This study aims to investigate the influence of alumina in-flight particle characteristics on coating properties and deposition efficiency. To this end, velocity and surface temperature measurements were carried out on the in-flight particles. Resulting coatings were characterized in terms of porosity, hardness, and related to particle properties. The final goal was to obtain an optimized coating with low porosity, high hardness, manufactured with a high powder flow rate and deposition efficiency.

Aerosol deposition (AD) is a novel method for producing dense nanocrystalline ceramic films at room temperature. Previous studies primarily used flat substrates with varying hardness and roughness. However, the development of micro-device applications is increasing the demand for deposition on structured/patterned surfaces. To investigate the impact of substrate patterns on coating microstructure and growth mechanisms in AD, alumina coatings were deposited on patterned Si substrates. Si wafers with patterns of micropillars were employed. The coatings were characterized using laser scanning microscopy, scanning electron microscopy, and x-ray diffraction. The microstructure and density of coatings in the valleys were influenced by the size of and the spacing between the patterns. The results revealed that coatings initially formed in the valleys before covering the entire pattern. Fragments of the initial powder particles became trapped between the patterns, adhering to the groove bottoms and pillar sides. Subsequent particle impacts and densification processes transformed these fragments, ultimately filling the gaps between the walls. With further deposition, a uniform coating surface was achieved.

This study assesses the quality of flame-sprayed alumina coatings produced from recently developed alumina cord using argon and compressed air as atomizing gases. Coatings of different thicknesses were deposited on aluminum substrates and then analyzed using optical microscopy, X-ray diffraction, and resistivity measurements. The coatings, particularly those sprayed with argon, had fine microstructure and higher surface and volume resistivity than flame-spray coatings made from alumina cord in the past. They were also found to have higher alpha phase content than plasma-sprayed coatings, regardless of the atomizing gas used. The effect of humidity and the possible formation of aluminum hydroxides are also addressed.

Molten calcium-magnesium-aluminum-silicate (CMAS) particles cause significant degradation of thermal barrier coatings (TBCs) in aero-engines. One way to protect TBCs against CMAS attack is through the application of a sacrificial topcoat. In this work, Al 2 O 3 coatings were deposited on top of EB-PVD 7wt% YSZ layers via suspension thermal spraying using an aqueous Al 2 O 3 suspension. Spray parameters were varied in order to obtain Al 2 O 3 layers with two different microstructures and porosity levels. The coating systems were evaluated by means of CMAS infiltration testing at 1250 °C. It was found that the porosity and morphology of Al 2 O 3 strongly influence CMAS infiltration and the formation of reaction products and that CMAS mitigation is a function of coating morphology and the speed at which Al 2 O 3 reacts with CMAS deposits.

This study compares the microstructure of Al 2 O 3 coatings produced by detonation gun spraying (DGS) and laser stereolithography (SLA). The SLA samples mostly consisted of alumina and voids, while the DGS-deposited alumina contained additional features such as splats, pores, cracks, and boundaries. DGS deposits were also denser with about 3% porosity compared to 8% porosity in the SLA samples. EDS analysis showed that both coatings contained only aluminum and oxygen, although additional carbon was detected in the SLA samples, indicating the presence of residual binder (resin based) material. XRD analysis revealed a mixture of α and γ-Al 2 O 3 phases in the DGS coatings, but no phase change in the SLA samples.

This study assesses the influence of particle size and spray parameters on the structural, mechanical, and electrical insulation properties of alumina coatings deposited by atmospheric plasma spraying. It has been found that the combination of a relatively fine feedstock powder and high velocity plasma spraying promotes the formation of denser coatings with high dielectric strength. Correlations between dielectric strength and deposition efficiency, coating hardness, crystal structure, and surface roughness are also assessed.

A novel composite MCrAlY coating with a fine dispersion of sub-micron Al 2 O 3 particles were sprayed by high velocity oxygen fuel (HVOF) thermal spraying for high temperature oxidation protections. The presence of Al 2 O 3 could reinforce the metallic MCrAlY coatings, in a similar way to ODS alloys (oxide dispersion strengthening). It could also enhance the oxidation resistance owing to the dispersion of Al 2 O 3 on the surface as nucleation sites, which promotes the early formation of a coherent α-Al 2 O 3 scale. This is essential for an effective protection against oxidation attack, especially for these applications at relatively lower temperatures (<900 °C) with slower growing Al 2 O 3 . In this study, a suspension route was employed to achieve a uniform dispersion of 0-10 wt.% Al 2 O 3 particles with commercial MCrAlY powders. A liquid fueled HVOF spray gun (MetJet IV) was used to deposit the composite MCrAlY-Al 2 O 3 coatings onto 304 stainless steels substrates. The composite coatings were examined thoroughly by field emission gun scanning electron microscopy (FEGSEM) with energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analysis. Isothermal oxidation at 900 °C was carried out to study the effect of Al 2 O 3 on the growth of oxide on the coatings surface. The composite coatings exhibited superior oxidation behavior against the conventional metallic coatings with the formation of nearly exclusively Al 2 O 3 on the coatings surface and inhibited NiO growth.

In the present work, three different APS alumina coatings were fabricated using three fused and crushed alumina powders of different particle size fine, medium and coarse. The influence of the particle size on thermal properties and micro-structural features of the produced coating were investigated by thermal insulation test and detailed image analysis technique, respectively. The analyzed micro-structural features include the total porosity, pore size (fine, medium, and large) and cracks. All types of cracks were considered in calculations as voids and were evaluated according to their sizes as pores. All spray parameters except the particle size were fixed throughout the spraying process. The results revealed that the fine starting powder has produced the densest coating with the lowest total porosity and that the total porosity increases with an increasing particle size. This was expected as powders of smaller particle size will reach a higher in-flight temperature and velocity than powders of bigger particle sizes as long as the same spray parameters are applied. However, a detailed image analysis investigation on the three produced coatings showed that the fraction of fine pores and cracks versus the total porosity is substantially higher in coatings produced by using fine starting powders than those produced using medium and coarse powders. In this work, a connection between the thermal insulation and the porosity fraction, which includes fine pores and cracks, was revealed.

Osteoarthritis in the hip or knee is one of the most common diseases in industrialized countries. The implantation of an endoprosthesis as a joint replacement represents the most effective way to treat serious pathological changes in these joints. The lifetime of an endoprosthesis can be shortened by aseptic inflammation and osteolysis. The main cause for the aseptic inflammations, osteolysis, and thus, the failure of the endoprostheses are abrasion particles of the acetabular cup inlays that are caused by the tribological load of the prosthesis. This research project aims at developing coatings with enhanced tribological behaviour for endoprostheses by an active hydrodynamic lubrication of the joint with synovial fluid. In addition, biocompatibility, as well as the increase of the strength under static and cyclic loading need be realized. In the current approach, a deterministic fluid flow tube structure is formed in a thermally sprayed alumina layer by the introduction of a leachable placeholder. This tube structure allows the transverse transport of the synovial fluid through the alumina layer. Furthermore the synovial fluid can be transported into the lubricating gap of the sliding surfaces by leaving the alumina layer through the porous surface. First results will be presented and the ramifications in correlation to applications will be discussed.

The study of both the interface strength and residual stresses within a plasma sprayed ceramic coating is of great interest which main purpose is a better understanding of the mechanical properties of metal/ceramic systems. In this work, experiments involving a LASAT facility (LASAT: Laser Shock adhesion test) were implemented in order to analyse the adhesion and the damaging behaviour (debonding and buckling) of alumina coatings onto Co-based alloy. Similar alumina coatings were deposited using same plasma parameters with various surface preparations: smooth or severe grit blasting, with and without pre-oxidation. The non-destructive analyses (Optical and IR imaging) of the buckled region after LASAT have allowed to compare and discuss the interface strength of the studied coated samples. Further discussion was carried out by analysing the blister, resulting from the release of residual stresses within the coating after LASAT. It was thus evidenced that the residual stress state is a key parameter on resulting adhesive properties. This explorating work suggests using the LASAT method to analyse the adhesion and residual stresses within thermal sprayings.

The lower wear and poor impact resistance of the amorphous coatings has been a great problem in the past few years for their use in industrial applications. Several research methods have been reported recently to overcome this issue. The present paper addresses the main work done recently on iron based amorphous composite coatings by the addition of 0-20% Alumina particles. These particles were homogeneously distributed in the amorphous matrix of the coatings which improved wear and impact resistance as compared to the monolithic coatings without any decrease in corrosion resistance. The hard alumina particles enhanced wear resistance to several times not only in air but also in salt water solution with a decrease in friction coefficient. The combined effect of wear and corrosion were also observed to become better by the alumina addition. Furthermore, the impact resistance was also improved three times by the addition of alumina particles. The hard second phase particles present in the amorphous coating matrix disperses the residual stresses generated during the impact loading. The brittle alumina particles absorb the impact energy by breaking itself which stop the initiation of cracks and also play a vital role in the crack arresting and blocking of the crack propagation.

Thermally sprayed Al 2 O 3 -TiO 2 coatings were in the area of interest over the last decade because they showed improved wear properties over conventional coatings. In this study, flexicord flame spray gun was used to deposit Al 2 O 3 -TiO 2 coatings at different spray parameters. The microstructural morphology variation and phase transformation of coatings were investigated. In addition, as one of the most important properties for ceramic coatings, hardness, solid particle abrasive wear resistance of coatings were measured before and after heat treated condition. Test results show that the higher mechanical properties and wear resistance by the heat treatment at elevated temperatures.

An alumina-silicon dioxide composite coating was fabricated by cold spraying on AZ31 magnesium alloy. The microstructure, mechanical properties (microhardness, bonding strength and tribological behaviour) and anticorrosion property of the coating as a function of the ceramic volume were investigated. The results show that the composite coating presents higher bonding strength and microhardness. Addition of silicon dioxide significantly enhances the anti-wear and anti-corrosion performances of AZ31 magnesium alloy.

Traditional low-cost bulk materials are unable to fulfil the increasing requirements of actual technology and functional coatings – e.g. APS alumina coatings on aluminium substrates for tribological properties – are a suitable alternative. The development of new thermal sprayings is usually based in experimental procedures which involve long development times and high costs. Nowadays, numerical simulation allows the researcher a better understanding of thermal spray processes as well as reducing the time and cost for the optimization of processes, but it requires a deep insight into the physics of the phenomenon. The submodelling approach allows the researchers to work with a local model, a thin layer – just a few microns – on the metal substrate surface, and more realistic boundary conditions, which requires a little specialised knowledge. The current study involves the workflow to manage the modelling at piece scale, and the transfer and interpolation of boundary conditions in each scale. The coating is divided into several layers, which represent the successive splats deposited during the process, and the heat flow from the torch is modelled by radiation and convection. A code is implemented in order to generate the routine needed by the FEM software, in which the results are processed and interpolated for the subsequent submodel. Furthermore, the material plasticity is considered and several tests are performed in order to check the simulation results.

This study assesses the influence of alumina particle additions on the impact and wear behavior of iron based amorphous coatings. Test results show that the presence of Al 2 O 3 particles improved the impact and wear resistance of the coatings by a factor of three. Deformation and fracture mechanisms under impact loading were also investigated. It was revealed via SEM analysis and finite element simulations that hard second phase particles in the amorphous coating matrix disperse residual stresses generated during impact loading and that brittle particles absorb impact energy by fracturing, which plays a vital role in crack prevention and arresting. Abstract only; no full-text paper available.

This work investigates the sliding wear resistance of alumina coatings deposited on stainless steel substrates by HVOF and air plasma spraying, using fine (1-5 μm) and conventional (10-45 μm) powders. Sliding wear tests were carried out using a sintered WC-Co ball as the counter-body and the wear tracks were examined to obtain a better understanding of wear mechanisms. HVOF coatings showed an order of magnitude improvement in wear resistance compared to their APS counterparts. The disparity in wear performance is correlated to differences in phase composition, porosity, hardness, and fracture toughness as revealed by SEM and XRD analysis and nanoindentation testing. The development of tribofilms and their role in wear behavior is also discussed.

This study investigates the feasibility of spraying aluminum nitride-alumina-yttria mixtures in a nitrogen plasma ambient. Coatings consisting of h-AlN, c-AlN, Al 5 O 6 N, and γ-Al 2 O 3 with small amounts of α-Al 2 O 3 and aluminum-yttrium oxide phases were produced. Although using the Y 2 O 3 additives significantly affected the process and microstructure, it did not achieve the high thermal conductivity desired in as-sprayed coatings. However, a high thermal conductivity (>90 W/m·K) AlN coating was fabricated by increasing the AlN content and enhancing sintering during heat treatment.

In this study, alumina coatings are deposited by atmospheric plasma spraying and sealed using an aluminum phosphate solution containing a small fraction of alumina nanoparticles. Potentiodynamic polarization, electrochemical impedance spectroscopy, and salt spray tests are used to evaluate the corrosion behavior of both the as-sprayed and sealed coating samples. Besides improving corrosion resistance, the sealing treatments are also shown to increase the electrical resistance of the coatings, making them better electrical insulators.

In this work, a mechanically clad NiCr powder feedstock was deposited on alumina substrates by atmospheric plasma arc spraying. The resultant splats were analyzed for features such as interfacial bonding, splat classification and, critically, Cr distribution. Using a slice-and-view sectioning technique in a dual-beam FIB-SEM system, a representative splat exhibiting discrete Ni and Cr regions was physically deconstructed then reconstructed with visualization software to analyze individual layers with the splat. Although the powder feedstock contained Ni particles clad with clusters of Cr, the splats solidified into distinct layers of Ni and Cr with no signs of interaction between them. A model formulated based on this observation shows that the distribution of Cr cladding on the Ni particulates influences the amount and location of Cr around the solidified Ni splats.

In this study, a water-based corundum suspension was used to deposit 60 μm alumina coatings onto carbon steel substrates by HVOF spraying. The aim was to develop thin coatings with superior wear properties. Hydrogen was used as a fuel gas and process parameters were varied to determine their effect on microstructure and properties. Coating microstructure was examined by SEM to assess particle melting and morphology and XRD was used to study the phase transformation of the feedstock suspension. At higher combustion flame energy, the coating transformed primarily to gamma alumina, while at lower energy, it was found to be a mixture of alpha and gamma alumina. Nanoindentation tests were used to measure the hardness and elastic modulus of individual phases. Ball-on-plate wear tests helped reveal the relationship between wear performance and the alpha-gamma ratios in the coatings.