Optimizing Nonlinear Properties of Thermal Sprayed Coatings through Processing Parameters
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Optimizing Nonlinear Properties of Thermal Sprayed Coatings through Processing Parameters Yajie Liu1, Toshio Nakamura1, Vasudevan Srinivasan2, Andrew Gouldstone2, and Sanjay Sampath2 1 Mechanical Engineering, State University of New York, Stony Brook, NY, 11794 2 Materials Science and Engineering, State University of New York, Stony Brook, NY, 11794 ABSTRACT Low-temperature thermal cycling of plasma sprayed zirconia coatings via curvature measurements is used to quantify their nonlinear mechanical behavior. The nonlinear feature arises from the unique layered, porous and cracked morphology of thermal sprayed ceramic materials. With this procedure, various specimens were tested to investigate the effects of processing conditions. The measured nonlinear properties are interpreted in the context of microstructural changes in the plasma sprayed coatings due to differences in particle state upon impact and coating build-up. The implications of this study are significant for thermomechanical design of strain-tolerant ceramic coatings in thermal barrier applications. INTRODUCTION Thermal spray (TS) is a well established processing technology to fabricate coatings through successive impingement of molten droplets on a prepared substrate resulting in a lamellar microstructure with pores and cracks. Accordingly, coating mechanical properties are highly dependent on the defect architecture and strongly related to the complex deposition processes and related conditions [1-6]. The feedstock powder also affects the nature of the deposit formation dynamics and the ensuing properties of coatings. Although it is well understood that desired or application-specific microstructures can be obtained by varying feedstock powder and processing parameters, a quantitative description in terms of properties has yet to be satisfactorily achieved. Past works (e.g., [7]) explained the source of macroscopic nonlinearity but a robust method of comparison of different TS coatings was yet to be developed. In this paper, we use a novel technique to extract nonlinear elastic coating properties via inverse analysis of substrate curvature measurements and present results on a range of plasma-sprayed (PS) yttria stabilized zirconia (YSZ) coatings made from a number of different processing conditions, and interpret them in the context of defect architecture. IDENTIFICATION OF NONLINEAR PROPERTY Constitutive Model In order to describe the nonlinear behavior, a phenomenological based constitutive model is introduced. The stress-strain relation is asymmetric under tension and compression and consists of linear and nonlinear parts.
Under a uniaxial stress condition, it is, n ⎧ σT σ ⎪ − ⎪⎪ E c E cσ Nn −1 ε =⎨ ⎪σ (σ − σ T ) n − σ T ⎪ + ⎪⎩ E c Ecσ Nn −1
for σ < σ T
(1)
n
for σ ≥ σ T
Here σT is the transitional stress where the change from linear to nonlinear occurs and assumed to be negative (compression). Also Ec is the Young’s modulus of the TS coating, n is the powerlaw exponent and σN is a reference stress. A small value of σΝ signifies
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