AS a result of their importance in industrial applications usually with respect to wear resistance requirements, WCCo coatings have been widely studied.The WC parti cles, with high hardness, provide a wear-resistant constit uent, while the Co binder contributes mechanical support, and hence, toughness in the coatings. Hardness, wear resis tance, and strength depend primarily on the size and volume fraction of WC particles. In a related, recent study, it was reported that a bulk nanostructured WC-Co material exhib ited higher hardness and toughness in comparison with the conventional counterpart material.
The abrasion resistance of sintered WC-Co materials is strongly dependent on hardness, while that of thermal sprayed WC-Co coatings is significantly related to the rela tive resistance to indentation fracture. Under a load of 1000 g, cracks caused by an indentation along the lamella structure are observed in the conventional coating. In sintered hard metal cements, four symmetrical indentation cracks were observed on the extended lines of the diagonals related to the quadrilateral pyramid indentation, where a maximum stress concentration caused by indentation is present. Whereas, in the present coatings, indentation cracks usually appear in the direction parallel to the coating surface, they are not present directly on the extended line of the diagonal of the indentation. No cracks are observed in the direction perpendicular to the coating surface. These results show that crack propagation within the coatings has a favored direction, and higher resis tance to indentation fracture is present in the direction per pendicular to the coating surface. The presence of a preferred direction for crack propagation is attributed to the thermal spray process. A complete coating was gradually produced by passing the thermal spray torch across the substrate 15 times. Because each layer had once been the outer sur face, there was a potential for contamination to occur and become covered by the next layer. Moreover, at layer boundaries, discontinuity in heat and mass transfer may have led to a discontinuity in the behavior of the coating.
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