Suspension High Velocity Oxy-fuel Research Articles

Suspension High‐Velocity Oxy‐Fuel–Sprayed Dense Vertically Cracked and Suspension‐Plasma‐Sprayed Columnar Yttria‐Stabilized Zirconia Coatings: Calcia Magnesia Alumino Silicates Infiltration and Thermal Cycling Performance

The quest to increase the surface temperatures and resistance to the corrosive environment of thermal barrier coatings topcoats mean that newer coating design strategies are needed. In this study, a performance evaluation of suspension high‐velocity oxy‐fuel (SHVOF)‐sprayed dense vertically cracked (DVC) and suspension‐plasma‐sprayed (SPS) columnar structure (CS) topcoats is conducted. The calcia magnesia alumino silicate (CMAS) evaluation is conducted at 1300 °C for 30 min, whereas the furnace cycling tests (FCT) is conducted at 1135 °C for 45 min cycle dwell time. The CMAS infiltrates down to the bond coat layer, but does not induce partial or complete topcoat spallation on all studied topcoat layers. In terms of CMAS infiltration, the CMAS appears to be restricted along the vertical cracks. The FCT of the SPS CS structure indicates a failure largely at the thermally grown oxide (TGO)–topcoat interface, while the DVC topcoat layers indicate a mix‐mode failure, i.e., both material‐associated cracking and localized spallations at the TGO–topcoat interface. Overall, the SHVOF‐sprayed ethanol‐based DVC topcoat seems to offer a balanced trade‐off, i.e., a majority of the topcoat is still intact after 100 thermal cycles and exceeds the material durability and performance offered by the SPS CS structure.

Enhancing wear performance by depositing alumina/GNPs coating on textured WC-TiC-Co substrates

Grid and line textures were made on the top surfaces of WC-TiC-Co cemented carbide substrates. Suspension High Velocity Oxy-Fuel (SHVOF) was utilized to deposit alumina and graphene nanoplatelets (GNPs) coating on the textured surfaces. The friction and wear behaviors of the Alumina/GNPs coating were evaluated using dry sliding wear tests against alumina balls. The friction reduction mechanism of alumina/GNPs coating was presented. GNPs in the alumina coatings played a significant role in reducing the coefficient of friction (CoF) up to 40 % and improving the wear resistance properties of the coating. A stress distribution model of the friction contact zone was provided to analyze the tribological performance of the textured sample. The friction reduction mechanism for alumina/GNPs coating was discussed. These results offer a benchmark for further studies combining microtextured surfaces and SHVOF thermal sprayed coatings as promising candidates for high wear resistant applications.

Tribological behavior of GNPs doped Al2O3 coating fabricated by SHVOF thermal spraying on cemented carbide

AbstractThis paper aims to experimentally investigate the effect of graphene nanoplatelets (GNPs) doped Al2O3 coating deposited on the surface of cemented carbide substrate using suspension high velocity oxy fuel (SHVOF) thermal spraying technique. Scanning electron microscopy was applied to characterize GNPs doped Al2O3 feedstock, the surface morphologies of cemented carbide before and after spraying, and the wear track morphology of cemented carbide after wear tests. The phases of GNPs doped Al2O3 feedstock, uncoated and coated cemented carbide were analyzed by X‐ray diffraction. The existence of GNPs was analyzed by Raman spectroscopy. A mixture of un‐molten and molten splats formed on the surface of cemented carbide substrate after SHVOF thermal spray. The average coefficient of friction (CoF) of coated samples was slightly lower than that of uncoated samples, which might be due to the friction‐reduction effect of GNPs. The wear rate of the samples was one order of magnitude higher than that of the alumina ball, showing that the wear of samples was the main wear between the friction couples. The wear mechanism of uncoated sample was mainly fatigue spalling, and that of cemented carbide substrate coated with GNPs doped Al2O3 coating was mainly plowing and abrasive wear.

Evolution of porosity in suspension thermal sprayed YSZ thermal barrier coatings through neutron scattering and image analysis techniques

Porosity is a key parameter on thermal barrier coatings, directly influencing thermal conductivity and strain tolerance. Suspension high velocity oxy-fuel (SHVOF) thermal spraying enables the use of sub-micron particles, increasing control over porosity and introducing nano-sized pores. Neutron scattering is capable of studying porosity with radii between 1 nm and 10 μm, thanks to the combination of small-angle and ultra-small-angle neutron scattering. Image analysis allows for the study of porosity with radii above ∼100 nm. For the first time in SHVOF 8YSZ, pore size distribution, total porosity and pore morphology were studied to determine the effects of heat treatment. X-ray diffraction and micro-hardness measurements were performed to study the phase transformation, and its effects on the mechanical properties. The results show an abundant presence of nano-pores in the as-sprayed coatings, which are eliminated after heat treatment at 1100 °C; a transition from inter-splat lamellar to globular pores and the appearance of micro-cracks along with the accumulation of micro-strains associated with the phase transformation at 1200 °C.

In-vitro cell interaction and apatite forming ability in simulated body fluid of ICIE16 and 13-93 bioactive glass coatings deposited by an emerging suspension high velocity oxy fuel (SHVOF) thermal spray

ICIE16 and 13-93 bioactive glasses have been proposed as alternative chemically stable compositions in physiological fluid keeping bioactivity comparable to Bioglass®. ICIE16 and 13-93 bioactive glasses coatings were produced via an emerging suspension high-velocity oxy-fuel (SHVOF) thermal spraying technique. Suspensions of ICIE16 and 13-93 with 10 wt% solid loading in isopropanol (IPA) and water were used to produce coatings on AISI304 stainless steel using a flame power of 50 kW and 75 kW. For both glass formulations, the coatings deposited at a lower flame power were more porous, less hard, and less rough (~6% porous and 242 HV) than the coatings obtained at a higher flame power (~4% porosity and 300 HV). ICIE16 coatings showed more dissolution in SBF (simulated body fluid) than the 13-93 coatings. Moreover, the 13-93 glass coating sprayed at 75 kW showed the highest stability in SBF since only 2% of the coating was resorbed in SBF after 7 days of immersion and revealed apatite precipitation after 7 days. In-vitro cell tests, using MG63 cells, showed good cell attachment and proliferation on the surfaces of the coatings, revealing good cytocompatibility. The 13-93 coating sprayed at 75 kW revealed the highest cell proliferation after 7 days of incubation. This can be attributed to the higher surface roughness of the coating (Ra = 6.5 ± 0.6 μm).

Microstructure and wear performance of alumina/graphene coating on textured Al2O3/TiC substrate composites

The incorporation of graphene-based nanodispersoids in ceramic coatings is known to enhance the wear behavior of these materials. In this study, for the first time, alumina/graphene nanoplatelets (GNPs) coatings were deposited through Suspension High Velocity Oxy-Fuel (SHVOF) thermal spray on Al2O3/TiC substrate composites, which were laser microtextured with grid and line surface patterns. Dense coatings with around 8−10 μm thickness were obtained by spraying alumina suspension containing 1 wt. % GNPs. The tribological performance of the alumina/GNPs coatings on the Al2O3/TiC substrate composites was evaluated using ball-on-disc sliding wear tests. The alumina/GNPs coated Al2O3/TiC substrate composites textured with grid pattern showed the best tribological performance reaching a steady coefficient of friction (CoF) at lower sliding distances and presenting the lowest wear rate of the Al2O3/TiC substrate composites and the counterbody.

A high-fidelity simulation of the primary breakup within suspension high velocity oxy fuel thermal spray using a coupled volume of fluid and discrete phase model

In the suspension high velocity oxy fuel (SHVOF) thermal spray, a suspension is injected into the combustion chamber where the jet undergoes primary and secondary breakup. Current knowledge of the primary breakup within the combustion chamber is very limited as experimental investigations are impeded due to direct observational inaccessibility. Numerical methods are also limited due to the computational costs associated with resolving the entire range of multiphase structures within SHVOF thermal spray. This paper employs a coupled volume of fluid and discrete phase model, combined with a combustion model, to simulate primary breakup at a fraction of the cost of a fully resolved simulation. A high-fidelity model is employed within this study to model the combustion chamber; the model shows a backflow region that will contribute to clogging within the nozzle. This study modifies the injector type for SHVOF thermal spray by introducing a co-flow around the liquid injection to reduce clogging within the combustion chamber. This study shows that introducing a co-flow of gas at a velocity of 200 m/s around the liquid injection reduces the backflow region by 40% within the combustion chamber. The addition of a gas co-flow results in a smaller region of backflow. Small suspension droplets with insufficient momentum are unable to overcome the backflow and will likely deposit themselves onto the wall of the combustion chamber. The deposition of the particles on the walls causes clogging of nozzles often seen in SHVOF thermal spray. The addition of a gas co-flow results in an increase in the velocity of droplets formed during primary breakup. The greater droplet velocity allows for small droplets to overcome the small backflow region near the liquid injection. The Sauter mean diameters predicted from the numerical model are compared to experimental measurements available within the literature and shows good agreement.

A suspension high velocity oxy-fuel thermal spray manufacturing route for silicon carbide – YAG composite coatings

Silicon carbide (SiC) coatings obtained through thermal spraying represent a major engineering challenge due to the high tendency of SiC to decompose during the spraying process. In order to avoid this decomposition, suspension high velocity oxy-fuel (SHVOF) thermal spray was used in combination with a novel SiC powder modified by addition of yttrium aluminium garnet (YAG) as the feedstock. The colloidal stability of the feedstock in water was optimised in order to obtain stable and well-dispersed suspensions suitable for SHVOF spraying. Dense SiC-YAG coatings with ~80 μm thickness were obtained and the coatings showed promising results in terms of porosity, microstructure, phase distribution and mechanical properties. Energy dispersive X-ray spectroscopy and X-ray diffraction showed a lack of degradation of the SiC phase in the coatings, thanks to the formation of a molten YAG phase during the spray process, which prevents SiC decomposition.

A novel approach to incorporate graphene nanoplatelets to Cr2O3 for low-wear coatings

Cr2O3 (chromia) coatings have been widely used in wear and corrosion resistant applications thanks to their good tribomechanical properties, and graphene nanoplatelets (GNPs) have been employed as nanofillers to further improve these properties. In this work, we propose a process to deposit chromia/GNPs composite coatings on stainless steel substrates using Suspension High Velocity Oxy-Fuel (S-HVOF) thermal spray. The coating showed good microhardness, with successful incorporation of GNPs showing no or minor spray-induced degradation. Compared to a chromia-only coating, the tribological performance improved: both coating and alumina counterbody specific wear rates lowered by 20 and 70% respectively and coefficient of friction decreased by 15%. This study shows a non-expensive and simple method to incorporate GNPs to improve material performance in large scale.

Modelling Knudsen number effects in suspension high velocity oxy fuel thermal spray

Suspension high velocity oxy fuel thermal spray is a system characterized by supersonic velocities and length scales of particles of the order of nm – µm. As the effects of rarefication become significant the assumptions within the continuum models begin to collapse, the effects of rarefication can be evaluated through the flow Knudsen number. Modifications to the numerical modelling must be made to incorporate the effects of rarefaction. This study looks to include the effects of rarefication into the computational fluid dynamics (CFD) models for the suspension high velocity oxy-fuel (SHVOF) thermal spray process. A model for the heat transfer coefficient that take into account the Knudsen and Mach number effects is employed. Finally, the Ranz-Marshall correlation for the Nusselt number is compared to the Kavanau correlation and a compressible Nusselt number correlation. The model is validated through comparisons of particle temperatures which are obtained from two colour pyrometry measurements using a commercially available Accuraspray 4.0 diagnostic system. This study shows that there is a significant improvement in the prediction of inflight particle temperatures when accounting for the effects of compressibility and the effects of rarefication on the Nusselt number.

A Review on Suspension Thermal Spray Patented Technology Evolution

Since the introduction of suspension thermal spray (STS) in 1997, the rapid evolution of this technology has resulted in a critical number of inventions justifying a comprehensive review. Today STS is one of the most important research and development topics in thermal spray. Due to the efforts of different industries, universities, and governmental institutions, STS has reached a certain maturity for implementation in different applications. STS includes any thermal spray process that is based on injection of a suspension of solid particles in a liquid carrier into the gas jet, such as suspension plasma spray, suspension high-velocity oxy-fuel, and high-velocity suspension flame spray. This review is on the state of the art of the STS technology and includes an overview of the current patent situation including the number of patents published in English language, patenting institutions and invention domains. Apparatus and methods, feedstock, and new material systems and applications were identified as the three main domains of focus for the STS inventions. The presented patents show a general perspective of the current situation and present the technology advancements in each domain. It also shows the potential of implementing this technology in new applications based on the needs of thermal spray market through further technology development.

Structure-Property Relationships in Suspension HVOF Nano-TiO2 Coatings

Hardness and tribological properties of microstructured coatings developed by conventional thermal spraying are significantly affected by the feedstock melting condition, however, their effect on the performance of nanostructured coatings by suspension high velocity oxy-fuel (HVOF) are inconclusive. In this work, nano-TiO2 coatings with different degrees of melting (12%, 51%, 81%) of nanosized feedstock were deposited via suspension HVOF spraying, using suspensions with a solid content of 5 wt.%. All the coatings produced had dense structures without visible pores and cracks. Two TiO2 crystal structures were identified in which the rutile content of the coatings increased with increased feedstock melting. Their mechanical, friction and wear behaviours largely relied on the extent of melting of the feedstock. The coating composed of mostly agglomerate particles (12% melted particles) had the lowest coefficient of friction and wear rate due to the formation of a smooth tribo-film on the wearing surface, while the coating composed of mostly fully melted splats (81% melted particles) presented the highest coefficient of friction and low wear rate, whose wear mechanism was dominated by abrasive wear and accompanied by the formation of cracks.

A Computational and Experimental Investigation into Radial Injection for Suspension High Velocity Oxy-Fuel (SHVOF) Thermal Spray

Suspension high velocity oxy-fuel thermal spray typically utilizes axial injections of suspension into the combustion chamber. There are certain cases where the oxygen-sensitive nanoparticles benefit from a reduction in the time and temperature spent in the gas flow. Therefore, a radial injection outside of the nozzle can enable deposition of oxygen-sensitive nanomaterials. This study investigated the effect of the suspension flow rate, angle of injection and the injector diameter on the in-flight particle conditions. The combustion reaction is modeled using the eddy dissipation concept model with a robust reaction mechanism and compared to the current approach within the literature. This approach has not been employed within SHVOF thermal spray and provides a robust treatment of the reaction mechanisms. The suspension was modeled using a two-way coupled discrete particle model. Experimental observations were obtained using high-speed imaging, and observations of the liquid jet were compared to the numerical values.

Effect of processing on microstructure, mechanical properties and dissolution behaviour in SBF of Bioglass (45S5) coatings deposited by Suspension High Velocity Oxy Fuel (SHVOF) thermal spray

Bioglass (45S5) is known to react with physiological media and has the unique characteristic of bonding with hard and soft tissues when implanted in the body. 45S5 coatings were deposited by Suspension High Velocity Oxy Fuel (SHVOF) thermal spraying to investigate the effect of combustion conditions on the coating microstructure and surface topography. Bioactive (45S5) coatings were deposited onto 304 stainless steel substrates via SHVOF thermal spray using a water + isopropanol (IPA) based suspension with 8 wt% particle loading. Selected flame powers of 25 kW (low), 50 kW (medium) and 75 kW (high) produced three different coatings of varying microstructure (porosity and thickness) and composition. At 25 kW flame power thin coating of <10 μm thickness was deposited; however, 25 ± 3 μm thick, well bonded coatings to the substrates were deposited at flame powers of 50 and 75 kW. The medium flame power coating was 16 ± 2% porous, while the high flame power coating was 10 ± 1% porous. All the coatings remained amorphous, as confirmed via X-ray diffraction. After immersing the coated samples in simulated body fluid (SBF), 50 and 75 kW coatings revealed hydroxyapatite (HA) deposition after 3 days. Also, no HA deposition was observed on 25 kW coating, even after 7 days of immersion in SBF. EDX analysis of the 50 kW coating after 7 days immersion in SBF showed that the initial coating thickness reduced from 25 μm to 6 μm, it means that this microstructure was highly reactive towards SBF and hence behaved like a resorbable coating. Through SHVOF spraying technique changing the flame power resulted in coatings of varying microstructure. These microstructures behaved differently in SBF, 50 kW coating showed more degradation than the 75 kW coating owing to the porosity.

Comparative Study of Corrosion Performance of HVOF-Sprayed Coatings Produced Using Conventional and Suspension WC-Co Feedstock

Corrosion properties of nanostructured coatings deposited by suspension high-velocity oxy-fuel (S-HVOF) via an aqueous suspension of milled WC-Co powder were compared with conventional HVOF-sprayed coatings. Microstructural evaluations of these coatings included x-ray diffraction and scanning electron microscopy equipped with an energy-dispersive x-ray spectroscopy. The corrosion performance of AISI440C stainless steel substrate and the coatings was evaluated in a 3.5 wt.% NaCl aqueous solution at ~ 25 °C. The electrochemical properties of the samples were assessed experimentally, employing potentiodynamic polarization and electrochemical impedance spectroscopy. The potentiodynamic polarization results indicated that coatings produced by S-HVOF technique show lower corrosion resistance compared with the coatings produced by HVOF-JK (HVOF Jet Kote) and HVOF-JP (HVOF JP5000) techniques. Results are discussed in terms of corrosion mechanism, Bode and Nyquist plots, as well as equivalent circuit models of the coating–substrate system.

Effect of nozzle geometry on the gas dynamics and evaporation rates of Suspension High Velocity Oxy Fuel (SHVOF) thermal spray: A numerical investigation

Thermally sprayed coatings formed from sub-micron or nanoparticles feedstock provide enhanced coating properties and Suspension High Velocity Oxy Fuel (SHVOF) thermal spray is a process that allows the deposition of dense coatings from fine particulates. However, in SHVOF thermal spray, submicron and nanoparticles are significantly influenced by fluctuations in the gas velocity and temperature field because of the turbulence and disturbance of the gas fields from the breakup of the suspension due to the small inertia of the particles. In this study, an ethanol suspension containing widely used engineering ceramic particles, TiO2, with four different particle concentrations was considered 0, 10, 15 and 25 wt%. The aim of this paper was to model four nozzle geometries and to investigate the influence the nozzle geometry has on the gas flow and the evaporation rate at four particle concentrations. Comparing four nozzle geometries it is found that the gas exits the longer barrel nozzles at a high velocity; while the velocity in the free jet region beyond the shock diamonds is greater for the shorter nozzles. It is seen that the shorter the nozzle the higher the gas temperature in the free jet; which is due to the lower contact time of the gas with the cooled nozzle walls. This study also considered the effect the nozzle geometry has on the evaporation of the suspension. It was found that the longer the combustion chamber the lower the maximum evaporation rate. Moreover, higher particle concentrations result in evaporation commencing closer to the nozzle inlet due to the lower volatile fraction of the suspension.

Suspension high velocity oxy-fuel (SHVOF) spray of delta-theta alumina suspension: Phase transformation and tribology

Suspension high-velocity oxy-fuel (SHVOF) thermal spray is an emerging spray technology that enables the processing of nanometric feedstock. Although SHVOF thermal sprayed alumina coatings prepared from alpha alumina feedstock have been widely reported, a metastable δ-θ Al2O3 feedstock has yet to be investigated despite its low cost and commercial availability. In this study, an aqueous δ-θ Al2O3 suspension was sprayed on to a stainless steel (SS 304) substrate via SHVOF thermal spraying using an internal injection UTP TopGun. X-ray diffraction (XRD) of the as-sprayed coating showed δ-θ Al2O3 to γ-Al2O3 transformation upon spraying, along with amorphous/nanocrystalline phase formation. Furthermore, post-spray heat treatment of the coatings was performed at 600–750 °C for 6 and 48 h. The microhardness and indentation fracture toughness of the heat treated coatings increased by a factor of two compared to the as-sprayed coatings: due to grain refinement, pore consolidation and phase transformation of amorphous and γ-Al2O3 to δ-Al2O3. Unlubricated sliding wear tests were conducted at room temperature (~25 °C, relative humidity ~60%) using α-Al2O3 balls (Ø 9.5 mm) as the counter body at a normal load of 16.8 N to study the wear performance of the coatings. The wear rate of the as-sprayed coating and the coating heat treated at 600 °C for 6 h is of the order of 10−9 mm3 (Nm)−1, a noticeable improvement over conventional alumina coatings. The coatings heat treated at 750 °C for both 6 and 48 h failed abruptly by severe wear due to their pre-cracked surface.

Robust Hydrophobic Surfaces from Suspension HVOF Thermal Sprayed Rare-Earth Oxide Ceramics Coatings

This study has presented an efficient coating method, namely suspension high velocity oxy-fuel (SHVOF) thermal spraying, to produce large super-hydrophobic ceramic surfaces with a unique micro- and nano-scale hierarchical structures to mimic natural super-hydrophobic surfaces. CeO2 was selected as coatings material, one of a group of rare-earth oxide (REO) ceramics that have recently been found to exhibit intrinsic hydrophobicity, even after exposure to high temperatures and abrasive wear. Robust hydrophobic REO ceramic surfaces were obtained from the deposition of thin CeO2 coatings (3–5 μm) using an aqueous suspension with a solid concentration of 30 wt.% sub-micron CeO2 particles (50–200 nm) on a selection of metallic substrates. It was found that the coatings’ hydrophobicity, microstructure, surface morphology, and deposition efficiency were all determined by the metallic substrates underneath. More importantly, it was demonstrated that the near super-hydrophobicity of SHVOF sprayed CeO2 coatings was achieved not only by the intrinsic hydrophobicity of REO but also their unique hierarchically structure. In addition, the coatings’ surface hydrophobicity was sensitive to the O/Ce ratio, which could explain the ‘delayed’ hydrophobicity of REO coatings.

Microstructure and phase stability of suspension high velocity oxy-fuel sprayed yttria stabilised zirconia coatings from aqueous and ethanol based suspensions

Two commercial 7–8wt.% Yttria Stabilised Zirconia (YSZ) suspensions were sprayed by Suspension High Velocity Oxy Fuel (SHVOF) thermal spraying for advanced high temperature coatings. Heat treatments of the free-standing coatings were conducted at 800°C and 1000°C for 72h. The SHVOF coatings using two liquid carriers: water and ethanol, behaved differently in terms of micro-structure and phase stability. The ethanol coatings retained a fully tetragonal composition after heat treatments; while the aqueous coatings, however, underwent the undesirable tetragonal to monoclinic phase transformation at 1000°C, which is lower than previously reported temperatures (>1200°C) in thermal sprayed YSZ coatings. The heat treatments not only resulted in densification of both coatings, but also caused excessive crystallite growth in aqueous coatings promoting the undesirable phase transformation. On the contrary, the ethanol suspension improved the phase stability by favouring the homogenization of yttrium during spraying.

Alumina-graphene nanocomposite coatings fabricated by suspension high velocity oxy-fuel thermal spraying for ultra-low-wear

An alumina coating containing 1wt.% graphene nanoplatelets (GNPs) is deposited for the first time by suspension high velocity oxy-fuel (SHVOF) thermal spraying – an emerging coating deposition technique. Raman spectroscopy shows the GNPs survive the process albeit with a two-fold increase in the density of defects in the graphitic lattice. Dry-sliding wear testing at a 10N load yields a two order of magnitude reduction in the specific wear rate for the alumina/GNP composite in comparison to the equivalent pure alumina coating. Testing with varying loads shows that the transition to the severe wear regime is prevented until between 30 and 35N for the alumina+GNP coating which otherwise occurs between 5 and 7.5N in the absence of GNPs. A reinforced microstructure, and specifically fracture toughness, explains the improved wear behaviour as opposed to tribofilm formation.