Low-oxygen heat treatments and internal platinum oxygen-diffusion barriers extend lifetimes.

Thermally sprayed dielectric ceramic coatings are the primary means of attaching strain and temperature gauges to hot-section rotating parts of turbine engines. As hot-section temperatures increase, lifetimes of installed gauges decrease, and seldom exceed one hour above 2,000 °F ( ≈1,100 °C). Advanced engine components are expected to operate at temperatures approaching 2,200 °F ( ≈1,200 °C), and the required high-temperature lifetime is 10 hours minimum.

Figure 1. A Test Fixture is depicted here during removal from a horizontal tube furnace maintained at a temperature of 1,100 °C. A coupon is fastened to the fixture with platinum wire.

The approach taken to increase the high-temperature lifetime of a gauge attached to an engine component by the method described above involves (1) selective oxidation of the bond coat by means of a heat treatment in reduced oxygen partial pressure followed by (2) the application of a noble-metal diffusion barrier. In experiments to test this approach, heat treatments of NiCoCrAlY bond coats were carried out in a tube furnace in which, in each case, the temperature was alternately (1) increased at a rate of 3 °C per minute and (2) held steady for one hour until the desired temperature was reached. The tube furnace was continuously purged with dry nitrogen gas. A final heat-treatment temperature range of 1,600 to 1,800 °F (871 to 982 °C) proved most beneficial.

Test coupons were made to enable evaluation of the cycle lives of various bond coats, including some made from the commercially available coating materials Praxair 171 (an NiCoCrAlY formulation) and Praxair 343 (an NiCrAlY formulation). Each test coupon included a base-metal coupon of Inconel™ 718 nickel alloy. One of the bond-coating materials to be tested was thermally sprayed on the metal, the coupon was subjected to the aforementioned heat treatment at reduced oxygen partial pressure, then a ceramic dielectric topcoat was thermally sprayed onto the bond coat. To provide a basis of comparison for evaluation of the relative merits of the various surface treatments and heat treatments, some of the NiCoCrAlY and NiCrAlY bond coats were incorporated into the coupons in the as-sprayed condition: that is, the affected coupons were not subjected to the heat treatment at reduced oxygen partial pressure.