B. Arsenault

Comparison of slurry and dry erosion behaviour of some HVOF thermal sprayed coatings

The performance of 10 high velocity oxy-fuel (HVOF) sprayed coatings has been evaluated under both dry particle and slurry erosion conditions at 90° and 20° impingement angles. The coatings were six WC cermets with Co- or Ni-based matrices, a Cr3C2–NiCr composite and three metallic alloys. Slurry jet erosion experiments were carried out using a 9 wt.% alumina/water slurry with 35 and 200 μm angular alumina particles, at an impingement velocity of 15 m/s and flow rate of 18 l/min in a re-circulating loop apparatus. Dry erosion was effected in a once-through mode with 50 μm diameter alumina particles at a velocity of 84 m/s and feed rate of 2 g/min. Dry erosion rates three orders of magnitude greater than slurry erosion rates were due mainly to the differences in actual particle-target impact velocities. Coating composition, microstructural integrity and hardness were the major determinants of relative erosion resistance in all tests, with matrix corrosion resistance influencing the surface damage resistance of cermets in longer duration aqueous slurry tests.

Evaluation of four high velocity thermal spray guns using WC-10% Co-4% Cr cermets

Four high velocity thermal spray guns were evaluated in the production of 10% Co-4% Cr tungsten carbide (WC) cermets. Three high velocity oxygen fuel guns (JP-5000, JP-5000ST, and Diamond Jet [DJ]-2700) and one plasma gun (Axial III) were used to spray the same angular, agglomerated, and crushed WC-10Co-4Cr powder. The DPV-2000 was used to monitor the in-flight velocity and temperature of the WC cermet-sprayed particles. From those values, spray conditions were selected to produce coatings that were evaluated in terms of porosity, hardness, and deposition efficiency. Results show that the plasma Axial III provides the highest particle temperature, between 2000 °C and 2600 °C, depending on the spray conditions. The JP-5000 imparts the highest velocity to the particles, between 550 and 700 m/s, depending on the spray conditions. The ST version of the JP-5000 provides the same velocity as the standard version but with lower particle temperature. The DJ-2700 sprays particles with temperature and velocity between those of the JP-5000 and the Axial III. Minimum porosity values of 2.1%, 3.7%, and 5.3%, respectively, were obtained for the JP-5000, the DJ-2700, and the Axial III guns. The porosity and carbide degradation are found to depend mostly on the particle velocity and temperature, respectively. The values for the Vickers microhardness number (200g) ranged from 950 to 1250. Measurements of the deposition efficiency indicated a variation between 10% and 80%, depending on the spray conditions and the gun used.

Sprayed titanium coatings for the cathodic protection of reinforced concrete

Thermally sprayed titanium coatings were investigated in the laboratory as anodes for the cathodic protection of reinforced concrete. Three proprietary catalyst systems were used to activate the titanium anode coatings. Some experiments were conducted that applied the catalyst as a precoat on the metallizing wires; in other experiments, the catalyst solution was applied onto concrete blocks before or after arc spraying with titanium. The coated reinforced concrete blocks were powered at a constant current density and in a 95% relative humidity for more than 95 days. The driving voltages measured across the samples demonstrated that precoating the catalyst on the titanium wires had little effect on the driving voltage over the recorded lifetime. In other experiments, where the catalyst was directly applied to the surface of the reinforced concrete blocks, only the cobalt oxide catalyst significantly reduced the driving voltage requirements. The cobalt oxide reduced the driving voltages regardless of whether it was applied on the concrete blocks before or after arc spraying with titanium.

Texturing of thick films on a metallic substrate

Low-temperature melt-texturing of a laminated composite tape was carried out. No reaction or contamination of the superconducting phase was detected and the interface was very clean. The application of the authors procedure for obtaining well-connected textured material is discussed.

Laser processing of plasma-sprayed R-Ba-Cu-O coatings

Thick Y-Ba-Cu-O and Er-Ba-Cu-O coatings were deposited by plasma spraying onto nickel substrates. These plasma-sprayed coatings were laser melted to modify their microstructures. The effects of primary processing conditions, such as linear energy and number of passes, on microstructural modifications were assessed. The microstructure of the as-sprayed coatings was largely transformed to produce a fine Y2O3 dispersion in a Ba-Cu-O matrix. A very low level of coating/substrate interactions can be maintained by appropriate laser processing conditions.

Aqueous corrosion behaviour of plasma sprayed coatings

ABSTRACT The aqueous corrosion degradation of ceramic and metallic coatings plasma-sprayed onto steel substrate were assessed by measuring their bond strengths as a function of salt spray exposure time. The bond strength-corrosion time dependence of Cr 2 O 3 , AlO 3 .TiO 2 , Zr0 2 .MgO, NiCr and Cr 2 0 3 /NiCr coatings were measured. The severity of the aqueous corrosion attack depends more upon the difference in the electrochemical potentials between the coating and the substrate than in the amount of the open porosity. For large differences in the electrochemical potentials, as in the case of Cr 2 O 3 and Cr 2 0 3 /NiCr, a severe drop in bond strength is observed. For small differences in the electrochemical potential, the corrosion is less severe and a reduction in bond strength with corrosion time is only observed after longer period of corrosion or this reduction is more gradual.