Nov_Dec_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 2 0 4 6 iTSSe TSS iTSSe TSS studies are underway to create porous vertically cracked (PVC) coatings (Fig. 2c) that combine the key benefits of both DVC and traditional porous TBC coatings. PVC coatings exhibit re- duced thermal conductivity compared to traditional segment- ed TBCs due to the introduction of fine porosity, but they also sustain an acceptable level of erosion resistance [6] . The SPS process has, thus far, shown its capability to pro- duce plasma-sprayed TBCs that aremore like EB-PVD coatings (Fig. 2d). This process has been showcased in various studies as a lower application-cost alternative toEB-PVDand therefore is an attractive process for coating applicators. Furthermore, the capital investment for SPS processing equipment is far less than that of the EB-PVD process. This process utilizes micron- or sub-micron-sized particles in an alcohol or water-based suspension as the feedstock. Although the process has been adapted for commercial use since the late 1990s, its evolution is still ongoing. Currently, the initial target for its application is on less-critical and smaller aerospace engine components. CASCADED ARC PLASMA TECHNOLOGY Today, the demands placed on IGT manufacturers to spray large parts require higher deposition rates and efficien- cies for coating application in a robust and reliable manner. Unlike legacy porous TBCs, segmented TBCs require very tight control of feedstock materials and the process to achieve con- sistent microstructures. For example, one of the design characteristics of seg- mented TBCs is to maintain a high vertical crack density. A slight deviation in the process can lead to changes in crack density and the formation of undesirable horizontal cracking, which can act as a weak link and cause coating spallation. Therefore, reliable and repeatable coating solutions are critical to the success of segmented TBC applications. Today, cascaded arc plasma torch technology surpasses its legacy counterparts. The cascaded arc approach was pio- neered to control and stabilize the arc within a plasma torch, which overall leads to improved repeatability and robustness of the plasma spray process [2,6,7] . Unlike legacy plasma spray devices that were designed to use a single anode-cathode set to create plasma, the cascaded arc chamber is characterized by fixing the length of the arc across a series of electrically isolated neutral rings, also known as neutrodes, within the arc chamber. This extended and fixed arc length stabilizes the plasma plume and eliminates high-amplitude power oscil- lations. The result is a number of additional advantages, in- cluding higher voltage/lower amperage operation, increased hardware life, reduced voltage oscillations, and the minimiza- tion of the influences of gas flow and type on arc behavior. Figure 3 shows the benefits of using cascaded arc plasma technology. It has been demonstrated that torch hardware life associatedwith this technology can function for over 100hours Fig. 3 — Benefits of cascaded arc plasma technology. FEATURE 10