Superior wear performance combined with excellent friction properties against metals makes chromium oxide (Cr 2 O 3 ) an interesting coating material for many industrial applications. However, Cr 2 O 3 is a challenging material for HVOF spraying due to its high melting temperature. Fracture toughness and lamella cohesion of a coating is limited and may be improved by using ceramic-ceramic –nanocomposite powders, which forms phases with improved properties. In this study Cr 2 O 3 -TiO 2 systems were selected aiming to improve the toughness and lamella cohesion of coating without reducing the excellent wear properties.

The potential of the high velocity oxy-fuel (HVOF) thermal spray process to produce coatings with reduced porosity is well known. The ability to produce high density ceramic coatings offers potential in high performance applications in the fields of wear, corrosion resistance and dielectric coatings. It has been, however, demonstrated that benefits from HVOF ceramic coatings can be obtained only if particles are melted enough and good lamella adhesion is produced. Therefore, due to the operational limits of the HVOF process, the process-structure- relationship must be well optimized. One strategy to improve melting of ceramic particles in the relatively low flame temperatures of the HVOF process is to modify particle crystal structure and composition. In this paper, the effects of the powder structure and the composition on coating microstructure and deposition efficiency of the HVOF spray process are studied. The effect of fuel gas, hydrogen vs. propane, was also demonstrated. The studied materials were agglomerated alumina- and titania-based pure and composite powders. Coating properties such as microstructure, hardness, and abrasive wear resistance, were compared to the coating manufactured by using conventional fused and crushed powders.

In this paper, different aluminum phosphate sealing treatment temperatures and substrate preheating temperatures are used to produce different coating properties. X -ray residual stress analyzer is used as a NDT inspection method to find out undesired coating defects. The wear properties of plasma-sprayed aluminum oxide and chromium oxide coatings are studied as a function of spray parameters and post-treatments. The residual stress states are associated with other coating properties such as dry wear resistance. Hardness and porosity as well as microstructure compared. The aim of this paper is to find connections between residual stresses and other coating properties. It was observed that there is good correlation between stress state, abrasion resistance and sealing treatment temperature when considering chromia coatings. Paper includes a German-language abstract.

Several recently published studies have shown remarkable improvements in dry abrasion resistance and corrosion resistance of aluminum phosphate sealed oxide coatings when compared to unsealed ones. There are numerous applications in chemical industry where a corrosive environment is accompanied with abrasive or erosive particles. In this study the wet abrasion resistance and slurry erosion resistance of aluminum phosphate-sealed and unsealed oxide coatings were studied and compared to their dry abrasion resistance. In wet abrasion tests kaolin and water mixture was used as the abrasive. In slurry erosion tests several abrasives in water with various pH values was used as the erosive medium. The coatings were characterized for microstructure and their wear mechanisms were analyzed using SEM. The results from wear tests are reported and correlated with coating properties. The influence of coating quality to the relative improvement achieved by sealing is presented and discussed.

Plasma spraying is a complicated process involving many partly interdependent parameters, which are in industrial spray environments difficult to optimise without laborious and time consuming experiments. In this work a non-intensified CCD camera without any external illumination is used for in-flight particle visualisation. Particle visualisation is based purely on the spontaneous light emitted by the hot particles. The motivation for this work is to outline the possibilities to develope a CCD based, low cost and rugged in-situ measurement system suitable also for industrial use. The measurement method has been tested with Plasmatechnik A3000S plasma spraying equipment using fused and crushed Al2O3 powder. Using digital image processing techniques relative particle concentrations and particle velocities have been calculated from the acquired images. These results have been correlated with wear resistance and deposition efficiency of the coatings produced with different powder feed rate and powder port adjustments. Coatings were also produced using both new and worn electrodes. The benefits and limitations of the method are discussed and the measurement results are compared against measurements made using laser sheet illumination, which can give information concerning also the colder and/or smaller particles not visible for the passive CCD system.

The deposition efficiency (DE) of a particular powder for a particular thermal spray process is very important factor when coating economics is being considered. There are many coating applications, however, where it is also important to know how the deposition efficiency changes during a longer coating process. Normally the DE is determined as mass ratio of powder fed into the process and corresponding weight gain of the sample. In this work the deposition efficiency has been determined for aluminum oxide powder in atmospheric plasma spraying using different spray parameters and electrode wear states. The coating process and in-flight particles were monitored using a fast non-intensified CCD-camera. Using digital image analysis the relative hot particle concentrations and velocity distributions were calculated from images. The possibility to use a CCD camera based monitoring system for in-situ measurement of DE is discussed. Additional laser illumination and PTV measurements were performed to verify the cold particle flux unseen by the plain CCD camera.

The plasma spraying process is controlled by various parameters that have an influence on powder particle velocities, temperatures and trajectories just before impact to the substrate. In order to fully utilize the thermal and kinetic energy of the plasma it is important to obtain information from these powder particle properties. In this work an intensified CCD camera has been used to detect in-flight particles in an atmospheric plasma spraying process. Plasma spraying was performed using fused and crushed AI2O3 powder. The powder carrier gas flow rate was varied during the spraying experiments. All the other deposition parameters were kept constant. Coatings produced using relatively new spraygun electrodes are compared with ones produced later with the same electrodes when they were worn out. The particle concentration is determined on a relative scale by the fraction of the area of a CCD camera frame covered by particle images. Further investigations necessary to clearify the relationship between the measured relative particle concentration and the true particle concentration are identified. The coatings are analyzed for wear resistance, degree of melting, deposition efficiency, hardness and porosity. The dependence of these coating properties on the relative particle concentration and the effect of electrode wear on the relative particle concentration are studied.