Screw element for the connection of tubes and tube connection device having this screw element

1. A union screw for the connection of tubes, for motor vehicle lines, wherein a threading and a bottom contact surface without threading are provided on the union screw, wherein the union screw comprises a through bore and a screw head, wherein the through bore extends through the screw head, wherein the screw head comprises an underside contact surface, wherein the union screw has an outer coating and an inner coating, wherein the inner coating lies between the outer coating and a metal substrate of the union screw, wherein the outer coating comprises a first polymer and nanoparticles of a first solid material, wherein the union screw has an outer surface,

wherein the outer coating covers at least 50% of the outer surface of the union screw, and wherein the friction coefficient μ 1 of the outer coating is greater than the friction coefficient μ 2 of the inner coating.

2. The union screw according to claim 1 , wherein the inner coating is disposed in a region of the bottom contact surface.

3. The union screw according to claim 1 , wherein the coating thickness of the outer coating is 0.5 to 10 μm.

4. The union screw according to claim 1 , wherein the coating thickness of the inner coating is 10 to 100 μm.

5. The union screw according to claim 1 , wherein the portion of the nanoparticles in the outer coating is 2% to 40% of the weight.

6. The union screw according to claim 1 , wherein the inner coating comprises a polymer.

7. The union screw according to claim 1 , wherein the inner coating comprises nanoparticles of a second solid material.

8. The union screw according to claim 7 , wherein the portion of nanoparticles in the inner coating is 2% to 40% of the weight.