This paper summarizes our knowledge at the beginning of 2003 about splat formation. First, the analytical and numerical models related to the impact and flattening of single particles on smooth or rough substrates with different tilting are recalled. Then, the different diagnostic methods, including imaging, are briefly described. The last part of the paper is devoted to the results and their discussion. Studies are related to the effect of various parameters on particle flattening. They include the characteristics of particles prior to impact: normal impact velocity, temperature, molten state, oxidation state, etc.; the parameters related to the substrate: tilting angle, roughness, oxide layer composition, thickness and crystallinity, desorption of adsorbates and condensates, wetting properties between impacting particle and substrate, etc.; and, finally, the parameters related to the heat exchange between the flattening particle and the substrate. They depend on previous parameters and control the propagation of the solidification front within the flattening particle, eventually modifying its liquid flow. It is obvious from this review that, if our understanding of the involved phenomena has been drastically improved during the last years, many points have still to be clarified. This is of primary importance because all the coating properties are linked to the particle flattening, splat formation, and layering.

Knowledge concerning splat formation : An invited review

Laser high heat flux test approaches have been established to obtain critical properties of ceramic thermal barrier coatings (TBCs) under near-realistic temperature and thermal gradients that may be encountered in advanced engine systems. Thermal conductivity change kinetics of a thin ceramic coating were continuously monitored in real time at various test temperatures. A significant thermal conductivity increase was observed during the laser-simulated engine heat flux tests. For a 0.25 mm thick ZrO2-8% Y2O3 coating system, the overall thermal conductivity increased from the initial value of 1.0 W/m K to 1.15, 1.19, and 1.5 W/m K after 30 h of testing at surface temperatures of 990, 1100, and 1320 °C, respectively, Hardness and elastic modulus gradients across a 1.5 mm thick TBC system were also determined as a function of laser testing time using the laser sintering/creep and microindentation techniques. The coating Knoop hardness values increased from the initial hardness value of 4 GPa to 5 GPa near the ceramic/bond coat interface and to 7.5 GPa at the ceramic coating surface after 120 h of testing. The ceramic surface modulus increased from an initial value of about 70 GPa to a final value of 125 GPa. The increase in thermal conductivity and the evolution of significant hardness and modulus gradients in the TBC systems are attributed to sintering-induced microporosity gradients under the laser-imposed high thermal gradient conditions. The test techniques provide a viable means for obtaining coating data for use in design, development, stress modeling, and life prediction for various TBC applications.

https://ntrs.nasa.gov/api/citations/19990047263/downloads/19990047263.pdf

Thermal Conductivity and Elastic Modulus Evolution of Thermal Barrier Coatings under High Heat Flux Conditions

Stereological Methods. Vol. 1: Practical Methods for Biological Morphometry@@@Stereological Methods. Vol. 2: Theoretical Foundations

http://me.eng.sunysb.edu/~compmech/downloads/N26.pdf

Determination of properties of graded materials by inverse analysis and instrumented indentation

In vitro studies of plasma-sprayed hydroxyapatite/Ti-6Al-4V composite coatings in simulated body fluid (SBF)

The elastic response behavior of thermal spray deposits at Knoop indentations has been investigated using indentation techniques. The ratio of hardness to elastic modulus, which is an important prerequisite for the evaluation of indentation fracture toughness, is determined by measuring the elastic recovery of the in-surface dimensions of Knoop indentations. The elastic moduli of thermal spray deposits are in the range of 12%-78% of the comparable bulk materials and reveal the anisotropic behavior of thermal spray deposits. A variety of thermal spray deposits has been examined, including Al2O3, yttria-stabilized ZrO2(YSZ), and NiAl. Statistical tools have been used to evaluate the error estimates of the data.

Elastic Response of Thermal Spray Deposits under Indentation Tests

http://me.eng.stonybrook.edu/~compmech/downloads/N20.pdf

Tensile toughness test and high temperature fracture analysis of thermal barrier coatings

The effect of spray angle on the properties of plasma spray deposits has been investigated. Two different materials, NiAl and Cr 3 C 2 –NiCr, were sprayed at angles from 50 to 90° with respect to the substrate. The spray angle influenced the microstructure and the properties of the deposits. The porosity increased as the spray angle decreased, i.e. as the angle was shifted away from the perpendicular position. The surface roughness of the Cr 3 C 2 –NiCr deposit was not sensitive to the spray angle, whereas NiAl exhibited an increase as the spray angle decreased. Microhardness, tensile adhesion strength, and interfacial fracture toughness decreased with spray angle. The change of these properties with spray angle is attributed to (1) the morphology of the splats, (2) the change of the local spray angle, and (3) the change of momentum of particles impacting on the substrate or previously deposited particles. The mechanical-property variations arise from porosity changes which occur from the spray angle variations.

Evaluation of off-angle thermal spray

The standard tensile adhesion test (TAT), ASTM C633, has been modified to perform multiple tests on flat and wide substrates. The TAT geometry, which specifies a 1-in. (25.4-mm)-diameter cylindrical substrate, has been used as the pull-off bar. Two renditions of this test were implemented and the Weibull moduli and characteristic stresses for both test methods obtained. The modified TAT, termed as the single-bar (SB) method, yields a higher Weibull modulus and characteristic strength than the other method, which is termed the double-bar (DB) method. It is believed that the different test results between the two methods arise from different stress distributions near the interface of the coating and substrate.

A test for coating adhesion on flat substrates—a technical note

The basic metallographic analysis leads to qualitative interpretation of the structural characteristics of a microstructure, for example the presence of phases, and the description of singularities such as inclusions. On the contrary, microstructural characterization which implements image analysis leads to a quantified analysis of structural characteristics. A method is described to assess thermal spray deposit microstructures using image analysis by means of a metallographic index. This index is based on the determination of several stereological and morphological parameters by primary referee to the size-shape distributions of the features, the fractal dimension of the deposit upper surface, and the Euclidean distance map of the bodies of interest. This work employs quantitative metallography on a much wider scale to provide better quality control of deposit microstructures.

https://researchbank.swinburne.edu.au/file/93a5add5-701e-4299-b48b-e08b992d2e2a/1/PDF (Published version).pdf

Sang Ha Leigh

Papers

Elastic Response of Thermal Spray Deposits under Indentation Tests

Tensile toughness test and high temperature fracture analysis of thermal barrier coatings

Evaluation of off-angle thermal spray

A test for coating adhesion on flat substrates—a technical note

Microstructural Index to Quantify Thermal Spray Deposit Microstructures Using Image Analysis