Trib-Joins vs. anaerobic adhesive joins in high temperature uses.
The significant difference between adhesive bonds and welds is that a cured adhesive forms a plastic inter-layer that bridges between and joins the metal surfaces whereas a weld is an intermixing of the basic metals. Therefore the weld is fundamentally stronger and more stable than an adhesive because metal is stronger and more stable than plastic. Most adhesives lose strength progressively as temperatures rise from ambient towards about 200 centigrade, beyond which plastics rapidly loose all their strength.
Trib-joins are a collection of asperity welds and are unaffected by temperature when joining similar metals because they are genuine metal to metal joins with no plastic interlayer. They form many small cohesive asperity welds between the surfaces and these welds are stronger and more stable than chemical adhesive bonds under high temperature and humidity. Trib-joins also develop mechanical interlocks between soft metal surfaces due to transfer of material between faces as the joins are made and these are stronger than the plastic wedge like interlocks formed with adhesive. Therefore when made with the proper pre-fit a Trib-join is stronger and more stable than an anaerobic adhesive join.
Trib-joins will join dissimilar materials providing the differential expansion does not exceed the elastic deformation holding the joined surfaces in firm face to face contact.
It is not generally realised that there is a high risk of THERMAL DEGRADATION in metal to metal slip fit and press fit sleeve joins made with ANAEROBIC (METHACRYLATE) ADHESIVES because of dissimilar expansion rates between the cured adhesive (plastic layer) and the joined metals.
According to published data the coefficient of thermal expansion of cured anaerobic materials is typically 9 times greater than steel and 5 times that of aluminium and more than double epoxy. This different rates of expansion would be expected to cause rapid failure of chemical adhesive bonds as a join is heated. But in sleeve joins made with anaerobic materials that only see a torsion load, such as close fitting cylindrical joins the chemical bonds may break, that means bonds between the adhesive and the substrate, but the join remains strong because of cured plastic interlocks formed with surface roughness. However there is a real risk that Anaerobic Adhesive interlocks will degrade as the cured adhesive material ages and degrades by fatigue after many repeated thermal cycles.
The degradation mechanism can be visualised by thinking of the cured material as a thin and irregular brittle layer of thermoset plastic trapped between the metal faces. If the joined metals are similar, upon heating they expand together. But the plastic expands up to 9 times more and tries to force the metal faces apart and will tend to extrude and fill any new gaps. The heavily cross-linked thermoset has very limited elasticity and cannot soften and flow laterally like a thermoplastic without damage. In service it suffers internal molecular damage which is cumulative and the plastic progressively loses strength until eventually the join loses all strength derived from the adhesive. However the join does not necessarily fail catastrophically because it may, as is the case with a shrink fit derive useful strength from the metal to metal elastic grip. However the joins will be much weaker than when originally made!
Static strength tests on anaerobic adhesive joins can be very misleading. Assemblies are typically tested at ambient 24 hours after assembly, when the join has reached its designed static shear strength. At this stage the plastic is still relatively soft and elastic and is more tolerant of overload because cross linking is incomplete, since it is unlikely to have been heated. Cross linking can continue for days, even weeks after assembly if it is not heated. As the cross linking proceeds so the plastic becomes more brittle. The inclusion of elastomers or other fibre fillers can make it more tolerant.
The actual risk of thermal degradation is difficult to quantify. Generally at temperature excursions above 15oC problems can occur. Serious problems are likely above 50oC and severe problems at engine operating temperatures. The rougher the surfaces the more uneven distribution of adhesive causing greater differential expansion. Slip fits are most at risk because these can fail completely and it was in these that the failure mechanism was first identified. Press fit or heat shrinks lose but retain some strength due to the elastic interference fit between the joined parts. Bearings retained with anaerobic adhesives are at risk of minor distortion due to uneven expansion.
Some thermal variation is unavoidable in most practical situations so this is a fundamental limitation on the safe use of anaerobic adhesive in static or dynamically loaded structural joins. Under these circumstances Trib joins are superior.
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