Stages of friction

There's some renewed talked on the slowtwitch forums about chain lubes and a claim that no lube is just as good as lube. I thought it would be good to provide a brief explanation of friction and wear. Perhaps the easiest way is to show what's going on with the qualitative picture below which is for repeated motion in a single wear track:

When you have fresh surfaces, either with or without lube, stage I exists. Even bare metal-on-metal contact exhibits relatively low friction early on. The duration of stage I is a function of many things, such as the materials, surface coatings, temperature, pressure, atmospheric conditions (air, pure nitrogen, argon, vacuum, etc), and liquid/solid lubricants. As wear particles develop, you enter into stage II. Stage III is a continuation of the wear particle buildup. Eventually, the number of wear particles leaving the wear track is balanced by the particles being generated and you reach a steady-state (stage IV).

The time is takes to go from stage I to stage IV is a function of many things, just like the duration of stage I. It may be anywhere from a single lap to many laps depending on the contact stress, materials, etc.

Some materials may exhibit a decrease in wear as signified by stages V and VI, though without active removal of wear particles, stage IV becomes the dominant player. From my previous entry you can see that for a sample metal-on-metal contact, stage I was short lived and stage II/III was also a short event. The metal pairing reached a steady state level quickly. It should be noted that had I taken a simple brush and wiped the surfaces down to remove the majority of the wear particles, the friction would have temporarily dropped down to stage I levels. Thus, active cleaning is a great way to keep friction low for bare metal-on-metal sliding contact.

In general, liquid lubes will keep the friction down to stage I levels. The lube acts as a transport agent for wear particles. It should be noted that you can still have wear of the materials, and indeed I have tested some VERY expensive grease (as in a few thousand dollars per pound) which continued to wear the surfaces significantly yet kept friction nice and low. However, any lubricant will have a finite life. Starvation occurs eventually and friction jumps up to metal-on-metal steady state levels as shown in Figure 4 of the link above (the June entry).

What does this mean for chains? When you hear that chain squeaking you can be pretty sure you are in stage IV and it's time to clean and relube. The surfaces are lube starved. Can you get a chain to have low friction without lube? Sure. But the chain has to be designed properly. That may mean proper material pairings, an active cleaning process (e.g., a debris wiper), or proper surface treatments (however, you could argue that something like gold plating is actually a form of lubrication).

So remember, it's not about the initial friction but how long the initial friction lasts. If it lasts a short time, then the question becomes how good is the life of my lube if one is used.