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The notched clamped beam geometry has proven to be a useful geometry in evaluating local mechanical properties like fracture and fatigue of thin graded platinum nickel aluminide bond coats. Cyclic fatigue tests performed on different zones of these coatings have shown that the structural stiffness of the beam is a useful parameter in estimating localized damage in front of the notch root, which can arise either due to cyclic softening from micro-crack nucleation and propagation in the crack wake or due to work hardening in the plastic zone surrounding the crack tip. This paper describes the use of finite element method-based analysis to quantify different geometrical factors that affect cyclic stiffness data in the micro-beam bending methodology employed in the present study. Variations in the stiffness values measured at the beginning of each loading cycle can occur due to factors such as the effect of loading line offsets (both lateral and angular) and the notch length to width ratio (a/W). Using this analysis, it was found out that lateral offsets affect the starting stiffness of the beam more profoundly compared to angular offsets. It has also been shown that for beams having lengths of ∼ 80 - 90 μ m and an a/W ratio of ∼ 0.3–0.4, the starting stiffness variations can be kept under 10%. The stiffness can also be used to estimate crack velocities (da/dN) in an approximate sense using the finite element method. Detailed finite element analysis has been performed to estimate through thickness crack lengths from cyclic stiffness drops. The applied stress intensity factor for real crack geometries observed during testing have been computed using the extended finite element methodology and da/dN vs Δ K plots have been generated for two cases. This paper highlights the importance of using computational analysis in optimizing and augmenting micro-scale fatigue testing data and thus providing for a richer dataset through such analysis.

A Correction to this paper has been published: https://doi.org/10.1007/s10704-020-00492-w

The field of micro-/nano-mechanics of materials has been driven, on the one hand by the development of ever smaller structures in devices, and, on the other, by the need to map property variations in large systems that are microstructurally graded. Observations of ‘smaller is stronger’ have also brought in questions of accompanying fracture property changes in the materials. In the wake of scattered articles on micro-scale fracture testing of various material classes, this review attempts to provide a holistic picture of the current state of the art. In the process, various reliable micro-scale geometries are shown, challenges with respect to instrumentation to probe ever smaller length scales are discussed and examples from recent literature are put together to exhibit the expanse of unusual fracture response of materials, from ductility in Si to brittleness in Pt. Outstanding issues related to fracture mechanics of small structures are critically examined for plausible solutions.

Stability of a fracture toughness testing geometry is important to determine the crack trajectory and R-curve behavior of the specimen. Few configurations provide for inherent geometric stability, especially when the specimen being tested is brittle. We propose a new geometrical construction called the single edge notched clamped bend specimen (SENCB), a modified form of three point bending, yielding stable cracking under load control. It is shown to be particularly suitable for small-scale structures which cannot be made free-standing, (e.g., thin films, coatings). The SENCB is elastically clamped at the two ends to its parent material. A notch is inserted at the bottom center and loaded in bending, to fracture. Numerical simulations are carried out through extended finite element method to derive the geometrical factor f(a/W) and K I for different beam dimensions. Experimental corroborations of the FEM results are carried out on both micro-scale and macro-scale brittle specimens. A plot of K I vs a/W, is shown to rise initially and fall off, beyond a critical a/W ratio. The difference between conventional SENB and SENCB is highlighted in terms of K I and FEM simulated stress contours across the beam cross-section. The K IC ’s of bulk NiAl and Si determined experimentally are shown to match closely with literature values. Crack stability and R-curve effect is demonstrated in a PtNiAl bond coat sample and compared with predicted crack trajectories from the simulations. The stability of SENCB is shown for a critical range of a/W ratios, proving that it can be used to get controlled crack growth even in brittle samples under load control.

The aim of this study was to identify the areal surface parameters that correlated with lowering of sliding friction. Different ground surfaces were created on stainless steel and the lubricated sliding friction generated at the contact interface with a flat-faced aluminum pin was studied. The frictional force encountered is an order of magnitude lower for a P1200-finished surface than the other ground surfaces. Using 3D surface profilometry, a unique surface parameter ratio “Spk/Sk” was found to predict the frictional performance of these surfaces. When this surface parameter ratio was less than 1, average sliding friction was close to 0.1. When this ratio was greater than 1, the coefficient was an order of magnitude lower. Using energy dispersive spectrometry, such surfaces after wear showed the presence of a uniform dispersed layer of iron oxide on the surface of the pin. This was absent on the surfaces having high friction, indicating the role of the steel counter surface in building this beneficial transfer layer. Scanning electron microscopy provided topography images to visualize the surface wear. The motivation for the authors was to use a commercially scaled process like precision grinding for the surface modifications on stainless steel.

The key objective of the presented study was to use a commercially feasible and scalable approach to modifying surfaces to reduce friction. In an industrial setting, surface grinding is commercially viable and scalable as compared to other surface modifying processes like laser surface texturing, plasma, or ion beam milling. Frictional force plots are generated from the lubricated contact interface between a flat-faced aluminum pin and a reciprocating stainless steel countersurface driven by a scotch yoke follower mechanism. Using a surface grinder and selecting coated abrasive sheets, different stainless steel surface specimens, classified as P320, P1200, and mirror were prepared and tested in this study. The frictional force encountered by the pin was recorded using a data acquisition system at discrete intervals in the reciprocating path and averaged along the sliding cycles. The shape of the frictional force plots thus generated were found to be different from each other. Various mechanisms of friction prevalent at the contact were presumed to influence the shape of these frictional plots. These mechanisms were tested by varying the sliding speeds, lubricating oil viscosities, and using tribofilm-forming additives. We used Group 1 base oil of two different viscosities in our tests. At lower speeds, the frictional force plot for the mirror-finished surface seemed to conform to the Stribeck curve, while in the same scale of reference, the P1200 surface had a force plot that was nearly flat and of very low magnitude. At the contact interface, there seemed to be a fine balance existing between adhesion and abrasion phenomena, while oil retention was promoted to achieve extremely low sliding friction.

In order to understand the influence of ductile metal interlayer on the overall deformation behavior of metal/nitride multilayer, different configurations of metal and nitride layers were deposited and tested under indentation loading. To provide insight into the trends in deformation with multilayer spacings, an FEM model with elastic-perfect plastic metal layers alternate with an elastic nitride on top of an elastic–plastic substrate. The strong strain mismatch between the metal and nitride layers significantly alters the stress field under contact loading leading to micro-cracking in the nitride, large tensile stresses immediately below the contact, and a transition from columnar sliding in thin metal films to a more uniform bending and microcracking in thicker coatings.

Gas turbine blades and disks undergo wear at high temperatures at dovetail joints where tolerances are very small. Thin hard coatings are known to enhance the wear resistance of the superalloy components minimally influencing the tolerance levels. However, fundamental understanding of the coating’s wear mechanisms operating in these harsh conditions is not well understood. In this study, wear tests are performed to understand the wear mechanisms that operate in the temperature range from RT up to 800 °C for thin hard TiAlN coating using simple wear geometry eliminating any influence of wear debris. It is challenging to measure wear of thin hard coatings especially at elevated temperatures but important nevertheless. A coated ball on disk geometry with rough alumina as counterface is used for wear studies to understand exclusively the influence of humidity and temperature coating wear behavior. Cathodic arc evaporation, a physical vapor deposition technique is used to deposit TiAlN coatings on heat-treated IN718 substrates and characterized with, XRD, EPMA, TEM, SEM, nanoindentation, and FIB. The wear at room temperature shows scatter which has been ascribed to seasonal fluctuations in relative humidity. Further, wear results are shown to correlate with Young’s equation for capillary condensation. Wear below 50 pct RH is essentially dry and constant up to 600 °C above which wear increases marginally upto 800 °C. The coefficient of friction shows a maximum at 400 °C, below which friction reduces due to increased adsorption of water vapor, while above 400 °C, TiO 2 forms on the surface to reduce the friction. The wear rate at 3 N load in the range of 50–800 °C is ~ 1 × 10 −6  mm 3 /m/N. For 5 N load, the wear rate is same as for 3 N load upto 600 °C but doubles above 700 °C. The average contact pressure through the test is ~ 550 and 650 MPa which is almost twice the design contact pressure. The wear debris gets richer in Ti with increase in temperatures. The Al-rich TiAlN coatings deposited by cathodic arc evaporation (CAE) technique show a low and constant wear behavior over a wide range of temperatures and are ideally suited for the protecting the dovetail joints in gas turbines.

The isothermal and thermal cyclic oxidation behaviour of NiCoCrAlY coatings sprayed using high-velocity air fuel (HVAF) and high-velocity air (oxy) fuel (HVA(O)F) processes was compared at 1100 °C and 1120 °C on Ni-based superalloy substrates. The evolution of the NiAl (β) phase and thermally grown oxide (TGO) was systematically analysed as a function of time, up to 100 h. The TGO grew from 2 to 3 μm (after 20 h) to nearly 25 μm after 40 h and comprised a mixed oxide layer of α-Al 2 O 3 and NiO after 40 h under isothermal oxidation. The Al content in the coating mid-zone remains unchanged at 8–10 wt%, however, the Al content in the β-depleted zone underneath the TGO drops to < 2 wt%, leading to a fully γ microstructure after 50 h of isothermal exposure. The β-depleted layer thickness increases with exposure time from 5 μm after 20 h to nearly 45 μm after 40 h. The key difference between HVAF and HVA(O)F coatings could be seen in the development, after isothermal exposure, of porosity in the case of HVAF, while the HVA(O)F remained dense. Thermal cyclic exposure after 20 h between room temperature and 1100 °C led to coating delamination in the case of HVAF coating, whereas HVA(O)F coatings remained intact. The delamination is attributed to the formation of pores in the vicinity of the grits. Overall, there seems little difference in the oxidation properties of the coatings between the two processes under isothermal exposure, whereas HVA(O)F shows better structural integrity after thermal cycling. The results in this manuscript are the first to show the behaviour of HVAF-based coating response to high-temperature heat treatment and the analysis thereof to look at the novel aspect of interchangeability of using these coatings for various high-temperature oxidation applications such as in gas turbines components.

The thermal stability of laser power bed fusion A205 was studied via transmission electron microscopy (TEM) following various aging heat treatments. The effects of direct aging at 160 °C and 205 °C were compared with solutionizing and aging. The aging response was evaluated at three different temperatures (170 °C, 190 °C, 205 °C) up to 100 h. Solutionizing above solvus of the alloy at 530 °C with water quenching, followed by aging at 205 °C for 11 h, gave the optimum combination of high yield strength (376 MPa) and ductility (10%) at room temperature, which was retained up to 150 °C (yield strength of 314 MPa). Detailed TEM analysis coupled with atom probe tomography was used to reveal 20 × 1 nm 2 clusters enriched in Cu and Ag in the As-printed condition, which evolved after aging as nanoscaled, Ω (Al–Ag–Cu–Mg) (55 × 7 nm 2 ), and θ′ (Al–Cu) (40 × 5 nm 2 ) precipitates, having an orientation relationship with α-Al, which resisted coarsening and were responsible for the thermal stability during aging up to 200 °C after aging for 100 h. Graphical abstract