Cold spray (CS) technology has proven an enormous potential in the production of composite coatings, enabling a production of materials with superior qualities such as enhanced tribological behavior. This study aims to investigate the tribological properties of CS Al-based composite coatings reinforced by quasicrystalline (QC) particles. Two different Al alloys were used as the matrix, AA 6061 and AA 2024, and mixed with Al-based QC particles (Al-Cr-Fe-Cu) at different Al/QC ratios. A room-temperature ball-on-disc test was then used to evaluate the wear resistance of the CS composite coatings in air and compared to those of the CS non-reinforced Al alloy coatings as well as cast counterparts (AA 6061-T6). We have demonstrated that CS could be employed to produce dense and thick Al-QC composites. Further, the addition of the QC particles into the structure increased the wear resistance of the matrix resistance up to 8 times.

Diamond-reinforced composites prepared by cold spray are emerging materials simultaneously featuring outstanding thermal conductivity and wear resistance. Their mechanical and fatigue properties relevant to perspective engineering applications were investigated using miniature bending specimens. Cold sprayed specimens with two different mass concentrations of diamond 20% and 50% in two metallic matrices (Al – lighter than diamond, Cu – heavier than diamond) were compared with the respective pure metal deposits. These pure metal coatings showed rather limited ductility. The diamond addition slightly improved ductility and fracture toughness of the Cu-based composites, having a small effect also on the fatigue crack growth resistance. In case of the Al composites, the ductility as well as fatigue crack growth resistance and fracture toughness have improved significantly. The static and fatigue failure mechanisms were fractographically analyzed and related to the microstructure of the coatings, observing that particle decohesion is the primary failure mechanism for both static and fatigue fracture.

This study developed microstructure-based finite element (FE) models to investigate the behavior of cold-sprayed aluminum-alumina (Al-Al2O3) metal matrix composite (MMCs) coatings subject to indentation and quasi-static compression. Based on microstructural features (i.e., particle weight fraction, particle size, and porosity) of the MMC coatings, representative volume elements (RVEs) were generated by using Digimat software and then imported into ABAQUS/Explicit. State-of-the-art physics-based modelling approaches were incorporated into the model to account for particle cracking, interface debonding, and ductile failure of the matrix. This allowed for analysis and informing on the deformation and failure responses. The model was validated with experimental results for cold-sprayed Al-18 wt.% Al2O3, Al-34 wt.% Al2O3, and Al-46 wt.% Al2O3 metal matrix composite coatings under quasi-static compression by comparing the stress versus strain histories and observed failure mechanisms (e.g., matrix ductile failure). The results showed that the computational framework is able to capture the response of this cold-sprayed material system under compression and indentation, both qualitatively and quantitatively. The outcomes of this work have implications for extending the model to materials design and under different types of loading (e.g., erosion and fatigue).

Super wear-resistant aluminum-based metal matrix composite (MMC) coatings were produced using cold spraying. Cu-Ni coated diamond and pure diamond particles were used as reinforcing agents. Test results show that the metallic Cu-Ni shell served as a buffer layer, preventing the fracture of diamond particles upon impact as occurred with the uncoated diamond. The coated diamond particles were also found to have a higher deposition efficiency due to metallurgical bonding between the Cu shell and Al matrix. Under tribological testing, all coatings performed well, but those reinforced with the coated diamond showed higher wear resistance due to higher diamond content and involvement of Cu and Ni.

For fabrication of high strength carbon nanotube (CNT) reinforced Al matrix composites, the uniform dispersion, strong interface bonding and high structural integrity of CNTs have been regard as the three most important issues. In this work, two distinct approaches, namely high shear dispersion (HSD) and shift-speed ball milling (SSBM), were applied to disperse CNTs (1.5 wt.%) into pure Al powders. These two kinds of CNTs/Al composite powders as well as pure Al powders (as comparison) were deposited onto stainless steel plates under the same processing parameters. The deposition efficiency, microstructure, as well as the structural integrity of CNTs in the coatings produced from different starting powders were comparatively investigated. According to the XRD and Raman analysis, the brittle Al 4 C 3 phase was not formed in both CNTs/Al composite coatings. Some structural damages of CNTs were found in both composite coatings, especially the one fabricated from HSD composite powder. The dispersion of CNTs onto Al particle surfaces by HSD approach did not achieve significant strengthening effect on the composite coatings, but adversely affect the metallic bonding of the particles. The microhardness of CNTs/Al composite coating produced from SSBM powders reached to ~115 HV0.1, showing a significant improvement compared to the pure Al coating. The strengthening mechanisms of the cold sprayed CNTs/Al composite coatings were also investigated.

The presence of defects such as voids, inter-lamellar porosities or cracks, provides a decrease of the effective thermal conductivity of plasma sprayed coatings as well as a decrease of the corresponding mechanical properties such as the Young’s modulus. In general, effective properties of thermal spray coatings are thus strongly different from that of the bulk material and have thus to be quantified to validate their in service performances. A complementary approach allowing understanding the relationships between the microstructure of a coating and its macro-properties is the use of Finite Element Modeling. The case of composite coatings is still more complicated due to the presence of different materials. In the present study, thermo-mechanical properties of a plasma sprayed composite coating were estimated by numerical modeling based on FEM. The applied method uses directly cross-sectional micrographs without simplification using a one-cell per pixel approach. Characteristics such as the thermal conductivity, the Young’s modulus, the Poisson ratio and the dilatation coefficient were considered. The selected example was an AlSi/polyester coating used as abradable seal in the aerospace industry.

The residual stresses present in coatings and layer composites are influenced not only by the thermal and mechanical loads generated during manufacturing, but also by the mechanical and thermophysical properties of the coating and substrate materials. In-process measurement of transient, process-induced stresses may thus enable the manufacturing of coated parts with a residual stress state that lies within a predefined application-oriented stress regime. This paper presents a quasi-nondestructive method by which such measurements may be obtained. A small amount of material is removed from the surface of a part by laser ablation, while optical interference sensors monitor surface deformation caused by stress relaxation and heating due to absorbed laser energy. The new method is evaluated by four-point bend testing using Al5754 plates coated with Al/TiO 2 by atmospheric plasma spraying.

This study investigates the effects of feedstock composition and annealing temperature on cold-sprayed aluminum-iron deposits. Commercially available Al and Fe powders mixed in ratios of 55:45, 75:25, and 85:15 were cold sprayed on aluminum substrates, producing dense coatings that were subsequently annealed at 500 and 550 °C. Solid diffusion reactions between Fe and Al produced Al 5 Fe 2 intermetallic compounds, the morphology and content of which were found to depend on annealing temperature and the composition of the as-sprayed deposit. The Fe particles in the Al matrix were fully consumed via compounding reaction with Al at 550 °C. At higher temperatures, however, the intermetallic particles begin to crack possibly due to large tensile stresses.

In this study, titanium and aluminum powders mixed in different ratios were deposited on stainless steel substrates by warm spraying. Microstructure and composition of as-sprayed and heat-treated samples were characterized and the effect of adding a third element was assessed. It was found that Al content has a major influence on the thickness and porosity of heat-treated Ti-Al coatings and that adding silicon to the powder mixtures reduces the melting point of Al, causing a loss of Al-Si particles during spraying.

A series of abradability tests were conducted on AlSi-hBN coatings, which are commonly used in the compressor section of aeroengines for clearance control. The coatings were sprayed on test plates to a thickness of 1.9-2.0 mm and ground to a finish of 10 μm with 400 grit paper. The tests were carried out in an automated test rig with adjustable temperature, blade tip velocity, and incursion rate. The rig is configured such that the coatings are exposed to rotating blades, making contact with the tips as they pass. In this study, investigators monitored the number of contacts, removing and examining abraded coating samples at a given count total ranging from 200 to 4000. It was found that wear characteristics change with each contact between the coating and blade tip, indicating that pass number is a factor that must be considered when testing abradable coatings.

To improve the mechanical properties of aluminum coatings, ceramic reinforcement may be added resulting in an aluminum matrix composite. Two processing routes were investigated to manufacture aluminum matrix composite powders for thermal spraying: ball milling and mixing. Three sizes of SiC reinforcement particles were used: 2, 15, and 25 µm. For the ball-milled powders, morphology and microstructure were investigated as a function of SiC grain size and milling time. It is shown that the hardness of the composite and the efficiency of the spray process depend on the size of the hard particles as well as the preparation method. Friction tests were also carried out and the results are shown to correlate with coating microstructure.

In this work, Al and Al-Al 2 O 3 coatings up to 8 mm thick were cold sprayed on AZ91D magnesium alloy substrates. Microstructure, microhardness, bond strength, and corrosion and wear resistance were studied to assess the viability of using these coatings to restore dimensionally degraded parts and protect them from further corrosion and wear.

This work investigates the effect of feedstock size on the hardness and wear resistance of metal-matrix composite coatings produced by cold spraying. Feedstocks consisting of Al and Al 2 O 3 powders were prepared for the study. The feedstocks, which differ in regard to Al 2 O 3 particle size (100, 50, 10 µm) and composition (25-50 vol%), were accelerated by compressed air through a Delaval-type nozzle positioned 10 mm from the target substrate. The morphology of the coating resembled that of an Al matrix with embedded Al 2 O 3 particles. Optical microscopy showed that large Al 2 O 3 particles (> 50 µm) fractured into small pieces and embedded in the matrix. It is likely that some of the fragmented particles bounced off, rather than adhering. These collisions (tamping effect) increase coating hardness and density. In the case of the feedstock with 10 µm Al 2 O 3 , particle sizes were unchanged during spraying and the benefits of work hardening were not achieved.

This paper describes the development of detonation-sprayed aluminum-matrix composite coatings reinforced with boron carbide. The goal is to achieve a homogeneous coating structure with low porosity, low oxide content, and high concentration of embedded carbides. Tensile tests of various types were conducted and different stages of deformation were analyzed using micro computed tomography, a 3D imaging technique that reveals the formation of cracks in real time.

This study evaluates a method for producing carbon fiber composite feedstocks suitable for cold gas spraying. Powders consisting of Al-Si particles and carbon nanofibers were attrition milled at 16.5 Hz and 27.5 Hz for up to 12 h at room and cryogenic temperature. Particle shape and size were examined every hour and carbon fiber integration in the Al-Si matrix was assessed. Detailed results are presented and discussed. In all cases, cryomilled powders had smoother surfaces.

In this study, Al-SiC composite coatings are produced by cold spraying ball-milled Al powders with different volume fractions of SiC particles. The morphology and microstructure evolution of the powder during ball milling are evaluated along with the effect of SiC content on the microstructure and wear behavior of the coatings. The results show that dense Al-SiC coatings with different volume fractions of SiC particles can be fabricated by cold spraying and that abrasive wear resistance is improved by raising the volume fraction of SiC particles. Wear surfaces indicate that the predominant wear mechanism is gouging of the soft Al matrix in the early stages and cracking and spalling of SiC particles in the latter stages. The dispersed SiC particles serve to protect the matrix from wear products thus raising the wear resistance of the coatings.

Cold gas dynamic spraying (“cold spraying”) at low pressure (150 psig) was used to fabricate Al-Al 2 O 3 metal-matrix composite (MMC) coatings onto 6061 Al alloy. The powder contained -45 µm Al stock powder admixed with -10 Al 2 O 3 in fractions ranging from 0-90 wt%. Scanning electron microscopy (SEM), Vickers microhardness testing, and image analysis were conducted on the as-sprayed coatings. The coatings were then friction-stir processed (FSP) using a milling machine and a 12 mm diameter cylindrical tool. Microhardness testing, SEM, and image analysis were then repeated to study the effect that FSP had on the MMC coating hardness. Hardness increased with increasing fraction of Al 2 O 3 in the feedstock powder, resulting in a maximum as-sprayed coating hardness of 85 HV when 90 wt% Al 2 O 3 is used. After FSP, the hardness of the MMC fabricated from a 90 wt% Al 2 O 3 powder blend increased to a maximum of 140 HV. SEM micrographs showed that the as-sprayed MMC coatings contained Al 2 O 3 particles that had been trapped between the larger Al particles. FSP succeeded in redistributing the Al 2 O 3 particles, decreasing the mean free interparticle distance and increasing the probability of load sharing between the reinforcing particles. It was suggested that this redistribution may be the primary reason for hardness improvement in the MMC coatings.

The final target of this study was to achieve a better understanding of the behaviour of thermally sprayed abradable seals such as AlSi/polyester composites. These coatings are used as seals between the static and rotating parts in gas turbine applications. The machinability of the composite coatings during the friction of the blades depends on their mechanical and thermal effective properties. In order to predict these properties from micrographs, numerical studies were performed with different software packages such as OOF developed by NIST and TS2C developed at the UTBM. In 2008, differences were reported concerning prediction of effective thermal conductivity obtained with the two codes. In the present paper, it is shown that a particular attention must be paid to the mathematical formulation of the problem. In particular, results obtained with a finite difference method using a cell centre approach or a nodal formulation, allow explaining the discrepancies previously noticed. A comparison of the predictions of computed effective thermal conductivities is thus proposed for different codes and different meshing methods. This study is part of the NEWAC project, funded by the European Commission within the 6th RTD Framework program (FP6).

Repair of damaged ion vapor deposition aluminum (IVD-Al) on aircraft components generally requires the use of brush plating with hazardous materials including cadmium. This paper describes a cold spray process that uses aluminum transition metals to make such repairs. The aluminum layers are applied with a handheld cold spray gun and tested according to JTP-2003 requirements for corrosion resistant coatings on steel components.

Post-annealing of cold spray coatings has great potential for wear applications because it produces intermetallic compounds at low temperature far below equilibrium. This study investigates the effects of spraying pressure on the intermetallics formed and their dispersion characteristics. In the experiments, Al and Al-Ni powders were sprayed on Ni and Al substrates at 0.7, 1.5, and 2.5 MPa and a portion of the coating samples were annealed in argon at 500, 550, and 600 °C. Detailed examinations showed that Al particles are subject to peening effects that can interfere with the formation of intermetallic compounds during annealing, but that the effects can be mitigated by controlling gas pressure. Spraying pressure was also found to have an effect on the formation of eutectic pores in Al-Ni composite coatings, with higher pressures corresponding to fewer pores.