Stress Generation during Oxide Growth Process The various factors - - PowerPoint PPT Presentation

Stress Generation during Oxide Growth Process

The various factors responsible for the accumulation of stress

Difference in the molar volumes of metal and the oxide scale formed on it, Pilling Bedworth Ratio (PBR)

Epitaxial stresses Mode of diffusing species Geometry of the sample Composition of the scale Differences in the thermal expansion coefficients of oxide and the metal.

Stress accumulation in the growing

• xide scale with time.

Maximum oxide thickness sustained before failure on cylinders of various diameters at different exposure temperatures.

a) Shear fracture during cationic oxidation on convex surfaces, and (b) decohesion during cationic oxidation on convex surfaces.

(a) Decohesion during anionic oxidation (b) on convex surfaces, and (b) tensile cracking during anionic oxidation on convex surfaces. Oxidation of tubular specimens of tantalum at 825?C.

Measurement of Stresses

E – Young’s modulus of metal t – thickness of the strip d – thickness of oxide r – radius of curvature

Stony’S Equation

Schematic diagram of apparatus used in the bending method.

A schematic diagram of the apparatus used for bending with inert gas protection of one side.

Spiral contractometer apparatus for stress measurement.

(a) Heating scheme and various acoustic emission parameters, (b) AE counts vs time, (c) AE event vs time, and (d) voltage level vs time,

• bserved during oxidation of 21/4 Cr-1Mo steel at 900?C.

Schematic of the experimental set up for time domain parameter measurement.

Frequency spectrum of signal during (a) prebreakaway, (b) breakaway, and (c) internal cracking of oxide layer during post-oxidation cooling of 21/4 Cr-1Mo steel.