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 acou stic microscope and two techniques based on resonant frequency to meas ure the elastic moduli of oxide systems. Measured values for bulk oxid es, namely Al2O3 and Cr2O3, have been used to compare the various meas urement systems. The comparison is then extended to measurements on ox ide scales. In general, hardness values measured using the MPM techniq ue agree with reported bulk values, although differences between labor atories have been identified which may be attributable to the position of indentation within the scales. Hardness values for scales are foun d 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 modul i for recrystallized Al2O3 have been measured using the mechanical pro perties microprobe, acoustic microscopy and resonance methods. Data de termined using the MPM technique give the highest values, up to 30% hi gher than values determined by acoustic microscopy or resonance method s. The last two methods agree well with published data. For chromia, Y oung's moduli measured using MPM techniques agree well with published data. For oxide scales there is good agreement between the MPM techniq ue and resonance techniques where measurements can be compared. For ba se 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 o xide. For alloy systems developing Cr2O3 scales, elastic moduli as det ermined by the MPM are in the range 327-202 GPa. Data measured using r esonance methods either fall into this range or are substantially high er. For alloys that develop a substantial internal oxide network, the values measured using resonance methods may well be double or triple t hose measured within the outer scale by the MPM technique. This is bel ieved to be due to surface interaction effects, possibly the added sti ffness provided by an internal oxide network. The resonance techniques are currently the only methods by which the change in modulus with te mperature can be investigated.