Answers to Frequently Asked Questions (FAQ's) on Trib-Joining and Trib-Bonding
No -Trib is not an "adhesive". Essentially it is a "cohesive" because it causes the metals to cohere and combine at asperity contact points and form genuine cold pressure welds at these points. Cohesion means an intermixing of molecules between the two metals to form solid phase cold welds. Once the join is made the Trib treatment plays no part in maintaining the coupling; whereas an adhesive would actually form an intermediate layer which couples the metal surfaces. Thus the strength of an adhesive join is determined first by the tenacity and stability of the adhesive binding to a surface and second by the strength of the cured polymeric coupling layer - which is a thermoset plastic. In contrast the strength of a Trib-Join is determined by the strength of the parent metals being joined and the number of individual asperity welds created. Trib-joins are generally much stronger than adhesive bonds.
The answer is to design the join so that it does not seize, then it behaves as a high resistance join that can be precisely positioned. If it seizes the join is too strong. Trib joins do behave as very high friction joins as they are formed- the result of thousands of micro welds and shears. The longer the rubbing overlap the stronger they get. Seizure occurs at the point where the join strength exceeds the applied force. If the force was greater it would travel further! In this sense seizure is not absolute but relative! Obviously if the forces keep rising at some point control will be lost and something will give and it most likely will be the parts will first distort then finally break.
Providing effective asperity contact is established, the friction produced between Trib treated surfaces is remarkably consistent so for a given length of rubbing a predictable resistance occurs which can be precisely calculated. Curiously in Trib-joins the frictional resistance is not very sensitive to contact pressure once the minimum threshold is reached to trigger the Trib action. This is explained by slip line theory where the additive effect of the pre-stress created by an interference fit and the new stress due to the assembly interact. Essentially the argument is that an asperity that is pre-stressed requires less additional stress to cause strain. Thus the minimum contact conditions are critical but generally above this pre-fit sizes are less critical - providing there is a minimum number of asperity contacts then the process is very reliable.
It is always best to start the assembly with untreated faces and apply the treatment some way along the over-lap as the parts are pressed together. This allows proper alignment to be established before the high friction occurs. If parts are fully treated before assembly it becomes impossible to start sliding them square because the first contact welds and throws (cocks) the parts and premature seizure and distortion results. Providing the assembly of the parts is started dry a true parallel fit can be guaranteed, then the sliding resistance is a reliable indicator of the joins final strength.
No - this is a common misconception. Metals have the unique property of being able to elastically and plastically deform under assembly loads without loosing strength. Press fit Trib-Joins are best made between pairs where at least one part can deform.
Successful Trib-Joining is dependant on bringing treated surface into good rubbing contact. It is surprising how many potential users fail to get a result at their first attempt. Just having metals touching is insufficient, they must have sufficient contact pressure to initiate Trib action when sliding, just pressing them together does not work. Typically the minimum sliding contact pressure needs to be 20 N/sq. mm (140 lbs/sq. inch) ranging up to the yield strength of the material.
Trib Joining works for most common metals so its easier to identify where Trib-joins do not work well.
The frictional increase on copper and it alloys is relatively small because copper is soft and is known to exhibit natural lubrication properties when running against steel. We see an increase in friction of typically 10% only for copper on steel (cpw 200 to 400% for most metal pairs) and this is not sufficient to make much difference. Hard bronzes and brasses will show rather more improvement (perhaps up to 30%). Copper will cold pressure weld to steel if the contact pressure can be raised sufficiently during sliding.
Trib works well on vermicular (nodular) iron providing reliable iron contact is established. Trib does not work well on flake cast iron however because flake graphite tends to orientate parallel with cast surfaces and acts as a solid lubricant and negates the friction inducing properties of Trib because Trib only works between clean sliding metals.
Trib does not work well on very soft metals like magnesium, zinc etc. because it is difficult to create sufficient contact pressure.
Trib is not recommended for use with parts that are plated with a soft material like, cadmium or zinc because it presents a soft layer on a hard substrate. The harder nickel rich plating materials may not have sufficient adhesion to the substrate metal to form Trib-joins. Generally for Trib to work on plated surfaces the plated layer needs to be penetrated.
Trib is inhibited by phosphating treatments and similar oxide enhancement treatments that act as sacrificial lubricants and these need to be abraded off prior to Trib-Joining.
It is not recommended that regular Trib-joins are disassembled because they tend to strengthen upon further rubbing or slip and they are likely to seize upon disassembly.
No the reverse occurs - they gain strength due to natural diffusion. Any increase in temperature also increases the likelihood of diffusion. Treated surface plays no part in the strength of the join once the join is formed - except if the join is overloaded and it yields, when any entrapped Trib material enables the join to reform without loss of strength. joins will actually often gain strength as a result of over-load yielding.
No - experience to date suggests they are not. The treated faces of a join after assembly tend to repel water preventing ingress and corrosion. Trib materials are non corrosive. High temperature salt spray tests showed no sign of attack within steel to steel joins.
Yes - all the dissimilar metal combinations that are listed in text books as being solid phase compatible should in theory work. We have not tested them all but certainly the common metals like steel (including stainless) aluminium's and nickel and chromium alloys work fine.
Laboratory studies confirm that Trib-joins behave in a similar way to heat shrink joins on which a great deal is published. In practice Trib joins have been shown not to be prone to torsional cyclic fatigue in applications like attaching cams to shafts or axial fatigue with pistons to rods in hydraulic cylinders. Trib-joins do have an energy absorbing capability providing they are designed to be less strong than the parts being joined. Trib-joins have the advantage that they are spread joins deriving their coupled strength from many small asperity welds distributed over relatively large areas hence the actual coupling is favourably spread unlike with fusion welding where the coupling is highly concentrated.
Trib-joins comprise of many small but genuine solid phase cold pressure welds which are distributed over much larger areas than a regular fusion weld. They are not adhesive bonds which rely on a layer of cured material between the metals to hold them together. What confuses most readers is the notion that a weld has to have a 100% intermolecular mixing and coupling across an entire overlap. This is not the case with a Trib-Join instead they are "distributed joins" with many individual weld sites spread over relatively large overlap areas. Trib-joins couple typically 10 to 30 % of an overlapping area with welds and most of the remainder of the overlap with mechanical interlocks.
A Trib-Join would be expected to create some useful tensile strength normal to the surface on which they are formed due to the many small asperity welds, formed as a result of rubbing - but this does not happen.
To explain this we must consider the conditions under which individual asperity welds are formed. The rubbing forces that create each weld also cause local plastic deformation in and around the weld area which results in a residual compressive elastic stress field in both of the joined parts adjacent to the weld. If this elastic stress is relieved the parts expand and the welds spontaneously crack and this is what happens when a join is cut open. Thus for a Trib-Join to retain useful strength the forces used to create it must be maintained to hold the faces together- as is the case when one part is forced into another. This concept causes some concern because it appears to indicate that Trib-joins are not true welds. They are technically known as cold pressure welds.
The natural instinct of a metallurgist when asked to examine a Trib-Join is to section it across the weld in other words along its axis. Several distinguished scientist have fallen into the trap of destroying the evidence by sectioning a Trib-Join in the wrong direction and thus breaking the weld! They then wrongly report that the join has failed! This is an essential learning experience and the joins had not failed in a functional sense. Therefore Trib-joins must be sectioned across their axis so they do not stress relieve.
Trib-joins cannot make butt welds for the reasons given above. However Trib-Bonds potentially can.
A Trib-Bond is an improved Trib-Join, made by heating a Trib-Join which drives diffusion bonding and it is well known that diffusion bonds make very satisfactory butt-welds. In this case providing the elastic residual stress due to deformation is relaxed during heating and diffusion the join will not crack when the forming forces are removed. Sleeve Trib-joins heated in a furnace showed parent metal tensile strength when sectioned along its axis. Upon sectioning and polishing the actual join between similar metals could not be seen because the metals had formed a complete molecular bond over the entire area. However in practice a complete bond my not be necessary perhaps as little a 40 to 50% bond would be useful and this can be achieved in open atmosphere using induction heating as shown in Application Note 5..
No - resistance welding depends on the resistance heating affect at the interface followed by forging (pressing together) of the heated (softened) parts. Trib does not increase resistance in a useful way and any oxide reduction would tend to reduce resistance so Trib is not helpful overall.
We believe the process relies upon single atoms of hydrogen being released onto the surface as the chemicals degrade. These atoms diffuse into and out of the metal matrix freely during join formation. There has been no evidence of detrimental effects such as hydrogen embritalment. For hydrogen embritalment to occur the hydrogen atoms must combine into H2 which are then too large to diffuse out from the metal matrix at room temperatures and thus become trapped at grain boundaries where they can initiate cracks.
Generally the smoother the surfaces the better. The ideal surfaces have been cold worked after machining to provide a work hardened skin. Thus if a shaft is roller burnished and the bore ballized then the asperity forms will be rounded and hardened. However these are refinements. Generally a clean "as machined" surface will join very well. Rough surfaces should be avoided because they provide insufficient asperity contacts to form an evenly distributed cold weld.
Trib treatment works well on most metals except zinc, copper and its alloys or flake cast iron. On soft metals like zinc the parts must be strong enough for some natural oxide to be mechanically scraped off as the join is made. Copper tends to act as a lubricant against metals like steel and the graphite in flake cast iron that tend to orientate parallel to surfaces. Nodular (vermicular) cast iron works well when the surfaces are cleaned after machining.
The most common reason for joins failing to "Trib" in steel or aluminium parts is poorly machined bores in which there is are insufficient asperity contacts. Generally one or both parts need to be able to plastically deform slightly to create a reasonable rubbing contact. Many drilling operations produce very erratic holes which are rarely straight or round by precision engineering standards. When a stiff round shaft is forced into a poorly machined bore the actual contact area may be a very small perhaps as little as 0.1% of the overlap area and it will be unevenly distributed. This creates a risk that the scattered asperities in contact simply wear down and reduce the contact pressure before the welds become established. The criteria for successful Trib-Joining is to make sure that evenly distributed asperity contact is established across the area to be joined and the average contact pressure needs to be at least 20N/mm˛. It is easier to ensure there is always an interference of 0.001% of the diameter of the join.
Other reasons are poor Trib action are likely to be related to surface cleanliness and finish. Any lubrication residues, hydrocarbon contamination, corrosion or surface coating treatment such as phosphating that prevent shiny metal contact will degrade the Trib action. For success surfaces should be dry and clean, "as machined" surfaces should be wiped to remove traces of cutting fluids.
No - a Trib join is quite different to a friction weld, which is essentially a butt join made by extensive rubbing of the parts to heat the metals to near melting point then forcing them together to forge a join. There is no detectable warming when making a Trib-Join.
No - Trib increase the friction levels far too much. The rubbing surfaces quickly cold weld and gall which damages and virtually destroys the surfaces being joined.
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