Abstract
Antibacterial surfaces such as copper coatings are able to reduce the growth of bacteria. In this study, copper coatings was deposited on the 316 stainless steel substrates by a handmade device operating as an atmospheric plasma spraying system. The chemical composition microstructure and morphology surface of the coatings are examined by x-ray diffraction (XRD) and scanning electron microscope (SEM) and back scattering electron microscope (BSE). Micro hardness as one of the key properties of the coating is characterized based on cross-section. We also evaluate the thickness and the adhesion strength of the coating. Dissection material of coating is performed by energy dispersive x-ray spectroscopy (EDX). Further, the antibacterial activity of our coatings is assessed by both gram negative Escherichia coli ATCC 10536 and gram positive Staphylococcus aureus PTCC 1112 bacteria. As the last step, the antibacterial performance of the coated stainless steel surface with copper are compared to uncoated one. Results confirm that the copper coatings improve the antibacterial property of substrates and owning fine antibacterial behavior compared to stainless steel.
Introduction
Thanks to its durability, ease of fabrication, cleanness, and its high resistance during welding, stainless steel may be used in medical and food processing and handling industries, abattoirs, hospital environments, public transit systems, drinking water systems and in domestic premises. On the other hand, bacterial infection is one of the significant clinical complications. Bacteria can survive for a long time on the stainless steel surfaces and increase infections.
A feasible way for killing infections is coating the surfaces with a bactericidal element. Cost considerations favor copper coating over solid copper. Thermal spray is one of the less expensive thermal spraying techniques that it can coat a broad variety of materials with high melting point without inflicting any damage to the properties of the substrate. It can produce dense coatings. Microstructure of these coatings mainly depends on the spray parameters. Copper is used for purification and eliminate Coliform bacteria in drinking water [1].
Antibacterial activity of Cu/SiO2 coating on glass substrate by the sol-gel method is increase with metal concentration [2]. HVOF-sprayed nanostructured TiO2 coating exhibits antibacterial effect on Pseudomonas bacteria. The presence of the anatase and rutile phases in the TiO2 coatings is responsible for the bacterial cell destruction [3]. Silver and copper are ion implanted into medical metals, 317L stainless steel, Titanium and Ti-Al-Nb. It show that silver ion implantation does not vary the corrosion resistance [4]. Films of TiO2 modified by deposition of nanostructured silver via thermal CVD method are show strong antibacterial behavior [5]. TiO2/Cu composite coatings by using Flame spraying combine performance of the photo catalytic TiO2 and antibacterial copper. It improves bactericidal performance when it exposes to light [6]. Antibacterial behavior of wire arc sprayed copper coating are compared to stainless steel and commercial copper that it is considerably more than others [7–13].
In the current study, atmospheric plasma spraying technique was utilized to deposit copper coatings on stainless steel substrates and the microstructure characteristics and antibacterial performance of the coatings was investigated. In section 2 the experimental method are explained the analytical method for investigation of results are given in section 3. The results and discussion are obtained in section 3. Some important results are given in section 4.
Experimental setup
Materials and methods
The spraying work is carried out by handmade atmospheric plasma spraying (ASP) system (Fig. 1a). The plasma torch system power is up to 30 kW. The diameter of nozzle anode is 8 mm and its length is 60 mm. The substrates are 316 stainless steel. The substrates are cut into 20×20 mm2 pieces with 2 mm thick. The powder is commercial copper powder with 99% purity. The particle size is less than 20 micrometer. The powder is fed into plasma jet at the exit nozzle and in radial direction. Here, Argon is used both as plasma gas and as powder feed gas. The schematic of the system is shown in Fig. 1b. Plasma spray parameters are represented in Table 1. Before spraying, the substrates are grit blasted by Alumina in order to increase surface roughness and therefore create a strong mechanical bond between the coating and the substrate [8]. Then, they are degreased by using methyl alcohol and acetone. Subsequently, they are cleaned by compressed air flow in order to remove grit residue left on the substrate and improve the adhesion strength.
Handmade atmospheric plasma spraying system.
Spray parameters
The phase composition of copper powder and copper coating are performed using X pert Philips x-ray diffraction (XRD) instrument with Cu Kα radiation. The microstructure of the deposited copper coating from top and polished cross-section by scanning electron microscopy (SEM, Philips XL 30) and back scattering electron (BSE) is examined to evaluate thickness, porosity and shape of splat. Energy dispersive x-ray spectroscopy (EDX) is performed to dissection material of coating. The etchant (50 mL HNO3 and 50 mL distilled water) is utilized to appear the microstructure of the copper coating. The adhesion strength of coating is assessed by use of scratch tape test. Using Vickers micro hardness tester at a load of 150 gram, hardness of the coating is inquired.
Antibacterial behavior
In order to determine the reduction of Escherichia coli and staphylococcus aureus cells on different samples, the samples are autoclaved (121°C, 15 min). After autoclaving, they are cooled to room temperature. The suspension of E. coli (ATCC 10536) and S. aureus (PTCC 1112) are prepared. The concentration of the cells is adjusted to about 7.00 log CFU/ml. The samples are covered with the suspension and the number of E. coli and S. aureus cells is enumerated after 24 hours. This process shows number of bacteria that survived in the suspension. With the help of relative error percent, antibacterial effect from below equation is obtained. Antibacterial effect = difference number of control and other cells/number of control cells.
Results and discussion
Study of XRD
The XRD pattern of copper powder and copper coating on substrate are shown in Fig. 2a and b, respectively. As seen in Fig. 2a, the structure of the copper powder is cubic and it orients along (111), (200), (220) directions. Figure 2b illustrates that the XRD pattern of copper coating in addition to pure copper peaks, shows copper monoxide and copper dioxide that is cubic and its orientations along (111), (200), (220) and (110). The variety of oxide occurs in atmospheric plasma spraying technique. Oxidation happens both during in-flight and after colliding to substrate and being cooled by atomization the atmosphere. The oxide content in the copper coating depends on spraying parameters.
The XRD pattern of copper powder and copper coating on stainless steel.
The back scattering electron (BSE) micrograph of the copper coating on the stainless steel substrate is shown in Fig. 3. As seen in Fig. 3, the coating is not homogeneous on the substrate. The electric arc moves inside the nozzle which results in temperature and velocity instabilities. Thus, the injection powder due to the variation of particle velocity and temperature in plasma jet appears in three states are molten, partial molten and solid particles. The SEM of the coating is shown in Fig. 4a. Figure 4a illustrates that the most of the splats deposited on stainless steel are flower shaped and few are pancake splats which depends on the spray condition. The diameter of pancake splat is estimated to be 70 micrometer. The magnification of Fig. 4a is shown in Fig. 4b. It is appeared that there are nanometer particles in the coating. Such fine grains may contribute to improvement of coating properties.
The BSE microstructure from top of copper coating.
The SEM microstructure from top of copper coating.
The EDX of microstructure is illustrated in Fig. 5. As seen in this figure the density of copper coating on the substrate is very good and the existence of nano size particles appears in the coating.
The EDX mapping of the as-sprayed copper coating.
As the temperature decrease a thin oxide layer create around particle before impact to substrate. The particles collide to the substrate due to deceleration it causes a pressure laterally imposed on the particle-substrate interface. They spread outward from the point of impact and after solidification they form a lamella. The partial molten and solid particles cannot adapt well to the previously deposited coating and it results in formation of pores. A set of lamellae form the coating. As seen in Fig. 6a, the thickness of coating is about 20 micrometer. The etched copper coating microstructure is shown in Fig. 6a, b. Figure 6b is the magnification of Fig. 6a. It is appear that there are some pores in the coating, mainly between the lamellae. The Lamellae are separated by alternating oxide layers. As seen in this figure, there are the oxide layers around particles. The oxygen not only surrounds particles but also it separates lamellae [9].
The SEM image of an etched cross-section of copper coating.
As seen in Fig. 7, the coating microstructure shows good interface and adhesion bonding splats locked onto surface roughness.
The SEM micrograph from cross-section of copper coating.
In order to analyze the antibacterial properties of coated layer, we consider three samples which the sample 1 is the control, the sample 2 is the stainless steel substrate and finally the sample 3 is the copper coated substrate. After performing the antibacterial test the results are illustrate in Table 2. Table 2 shows that the initial number of E. coli and S. aureus cells are increased for sample 1 from 7.00±0.03 log CFU/m to 8.20±0.61 log CFU/m and 8.10±0.55 log CFU/m respectively as seen in this Table. E. coli cells are reduced to 6 log CFU/ml and S. aureus cells are reduced to 6.4 log CFU/ml for sample 2. The results for sample 3 show that the E. coli cells are reduced to 3.1 log CFU/ml and S. aureus cells are reduced to 3.7 log CFU/ml. The antibacterial effect stainless steel against E. coli 27% and against S. aureus is 21%. However antibacterial rate copper coating against E. coli 62% and against S. aureus is 54%. It is evident that the concentration of S. aureus cells is higher than E. coli cells in all samples namely S.aureus need longer time to find a similar antibacterial effect. The antibacterial of the copper coating with very low thickness on stainless steel is more than two time of stainless steel. Atmospheric plasma sprayed copper coating has defects such micro pores and micro cracks that are motive factors dissolution of metal ions in aqueous media. Further, the nature crystalline copper and its bonding with oxides play a key role in growth of antibacterial behavior [7]. It punches a hole in the cell membrane, something like a balloon, and therefore the bacteria collapses. It stops the bacteria respiring, goes into the cell and destroys their DNA [10].
Reduction of E. coli and S. aureus (log CFU/ml) on different surfaces after 24 h
Reduction of E. coli and S. aureus (log CFU/ml) on different surfaces after 24 h
Sample 1: control, Sample 2: steel, Sample 3: copper coating on steel.
Using handmade device operating as an atmospheric plasma spraying system, copper powder was deposited on 316 Stainless steel surfaces. Microstructure of coatings depends on spray conditions. By varying of spray parameters, microstructure of coatings (pores and oxidation content) can be controlled and also the properties of coating (hardness and adhesion strength) would be improved. The copper coatings are not only stable, but adhere to the substrate. The micrograph shows that the nano size particles exist in the coated layer. It is shown the density of coated copper on substrate is high. The copper coatings improve the antibacterial property of stainless steel. The antibacterial properties of the copper coated surface is more than two time of stainless steel. Coated stainless steel surfaces with its high antibacterial effect can be applied in hospital environments, food industries surgical instruments and suchlike. They reduce the spread of infections and help to cure and prevent diseases.