Residual stresses in high-velocity oxy-fuel thermally sprayed coatings – Modelling the effect of particle velocity and temperature during the spraying process

The application of thick thermally sprayed coatings on metallic parts has been widely accepted as a solution to improve their corrosion and wear resistance. Key attributes of these coatings, such as adherence to the substrate, are strongly influenced by the residual stresses generated during the coating deposition process. In high-velocity oxy-fuel (HVOF) thermal spraying, due to the relatively low temperature of the particle, significant peening stresses are generated during the impact of molten and semi-molten particles on the substrate. Whilst models exist for residual stress generation in plasma-based thermal spray processes, finite element (FE) prediction of residual stress generation for the HVOF process has not been possible due to the increased complexities associated with modelling the particle impact. A hybrid non-linear explicit–implicit FE methodology is developed here to study the thermomechanical processes associated with particle impingement and layer deposition. Attention is focused on the prediction of residual stresses for an SS 316 HVOF sprayed coating on an SS 316 substrate.     

Residual Stresses Distribution through Thick HVOF Sprayed Inconel 718 Coatings

Residual stress buildup in thick thermal spray coatings is a property of concern. The adhesion of these coatings to the substrate is influenced by residual stresses that are generated during the coating deposition process. In the HVOF spray process, significant peening stresses are generated during the impact of semimolten particles on the substrate. The combination of these peening stresses together with quenching and thermal mismatch stresses that arise after deposition can be of significant importance. Both numerical method, i.e., Finite Element Method (FEM), and experimental methods, i.e., the Modified Layer Removal Method (MLRM) and Neutron Diffraction, to calculate peening and quenching stresses have been utilized in this work. The investigation was performed on thick Inconel 718 coatings on Inconel 718 substrates. Combined, these numerical and experimental techniques yield a deeper understanding of residual stress formation in the HVOF process and thus a tool for process optimization. The relationship between the stress state and deposit/substrate thickness ratio is given particular interest.     

Adhesion Strength of HVOF Sprayed IN718 Coatings

The adhesion strength of high-velocity oxyfuel thermally sprayed coatings is of prime importance when thick coatings are to be sprayed in repair applications. In this study, relationships between process parameters, particle in-flight characteristics, residual stresses, and adhesion strength were explored. The most important process parameters that influence HVOF sprayed IN718 coating adhesion strength on IN718 substrate material were identified. Residual stress distributions were determined using the modified layer removal method, and adhesion strength was measured using an in-house-developed tensile test. Relationships between process parameters, particle in-flight characteristics, coating microstructure, and adhesion strength were established. Particle temperature, particle velocity, substrate preparation, and deposition temperature were identified as critical parameters to attain high adhesion strength. Controlling these parameters can significantly improve the adhesion strength, thus enabling thick coatings to be sprayed for repair applications.     

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.     

Effect of particle state on the adhesive strength of HVOF sprayed metallic coating

NiCrBSi and Ni-50Cr coatings were deposited using the high velocity oxygen fuel (HVOF) spray process under different spray parameters with two powders of different sizes to clarify the influence of the melting state of spray particles on the adhesive strength of the coating. The adhesive strength of the coating was estimated according to the American Society for Testing and Materials (ASTM) C633-79. The melting state of the spray droplet was examined from the coating microstructure. It was found that the melting state of spray particles had a significant effect on the adhesive strength of HVOF sprayed Ni-based coatings. The significant melting of the spray particle did not contribute to the increase in the adhesion of HVOF metallic coatings. On the other hand, the deposition of a partially melted large particle contributed to the substantial improvement of adhesive strength of the HVOF coating. The subsequent coating presented a dense microstructure and yielded an adhesive strength of more than 76 MPa, which was double that of the coating deposited with completely molten particles. It can be suggested that the good melting of the spray particle is mainly related to the mechanical interlocking effect, which reaches the limited and approximately defined adhesive strength up to 40–50 MPa.     

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.     

FeAl and Mo–Si–B intermetallic coatings prepared by thermal spraying

FeAl and Mo–Si–B intermetallic coatings for elevated temperature environmental resistance were prepared using high-velocity oxy-fuel (HVOF) and air plasma spray (APS) techniques. For both coating types, the effect of coating parameters (spray particle velocity and temperature) on the microstructure and physical properties of the coatings was assessed. Fe–24Al (wt%) coatings were prepared using HVOF thermal spraying at spray particle velocities varying from 540 to 700 m/s. Mo–13.4Si–2.6B coatings were prepared using APS at particle velocities of 180 and 350 m/s. Residual stresses in the HVOF FeAl coatings were compressive, while stresses in the APS Mo–Si–B coatings were tensile. In both cases, residual stresses became more compressive with increasing spray particle velocity due to increased peening imparted by the spray particles. The hardness and elastic moduli of FeAl coatings also increased with increasing particle velocity. For Mo–Si–B coatings, plasma spraying at 180 m/s resulted in significant oxidation of the spray particles and conversion of the T1 phase into amorphous silica and -Mo. The T1 phase was retained after spraying at 350 m/s.     

Effect of substrate hardness on splat morphology in high-velocity thermal spray coatings

In this study, Ni-chrome alloy particles were thermally sprayed onto a variety of substrate materials using the high-velocity air fuel (HVAF) technique. Although the various substrate materials were sprayed using identical powder material and thermal spray conditions, the type and variation of splat morphologies were strongly dependent on the substrate material. Predominantly solid splats are observed penetrating deeply into softer substrates, such as aluminum, whereas molten splats were observed on harder substrates, which resisted particle penetration. The observed correlation between molten splats and substrate hardness could be due a dependency of deposition efficiencies of solid and molten splats on the substrate material. However, it was found that conversion of particle kinetic energy into plastic deformation and heat, dependent on substrate hardness, can make a significant contribution towards explaining the observed behavior. 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.     

Residual stress development in cold sprayed Al,Cu and Ti coatings

Residual stresses play an important role in the formation and performance of thermal spray coatings. A curvature-based approach where the substrate–coating system deflection and temperature are monitored throughout the coating deposition process was used to determine residual stress formation during cold spray deposition of Al, Cu and Ti coatings. The effect of substrate material (carbon steel, stainless steel and aluminium) and substrate pre-treatment (normal grit blasting, grit blasting with the cold spray system and grinding for carbon steel substrate) were studied for all coating materials with optimized deposition parameters. Mainly compressive stresses were expected because of the nature of cold spraying, but also neutral as well as tensile stresses were formed for studied coatings. The magnitudes of the residual stresses were mainly dependent on the substrate/coating material combination, but the surface preparation was also found to have an effect on the final stress stage of the coating.     

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.     

Microstructural characterization and hardness evaluation of HVOF sprayed Ni–5Al coatings on Ni- and Fe-based superalloys

HVOF sprayed Ni–5Al coatings on Ni- and Fe-based superalloy substrates were characterized to assess the microstructural features and strength in the as deposition condition for their applications in high-temperature corrosive environment of gas turbine. X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDAX), and X-ray mapping analysis are used to characterize the Ni–5Al coatings. The dense coatings with less porosity and inclusions were produced using HVOF process. The deposited Ni–5Al coatings exhibited splat like layered morphologies due to deposition and resolidification of successive molten and semi molten powder particles. The hardness of coatings on three different superalloy substrates was measured and it was in the range of 210–272 Hv. The average bond strength and surface roughness of the as-sprayed coatings were 42.62 MPa and 9.22–9.45 μm, respectively. Diffusion of alloying elements from the substrate into the coating has occurred in all the three superalloy substrates as observed from the X-ray mapping analysis.     

Process Control and Characterization of NiCr Coatings by HVOF-DJ2700 System: A Process Map Approach

The concept of ‘process maps’ has been utilized to study the fundamentals of process-structure-property relationships in high velocity oxygen fuel (HVOF) sprayed coatings. Ni-20%Cr was chosen as a representative material for metallic alloys. In this paper, integrated experiments including diagnostic studies, splat collection, coating deposition, and property characterization were carried out in an effort to investigate the effects of fuel gas chemistry (fuel/oxygen ratio), total gas flow, and energy input on particle states: particle temperature (T) and velocity (V), coating formation dynamics, and properties. Coatings were deposited on an in situ curvature sensor to study residual stress evolution. The results were reconciled within the framework of process maps linking torch parameters with particle states (1st order map) and relating particle state with deposit properties (2nd order map). A strong influence of particle velocity on induced compressive stresses through peening effect is discussed. The complete tracking of the coating buildup history including particle state, residual stress evolution and deposition temperature, in addition to single splat analysis, allows the interpretation of resultant coating microstructures and properties and enables coating design with desired properties.     

Adhesion improvements of Thermal Barrier Coatings with HVOF thermally sprayed bond coats

Thermal barrier coatings (TBC) are an effective engineering solution for the improvement of in service performance of gas turbines and diesel engine components. The quality and further performance of TBC, likewise all thermally sprayed coatings or any other kind of coating, is strongly dependent on the adhesion between the coating and the substrate as well as the adhesion (or cohesion) between the metallic bond coat and the ceramic top coat layer. The debonding of the ceramic layer or of the bond coat layer will lead to the collapse of the overall thermal barrier system. Though several possible problems can occur in coating application as residual stresses, local or net defects (like pores and cracks), one could say that a satisfactory adhesion is the first and intrinsic need for a good coating. The coating adhesion is also dependent on the pair substrate-coating materials, substrate cleaning and blasting, coating application process, coating application parameters and environmental conditions. In this work, the general characteristics and adhesion properties of thermal barrier coatings (TBCs) having bond coats applied using High Velocity Oxygen Fuel (HVOF) thermal spraying and plasma sprayed ceramic top coats are studied. By using HVOF technique to apply the bond coats, high adherence and high corrosion resistance are expected. Furthermore, due to the characteristics of the spraying process, compressive stresses should be induced to the substrate. The compressive stresses are opposed to the tensile stresses that are typical of coatings applied by plasma spraying and eventually cause delamination of the coating in operational conditions. The evaluation of properties includes the studies of morphology, microstructure, microhardness and adhesive/cohesive resistance. From the obtained results it can be said that the main failure location is in the bond coat/ceramic interface corresponding to the lowest adhesion values.     

Effect of reinforcement size on the scratch resistance and crystallinity of HVOF sprayed nylon-11/ceramic composite coatings

The high-velocity oxyfuel (HVOF) combustion spraying of dry ball-milled nylon-11/ceramic composite powders is an effective, economical, and environmentally sound method for producing semicrystalline micron and nanoscale reinforced polymer coatings. Composite coatings reinforced with multiple scales of ceramic particulate material are expected to exhibit improved load transfer between the reinforcing phase and the matrix due to interactions between large and small ceramic particles. An important step in developing multiscale composite coatings and load transfer theory is determining the effect of reinforcement size on the distribution of the reinforcement and the properties of the composite coating. Composite feedstock powders were produced by dry ball-milling nylon-11 together with 7, 20, and 40 nm fumed silica particles, 50 and 150 nm fumed alumina particles, and 350 nm, 1, 2, 5, 10, 20, 25, and 50 μm white calcined alumina at 10 vol.% overall ceramic phase loadings. The effectiveness of the ball-milling process as a function of reinforcement size was qualitatively evaluated by scanning electron microscopy+energy dispersive x-ray spectroscopy (SEM+EDS) microanalysis and by characterizing the behavior of the powder during HVOF spraying. The microstructures of the sprayed coatings were characterized by optical microscopy, SEM, EDS, and x-ray diffraction (XRD). The reinforcement particles were found to be concentrated at the splat boundaries in the coatings, forming a series of interconnected lamellar sheets with good three-dimensional distribution. The scratch resistance of the coatings improved consistently and logarithmically as a function of decreasing reinforcement size and compared with those of HVOF sprayed pure nylon-11. 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.     

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.     

Process and microstructure simulation in thermal spraying

Thermally sprayed coatings are widely used to improve wear and corrosion behavior of mechanical parts. To develop new coating processes and applications - a detailed knowledge of process-material interaction is necessary. According to the progress in computing and commercial software today simulation can be used to understand the interaction of process parameters with the material behavior like solidification, microstructure and residual stresses within the coating. Under commercial aspects the process parameters can be optimized to increase the deposition rate and minimize the residual stresses simultaneously. For plasma spraying the simulation of the coating process from plasma generation, particle injection, heating and acceleration to particle impact on the substrate, solidification and residual stresses and the influence of different parameters will be demonstrated and correlated to the experimental results.     

Hypersonic plasma particle deposition—A hybrid between plasma spraying and vapor deposition

In the hypersonic plasma particle deposition process, vapor phase reactants are injected into a plasma and rapidly quenched in a supersonic nozzle, leading to nucleation of nanosize particles. These particles impact a substrate at high velocity, forming a coating with grain sizes of 10 to 40 nm. As previously reported, coatings of a variety of materials have been obtained, including silicon, silicon carbide, titanium carbide and nitride, and composites of these, all deposited at very high rates. Recent studies have shown that slight modifications of the process can result in nanosize structures consisting of single crystal silicon nanowires covered with nanoparticles. These nanowires are believed to grow in a vapor deposition process, catalyzed by the presence of titanium in the underlying nanoparticle film. However, simultaneously nanoparticles are nucleated in the nozzle and deposited on the nanowires, leading to structures that are the result of a plasma chemical vapor deposition (CVD) process combined with a nanoparticle spray process. The combination of these two process paths opens new dimensions in the nanophase materials processing area. 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.     

Innovation of Ultrafine Structured Alloy Coatings Having Superior Mechanical Properties and High Temperature Corrosion Resistance

High temperature protection requires full coating density, high adhesion, minor oxide inclusions, and preferably fine grains, which is not achievable in most thermal spray processes. High velocity oxygen fuel (HVOF) thermal spray process has been applied extensively for making such coatings with the highest density and adhesion strength, but the existence of not melted or partially melted particles are usually observed in the HVOF coatings because of relatively low flame temperature and short particle resident time in the process. This work has investigated the development of an innovative HVOF process using a liquid state suspension/slurry containing small alloy powders. The advantages of using small particles in a HVOF process include uniform coating, less defective microstructure, higher cohesion and adhesion, full density, lower internal stress, and higher deposition efficiency. Process investigations have proven the benefits of making alloy coatings with full density and high bond strength attributing to increased melting of the small particles and the very high kinetic energy of particles striking on the substrate. High temperature oxidation and hot corrosion tests at 800 °C have demonstrated that the alloy coatings made by novel LS-HVOF process have superior properties to conventional counterpart coatings in terms of oxidation rates and corrosion penetration depths.     

Residual Strain and Fracture Response of Al2O3 Coatings Deposited via APS and HVOF Techniques

The aim of this investigation was to nondestructively evaluate the residual stress profile in two commercially available alumina/substrate coating systems and relate residual stress changes with the fracture response. Neutron diffraction, due to its high penetration depth, was used to measure residual strain in conventional air plasma-sprayed (APS) and finer powder high velocity oxy-fuel (HVOF (θ-gun))-sprayed Al₂O₃ coating/substrate systems. The purpose of this comparison was to ascertain if finer powder Al₂O₃ coatings deposited via θ-gun can provide improved residual stress and fracture response in comparison to conventional APS coatings. To obtain a through thickness residual strain profile with high resolution, a partially submerged beam was used for measurements near the coating surface, and a beam submerged in the coating and substrate materials near the coating-substrate interface. By using the fast vertical scanning method, with careful leveling of the specimen using theodolites, the coating surface and the coating/substrate interface were located with an accuracy of about 50 μm. The results show that the through thickness residual strain in the APS coating was mainly tensile, whereas the HVOF coating had both compressive and tensile residual strains. Further analysis interlinking Vickers indentation fracture behavior using acoustic emission (AE) was conducted. The microstructural differences along with the nature and magnitude of the residual strain fields had a direct effect on the fracture response of the two coatings during the indentation process.