Synthesis of nanostructured WC-12 pct Co coating using mechanical milling and high velocity oxygen fuel thermal spraying

Abstract

A nanostructured WC-12 pct Co coating was synthesized using mechanical milling and high velocity oxygen fuel (HVOF) thermal spraying. The variation of powder characteristics with milling time and the performance of the coatings were investigated using scanning electron microscope (SEM), X-ray, transmission electron microscope (TEM), thermogravimetric analyzer (TGA), and microhardness measurements. There is no evidence that indicates the presence of an amorphous phase in the sintered WC-12 pct Co powder, and the binder phase in this powder is still crystalline Co. Mechanical milling of up to 20 hours did not lead to the formation of an amorphous phase in the sintered WC-12 pct Co powder. During the initial stages of the milling, the brittle carbide particles were first fractured into fragments and then embedded into the binder phase. This process gradually formed polycrystal nanocomposite powders of the Co binder phase and W carbide particles. The conventional cold welding and fracturing processes primarily occurred among the Co binder powders and polycrystal composite powders. The nanostructured WC-12 pct Co coatings, synthesized in the present study, consist of an amorphous matrix and carbides with an average particle diameter of 35 nm. The coating possesses an average microhardness of 1135 HV and higher resistance to indentation fracture than that of its conventional counterpart.

Nanocrystalline materials and coatings

“…The nanoscale microstructure can be retained in the coatings despite the high temperatures and melting involved in forming a dense coating. Plasma spraying [393][394][395][396][397][398][399] and HVOF spraying [400][401][402][403] of nanostructured feedstock powders of metals or ceramics proved to be potential methods to form nanocrystalline coatings. The HVOF method is preferred to plasma spraying because of its higher droplet velocity and lower thermal energy levels, yielding denser structure, and higher bond strength between the coating and the substrate.…”

Preparation and Properties of a Nanostructured NiCrC Alloy Coating for Boiler Tubes Protection

“…The characteristics of ultra high flying velocity and relatively low temperature for injected feedstock powders favor depositing of nanostructured coatings, 6,10) as the nanofeatures of the powders could remain well after the spraying process. HVOF/HVAF sprayed nanostructured coatings have prove great success in improving the performances of existing material systems, and are believed to be one of the most possible ways for commercial applications of nanomaterials.…”

Synthesis and Characterization of Nanocomposite Coatings

“…This indicates that the coating has a nanostructured microstructure. In the nanostructured WC-12%Co coating [20], TEM examination revealed a microstructure consisting of fano sized WC carbide particles in an amorphous matrix phase. While in the nanostructured Cr 3 C 2 -NiCr coatings, the diffraction pattern does not exhibit clearly the presence of an amorphous matrix phase.…”

“…During mechanical milling, particle welding and fracturing result in severe plastic deformation. Using mechanical milling, Cr 3 C 2 -NiCr and WC-Co nanocomposites were successfully synthesized [19][20][21] in our laboratories.…”