dpryl,

Thank you for the response. Mabey, you can correct me in the follwong steps:

1. Made one big block and one smaller in the middle-ouside face of the bigger block

2. Copy the smaller surface inside the bigger one with the distance 0.1mm

3. Geometry->create->contact volume => selected the two smaller surfaces => two new volumes created

4. Fixed contact  for Surface -> Master the big surface -> Slave the smaller surface inside the bigger volume 

5. Assign material gap to the new contact voumes

6. Assign material to the blocks

7. Assign constraint on the lower face of the big block (not were the contact is), so it doesn't slip

8. Assign displacement on the top face of the small block (not were the contact is), so I could see the sliding

9. Mesh and run

Thank you in forward

Well, I did as you now described earlyer, than looked at the section provided, with this:

GiD only allows [prism] contact elements between surfaces of the same size and mesh
settings. Therefore, if the two surfaces (lines) to be connected are of different sizes
(partial contact) or with differing meshes, an extra surface (line) needs to be defined of
the size of the smaller of the two, located a small distance, e.g., 0.1mm, inside the
volume the bigger surface belongs to. Please keep in mind the 3 surfaces (lines) can not
share any lines or points (points). The easiest way usually is to copy the smaller surface.
Then, create a contact volume from the two smaller surfaces and assign the desired
interface (GAP) material to it. Finally, connect the additional surface to the bigger

But, here is the how I did it before:

1. Created two blocks one bigger one smaller in the middle

2. Copyed the smaller surface on its place and checked the normals

3. Create volumes, select two small surfaces, one contact volume is made

4. Fixed contact for surface : Master the big surface, and slave the new smaller surface

5. Add Interface to the contact volume

6. Add the other materials

7. Run, and the blocks are still stuck together