Ceramic material uncertainties

The interfacial resistance between the coating and the lower thermocouple plate is different from the interfacial resistance between the substrate-measurement plate due to light oxidation of the steel substrate, which is not observed for the coating. We have observed that the interfacial resistance of ceramics depends primarily on the surface finish of the ceramic material, since ceramics are chemically stable. Therefore, we use an interfacial resistance that was previously measured between Pyroceram 9606, of the same surface finish as the FGM coating, and the upper thermocouple plate as our interfacial resistance function for the coating-measurement plate interface [6]. Thermal conductivity of the FGM coating can then be extracted from the total conductance data by using these interfacial resistances and the thermal conductivity data for 410 stainless steel. The estimated uncertainty of the measurement system is 5%. 

The SRM 2100 results are reassuring in the sense that they are extraordinarily consistent and independent of test method. Claims of ceramic fracture toughness measurements that were accurate and precise to within a few percent were unheard of in the 1980 s and early 1990 s. It would seem that the experimental errors now have been wrung out of the individual test methods to the extent that the methods are reliable and the test method uncertainties are of the order of a few percent. The scatter of the SRM data base (Table 6) is smaller than the scatter in the VAMAS round robin (Table 1). This is not surprising since the round robin scatter included material variability, within-laboratory test method uncertainty, plus the between-laboratory method uncertainties. 

There are important issues that must be discussed before presenting composite properties. The traditional structural materials are primarily metal alloys for most of which there are industry and government standards. The situation is very different for composites. Most reinforcements and matrices are proprietary materials for which there are no standards. In addition, many processes are proprietary. This is similar to the current situation for most polymers and ceramics. The matter is further complicated by the fact that there are many test methods in use to measure mechanical and physical properties. As a result, there are often conflicting material property data in the usual sources used by engineers, published papers, and manufacmrers literature. The data presented in this chapter represent a carefully evaluated distillation of information from many sources. However, in view of the uncertainties discussed, the properties presented in this chapter should be considered approximate values. 

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