Under load and stress thermal spray coatings have unique behaviour as compared to the bulk materials. The aim of this study was to define the mechanical properties of thermal spray coatings and consider the effect of external stress on coatings under test conditions. Five HVOF or Plasma spray coatings: NiCr, WC-CoCr, Al 2 O 3 -TiO 2 , Al 2 O 3 , and Cr 2 O 3 were studied. Mechanical properties, such as elastic modulus (E), and tensile strength (a) were measured. Behaviour of some coatings in different loading conditions (tension and compressive) was studied.

Thermal spraying is a widespread coating process with various applications in many industry branches. To be able to spray successfully on the sensitive materials an in-situ temperature control is most beneficial. The objective of this paper is to find out the correlation between the mechanical properties of some coating-substrate variations and temperature history of the coating made by HVOF. The paper describes the processes involved in implementation of the thermographic measurement systems into the HVOF spray system to measure the temperature of the coating on aluminum and steel during spraying. It was observed that when spraying on aluminum, the temperature of the coating has a marked influence on the adhesion. Tests with a steel substrate show how the hardness of the WC-CoCr depends on the quenching speed of the coating during spraying. Paper includes a German-language abstract.

In this paper, the Young's modulus, Poisson's ratio, and ultimate tensile strength of HVOF-sprayed NiCr (80/20) coating are evaluated by a uniaxial tensile test parallel to the coating plane. The effect of stand off distance is also considered. It is observed that, in the tensile test, the alloy NiCr (80/20) showed moduli of elasticity between 130 and 155 GPa and tensile strengths between 185 and 210 MPa. For the WC-CoCr alloy, the modulus of elasticity was between 185 and 210 MPa and the tensile strength was around 80 MPa. Paper includes a German-language abstract.

The high quality of the thermally sprayed tungsten carbide coatings has been attributed to high particle velocity and relatively low particle temperature. Such thermal spray conditions can be obtained with the HVOF spray process. In comparison to the plasma spray process, in the HVOF spray process the high particle velocity and optimum particle temperature have been associated with very high gas velocity (>1000 m/s) and a relatively low gas temperature (< 2700 °C). In this work tungsten carbide coatings (WC-17Co) were sprayed by the HVOF process with a low and a high gas velocity of 1050 and 1560 m/s, respectively. The spray tests were carried out also with different hydrogen/oxygen ratios. The coatings were abrasion tested in order to find out how gas velocity and the fuel/oxygen ratio affect the coating quality and wear rate. Wear rates of the HVOF sprayed coatings were found to decrease with the higher combustion gas velocity. The coating quality and wear rate became also less sensitive to gas parameters with the increasing gas velocity. The coating microhardness and wear rate were also compared to hot isostatic pressed (HIP) reference material from the same spray powder lot. The HIP sintered test piece was found to be less wear resistant than the corresponding thermally sprayed coatings.

An important aspect of the APS plasma spray process is the turbulent mixing of the spray jet with the surrounding air. The air mixing into the jet causes undesirable oxidation of the sprayed coating. In this work the air mixing in the plasma jet was determined by direct measurement of the oxygen content. The measuring method is based on electrochemical determination of the oxygen potential using a solid electrolyte cell. The partial pressure of oxygen along the centreline of the plasma jet was measured with the hydrogen/argon and helium/argon ratio, the gas flow rate and the stand-off distance as experimental parameters. The oxygen content of the plasma tail flame was found to vary between 13.6 to 19.3 % depending on the hydrogen to argon ratio and the stand-off distance. Such high oxygen contents are far too high to avoid serious oxidation of metal coatings. The plasma spraying tests were carried out with WC-Co 17 coating powder. The plasma gases were Ar/H 2 and Ar/He. The respective oxygen contents by air mixing were measured to be 18.9 and 17.3 %. The WC-Co was observed to be decarburised more in the Ar/H 2 mixture than in the Ar/He mixture, which was attributed to the higher oxygen concentration, higher particle temperature and longer flight time in the plasma jet. Solid electrolyte cell technique was applied to this type of measurements and it proved to be a convenient way to determine the oxygen mixing in the plasma jet.