The physicomechanical properties of siloxane coatings on polymer substrates

Transcription

1 Plasticheskie Massy, No. 4, 2012, pp The physicomechanical properties of siloxane coatings on polymer substrates A.S. Useinov, 1 S.A. Radzinskii, 2 K.S. Kravchuk, 1 I.Yu. Zolkina, 2 T.I. Andreeva, 2 and I.D. Simonov-Emel yanov 3 1 Technological Institute of Ultrahard and New Carbon Materials, Troitsk 2 OAO Institut Plastmass (Institute of Plastics OJSC), Moscow 3 M.V. Lomonosov State University of Fine Technical Technology, Moscow Selected from International Polymer Science and Technology, 39, No. 8, 2012, reference PM 12/04/14; transl. serial no Translated by P. Curtis Summary An investigation was made of the physicomechanical surface properties of polycarbonate, polymethyl methacrylate, and polymers with an applied heatcurable siloxane coating by methods of scanning probe microscopy, nanoindentation, and sclerometry. It was established that high resistance of the siloxane coating to abrasive wear is achieved as a result of a reduction in the average surface roughness and increase in the ratio of hardness to elastic modulus of the material, and also the high degree of elastic recovery. The results obtained were compared with data obtained using traditional methods of polymer study. INTRODUCTION At the present time, the production and application of polymeric materials are increasing at high rates [1]. The widely used polymeric materials have a wide range of advantages, but a general shortcoming that reduces the areas of their application is the low abrasion resistance (scratch resistance) of the surface of polymers. To increase the wear resistance and improve the physicomechanical properties of polymeric materials, different methods are used, such as modification of the structure and surface of the polymer [2], the creation of polymer composites, etc. [3 5]. The abrasion resistance of polymers can also be controlled by changing the properties of the thin near-surface layer, for example by ion implantation [6]. Polymer protective coatings are most commonly used for improving the abrasion resistance of the surface of polymeric materials and resistance to light, heat, moisture, air oxygen, corrosive chemical substances, and so on. Increase in the surface strength and resistance to abrasive wear of articles manufactured from polymeric materials by the application of coatings, in particular polymer coatings, has been paid a great deal of attention in recent years, as indicated by the considerable number of publications in the patent and scientific and technical literature [7 12]. In spite of the entire diversity of polymer protective coatings used, a detailed study of their physicomechanical properties has been presented only in work by Charitidis et al. [13] and Rodriguez et al. [14] who investigated optical discs based on polycarbonate (PC). This is due either to the absence of methods for studying thin (nanosized) layers of polymer coatings or to the inadequate application of existing methods of micro- and nanoindentation for polymeric materials. To obtain objective and sufficiently complete information on the physicomechanical properties of the surface layers of polymeric materials, it is necessary to use a complex approach using a set of special procedures such as surface profilometry, micro- and nanoindentation, sclerometry (scratching), friction and wear tests, and also a number of other traditional methods for investigating polymers. The combined use of different investigation methods makes it possible to obtain objective data on the surface properties of polymeric material and to determine features of its behaviour that cannot be established using traditional investigation methods only Smithers Rapra Technology T/39

2 In this work, for the first time, by methods of profilometry, scanning probe microscopy (SPM), nanoindentation, sclerometry, and traditional methods of investigation, data are obtained on the physicomechanical properties of the surface of specimens of PC and polymethyl methacrylate (PMMA), and also an abrasion-resistant polymer coating based on a heat-curable siloxane composite applied from solution onto the surface of PMMA and PC specimens obtained by extrusion. DESCRIPTION OF SPECIMENS The investigation was conducted on sheet specimens of PMMA of 4 mm thickness with a weight-average molecular weight M w of and on sheet specimens of PC of 4 mm thickness with M w = A heat-curable siloxane composite in an alcohol solution was applied as a coating onto PMMA and PC specimens by the casting method, and then the solvent was removed and the composite was cured at 90 C for 2 h (until there was no effect from rubbing with steel wool No. 00). The thickness of the heat-curable siloxane coating (HCSC) on PC and PMMA amounted to ~7 µm, and the thickness of the intermediate polymer layer (primer) on the surface of the PC was 2 µm. In this work, an investigation was made of the following polymer systems: specimen 1 PC; specimen 2 PMMA; specimen 3 PC + primer + HCSC; specimen 4 PC + HCSC; specimen 5 PMMA + HCSC. DESCRIPTION OF THE INSTRUMENT AND PROCEDURES This work sets out results obtained using a NanoScan-3D scanning nanohardness meter [15 18]. The sensitive element of the instrument is a piezoresonance cantilever sensor of tuning-fork design with high flexural rigidity of the arm (of the order of 20 kn/m). The design of the sensor makes it possible to implement several procedures at the same time in the same instrument: measurement of the surface relief in a semi-contact regime of scanning probe microscopy and measurement of the physicomechanical properties (hardness, Young s modulus, etc.) by methods of nanoidentation (in accordance with ISO 14577) and force spectroscopy [15]. Likewise, a hardness measurement procedure was implemented by sclerometry (application and profile analysis of scratches) with a constant or variable normal load, with subsequent surface scanning in the region of indentation and application of scratches [16 18]. A Berkovich three-face diamond pyramid with an apex angle of 65 was used as the probe tip. The effective size of the indentor point was 100 nm. The pencil hardness of specimens was determined under a load of 7.5 N using pencils of increasing hardness: 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H, 7H, 8H, 9H, and 10H (in accordance with ISO 15184) on an automatic Elcometer 3086 hardness meter (Elcometer Ltd, UK). The abrasion resistance of the surface of the polymeric materials was tested with steel wool No. 00 under a load of 7 N for ten cycles. The strength of adhesion of the HCSC to PC, PMMA, and primer was determined by cross-hatching using an Elcometer 1542 adhesion tester. The adhesion strength was measured according to the following scale: from a maximum rating of 0 to a minimum rating of 5 (in accordance with ISO 2409). The adhesion strength (rating) depends on the area of the coating that is separated from the substrate, rating 0 corresponding to a completely smooth surface with no separation, and rating 5 corresponding to the separation of over 65% of the coating from the substrate. EXPERIMENTAL The hardness, Young s modulus, and coefficient of elastic recovery (relaxation) were determined by measuring nanoindentation [19]. The maximum load under nanoindentation amounted to 10 mn, the loading rate to 50 nm/s in a regime of linear displacement control, and the holding time under maximum load to 10 s. Figure 1 shows the dependence of the load (P) on the indentor on the depth (h) of its impression into the polymer specimen in a loading unloading regime. The graphic dependence of load on depth of introduction of the indentor into the surface of the polymer makes it possible to calculate the hardness and Young s modulus. The main parameters for calculation are: the maximum load (P max ), the maximum depth of introduction of the indentor under this load (h max ), and the rigidity of contact of the specimen with the indentor (S), which is Figure 1. General form of the loading unloading curve and a diagram of the contact of the indentor with the surface: a initial surface; b surface profile after load removal; c indentor; d surface profile under load T/40 International Polymer Science and Technology, Vol. 40, No. 8, 2013

3 determined as the slope tangent to the unloading curve at the point of P max : S = dp dh P=P max Within the framework of the given method, the hardness (H) of the surface of the specimen is calculated by means of the formula: H = P max A where A is the area of projection of the indentor impression on the material at P max. The depth of penetration of the indentor into the specimen (h c ) can be determined from the unloading curve and calculated by means of the formula: h c = h max ε P max S where ε is a constant depending on the geometry of the indentor (for the Berkovich diamond pyramid, ε = 0.75), and h max is the maximum depth of introduction of the indentor into the specimen at P max. From the depth of contact h c, the projection of the contact area A and the reduced elastic modulus E eff are calculated: A = 24.5h c 2 E eff = 1 β π 2 S A where b is a constant that depends on the shape of the indentor (for the Berkovich diamond pyramid, b = 1.034). After physicomechanical tests, the surfaces of the specimen were scanned in an SPM regime in the region of indentation with the aim of obtaining information about the relief of residual impressions and the change in this relief when the load is removed (relaxation). Examples of the images of these impressions and the relief for specimens of PC, PMMA, and PC + primer + HCSC are given in Figure 2. The PC + HCSC and PMMA + HCSC specimens behave similarly to the PC + primer + HCSC specimen. From Figure 2 it follows that, after removal of the load, the greatest depth of introduction of the indentor is observed for PC (up to 500 nm); for PMMA it decreases almost twofold. For the siloxane coating in the PC + primer + HCSC system the impression of the indentor is blurred and the depth amounts to no more than 3 nm. Such behaviour of the surface of the materials is determined by their elastic modulus, hardness, relaxation effects, and the magnitude of the reversible strain. Typical diagrams of nanoindentation of the investigated polymeric materials and coating, comprising the dependences of the applied load on the displacement of the indentor during loading and subsequent load removal (unloading) are given in Figure 3. The P(h) curves for PC and PMMA differ considerably, and here, for PMMA the curve is shifted towards lower strains. For PC and PMMA there is a change in the rates of elastic recovery (the unloading curve) and also in the value of residual strain (h rec ), which is due to the relaxation processes occurring in the polymers. The siloxane coating in the PC + primer + HCSC system approaches PMMA in its behaviour, but the relaxation processes in this case proceed at a greater rate, and the residual strain tends to zero. These data are in good agreement with the results given in Figure 2. From the data presented in Figure 2, the coefficients of elastic recovery of the investigated specimens were calculated by means of the formula: Figure 2. Impressions and surface relief of specimens of PC (a), PMMA (b), and PC + primer + HCSC (c) after indentation. Maximum load 10 mn 2013 Smithers Rapra Technology T/41

4 K = [(h max h rec ) / h max ] 100% Figure 3. Loading unloading curves for specimens of PC (1), PMMA (2), and PC + primer + HCSC (3) Figure 4. Surface relief of specimens of PC (a) and PC + primer + HCSC (b) after the application of scratches with increasing load from 0 to 30 mn where h max is the maximum depth of indentation at P max, and h rec is the depth of impression after indentation. To investigate the behaviour of polymeric materials and coatings when scratches are applied, the method of sclerometry with variable load (P) was used. The conditions of the experiment were as follows: the initial scratch length was 60 µm, the scratching rate was 0.2 µm/s, the load during scratching was varied from 0 to 30 mn, and the direction of scratching was edge first. After the tests, the surface of the specimens was scanned in an SPM regime with the aim of obtaining an image of the relief of the residual trace after application of the scratch. Examples of scratch reliefs obtained for PC and PC + primer + HCSC specimens are given in Figure 4. The results of measuring the physicomechanical characteristics of the investigated specimens by nanoindentation are given in Table 1. It should be pointed out that, when a siloxane coating is applied to the surface of polymeric materials (PC and PMMA), there is a considerable reduction in their roughness, there is a slight reduction in the elastic modulus with increase in hardness according to ISO 14577, and the coefficient of elastic recovery reaches 99%. The results of determining the adhesion strength and abrasion resistance of the investigated polymer specimens by traditional methods are given in Table 2. Table 1. Physicomechanical characteristics of the surface of polymer specimens No. Specimen Roughness R a (nm) Young s modulus (GPa) Hardness (ISO 14577) (GPa) H/E Coefficient of elastic recovery K (%) 1 PC PMMA PC + HCSC PMMA + HCSC PC + primer + HCSC Table 2. Adhesion strength and abrasion resistance of polymeric materials No. Specimen Adhesion strength of HCSC to substrate (rating) (ISO 2409) Pencil hardness (ISO 15184) Testing with steel wool No PC 3B Rubbing effect 2 PMMA 4H Rubbing effect 3 PC + primer 0 H Rubbing effect 4 PC + HCSC 3 H No rubbing effect 5 PMMA + HCSC 0 7H No rubbing effect 6 PC + primer + HCSC 0 4H No rubbing effect T/42 International Polymer Science and Technology, Vol. 40, No. 8, 2013

5 RESULTS AND DISCUSSION As follows from the data in Table 2, a certain correlation is observed between the protective properties of the coating (hardness, abrasion resistance, rubbing resistance) and its adhesion strength to the substrate. Thus, a PC + HCSC specimen has a low adhesion strength of the siloxane coating to the PC (rating 3), which leads to its partial separation during pencil hardness tests. Owing to this, the hardness for this specimen has a minimum value equal to H for the investigated specimens with a coating. With increase in adhesion strength, the pencil hardness increases to 4H 7H (for specimens 5 and 6). Note that, when a siloxane coating is applied to polymeric materials, their surface is not affected by rubbing with steel wool (it does not become dull). Specimens 4, 5, and 6 have an identical upper protective layer HCSC, and the pencil hardness of specimens differs considerably. It is evident that the pencil hardness is determined not only by the hardness of the siloxane protective coating but also by the hardness of the polymer substrate. However, the hardness of the material is not a determining factor characterising its abrasion resistance, and in particular its rubbing resistance. The rubbing or clouding effect of materials is normally connected either with light scattering on the surface or volume mechanical defects or with the absorption of some of the light flux of the impurity layer formed on the surface. In the former case, increase in wear resistance is realised by increase in the physicomechanical characteristics of the surface of the polymeric material, and in the latter case it is realised by improvement in the surface relief, primarily by reduction in its roughness. As can be seen from the data in Table 1, the siloxane coating promotes the formation of a smoother surface, which promotes an increase in abrasion resistance. The roughness of specimens of PC and PMMA is an order of magnitude greater than the roughness of specimens with a siloxane coating, which makes a definite contribution to increase in the resistance of the surface of materials to mechanical contact damage with an identical pencil hardness. It is necessary to consider the question of the hardness of polymeric materials with a siloxane coating, the values of which were obtained by nanoindentation. When a siloxane coating is applied, the coefficient of elastic recovery reaches 99%. For such materials, the concept of hardness as a measure of the resistance of the material solely to plastic strain becomes inapplicable, as the plastic strain tends to zero, while the hardness in this case tends to infinity. To describe their behaviour in a loading unloading regime, the concept of deformation or universal hardness, determined by the nanoindentation method, was proposed [20]. Within the framework of this concept, hardness is a measure simultaneously of elastic and plastic strain. It is evidently not quite correct to compare traditional and universal hardness, although for materials with plastic strain predominating over elastic strain these two values are in fairly good agreement. It should be pointed out that a similar situation with the measurement of hardness is also characteristic of other classes of materials such as carbon-fibre-reinforced composites [21], ultrahard films based on titanium nitrides and carbides [22], and carbon materials based on fullerenes [23]. In the creation of nanostructured protective coatings based on metals, modern materials science uses the concept of the optimisation of the two main quantities, which reduces to the requirement of increase in hardness H with a simultaneous reduction in elastic recovery E [24]. Such a combination reduces to a known law increase in the magnitude of the ratio H/E, which makes it possible to create special materials for coatings and to obtain interesting and promising nanostructured coatings ensuring the required mechanical and tribological characteristics. These concepts, obtained earlier for metals, are in good agreement with the results of the present work for polymeric materials (PC and PMMA) with a siloxane protective coating. The results obtained by the nanoindentation method indicate that the hardness of polymer specimens with a siloxane coating increases considerably with a small reduction in elastic modulus, and here the ratio H/E increases, which leads to an increase in the tribological characteristics of the investigated materials. CONCLUSIONS In this work, for the first time, the results are given of an investigation of the physicomechanical properties of PC and PMMA, and also a heat-curable siloxane protective coating applied to substrates of polycarbonate and polymethyl methacrylate, by methods of scanning probe microscopy, nanoindentation, and sclerometry with a variable load with the aid of a NanoScan-3D scanning nanohardness meter. It was shown that increase in the abrasion resistance of the investigated polymer specimens with a siloxane coating is due mainly to the greater coefficient of elastic recovery and ratio H/E, and also to the considerably lower roughness of the surface with a coating. The use of methods for determining the adhesion strength of siloxane coatings to polymer substrates and the pencil hardness showed the important role of the substrate in the coating substrate system. Increase in the adhesion strength of the siloxane coating to the polymer substrate and in the hardness of the substrate leads to a considerable increase in the protective 2013 Smithers Rapra Technology T/43

6 properties of the coating to mechanical contact damage. Only by combining special and traditional methods of investigation is it possible to obtain objective and reliable data on the surface properties of the polymeric materials and coatings, and also on the mechanism of the protective action of coatings on the polymeric materials. Thus, as a result of the conducted investigations, it proved possible to increase considerably the abrasion resistance and hardness of the surface of PC (from 3B to H 4H) by applying a siloxane coating and creating a layered PC + primer + HCSC structure, thereby achieving a surface hardness equal to that of PMMA but with a superior abrasion resistance (rubbing resistance). ACKNOWLEDGEMENTS This work was supported financially by the Russian Ministry of Education and Science within the framework of State Contract No The authors are grateful to colleagues of Laboratory No. 17 of the GNIIKhTEOS Federal State Unitary Enterprise for supplying the HCSC specimen. REFERENCES 1. Mills N.J., Plastics. Microstructure and Engineering Applications, 3rd edition. Elsevier, Oxford, UK (2005). 2. Kim S.J. and Ho J., Tribol. Int., 33(7): (2000). 3. Burris D.L. et al., Macromol. Mater. Eng., 292: (2007). 4. Sawyer W.G. et al., Wear, 254: (2003). 5. Sinha S.K., Wear failures of plastics, in ASM Handbook. Vol. 11. Failure Analysis and Prevention. ASM International, Materials Park, OH, pp (2002). 6. Sana J. et al., Surf. Coat. Technol., 138: (2001). 7. US , C 08 F 2/ EP , C 08 J 7/04, C 08 L 69/ EP , C 08 J / WO , C 08 J 3/ JP , C 08 J 7/ International Coating for Plastics Symposium, Troy, 4 6 June, M1 (2001). 13. Charitidis C. et al., Superlattices Microstruct., 36: (2004). 14. Rodriguez J. et al., Wear, 263: (2007). 15. Useinov A.S., Pribory Tekh. Eksp., (1):134 (2004). 16. Useinov A. et al., Int. J. Mater. Res., (7):968 (2009). 17. Useinov A. and Gogolinskii K., Nanoindustriya, (5):54 (2010). 18. Useinov A. and Useinov S., Nanoindustriya, (6):28 (2010). 19. Oliver W.C. and Pharr G.M., J. Mater. Res., 19:3 (2004). 20. Cheng F.T., J. Mater. Sci. Technol., 20(6):700 (2004). 21. Useinov A.S. et al., Nanoindustriya, (6):24 (2011). 22. Veprek S. and Argon A.S., Surf. Coat. Technol., :175 (2001). 23. Tchernogorova O.P. et al., Mater. Sci. Eng., A299(1 2):136 (2001). 24. Cavaleiro A. and de Hosson J.T., Nanostructured Coatings. Springer, New York, NY, p. 598 (2006). T/44 International Polymer Science and Technology, Vol. 40, No. 8, 2013