•We examined long-term atmospheric corrosion resistance of thermally sprayed coatings.•The 100 μm thermally sprayed coatings showed high corrosion performance.•The corrosion products formed on the thermally sprayed coatings were identified.•The distribution of elements of S and Cl was observed in the corrosion products.•The surface structures of thermally sprayed coatings were proposed.The long-term atmospheric corrosion properties of thermally sprayed Zn, Al and Zn–Al coatings have been evaluated using an electrochemical impedance measurement and several analytical techniques. All the thermal-sprayed specimens with 100 μm coating thickness have protected the steel substrates. In case of the Zn–Al coating, the red rust was not observed regardless of the coating thickness. The corrosion products were identified by the XRD analysis. Preferential dissolution of zinc was observed on the Zn–Al coating by EPMA analysis. The electrochemical impedance results provided an insight about the surface structures of each thermally sprayed coating.

Thermal barrier coatings (TBC) have been applied extensively onto the high-temperature components in turbine engines to prolong their service life in extremely harsh environments. TBCs are typically composed of a ceramic top coating for thermal insulation and a metallic bond coating (BC) for oxidation resistance and providing adhesion to the top coating. MCrAlY, where M stands for Co, Ni or an alloy of these elements, is a widely used material for BC and usually produced by low pressure plasma spraying (LPPS) in industry. Recently high velocity oxy-fuel (HVOF) spraying is attracting significant attention as a more economical alternative procedure to LPPS. In terms of the quality of sprayed coatings, however, LPPS is still superior in terms of oxygen pick-up during coating preparation, which should affect the performance as a bond coating in service. In this study, a modified HVOF process, so called 2-stage HVOF or warm spray (WS) was applied to deposit a CoNiCrAlY alloy. Comparisons between BCs fabricated by HVOF and WS were made in terms of microstructure, surface morphology, and cyclic and isothermal oxidation behaviors in air at 1423 K up to 100 cycles and 100 h respectively. The results showed that rougher and less oxidized BCs were deposited by the WS process, which exhibited slower kinetics of β-phase depletion during oxidation. A simple Al diffusion model revealed that apparently a small difference in the initial oxidation between the two spraying processes had significant influence on the β-depletion phenomena, which may influence the life time as a bond coating.Highlights► A modified HVOF process called “Warm Spray” was used to deposit CoNiCrAlY alloy. ► This alloy is important as a bond coat (BC) material for thermal barrier coatings. ► Comparison was made between bond coatings by HVOF and WS. ► Rougher and less oxidized BCs were deposited by the WS process. ► A simple diffusion model showed that initial oxidation determines service life time.

A modification of high-velocity oxy-fuel (HVOF) thermal spray process named as warm spray (WS) has been developed. By injecting room temperature inert gas into the combustion gas jet of HVOF, the temperature of the propellant gas can be controlled in a range approximately from 2300 to 1000 K so that many powder materials can be deposited in thermally softened state at high impact velocity. In this review, the characteristics of WS process were analyzed by using gas dynamic simulation of the flow field and heating/acceleration of powder particles in comparison with HVOF, cold spray (CS), and high-velocity air-fuel (HVAF) spray. Transmission electron microscopy of WS and CS titanium splats revealed marked differences in the microstructures stemming from the different impact temperatures. Mechanical properties of several metallic coatings formed under different WS and CS conditions were compared. Characteristics of WC-Co coatings made by WS were demonstrated for wear resistant applications.

A new testing procedure to evaluate the interfacial toughness of thermal-sprayed coatings has been developed. The newly designed test specimen is a modification of the pin test with an artificially introduced weak interface, which is expected to open up easily under tensile loading and act as a circumferential precrack along the interface between a coating and the substrate. This configuration makes it possible to calculate the stress intensity factor KInt at the tip of the precrack, which can be expressed as \( K_{{{\text{Int}}}} = \upsigma_{{\text{0}}} {\sqrt { \uppi a}}F_{{\text{I}}} (a{\text{/}}R) \), where σ0 is the apparent average stress, a the crack length, R the specimen radius, and FI the geometrical correction function. Finite-element analysis was carried out to calculate the correction function FI for various values of a/R. In the experiments, the flat surface of a pin was grit-blasted and a ring-shaped area from the periphery was covered with carbon using a pencil and set into a mating dice. SUS316L stainless steel was plasma-sprayed onto the flat surface of the pin and the dice. Then, tensile load was applied to the pin to break the weak interface containing the carbon and finally the unmodified coating-substrate interface. The load required to pull out the pin was measured for various specimen parameters such as a and R. The results indicate that the adhesion of the tested coatings can be represented by interface toughness of 1.9 ± 0.1 MPa m1/2. As a consequence, this testing procedure can be considered as a viable method to evaluate adhesion of a thermal-sprayed coating on a substrate.