The use of thermal spray technology to prepare WC/Co-based cermet coatings is currently a research hotspot in the field of materials surface and engineering. By employing thermal spray technology to produce tungsten carbide/cobalt cermet coatings, a wide range of coating and substrate materials can be selected, the coating thickness can be varied over a broad range, deposition efficiency is high, and the resulting coatings exhibit excellent wear resistance.
The properties and structural characteristics of coatings prepared by thermal spray technology are largely determined by the morphology of the feedstock powder, the energy flux applied to the feedstock, and the spray process parameters. In fact, the morphology and particle size distribution of the powder feedstock are closely related to the preparation method. In recent years, WC-based thermal spray powders have been used to produce dense coatings with high hardness and high wear resistance via high-velocity oxy-fuel (HVOF) spraying, making them the most promising candidates to replace electroplated hard chromium coatings. In particular, WC-Co coatings have already been adopted as substitutes for electroplated hard chromium layers.
Preparation Technologies for WC-Co Composite Powders
There are various methods for preparing WC-Co composite powders. Depending on the preparation process, WC-based powders used for HVOF spraying can exhibit morphologies ranging from spherical to irregular or blocky shapes. Currently, a popular approach is the direct synthesis of composite powders, utilizing the fluidization characteristics of a fluidized bed medium to continuously reduce and carburize chemically uniform tungsten-cobalt compounds within the bed, ultimately yielding WC-Co composite powders with fine particle size and uniform distribution. The preparation methods for WC/Co-based powders can be classified into solid-phase methods, gas-phase methods, liquid-phase methods, and combinations thereof (such as gas-solid phase methods). Representative techniques include the plasma method, gas-phase carburization, co-precipitation, mechanical alloying, and thermochemical synthesis (spray drying). However, the composite powder particles produced by these methods are generally fine in size, have large specific surface areas, and exhibit very low flowability and poor tap density, which can easily lead to nozzle clogging during spraying. Moreover, they lack sufficient inertia for effective deposition onto the substrate surface. Therefore, these powders must be agglomerated into micron-sized powders ranging from 15 to 100 μm for use as thermal spray feedstocks. Typically, processing methods such as sintering-crushing, agglomeration-sintering, mixing, cladding, and fusion-crushing are employed to agglomerate the powders to meet the particle size requirements for thermal spraying.
Powders prepared by the sintering-crushing method are generally irregular in shape with multiple sharp edges, possessing relatively smooth surfaces, a high degree of alloying, and dense internal microstructures. A large number of WC particles are distributed within the binder phase, and the individual WC particles exhibit good bonding with the binder.
The fusion-crushing method involves mixing different powders and melting them in a specialized furnace, followed by crushing the cooled sintered blocks using various crushers. The powders produced by this method are characterized by a dense, blocky, and angular morphology. They are considerably more difficult to crush than powders prepared by the sintering-crushing method.
Powders produced by the agglomeration-sintering method have a regular morphology. This technique uses spray drying to agglomerate powders from a self-suspending slurry; after spray drying and sintering into agglomerated particles, the resulting powders are nearly spherical, with rough and porous surfaces, a narrow particle size distribution, and good flowability. However, preparing suspensions for cermets such as WC-Co is challenging due to the high density of WC-Co and the differences in acidity/basicity among the constituent particles.
The cladding method typically employs mechanical fusion, in which raw powders of different particle sizes are thoroughly mixed and heated. During this process, the lower-melting-point material is brought to a plastic state (or even melted in some cases), while the higher-melting-point material remains unmelted. Mechanical mixing then causes the two powders to weld together. Composite powders produced by this method exhibit a core-shell structure, with good particle strength and toughness.
The mixing method produces WC/Co powders consisting of two phases, WC and Co, in which WC particles are loosely held together by cobalt.
There are relatively few reports on the preparation of WC/Co-based thermal spray powders, and even fewer on the preparation of WC-CoCr powders containing added Cr elements. Only a handful of companies worldwide produce such products. The addition of an appropriate amount of metallic Cr to the Co binder phase can significantly improve the corrosion resistance and oxidation resistance of the coatings, resulting in superior overall performance that can replace toxic electroplated hard chromium. As a result, their application fields are expanding, and they are attracting increasing industrial attention. At present, most domestic production in China relies on the sintering-crushing and agglomeration-sintering methods.