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The powder microstructure and morphology has significant influence on the cold sprayability of Ti6Al4V coatings. Here, we compare the cold sprayability and properties of coatings obtained from Ti6Al4V powders of spherical morphology (SM) manufactured using plasma gas atomization and irregular morphology (IM) manufactured using the Armstrong process. Coatings deposited using IM powders had negligible porosity and better properties compared to coatings deposited using SM powders due to higher particle impact velocities, porous surface morphology and more deformable microstructure. To evaluate the cohesive strength, multi-scale indentation was performed and hardness loss parameter was calculated. Coatings deposited using SM powders exhibited poor cohesive strength compared to coatings deposited using IM powders. Images of the residual indents showed de-bonding and sliding of adjacent splats in the coatings deposited using SM powders irrespective of the load. Coatings deposited using IM powders showed no evidence of de-bonding at low loads. At high loads, splat de-bonding was observed resulting in hardness loss despite negligible porosity. Thus, while the powders from Armstrong process lead to a significant improvement in sprayability and coating properties, further optimization of powder and cold spray (CS) process will be required as well as consideration of post-annealing treatments to obtain acceptable cohesive strength.

The poor wear resistance of titanium and its alloys often results in galling and high wear rates. One method to improve their wear resistance is by hard secondary phase reinforcement to create a metal matrix composite, which can be utilized in the bulk or as a coating. In the present study, Ti6Al4V coatings and Ti6Al4V-TiC metal matrix composite coatings were deposited on mild steel substrates using cold spray process and their dry sliding wear behaviour was studied over a static temperature range of 25-575 º C. Tests were performed using a unidirectional ball-on-disc tribometer with WC-Co sphere as the counterface at a load of 2.5 N and sliding velocity of 2.1 cm.s -1 . Wear rate and coefficient of friction (CoF) decreased with an increase in temperature and composite coatings exhibited higher wear resistance at all temperatures. Below 200 º C, abrasive wear characterized by ploughing by wear debris resulted in high wear of Ti6Al4V coatings, whereas formation of tribolayers led to lower wear of composite coatings. At elevated temperatures (> 200 º C), oxide glaze layers formed on both coatings were composed of WO3, TiO2 and CoWO4. Electron channel contrast imaging of the wear track cross-sections showed no splat debonding, less extent of recrystallization and larger grain sizes (at 575 º C) in composite coatings compared to Ti6Al4V.

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The adhesion strength and microstructure of single splats significantly influence the properties of cold sprayed coatings. Here, we compare the adhesion strength and microstructural changes of Ti6Al4V spherical powder (SP) particles with a martensitic microstructure and irregular powder (IP) particles with an equiaxed microstructure deposited by cold spray (CS). Splat adhesion tests were performed to determine the adhesion strength and electron channelling contrast imaging was done for microstructural analysis of splat cross-sections. IP splats formed a continuous bonded interface with the substrate resulting in a greater adhesion strength when compared to SP splats.IP splat cross-sections revealed ultrafine grains (UFG) near the interface followed by a highly deformed microstructure. SP splat cross-sections also showed UFG at the interface but largely retained the initial microstructure in the top portion of the splat due to poor deformability. The irregular morphology of the IP led to more adherent deposits while the equiaxed microstructure resulted in highly deformed post spray microstructures.

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Ti6Al4V-TiC metal matrix composite (MMC) coatings were cold sprayed using spherical and irregular Ti6Al4V powders manufactured with plasma gas atomization and the Armstrong process, respectively. Composite coatings deposited using irregular powders showed higher ceramic retentions and lower porosity compared to spherical powders coatings. Sliding wear tests were performed at two different normal loads using a spherical WC-Co ball as counterface. Wear test results showed that at similar ceramic contents, spherical powder (SP) composites exhibited abrasive wear mechanisms with high coefficients of friction (CoF) and wear rates. In irregular powder (IP) composites, islands of tribolayers, comprised of fragmented TiC and TiO2 particles, resisted the localized shear deformation leading to low wear, and with free carbon from the TiC particles also lowered the CoF. Increase in ceramic content in the IP MMCs to 23% led to higher coverage of wear track area with tribolayers and further lowered the wear rate. At higher load, 23% TiC composite coatings showed a more continuous tribolayer, whereas lower ceramic content MMCs showed no significant formation of tribolayers. The formation of a highly continuous tribolayer at higher load (in 23% TiC MMC), led to extremely low CoF (~ 0.25) compared to other coatings along with low wear rate. Electron channel contrast imaging and transmission electron microscopy analysis of the worn subsurface under the tribolayers showed coarse-grain microstructures compared to the bare regions of the wear track (i.e. without tribolayer coverage).The formation of coarser grain microstructures indicated less stress transfer to the subsurface both due to the high hardness and lubricating nature of the tribolayers.

Single splats of commercially pure (CP) Ti are deposited onto sapphire by cold spray under two spray conditions to achieve different in-flight powder velocities. The powders used have two morphologies: spherical powder (SP), manufactured by plasma gas atomization and irregular powder (IP), manufactured by the Armstrong process, with a coral-like morphology. The adhesion strength of the single splats is measured by splat adhesion testing. By use of a specialized in situ scratch tester, interface failure during splat adhesion testing is observed through the sapphire substrate. Particle velocity does not significantly influence the adhesion strength and failure mechanism of SP splats. After deposition, the SP splat has an interface pore in its center which acts as an initiation site for crack propagation during splat adhesion testing. After failure, a well bonded portion of Ti remains on the substrate in the shape of a ring. IP splats deposited at low velocity show similar, well adhering, rings on the surface in localized locations scattered throughout the interface. An increase in velocity for IP splats led to an increase in adhesion strength and a nearly continuous well adhering interface. The behaviour of IP splats is understood by electron channelling contrast images of cross-sections where low velocities resulted in little change in microstructure while high velocities led to a highly deformed microstructure at the interface.