In this study, an internal injection plasma torch is used to deposit nano-agglomerated YSZ feedstock powders on superalloy substrates at low ambient pressures ranging from 5000 Pa to 6000 Pa. Coatings with unique fully nano-equiaxed structures were obtained when operating the torch below 300 A. With increasing current, up to 700 A, coatings with mixed-grain and eventually large-grain structures were produced. Experimental results suggest that the equiaxed nanoscale structure derives from the original agglomerated nanoparticles that had undergone melting while inside the nozzle of the plasma torch and were subsequently solidified or sintered in the coating. Coating hardness and elastic modulus were also measured and are shown to correspond with microstructure.

Nondestructive techniques for evaluating and characterizing coatings have been extensively demanded by the thermal spray community; nonetheless, few results have been produced in practice due to difficulties in analyzing the complex structure of thermal spray coatings. Of particular interest is knowledge of the elastic modulus values and Poisson’s ratios, which are very important when seeking to understand and/or model the mechanical behavior or develop life prediction models of thermal spray coatings employed in various applications (e.g., wear, fatigue and high temperatures (TBCs)). In the present study, two techniques, laser-ultrasonics and Knoop indentation, were used to determine the elastic modulus of thermal spray coatings. Laser ultrasonics is a non-contact and nondestructive evaluation method that uses lasers to generate and detect ultrasound. Ultrasonic velocities in a material are directly related to its elastic modulus value. The Knoop indentation technique, which has been widely used as a method for determining elastic modulus values, was employed in order to compare and validate the measurements of the laser-ultrasonic technique. The determination of elastic modulus values via the Knoop indentation technique is based on the measurement of elastic recovery of the dimensions of the Knoop indentation impression. The approach used in the present study was to focus on evaluating the elastic modulus of very uniform, dense and near-isotropic titania and WC-Co thermal spray coatings using these two techniques. Four different coatings were evaluated: two titania coatings produced by APS and HVOF and two types of WC-Co coatings, conventional and multimodal (nanostructured and micro-sized particles), deposited by HVOF.

Yttria partially stabilized zirconia (YSZ) was atmosphere plasma sprayed on to mild steel substrates. The spray parameters were varied to determine their effects on the elastic modulus of the coating. The parameters were (i) continuous spray vs. paused spray, (ii) bond coat vs. no bond coat, and (iii) cooled vs. not-cooled. The elastic modulus was measured using laser ultrasonics and Knoop Indentation. Using indentation, the continuous/ paused spray exhibited the greatest effect with the paused spray samples having a lower elastic modulus value regardless of the condition of the other parameters. The other parameters did not reveal any statistically significant effect. The laser ultrasonics measurements showed that cooling and no-cooling had a greater effect on elastic modulus, with the other parameters having little effect. Laser ultrasonics detected parameters whose influence can be detected near the surface (in this work the cooling and no-cooling), but did not detect those parameters that influence the properties throughout the coating. Indentation detected the parameters that influence the properties throughout the coating, in this work continuous and paused spraying.

The development of thermal barrier coatings (TBCs) for diesel engines has been driven by the potential improvements in engine power and fuel efficiency that TBCs represent. TBCs have been employed for many years to reduce corrosion of valves and pistons because of their high temperature durability and thermal insulative properties. There are research programs to improve TBCs wear resistance to allow for its use in tribologically intensive areas of the engine. This paper will present results from tribological tests of ceria stabilized zirconia (CeSZ). The CeSZ was applied by atmospheric plasma spray process. Various mechanical and thermal properties were measured including wear, coefficient of thermal expansion, thermal conductivity, and microhardness. The results show the potential use of CeSZ in wear sensitive applications in diesel applications. Keywords: Thermal Barrier Coating, Diesel Engine, Wear, Thermal Conductivity, and Thermal Expansion