LASER CVD FOR THERMAL BARRIER COATINGS DEVELOPEDIn gas turbine technology ceramic coatings play a key role as far as thermal barrier
coatings (TBCs) are concerned. Gas turbine blades are usually constructed using a nickelbased superalloy substrate, a bond coat layer that acts as an intermediate and ceramic
thermal barrier coatings. Gas turbines operate at high temperatures and in order toimprove the performance of gas turbines advanced TBCs are required. Recent advancesin TBCs have enabled the operation of the gas turbine inlet at high-temperatures in therange of 1500 degrees C. Further improvement in the coating process and the TBC
nanostructure would enable operation at higher temperatures.
The ceramic material used for TBCs must have properties like high mechanical properties,
low thermal conductivity and thermal shock resistance. For this yttria stabilized zirconia(YSZ) has been widely used in practical applications. But, due to a slightly lower thermal
coefficient of YSZ than nickel-based super alloy substrates, significant thermal stresses
and cracks result in the TBCs that result in disastrous peeling off from the substrate.
Using various coating processes can help relax the thermal stress by controlling themicrostructures of YSZ. Electron beam physical vapor deposition (EBPVD) andatmospheric plasma spray (APS) are the most commonly employed coating processes inpractical applications. Chemical vapor deposition (CVD) is also another process that canbe considered but thick coatings cannot be obtained fast using this processing technique.
Laser CVD processes however have a high-deposition rate for YSZ and this has been thefocus of study for Takashi Goto, a researcher at the Institute for Materials Research,Tohoku University, Japan.
"We have obtained very thick (more than several hundreds micrometers in thickness andseveral cm by cm substrates) oxide coatings, particularly yttria stabilized zirconia films,by laser assisted CVD. Many other oxides like aluminium oxide (Al2O3), yttrium oxide(Y2O3), titanium dioxide (TiO2) and others were also fabricated at deposition rates of 0.7mm/h to 3.0 mm/h," says Goto. The high deposition rates have resulted in a large numberof nanopores in the grains that has lead to a smaller thermal conductivity of 0.7 W/mK.
This process appears to be very promising for TBCs as the researchers have alreadycoated YSZ films on practical gas turbine blades. The researcher explains that TBCs areused not only for gas turbines but also missile, spacecraft, cutting tools and others. Otheroxides are also promising for a number of applications, for example, Al2O3 for cutting
tools, Y2O3 for anti-plasma etching coatings and TiO2 for photo catalytic films.
What makes this work unique is that the wide-area substrates can be coated at very highdeposition rates. It is also possible that using this technique very specific areas can becoated that would be useful for the repairing of TBCs. There seems to be no significant
obstacles to the development of the laser CVD as the price of the laser is decreasing
significantly and the equipment may become cheaper. The researcher says the price ofCVD precursors used to be rather expensive, but due to several technical developments,
the price of metal organic (MO) precursors have been decreasing.
The research at this moment is supported by the government, which was performed as apart of a nano-coating project sponsored by the New Energy and Industrial Technology
Development Organization, Japan. The researchers are also collaborating with companies
mainly TBC related companies, cutting toolmakers and MO precursor makers. The next
step in development with regard to this work could be the scale-up of the process by
several times as this work has gone past the university level research stage. Additionally,
the researcher has patented the laser CVD process.
Details: Takashi Goto, Institute for Materials Research, Tohoku University, 2-1-1
Katahira Aoba-ku Sendai 980-8577, Japan. Phone: +81-22-215-21105. Fax: +81-22-2152091. E-mail: goto@imr.tohoku.ac.jp.