Can it be measured - Fracture Toughness from Repetitive Nano-impacts Test?

The design of coatings in the field of engineering applications aims at a progressive shift to the development of “hard but tough” coatings. The difficulty in assessing their mechanical behaviour by conventional methods is behind the growing relevance of “in situ” experiments using instrumented microindentation technique. This technique is one of the few able to investigate the mechanical properties along of whatever length range, i.e. from small volume to bulk material. Information from the load-depth curve of indentation has proven to be abundant and varied and it can be used to determine several mechanical properties such as: hardness, Young’s modulus, yield strength, viscoelastic properties, etc [[1]].In fact, from this technique the evaluation of wear-resistance by hardness and/or scratch testing have become in a routine in a multitude of different materials [[2]]. Nevertheless, the results are not always accurate, particularly when the surfaces are subjected to erosive wear during service and fail by a fatigue process. For the purpose of providing a solution, impact technique have been developed to extend the capability of depth-sensing indentation/scratch instrumentation to perform fatigue testing in a wide variety of surfaces, such as DLC and amorphous carbon [[3]], plasma electrolytic oxidation surface[[4]], polymers [[5]] and numerous coatings for cutting tools [[6]]. Along the experiment, this technique produces a repetitive impact with a high stresses and high strain rates, simulating the fatigue conditions under repetitive contact conditions, at the nano or micro-scale. Depending on the material ductility and the load magnitude, fracture of the surface may be achieved. Therefore, the question is whether from these tests it is possible to calculate fracture toughness values, KC, since conventional ways to determine fracture toughness by single edge notched beam (SENB), chevron notched beam (CVNB) and double cantilever beam (DCB) are not applicable for systems whose dimensions are between a few hundreds of nanometers to a few microns. Along this work, it will be studied the feasibility of using impact tests with a cube-corner tip using low loads, assuming that the indented coatings resembles the pattern for the fracture mode type I considered in the classical fracture toughness tests. For this purpose, fracture toughness calculation by impact test will be analysed first in a brittle bulk Al2O3 material, since its mechanical properties are well known. After this, fracture toughness of Al2O3 coating on ductile metallic substrate will be analysed for chequing the substrate contribution and the applicability of the classical indentation models, IM, in case of impact done at low loads. Finally, a novel method to determine the fracture toughness in metallic nanomultillayer coatings by impact test will be show. The design of coatings in the field of engineering applications aims at a progressive shift to the development of “hard but tough” coatings. The difficulty in assessing their mechanical behaviour by conventional methods is behind the growing relevance of “in situ” experiments using instrumented microindentation technique. This technique is one of the few able to investigate the mechanical properties along of whatever length range, i.e. from small volume to bulk material. Information from the load-depth curve of indentation has proven to be abundant and varied and it can be used to determine several mechanical properties such as: hardness, Young’s modulus, yield strength, viscoelastic properties, etc [[1]].In fact, from this technique the evaluation of wear-resistance by hardness and/or scratch testing have become in a routine in a multitude of different materials [[2]]. Nevertheless, the results are not always accurate, particularly when the surfaces are subjected to erosive wear during service and fail by a fatigue process. For the purpose of providing a solution, impact technique have been developed to extend the capability of depth-sensing indentation/scratch instrumentation to perform fatigue testing in a wide variety of surfaces, such as DLC and amorphous carbon [[3]], plasma electrolytic oxidation surface[[4]], polymers [[5]] and numerous coatings for cutting tools [[6]]. Along the experiment, this technique produces a repetitive impact with a high stresses and high strain rates, simulating the fatigue conditions under repetitive contact conditions, at the nano or micro-scale. Depending on the material ductility and the load magnitude, fracture of the surface may be achieved. Therefore, the question is whether from these tests it is possible to calculate fracture toughness values, KC, since conventional ways to determine fracture toughness by single edge notched beam (SENB), chevron notched beam (CVNB) and double cantilever beam (DCB) are not applicable for systems whose dimensions are between a few hundreds of nanometers to a few microns. Along this work, it will be studied the feasibility of using impact tests with a cube-corner tip using low loads, assuming that the indented coatings resembles the pattern for the fracture mode type I considered in the classical fracture toughness tests. For this purpose, fracture toughness calculation by impact test will be analysed first in a brittle bulk Al2O3 material, since its mechanical properties are well known. After this, fracture toughness of Al2O3 coating on ductile metallic substrate will be analysed for chequing the substrate contribution and the applicability of the classical indentation models, IM, in case of impact done at low loads. Finally, a novel method to determine the fracture toughness in metallic nanomultillayer coatings by impact test will be show. [1] W. C. Oliver, G. M. Pharr. J. Mater. Res. 1992;7:1564. [2] J.L. Meneve, J.F. Smith, N.M. Jennett, S.R. Saunders. Appl. Surf. Sci. 1996;100/101:64 [3]B. Beake. Surf. Coat. Technol. 2005:198:90 [4]J.M. Wheeler, C.A. Collier, J.M. Paillard, J.A. Curran. Surf. Coat. Technol. 2010;204:3399. [5]B. Beake, S. Goodes, J. Smith, F. Gao. J. Mater. Res. 2004;19:237. [6]B.D. Beake, V.M. Vishnyakov, J.S. Colligon. J. Phys. D Appl. Phys. 2011;4

Structural and mechanical properties of ?-irradiated Zr/Nb multilayer nanocomposites

Zr/Nb multilayers with periodicities of 10, 30 and 60 nm were prepared by magnetron sputtering and irradiated for prolonged time (1311 h) by ?-rays with energy of 1.25 MeV and a dose of 510 kGy. A qualitative comparison between XRD patterns acquired before and after irradiation revealed a progressive increase of compressive stress, especially in Nb layers, for smaller periodicities with a consequent increase in hardness measured by nanoindentation. The combination of smaller grain size and radiation-induced defect density distribution, primarily in Nb layers, was found to be responsible for the observed radiation hardening effect

Thermal stability of reactive sputtered tungsten oxide coatings

The thermal stability of different W-O coatings, W100, W90O10, W54O46, W30O70 and W25O75, were studied by in-situ X-ray diffraction at elevated temperatures up to 900 °C. The coatings were deposited by DC reactive magnetron sputtering from a pure tungsten target in an Ar + O2 atmosphere onto Fecralloy alloy. The evolution of the structure of the coatings was studied in both protective and oxidant atmospheres. Three groups of films were identified: (1) W25O75 which showed structural evolution following the W-O phase diagram; (2) amorphous O-deficient WO3 whose structure followed the W-O phase diagram either as WO3 or as its chemical composition depending on annealing in oxidant or protective atmospheres, respectively; and (3) low O-content crystalline films that oxidized from 500 °C.http://www.sciencedirect.com/science/article/B6TVV-4MV74W9-1/1/f13fd19171ebf6554fff185464ae87e

Structural and mechanical properties of nanocrystalline Zr co-sputtered a-C(:H) amorphous films

The aim of this study was to investigate the effect of Zr as alloying element to carbon films, particularly in respect to film structure and mechanical properties. The films were deposited by magnetron sputtering in reactive (Ar + CH4) and non-reactive (Ar) atmosphere with different Zr contents (from 0 to 14 at.%) in order to achieve a nanocomposite based films. With an increase of Zr content a broad peak was observed in X-ray diffraction spectra suggesting the presence of nanocrystalline (nc) ZrC phase for the coatings with Zr content higher than 4 at.%. The application of Scherrer formula yielded a grain sizes with a dimension of 1.0–2.2 nm. These results were supported by X-ray photoelectron spectroscopy showing typical charge transfer at Zr-C nanograins and carbon matrix interface. The nc-ZrC phase was also observed by transmission electron microscopy. The hardness of the coatings was approximately independent of Zr content. However, the Young modulus increased linearly. The residual stress of the coatings was strongly improved by the presence of nc-ZrC phase embedded in the a-C matrix. Finally, the incorporation of H into the matrix led to denser and harder films

Lubricious TiSi(V)N coatings for extreme temperature application deposited by Deep Oscillation Magnetron Sputtering (DOMS) mode

Development of new self-lubricant coating systems with control of the lubricous species diffusion have enormous potential to be used in the protection of surfaces in components for machining. In the last years, vanadium rich coatings have been introduced as possible candidates for self-lubrication due to their optimum tribological properties. This work is devoted on the study of V additions on the morphology, structure and on the tribological properties of TiSiN films deposited by HiPIMS in Deep Oscillation Magnetron Sputtering (DOMS) mode. Two different peak powers were used on the deposition of coatings in order to produce films with different morphology. The tribological properties of coatings were evaluated at room temperature against Al2O3 balls on a pin-on-disc apparatus. Increasing of peak power showed to change the cross section morphology of films from columnar type to compact structure. V additions improved the tribological properties of TiSiN coatings.This research is sponsored by FEDER funds through the program COMPETE – Programa Operacional Factores de Competitividade – and by national funds through FCT – Fundação para a Ciência e a Tecnologia, under the projects: PTDC/EMETME/ 122116/2010 and PTDC/EMS-TEC/1805/2012. The authors acknowledge the Master student Melkamu A. Mekicha (co-author of this paper) from the International Joint European Master in Tribology of Surfaces and Interfaces (TRIBOS) program, doing his thesis in the mechanical engineering department of the University of Coimbra, for his special dedication on the development of this research

PVD-grown antibacterial Ag-TiN films on piezoelectric PVDF substrates for sensor applications

This work reports on Ti1-xAgx and Ag-TiNx electrodes deposited by dc/pulsed dc magnetron sputtering at room temperature on poly (vinylidene fluoride) (PVDF) with the purpose to develop sensors for prosthesis. In Ti1-xAgx electrodes, silver content was varied from 0 to 100 at.%; and for Ag-TiNx electrodes, the nitrogen content was varied between 34 to 43 at.% and the Ti/Ag ratio changed from 12.2 to 5.2. The antibacterial activity of the samples was assessed by agar diffusion method. Interestingly, samples from the Ag-TiNx series presented antibacterial activity, in contrast to the samples from Ti1-xAgx series. XPS results showed that the as-deposited samples from the Ag-TiNx series exhibited silver clusters smaller than 4 nm. In order to deepen the results obtained, samples were analysed after the Halo test (antibacterial test), and XPS analyses showed the disappearance of these Ag clusters in agreement with the SEM results, which displayed a visible aggregation and a significant decrease in the number of Ag clusters, suggesting that silver nanoparticles aggregated and diffused through the agar killing bacteria or inhibiting their grown in the vicinity. All results show that Ag-TiNx electrodes are more promising candidates to be used in PVDF sensor protection than Ti1-xAgx electrodesS. M. Marques gratefully acknowledges the funding from the Portuguese National funds through the FCT - Fundacao para a Ciencia e a Tecnologia, (project SFRH/BD/71259/2010). Also, the authors thank the support of FEDER through the COMPETE Programme and of the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2014 and the project Matepro-Optimizing Materials and Processes, ref. NORTE-07-0124-FEDER-000037", co-funded by the "Programa Operacional Regional do Norte" (ON.2 - O Novo Norte), under the "Quadro de Referencia Estrategico Nacional" (QREN), through the "Fundo Europeu de Desenvolvimento Regional" (FEDER).The authors thank the FCT Strategic Project of UID/BIO/04469/2013 unit, the project RECI/BBB-EBI/0179/2012 (FCOMP-01-0124-FEDER-027462) and the project "BioInd - Biotechnology and Bioengineering for improved Industrial and Agro-Food processes", REF. NORTE-07-0124-FEDER-000028 co-funded by the Programa Operacional Regional do Norte (ON.2 - O Novo Norte), QREN, FEDER.The authors thank the FCT Strategic Project of UID/BIO/04469/2013 unit, the project RECI/BBB-EBI/0179/2012 (FCOMP-01-0124-FEDER-027462) and the Project "BioHealth Biotechnology and Bioengineering approaches to improve health quality", Ref. NORTE-07-0124-FEDER-000027, co-funded by the Programa Operacional Regional do Norte (ON.2 - O Novo Norte), QREN, FEDER.The authors thank the FCT Strategic Project of UID/BIO/04469/2013 unit, the project RECI/BBB-EBI/0179/2012 (FCOMP-01-0124-FEDER-027462) and the Project "BioEnv Biotechnology and Bioengineering for a sustainable world", REF. NORTE-07-0124-FEDER-000048, co-funded by the Programa Operacional Regional do Norte (ON.2 0 Novo Norte), QREN, FEDER

Influence of Al content on the mechanical properties and thermal stability in protective and oxidation atmospheres of Zr–Cr–Al–N coatings

Tools have been used under very high temperature in recent decades. Therefore, coatings on tools are required to be thermally stable and excellent in oxidation-resistance. The introduction of Al into nitride coatings was considered as an effective way to improve the coating oxidation-resistance and thermal stability. Quaternary Zr–Cr–Al–N coatings were deposited with increasing Al contents by d.c. reactive magnetron sputtering. The coatings were annealed at 800 and 900 °C for 1 h in Ar + H2 atmosphere and exposed at 700 °C for 2 h and 1200 °C for 1 h in air, respectively. The hardness, Young's modulus, adhesive/cohesive strength and residual stress of as-deposited and annealed coatings were characterized by nanoindentation, scratch test and curvature measurement method. The results showed that the coating hardness and Young's modulus increased after post-annealing at 800 °C or 900 °C; the adhesive/cohesive strength decreased after annealing due to the substrate softening. The residual stress decreased after annealing and the lowest stress status depended on the crystallinity degree of the coating. XRD patterns showed mainly Cr2O3 formed in the Al-free, and a mixture of oxides of ZrO2 and Al2O3 in the high Al coatings after 1200 °C/1 h oxidation. N is still detected in the coatings, which indicates that the coatings are not fully oxidized being exposed to 700 °C for 2 h

Development of Ag/TiO2NM-treated leathers with antibacterial activity for footwear industry

This work aims to contribute to the industrial development of safer and advanced leathers using nanomaterials (NMs) that are based in titanium oxide doped with silver (Ag-TiO2NMs). These NMs present an ecological alternative to volatile organic biocides and organic solvents, frequently used nowadays. Safe application of these NMs brings added value to footwear and leather products and reduces the bulk chemical wide pollution. These newly developed materials are expected to be useful for footwear manufacturing (for common or professional use) with advanced protection against bacteria and fungus, which are the most common cause of feet infections, for either common or professional use. Hence, the main aim of this work is the improvement of leather footwear in order to overcome the feet infections and ensure minimum risk of human skin penetration. In order to accomplish the main aim, the cytotoxicity, antimicrobial and self-cleaning properties of leather surface, treated with Ag-TiO2NMs by sputtering deposition were assessed. The Ag-TiO2 coatings were deposited onto leather substrates by DC-pulsed reactive magnetron sputtering using two targets, Ti and Ag, in an Ar + O2 atmosphere. Compositional analysis was achieved by X-ray energy dispersive spectrometry (EDX). Contact angle of the coatings was measured and the results show that with the introduction of silver in TiO2 microstructure promotea an increase in hydrophobicity. The antibacterial activity of the samples was assessed by agar diffusion method and the results point out to a silver antibacterial activity

PVD-grown antimicrobial thin films on PVDF substrates for sensors applications

Electroactive polymers are the most interesting class of polymers used as smart materials in various applications, such as the development of sensors and actuators for biomedical applications in areas as smart prosthesis, implantable biosensors and biomechanical signal monitoring, among others. For acquiring or applying the electrical signal from/to the piezoelectric material, suitable electrodes can be produced from Ti based coatings with tailored multifunctional properties, as conductivity and antibacterial characteristics, obtained by the inclusion of Ag. This work reports on Ti1-xAgx electrodes and Ag-TiNy electrodes deposited by d. c. and pulsed magnetron sputtering at room temperature on poly(vinylidene fluoride)(PVDF). In the first system (Ti1-xAgx electrodes), silver content was varied from 0-100 at. %. For the second system (Ag-TiNy electrodes), the nitrogen content changed between 0 to 40.3 at. % by increasing the nitrogen gas flow between 0 sccm and 15 sccm and the ratio Ti/Ag changed from 13.4 to 2.2 being clearly the visible decrease on the Ti content in the reactive mode. The X-Ray Diffraction (XRD) results revealed that the deposition conditions preserve the polymer structure and suggested the presence of crystalline Ti phase in pure titanium coating and fcc-Ag phase in pure silver coating for the Ti1-xAgx system. For the Ag-TiNy system it is possible to detect a fcc TiN structure and a fcc Ag phase. Sheet resistivity values show a typical behavior of a binary alloy system, varying between 0.12 and 28.5 /sq for the Ti1-xAgx electrodes. For the second system the sheet resistivity decrease with the nitrogen content from 12.0 /sq with 0 at. % to 2.8 /sq for 40.3 at. % of N. The piezoelectricity of the different samples show similar values, showing values from 19.6 to 27.6 pCN-1 for the Tix-1Agx system and 13.6 pCN-1 as minimum for the Ag- TiNy system, achieved for the highest N content. In order to assess the mechanical behavior of the as-sputtered films, the film/substrate system was loaded unidirectionally using a tensile machine. The stress-strain curves were analyzed and correlated with the structural data. Moreover, the antibacterial activity of the samples was assessed and it was verified that samples from the second series (Ag-TiNy) present antibacterial activity, in contrast of the first series (Ti1-xAgx)