In an atmospheric plasma spray (APS) process, in-flight powder particle characteristics, such as the particle velocity and temperature, have significant influence on the coating formation. The nonlinear relationship between the input process parameters and in-flight particle characteristics is thus of paramount importance for coating properties design and quality control. It is also known that the ageing of torch electrodes affects this relationship. In recent years, machine learning algorithms have proven to be able to take into account such complex nonlinear interactions. This work illustrates the application of ensemble methods based on decision tree algorithms to evaluate and to predict in-flight particle temperature and velocity during an APS process considering torch electrodes ageing. Experiments were performed to record simultaneously the input process parameters, the in-flight powder particle characteristics and the electrodes usage time. Various spray durations were considered to emulate industrial coating spray production settings. Random forest and gradient boosting algorithms were used to rank and select the features for the APS process data recorded as the electrodes aged and the corresponding predictive models were compared. The time series aspect of the data will be examined.

A novel powder modification method based on the simultaneous softening and agglomeration of steel powders via heat treatment in a rotary tube furnace has been investigated as a means to improve the cold sprayability of H13 tool steel powder. By adjusting starting powder size and shape as well as heat treatment conditions (maximum temperature, cooling rate, and atmosphere), cold spray of H13 powder went from virtually no deposition to the production of thick dense deposits with a deposition efficiency of 70%. Powder agglomeration, surface state, microstructure evolution, and softening are identified as key factors determining powder deposition efficiency and resulting deposit microstructure.

Cold spray processing of stainless steel coatings, which represent a cost-effective method for wear and corrosion resistance, has been demonstrated as technically feasible. However, these coatings have very low tensile strength in the as-sprayed condition and may also exhibit a marginally higher wear rate. In this study, the cold spraying of 316L stainless steel coatings was investigated to assess the effect of powder size distribution and post-spray heat treatment on strength and wear properties. Coatings on aluminum and steel substrates were produced with a feedstock powder obtained in three particle size distributions. All coatings were deposited under the same conditions using nitrogen as the propellant gas, and then annealed at the optimum temperature. The microstructure and mechanical properties of both as-sprayed and heat-treated coatings were evaluated and the results are presented in the paper.

The processing conditions, microstructural and tribological characterizations of plasma sprayed CoNiCrAlY-BN high temperature abradable coatings are reported in this manuscript. Plasma spray torch parameters were varied to produce a set of abradable coatings exhibiting a broad range of porosity levels (34-62%) and superficial Rockwell hardness values (0-78 HR15Y). Abradability tests have been performed using an abradable-seal test rig capable of simulating operational wear at different rotor speeds and seal incursion rates. These tests allowed determining the rubbing forces and quantifying the blade and seal wear characteristics for slow and fast seal incursion rates. Erosion wear performance and ASTM C633 coating adhesion strength test results are also reported. For optimal abradability performance, it is shown that coating hardness needs to be lower than 70 and 50 HR15Y for slow and fast blade incursion rate conditions, respectively. It is shown that the erosion wear performance, as well as, the coating cohesive strength is a function of the coating hardness. The current results allow defining the coating specifications in terms of hardness and porosity for targeted applications.

Thermal sprayed coatings are often used for high temperature applications and, per se, are subjected to transient temperature gradients during operation. The recurrent temperature changes generate stresses that damage the coating with time, and can even lead to its delamination. The most common methods to evaluate coating behavior under thermal cycling are furnace testing or burner rigs. Both approaches cannot match the conditions reached in service for several applications, in terms of the achievable heating rates for instance. As a consequence, a versatile and robust method to evaluate coating resistance to spalling under thermal cycles is still to be found. This paper presents the development of a thermal cycling rig where the heat input is provided by a laser. This rig allows easy testing of several samples jointly for heating rates as high as 55°C/s and for thousands of thermal cycles. Preliminary trials have allowed the development of different spalling criteria. Finally, it was found that SS430-based materials arc-sprayed on Al substrates exhibit higher delamination resistance (life) under rapid heating/cooling cycles than SS304 coatings on the same substrate. For such high heating rates, the thermal stresses generated in the coating would be more critical than the thermal mismatch at the interface coating/substrate.

In this study, the temperature distribution of the surfaces of several substrates under an impinging gas jet from a cold spray nozzle was determined. A low-pressure cold-gas dynamic spraying unit was used to generate a jet of hot compressed nitrogen that impinged upon flat substrates. Computer codes based on a finite differences method were used to solve a simplified 2-D temperature distribution equation for the substrate to produce non-dimensional relationships between the surface temperature and the radius of the impinging fluid jet, the substrate thickness, and the heating time. It was found that a single profile of the transient non-dimensional maximum surface temperature could be used to estimate the dimensional maximum surface temperature, regardless of the value of the compressed gas temperature. It was found further that as the thermal conductance of the substrate increased, the maximum surface temperature of the substrate beneath the gas jet decreased. The close agreement of the numerical results with the experimental results suggests that the non-dimensionalized results may be applied to a wide range of conditions and materials.

In this investigation, Inconel 718, a material known to cause nozzle clogging upon cold spraying, was cold spray formed to 6 mm-thick using the Plasma Giken cold spray system PCS- 1000. This was made possible due to the novel non-clogging nozzle material combined with a nozzle water cooling system. Coatings were as-spray formed using both nitrogen and helium as the propelling gasses. The resulting microstructures as well as the corresponding mechanical properties were studied. In addition, the effect of post-heat treatments was also investigated. It was found that for a given propelling gas used, the coating porosity level remained relatively similar (about 2.4% for nitrogen and 3.6% for helium) regardless of the coating treatment (as-sprayed or heat treated). Visual inspection from SEM micrographs showed a higher fraction of inter-particle metallurgical bonds for nitrogen gas sprayed coatings heat treated at 1250°C for 1 hour due to some sintering effect. This significantly affected its tensile properties with an average resulting ductility of 24.7%.

This research systematically examines the effect of heat treatment on the microstructural properties of cold sprayed titanium coatings. Heat treatments were performed on as-sprayed coatings at 200, 400, 600, and 800°C for four hours under argon atmosphere. Vickers microhardness, microstructural investigation using FEG-SEM, structural characterization using XRD, and porosity evaluation using SEM image analysis were performed on as-sprayed and heat treated coatings. Results demonstrated that static recovery and static recrystallization may have occurred for heat treated coatings at 600 and 800°C. In addition, for the heat treated coating at 800°C, significant oxidation occurred and a slight decrease in porosity took place. Furthermore, a thin metallic layer characteristic of a solid solution or an intermetallic compound, was found at the coating/substrate interface.

This study reports on the effect of combined pulsed laser ablation and laser pre-heating surface pre-treatments to cold spraying Ti and Ti-6Al-4V on coatings’ microstructure, bond strength and cohesive strength. The Ti and Ti-6Al- 4V coatings were sprayed on pure titanium and Ti-6Al-4V substrates, respectively. Coatings were characterized by SEM and porosity level was evaluated through image analysis. Bond strength was evaluated by standard ASTM C633 pull tests and by the laser shock (LASAT) technique. Cohesive strength was evaluated by the cross-section scratch test method. Results show that among the spray conditions used in this study, laser pre-treatment yielded high bond strength (such that all cases had higher cohesive strength than the epoxy glue). The LASAT technique provided a means to evaluate the influence of the laser ablation energy density and the laser pre-heating temperature. For both Ti and Ti-6Al-4V coatings, surface pre-heating increased the coating bond strength to the substrate. The laser ablation process would either increase or decrease the bond strength of the coating to the substrate depending on the laser energy density. The laser energy density needs to be adjusted as a function of the surface pre-heating temperature in order to optimize bond strength improvement. Coating cohesion did not improve with continuous laser pre-treatment in-between passes. However, the laser pre-heating helped reduce the coating porosity.

This paper reports on the influence of the He to N 2 ratio on the properties of low pressure cold sprayed titanium coatings and on the characteristics of the generated supersonic two-phase flow. Experiments were carried out varying the He to N 2 concentration ranging from pure He to pure N 2 . Samples were characterized by their microstructural properties (i.e. microhardness and porosity). Deposition rate was evaluated and particle velocities were measured for all conditions. Deposition efficiency, coating density, and microhardness were found to be a function of particle impact velocity. Velocity data were used to validate a computational fluid dynamic model. The numerical solution of the flow inside the nozzle was obtained from the Euler equations for the various He to N 2 concentrations. Particle tracking was carried out by using the computed distribution of density, Mach number, temperature, viscosity, and a second order Runge-Kutta scheme. In addition, mean particle velocities at the exit of the nozzle were determined. Computed velocities were found to be in good agreement with measured ones. The model was then used to calculate nozzle dimensions that would maximize particle velocity. Optimized dimensions are proposed.

The cold spray, for its peculiarity, is becoming increasingly in the reconstruction or repair of damage aluminium alloy components, especially in the aviation industry. Both thin (<0.5mm) and thick (up to centimeters) coatings are necessary in order to achieve dimensional recovery of the components. Contrary to thin, thick coatings can be deposited in single-pass or in multi-pass giving different thermal and stress contribution to the components and coatings itself. The thermal input, the amount and the type of residual stresses (compressive or tensile) confer appreciable or depreciable characteristics to the coatings adhesion, the crack propagation and the coating mechanical property. In this study two sets, single and multi-pass aluminium alloy coatings of different thickness are deposited into Al6061 substrate. The metallographic analysis by electronic and optical microscopes, the four-point bending test and the Vickers microhardness are performed; also the multi-pass coatings were characterized by fractographic analysis. Finally the different coating adhesions to substrate and cohesions are compared by standard ASTM C633 adhesion and cohesion tests.

This study investigated the effect of the type of gas used, nitrogen and helium, during cold spraying of titanium coatings. In all conditions, the propelling gases’ temperature and pressure were attuned to attain three similar particle velocities for each gas. Coatings were characterized by SEM and XPS. Deposition efficiency, coating microhardness, and porosity were evaluated for all conditions. Results show that for the same particle impact velocity, the deposition efficiency and coating density were mostly the function of the surface temperature, which in turn was influenced by spray parameters. It is shown that loosely-bonded particles at the surface can be detached by the passage of high pressure supersonic gas stream. In addition, a thick and fully dense cold sprayed titanium coating was achieved with optimized spray parameters with He and the corresponding average particle velocity was measured at 1173 m/s.

In this work, the influence of the substrate temperature on the deposition efficiency and on the coating properties and residual stress was investigated. Pure Al coatings were deposited on Al 6061 alloy substrates using CGT Kinetics 3000 deposition system. The substrate temperature was ranged between 20°C (room temperature) and 375 °C and was kept nearly constant during the deposition while all the other deposition parameters were unchanged. The deposited coatings were quenched in water (within one minute from the deposition) and then characterized. The residual stress was determined by Almen gage method (Ref 1, 2, 3), Modified Layer Removal Method (Ref 4, 5, 6), and XRD (Ref 7) in order to identify both the mean coating stress and the stress profile through the coating thickness from the surface to the coating- substrate interface. The residual stress results obtained by these three methods were compared and discussed. The coating morphology and porosity were investigated using optical and scanning electron microscopy.

Cold gas dynamic spray (CGDS) utilizes a supersonic gas jet to accelerate fine solid powders above a critical velocity at which particles impact, deform plastically, and bond to the substrate material in the ambient environment. This process is potentially beneficial for thermal barrier coating (TBC) bond coat deposition because it would avoid oxidation of the feedstock powder that normally occurs when higher temperature thermal spray processes are employed. Therefore, there would be no prior aluminum depletion in as-deposited bond coats produced by the CGDS technique. This paper presents the oxidation behaviour of a TBC with CGDS-produced CoNiCrAlY bond coat, in comparison with TBCs with APS- and HVOF-CoNiCrAlY bond coats. Oxidation behaviors of these TBCs were evaluated in terms of microstructural evolution, kinetics of thermally-grown-oxides (TGO), as well as cracking behaviour during thermal exposure at 1050 °C.

This paper describes and evaluates the performance of a Helium Recovery System (HRS) designed for cold spraying. A flexible, automated, full scale HRS system has been designed and installed in the McGill Aerospace Materials & Alloy Development Center Cold Spray Facility, located at and in collaboration with the National Research Council of Canada. The fully automated HRS has been designed to recover helium from the cold spray chamber with sufficient purity (>99%) and flow capacity (5 to 220 Nm 3 /h), allowing for a cost-effective operation by insuring a recovery rate of above 85%. In addition, a comparison of titanium coating properties obtained by using both He and N 2 as propellant gas is presented.

Cold spraying is particularly suitable for elaborating heat and oxidation sensitive coatings. Due to the fact that the particles are not melted during the spraying process, it is thus possible to elaborate coatings without chemical modifications. Nevertheless, according to the materials considered, some interface defects can be detected inducing an inadequate adhesion between the substrate and the coating. Bonding mechanisms are not only strongly dependent on the particle velocity but also on the state of the surfaces. By this way, surface pre-treatments can be necessary to improve adhesion. From all the surface modification technologies, laser ablation process is very interesting due to its flexibility by using optical fibers and due to the perfect control over the treated area. It is then possible to interact with the material during all the spraying process on the substrate surface as well as on the interface layers. This is particularly the aim of this study which consists in exploring the laser influence, implementing the PROTAL process, on the different interfaces quality for coatings elaborated by cold spray on metallic substrates. By controlling the chemical composition of the materials, the coating cohesion as well as the adhesion level, coatings were sprayed on pure titanium and titanium and nickel based alloy substrates.

This work investigates the influence of nitrogen gas pressure and temperature on the structure and properties of cold sprayed titanium coatings. Two guns were used to assess the effect of impinging particle temperature. Particle speed was measured and used to calculate critical velocity for selected experimental conditions. The results show that increasing the temperature and pressure of the gas propellant reduces coating porosity and increases hardness, flattening ratio, and deposition efficiency. At the maximum pressure and temperature (40 bar and 800 °C) for nitrogen gas, coating density was close to the value reported for cold sprayed titanium produced using helium as the propellant.

In the present work, pure Al and Al-Al 2 O 3 composite coatings are deposited by cold spraying while measuring in-flight particle velocities. Residual stresses, evaluated using the Almen curvature method, X-day diffraction, and modified layer removal, are correlated with particle velocity, coating thickness, and alumina content. Peening stresses due to plastic deformation were estimated to be less than 100 MPa and are shown to be nearly constant through the thickness of the coatings.

In this study, fine aluminum powder was cold sprayed onto aluminum substrates, some of which were polished, some grit blasted, and some pretreated using a nano-pulsed Nd:YAG laser. In the latter case, the laser is coupled with the cold spray gun and the irradiation treatment occurs just prior to deposition. To better understand the interaction mechanisms involved with laser pretreating, coating-substrate interfaces were examined on thin-foil specimens and adhesion strength was determined by laser shock testing. The results show that substrate pretreatment with a nano-pulsed laser significantly improves the coating-substrate interface as well as coating adhesion.