Grinding effects on surface integrity, flexural strength and contact damage resistance of coated hardmetals
This thesis assesses the influence of substrate surface integrity on different mechanical (flexural strength and contact damage resistance under spherical indentation) and tribological (scratch resistance as well as cracking and delamination response under Brale indentation) properties for a TiN-coa...
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| Formato: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2016 |
| País: | España |
| Recursos: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/396135 |
| Acesso em linha: | http://hdl.handle.net/10803/396135 https://dx.doi.org/10.5821/dissertation-2117-96364 |
| Access Level: | acceso abierto |
| Palavra-chave: | Àrees temàtiques de la UPC::Enginyeria dels materials 620 66 |
| Resumo: | This thesis assesses the influence of substrate surface integrity on different mechanical (flexural strength and contact damage resistance under spherical indentation) and tribological (scratch resistance as well as cracking and delamination response under Brale indentation) properties for a TiN-coated fine-grained hardmetal grade (WC-13 wt.%Co). In doing so, three different surface finish conditions are studied: as-sintered (AS), ground (G), mirror-like polished (P) and ground plus thermal annealed (GTT). Moreover, a relevant part of the work is devoted to nude hardmetal substrates. The result indicates that grinding induces significant alterations in the surface integrity. It yielded in high roughness and emergence of a topographic texture; anisotropic distribution of microcracks within a thin subsurface layer; severe deformation, microstructure refinement and phase transformation of binder regions; and large compressive residual stresses. Subsequent ion etching and coating deposition resulted in a significant residual stresses decrease while damage induced by grinding was not completely removed in the substrate surface. On the other hand, high temperature annealing (GTT condition) completely relieved the referred residual stresses, but without inducing any additional change in terms of existing damage. This was not the case for the metallic binder phase where such treatment induced an unexpected microporosity, development of a recrystallized subgrain structure and reversion of grinding-induced phase transformation. Strength of hardmetals was significantly enhanced by grinding, as compared to AS and P conditions. Such beneficial effect is partly lost during the subsequent ion etching and coating deposition stages. On the other hand, strength of coated GTT condition increases compared to that of the corresponding uncoated one. Systematic residual stress analysis combined with extensive fractographic inspection reveal that strength variations measured after individual manufacturing chain or heat treatment steps (grinding, ion-etching, coating and/or thermal annealing) may be rationalized on the basis of effective residual stress state and location, either at the surface or at the subsurface, of strength-controlling flaws. Independent of substrate surface finish, coated AS, G and P samples exhibit similar critical load for initial substrate exposure as well as same predominant failure mode as they get scratched. However, clear differences in the failure scenario were evidenced. Scratch track for G samples exhibited discrete and localized substrate exposure, compared to the more pronounced and continuous exposure for AS and P ones. On the other hand, GTT samples showed lower critical load and changes in the mechanisms for the scratch-related failure; the latter depending on the relative orientation between scratch and grinding directions. Coated hardmetals exhibit more brittleness and lower adhesion strength, under Brale indentation testing conditions, with decreasing binder content. Grinding is discerned to promote delamination, compared to the polished condition, but also to strongly inhibit radial cracking. Such a response is analyzed on the basis of the interaction between elastic-plastic deformation imposed during indentation and several grinding-induced effects: remnant compressive stress field, pronounced surface texture and microcracking within a thin microcracked subsurface layer. Contact damage resistance of coated hardmetals, subjected to spherical indentation, is enhanced by grinding of the substrate previous to coating stage. Such beneficial effects are discerned regarding both crack nucleation at the coating surface and subsequent propagation into the hardmetal substrate. The grinding-induced compressive residual stresses are pointed out as the main reason for the improved response against contact loading. Such statement is sustained by the lower damage resistance evidenced in coated GTT specimens. |
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