Near-nanostructured WC-18 pct Co coatings with low amounts of non-WC carbide phase: Part I. Synthesis and characterization

Abstract

Near-nanostructured WC-18 pct Co coatings, with low amounts of non-WC carbide phases, have been synthesized using high velocity oxygen fuel (HVOF) thermal spraying under spraying conditions of varying fuel chemistry, fuel-oxygen ratio, and powder particle size. The results show that the temperature the particles experience during spraying depends on the preceding parameters. Compared to available published results on WC-Co system coatings, nanostructured WC-18 pct Co coatings, synthesized in these experiments, contain very low amounts of non-WC carbide phase (less than 10 pct vol). This is comparable to that of the conventional WC-12 pct Co coating, prepared in the present study for comparison purposes. Regardless of whether the binder phase in the agglomerated feedstock powder particles melt or not, the WC particles do not appear to experience significant growth as a result of the spraying. The size of WC particles remains in the 200 to 500 nm range, consistent with that present in the feedstock powder. The as-received nearnanostructured WC-18 pct Co feedstock powder exhibits morphological characteristics that lead to low amounts of non-WC carbide phases in the coatings. The microstructure and phase constitution of the coatings depend on particle size of the feedstock powder and flame characteristics of the fuels during spraying. A higher particle temperature causes more decomposition of the WC phase but reduces porosity in the coatings, this occurs with higher flame temperature and smaller particle sizes. Propylene fuel produces less decomposition of the WC phase despite the higher flame temperature and, thus, provides the best combination of dense coating with low amount of non-WC phase.

A comparative study on the microstructure and surface property evaluation of coatings produced from nanostructured and conventional WC–Co powders HVOF-sprayed on Al7075

“…Hence, the sprayed particles should be heated just to melting of the binder phase (through spray parameter control); this way, the spray particles will obtain the necessary deformability upon impact to conform to the surface of the previously deposited layer and fill in pores, cavities and other defects [20]. A low coating porosity is also benefited by a dense and uniform powder feedstock [21].…”

Nanocrystalline Ni coatings strengthened with ultrafine particles

“…The porosity in the coatings depends primarily on the temperature that the powder experiences during spraying, and the higher the powder temperature, the lower the porosity in the coatings and the higher the wear resistance. [37,38] Thus, unique characteristics of the agglomerate size of the (Ni ϩ 2 wt pct AlN) powder may (b) benefit the achievement of coatings having a high fracture toughness.…”

Comparative Abrasive Wear Study of HVOF Coatings Obtained by Spraying WC-17Co Microcrystalline and Duplex Near-Nanocrystalline Cermet Powders

“…It is generally accepted that the degree of powder particle melting and the presence of liquid fraction in the sprayed droplet influence the extent of WC dissolution in the Co matrix and the development of splat morphology [10]. The cross-section of the "duplex Co coated" coating showed coarser splats from impacting droplets, indicating a lower degree of powder melting on impact to form the lamellae (see Fig.…”