ENHANCEMENT OF MECHANICAL AND TRIBOLOGICAL PROPERTIES OF EPOXY BASED POLYMER WITH TALC & PTFE AS FIL

International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN (P): 2249–6890; ISSN (E): 2249–8001 Vol. 10, Issue 3, Jun 2020, 137–146 © TJPRC Pvt. Ltd.

ENHANCEMENT OF MECHANICAL AND TRIBOLOGICAL PROPERTIES OF EPOXY BASED POLYMER WITH TALC & PTFE AS FILLERS MOHAN KUMAR K1 & Dr. V. VELMURUGAN2 1 2

Associate Professor, Dr. T. Thimmaiah Institute of Technology, KGF, Karnataka, India

Associate Professor, Centre for Nanotechnology Research (CNR), VIT- University, Vellore, Tamil Nadu, India

ABSTRACT SU-8, an epoxy based polymer is a promising material for fabricating MEMS 3D structures. However, they have poor tribological properties, which needs to be addressed for optimal performance. Many researchers have shown that PTFE (polytetrafluoroethylene) has low coefficient of friction, and Talc is been used for better wear durability; and hence this combination with any of the polymers has never been studied. To enhance the mechanical and tribological properties of SU-8, different weight percentage of Talc and PTFE concentration has been studied; and the results have shown that the right weight percentage of these two fillers (10P10T) have provided superior properties (Improved hardness and young's modulus with reduced friction coefficient) compared to pure SU-8 and hence can be used as coatings in MEMS devices that experiences lower contact stresses.

Received: Mar 20, 2020; Accepted: Apr 10, 2020; Published: Apr 28, 2020; Paper Id.: IJMPERDJUN202013

1. INTRODUCTION With an improvement in nanotechnology over a few decades, there has been an advancement in miniaturized

Original Article

KEYWORDS: SU-8 Polymer, PTFE Powder, Talc Powder, Micro-Composites & Reciprocating Tribometer

components in the field of MEMS/NEMS devices due to its small size with high performance at relatively low cost; and hence finds its market in an immense number of applications including automobile and aerospace industries.

Microsensors and microactuators are the most functional components in MEMS devices apart from microelectronics and microstructures; of which only MEMS microsensors are commercially available, while the MEMS microactuators are not yet commercialized due to high friction which hinders the smooth movement between two surfaces having relative motion

[1]

. MEMS devices have a high surface to volume ratio, because of its

small size that varies from a single micrometer to few millimeters, the forces associated with the surface dominates the forces associated with the volume

[3,10]

. Between the contacting surfaces, surface forces like adhesion and

friction play a predominant role leading to failure of the device functionality due to wear.

Silicon is the most conventionally used materials for MEMS devices for the past few decades, due to wellestablished design and process fabrication related to IC technology. But in the recent years, silicon is been replaced

by SU8 (an UV curable epoxy based negative photoresist) as a structural material for most of the applications as compared to poly dimethylsiloxane (PDMS) & poly methylmethacrylate (PMMA)[2], due to its superior properties such as biocompatibility, low surface energy and compatibility of fabricating high aspect ratio componenets at extremely low cost. Despite the above advantages, there are many researchers across the globe who have put efforts in enhancing the poor mechanical and tribological properties of SU8 material. Texturing of SU-8 surfaces and coating with polymers have shown improvement in tribological properties. A cost effective SU-8 textures (3D

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negative finger print and honeycomb) on silicon surface have shown decreased friction coefficient (COF) with increased load [3]. A combination of lithography to fabricate SU-8 micropillars on silicon, and spray coating with submicron PTFE & nano roughness Iron oxide has resulted in superhydrophobic surfaces by lowering the surface energy, but there was no tribological study being made

[4]

. However, a hierarchical structure was fabricated on silicon sample by simple spray

coating with CNT and PTFE resin which resulted in superhydrophobic surface with enhanced wear resistance [5].

For the fabrication of MEMS structures, the SU-8 in its bulk form or by adding fillers (nanoparticles or liquid lubricants) can be coated on the silicon surfaces or glass [2]. High young’s modulus was observed in a composite of SU-8 and multi walled carbon nanotubes (MWCNTs)[6]. Heat treated surface of SU-8 composite with filler materials (quartz, graphite, silica nanoparticles, aluminium and SiO) at different weights, resulted in reduced COF by 4 times and increased wear life by 2 times, apart from reducing the internal stress[7]. Carbon fillers (graphite, graphene & MWCNTs) were added to SU-8, and found that 10 wt % of graphite has resulted in high wear resistance at high speeds and high loads [8]., further in his other study, he added talc in graphite at varying proportions and found that 15 wt% of talc + 15 wt% of graphite has resulted in low COF due to high mechanical strength [9]. Hybrid SU-8 nanocomposites using nanoparticles (CNT, graphite & SiO2) and PFPE nano lubricant has reduced COF by 6 times and mechanical properties by 1.4 times as compared to pristine SU-8

[10]

. The

combination of fixed amount of talc (30 wt %) with PFPE has resulted in a lubricating mechanism being responsible for COF reduction[11]. Self lubricating SU-8 composite with PFPE has resulted in higher wear life at all weight proportionates, as compared with other liquid lubricants like Multiply Alkylated Cyclopentanes (MAC) lubricant and SN 150 oil [12].

PTFE particles at varying proportionate were added in PDMS films on aluminium substrate to achieve a superhydrophobic surface of WCA = 163°, in addition to good mechanical abrasion resistance

[13]

. PTFE particles at

different sizes were incorporated in PDMS films on top of the silicon wafer to study the wetting behaviour of the surfaces [14]

, further in his other study, he investigated that PTFE/PDMS by spin coating produced good results compared to dip

coating methods

[15]

. Polyurethane/Talc compositewas prepared by varying the proportionate of Talc with 7% hydroxyl-

terminated PDMS as modifiers to study the mechanical, tribological and wetting behaviours of the composite. The results revealed that talc has much contribution on the tribological properties [16].

Therefore from the above literatures, it is clear that talc is been recently tried to improve the wear resistance, and PTFE has shown extremely low COF because of strong bonding between carbon and fluorine atoms and also has low surface energy with good chemical resisitance[17]; Since there is no comprehensive work reported with the combination of polytetrafluoroethylene (PTFE or Teflon) and talc particles, this will remain as a main objective in the fabrication of SU-8 composites, for the improvement in tribological and mechanical properties.

2. EXPERIMENTAL METHODOLOGY 2.1 Materials Microscopic glass slides were used as a substrate, SU8-2025 (viscosity-2500cSt) as a base material for coatings (Microchem Ltd, USA), with Poly (tetrafluoroethylene) and Talc (Sigma Aldrich – India) as filler material with particle size of 1µm and 10µm respectively. All these above were used as received.

2.2 Sample Preparation Microscopic glass slides of 3mm thickness were cut to ~2.5 x 1.5 cm size samples, and then the samples were cleaned sequentially with soap water, DI water, followed by acetone for 30 minutes, and finally dried at room temperature. Then the sample surfaces were Impact Factor (JCC): 8.8746

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exposed to oxygen plasma treatment for 3-4 minutes at 45 Watts using Harrick plasma Cleaner (PDC-002) to remove foreign contaminants and to generate –OH functional group to have better adhesion between the coatings and the substrate.

Thin films of SU-8 composites were coated on plasma treated samples by using a spin coater unit (SUSS Microtec) at an initial speed of 500 rpm for duration of 30 sec with 200rpm/s acceleration, followed by rotational speed of 1000 rpm for 30 sec with an acceleration of 500rpm/s; which will result in SU-8 films of ~45 - 50µm thickness. Prior to spin coating, SU-8 composite was prepared by adding different weight proportionate of fillers (2wt%, 4wt%, 6wt%, 8wt% and 10wt% of both PTFE and talc in 1:1 ratio) in SU-8 and stirred mechanically in both clockwise and counterclockwise directions for 25 to 30 minutes until a homogeneous mix without any particle agglomeration. Table 1 provides the details of fabricated sample compositions with abbreviations.

Table 1: Sample Composition with Abbreviations Sample Composition in weight percentage Abbreviation Pure SU-8 SU-8 SU-8 + 2 % Talc +2 % PTFE 2P2T SU-8 + 4 % Talc + 4 % PTFE 4P4T SU-8 + 6 % Talc + 6 % PTFE 6P6T SU-8 + 8 % Talc + 8 % PTFE 8P8T SU-8 + 10% Talc + 10 % PTFE 10P10T SU-8 + 12 % Talc + 12 % PTFE 12P12T SU-8 + 10 % Talc 10T SU-8 + 10 % PTFE 10P

The spin coated samples were prebaked for 30 min at 65°C, followed by 10 min at 95°C to densify the film by evaporating the solvent, and then the samples were polymerized by exposing it with UV rays for 30 seconds using mask aligner - SUSS Microtec with a wavelength of 365nm and power 350watts. To achieve complete cross linking of SU-8 composite, the UV exposed samples were post baked for 20 min at 95°C, and then the samples were let to cool down for having better adhesion of SU-8 composite to the glass slide. Finally, the samples were stored in a decicator for further analysis. For coating pure SU-8 and SU-8 composites on the samples, the process and the parameters of spin coating, baking (pre and post) and UV exposure were kept same. The spin coating, curing process and UV exposure were performed in a class 10,000 clean room.

2.3 Surface Characterization 3D optical profilometer (Taylor Hobson Talysurf CCI) was used to determine the roughness of the coated surface. And the same instrument was used to measure the thickness of the coatings by removing the strip of cellophane tape attached at the end of the sample while spin coating; the step height between the coated and uncoated sample surface represents the thickness of the coatings.

The mechanical properties like hardness and young’s modulus were measured on the coated samples using Nanoindenter (Hysitron USA). The indentation load was applied using berkovich diamond indenterof triangular pyramid with an indentation load of 8mN and nanoindentation depth around 1µm. The data reported are the average of 4 indents on each sample with standard deviations indicated as error bars.

Wettability is an important surface property, and it is characterized as a measure of static contact angle of a liquid droplet on the surface of the sample. The static contact angle (WCA) was measured using goniometer (Holmarc Opto

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mechatronics Pvt Ltd). The results indicated are the average of three readings with standard deviations indicated as error bars, taken at different locations on the samples of SU-8 composite using DI water (volume of droplet = 5µl) at room temperature.

Friction and wear test were performed on all coated samples using a reciprocating ball on flat tribometer (TR 282M111, M/s Ducom Instruments, Bangalore – India). The schematic diagram of reciprocating tribometer is shown in figure 1.

The counter surface is the Si3N4 ball of 6mm diameter with roughness ~5nm, which is mounted to the reciprocating arm through a ball holder. To generate linear reciprocating motion for a smaller stroke of 1mm, with a frequency of reciprocation ~6Hz, the tribometer is modified with scotch yoke mechanism. The slider is loaded with a dead weight to apply the normal load of 1N & 3N, while the range of load for the tribometer varies from 0.98 to 9.81 N. The tangential force is measured by piezoelectric force sensor at the rate of 1000 data/s (PCB, Model 208C01), which has a resolution of 0.45 mN, and stiffness of 1.05 kN/µm. The initial COF and steady state COF were obtained from the average values of first 300 and last 600 cycles respectively, for each test been carried out for 3600 cycles with a new ball for every trial in a test. The data reported here are the mean of three repeatitive trials for every test, and the standard deviations was calculated and were indicated as error bars.

All tests were carried out at laboratory conditions with a temperature of 25±2°C and humidity of 60±5%.

Displacement sensor Balancing weight + Normal load

Pivot

Motor with Cam

37,28

Counterface (Ball)

Reciprocating Arm

Piezosensor Modified Scotch Yoke Mechanism

Sample

Linear Bearing

Rigid Frame

Figure 1: Schematic Diagram of a Reciprocating Tribometer.

3. RESULTS AND DISCUSSIONS 3.1 Surface Roughness and Water Contact Angle Figure 2 shows the plot of surface roughness and WCA of SU-8 and its composites. It is observed that the surface roughness has increased by adding the fillers to a certain concentration, and on further increase in concentration of fillers the surface roughness decreases. 10P10T has more roughness as compared to 12P12T composite, as higher concentration of fillers increases the viscosity of the mixture and limits the flowability during mixing and coating. Also the roughness of PTFE composite (10P) was found to be comparatively less than talc composite (10T), because PTFE has particle size of 1µm and for talc it is 10µm, which is almost 10 times lesser than talc particles; higher particle size of talc has given higher roughness in comparison with PTFE composite surface. The density of talc and PTFE are known to be 2.75 and 2.2 grams/cm3 respectively, hence the volume of PTFE is almost 25% more than the volume of talc for the same weight in the composite. Greater volume of PTFE in SU-8 combined with particle size reduces the flowability of the mixture while spin coating, and this would have resulted in lesser roughness [9].

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Figure 2: Surface Roughness and Water Contact Angle for SU8 and its Composites as a Function of Concentration of Fillers in wt %.

The measurement of water contact angle is to differentiate between the surfaces that wets known as hydrophilic surface (WCA <90°), and the surfaces that repels water known as hydrophobic surfaces (WCA > 90°). Hydrophilic surfaces supports high friction and adhesion as they posses high surface energy, while hydrophobic surfaces are lower surface energy surfaces

[19]

. All the SU-8 composite coated samples showed WCA less than 90°, indicating that the

surfaces are not very hydrophobic. On comparison with SU-8 and its composite, talc at 10 wt% concentration with SU-8 (10T) has shown higher WCA indicating hydrophobic behavior, while PTFE at 10 wt% concentration (10P) has shown hydrophilic nature because of decreased WCA, this decrease is due to incomplete curing of SU-8 because of impenetrability of UV radiations. It is observed that SU-8 composite at higher concentrations (10P10T and 12P12T) shows decreased WCA. This is due to improper curing caused by the presence of large volume of PTFE, which is in close agreement with WCA of 68° which was obtained for pre-baked SU-8 without UV exposure [9].

3.2 Mechanical Properties

Figure 3: Modulus of Elasticity and Hardness of SU-8 and its Composites as a Function of Concentration of Fillers in wt %. www.tjprc.org

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Figure 3 shows the mechanical properties of SU-8 and its composites tested by nanoindentation technique. The mechanical properties (both modulus of elasticity and hardness) were found to be increased after adding the filler materials in SU-8. The modulus of elasticity and hardness follows the same trend. By adding fillers with 2 wt%, the mechanical properties have increased tremendously as compared to pure SU-8, while on further increase of wt % of filler material till 8 wt%, the modulus of elasticity was almost same, while there was a gradual drop with hardness. This may be due to errors in manual stirring of SU-8 composites. But at 10 wt% concentration (10P10T), there was highest modulus of elasticity and hardness. It was observed further at 12 wt% (12P12T), there was a sudden decrease of elastic modulus and hardness because of poor bonding of fillers with SU-8 and insufficient homogeneity. Also, the viscosity of the SU-8 composite at higher concentration of fillers may be another reason for weak mechanical property because of limited flowability of SU-8 composite during mixing and spin coating.

The modulus of elasticity and hardness of 10T and 10P were found to be less when compared to SU-8 composites and more than the pure SU-8, and hence the combination of talc and PTFE as fillers has resulted in improved mechanical properties of SU-8 composite. Also talc has slightly higher elastic modulus compared to PTFE with SU-8, because PTFE is less dense and hence has large volume of material in comparison with talc for the same wt%. The presence of PTFE at higher concentration (12 wt%) results in incomplete curing of SU-8 composite, due to blockage of UV radiations at deeper ends. The another reason for talc is that it has -OH group, and can make better bonding with SU-8 by etherification [7,18].

Table 2: Hertzian Contact Pressure (MPa) for SU8 and its Composite Samples Hertzian Contact Pressure (MPa) Sample Abbreviation Lower Load (100 gms = 0.98N) Higher Load (300gms = 2.942N) SU-8 23.29 33.59 2P2T 42.96 61.97 4P4T 42.64 61.51 6P6T 41.53 59.91 8P8T 41.41 59.73 10P10T 48.73 70.30 12P12T 41.92 60.48 10T 37.55 54.17 10P 35.96 51.87 Hertzian contact pressure for SU-8 and its composites samples were calculated, by considering the above values of elastic modulus and the properties of material in contact (Ball) as per the literature. The following equation was used to calculate the Hertzian contact pressure [21] between a spherical ball (Si3N4) and the flat surface.

P=

(1)

where P, L, D and A are the Hertzian contact pressure, normal load, diameter of spherical ball and herzian contact area respectively.

E is the equivalent elastic modulus, given as

E=

(2)

Where E1, υ1 are the elastic modulus and poisson’s ratio of ball material, E2, υ2 are the elastic modulus and poisson’s ratio of SU8 and its composite. As per the supplier data, the values of E1 and υ1 for a spherical ball (Si3N4 ball of 6mm diameter) are 310Gpa and 0.27 respectively, and the value for υ2 was taken as 0.22 [3 ].

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The Hertzian contact pressure for SU-8 and its composites are reported in the table 2. From this we can observe that the values varies from 23.29 for pure SU-8 to ~48.73 and for 10P10T concentration at a normal load of 0.98N ≃ 1N. It varies from 33.59 for pure SU-8 to ~70.30 for 10P10T concentration at a normal load of 2.942N ≃ 3N. The values reported are in the range of 27 – 140 Mpa, and hence these composites can be used in polymer MEMS applications as they experience lower contact stresses [20].

3.3 Tribology Tests To evaluate the coefficient of friction (COF) of SU-8 and its composites, tribological tests were performed for 3600 sliding cycles at 1N & 3N normal applied load with sliding frequency of 6Hz. Figure 4a shows the plot of coefficient of friction versus number of reciprocating cycles for 1N load. To observe the COF before and after the initiation of wear at the interface, initial and steady state friction coefficient need to be determined. The initial and steady state friction coefficient were obtained from the average values of first 300 cycles and average of last 600 cycles respectively, for each test being conducted for 3600 sliding cycles. Fig 4b presents the data of initial and steady state friction coefficient for SU8 and its composites for 0.98N load. It is observed from figure 4a that all the composites shows the same profile with COF less than 0.12, except 4P4T which shows 0.20 at the end of the cycles; which may be due to errors in manual stirring.

Figure 4: (a) Raw Data of COF for 3600 Sliding Cylces.

(b)Initial and Steady State COF at 1N load for SU8 and its Composites.

2P2T and 10P10T shows the lowest COF profile compared to all other composites; with 10P10T maintaining the uniformity throughout the cycles. The data summarized in figure 4b shows the averaged values, with standard deviation as error bars. The initial COF for all samples are less than 0.075, while the final COF are less than 0.13, this is mainly because of the contribution of PTFE which has low COF. Higher concentration of fillers have shown lesser steady state COF in comparison with smaller concentration of fillers. The lowest initial and the steady state COF for the 10P10T composite is 0.075 & 0.088 respectively. 12P12T composite sample has resulted with more COF compared to all other composite and the data obtained for three trials at different locations produced large errors, and hence it is not shown in figure; the reason could be due to non flowabitlity of composite while spinning, because of large viscosity of SU8 composite.

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Figure 5: (a) Raw Data of COF for 3600 Sliding Cylces.

(b)Initial and Steady State COF at 3N Load for SU8 and its Composites.

Similar experiment was conducted for a applied normal load of 3N with sliding frequency of 6Hz and are shown in Figure 5a & 5b. As a general trend, the COF (both initial & steady state) should decrease with increase in normal load. But from figure for 2P2T concentration, the COF is almost the same like in case of lower load, while for the larger concentration the COF increases after few initial cycles, which may be due to the wear of the top layers of coatings at higher loads. The higher concentration of fillers at higher loads resulted in increase in COF when compared to smaller concentration. The coatings with COF greater than 0.3 are considered to be failed for wear analysis. All composite coatings except 6P6T has shown the COF lesser than 0.3, and hence the fillers with SU8 have contributed for the reduction of COF. At higher concentration 10P10T has also shown to exhibit lower steady state COF. Good mechanical properties was achieved only with talc as a composite in SU-8, however abrasive wear occurs due to higher friction [9]. With PTFE only as a composite, the COF is reduced. Therefore it is clear from the above study that the combination of PTFE and talc has improved both mechanical and tribological properties of SU-8, of which 10P10T concentration is most favourable because of higher roughness and lower surface energy.

4. CONCLUSIONS SU-8 based composite consisting of PTFE and talc as fillers, were coated on glass substrate. Surface characterization, mechanical and tribological properties were studied on the coated surfaces. The following are the conclusions made by adding fillers to pure SU-8. 

Surface roughness is increased by ~3 to 21 times as compared to pure SU-8, with highest change been observed for 10P10T concentration.



Increased elastic modulus by ~2.1 times and hardness by ~1.7 times could be achieved for 10P10T concentration.

For higher concentration of 12P12T, the mechanical properties decreased due to improper curing upon UV exposure by large volume of PTFE in the composite.



The combination of fillers at different wt % in SU-8 resulted in initial COF less than 0.08 and steady state COF less than 0.2 at lower load (1N), while at higher load both initial and steady COF were found to be increasing because of the failure of the top layer of coatings.



10 wt% of PTFE and talc (10P10T) were found to be favourable in increasing the mechanical and tribological properties of SU-8 polymer and can be used as coatings in MEMS devices that experiences lower contact stresses.

Equal proportion of fillers in SU8 composite is examined in this study, but to know the exact influence of filler

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(either PTFE or Talc) for the enhancement of mechanical and tribological properties; another examination at varying proportionate of filler keeping the other filler as constant (10%) may be studied.

ACKNOWLEDGEMENTS Author expresses sincere thanks to Prof. M S Bobji, Prof. M Muthu kumar and Mr. Pratheek of IISc Bengaluru for their help in providing tribometer facility.

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AUTHORS PROFILE

Mohan Kumar K is an Associate Professor in the Department of Mechanical Engineering at Dr. T. Thimmaiah Institute of Technology – KGF, Karnataka – India. He has almost 14 years of teaching experience. His area of interest includes FEM, Micro composites & MEMS tribology.

V Velmurugan is an Associate Professor in the Centre for Nanotechnology Research at VIT University, Vellore and has 15 years of teaching and research experience in the field of Nanotechnology. His research interests include Optical Interference Lithography for sub-wavelength structures, Nanomaterials for environmental applications, modelling, simulation of MEMS and Nanosensors.

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