New attachment for controlling gas flow in the HVOF process

During the decade, the high-velocity oxyfuel (HVOF) process proved to be a technological alternative to the many conventional thermal spray processes. It would be very advantageous to design a nozzle that provides improved performance in the areas of deposition efficiency, particle in-flight oxidation, and flexibility to allow deposition of ceramic coatings. Based on a numerical analysis, a new attachment to a standard HVOF torch was modeled, designed, tested, and used to produce thermal spray coatings according to the industrial needs mentioned above. Performance of the attachment was investigated by spraying several coating materials including metal and ceramic powders. Particle conditions and spatial distribution, as well as gas phase composition, corresponding to the new attachment and the standard HVOF gun, were compared. The attachment provides better particle spatial distribution, combined with higher particle velocity and temperature. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), 5–8 May, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

Adhesive/cohesive properties of thermally sprayed functionally graded coatings for polymer matrix composites

High-velocity oxyfuel (HVOF) sprayed polyimide/WC-Co functionally graded (FGM) coatings with flame-sprayed WC-Co topcoats have been investigated as solutions to improve the solid-particle erosion and oxidation resistance of polymer matrix composites (PMCs) in the gas flow path of advanced turbine engines. Porosity, coating thickness, and volume fraction of the WC-Co phase retained in the graded coating architecture were determined using standard metallographic techniques and computer image analysis. The adhesive bond strength of three different types of coatings was evaluated according to ASTM D 4541. Adhesive/cohesive strengths of the FGM coating were measured and compared with those of pure polyimide and polyimide/WC-Co composite coatings and also related to the tensile strength of the uncoated PMC substrate perpendicular to the thickness. The FGM coatings exhibited lower adhesive bond strengths (∼6.2 MPa) than pure polyimide coatings (∼8.4 MPa), and in all cases these values were lower than the tensile strength (∼17.6 MPa) of the reference uncoated PMC substrate. The nature and locus of the failures were characterized according to the percent adhesive and/or cohesive failure, and the interfaces tested and layers involved were analyzed by scanning electron microscopy. The original version of this paper was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), 5–8 May, 2003, Basil R. Marple and Christian Moreau, Eds., ASM International, 2003.     

Biocompatible nanostructured high-velocity oxyfuel sprayed titania coating: Deposition, characterization, and mechanical properties

Nanostructured titania (TiO₂) coatings were produced by high-velocity oxyfuel (HVOF) spraying. They were engineered as a possible candidate to replace hydroxyapatite (HA) coatings produced by thermal spray on implants. The HVOF sprayed nanostructured titania coatings exhibited mechanical properties, such as hardness and bond strength, much superior to those of HA thermal spray coatings. In addition to these characteristics, the surface of the nanostructured coatings exhibited regions with nanotextured features originating from the semimolten nanostructured feedstock particles. It is hypothesized that these regions may enhance osteoblast adhesion on the coating by creating a better interaction with adhesion proteins, such as fibronectin, which exhibit dimensions in the order of nanometers. Preliminary osteoblast cell culture demonstrated that this type of HVOF sprayed nanostructured titania coating supported osteoblast cell growth and did not negatively affect cell viability. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

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. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

Microstructural and mechanical characterization of Ni-base thermal spray coatings deposited by HVOF

The present work has been conducted in order to determine the microstructural features, hardness and elastic modulus of two different Ni-base coatings deposited by means of HVOF thermal spray, onto a SAE 1045 plain carbon steel substrate. The morphology and chemical composition of the phases that are present in the coatings were characterized by means of SEM, EDS and XRD techniques. Image analysis was used for the evaluation of the coatings porosity. Both conventional and instrumented indentation tests were also carried out on the surface and cross section of the coatings, in order to evaluate the effect of coating microstructure on hardness and elastic modulus. Conventional indentation tests were conducted using a Knoop indenter and a maximum load of 9.8 N. Instrumented indentation tests, in which the indenter depth and applied load were recorded continuously, were carried out employing a Vickers indenter and maximum loads of 0.49, 0.98, 1.96, 4.9 and 9.8 N. Instrumented nanoindentation tests (in a continuous stiffness measurement mode) were also conducted employing a Berkovich indenter with a maximum load of 9.8 N. The elastic modulus was computed by means of the Oliver and Pharr method and compared with the values determined by means of the method earlier advanced by Marshall et al. The results obtained indicate that the elastic modulus values determined on the cross section of the coatings are higher than those obtained on the surface, clearly indicating the anisotropy of the structure. Also, the values found employing a Berkovich indenter are very similar to those derived by means of the Vickers indenter. In addition, the these values are in agreement with those determined by taking into consideration the elastic recovery of the short Knoop diagonal after removal of the load.     

Microstructure and properties of tungsten carbide coatings sprayed with various high-velocity oxygen fuel spray systems

This article reports on a series of experiments with various high-velocity oxygen fuel spray systems (Jet Kote, Top Gun, Diamond Jet (DJ) Standard, DJ 2600 and 2700, JP-5000, Top Gun-K) using different WC-Co and WC-Co-Cr powders. The microstructure and phase composition of powders and coatings were analyzed by optical and scanning electron microscopy and x-ray diffraction. Carbon and oxygen content of the coatings were determined to study the decarburization and oxidation of the material during the spray process. Coatings were also characterized by their hardness, bond strength, abrasive wear, and corrosion resistance. The results demonstrate that the powders exhibit various degrees of phase transformation during the spray process depending on type of powder, spray system, and spray parameters. Within a relatively wide range, the extent of phase transformation has only little effect on coating properties. Therefore, coatings of high hardness and wear resistance can be produced with all HVOF spray systems when the proper spray powder and process parameters are chosen. This paper originally appeared in Thermal Spray: Meeting the Challenges of the 21st Century; Proceedings of the 15th International Thermal Spray Conference, C. Coddet, Ed., ASM International, Materials Park, OH, 1998. This proceedings paper has been extensively reviewed according to the editorial policy of the Journal of Thermal Spray Technology.     

Indentation size effect on mechanical properties of HVOF sprayed WC based cermet coatings for a roller cylinder

The present paper concerns the determination of mechanical properties such as hardness, elastic modulus and yield strength of WC-based cermet coatings for a roller cylinder. With this regard, Co and Ni containing WC-based coatings were sprayed on Ni-Al deposited 316 L stainless steel substrates by using High Velocity Oxygen Fuel (HVOF) technique. These HVOF sprayed coatings were analyzed by Scanning Electron Microscopy (SEM) with an Energy Dispersive Spectroscopy (EDS) system attachment. Mechanical properties of the coatings were examined by Shimadzu Dynamic Ultra-micro hardness test machine in order to determine the Young's modulus through load-unload sensing analysis. In addition to mechanical investigation, hardness-depth and hardness-force curves of WC-based coatings were investigated. It was found that both of these characteristics exhibit significant peak load dependency. Experimental indentation studies were carried out to determine load-unload curves of WC-Co and WC-Ni based coatings under 300 mN, 350 mN, 400 mN and 450 mN applied peak loads. Hardness and Young's modulus of WC-based coatings were calculated from experimental indentation test data of samples. It has been observed that the hardness and Young's modulus of the coating depends on the contact area and indentation size. The originality of this study is to determine the indentation size effect and contact area variations on mechanical properties of HVOF sprayed WC-based coatings.     

Towards engineering isotropic behaviour of mechanical properties in thermally sprayed ceramic coatings

It is widely recognized by the scientific community that thermal spray coatings exhibit anisotropic behaviour of mechanical properties, e.g., the elastic modulus values of the coating in-plane (i.e., parallel to the substrate surface) or through-thickness (i.e., perpendicular to the substrate surface) will tend to be significantly different due to their anisotropic microstructures. This work shows that thermally sprayed ceramic coatings may exhibit isotropic mechanical behaviour similar to that of bulk materials even when exhibiting the typical anisotropic coating microstructure. Elastic modulus values on the in-plane and through-thickness directions were measured via Knoop indention and laser-ultrasonic techniques on a coating produced via flame spray (FS) using a nanostructured titania (TiO₂) powder. No significant differences were found between the coating directions. In addition, four major cracks with similar lengths were observed originating near or at the corners of Vickers indentation impressions on the coating cross-section (i.e., a typical characteristic of bulk ceramics), instead of two major cracks propagating parallel to the substrate surface, which is normally the case for these types of coatings. It was observed by scanning electron microscopy (SEM) that coatings tended to exhibit an isotropic behaviour when the average length of microcracks within the coating structure oriented perpendicular to the substrate surface was about twice that of the microcracks aligned parallel to the substrate surface. Modelling, based on scalar crack densities of horizontal and vertical cracks, was also used to estimate when thermal spray coatings tend to exhibit isotropic behaviour.     

Synthesis and oxidation behavior of nanocrystalline MCrAlY bond coatings

Thermal barrier coating systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO₂/Y₂O₃). In this work, the oxidation behavior of conventional and nanostructured high-velocity oxyfuel (HVOF) NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Freestanding bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at 1000 °C for different time periods to form the thermally grown oxide layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared with that of the conventional one. The observed behavior is a result of the formation of a continuous Al₂O₃ layer on the surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

Comparison of Isolated Indentation and Grid Indentation Methods for HVOF Sprayed Cermets

This paper compares the results of two approaches of instrumented indentation for characterization of mechanical properties of HVOF coatings. Three types of HVOF sprayed coatings (Cr₃C₂-NiCr, WC-Co, (Ti, Mo)(C,N)-NiCo) were investigated by the means of isolated nanoindentation and grid indentation methods. The results of the isolated indentation revealed hardness and elastic modulus of the individual phases in very good agreement with the corresponding bulk material. The grid indentation method, based on statistical evaluation of a large number of indentations, was influenced by the carbide-matrix interface, which gave rise to a third peak apart from the two peaks corresponding to the carbides and metallic matrix. As a consequence, the bimodal Gaussian fit was insufficient and a trimodal fit had to be used. The results extracted from low load grid nanoindentations were quite close to the results of isolated indentations whereas higher load grid nanoindentation revealed overall properties of the coating.     

A new high-velocity oxygen fuel process for making finely structured and highly bonded inconel alloy layers from liquid feedstock

High-velocity oxygen fuel (HVOF) thermal spray processes are used in applications requiring the highest density and adhesion strength, which are not achievable in most other thermal spray processes. Similar to other thermal spray processes, however, a normal HVOF process is unable to apply fine powders less than 10 μm via a powder feeder. The advantages of using smaller and even nanosized particles in a HVOF process include uniform microstructure, higher cohesion and adhesion, full density, lower internal stress, and higher deposition efficiency. In this work, a new process has been developed for HVOF forming of fine-grained Inconel 625 alloy layers using a liquid feedstock containing small alloy particles. Process investigations have shown the benefits of making single and duplex layered coatings with full density and high bond strength, which are attributed to the very high kinetic energy of particles striking on the substrates and the better melting of the small particles. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

High-speed visualization and plume characterization of the hybrid spray process

The hybrid spray process that combines arc spray with a high-velocity oxyfuel (HVOF)/plasma jet has recently demonstrated its effectiveness in deposition of functionally gradient coatings. This approach aims at exploiting the combined attributes of the arc-spray technique and the HVOF/air plasma spraying (APS) technique. This paper presents high-speed visualization and plume characterization of an arc/HVOF hybrid spray gun as well as a twin-wire arc-spray gun. The physics of atomization in the hybrid spray process is examined using a high-speed camera. A DPV/CPS-2000 (Tecnar, St-Bruno, QC, Canada) particle diagnostics sensor is used to measure particle velocity, temperature, size, and distribution. The influence of feed material, arc parameters, and HVOF parameters on the particle characteristics is presented. Differences in the in-flight characteristics between the hybrid and the twin-wire arc process are discussed aided by the observed atomization phenomena with the high-speed camera. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Determining microhardness and elastic modulus of plasma-sprayed Cr3C2–NiCr coatings using Knoop indentation testing

In this paper, the microhardness and elastic moduli of plasma-sprayed Cr₃C₂–NiCr coatings were measured using the Knoop indentation technique, and the effects of coating process, measurement directions and indenter loads were investigated. The measured data sets were then statistically analyzed employing the Weibull distribution to assess their variability within the coatings. It was found that the measured Knoop hardness and elastic moduli of the coatings were related to microstructure and displayed highly anisotropic behavior of the coatings. In particular, the Knoop hardness, when measured with the major diagonal of the indenter parallel to the interlamellar boundaries of the coatings, was obviously lower than that done where the major diagonal was vertical to the boundaries for an identical indenter load. The measured elastic modulus data exhibited much more scatter than those of the corresponding Knoop hardness. Because it depends on the applied indenter load and it contains systematical error, the Knoop indentation test is not practicable to measure the elastic modulus of thermal sprayed coatings when the major diagonal of the indenter is parallel to the interlamellar boundaries. The results are mainly explained in terms of interlamellar boundary sliding caused by the indenter loads.     

Near-isotropic air plasma sprayed titania

A titania feedstock was air plasma sprayed on low carbon steel substrates. In-flight particle temperature, velocity and diameter were monitored in order to find a parameter set that resulted in high particle temperature and velocity. Coatings were produced using the chosen parameter set, and certain mechanical properties (Vickers microhardness, Knoop microhardness and elastic modulus) of these deposits were measured on the cross-section and in-plane (top surface). The microstructure was evaluated using image analysis (porosity) and scanning electron microscopy. Thermal spray coatings are widely known for their anisotropic character, however, in this work, the mechanical properties of the titania coatings exhibited very similar values on the cross-section and in-plane regions. The reasons why this near-isotropic character is present are suggested and discussed.     

Microstructural evaluation of tungsten carbide-cobalt coatings

Tungsten carbide-12 wt.% cobalt coatings were deposited using optimized high-energy plasma (HEP) and high-velocity oxygen fuel (HVOF) thermal spray techniques. The coatings were evaluated using transmission electron microscopy, differential thermal analysis, X-ray diffraction, and subjected to wear tests to relate the coating structure to wear performance. Coatings were evaluated in the assprayed condition, as well as after heat treatments in inert atmosphere. The results indicate that a substantial amount of amorphous matrix material is created during the thermal spray process. Carbon and tungsten, liberated through the dissociation of the WC, combine with cobalt present in the starting powder to form amorphous material on solidification. Differential thermal analysis revealed an exothermic reaction for both the HVOF and HEP coatings at approximately 853 and 860 °C, respectively, which did not occur for the powder. Post-coating heat treatment in an inert atmosphere resulted in the recrystallization of the amorphous material into Co₆W₆C and Co₂W₄C, which was dependent on the time and temperature of the heat treatment. Wear testing showed improvement in the wear performance for coatings that were subjected to the heat treatment. This was related to the recrystallization of the amorphous matrix into eta phase carbides. Editor’s Note: This paper was presented at the 4th National Thermal Spray Conference, Pittsburgh, 6-10 May, 1991. The proceedings of this conference will be published by ASM International. Dr. T.F. Bernecki is the Editor of these proceedings.     

Characterization of thermal sprayed nanostructured WC-Co coatings derived from nanocrystalline WC-18wt.%Co powders

Nanostructured WC-Co coatings were synthesized using high velocity oxygen fuel (HVOF) thermal spray. The nanocrystalline feedstock powder with a nominal composition of WC-18 wt.%Co was prepared using the novel integrated mechanical and thermal activation (IMTA) process. The effects of HVOF thermal spray conditions and powder characteristics on the microstructure and mechanical properties of the as-sprayed WC-Co coatings were studied. It was found that the ratio of oxygen-to-hydrogen flow rate (ROHFR) and the starting powder microstructures had strong effects on decarburization of the nano-coatings. Decarburization was significantly suppressed at low ROHFR and with the presence of free carbon in the powder. The level of porosity in the coatings was correlated with the powder microstructure and spray process conditions. The coating sprayed at ROHFR=0.5 exhibited the highest microhardness value (HV_300g=1077), which is comparable to that of conventional coarse-grained coatings.     

Optimized HVOF titania coatings

A series of spray parameters was tested for a titania (TiO₂) feedstock, and the in-flight particle temperature was measured for each setting combination. The parameter set that resulted in the highest particle temperature was selected for producing coatings for further study and analysis. With this parameter set, the majority of the sprayed particles had temperatures (at least superficially) above that of the melting point of titania. The hardness (H), elastic modulus (E), and elasticity index (H/E ratio) on the cross section and top surface of these HVOF-sprayed titania coatings were evaluated using the Knoop technique and Vickers hardness measurements. The distribution of elastic modulus and hardness values was analyzed via Weibull statistics. The coating microstructure and phase composition were evaluated using scanning electron microscopy (SEM) and x-ray diffraction (XRD) analysis, respectively. The porosity level was determined via image analysis. It was observed that the coatings were uniform and very dense, consisting of rutile as the major phase. The optimized spray conditions allowed the production of thick coatings (∼740 μm), which were shown to be in a state of residual compressive stress using Almen strip measurements.     

Influence of the HVOF Gas Composition on the Thermal Spraying of WC-Co Submicron Powders (−8 + 1 μm) to Produce Superfine Structured Cermet Coatings

Thermal spraying technology represents a novel and promising approach to protect forming tools with complex surfaces and highest shape accuracy against abrasive wear and galling. However, due to high or nonuniform layer thicknesses or inappropriate surface roughness conventional coarse-structured coatings are not suitable to achieve this aim. The application of novel submicron or nanoscaled feedstock materials in the thermal spray process can provide the deposition of cermet coatings with significantly improved characteristics and is recently of great interest in science and industry. In this collaborative study, the feeding and HVOF spraying of WC-Co submicron powders (?8 + 1 μm) have been investigated to manufacture superfine structured, wear resistant, near-net-shape coatings with improved macroscopic properties and smooth surfaces. The influences of varying HVOF gas compositions on the spray process and the coating properties have been analyzed.     

Effect of spraying distance on the microstructure and mechanical properties of a Colmonoy 88 alloy deposited by HVOF thermal spraying

The present work has been conducted in order to determine systematically the influence of the spraying distance on the microstructure and mechanical properties of a Colmonoy 88 alloy deposited by means of HVOF thermal spray onto a SAE 1045 steel substrate. The spray distance varied between 380-470 mm and the evaluation of the deposits characteristics and properties was carried out both on their surface and on cross section. Both hardness and elastic modulus of the coatings were determined according to the model of Oliver and Pharr. The yield strength of the coatings was also estimated following the methodology developed by Zeng and Chiu for the analysis of the loading and unloading curves obtained from nanoindentation experiments, as well as from classical static spherical indentation tests. The microstructural analysis indicated a significant increase in the unmelted particles volume fraction and the development of interlamellar microcracks as the spraying distance increases, leading to a decrease in the elastic modulus of the coatings. Both hardness and elastic modulus showed an anisotropic behavior and were found to be higher on the cross section of the coating than on the deposition plane. A satisfactory comparison between the predicted and experimental values of the coatings yield strength was observed for all the conditions investigated.     

High-velocity oxyfuel reactive spraying of mechanically alloyed Ni-Ti-C powders

Recently, there has been considerable interest in producing cermet coatings with nanoscale carbide grains in the size range 50 to 500 nm. In this article, the production of nanoscale TiC grains in a Ni-based alloy matrix by reactive high-velocity oxyfuel (HVOF) spraying of metastable Ni-Ti-C powder is reported. Mechanical alloying of a Ni(Cr) prealloyed powder and Ti and C elemental powders was performed in a planar-type ball mill, and materials were characterized in detail using x-ray diffraction (XRD) and scanning electron micros-copy (SEM). Phase changes were correlated with milling time and other processing conditions. Results show that, by the selection of appropriate conditions, a metastable Ni-Ti-C powder could be obtained with the nominal composition 50wt.%Ni-40wt.%Ti-10wt.%C. Following sieving and classification, powder was produced with a particle size range of −38 to 8 μm, which is suitable for HVOF spraying. Coatings, approximately 250 μm thick, were deposited by HVOF spraying onto mild steel substrates, and the microstructures formed were investigated. XRD showed that a self-propagating high-temperature synthesis (SHS) reaction had occurred in the powder particles during spraying and that the principal phases present in the coating were TiC and a Ni-rich solid solution; small quantities of NiTi, TiO₂, and NiTiO₃ were also present. SEM revealed that the coatings had a characteristic, splatlike morphology and that TiC formed as a nanoscale dispersion, with a size range of ∼50 to 200 nm, within solidified splats. The microstructures of these reactively sprayed Ni-TiC coatings are briefly compared with those observed in HVOF-sprayed coatings deposited using prereacted SHS powder. The original version of this paper was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.