On understanding the microstructure of SiC/SiC Ceramic Matrix Composites (CMCs) after a material removal process

The unique material nature (e.g. hard, brittle, heterogeneous and orthotropic) of SiC-based Ceramic Matrix Composites (CMCs) highly affects the outcomes of machining process by inducing high thermo-mechanical loads during material removal. This can result in severe material damage which in turn causes a reduction of the in-service life of critical structural ceramic components (such as in aero-engines or nuclear reactors). In this study, the phenomenon by which the material removal mechanism during drilling influences the CMC surface integrity are discussed by characterising the fracture and deformation phenomena on the CMC's constituents - i.e. SiC and Si materials. Moreover, the strain induced to the surface, together with the changes in chemical composition are characterised via micro Raman spectroscopy and related to the principles of residual stresses upon cutting. This results in a novel understanding of the material removal process that governs cutting of SiC-based CMCs while emphasising how the different microstructure, morphology and nature of ceramics behave under the same cutting conditions. This study has therefore led to a comprehension of how the microstructure of complex hierarchical ceramic materials such as SiC/SiC CMCs is affected by a mechanical cutting process and opens avenues to understand the structure damage under other machining operations (e.g. milling, grinding)

An assessment of the wear characteristics of microcutting arrays produced from polycrystalline diamond and cubic boron nitride composites

The current methods for manufacturing super-abrasive elements result in a stochastic geometry of abrasives with random three-dimensional abrasive locations. This paper focuses on the evaluation of wear progression/failure characteristics of micro-abrasive arrays made of ultrahard composites (polycrystalline diamond—PCD; polycrystalline cubic boron nitride—PCBN) in cutting/wear tests against silicon dioxide workpiece. Pulsed laser ablation (Nd:YAG laser) has been used to manufacture repeatable patterns of micro-abrasive edges onto microstructurally different PCD/PCBN composites. Opposing to these highly engineered micro-abrasive arrays, conventional electroplated abrasive pads containing diamond and CBN abrasives, respectively, have been chosen as benchmarks and tested under the same conditions. Contact profiling, optical microscopy, and environmental scanning electron microscopy have been employed for the characterization of the abrasive arrays and electroplated tools before/during/after the wear/cutting tests. For the PCD abrasive micro-arrays, the type of grain and binder percentage proved to affect the wear performances due to the different extents of compressive stresses occurring at the grain boundaries. In this respect, the micro-arrays made of PCD with mixed diamond grain sizes have shown slower wear progression when compared to the electroplated diamond pads confirming the combination of the high wear resistance typical of the fine grain and the good shock resistance typical of the coarse grain structures. The micro-arrays made of fine grained diamond abrasives have produced lower contact pressures with the workpiece shaft, confirming a possible application in polishing or grinding. As for the PCBN abrasive micro-arrays, the increase of metallic binder and the presence of metalloids in the medium content-CBN specimens have shown to produce higher contact pressure with the workpiece when compared to the electroplated specimen, causing fracturing as the main wear mechanism; while the PCBN micro-array with purely a metallic binder phase has shown slower wear and lower contact pressure in comparison to the electroplated CBN specimen. Among all of the tested arrays, the mixed grained PCD and the purely metallic binder phase PCBN micro-arrays have shown slower wear when benchmarked to the electroplated pads, giving a possible application of their use in the cutting tool industry

Pulsed laser ablation as a tool for in-situ balancing of rotating parts

The balancing of complex rotating systems is a challenging task as it may require repetitive (dis)assembly to enable mass adjustments; thus, developing methods for in-situ dynamic balancing of rotatives is regarded as a key technology enabler. In this context laser balancing with its high flexibility in adjusting its firing frequency (to match that of the rotating part) and pulse energy (to vary the material removal) could offer significant advantages from both precision and cost point of view. In this paper, a laser balancing system is developed to continuously remove material from a target part in a controlled and automated manner. The amount of material ablated can be controlled by an influence coefficient, which is related to the change in vibration amplitude for a predefined amount of pulses at a given operational balancing speed, material, and geometry of the rotative part. The proposed system features a three-layered case-driven programmatic approach to optimize single-plane balancing process duration in a fully automated system. This enables the use of prioritization to avoid misfire and therefore, structural damage to the targeted part. Furthermore, the application allows the component to be balanced to all common balancing grades as specified in the ISO 1940/1 standard. Thus, validation trials involved balancing an Inconel 718 rotative to a preliminarily specified balancing grade by extracting the acceleration signals using an IIR peak filter. A computer simulation encompassing the rotor bearing state space system, a model of the laser and the adapted peak detection algorithm, has been developed and used to validate the trials conducted. Henceforth, a maximum deviation from the desired correction position of less than 1 mm has been recorded. Moreover, it has been shown that the detection and correction of imbalances can be reliably achieved by reducing the vibration level of a rotor from G 22.5 to G 19.5

Combustion flame spray of CoNiCrAlY & YSZ coatings

The properties of CoNiCrAlY and ∼7–8%YSZ layers, used as thermal barrier coatings (TBC) to protect hot gas paths of power generation and aerospace gas turbines, that have been deposited through the Combustion Flame Spray (CFS) process, are assessed and compared to coatings of the same materials deposited through Atmospheric Plasma Spray (APS). Fuel-to-oxygen equivalence ratio, combustion and carrier gases flows, torch standoff distance and powder feed rate values have been varied during the CFS tests in order to assess their effect on microstructural characteristics, i.e. thickness, total porosity, oxide level and microhardness. Results show that, in CFS-deposited coatings, although a higher content of oxide strings and porosity is observed compared to APS, also comparable phase transformations and a higher thermal cyclic lifetime can be achieved with an appropriate tuning of the deposition parameters. Thus, the study demonstrates the excellent capability of the CFS process in depositing thermal barrier coating systems, providing a viable alternative deposition technology for this class of materials at significant hardware simplicity. As the CFS setup has a simple design, this research stimulates a miniaturisation concept of the combustion flame spray torch for allowing its deployment into highly restricted workspaces

Continuous trench, pulsed laser ablation for micro-machining applications

The generation of controlled 3D micro-features by pulsed laser ablation in various materials requires an understanding of the material's temporal and energetic response to the laser beam. The key enabler of pulsed laser ablation for micro-machining is the prediction of the removal rate of the target material, thus allowing real-life machining to be simulated mathematically. Usually, the modelling of micro-machining by pulsed laser ablation is done using a pulse-by-pulse evaluation of the surface modification, which could lead to inaccuracies when pulses overlap. To address these issues, a novel continuous evaluation of the surface modification that use trenches as a basic feature is presented in this paper. The work investigates the accuracy of this innovative continuous modelling framework for micro-machining tasks on several materials. The model is calibrated using a very limited number of trenches produced for a range of powers and feed speeds; it is then able to predict the change in topography with a size comparable to the laser beam spot that arises from essentially arbitrary toolpaths. The validity of the model has been proven by being able to predict the surface obtained from single trenches with constant feed speed, single trenches with variable feed speed and overlapped trenches with constant feed speed for three different materials (graphite, polycrystalline diamond and a metal-matrix diamond CMX850) with low error. For the three materials tested, it is found that the average error in the model prediction for a single trench at constant feed speed is lower than 5 % and for overlapped trenches the error is always lower than 10 %. This innovative modelling framework opens avenues to: (i) generate in a repeatable and predictable manner any desired workpiece microtopography; (ii) understand the pulsed laser ablation machining process, in respect of the geometry of the trench produced, therefore improving the geometry of the resulting parts; (iii) enable numerical optimisation for the beam path, thus supporting the development of accurate and flexible computer assisted machining software for pulsed laser ablation micro-machining applications

CONSIDERATIONS ON THE SURFACE WATER QUALITY CONDITION IN THE SIRET HYDROGRAPHIC BASIN

Considerations on the surface water quality condition in theSiret hydrographic basin. The surface water resources in the Siret river basin areabout 17% of total water resources of the country and consist mainly of the SiretRiver, its tributaries and in a very small extent, from lakes and ponds. The study ofthe water quality variation of Siret River is done according to the organoleptic,physical, chemical, biological and bacteriological characteristics which aredetermined by analysis of groups of qualitative and quantitative indicators thatvary with specific environmental conditions. Thus, the authors follow, through studieson the evolution of state parameters of the Siret river water quality in differentsections of monitoring,to assess the degree of pollution and its impact on river

Investigation of the microstructure change due to phase transition in nanosecond pulsed laser processing of diamond

Experiments and theory are employed to investigate the thermal damage induced by infra-red nanosecond pulses in atmospheric air into a boron-doped diamond target. Micro-Raman spectroscopy, Transmission Electron Microscopy (TEM) analysis and surface topography measurement are used to investigate the carbon phase created during the rapid heating and cooling of diamond, as well as the amount of material ablated during the interaction with the laser. The analysis provides insight into the phenomena occurring for the rapid graphitisation of diamond during pulsed laser ablation, and also the microstructural disorder induced by the thermal and pressure fields at level of energy below and above the melting threshold. To support the understanding from the experimental investigations, a model is constructed for the graphitisation and ablation of diamond coupled with a collisional radiative model for the plasma evolution. The one-dimensional system of non-linear equations that model the physical processes provides an insight into the dynamics of the phenomena leading to the creation of disturbed graphite during pulsed laser ablation. Furthermore, the model helps to identify the main physical processes leading to the creation of disordered graphite, suggesting that plasma evolution does not follow a Boltzmann-Saha equilibrium and that radiative recombination is a main factor influencing the thermal evolution of the plasma and the diamond target. Finally, a good agreement with experimental findings is obtained, particularly in regards to the amount of material ablated, the thickness of the graphite layer and the processes leading to the melting of graphite