Microlaminate composites: An alternate approach to thermal barrier coatings

Ceramic thermal barrier coatings suffer from a major drawback, i.e., brittle behavior. An alternate approach is microlaminate composite coatings consisting of alternate layers of metal and oxide. As the thickness of the individual laminae decrease while keeping the total thickness of the coating constant, the thermal conductivity drops markedly. Data on the Fe-Cu system will be presented. A model is proposed for an MCrAlY-Al2O3 microlaminate coating for thermal barriers. The methods of fabrication will also be discussed

Second harmonic generation from metallo-dielectric multilayer photonic band gap structures

We experimentally and theoretically investigate the second order nonlinear optical response of metallo-dielectric multilayer structures composed of Ag and Ta2O5 layers, deposited by magnetron sputtering. Second harmonic generation measurements were performed in reflection mode as a function of incidence angle, using femtosecond pulses originating from a Ti:Sapphire laser system tuned at 800 nm. The dependence of the generated signal was investigated as a function of pump intensity and polarization state. Our experimental results show that the conversion efficiency from a periodic metallo-dielectric sample may be enhanced by at least a factor of 30 with respect to the conversion efficiency from a single metal layer, thanks in part to the increased number of active surfaces, pump field localization and penetration inside the metal layers. The conversion efficiency maximum shifts from 70 degrees for the single silver layer down to approximately 55 degrees for the stack. The experimental results are found to be in good agreement with calculations based on coupled Maxwell-Drude oscillators under the action of a nonlinear Lorentz force term

Thin film temperature sensors for gas turbine engines: Problems and prospects

The increasing trend towards high-temperature, fuel efficient jet engines has led to the development of complex cooling schemes for the turbine blades. The measurement of temperature of the blade during operation, which is accomplished in conventional blade design by embedding wire thermocouples in the blade wall, causes serious structural and aerodynamic problems in the case of cooled turbines. In order to meet the requirement of temperature measurement in cooled turbines, it is desirable to develop surface-mounted thin-film thermocouples or a resistance thermometer. In the current state of the art of thin-film thermocouples, the sensing element consists of 2-μm-thick Pt and Pt 10% Rh thin-film elements deposited on the insulating surface of the blades and vanes. The insulator is developed by thermal oxidation of a MCrAlY coating which is deposited on the blade and vane surface in the current state of turbine technology. The understanding of the structural and thermoelectric stability of the sensor elements and of the insulating layer of Al<SUB>2</SUB>O<SUB>3</SUB> in the hostile environment of a gas turbine requires an in-depth study of the metallurgical reactions occurring at the thin-film Al<SUB>2</SUB>O<SUB>3</SUB> and Al<SUB>2</SUB>O<SUB>3</SUB>-MCrAlY interfaces and of the corrosive reactions on the surface of the metal film. The work presented in this review addresses the problems associated with obtaining highly adherent and insulating Al<SUB>2</SUB>O<SUB>3</SUB> on the MCrAlY surfaces, adhesion of the sensor elements, thermoelectric stability of the sensors on contamination, and finally the development of a corrosion protection coating. The desired quality Al<SUB>2</SUB>O<SUB>3</SUB> has been grown on NiCoCrAlY-coated nickel-based superalloy substrates by a combination of oxidation treatments. The interface-modified Pt and Pt/Rh films are deposite- - d on the oxide by a dc magnetron sputtering technique. The corrosion protection requirements involve deposition of Si-O-N and Si<SUB>3</SUB>N<SUB>4</SUB> graded structures on the sensors by the plasma-assisted chemical vapor deposition process. Details of the electrical and metallurgical characteristics of the device at each stage of the coating/film growth have been analyzed by a number of surface sensitive and bulk analytical techniques

Effects of substrate bias on the resistivity and microstructure of molybdenum and molybdenum silicide films

A continuous series of Mo-Si alloys (Mo<SUB>x</SUB>Si<SUB>1-x</SUB>), with 0.3 &#8804; x &#8804; 1, have been deposited by reactive magnetron sputtering of molybdenum in silane and argon plasma. The effects of sputter deposition parameters such as the deposition rate, substrate temperature, substrate bias and gas flow conditions on the resistivity of molybdenum and molybdenum silicide films are reported. The resistivity of molybdenum films shows a pronounced decrease with an increase in the negative substrate bias and the rate of film growth. An electrical resistivity as low as 7.8 &#956;&#937; cm has been obtained in molybdenum films by optimizing the deposition conditions. The resistivity of MoSi<SUB>2</SUB> films, measured after annealing them at 1000&#176;C for 1 h in nitrogen ambient, shows a broad minimum at -75 V bias. However, the effect of bias is not as pronounced as for molybdenum. The changes in the resistivity of molybdenum and MoSi<SUB>2 </SUB>films are attributed to the degree of oxygen contamination and microstructural modifications during growth

Kinetics of structural relaxation and hydrogen evolution from plasma deposited silicon nitride

Infrared absorption measurements and the temperature dependence of stress have been used to establish the kinetics of structural relaxation and hydrogen evolution from plasma deposited a-SixNy :H films. The Arrhenius rate law describes the dissociation of N-H and Si-H bonds which occurs on annealing the films above 600 &#176;C. The activation energies deduced from the infrared data are lower than the respective bond dissociation energies. The films undergo a rapid stress relaxation in the temperature range 400-650 &#176;C. The discussion of the experimental results highlights possible mechanisms for the evolution of hydrogen from a-SixNy@B:H networks

Devolopment of thin film temperature sensors for high performance turbo jet engines

The introduction of ceramic thermal barrier coatings, then monolithic ceramic blades and finally blades made from ceramic-metal composites are expected to have dramatic effects on the operating temperatures of gas turbines. Successful functioning of these components in the hostile environment of a gas turbine depends partly on the accurate measurement of their surface temperature, and for this, the development of appropriate technology is needed. This paper deals with the two phase research program started by us at UCLA on the development of Pt/Pt+Rh thin film thermocouples on nickel based superalloys and monolithic ceramic type blades and vanes. In the case of super-alloy substrates, the thermocouple consists of 10 micron thick Pt and Pt + 10% Rh thin film elements deposited on the insulating surface of the blades and vanes. The insulator is developed by a combination of treatments given to the coated blade. Thermocouples, stable up to 50 hrs. in an oxyacetylene flame (1050°C) with less than 5 percent drift in temperature, have been fabricated. Pt and Pt + Rh thin films are currently being deposited on sintered SiC and Si<SUB>3</SUB>N<SUB>4</SUB> substrates. Some results of thermo-emf measurements for thermocouples deposited on Si<SUB>3</SUB>N<SUB>4</SUB> blades are also reported here, especially in regard to emf-drift due to Pt-Si interdiffusion and oxidation of rhodium