As a result of the rise in processing power demands of today’s personal computers, water cooled pin fin heat sinks are increasingly being employed for the cooling of graphical processing units. Currently, these high performance devices are manufactured through high-cost, high-waste processes. In recent years, a new solution has emerged using the cold gas dynamic spray process, in which pin fins are directly manufactured onto a base plate by spraying metallic powder particles through a mask. This process allows for a high degree of adaptability to different graphics processing unit shapes and sizes not achievable by any other process to date. One drawback of this new additive manufacturing process is reduced deposition efficiency, resulting in a fair portion of the feedstock powder being wasted as substrate sensitivity to heat and mechanical residual stresses requires the use of reduced spray parameters. This work aims to demonstrate the feasibility of using powder recycling to mitigate this issue and compares coatings sprayed with reclaimed powder to their counterparts sprayed with as-received powder. In so doing, cold gas dynamic spray is shown to be a highly flexible and economically competitive process for the production of pin fin heat sinks even when spray parameters result in reduced deposition efficiency.

This work compares the oxidation behaviour of CoNiCrAlY coatings manufactured by the HVOF, APS and CGDS processes when submitted to temperatures of 1000°C and 1100°C. The as-sprayed coating microstructural features were characterised using SEM and XRD analysis before being subjected to isothermal heat treatments in an air furnace. Oxide scale composition was determined using XRD, SEM and EDS analysis while the oxide growth rates were obtained using mass gain measurements. The as-sprayed HVOF and CGDS coatings exhibit similar microstructures while the APS samples have a significantly higher porosity and oxide content levels. Results from the oxidation experiments indicated that the oxide growth rate of HVOF and CGDS were lower than that of the APS samples. The results also indicated that samples oxidised at 1100°C have a lower oxide growth rate than those oxidised at 1000°C. Analysis of the oxidation process up to 100 hours indicates that the formation of dense α-Al 2 O 3 is more favourable at 1100°C while a transition alumina, θ-Al 2 O 3 is more favourable at 1000°C. Furthermore, the surface profile of the samples oxidised at 1100°C were more uniform and free of protrusions.

Heat exchangers play a vital role in ongoing efforts to conserve energy. Plate-type heat exchangers typically consist of two flat separated flow paths in which heat transfer enhancing matrices are inserted. The combined effects of small irregular hydraulic diameters along with elevated heat transfer areas results in highly-efficient heat transfer to the external fluid. This allows for very versatile and compact heat exchanger designs. Typical plate-type heat exchanger fabrication methods such as brazing are labour intensive and limit post-processing operations like welding. In this paper, a novel micro-heat exchanger fabrication method using recently patented technologies is presented. The approach uses thermal spray processes such as Pulsed Gas Dynamic Spraying (PGDS) as an alternative to brazing for the production of a pressure barrier and integration of flow headers. Mesh wafer surfaces sealed using PGDS

This paper examines the oxidation behaviour of CoNiCrAlY coatings manufactured by APS, HVOF and CGDS deposition techniques when subjected to isothermal heat treatments. Comparison of the as-deposited coating microstructures is achieved by means of scanning electron microscopy (SEM). Investigation of the oxide compositions and growth dynamics is achieved by SEM, X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). Oxide growth rates were determined by a series of mass gain measurements. Results from this study demonstrate that lower coating porosity and oxide contents lead to lower oxide growth rates. Results also demonstrate that low-temperature processing of CoNiCrAlY bond coats is beneficial to their oxidation behaviour as it favours the formation of alumina in preference to other detrimental fast-growing mixed oxides.

This study is part of an ongoing effort to develop high performance CoNiCrAlY bond coats by means of cold gas dynamic spraying in order to have complete control of bond coat microstructure. As such, the objective of the work presented in this paper is to perform a detailed analysis of cold sprayed CoNiCrAlY layers to assess any changes in microstructure, relative to the original feedstock powder, that may have occurred during layer build-up and compaction. Based on SEM, TEM, and XRD examination results, it is shown that, contrary to the generally accepted theory, important microstructural changes occur during the cold spray deposition process. In the coating samples examined, investigators observed evidence of grain refinement in the CoNiCr γ-phase matrix and partial dissolution of NiAl β-phase precipitates. These changes are a result of severe plastic deformation due to the high-velocity impact of sprayed particles.

Aluminum-based composite coatings reinforced with different volume fractions of SiC particles were deposited on aluminum substrates by means of pulsed gas dynamic spraying using a mechanically mixed composite feedstock powder. Microstructural features of the coatings are examined and their hardness is reported. The results show that the high fraction of SiC particles in the feedstock powder are retained in the coatings and that increasing SiC content in the aluminum matrix significantly improves coating hardness. The highest hardness value was obtained for a coating with 28 vol% SiC. Beyond that, coating hardness decreased, which is attributed to increasing porosity and decreasing cohesion between deposited aluminum-based particles.

A new process, the Pulsed-Cold Gas Dynamic Spraying process is presented in this work. A description of the process and of the experimental set-up developed at the University of Ottawa Cold Spray Laboratory is presented. It is envisioned that this process could allow for the feedstock particles to be accelerated to high impact velocities and intermediate temperatures, in a non-reacting gas. That way, the intermediate particle impact temperature would lead to a lower critical velocity as compared with CGDS that could be easily reached while preserving the chemical and microstructural composition of the feedstock particles in the coating. Selected examples of the variety of coatings produced with the system are presented, illustrating the potential of the process to deposit various materials as well as its main benefits.

Thermal barrier coatings (TBCs) with nanostructured bond coats have shown significant thermal cycling enhancements over their conventional microstructured counterparts; however, the high cost inherent to the cryomilling processing of the MCrAlY powder limits commercial application. Hence, this study characterizes and evaluates nanostructured bond coats derived from non-cryogenically milled MCrAlY powder with emphasis placed on reduced processing cost and scale-up capability. After extensive development of both a high-energy planetary mill and its operating parameters, fine-grained equiaxed NiCrAlY powder has been produced. XRD and SEM characterization of the milled powder will be presented. Microstructural analyses of the coatings sprayed via the HVOF and cold spray processes will also be carried out, in addition to some preliminary static oxidation test results of the conventional and milled NiCrAlY.

Due to their mechanical properties, WC-based cermet coatings are extensively used in industrial wear-resistant applications. These coatings are usually prepared using thermal spray processes. However, due to the nature/environment of the spraying processes, the feedstock powder structure and properties suffer from severe decomposition, which subsequently degrade the performance of the coatings produced. The cold spray process appears to be a promising process alternative to preserve the properties of the feedstock powder during the coating preparation. Although, the later technique can eliminate or minimize the degradation of the material, the deposition of cermet is a complex process. In this study, nanocrystalline WC-15Co and conventional WC- 10Co4Cr coatings were deposited using both the continuous and the pulsed cold spray processes. Microstructures of the feedstock powders and the coated layers were investigated by OM, SEM and XRD. The results revealed the possibility of depositing cermet coatings onto aluminum substrates by both processes without any degradation of the phase composition of the feedstock powder. However, the continuous process experienced difficulty in depositing and building up dense coatings without major defects. Meanwhile, the new pulsed process revealed the capacity of depositing thick cermet (conventional and nanocrystalline) coatings with low porosity as long as the feedstock powder was preheated above 573 K.

This paper describes recent effort to synthesize Fe-based amorphous alloys coatings using Cold Gas Dynamic Spraying. Characterization of the gas atomized Fe-based (Fe-Cr-Mo-WC-Mn-Si-Zr-B) powder shows that fully amorphous powder is found when particle diameter is below 20 µm. The coatings produced were composed of the same microstructure as the one observed in the feedstock powder. The overall deformation suggests the occurrence of a localized deformation process at the particle/particle boundary and possible adiabatic deformation softening inside the powder particles during splat formation. The influence of the substrate material on the coating deposition process was also investigated. The synthesis of fully amorphous, porous free coatings using Cold Spray was demonstrated in this work.

Aluminum alloy powders of different compositions and phases, Al/B 4 C, Al-Co-Ce, and Al 5083, were sprayed using the Cold Spray deposition process. The resulting coatings and the effects of several process parameters were evaluated using scanning electron microscopy and bond strength tests. The results show that the bond strengths depend on the powder composition but do not vary significantly with the powder feed rate. Adhesion strength values were obtained for Al/B 4 C and Al 5083 coatings. The Al-Co-Ce coatings failed at the coating-adhesive interface, indicating a superior adhesion strength than what was achieved in the bond strength tests.

Nickel based alloys used in coating applications have been the focus of many studies, particularly in the aerospace industry. Their inherent corrosion and oxidation resistant properties have made them especially attractive for use as the metallic bond coat found in thermal barrier coating systems. Cold Spray is an emerging coating technology in which fine powder particles are accelerated in a supersonic flow and then deposited onto a substrate by means of plastic deformation. In this study, conventional CoNiCrAlY coatings and nanocrystalline nickel coatings are produced using the Cold Spray deposition technique. The coating quality is evaluated using scanning electron microscopy as well as porosity and microhardness measurements.

The cold spray process is a relatively new coating process by which protective coatings have been successfully applied. In this process, fine powder particles are accelerated in a supersonic gas jet towards a substrate. Experimental investigations show that successful bonding of the particles to the substrate is achieved only above a critical particle velocity at which point the particles possess sufficient kinetic energy to plastically deform upon impact. Consequently, one of the most important parameters in the cold spray process is the particle velocity prior to impact. In the present investigation, the Particle Image Velocimetry (PIV) technique have been implemented to a cold spray setup. The objective was to quantify the effects of the powder feed rate on the particle velocity. Fine metal particles were cold sprayed and in-flight particle velocities were measured using the PIV technique. This paper will discuss the implementation of the systems and present results illustrating the effects of powder feed rate on the particle in-flight velocity. Abstract only; no full-text paper available.

Cold Gas Dynamic Spray is a line of sight, high rate material deposition process that uses a supersonic flow to accelerate small particles (micron size powder) above a material dependent critical velocity. When the particles impact the substrate, they plastically deform and bond to form a coating. The objective of this research is to investigate the influence of the particle mass flow rate on the properties of coatings sprayed using the Cold Spray process. Varying the mass flow rate at which the feedstock particles are fed into the carrier gas stream can change the thickness of the coating. It was shown that poor coating quality (peeling) was not a result of flow saturation but instead, excessive particle bombardment per unit area on the substrate. This can be overcome by increasing the relative velocity between the substrate and the spraying nozzle. The results are dense coatings, even at higher powder mass flow rates.

Cold Gas Dynamic Spraying is a relatively new high rate deposition process that uses a supersonic gas flow to accelerate fine powder particles (micron size) above a critical velocity. Upon impact, the particles deform plastically and bond to the substrate to form a coating. In this study, nanocrystalline Al-Mg coatings are produced using the Cold Spray technology. In an attempt to improve the understanding and optimize the process, the effects of substrate preparation and substrates thickness on the overall quality of the coatings are investigated. Two different grit materials are used to prepare the substrates with simple grit-blasting. Results show that the use of different grit sizes leads to changes in the mass deposited on the substrate (deposition efficiency) but has no significant effect on the coating microstructure. Other trials are conducted on samples of different thickness to verify the applicability of the Cold Spray process on thin surfaces. Results show that the Cold Spray process can be used to produce coatings on thin surfaces without noticeable damage to the substrate and with the same coating quality.