Mechanical properties of WC-Co coatings prepared by cold spraying (CS) and warm spraying (WS) have been studied with changing material parameters of Co content (12~25%), powder size (-45+15 and -20+5 µm) and WC particle size (0.2 and 1.8 µm) in this paper. The study reveals that a formation of undesirable phases such as W 2 C, W, and amorphous or nanocrystalline Co-W-C (eta) phase has been suppressed in the CS and WS coatings. Both coatings have high hardness, which is comparable to or superior to HVOF coatings as well as higher density (low porosity) than the HVOF. Abrasion wear test has shown that WS coatings has higher resistance than CS coatings within this study. As for powder properties, smaller powder and smaller WC particle sizes are effective to produce hard and dense coatings leading to higher wear resistance.

Warm Spray (WS) process, which can control the temperature of a combustion gas jet used to propel powder, has been successfully applied to deposit WC-Co coatings. Detrimental reactions resulting from dissolution of WC into Co binder and decarburization were suppressed effectively by keeping the WC-Co particles’ temperature below the m.p. of the binder phase. In this study, three nano-structured WC-12Co powders with different particle strength were prepared by changing the sintering conditions of spray-dried powder and were deposited by WS. The deposition efficiency and porosity of the coatings decreased with increasing the particle strength. The coating deposited from the powder with very low particle strength showed significant phase changes, while those deposited from the higher particle strengths showed almost no change. Particle Image Velocimetry revealed significant disintegration of the weakest powder, which explains the changes observed. The hardness and wear properties of the former coating, therefore, were inferior to the other two.

In this present work, WC-Co coatings with different Co contents were deposited by warm spraying using two different powder sizes and their microstructure, hardness, fracture resistance, and wear properties were investigated. The coatings produced from fine powders showed higher hardness and better wear behavior for all Co contents than those deposited from coarse powders, which is attributed to improved splat-splat bonding and a reduction in porosity that comes with using fine powder.

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 the spraying. To eliminate these degradation, the warm-spray (WS) process originally developed by our group, which provides a possibility to control the flame temperature and the fabrication of WC-Co coatings can be made at lower temperature ranges that those of HVOF process, was applied to deposit WC-Co coatings. Microstructural characterization and phase analysis were carried out on deposited coatings by SEM and XRD. The mechanical properties such as hardness, fracture toughness, and wear properties were investigated. The results showed that WS coatings did not contain any detrimental phase such as W 2 C and W, which are usually observed in HVOF coatings. The hardness of WS coatings were lower than those of HVOF coatings, however, the relation of hardness-Co content of WS coatings showed the similar trend as that of the sintered WC-Co. The improvement of wear behavior was also observed in WS coatings.

In recent years, nano alumina/titania ceramic composite coatings have been investigated and exhibited very attractive properties in several mechanical applications such as wear resistance. One of the reasons for significant improvement of those coatings properties is considered to be “nanostructure” preserved in the coating during depositions. Thus, it is of interest to introduce nanostructure with high aspect ratio into coatings and to investigate the effects on the coating properties, especially fracture properties. In this study, alumina/titania composite powders were fabricated by spray dry procedure, which consist of the alumina nano particle (~50nm) and the titania nano fibers (diameter 100~200nm, length 2~3µm). The developed powders were sprayed by APS process. The deposited coatings contain fibrous titania structures which were not melted during deposition. Fracture resistance was evaluated as a function of a crack length by Double Cantilever Beam (DCB) Test for the coatings fabricated under various spray conditions and corresponded to the microstructure.

Twelve commercially available WC-Co powders with different average carbide sizes (0.2, 2, and 6 µm) and cobalt contents (8, 12, 17 and 25 wt.%) were sprayed on carbon steel substrates using High Velocity Oxy-Fuel (HVOF) spraying Characterization of the coatings showed that the average carbide sizes and carbide volume contents in the coatings were lower than those of feedstock powders. Hardness and fracture toughness of the coatings were investigated using indentation techniques. Young’s modulus was measured by an ultrasonic technique. The hardness and Young’s modulus decreased with increasing cobalt content, while fracture toughness slightly increased. The effect of carbide size on the hardness showed no specific trend. These behaviors were discussed with the help of microstructure observations of the coatings by scanning electron microscopy, X-ray diffraction and chemical analysis. Using an improved HVOF process with a gas shroud could result in less decomposition of the powder and higher fracture toughness.