The mechanical properties of ceramic matrix composites (CMCs) are governed by the relationships between the matrix, the interface material, and the fibers. In non-oxide matrix systems the use of compliant pyrolytic carbon or BN have been demonstrated to be effective interface materials, allowing for absorption of mismatch stresses between fiber and matrix and offering a poorly bonded interface for crack deflection. The resulting materials have demonstrated remarkable strain/damage tolerance together with high strength. Carbon or BN, however, suffer from oxidative loss in many service environments, and thus there is a major search for oxidation resistant alternatives. This paper will review the issues related to developing a stable and effective interface material for non-oxide matrix CMCs.

The mechanical properties of unidirectional Nicalon SiC fiber reinforced calcium aluminosilicate (CAS/SiC) and magnesium aluminosilicate (MAS/SiC) glass-ceramic composites have been investigated by tensile testing and a nondestructive laser-ultrasound technique. The barium-stuffed MAS was either undoped or doped with 5% borosilicate glass. The degradation of the elastic stiffness constant Cu in the transverse direction due to interface damage was monitored in-situ by measuring the laser-generated ultrasound wave velocity. The three composite materials show distinctly different macroscopic deformation characteristics, which are correlated strongly to the interface degradation. A stronger reduction trend of the elastic constant ?? is associated with a larger degree of inelastic deformation. Observations of the fracture surfaces also reveal the close relation between fiber pullout length and interfacial characteristics. Interfaces of these composites have been studied by TEM, and their influence on inhibiting and deflecting matrix cracks is discussed.

The performance of reinforced ceramics, particularly the toughness and creep resistance, is often determined by the nature of the interface between the reinforcement and the ceramic matrix. Specially-designed experiments to investigate the role of the interfacial characteristics on toughening mechanisms and crack propagation in reinforced (silicon carbide whisker reinforced alumina) and self-reinforced (silicon nitride) ceramic composites will be described. In the whisker-reinforced composites, the interfacial topography and chemistry were of primary importance, whereas in the silicon nitride materials the formation of interfacial phases and glassy-phase chemistry influenced the interfacial debonding process. The composite interfaces were characterized by high resolution electron microscopy and high spatial resolution microchemical analysis, including energy-dispersive X-ray and electron energy loss spectroscopy. Results from energy-filtered images from ceramic interfaces will also be shown.

Interfaces between glass and crystalline grains have been examined using a thin-film geometry which allows the use of newly developed experimental methods for micromechanical testing of interfaces. In this approach, continuous films of thicknesses ranging 100–200 nm of anorthite (CaAl2Si2O8), celsian (BaAl2Si2O8), and monticellite (CaMgSiO4) are deposited onto single-crystal Al2O3 (α-structure) surfaces of different crystallographic orientations by pulsed-laser deposition (PLD).

Mechanical properties such as hardness, stiffness, and reduced Young's modulus were probed with a newly developed high-resolution depth-sensing indentation instrument. Emphasis has been placed on examining how changes in the glass composition will affect the mechanical properties of the single-crystal Al2O3/silicate-glass interfaces. The indentation data obtained from these experiments correlate directly to the morphology of the deformed regions imaged with atomic force microscopy (AFM). Nanomechanical tests combined with AFM imaging of the deformed regions allow force-displacement measurements and in-situ imaging of the same regions of the specimen before and immediately after indentation. This new technique eliminates the uncertainty of locating the indenter after unloading.

The effects of the interface on the tension and fatigue crack growth behavior of fiber-reinforced titanium matrix composites were studied using single-ply and single-fiber SiC/Ti-6Al-4V mini-composites with different SiC fibers and coatings. Attention was focused on fiber failure mechanisms in the absence of a matrix crack (tension loading), and on fatigue crack deceleration mechanisms. In contrast to established models of tensile failure, local load sharing behavior was observed for very weak interfaces, with plasticity playing a major role in enhancing stress concentration effects. The approach included statistical evaluation of fiber breaks, their locations, and comparisons between single-fiber and single-ply composites. Under fatigue loading, crack deflection behavior was found to be consistent with a strength based interface debonding model. Crack shielding through modulus mismatch was found to have a significant effect on crack growth rates, independent of bridging conditions, with a stronger interface performing better under such conditions. The ramifications of the different mechanisms on interface optimization for longitudinal and transverse properties of composites are indicated.

The effect of interface composition on the fracture energy and fracture toughness of the niobium/sapphire system was studied. Niobium films with thickness of 100 nm were deposited on (0001) sapphire substrates by physical vapor deposition (PVD). The interface was doped with silver to reduce the interfacial cohesion. The amount of silver ranged from less than 2.1 monolayers to 6.4 monolayers as measured by Rutherford backscattering spectrometer (RBS). The microscratch test was used in determining the interfacial fracture energy and the interfacial fracture toughness. Both interfacial fracture energy Gc and fracture toughness Kc decrease non-linearly with the amount of silver, and level off for samples with larger silver thickness ( > 4.2 monolayers). The values of interfacial fracture energy range from 3.43 J/m2 - 9.20 J/m2 for the sample doped with 2.1 monolayer silver (sample B) to 0.05 J/m2 - 0.5 J/m2 for samples with silver levels above 4.2 monolayers (samples D and E). The corresponding fracture toughnesses are 0.80 MPa-m½ - 1.29 MPa-m½ for sample B and 0.09 MPa-m½ - 0.30 MPa-m½ for samples D and E.

Several techniques of grain boundary engineering for improving ductility in polycrystalline Ni3Al are discussed with the plausible mechanism. Single crystals of Ni3Al are ductile at room temperature, but polycrstalline Ni3Al fails intergranularly with little elongation. The grain boundaries are also embrittled by environmental moisture. The weakness of the boundaries is attributed to the presence of crack-like microcavities at the boundaries. Microalloying with B closes up the microcavities by enhancing relaxation of Ni and Al atoms, thereby improving ductility. In directionally solidified Ni3Al containing low-angle boundaries, large ductility has been reported. Polycrystalline N?3AI produced by recrystallization of rolled single crystals contains a large fraction of Σ3 boundaries and exhibits large ductility. Due to good matching, the low-angle and Σ3 grain boundaries do not contain microcavities and consequently are strong.

The effects of surface films and second phase interfaces on the room temperature ductility of <001> oriented β (B2) NiAl-based alloys were investigated. Significant reduction in flow stress and enhancement in plasticity, as compared to the monolithic β phase, were seen in both film-coated and ductile γ (fcc)/γ' (Ll2) second-phase-containing alloys. The constrained deformation at the film/substrate and interphase interfaces was effective in nucleating mobile a<100> dislocations in the β phase even though the loading axis was approximately parallel to <001>. For the case of film-coated NiAl single crystals deformed in compression, a micro-kinking model is proposed to explain the propagation of a<001> dislocations generated at the film/substrate interface. For multiphase β/γ+γ' alloys where 10–12% tensile ductility was observed, the role of crystallographic orientation relationships in promoting slip transfer is highlighted. In these multiphase alloys, the intrinsic tensile ductility of β phase by {011}<100> slip when deformed approximately along the <001>β orientation is explained using lattice rotation concepts.

Modeling of grain boundary (GB) relaxation during ideal fracture, and the fracture energetics of a Σ3 (111) GB in Fe was performed using the modified Finnis-Sinclair semi-empirical method, and utilizing the so-called “environment-sensitive embedding energies” of impurity atoms, introduced earlier by the author. The calculations were done for both the clean GB, and GB with the following impurity atoms: H, B, C, N, O, P and S. The ideal fracture was modeled by separating the two halves of crystal normal to the GB, step-wise, minimizing the total crystal energy at every step. The interplanar distances were varied, while the Fe interatomic spacing within the hexagonal planes was held fixed. When the distance between the two crystal halves: one with the impurity and another without, exceeded the interatomic interaction cut-off radius (3.6 Å), two different free surfaces - with and without the impurity - emerged. The GB and surface energies were found both for the pure Fe, and that with impurity atoms at the GB or free surface. Both the (111) GB energy and the (111) surface energy of pure Fe agree well with experimental data and results of previous semi-empirical modeling. In general, the correlation between the embrittling/ cohesion enhancing effect of impurities in GB and the difference between the GB and free surface energies agrees with the thermodynamic criterion of embrittlement.

Alumina based composite ceramics with silicon carbide whisker were fabricated by hot-press sintering. Their abrasive or erosive wear properties were measured by abrasive waterjet method and micro-sandblast method, and the results were compared with those of normally sintered engineering ceramics. In the case of normal impact of abrasive waterjet, where micro-fracture may be the mam wear mechanism, the whisker composite showed considerable improvement of wear properties because of its higher toughness. In the case of low angle impact, where cutting is the mam wear mechanism, only a small improvement were observed by a small addition of whisker (5wt%SiC), but larger addition (30wt%SiC) resulted in higher wear. No effect of whisker was observed with the micro-sandblast method, perhaps because of its rather mild abrasive conditions.

A tension adhesion test is developed to measure the adhesion of a polymer interlayer to the glass plates in laminated glass. The polymer adhesive is bonded between two cracked glass plates and is loaded under a remote tension P. Typically, a peak load P* exists and its value depends on the adhesion between the polymer interlayer and the glass. The peak load P* is often used to characterize the adhesion strength of the interface. The experimental data is analyzed using a micromechanical model of debonding and simulated using finite element computation. Our analysis allow us to determine the shear and normal strength of the interface and the relation between the peak load and the mechanism of debonding.

Unlike the iron based alloys, the embrittlement behavior of aluminum based alloys due to impuri ty segregation at the grain boundaries have not previously studied. Recently, experiments were performed on the Al-Mg alloys and it was found that more than 0.6ppm Na impurity atoms will cause embrittlement of these alloys. In the present study, we have created aluminum clusters containing about 20 atoms based on the structure of tilt grain boundries and then performed the first-principle calculations on these clusters with or without alkalline metal impurity atom(Li, Na, K,). The overlap populations between aluminum atoms were calculated so as to find the effect of impurity atoms on the bonding between aluminum atoms. Na impurity atom was found to weaken the bonding between the aluminum atoms around it.

A series of metals was examined for suitability for the Wheel Abrasion Experiment, one of ten microrover experiments of the Mars Pathfinder Mission. The seven candidate metals were: Ag, Al, Au, Cu, Ni, Pt, and W. Thin films of candidate metals from 0.1 to 1.0 micrometer thick were deposited on black anodized aluminum coupons by e-beam and resistive evaporation and chemical vapor deposition. Optical, corrosion, abrasion, and adhesion criteria were used to select Al, Ni, and Pt. A description is given of the deposition and testing of thin films, followed by a presentation of experimental data and a brief discussion of follow-on testing and flight qualification.

The dissociation of extrinsic grain boundary dislocations in Al, Al-Mg and Al-Mg-Sn alloys was studied at various temperatures to examine the compositional effect on the dissociation process. It was found that both alloying elements (Mg and Sn) hinder the dissociation, however the effect of Sn is more significant even at a very low concentration due in part to grain boundary segregation.

The present work reports oii the effect of grain boundary structure on intergranular corrosion and creep by grain boundary diffusion in polycrystalline lead (Pb). “Special” boundaries characterized by low-Σ misorientations in the Coincident Site Lattice (CSL) model have been shown to exhibit resistance to grain boundary sliding and corrosion. PbCa and PbCaSn alloys having “special” grain boundary frequencies greater than 50% showed a 30 fold reduction in steady-state creep compared with corresponding as-received alloys containing 10% to 28% of low-Σ CSL boundaries in the microstructure of comparable grain size. At the same time, increasing the frequency of “special” boundaries enhanced alloy ductility with no compromize in tensile strength. Results from electrochemical corrosion tests on polarizied Pb alloys immersed in H2SO4 (S.G.=1.28) at 70°C indicate a clear correlation between the frequency of “special” boundaries and the prolifíeration of intergranular corrosion resulting in bulk weight loss from grain-dropping. Collectively, these results advocate adopting a “gram boundary design” approach in developing advanced lead-acid battery electrodes.

Structure of interfaces in as-deposited state and after annealling at elevated temperatures was studied in variety of dc-magnetron sputtered multilayers by X-ray diffraction and high resolution electron microscopy. Interfaces in multilayers from materials without phase equilibrium (Mo/Si, W/Si, MO/B4C, Cr/C, Mo/(B+C)) are unstable and has the trend to evolution due to interlayer redistribution of chemical elements. Formation of the extended intermixed transition zones at interfaces is characteristic for multilayers from materials without phase equilibrium with each other. Unhomogeneous structure of materials along and across layers is responsible for nonuni-form proceeding of layer intermixing, development of interface roughness and differences of neighbour interfaces. It was shown that thickness asymmetry of intermixed transition zones at Mo/Si and Si/Mo interfaces in large-period Mo/Si multilayers is caused by different mechanisms of growth of Mo and Si on each other and by different structure of lower and upper parts of polycrystalline Mo-layers. Interfaces in multilayers from materials with phase equilibrium MoSi2/Si, WSi2/Si, OB2/C, TiC/C, Ni/C, MO2B5/B4C) are sharp and stable at elevated temperatures which gives the possibility to treat them for modification of their structure (smoothening, restoring sharp profiles of chemical elements). Multilayers W/Ti and W/TiN showed trend to formation of epitaxial superlattices with coherent or partially coherent (with misfit dislocations) interfaces and pseudomorphic state of layers.

Plasma spraying is being investigated as a method to fabricate first wall Be armor for the International Thermonuclear Experimental Reactor. Diffusion barriers of Ti, V, and W were deposited between the Be and the Cu substrate, also by plasma spray, to prevent formation of brittle Be-Cu intermetallics. Interface fracture toughness tests revealed loading mode-dependent crack behaviors and interface fracture toughnesses.

The key role of interfaces in the initiation and propagation of fatigue damage is elucidated for a model eight ply [0/90]2s Ti-15V-3Cr-3Al-3Sn (Ti-15–3) composite reinforced with SiC (SCS-6) fibers. Interfacial strengths obtained from fiber push-out tests are used in the quantification of shielding due to crack bridging. Composite fatigue tests are predicted using an idealized fracture mechanics model. The possible effects of interfacial strength variation on the predicted fatigue lives are also elucidated.

Creep occurs in two phase α/β Ti alloys at room temperature and at stresses below the yield strength. Creep strains in the Widmanstatten structure are affected by colony size and by sliding at the α/β interfaces. This paper reports the results of an investigation on single colony crystals of a near a Ti alloy to understand the dependence of creep on the orientation of the α/β interface and the operative slip system. Associated mechanisms of slip transfer across the α/β interface and possible interface sliding are investigated and the results obtained are presented.

Intergranular degradation processes, (e.g., corrosion, stress corrosion, cracking, creep cracking) are a frequent cause of premature and unpredictable service failure of engineering components. Recent advances in (1) understanding structure-property relationships for grain boundaries, and (2) characterization techniques for grain boundaries in polycrystalline materials, have provided the means for improved component lifetime prediction, and the opportunity to engineer intergranular-degradation resistant microstructures.

In this work, we present our previously developed geometric models for grain boundary structure and grain size effects on intergranular degradation susceptibility. Specific examples are presented of the successful application of the ‘grain boundary engineering’ approach to the prediction and mitigation of intergranular corrosion, stress corrosion cracking, and creep cracking in Ni-based materials.