Cyclic Oxidation Testing and Modelling: A NASA Lewis Perspective
The Materials Division of the NASA Lewis Research Center has been heavily involved in the cyclic oxidation of high temperature materials for 30 years. Cyclic furnace and burner rig apparati have been developed, refined, and replicated to provide a large scale facility capable of evaluating many materials by a standard technique. Material behavior is characterized by weight change data obtained throughout the test, which has been modelled in a step-wise process of scale growth and spallation. This model and a coupled diffusion model have successfully described cyclic behavior for a number of systems and have provided insights regarding life prediction and variations in the spalling process. Performance ranking and mechanistic studies are discussed primarily for superalloys and coating alloys. Similar cyclic oxidation studies have been performed on steels, intermetallic compounds, thermal barrier coatings, ceramics, and ceramic composites. The most common oxidation test was performed in air at temperatures ranging from 800 deg. to 1600 C, for times up to 10000 h, and for cycle durations of 0.1 to 1000 h. Less controlled, but important, test parameters are the cooling temperature and humidity level. Heating and cooling rates are not likely to affect scale spallation. Broad experience has usually allowed for considerable focus and simplification of these test parameters, while still revealing the principal aspects of material behavior and performance. Extensive testing has been performed to statistically model the compositional effects of experimental alloys and to construct a comprehensive database of complex commercial alloys.
A deterministic interfacial cyclic oxidation spalling model
A series summation has been developed to model the iterative scale growth and spalling process of cyclic oxidation. Parabolic scale growth has been assumed. Interfacial spallation of a constant area fraction was stipulated to occur only at the thickest portions. Inputs are the parabolic growth rate constant, spall area fraction, oxide stoichiometry, and cycle duration. Outputs include the net weight change, amount of oxygen and metal consumed, and amount of oxide spalled. Classic weight change curves are produced with an initial maximum and final linear weight loss rate. This simplicity allowed for representation by explicit algebraic functions for all outputs and characteristic features. The maximum in weight change varies directly with the parabolic rate constant and cycle duration and inversely with the spall fraction, all to the 1/2 power. The ratio of the number of cycles to reach maximum and zero weight change is exactly 1:3, and these vary only with the inverse of the spall fraction. Many similarities to and some differences with previous cyclic models are identified.
Static and dynamic cyclic oxidation of 12 nickel-, cobalt-, and iron-base high-temperature alloys
Twelve typical high-temperature nickel-, cobalt-, and iron-base alloys were tested by 1 hr cyclic exposures at 1038, 1093, and 1149°C and 0.05 hr exposures at 1093°C. The alloys were tested in both a dynamic burner rig at Mach 0.3 gas flow and in static air furnace for times up to 100 hr. The alloys were evaluated in terms of specific weight loss as a function of time, and X-ray diffraction analysis and metallographic examination of the posttest specimens. A method previously developed was used to estimate specific metal weight loss from the specific weight change of the sample. The alloys were then ranked on this basis. In general the burner-rig test was more severe than a comparable furnace test and resulted in an increased tendency for oxide spalling due to volatility of Cr in the protective scale and the more draştic cooling due to the air-blast quench of the samples. Increased cycle frequency also increased the tendency to spall for a given test exposure. The behavior of the alloys in both types of tests was related to their composition, particularly their Cr and Al contents and their tendency to form four types of scales: NiO or CoO, Cr2O3-chromite spinel, α-Al2O3-aluminate spinel, or ThO2-blocked Cr2O3. The alloys with the best overall behavior formed α-Al2O3-aluminate spinels.