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Student Number : 0109917P - PhD thesis - School of Process and Materials Engineering - Faculty of Engineering and the Built Environment / The aim of this project was to characterise new WC-10VC-Co powders, and to deposit WC-10VC-Co thermal spray coatings from these powders for characterisation and testing in adhesion, wear and corrosion tests. Throughout the project, the new powders and coatings were compared to commercial WC-Co powders of the same binder content and commercial WC-Co thermal spray coatings. All the powders i.e WC-10VC-Co and WC-Co powders, were produced by agglomeration (by spray drying) and sintering and characaterised by determining the sizes and size distributions of the powders' particles, the morphology, the flowability and the phase composition and grain size and size distribution of carbide grains. The vanadium carbide in the WC-10VC-Co powders occurred in the solution as the double carbide (V,W)C and the carbides present in the WC-10VC-Co powders were WC and (V,W)C. None of the starting VC was left in the powders. Coatings were deposited using high pressure high velocity oxy-fuel (HP/HVOF) spraying systems, and characterized by determining the microstructures, the phase compositions and the carbide grain sizes, as had been done for the powders. Three types of tests were done on the coatings: adhesion tests, (according to standard SNECMA 14 -008); dry abrasion, wet abrasion and slurry erosion tests; and corrosion tests, in synthetic mione water. Thermal spraying lead to some WC decarburization to W2C and eta phase, and to the formation of amorphous binder. The W2C grains from the WC decarburization formed in the amorphous binder matrix of coatings. All the coatinge were porous, but the new WC-10VC-Co coatings were more porous than the commercial Wc-Co coatings because the spray parameters had only been optimized for the WC-Co coatings. The carbide grains decreased in size by as much as 50% during decomposition. Evidence suggested that the WC grains in the coatings were subjected to different residual stresses that in the powders, probably due to the formation of the amorphous binder. Vanadium carbide in the Wc-10VC-Co coatings occurred as (V,W)C, just as in the powders, with as distribution that was reasonably homogeneous. The apparent hardness of the new Wc-10VC-Co coatings was slightly lower than that of WC-Co coatings of the same cobalt content, due to their higher porosity. The adhesion of the new Wc-10VC-Co coatings was as good as that of the Wc-Co coatings. The dry and wet abrasion resistance of the new Wc-10VC-Co coatings was better that for the Wc-Co coatings of equal Co wt%, on account of the Wc-10VC-Co coatings having a lower binder volume fraction, finer carbide grains, and (V,W)C grains. The (V,W)C grains are harder than WC grains and apparently slowed down the overall abrasion rate. In slurry erosion, the best performance of the Wc-10VC-Co coatings was as good as that of the commercial WC-Co coatings at equal cobalt mass content, due to the higher porosity of the Wc-10VC-Co coatings, apparent faster erosion of the harder but brittle (V,W)C grains, and, from what evidence appreared to suggest, generally slightly poorer erosion resistance of the fine WC grains under the test conditions used. Polishing the slurry erosion test specimens reduced mass losses in slurry erosion by factor of up to 10 compared to the unpolished specimens, and led to better erosion resistance of the WC-10VC-Co coating compared to the WC-12Co coating. The results of the tests done to investigate the corrosion properties of the coatings were conclusive. This is because the effects of cleaning procedures on mass loss after immersion corrosion were not explored, and it appeared, for some coatings, that the corrosion mechanisms in immersion corrosion could not be reproduced in electrochemical testing.

This Thesis presents two different deposition techniques for the synthesis of Ti2AlC coatings. First, I have fabricated Ti2AlC coatings by high velocity oxy-fuel (HVOF) spraying. Analysis with scanning electron microscopy (SEM) show dense coatings with thicknesses of ~150 µm when spraying with a MAXTHAL 211TM Ti2AlC powder of size ~38 µm in an H2/O2 gas flow. The films showed good adhesion to stainless steel substrates as determined by bending tests and the hardness was 3-5 GPa. X-ray diffraction (XRD) detected minority phases of Ti3AlC2, TiC, and AlxTiy alloys. The use of a larger powder size of 56 µm resulted in an increased amount of cracks and delaminations in the coatings. This was explained by less melted material, which is needed as a binding material. Second, magnetron sputtering of thin films was performed with a MAXTHAL 211TM Ti2AlC compound target. Depositions were made at substrate temperatures between ambient and 1000 °C. Elastic recoil detection analysis (ERDA) shows that the films exhibit a C composition between 42 and 52 at% which differs from the nominal composition of 25 at% for the Ti2AlC-target. The Al content, in turn, depends on the substrate temperature as Al is likely to start to evaporate around 700 °C. Co-sputtering with Ti target at a temperature of 700 °C, however, yielded Ti2AlC films with only minority contents of TiC. Thus, the addition of Ti is suggested to have two beneficial roles of balancing out excess of C and to retain Al by providing for more stoichiometric Ti2AlC synthesis conditions. Transmission electron microscopy and X-ray pole figures show that the Ti2AlC grains grow in two preferred orientations; epitaxial Ti2AlC (0001) // Al2O3 (0001) and with 37° tilted basal planes of Ti2AlC (101̅7) // Al2O3 (0001). / Report code: LIU-TEK-LIC-2008:15.

In thermal spraying, semi-molten (or partially-melted) particles are likely to form when the sprayed particles are insufficiently heated, or when a composite material is deposited. The present 2D model serves to begin to assess the spreading behavior of a semi-molten particle when impacting a solid substrate. An Immersed-Boundary (IB) scheme was implemented in an axisymmetric fluid model to simulate fluid flow in the presence of a solid core. The IB method calculates a forcing term, which is added to the momentum equation, to enforce the no-slip boundary condition at the core surface. Results are presented for the impact of a semi-molten tin droplet of radius R for a wide range of solid core radii r, varying the drop size ratio r/R, and the impact velocity Uo.

In thermal spraying, semi-molten (or partially-melted) particles are likely to form when the sprayed particles are insufficiently heated, or when a composite material is deposited. The present 2D model serves to begin to assess the spreading behavior of a semi-molten particle when impacting a solid substrate. An Immersed-Boundary (IB) scheme was implemented in an axisymmetric fluid model to simulate fluid flow in the presence of a solid core. The IB method calculates a forcing term, which is added to the momentum equation, to enforce the no-slip boundary condition at the core surface. Results are presented for the impact of a semi-molten tin droplet of radius R for a wide range of solid core radii r, varying the drop size ratio r/R, and the impact velocity Uo.

<p>This Thesis presents two different deposition techniques for the synthesis of Ti₂AlC coatings. First, I have fabricated Ti₂AlC coatings by high velocity oxy-fuel (HVOF) spraying. Analysis with scanning electron microscopy (SEM) show dense coatings with thicknesses of ~150 µm when spraying with a MAXTHAL 211^TMTi₂AlC powder of size ~38 µm in an H₂/O₂ gas flow. The films showed good adhesion to stainless steel substrates as determined by bending tests and the hardness was 3-5 GPa. X-ray diffraction (XRD) detected minority phases of Ti₃AlC₂, TiC, and Al_xTi_y alloys. The use of a larger powder size of 56 µm resulted in an increased amount of cracks and delaminations in the coatings. This was explained by less melted material, which is needed as a binding material. Second, magnetron sputtering of thin films was performed with a MAXTHAL 211^TM Ti₂AlC compound target. Depositions were made at substrate temperatures between ambient and 1000 °C. Elastic recoil detection analysis (ERDA) shows that the films exhibit a C composition between 42 and 52 at% which differs from the nominal composition of 25 at% for the Ti₂AlC-target. The Al content, in turn, depends on the substrate temperature as Al is likely to start to evaporate around 700 °C. Co-sputtering with Ti target at a temperature of 700 °C, however, yielded Ti₂AlC films with only minority contents of TiC. Thus, the addition of Ti is suggested to have two beneficial roles of balancing out excess of C and to retain Al by providing for more stoichiometric Ti₂AlC synthesis conditions. Transmission electron microscopy and X-ray pole figures show that the Ti₂AlC grains grow in two preferred orientations; epitaxial Ti2AlC (0001) // Al2O3 (0001) and with 37° tilted basal planes of Ti₂AlC (101̅7) // Al₂O₃ (0001).</p> / Report code: LIU-TEK-LIC-2008:15.

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Thermal barrier coatings (TBC) with MCrAlY (M=Co and/or Ni) bond coats have been widely used in hot sections of gas turbines to protect underlying superalloys from high temperatures, oxidation, and hot corrosion. Deposition of MCrAlY bond coats using atmospheric plasma spray (APS), as oppose to conventionally employed vacuum/low-pressure plasma spray and high velocity oxy-fuel deposition, allows greater flexibility in ability to coat economically and rapidly for parts with complex geometry including internal surfaces. There were three objectives of this study. First, relationships between APS spray parameters and coating microstructure was examined to determine optimum spray parameters to deposit APS CoNiCrAlY bond coats. Second, free-standing APS CoNiCrAlY coatings were isothermally oxidized at 1124ºC for various periods to examine the evolving microstructure of internal oxidation. Third, as a function of time of isothermal oxidation (i.e., internal oxidation), thermal conductivity and coefficient of thermal expansion were measured for free-standing APS CoNiCrAlY bond coats. Thirteen CoNiCrAlY coatings were deposited on steel substrates by APS using the F4-MB plasma torch. APS CoNiCrAlY bond coats were produced by incremental variation in the flow rate of primary (argon) gas from 85 to 165 SCFH and the flow rate of secondary (hydrogen) gas from 9 to 29 SCFH. Optimum coating microstructure was produced by simultaneously increasing the flow rate of both primary and secondary gas, so that the particle temperature is high enough for sufficient melting and the particle velocity is rapid enough for minimum in-flight oxidation. Optimum spray parameters found in this study were employed to deposit free-standing APS CoNiCrAlY coatings that were isothermally oxidized at 1124ºC for 1, 6, 50,100, and 300 hours. Extent of internal oxidation was examined by scanning electron microscopy and image analysis. Internal oxidation occurred by a thickening of oxide scales segregated at the splat boundaries oriented parallel to the coating surfaces. Thermal conductivity and coefficient of thermal expansion (CTE) of the free-standing APS CoNiCrAlY coatings were measured as a function of internal oxidation (i.e., time of oxidation or extent of internal oxidation). Thermal conductivity of free-standing APS CoNiCrAlY was found to decrease with increasing internal oxidation from 28 to 25 W/m-K. This decrease is due to an increase in the amount of internal oxides with lower thermal conductivity (e.g., Al2O3). CTE of free-standing APS CoNiCrAlY, measured in temperature range of 100°~500°C, was also found to decrease with increasing internal oxidation. Internal oxides have lower CTE than metallic CoNiCrAlY coatings. These evolving properties of APS CoNiCrAlY should be beneficial to the overall performance of TBCs in gas turbine applications.