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.

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.

SiC-reinforced Al-12Si alloy coatings were produced using the Cold Gas Dynamic Spraying deposition process. Feedstock powder mixtures containing 20% and 30% vol. of particulate SiC were used. The composite coatings’ bond strengths and microstructures were evaluated, as well as the porosity and the SiC content. It was found that approximately 45% of the SiC particulate blended with the aluminum alloy was embedded in the coatings. The SiC was homogeneously distributed inside the Al-12Si matrix. Particle velocity measurements revealed that the addition of up to 30% vol. of SiC did not change the Al-12Si particle velocities.

The proposed work describes recent efforts to develop metastable Al-Fe-V-Si coatings for internal-combustion engine applications with higher mechanical properties at high operating temperatures. To do so, the metastable solid powder alloy was engineered from rapid solidification technique (namely gas atomization). The metastable alloy powder was deposited using the Cold Spray process in order to produce protective coatings on top of existing parts. The critical velocity, above which Cold Spray deposition takes place, was successfully achieved and Al-Fe-V-Si coatings were produced. The microstructure of the feedstock powder was retained in the coatings produced, showing the potential of the Cold Spray process to produce metastable coatings for internal combustion engine applications.

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.

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.

This work describes recent progress in Cold Spray of 2618 (Al-Cu-Mg-Fe-Ni) aluminum alloy with Sc addition. Atomized 2618+Sc aluminum powder was sprayed onto aluminum substrates. 2618Al alloy is used for intermediate to high temperature applications in aerospace and automotive industry. The addition of Sc provides higher thermal stability to this alloy by the formation of fine Al 3 Sc particles. The Cold Spray process allows the fabrication of refined microstructure when compared to conventional manufacturing techniques such as casting. The microstructure of the powder and the resulting coating was analyzed using scanning electron microscopy. The mechanical behavior of the powder and the coating was studied using micro indentation measurements. The influence of different heat treatment conditions on the coating was also analyzed. This work shows that Al-Cu alloys with a refined grain structure microstructure can be successfully produced by the Cold Spray process.

This work describes recent progress in cold spray deposition of nanocrystalline metals and alloy. Gas-atomized powders (Ni and Aluminum alloys powder) were mechanically milled under liquid nitrogen to achieve a nanocrystalline grain size of the order of 10-30 nm. The powders were subsequently sprayed using a nozzle designed with a validated numerical model for cold spray technology. The resulting coatings were evaluated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), micro- and nanoindentation. TEM analysis shows that the nanocrystalline grain structure of the cryomilled feedstock powder was retained after the cold spray process. A significant increase in hardness was observed for all the cryomilled material when comparing with the conventional gas-atomized powder. The ability to use cold spray to produce nanocrystalline large deposits with low porosity (<1%) was also demonstrated in this work. Abstract only; no full-text paper available.

Thermal cycle lifetimes of two thermal barrier coating (TBC) systems with the same plasma sprayed yttria-stabilized- zirconia (YSZ) topcoat but different low pressure plasma sprayed (LPPS) bond coats, conventional and cryomilled NiCrAlY feedstock powder, were studied. Thermal cycling tests consisted of 50 min at 1121C followed by 10 min air-cooling to room temperature. The coating produced with the cryomilled powder showed a 300% increase in lifetime when compared to the conventional one. Both TBCs failed as a result of delamination and spallation of the ceramic top coats. Several factors like thermally grown oxide (TGO) thickness, TGO composition, CTE mismatch, creep resistance of the NiCrAlY bond coat, and others that affected the thermal cycling life of the system, were analyzed in this work. Abstract only; no full-text paper available.

This work describes the synthesis and characterization of nanostructured coatings produced by low pressure plasma spraying (LPPS) of cryomilled NiTi powder. Ni and Ti powders (60 and 40 wt %, respectively) were cryomilled together and LPPS sprayed onto stainless steel substrates. The elemental powders reacted and alloyed during cryomilling forming a nanocrystalline grain structure with nanodispersed oxide and nitride phases. These nanodispersoids are formed due to contamination by the milling media (liquid nitrogen). After spray deposition, the coatings presented a nanostructured microstructure with enhanced mechanical properties when compared with conventional NiTi coatings sprayed under the same conditions. High hardness and toughness values together with intrinsic corrosion resistance of the NiTi alloy lead to the formation of an attractive coating material for applications where corrosion and wear resistance are required. The ability to synthesize the NiTi from elemental Ni and Ti powders and the refinement of the microstructure achieved during milling makes the cryomilling process together with thermal spray of the nanostructured NiTi coatings a unique process and coating to be used in engineering applications. Abstract only; no full-text paper available.

A mathematical model was developed and used to design and optimize a supersonic nozzle for a cold spray system. The objective was to spray 20 micron-size aluminum particles. Conventional and agglomerated nanostructured powders were used and successfully sprayed using a radial injection port. The microstructure of the coatings revealed that the nanocrystalline structure is preserved. An increase of 100% of the coating hardness was found for nanostructured coatings compared to conventional coatings. Further work needs to be done to improve the porosity of the coatings by changing some of the process parameter.

WC-Co has been extensively investigated for use in wear resistant coatings for engineering applications. In principal, when the WC particle size decreases in the starting powder, the decomposition of WC increases, and therefore, significant amounts of W 2 C and W 3 C, and even metallic phases, are observed in nanocrystalline WC-Co coatings. The reported increase in hardness of nanostructured materials is generally attributed to the significant decrease in grain size or particle size. However, the presence of brittle, non-WC phases in nanostructured WC-Co coatings leads to sliding and abrasive wear by removal of large plates of the coating. Concurrently, the greater degree of decomposition suffered by the nanostructured powder during spraying leads to a reduction in the volume fraction of the wear-resistant primary WC phase. For the reasons presented above, the present efforts are directed towards the synthesis of a wear-resistant coating using a multimodal WC size distribution of particles in the starting powder. The multimodal distribution is characterized by small WC particle(~50 nm) and coarse WC particles (1.7µm). In addition, the distribution of Co also spanned an order of grain size, hence the name multimodal. The coatings were deposited using HVOF technology.

Ni powders prepared by mechanical milling under liquid nitrogen for 15 hr were sprayed using two stoichiometric ratios of the oxygen-fuel mixture in an effort to promote the formation of fine oxide phases. The oxide phases were introduced in an effort to improve mechanical properties and thermal stability of the coatings, via chemical reaction between oxygen and milled powders during flight and after impingement. The microstructure and properties of the milled powders and as-sprayed coatings were characterized by scanning electron microscopy, transmission electron microscopy and nanoindentation. The average grain size of the milled powders was 15.7 ± 5.1 run and ultrafine NiO and Ni 3 N particles with a size less than 5 run were distributed in the milled powders. These fine oxide and particles distributed in the powders were formed as a result of interaction between Ni, N from the milling slurry, and O from the surrounding environment under the energetic milling conditions. The coating microstructure was composed of nanocrystalline grains with an average grain size of 92.5 + 41.6 nm and extremely fine NiO particles of ~5 nm distributed homogeneously inside the grains. Ni 3 N phase was not found in the coating as it appears to have decomposed during HVOF thermal spraying. The coating sprayed with higher oxygen fraction in a hydrogen-oxygen mixture showed no significant increase in hardness and elastic modulus when compared to those of the coating sprayed with lower oxygen fraction in hydrogen-oxygen mixture. This was attributed to the small difference in the volume fraction of NiO particles between the coatings. These results indicate that new techniques of ultrafine dispersoid introduction in nanocrystalline coatings are potentially attractive as a means to improve the mechanical properties of the coating through reactive HVOF spraying.