Through the measurement results of the residual strains of Hydroxyapatite coatings (HACs) and the values of Young’s moduli, the initial vacuum heating treatment (<600°C) can release the compressive residual strain of HAC after plasma spraying, so enabling HACs to obtain the optimal bond strength at the heating temperature of 500~600°C. Nonetheless, the post-vacuum heating temperature higher than 600°C led to the deterioration of the bond strength of HA/Ti-substrate. With the increasing temperature, the increased compressive residual strain will weaken the interfacial adhesive force of coating to substrate, resulting in the lower bond strength with higher temperature. The results of this paper demonstrated that the proper temperature of post-vacuum heating treatment improved effectively the bonding strength between HAC and Ti-alloy substrate.

In recent years, laser-based post-processing of thermally sprayed coatings has gained significant attention as an alternative post-processing route; to mitigate the microstructural defects such as pores, microcracks, and splat boundaries associated with thermally sprayed coatings. Optimisation of the parameters for the laser post-processing is of paramount importance to maintain the required properties of these coatings. The current thermo-mechanical model simulates the impact of laser heat treatment on thermally sprayed Tungsten Carbide Cobalt (WC-17Co) coating and AISI 316L as substrate. A sequentially coupled transient thermal and structural analysis is performed. Transient temperature field from thermal analysis due to laser source will become input loads for the subsequent stress-strain analysis with appropriate boundary conditions. Both the coating and substrate are given temperature-dependent material properties. A gaussian heat flux distribution is used to model the laser source. The finite element analysis results underline the importance of temperature gradients and the presence of thermally induced stress-strain fields responsible for promoting coating degradation. The obtained results also revealed that heat input and dimensional characteristics play a vital role in the annealing treatment's efficacy. Three separate test cases were considered wherein the hatch spacing was varied, keeping the other parameters (scan speed, laser power, and laser spot diameter) constant. The impact of hatch spacing on the temperature and residual stress distribution across the coating was assessed by this simulation. Residual compressive stress was observed in the coating for two out of the three test cases, which further improved the durability of the coating.

This paper describes the influence of post-spray heat treatment parameters on mechanical properties of Ni-TiC composite coatings. Thin Ni-TiC composite coatings were produced by low pressure cold gas dynamic process (also referred as cold spray or kinetic spray process) on an Inconel alloy substrate. In the coating process, mechanically mixed micron-sized Ni-TiC powders (~50 µm) were injected into a de-Laval nozzle propelled by a supersonic gas stream to high velocity (>300 m/s) to impinge upon a substrate. The coatings are formed subsequently as the metallic particles are severely deformed plastically and bonded to both the substrate and to one another. However the tensile adhesion strength levels were determined to be in the range of 10-14 MPa. A subsequent post-spray heat treatment in vacuum was found to enhance the bond strength of the coated particles with the substrate due to good metallurgical bonding caused by diffusion mechanism.

Large pure aluminum powders were deposited on as cast-, T4- and T6-AZ91D magnesium substrates using cold spray. Heat treatment was applied to the coated components under vacuum at 400°C for different holding time. The effects of the heat treatment on the microstructure as well as the coating/substrate adhesion strength were investigated. Thick (~ 400µm) and dense (<1% porosity) Al coatings have been obtained on the three different substrates. During heat treatment, Mg 17 Al 12 (β) and Al 3 Mg 2 (γ) intermetallic phases were formed at the Al/Mg interface and the thickness of the intermetallics layers increased with the holding time. No significant thickness difference of the intermetallics layers were observed on as cast- and T6-AZ91D substrates, while thicker layers took place on the T4- substrate. It is believed that the higher Al concentration within the T4-AZ91D material could be beneficial for intermetallic growth because less enrichment is required to reach the critical level for intermetallic formation in the substrate. Shear strength tests were performed on the as sprayed and after heat treatment coatings. The results revealed lower adhesion strength in the samples after heat treatment than the as sprayed ones which is attributed to the presence of brittle intermetallics layers at the coating/substrate interface.

The most recent increase in turbine entry temperature (TET) can be obtained by the use of Thermal Barrier Coatings (TBCs) on cooled hot section components. The TBC systems are comprised of Ni-based super-alloy substrate which provides certain mechanical properties and blade geometry consisting of a ceramic top coat with low thermal conductivity applied to a metallic bond coat resulting in a significant temperature drop across the coating. The bond coat provides oxidation resistance and adherence of the top coat to the substrate. In this study different bond coat layers were applied on carbon steel substrates which were covered by yttria stabilized zirconia (YSZ) as a top coat layer using atmospheric plasma spray technique (APS). Al-12%Si and Al 99% were deposited by cold gas dynamic spray technique (CGDS) while Ni-5%Al layer was deposited by high velocity oxy fuel technique (HVOF). Heat treatment was performed on the samples under controlled atmosphere for 15 hrs. The microstructure and micro hardness of as sprayed and after heat treatment samples were investigated. Adhesion strength for top coat / bond coat interface and bond coat / substrate interface were investigated. The residual stresses for as sprayed and after heat treatment was estimated by XRD measurement on the top coat layer with different bond coat material. The results indicate that the adhesion strength either for as sprayed or after heat treatment was enhanced using this new bond coat materials compared to the traditionally as deposited Ni Co Cr Al Y bond coat material.

This research systematically examines the effect of heat treatment on the microstructural properties of cold sprayed titanium coatings. Heat treatments were performed on as-sprayed coatings at 200, 400, 600, and 800°C for four hours under argon atmosphere. Vickers microhardness, microstructural investigation using FEG-SEM, structural characterization using XRD, and porosity evaluation using SEM image analysis were performed on as-sprayed and heat treated coatings. Results demonstrated that static recovery and static recrystallization may have occurred for heat treated coatings at 600 and 800°C. In addition, for the heat treated coating at 800°C, significant oxidation occurred and a slight decrease in porosity took place. Furthermore, a thin metallic layer characteristic of a solid solution or an intermetallic compound, was found at the coating/substrate interface.

A novel process to produce dense, well adherent aluminium coatings on ceramic materials is cold gas spraying (CGS). The mechanical, physical and chemical processes leading to the bonding of cold-sprayed coatings on ceramic substrates have only been described rudimentarily. A survey of the literature on adhesion mechanisms of cold spray coatings is given, where influences on bond strength are discussed and parameters identified. Using the example of coating Al 2 O 3 and AlN substrates with pure aluminium via cold gas spraying, a process and substrate parameter variation is presented. A significant raise in tensile coating strength was seen at elevated substrate temperatures and after subsequent annealing. Tensile strength also depended on chemical composition and roughness of the substrate. The results allow the discussion of the bonding mechanisms of cold spray aluminium on ceramic substrates as a function of deposition and annealing parameters.

This work assesses the effect of heat treatment on the oxidation resistance of blended-powder CrC-NiCr coatings produced by HVOF spraying. The as-sprayed coating phases oxidized independently with NiCr rapidly forming nickel oxide, which was subsequently consumed by a solid-state reaction with Cr 2 O 3 . The NiO formed two distinct morphologies: large bulbous oxides on thin regions of exposed alloy and thin, flat oxide layers on large alloy regions. Heat treatment led to sintering of the splats and diffusion of Cr from the carbide phase into the NiCr alloy, increasing Cr to the point where Cr 2 O 3 dominated the oxide composition from the earliest exposure time. The implications of the change in oxidation mechanism and oxide morphology are discussed.

To improve the mechanical properties of 316L stainless steel coatings prepared by plasma spraying, post-spray heat treatments were conducted at 600-800 °C for 1-2 hours. The effect on microstructure and hardness was assessed via XRD and SEM analysis and microhardness measurements. The results show that heat treatment at various temperatures improved coating hardness as well as fracture behavior.

In this study, Ti-6Al-4V coatings were deposited by low-pressure plasma spraying (LPPS) using different powder sizes and spray currents. The coatings were also heat treated at different temperatures, after which their microstructure and properties were assessed. The results show that finer powders are more conducive to the preparation of dense coatings and that porosity is reduced by increasing plasma current. As for the effects of heat treating temperature, at 870 °C, the lamellar structure of the coating disappeared and was found to be fully equiaxed with a grain size of 5-10 µm at 1100 °C. Hardness also increased, becoming significantly higher than that of forged TC4 alloy following treatment at 1100 °C.

Thermal barrier coating systems have been heat treated in order to study the oxidation kinetics of the bond coat. All the surfaces of Ni superalloy substrates were sprayed with ~100 μm of a NiCrAlY bond coat, with or without ~250 μm of a ZrO 2 top coat. Thermogravimetric analysis (TGA) was used to monitor continuously the mass change as a result of oxidation of the bond coat during heating at 1000°C for 100 hours in flowing air. In addition, some specimens were heated to 1000°C in static air, cooled to room temperature, weighed and re-heated cyclically. The total exposure time was 1000 hours. Rates of weight gain were found to be higher for the cycled specimens, despite the absence of air flow. This is attributed to damage to the oxide film, which was predominantly α-Al 2 O 3 , as a consequence of differential thermal contraction stresses. The changing residual stress state during heat treatment was predicted using a previously-developed numerical model. A thin (1 mm) substrate with ~100 μm bond coat and ~250 μm ZrO 2 top coat was used in these simulations, which incorporated creep of the bond coat and the lateral strain associated with oxidation. It is concluded from these computations that, while high stresses develop in the oxide layer, the associated driving forces for interfacial debonding remain relatively low, as do specimen curvature changes. It seems likely that coating spallation after extensive oxide layer formation arises because the interface is strongly embrittled as the layer thickens.

This paper presents the results of mechanical and tribological property measurements obtained from NiCrBSi alloy coatings and evaluates the effect of different heat treatments. Coating specimens were deposited by means of flame spraying and heat treated using an acetylene torch. HVOF samples were also prepared as a reference for comparison. The microstructure of as-sprayed and heat-treated coating samples were compared, hardness and surface roughness were measured, and erosion resistance tests were performed. The processes and procedures used are described and the results obtained presented and discussed.

The aim of this work is to fabricate nanostructured ceramic coatings using unsintered agglomerated powder and to characterize differences in microstructure, especially those at the nanoscale, due to spraying conditions. Feedstock powders were prepared from commercial YSZ nanoparticles that were reconstituted into solid spheres (70-100 μm) by spray drying. The surface morphology of sintered and unsintered agglomerates was examined by FE-SEM prior to deposition by atmospheric plasma spraying using two gun configurations, one with a lengthened barrel and one with water cooling. YSZ coating cross-sections were examined by optical and electron microscopy, revealing details at the micro and nano scale. The results show that the unsintered agglomerates, which were successfully deposited using both spray guns, are favorable for developing bimodal coating structures with fine grains and porous nano zones.

The Cold spray (CS) is a promising solid-state additive manufacturing method. The interesting physics involved in the CS process including cold, high strain rate, adiabatic and severe plastic deformation results in a unique and complex structure of CS deposits at different length scales that directly determines the properties of the deposits. Therefore process- structure properties (performance) (PSP) linkages explorations are pivotal. Integrated computational materials engineering (ICME) methods in complement with experimental analyses are required to evaluate materials properties and behaviour in PSP links exploration. Finite element modelling is used to simulate the thermomechanical response of materials and evolution of field variables in CS, i.e stress, strain, strain rate, and temperature, at structural scales. Molecular dynamics modellings of nano-particle impact have provided useful insights into atomic-scale phenomena of individual particle impact while the modelling of microstructure evolution in micro and mesoscale has yet to be investigated. In this study, we developed and implemented a thermodynamic phase field simulation method to capture the structure evolution of CS composite Ni-Ti deposit upon post-spray heat treatment (PSHT) in microstructure scale. The external or internal stimuli such as heat and strain either generated in the system because of phase transformation or stored as internal energy upon CS process are accounted for. The interface mobility and microstructure development are calculated by minimization of Gibbs free energy of the system. The comparison of the simulated microstructure with experimental results confirms that the phase field modelling precisely predicts the microstructure evolution of the CS deposits upon PSHT.

The application of cold spray (CS) for additive manufacturing (CSAM) of structural components using metallic materials has recently attracted much attention. However, there are limited reports on developing thick deposits or components with high entropy alloys (HEAs) via CSAM and investigating the microstructural evolution and mechanical properties after deposition and subsequent annealing heat-treatment. This work investigated the microstructure and mechanical properties of asdeposited and heat-treated thick CoCrFeNiMn HEA deposit fabricated via CSAM. The microstructure of the HEA deposit and after heat-treatment were characterised using scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), and x-ray diffraction (XRD). The microstructural analysis reveals heterogeneous grain size distribution with ultrafine grains at the particle-particle interfacial regions and coarse grains at the particle interiors in the as-deposited sample. The as-deposited sample, characterised by moderate porosity, was consolidated following the heat treatment at different temperatures. Additionally, increasing the temperature increases grain sizes resulting from static recovery and recrystallisation, with annealing twin formed at higher temperatures. Most notably, phase decomposition of the deposit microstructure occurs at 600 ºC, with Cr-rich phase particles formed at regions of high dislocations and grain boundaries. Nano-and micro-hardness and tensile testing of micro-flat dogbones samples were performed on the as-deposited and heattreated samples. The effect of heat-treatment on the microstructure and mechanical properties of the cold-sprayed HEA deposit were analysed and discussed.

In this present work, investigators determine how particle temperature, combustion pressure, and heat treatment affect the porosity, oxide content, and tensile properties of warm-sprayed titanium. Coatings were deposited with nitrogen flow rates ranging from 0.5 to 1.5 m 3 /min and combustion pressures of 1 and 4 MPa. Optimal coating properties were found for specimens formed at a nitrogen flow rate of 0.75 m 3 /min and a combustion pressure of 4 MPa. Post-spray heat treatment was found to improve bonding between deposited particles, significantly increasing the strength and ductility of the titanium coatings.

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.

Aluminum-titanium powder mixtures were deposited on γ-TiAl alloy substrates by cold spraying then heat treated for 5 h at 600, 650, and 700 °C. SEM and XRD examination showed that the treatment caused Al to diffuse into the substrate where it reacted with Ti, resulting in changes in microstructure. The diffusion of Al left pores in the fringes of the TiAl 3 phase, increasing the porosity of the coatings. A surplus of Al remained in the coatings after heat treatment at 600-650 °C, but at 700 °C, all Al was consumed, contributing to the formation of a continuous TiAl 3 layer.

This study demonstrates the feasibility of an in-situ heat treatment for cold spray coatings. By controlling heat flow, temperature gradients are maintained in the coatings, leading to the development of graded mechanical properties. Initial experimental results validate both the starting idea and the first results of numerical simulations.

In this study, Al-Sn binary alloy coatings were prepared with Al-5wt.%Sn (Al-5Sn) and Al-10wt.%Sn (Al-10Sn) gas atomized powders by low pressure and high pressure cold spray process. The microstructure and microhardness of the coatings were characterized. The as-sprayed coating were heat treated at 150, 200, 250 and 300 °C for 1 hour, respectively. The effect of heat treatment on microstructure, microhardness and content of Sn phase of the coatings were investigated. The bonding strength of as-sprayed and heat treated Al-Sn coatings were also studied. The results show that the dense and well-bonded Al-10Sn coating can be deposited by low pressure with helium gas while Al-5Sn coating by high pressure cold spray with nitrogen gas. The content of Sn for both Al-5Sn and Al- 10Sn in as-sprayed coatings are consistent with that in feed stock powders. The coarsening and/or migration of Sn phase in both coatings were observed when the annealing temperature exceeds 200 °C. Furthermore, the microhardness of the coatings decreased significantly under the annealing temperature of 250 °C. EDAX analysis shows that the heat treatment has no significant effect on content of Sn phase in Al-5Sn coatings. Bonding strength of as-sprayed Al-10Sn coating is slightly higher than that of Al-5Sn coating. Annealing under 200 °C can increase the bonding strength of Al-5Sn coatings.