The result indicates that the UCS p

The result indicates that the UCS physical conditions during Stage II, in the case of pores not completely filled with water, are not consistent with the presence of an interparticle liquid film on the nanometric scale that would be thick enough to allow dissolution and ion solvatation. It seems therefore that UCS cannot be driven by a simple LPS, when the porosity is only partially filled with water. Nevertheless, deq dif is derived from a classical theory of the lubricant, and somehow loses its physical meaning at the atomic/molecular level, thus does not exclude the presence of water molecules adsorbed on the ceramic surface. In other words, especially in ionic compounds which are strongly hydrophilic, water molecules might remain attached within the grain boundaries and form a sort of grain boundary saturated brine, which has been reported in creep geological studies on bulk NaCl samples [44–46]. This brine can still cause a sort of LPS, but if this were the case LPS would take place through an interparticle liquid film with a thickness of the same order of magnitude as the water molecule, thus its properties (viscosity, ions solubility and diffusivities) are expected to largely differ from those measured under standard conditions. One should however be very careful when comparing creep studies on bulk poly-crystals [44–46] and cold sintering; as amatter of fact UCS usually employs powders (10−7~10−5 m) and pressures of the order of 102 MPa (nominal pressure). On the other hand, creep studies are carried out on bulk samples (10−2 m) and at a few MPa. If we consider Eq. (15), the squeezing velocity is proportional to the pressure and inversely proportional to the square of the approaching regions radius, therefore the squeezing speed is expected to be several orders of magnitude larger in UCS than in any creep tests.