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Improvement effects of CaO nanoparticles on tribological and microhardness properties of PMMA coating.

Abstract Poly(methyl methacrylate)/calcium oxide (PMMA/CaO) nanocomposite in the form of solution has been successfully prepared by mixing CaO nanoparticles and PMMA obtained by a free radical polymerization. For comparative studies, pure PMMA coating was also prepared to determine the influence of CaO nanoparticles incorporation into PMMA matrix on the properties behavior. Fourier transform infrared technique was used to study the synthesis of PMMA and PMMA/CaO nanocomposite. The coating thickness was determined by profilometry; it was increased from 97 to 107 [micro]m, when CaO nanoparticles were added. The surface morphology of the coatings was characterized by scanning electron microscopy. The wear tests for PMMA/CaO and PMMA coatings on ultra-high molecular weight polyethylene substrates were carried out on a pin-on-disk tribometer in dry conditions with normal loads ranging from 2 to 10 N. The friction coefficient behavior of PMMA coating was improved by the addition of CaO nanoparticles; however, volume loss and wear rate values were slightly higher for PMMA/CaO coatings than those corresponding to PMMA coatings. Microhardness of PMMA and PMMA/CaO coatings was also evaluated by Vickers measurements, revealing an increase of one order of magnitude for PMMA/CaO coating.

Keywords PMMA/CaO nanocomposites, Coating, Wear, Microhardness

Introduction

The possibility of combining properties of organic and inorganic materials was explored several years ago. By introducing an inorganic component into the polymer network, reinforcement is expected when the miscibility between the two components is promoted, resulting in a hybrid matrix with complementary properties between the organic and inorganic constituents. (1) These hybrid materials sometimes lead to unexpected new properties, which are often not exhibited by their individual compounds. One of the main advantages in the field of the organic-inorganic hybrid materials is the possibility of tailoring their properties by controlling the contents of the organic and inorganic constituents. (1)

One of the most widely studied hybrid materials is poly(methyl methacrylate) (PMMA)/inorganic fillers. PMMA is an important commercial plastic that finds applications in many engineering areas, as an organic matrix has been of special interest for its exceptional thermal and mechanical stability that has been applied to a wide range of industries such as bone substitution and optical devices. (2,3) Also, it has been largely used as bone cement due to its self-hardening property and excellent mechanical properties compared to other polymers. (4) Unfortunately, PMMA is characterized by a poor adhesion and abrasion resistance; this is one of the reasons for its limited use in other fields such as dental applications. One way to improve the performance of polymers is the addition of inorganic particles. To this point, calcium oxide (CaO) as an alkaline earth metal oxide is an important material because it has many applications such as catalyst, (5) toxic-waste remediation agent, additive in refractory, (6,7) doped material to modify electrical and optical (dielectric) properties, (8) crucial factor for C[O.sub.2] capture, (9,10) flue gas desulfurization, and pollutant emission control. (11) Ultra-fine metal oxide particles can be used as bactericide, adsorbent, and catalyst. (12) CaO, in particular, has been applied in the biomedical field, (13) as a precursor of bioceramics (14,15) and its bactericidal effects have been investigated. (16) On the other hand, it was found that some inorganic fillers show distinct effects on the friction and wear behaviors of PMMA composites, and many researchers have also studied the mechanism of filler action in reducing the wear of PMMA polymer. Jui-Ming et al. (17) prepared PMMA-Si[O.sub.2] hybrid coatings by using 3-(trimethoxysilyl) propyl methacrylate (MSMA) as a coupling agent. They showed an advanced corrosion protection effect on cold-rolled steel (CRS) coupons when compared to the pure PMMA. Kyu-Hyeon and Sang-Hoon (18) investigated the mechanical behavior and bioactivity of PMMA with the incorporation of Si[O.sub.2]-CaO nanoparticles, determining that the fracture toughness of PMMA/Si[O.sub.2]-CaO composite was improved. Avella et al. (19) evaluated the influence of CaC[O.sub.3] nanopowders on the abrasion and chemical-physical properties of the PMMA matrix by performing thermal, morphological, and mechanical analysis. The abrasion resistance increased as the filler content was increased, improving by a factor of 2 with 3% CaC[O.sub.3] by weight. Brostow et al. (20) evaluated the tribological performance of nanocomposites of PMMA and montmorillonite Brazilian clays (PMMA/MMT). They found that friction coefficient and wear rate increased when the content of MMT Brazilian clay increased. Bin et al. (21) performed friction and wear tests on PMMA/MWNTs composites with different concentrations of MWNTs. It was found that MWNTs, significantly decreased the friction coefficient and increased the wear resistance of composites. Thus, it can be expected that the friction, wear, and hardness properties of PMMA/CaO nanocomposites would be also improved with respect to pure polymer matrix. The purpose of this work is to study the influence of CaO nanoparticles as filler particles in order to improve the PMMA performance as a coating. In this study, we reported the PMMA/CaO coating evaluation by its wear, friction, and microhardness on ultra-high molecular weight polyethylene (UHMWPE) substrates.

Experimental

Chemical

In order to synthesize the nanocomposite, calcium oxide (CaO) nanoparticles with an average size of 16 nm were previously obtained from egg shells as described elsewhere (22) by thermal decomposition. Methyl methacrylate (MMA, 99% Sigma-Aldrich), benzoyl peroxide (BPO, Sigma-Aldrich), acetic acid (C[H.sub.3]COOH, 99.8%), sodium hydroxide (NaOH, J.T.Baker), and distilled water were used as received.

PMMA-CaO composite preparation

Before the reaction, 75.35 x [10.sup.-3] mol of MMA were subjected to a continuous magnetic stirring with 1.18 x [10.sup.-3] mol of NaOH during 30 min to remove the inhibitor agent and the solution was then filtered. Simultaneously, a 1.485 x [10.sup.-4] M solution of CaO with acetic acid was prepared and mixed under continuous magnetic stirring during 10 h. Preliminary studies of our research group were carried out in order to analyze the optimal concentration, and finding this concentration to be suitable for obtaining a stable solution applying these synthesis conditions. Then, 5 ml of the filtered solution of MMA was added. The solution was kept in continuous magnetic stirring for 5 min; afterward 5.161 x [10.sup.-5] mol of BPO were added to initiate the reaction keeping the continuous magnetic stirring during 90 min at room temperature. In order to analyze the polymer and the resulting nanocomposite formation, a comparison was performed by a Fourier transform infrared (FTIR) spectral analysis. FTIR transmittance spectra of the samples were obtained in the 4000-400 [cm.sup.-1] region using a Perkin Elmer Spectrometer 400 with an ATR coupled. An electronic microscope FEI Nova nano SEM 200 equipped with STEM mode was used to observe the CaO particles morphologies.

Coating deposition

UHMWPE substrates, used in the present investigation, were machined with dimensions of 25 mm x 25 mm and polished until a surface roughness of [R.sub.a] < 0.4 [micro]m was obtained. Prior to the deposition of the coating, the UHMWPE substrate surface was thoroughly cleaned; for this purpose, an organic solvent mixture composed of ethanol and water was employed. The coatings were prepared by immersion of UHMWPE substrates in the PMMA/CaO solution, as described in previous works. (23,24) For comparative studies, the same methodology was applied to prepare the PMMA coatings on UHMWPE substrates.

Evaluation of coatings

The surface morphology of the samples was analyzed by a scanning electron microscope FEI Nova nano SEM 200. The average surface roughness ([R.sub.a]) and the thickness of the coatings on UHMWPE substrates, were determined using a profilometer VeccoDektak 150.

Wear tests were carried out on a CSM Instruments Tribometer with a pin-on-disk configuration in dry conditions. The values of kinetic friction coefficient ([[mu].sub.k]) were obtained directly from the Tribox 4.1 software. A tungsten carbide (WC) ball, with a diameter of 6 mm, a roughness average [R.sub.a] = 0.02 [micro]m and hardness of 1770 HV, slid against the coated UHMWPE substrate with the PMMA/CaO composite. For the test, the WC ball was fixed on the load arm and the sample was placed on a rotating disk with a rotating radius of 1.74 mm. The standard contact loads applied were 2, 4, 6, 8, and 10 N, a sliding speed of 0.10 m/s, a sliding distance of 300 m, and an acquisition rate of 2.0 Hz, for the complete test. The temperature during the test was maintained at 26 [+ or -] 1[degrees]C with a relative humidity of 30-40%. Since the wear mass loss values of the samples were inconsistent and the difference between the weight loss was negligible, we instead determined the volume loss (V) values using a standard test method (ASTM G99-05). (25) Assuming that there was no significant pin wear, the volume loss was determined using the following equation (26):

V = ([pi]R[D.sup.3])/6r, (1)

where V is the wear volume ([mm.sup.3]), R is the rotating radius (mm), D is the wear trace width (mm), and r is the ball radius (mm). Then, the wear rate (k) was calculated using the equation given in reference (27):

k = V/Lx, (2)

where V is the wear volume ([mm.sup.3]), L is the applied load (N), x is the sliding distance (m), and k is the wear rate ([mm.sup.3]/Nm). UHMWPE substrates coated with PMMA were also analyzed to study the influence of CaO particles incorporation on the tribological behavior of the composite.

The microhardness of PMMA/CaO coatings on UHMWPE substrates was evaluated by using a Vickers hardness tester. Tests were carried out on a microhardness apparatus Matsuzawa MMT-X7 with Clemex CMT Software, under an indentation load of 0.098 N. Ten indentations were performed, yielding ten diagonal indentation measurements from which the average microhardness was calculated by using the microhardness software. The theoretical depth indentation (An) was determined according to the equation reported by Chicot et al. (28) for a Vickers pyramidal indenter, where d is the indent diagonal:

[D.sub.th] = d/7. (3)

Results and discussion

Figure 1 shows the FTIR spectra comparison, as evidence of formation, between the PMMA (Fig. 1a) and PMMA/CaO (Fig. lb) solutions. The strong vibration bands characteristic to PMMA appear at 1723 [cm.sup.-1] (a) and 1718 [cm.sup.-1] (b) which corresponds to the stretching of the C=0 group (29) characteristic of PMMA polymer. The bands around 2992 and 2947 [cm.sup.-1], in both specimens, correspond to the C-H stretching of methyl group (C[H.sub.3]), while the bands at 1200 [cm.sup.-1] are assigned to torsion of methylene group (C[H.sub.2]) and the bands around 1145-1157 [cm.sup.-1] corresponds to vibration of the ester group C-O. (27) Also, a stretching vibration band appears at 1635 [cm.sup.-1], which represents the C=C stretching vibration, indicating a partial polymerization of MMA. (29) The average roughness ([R.sub.a]), the average of the individual heights (asperities) and depths from the arithmetic mean elevation of the profile, was obtained through profilometry; the roughness for PMMA coating was calculated in 0.62 [micro]m [+ or -] 0.06, while the calculated roughness of PMMA/CaO coating was 0.74 [micro]m [+ or -] 0.10. Moreover, the average thickness of PMMA/CaO coating was 107 [micro]m [+ or -] 2.3 and 97 [micro]m [+ or -] 0.8 for PMMA coating. Figure 2 shows the SEM micrographs obtained by STEM mode, of the CaO nanoparticles used in the nanocomposites preparation. Figure 2 shows two representative images of CaO nanoparticles used in this study, which exhibited a group of agglomerated particles with nanometric sizes and spherical morphologies.

Figure 3 shows the variation of kinetic friction coefficient ([[mu].sub.k]) with respect to the sliding distance for PMMA (a) and PMMA/CaO (b) coatings on UHMWPE substrates, under different loads. From Fig. 3a, it can be seen that PMMA coating exhibited an unstable behavior of friction coefficients for the lowest loads, showing high fluctuations at 2 and 4 N, whereas at high loads, it shows stable friction coefficients almost constant during the sliding progress, reaching the lowest friction coefficient at 6 and 8 N with an average value of [[mu].sub.k] = 0.13. It can be seen that friction coefficients at these loads overlap from 50 to 200 m and then decrease for 8 N, reaching a minimum value of [[mu].sub.k] = 0.10. Figure 3b exhibits the friction behavior of PMMA/CaO coating, showing lower friction coefficients but higher fluctuations in all loads than the ones observed for the PMMA coating. It can be observed that friction coefficients of PMMA/CaO coating showed a tendency to increase with the sliding distance at all loads. In general, PMMA/CaO coating shows a lower mean friction coefficient value ([[mu].sub.k] = 0.08), as average for all loads, as compared to the PMMA coating ([[mu].sub.k] = 0.15). The low friction coefficient behavior for PMMA/CaO coating could be related to the presence of CaO nanoparticles on the coating surface which can act as solid lubricant; however, this effect could lead to a rapid increase in the numbers of wear particles entrapped between the sliding surfaces, which would promote higher wear rate values as a consequence of the increase in the contact area. Also, it can be seen that, in general, kinetic friction coefficient ([[mu].sub.k]) values for PMMA and PMMA/CaO coatings decreased with the increase of load, except for a 10 N load where kinetic friction value increased for both coatings. Generally, an increase in the load could increase the contact area on the soft substrate used, decreasing the friction coefficients. Such behavior also has been related to the elastic deformation of some polymers. (30-33) The average friction coefficients ([[mu].sub.k]) and the volume loss values are listed in Table 1, along with the average wear rates for each load. Samples in this study have lower friction coefficients than the other similar materials improved with filled particles reported by Sawyer et al., (34) and Bin et al., (21) where friction coefficient values obtained were [[mu].sub.k] = 0.16 and [[mu].sub.k] = 0.20, respectively. Avella et al. (19) filled PMMA with CaC[O.sub.3] nanoparticles, finding that the abrasion resistance increased as the filler content was increased to 3 wt% of CaC[O.sub.3], whereas, in our study we used 0.05 wt% of CaO. Another investigation with lower filler content was reported by Sawyer et al. (34) which filled PTFE with alumina nanoparticles, finding the lowest friction coefficient ([[mu].sub.k] [approximately equal to ] 0.15) to 0.04 wt% of nanoparticles. However, the lowest wear rate (Ac = 1.2 x [10.sup.-6] [mm.sup.3]/Nm) result in that study was for PTFE, with almost 20 wt% of alumina nanoparticles with an average friction coefficient of [[mu].sub.k] = 0.19. Figure 4 shows the wear rate (k) and volume loss (F) values for PMMA and PMMA/CaO coatings on UHMWPE substrates, as a function of applied load. PMMA/CaO coating exhibits volume loss values higher than PMMA coating. Although wear rate values of PMMA/CaO coating are slightly higher than PMMA coating, wear rate values of PMMA/CaO are lower than those reported before for polymers filled with harder particles. (31) It is reported that the improvement observed in the wear resistance of polymer-filled composites is related to the fact that the particles help to support a part of the applied load, reducing the penetration into the polymer surface leading to lower wear rates. (20)

Figure 5 shows the SEM micrographs of the PMMA coating surfaces, outside the wear track in an area without wear (a, b) and the inside the wear track (c, d) showing the wear surface, tested at 10 N in order to observe the changes in the surface topography of the coating after wear measurements. The area of PMMA coating without wear shows a flat homogeneous surface. Also, some brightness lines can be seen that can be related to the poor homogeneity of PMMA coating during the dipping process; such lines are more evident at higher magnification; while the worn surfaces exhibit galling forms of adhesive wear. (30,35) The corresponding worn area shows a deformed surface, displaying fractures on the coating surface, which means poor wear resistance at this load when it is sliding against the WC counterface. Bin et al. (21) have reported similar adhesive wear for PMMA sliding against a counterface with lower hardness than WC.

Figure 6 shows the corresponding SEM micrographs for PMMA/CaO coating surfaces, outside the wear track in an area without wear (a, b) and the inside wear track (c, d) showing the wear surface; the images exhibit the changes in the surface topography of the coating after wear measurements. They exhibit an evident change due to the CaO nanoparticles incorporation. The surfaces without wear show flake-like features with soft appearance due to the incorporation of inorganic material into the polymeric coating; whereas after the wear test at 10 N, the worn surfaces show adhesive wear resulting from the shear of the friction junctions. Higher detached material can be seen and, at higher magnification, some traces spreading on the surface coating can be observed. It is well known that a principal feature of adhesive wear is the transfer of material from one surface to another due to localized bonding between the contacting solid surfaces. (30) Also, it has been reported that, under certain conditions, the transfer material can produce an influence on tribological behavior, i.e., transfer material of a thin soft film onto the harder surface increasing the wear rate. (30) Thus, the wear increment observed for PMMA/CaO composite could be explained on the basis of this statement.

Table 2 shows the hardness and depth indentation values for the PMMA and PMMA/CaO coatings on UHMWPE substrates. As expected, PMMA/CaO coating exhibited higher hardness value than PMMA coating due to CaO nanoparticles incorporation into PMMA matrix. The microhardness values found in this work are higher than those reported by Kyu-Hyeon and Sang-Hoon (18) for poly(methyl methacrylate)/Si[O.sub.2]-CaO nanocomposites testing at 98 N. They reported hardness values of 20.5 [+ or -] 0.6 and 140.5 [+ or -] 1.2 MPa (=0.02 and 0.14 GPa, respectively). Although the influence of substrate on the coated systems hardness has been widely reported, there is general agreement that the coating-only hardness response may be observed at very low loads, when the indentation depth does not exceed one-tenth of the film thickness. (36,37) Then, we can discuss the coatings hardness through the value of the relative indentation depth ([beta]), which is given by the ratio between the indentation depth and the coating thickness. Values of [beta] > 1 signify that the indenter severely deformed and penetrated the coating, and reached the underlying substrate material. At the other extreme, when [beta] < 0.1, the influence of the substrate on the deformation is small, and for this and lower indentation depths, the coating-only response is observed. (38) The determined values of [beta] for PMMA and PMMA/CaO coatings were 0.08 and 0.06, respectively; therefore, on the basis mentioned above, we can support that the hardness measurements in this work are not influenced by the substrate.

Conclusions

PMMA/CaO nanocomposite with 0.05 wt% of CaO was prepared by in situ polymerization process for coating applications on UHMWPE substrate. The mixture between poly(methyl methacrylate) and calcium oxide nanoparticles resulted in a rough and soft coating. PMMA/CaO showed lower friction coefficients than PMMA coating at all loads. In addition, PMMA/CaO coating exhibited lower friction coefficients than other similar materials improved with filled nanoparticles reported; however, PMMA/CaO coating exhibited volume loss and wear rate values higher than PMMA coating. Although the wear rate values of PMMA/CaO coating were slightly higher than PMMA coating, wear rate values of PMMA/CaO were lower than those reported before for polymers filled with harder particles. We also observed adhesion damage as the main wear mechanism.

DOT 10.1007/sl1998-014-9639-y

L. D. Aguilera-Camacho, C. Hernandez-Navarro, K. J. Moreno (El), J. S. Garcia-Miranda

Tecnologico Nacional de Mexico, Instituto Tecnologico de Celaya, A.P. 57, 38010 Celaya, Guanajuato, Mexico e-mail: kjmb@live.com.mx; karla.moreno@yahoo.com.mx

A. Arizmendi-Morquecho

Cimav-Unidad Monterrey, Apartado Postal, 66600 Apodaca, Nuevo Leon, Mexico

Acknowledgments The authors gratefully acknowledge the financial support of DGEST-Mexico. Also we would like to express our gratitude to Miguel Esneider and Nayeli Pineda from CIMAV Monterrey for their help with the experimental procedures.

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Table 1: Wear factors of PMMA and PMMA-CaO coatings against WC ball
on UHMWPE substrate

Load                               PMMA
(N)
            Friction           Volume loss           Wear rate,
          coefficient        ([mm.sup.3]) (a)       [10.sup.-5]
       ([[mu].sub.k]) (a)                         ([mm.sup.3]/Nm)

2      0.18 [+ or -] 0.02   0.03 [+ or -] 0.01   4.95 [+ or -] 0.14
4      0.19 [+ or -] 0.01   0.04                 3.27 [+ or -] 0.09
6      0.13                 0.07                 3.93 [+ or -] 0.45
8      0.13                 0.10                 4.08 [+ or -] 0.12
10     0.14                 0.13                 4.23 [+ or -] 0.26

Load                             PMMA-CaO
(N)
            Friction           Volume loss           Wear rate,
          coefficient        ([mm.sup.3]) (a)        [10.sup.-5]
       ([[mu].sub.k]) (a)                          ([mm.sup.3]/Nm)

2      0.12 [+ or -] 0.01   0.01                  2.75 [+ or -] 0.10
4      0.10                 0.07                  5.90 [+ or -] 0.25
6      0.05                 0.06                  3.30 [+ or -] 0.16
8      0.06                 0.17                  7.20 [+ or -] 0.35
10     0.09                 0.36 [+ or -] 0.01   12.10 [+ or -] 0.59

(a) Standard deviation to some values for friction coefficients and
volume loss were negligible

Table 2: Hardness and depth indentation of PMMA and
PMMA/CaO coatings on UHMWPE

Coating    Vickers hardness   Depth indentation, [D.sub.th]
                (GPa)                   ([micro]m)

PMMA             0.06                      7.9
PMMA/CaO         1.72                      7.3


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Title Annotation:Poly(methyl methacrylate)
Author:Aguilera-Camacho, L.D.; Hernandez-Navarro, C.; Moreno, K.J.; Garcia-Miranda, J.S.; Arizinendi-Morque
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:Mar 1, 2015
Words:4678
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