No abstract

Fracture toughness is one the most important parameters for design applications and performance assessment of WC-Co cemented carbides (hardmetals). Different from hardness, fracture toughness is commonly a property more difficult to evaluate, particularly in brittle materials. A large number of different testing methods have been introduced to evaluate toughness of hardmetals, but in general all of them have either theoretically debatable issues or important experimental difficulties. In this study, three different fracture toughness testing methodologies are investigated: three-point bending on Chevron notched specimen ("reference" baseline), Palmqvist indentation test, and Hertzian indentation method. The work is conducted in several cemented carbide grades with different microstructures, in terms of both WC grain size and Co binder content. It is found that Chevronnotched three-point bending test yields suitable fracture toughness values for a wide range of cemented carbide grades with varying hardness. Concerning indentation methods, the Hertzian one may be particularly recommended, as compared to Palmqvist method, as far as hardness (HV30) drops below 1300. On the other hand, if HV30 is higher than 1300 Palmqvist indentation procedure yields reliable fracture toughness measurements. Experimental findings are finally analyzed and discussed on the basis of two theoretical models proposed in the literature.

No abstract

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...

No abstract

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.

The cracking and delamination of TiN-coated hardmetals (WC-Co cemented carbides) when subjected to Brale indentation were studied. Experimental variables were substrate microstructure related to low (6 wt% Co) and medium (13 wt% Co) binder content, and surface finishes associated with grinding and polishing stages before film deposition. Brale indentation tests were conducted on both coated and uncoated hardmetals. Emphasis has been placed on assessing substrate microstructure and subsurface finish effects on load levels at which cracking and delamination phenomena emerge, the type of cracking pattern developed, and how fracture mechanisms evolve with increasing load. It is found that polished and coated hardmetals are more brittle (radial cracking) and the adhesion strength (coating delamination) diminishes with decreasing binder content. Such a response is discussed on the basis of the influence of intrinsic hardness/brittleness of the hardmetal substrate on both cracking at the subsurface level and effective stress state, particularly regarding changes in shear stress component. Grinding promotes delamination compared to the polished condition, but strongly inhibits radial cracking. This is a result of the interaction between elastic-plastic deformation imposed during indentation and several grinding-induced effects: remnant compressive stress field, pronounced surface texture and micro cracking within a thin altered subsurface layer. As a consequence, coating spallation prevails over radial cracking as the main mechanism for energy dissipation in ground and coated hardmetals. (C) 2016 Elsevier B.V. All rights reserved.

Funding Agencies|Spanish MINECO [MAT 2012-34602]; Erasmus Mundus joint European Doctoral Programme DocMASE

No abstract