RUBBER FRICTION AND 3D-PRINTED PAVEMENT
With a doctorate in vehicle engineering Mona Mahboob Kanafi, who has also specialized in data science and tribology, is working for Eniram, a global leader in energy management, which is a Finland-based technology company-owned by Wärtsilä. She has done considerable work researching application of 3D-printing technology to replicate the surface of road pavements for rubber friction experiments. In an interview to Tyre Asia, she speaks of her research that would have a lot of implications in tyre development and pavement design for optimum gains in mobility
TA News Bureau
Tyre researchers and pavement engineers have been closely working to understand rubber friction when the tyre hits the road. Surface roughness of road pavements and their impact on rubber friction are complex issues that need to be understood for optimal development of mobility.
The challenge before Mona Mahboob Kanafi, while working for her doctorate programme at Finland’s prestigious Aalto University, was to unravel the application of 3D-printing or additive manufacturing technology in tyre–road friction and pavement engineering.
“Additive manufacturing, or three-dimensional (3D) printing is a fast-growing technology which seems to induce fundamental changes in the manufacturing processes,” she said in an interview with Tyre Asia.
“This technique allows to synthesize customized designs, with customized selection of materials. However, up to now, the technology is limited in the resolution of its final products as well as the available range of materials.”
On the other hand, tyre-road friction or in general tyre-road contact mechanics is a complex research topic. Despite much advancement in rubber friction theories, they cannot yet adequately predict friction even in laboratory experiments.
“This arises from the complexity of filled rubbers, the multi-scale road surface roughness and the partial contact of the tyre with the road,” she explains.
Given these two issues, it is understandable that introducing 3D-printing technology with known limitations (failure to replicate micro-roughness and printing with durable plastics not rock) to such a complex topic with many unanswered questions will encounter negative views and major doubts.
Tyre-road friction or in general tyre-road contact mechanics is a complex research topic. Despite much advancement in rubber friction theories, they cannot yet adequately predict friction even in laboratory friction experiments
“Therefore, for the first time in the literature of this topic, we decided to investigate scientifically the applicability of the 3D-printing technology to replicate the surface of road pavements for rubber friction experiments,” Dr Kanafi said.
The objectives were first to quantify the current limitation in replicating the multi-scale surface roughness of the road pavements, and second, after knowing the limits properly, find out how researchers can take advantage of the situation and get closer to understand roughness impacts on rubber friction.
“I must admit that in the course of the research, we also had doubts if the research might have further extensions, but the results proved very positive,” she points out.
When asked about the advantages of utilizing 3D printing for replicating pavement texture roughness patterns, Dr Kanafi said that the research objective was not to make any claim of replacing asphalt with 3D-printed road surfaces.”
“Before digging into the advantages of the 3D-printed substrates, I wish to state that the aim of our research was never to claim 3D-printed road surfaces must replace the asphalt samples in laboratory rubber friction experiments at least for the time being.”
The research aim was to gain insight on the physics behind this topic. “Keeping this in mind, the results of our experiments have shown that one of the highest resolution 3D-printers currently on the market can only replicate the surface roughness of the road pavements for wavelengths down to 0.5 mm.”
This may sound bad news at first glance, since many studies have demonstrated that micrometer roughness (and probably even smaller scales) has significant impact on rubber friction. However, the possible influences of macro-level texture to rubber friction are often overlooked.
The performance optimization of the tyre rubber compounds is one of the central areas of research and development by the tyre industry. Among the research tools, controlled laboratory rubber friction experiments are of foremost importance to test the performance of different compounds.
Utilizing 3D-printing technology for the first time, Dr Mona Kanafi’s research demonstrated the distinct influence of surface roughness at scales close to the aggregate size on rubber friction, which is typically ignored in dry friction studies
However, the surface roughness of the asphalt test specimens in use is constantly evolving through the course of the measurements, which makes the interpretation of the results challenging. Above all, the exploration of the exact link between surface roughness and rubber friction is still an open question in science.
“Let’s consider rubber friction experiments on two asphalts with different nominal aggregate size. Here, still the difference in the friction coefficients cannot directly be linked to their difference in macro-roughness, because usually the roughness at other length scales may differ, too. In this regard, the solution only lies in rubber friction experiments on counter-surfaces with controlled surface roughness at different length scales,” Dr Kanafi emphasized.
This is where the unique advantage of the 3D-printed substrates proves itself. Utilizing this technology, for the first time, her research demonstrated the distinct influence of surface roughness at scales close to the aggregate size on rubber friction, which is typically ignored in dry friction studies.
To say it in a more explicit way, when you print asphalt pavements of different aggregate size, with the same 3D printer and consequently same short-scale roughness or resolution, it is realistic to assume that the printed substrates have the same level of micro-roughness, but different macro-level roughness. Such a substrate is so interesting to study, since researchers can investigate the sole effect of macro-texture and the physics behind it on rubber friction.
“Having used these printed substrates in our experiments, we concluded that the macro-texture seems to affect rubber friction, since it influences frictional heating which tends to reduce the friction coefficient of rubber.”
This was the first rubber friction experiment to confirm the theoretical predictions of Persson’s friction theory which says macro-texture impacts on rubber friction must be due to frictional heating.
When asked in what way can tyre designers utilize the knowledge about artificial surface patterns/printed topography on rubber wear in order to come up with better tread, Dr Kanafi has some key observations to make.
“I wish to emphasize that although rubber friction and wear are intertwined topics, the focus of our studies were on rubber friction rather than rubber wear. Especially on the 3D-printed substrates, the rate of wear of the rubber samples is much lower than that of the experiments on the asphalt surfaces, since the printed substrates are much smoother than the real asphalts.”
Saying this, she restated the unique power of 3D-printing technology in fabricating customized surface roughness patterns. It is possible to statistically manipulate surface characteristics or even synthesize artificial geometries and then print them as substrates for rubber friction experiments.
This will help researchers to study the physics behind the effects of roughness at large-length scales on rubber friction. The 3D-printed artificial surfaces give researchers the control that is needed in the experiments.
“We can control what roughness length scales are contributing to the observed friction, since we artificially generated those length scales. Thus, we are sure about their magnitudes.”
For instance, in her studies, Dr Kanafi first mathematically generated three substrates with same level of micro-roughness, but with different aggregate size, and then 3D-printed the surface models. Performing rubber friction experiments on these printed substrates at many different test conditions, she could conclude that macro texture in general tends to reduce the rubber friction.
This reduction was sometimes reported to be as large as 0.2 in the friction coefficients for rubber sliding on the printed substrates. Based on the observations, macro-texture effect seems to be more visible where the hysteresis contribution along with the frictional heating effects is dominant phenomena.
“So far, we have introduced 3D-printing technology for fundamental studies about roughness impacts on rubber friction and the bottom line is that the potential of this technology in the research field is very promising.”
However, although the research showed the impact of macro-roughness on the dry rubber friction, further research is still required to quantify the impact of pavement macro-roughness on tyre/road friction. If the macro-roughness influences rubber friction mainly via frictional heating and heat diffusion, then similar controlled experiments on 3D-printed samples with different material properties would be needed to judge the importance of the macro-roughness in practical tyre/road applications.
To come up with a better tyre tread, we first need to understand how various factors contribute to tyre-road friction. We would like to be able to theoretically model tyre-road friction without having to go through many costly measurements, and trials and errors
Moreover, macro-roughness is a measure of both roughness wavelength and amplitude. “In our studies, it seems that the macro-roughness wavelength is the dominant parameter for the dry rubber friction, i.e. smaller aggregate size results in less frictional heating.”
However, proper macro-roughness amplitude is a relevant parameter, for example, for the tyre/road wet friction to prevent aquaplaning. Therefore, extended research is required to conclude about the optimum macro-roughness in terms of both wavelength and amplitude on dry/wet grip, rolling resistance, noise, hydroplaning, durability/wear, energy consumption and even air pollution.
“All in all, to come up with a better tyre tread, we first need to understand how various factors contribute to tyre-road friction. We would like to be able to theoretically model tyre-road friction without having to go through many costly measurements, and trials and errors,” Dr Kanafi observed.
3D-printed substrates with predefined surface roughness characteristics are ideal surfaces for laboratory rubber friction experiments. “By comparing the results of such experiments with the theoretical predictions of the rubber friction models, it is hoped that we would be able to come up in future with an experimentally verified rubber friction theory,” she says.
(Appeared in February-March 2018 issue of Tyre Asia)