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Effects of roughness on interfacial performances of silica glass and non-polar polyarylacetylene resin composites
Applied Surface Science 253 (2007) 9357–9364 www.elsevier.com/locate/apsusc
Effects of roughness on interfacial performances of silica glass and non-polar polyarylacetylene resin composites Z.X. Jiang, Y.D. Huang *, L. Liu, J. Long Department of Applied Chemistry, Faculty of Science, Harbin Institute of Technology, PO Box 410#, Harbin 150001, People’s Republic of China Received 27 April 2007; received in revised form 28 May 2007; accepted 28 May 2007 Available online 6 June 2007
Abstract The influence of roughness on interfacial performances of silica glass/polyarylacetylene resin composites was investigated. In order to obtain different roughness, silica glass surface was abraded by different grits of abrasives and its topography was observed by scanning electron microscopy and atomic force microscopy. At the same time, the failure mechanisms of composites were analyzed by fracture morphologies and the interfacial adhesion was evaluated by shear strength test. The results indicated that shear strength of silica glass/polyarylacetylene resin composites firstly increased and then decreased with the surface roughness of silica glass increased. The best surface roughness range of silica glass was 40– 60 nm. The main mechanism for the improvement of the interfacial adhesion was physical interlocking at the interface. # 2007 Elsevier B.V. All rights reserved. PACS : 68.35.Np Keywords: Polyarylacetylene resin; Interlocking; Scanning electron microscopy
1. Introduction The addition of silica fiber reinforcement is known to improve the stiffness, strength and the high temperature performance of polymeric materials. The mechanical properties of the resulting silica fiber reinforced composite materials depend not only on the properties of each primary component but also on the nature of the fiber-matrix interface. A strong interface generally leads to the best composite properties, so many effective interfacial modifications for silica fiber that forming strong interface has been investigated [1–3]. Among all the available surface treatments, the most widely used modification method for silica fiber is sizing with silane coupling agents [4,5]. Common silane coupling agents used to increase the interfacial adhesion of fiberreinforced composites incorporate epoxy, amine, or anhydride functional groups as the reactive organic component, depending on the chemistry of the matrix resin [6,7]. Moreover, other techniques such as corona discharge or plasma treatment can provide a superficial cleaning and/or chemical modification * Corresponding author. Tel.: +86 451 86414806; fax: +86 451 86413711. E-mail addresses: [email protected] (Z.X. Jiang), [email protected] (Y.D. Huang). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.05.075
[8,9]. The corresponding surface modification enhances the physical and/or chemical adhesion between the matrix resin and the treated surface [10]. Laser treatment, which is studied in recent years, can also provide adhesion improvement as a result of physical modification at the surface [11,12]. Though there are many surface modification methods for silica fiber, these treatments are believed to improve interfacial performances of fiber/matrix composites in the following two main ways: the introduction of reactive groups onto fiber surface and the increase of roughness of fiber surface [13,14]. The effects of the reactive groups on the interfacial performances of composites have been widely studied, while the effects of roughness on the interfacial performances of composites have not been paid enough attention. Hence, the aim of this paper is to individually study the influence of roughness on interfacial performances of silica fiber reinforced matrix resin composites. Due to the difficulty of controlling roughness when use silica fiber as substrate, silica glass discs are chosen to instead of silica fiber [15]. The resin chosen is non-polar polyarylacetylene (PAA) resin [16,17]. The main reason for choosing the PAA resin is that the molecular structure of all benzene ring of PAA resin can avoid chemical interaction between PAA resin and the silica glass surface. Even
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Table 1 Diameter of abrasives used Grit of abrasives
Diameter of abrasives (mm)
#2000 #1500 #800 #150
6–8 8–12 14–20 70–80
so it is also difficult to avoid all weak interaction between silica glass discs and PAA resin except the interaction caused by roughness. However, we use the same substrate material and resin in order to obtain same interaction between substrate and resin except the interaction caused by roughness. In this way, we can ignore the effect of these interactions mentioned above, and individually study the effects of roughness on the interfacial performances of silica glass and PAA resin composites. 2. Experimental Fig. 2. Schematic illustration of sample for mechanical testing.
2.1. Materials 2.1.1. Surface pretreatment of silica glass The silica glass discs (Education Optics Lens Factory of Haian City, Jiangsu Province, PR China) were fabricated into 20 mm 10 mm 2 mm, and were abraded on H-015 two-axle lens lapping machine (Nanjing Instrument Machine Tool Works, PR China) with different grits of abrasives (supplied by Minshan Abrasives Factory, PR China). Grits of abrasives used were #2000, #1500, #800 and #150, which are shown in Table 1. Before use, the discs were washed in acetone for 15 min, and then were washed with ultrapure water in ultrasonic bath for 20 min, in the end, were dried in oven for 10 h at 120 8C. 2.1.2. PAA resin PAA resin is a brown liquid with the density of 1.05 g/cm3 and the viscosity of 200–3000 mPa s (corresponding to a temperature changing from 90 8C to 30 8C) (supplied by Beijing Aerospace Research Institute of Material and Processing Technology, PR China). The molecular structure of PAA resin and its prepolymer is shown in Fig. 1. 2.1.3. Sample for shear strength test To prepare sample for shear strength test, two silica glass discs abraded by the same grit of abrasive were used. In vacuum
condition, about 25 ml PAA resin were dropped on one of silica glass disc which was kept flat in container, and then the other silica glass disc was put on top of it and kept for 10 min. Subsequently, the specimen was fastened on a mould and cured at 120 8C for 2 h, 140 8C for 2 h, 180 8C for 2 h, 200 8C for 2 h, and post-cured at 250 8C for 10 min, respectively. After cooling to room temperature naturally, the specimen was removed from the mould and affixed to a set of holding device by epoxy adhesive. Then the sample was under mechanical testing. The schematic plan of the sample is given in Fig. 2. 2.2. Analysis methods 2.2.1. SEM observation SEM study for samples with gold coated were carried out using a FEI Sirion 200 scanning electron microscope (Royal Dutch Philips Electronics Ltd., The Netherlands) connected with an energy-dispersive X-ray microanalyzer (EDAX Inc., USA). 2.2.2. AFM measurement An atomic force microscopy (NT-MDT Co., Zelenograd Research Institute of Physical Problems, Moscow, Russia) was used to observe the topographies of sample surface. Images were obtained in non-contact mode with a silicon cantilever (nominal spring constant of 3 N/m, minimum tip radius of 10 nm) and the observed area was 4 mm 4 mm. Parameters of average roughness (Ra) and roughness factor (r) were used to determine topography of sample surface. Average roughness Ra is defined as: Ny Ny Nx X Nx X 1 X 1 X zi j zi j Ra ¼ N x N y i¼1 j¼1 N x N y i¼1 j¼1 where Nx, Ny is the number of points along axis X and Y. The roughness factor r is defined as:
Fig. 1. Molecular structure of PAA resin. (a) Prepolymer of PAA resin; (b) PAA resin.
r¼
A A0
Z.X. Jiang et al. / Applied Surface Science 253 (2007) 9357–9364 Table 2 Roughness parameters of sample surface No.
Treated methods
Ra (nm)
Roughness range (nm)
Roughness factor
1# 2# 3# 4# 5#
Untreated #2000 #1500 #800 #150
1.9 35.1 47.4 69.6 93.3
0.5–3 20–40 40–60 60–80 80–110
0.03 0.11 0.12 0.15 0.20
where A is the ‘true’ surface area and A0 is the apparent or projected area. The results of performed experiments are shown in Table 2. The standard deviation of Ra was 1.3–3.6 nm.
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2.2.3. Shear strength test At least 24 specimens for each sample type were subjected to shear strength test with the aid of a WD-1 type universal testing machine (Changchun Second Material Testing Machine Factory, PR China). 3. Results and discussion 3.1. Surface topography analysis used SEM and AFM The topographies of sample surface are shown in Figs. 3 and 4. As shown, sample surface becomes rougher with the increase of abrasive diameter. It is clear that the 2D surface topography of sample 1# is the smoothest. The 2D surface
Fig. 3. SEM micrographs of silica glass surface. (a) Sample 1# (roughness range: 0–3 nm); (b) sample 2# (roughness range: 20–40 nm); (c) sample 3# (roughness range: 40–60 nm); (d) sample 4# (roughness range: 60–80 nm); (e) sample 5# (roughness range: 80–110 nm).
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Fig. 4. Two- and three-dimensional AFM micrographs of silica glass surface. (a) Sample 1# (roughness range: 0–3 nm); (b) sample 2# (roughness range: 20–40 nm); (c) sample 3# (roughness range: 40–60 nm); (d) sample 4# (roughness range: 60–80 nm); (e) sample 5# (roughness range: 80–110 nm).
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Fig. 5. Schematic illustration for the characteristics of the sample surface. (a) Sample 1#; (b) sample 2#; (c) samples 3# or 4#; (d) sample 5#.
Fig. 6. Representative images of the breakage region of samples after shear strength test. (a) Sample 1# (roughness range: 0–3 nm); (b) sample 2# (roughness range: 20–40 nm); (c) sample 3# (roughness range: 40–60 nm); (d) sample 4# (roughness range: 60–80 nm); (e) sample 5# (roughness range: 80–110 nm).
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topography of sample 2# also seems relatively smooth, and the holes on the surface are shallower compared with other abraded samples. The 2D surface of samples 3# and 4# has some moderate scale of holes, and the sample 5# has the deepest
holes. Furthermore, these differences are also presented in 3D AFM images in Fig. 4. The 3D surface topography of sample 2# seems planar, while surface topographies of samples 3#, 4# and 5# seem rugged. Surface of samples 3# and 4# has lots of cliffy
Fig. 7. EDS X-ray analysis for fracture morphologies of samples. (a) Sample 1# (roughness range: 0–3 nm); (b) sample 2# (roughness range: 20–40 nm); (c) sample 3# (roughness range: 40–60 nm); (d) sample 4# (roughness range: 60–80 nm).
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‘‘hills’’ with moderate height on surface, and surface of sample 5# has many gentle ‘‘hills’’ with relatively high height. Schematic illustration for the characteristics of the sample surface is shown in Fig. 5. 3.2. Fractographic analysis According to the Shear-Lag theory [18–23], stress is transferred from matrix to substrate when composites are loaded. As breakage happens, there are two different failure mechanisms: failure of the interfacial bond between matrix and substrate, and matrix shear yield failure. When the interfacial bonding strength is higher than the matrix shear yield strength, matrix failure occurs around the substrate, and many resins adhere on the substrate, otherwise the de-bonding happens at the interface, and few resins adhere on the substrate. From Fig. 6, it can be seen that there are little remnants of PAA resins on surface of samples 1# and 5#, while the cases of samples 2#, 3# and 4# are opposite. So the failure mechanism for samples 1# and 5# is main failure of the interfacial bonding between matrix and substrate, and the samples exhibit poor interfacial adhesion. While for samples 2#, 3# and 4#, the fracture mode is main matrix shear yield failure and samples show strong interfacial bonding. From EDS X-ray microanalysis, it is possible to clearly see the existence of remnants on surface of samples 2#, 3# and 4#, especially the remnants in holes on the surface of samples 3# and 4#. EDS X-ray spectrum is shown in Fig. 7. All the results illustrate that samples 2#, 3# and 4# have good interfacial bonding, especially samples 3# and 4#. 3.3. Mechanical properties: shear strength test The values of sample shear strength are shown in Fig. 8. As shown, the roughness is found to have remarkable influence on
Fig. 8. Effect of roughness on shear strength of samples.
interfacial adhesion of silica glass/PAA resin composites. As the surface roughness of silica glass increase, the shear strength of samples firstly increases rapidly, and then decreases. The maximum shear strength of 19.6 MPa is obtained with the sample 3# (roughness range: 40–60 nm) and the increase of adhesion has reached about 369% compared with the sample 1#. Furthermore, the minimum of average roughness (Ra) that can influence the shear strength of sample is about 1 nm which is calculated from the size of prepolymer of PAA resin used, i.e. the roughness has influenced the shear strength of sample 1# (roughness range: 0.5–3 nm). It is well known that surface area of rough substrate has significant influences on the interfacial performances. From Table 2, the sample 5# has larger surface area, which is supposed to have better mechanical property. However, from the values of shear strength of samples, it can be seen that the sample 5# does not have the expected mechanical property.
Fig. 9. Schematic illustration for interlocking mechanism. (a) Sample 1#; (b) sample 2#; (c) sample 3# or 4#; (d) sample 5#.
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Thus, in this experiment, surface area has less contribution to the improvement of interfacial properties, and the interlocking at the interface should be responsible for the improvement of interfacial performances. Furthermore, the different extent of interlocking at the sample interface may be due to the different morphologies of sample surface. According to the analysis of surface topographies in Section 3.1, lots of holes (SEM images and 2D AFM images), and ‘‘hills’’ (3D AFM images) presented on the sample surface after being abraded, while the untreated one seems smooth and planar. These conduce to treated ones have stronger mechanical anchor effect on the interface than the untreated one, which make abraded ones have better interfacial properties. However, the surface topography of sample 2# also seems smooth and planar compared to other treated ones, so the anchor effect is infirm and exhibits poor interfacial adhesion. In addition, the surface of sample 5# has deeper holes (SEM images and 2D AFM images) and higher ‘‘hills’’ (3D AFM images), but the ‘‘hillside’’ is gentle, which is not advantageous for the formation of fast physical interlocking at the interface. Thus, the value of shear strength of sample 5# is also low. While the surface topographies of samples 3# and 4# have lots of cliffy ‘‘hills’’ with moderate height on surface, these are relatively beneficial for PAA resin physical anchor in the substrate surface to form firm interlocking at the interface, so the interlocking is stronger and the values of shear strength are higher. The schematic illustration for above interlocking mechanism is presented in Fig. 9. 4. Conclusions With the increase of the surface roughness of silica glass, the shear strength of samples firstly increases, and then decreases. The maximum value of shear strength of 19.6 MPa is achieved by sample 3# with roughness range of 40–60 nm and the dramatic increment has reached about 369% comparing with the untreated sample 1#. The main mechanism for the improvement of the interfacial adhesion is physical interlocking at the interface.
Acknowledgements The authors would like to thank the National Natural Science Foundation of China (Nos. 50333030 and 50603004) and the National Science Foundation of Heilongjiang for Distinguished Young Scholars (No. JC04-12) for financial supports. References [1] C. Dilek, E. Selim, Z. Jolef, Dent. Mater. 23 (2007) 335. [2] J.K. Kim, J. Adhesion 79 (2003) 383. [3] Q. Benard, M. Fois, M. Grisel, P. Laurens, Int. J Adhes. Adhes. 26 (2006) 543. [4] S.J. Park, J.S. Jin, J. Colloid Interf. Sci. 242 (2001) 174. [5] J.G. Iglesias, J.G. Benito, A.J. Aznar, J. Colloid Interf. Sci. 250 (2002) 251. [6] D.M. Laura, H. Keskkula, J.W. Barlow, D.R. Paul, Polymer 43 (2002) 4673. [7] L. Salmin, F. Thominette, M.F. Pays, J. Verdu, Compos. Sci. Technol. 57 (1997) 1119. [8] J. Comyn, L. Mascia, G. Xiao, B.M. Parker, Int. J Adhes. Adhes. 16 (1996) 301. [9] M.E.R. Shanahan, C.B. Monnier, Int. J Adhes. Adhes. 16 (1996) 129. [10] J.K. Kim, D.G. Lee, Compos. Struct. 36 (2002) 37. [11] G.W. Crichtlow, C.A. Cottam, D.M. Brewis, D.C. Emmony, Int. J Adhes. Adhes. 17 (1997) 143. [12] P. Laurens, M.B. Ould, F. Meducin, B. Sadras, Appl. Surf. Sci. 168 (2000) 211. [13] N. Dilsiz, E. Ebert, W. Weisweiler, G. Akovali, J. Colloid Interf. Sci. 170 (1995) 241. [14] H. Zhang, Z. Zhang, C. Breidt, Comp. Sci. Technol. 64 (2004) 2021. [15] Z.X. Jiang, L.H. Meng, Y.D. Huang, Appl. Surf. Sci. 253 (2007) 4338. [16] D.Y. Zhang, Y. Zhang, J. Solid Rocket Technol. 24 (2001) 53. [17] K.M. Jones, Sci. Eng. 68 (1993) 97. [18] D. Tripathi, J.R. Jones, J. Mater. Sci. 33 (1998) 1. [19] H.L. Cox, Br. J. Appl. Phys. 3 (1952) 72. [20] C. Galiotis, R.J. Young, P.H.J. Yeung, D.N. Batchelder, J. Mater. Sci. 19 (1984) 3640. [21] E.M. Asloun, M. Nardin, J. Schultz, J. Mater. Sci. 24 (1989) 1835. [22] Y. Termonia, J. Mater. Sci. 22 (1987) 504. [23] A. Kelly, W.R. Tyson, J. Mech. Phys. Solids 13 (1969) 329.
Effects of roughness on interfacial performances of silica glass and non-polar polyarylacetylene resin composites Z.X. Jiang, Y.D. Huang *, L. Liu, J. Long Department of Applied Chemistry, Faculty of Science, Harbin Institute of Technology, PO Box 410#, Harbin 150001, People’s Republic of China Received 27 April 2007; received in revised form 28 May 2007; accepted 28 May 2007 Available online 6 June 2007
Abstract The influence of roughness on interfacial performances of silica glass/polyarylacetylene resin composites was investigated. In order to obtain different roughness, silica glass surface was abraded by different grits of abrasives and its topography was observed by scanning electron microscopy and atomic force microscopy. At the same time, the failure mechanisms of composites were analyzed by fracture morphologies and the interfacial adhesion was evaluated by shear strength test. The results indicated that shear strength of silica glass/polyarylacetylene resin composites firstly increased and then decreased with the surface roughness of silica glass increased. The best surface roughness range of silica glass was 40– 60 nm. The main mechanism for the improvement of the interfacial adhesion was physical interlocking at the interface. # 2007 Elsevier B.V. All rights reserved. PACS : 68.35.Np Keywords: Polyarylacetylene resin; Interlocking; Scanning electron microscopy
1. Introduction The addition of silica fiber reinforcement is known to improve the stiffness, strength and the high temperature performance of polymeric materials. The mechanical properties of the resulting silica fiber reinforced composite materials depend not only on the properties of each primary component but also on the nature of the fiber-matrix interface. A strong interface generally leads to the best composite properties, so many effective interfacial modifications for silica fiber that forming strong interface has been investigated [1–3]. Among all the available surface treatments, the most widely used modification method for silica fiber is sizing with silane coupling agents [4,5]. Common silane coupling agents used to increase the interfacial adhesion of fiberreinforced composites incorporate epoxy, amine, or anhydride functional groups as the reactive organic component, depending on the chemistry of the matrix resin [6,7]. Moreover, other techniques such as corona discharge or plasma treatment can provide a superficial cleaning and/or chemical modification * Corresponding author. Tel.: +86 451 86414806; fax: +86 451 86413711. E-mail addresses: [email protected] (Z.X. Jiang), [email protected] (Y.D. Huang). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.05.075
[8,9]. The corresponding surface modification enhances the physical and/or chemical adhesion between the matrix resin and the treated surface [10]. Laser treatment, which is studied in recent years, can also provide adhesion improvement as a result of physical modification at the surface [11,12]. Though there are many surface modification methods for silica fiber, these treatments are believed to improve interfacial performances of fiber/matrix composites in the following two main ways: the introduction of reactive groups onto fiber surface and the increase of roughness of fiber surface [13,14]. The effects of the reactive groups on the interfacial performances of composites have been widely studied, while the effects of roughness on the interfacial performances of composites have not been paid enough attention. Hence, the aim of this paper is to individually study the influence of roughness on interfacial performances of silica fiber reinforced matrix resin composites. Due to the difficulty of controlling roughness when use silica fiber as substrate, silica glass discs are chosen to instead of silica fiber [15]. The resin chosen is non-polar polyarylacetylene (PAA) resin [16,17]. The main reason for choosing the PAA resin is that the molecular structure of all benzene ring of PAA resin can avoid chemical interaction between PAA resin and the silica glass surface. Even
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Table 1 Diameter of abrasives used Grit of abrasives
Diameter of abrasives (mm)
#2000 #1500 #800 #150
6–8 8–12 14–20 70–80
so it is also difficult to avoid all weak interaction between silica glass discs and PAA resin except the interaction caused by roughness. However, we use the same substrate material and resin in order to obtain same interaction between substrate and resin except the interaction caused by roughness. In this way, we can ignore the effect of these interactions mentioned above, and individually study the effects of roughness on the interfacial performances of silica glass and PAA resin composites. 2. Experimental Fig. 2. Schematic illustration of sample for mechanical testing.
2.1. Materials 2.1.1. Surface pretreatment of silica glass The silica glass discs (Education Optics Lens Factory of Haian City, Jiangsu Province, PR China) were fabricated into 20 mm 10 mm 2 mm, and were abraded on H-015 two-axle lens lapping machine (Nanjing Instrument Machine Tool Works, PR China) with different grits of abrasives (supplied by Minshan Abrasives Factory, PR China). Grits of abrasives used were #2000, #1500, #800 and #150, which are shown in Table 1. Before use, the discs were washed in acetone for 15 min, and then were washed with ultrapure water in ultrasonic bath for 20 min, in the end, were dried in oven for 10 h at 120 8C. 2.1.2. PAA resin PAA resin is a brown liquid with the density of 1.05 g/cm3 and the viscosity of 200–3000 mPa s (corresponding to a temperature changing from 90 8C to 30 8C) (supplied by Beijing Aerospace Research Institute of Material and Processing Technology, PR China). The molecular structure of PAA resin and its prepolymer is shown in Fig. 1. 2.1.3. Sample for shear strength test To prepare sample for shear strength test, two silica glass discs abraded by the same grit of abrasive were used. In vacuum
condition, about 25 ml PAA resin were dropped on one of silica glass disc which was kept flat in container, and then the other silica glass disc was put on top of it and kept for 10 min. Subsequently, the specimen was fastened on a mould and cured at 120 8C for 2 h, 140 8C for 2 h, 180 8C for 2 h, 200 8C for 2 h, and post-cured at 250 8C for 10 min, respectively. After cooling to room temperature naturally, the specimen was removed from the mould and affixed to a set of holding device by epoxy adhesive. Then the sample was under mechanical testing. The schematic plan of the sample is given in Fig. 2. 2.2. Analysis methods 2.2.1. SEM observation SEM study for samples with gold coated were carried out using a FEI Sirion 200 scanning electron microscope (Royal Dutch Philips Electronics Ltd., The Netherlands) connected with an energy-dispersive X-ray microanalyzer (EDAX Inc., USA). 2.2.2. AFM measurement An atomic force microscopy (NT-MDT Co., Zelenograd Research Institute of Physical Problems, Moscow, Russia) was used to observe the topographies of sample surface. Images were obtained in non-contact mode with a silicon cantilever (nominal spring constant of 3 N/m, minimum tip radius of 10 nm) and the observed area was 4 mm 4 mm. Parameters of average roughness (Ra) and roughness factor (r) were used to determine topography of sample surface. Average roughness Ra is defined as: Ny Ny Nx X Nx X 1 X 1 X zi j zi j Ra ¼ N x N y i¼1 j¼1 N x N y i¼1 j¼1 where Nx, Ny is the number of points along axis X and Y. The roughness factor r is defined as:
Fig. 1. Molecular structure of PAA resin. (a) Prepolymer of PAA resin; (b) PAA resin.
r¼
A A0
Z.X. Jiang et al. / Applied Surface Science 253 (2007) 9357–9364 Table 2 Roughness parameters of sample surface No.
Treated methods
Ra (nm)
Roughness range (nm)
Roughness factor
1# 2# 3# 4# 5#
Untreated #2000 #1500 #800 #150
1.9 35.1 47.4 69.6 93.3
0.5–3 20–40 40–60 60–80 80–110
0.03 0.11 0.12 0.15 0.20
where A is the ‘true’ surface area and A0 is the apparent or projected area. The results of performed experiments are shown in Table 2. The standard deviation of Ra was 1.3–3.6 nm.
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2.2.3. Shear strength test At least 24 specimens for each sample type were subjected to shear strength test with the aid of a WD-1 type universal testing machine (Changchun Second Material Testing Machine Factory, PR China). 3. Results and discussion 3.1. Surface topography analysis used SEM and AFM The topographies of sample surface are shown in Figs. 3 and 4. As shown, sample surface becomes rougher with the increase of abrasive diameter. It is clear that the 2D surface topography of sample 1# is the smoothest. The 2D surface
Fig. 3. SEM micrographs of silica glass surface. (a) Sample 1# (roughness range: 0–3 nm); (b) sample 2# (roughness range: 20–40 nm); (c) sample 3# (roughness range: 40–60 nm); (d) sample 4# (roughness range: 60–80 nm); (e) sample 5# (roughness range: 80–110 nm).
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Fig. 4. Two- and three-dimensional AFM micrographs of silica glass surface. (a) Sample 1# (roughness range: 0–3 nm); (b) sample 2# (roughness range: 20–40 nm); (c) sample 3# (roughness range: 40–60 nm); (d) sample 4# (roughness range: 60–80 nm); (e) sample 5# (roughness range: 80–110 nm).
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Fig. 5. Schematic illustration for the characteristics of the sample surface. (a) Sample 1#; (b) sample 2#; (c) samples 3# or 4#; (d) sample 5#.
Fig. 6. Representative images of the breakage region of samples after shear strength test. (a) Sample 1# (roughness range: 0–3 nm); (b) sample 2# (roughness range: 20–40 nm); (c) sample 3# (roughness range: 40–60 nm); (d) sample 4# (roughness range: 60–80 nm); (e) sample 5# (roughness range: 80–110 nm).
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topography of sample 2# also seems relatively smooth, and the holes on the surface are shallower compared with other abraded samples. The 2D surface of samples 3# and 4# has some moderate scale of holes, and the sample 5# has the deepest
holes. Furthermore, these differences are also presented in 3D AFM images in Fig. 4. The 3D surface topography of sample 2# seems planar, while surface topographies of samples 3#, 4# and 5# seem rugged. Surface of samples 3# and 4# has lots of cliffy
Fig. 7. EDS X-ray analysis for fracture morphologies of samples. (a) Sample 1# (roughness range: 0–3 nm); (b) sample 2# (roughness range: 20–40 nm); (c) sample 3# (roughness range: 40–60 nm); (d) sample 4# (roughness range: 60–80 nm).
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‘‘hills’’ with moderate height on surface, and surface of sample 5# has many gentle ‘‘hills’’ with relatively high height. Schematic illustration for the characteristics of the sample surface is shown in Fig. 5. 3.2. Fractographic analysis According to the Shear-Lag theory [18–23], stress is transferred from matrix to substrate when composites are loaded. As breakage happens, there are two different failure mechanisms: failure of the interfacial bond between matrix and substrate, and matrix shear yield failure. When the interfacial bonding strength is higher than the matrix shear yield strength, matrix failure occurs around the substrate, and many resins adhere on the substrate, otherwise the de-bonding happens at the interface, and few resins adhere on the substrate. From Fig. 6, it can be seen that there are little remnants of PAA resins on surface of samples 1# and 5#, while the cases of samples 2#, 3# and 4# are opposite. So the failure mechanism for samples 1# and 5# is main failure of the interfacial bonding between matrix and substrate, and the samples exhibit poor interfacial adhesion. While for samples 2#, 3# and 4#, the fracture mode is main matrix shear yield failure and samples show strong interfacial bonding. From EDS X-ray microanalysis, it is possible to clearly see the existence of remnants on surface of samples 2#, 3# and 4#, especially the remnants in holes on the surface of samples 3# and 4#. EDS X-ray spectrum is shown in Fig. 7. All the results illustrate that samples 2#, 3# and 4# have good interfacial bonding, especially samples 3# and 4#. 3.3. Mechanical properties: shear strength test The values of sample shear strength are shown in Fig. 8. As shown, the roughness is found to have remarkable influence on
Fig. 8. Effect of roughness on shear strength of samples.
interfacial adhesion of silica glass/PAA resin composites. As the surface roughness of silica glass increase, the shear strength of samples firstly increases rapidly, and then decreases. The maximum shear strength of 19.6 MPa is obtained with the sample 3# (roughness range: 40–60 nm) and the increase of adhesion has reached about 369% compared with the sample 1#. Furthermore, the minimum of average roughness (Ra) that can influence the shear strength of sample is about 1 nm which is calculated from the size of prepolymer of PAA resin used, i.e. the roughness has influenced the shear strength of sample 1# (roughness range: 0.5–3 nm). It is well known that surface area of rough substrate has significant influences on the interfacial performances. From Table 2, the sample 5# has larger surface area, which is supposed to have better mechanical property. However, from the values of shear strength of samples, it can be seen that the sample 5# does not have the expected mechanical property.
Fig. 9. Schematic illustration for interlocking mechanism. (a) Sample 1#; (b) sample 2#; (c) sample 3# or 4#; (d) sample 5#.
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Thus, in this experiment, surface area has less contribution to the improvement of interfacial properties, and the interlocking at the interface should be responsible for the improvement of interfacial performances. Furthermore, the different extent of interlocking at the sample interface may be due to the different morphologies of sample surface. According to the analysis of surface topographies in Section 3.1, lots of holes (SEM images and 2D AFM images), and ‘‘hills’’ (3D AFM images) presented on the sample surface after being abraded, while the untreated one seems smooth and planar. These conduce to treated ones have stronger mechanical anchor effect on the interface than the untreated one, which make abraded ones have better interfacial properties. However, the surface topography of sample 2# also seems smooth and planar compared to other treated ones, so the anchor effect is infirm and exhibits poor interfacial adhesion. In addition, the surface of sample 5# has deeper holes (SEM images and 2D AFM images) and higher ‘‘hills’’ (3D AFM images), but the ‘‘hillside’’ is gentle, which is not advantageous for the formation of fast physical interlocking at the interface. Thus, the value of shear strength of sample 5# is also low. While the surface topographies of samples 3# and 4# have lots of cliffy ‘‘hills’’ with moderate height on surface, these are relatively beneficial for PAA resin physical anchor in the substrate surface to form firm interlocking at the interface, so the interlocking is stronger and the values of shear strength are higher. The schematic illustration for above interlocking mechanism is presented in Fig. 9. 4. Conclusions With the increase of the surface roughness of silica glass, the shear strength of samples firstly increases, and then decreases. The maximum value of shear strength of 19.6 MPa is achieved by sample 3# with roughness range of 40–60 nm and the dramatic increment has reached about 369% comparing with the untreated sample 1#. The main mechanism for the improvement of the interfacial adhesion is physical interlocking at the interface.
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