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TOWARDS DURABLE THERMAL BARRIER COATINGS WITH NOVEL MICROSTRUCTURES DEPOSITED BY SOLUTION- PRECURSOR PLASMA SPRAY

The feasibility of a new processing method—solution precursor plasma spray (SPPS)—for the deposition of ZrO2-based thermal barrier coatings (TBCs) with novel structures has been demonstrated. These desirable structures in the new TBCs appear to be responsible for their improved thermal cycling life relative to conventional plasma-sprayed TBCs. Preliminary results from experiments aimed at understanding the SPPS deposition mechanisms suggest that nanometer-scale particles form in the plasma flame, followed by their deposition by sintering onto the substrate in the intense heat of the plasma flame. The SPPS method, which offers some unique advantages over the conventional plasma-spray process, is generic in nature and can be potentially used to deposit a wide variety of ceramic coatings for diverse applications.

Mechanisms of ceramic coating deposition in solution-precursor plasma spray

The solution-precursor plasma spray (SPPS) method is a new process for depositing thick ceramic coatings, where solution feedstock (liquid) is injected into a plasma. This versatile method has several advantages over the conventional plasma spray method, and it can be used to deposit nanostructured, porous coatings of a wide variety of oxide and non-oxide ceramics for a myriad of possible applications. In an effort to understand the SPPS deposition process, key diagnostic and characterization experiments were performed on SPPS coatings in the Y 2 O 3 -stabilized ZrO 2 (YSZ) system. The results from these experiments show that there are multiple pathways to SPPS coating formation. The atomized precursor droplets undergo rapid evaporation and breakup in the plasma. This is followed by precipitation, gelation, pyrolysis, and sintering. The different types of particles reach the substrate and are bonded to the substrate or the coating by sintering in the heat of the plasma. The precursor also reaches the substrate or the coating. This precursor pyrolyzes in situ on the substrate, either after it reaches a “cold” substrate or upon contact on a “hot” substrate and helps bond the particles. The coating microstructure evolves during SPPS deposition as the coating temperature reaches approximately 770 °C.

The Solution Precursor Plasma Spray Process for Making Durable Thermal Barrier Coatings

The Solution Precursor Plasma Spray (SPPS) process involves the injection of atomized droplets of precursor into the plasma plume, instead of powder that is used in conventional plasma spray. The resultant thermal barrier coating (TBC) microstructure consists of (1) through-coating-thickness cracks, (2) ultra-fine splats, and (3) nanometer and micrometer-sized dispersed pores. These unique SPPS microstructural features provide highly durable TBCs. The SPPS TBCs in 1121°C /1 hour cyclic furnace tests exhibit a significantly improved spallation life compared to APS, DVC, and EB-PVD/Pt-Al TBCs. Extensive process diagnostic and modeling studies have been conducted to provide a foundation for understanding and control of the process. Process/microstructure/property relationships have been defined. Extension of the process for making thick coatings (> 3mm) and low thermal conductivity coatings are described.© 2005 ASME