Quenching stress and fracture of paraffin droplet during solidification and adhesion on metallic substrate

Significance 

One major application of the thermal barrier coatings is the protection of metallic substrate from high-temperature combustion gases. Presently, the air plasma spraying method is widely used in the manufacturing of thermal barrier coatings. It involves spraying and depositing the coating materials on metal substrates. However, residual stress built up in the coatings has been observed during the deposition, solidification and quenching processes of the sprayed particles. Considering the critical role of the residual stress in inducing cracking and separation of thermal barrier coatings during operation, it is wise to develop effective ways of clarifying the development of residual stress during thermal particle spraying.

Herein, researchers at Tokyo Institute of Technology: Mr. Chao Kang (PhD candidate), Dr. Motoki Sakaguchi, Ms. Ayumi Amano, Dr. Yu Kurokawa, and Professor Hirotsugu Inoue developed a model experiment to evaluate the strain/stress development behavior during thermal spraying processes. In particular, the adhesion and solidification processes of molten droplets were discussed. Their work is currently published in the journal, Surface and Coatings Technology.

In brief, the experiment model entailed a molten paraffin droplet and a pre-cooled 430 stainless steel substrate. First, the molten paraffin droplet was dropped from a specified height onto the pre-cooled stainless-steel substrate to enable the measurement of the stain at the substrate back surface using a bi-axial strain gauge. As such, the authors further investigated the factors affecting the quenching strain including the substrate pre-set temperature, drop height and paraffin materials properties.

Vertical cracking in splats, debonding at the splat/substrate interface and delamination between the splats were observed during the experiment. These fracture behaviors depended on the experimental conditions. Therefore, it was necessary to examine the driving forces of the highlighted fracture behavior based on the finite element analysis taking into consideration the experimentally measured quenching strains, material properties and the geometries of the substrates and splats.

Splat and interfacial stresses were specifically identified as the driving forces for cracking and debonding. This information provided more insights into the cracking behavior observed in the experiments. On the other hand, the shrinkage of the splat during solidification and cooling resulted in the generation of the tensile quenching strain at the substrate back surface. The quenching increased with an increase in the number of droplets. For candle wax, larger quenching strains were measured without the occurrence of cracking. This was attributed to the difference in the splat geometries and material properties. As the substrate pre-test temperature decreased, an increase in the splat stress was observed. This led to cracking especially when the wax tensile strength was exceeded.

Depending on the experimental conditions, the interfacial separation was classified into two: debonding at the splat-substrate interface in the candle wax and delamination at the interface of the first and second splat in HNP-9. The two were easily initiated and propagated at lower substrate temperatures.

In summary, Tokyo Institute of Technology scientists successfully brought a solution for to explain the fracture behaviors during the solidification and adhesion on the metallic substrate based on a molten paraffin droplet. Based on the experimental results, as highlighted by Dr. Motoki Sakaguchi (the corresponding author), the study provides essential information that will advance the optimization of thermal spraying processes for better applications.

Quenching stress and fracture of paraffin droplet during solidification and adhesion on metallic substrate - Advances in Engineering

About the author

Mr. CHAO KANG is currently a Ph.D. candidate in Department of Engineering, Tokyo Institute of Technology (Tokyo Tech), Japan. He received his Bachelor’s degree (2017) in mechanical engineering from Dalian University of Technology, China and Master’s degree in Mechanical Engineering from Tokyo Tech (2019).

His research is mainly focused on mechanics of materials and fracture mechanics especially for experimental and numerical analysis of stress and fracture behaviors in thermal barrier coating system.

About the author

Dr. Motoki SAKAGUCHI is an Associate Professor in Department of Mechanical Engineering, Tokyo Institute of Technology (Tokyo Tech). He graduated from The University of Tokyo in 2002, and received his Dr. Eng. in materials science from Nagaoka University of Technology in 2007. He was an Assistant Professor at Nagaoka University of Technology from 2007 to 2012, a visiting professor at University of Siegen supported by Alexander von Humboldt Foundation in Germany, and is an Associate Professor at Tokyo Tech from 2012. He was awarded JSMS Award for Scientific Paper in 2006, JSMS Award for Promising Researchers in 2018, The Young Scientists’ Prize in MEXT and JSME Medal for Outstanding Paper in 2019.

His research is mainly focused on mechanics of materials and fracture mechanics especially for high temperature materials including Ni-base superalloy, Ti-Al alloy and coating system in jet engine application and land base power generation.

About the author

Ms. Ayumi AMANO graduated Tokyo Institute of Technology (Tokyo Tech) in 2016, and completed a master’s degree in mechanical engineering from Tokyo Tech in 2018. She got Best Presentation Awards in 2017 M&M conference and 55th Symposium on Strength of Materials at High Temperature and Fracture Mechanics. She is now a researcher at Railway Technical Research Institute (RTRI), Japan.

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About the author

Dr. Yu KUROKAWA is an Assistant Professor in Department of Mechanical Engineering, Tokyo Institute of Technology (Tokyo Tech) from 2008. He received his Ph. D (Eng.) and Master degree from department of mechanical sciences and engineering, Tokyo Tech.

His research is mainly focused on non-destructive testing and evaluation using ultrasonic. Also non-destructive testing using infrared thermography, stress analysis using infrared thermography, fatigue limit evaluation using infrared thermography, and strain field measurement using digital image correlation are his research topic.

About the author

Prof. Hirotsugu INOUE is a Professor at the Department of Mechanical Engineering, Tokyo Institute of Technology (Tokyo Tech), Japan. He graduated from Tokyo Tech in 1985 and received his D.Eng. in mechanical engineering also from Tokyo Tech in 1993. He has been working at Tokyo Tech for more than 30 years as a Research Associate (1987-1996), an Associate Professor (1996-2009) and a Professor (2009-present). His research areas include classical theory of elasticity, impact mechanics of solids, stress/strain measurement techniques, ultrasonic and thermographic non-destructive testing, inverse problems and other topics in the field of mechanics of materials. His representative papers include “Time frequency analysis of dispersive waves by means of wavelet transform” (Trans ASME, J Appl Mech, 1995) and “Review of inverse analysis for indirect measurement of impact force” (Appl Mech Rev, 2001).

He is recently interested in development of measurement techniques using infrared thermography in the field of mechanics of materials with clarifying theoretical background of these techniques. He served as a member of many committees in several academic societies such as Chair of Materials & Mechanics Division, Japan Society of Mechanical Engineers (JSME).

He is now serving as a director of Japanese Society for Non-Destructive Inspection (JSNDI) and Japan Society for Computational Methods in Engineering (JASCOME). He was awarded several awards from JSME, JSNDI and JACM (Japan Association for Computational Mechanics).

Reference

Kang, C., Sakaguchi, M., Amano, A., Kurokawa, Y., & Inoue, H. (2019). Quenching stress and fracture of paraffin droplet during solidification and adhesion on metallic substrate. Surface and Coatings Technology, 374, 868-877.

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