Fracture mechanics analysis of hardmetals by using artificial small-scale flaws machined at the surface through short-pulse laser ablation

Laser ablation has become an innovative treatment for cemented carbides, regarding edge rounding and surface modification, aiming to improve their tribomechanical performance. Meanwhile, the precision offered for this technique has also positioned it as an effective mean to generate micronotches used for evaluation of mechanical properties in structural materials. However, similar approach has not been attempted for hardmetals; thus, it becomes the main objective of this work. Dimple-like and elongated micronotches are introduced in one fine-grained WC-11%wtCo grade. In doing so, laser processing parameters are first optimized to attain micronotches with appropriated geometry and size, i.e. similar to critical flaws identified in broken pristine specimens. Success of the implemented approach is then validated through subsequent flexural testing, fractographic inspection and fracture mechanics analysis of the results attained on samples containing surface micronotches, as far as laser-induced residual stresses are taken into consideration. In this regard, elongated micronotches are found to exhibit lower residual stresses, and postulate themselves as the optimal option of the two micronotch types studied. The suitability of laser ablation for shaping artificial small-scale flaws opens a new route for introducing controlled defects, alike those intrinsic to processing or induced during service, key aspect for further understanding damage tolerance issues in cemented carbides.Peer ReviewedPostprint (published version

Simulation of metal punching and trimming using minimal experimental characterization

This paper presents a validated finite element modeling approach for simulating shear cutting, needing a minimal amount of experimental characterization. Only one uniaxial tensile test and one force–displacement relationship from a punching experiment are needed for calibration, with maintained prediction accuracy compared to more experimentally demanding approaches. A key ingredient is the observation that the Lode angle parameter is close to zero in the fracture region, postulating that the fracture strain only depends on stress triaxiality, with one free calibration parameter. The true stress–strain behavior is provided from inverse modeling of the tensile test, whereas the fracture model is calibrated using the punching test. The model is verified for different materials by comparing force–displacement curves for punching experiments not used in the calibration. The prediction error for the intrusion is below 4%. A validation is made for two setups. The local residual stresses are measured using Focused Ion-Beam Digital Image Correlation (FIB-DIC). The simulated values are within the experimental bounds. Cut edge morphology and plastic strains obtained by nano-indentation mappings are compared to simulation results, showing a decent agreement. For trimming, the cut edge morphology prediction performance decreases at 17% cutting clearance while it is maintained over the whole range for punching. The predicted hardness values have a mean absolute percentage error below 7.5%. Finally, the effect of element size and remeshing is discussed and quantified. The minimal experimental characterization and simulation effort needed, enables an efficient optimization of the cutting process in the industry.Peer ReviewedPostprint (published version

Functionally graded ultra-high temperature ceramics: From thermo-elastic numerical analysis to damage tolerant composites

To maximize the toughening contributions due to fiber bridging and residual stresses upon layering, ultra-high temperature ceramics containing variable amounts of short carbon fiber in functionally graded stacking sequences were designed and characterized. Stress fields evaluated by finite element model on (AB)nA and more complex asymmetric architectures were compared to the experimental fracture toughness pointing to an effective toughness increment in those structures where the notch fell in zones of residual compression. For the best composite, toughness at room temperature achieved 7 MPa·m0.5 and further increased to 10 MPa·m0.5 when tested at 1500 °C within a light ZrB2-based composite with density below 4 g/cm3. According to the numerical simulations and the effective microstructural features of the composites, the main guidelines for the realization of ceramics with simultaneous failure tolerance and ablation resistance were established.Peer ReviewedPostprint (published version

Multi-phase (Zr,Ti,Cr)B2solid solutions: Preparation, multi-scale microstructure, and local properties

Multi-phase ceramics based on ZrB2, TiB2 and doped with CrB2 and SiC were prepared by powder metallurgy and hot pressing to explore the possibility of obtaining multi-scale microstructures by super-saturation of complex (Zr,Ti,Cr)B2 solid solutions. Core–shell structures formed in TiB2 grains, whereas ZrB2 appeared to form a homogeneous solid solution with the other metals. Precipitation of nano-inclusions within both micron-sized borides was assessed by transmission electron microscopy and thermodynamics elucidated the preferential formation of boride inclusions due to the specific sintering atmosphere. In addition, atomic size factors explicated the precipitation of CrB2 nano-particles into ZrB2-rich grains and of ZrB2 nano-particles into TiB2-rich grains. The hardness of the constituent phases measured by nanoindentation ranged from 36 to 43 GPa

Influence of ECAP process on mechanical, corrosion and bacterial properties of Zn-2Ag alloy for wound closure devices

Actual polymeric wound closure devices are not optimal for load-bearing applications due to the low mechanical properties and the risk of inflammation and bacterial infection mainly produced by multifilament and braided configurations. Biodegradable metallic Zn alloys are promising materials candidates; however, mechanical performance, corrosion behaviour, and biological response should be controlled in order to inhibit the risk of inflammation and bacterial infection. To this end, a Zn-2Ag (2 wt% Ag) alloy was processed by ECAP to evaluate the concurrent combined effect of grain refinement and Ag alloying on biodegradation and antibacterial activity. Two ECAP cycles were successfully applied to a Zn-2Ag alloy obtaining a homogeneous ultra-fine-grained structure in which nanoindentation maps suggested isotropic mechanical properties. Lower UTS and YS with higher elongation was reported after ECAP with similar corrosion rates as before processing. ECAP processed samples showed a homogeneous Ag+ release below the minimum inhibitory concentration for S. Aureus and no antibacterial effect was observed by diffusion. As expected, the presence of Ag in Zn-Ag alloys reduced bacterial attachment. Nevertheless, ECAP processed Zn-2Ag provided an excellent antibacterial activity after 3 h probably caused by the uniformly degraded and thus, non– stable, surface observed after bacterial adhesion.Peer ReviewedPostprint (published version

Mechanical performance of AlCrSiN and AlTiSiN coatings on inconel and steel substrates after thermal treatments

The objective of this study was to explore the mechanical properties of AlCrSiN and AlTiSiN coatings deposited on Inconel and steel substrates after thermal treatments of 500 °C and 800 °C. Nanoindentation was used to measure the hardness and elastic modulus of the coatings, and microindentation was used for observing the contact damage with Hertzian contact loadings. Microscratch and Mercedes tests were used to evaluate the adhesive strength between coating and substrate with both progressive and static loads, respectively. The surface damage was inspected by optical microscopy and scanning electron microscopy (SEM). Focus ion beams (FIB) were used to mill the cross-sections in order to detect the extent and mode of failure. The results show that AlCrSiN coatings and Inconel substrates exhibit better mechanical performance, even after thermal treatments.Peer ReviewedPostprint (published version

Improved adhesion of cathodic arc PVD AlCrSiN coating on ion-implanted WC-Co substrates

Ion implantation has been shown to improve adhesion strength of AlCrSiN coatings due to a synergic enhancement on fracture toughness and load bearing capability of the substrate that can potentially increase the in-service efficiency of coated cutting tools. In this work, AlCrSiN coatings deposited by PVD on WC-Co substrates implanted with Ti, Cr and N ion species have been processed. The mechanical properties and adhesion have been characterized by contact techniques and the residual stress of the coatings and substrates have been evaluated using FIB-DIC technique and Vickers indentation tests, respectively. An improvement of adhesion strength is obtained for treated substrates, especially for those implanted with titanium and chromium ions. This improvement is attributed to the introduction of residual stresses in the substrate, which increases its fracture toughness and enhances its load bearing capability.Work funded through The Spanish Ministry of Science, Innovation and Universities through grants PGC-2018-096855-B-C41, PGC-2018-096855-B-C42 and PGC-2018-096855-A-C4

Bimetallic NiFe nanoparticles supported on CeO2 as catalysts for methane steam reforming

Ni-Fe nanocatalysts supported on CeO2 have been prepared for the catalysis of methane steam reforming (MSR) aiming for coke-resistant noble metal-free catalysts. The catalysts have been synthesized by traditional incipient wetness impregnation as well as dry ball milling, a green and more sustainable preparation method. The impact of the synthesis method on the catalytic performance and the catalysts’ nanostructure has been investigated. The influence of Fe addition has been addressed as well. The reducibility and the electronic and crystalline structure of Ni and Ni-Fe mono- and bimetallic catalysts have been characterized by temperature programmed reduction (H2-TPR), in situ synchrotron X-ray diffraction (SXRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Their catalytic activity was tested between 700 and 950 °C at 108 L gcat-1 h-1 and with the reactant flow varying between 54 and 415 L gcat-1 h-1 at 700 °C. Hydrogen production rates of 67 mol gmet-1 h-1 have been achieved. The performance of the ball-milled Fe0.1Ni0.9/CeO2 catalyst was similar to that of Ni/CeO2 at high temperatures, but Raman spectroscopy revealed a higher amount of highly defective carbon on the surface of Ni-Fe nanocatalysts. The reorganization of the surface under MSR of the ball-milled NiFe/CeO2 has been monitored by in situ near-ambient pressure XPS experiments, where a strong reorganization of the Ni-Fe nanoparticles with segregation of Fe toward the surface has been observed. Despite the catalytic activity being lower in the low-temperature regime, Fe addition for the milled nanocatalyst increased the coke resistance and could be an efficient alternative to industrial Ni/Al2O3 catalysts.Peer ReviewedPostprint (published version