Advances in Engineering Advances in Engineering features breaking research judged by Advances in Engineering advisory team to be of key importance in the Engineering field. Papers are selected from over 10,000 published each week from most peer reviewed journals.
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.
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.
