Plasma Spray Processing And Bench Test Evaluaiion

United Technologies Research Center (UTRC) has fabricated basic strength test panels under robot motion control and in the Controlled Process Plasma Spray Facility (CPPS). A set of increasingly more severe rig test thermal cycles were established and calibrated using the UTRC layered coating on superalloy test panel that had been instrumented with embedded thermocouples. A CI test cycle, which most closely reflects the predicted diesel engine thermal environment was created with a maximum thermal gradient across the coating of approximately 510°C in seven seconds and a top surface temperature of 6/5°C. Representative test panels from each of the spray fabrication methods including superalloy, ductile iron, and fiber/aluminum substrates survived this cycle. The two additional test cycles, C2 and C3, representing maximum thermal gradients of 555°C and 700°C respectively were used to screen thermal strain capability at higher temperatures. The three rig test cycles are shown in Figure 9. All test results are documented in Table 2.

Panels fabricated under a medium prestress in the robot motion control and CPPS facilities were then run through five CI cycles in succession, with the panel being allowed to reach the initial soak temperature of 65°C before being recycled to the maximum top surface temperature. Examination of both test panels after the five cycles yielded no visible damage. These test panels were then subjected to the more severe C2 and C3 test cycles. No damage was noted to have occurred after the C2 cycle; however, after the C3 cycle the panel fabricated in the robot facility had severe cracking at the primary crack location of the bond coat to 40/60 layer while the panel fabricated in the CPPS facility showed moderate cracking of a secondary nature at the 40/60 to 85/15 interface. The remaining panels from the medium fabrication temperature spray trial conducted in the CPPS facility represent varying base metal material/thickness combinations with layered coating. All were tested at the CI rig cycle only and found to survive, lest panels fabricated with the medium prestress (hybrid) temperature profile when exposed to the C3 rig cycle showed minimum cracking sensitivity. It is expected, given our experience, that this coating system would have survived the C2 rig cycle and have acceptable performance up to a 675°C maximum thermal gradient. These test results confirm the benefits derived from coating application under closely monitored process control.

A third spray fabrication trial was conducted in the CPPS facility to produce a coating system with minimum strain discontinuities at the bond coat to 40/60 interface. A representation of this temperature profile which incorporates a localized high temperature spike through the bond and 40/60 layer is shown in Figure 10. It is noteworthy that the medium prestress (hybrid) temperature control test specimens showed superior thermal strain capacity in comparison to all other coating configurations up to the C3 test cycle. Thermal cyclic test results of the multilayer coating sprayed on both superalloy and ductile iron substrate materials were equivalent.

Additionally, parts sprayed by the CPPS technique out-performed those produced under robot control.

Efforts continued to improve the quality of the coating produced in the robot facility. Upgrades to the powder feed system have been implemented to continuously spray the constituent layers of the coating system. Simultaneously, plasma spray parameters have ' been modified to increase the density and the strength of the coating layers. This was accomplished for the top 88-92% dense zirconia layer by the combination of reducing the gun-to-part standoff distance and increasing the robot traversing speed by a factor of two.

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