For the engines used in small turboprop aircrafts, the introduction of abradable coatings represents a feasible way to reach higher levels of overall engine efficiency, specifically by improving the fuel consumption and increasing the inter turbine temperature margin. Abradable coatings on seals also contribute to improved hot restarts capability of an engine and lead to substantial extension of service life of the rotating counter bodies. In our contribution, we concentrate on flame sprayed nickel graphite abradable coating that can be used in turboprop engines both for seals and clearance control. The focus is the impact of spraying parameters on the physical and function properties of the abradable coating.

Thermally sprayed hard metal coatings are the industrial standard solution for numerous demanding applications. Often the performance of thermally sprayed coatings is improved by using finer particle sizes due to improved surface finish and decreased defect sizes. In the aim of utilizing finer particle and primary carbide sizes in thermal spraying of hard metal coatings, several approaches have been studied to control the spray temperature. The most viable solution is to use the modern high velocity air-fuel (HVAF) spray process, which has already proven to produce high quality coatings with dense structures. In HVAF spray process, the particle heating and acceleration can be efficiently controlled by changing the nozzle geometry. In this study, fine WC-10Co4Cr powder (-25+5 µm) was sprayed with three nozzle geometries to investigate their effect on the particle temperature, velocity and coating microstructure. The study demonstrates that the particle melting and resulting W2C formation can be efficiently controlled by changing the nozzle geometry from cylindrical to convergent-divergent. Moreover, the average particle velocity was increased from 780 to over 900 m/s. This increase in particle velocity significantly improved the coating structure and density while deposition efficiency decreased slightly. Further evaluation was carried out to resolve the effect of particle in-flight parameters on coating structure and cavitation erosion resistance, which was significantly improved with the increasing average particle velocity.

Measuring the cohesive strength of thermally sprayed coatings is relatively difficult matter, which can be accessed in many directions. This issue is nowadays solved by use of Scratch test method. This method is not completely sufficient for the cohesive strength testing because the coating is under load of combined stresses during the Scratch test. The reason to develop this method was need for exact measurement of tensile cohesion toughness of thermally sprayed coatings, which could provide results as close to a classic tensile test as possible. Another reason for development of this method was the impossibility of direct comparison with results obtained by other methods. Tested coatings were prepared using HP / HVOF (Stellite 6, NiCrBSi, CrC-NiCr and Hastelloy C-276). These coatings were selected as commonly used in commercial sector and on because of rising customer demand for ability to provide such coating characteristics. The tested coatings were evaluated in terms of cohesive strength (method based on tensile strength test). Final fractures were evaluated by scanning electron microscopy and EDS analysis. As expected higher cohesive strength showed metallic coatings with top results of coating Stellite 6. Carbide coatings showed approximately third of the cohesion strength in comparison with metal based coating.

Cr 3 C 2 -25%NiCr, Stellite 6, NiCrBSi and Hastelloy C-276 coatings were deposited on substrate material P91 by HP/HVOF (High Pressure / High Velocity Oxygen Fuel) thermal spraying technology. The resistance against high temperature corrosion was evaluated exposition of coatings to corrosive-aggressive environment in the form of molten salts mixture with composition of 60% V 2 O 5 and 40% Na 2 SO 4 at temperature of 750 °C. Further, coatings were exposed to cyclic conditions. After the corrosion tests, all coatings were analyzed using scanning electron microscope (SEM), and analysis of elemental composition (EDX). Alloys-based coatings showed very similar corrosion mechanism in the selected aggressive environment and the same can be stated about cermet coatings. The obtained results prove that HVOF deposited coatings can replace current surface protection of components in power equipment such as nitriding.

This paper summarizes the results of high-temperature corrosion and erosion tests conducted on a wide range of coating materials, including Cr 3 C 2 -NiCr, Cr 3 C 2 -CoNiCrAlY, TiMoCN-Ni, Stellite 6, NiCrBSi, and Hastelloy C-276. All coatings were deposited on stainless steel substrates by HVOF spraying, and after high-temperature testing, were evaluated by means of SEM and EDX analysis. Of the coating materials evaluated, Hastelloy C-276 provided the best protection against high-temperature corrosion. It also exhibited the highest erosion resistance at a particle impact angle of 90°, but at the sharpest impact angle of 15°, Cr 3 C 2 -NiCr coatings were found to be the most erosion resistant, likely due to the strong bonding of carbide particles in matrix. NiCrBSi coatings, on the other hand, exhibited the highest values of volume loss.

This paper presents the results of mechanical and tribological property measurements obtained from NiCrBSi alloy coatings and evaluates the effect of different heat treatments. Coating specimens were deposited by means of flame spraying and heat treated using an acetylene torch. HVOF samples were also prepared as a reference for comparison. The microstructure of as-sprayed and heat-treated coating samples were compared, hardness and surface roughness were measured, and erosion resistance tests were performed. The processes and procedures used are described and the results obtained presented and discussed.

This paper evaluates and compares five wear-resistant coatings produced by HVOF spraying for high-temperature use. CrC-NiCr, CrC-CoNiCrAlY, Stellite 6, NiCrBSi, and TiMoCN-Ni coatings were sprayed on grit-blasted carbon steel substrates. Abrasive, sliding, and fretting wear resistance were measured and changes in microstructure and hardness due to high-temperature exposure were recorded. CrC-NiCr coatings exhibited the best wear properties, but the oxidation of carbides at high temperatures proved to be a problem. Based on test results, alternative coatings would include CrC-CoNiCrAlY for abrasive wear, Stellite 6 for erosive wear, and NiCrBSi for sliding wear.

This study evaluates the base mechanical properties of Hastelloy C-276, WC-Co, and Cr 3 C 2 -NiCr coatings produced by HVOF spraying at angles ranging from 60° to 90°. Examinations and tests were conducted to characterize changes in microstructure, density, hardness, wear resistance, surface roughness, and deposition efficiency based on spraying angle. Only minor spray-angle dependencies were found for base mechanical properties, although thickness, roughness, and deposition efficiency varied greatly.