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In high-velocity oxy-fuel (HVOF) spraying of WC-Co coatings, the decomposition and decarburization of WC during deposition are responsible for their much lower toughness compared with a sintered bulk WC-Co. In a previous study, Warm Spray (WS) process, which is capable to control the flame temperature used to propel powder particles, was successfully applied in an attempt to suppress such detrimental reactions by keeping particlesÕ temperature lower than their melting point. The coatings deposited by WS process showed no or little formation of W 2 C and g phases and demonstrated moderately improved fracture properties. However, there is still a gap in fracture toughness between WS coatings and the corresponding sintered bulk. In order to optimize the properties of the WS coatings, the effect of original powder sizes were investigated. Microstructural characterization and phase analysis were carried out on deposited coatings by SEM and XRD. The results show that the feedstock powder size has substantial effects on the properties of the coatings, i.e., the smaller powder showed improved properties.

The high-velocity oxy-fuel (HVOF) process is commonly used to deposit WC-Co coatings. There are some problems with this process; especially the decomposition and decarburization of WC during spraying make a coating brittle. To suppress such degradation, the warm spray (WS) process was applied to deposit WC-Co coatings, which is capable of controlling the flame temperature in the range of 500-2000°C. The microstructure and phases of the deposited coatings were characterized by using SEM and XRD, and the mechanical properties such as hardness, fracture toughness, and wear properties were also investigated. WS process successfully suppressed the formation of the detrimental phases such as W 2 C and W, which are usually observed in HVOF coatings. The WS coatings showed the similar trend of the hardness variation for Co content with a sintered bulk material. Improvement of toughness and wear behavior was also observed in WS coatings.

In this work, acoustic emission (AE) monitoring was correlated with in-situ optical microscopy observation and electron backscatter diffraction (EBSD) measurements to investigate the evolution of a single stress corrosion crack in SUS420J2 stainless steel subjected to chloride droplet corrosion. A single dominant crack evolution was observed to transition from a slow initiation of active path corrosion-dominant cracking to a rapid propagation of hydrogen-assisted cracking. The initiation-to-propagation was concomitant with a significant increase in the number of AE events. In addition, a cluster analysis of the AE features including traditional waveform parameters and fast Fourier transform (FFT)-derived frequency components was performed using k-means algorithms. Two AE clusters with different frequency levels were extracted. Correlated with the EBSD-derived kernel average misorientation (KAM) map of crack path, low-frequency AE cluster was found to correspond with the location of plastic deformation in the propagation region. High-frequency AE cluster is supposed to be from the cracking process. The correlation between AE feature and SCC progression is expected to provide an AE signals-based in-situ insight into the SCC monitoring.

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