Abstract In this work; a novel liquid fuel HVOF process fueled with ethanol was used to prepare 75wt%Cr3C2–25wt%NiCr coatings on AISI304 stainless steel substrate. Taguchi method was employed to optimize the spray parameters (ethanol flow rate; oxygen flow rate; powder feed rate and standoff distance) to achieve better erosion resistance at 90° impact angle. The results indicated that ethanol flow rate and oxygen flow rate were identified as the highly contributing parameters on the erosion wear loss. The important sequence of the spray parameter is ethanol flow rate > oxygen flow rate > standoff distance > powder feed rate. The optimal spray parameter (OSP) for minimum erosion wear loss was obtained under ethanol flow rate of 28slph; oxygen flow rate of 420slpm; powder feed rate of 76.7 g/min and standoff distance of 300mm. The phase composition; microstructure; hardness; porosities; and the erosion wear behaviors of the coatings have been studied in detail. Besides; erosion wear testing of the optimized coating was conducted at 30°; 60° and 90° impact angle using air jet erosion testing machine. The SEM images of the erodent samples were taken to analyze the erosion mechanism.

Abstract Three different coatings were deposited using the Detonation Gun Spraying (DGS) technology from steel powders alone; and steel powers mixed with Fe3C and SiC particles; respectively. The microstructural characteristics of these coatings were examined and the hardness of each type of coating was studied. The morphology and structure of the feedstock powders were affected by the exposure to high temperature during the spraying process and rapid solidification of steel powders that resulted in the formation of an amorphous structure. The unreinforced steel coating had the highest hardness among the three types of coatings; possibly due to a higher degree of amorphization in the coating compared to the other two samples. The microstructural observation confirmed the formation of dense coatings with a layered structure with good connectivity between layers with minimum defects and porosities in the interfacial regions.

Abstract In previous studies at McGill University; tin was successfully cold sprayed onto carbon fiber reinforced polymers (CFRPs). A “crack-filling” mechanism was described as the deposition mechanism that allowed deposition of tin onto the CFRP. Improving the coating conductivity for lightning strike protection (LSP) purposes was achieved by adding other metal powders (aluminum; copper; zinc) to tin and cold spraying on the CFRP. At the same time; it was noticed that the addition of this secondary component (SC) provided an increase in deposition efficiency (DE); tamping was initially hypothesized to explain this improvement; thus prompting a study solely on the effect of SC hardness; which is reported elsewhere in this conference. However; it is recognised that other powder characteristics may also be influencing the DE. Thus; in this study; SCs with a wider variety of particle sizes; morphologies; densities and hardness values were mixed with tin and sprayed on CFRPs. The effect of SC properties on tin deposition is discussed and an optimal combination of SC properties for cold spraying of tin is suggested.

Abstract Industries developing cold-spray processes aim at producing dense and resistant coatings. Controlling microstructure and inter-particular fracture characteristics of sprayed coatings is essential to improve their properties. To do so; post-spraying heat treatment is a promising approach. This work addresses the development of such heat treatments and focuses on the analysis of recovery and recrystallization. Different heat treatment parameters were explored; namely; holding temperature and time; heating rate; and heating method. This approach revealed a competition between recrystallization and other microstructural evolution mechanisms; such as precipitation and porosity coalescence. An optimized heat treatment; allowing microstructural softening and adequate mechanical properties; was sought after. First; differential scanning calorimetry measurements applied to as-sprayed coatings enabled to identify recovery and recrystallization temperature ranges. Then; a variety of heat treatments was applied; involving long-time isothermal holdings as well as shorter cycles. Microstructure analysis and hardness measurements allowed making a first selection of treatment conditions.

Abstract Although Ni-Al intermetallics are of excellent corrosion and oxidation resistance; their oxidation during thermal spraying limits the applications as thermal spray coating. Thus; controlling the oxidation is a necessary approach to improve their performances as the coatings. In this study; a novel approach is proposed to exclude oxide of in-flight Ni-Al droplets by introducing diamond as a deoxidizer in Ni/Al powder during plasma spraying. Ni-Al coatings were deposited with Ni/Al/diamond composite powder prepared by mechanical alloying by atmospheric plasma spraying at different spray conditions to study the effect of the diamond addition on the oxidation behavior of in-flight droplets. The phase composition; microstructure; porosity and microhardness of the coatings were characterized by X-ray diffraction; scanning electron microscopy; image analyzing and hardness test; respectively. The oxygen content of the coatings was measured quantitatively. The results show that APS Ni-Al coating prepared by Ni/Al/diamond powder presents significantly lower oxygen content. The measurement yields particle temperatures higher than 2300 °C. The low oxygen content in the coating is attributed to the in-situ deoxidizing effect of ultra-high temperature contained droplets. Moreover; the oxide-free droplets promote the deposition of Ni-Al coatings with lower oxygen content and porosity than the conventional Ni-Al coatings.

Abstract The cascade plasma torches should gradually replace the standard F4 type torches consisting of two electrodes without neutrodes. For industry; the advantages are a better lifespan of these electrodes but also a less expensive use of the process in terms of the type of gas to be used and of the power to be provided to the plasma; as well as a possible flowrate of injected powder increased. However; the configuration of a cascade torch is still poorly studied and notably the influence of the length of the body of the anode segmented by neutrodes. This work consists in studying the effect of the anode-cathode distance on the voltage fluctuations. The jet stability is clearly demonstrated with the cascaded torch. Indeed; the neutrodes can provide a longer and more stable jet compared to F4 plasma jet. That means that the length of the arc is much higher with the cascaded torch. The in-flight characteristics of alumina particles for different cascading lengths and operating parameters are studied. Particle velocities and temperatures are measured using diagnostic tools. Some alumina coatings are manufactured in order to address a correlation between the different configurations and the microstructure of the coating obtained. Good performances are obtained with microhardness of 1075HV0.3 for a porosity of 4.6 %but with a pure Ar plasma and a 350 A low amperage.

Abstract Outstanding wear resistance; excellent abrasion; and corrosion protection under heavy loads characterize thermally sprayed WC-Co coatings. However; the good wear resistance of these coatings requires high post-processing costs (grinding; polishing) which are estimated to be up to 3-4 times higher than the spray coating process itself. Thus; a great interest of spraying smooth near-net-shape carbide coatings exists. In addition; less of the coating must be removed and the required sprayed coating thicknesses can be reduced; offering saving of costs and resources at the same time. This contribution is an investigation on the development of WC-12Co coatings obtained by suspension HVOF spraying technology. Significant work was devoted to the development and characterization of appropriate water-based hardmetal suspensions starting from commercially available WC and Co raw powders feedstock. The developed suspensions were sprayed using HVOF process to produce fine-structured carbide coatings. The S-HVOF coatings properties; i.e.; microstructure; phase composition; and microhardness were evaluated and discussed.

Abstract Twin wire arc is a commonly used thermal spray technology for application of steel coatings to cast iron components. Hardness and adhesion strength are critical properties of such coatings; and significant research is available reporting these properties. However; residual stresses and the anisotropic structure of the coatings leads to significantly different behavior in bending applications than in the purely tensile loading of the standard adhesion test. In addition; microstructural features that are controlled by certain process parameters during deposition of the coating can have a significant effect on these properties. This work seeks to relate the hardness and pull-off adhesion strength to the coating microstructure; and to assess the related bending strength and failure mode. Comparisons between bend tests and pull-off adhesion tests show significant differences to consider when evaluating twin wire arc coatings.

Abstract In suspension spraying; two most frequently used solvents are water and ethanol. In this study; we test the potential of using alternative; high-molecular weight solvent and demonstrate the associated advantages. Two organic solvents are directly compared: ethanol (serving as a benchmark; suspension formulated at 10 wt.% solid load) and di-propylene glycol methyl ether (two suspensions at 10 wt.% and 20 wt.%). Submicron 100% alpha-alumina powder is used as a model material to formulate the suspensions. It is shown that the ethanol- and ether-based-feedstock coatings are fully comparable in terms of their microstructure; porosity content; surface roughness as well as hardness. Importantly; the ether-based coatings exhibit slightly higher levels of α-Al2O3 phase when compared to their ethanol-based counterpart (17 wt.% vs. 6 wt.%). The use of 20 wt.% solid load in the ether solvent leads to twofold increase in the deposition rate while - as opposed to ethanol - successfully retaining a dense microstructure. Lastly; the ether is significantly cheaper and safer to handle than ethanol.

Abstract A previous study showed that Cu can be cold sprayed onto carbon fiber-reinforced polymers (CFRPs) if a Cu interlayer is deposited prior to low-pressure cold spraying. In this present study; one layer of Cu powders was cold sprayed onto a Sn cold sprayed CFRP and a Ni electroplated interlayer. Two layers of Cu powders were also cold sprayed onto a Cu electroplated CFRP to investigate the effect of second particle layer on deposition efficiency/behavior of the impacting particles. Deposition efficiency (DE) of the cold-sprayed coatings was measured and characterization of the coatings with scanning electron microscopy and microhardness testing were performed. The results showed that relative hardness of the particle to the substrate played a significant role in deformability and impact mode of the particle; as well as the subsequent DE. Therefore; cold spray deposition of Cu particles onto a soft Sn interlayer and a hard Ni interlayer was not possible due to interlayer erosion and the presence of a very hard substrate; respectively. DE of Cu-on-Cu coated CFRP decreased as compared to one pass spraying; due to rebounding of the particles from a harder interlayer (first-layer coating) than the electroplated layer underneath. These results suggest that for a low-pressure cold spraying; Cu can be successfully cold sprayed onto an electroplated Cu interlayer due to the similarity in their hardness values while spraying one layer. However; spraying further passes resulted in rebound of the particles and decrease in overall DE.

Abstract The FeMnCrSi and 316L alloys coatings were obtained by high pressure Cold Gas Spray process on a carbon steel substrate; their microstructure; hardness; and wear resistance were obtained. The ball-on-disk testing; ASTM G99; was used to evaluate the coatings and the substrate sliding wear behavior. The mechanism of wear was the same for both coatings and FeMnCrSi had higher coefficient of friction; but lower volume loss than 316L.

Abstract Cold spray materials deposition is being investigated for mitigation of chloride-induced stress corrosion cracking (CISCC) in Dry Cask Storage Systems (DCSS) for used nuclear fuel. The welded regions of austenitic stainless-steel canisters in DCSS are under tensile stress and susceptible to environmental chloride corrosion; which can potentially lead to the formation of CISCC. The low thermal input and high throughput nature of the cold spray have led to its active consideration as a viable repair and mitigation approach to manage potential CISCC. The cold spray coatings are under compressive stresses and act as environmental barriers against a Cl-rich environment. Characterization data including thickness; microstructure; adhesion; hardness; and residual stress in the cold spray stainless steel coatings on stainless steel samples with prototypical CISCC are presented. Gas permeability testing of the cold spray coatings are also also discussed.

Abstract Hydraulic turbines are strongly affected by cavitation; which damages the surface and can change the original profile. The study of thermal spray processes and materials becomes relevant to improve surface properties and cavitation resistance of turbines. The main objective of this work is to evaluate the influence of the Fuel/oxygen ratio on the HVOF deposition of WC–20Cr3C2-7Ni and Cr3C2-37WC-18M coatings. The velocity and temperature of the particles were measured; as well as the coatings properties as hardness; porosity and cavitation resistance were evaluated. The HVOF F/O ratio was performed at two different levels; 0.33 and 0.43; using Kerosene as liquid fuel. It was observed that the higher velocity of the particles was obtained with the WC–20Cr3C2-7Ni powder and higher F/O ratio; meanwhile the powder Cr3C2- 37WC-18M showed higher temperature. The samples with the lowest cavitation resistance were the alloys deposited with the lower fuel ratio; which also showed lower micro-hardness and porosity. The samples with the better cavitation resistance were deposited with higher F/O ratio. The wear mechanism started with the nucleation of the cavitation that occurred in the pores forming craters and the detachment of the lamellae was also observed; indicated by the smoothness of the surface. The results show that; regardless of the coating material; the total ratio level had significant influence on cavitation resistance of these materials.

Abstract Two kinds of cermet powders; namely WC-10Co4Cr and WC- 20CrC-7Ni; were deposited on an AISI 1040 steel substrate using a high velocity air fuel thermal spraying process (HVAF) to evaluate resistance in cavitation erosion conditions with additional electrochemical effects. The microstructure of the two HVAF WC-based coatings were observed by a scanning electron microscope (SEM); while the phase composition was analyzed using an energy-dispersive microanalysis system (EDS). The microhardness (HV0.5) of the coatings was measured. In addition; the surface topography of the eroded surface layers was observed using a 3D optical profilometer. The results reveal that the cavitation resistance of the WC-20CrC-7Ni coating was approximately 1.3 times greater than that of the WC-10Co4Cr coating. The study of the structure and surface topography allows us to identity the reasons for the differences between the cavitation resistance of two coatings: the WC-20CrC-7Ni coating has a finer grain structure and a lower pore density level. These differences; along with the presence of high Cr and Ni content in the feedstock powder of this coating (responsible for strengthening the matrix); contribute to improving cavitation resistance and reducing material loss.

Abstract Coatings prepared from chromia-rich (Al;Cr)2O3 solid solution feedstock powders are rarely studied so far. In this work; the processability of a commercial (Al;Cr)2O3 solid solution (ss) powder containing 78 wt.-% Cr2O3 by atmospheric plasma spraying (APS) and the corresponding coating microstructures and hardness were studied; including the effect of blending with 2; 23 and 54 wt.-% TiO2. The microstructures were analyzed using SEM; EDS and XRD measurements. It was shown that blending with TiO2 reduces the porosity and increases the deposition efficiency. Due to the lower porosity; the Vickers microhardness of the coatings was increased. Small amounts of Ti in ss (Al;Cr)2O3 splats were detected in coatings prepared from blends with higher TiO2 contents. The ss (Al;Cr)2O3 areas also vary after the spraying process in terms of their aluminum and chromium content.

Abstract The effect of an alumina shell on stainless steel particles used in plasma spraying has been studied. The mean size of the injected particles is about 65 nm and the thickness of the alumina shell is 3 µm. The composite powder is plasma sprayed using a PTF4 type plasma gun with an internal injection 3 mm upstream of the nozzle exit. The results show that without preheating the substrate splats are extensively fingered and become circular when the substrate surface is preheated over 200°C. EDS analysis of the distribution of the various elements shows that the alumina either uniformly covers the stainless steel splat or is distributed in pieces over the surface. This behavior has been explained by collecting particles in flight and analyzing them. A composite stainless steel/alumina coating sprayed on a rough stainless steel substrate preheated to 400°C has been examined and compared with a pure stainless steel coating. Both hardness and cohesion are improved for the alumina coated particles due to the random distribution of alumina within the steel matrix.

Abstract Quasicrystalline phases improve many alloy properties such as thermomechanical stability, thermal and electrical conductivity, and tribological performance. High hardness, however, is accompanied by brittleness, an undesired property in many applications. Reduced brittleness can be achieved by embedding quasicrystalline phases in a more ductile material, forming a metal-matrix composite that retains some quasicrystalline properties. This study evaluates thermally sprayed coatings made from different compositions of such composites. The coatings assessed were produced by arc-wire, HVOF, and atmospheric plasma spraying using various forms of feed material, including blended, agglomerated, chemical encased, and attrition-milled powders and filled wires. The investigation involved metallurgical analysis, proving the existence of quasicrystal content and assessing the matrix phase, and tests showing how sliding wear is influenced by the composition of quasicrystalline phases.

Abstract A brief feasibility study was performed to produce thermal spray coatings using gas atomized powders of Cu47Ti34-xZr11Ni8Six, where x=0 and 1. These alloys have previously been shown to be capable of forming metallic glasses having thick (1-2 cm) cross sections because they can be cooled from the melt at relatively low cooling rates (e.g., 100-102Ks-1). The properties of these metallic glasses include high strength, high elasticity and high fracture toughness. Amorphous plasma arc sprayed coatings were produced which were close in composition to the starting powders, and exhibited comparable glass transition and crystallization behavior. The amorphous structure of the as-sprayed coatings was used as a source for forming a range of partially devitrified and fully crystallized structures. The average hardness of the coatings increased from around 6 GPa to near 10 GPa as the degree of crystallization increased.

Abstract WC-Co based cermets are extensively used in wear applications due to their hardness and toughness. Recent work has demonstrated the potential for using nanoscale constituents to improve the wear properties of these materials. In the present study, two WC-Co powders containing a nanosized WC phase were used to produce coatings by HVOF thermal spraying. These powders had similar properties except for the volume percent binder present: WC-8C0 and WC-12Co. The thermal spraying conditions were varied in order to identify their effect on the microstructure, properties and phase composition of the sprayed coatings. The as-sprayed coatings possess porosity values ranging between 1% and 2% and microhardness values (HV100) from 1150 to 1550, which are quite similar to values obtained for conventionally sized WC-based coatings. For all the coatings, phase analysis indicated significant degradation of the WC phase to produce W2C, W, CO3W3C and Co6W3C. For some spray conditions, even WO3 phase was found in the coatings. The JP-5000 HVOF system produces coatings with lower porosity, similar microhardness values and, more importantly, with lower WC degradation than the coatings produced with the DJ-2700.

Abstract Wire flame sprayed molybdenum is a wide used procedure for manufacturing of wear resistance coatings. The properties of thermal sprayed coatings depend mainly on the kinetic and thermal energy of sprayed particles, i.e., a higher particle velocity causes an increase of coating quality. The now available high velocity spray system from Praxair which is used within this work is capable to realise the aim of high particle velocities. The coating properties presented in this work are analysed in comparison to conventional wire and powder plasma spray processes. HVWFS molybdenum coatings show lower porosity, higher adhesion and cohesion and better wear properties. To explain the results, particle size distribution, oxygen/carbon content and structure are analysed. Hardening mechanisms of coatings and their adhesion/cohesion properties are discussed based on light microscopy, SEM, XRD and TEM investigations.