(Invited) Potential of Double Network Gel as a Tribological Material Realizing Low Friction in Water
Authors: Kanda Koki , Adachi Koshi 
Published: February 2020

Introduction

Soft matter enables stress relaxation at contact point and hydrogel, one of soft matter which consists of gel network and water, is expected as sliding material in water due to its high water content since hydrophilic surface is known to form water film between sliding parts and thus realizes low friction in water. Especially, double network (DN) gel is highlighted since mechanical strength of DN gel is tough, realized by combination of hard network and soft network, compared to other hydrogel gel such as PVA gel. In this study, effect of properties of counter material, such as surface roughness or wettability, is examined to its potential of practical usage. Also, friction property of DN gel against silicon carbide (SiC) in water is examined under varied sliding condition such as sliding speed and load.

Specimen and experimental setup

DN gel sheet is fixed and slid against SiC disk rotated by servo motor. Friction experiments were carried out in water and friction force is measured by load cell and thus friction coefficient was calculated with applied load. Friction tests using PVA gel sheet, its tensile strength is lower by 1 digit than that of DN gel, were also carried out to investigate effect of tensile strength of gel material on friction property. All experimental procedures were done at room temperature.

Effect of wettability of counter surface on friction property

Varied wettability of sliding counterpart indicates that hydrophilic disk slid against DN gel realizes lower friction coefficient around 0.01 while hydrophobic disk increases friction coefficient up to 0.34. Roughened DN gel surface was fabricated by means of laser cutter and textured DN gel realized lower friction coefficient around 0.01 even disk is hydrophobic. These results indicate that maintenance of water between DN gel and sliding counterpart is crucial at practical usage to realize low friction in water.

Generation of low friction by formation of damaged layer

Varied sliding speed and load exhibits that friction coefficient of DN gel/SiC in water shows running-in behavior which consists of twice decrease, then finally induces low friction coefficient less than 0.001 under medium sliding severity between boundary lubrication and hydrodynamic lubrication. On the other hand, PVA gel does not show such running-in behavior within examined sliding condition and severe wear occurs after sliding test.
In-situ friction apparatus revealed that DN gel surface is roughened during 1st decrease and 2nd decrease starts as soon as roughening process is finished. Also, when green DN gel and red water, DN gel surface become darker during 2nd decrease, indicating that water between DN gel and sliding counterpart intrude in DN gel surface during 2nd decrease. Furthermore, cross section of DN gel indicated that moist layer with thickness of several-tens mm is formed on DN gel surface by friction.
It is known that applied tensile stress causes destruction and elongation of each network to the depth of 100 mm approximately, called as damaged region, on DN gel surface. Also, mechanical property of damaged region is softer and sparser compared to original DN gel. Therefore it is estimated that damaged region is formed beneath DN gel surface during 1st decrease. Also, water between DN gel and sliding counter material penetrated into damaged region on DN gel surface and formed thin water film enables generation of lower friction coefficient than 0.001. It is understood that higher tensile strength of DN gel maintained damaged region formed beneath DN gel surface and kept damaged region played a role as water reservoir to form water film when DN gel experiences higher contact pressure, as a result, adhesion between DN gel and sliding counterpart is prohibited and low friction coefficient is realized.

Conclusions

Friction tests using DN gel sheet and SiC disk were carried out in water and following conclusions were achieved.

Roughened DN gel decreases friction coefficient around 0.01 with hydrophobic disk where non-textured DN gel shows higher friction coefficient around 0.04.

DN gel/SiC shows running in behavior which consists of decrease of friction coefficient twice in water, then friction coefficient exhibits lower than 0.001.

Damaged region formed beneath DN gel surface during 1st decrease and water between DN gel and sliding counterpart penetrates into damaged region.