr alloy. No APS-deposited coatings were suitable for the intended application due to poor substrate adhesion (SA) and very high surface roughness (SR). The DJ2700 deposited coating properties were inferior to the coatings deposited using other HVOF systems and thus a Taguchi L18 five parameter, three-level optimization was used to optimize SA of CRC-1 and FE-1. Significant mean increases in bond strength were achieved (147±30% for FE-1 [58±4 MPa] and 12±1% for CRC-1 [67±5 MPa]). An analysis of variance (ANOVA) indicated that the coating bond strengths were primarily dependent on powder flow rate and propane gas flow rate, and also secondarily dependent on spray distance. The optimal deposition parameters identified were: (CRC-1/FE-1) O2 264/264 standard liters per minute (SLPM); C3H8 62/73 SLPM; air 332/311 SLPM; feed rate 30/28 g/min; and spray distance 150/206 mm.

Process Temperature/Velocity-Hardness-Wear Relationships for High-Velocity Oxyfuel Sprayed Nanostructured and Conventional Cermet Coatings

High-velocity oxyfuel (HVOF) spraying of WC-12Co was performed using a feedstock in which the WC phase was either principally in the micron size range (conventional) or was engineered to contain a significant fraction of nanosized grains (multimodal). Three different HVOF systems and a wide range of spray parameter settings were used to study the effect of in-flight particle characteristics on coating properties. A process window with respect to particle temperature was identified for producing coatings with the highest resistance to dry abrasion. Although the use of a feedstock containing a nanosized WC phase produced harder coatings, there was little difference in the abrasion resistance of the best-performing conventional and multimodal coatings. However, there is a potential benefit in using the multimodal feedstock due to higher deposition efficiencies and a larger processing window.

Sliding wear behavior of WC–12% Co coatings at elevated temperatures

WC–Co coatings with low degree of decomposition were thermally sprayed by a high velocity oxy-fuel system from three agglomerated WC–12 mass% Co powders with different carbide grain size distributions. Dry sliding friction and wear behavior of the WC–12% Co coatings was investigated by using sintered alumina (Al2O3) as the mating material at 200 °C, 300 °C and 400 °C. The friction coefficient of these WC–Co/alumina pairs was nearly constant regardless of the test temperature and the carbide grain size of the coatings. The specific wear rate of the coatings increased with increasing the carbide grain size at a given testing temperature. It decreased with increasing the temperature for a given carbide grain size. The specific wear rate was reduced by more than one order of magnitude when the test temperature was increased from room temperature to 400 °C. The microscopic analyses of the worn tracks showed that the tribofilm formed at higher temperature is denser and more adhesive to the underlying surface, thus providing more surface protection against wear. The results show that the formation of dense and adhered tribofilms plays an important role on the low sliding wear rate of the coatings at elevated temperatures.

High velocity oxy-fuel thermal spraying of a NiCoCrAlY alloy

During the last few years third-generation high velocity oxy-fuel (HVOF) spray systems have gained interest concerning the deposition of sensitive metal alloys such as MCrAlYs. This work deals with the HVOF thermal spraying of a NiCoCrAlY alloy and its isothermal oxidation behaviour. Two HVOF systems were investigated, DJ 2600 and JP 5000. For both spray systems, the influence of the spray parameters on the oxidation during spraying as well as on the coating porosity was investigated. The coatings with the lowest oxygen content and the lowest porosity were subjected to heat-treatment in vacuum and oxidation at 1100 °C in air. The microstructure of the coatings was studied as-sprayed as well as following heat-treatment. Moreover, the morphology and thickness of the oxide scales were characterized by metallography, scanning electron microscopy, energy dispersive X-ray spectrometry and XRD. The oxidation kinetics were determined by use of thermogravimetry. The DJ system led to the lowest coating porosities after thermal treatment as well as to the lowest oxidation rates. The oxide scale on the NiCoCrAlY, formed at 1100 °C, consisted mainly of α-Al 2O 3, which was also seen after short oxidation times (2 h).

HVOF system definition to maximise the thickness of formed components

The present study aims to establish the potential of producing various hard metal industrial components using the high-velocity oxy-fuel (HVOF) thermal spray process, rather than by the sintering techniques currently used. The HVOF technique is generally used as a coating process, but previously has been employed to spray-form thin tungsten carbide–cobalt (WC–Co) components, flat and cylindrical in shape [Proceedings of Advances in Powder Metallurgy and Particulate Materials, Washington DC 5 (1996) 18.41]. The present work is focused on maximising the thickness of such components. The difficulty in producing thick-formed components using this technique arises from residual stress in the sprayed material. In coatings this stress leads to adhesion loss and interlaminar debonding and, in formed components, cracking or buckling. Residual stress that arises during spraying can be reduced by limiting the rise and fluctuation of the die temperature, and this was carried out in the present study using a carbon dioxide cooling system. This enabled continuous deposition at a steady temperature, and led to the successful production of thick-formed WC–Co components.

Parameter studies on high-velocity oxy-fuel spraying of MCrAlY coatings

Todays thermally sprayed MCrAlY coatings are commonly manufactured by the vacuum plasma spraying (VPS) process. This technique provides dense and oxide-free coatings. However, mainly due to the vacuum procedures this production is cost intensive and time consuming. The third generation of high-velocity oxy-fuel (HVOF) systems nowadays offer processing of materials that are sensitive to oxidation even in atmosphere. This is mainly due to the achievement of higher kinetic energy of the particulates and lower melting degrees which enables particle flattening in a plastic state. The work presented here focuses the influences of process parameters of a gas-driven HVOF system on the microstructure and oxygen content of MCrAlY coatings. The major parameters were subjected to DOE investigation to estimate both single and interacting effects. It was found that spray distance, fuel/oxygen ratio and powder feed rate exert a major influence on microstructure and oxygen content, whereas powder feed gas rate is not significant. Further parameters of significance are substrate temperature, shroud-gas type, fuel-gas type and powder size fraction. The coatings with the lowest oxygen contents were subjected to heat-treatment in vacuum and oxidation in air. Subsequently they were metallographically investigated. A comparison with VPS coatings showed that HVOF-sprayed MCrAlY coatings show similar oxidation behavior.

Mathematical modeling of the gas and powder flow in HVOF systems

A mathematical model was developed to describe the gas dynamics and heat-transfer mechanism in the gas/particle flow of high- velocity oxyfuel (HVOF) systems. A numerical solution was carried out using a PC- based computer program. One- dimensional predictions of the temperature and velocity profiles of gas and particles along the axis of flow were obtained to conduct cost- effective parametric studies and quality optimization of thermal spray coatings produced by HVOF systems. The numerical computer model allows for the variation of the HVOF system parameters, such as air/fuel ratio and flow rates, cooling water inlet temperature and flow rate, barrel length, standoff distance, particle size, and gun geometry. Because of the negligible volume of the powder relative to the gas, the gaseous phase was modeled as continuous nonadiabatic, and friction flow with variable specific heats and changing cross- sectional areas of flow. The generalized continuity, momentum, and energy equations with the influence parameters were used to model the gaseous flow regime and predict its thermodynamic properties. Empirical formulas for the mean axial decay of both velocity and temperature in the supersonic jet plume region were generated from published measurements of these parameters using laser Doppler velocimeter and Ray leigh scattering techniques, respectively. The particle drag and heat- transfer coefficients were calculated by empirical formulas in terms of Reynolds, Nusselt, and Prandtl numbers to evaluate both the momentum and heat transferred between the combustion gases and the powder particles. The model predictions showed good agreement with the particle and gas temperature and velocity measurements that are available in the literature.