HARDNESS AND MODULUS MEASUREMENTS ON OXIDE SCALES

This paper provides a comparison between hardness and elastic modulus data measured using a mechanical properties microprobe (MPM), the acoustic microscope and two techniques based on resonant frequency to measure the elastic moduli of oxide systems. Measured values for bulk oxides, namely Al2O3 and Cr2O3, have been used to compare the various measurement systems. The comparison is then extended to measurements on oxide scales. In general, hardness values measured using the MPM technique agree with reported bulk values, although differences between laboratories have been identified which may be attributable to the position of indentation within the scales. Hardness values for scales are found to be similar to hardness values for the bulk, lying in the range 21-30 GPa for Al2O3 scales and 18-33 GPa for Cr2O3 scales. Young's moduli for recrystallized Al2O3 have been measured using the mechanical properties microprobe, acoustic microscopy and resonance methods. Data determined using the MPM technique give the highest values, up to 30% higher than values determined by acoustic microscopy or resonance methods. The last two methods agree well with published data. For chromia, Young's moduli measured using MPM techniques agree well with published data. For oxide scales there is good agreement between the MPM technique and resonance techniques where measurements can be compared. For base metal oxides, elastic moduli data are in the range 151-192 GPa for iron oxides, 205-315 GPa for nickel oxide and 116-163 GPa for cobalt oxide. For alloy systems developing Cr2O3 scales, elastic moduli as determined by the MPM are in the range 327-202 GPa. Data measured using resonance methods either fall into this range or are substantially higher. For alloys that develop a substantial internal oxide network, the values measured using resonance methods may well be double or triple those measured within the outer scale by the MPM technique. This is believed to be due to surface interaction effects, possibly the added stiffness provided by an internal oxide network. The resonance techniques are currently the only methods by which the change in modulus with temperature can be investigated.