Friction, wear and tangential stiffness of metal surfaces under fretting conditions

Bladed disk vibrations in turbomachinery can lead to failure due to High Cycle Fatigue. One way in which vibration may be reduced is by dry friction damping. Frictional damping originates from micro and macro slip in the contacting interfaces (“joints”) and is controlled by the relationship of the applied load and tangential displacement. In order to predict the dynamic response of the structure, knowledge of the coefficient of friction and the tangential contact stiffness of the contact are crucial. Vibration induced slip and the consequent damage in contacting surfaces has been widely studied and is usually called fretting. However, little is known about the effect of the changing interface during fretting on the coefficient of friction and the tangential contact stiffness, which is required when trying to predict these parameters. This study seeks an improved understanding of the effects of surface topography, surface chemistry, and elastic and plastic material properties on the friction and damping performance of joints under fretting conditions. In the present study experiments were conducted to measure the coefficient of friction and the tangential contact stiffness of different metals under different test conditions. Fretting damage mechanisms were investigated using metallography, SEM, EBSD, TEM and XRD techniques. The evolution of roughness and conformity was investigated by using interferometric profiling systems and image registration via cross correlation. An infrared radiation measuring system was employed to measure the dissipated radiation and frictional power in fretting which was then compared with calculated energy dissipation maps. Experimental results were used to validate models predicting contact stiffness which have been developed throughout the project by collaborating researchers. This study highlighted real contact conditions and their dependence on running time, which need to be taken into account when modelling friction contacts

Defect tolerance in as-deposited selenium-alloyed cadmium telluride solar cells

The efficiency of cadmium telluride (CdTe) solar cells is limited primarily by voltage, which is known to depend on the carrier concentration and carrier lifetimes within the absorber layer of the cell. Here, cathodoluminescence measurements are made on an as-deposited CdSeTe/CdTe solar cell that show that selenium alloyed CdTe material luminesces much more strongly than non-alloyed CdTe. This reduction in non-radiative recombination in the CdSeTe suggests that the selenium gives it a certain defect tolerance. This has implications for carrier lifetimes and voltages in cadmium telluride solar cells

The strain fields present during the bending of ultra-high strength steels

Ultra high strength steels (UHSS) have an ultimate tensile strength of greater than 1GPa. Typically, their ambient temperature elongation is less than 10% and as a result, they are rarely used in stamping applications. However, the continuous demand for the weight reduction of structures built for the transport sector means that such materials are attractive because they can be used for parts with thinner cross-sections while maintaining required in-service performance. One way to overcome the ambient temperature ductility of these materials is to roll-form them, particularly with emerging flexible roll forming technology. Using numerically-controlled actuators, the rolls on each stand are designed with sufficient degrees of freedom to form parts that curve, vary in depth and width along their lengths. This makes flexibly roll-formed parts attractive to the transport, particularly the automotive, sector. Roll forming deforms a material through incremental, localised bending, which is known to suppress the necking response, resulting in deformations that are higher than in stretch deformation. Recent work, such as Le Maoût, Thuillier & Manach, Eng. Frac. Mech., Vol. 76, p.1202 (2009), focussed on the development of ductile fracture models to explain failure but their validation was limited to load displacement and surface strain data. This work aims to characterise the strain field during bending more comprehensively. Using the digital image correlation technique, the macroscopic strain distribution in UHSS in the thickness of the sheet and the strain partitioning in its microstructure is measured during bending. The data provides a detailed explanation of the strain distribution during bending

Effects of oxygen-related damage on dwell-fatigue crack propagation in a P/M Ni-based superalloy : from 2D to 3D assessment

Effects of oxygen-related damage (i.e. oxidation and dynamic embrittlement) on fatigue crack propagation behavior in an advanced disc alloy have been assessed in air and vacuum under dwell-fatigue conditions at 725 oC. The enhanced fatigue crack propagation is closely related to oxygen-related damage at/ahead of the crack tip, which is determined by the testing environment, the dwell period and the crack propagation rate itself based on two dimensional (2D) observation of the crack tip in an optical microscope and scanning electron microscope. X-ray computed tomography has also been employed to examine the differences between three dimension (3D) crack morphology in air and vacuum conditions, and the crack features have been quantified in terms of crack opening displacements, secondary cracks and uncracked bridging ligaments. The results show that the fatigue crack propagation rate is related to the amount of secondary cracks, and the crack length increment in a loading cycle is related to the breaking/cracking of the uncracked bridging ligaments within the discontinuous cracking zone ahead of the crack tip as oxygen-related damage preferentially occurs in these highly deformed regions. By combination of 3D X-ray computed tomography and traditional 2D observation, a deeper understanding is provided of the mechanisms of oxygen-enhanced fatigue crack propagation behavior

Effects of oxidation on fatigue crack initiation and propagation in an advanced disc alloy

Powder metallurgy Ni-based superalloys are widely used for aeroengine turbine disc application due to their exceptional strength properties at elevated temperatures, good fatigue and creep performance as well as excellent corrosion and oxidation resistance. However, oxygen enhanced fatigue crack initiation and intergranular propagation at elevated temperatures in air is commonly observed in aeroengine turbine disc superalloys under dwell fatigue testing conditions [1-7], and this phenomenon is usually ascribed to either decohesion/reduction in cohesion strength of grain boundary (GB) due to dynamic embrittlement [8, 9] or GB oxide cracking caused by stress assisted grain boundary oxidation (SAGBO) [5, 10-12]. Although the influence of oxygen on fatigue crack initiation and propagation has been intensively studied, the underlying mechanism for the oxygen-assisted fatigue failure process is still not clear due to the complex composition of disc alloy and the interaction between environmental attack and mechanical load. In this study, fatigue tests were conducted on the Low Solvus, High Refractory (LSHR) alloy designed by NASA for turbine disc application, with a particular focus on studying the influence of the formation of GB oxides on fatigue crack initiation and propagation processes

Fatigue crack growth in a nickel-based superalloy at elevated temperature - experimental studies, viscoplasticity modelling and XFEM predictions

Experimental studies and computational modelling of crack deformation and growth in a nickel-based superalloy at elevated temperature have been carried out for a threepoint bending specimen subjected to fatigue loading condition. In order to remove the influence of oxidation which can be considerable at elevated temperature, crack growth was particularly tested in a nominal vacuum/minimal oxidation environment with a focus on dwell effects. For simulation, the material behaviour was described by a cyclic viscoplastic constitutive model with nonlinear kinematic and isotropic hardening rules. Computational analyses of a stationary crack showed the progressive accumulation of plastic strain near the crack tip, which has been subsequently used as a fracture criterion to predict crack growth using the extended finite element method (XFEM). The crack was assumed to grow when the accumulated plastic strain ahead of the crack tip reached a critical value which was calibrated from crack growth test data in vacuum. During the simulation, the crack length was recorded against the number of loading cycles, and the results are in good agreement with the experimental data which proves the model’s capability to predict fatigue crack growth in nickelbased superalloys at high temperature. It is also shown, both experimentally and numerically, that an increase of dwell period leads to an increase of crack growth rate due to the increased creep deformation near the crack tip, but this effect is marginal when compared to the dwell effects under fatigue-oxidation conditions

Role of oxygen in enhanced fatigue cracking in a PM Ni-based superalloy : stress assisted grain boundary oxidation or dynamic embrittlment?

The role of oxygen in enhanced fatigue cracking in an advanced Ni-based superalloy for turbine disc application has been evaluated in fatigue crack initiation and propagation stages along with static oxidation tests. It is found that the grain boundary oxide intrusion has a layered structure. The microstructure- and deformation-dependent grain boundary oxidation dominates the fatigue crack initiation and early propagation processes. As the crack propagates, this contribution arising from oxidation damage may gradually be overtaken by dynamic embrittlement processes until the mechanical damage outstrips the oxygen-related damage, resulting in a transition from intergranular to transgranular crack propagation

An alternative approach to the analysis of Si-doped DLC coatings deposited with different bias voltage

A series of diamond-like carbon (DLC) coatings were deposited with increasing bias voltage using magnetron sputtering techniques. Structural changes were observed in the sp2-configuration across the films which were accompanied by a slight increase in the sp3 fraction. With an increasing bias voltage, the thermal stability of the coatings increased from 300 to 450 °C. Oxygen diffusion was observed through the coating as a result of the high-temperature annealing and found to slow down with increasing bias voltage. Coefficients of friction (COF) remained stable with temperature for the individual coatings, with the softer films reporting the lowest COF. Our approach employed Raman spectroscopy to map the wear tracks at different temperatures, providing a deeper understanding of the coating performance and suggested maximum flash temperatures endured during testing

Effects of oxidation on fatigue crack initiation and propagation in an advanced disc alloy

Powder metallurgy Ni-based superalloys are widely used for aeroengine turbine disc application due to their exceptional strength properties at elevated temperatures, good fatigue and creep performance as well as excellent corrosion and oxidation resistance. However, oxygen enhanced fatigue crack initiation and intergranular propagation at elevated temperatures in air is commonly observed in aeroengine turbine disc superalloys under dwell fatigue testing conditions [1-7], and this phenomenon is usually ascribed to either decohesion/reduction in cohesion strength of grain boundary (GB) due to dynamic embrittlement [8, 9] or GB oxide cracking caused by stress assisted grain boundary oxidation (SAGBO) [5, 10-12]. Although the influence of oxygen on fatigue crack initiation and propagation has been intensively studied, the underlying mechanism for the oxygen-assisted fatigue failure process is still not clear due to the complex composition of disc alloy and the interaction between environmental attack and mechanical load. In this study, fatigue tests were conducted on the Low Solvus, High Refractory (LSHR) alloy designed by NASA for turbine disc application, with a particular focus on studying the influence of the formation of GB oxides on fatigue crack initiation and propagation processes

Effect of chromium doping on high temperature tribological properties of silicon-doped diamond-like carbon films

Amorphous carbon films were deposited by means of closed-field unbalanced magnetron sputtering (CFUBMS). The silicon content was fixed at 1.3 at. %, while the chromium content was increased by modification of the current applied to the chromium magnetrons, with two doping levels, 0.3 and 2.7 at. %. Both, hardness and thermal stability were found to decrease as result of increasing chromium. Ball-on-disk tests revealed friction coefficients of 0.06 at room temperature with similar specific wear rate in all films (~4 × 10−13 m3 N−1 m−1). Increasing annealing temperatures were found to reduce the coefficient of friction compared to room temperature values, while increasing the specific wear rate for all films