Eric Irissou

Effect of Particle Morphology and Size Distribution on Cold-Sprayed Pure Titanium Coatings

The effects of commercially pure titanium particle morphology (spherical, sponge, and irregular) and size distributions (mean particle sizes of 20-49 μm) on the cold spray process and resulting coating properties were investigated. Numerous powder and coating characterizations were performed including: powder oxygen and nitrogen contents, powder flowability, powder compressibility, coating microhardness, coating porosity, LOM/SEM analyses, and XRD. Compared to spherical powders, the sponge and irregular CP-Ti powders had higher oxygen content, poorer powder flowability, higher compression ratio, lower powder packing factor, and higher average particle impact velocities. XRD results showed no new phases present when comparing the various feedstock powders to corresponding coatings. A higher particle temperature was also obtained with larger particle size for all feedstock powder morphologies processed with the same set of spray parameters. A spherical powder with 29 μm mean particle size was found to have the lowest porosity coating and best cold sprayability. The relationships of several as-cold sprayed coating characteristics to the ratio of particle impact and critical velocities were also discussed.

The 2016 Thermal Spray Roadmap

Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.

Cold Spray Characteristics of Commercially Pure Ti and Ti-6Al-4V

Ti and Ti-6Al-4V coatings were deposited by cold gas dynamic spray process using nitrogen as propellant gas. For Ti, the inlet gas temperature and pressure were varied for two different powder morphologies in such a way that the average particle velocity ranged from ~600 to 850 m/s. In addition, the nozzle traverse speed was varied. For all conditions, the deposition efficiency, the porosity, and the microhardness were measured. It is shown that the porosity level decreased as the gas temperature and pressure increased, whereas the velocity, deposition efficiency, and microhardness increased. Furthermore, it is observed that a lower nozzle traverse speed engendered a softer coating. The coating adhesion on a grit 24 Al2O3 blasted mild steel surface was established to be greater than 78.8 MPa. In addition, a Ti-6Al-4V coating was produced and was determined to be slightly more porous compared to a CP Ti coating.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of Stainless Steel 316L Coatings Produced by Cold Spray for Biomedical Applications

In this study, the effects of heat treatment on the microstructure and mechanical properties of cold sprayed stainless steel 316L coatings using N2 and He as propellant gases were investigated. Powder and coating characterizations, including coating microhardness, coating porosity, and XRD phase analysis were performed. It was found that heat treatment reduced porosity, improved inter-particle bonding, and increased ductility. XRD results confirmed that no phase transformation occurred during deposition. Significant increase in UTS and ductility was observed for the annealed specimens obtained with nitrogen propellant, whereas little changes were observed for the helium propellant produced specimen.

Third Body Behavior During Dry Sliding of Cold-Sprayed Al-Al2O3 Composites: In Situ Tribometry and Microanalysis

Abstract Reciprocating sliding wear experiments were conducted on cold-sprayed pure aluminum and Al–22.6 wt% Al2O3 coatings using a custom-built in situ tribometer. Using a transparent sapphire counterface for the wear tests, the dynamic behavior of third body material in the contact was optically observed. The presence of Al2O3 particles led to greater stability of the transfer films adhering to the sapphire counterface, as well as greater stability of the friction coefficient and lower wear rates. Ex situ microanalysis of material in the wear tracks and transfer films suggests that the presence of Al2O3 particles promoted strain localization during sliding. This produced more uniform third body microstructures and protected the underlying aluminum matrix from deformation, which slowed the rate of transfer to the counterface.

Effect of Pulsed Laser Ablation and Continuous Laser Heating on the Adhesion and Cohesion of Cold Sprayed Ti-6Al-4V Coatings

The individual and cumulative effects of in situ pulsed laser ablation and continuous laser pre-heating on adhesion and cohesion strength of cold sprayed Ti-6Al-4V coatings are investigated. Laser beams were coupled to a cold spray gun in order to ablate and pre-heat the substrate surface a few milliseconds prior to the impact of the spray particles. Cohesion and adhesion strength were evaluated by scratch test, standard ASTM C633 pull test and laser shock (LASAT) technique. The effects of laser ablation before and during cold spray operations were investigated. Results demonstrate that laser ablation of the substrate before cold spraying led to a smooth surface which improved adhesion strength. However, when laser ablation was maintained throughout the cold spray process, i.e., in between the coating layers, a reduction of cohesion and adhesion was observed. These negative effects were circumvented when laser ablation and laser pre-heating were combined.

Tribological Characterization of Plasma-Sprayed CoNiCrAlY-BN Abradable Coatings

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 (SIRs). These tests allowed determining the rubbing forces and quantifying the blade and seal wear characteristics for slow and fast SIRs. 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.

Gas-Substrate Heat Exchange During Cold-Gas Dynamic Spraying

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 2D temperature distribution equation for the substrate to produce nondimensional relationships between the surface temperature and the radius of the impinging fluid jet, the axial velocity of the cold spray nozzle, the substrate thickness, and the heating time. It was found that a single profile of the transient nondimensional 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. Heat exchange between the substrate and the compressed gas jet during motion of the nozzle to produce heat conduction within the substrate was characterized by the nondimensional Peclét number. It was found that lower Peclét numbers produced higher temperatures within the substrate. The close agreement of the numerical results with the experimental results suggests that the nondimensionalized results may be applied to a wide range of conditions and materials.

Powder Processing and Coating Heat Treatment on Cold Sprayed Ti-6Al-4V Alloy

This study investigates the effect of powder processing on powder flowability, compact ability, and the heat treatment of the resulting coatings on the mechanical properties of cold gas dynamic sprayed Ti-6Al-4V alloy. Nitrogen gas was used throughout the coating deposition process. Propellant gas temperature and pressure were attuned to maximize particle impact velocity. Three powder processing conditions were used in this study: as received (AR), low-energy ball milled (BM), and argon atmosphere heat treated (HT). Results showed coating porosities of around 6 to 7%, regardless of the feedstock powder used or the heat treatment performed. It was observed at 600 and 800°C anneals that a coating hardness reduction occurred, possibly due to static recovery and recrystallization, with minor sintering possibly occurring at the 800°C anneals. In addition, micro tensile tests showed an increase in cohesion strength at higher heat treatment temperatures.

Lasers and Thermal Spray

Basically, thermal spray and laser processing can be considered as half brothers since they show many common features due to the use of a (more or less) high-energy source for both. Their combination can therefore be very fruitful and prominent to achieve coatings, which results in their most recent and advanced applications. In the materials processing development story, the laser will thus have moved from cutting to coating. This keynote presentation focuses on the recently-developed coupling of laser processing to cold spray). In this dual process, a cold spray gun is combined to a laser head in a single device, e.g. on a robot. Series of coating experiments using various laser irradiation conditions, primarily pulse frequency, were carried out for Al-based and Ni-based alloys. Laser pre-treatment of the substrate just prior to cold spray, was shown to be beneficial for adhesion of cold-sprayed coatings. Adhesion improvement was exhibited and studied from LASATesting (LASAT for “LAser Shock Adhesion Test”). Incidentally, through LASAT also, the role of lasers in the development of thermally-sprayed coatings can be considered as major. Results are discussed in the light of a TEM (Transmission Electron Microscope) study of the coating-substrate interface with and without laser pre-treatment.