Thermal spray technology keeps attracting several industries in both the manufacturing and repair sectors, thanks to its practicability and its reasonable processing time. Moreover, different kinds of materials can be successfully deposited to form coatings with potential excellent thermo-electro-mechanical properties. The resultant coating microstructure is completely different from the wrought powder material before the deposition process. In the case of metallic materials, the thermomechanical characteristics are quite dependent on the deposition conditions monitored from the spraying setup. One can mention gas temperature, impact velocity and angle, material combination, surface state, particles size, etc. Hence, one major factor which influences the final coating microstructural state is kinetic energy. In fact, in such processes where high velocity deposition is observed, intense grain refinement and sharp increase of the dislocation density are an outcome that is tightly related to high temperature and severe plastic deformation. Prediction of the mechanical properties of the produced coating is usually carried out using phenomenological models that describe very well the relationship between stress and strain under different conditions of temperature and strain rates. Most of these models fail, however, to describe the effect of the deformation mechanisms observed at ultra-high strain rates such as the viscous drag regime of dislocations or further the weak shock load regime, scenarios commonly observed in such processes. In the present paper, we present an enhanced physics-based model to describe the stress strengthening of metals upon impact and associated microstructure changes. We show that the model can accurately represent the desired effect of the dislocation drag. Modelling of the impact of a single copper particle onto a copper substrate is carried out to show the capability of the model to predict grain refinement and dislocation network modification.

Thermal insulation performance is a measurement of the thermal protection offered by the thermal barrier coatings (TBCs) to the substrate, therefore, it is essentially important to compare different double ceramic layer (DCL) TBCs on the premise of the same thermal resistance. In this study, a series of LZO/YSZ DCL-TBCs, with the equivalent thermal insulation to 500 µm thick YSZ TBCs, were prepared, and their lifetimes were evaluated by thermal gradient cyclic test at the top coat surface temperature of 1300°C. Result show that, the lifetime of DCL-TBCs was more than doubled compared to 500 µm thick YSZ TBCs, when 100µm thick YSZ coating was substituted by LZO coating. In addition, the lifetime of DCL-TBCs decreased with the increase of LZO substitutional ratio. X-ray diffraction analysis revealed that LZO maintains the pyrochlore structure after thermal cyclic test. Microstructure examination demonstrated that, with the increase of LZO substitutional ratio, the delamination position transferred from near top/bond coating interface to near LZO/YSZ interface and finally to the inside of LZO coating. Therefore, this study would shed light to further coating structure optimization towards the next generation advanced DCL-TBCs.

Thermal Inkjet printhead heater chips have a unique construction; getting quality cross-sections using traditional methods is very difficult. This paper describes the development of a new protective coating material, which enables the application of Ion cross section polishing technique for Inkjet heater chip analysis. Applications of this technique are also discussed.

The deposition behavior of particles during cold spraying is determined by plastic deformation of both substrate and impinging spray particles. In this paper, the in-situ heating and subsequent softening of the local substrate were examined to reveal their influence on the deposition behavior of spray particles and the microstructure and property of the cold-sprayed 316L stainless steel and copper coatings. Results show that the temperature of the substrate surface, where the spray gas stream and high velocity particles were projected on, increased to 300°C when the gas temperature was 500°C. Such effect is referred to as the in-situ substrate heating. The in-situ heating of the substrate surface was enhanced with the decrease in the gun nozzle traverse speed. With the decrease of nozzle traverse speed from 100 to 20 mm/s, the relative deposition efficiency significantly increased and the porosity of cold-sprayed 316L coatings decreased from 5.6% to 2.5%, and the micro-hardness of the coatings increased from 283 Hv to 351 Hv. The influence of the nozzle traverse speed on the microstructure and property of cold-sprayed coatings is discussed based on the influence of the in-situ heating and softening effect of substrate surface on its deformation behavior and particle deposition upon the impact of spray particles. The in-situ substrate surface heating is proposed as an essential processing parameter as a function of gun traverse speed during cold spraying.

Surface morphology of MCrAlY bond coats is one of the factors that affect the oxidation behavior and the thermal stability of thermal barrier coatings. In the present study, the isothermal oxidation behavior of cold sprayed MCrAlY bond coat was investigated at three surface conditions: as-sprayed, polished and shot-peened conditions. The MCrAlY bond coat with sputters adhered weakly on the surface prepared by low-pressure plasma spraying was also employed. The above four types of surface conditions were employed to investigate the effect of the surface morphologies of MCrAlY (M=NiCo, Ni) bond coats on their oxidation behavior. MCrAlYs with the compositions of Ni20Cr10AlY and Ni23Co20Cr8.5Al5.0Ta0.6Y were employed for bond coat deposition. Cold-sprayed Ni20Cr10AlY exhibited a higher oxidation rate than that of the cold-sprayed Ni23Co20Cr8.5Al5.0Ta0.6Y bond coats. After 10 hrs oxidation, the TGO on the as-cold-sprayed bond coat surface was merely constituted by Al 2 O 3 , while the TGO on the bond coat surface attached with sputters was composed of Al 2 O 3 and spinel. After 500 hrs oxidation, Cr 2 O 3 and porous spinel appeared in the TGO on the two surfaces of as-cold-sprayed bond coat. The growth of Ni/Cr-oxides was attributed to the Al depletion. The content of spinel on cold-sprayed NiCrAlY decreased after shot-peening.

This study investigates the influence of thermally grown oxide (TGO) on the lifetime of thermal barrier coatings (TBCs). In the experiments, cold sprayed NiCoCrAlY powder was deposited as the bond coat and plasma sprayed YSZ was deposited as the topcoat. The TBCs were subjected to various treatments to induce different types of TGOs at the bond coat-topcoat interface. Three types of oxide were detected, including spinel, chromium oxide, and alumina. Thermal cycling tests show how the oxides contribute to cracking and spalling in the topcoat layer and how the morphologies and constituents of the oxides can change with time and temperature.

Critical velocity is an important parameter in cold spraying. It determines the deposition efficiency under a given spray condition. It depends not only on material types, but also particle temperature and oxidation conditions. In this present work, three types of materials including copper, 316L stainless steel, and Monel alloy were used to deposit coatings by cold spraying. The critical velocities of spray materials were determined using a novel measurement method. Oxygen content in three powders was changed by isothermal oxidation at ambient atmosphere. The effect of oxygen content on the critical velocity was examined. It was found that critical velocity was significantly influenced by particle oxidation besides material properties. The critical velocity of Cu particles increased from about 300 m/s to over 610 m/s with a change in oxygen content in the powder. The results suggest that with a severely oxidized powder, critical velocity tends to be dominated by the oxide on the powder rather than material properties.

Nanostructured YSZ is expected to exhibit a high strain tolerability due to its low Young’s modulus and consequently high durability. In this study, a porous YSZ as the thermal barrier coating was deposited by plasma spraying using an agglomerated nanostructured YSZ powder on a Ni-based superalloy Inconel 738 substrate with a cold-sprayed nanostructured NiCrAlY as the bond coat. The heat treatment in Ar atmosphere was applied to the cold-sprayed bond coat before deposition of YSZ. The isothermal oxidation and thermal cycling tests were applied to examine failure modes of plasma-sprayed nanostructured YSZ. The results showed that YSZ coating was deposited by partially melted YSZ particles. The nonmelted fraction of spray particles retains the porous nanostructure of the starting powder into the deposit. YSZ coating exhibits a bimodal microstructure consisting of nanosized particles retained from the powder and micro-columnar grains formed through the solidification of the melted fraction in spray particles. The oxidation of the bond coat occurs during the heat treatment in Ar atmosphere. The uniform oxide at the interface between the bond coat and YSZ can be formed during isothermal test. The cracks were observed at the interface between TGO/BC or TGO/YSZ after thermal cyclic test. However, the failure of TBCs mainly occurred through spalling of YSZ within YSZ coating. The failure characteristics of plasma-sprayed nanostructured YSZ are discussed based on the coating microstructure and formation of TGO on the bond coat surface.

Ni/Al alloy powders were synthesized by ball milling of nickel-aluminum powder mixture with a Ni/Al atomic ratio of 1:1. Ni/Al alloy coating was deposited by cold spraying using N 2 as accelerating gas. NiAl intermetallic compound was evolved in-situ through post-spray annealing treatment of cold-sprayed Ni/Al alloy coating. The effect of annealing temperature on the phase transformation behavior from Ni/Al mechanical alloy to intermetallics was investigated. The microstructure of the mechanically alloying Ni/Al powder and NiAl coatings was characterized by scanning electron microscopy and X-ray diffraction analysis. The results show that a dense Ni/Al alloy coating can successfully be deposited by cold spraying using the mechanically alloyed powder as feedstock. The as-sprayed alloy coating exhibited a laminated microstructure retained from the mechanically alloying powder. The annealing of the subsequent Ni/Al alloy coating at a temperature higher than 850°C leads to the complete transformation from Ni/Al alloy to NiAl intermetallic compound.

FeAl Intermetallic compounds have excellent wear resistance and high temperature oxidation resistances. The low temperature brittleness makes intermetallic compound materials more suitable to be applied in the form of coating to protect materials from high temperature oxidation and wear. In the present study, a iron/aluminum composite coating was produced by cold spraying of iron and aluminum powder mixtures and then was annealed at different temperatures to aim at forming an iron aluminide intermetallic based coating. The deposition behavior of iron and aluminum powder mixtures and microstructural characteristics of the as-sprayed deposit were examined by scanning electron microscopy (SEM). The kinetics of the phase transformation of the as-sprayed iron/aluminum composite deposit to iron aluminide was characterized by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The results showed that after heat treatment at a temperature of 600°C, intermediate phase Al 5 Fe 2 coexisted in the deposit with remaining Fe and Al. With increasing heat treatment temperature to 900°C, the deposits consisted of mainly FeAl phase and a trace of remaining Fe phase.

Nickel titanium is promising cavitation erosion resistant material. Using NiTi in bulk for components might not be feasible due to its poor workability, as well as the high material and processing costs. Surfacing components with its coating is effective for utilizing the good erosion properties of NiTi intermetallic compounds. In this study, a method to prepare NiTi intermetallic compound coatings in-situ through annealing of the cold-sprayed Ni(Ti) metastable coating was investigated. A nanostructured Ni(Ti) solid solution alloy powder was prepared by ball-milling process. The cold sprayed Ni(Ti) alloy coating was used as the precursor coating. The effect of annealing temperature on the microstructure in-situ evolution of Ni-Ti intermetallic compound in cold-sprayed coating was investigated. The morphology and phase composition of the powders milled for different durations and the microstructure of the as-sprayed coating were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that after annealing at 950°C the Ni(Ti) alloy was transformed to intermetallic phases. NiTi, Ni 3 Ti and NiTi 2 intermetallic phases coexisted in the annealed coating.

During plasma spraying, high substrate temperature contributes to increase the interface temperature between flattening droplet and substrate and subsequently promotes the through-lamella grain growth in the coating. In this study, yttria stabilized zirconia (YSZ) coatings are prepared by atmospheric plasma spray (APS) on the stainless steel substrate preheated to different temperatures from room temperature to 1100°C. The microstructure of the coatings is characterized from polished and fractured cross sections by SEM. The ionic conductivities of the coatings are measured using both DC and AC methods, and the relationship between ionic conductivity and microstructure of coatings is examined. SEM observation shows that the coatings exhibit different microstructures with different substrate temperatures. With the increase of substrate temperature, the columnar grain growth continuously across lamellar interfaces is enhanced and subsequently the intersplat bonding ratio in the coating is increased. The ionic conductivity of YSZ coatings at the direction perpendicular to coating surface is significantly increased through the microstructure development by increasing deposition temperature.

It is difficult to deposit dense intermetallic compound coatings by cold spraying directly using the compound feedstock powders due to their intrinsic low temperature brittleness. A method to prepare intermetallic compound coatings in-situ employing cold spraying was developed using a metastable alloy powder assisted with post-annealing. In this study, a nanostructured Fe/Al alloy powder was prepared by ball-milling process. The cold sprayed Fe/Al alloy coating was evolved in-situ to intermetallic compound coating through a post-annealing treatment. The microstructural evolution of the Fe-40Al powder during mechanical alloying and the effect of the post-annealing on the microstructure of the cold sprayed Fe(Al) coatings were characterized by optical microscopy, scanning electron microscopy and X-ray diffraction analysis. The results showed that the milled Fe-40Al powder exhibits lamellar microstructure. The microstructure of the as-sprayed Fe(Al) coating depends significantly on that of the as-milled powder. The annealing temperature significantly influences the in-situ evolution of the intermetallic compound. The annealing treatment at a temperature of 500oC results in the complete transformation of Fe(Al) solid solution to FeAl intermetallic compound.

Nanostructured NiCrAlY coating was deposited by cold spray, using a milled powder for applications as a bond coat to thermal barrier coating. A shot-peening treatment was then applied to the as-sprayed coating to modify the coating surface morphology. The oxidation behavior of the coating with the shot-peened surface was investigated under isothermal oxidation at 900°C and 1,000°C for different times. The oxidation behavior of the coating was characterized through surface morphology and cross-sectional microstructure by scanning electron microscopy and X-ray diffraction analysis. It was found that a uniform oxide layer was formed on the surface of the shot-peened nanostructured NiCrAlY coating during oxidation at temperatures of 900°C and 1,000°C. The nanostructure of the initial coating possibly promoted rapid formation of α-Al 2 O 3 oxide. It was clearly revealed that the surface morphology of the coating significantly impacted the morphology of the oxide. The surface geometry of the cold-sprayed MCrAlY coating must be modified to promote formation of a protective oxide film during oxidation, through application of a post-treatment process such as shot-peening.