Fortunately, a low oxygen partial pressure favors a more thermodynamically preferential oxidation of Al atoms in Ti 3AlC 2 grains ( Wang and Zhou, 2003).
Meanwhile, the preceded healing areas of the rough crack surface also will decrease the oxygen supplement for the subsequent healing of the isolated crack zones, which means the rapid channels for oxygen supplement are blocked by the preceded healing places. If the healing at the crack mouth precedes the healing of the crack inside, the supply of oxygen from the outside into the crack inside will be disturbed, which results in a low oxygen partial pressure in the crack inside. Oxygen can not be supplied sufficiently inside the crack due to the narrow zigzag crack path which is frequently bridged by the lamellar Ti 3AlC 2 grains, whereas oxygen is provided sufficiently at the sample surface, which results in a fast oxidation rate at the sample surface.
The oxygen partial pressure and the limited space in the crack gap will influence the healing rate and eventually the nature of oxidation products. Such a high volume expansion is responsible for the crack filling with oxidation products. 11.3 and the densities of these phases, which results in 50% volume expansion in solid state and mainly outward growth of the oxide scale. Thus, with the consumption of one unit volume Ti 3AlC 2, 0.28 unit volume Al 2O 3 and 1.22 unit volume TiO 2 will be created according to Eq. The mass densities of Ti 3AlC 2, TiO 2, and Al 2O 3 are 4.25, 4.27, and 3.99 g/cm 3, respectively ( Wang and Zhou, 2003 Zhou, 1995). With prolonging oxidation time, a thick TiO 2 layer finally develops as the outermost layer in the oxide scale. The subsequent TiO 2 particles will mix with partial Al 2 O 3 particles to form a (TiO 2 + Al 2O 3)-mixed layer on top of the Al 2O 3 layer. 4 T i 3 Al C 2 + 23 O 2 = 12 Ti O 2 + 2 A l 2 O 3 + 8 C O 2 g The theory of coalescence required two stages: the first was an agglomeration step which was driven by drying of the latex film, allowing the particles to be drawn together into close-packed adhesive contact, but with small adhesion because of the presence of water the second was a contact healing step in which the work of adhesion increased as the last water was removed and elastic deformation occurred with considerable shrinkage.
2.19A), together with the macroscopic polymer sphere contact diameter, and plotted them against JKR theory to show that elastic crack healing at the sphere contacts was causing the effect, taking the elastic modulus to be 5.64 MPa and the work of adhesion to be 26.5 J m −2, too high to be an equilibrium value, most probably due to the viscoelastic nature of the polymer. Kendall measured the contact spot sizes between single polymer spheres of several sizes ( Fig. 2.19B) of dried monosize rubbery polymer latex using the SEM. This crack healing mechanism was first proved in 1982 when John Padget observed the hexagonally packed structure ( Fig.