Fracture and fatigue behavior of electrical-discharge machined cemented carbides

Surface and Coatings Technology

Roa

Odén

et al. 2015

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In this study, the influence of substrate surface finish on scratch resistance and associated failure mechanisms is investigated in the case of a TiN-coated hardmetal. Three different surface finish conditions are studied: as-sintered (AS), ground (G), and mirror-like polished (P). For G conditioned samples, scratch tests are conducted both parallel and perpendicular to the direction of the grinding grooves. It is found that coated AS, G and P samples exhibit similar critical load for initial substrate exposure and the same brittle adhesive failure mode. However, the damage scenarios are different, i.e. the substrate exposure is discrete and localized to the scratch tracks for G samples while a more pronounced and continuous exposure is seen for AS and P ones. Aiming to understand the role played by the grinding-induced compressive residual stresses, the study is extended to coated systems where ground substrates are thermal annealed (for relieving stresses) before being ion etched and coated. It yielded lower critical loads and changes in the mechanisms for the scratch-related failure; the latter depending on the relative orientation between scratching and grinding directions. Please find attached electronic files corresponding to our contribution on influence of substrate surface finish on scratch response and induced failure modes for TiN-coated hardmetals, which we (all authors do agree to the submission of the manuscript) offer for publication in Surface and Coatings Technology.I hope it is found satisfactory. Regarding reviewer 1's suggestion for improvement of the manuscript, although it seems rational and suitable, we feel that an additional figure (sketch) in a paper including already 10 other Figures (and most of them with multiple captions) may go beyond the (not written) limit associated with space limitation. Sincerely yours, Luis LlanesAnd aiming to clarify locations in the various surfaces where residual stresses occur, text has been slightly modified (Within last paragraph in section 3.1):… The coated G condition has a maximum compressive stress of about -1.0 GPa in the substrate surface (i.e. just at the coating-substrate interface). As expected, this value is one order of magnitude higher than those assessed for the coated AS and P conditions at similar substrate surface location, i.e. -0.2 and -0.1 GPa respectively. However, it should be noted that such residual stress level at the substrate surface for coated G specimen is lower ( (1) Several authors (Steinmann et. al., Thin Sol. Film., 1987; Bromark et. al., Surf. & Coat. Technol. 1992) We do have the data for plotting the requested curves but we do not see that they provide any additional useful information to the understanding of the results presented. In addition, the length of the scratch grooves is much smaller than the scratch track and sliding distance (as it may be seen in Figures 3, 4, 7 and/or 8). To provide the requested overlays would therefore require additional figures and corresponding text and consequently extend...

Section: Heat Treatment Of Ground Hardmetals (Prior To Tin Coating Dementioning

confidence: 99%

Substrate surface finish effects on scratch resistance and failure mechanisms of TiN-coated hardmetals

Surface and Coatings Technology

Roa

Odén

et al. 2015

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“…In an attempt to provide such information, a residual stress-free ground substrate was studied as the third surface finish condition. High temperature annealing of hardmetals has been validated as a successful protocol for relieving residual stresses, independent of nature (tensile/compressive) or source (mechanical abrasion [27][28][29] or electrical discharge machining [30][31][32]). Hence, ground specimens were heat treated at 920 °C for 1 h in vacuum, and the resulting surface finish condition is here referred to as GTT.…”

Section: Materials and Substrate Surface Finish Conditionsmentioning

confidence: 99%

Contact damage resistance of TiN-coated hardmetals: Beneficial effects associated with substrate grinding

Surface and Coatings Technology

Marro

Trifonov

et al. 2015

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Contact loading is a common service condition for coated hardmetal tools and components. Substrate grinding represents a key step within the manufacturing chain of these coated systems. Within this context, the influence of surface integrity changes caused by abrasive grinding of the hardmetal substrate, prior to coating, is evaluated with respect to contact damage resistance. Three different substrate surface finish conditions are studied: ground (G), mirror-like polished (P) and ground plus heat-treated (GTT). Tests are conducted by means of spherical indentation under increasing monotonic load and the contact damage resistance is assessed. Substrate grinding enhances resistance against both crack nucleation at the coating surface and subsequent propagation into the hardmetal substrate. Hence, crack emergence and damage evolution is effectively delayed for the coated G condition, as compared to the reference P one. The observed system response is discussed on the basis of the beneficial effects associated with compressive residual stresses remnant at the subsurface level after grinding, ion-etching, and coating. The influence of the stress state is further corroborated by a lower resistance against damage for the coated GTT specimens. Finally, it is proposed and preliminary validated that substrate grinding also enhances damage tolerance of the coated system when exposed to contact loads. M.P. Johansson-Jõesaar and L. LlanesDear Professor Matthews, Please find attached electronic files corresponding to our contribution on contact damage resistance of TiN-coated hardmetals: beneficial effects associated with substrate grinding, which we (all authors do agree to the submission of the manuscript) offer for publication in Surface and Coatings Technology.I hope it is found satisfactory. (Procedia CIRP, Volume 13, 2014, Pages 257-263).(Authors) R1 is right about connection between results published in our contribution to the 2nd CIRP Conference on Surface Integrity (reference [27] in the submitted manuscript) and those presented in this paper. However, the two papers are distinctly different and cover completely different research topics and contain different data. The former focuses on "ground hardmetals" and "flexural strength/fractography", whereas the current one deals with "coated hardmetals" and "contact damage". Hence, we find R1's comment regarding "similar results" inappropriate. (Authors) Co binder effects on residual stresses are not neglected, and our writing may be blamed for such misunderstanding. Residual stresses induced by grinding are "macrostresses" evaluated in the WC phase and assumed to be representative of the whole WC-Co composite. They are different from the "microstresses" (different for each individual phase) that arise due to the difference in thermal expansion between the binder and the carbide as the material cools from liquid-or solid-phase sintering. In cemented carbides with WC as the carbide, the WC is taken as a reliable reference phase because it remains stoichiometric...

“…electrical discharge machining) and coating stages, especially in term of induced subsurface damage and residual stresses. Accordingly, strength is effectively defined by the combined influence of the depth and magnitude of the residual stresses induced within the subsurface, geometry of the induced cracks, and the synergic interaction of these features with microstructure and processing defects [22][23][24][25][26][27]. Studies reporting the combined influence of grinding and coating on induced changes in residual stress state, adhesion strength, and contact damage resistance exist [20,[28][29][30][31].…”

Section: Introductionmentioning

confidence: 99%

Mechanical strength of ground WC-Co cemented carbides after coating deposition

Materials Science and Engineering: A

Odén

Johansson-Jöesaar

et al. 2017

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Manufacturing of hardmetal tools often involves surface grinding, ion etching and final coating.Each stage throughout the manufacturing chain introduces surface integrity changes which may be critical for defining the final mechanical behavior of the coated tools. Within this context, an experimental test program has been developed to assess the influence of a coating (TiN) deposition on surface integrity and transverse rupture strength of a previously ground finegrained WC-Co grade substrate. Four different substrate surface finish conditions (prior to ion etching and coating) were evaluated: as sintered (AS), ground (G), polished (P), and ground plus high temperature annealing (GTT). Surface integrity and fracture resistance characterization, complemented with a detailed fractographic analysis, were performed on both uncoated and coated samples. Results show that the surface integrity after grinding has been partly modified during the ion etching and film deposition processes, particularly in terms of a reduced compressive residual stress state at the substrate surface level. Consequently, the grinding induced strength enhancement in hardmetals is reduced for coated specimens. Main reason behind it is the change of nature, location and stress state acting on critical flaw: from processing defects existing at the subsurface (uncoated G specimens) to grinding-induced microcracks located close to the interface between coating and substrate, but within the subsurface of the 1 Current address: AMES-Sintered Metal Components, Camí de Can Ubach, 8, Pol. Ind. ¨Les Fallulles¨, 08620 -Sant Vicenç dels Horts, Barcelona -Spain latter (coated G specimens). This is not the case for AS and P conditions, where flexural strength does not change as a result of ion etching and coating. Finally, fracture resistance increases slightly for GTT specimens after coating process, possibly caused by a beneficial effect of the deposited film on the residual stress state at the surface.