Velocity Oxy-fuel Thermal Spraying Research Articles

Exhaust gas recirculation for enhancing performance of HVOF sprayed zirconium dioxide & aluminium oxide coated internal combustion engine

In the present times, a lot of research are being conducted in automobile sector for improving the performance of Internal Combustion Engines. In this paper, an attempt has been made to enhance the performance aspects and reduce emissions of High Velocity Oxy Fuel thermal spray coated piston incorporated Internal Combustion engine, by using Exhaust Gas Recirculation. By using Zirconium Dioxide and Aluminium Oxide, the piston was coated and used in the combustion evaluation experiments. The important internal combustion engine process parameters were optimized by using a central composite design model, which was evaluated using Analysis of variance. The combustion process parameters were optimized so as to enhance the Brake Thermal Efficiency to maximum extent and reduce emission of Oxides of Nitrogen (NOx) to minimum. At optimized combustion process parameters maximum Brake Thermal Efficiency of 29.1% and minimum NOx at 688 ppm, 21.8 ppm of unburnt hydro carbons and 11.08% smoke opacity was observed, which was validated to a very high level of predictability.

Modelling Knudsen number effects in suspension high velocity oxy fuel thermal spray

Suspension high velocity oxy fuel thermal spray is a system characterized by supersonic velocities and length scales of particles of the order of nm – µm. As the effects of rarefication become significant the assumptions within the continuum models begin to collapse, the effects of rarefication can be evaluated through the flow Knudsen number. Modifications to the numerical modelling must be made to incorporate the effects of rarefaction. This study looks to include the effects of rarefication into the computational fluid dynamics (CFD) models for the suspension high velocity oxy-fuel (SHVOF) thermal spray process. A model for the heat transfer coefficient that take into account the Knudsen and Mach number effects is employed. Finally, the Ranz-Marshall correlation for the Nusselt number is compared to the Kavanau correlation and a compressible Nusselt number correlation. The model is validated through comparisons of particle temperatures which are obtained from two colour pyrometry measurements using a commercially available Accuraspray 4.0 diagnostic system. This study shows that there is a significant improvement in the prediction of inflight particle temperatures when accounting for the effects of compressibility and the effects of rarefication on the Nusselt number.

Cavitation erosion behaviour and mechanism of HVOF-sprayed NiCrBSi–(Cr3C2–NiCr) composite coatings

ABSTRACTNiCrBSi–(Cr3C2–NiCr) coatings were prepared by high velocity oxy-fuel (HVOF) thermal spraying technique, and their cavitation erosion behaviour was investigated by a magnetostrictive cavitation method. It was observed that in the initial 8 h, cavitation occurred at higher rate, and cumulative mass loss of the two coatings was apparently higher than that of the substrate. Conversely, beyond 12 h, the former was lower than the later, indicating that the coatings depressed the cavitation erosion effectively. This study verified that the cavitation is due to mechanical shock, turbulence of liquid with bubbles and residual stresses, and therefore the formation of cracks and the delamination of coating are induced. Furthermore, an increase in the number of pores results in an increase in damage to the coatings, taking this into account distributions of pores can be termed as a key factor in determining the resistance of cavitation damage.

The High Temperature Creep Properties of a Thermally Sprayed CoNiCrAlY Coating via Small Punch Creep Testing

Thermal barrier coatings (TBC’s) protect superalloy components from excessively high temperatures in gas turbines. TBC’s comprise of a ceramic top coat, a metallic bond coat and a thermally grown oxide (TGO). The creep behaviour of the MCrAlY bond coat, which is sensitive to the composition and the method of deposition, has a significant effect on the life of the TBC. High velocity oxy-fuel (HVOF) thermal spraying is a popular deposition method for MCrAlY bond coats however the creep properties of HVOF MCrAlY coatings are not well documented. The creep behaviour of a HVOF thermally sprayed CoNiCrAlY coating has been determined by small punch creep (SPC) testing. Tests were conducted between an equivalent uniaxial stress range of 37-80 MPa at 750 °C on two different SPC rigs and between 30-49 MPa at 850 °C on a single SPC rig. The measured steady-state creep deformation rates at 750 °C were consistent across the two rigs. A comparison with previous work demonstrated that the creep behaviour of HVOF CoNiCrAlY coatings is not sensitive to the manufacturing variability associated with HVOF thermal spraying. The CoNiCrAlY coating exhibited typical SPC deformation at 750 °C. At 850 °C the CoNiCrAlY coating showed significantly different creep behaviour which could be attributed to the onset of superplasticity.

Characterisation and corrosion performance of HVOF sprayed Ni–10 wt-%Ti coatings

Mechanically alloyed Ni–10 wt-%Ti powders were deposited on 1045 steel substrate using high velocity oxy-fuel (HVOF) thermal spraying process. The microstructure and phase composition of coatings were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The results showed that the phase composition of feedstock powders was preserved during spraying and only supersaturated Ni(Ti) solid solution was present in the coatings. The hardness measurements showed that the hardness of coatings was about 510 HV (50 g). Tafel polarisation tests performed after 1 h immersion revealed that the corrosion behaviour of HVOF Ni–Ti coatings was not affected by substrate. However, delamination of air plasma sprayed (APS) Ni–10 wt-%Ti coatings and significant corrosion of substrate was observed in similar conditions. For HVOF Ni–Ti coating and after 24 h of immersion time, the corrosion of underlying substrate was observed after polarisation measurements but no delamination of coating occurred.

Micro-Scale Abrasion of WC-Based Coatings with Different Abrasive Type

Various research programmes have been conducted examining cermet coatings regarding wear, corrosion and the combination of both (erosion-corrosion and abrasion-corrosion). Several methods have been used to deposit cermet coatings, the most common being thermal spraying or hard facing (weld overlaying). In the current work, the micro-scale abrasion of coatings deposited using both high velocity oxy-fuel (HVOF) thermal spraying and weld overlay techniques are compared. The weld-overlayed WC-nickel alloy systems have the carbide particles are typically two orders of magnitude larger than in the sprayed coatings. Micro-scale abrasion tests were performed using silicon carbide, alumina and silica particle slurries with abrasive particle sizes in the range of 2-10 μm in all cases. Wear rates were determined and the wear scars were examined using SEM to elucidate the dominant wear mechanisms. The wear rate is generally observed to decrease with decreasing abrasive hardness from silicon carbide, followed by alumina and silica.

3E5 OpenFOAMによる超音速流れを用いる高速フレーム溶射の数値シミュレーション(<OS4・2>熱流体フロンティア(圧縮性流体))

3E5 OpenFOAMによる超音速流れを用いる高速フレーム溶射の数値シミュレーション(<OS4・2>熱流体フロンティア(圧縮性流体))

G051042 超音速流れを用いる高速フレーム溶射のOpenFOA]VIによる乱流数値シミュレーション([G051-04]流体工学部門,一般セッション : 乱流・超音速・制御)

G051042 超音速流れを用いる高速フレーム溶射のOpenFOA]VIによる乱流数値シミュレーション([G051-04]流体工学部門,一般セッション : 乱流・超音速・制御)

THE EFFECT OF FUEL TO OXYGEN RATIOS ON THE PROPERTIES OF HIGH VELOCITY OXY-FUEL TiO2 NANO-PHOTOCATALYST COATINGS

A liquid fuel high velocity oxy-fuel (HVOF) thermal spray process has been used to deposit TiO 2 photocatalytic coatings utilizing a commercially available anatase/rutile nano-powder as the feedstock. The coatings were characterized in terms of the phases present, its crystallite size and coating morphology by means of X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy, respectively. The results indicate that the sprayed TiO 2 coatings were composed of both TiO 2 phases, namely anatase and rutile with different phase content and crystallite size. A high anatase content of 80% by volume was achieved at 0.00015 fuel to oxygen ratio with nanostructure coating by grain size smaller than feedstock powder. It is found that fuel to oxygen ratio strongly influenced on temperature and velocity of particles in stream jet consequently on phase transformation of anatase to rutile and their crystallite size and by optimizing the ratio which can promote structural transformation and grain coarsening in coating.

Design and optimisation of a multi-powder feed system for the HVOF deposition process

Currently no specific method exists for the deposition of High Velocity Oxy-Fuel (HVOF) thermal spray functionally graded coatings. This paper investigates the design and optimisation of a multi-powder HVOF thermal spray device in order to deposit aluminium/tool-steel functionally graded coatings. A multi-powder feed device concept was developed. The concept was based on a stand alone two powder chamber device which integrates with common hopper systems to allow the mixing of two powders during thermal spray deposition. This concept was verified by simulation the design of this device for multi-powder flow using Finite Element Analysis (FEA) to arrive at the optimum dual feed design. The FEA model predicted the mixing and flow of two powders of various ratios' of mass flow rate and velocity based on an optimum designed shape and pressure ratios' of nitrogen gas in the chamber to pick-up shaft of 2.25:1. This yielded the best results in terms of carrying the powders from the mixing zone into the nitrogen gas flow path, inside the pick-up shaft and on towards the HVOF gun. Post finite element analysis the device was manufactured for the utilisation within the HVOF process. Optimisation tests of the device included; powder flow bench tests and HVOF thermal spraying of graded deposits. The results revealed a calibration graph for the two powders in question and the compositional variation across the deposit during functionally graded deposition. The composition of the graded deposits were close to that anticipated hence this showed the suitability of the newly designed multi-powder deposition system in mixing two powders for the purpose of producing HVOF graded coatings.

Microstructure and thermal cycling behavior of thermal barrier coating on near-α titanium alloy

Two kinds of thermal barrier coatings (TBCs), consisting of NiCoCrAlY bond coats (BCs) deposited by electron beam-physical vapor deposition (EB-PVD) and high velocity oxy-fuel (HVOF) thermal spraying, respectively, and top 8 wt%Y2O3–ZrO2 (8YSZ) ceramic layers deposited by EB-PVD were prepared on near-α titanium alloys. The field emission scanning electronic microscopy and microhardness indentation are used to study the microstructure and microhardness. Different failure features including cracking patterns and the delamination degree of these two TBCs are discussed according to the thermal cycling tests in the atmosphere. It is found that the morphology of the two BCs deposited by different methods (EB-PVD and HVOF) determines the microstructure and microhardness of their corresponding top 8YSZ layers. The BC prepared by EB-PVD is dense and homogeneous, which leads to a dense and hard 8YSZ with clustered slim columnar grains. The BC prepared by HVOF, however, is porous and inhomogeneous in microstructure and, as a result, the top ceramic layer is loose with low microhardness and clustered coarse columnar grains.

Synthesis and processing of nanostructured WC-Co materials

In this study a novel approach, termed the integrated mechanical and thermal activation (IMTA) process, was used to synthesize nanostructured WC-Co powder. As a result of the integration of mechanical and thermal activation, nanostructured WC-Co powder was synthesized below 1000°C, starting from WO3, CoO and graphite powder mixtures. Furthermore, consolidation of the nanostructured WC-Co powder via high velocity oxy-fuel (HVOF) thermal spraying and solid state sintering was investigated. The results demonstrated the feasibility of converting the nanostructured WC-Co powder to coatings and bulk components, the properties of which are either comparable to or better than that of the conventional coarse-grained counterparts.