Eric H. Jordan

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/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.

Mechanisms of ceramic coating deposition in solution-precursor plasma spray

The solution-precursor plasma spray (SPPS) method is a new process for depositing thick ceramic coatings, where solution feedstock (liquid) is injected into a plasma. This versatile method has several advantages over the conventional plasma spray method, and it can be used to deposit nanostructured, porous coatings of a wide variety of oxide and non-oxide ceramics for a myriad of possible applications. In an effort to understand the SPPS deposition process, key diagnostic and characterization experiments were performed on SPPS coatings in the Y 2 O 3 -stabilized ZrO 2 (YSZ) system. The results from these experiments show that there are multiple pathways to SPPS coating formation. The atomized precursor droplets undergo rapid evaporation and breakup in the plasma. This is followed by precipitation, gelation, pyrolysis, and sintering. The different types of particles reach the substrate and are bonded to the substrate or the coating by sintering in the heat of the plasma. The precursor also reaches the substrate or the coating. This precursor pyrolyzes in situ on the substrate, either after it reaches a “cold” substrate or upon contact on a “hot” substrate and helps bond the particles. The coating microstructure evolves during SPPS deposition as the coating temperature reaches approximately 770 °C.

Thermal cycling of EB-PVD/MCrAlY thermal barrier coatings: I. Microstructural development and spallation mechanisms

Microstructural changes, damage initiation and spallation of a production TBC, which consists of: an electron beam physical vapor deposited ZrO2–7 wt.% Y2O3 (YSZ); thermally grown oxide (TGO); MCrAlY bond coat; and a polycrystalline IN-738 superalloy substrate, were examined as a function of thermal cycles at 1121°C. Thermal cycling for TBC specimens in air consisted of a 10-min heat-up, a 40-min hold at 1121°C and a 10-min quench. Microstructural characterization was carried out by: photo-stimulated luminescence piezo-spectroscopy (PLPS); X-ray diffraction (XRD); scanning electron microscopy (SEM); and energy dispersive spectroscopy (EDS). Development of microstructure in the YSZ coatings, growth of the thermally grown oxide (TGO) and its constituents, and phase transformations in the MCrAlY bond coat were examined as a function of thermal cycles. Instability of the TGO/bond coat interface (i.e. rumpling), leading to localized cracking at the YSZ/TGO interface and within the YSZ, was observed after as few as five thermal cycles. Spallation of YSZ coatings occurred after approximately 400 cycles. Significant void formation at the TGO/bond coat interface and formation of a Ni/Co rich oxides at the TGO/bond coat interface due to rapid internal oxidation of Al-depleted MCrAlY bond coat were strong contributors to spallation failure of the TBC. The final spallation of TBC results from a ‘link-up’ of damage at the TGO/bond coat interface with the rumpling-induced micro-cracking at the YSZ/TGO interface.

Application of Cr3+ photoluminescence piezo-spectroscopy to plasma-sprayed thermal barrier coatings for residual stress measurement

Abstract Cr 3+ photoluminescence piezo-spectroscopy (CPLPS) is being developed as a non-destructive inspection technique for the measurement of residual stresses within the thermally grown oxide (TGO; consisting of α-Al 2 O 3 with Cr 3+ solute) layer buried under Y 2 O 3 -stabilized ZrO 2 (YSZ) thermal barrier coatings (TBCs). In this study, CPLPS experiments were performed to measure residual stresses in TGOs buried under four different types of plasma-sprayed TBCs, as a function of TBC thickness (from 0 to full-thickness of 250 μm) using ‘taper-polishing’. In one type of TBC, the CPLPS technique could be used to measure TGO residual stresses, but was limited to a TBC thickness of less than 170 μm due to severe attenuation of the Cr 3+ photoluminescence signal through the YSZ. In the other three types of TBCs, that thickness was limited to about 50 μm. However, non-TGO Cr 3+ photoluminescence signals were obtained through thicker TBCs of these latter types. To identify the source of this non-TGO signal, chemical analyses and CPLPS of as-received and heat-treated plasma-spray feedstock powders were performed. It was found that these powders contained both Al and Cr, which upon heat-treatment created conditions for Cr 3+ photoluminescence. To identify the sources of attenuation of the Cr 3+ photoluminescence signal intensity in all types of TBCs, CPLPS was performed on a set of ‘model coatings’. These ‘model coatings’ consisted of monolithic ceramics, where a polycrystalline α-Al 2 O 3 slab was placed underneath a variety of thin YSZ plates containing varying amounts of porosities, pore sizes, Y 2 O 3 contents, and grain boundaries. It was found that porosity, grain boundaries, and most importantly splat boundaries, were the key factors that obstructed the observation of CPLPS from α-Al 2 O 3 through YSZ. In order to alleviate this signal attenuation by pores and microcracks, plasma-sprayed TBCs were vacuum-impregnated with high-refractive-index materials (mineral oil or Stycast™ epoxy), allowing us to measure, for the first time, TGO residual stresses through full-thickness plasma-sprayed TBCs using CPLPS.

Stress variation with thermal cycling in the thermally grown oxide of an EB-PVD thermal barrier coating

Abstract A measurement approach for detecting thermal barrier coating (TBC) damage and assessing remaining life is described in this study. This is based on an experimental study of the oxide stress evolution as a function of thermal cycling. Residual stress in the thermally grown oxide (TGO) has been measured using photo-stimulated luminescence piezo spectroscopy (PLPS) for electron beam physical vapor deposited TBCs that were thermally cycled at 1100, 1121 and 1151 °C using 1-h and 24-h cycling periods. The observed monotonic change in the oxide stress level with life fraction is systematic with cycle time and nearly independent of exposure temperatures. The temperature independence of the measured stress with life fraction greatly simplifies the use of this technology for non-destructive evaluation of this TBC coating.