Plasma-sprayed ZrO2 Research Articles

Influence of transparency and grinding on the thermal diffusivity of plasma sprayed ZrO 2 with 8 wt.% Y 2O 3 coatings

Plasma-sprayed ZrO 2 with 8 wt.% Y 2O 3 coatings were found to be partly transparent to impinging radiation. The partial transparency affects the determination of thermal diffusivity by the flash method. The thermal diffusivity of the coatings decreases when their surfaces are ground. From the present work, it ensues that the thermal diffusivity of the coatings is affected by their surface properties.

Electric-field-assisted treatment of plasma-sprayed ZrO 2 coatings

A systematic experimental investigation was carried out to clarify the phenomena which occur during the electric-field-assisted treatment of ZrO 2 (8% Y 2O 3) coatings sprayed on copper and SS41 steel substrates, and the possibility of improving the adhesive strength between substrate and coating by the electric field assisted treatment was examined. It was found that the occurence of a Zr-Cu-O layer was clearly recognized at the interface between the ceramic coating and the copper substrate, which was arranged as the cathode in the experiment. It was revealed that the adhesive strength of a coating can be raised by the electric-field-assisted treatment to over three times that of a sprayed coating. The electric-field-assisted treatment was also applied to ZrO 2 sprayed onto mild steel but no improvement in adhesive strength resulted. However, when the coated steel was heated in air to form an iron oxide at the coating-substrate interface, the adhesive strength was nearly doubled by a subsequent electric-field-assisted treatment.

PLASMA SPRAYED ZrO2THICK THERMAL BARRIER COATINGS

ABSTRACT ZrO2 thermal barrier coatings have been plasma sprayed onto steel substrates with a FeCrAlY bond layer. During spraying the conditions have been varied by cooling the back side of the substrate in ice-water or heating it on a hot plate, and simultaneously cooling the sprayed surface with air at various pressures. The residual strain distributions have been measured in the as-sprayed couples using the incremental blind-hole method. The measurements show that residual strains can be controlled through temperature manipulation, and the method can be used to determine the optimum spraying conditions. The microstructure has been observed in a transmission electron microscope, and the effect of microstructural features on the microstrains are briefly discussed.

レーザー局所加熱試験による傾斜機能材料の熱衝撃破壊及び熱応力緩和特性評価に関する研究

This paper presents some results of transient thermal transfer and thermal stress analyses of thermal barrier coatings subject to rapid heating. The objective of this study is to develop a laser heating test for evaluating the thermal shock resistance of functionally gradient materials (FGMs). The numerical calculations are conducted on plasma-sprayed single-layer ZrO2 (stabilized with 8% Y2O3), and FGM which consists of multilayer with varying mixture ratios of ZrO2 and NiCrAIY deposited on a stainless steel substrate. The thermal barrier coatings with temperature dependent thermal and mechanical properties are heated by laser impingement. The transient thermal stress distribution is determined based on temperature profile using a finite element method. The intensity distribution within laser beam is taken into account. Based on the numerical results, the effect of the gradient composition in the FGM on the thermal shock resistance is examined. It is shown that the numerical analysis can provide a quantitative description of the enhanced thermal shock resistance of the FGM.

Improvement of Plasma Sprayed ZrO<sub>2</sub> Properties by Laser Processing

The surface of the Y2O3 partially stabilized ZrO2 thermal barrier coatings (TBC), which was formed about 300μm thick by plasma spraying, was densified and flattened by laser processing. The laser processed TBC properties were evaluated and compared with those before laser processing. The results were as follows. Erosion resistance was improved threefold by the laser processing. More than sevenfold improvement in the TBC thermal fatigue life was expected by the laser processing. It was confirmed that hot corrosion resistance was improved by the laser processing. But, the laser processing reduced the TBC shear bond strength by 40%. The principal reasons for the TBC property improvements were confirmed to be the densification and flattening of the TBC surface by laser processing. As for the thermal fatigue life, the effect of cracks, which were induced during laser processing, was found to be important. These cracks caused shear strength reduction, but no bad influence of the cracks was observed for other TBC properties.

Microlaminate composites as thermal barrier coatings

At present, plasma-sprayed ZrO 2 coatings are used as thermal barrier coatings. Although these coatings exhibit excellent thermal barrier properties, their brittle behavior puts significant limitations on their use. Microlaminate composites offer an excellent alternative to sprayed ZrO 2 coatings because in such composites the thermal conductivity in the plane perpendicular to the laminate is markedly low. Moreover, the thermal conductivity can be tailored by choosing appropriate metal/metal, metal/ceramic combinations, as well as the thickness of individual layers. Presently, we are exploring the Ni/NiCoCrAlY and Ti/CoCrAlY microlaminate composites as thermal barrier coatings. The method of fabrication of these laminates and the measurement technique used to determine thermal diffusivity and thermal conductivity of these coatings will be discussed.

A New Approach to Low Thermal Inertia Molding

Abstract Processing of polymers by the injection molding process has some inherent problems associated with the coupling of the filling and cooling processes. Kim and Suh [11 introduced the low thermal inertia molding (LTIM) concept to uncouple the filling process from the cooling process without appreciably increasing the total cycle time. A new injection mold was developed to control the surface temperature of the mold rapidly. The mold consisted of a woven graphite fiber fabric as a heating element, RTV silicon rubber as an adhesive, and a plasma-sprayed ZrO2 layer on the mold surface as an electrical insulation. This mold provided the means to show the potential of the low thermal inertia molding technique in terms of improved part quality, reduction in energy input due to lower requirement of injection temperature, injection pressure, packing pressure, and ease of multicavity mold balancing.

Current status of thermal barrier coatings — An overview

Abstract The science and technology of thermal barrier coatings has advanced considerably since reports of the first test on turbine blades in a research engine in 1976. Today thermal barrier coatings are flying in revenue service in a low risk location within the turbine section of certain gas turbine engines. The state-of-the-art coating system for gas turbine applications is currently a plasma-sprayed ZrO 2 -(6%–8%) Y 2 O 3 ceramic layer over an MCrAlY (M ≡ Ni, Co or NiCo) bond coat layer plasma sprayed at low pressure. Although the potential for meeting current and short-term goals is high, longer-range goals may not be attainable with current coating concepts. These longer-range goals will involve high risk designs where coating loss could lead directly to component loss. Several steps must be taken to help meet these goals. Improved understanding of coating failure mechanisms is required. Models are needed to predict lifetimes. Process automation and quality control procedures must be instituted. Finally, new concepts in plasma-sprayed coatings must be developed and alternatives to the plasma- spraying process may be required. The current status of thermal barrier coatings and prospects for future progress in the above areas are summarized.

Long-term exposure of plasma-sprayed ZrO 2 in high temperature reactor helium

Plasma-sprayed coatings will be used in future high temperature reactors (HTRs) to help overcome friction and wear problems. Extensive investigations have shown that an air-plasma-sprayed Ni-Cr-Al-Y/ZrO 2-Y 2O 3 duplex coating offers effective protection against thermal shock, non-self-welding, sliding wear, strain tolerance and short-term exposure to HTR helium. A report is given on the response of a duplex coating prepared by reduced pressure plasma spraying (composition as above) to high temperature wear testing and long-term exposure in HTR helium.

Failure during thermal cycling of plasma-sprayed thermal barrier coatings

The thermal cycling behavior of plasma-sprayed ZrO 2−12wt.%Y 2O 3 coatings was studied. Coatings were produced with and without bond coats of Ni-Cr-Al-Zr and in some cases the substrates were heated to above the optimum temperature prior to spraying. The coatings (attached to the substrate) were thermal cycled to 1200 °C and their cracking behavior was followed by acoustic emission (AE) techniques. It was possible to examine the failure mechanisms by statistical analysis of the AE data and to evaluate the influence of preheating and bond coating. It is shown that the AE spectrum changes when a bond coat is used because of the presence of microcracks which, in turn, dissipate energy and improve the coating integrity. The preheating effect is reflected by a decrease in the peak count rate and an increase in the temperature at which AE activity is initiated.

Effect of thermal cycling on ZrO 2Y 2O 3 thermal barrier coatings

A study was made of the comparative lifetimes of plasma-sprayed ZrO 2Y 2O 3 thermal barrier coatings on NiCrAlY bond coats on René 41 in short (4 min) and long (57 min) thermal cycles to 1040°C in a 0.3 Mach flame. Short cycles greatly reduced the life of the ceramic coating in terms of time at temperature as compared with longer cycles. The appearance of the failed coating indicated compressive failure. Failure occurred at the bond coat-ceramic coat junction. At heating rates greater than 550 kW m −2, the calculated coating detachment stress was in the range of literature values of coating adhesive/cohesive strength. Methods are discussed for decreasing the effect of high heating rate by avoiding compressive stress.