Surface Charge Density is the electrical charge present on the Interface of two or more materials, organic or inorganic. There are variances of surface charge depending on whether the materials are solid, fluid, or gaseous. For example, if you mix oil and water in a jar, they will not mix and form an interface with a specific surface charge density; if you mix water and salt, the salt will break down and attach to the water, forming a different surface charge density.
Ionic Bond Strength is the measurement of the attraction between oppositely-charged metallic and non-metallic Ions. One ion loses one or more electrons, which transfer to the other ion, causing an attraction and an electrostatic bond between two differently charged ions. Salt itself is an example of an ionic bond between Sodium and Chlorine; salt in water is a different ionic bond, this time between two substances. Also, although Ionic Bonds are technically different from Covalent Bonds (sharing electrons vs. attractions between differently charged ions), all ionic bonds are covalent to some degree.
Briefly, you would use a measurement of Surface Charge Density to measure the strength of an Ionic Bond, and to figure out if two or more materials are similar or dissimilar in their individual electrical charges. Any two materials will have a surface charge at their interface, and substances have charge at the molecular or atomic level, depending on their covalency.