A grain-size-dependent reduction in the room-temperature thermal conductivity of nanocrystalline yttria-stabilized zirconia is reported for the first time. Films were grown by metal-organic chemical vapor deposition with controlled grain sizes from 10 to 100 nm. For grain sizes smaller than approximately 30 nm, a substantial reduction in thermal conductivity was observed, reaching a value of less than one-third the bulk value at the smallest grain sizes measured. The observed behavior is consistent with expectations based on an estimation of the phonon mean-free path in zirconia. © 2000 American Institute of Physics. ͓S0003-6951͑00͒05034-8͔The efficiency of gas turbine engines is dictated by the maximum sustained operating temperature of their typically Ni-or Co-based alloy turbine rotors. The development of new, higher temperature, high-strength, lightweight alloys is desirable. 1 However, recent studies have concluded that significant near-term progress in increasing turbine engine operating temperatures is more likely to come from the development of improved thermal barrier coatings ͑TBCs͒, typically yttria-stabilized zirconia ͑YSZ͒, than from the design of new alloys. 2 New processing techniques that result in TBC microstructures with lower thermal conductivity could lead either to higher operating temperatures of turbine engines, resulting in greater efficiency, or thinner coatings for the same operating temperature, which would reduce overall weight. Nanocrystalline YSZ coatings are of interest because they offer the possibility of lowering thermal conductivity, and may also provide additional benefits for TBC applications because of the possibility of improved toughness and ductility compared to that of coarser-grained ceramics. 3,4 The low thermal conductivity of YSZ ͑ϳ2.3 W/mK for high-density, polycrystalline material with a yttria-content of 10 mol. % at 20°C 5 ͒ is due primarily to phonon scattering by vacancies on the material's highly defective oxygen sublattice. 6 The potential for reduced thermal conductivity in nanocrystalline coatings arises from the predicted enhanced phonon scattering due to the presence of numerous closely spaced grain boundaries. For example, Klemens and Gell 6 have theoretically predicted that the room temperature thermal conductivity of 10 nm grain-sized YSZ containing 7 wt. % Y 2 O 3 will be decreased more than 50% compared to 1 m grain-sized YSZ of the same composition. The goal of the present study was to experimentally determine the effect of grain size on the room-temperature thermal conductivity of YSZ, thus contributing to the fundamental understanding of grain-size-dependent phonon scattering processes.Nanocrystalline YSZ films were grown by metal-organic chemical vapor deposition ͑MOCVD͒ using a low-pressure, horizontal, cold-walled deposition system. Yttrium b-diketonate ͓Y͑thd͒ 3 ͔ and zirconium t-butoxide ͓ZrOC͑CH 3 ͒ 4 ͔ 7 were chosen as precursor materials. Highpurity nitrogen was used as the precursor carrier gas. The precursors were mixed with high-purity ...

by the University of Chmago as Operatoro Argonne National Laboratory ("+lrgonneũ nder Contract No. W-31-109-ENG-38 wit Government' retains for itsel~" and other ! acting on its behalf, a paid-up, no exclusive, irrevocable worldwide duce, prepare derivative works, distribute copie to the public, and perform publicly and display pubhcly, by or on behalf of th Government.

by the University of Chmago as Operatoro Argonne National Laboratory ("+lrgonneũ nder Contract No. W-31-109-ENG-38 wit Government' retains for itsel~" and other ! acting on its behalf, a paid-up, no exclusive, irrevocable worldwide duce, prepare derivative works, distribute copie to the public, and perform publicly and display pubhcly, by or on behalf of th Government.

The rubrnitted rnanuzcript has &en authored bv a contractor of the U. S MITIGATION OF SUB-SURFACE CRACK PROPAGATION IN RAILROAD RAILS BY LASER SURFACE MODIFICATION AbstractWe address the mitigation of sub-surface crack propagation in railroad rails via laser surface modification. The goal is to reduce the shear forces from rail-wheel friction, which contribute significantly to the nucleation and propagation of cracks in the sub-surface region at rail gage comers. Microhardness scans and tensile tests were performed on samples from cross-sections of unused and heavily used rail heads. The results of these tests indicate that the severe cyclic plastic deformation that occurs at the gage corners, during service, significantly hardens the sub-surface region there, which leads to cracking. Laser glazing, the rapid melting and rapid solidification of a thin surface layer, was used to reduce the friction coefficient of rail steel. The advantages of this process are that specific regions of the rail surface can be targeted; the treatment does not wash away as the currently used liquid lubricants do; it is more environmentally sound than liquid lubricants;and it can beapplied in service, during re-work or during rail fabrication.' Laser processing was conducted at Argonne National Laboratory's Laser Applications Laborbtury, using a 1.6 kW Nd:YAG laser. Preliminary glazing treatments, involving a single pass of the laser beam over the surface of a rail head, produced a thin ( 4 0 0 pm) glazed surface layer, intimately bonded to a martensitic heat-affected-zone that is, itself, well bonded to the pearlitic rail-steel substrate. The glazed layer possesses an unfamiliar microstructure. The microhardness (Vickers) of the glazed layer is Hv800, while that of the heat-affected-zone and substrate are Hv840 and Hv300, respectively. A number of laser treatments were conducted on AIS1 1080 steel plates, similar to rail steel, from which friction samples were extracted. Treatments involving a single pass of the laser beam produced a similar glazed layer (Hv689) intimately bonded to an untempered martensite heataffected-zone (Hv1072), which is well-bonded to the substrate (Hv300). Treatments involving multiple passes of the laser beam, overlapping each other but rastered to produce a wider glazed region on the steel plates, produced a similar glazed layer (Hv655) bonded to a tempered martensite heat-affected-zone (H,470). Tempering occurs because each subsequent laser pass tempers the martensite produced by the previous one. Static "block-on-ring" friction experiments performed at Falex Corporation on a variety of laser treated surfaces, involving both single and multiple pass treatments, showed reductions in the friction coefficient by about 25% relative to untreated surfices at loads corresponding to prototypic rail service loads. X-ray scans of treated surfaces were inconclusive regarding the nature of the glazed layer. We laser-glazed two areas on the top surface of a six-foot len,oth of rail with multiple pass treatments, one w...

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