The presence of defects such as voids, inter-lamellar porosities or cracks, provides a decrease of the effective thermal conductivity of plasma sprayed coatings as well as a decrease of the corresponding mechanical properties such as the Young’s modulus. In general, effective properties of thermal spray coatings are thus strongly different from that of the bulk material and have thus to be quantified to validate their in service performances. A complementary approach allowing understanding the relationships between the microstructure of a coating and its macro-properties is the use of Finite Element Modeling. The case of composite coatings is still more complicated due to the presence of different materials. In the present study, thermo-mechanical properties of a plasma sprayed composite coating were estimated by numerical modeling based on FEM. The applied method uses directly cross-sectional micrographs without simplification using a one-cell per pixel approach. Characteristics such as the thermal conductivity, the Young’s modulus, the Poisson ratio and the dilatation coefficient were considered. The selected example was an AlSi/polyester coating used as abradable seal in the aerospace industry.

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.

In situ values of Young's modulus and Poisson's ratio for thermal spray coatings are needed to evaluate properties and characteristics of thermal spray coatings such as residual stresses, in-service stresses, bond strength, fracture toughness, and fatigue crack growth rates. It is important to have methods documented in detail so that people can follow the document and use the methods. Such a document requires more pages than are allowed in conference proceeding and journal papers. Thus, Recommended Practices and Standards describing these methods are needed. Currently, there is not a recommended practice or standard for evaluating Young's modulus and Poisson's ratio for thermal spray coatings. The ASM International Thermal Spray Society has recognized this need and formed a committee on Recommended Practices for Thermal Spray Coatings. This paper describes one of the recommended practices being written by the Mechanical Properties Evaluation Subcommittee of the Recommended Practices Committee. The specimen is a coated substrate in the form of a cantilever beam. The method is easy to use and inexpensive. The equipment needed is a vise or clamping fixture, strain gages, a strain indicator, a micrometer, a ruler, a hanger, and a set of weights. The specimen is easy to machine and spray. The loading is easy to apply and remains constant during readings. The method can be used to evaluate Young's modulus and Poisson's ratio in tension or compression. A description of the method, a verification, and a sensitivity analysis was done and published in Reference [1]. Some of the details of implementing the method and the data sheet are presented here.

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.