AbstractThermally formed oxides scales can protect metals from aggressive environments at high temperatures. However the barrier function is destroyed when the oxide fails. Therefore a new 4-point bend test technique has been developed to measure the failure strains and to study the failure mechanisms of brittle layers in tension and compression. Tests were made with iron oxide and nickel oxide at room temperature and 550 or 900 °C, respectively, using strain rates of 10~* and 10"5 s"1 . Brittle lacquer was used as a model layer. Acoustic emission (AE) was employed to monitor and interpret failure mechanisms together with post test metallography.
Equi-distant cracks were formed during failure in tension. Further cracking was affected by elastic and plastic stress relaxation processes, and interface delamination only started after these processes were exhausted. The crack spacing increased with oxide thickness and the results indicated that plastic relaxation processes were dominant at growth temperature conditions. The shear strength of the interface was lower at growth temperature.
Shear failure within the layer was found in NiO and brittle lacquer when tested in compression. Failure in iron oxide under compression always started at the interface. The failure mechanism and initiation in compression was determined by the relative shear strength of interface, the shear strength of the layer and the buckling stability of the layer. However, spallation always required crack growth at the interface.
Measured failure strains in tension and compression agreed well with the predictions of a model incorporating the fracture mechanics condition for tensile cracking or interface crack growth respectively and factors like residual strains, oxide creep and lateral oxide growth which accounted for the behaviour of a thin growing scale on a thick substrate. The critical fracture mechanics parameter in tension was the composite void size.
A K1C value of ~ 1.1 MN nv3/2 was obtained for iron oxide for room temperature and 550 °C. Values of 0.41 and -1.61 MN m3/2 were found for NiO at room temperature and at 900 °C, respectively. The residual growth stresses in iron oxide were determined as approximately zero and the cooling strain from 550 °C was -0.05 - 0.06%. The residual stresses in NiO were -175 MPa at room temperature. The strain energy release rate for interfacial failure in iron oxide was 27 J m-2 and the fracture surface energies were 3.4 and 0.8 J m-2 for iron and nickel oxide, respectively.
AE was a useful tool for explaining the failure mechanisms and a numerical analysis showed a slight difference in the AE signal released during tensile and compressive failure.
|Date of Award||Sep 1992|