WC-Co/copper coatings with 8 layers were fabricated by warm spray deposition in order to investigate the effect of ductile layer inclusion on their fracture behavior. Bending strength, work of fracture, and surface hardness of freestanding coatings were examined by three point bending tests after removal of the substrates. The multilayered samples showed nonlinear stress-strain curves due to cracks in the WC-Co layers and plastic elongation of the copper layers. The multilayered samples with lower volume fraction of copper showed even lower bending strength than the monolithic samples of WC-Co and copper and no beneficial feature in mechanical performance was found. On the other hand, the samples containing higher volume fraction of copper exhibited more than twice higher work of fracture and moderately better bending strength than the monolithic WC-Co coatings, while the surface hardness was almost identical among all samples instead of the monolithic copper. The ductility of copper layers and the plastic constraint by the intact WC-Co layers attributed to enhance their mechanical properties. It has been concluded that cermet/metal laminate coatings can be one alternative approach to further improvement of the mechanical properties of thermal sprayed cermet coatings.

Oxidation is a serious issue for thermally sprayed alloy coatings, especially in atmospheric plasma spray (APS) process, by which it is believed to be impossible to obtain alloy coatings with low oxide content. Oxidation behavior of Ni20Cr alloy and NiCrSiBC alloy in APS were compared by evaluating collected in-flight particles and sprayed coatings in our previous study. The results clearly show that the oxide content in the in-flight particles and coating of NiCrSiBC alloy was significantly lower than that of Ni20Cr alloy. In the present study, further study of the surface oxide layer of NiCrSiBC coating was carried out by TEM, which showed 10 nm thick oxygen enriched layer. The effects of particle temperature and velocity on the oxidation of the NiCrSiBC alloy were investigated. It was found that increasing particle temperature and velocity did not affect the oxidation of inflight particle of the NiCrSiBC alloy but significantly increased the oxidation after particle impacting substrate. The results are discussed in conjunction with effectiveness of preferential oxidation of Si, B, C and simultaneous vaporization of the formed oxides on suppressing in-flight oxidation and influence of a large amount of sub-particles produced upon impacting substrate on oxidation characteristics when increasing plasma power.

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.

In Warm Spraying (WS), the temperature of the combustion flame is reduced and controlled by injecting nitrogen gas into the combustion flame before the injection of spray powders. Thus, temperatures of spray particles are kept under their melting points with moderately heated and thermally softened states. As compared to HVOF-spraying, the oxidation of particles can be significantly suppressed due to lower deposition temperatures, whereas, as compared to cold spraying, the degree of particle deformation upon impact can be enhanced by attaining higher particle temperatures. In the present study, Ti, Cu, and Al coatings were fabricated by WS under various nitrogen flow rates. The mechanical properties of the coatings were evaluated by tubular coating tensile (TCT) and micro flat tensile (MFT) tests. For the lower impact temperature regime, the coatings became denser and the ultimate strengths of Ti or Cu coatings increased reaching a maximum by decreasing the nitrogen flow rates. A further decrease of nitrogen flow rates and reaching the upper temperature regime reduced the coating strength. These results clearly demonstrate how particle temperatures affect the microstructures and mechanical properties of WS coatings and that optimum spray conditions have to be balanced between softening and oxidation by adjusting particle temperatures.

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.

WC-Co thermal sprayed coatings are mainly used for wear protecting functions in various industries, for which high velocity oxy fuel (HVOF) spray is considered to be the best suited process. However, WC-Co HVOF coatings still have some defects as compared with sintered bulk, such as decarburization of WC and porous structure. Recently, experiments of WC-Co coatings using warm spray (WS) and cold spray processes have demonstrated some improvements in reduction of these defects. In particular, WS process seems to be a more promising process for WC-Co coatings from the previous work. In this study, wear resistant functions of WC-12%Co coatings prepared by HVOF and WS were investigated by abrasion and erosion tests. In addition, in-flight particles were captured and their characteristics such as the amount of decarburization, crystal phase, particle strength and particle size distribution were investigated to clarify the difference between HVOF and WS processes. The result shows that the wear resistances of the WC-Co WS coatings are comparable or superior to those of the HVOF coatings, which can be attributed to the difference in the amount of W 2 C and coatings porosity revealed by the in-flight particles and the coating microstructure. The result of the in-flight particle analysis also indicates that wear resistance of WS coatings can be further improved by optimizing the powder shape and chemical composition.

WC-Co cermet coatings were fabricated by using Warm Spraying, which is a modification of HVOF spraying to lower the temperature of the propellant gas below the melting point of Co. By changing the processing parameters, specimens were prepared for hardness, abrasion wear and particle erosion tests. Their microstructures were examined by SEM and XRD. The microstructure clearly showed the effects of suppression of the dissolution of WC into the Co phase, which is the major cause of embrittlement of the conventional HVOF sprayed WC-Co coatings. By combinations of adequate feedstock powder and processing parameters, it was possible to take advantage of fine WC grain size to prepare coatings with higher hardness (HV > 1400), smoother surface (Ra < 2 μm), and moderately improved wear performances compared with conventional HVOF coatings.

This study investigates the effect of alloying additions on the oxidation behavior of iron (Fe) and nickel-chromium (NiCr) powders during atmosphere plasma spraying. The chemical composition and phases of oxides in the particles as well as in the coatings are assessed for different powder mixtures and spraying parameters. The results show that oxygen content can be significantly reduced by adding silicon (Si) and boron (B) to iron powders and Si, B, and carbon (C) to NiCr. The preferential oxidation and subsequent vaporization of Si, B, and C from the surface of the sprayed particles are believed to play a major role in controlling oxidation in the APS process.

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.