In the face of adversity and further pursuit of engineering plastics betterment, this week we became scientists in all but name and salary to learn all about the Holy Grail of engineering plastics; wear and wear resistance. We all know that for most of us, science can trigger an immediate attack of sleep apnea but we persevered in our journey with the help of match sticks and numerous jars of Nescafe to return with the answers to these intriguing mysteries. We also discovered some new, long and technical sounding words which we’ll also chuck in for you to enjoy.
What is this “wear” anyway?
Well, the wearing away of a material as described by a boffin would be; the action of two properties wear and friction. Wear and friction are not material properties but the properties of a tribological system. Tribology is that field of science and technology concerned with “interacting surfaces in relative motion” and is a branch of mechanical engineering and materials science. But let’s break it down and make it simple:
In materials science, wear is defined as erosion or sideways displacement of material from its "derivative" and original position on a solid surface performed by the action of another surface. In other words, it is simply the deforming of a component or loss of material when a part is moving or rubbing against another part. Wear also occurs when two materials under load slide against each other. This wear occurs in such devices as rotating shafts. Three common types of wear we encounter:
- Abrasive wear – “cutting” caused by hard irregularities on the countersurface.
- Fatigue wear – failure of the polymer due to repeated stressing from hard irregularities on the countersurface.
- Adhesive wear – loss of polymer by transfer and adhesion to the countersurface.
And Friction, on the other hand, is a measure of the resistance to motion (and loss of energy) of two interacting surfaces. The friction is measured by the term coefficient of friction. So It is therefore a result of the real contact area between the two surfaces and the force or pressure applied to it.
Wikipedia describes coefficient of friction (COF) as, “a dimensionless scalar value, often symbolized by the Greek letter µ, which describes the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction depends on the materials used; for example, ice on steel has a low coefficient of friction, while rubber on pavement has a high coefficient of friction. Coefficients of friction range from near zero to greater than one.”
So, after that heavy dose of science stuff, if you’re still awake we shall move swiftly on to where “the rubber meets the road” – wear resistance!
The Resistance and does anyone really know?
The technical definition of wear resistance of any material is; the measurement of material loss, under recorded loads, speeds, temperatures and surface roughness. There are a few tests and instruments that are typically used to measure and compare the ability of different plastics to withstand the friction and wear that can applied to them.
But all these test methods seem to have their limitations for us and don’t give us a fully accurate picture for what we’re looking for. This is because of the complex nature of wear, in particular "industrial wear", and the difficulties associated with anyone accurately re-creating the environments our materials are expected to endure. In addition, in real-world applications, variables such as lubricants, moisture, temperature fluctuations and even the quality of component machining can decrease the actual wear resistance of a material as well.
So then what makes for a “hard wearing” plastic?
That seems to be the million dollar Question, and we wish it was as straight forward as that, the variables are where the sticking points lie.
- What is the application?
- Is water absorption a consideration?
- Will there be high sliding speeds for the components?
- Does the material require impact resistance, as well
- How much abrasion is there?
Most of our engineering plastic manufacturers provide some type of comparison charts as a guide for engineers in determining which material may be best suited for a particular component that requires high wear resistance. Typical materials that are at the top of any scale include PTFE,HDPE, UHMWPE and Nylon for their very low coefficient of friction (COF), and PETP, PPS, PEEK, and Nylon for their high wear resistance.
Based on this short list one might assume the Nylon, for example, provides the best combination of “slipperiness” – low friction – and toughness. Perhaps, but again, the application and other factors must be allowed for.
Is harder, better?
In some ways not really; hard plastics don’t always mean higher wear resistance, HDPE is softer than PVC but HDPE is much better at wear resistance. Polyurethane would also be described as being pretty soft, almost like a hard rubber some would say but the bottoms of snow ploughs are often protected with a strip of polyurethane and that stuff is rubbing along the road for mile after mile now that our climate seems to have turned Scandinavian
Asking the experts
Ultimately, the best course is to ask one of our technical team who can delve into your specific application and needs and recommend the best engineering plastic for this or that particular application and then you don’t have to become a professor of tribology to find the answers