Thick metal/diamond composite coatings on Al substrate were deposited by cold spray process with in-situ powder preheating. Microstructural characterization of the as-sprayed coatings with different diamond size, strength and with/without Ti clad coating was carried out by OM and SEM. Assessment of basic properties such as tensile bond strength, porosity, and hardness of the coatings and the deposition efficiency was also carried out. Particular attention on the coatings was on the diamond fracture phenomenon during the cold spray deposition and the interface bonding between diamond and metal matrix. The potentials and the problems of metal/diamond composite coatings by cold spray are discussed.

This study evaluated the effects of particle temperature on deposition efficiency and critical velocity in kinetic spraying. A wide range of pressures and preheat temperatures of the process gas were used in these experiments to vary both particle velocity and temperature, and a bronze (Cu-Sn alloy) powder was deposited onto aluminum and bronze substrates. The deposition efficiency of the coatings was measured, and the critical velocity was estimated. The experimental results showed that the critical velocities of the bronze feedstock deposited onto either an aluminum or bronze substrate were different and that the critical velocity was strongly dependent on the particle temperature, which when increased, caused the critical velocity to decrease. Increasing the gas pressure caused an increase in particle velocity, while increases in the gas temperature not only affected the particle velocity but also the particle temperature. In our experiments, the critical velocity decreased by 50 m/s when the process gas temperature increased by 100 °C.

Critical velocity has been accepted as a characteristic property of kinetic spraying (or cold gas dynamic spraying), which works by accelerating small solid particles to supersonic velocities and then impacting them onto a substrate. However, there is a lack of information about the impact of individual particles and their deposition behavior over a large range of impact velocities. To probe into the impact behavior of the particles and to elucidate the deposition mechanism, individual particle impaction tests have been carried out. A rebound phenomenon was found to occur at a high impact velocities, in which a large fraction of the particles rebounded. Based on experimental results, a model of a plastic particle impacting onto an un-deformed substrate was developed. The adhesion and rebound energies were calculated to estimate the particle/substrate interactions. A maximum impact velocity was found for particle deposition onto the substrate. The particle deposition behavior was controlled by the adhesion and rebound energies.

The cold spray process is a relatively new process using high velocity metallic particles for surface modifications. Metallic powder particles which are injected into a converging-diverging nozzle are accelerated to supersonic velocities. In this study two-dimensional temperature and velocity distributions of gas along the nozzle axis are calculated and the effects of gas pressure and temperatures on particle velocities and temperatures inside and outside of the nozzle are also investigated. It is found that the acceleration of the gas velocity takes place in the area of the nozzle throat and it increases and reaches maximum value at the nozzle exit. Due to compression shocks in the area after the nozzle, the gas jet properties show irregular shape and these result in the existence of the maximum particle velocity by the change of particle size at a given gas pressure and temperature.

WC-Co powders with nano-sized WC were deposited by dynamic powder deposition process. Microstructural characterization and phase analysis of feedstock powders with different compositions and as-deposited coatings with various substrates were carried out by SEM and XRD. The results show that there is no detrimental phase transformation and/or decarburization of WC by dynamic powder deposition. It is also observed that nano-sized WC in the feedstock powder is maintained in the deposited coatings. It is demonstrated that it is possible to fabricate the nano-structured WC-Co coatings with low porosity and very high hardness by dynamic powder deposition.

Characterization of flame sprayed and furnace fused NiCrBSiC alloy coatings with two different carbon contents and 15~45 wt.% WC-Co addition is described in terms of microstructure, microhardness, and differential thermal analysis. Microstructural development of these coatings before and after fusing treatment is discussed to identify the precipitates in the coatings. Optimum fusing conditions (time and temperature) for wear testing sample are investigated in terms of microhardness and porosity of the coatings. Wear performance of these coatings is also investigated by two-body and three-body abrasive and dry sliding wear experiments. Optimum tungsten carbide content of the coatings is also selected to improve wear performance and thus enhance the service life of the process roll for cold rolling steel plant. Finally, microstructure and microhardness of the furnace and induction fused coatings are compared with emphasis on the interface between the coating and the steel substrate.

The isothermal oxidation behaviors of plasma sprayed NiCrAlY bond coatings were evaluated. Two unique microstructures, characterized by oxide stringers and improperly flattened zones can be observed in the APS bond coatings. Structure and chemical composition of the oxide stringers were examined by TEM and EDS. Improperly flattened zones had a high density of open porosities. These features affected the oxidation behaviors at both transient and steady state stages. Under transient oxidation, NiO, Cr2O3, and α-Al2O3 were present together on the surface. Oxide stringers and improperly flattened zones had an especially high density of NiO and Cr2O3, which implied that these regions had inherently lower contents of Al. During steady state oxidation, continuous α-Al2O3 reduced the diffusion rate of oxygen and, in turn, the rate of isothermal oxidation. Over longer oxidation times, the depletion of Al led to the formation of NiAl2O4 and Cr2O3 layers along with hemispherical protrusions.

This paper aims to clarify the thermal and mechanical properties of zirconium dioxide-cerium(IV) oxide-yttrium oxide and zirconium dioxide-yttrium oxide plasma sprayed coatings. such as phase transformation, bond coat oxidation, and thermal expansion mismatch. Both coatings showed a 7 to 11% porosity fraction and typical lamellar structure, which formed due to continuous wetting by liquid droplets. It is observed that the zirconium dioxide-cerium(IV) oxide-yttrium oxide coating turned out to be better than the zirconium dioxide-yttrium oxide because there was no phase change from the tetragonal to the monoclinic phase (which leads to a spatial expansion), because there was a smaller thermal expansion offset and the effect of the oxide growth stress the better thermal insulation was relatively small. Paper includes a German-language abstract.

Air plasma spray coatings of three different WC-12%Co powders based on the powder manufacturing methods were conducted. S&F (Sintered and Crushed) WC-12%Co and NiCrSiB mixed powder with different ratio were also sprayed. The best wear resistance of S&F WC-12%Co coating was mainly due to its high hardness related with large amount of homogeneously and uniformly distributed retained carbides. The wear resistance of blend coatings increased with increasing WC-12%Co weight percent which was associated with the fact that the wear resistance of all coatings increased with increasing hardness and decreasing friction coefficient. But the exact relation of wear resistance with cohesive bond strength could not be found. The dominant wear mechanisms of WC-12%Co coatings were adhesive wear, tribo-film formation by material transfer, fatigue crack, particle (splat) fracture and delamination (spallation of coating layers). In the case of blend coatings, when the weight percent of WC-12%Co was small plastic deformation and abrasive mark (or scratching) by asperity and/or hard particle were observed. with increased weight percent of WC-12%Co the wear mechanism was closer to that of WC-12%Co coatings where no plastic deformation and abrasive mark was observed.