Title

High-velocity oxygen fuel thermal spray deposition of nanostructured tungsten carbide-cobalt composites

Date of Completion

January 2001

Keywords

Engineering, Metallurgy

Degree

Ph.D.

Abstract

The consolidation of WC-Co powder with a ∼50 nanometer (nm) WC grain size (nWC) was investigated using high-velocity oxygen-fuel (HVOF) thermal spray deposition. Initial baseline data representative of the state-of-the-art of thermal spray deposition of WC-Co coatings was established using Diamalloy™ 2003, a premium grade WC-Co powder, and a Sulzer-Metco Diamond Jet™ (DJ) system. An increased understanding of the HVOF thermal spray process was established via a correlation of DJ process variables to the mechanisms of WC-Co coating defect formation. The critical HVOF process variables were identified as WC-Co particle size, DJ gun-to-substrate distance, DJ gun translation velocity, and powder feed rate, and a mechanism for controlled porosity formation in coatings was identified. These data were used to formulate a HVOF optimizing procedure which was used for DJ thermal spray of nWC-Co coatings. ^ As-synthesized nWC-Co particles are hollow shells with diameters of 25–125 μm. Incomplete matrix melting of as-synthesized nWC-Co occurred during W thermal spray and indicated the need for higher density particles. Heat treatments at 900–1400°C reveal that insignificant densification of the hollow shell nWC-Co particles occurred prior to their sintering at contact points, and indicated that mechanical reprocessing was required. To this end a process using ultrasonic disintegration of nWC-Co powder and spray dry re-agglomeration was developed. ^ DJ thermal spray of reprocessed nWC-Co and nWC-Co+VC (vanadium carbide is a WC grain growth inhibitor) powders demonstrated that VC did not effect nWC grain size of coatings. DJ thermal sprayed nWC-Co coatings were near fully dense with a mean WC grain size of ∼200 nm, but displayed extensive nWC oxidation. The addition of 2.5 and 5 wt % C during reprocessing of nWC-8Co powder significantly lowered the oxidation of nWC during DJ thermal spray. Addition of 5 wt % C produced a slightly lower nWC-8Co coating hardness than 2.5 wt% C, due to η carbide formation. The W thermal spray of nWC-8Co-2.5C powder using nitrogen DJ gun cooling demonstrated that exclusion of oxygen from the cooling gas caused a significant additional reduction in the oxidation of nWC, yielding a coating hardness value superior to Diamalloy 2003 coatings. ^