Pressure/ friction welding of two metals

I am working on some soft metal thin film and wire, i.e. gold wire
bonding to gold thin film. We all know that when two soft metals put
together, they may easily "bonded" to each other by simply applying
some presurre.
Meanwhile, I need some more therectical background/mechanism of this
"presurre welding" phenomenon. Would anyone give me some references and
suggestions?