Evaluation of elastoplastic properties and fracture strength of thick diamond like carbon film by indentation

The elastoplastic properties and fracture strength of thick diamond like carbon films (with thickness over 10 µm) are studied. An indentation-based framework is outlined where the dual sharp shallow micro indentation method is employed to measure the elastoplastic properties of the film and the substrate; next, integrated acoustic emission and corrosion potential fluctuation techniques are employed to characterize the ring cracks formed due to deep spherical indentation. The fracture strength of the DLC film is obtained from parallel numerical simulations with the identified elastoplastic properties. It is found that the thick DLC film prepared by using the plasma-based ion implantation method has better fracture property than thin DLC films in previous studies.     

Fracture Toughness of Chemically Vapor-Deposited Diamond

The fracture toughness of chemically vapor-deposited diamond is estimated by a Vickers indentation method. Freestanding diamond films of 400-μm thickness are produced with plasma-enhanced chemical vapor deposition and highly polished for indentation testing. Indentation testing was performed with a microhardness tester using a load range of 5 to 8 N. The average fracture toughness is estimated as 5.3 ± 1.3 MPA · m^1/2.     

Nanoindentation-induced delamination of submicron polymeric coatings

An elastic model is developed to estimate the interfacial strength between a submicron surface coating and a compliant substrate. The analysis uses a shear-lag model and assumes the plane-stress state in the surface coating. The critical indentation load for the indentation-induced delamination of the coating from the substrate increases with the third power of the indentation depth and is a linear function of the reciprocal of the coating thickness. The indentation-induced delamination of SR399 ultrathin surface coatings over acrylic substrate has been evaluated, using the nanonindentation technique for coating thicknesses of 47, 125, 220 and 3000 nm. For the submicron coatings, the dependence of the critical indentation load on the coating thickness supports the elastic model. The interfacial strength is found to be 46.9 MPa. In contrast, the polymeric coating of 3000 nm displays multiple “excursions” in the loading curve, and the critical indentation load is a linear function of the indentation depth.     

Fracture Toughness of Porous Material of LSCF in Bulk and Film Forms

Fracture toughness of La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ (LSCF) in both bulk and film forms after sintering at 900°C to 1200°C was measured using both single‐edge V‐notched beam (SEVNB) 3‐point bending and Berkovich indentation. FIB/SEM slice‐and‐view observation after indentation revealed the presence of Palmqvist radial crack systems after indentation of the bulk materials. Based on crack length measurements, the fracture toughness of bulk LSCF specimens was determined to be in the range 0.54–0.99 MPa·m^1/2 (depending on sintering temperature), in good agreement with the SEVNB measurements (0.57–1.13 MPa·m^1/2). The fracture toughness was approximately linearly dependent on porosity over the range studied. However, experiments on films showed that the generation of observable indentation‐induced cracks was very difficult for films sintered at temperatures below 1200°C. This was interpreted as being the result of the substrate having much higher modulus than these films. Cracks were only detectable in the films sintered at 1200°C and gave an apparent toughness of 0.17 MPa·m^1/2 using the same analysis as for bulk specimens. This value is much smaller than that for bulk material with the same porosity. The residual thermal expansion mismatch stress measured using XRD was found to be responsible for such a low apparent toughness.     

Nanotribological and nanomechanical properties of 5–80 nm tetrahedral amorphous carbon films on silicon

Nanoscratch testing has been used to investigate the tribological behaviour of 5, 20, 60 and 80 nm tetrahedral amorphous carbon (ta-C) thin films deposited on silicon by the filtered cathodic vacuum arc method. The nanoscratch behaviour of the films was found to depend on the film thickness, with 60 and 80 nm films undergoing border cracking and then at higher critical load a dramatic delamination event. 5 and 20 nm films have a lower critical load for onset of border cracks but do not undergo a clear dramatic failure, and instead are increasingly worn/ploughed through until film removal as confirmed by microscopic analysis. This is consistent with the thinner films having lower stress and reduced load-carrying ability. Nanoindentation confirms that the thicker films have enhanced load support and higher measured composite (film + substrate) hardness. The 80 nm film in particular can retain appreciable load support whilst deformed during indentation, as shown by its ability to alter the critical loads for contact-induced phase transformations in the Silicon substrate during unloading.     

Delamination toughness of fiber-reinforced composites containing a carbon nanotube/polyamide-12 epoxy thin film interlayer

We present a simple, out-of-autoclave approach to improve the delamination toughness of fiber-reinforced composites using epoxy interlayers containing 20 wt.% polyamide-12 (PA) particles and 1 wt.% multi-walled carbon nanotubes (MWCNTs). Composites were prepared by integrating partially cured thin films at the laminate mid-plane using vacuum-assisted resin transfer molding. The introduction of epoxy/PA interlayers increased fracture toughness due to the ductile deformation and crack bridging of PA particles within an interlaminar damage zone with uniform thickness of about 20 μm. Composites interlayered with epoxy/PA/MWCNT exhibited nearly 2.5 and 1.5 times higher fracture toughness than composites containing neat epoxy and epoxy/PA interlayers, respectively, without an observable increase in interlaminar thickness. The fracture surface was analyzed to identify failure modes responsible for the fracture toughness improvement. The MWCNTs are proposed to inhibit critical loading of defects by minimizing stress concentration within the interlaminar region, thereby enabling greater deformation of the PA particles during fracture.     

Effect of interface roughness and coating thickness on interfacial shear mechanical properties of EB-PVD yttria-partially stabilized zirconia thermal barrier coating systems

The effect of interface roughness and thickness of thermal barrier coating (TBC) on the interfacial shear mechanical properties of electron beam-physical vapor deposited (EB-PVD)-TBC was examined using as-sprayed and polished bond coats (BC) 200 μm and 500 μm TBC thickness systems, by using a barb test method. The residual compressive stress in the TBC layer from the interface to the top surface was measured, by using Raman spectroscopy. The interface toughness related to the interface roughness and the thickness of the TBC. The interface toughness was larger for the BC as-sprayed TBC system than for the BC polished TBC system. The delamination of the TBC propagated within the TBC layer adjacent to the interface for the BC as-sprayed TBC; for the BC polished TBC, this occurred at the interface between the TGO and the BC. Moreover, the interface toughness was larger in the 500 μm thickness TBC than in the 200 μm thickness TBC. The relation of interface toughness to interface roughness and thickness of the TBC was associated with the interface residual compressive stress and with the interface sliding friction during the delamination of TBC.     

Mechanical properties of porous methyl silsesquioxane and nanoclustering silica films using atomic force microscope

Mechanical properties of porous methyl silsesquioxane samples with dielectric constant 2.4 and 2.0 and a recently developed nanoclustering silica film samples with dielectric constants 2.3 and 2.0 were evaluated using an atomic force microscope based nanoindentation. It was found that the Young’s modulus and the hardness decrease while the fracture toughness increases with a decrease in the dielectric constant in the same type of material. Moreover, the Young’s modulus and the hardness of the nanoclustering silica films were observed to be at least twice and fracture toughness values ~1.3–1.5 higher than those for methyl silsesquioxane films with similar dielectric constants. The high resolution topographic imaging capability of atomic force microscope was shown to be particularly useful in the measurement of cracks generated by the ultra-low indentation loads, and the evaluation of the fracture toughness of the nanoscale volumes of materials.     

Effect of the tie‐layer thickness on the delamination and tensile properties of polypropylene/tie‐layer/nylon 6 multilayers

This study examined the effect of the tie‐layer thickness on the delamination behavior of polypropylene/tie‐layer/nylon 6 multilayers. Various maleated polypropylene resins were compared for their effectiveness as tie‐layers. Delamination failure occurred cohesively in all the multilayer systems. Two adhesion regimes were defined according to the change in the slope of the linear relationship between the delamination toughness and the tie‐layer thickness. The measured delamination toughness of the various tie‐layers was quantitatively correlated to the length of the damage zone that formed at the crack tip. In addition, the effect of the tie‐layer thickness on the multilayer tensile properties was correlated with the delamination behavior. The fracture strain of the multilayers decreased with decreasing tie‐layer thickness. An examination of the prefracture damage mechanism of the stretched multilayers revealed a good correlation with the delamination toughness of the tie‐layers. In thick tie‐layers (>2 μm), the delamination toughness was great enough to prevent the delamination of the multilayers when they were stretched. In thin tie‐layers (<2 μm), the delamination toughness of all the tie‐layers was low; consequently, delamination led to premature fracture in the stretched multilayers. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Interplay between adhesion and interfacial properties of diamond films deposited on WC-10%Co substrates using a CrN interlayer

In this study, the effect of deposition temperature on the adhesion of diamond films deposited on WC-10%Co substrates with a Cr-N interlayer is investigated. Diamond films were deposited at different temperatures (550, 650 and 750 °C), using a hot filament chemical vapor deposition reactor. It was found that the optimal adhesion is obtained for the film deposited at 650 °C. The interplay between carbon interfacial diffusion and the adhesion of diamond films deposited at different deposition temperatures were investigated. The combined use of different characterization techniques (Indentation tests, SIMS, XPS, XRD and SEM) shows that the adhesion strength depends on the thickness of Cr-C layer formed at the interface during diamond deposition, which is strongly influenced by the deposition temperature. It is suggested that at the optimum deposition temperature, thickness of the Cr-C layer is too low to introduce a large thermal stress at the interface and sufficiently thick enough to withstand the propagation of indentation induced cracks.     

Mechanical properties of a-C:H multilayer films

Multilayered amorphous hydrogenated carbon (a-C:H) films consisting of alternating sublayers with different mechanical properties have been deposited by an electron cyclotron resonance microwave-plasma chemical vapor deposition (ECR MP-CVD) system and modulating substrate bias voltage. The mechanical properties of the multilayer films were determined using nanoindentation and nanoscratch experiments with reference to single a-C:H layers of which the multilayer structure were composed. In nanoindentation tests, the relationship between the film hardness and indentation depth has been obtained over an indentation depth range of 20–500 nm. Since the films tend to fracture under high load in nanoindentation tests, their critical fracture loads were determined. The critical loads for fracturing the multilayered a-C:H films were higher than those of single a-C:H layers. The nanoscratch tests also showed that the multilayered a-C:H films required a higher critical load for scratching fracture. This study implies that the mechanical properties of a-C:H film can be improved by engineering suitable multilayer structures.     

Low‐velocity impacts in continuous glass fiber/polypropylene composites

The low‐velocity impact behavior of a continuous glass fiber/polypropylene composite was investigated. Optical microscopy and ultrasonic scanning were used to determine the impact‐induced damage. At low impact energy, the predominant damage mechanism observed was matrix cracking, while at high energy the damage mechanisms observed were delamination, plastic deformation, which produced a residual specimen curvature, and a small amount of fiber breakage at the edge of the indentation on the impacted face of the specimens. The impact load vs. time signals were recorded during impact and showed that the load corresponding to the onset of delamination was independent of the impact energy in the range tested. The load at which the onset of delamination occurred corresponded to the values obtained by performing a linear regression of the delaminated area, obtained by ultrasonic scanning, as a function of the impact force. Tensile and flexural tests performed on impacted specimens showed that the tensile and flexural residual strengths and the flexural modulus decreased with increasing incident impact energy, while the post‐impact residual tensile modulus remained constant. The dynamic interlaminar fracture toughness was evaluated from the critical dynamic (impact) strain energy release rate of specimens with a delamination simulated by an embedded insert. The results are compared with the interlaminar fracture toughness values obtained during subcritical steady crack growth.     

Steady-State Mechanics of Delamination Cracking in Laminated Ceramic-Matrix Composites

A fracture mechanics delamination cracking model has been developed for brittle-matrix composite laminates. The near-tip mechanics is discussed in the context of material orthotropy and composite material inhomogeneities. A fracture mechanics framework based on the near-tip energy release rate and the associated phase angle Ψ has been adopted. In the case of steady-state delamination cracking in a prenotched cross-ply symmetric laminated beam, analytical expressions for the steady-state energy release rate, _ss, have been obtained for the combined applied loading of an axial force and a bending moment. Parameter studies assessing the effects on _ss of load coupling, crack location, and lamination morphology which includes the total number of layers, layer thickness, and material properties are presented. Thus, composite homogenization criteria with respect to the total number of layers placed along the beam height can be obtained for a wide range of material selection. The associated phase angle Ψ at the delamination crack tip is discussed in the context of existing solutions. The analysis has been developed based on a theory for structural laminates. The delamination model can be used in conjunction with experimental data obtained from model geometries to extract the mixed-mode transverse composite fracture toughness. Thus, conditions for stable delamination crack growth can be established and design criteria based on toughness for composite laminates and composite fasteners can be obtained.     

Room temperature deposition of functional ceramic films on low-cost metal substrate

In various practical applications, such as high power actuators, high sensitivity sensors, and energy harvesting devices, polycrystalline piezoelectric films of 1–100?µm thickness and sizes ranging from several µm² to several cm² are required. With conventional film deposition processes, such as sol-gel, sputtering, chemical vapor deposition, or pulsed laser deposition, it is difficult to fabricate films with higher thickness due to their low deposition rate and high interfacial stress. The aerosol deposition method (AD), a relatively new deposition technique, can be used to fabricate highly dense thick films at room temperature by the consolidation of submicrometer-sized ceramic particles on various ceramic, metal, glass, and polymer substrates. Ferroelectric BaTiO₃ ceramic films of different thicknesses ranging from 1 to 30?µm were fabricated on a low-cost metallic substrate at room temperature using the AD method. Surface morphology and adhesion of the film were analyzed. Analysis of internal residual stresses revealed an equibiaxial compressive stress state in the as-processed film. Electrical characterization of films annealed at 500?°C shows an enhanced polarization value of ~?14?µC/cm² over that of the as-processed film. This improved property is related to the decreasing internal residual stress. In addition, the BT films prepared in this work were found to withstand electric fields greater than 100?kV/mm, which is possibly related to the inherent relatively defect-free structure of AD films.     

Comparison of the adhesion of diamond films deposited on different materials

Indentation tests combined with acoustic emission spectra were used to compare the adhesion of diamond films deposited on various substrates, including Ti, Cr, Si and Ti coated Cu. We show that indentation in the diamond coatings may cause the following failure modes: (a) the substrate cracking; (b) the film cracking and localised detachment; and (c) the film delamination and the delamination propagation. Acoustic emission during indentation loading provided essential information in predicting what mode of failure occurs. Combined with the acoustic emission spectra, the indentation tests are reliable in comparing the adhesion of diamond films deposited on the same or similar substrate materials. However, the comparison of the film adhesion on very different substrates, like Cu and Ti, is not so straightforward. Acoustic emission spectra also revealed that indentation caused substrate cracking prior to the failure of the film/substrate interface for diamond coatings on Si. In this case, the indentation tests are not valid to compare the coating adhesion.     

Effect of Adhesion, Film Thickness, and Substrate Hardness on the Scratch Behavior of Poly(carbonate) Films

The effect of adhesion, film thickness, and substrate hardness on the scratch behavior of poly(carbonate) (PC) films was investigated. Films of various thickness were prepared by spin-coating solutions of PC in chloroform onto glass, ferroplate, Al 1100, Al 6022, and Al 6111 substrates. Adhesion between the films and the substrates was controlled by pretreatment of the substrates and the thickness of the films was controlled by the concentration of the PC solutions. Adhesion of the films to the glass substrates was measured by a blister test. Scratch tests were performed using a custom-built, progressive-load scratch tester with interchangeable diamond indenters; the resulting scratches were observed by optical microscopy, atomic force microscopy (AFM), and environmental scanning electron microscopy (ESEM). The critical normal load (i.e., the smallest applied normal load for which delamination of the film from the substrate was observed) was used as a criterion to determine the scratch resistance of the films. It was found that better film/substrate adhesion resulted in a higher critical load for delamination. As film thickness increased, the critical load and, thus, scratch resistance also increased. Substrate hardness had a strong influence on the scratch behavior of the PC films. For a low-hardness substrate (i.e., Al 1100), the work from scratching was mainly consumed by deforming the substrate. In the case of substrates with intermediate hardness (i.e., Al 6022, Al 6111, and ferroplate), the substrates were more resistant to the stresses that were generated in the films; hence, the deformation of the substrates was less severe. A high-hardness substrate (i.e., glass) resisted the applied load and resulted in higher stress concentrations in the films and at the interface. Consequently, a rougher surface inside the scratch track was observed.     

Effect of tie-layer thickness on the adhesion of ethylene-octene copolymers to polypropylene

The adhesion of some ethylene-octene copolymers to polypropylene (PP) and high density polyethylene (HDPE) was studied in order to evaluate their suitability as compatibilizers for PP/HDPE blends. A one-dimensional model of the compatibilized blend was fabricated by layer-multiplying coextrusion. The microlayered tapes consisted of many alternating layers of PP and HDPE with a thin tie-layer inserted at each interface. The thickness of the tie-layer varied from 0.1 to 15 μm, which included thicknesses comparable to those of the interfacial layer in a compatibilized blend. The delamination toughness was measured in the T-peel test. Generally, delamination toughness decreased as the tie-layer became thinner with a stronger dependence for tie layers thinner than 2 μm. Inspection of the crack-tip damage zone revealed a change from a continuous yielded zone in thicker tie layers to a highly fibrillated zone in thinner tie layers. By treating the damage zone as an Irwin plastic zone, it was demonstrated that a critical stress controlled the delamination toughness. The temperature dependence of the delamination toughness was also measured. A blocky copolymer (OBC) consistently exhibited better adhesion to PP than statistical copolymers (EO). A one-to-one correlation between the delamination toughness and the reported performance of the copolymers as compatibilizers for PP/HDPE blends confirmed the key role of interfacial adhesion in blend compatibilization.     

Delamination failure mechanisms in microlayers of polycarbonate and poly(styrene-co-acrylonitrile)

The peel strength and delamination failure mode of coextruded microlayer sheets consisting of alternating layers of polycarbonate (PC) and poly(styrene-co-acrylonitrile) (SAN) were studied with the T-peel test. Four delamination modes were observed: two modes where the crack propagated along the PC–SAN interface and two other modes where the crack propagated through crazes in the SAN. The SAN layer thickness determined whether crack propagation was interfacial or through crazes. Crazing and crack propagation through crazes were observed only if the SAN layer was thicker than 1.5 μm. As the thickness of the SAN layer increased, the amount of crazing in front of the crack tip and the amount of craze fracture gradually increased; the peel strength increased accordingly. If the SAN layers were thinner than 1.5 μm and the PC layers were relatively thick, the crack propagated along a single interface. The peel strength for this delamination mode was the lowest and equal to about 90 J/m², independent of layer thicknesses. This delamination mode came closest to providing a ”real” measure of the adhesive toughness of PC to SAN. With both interfacial and craze delamination, the crack could move from layer to layer if the PC was thin enough. Tearing of the relatively thin PC layers increased the peel strength of the multiple-layer delamination modes. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:793–805, 1998     

Measurement of the Fracture Toughness of Ceramic Materials Using a Miniaturized Disk-Bend Test

The fracture toughness of several ceramic materials has been measured using a miniaturized disk-bend test apparatus and methodology based on small disk-shaped samples 3 mm in diameter. The method involves the Vickers indentation of specimens ranging in thickness from 300 to 700 μm, and testing them in a ring-on-ring bending mode. New experiments on a glass-ceramic (GC) and Si₃N₄ have been performed to demonstrate the validity of the technique, supplementing the original work on ZnS. The fracture resistances of these materials increase with increasing crack length ( R -curve behavior). The data are analyzed using a specific model for the relationship between fracture resistance and crack length; this model enables the R -curve behavior to be treated analytically, and the fracture resistance at "infinite" crack length to be evaluated using a straightforward graphical procedure. The resulting values of the fracture toughness for ZnS, GC, and Si₃N₄ are 0.74 ± 0.02, 2.18 ± 0.09, and 4.97 ± 0.07 MPa-m^1/2, respectively, which are all in very good agreement with values obtained from conventional fracture toughness tests on large specimens. The results verify the utility of the miniaturized diskbend method for measuring the fracture toughness of brittle materials.     

Effect of Indenter Geometry on Controlled-Surface-Flaw Fracture Toughness

Fracture toughness values obtained using both Knoop and Vickers-indentation-produced controlled surface flaws were compared as a function of indentation load for a well-characterized glass-ceramic material. At the same indentation load, Knoop cracks were larger than Vickers. As-indented K_c values calculated from fracture mechanics expressions for surface flaws were higher for Knoop flaws than Vickers, but both types gave low K_c values due to indentation residual stress effects. Analysis suggested that theoretical formalisms for indentation residual stress effects based on fracture mechanics solutions for a center-loaded penny crack in an infinite medium should apply to both indentation types. K_c values calculated using the residual stress approach were identical for Knoop and Vickers controlled surface flaws when a "calibration" value for a constant term in the expression for K_c was used for both indentation types.