Janet M. Hampikian

Elevated Temperature Coatings: Science and Technology IV

Oxidation resistance and thermal barrier coatings for components on the hot section of gas turbine engines are desired to have lifetimes on the order of tens of thousands of hours. This presents a problem in evaluating new coatings and modifications to existing coatings tests, which completely replicate the operating conditions, could take years to complete. Therefore, a reliable accelerated testing protocol is required. In this paper efforts directed toward developing a mechanism-based protocol for evaluating the life times of oxidation resistant coatings under thermal cyclic and hot corrosion conditions and thermal barrier coatings under thermal cyclic conditions is described. The cyclic lifetimes of oxidation resistant and thermal barrier coatings are determined by spalling behavior. Spallation is a function of oxide thickness and stress level, which control the elastic energy available to drive spallation, and the structures and morphologies of the various layers and interfaces in a given system, which control the fracture toughness at possible planes of weakness. Efforts to evaluate these quantities in relatively short duration tests are described. Specific techniques include acoustic emission studies, indentation techniques, and detailed metallographic observations. The extrapolation of results from high temperature tests, where failure can be achieved in relatively short times, to lower temperatures, which are characteristic of service conditions is also described. An approach to control these variables in a manner to produce accelerated failures under conditions, which allow estimation of lifetimes under typical operating conditions, are described and preliminary results are presented.

The combustion chemical vapor deposition of high temperature materials

Abstract Combustion chemical vapor deposition (combustion CVD) has been used to deposit a number of different high temperature oxide coatings, including Al 2 O 3 , Cr 2 O 3 , SiO 2 , CeO 2 , some spinel oxides (MgAl 2 O 4 , NiAl 2 O 4 ), and yttria stabilized zirconia (YSZ). The materials presented in this paper were selected because of their intrinsic merit as protective materials at elevated temperature. This paper reviews the technique of combustion CVD, and summarizes both the methods used and key results from high temperature materials which have been deposited using this technique. The ability of Al 2 O 3 , Cr 2 O 3 , and SiO 2 to provide oxidation protection to a prototypical superalloy substrate, Ni-20Cr, is reported, using thermogravimetric analysis (TGA) to quantify the alloy’s oxidation response; all three coatings reduce the oxidation kinetics of Ni-20Cr.

The effect of ion-implanted yttrium on the oxidation of nickel

Abstract Yttrium ions with 150 keV energy were implanted to their steady state concentration into nickel and the nickel was then annealed in a vacuum of 5 × 10 −7 Torr for 1 h at selected temperatures. Yttrium diffused to the surface of nickel to form randomly oriented grains of Y 2 O 3 at temperatures between 600 and 1000°C. The oxidation kinetics of nickel and yttrium-implanted nickel were measured at 990°C in air at 1 atm, and the oxidation rate was found to be less in the implanted nickel than in the unimplanted nickel. The kinetic behavior was parabolic in the latter case but non-parabolic in the former case. The surface microstructures of implanted and unimplanted specimens were studied as a function of time at 900°C in air; the yttrium-implanted specimens yielded very uniform surface microstructures which reflected a lack of an orientation relationship between the oxide and the nickel substrate. For short oxidation times (12 h and less), the oxide layer was adherent to the nickel substrate. However, after oxidation for 24 h, the oxide layer was non-adherent.

Combustion CVD of magnesium spinel and nickel spinel

Combustion chemical vapor deposition (combustion CVD) is being developed to deposit magnesium spinel, MgAl 2 O 4 , and nickel spinel, NiAl 2 O 4 , onto silica substrates. A possible use of the magnesium spinel is as an overlayer on thermal barrier coatings (TBCs) used in the hot sections of gas turbines. The deposition of nickel spinel is being developed to explore its role in the thermomechanical spallation of TBCs. Combustion CVD uses a flame to provide the heat for the chemical reactions and the medium for the deposition of the ceramic film. Combustion CVD is performed in the open atmosphere and requires relatively little capital investment compared to conventional CVD processes. Precursors of Mg naphthenate and Al acetylacetonate dissolved in toluene were used to successfully deposit MgAl 2 O 4 with the face centered cubic spinel crystal lattice. Ni acetylacetonate and Al acetylacetonate, dissolved in toluene, were used to deposit NiAl 2 O 4 , with the face centered cubic spinel crystal lattice. Total metal ion concentrations in the flammable solutions ranged from 0.002 to 0.004 M. Flame temperatures at the surface of the substrate during deposition were approximately 1100°C. Dense, nodular films up to 1 μm thick were produced at deposition rates of 1.2 to 2.0 μm/h. Transmission electron microscopy (TEM) dark-field imaging indicates that projected grain sizes range from 8 nm to 88 nm. Electron diffraction performed with TEM verifies the spinel crystal structure and quantitative EDS shows weight percents of the constituent elements close to the stoichiometric values.

Silica thin films applied to Ni-20Cr alloy via combustion chemical vapor deposition

Amorphous silica thin films (<0.5 μm) have been deposited onto Ni-20Cr via combustion chemical vapor deposition (CVD) using two different nozzle assemblies, an oscillating capillary nebulizer (OCN) and a simple nebulizing needle. The deposition conditions were similar for the two different nozzles, although slightly different coating morphologies were observed. The OCN-deposited material was deposited onto rotating samples (4 rpm), and showed a morphology striated in thickness, presumed to be a result of thermal gradients present across the rotating sample. The samples deposited with a nebulizing needle did not show these striations, but rather, showed a nodular morphology. Protection to Ni-20Cr from the flame during deposition is reported, in addition to isothermal oxidation kinetic measurements which quantify the degree of protection provided by silica. Silica reduces the oxidation rate of Ni-20Cr, with less transient oxidation of Ni reported for silica-coated samples, as well as the rate constants of Ni-20Cr being reduced at all test temperatures by more than an order of magnitude. These reductions are attributed to the deposited oxides serving as reactive elements in the chromia scale.

Electron diffraction measurements on chromium oxide

Abstract Bloch-wave analysis of partial waves formed by 300 keV electrons in chromium oxide illustrates how an absorptive potential affects convergent-beam contrast in higher-order Laue zones (HOLZs). Mean square displacements 〈u 2〉 of chromium and oxygen atoms are measured from intensities in the series of HOLZ lines containing information on excitation and absorption of each partial wave. Data obtained at 110 and 300K implies a strain contribution to 〈u 2〉 for chromium. Correlation between experiment and theory for HOLZ, zeroth-order Laue zone convergent-beam electron diffraction and large-angle convergent-beam electron diffraction measurements from crystals 74-230 nm thick are reported.

A PROTOTYPE CONTINUOUS FLOW POLYMERASE CHAIN REACTION LTCC DEVICE

There is a growing need for remote biological sensing in both laboratory and harsh field environments. Sensing and detection of biological entities such as anthrax, Ebola and other micro-organisms of interest involves sampling of the environment, amplification, analysis and identification of the target DNA. A key component of such a sensor is a low cost, portable, reusable, continuous flow polymerase chain reaction (PCR) thermal cycler. Fabrication with low temperature co-fired ceramics (LTCC) can provide a reusable low cost device capable of operating in a wide range of environments The design and manufacture of a prototype continuous flow micro-fluidic PCR device using low temperature co-fired ceramic is presented. Initial modeling of flow characteristics and heat transfer was carried out in SolidWorks™. The prototype device employs resistance heaters below the channels, buried and surface thermocouples for temperature monitoring, and air gaps for thermal isolation.

Improving tantalum's oxidation resistance by Al+ ion implantation

Tantalum was implanted with 180 keV Al+ ions to fluences up to 3×1018 Al+/cm2. Subsequent microchemical and microstructural observations showed that an amorphous layer covered the surface and extended to depths near 3000 Å for fluences above 2.4×1018 Al+/cm2. The layer, comprised of ∼70 at. pet Al and ∼30 at. pet Ta, crystallized at temperatures above 500°C. Oxidation measurements, performed in one atmosphere of air and at temperatures below 600°C, showed that the layer stopped oxidation of the implanted tantalum, while unimplanted tantalum oxidized rapidly. The protection provided by the implantation deteriorated somewhat by temperatures near 735°C but still reduced the oxidation rate by a factor of 5. The deterioration is caused by localized rupturing of the implanted layer and the resulting oxidation of the underlying tantalum. At 910°C, the implanted tantalum oxidized almost as rapidly as unimplanted tantalum.

Improved oxidation resistance of group VB refractory metals by Al+ ion implantation

Aluminum ion implantation of vanadium, niobium, and tantalum improved the metals’ oxidation resistances at 500 °C and 735 °C. Implanted vanadium oxidized only to one-third the extent of unimplanted vanadium when exposed at 500 °C to air. The oxidative weight gains of implanted niobium and tantalum proved negligible when measured at 500 °C and for times sufficient to fully convert the untreated metals to their pentoxides. At 735 °C, implantation of vanadium only slightly retarded its oxidation, while oxidative weight gains of niobium and tantalum were reduced by factors of 3 or more. Implanted niobium exhibited weight gain in direct proportion to oxidation time squared at 735 °C. Microstructural examination of the metals implanted with selected fluences of the 180 kV aluminum ions showed the following. The solubility limit of aluminum is extended by implantation, the body centered cubic (bcc) phases being retained to ∼60 at. pct Al in all three metals. The highest fluence investigated, 2.4 × 1022 ions/m2, produced an ∼400-nm layer of VAl3 beneath the surface of vanadium, and ∼300-nm layers of an amorphous phase containing ∼70 at. pct Al beneath the niobium and tantalum surfaces. All three metals, implanted to this fluence and annealed at 600 °C, contained tri-aluminides, intermetallic compounds known for their oxidation resistances. Specimens implanted to this fluence were thus selected for the oxidation measurements.

Thin layers near surfaces by ion implantation

Ion implantation produces thin films or layers that reside within a substrate rather than on top of it. The layers extend to the surface, differ in chemical composition and/or microstructure as compared with the substrate, and often modify the surface-dependent properties of the substrate material. Selected examples of modified properties are presented in this article and include improvements in oxidation and wear resistance from ion implantation. The shallow depths of the modified layers and the small scales of the implanted microstructures require the use of several modern analytical techniques for their characterization. Examples of such characterizations are also presented in this article. The implanted layers are examined for chemical concentrations versus depth with Auger electron spectroscopy and controlled sputtering, and with Rutherford backscattering spectrometry. Scanning electron microscopy and electron backscattering channeling analysis yield, respectively, information regarding products formed during oxidation and amorphous phase formed during implantation. Transmission electron microscopy and diffraction quantify the microstructures within the implanted layer and identify crystalline, nanocrystalline, and amorphous phases. Finally, it is shown that lattice imaging delineates atomic level defects, including stacking faults in ∼3 nm implanted rare gas precipitates, and amorphous phase formed during deceleration of the incident ions within intermetallic NiAl3 phase.