Thermal Expansion Mismatch Stresses Research Articles

The oxide pegging spalling mechanism and spalling modes of ZrO2 8 wt% Y2O3/Ni-22Cr-10Al-1Y thermal barrier coatings under various operating conditions

Mar-M247 superalloy coated with ZrO2 8 wt% Y2O3/Ni-22Cr-10Al-1Y duplex thermal barrier coatings were tested under various operating conditions within the temperature range from 1000 to 1150°C. The spalling modes of the top coat were classified by fractographic observations on all spalled specimens. The postulated degradation mechanism is oxidation of the bond coat, coupled with unrelieved coefficient of thermal expansion mismatch stress caused by the pegging effect of protruded bond-coat out-grown oxides growing into radial cracks of the top coat.

Evaluation of thermal shock fracture of plasma-sprayed thermal barrier coatings by means of a laser technique.

This paper presents some results of laser heating thermal shock tests performed on plasma-sprayed thermal barrier coatings. The coatings tested were single-layer ZrO2 (stabilized with 8% Y2O3), and functionally gradient materials (FGMs) which consisted of multi-layers with varying mixture ratios of ZrO2 and Ni-based superalloy (NiCrAlY). FGMs have recently been proposed as coating materials which aim at mitigating thermally induced stresses in the coating. The substrate was a stainless steel (SUS304), and its thickness was 3.4mm. A CO2 laser (maximum output=50W) was used for the thermal shock tests, where a laser beam with a preset spot size, duration and intensity was irradiated onto the coating surfaces. Concurrently with the laser experiments, acoustic-emission (AE) monitoring was carried out to detect the onset of the thermal shock fracture. In this investigation, critical laser power density at the onset of coating failure was defined in order to characterize the thermal shock resistance of the coatings. It was shown that the critical power density of the FGMs is significantly higher than that of the single-layer coating, indicating the effectiveness of the FGMs to reduce the thermal expansion mismatch stress. The single-layer coating was also subjected to thermal cycling using the laser method, either in the as-sprayed condition or after preoxidation treatment (1000°C, 10days in air). Two different fracture patterns were identified which led to coating spallation or exposure of the substrate; one is the extensive coalescence of vertical cracks, and the other the delamination growth. In paticular, the oxidation at the ceramicmetal interface was shown to control the thermal cycling life of the ceramic coating.

傾斜機能材料の耐熱衝撃特性評価に関する研究

This paper presents some results of laser heating thermal shock tests performed on plasmasprayed thermal barrier coatings. The coatings tested were single-layer ZrO2 (stabilized with 8% Y2O3), and Functionally Gradient Materials (FGMs) which consisted of multi-layer with varying mixture ratios of ZrO2 and Ni-based superalloy (NiCrAlY). FGMs are recently proposed coating materials which are aimed at mitigating thermally induced stresses in the coating. A CO2 laser was used for the thermal shock tests, where a laser beam with a preset spot size, duration and intensity was irradiated onto the coating surfaces. Concurrently with the laser experiments acoustic-emission (AE) monitoring was made to defect the onset of the thermal shock fracture. In order to evaluate the stress state of coatings induced by the laser heating and to discuss the fracture process, numerical computations of transient thermal stresses was made using the finite element method. Based on the results, critical laser power density at the onset of coating failure was defined in order to characterize the thermal shock resistance of the coatings. It was shown that the critical power density of the FGMs is significantly higher than that of the single-layer coating, indicating the effectiveness of the FGMs to reduce the thermal expansion mismatch stress.

Role of interfacial grain-bridging sliding friction in the crack-resistance and strength properties of nontransforming ceramics

A grain-bridging model of crack-resistance or toughness (R-curve, or T-curve) properties of nontransforming ceramics is developed. A key new feature of the fracture mechanics treatment is the inclusion of internal residual (thermal expansion mismatch) stresses in the constitutive stress-separation relation for pullout of interlocking grains from an embedding matrix. These internal stresses play a controlling role in the toughness properties by determining the scale of frictional tractions at the sliding grain-matrix interface. By providing a physical account of the underlying micromechanics of the bridging process the analysis allows for predetermination of the material factors in the constitutive relation, thereby reducing parametric adjustments necessary in fitting the theoretical toughness curve to experimental data. The applicability of the model is illustrated in a case study on indentation-strength data for a “reference” polycrystalline alumina with particularly strong T-curve characteristics. From theoretical fits to these data the constitutive relation, and thence the entire T-curve, can be deconvolved. This “parametric calibration”, apart from demonstrating the plausibility of the model, allows for quantitative predictions as to how the toughness and strength characteristics of ceramics depend on such microstructural variables as grain size and shape, grain boundary energy, level of internal stress and sliding friction coefficient. An indication of this predictive capacity is provided by a preliminary calculation of the grain-size dependence of strength, using some existing data for other aluminas as a basis for comparison.

Grain‐Size and R‐Curve Effects in the Abrasive Wear of Alumina

Results of sliding wear tests on three alumina ceramics with different grain sizes are discussed in the light of crackresistance (R‐curve, or T‐curve) characteristics. The degree of wear increases abruptly after a critical sliding period, reflecting a transition from deformation‐controlled to fracture‐controlled sulface removal. This transition occurs at earlier sliding times for the aluminas with the coarser‐grained microstructures, indicative of an inherent size effect in the wear process. A simplistic fracture mechanics model, incorporating the role of internal thermal expansion mismatch stresses in the crack‐resistance characteristic, is developed. The results suggest an inverse relation between wear resistance and large‐crack toughness for ceramics with pronounced Rcurve behavior.