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Abstract Oxidation behaviors of carbon fiber reinforced SiC matrix composites (C/SiC) are one of the most noteworthy properties. For C/SiC, the oxidation behavior was controlled by matrix microcracks caused by the mismatch of coefficients of thermal expansion (CTEs) and elastic modulus between carbon fiber and SiC matrix. In order to improve the oxidation resistance, multilayer SiC-Si3N4 matrices were fabricated by chemical vapor infiltration (CVI) to alleviate the above two kinds of mismatch and change the local stress distribution. For the oxidation of C/SiC with multilayer matrices, matrix microcracks would be deflected at the transition layer between different layers of multilayer SiC-Si3N4 matrix to lengthen the oxygen diffusion channels, thereby improving the oxidation resistance of C/SiC, especially at 800 and 1000 °C. The strength retention ratio was increased from 61.9% (C/SiC-SiC/SiC) to 75.7% (C/SiC-Si3N4/SiC/SiC) and 67.8% (C/SiC-SiC/Si3N4/SiC) after oxidation at 800 °C for 10 h.

With the addition of Al element, the microstructure of high entropy alloy Al x CoCrCuFeNi ( x = 0.5, 1.5 and 3.0) changes from single FCC to FCC + BCC and then to single BCC phase. Hardness and elastic modulus of different phases in the HEAs are measured utilizing nanoindentation method. It is found that with the increase of the Al element, the hardness rises more significantly in the BCC phase than that in the FCC phase. The reason is attributed to that the Al element has a stronger solid-solution strengthening effect in the BCC phase, due to the larger elastic modulus mismatch. The precise characterization of local mechanical properties of different phases in the HEAs provides us opportunity to design alloys with desired performance. [ABSTRACT FROM AUTHOR]

Previous work by the authors (Harvey et al., 2015) on brittle interfacial cracking between two dissimilar elastic layers is extended to accommodate Poisson’s ratio mismatch in addition to the existing capability for elastic modulus mismatch. Under crack tip bending moments and axial forces, it is now possible to use a completely analytical 2D elasticity-based theory to calculate the complex stress intensity factor (SIF) and the crack extension size-dependent energy release rates (ERRs). To achieve this, it is noted that for a given geometry and loading condition, the total ERR and bimaterial mismatch coefficient are the two main factors affecting the partitions of ERR. Based on this, equivalent material properties are derived for each layer, namely, an equivalent elastic modulus and an equivalent Poisson’s ratio, such that both the total ERR and the bimaterial mismatch coefficient are maintained in an alternative equivalent case. Cases for which no analytical solution for the SIFs and ERRs currently exist can therefore be ‘transformed’ into cases for which the analytical solution does exist. The approach is verified against results from 2D finite element method simulations in which excellent agreement is observed for cases of plane stress and plane strain with a variety of loading conditions. [ABSTRACT FROM AUTHOR]

Abstract: Enamel and dentin compose the crowns of human teeth. They are joined at the dentinoenamel junction (DEJ) which is a very strong and well-bonded interface unlikely to fail within healthy teeth despite the formation of multiple cracks within enamel during a lifetime of exposure to masticatory forces. These cracks commonly are arrested when reaching the DEJ. The phenomenon of crack arrest at the DEJ is described in many publications but there is little consensus on the underlying cause and mechanism. Explanations range from the DEJ having a larger toughness than both enamel and dentin up to the assumption that not the DEJ itself causes crack arrest but the so-called mantle dentin, a thin material layer close to the DEJ that is somewhat softer than the bulk dentin. In this study we conducted 3-point bending experiments with bending bars consisting of the DEJ and surrounding enamel and dentin to investigate crack propagation and arrest within the DEJ region. Calculated stress intensities around crack tips were found to be highly influenced by the elastic modulus mismatch between enamel and dentin and hence, the phenomenon of crack arrest at the DEJ could be explained accordingly via this elastic modulus mismatch. [Copyright &y& Elsevier]

Effect of elastic modulus mismatch on the contact crack initiation is investigated to find major parameters in designing desirable surface-coated system. Silicon nitride coated soft materials with various elastic modulus mismatch, E c / E s = 1.06 — 356 are prepared for the analysis. Hertzian contact test is conducted for producing contact cracks and the acoustic emission detecting technique for measuring the critical load of crack initiation. The implication is that coating thickness and material strength are controllable parameters to prevent the initiation of contact cracks resulted from the elastic modulus mismatch in the hard ceramic coating layer on the soft materials. [ABSTRACT FROM AUTHOR]

A study is made on the characterization of damage tolerance by spherical indentation in hardly coated layer structure with modest elastic modulus mismatch. A hard silicon nitride is prepared for the coating material and silicon nitride with 5wt% of boron nitride composites for underlayer. Hot pressing to eliminate the effect of interface delamination during the fracture makes strong interfacial bonding. The elastic modulus mismatch between the layers is not only large enough to suppress the surface crack initiation from the coating layer but sufficiently small to prevent the initiation of radial crack from the interface. The strength degradation of the layer structure after sphere contact indentation does not significantly occur, while the degradation of silicon nitride-boron nitride composite is critical at a high load and high number of contacts. [ABSTRACT FROM AUTHOR]

In this paper, the different performances of duplex (i.e. with a prior plasma nitriding treatment) and non-duplex PVD coatings in micro-abrasive wear tests and impact tests are discussed, taking into account some key properties, such as nanohardness (H), elastic modulus (E) and the H/E ratio. The nanohardness and elastic modulus of the coatings were determined by nanoindentation and were characterised by means of several analytical methods, such as glow discharge optical emission spectroscopy (GDOES), X-ray diffraction (XRD), surface profilometry, Knoop hardness measurements, scratch tests and scanning electron microscopy (SEM). Similar H/E ratios were found for all coatings under investigation. Duplex (Ti,Al)N coatings exhibited the best wear resistance in micro-abrasive wear tests. The results obtained in such tests suggested the occurrence of abrasive wear by plastic deformation, with the hardest coatings displaying the lowest wear rates. The highest wear rates were recorded for duplex and non-duplex Cr–N coatings. Conversely, a duplex Cr–N coating exhibited the best performance in impact tests using a tungsten carbide ball. The duplex Cr–N coating displayed the lowest elastic modulus among all coatings tested, which can be attributed to the presence of a small amount of α-Cr metallic phase. This coated system also showed the smallest difference between the substrate and coating elastic moduli, indicating that low elasticity mismatch between the coating and substrate is desirable for achieving good performance in impact tests. The differing behaviours exhibited by duplex (Ti,Al)N and Cr–N coatings in different tribological tests reveal the importance of selecting the best coating for a given tribological application. [Copyright &y& Elsevier]