In this work the sliding wear resistance of HVOF and APS-sprayed coatings from two experimental TiO 2 -10-20 wt.%Cr 2 O 3 fused and crushed feedstock powders was investigated. During the spraying process the particle temperatures and velocities were mapped in the cross section of the spray jet at different spray distances. In addition, the coating surface temperature was monitored. Coating microstructures and phase composition were studied by SEM and XRD, respectively. The elastic modulus (E) and hardness (H) values were measured via in-depth sensing indentation. The unidirectional ball-on-disk sliding wear test was performed at room temperature against a tungsten carbide ball (sliding speed 0.05 m/s, normal force 90 N). The increase of the particle temperature and velocity leads to an increase of hardness and Young’s modulus of the coatings, thereby increasing their sliding wear resistance. The sliding wear resistance can be correlated with the H 3 /E 2 ratio better than with the hardness alone. The sliding wear resistance was also sensitive to the coating surface temperature reached during deposition.

In this study, the deposition, microstructure, and resistivity of APS and HVOF sprayed Cr 2 O 3 -TiO 2 coatings is systematically investigated. Commercially available Cr 2 O 3 -rich feedstock powders are used along with five agglomerated and sintered experimental powders on the TiO 2 -rich side. Both processes are found to produce homogeneous, low-porosity coatings with phase compositions that can be changed by adjusting process parameters. Coating hardness and electrical resistivity are found to depend heavily on Cr 2 O 3 content.

The composition WC-(W, Cr) 2 C-Ni is one of the standard hardmetal compositions used for the preparation of thermally sprayed coatings by high velocity oxy-fuel (HVOF) spraying. Surprisingly, this composition has been poorly investigated in the past. Frequent use of the commercial designations WC-“CrC”-Ni, WC-Cr 3 C 2 -Ni, and WC-NiCr indicate the insufficient knowledge about the phase compositions of the powders and coatings. In this paper, the processability of five commercial feedstock powders was studied. These feedstock powders were of different origin and were sprayed with two different liquid-fueled HVOF systems (K2 and JP-5000). The microstructures and phase compositions of the powders and the coatings were studied. Focus was on the appearance, composition, and distribution of the (W, Cr) 2 C phase, which is either formed or changes its Cr/W ratio during the spray process. The composition of the (W, Cr) 2 C phase was estimated from the lattice parameters. Hardness, density, and Young’s modulus were determined for the coatings. Additionally, the abrasion wear resistance of the coatings was studied. Unlike WC-Co and Cr 3 C 2 -NiCr, WC-(W, Cr) 2 C-Ni is not a simple binary hard phase-binder metal composite. The excellent properties of this composition, for instance, its oxidation and corrosion resistance, will surely attract more attention to this composition than it has in the past.

Thermally sprayed titanium suboxide (TiOx) coatings are widely used in industrial applications due to their good tribological properties and their electrical conductivity. These properties are mainly dependent on the amount of oxygen in the crystal lattice. Oxygen defects lead to the formation of so-called Magnéli phases. The range of applications is limited by the fact that TiOx tends to reoxidize in many service conditions, especially at elevated temperatures. Also, the extreme conditions in the flame or torch used in the thermal spray process lead to undefined phase changes and defects. In the TiO 2 -Cr 2 O 3 system, Magnéli phases are also formed, but it is assumed that the properties do not change due to oxidation during spraying and subsequent use. This work shows the possibilities of the new coating materials. Powders with different TiO 2 and Cr 2 O 3 contents and prepared by different technologies were used for the investigation of coating properties. Experimental powders with defined phase compositions were prepared. The powders were thermally sprayed and the coatings investigated in terms of phase composition, microstructure, hardness, and abrasion wear resistance.

In result of the spray process normally a phase change from α-alumina (corundum) in the feedstock powder to predominantly α-alumina in the coating takes place. This is a well known but often neglected fact in the preparation and use of alumina coatings. This is of special interest since the high usage properties of thermally sprayed alumina coatings are generally derived from sintered alumina ceramics which consist of corundum. It is expected that the prevention of this phase transformation will significantly change the mechanical, electrical and other properties of thermally sprayed alumina coatings. There is controversial information about the possibility of stabilization of α-alumina by additions of chromia in the literature. In this work the stabilization using different spray processes (water stabilized plasma WSP, gas stabilized plasma APS and HVOF) was studied. Mechanical mixtures of alumina and chromia were used, as well as pre-alloyed powders consisting of solid solutions. The coatings were studied by X-ray diffraction and metallographic cross-sections. It was shown that in the case of the mechanically mixed powders the stabilization predominantly depends on the applied spray process. The stabilization by use of the WSP process was confirmed.

In the current study, the oxidation of hardmetal coatings (WC-12%Co, WC-17%Co, WC-10%Co4%Cr, WC-20%“CrC”-7%Ni, Cr 3 C 2 -25%NiCr, (Ti,Mo)(C,N)-29%Ni and (Ti,Mo)(C,N)-29%Co) in the temperature range 350- 900°C was studied for test durations ranging from 2 h to 128 h. The formation of oxide scales was investigated by X-ray diffraction, as well as by optical microscopy and SEM (including EDX) of coating cross sections. For coatings obtained by spraying with DJH 2700 and TopGun HVOF systems, the phase composition had only a moderate influence on high-temperature oxidation behavior in atmospheric conditions. The first oxides detectable by X-ray diffraction appeared on the coating surfaces after oxidation at 350°C for 128 h for all coatings. Pronounced oxidation (formation of oxide scales with thicknesses of greater than 10 µm) started at 600°C. Oxide scale growth differed significantly above this temperature among the hardmetal compositions studied here. WC-20%"CrC"-7%Ni and Cr 3 C 2 -NiCr had the highest oxidation resistance, with the oxide scale thicknesses lying below 10 ìm after oxidation at 800°C and 900°C for the two materials, respectively.