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American Society for Testing and Materials

Cold gas dynamic spray or simply cold spray (CS) is a process in which solid powders are accelerated in a de Laval nozzle toward a substrate. If the impact velocity exceeds a threshold value, particles endure plastic deformation and adhere to the surface. Different materials such as metals, ceramics, composites and polymers can be deposited using CS, creating a wealth of interesting opportunities towards harvesting particular properties. CS is a novel and promising technology to obtain surface coating, offering several technological advantages over thermal spray since it utilizes kinetic rather than thermal energy for deposition. As a result, tensile residual stresses, oxidation and undesired chemical reactions can be avoided. Development of new material systems with enhanced properties covering a wide range of required functionalities of surfaces and interfaces, from internal combustion engines to biotechnology, brought forth new opportunities to the cold spraying with a rich variety of material ...

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Cold spray coating: review of material systems and future perspectives

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Binder jet 3D printing—Process parameters, materials, properties, modeling, and challenges

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The growth of thermally grown oxide (TGO) layers and their influence on crack formation were studied for two thermal barrier coating (TBC) systems with CoNiCrAlY bond coats produced by (i) air plasma spray (APS) and (ii) high-velocity oxy-fuel (HVOF) techniques. All samples received a vacuum heat treatment and were subsequently subjected to thermal cycling in air. The TGOs were predominantly comprised of layered alumina, along with some oxide clusters of chromia, spinel and nickel oxide. However, after extended oxidation, the alumina layer formed in the APS-CoNiCrAlY bond coat transformed to chromia/spinel, while that formed in the HVOF-CoNiCrAlY bond coat remained stable. TGO thickening in the APS-CoNiCrAlY bond coat generally exhibited a three-stage growth behavior, which resembles a high temperature creep curve, whereas growth of the alumina layer in the HVOF-CoNiCrAlY bond coat showed an extended steady-state stage. Crack propagation in these two TBCs was found to be related to the growth and coalescence of oxide-induced cracking, connecting with pre-existing discontinuities in the topcoat. Hence, crack propagation during thermal cycling appeared to be controlled by TGO growth.

TGO growth behaviour in TBCs with APS and HVOF bond coats

The oxidation behavior of an air-plasma-sprayed thermal barrier coating (APS-TBC) system was investigated in both air and low-pressure oxygen environments. It was found that mixed oxides, in the form of (Cr,Al)2O3·Ni(Cr,Al)2O4·NiO, formed heterogeneously at a very early stage during oxidation in air, and in the meantime, a layer of predominantly Al2O3 grew rather uniformly along the rest of the ceramic/bond coat interface. The mixed oxides were practically absent in the TBC system when exposed in the low-pressure oxygen environment, where the TBC had a longer life. Through comparison of the microstructures of the APS-TBC exposed in air and low-pressure oxygen environment, it was concluded that the mixed oxides played a detrimental role in causing crack nucleation and growth, reducing the life of the TBC in air. The crack nucleation and growth mechanism in the air-plasma-sprayed TBC is further elucidated with emphasis on the Ni(Cr,Al)2O4 and NiO particles embedded in the chromia.

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Oxidation and crack nucleation/growth in an air-plasma-sprayed thermal barrier coating with NiCrAlY bond coat

Vibrational response of a beam with a breathing crack

The growth of a thermally grown oxide (TGO) layer and its influence on cracking was studied in an air-plasma sprayed (APS) thermal barrier coating (TBC) following thermal cycling. The TGO that formed upon thermal exposure in air was comprised predominantly of layered chromia and spinels, as well as some oxide clusters of chromia, spinel and nickel oxide. The increase in thickness of the TGO exhibited a three-stage growth phenomenon. Cracks formed mostly at oxide clusters as well as at the opening of discontinuities. Crack propagation during cyclic oxidation appeared to be related to TGO growth, with a nearly linear relationship between crack length and TGO thickness.

The growth and influence of thermally grown oxide in a thermal barrier coating

In thermal barrier coating (TBC) systems, spinel and nickel oxide formed in an oxidizing environment are believed to be detrimental to TBC durability during service at high temperatures. The present study shows that in an air-plasma-sprayed (APS) TBC with Co–32Ni–21Cr–8Al–0.5Y (wt.%) bond coat, pre-oxidation treatments in low-pressure oxygen environments can suppress the formation of the detrimental oxides by promoting the formation of an Al 2 O 3 layer at the ceramic topcoat/bond coat interface. The development of the thermally grown oxide (TGO) layer generally exhibits a three-stage growth phenomenon that resembles high temperature creep. The pre-oxidation treatments reduce the growth rate and extend the steady-state growth stage, leading to an improved durability. Crack propagation in the TBC proceeds via opening and growth of pre-existing discontinuities in the ceramic topcoat, assisted by crack nucleation and growth associated with the TGO. Crack propagation during thermal cycling appeared to be controlled by TGO growth, and the maximum crack length and TGO thickness generally have a power law relationship.

Xijia Wu

Papers

TGO growth behaviour in TBCs with APS and HVOF bond coats

Oxidation and crack nucleation/growth in an air-plasma-sprayed thermal barrier coating with NiCrAlY bond coat

Vibrational response of a beam with a breathing crack

The growth and influence of thermally grown oxide in a thermal barrier coating

Pre-oxidation and TGO growth behaviour of an air-plasma-sprayed thermal barrier coating