ASTM C633 has been an industry standard for determining thermal spray coating adhesion and cohesion strengths for nearly 40 years. The test, however, has several drawbacks that can greatly affect the results. The epoxies used cannot withstand stresses greater than 15,000psi, producing data that may suggest coatings cannot function beyond the epoxy threshold under uniaxial tensile loading, resulting in data that can only be used for general quality control or acceptance testing. Previously published data shows coatings functioning beyond C633 limits, yet there is no standardized test to show true functional stress limitations. A four-point bend test method with an instrumented strain-gage has been used to show coating adhesion well beyond the yield point of the steel substrates and beyond the C633 limits for three materials and thermal spray processes: electric arc sprayed aluminum bronze, plasma sprayed alumina, and HVOF WC/Co/Cr. A strain-gage is applied to a prepared coating surface on a bend bar and loaded under tension or compression. The MTS universal load frame force data is used to calculate the stress at the coating/substrate interface by beam theory equations, allowing for stress and strain vs displacement curves to be generated and directly compared against C633 data for coating adhesion strengths. The resulting data can indicate microscopic coating behavior (cracking, de-bonding) as a result of the test sensitivity and can ultimately be used as design data for the practicing engineer.

Using cold spraying (CS), a surface layer with a modified microstructure and enhanced mechanical properties was formed on a 3.2 mm thick friction stir welded (FSW) AA2024-T3 joint. The combined effect of “shot peening effect (SPE)” and “heat flow effect (HFE)” during CS were used to enhance joint mechanical properties. The microstructure evolution of the FSW AA2024-T3 joints in the surface layer following CS coatings and their effect on mechanical properties were systematically characterized with electron back-scattered diffraction, transmission electron microscopy, differential scanning calorimetry and mechanical tests. Based on these experiments, a grain refinement, finer and more S phases, and improved amount of Guinier-Preston-Bagaryatsky (GPB) zones produced by CS treatments are proposed. The deposition of aluminium coating on the joint, lead to hardness recovery in the stir zone and the development of two low hardness zones as the density of GPB increased. The tensile properties of FSW AA2024-T3 joints improved with the application of the aluminium coatings. Experiments and analysis of the enhanced mechanical properties mechanism indicate that SPE with a high plastic deformation and HFE with an intensive heat flow are necessary for the production of refined grains and increased numbers of GPB zones.

Presently one of the most important tendencies is the use of tungsten (W) monoblock material for the first wall and other plasma facing components (PFCs) in tokamak. The use of low Z materials such as B 4 C for protection of PFCs is a conventional method to decrease heavy impurity influx into tokamak plasma. This study involves the fabrication and characterization of inductively coupled plasma (ICP) thermal sprayed B 4 C coating on tungsten monoblock. Thickness of the coating was about 120μm. Surface morphology of the coating is presented with scanning electron microscope and metallographic microscope analyses. X-ray diffraction analysis and X-ray photoelectron spectroscopy showed that the main phase and chemical composition of the coatings were preserved when compared with that of the initial B 4 C powder. Adhesion test result revealed that the adhesion/cohesion strength of the coating was above 13.1 MPa. This work is innovative not only for the ICP thermal sprayed method for the B 4 C coating fabrication but for the plasma sprayed B 4 C on tungsten substrate.

Fatigue crack growth in self-standing plasma sprayed tungsten and molybdenum beams with artificially introduced notches subjected to pure bending was studied. Beams width, thickness and length was 4 mm, 3 mm and 32 mm respectively. Fatigue crack length was measured using the differential compliance method and fatigue crack growth rate was established as a function of stress intensity factor. Unusual crack opening under compressive loading part of the cycle was detected. Fractographic analysis revealed the respective crack formation mechanisms. At low crack propagation rates, the fatigue crack growth takes place by intergranular splat fracture and splat decohesion for Mo coating. In W coating, intergranular splat fracture and void interconnection formed the fatigue crack. Frequently, the crack deflected from the notch plane being attracted to stress concentrators formed by porosity. At higher values of the stress intensity factor, the splat intergranular cracking become more common and the crack propagated more perpendicularly to the specimen surface.

In this study, the mechanisms responsible for enhancing the adhesion strength of thermally sprayed metallic coatings subjected to vacuum heat treatment were investigated using atmospheric plasma sprayed (APS) CoNiCrAlY coatings as an example. The formation of metallurgical bonding between the coating and the substrate, which determined the increase in the adhesion strength of the coatings, was studied by analyzing the effect of morphological changes of the oxide film in the coating. The results showed that during the vacuum heat treatment process, the oxide film formed during the coating deposition gradually broke down and subsequently shrank into round oxides. After vacuum heat treatment, the adhesion strength of the coating improved significantly, and there was a positive nonlinear relationship between the treatment time and the adhesion strength. The increase in the adhesion strength was caused by the formation of metallurgical bonding between the coating and the substrate. However, the prerequisite for the formation of metallurgical bonding was that the oxide film had to break during the vacuum heat treatment process. A thermodynamic 2D model based on the thermal grooving theory was proposed to explore the essential conditions for the breaking and shrinking of the oxide film.

Common issues such as ice formation on wind turbine blades and lightning strikes on airplanes can be mitigated by metallizing polymers and composites used on the outer surface of the component. Cold gas dynamic spray is a novel process that has the potential to be used for metallization of polymer and composite surfaces to produce electrically and/or thermally conductive components. In this study, mixed Cu-Zn and Al-Zn feedstock powders were deposited onto polypropylene and nylon-6 substrates to investigate the viability of metallizing nonmetallic surfaces using a commercially available low-pressure cold spray process. The behavior of the individual metallic particles upon impact on the polymers and the deformation of the substrate were characterized by coating the two feedstock powders onto a nylon-6 substrate over a wide temperature range. The Cu-Zn coating was deposited in thicknesses up to 1 mm onto the nylon-6 substrate using optimized parameters. To understand the deposition of the metallic powder onto the polymers, the process was modeled using computational fluid dynamics methods. The correlation of the gas and particle modeling with examination of the coating microstructure highlighted the major importance of the particle velocity during cold spray deposition.

The present study aims to elaborate the particle in-flight behavior during plasma spraying and its significance in determining the microstructure and mechanical properties of plasma sprayed yttria partially stabilized zirconia (YPSZ) based thermal barrier coatings (TBCs). The as-sprayed YPSZ coatings were characterized in terms of the defects (such as porosity, unmelted particles, cracks and micro-cracks), hardness, elastic modulus and fracture toughness. The results showed that the total defects percentage, porosity, unmelted particles and crack content were found to decrease significantly with the improvement of temperature of in-flight particles. The mechanical properties were associated with the microstructure of these coatings, such as total defects, porosity, unmelted particles and cracks. It was confirmed that the mechanical properties, including hardness, elastic modulus and fracture toughness, notably enhanced with the total defects, porosity, unmelted particles and cracks decreased. The SAPS (supersonic atmospheric plasma spraying) coatings sprayed at 3401 } 3.76 °C and 482 ± 2.18 m/s and a spraying distance of 100 mm possessed the lowest microstructural defects percentage and the most favorable mechanical properties among the 15 coatings.

Pure metal coatings have successfully been cold sprayed on to carbon fiber reinforced polymers (CFRPs) in previous studies at McGill University. As a means to improve coating conductivity for lightning-strike protection (LSP) purposes, coatings with mixed metal powders were sprayed. There is also the possibility of improving the deposition efficiency (DE) since single component tin coatings previously had a maximum DE of only 20%. The studied coatings were based on a mix of tin and aluminum powders, the latter being a metal commonly used in the aerospace industry for its lightweight properties. The different coatings were characterized and compared to results on pure tin coatings and on mixed Sn-Cu and Sn-Zn coatings. The DE was measured for different conditions and compared to those of previous studies. Mixing tin and aluminum powders is discussed and various mechanisms related to cold spraying mixed powders on CFRPs are explored.

Cold spray has been proved to be a viable method for metallization of polymers and polymer composites. It has been reported that the mechanism of cold spray on polymeric substrates is different from the conventional mechanism on metallic substrates (i.e. adiabatic shear instability). In this work, single particle impact experiments were performed on polymeric substrates as well as mild steel. The particle-substrate interactions on different substrates were analyzed. Based on the results, the mechanism of cold spray on polymeric substrates is discussed and compared to that on metallic substrates.

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.

Cold spray (CS) is characterized as a solid-state process of high deposition efficiency for metallic coatings as well as additive manufacturing of metals. However, due to high velocity impact and extensive deformation of particles during CS, the as-received coatings or deposits may present anisotropic characteristics which could influence the performance of deposits. Hence this study aims to investigate the anisotropic behaviors of CS copper deposits in a systematic way. The microstructure and micromechanical properties of the deposits both in the cross-section (v-face) and in the parallel plane to the surface (p-face) were characterized. Tensile tests were performed at various loading angles with respect to the nozzle moving direction in the p-face. It is revealed that there exist strong microstructural and mechanical anisotropies in CS deposits. Different interparticle interaction results in more severe particle impact deformation in v-face than p-face, with larger elastic modulus and microhardness values. The tensile tests show an unexpected anisotropy in both ultimate tensile strength and elongation, with the highest performance occurring at the angle of 20°. The in-plane tensile anisotropy could be attributed to the parallel multiple passes. Therefore, a novel weave-spraying method was proposed, which can greatly reduce the tensile anisotropy of CS deposits.

In this work, advanced Al/diamond wear-resistance composites were fabricated by solid-state cold spray additive manufacturing using core-shell-structured diamond powders. Based on the experimental results and thorough discussion, it is found that core-shell-structured diamond powders were much easier to deposit than pure Al by cold spray, showing great potentials as feedstock for cold spraying. The deposition mechanism of the Al/diamond composites were dominated by the true metallic bonding between Al matrix and Cu layer, which is different from other conventual cold sprayed metal matrix composites. Tensile tests indicated that the tensile strength of the Al/diamond composites can be higher than cold sprayed pure Al. In addition, the Al/diamond composites had super wear-resistance performance. The wear rate was reduced by 17.8 times for the N 4-1 composite and by 37.5 times for the N 1-1 composite as compared with cold sprayed Al/Al 2 O 3 composite.

In this study, high pressure cold spray (HPCS) process was used to metallize the surface of polymeric substrates to improve their mechanical performance, such as erosion, wear, and strength. Thermoplastic polymer materials (PEEK, PEI, and ABS) were used as substrate. Commercially pure (CP) Al and 7075 Al were cold sprayed onto the polymeric substrates. Good quality defect-free coatings were achieved in all combinations except with ABS substrates, which suffered from distortion during CS process due to stored thermal energy. 7075Al coatings showed high adhesion strength but low thickness (low deposition efficiency (DE)), whereas CP Al coatings revealed high thickness (high DE) but poor adhesion strength. Based on the obtained results, the DE and bonding strength are not only highly sensitive to properties of the substrate, but also to the applied process parameters as well as powder morphology. It is concluded that two separate sets of spray parameters should be applied for 7075 Al and CP Al deposition otherwise, either more damage or less bonding is achieved to the substrate. Also, for each one of these powders, the first layer of metal/polymer should be deposited with a separate recipe than the subsequent metal/metal layers. Coefficient of thermal expansion and hardness difference between the coating material and the substrate were also found to be key factors to developing continuous coatings on the polymeric substrates with the HPCS process.

Experimental investigation supported by numerical modeling was conducted to explore the formation mechanism of intertwining interface in cold spray. The result revealed that low particle impact velocity and the consequent low deposition efficiency were the essential reason for inducing intertwining interface. In addition, intertwining interface was found to generate at very beginning of coating deposition; further particle deposition posed negligible effect on the formation of intertwining interface. Based on the experimental and numerical analysis, for the first time, the formation mechanism of intertwining interface was concluded and proposed in this paper. Low deposition efficiency led to slow coating growing rate. Therefore, at the beginning of the coating deposition, a large number of rebound particles repetitively hit the very thin single-layer or double-layer coating, forming a shot-peening effect. Such effect resulted in periodic shear stress and plastic strain in the first-layer coating. Particles of the first-layer coating were elongated and fractured and mixed with the substrate material to form the intertwining structure.

Cold spray is a technology with great potential for additive manufacturing applications. Due to the high levels of plastic deformation experienced by the powder during the coating process, any deposit will require heat treatment post-spraying to improve ductility and fatigue strength. In extreme cases, the residual stresses from coating can cause delamination or compromise the bond strength when subsequent cold spray layers are deposited. This work details the use of a commercial CO 2 laser cutter to perform a surface heat treatment on single lines of cold sprayed aluminium, to relieve residual stresses. The effect of laser power and traverse speed on material hardness is quantified, and compared with as sprayed deposits. The results shown in this work demonstrate the potential for in-process heat treatment to reduce post-processing time and improve coating quality by reducing residual stresses.

In this study, pure Al coating was deposited via in-situ shot-peening-assisted cold spray method in order to study the effect of the in-situ tamping effect which was caused by the impact of large sized shot-peening particles on grains size evolution of coatings. The microstructures of the as-sprayed Al coating were observed by using Scanning Electron Microscope and Electron Backscatter Diffraction. A commercial gas atomized Al powder with a grain size range of 10-20 μm was used as the spraying powder. The cross section of the as-sprayed Al particles presented elongated rectangular morphologies, which indicated that the nearly spherical particles experienced severe plastic deformation by the impact of large sized shot-peening particles. It was found that dynamic recrystallization of dislocations-ridden regions was responsible for the grain refinement of cold sprayed coating. Aluminum grains with size of several tens to several hundred of nanometers can be apparently recognized at the whole cross section of the particle. Therefore, in-situ shot-peening-assisted cold spray method can deposit completely nanocrystalline coating using micrometer-grain powder, and thus can be employed to develop high quality coatings of commercial importance.

To understand the adhesion mechanism of cold spraying, the characteristics of a newly formed cold spray surface are essential. This surface is formed by the dynamic plastic deformation of the substrate and particles during cold spray impact. Over the surface, the amount of newly generated surface, bonding state, and strength can differ. Even within an individual attached particle, the amount of plastic deformation also differs. To determine the relationship between the coating deposition mechanism, microstructure, and adhesion strength, tensile adhesion strength tests of cold sprayed copper coatings on an aluminum substrate were carried out. Then, using an Auger electron spectroscopic analyzer, the remained oxide film at the fracture surface, which is the bonding interface, was analyzed. The natural oxide film that covers the surface of the substrate before the impact, which is broken by plastic deformation during the spray process. However, the results show that it is not broken at the center of the collision crater, where the amount of plastic deformation of the substrate material is small. Hence, at the center of the collision crater, the oxide film still covers the substrate. Moreover, the results reveal that the adhesion strength is not uniform but is strong at the edge of the crater, where the oxide film has been removed by the colliding particle. These results reveal insights that will be valuable for future improvements in the adhesion strength of cold spray coatings.

Polymer metallization using cold spray method, due to low process temperature, is a potential candidate to form electrically conducting polymers as well as composites and improve the mechanical properties of their surface (abrasion, corrosion, etc.). Low Pressure Cold Sprayed copper coatings on PEEK (Poly-Ether-Ether-Ketone) based composites reinforced by carbon fibers have been investigated. Cold Spraying involves high erosion on composite materials due to solid state and high velocity particles thus a new way has been developed. Based on the elastic behaviors of organic materials, pure PEEK matrix has been added on the composite surface to behave as an interfacial layer between the composite and the coating. Optimization of the LPCS parameters has then been carried out using a careful choice of powder size distribution in order to avoid substrate destruction, erosion and delamination of the coating. Consequently, dense thick copper coatings have been obtained and analyzed in terms of microstructure implementing SEM observations. Finally, electric measurements have been performed in order to check the efficient metallization of the composites. A new way for metallic coating on organic composites using Low Pressure Cold Spraying is then demonstrated.

Repairing of Ni-alloy components using cold spray is being increasingly considered as an option in the aerospace industry. To further the understanding of the microstructure of Ni-alloy coatings and the bonding mechanism, gas atomised alloy 718 was sprayed onto carbon steel substrates to form 0.5mm thick coatings and single particle impacts. Spray trials were performed with different process parameters to compare the splat and coating morphology/microstructure and to optimise the parameters. The powder consumable, single particle impacts and coatings were characterised using SEM, EBSD, TEM and nanoscale XRF and XRD. Four-point bend tests were performed to test strength, ductility, cracking and de-bonding. Fine grains were observed in the substrate-particle interfaces caused by particle fragmentation, deformation and dynamic recrystallisation. Low angle grain boundaries and sub-grains form in the substrate due to strain induced by high energy impacts. The deposition efficiency, thickness, porosity, hardness and surface roughness of the coatings were measured and compared across all parameters. The porosity decreases notably (1.2% to 0.25%) and the hardness increases (410HV to 465 HV) with the increase in gas temperature and pressure. The results indicate that temperature has a larger effect on the coating properties compared to the pressure and that deformation has an important role in bonding.

In this study, 43 μm 316L stainless steel and 23 μm commercial purity Fe feedstocks were used. The following coatings were made by cold spray: single component 316L, Fe, and their binary composites with nominal compositions of 20 wt.% Fe (20Fe), 50 wt.% Fe (50Fe) and 80 wt.% Fe (80Fe). The coatings were characterized (microstructure, flattening ratio, composition) and the cold sprayability metrics (DE, porosity, coating cohesion strength) were analyzed. Results show that the single component 316L coating has a much better DE and coating cohesion strength, and a slightly lower porosity as compared with the Fe coating, whereas all the composite coatings have the similar cohesion strength. Moreover, the 20Fe coating features the highest porosity and the lowest DE; 50Fe coating features the lowest porosity; and the 80Fe coating features the highest DE. To characterize the feedstock mixture composition, in addition to the usual approach of weight or volume fraction, the ratio of the 316L and Fe particle numbers in a mixture (i.e. particle number fraction), was calculated. Using this metric, the effects of the feedstock mixing composition on the cold sprayability of bimodal size 316L/Fe powder mixtures can be better explained.