Microstructures and friction-wear characteristics of bivalve shells

Microstructures and friction-wear characteristics of bivalve shells

Tribology International 39 (2006) 657–662 www.elsevier.com/locate/triboint Microstructures and friction-wear characteristics of bivalve shells Xian J...

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Tribology International 39 (2006) 657–662 www.elsevier.com/locate/triboint

Microstructures and friction-wear characteristics of bivalve shells Xian Jiaa,*, Xiaomei Lingb, Donghai Tanga b

a Department of Science and Technology, University of Science and Technology Beijing, Beijing 100083, China Department of Analytical Chemistry, School of Pharmaceutical Science, Peking University, Beijing 100083, China

Received 16 August 2004; received in revised form 27 April 2005; accepted 3 May 2005 Available online 14 July 2005

Abstract All structural biological materials are nearly composite materials and often exhibit some superior properties. In order to obtain useful information for the design and manufacture of composite materials, the microstructures and the friction-wear properties of three species of bivalve shells were studied in this paper. The results showed that the microstructure of tested bivalve shells I (Meretrix meretrix) and II (Saxidomus purpuratus) are constructed of about 10–100 mm small platelets stacked in brick-and-mortar fashion and small platelets consist of lamella clusters. The lamellae are parallel inside the individual cluster. The orientations of adjacent lamellae clusters form the angle of about 70–908. The thickness of lamellae is about about 0.2–0.6 mm. The organic component and the calcium carbonate form the threedimensional net-like microstructure, respectively, which interlaces each other to form the microstructure of the bivalve shells. The microstructure of bivalve shells III (Periglypta chemnitzii) is constructed of about 10–40 mm particles with the internal structure of lamellae clusters as the above mentioned. Under the experimental conditions of medium-carbon steel counterpart and sliding dry friction, the friction coefficient of the bivalve shells is lower than that of grey cast iron HT200. The organic component of the bivalve shells can transfer to the friction interface and form organic film, which not only lubricates the friction interface but also protects the friction surface. The organic component is very important to the friction-wear property of the shells. q 2005 Elsevier Ltd. All rights reserved. Keywords: Friction; Wear; Biomaterials; Microstructure; Composite

1. Introduction The research and the development of modern materials have entered an era of composite materials, function materials and environmental protection materials. The biomimetic design and exploration of composite materials exactly reflects this new trend in materials research [1–3]. It is well known that biomaterials, such as bamboo, wood, and mollusc shell, are all natural composite materials and have very complicated microstructures resulting from the biotic evolution for thousand millions of years [4,5]. The investigation of the microstructure and composite mechanism of these biomaterials can undoubtedly inspire scientific * Corresponding author. Address: Department of Science and Technology, University of Science and Technology Beijing, 30 Xueyuan Road, Beijing 100083, China. Tel.: C86 10 623 32875. E-mail address: [email protected] (X. Jia).

0301-679X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2005.05.001

researchers to create artificial composite materials with excellent properties. The reported results [1] have shown that mollusc shells consist primarily of calcium carbonate, together with a relatively small amount of organic matrix material (less than 5 wt%). Calcium carbonate is an ionic crystal with relatively low modulus and strength compared with oxide or carbide ceramics. Yet incorporated into the architecture of shells the chalky substance exhibits remarkably high flexural and compressive strength, and unlike man-made ceramics, it is very resistant to fracture. Furthermore, the composite hardness can exceed that of the consistent calcium carbonate minerals (calcite and aragonite) depending on orientation. Mollusc shells have various architectures, such as a well-ordered crossed-lamellar structure with plywood-like lamellar 10–40 mm, a small platelet structure of calcium carbonate stacked in brick-and-mortar fashion and the platelets are about 300 mm thick with an aspect ratio of about 8, a forliated structure, a simple prism structure, and a complex structure.

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The mechanical properties of mollusc shells vary widely, but the nacre (mother of pearl) often found at the inner layers of molluscan shells has been reported to have a fracture toughness three to four orders of magnitude higher than calcium carbonate [6], a tensile strength of 140– 170 MPa, a Young’s modulus of 60–70 GPa, and a three point bending fracture work of 350–1240 J/m2 depending on its hydration state [7]. The presence of water increased ductility and toughness by approximately an order of magnitude. The mechanical properties of wet nacre of the red abalone shell were pffiffiffiffi reported as follows: fracture toughness, 7G3 MPa m and tensile strength, 180G 20 MPa. Wet samples of nacre from Pinctada pffiffiffiffi margaritifea had a fracture toughness of 10G6 MPa m and a tensile strength of 220G60 MPa [8]. Currently, the methods used to generate ceramic materials are unable to control crystal density, orientation, and morphological uniformity to the degree observed in mollusc shell composites. Understanding of the structure and formation mechanisms of bioceramic composites will lead to enhancing mechanical properties as toughened ceramics. These are important in biological systems to achieve functional biological ceramic composites and suggest that a similar benefit can be attained with biomaterial designs for enhanced performance. At present, many novel approaches are being explored through the application of biomimetic principles elucidated from the assembly and structure of mollusc shells. However, there remains a large gap between the control of the morphological and chemical features of these material systems and those observed in mollusc shell structures. This gap suggests that a great deal of improvement in biomaterials performance can be anticipated as a more comprehensive understanding of these interactions is developed and utilized. These mollusc shells are all toughened nanoscale composites with mechanical properties optimized for a specific repair function, although some mechanical properties have been tested, there is not a report on the friction-wear property. In this study, the microstructures of three species of bivalve shells were observed by SEM and EDX, their friction-wear behaviors were investigated by block-on-ring tribometer MM200 compared with those of grey cast iron HT200.

2. Experimental Three species of bivalve shells and grey cast iron HT200 were used as tested materials. Because grey cast iron has better sliding friction-wear property, HT200 was selected as a comparative material. First, the samples of bivalve shells for microstructure observation were ground by 150–1000# emery paper, polished on polishing machine and etched with 95% concentrated sulphuric acid, then the microstructure was observed by SEM (S250MK3). For friction-wear test, the samples of bivalve shells were machined into

Fig. 1. Schematic of friction-wear test.

the size of 15 mm!10 mm and adhered onto grey cast iron HT200 substrate, the final size of the samples was 15 mm! 10 mm!10 mm. The size of grey cast iron sample was also 15 mm!10 mm!10 mm. Counterpart samples were made of hardened and low-temperature-tempered steel 45 and their shape was a ring of 50 mm outside diameter, 10 mm internal diameter and 10 mm thickness. Friction-wear experiment was carried out on block-on-ring tribometer MM200 under sliding dry friction as shown in Fig. 1. The counterpart steel 45 ring rotated at 200 r/min. Correspondingly, sliding friction velocity was 0.52 m/s. The wear time was 40 min and the sliding friction distance of each sample was 1256 m. Normal load acting on the block sample was 68.6 N. The worn surfaces were observed by SEM.

3. Results and discussion 3.1. Microstructures of bivalve shells The tested bivalve shells have different microstructures. The results are shown in Figs. 2 and 3. Fig. 2(a) presents the microstructure of the shell I, Fig. 2(b) gives the surfacedistribution of calcium about Fig. 2(a), Fig. 2(c) is the cross section structure of the shell I and Fig. 2(d) shows the local magnification of the small platelet in Fig. 2(a). It can be found that the microstructure of the shell I is composed of about 10–60 mm small platelets which are separated by thick dark strips (see Fig. 2(a)) which is indicative of an organic composition [9]. The quantities of organic composition, i.e. matrices, are variable. All the soluble matrices are glycoproteins, with low S contents. Their molecular weights, the protein-sugar ratios and acidities are variable. Neither a gastropod nor a bivalve pattern is recognized. The diversity of the organic matrices probably plays a main role in the fossilization processes of mollusc shells [10]. Highspatial-resolution XANES maps also showed that sulfate is the principal component in organic matrices of mollusc shells [11]. And Fig. 2(b) proves that there is a great deal of calcium at the surface of the shell I. This is an agreement with the reported results [1,3], i.e. mollusc shells are composed of more than 95% calcium carbonate, which is either calcitic, aragonitic or consists of a mixture of aragonite and calcite [9]. So it can be concluded that the small platelets of the shell I consist of calcium carbonate.

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Fig. 2. Microstructure of the shell I. (a) surface, (b) surface-distribution of calcium for (a), (c) cross section, and (d) local magnification of small platelet in the microstructure (a).

The layered composite shells are characterized by a calcium carbonate inorganic ‘brick’ system with a biopolymer ‘mortar’. Fig. 2(c) shows that the small platelets were 0.5–3.0 mm thick. The size of these small platelets is obviously larger than that of the ‘bricks’ in mollusc shells reported by Jackson et al. [7], i.e. 0.2–0.6 mm thick and 5–10 mm wide. These small platelets are stacked in brickand-mortar fashion, the calcium carbonate inorganic ‘brick’ system is bonded together with a biopolymer ‘mortar’ organic ‘adhesive’ between the bricks in the range of about 0.1–1.0 mm thick, this thickness is larger than 20–30 nm [7]. The mass fraction of organic material in bivalve shells is less than 5% [3]. These small platelets consist of different lamella clusters. The basic structure features of different lamella clusters are similar; only the orientation of lamella clusters changes. The angle between adjacent lamella

clusters is about 70–908. The lamellae are parallel within the individual cluster. Between the calcium carbonate lamellae, not only there exits organic binding, but also there exits calcium carbonate crystal connecting as show in Fig. 2(d). In Fig. 2(d), the dark area is the organic component and the bright area the calcium carbonate crystal. The organic component and the calcium carbonate crystal form the three-dimensional net-like microstructure, respectively. The two kinds of the three-dimensional netlike microstructure interlace each other to form the microstructure of the bivalve shells. Fig. 3(a) displays the structure of the shell II. It is constructed of small platelets too. But the small platelet size of the shell II (about 30–100 mm) is larger than that of the shell I (about 10–60 mm). The internal structure of small platelets is also lamellae clusters, which become the angle

Fig. 3. Microstructure of bivalve shells. (a) the shell II and (b) the shell III.

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of 70–908 each other. Their microstructures are very similar to that of the shell I, and the only difference is that the size of small platelets changes for different shells. Fig. 3(b) shows the microstructure of the shell III, which consists of about 10–40 mm particles (calcium carbonate crystal) and organic component matrix. The particles are surrounded by sinuous organic component matrix. This kind of microstructure is not discovered in the shells I and II, and so far has not been reported in other literatures. The previous investigations [3,10,11] have indicated that the crossed lamellar layer is the most widespread and its basic structure is similar in all the studied species of mollusc shells. Fig. 3(b) also suggests that the particles of calcium carbonate crystal are distributed homogenously in the organic component matrix, i.e. this kind of microstructure is a typical particlereinforced composite material. The internal structure of particles also exhibits lamellae clusters as the above mentioned. The lamellae within each lamellae cluster align in the parallel rows. Some particles are composed of different lamellae clusters, these contiguous lamellae clusters cross each other in the angle of about 70–908, and other some only contain one lamellae cluster. The orientations of lamellae clusters inside different particles are random and have not the relation between them. Between the lamellae inside lamellae cluster, there is the presence of organic membranes whose thickness is approximately 0.3 mm. Recent studies have focused on the molecular interfaces between the ‘bricks’ and the mode by which overlying layers of bricks are interconnected. The organic matrix provides the molecular template for inorganic nucleation and crystal growth, as well as the scaffold, which controls the layer thickness. Registry between the ‘bricks’ appears to be influenced by nanopores in the organic sheets (‘mortar’) [12,13]. These pores provide connections between bricks and localize new layers of growth relative to the previous layers [13], they also provide a network for interlayer crystal growth to produce a continuum crystal morphology in the shell structure. Currently, the formation mechanisms of these microstructural characteristics constructed of the calcium carbonate particles and the organic component matrix have not been entirety known, but its understanding will provide new information on developing biomimetic composite materials.

Fig. 4. Friction coefficient of samples.

Fig. 6. Because the microstructure of bivalve shells is constructed of small platelets or small particles of calcium carbonate and organic matrix material, the inorganic component and the organic component divide each other in nanometer order, and the organic component has a good deformability, during friction, the squeezing action of friction force and normal load will make the organic matrix material transfer onto the friction interface and form organic film at the friction interface. Because the organic component contains element S, in order to identify whether there is the organic component at the friction interface, it can be examined by EDX whether element S exits at the worn surface of the counterpart. The EDX experimental results shows that there is the presence of element S at the worn surface of the carbon steel counterparts and the distribution of element S is very homogenous. Fig. 6(e) gives a typical image about the distribution of element S at the worn surface of the counterpart. The organic film not only lubricates the friction interface but also protects the friction surface and improves the contact state of the friction interface. From Fig. 6, it can be found that the worn surfaces of shells are very smooth, but there is a lot of evident strip-shaped wear marks at the worn surface of the counterpart. Meanwhile, friction will produce a lot of heat, which can lead the temperature to rise at the friction interface. Because the organic component of bivalve shells is very sensitive to heat, its pyrolysis will take place at the friction interface and in the worn surface layer, and produce tar-like substance and some gases, such as CO, CO2, H2 and CH4. The tar-like substance can lubricate the friction interface and gases form gas-cushion [14].

3.2. Friction and wear properties of bivalve shells The friction coefficient of three species of bivalve shells and grey cast iron HT200 is shown in Fig. 4 and the volume wear loss in Fig. 5. Fig. 4 shows that the friction coefficient of the shells is much smaller than that of grey cast iron HT200, but only is the volume wear loss of the shell III smaller than that of grey cast iron HT200, the volume wear loss of the shells I and II is larger than that of grey cast iron HT200 shown in Fig. 5. The worn surfaces were observed by SEM, the results showed that the worn surface of bivalve shells are more smooth than that of grey cast iron HT200 shown in

Fig. 5. Volume wear loss of samples.

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Fig. 6. Worn surface morphologies (SEM). (a) the shell I, (b) the shell II, (c) the shell III, (d) grey cast iron, and (e) surface-distribution of element S at the worn surface of counterpart steel 45.

Owing to the beneficial action of organic component, the shells have better self-lubricative function and lower friction coefficient compared with grey cast iron HT200, and at the same time, have a small wear rate which is close to 10K4 mm3/N/m. But the pyrolysis of organic component in the surface layer of shells makes the strength of layer lower and easily break, which makes the shells wear more seriously than grey cast iron HT200 does. Therefore, the ultrastructure of these shells is an important to their mechanical properties. The organic component of these structures is particularly beneficial to the friction-wear property of the shells despite being only a few percent of the total shell materials on a weight basis. 4. Conclusions (1) The tested bivalve shells have two kinds of microstructures, the first is about 10–100 mm small

platelets stacked in brick-and-mortar fashion and small platelets consist of lamella clusters. The lamellae are parallel within the individual lamellae cluster, the orientation between adjacent lamellae clusters form the angle of about 70–908 and the thickness of lamellae is about 0.2–0.6 mm, there exits the organic material of 0.1–1.0 mm thickness between lamellae. The organic component and the calcium carbonate crystal form the three-dimensional net-like microstructure, respectively. The two kinds of the three-dimensional net-like microstructure interlace each other to form the microstructure of bivalve shells. The second is about 10–40 mm particles with the same internal structure of above mentioned lamellae clusters, and the particles are surrounded by the organic material. The formation mechanisms of the second need to be further studied, and its understanding will provide new information on

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developing biomimetic composite materials, which are reinforced by particles with lamellar internal structure. (2) Under experimental conditions, the friction coefficient of bivalve shells is evidently smaller than that of grey cast iron HT200. Except the shell III, the shells I and II have the larger volume wear loss than grey cast iron HT200. During friction, friction action makes the organic matrix material transfer to the friction interface and form organic film. The organic film not only lubricates the friction interface but also protects the friction surface, which makes the bivalve shells have the lower friction coefficient than grey iron cast HT200 does. The organic component of these structures is particularly significant to the friction-wear property of the shells despite being only a few percent of the total shell material on a weight basis. This will provide very useful information for designing a new self-lubricative material.

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