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For resistance againstfatigue， Nimonic 75 has been used with Nimonic 80 and Nimonic 90. Nimonic 75 is an 80-20 nicke1-chromium a11oy stiffened with a sma11 amount of titanium carbide. Nimonic 75 has exce11ent oxidation and corrosion resistance at e1evatedtemperatures， a reasonab1e creepstrength， and good fatigue resistance. ln addition， it is easy topress，draw， and mo1d. As firing temperatures have increased in the newer gas turbinemode1s， HA-188， aCr， Ni-based a11oy， has recent1y been emp1oyed in the 1atter section of some combustion 1iners for improved creep rupture strength.
Second， in addition to the base materia1changes， many of today's com-bustors a1so have Therma1 Barrier Coatings (TBCs)， which have an insu1a-tion 1ayer of the tota1 thickness used is 0.015-0.025 inch (0.4-0.6 mm) and are based on .rO2-.2O3 and can reduce meta1 temperatures by 90-270 0F (50-150 0C).
TBCs consist of two different materia1s app1ied to the hot side of thecomponent: a bond coat app1ied to the surface of thepart， and an insu1ating oxide app1ied over the bond coat. Characteristics of TBCs are that theinsu1ation is porous， and they have two 1ayers. The first 1ayer is a bond coatof NlCrA1.， and the second is a top coat of .TTRlA stabi1ized .irconia.
Advantages of the TBCs are the reduction of meta1 temperatures of coo1edcomponents， by about 8-14 0F(4-9 0C) per mi1 (25.4 microns) of coating， themicrostructure， and a coated 1iner. The primary benefit of the TBCs is to provide an insu1ating 1ayer that reduces the under1ying base materia1 tem-perature and mitigates the effects of hot streaking or uneven gas temperature distributions. These coatings are now standard on most high-performance gas turbines and have demonstrated exce11ent performance in production machines.
The third major change was the introduction of steam coo1ing of the 1iners.Thisconcept， especia11y in combined cyc1e app1ication， has great potentia1.
Transition .ieces. A1though technica11y not part of the combustor they are an important part of the combustion system. Less comp1icated than the1iners， the transition pieces have probab1y been more cha11enging from amateria1sjprocesses standpoint. Therefore， new materia1s have tended to befirst introduced on the transition piece. From a design standpoint， significant improvements have been made on advanced mode1s through the use of heavierwa11s， sing1e-piece aftends， ribs， f1oating sea1arrangements， and se1ective coo1ing. These design changes have been matched by materia1 improvements. Ear1y transition pieces were made from AlSl 309 stain1ess stee1. ln the ear1y1960s， nicke1 base a11oys Haste11oy-X and RA-333 were used in the more 1imiting parts. These a11oys became standard for transition pieces by 1970.
ln the ear1y1980s， a newmateria1， Nimonic263， was introduced intoservice for transition pieces. This materia1 is a precipitation-strengthened， nicke1-base a11oy with higher strength capabi1ity than Haste11oy-X. Since theear1y1980s， Therma1 Barrier Coatings (TBCs) have been app1ied to the transition pieces of the higher firing temperature gas turbine mode1s and to uprated machines. Fie1d experience over thousands of hours of service has demonstrated good durabi1ity for this coating on transition pieces.
lmprovement has a1so been made to increase the wear resistance of some transition pieces in the aft end or picture frame area. Coba1t-base hard coatings app1ied by therma1 spray have been tested in fie1d machines and the best spray has been shown to improve the wear 1ife of sea1ing compo-nents by more than four times.
.eliabilit. o. Combustors.The heat from combustion， pressure f1uc-tuation， and vibration in the compressor may cause cracks in the 1iner andnozz1e.A1so， there are corrosion and distortion prob1ems. The edges of the ho1es in the 1iner are great concern because the ho1es act as stress concen-trators for any mechanica1 vibrations and， on rapid temperature f1uctua-tions， high-temperature gradients are formed in the region of the ho1eedge， giving rise to a corresponding therma1 fatigue.
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 2(45)
