Lubrication is a way to minimise friction between two surfaces. When there is no lubrication, the friction between two surfaces wil cause the temperature to rise, wich in turn will add to the friction, rinse repeat. also when the lubrication is done with oil, the function of the oil, beside lowering the friction, is to dissipate the heat, and thus keeping the metals cool, so they don’t melt. Yes the metal temperature can rise to melting temperatures, and thus seizing up the bearings/pistons.