The aim of this study is to investigate the effect of multimodal powders and annealing on cold sprayed deposits. In the experiments, a multimodal WC-Co powder containing micro- and nano-sized particles was prepared by ball milling and deposited on stainless steel substrates. The hardness of the resulting layer was comparable to that of the nanostructured WC-Co bulk and its fracture toughness reached a level close to that of the microstructured bulk. It is thus shown that with multimodal particle distribution cold-sprayed WC-Co can be simultaneously strengthened and toughened. Investigators also show that interface cohesion between WC and cobalt phases can be improved by annealing, which leads to further toughening as well.

In this study, WC-12Co powders with multimodal-sized WC particles were prepared by ball milling and deposited onto stainless steel substrates by cold spraying. Microhardness and fracture toughness were measured on cross-sections by the indentation approach. Coatings produced from powders with small carbide particles, resulting from long milling times, were found to exhibit high microhardness and high fracture toughness. Deposits sprayed with high porosity powders produced by cold compaction had low microhardness, but high fracture toughness.

Deposition of a dense coating with solid particles by cold spraying requires sufficient deformation of impacting particles and previously deposited underlying particles. The cermet particles and subsequent coating with a high hardness are difficult to deform upon impact. To increase the ability of deformation, the cermet spray particles with a porous structure design is proposed to fulfill the requirements of deformation on impact. To understand the deposition mechanism, the deposition behavior of single WC-Co spray particles impacting on the substrates with different hardnesses during cold spraying were examined using WC-12Co powders with different porosity. The substrates include stainless steel, nickel-based self-fluxing alloy coatings were employed to examine the effect of substrate deformation on the cermet particle deposition. It was found that using two porous powder of the porosity of 30% and 44% the WC-Co cermet particles were deposited on the substrate of different hardness from 200Hv to 800Hv. The deposition of the particles is mainly attributed to the deformation of powders themselves. The properly designed porous cermet powder with certain hardness is necessary condition to deposit hard WC-Co cermet coating.

The carbide decomposition and the dissolution of carbide into the binder accompanying with thermal spraying of cermets can be eliminated during cold spray deposition of WC-Co. However, the limited deformation of hard impacting cermet particles and impacted coating makes it difficult for conventional thermal spray powders to continuously build up on impact in cold spraying. The porous structure of WC-Co powders provides the powder particles with certain deformability on high velocity impact, which benefits the continuous building-up of coating. In this study, three nano-structured WC-12Co powders with different porous structures and apparent hardness were employed to deposit WC-Co coatings on stainless steel substrate by cold spraying. The deposition characteristics of three powders of different porosity levels of 44%, 30% and 5% were investigated. It was found that WC-Co coating is easily built-up using the porous powders with WC particles bonded loosely and a low hardness. The microhardness of WC-12Co coatings varied from Hv400 to Hv1790 with powders and spray conditions, which depends on the densification effects by impacting particles. With porous WC-Co powders, the fracture of particles on impact may occur and low deposition efficiency during cold spraying. The successful coating building up at high deposition efficiency depends on the design of powder porous structure.

Nickel titanium is promising cavitation erosion resistant material. Using NiTi in bulk for components might not be feasible due to its poor workability, as well as the high material and processing costs. Surfacing components with its coating is effective for utilizing the good erosion properties of NiTi intermetallic compounds. In this study, a method to prepare NiTi intermetallic compound coatings in-situ through annealing of the cold-sprayed Ni(Ti) metastable coating was investigated. A nanostructured Ni(Ti) solid solution alloy powder was prepared by ball-milling process. The cold sprayed Ni(Ti) alloy coating was used as the precursor coating. The effect of annealing temperature on the microstructure in-situ evolution of Ni-Ti intermetallic compound in cold-sprayed coating was investigated. The morphology and phase composition of the powders milled for different durations and the microstructure of the as-sprayed coating were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that after annealing at 950°C the Ni(Ti) alloy was transformed to intermetallic phases. NiTi, Ni 3 Ti and NiTi 2 intermetallic phases coexisted in the annealed coating.

Nano-structured WC-Co coating was deposited by cold spraying using a nanostructured WC-12Co powder. The critical velocity for the particle to deposit was measured. The coating microstructure was characterized by X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy. The coating hardness was tested by using a Vickers hardness tester. The deposition behavior of single WC-Co particles was examined. WC particle size was also measured for comparison of deposit properties to that of sintered bulk. The result shows that the nanostructured WC-Co coating can be successfully deposited by cold spraying using nanostructured powders. The coating exhibited a dense microstructure with full retention of the original nanostructure in the powder to the coating. The test of microhardness of the coating yielded a value of over 1820 Hv0.3, which is comparable to that of sintered nanostructured WC-Co. The deposition behavior of WC-Co powders as superhard cermet materials in cold spraying and powder structure effects are discussed.