Maurice Gell

Ceramic materials for thermal barrier coatings

A method for identifying ceramics suitable for use as thermal barrier coatings is presented, based on parameters associated with thermal conductivity, oxygen diffusivity, thermal expansion coefficient, maximum temperature capability, hardness, elastic modulus, density, and chemical reactivity. A ceramic thermal barrier coating and method of manufacture is further presented, the ceramic comprising yttrium aluminum garnet (Y3 Al5 O12, or YAG)-based ceramics. Such ceramics are based on yttrium aluminum garnet or other ceramics with the garnet structure and alloys thereof. The ceramics in accordance with the present invention have low thermal conductivity, and are more potentially durable than prior art zirconia based ceramics.

The dependency of microstructure and properties of nanostructured coatings on plasma spray conditions

Abstract In this paper, Al2O3-13 wt.% TiO2 coatings formed via a plasma spray approach using reconstituted nanosized Al2O3 and TiO2 powder feeds are described. Effects of various plasma spray conditions on the microstructure, grain size, phase content and microhardness of the coatings have been evaluated. It is found that phase transformation of nanosized Al2O3 and TiO2 during heat treating, sintering and thermal spraying is, in general, identical to that of micrometer-sized counterparts. Furthermore, the particle temperature during thermal spray could be divided into three regimes, i.e. low, intermediate and high temperature regimes, according to the characteristics of the coating produced from the nanopowder. The hardness and density of the coating increase with the spray temperature. The phase content and grain size of the coating also exhibits a strong dependency on the spray temperature. The coating sprayed using nanopowder feed displays a better wear resistance than the counterpart sprayed using commercial coarse-grained powder feed. The observed phenomena are discussed in terms of physics of thermal spraying, mechanisms of coating growth and phase transformation of the oxides.

Development and implementation of plasma sprayed nanostructured ceramic coatings

Abstract A broad overview of the science and technology leading to the development and implementation of the first plasma sprayed nanostructured coating is described in this paper. Nanostructured alumina and titania powders were blended and reconstituted to a sprayable size. Thermal spray process diagnostics, modeling and Taguchi design of experiments were used to define the optimum plasma spray conditions to produce nanostructured alumina–titania coatings. It was found that the microstructure and properties of these coatings could be related to a critical process spray parameter (CPSP), defined as the gun power divided by the primary gas flow rate. Optimum properties were determined at intermediate values of CPSP. These conditions produce limited melting of the powder and retained nanostructure in the coatings. A broad range of mechanical properties of the nanostructured alumina–titania coatings was evaluated and compared to the Metco 130 commercial baseline. It was found that the nanostructured alumina–titania coatings exhibited superior wear resistance, adhesion, toughness and spallation resistance. The technology for plasma spraying these nanostructured coatings was transferred to the US Navy and one of their approved coating suppliers. They confirmed the superior properties of the nanostructured alumina–titania coatings and qualified them for use in a number of shipboard and submarine applications.

Failure modes in plasma-sprayed thermal barrier coatings

Commercial plasma-sprayed thermal barrier coatings (TBCs) were investigated in an effort to elucidate the failure modes during thermal-cycling. Residual stresses in the thermally grown oxide (TGO) was measured using the Cr3+ photoluminescence piezo-spectroscopy (PLPS) method and the microstructures of the TBCs were characterized as a function of thermal cycles. The average residual stress in the TGO was found to be of the order of 1 GPa. The average thermal-cyclic life of the TBCs was found to be ∼350 cycles. Microstructural observations revealed that as the TGO thickened, cracking occurred at the bond-coat/TGO interface, and in some instances cracking also occurred at the TGO/top-coat interface, but primarily at crests of bond-coat undulations. The bond-coat-TGO separation resulted in ‘layering’ of the TGO at crests due to enhanced TGO thickening in those regions. In the troughs of bond-coat undulations, cracking occurred within the top-coat when the TGO was thick. Thus, the primary failure modes in these TBCs were: (i) cracking of the bond-coat/TGO interface; (ii) cracking within the top-coat; and (iii) linking of these microcracks by fracture of the TGO. A semi-quantitative failure model has been used to rationalize some of the observed cracking modes. Based on this analysis some suggestions are made for improving TBC durability.

Fabrication and evaluation of plasma sprayed nanostructured alumina-titania coatings with superior properties

Reconstituted nanostructured powders were plasma sprayed using various processing conditions to produce nanostructured alumina‐titania coatings. Properties of the nanostructured coatings were related to processing conditions through a critical plasma spray parameter (CPSP) that in turn, can be related to the amount of unmelted powder incorporated into the final coating. Those coatings that retain a significant amount of unmelted powder show optimum microstructure and properties. Selected physical and mechanical properties were evaluated by X-ray diffraction (XRD), optical and electron microscopy, quantitative image analysis and mechanical testing. Constituent phases and the microstructure of the reconstituted particles and plasma sprayed coatings were examined with the aid of quantitative image analysis as a function of processing conditions. Mechanical properties including hardness, indentation crack growth resistance, adhesion strength, spallation resistance during bend- and cup-tests, abrasive wear resistance and sliding wear resistance were also evaluated. These properties were compared with a commercial plasma sprayed alumina‐titania coating with similar composition. Superior properties were demonstrated for nanostructured alumina‐titania coatings plasma sprayed at optimum processing conditions.

TOWARDS DURABLE THERMAL BARRIER COATINGS WITH NOVEL MICROSTRUCTURES DEPOSITED BY SOLUTION- PRECURSOR PLASMA SPRAY

The feasibility of a new processing method—solution precursor plasma spray (SPPS)—for the deposition of ZrO2-based thermal barrier coatings (TBCs) with novel structures has been demonstrated. These desirable structures in the new TBCs appear to be responsible for their improved thermal cycling life relative to conventional plasma-sprayed TBCs. Preliminary results from experiments aimed at understanding the SPPS deposition mechanisms suggest that nanometer-scale particles form in the plasma flame, followed by their deposition by sintering onto the substrate in the intense heat of the plasma flame. The SPPS method, which offers some unique advantages over the conventional plasma-spray process, is generic in nature and can be potentially used to deposit a wide variety of ceramic coatings for diverse applications.

Microstructure development of Al2O3-13wt.%TiO2 plasma sprayed coatings derived from nanocrystalline powders

The development of constituent phases and microstructure in plasma sprayed Al2O3–13wt.%TiO2 coatings and reconstituted nanocrystalline feed powder was investigated as a function of processing conditions. The microstructure of the coatings was found to consist of two distinct regions; one of the regions was completely melted and quenched as splats, and the other was incompletely melted with a particulate microstructure retained from the starting agglomerates. The melted region predominantly consisted of nanometer-sized γ-Al2O3 with dissolved Ti4+, whereas the partially melted region was primarily submicrometer-sized α-Al2O3 with small amounts of γ-Al2O3 with dissolved Ti4+. The ratio of the splat microstructure to the particulate microstructure and thus the ratio of the γ-Al2O3 to α-Al2O3 can be controlled by a plasma spray parameter, defined as the critical plasma spray parameter (CPSP). This bimodal distribution of microstructure and grain size is expected to have favorable impact on mechanical properties of nanostructured coatings, as has been observed before.

Thermal conductivity of thermal barrier coatings

Abstract In thermal barrier coatings and other ceramic oxides, heat is conducted by lattice waves, and also by a radiative component which becomes significant at high temperatures. The theory of heat conduction by lattice waves is reviewed in the equipartition limit (above room temperature). The conductivity is composed of contributions from a spectrum of waves, determined by the frequency dependent attenuation length. Interaction between lattice waves (intrinsic processes), scattering by atomic scale point defects and scattering by extended imperfections such as grain boundaries, each limit the attenuation length in different parts of the spectrum. Intrinsic processes yield a spectral conductivity which is independent of frequency. Point defects reduce the contribution of the high frequency spectrum, grain boundaries and other extended defects that of the low frequencies. These reductions are usually independent of each other. Estimates will be given for zirconia containing 7wt% Y 2 O 3 , and for yttrium aluminum garnet. They will be compared to measurements. The effects of grain size, cracks and porosity will be discussed both for the lattice and the radiative components. While the lattice component of the thermal conductivity is reduced substantially by decreasing the grain size to nanometers, the radiative component requires pores or other inclusions of micrometer scale.

Application opportunities for nanostructured materials and coatings

Abstract Nanostructured materials have the potential to change materials science as we know it today significantly, as well as to provide a new generation of materials with a quantum improvement in properties. While many interesting properties have been generated in the laboratory, there is still much work to be done before there are production applications for nanostructured materials and coatings in gas turbine engines and similar demanding strength- and temperature-limited applications. This paper (1) describes the need for improved materials in gas turbine engines, (2) summarizes the improved physical and mechanical properties that have been reported for nanostructured materials, (3) discusses a research and development methodology that has the potential for accelerating technology implementation, and (4) describes high pay-off applications.

Thermal/residual stress in an electron beam physical vapor deposited thermal barrier coating system

Abstract Elastic–plastic finite element models are used to define the thermal/residual stress state responsible for the observed failure behavior of an electron beam physical vapor deposited yttria stabilized zirconia thermal barrier coating on a Pt–Al bond coat. The failures were observed to start at grain boundary ridges, some of which evolved into oxide filled cavities. Finite element models are made of the actual interface geometries through the use of metallographic sectioning and image processing. There is a one to one correspondence of calculated tension in the oxide layer and the observed localized damage. Purely elastic analysis failed to show some important tensile regions associated with the observed failure.