The Hammerless Nail

This is Theory, until ways are found to test it.
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Paper is held together by friction.

Thread and yarn are held together by friction.

Even The metal in swords is heated up and pressed together and the molecules mesh together and when cooled are held together with friction.

As well as heated plastic bonds work on the same principle except for chemical plastic bonds such as epoxy.

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Also it is known that there are 3-4 phases of matter. Solid, liquid, gas, plasma. But there are also intermediate forms of matter such as glass which is strong yet still a liquid. The only reason dense materials stick together is because they have less molecular movement. Even adobe which is a mixture of only sand and clay is just very small particles that make up a whole and when mixed together with water in an expanded slurry it will dry later and compress into a substance and structure almost as solid as rock without any chemical bond. Only held together by friction until water washes it away. So if you were to expand substances you can make them mesh together and when they contract they are held together by friction. Although chemical bonds may be stronger they are unneeded for glass and other strong bonds.

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So if you could expand the space between molecules by either expanding space itself or just separating the particles in another way. Then you could simply push 2 materials together and when recompressed they would be held together as if welded by only friction. Or if you could alternatively press 2 materials together from all directions at once with extreme force but in some way have no heat generated that would also work.

If you could do this without heat then you would be able to combine any 2 materials and even dissimilar materials. And you would not have to worry about deforming the material or burning from heat.

This would also be useful in the medical industry because you would not need stitches anymore but instead would just combine the skin together.

Also you could injection mold every single aspect of modern life into objects that would never break in 1000 years. From cell phones to TV's to lights. By combining materials instead of using nails or screws cars would be less likely to break. Our houses would be solid and never break if made out of the right material for insulation, sound proofing, and structural integrity, and weather proofing, all in one. Stone is such a substance but for now we only use expensive cement to combine it together.

Also aircraft and space craft could be seamlessly manufactured with no flaws and smooth surfaces with less likelihood of structural failure. But for movable parts you'd need new smart materials that bend if you wanted to still have a seamless surface.

This could be done with a gravity well or anti-gravity well in the best of circumstances. But we don't have the ability to control gravity yet. Another option is to press 2 materials together with intense pressure but also to have it be supercooled at the same time in order to eliminate the heat bi-product. Or liquefy or expand a substance in another way such as how it is done with solvent to eliminate heat. This is already done with resins like acrylic when used as paint instead of as epoxy glue.

But that is the basic theory. Now it needs to be tested somehow.