When working through the design process, both friction and wear are commonly overlooked or underestimated when deciding the selection of engineering materials. As discussed in other articles, wear can lead to excessive maintenance requirements and even replacement costs, having a negative impact financially.
In addition, friction leads to a diminished efficiency for the operation of machines, having a knock-on effect of excessive operating temperatures.
Early tribological failure can lead to severe detrimental failure of systems and components, especially when improperly predicted. For this reason, tribological performance needs considering during the design phase in any mechanical system as problems can quickly occur during service and trials that can lead to high expenses.
The main concern is determining whether significant wear on the system will be present, and if so, what steps are necessary to reduce it to an acceptable level.
All wear mechanisms should be considered, ranging from fretting to abrasive wear. In addition to this, rolling contact fatigue should also be considered.
Estimating wear rates
There are three methods available to the designer to predict/estimate the wear rate for the system.
The first method, which I have found applicable so far in my career, is to measure the wear rate of the system already in service or a system that has a close representation. This method allows practical rates to be collected and predict realistic wear rates.
On the other hand, there are downfalls to this method. The wear rate of a system will vary with time. If the wear rate of the system has 5 years of service, it will differ from wear rates witnessed on a new system. The cause of this may be due to a running-in period, where a high wear rate will occur as the system finds a balance.
Understanding the failure modes of the system will help the designer propose an expected life of the system. If a system fails in a particular area, the wear rate at the failure point is required, not an average wear rate across the system. If not, premature failure can occur relative to the design predictions.
The second method is to perform testing on the components, setting the environment to represent in-service behaviour. This method is widely used on bearings when tested in labs. The loads, speeds, lubrication and temperatures are set to represent service conditions.
However, the wear mechanism must not changes throughout the testing because it will affect the outcome.
If testing isn’t possible, performing theoretical calculations is the next option. There are simple equations for the various wear mechanisms, and these can provide essential information at the design phase.
Although, for an accurate representation of actual wear rates in service, this option may not be the best.
Effects of Lubrication
Lubrications provides the benefit of reducing the wear rate of a system along with friction. The lubrication regime with the lowest wear rate is hydrodynamic lubrication (see article).
Maintaining hydrodynamic lubrication for the duration is not realistic. During non-operation times, boundary lubrication will be present, resulting in high wear rates.
The optimum is to have the system in hydrodynamic lubrication for as long as possible. The best indicator for this is establishing λ, the ratio of minimum lubricant thickness to surface roughness.
The lubricant viscosity, the load and the entrainment speed can be varied to alter the lubrication film thickness.
Selection of Engineering Materials
Although we have spoken about tribological performance thus far, this is not a restrictive factor. Cost is commonly a restrictive factor during the material selection process. Other factors include mass, the strength of the material, stiffness and toughness.
In some cases, you will find that electrical and magnetic properties are important.
Metals are the primary material used for components in mechanical systems. Metals microstructure and composition are standard in the industry through set practises and specifications. Alongside this, the mechanical properties are accurately controlled and predicted.
On the other hand, non-metals are less standardised, and you will find more variability, even if the structure of the material is the same.
Sliding Wear
The main property affecting sliding wear is the hardness of the softer material between the two contacting surfaces. The other property affecting the wear rate is the dimensionless wear coefficient K.
Higher values of K occur when the two contacting materials are the same materials or metal and metal compared to metal and non-metal.
Therefore, similar materials should be avoided when encountering sliding wear.
The range of surface engineering methods available allows the tribological properties of a metal to be selected independently from the bulk of the material.
The different methods will give different thicknesses, which is of high importance. Materials with a thin layer are only suitable in scenarios where the wear rate is comparably small, and the subsurface stress lies close to the surface.
For cases that involve surface stresses imposing deeply into the material, larger layers are required. A great example is a highly loaded gear, which needs high yield stress to withstand the high contact stresses and minimise wear. On the other hand, the rest of the material requires a high fracture toughness and resistance to fatigue cracking to withstanding the cyclic loading.
In steel, the two requirements of yield stress or hardness at the surface along with a high fracture toughness cannot be achieved together. To obtain this economically, a material with high yield stress will require surface treatment to increase its hardness. Most likely via nitriding or carburising, which gives a hard outer layer and a high toughness core.
Metals that have had hard surface layers created via surface treatment tend to have excellent wear resistance.
Fretting Wear
Some metals are susceptible to fretting, which include titanium, and these types of metals should be avoided in fretting environments.
As discussed above, surface treatments will increase wear resistance, which will reduce fretting.
An additional benefit of surface treatments in this application is that compressive residual stress is induced, reducing the fatigue crack growth from fretting.
Although, the hardening process should be selected carefully, as it could increase friction.
Abrasive Wear
To obtain the lowest wear rates from abrasive wear it is necessary that the hardest material is chosen, either in bulk or surface coating.
With cost constraints, the bulk material may not be feasible, which will lead to surface coating.
As with sliding wear, the depth of the casing is an extremely important consideration. The rate of wear and the depth of significant stresses induced into the material must also be considered.
Summary
Here we have covered key factors to consider in deciding the selection of engineering materials for tribological selection. The selection of engineering materials requires a lot of considerations, with wear being a predominant factor in the life of the system.
We discussed various methods to help estimate wear rates of systems, along with the pros and cons of each method.
Case hardening is a common method used to get desirable mechanical properties of engineering materials within tribological applications, as seen in this article. The method provides a hard case for the exterior of the material whilst maintaining ductility in its core, which is the best combination for wear environments.
This article has relied on the book Tribology: Friction and Wear of Engineering Materials, by Ian Hutchings and Philip Shipway therefore I give credit to this book for this article, not myself. It is a fantastic book to boost your understanding of tribology and a must-read for tribologists.
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