Development of improved cold spray and HVOF deposited coatings

The overall aim of this research project was to expand the understanding of the deposition of titanium and the nickel-based superalloy Inconel 718 by spray deposition methods. The spray processes employed were cold spraying and high velocity oxy-fuel (HVOF) thermal spraying. The first part of the work was undertaken to expand the understanding of the deposition of titanium by cold spraying; the HVOF process is unsuitable for Ti because of the metal's high reactivity. The deposits were produced from commercially pure titanium using cold spray equipment designed in the University. Using helium gas, the effects of different powder particle size ranges, types of substrate, substrate preparation methods, and spray parameter conditions on deposit formation were investigated. Using a simple one-dimensional model of compressible gas flow and particle acceleration, particle velocity distributions were calculated to aid interpretation of experimental data. Results show that titanium can be successfully cold sprayed onto substrates of Ti6AI4V and mild steel, with the critical velocity for deposition of this powder type of approximately 690 m s-1. The level of porosity was generally in the range of 13-23% and the adhesive bond strength was dependent on surface preparation but independent of gas pressure with values ranging from 22 MPa to 10 MPa for ground and grit blasted substrates respectively. This compares with a value of around 80 MPa which is typical for well adhered HVOF sprayed coatings. The second part of the study was concerned with comparing the deposition of Inconel 718 by cold spraying and HVOF thermal spraying; the latter employed a JP5000 liquid fuel gun. A Tecnar DPV-2000 instrument was used to systematically investigate the effect of changes in spray parameters (spraying stand-off distance, oxygen/fuel ratio, total mass flow rate, combustion pressure), on particle velocity and temperature during HVOF spraying. It was found that generally the particle velocity was more strongly affected by the stand-off distance and combustion pressure of the spraying gun whereas the particle temperature was mostly influenced by the particle size and combustion pressure. The microstructures of coatings sprayed under 4 different well controlled conditions were investigated and changes in the morphology of splats and partially melted particles in the coating were related to the particle temperature and velocity at impact. The HVOF had high bond strength and low oxygen level of typically 0.45 wt% (corresponding to an oxide content of less than 1.6 wt.%). By contrast, in the cold sprayed coatings, the bonding was considerably low (-14 MPa), independently from the process conditions. It was found that the process parameter that mainly affected the properties of the cold sprayed deposits was the gas pressure. More specifically, the microhardness of the coatings increased with the pressure whereas the relative porosity decreased

Optical filter based on a coupled bilayer photonic crystal

We report on the fabrication of an ultra-compact optical filter based on photonic crystal free-standing membranes in bi-layer configuration. The basic heterostructure consists of two 376nm-thick GaAs-membranes sandwiched between air on a GaAs substrate. The air gap between the two membranes is 520nm thick. The normal-incidence reflectance measurements and the numerical simulation of reflection spectra show a high sensitivity to the holes diameter

Development of improved cold spray and HVOF deposited coatings

The overall aim of this research project was to expand the understanding of the deposition of titanium and the nickel-based superalloy Inconel 718 by spray deposition methods. The spray processes employed were cold spraying and high velocity oxy-fuel (HVOF) thermal spraying. The first part of the work was undertaken to expand the understanding of the deposition of titanium by cold spraying; the HVOF process is unsuitable for Ti because of the metal's high reactivity. The deposits were produced from commercially pure titanium using cold spray equipment designed in the University. Using helium gas, the effects of different powder particle size ranges, types of substrate, substrate preparation methods, and spray parameter conditions on deposit formation were investigated. Using a simple one-dimensional model of compressible gas flow and particle acceleration, particle velocity distributions were calculated to aid interpretation of experimental data. Results show that titanium can be successfully cold sprayed onto substrates of Ti6AI4V and mild steel, with the critical velocity for deposition of this powder type of approximately 690 m s-1. The level of porosity was generally in the range of 13-23% and the adhesive bond strength was dependent on surface preparation but independent of gas pressure with values ranging from 22 MPa to 10 MPa for ground and grit blasted substrates respectively. This compares with a value of around 80 MPa which is typical for well adhered HVOF sprayed coatings. The second part of the study was concerned with comparing the deposition of Inconel 718 by cold spraying and HVOF thermal spraying; the latter employed a JP5000 liquid fuel gun. A Tecnar DPV-2000 instrument was used to systematically investigate the effect of changes in spray parameters (spraying stand-off distance, oxygen/fuel ratio, total mass flow rate, combustion pressure), on particle velocity and temperature during HVOF spraying. It was found that generally the particle velocity was more strongly affected by the stand-off distance and combustion pressure of the spraying gun whereas the particle temperature was mostly influenced by the particle size and combustion pressure. The microstructures of coatings sprayed under 4 different well controlled conditions were investigated and changes in the morphology of splats and partially melted particles in the coating were related to the particle temperature and velocity at impact. The HVOF had high bond strength and low oxygen level of typically 0.45 wt% (corresponding to an oxide content of less than 1.6 wt.%). By contrast, in the cold sprayed coatings, the bonding was considerably low (-14 MPa), independently from the process conditions. It was found that the process parameter that mainly affected the properties of the cold sprayed deposits was the gas pressure. More specifically, the microhardness of the coatings increased with the pressure whereas the relative porosity decreased.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Microstructural evaluation of cold spray deposited WC with subsequent friction stir processing

Friction Stir Processing (FSP) has been shown to improve the strength, ductility and toughness of both aluminium and steel materials through grain refinement and the even distribution of precipitates within the substrate matrix. This article presents the application of FSP of Cold Spray deposited Tungsten Carbide – Cobalt (WC-Co) of two distinct types, on a series of aluminium substrates. Microstructural investigations of FSP processed samples exhibit interaction between the deposited WC-Co particles and aluminium alloy and show the homogeneous dispersion of deposited particles through the metal matrix. Results show that the dispersion of these particles varies with powder type, FSP parameters and substrate characteristics. A parallel study focusing on the hardness of the FSP generated Metal Matrix Composite (MMC) further demonstrate the potential of combining Cold Spray and FSP technologies to tailor surface properties for specific applications

Fabrication of high efficiency compact 90° bend waveguide by using a dielectric 2D-PC structure

In this paper we propose the design and the fabrication of 90° bend ridge waveguide (WG) assisted by a two-dimensional photonic crystal (2D-PC). 2D-PCs act as efficient mirrors along the boundaries of the bend ridge thus reducing the in-plane losses. The ridge waveguide consists of a 3 μm x 0.75 μm titanium dioxide core on a silica bottom cladding. The 2D-PC structure surrounding the bend waveguide is composed of a triangular array of circular dielectric pillars having a height of 0.75 μm. The titanium dioxide waveguiding core layer is covered with PMMA in order to create a quasi-symmetric structure. A photonic band gap centered around 1.3 μm is obtained by a PC radius r = 0.33a and lattice period a = 0.450 μm. The design of the whole structure is subsequently optimized by using a 3D Finite Difference Time Domain based computer code. The ridge waveguide assisted by a 2D-PC has been fabricated by using electron beam lithography and reactive ion etching. For the pattern transfer we have used about 50 nm thin layer Cr metal etch mask obtained by means of a lift-off technique based on the use of bi-layer resist (PMMA/MMA). The presence of the 2D-PC around the bend waveguide leads to a sharp increase of the transmission efficiency around 1.3 μm for curvature radius of 2.5 μm. The bend transmission results to be in the range between 0.76 and 0.85 when the thickness of the ridge WG and of the 2D-PC pillars is between 0.75 and 1.3 μm. This value is more than twice with respect to the bend waveguide without 2D-PC

The influence of load on dry and tribocorrosive sliding of AISI 4330 and 15-5PH against cast iron

Environmental conditions, testing variables, and material properties significantly influence the sliding wear behavior of all tribosystems. Such parameters affect the mechanism of wear occurring, govern how the wear scar will be categorized, and control whether transitions occur throughout the test. The present study investigated this through sliding wear tests of AISI 4330, 15-5PH, and heat-treated cast iron in the dry and NaCl regime. Loads of 2, 4, and 6 kg were applied to the cast iron pin in both regimes, with the volume loss results, wear scar morphology, and microstructural evolution analyzed. It was found that the AISI 4330 and 15-5PH discs produced higher volume losses in the dry regime than in the NaCl regime, indicating that the solution had a beneficial effect rather than a detrimental effect versus the dry tribosystem. Beneficial oxidative wear allowed for AISI 4330 to lose less than 15-5PH in the dry regime, with their respective cast iron pins following the same result. Microstructurally, the 15-5PH wear scar cross sections exhibited a mechanically mixed surface layer of refined material mixed with oxides above a layer of grains distorted in the direction of sliding. This highlighted the significant difference between the wear of 15-5PH and AISI 4330, because the latter did not exhibit any areas of interest

No red blood cell damage and no hemolysis in G6PD-deficient subjects after ingestion of low vicine/convicine <em>Vicia faba</em> seeds

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