Improvement of microwear resistance of PDA/PTFE coatings by annealing

Yan Jiang ^a), Dipankar Choudhury ^{b, c)}, and Min Zou^{*, b, c)}

^a) School of Materials Science and Engineering, Jiangsu University, Zhenjiang, Jiangsu, China

^b) Department of Mechanical Engineering, University of Arkansas, Fayetteville, AR 72701, USA

^c) Center for Advanced Surface Engineering, University of Arkansas, Fayetteville, AR 72701, USA

INTRODUCTION: Polytetrafluoroethylene (PTFE) is widely used for engineering applications due to its low coefficient of friction, high-temperature stability, and excellent chemical resistance.¹ Unfortunately, PTFE coatings are very easily worn due to their poor mechanical property and weak adhesion to substrates. To increase the adhesion of PTFE coatings to substrates, a polydopamine (PDA) adhesive underlayer has been added.² This layer significantly improved the adhesion of PTFE to substrates. The formed PDA/PTFE dual-layer coating exhibited stronger wear resistance than a PTFE coating alone while maintaining the similar coefficient of friction.

Heat treatment is an effective method to improve the mechanical and tribological properties of polymers. Some researchers have investigated the effect of annealing temperature and duration on the properties of PTFE coatings.^3-6. However, the relationships between the annealing condition and the wear were not reported for the PDA/PTFE multilayer coatings, which is important for the application of PDA/ PTFE as a solid lubricant coating. Here, we study the effects of annealing on the microwear properties of PDA/PTFE by scratching the coatings surface at different normal loads using an atomic force microscope tip.

METHODS:  Clean stainless-steel sheets (Grade-316) were placed into tris(hydroxymethyl)aminomethane buffer solution (pH ≈ 8.5) followed by adding dopamine hydrochloride for the PDA deposition. The PDA deposition was continued for 45 minutes at 25 rpm in a rocker with 10^o rocking angles, while the temperature of buffer solution was maintained 60 ^oC. Immediately after the PDA deposition, the samples were taken out of the bath, rinsed with deionized water and dried with nitrogen gas. PTFE aqueous dispersion (60 wt%) was deposited on the PDA coating by a dip coater at 10 mm/min dipping and withdrawing speed to make the PDA/PTFE coatings. Three groups of samples were prepared. All samples were first heated to 120 ^oC for 3 minutes to remove the water from the coatings. Next, the samples were heated to 300 ^oC for 4 minutes to remove the surfactant. Finally, to study the effect of the annealing, the samples were heated at 372 ^oC for three different durations: 0 minute, 4 minutes and 8 minutes. Microwear testing was performed with AFM in contact mode, using a silicon cantilever with a triangular pyramid tip to scratch the sample surface at different normal loads.

RESULTS: Figure 1A, B, and C show the morphologies of the PDA/PTFE coatings after scratching the center area. The applied load during the scratching was 167 nN for 0 min annealing sample and 334 nN for 4 min and 8 min annealing samples. After scratching, a serious worn area was observed on the coatings without the final annealing step, and a large amount of loose wear debris piled up on the left side of wear scar (Figure 1A). For the coatings annealed for 4 and 8 min, no obvious worn scar was found at 167 nN load. When the load was increased to 334 nN, worn scars were visible (Figure 1 B and C), but the wear was much less visible than that in the coating without the final annealing step. This indicates that the final annealing step significantly increased the wear resistance of the PDA/PTFE coatings.

Figure 1D shows the bearing area ratio curves of 4 min and 8 min annealing samples after scratching 1 and 15 cycles at 800 nN. At the same number of scratching cycles, the curves of the 8 min annealing sample are less steep than those of 4 min annealing

sample and have a larger height range, indicating the 8 min annealing sample had a larger wear depth and pileup height than the 4 min annealing sample. Therefore, 4 min annealing sample has better wear resistance.

Figure 1 AFM morphologies of the samples after area scratching for 1 cycle: A) 0 min, B) 4 min, and C) 8 min annealing. D) Bearing area ratio histograms of the scratched areas on the 4 min and 8 min annealing samples.

DISCUSSION:

PTFE particles are composed of a ribbon-like crystalline structure that is folded compactly through weak attraction force into a globular shape.⁷ Annealing above the melting point of PTFE causes the polymeric chains in the PTFE particles to stretch along the axial direction of the columnar particles. The particle is elongated to transform into spindle shape with a looser structure. When the end of one PTFE particle reaches another particle during the elongation, they will bind together to form a connected network structure between the PTFE particles, resulting in more wear resistance in the annealed samples.

With increasing annealing duration, more PTFE particles are elongated further and they tend to orient themselves normal to the substrate, resulting in some protrusions on the coating surface and thus increasing the surface roughness of the coatings. When scratching, most of the wear occurs on the protrusions of the coating surface. The top layer of the coatings having a higher roughness wears off easily.⁸

Moreover, some fibril structures are also observed on the wear scar of 4 min and 8 min annealing samples (Figure 1B and C). They are from the extending of PTFE particles induced by AFM tip scratching. The folded structure of PTFE particles becomes loose after annealing. When the AFM tip scratches on the PTFE particles, at a certain load, the tensile stress produced by the moving tip overcomes the attraction force in the folded crystalline structure, in which case, the ribbon-like crystalline structure is pulled out of the particles to form fibril structure and cause wear when the fibrils break.