188.8.131.52 Surface Contaminants.
Surface films will be present on practically every bond surface. The more active metals such as iron and
aluminum readily oxidize to form surface films while the noble metals such as gold, silver, and nickel are less
affected by oxide films. Of all metals, gold is the least affected by oxide films. Although silver does not
oxidize severely, silver sulfide forms readily in the presence of sulfur compounds.
If the surface films are much softer than the contact material, they can be squeezed from between the
asperities to establish a quasi-metallic contact. Harder films, however, may support all or part of the applied
load, thus reducing or eliminating the conductive contact area. If such films are present on the bond surfaces,
they must be removed through some thermal, mechanical, or chemical means before joining the bond members.
Even when metal flow processes are used in bonding, these surface films must be removed or penetrated to
permit a homogeneous metal path to be established.
Foreign particulate matter on the bond surfaces will further impair bonding. Dirt and other solid matter such
as high resistance metal particles or residue from abrasives can act as stops to prevent metallic contact.
Therefore, all such materials must be thoroughly removed from the surfaces prior to joining the bond members.
184.108.40.206 Surface Hardness. The hardness of the bond surfaces also affects the contact resistance. Under a
given load, the asperities of softer metals will undergo greater plastic deformation and establish greater
metallic contact. Likewise, at a junction between a soft and a hard material, the softer material will tend to
conform to the surface contours of the harder material and will provide a lower resistance contact than would
be afforded by two hard materials. Table 7-1 shows how the resistance of 6.45 square cm (1 square inch) bonds
varies with the type of metals being joined.
220.127.116.11 Contact Pressure. The influence of mechanical load on bond resistance is illustrated by Figure 7-4.
This figure shows the resistance variation of a 6.45 square cm (1 square inch) bond held in place with a 1/4-20
steel bolt as a function of the torque applied to the bolt. The resistance variation for brass is lowest due to its
relative softness and the absence of insulating oxide films. Even though aluminum is relatively soft, the
insulating properties of aluminum oxide cause the bond resistance to be highly dependent upon fastener torque
up to approximately 40 in.-lb torque (which corresponds to a contact pressure of about 1200 psi). Steel, being
harder and also susceptible to oxide formations, exhibits a resistance that is dependent upon load below
80 in.-lb or about 1500 psi (for mild steel). Above these pressures, no significant improvement in contact
resistance can be expected.