Bioinspired Smart Coating with Superior ... - ACS Publications

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Bio-inspired Smart Coating with Superior Tribological Performance Shuangjian Li, Yulong An, Xiaoqin Zhao, Huidi Zhou, Jianmin Chen, and Guoliang Hou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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ACS Applied Materials & Interfaces

Bio-inspired Smart Coating with Superior Tribological Performance Shuangjian Li,†,§Yulong An,

†,*

Xiaoqin Zhao,

†

Huidi Zhou† Jianmin Chen,†,* and

Guoliang Hou† †

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,

Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China §

University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of

China

KEYWORDS: bio-inspired; smart; self-healing; friction and wear; thermal spray; engineering application

ABSTRACT: Inspired by the structure of cancellous bone and the nutrition metabolism of articular cartilage, we present a novel concept for a synthetic articular-cartilage-like material. The bio-inspired material possesses a low coefficient of friction even under ultra-high loads and has an extremely long lifetime. Furthermore, the composite shows zero-wear behavior and causes negligible wear damage to the friction pair. The superior tribological performance is attributed to the spontaneously generated articular-cartilage-like layer, which is constantly replenished by frictional heat and pressure. Our findings open a new area for industrial scale 1 ACS Paragon Plus Environment


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engineering applications to improve the friction and wear properties of moving components.

Mimicking the structures or replicating the design principles of materials in biological systems is favorable for the development of advanced materials with diverse properties.1-6 Articular cartilage is an example material that is often imitated. Articular cartilage is a lubricious load-bearing tissue that covers and protects the ends of long bones at the joints.7-8 However, due to its avascular and aneural nature, its nutrition can only be supported by chondrocytes via diffusion, and metabolic waste can only be discharged into the synovial fluid. Interestingly, the particular nutrition metabolism of articular cartilage relies on articular movement, which supplies a pressure stimulus on the articular cartilage.

Biologists and materials scientists have shown significant research interest in identifying the structure and understanding the mechanisms of articular cartilage and have attempted to replicate the cartilage using synthetic materials with superior friction properties.4,

8-11

However, previous work focused on the application of

artificial articular cartilage materials in biology, and only a few applications of such materials have been examined on the industrial scale, in which lowing the coefficient of friction (COF) and reducing wear on the components is of great technological significance for improving the reliability, security and efficiency.12-14 If bionic artificial articular cartilage materials are applied in engineering, several considerations immediately arise from the evaluation of the extensive effort exerted in related work. 2 ACS Paragon Plus Environment

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The first concern is that the strong hydration ability of hydrogels, the most common synthetic articular cartilage materials, provides low friction at the cost of reduced load bearing capability.4, 8, 15 Meanwhile, the excellent friction performance of reported artificial articular cartilages can only be exhibited in a sufficient amount of water4, 8 or other liquid media.9 Moreover, satisfactory performance is maintained for an unsatisfactory amount of time. Furthermore, most (if not all) previous studies simply mimicked the articular cartilage structure and did not focus on replicating its design principles. However, the optimum structure may vary with the boundary conditions and engineering application demands.16 Finally, despite the usefulness of the reported fabrication, the scalability, complexity, dimensionality, monetary and time costs are issues that material and engineering scientists need to address.

For engineering applications on the industrial scale, to improve the friction and wear properties of moving components, we developed a new artificial analogue using the common and highly effective surface engineering technology of thermal spraying (TS). Unlike previously reported processes that fabricated the articular-cartilage-like material before testing it, we used its working conditions to active the formation of an articular-cartilage-like layer that can be constantly repaired. In detail, inspired by the nutrition metabolism principles of articular cartilage and by combining the ubiquitous accompanying phenomena of friction and the significantly different response to thermal stimuli of ceramics and polymer materials (the thermal expansion of polymers is 22 times higher than that of ZrO2 ceramics, see Supporting Information

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Figure S2), we utilized friction-induced heat and pressure as the formation conditions and repair force of a cartilage-like hybrid coating.

Given the light weight, high hardness, and stiffness, combined with low thermal expansion, ZrO2 coating fabricated through TS presents as a hard phase, which is analogous to bone, which is light weight, high strength, and hard. In the TS process used in this work, powder feedstocks were introduced into a heat source via an inset carrier gas and were heated to the molten or semi-molten state. A “stream” of the resulting particles was accelerated and propelled toward the workpiece by high-velocity gas jet (Figure 1a). After impact with the substrate, these particles rapidly quenched in excess of 106 K/s. For brittle ceramic materials, ultra-high cooling rates cause micro-cracks and a nanoscale grain, columnar structure (Figures 1b and 1c), which favors the diffusion of infiltrated soft material in response to an environment stimulus. The impingement of thousands of subsequent particles results in the buildup of a thick coating.

The prominent feature in the scanning electron microscope (SEM) images of cancellous bone proximal to the articular cartilage (Figures 1d and 1e) is the presence of many disordered holes with a large range of diameters. These holes are aligned toward the mechanical load distribution. Inspired by the natural structure, an analogous cancellous bone host with a total diameter of approximately 200 µm was produced via laser texturing (Figures 1f–1h). To enhance the load capacity, a margin region with a width of approximately 40 µm was non-textured between two analogous 4 ACS Paragon Plus Environment

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cancellous bones. That was followed by the vacuum impregnation of modified polytetrafluoroethylene (M-PTFE), which served as the soft material. Thus, the precursor of the artificial articular cartilage was obtained. Cross-sectional analysis indicated that the holes were full of M-PTFE, and the depth of the textured holes was approximately 70 µm (Figure 1i).

Tribological measurements were conducted to reveal the superior friction properties. Compared with the COF of thermally sprayed ZrO2 ceramic coatings, an order of magnitude reduction was obtained for the bio-inspired coatings (inset in Figure 2a). Remarkably, the hybrid coatings exhibited a low friction (< 0.065) and an extremely long lifetime (>1×106 cycles) under ultra-high initial Hertzian contact pressure (1411 MPa). A slight decrease in the COF as the load increased is clearly shown in Figure 2b. The COF decreased from 0.070 to 0.048 as the load increased from 1 N to 30 N, which corresponds to a maximum Hertzian contact pressure of 654.9 MPa to 2035 MPa, respectively. The bearing load was much higher than that of the PTFE and ceramics composites prepared through a conventional process (approximately 6.3MPa17-18). Compared with the applied load, the sliding velocity exerted a greater influence on the COF of the hybrid coatings. An obvious decrease is depicted in Figure 2c. The COF decreased from 0.089 to 0.048 when the sliding velocity increased from 1 cm/s to 15 cm/s.

The wear performance, another critical factor for tribological properties, was also evaluated. An unexpected result was obtained. The near-constant volume loss of the 5 ACS Paragon Plus Environment


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hybrid coatings is shown in Figure 3a. In other words, the coating volume was lost only in the initial sliding period. Another surprising result was that the weights of the bio-inspired coatings did not decrease during the extremely long sliding test (Figure 3b). We attribute the volume loss to frictional pressure compacting a small notch rather than wear. That is, the bio-inspired coatings possess zero-wear property. For comparison, the sprayed ceramic coating showed a linearly increasing trend for the weight loss, which is also included in Figure 3b. Severe wear damage occurred on the surface of the ceramic ball sliding against the sprayed ceramic coating (Figure 3c1), whereas almost no wear was exhibited on the ceramic ball that rubbed against the bio-inspired coating. Surprisingly, no obvious damage was observed on the surface of the ceramic ball sliding against the composites after 1,000,000 cycles (Figure 3c3) compared with that after 500 cycles (Figure 3c2).

The cross-sectional transmission electron microscope (TEM) image of the worn surface onto the synthetic cancellous bone analogue shows a compact layer with approximately 200 nm in thickness covering an artificial spongy bone (Figure 4a). With textured coating as the “cancellous bone” (Figure 1h), the compacted layer and the cross-section of the bone (shown in the red circle in Figure 4c) show a striking similarity. According to the point element composition in Figure 4a, the formed articular-cartilage-like layer mainly consists of C, F, O and Zr. The selected area electron diffraction (SAED) pattern in Figure 4a shows that the analogue of the articular cartilage contains nanocrystallites and amorphous components. The high-resolution TEM image (Figure 4b) clearly shows the ZrO2 nanocrystallites with 6 ACS Paragon Plus Environment

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a d-spacing of 0.296 nm. Furthermore, a characteristic amorphous region (disordered dot contrast) surrounds the nanocrystallites. Analogous to the function of the articular cartilage, the formed articular-cartilage-like layer with nanocrystallites surrounded by an amorphous material is responsible for the low COF of the hybrid coating.

The SEM images of the worn tracks of the composite coatings after different sliding cycles shown in Figure 5 reveal that the microcracks induced by friction went through non-existence (Figure 5a), generation (Figure 5b), extension (Figure 5c), shrinkage (Figure 5d), and healing (Figure 5e). Figure S5 clearly shows the generation of micro-cracks on the articular cartilage analogue. As depicted in the elemental distribution images of the worn track after 500,000 cycles, the F:Zr ratio of the area with numerous cracks is much higher than that with a few cracks (Figure 5g), with Zr having an almost homogeneous distribution (Figure 5i). The F element distribution image (Figure 5h) indicates that F accumulated in the region with many cracks, although it can be found over the entire worn surface. In contrast, Figure S6 and Table S2 indicated that F only existed in the textured region. Interestingly, many bright yellow dots appeared around the microcracks (except in the undetected area), suggesting that microcracks are the channels from which M-PTFE emerges. Given the healing phenomenon of the cracks, we conclude that the much higher thermal expansion of M-PTFE, compared with ZrO2 ceramics, induced by frictional heat resulted in its emergence on the worn surface; under the influence of frictional force, the overflowed polymer brought to the entire worn track and spontaneously formed and repaired the articular cartilage analogue. The repair mechanism is analogous to 7 ACS Paragon Plus Environment


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the nutrition metabolism of the articular cartilage, which relies on articular movement to impose a pressure stimulus on the articular cartilage. Meanwhile, the “nutrition” (M-PTFE) for the articular-cartilage-like layer relies on the friction behavior, which imposes pressure and heat stimuli on the composite coating. The constant self-repairing behavior is responsible for the ultra-long operation lifetime and excellent wear resistance.

In summary, inspired by the structure of cancellous bone and the nutrition metabolism principles of articular cartilage, we utilized friction-induced heat and pressure as a trigger to form and repair an analogue of articular cartilage. The low COF of the bio-inspired coatings can be maintained under ultra-high initial Hertzian contact pressure (1411 MPa) with an extremely long lifetime (>1×106 cycles). Moreover, the low friction properties can be maintained over a large range of sliding velocities and applied loads. The composites also show zero-wear properties during ultra-long operation and cause negligible wear damage to the surface of ceramic balls rubbing against them. The superior tribological performance of the hybrid coatings is attributed

to

the

spontaneously

generated

and

constantly

repaired

articular-cartilage-like layer by heat and pressure generated by friction. Our findings represent a new area for industrial-scale engineering applications to improve the friction and wear properties of components, given that TS is a scalable, highly efficient and economical process. The bionic concept, utilizing the serving conditions of components as a trigger and repairing force, provides important inspiration for the development of biomimetic materials. 8 ACS Paragon Plus Environment

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Figure 1. a) Schematic of thermal spray technology. b) SEM images of the sprayed ceramic surface. c) Higher magnification image of the sprayed ceramic surface, showing the nanoscale grain, columnar structure. d) Sketch map of a knee cap. e) SEM image of the cancellous bone of a chicken. f) Sketch map of the laser surface texturing process. g) SEM image of the textured sprayed ceramic coating surface. h) Magnified view of the morphology of the textured sprayed ceramic coating surface. i) BSE image of the cross-sectional view of the ceramic coatings infiltrated with M-PTFE (gray zones with red pointed arrows)



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Figure 2. a) COF of both coatings as a function of sliding cycle at a fixed velocity of 10 cm/s and a normal load of 10 N (corresponding to a theoretical initial Hertzian contact pressure of 1411 MPa). COF of the composite coatings b) as a function of applied normal load (at a velocity of 10 cm/s) and c) for different sliding velocities (at an applied load of 10 N).


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Figure 3. a) Volume loss of the composite coatings for different sliding cycles (at an applied load of 10 N). b) Weight loss of both coatings for different sliding cycles (at an applied load of 10 N). c) Optical micrographs revealing the wear damage on the surface of ZrO2 balls sliding against c1) the sprayed ceramic coatings after 500 cycles and the hybrid coatings after c2) 500 cycles and c3) 1,000,000 cycles.



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Figure 4 a) TEM image of the articular cartilage analogue on synthetic cancellous bone. Insets are the elemental composition at selected points (right) and SAED pattern (left). b) High resolution TEM image of the articular-cartilage-like layer. c) Cross-section of bone, as depicted by Wikipedia.


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Figure 5 (a–e) SEM images of the worn tracks of the composite coatings after different numbers of sliding cycles: a) 10,000 cycles, b) 20,000 cycles, c) 100,000 cycles, d) 200,000 cycles, e) 500,000 cycles. f) Magnified view of e). g) Live elemental map of f), where the atomic ratio of F and Zr in the blue and red areas is 1:2 and 1:16, respectively, and the gray region was undetectable due to the depth of the microcracks. Distribution of elements h) F and i) Zr.



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ASSOCIATED CONTENT

Supporting Information.

Experimental and characterization details; descriptions of M-PTFE; thermal behavior of M-PTFE and the ZrO2 ceramic coating; nominal contact friction-induced temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author E-mail:[email protected]; and [email protected]

Funding Sources The work was financially supported by the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. 2014378) and the West Light Foundation of the Chinese Academy of Sciences.

Notes The authors declare no competing financial interest.


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