The microstructure of thermally sprayed ceramic coatings is characterized by the existence of various pores and microcracks. The porous microstructure makes coating desirable for thermal insulation, but this unique microstructural feature also gives rise to anelastic response under tension and compression loads. Detail investigations of curvature measurements of ceramic coated substrate indicate the coatings to exhibit anelastic behavior composed of nonlinear and hysteresis characteristics. In this paper, the mechanisms of such behaviors were studied from curvature-temperature measurements and finite element analysis through modeling the microstructure of yttria stabilized zirconia (YSZ) coating. Computational models contain numerous randomly distributed pores and microcracks with various sizes, aspect ratios, locations and orientations. The effects of such attributes of pores and microcracks on coating anelastic behavior were studied by simulations of curvature change during thermal cycles.

The concept of ‘process maps’ has been utilized to study the fundamentals of process–structure–property relationships in high velocity oxygen fuel (HVOF) sprayed coatings. Ni- 20%Cr was chosen as a representing material of metallic alloys. In this paper, concurrent experiments including diagnostic studies, splat collection, and deposition of coatings were carried out to investigate the effects of fuel gas chemistry (fuel gas/oxygen ratio), total gas flow, and energy input on particle temperature (T) and velocity (V), and coating microstructure formation and properties. Coatings were deposited on an ‘in situ’ curvature monitoring sensor to study residual stress evolution. A strong influence of particle velocity on induced compressive stresses through peening effect is discussed. The complete tracking of the coating buildup history including residual stress evolution and temperature deposition, in addition to single splat analysis allows the interpretation of resultant coating microstructures and properties, and enables coating design with desired properties.

Over the last decade there has been an explosion in terms of available tools for sensing the particle spray stream during thermal spray processes. This has led to considerable enhancement in our understanding of process reproducibility and process reliability. However, in spite of these advances, the linkage to coating properties has continued to be an enigma. This is partially due to the complex nature of the build-up process and the associated issues with measuring properties of these complex coatings. In this paper, we present an integrated strategy, one that combines process sensing, with process modeling and extracting coating properties in situ through the development of robust and advanced curvature based techniques. These techniques allow estimation of coating modulus, residual stress and non-linear response of thermal sprayed ceramic coatings all within minutes of the deposition process. Finally, the integrated strategy examines the role of process maps for control of the spray stream as well as design of thermal spray coatings. Examples of such studies for both MCrAlY and YSZ coatings will be presented.

The mechanical behavior of single splats on substrates can provide insight into a number of critical coating aspects, including stress evolution, inelastic behavior, and adhesion. Single splat studies provide a theoretically sound base for the understanding of properties, as they are essentially thin film structures on substrates – a geometrically simple arrangement. However, the experimental measurement of splat properties is non-trivial. Previous work has shown that residual stresses and hardness of splats can be measured successfully using X-ray diffraction and nano-indentation, respectively. Here we present the development of a new technique for the determination of in-plane splat properties, including modulus and flow stress, as well as splat-substrate adhesion. This method is based on the widely-used substrate curvature technique, and adaptations of the Stoney formula for electronic thin films. Critical aspects of the continuum-based analysis, including the effects of splat geometry and/or partial debonding, are discussed.

The adhesion at the interface between coating and substrate is a key factor for the reliability and performance since the main problem in the application of coating systems is the delamination at the interface. Finite element models are developed to find a way to predict the adhesion strength of a thermal sprayed coating system from simply designed indentation test. Large depth indentation behavior is simulated to study shear induced delamination beneath the indenter. The interface between the coating and the substrate is modeled by three different bonding characters to investigate the effect of interfacial bonding on indentation test. Pressure-strain relations based on Tabor’s suggestion are observed for various combinations of material properties and interfacial bonding characteristics. The growth of a crack in the interface plane leaves a clear imprint on indentation load-depth curve in case of soft coating on hard substrate. Tabor curve also shows potential ability to detect interface bonding strength under indentation test.

Performance and reliability of TS coatings are strongly dependent upon their inelastic behavior. To that end, continuum-level flow stress and hardening have been explored by recourse to e.g., indentation, substrate curvature and tensile measurements. While the results of such efforts have great implication for process and quality control, further refinement is necessary for incorporation into models of wear, fatigue or delamination. Here we present results and analysis of mechanical testing on metallic TS coatings deposited by different methods, emphasizing particular characteristics under localized and cyclic loading.

The general method of process maps to understand and control thermal spray processes has been applied to monitor the deposition of WC-Co by high velocity oxygen fuel (HVOF). A selected number of particle state conditions (velocity and temperature) has been performed to produce a variety of coatings. Microstructure, mechanical properties, and wear resistance were evaluated and compared. A second order process map for sliding wear, impacting particle erosion and abrasive wear control can be constructed from the process map to provide the limits within which the particle state can be changed to achieve a predefined coating specification. The mechanisms behind the wear resistance are discussed within the framework of wear maps –third order process map-in the context of analysis of inter splat de-bonding, mechanical properties of the coating, and delamination failure.

Thermal spray coatings exhibit a wide variety of microstructural characteristics that lead to variation in their functional properties. A complete understanding of the plasma spray process includes examination of the particle-flame interaction, particle impact (to form the splats), and the particle-substrate interaction during coating deposition. The links between these process parameters and coating properties has been established by using diagnostic tools in conjunction with a splat collection shutter and an in-situ curvature measurement instrument. In this study, a commercial grade molybdenum (Mo) powder was plasma sprayed; the spray stream was characterized in relation to the resulting particle state. A "splat map" was deposited through a "spray stream guillotine" to capture the fingerprint of the plume cross section. Subsequently, coatings were deposited at these spray conditions on a newly developed in-situ curvature measurement instrument to measure coating stresses and to estimate the coating modulus. Splats and coatings were subsequently characterized by micro-diffraction (for splat residual stresses), by nano and micro-indentation for elastic and elastic-plastic properties, and by electron microscopy. This complete history of the process followed by splat and coating characterization provides insight into the correlation between processing parameters, resultant particle states, and final coating properties. The role of particle temperature and velocity on the splat (and coating) morphology and residual stress is explained in the results.