Effect of Sputtering Power on the Microstructure and Antimicrobial Capabilities of the
ZrAlNiCuSi Thin Film Coating
P.T. Chiang1,2, J.Y. Yu1, J.S.C. Jang1, G. J. Chen1, Y.H. Shih1, and S.H. Tseng1
1 Department of Materials Science and Engineering, I-Shou University, Kaohsiung, Taiwan
2 Department of Plastic Surgery, E-Da Hospital, Kaohsiung, Taiwan, ROC
Address of 1: #1,section 1, Hsueh-Cheng Road, Ta-Hsu Shiang, Kaohsiung county, 840
Address of 2: #1, E-Da Road, Jiau-Shu Tsuen,Yan-Chau Shiang, Kaohsiung County,
TAIWAN, 824
1
Abstract
Direct current (DC) sputtering is a good method for thin film coating, such as making
ZrAlNiCuSi bulk amorphous alloys fabricated to thin film. A series of thin ZrAlNiCuSi films
were coated on a 304 stainless steel substrate by a DC sputtering method at varying sputtering
power. The amorphous feature of these thin ZrAlNiCuSi films was confirmed using glancing
incident x-ray diffraction (GIXRD). The porosity and surface roughness were observed using
field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM),
respectively. The surface profile of the ZrAlNiCuSi film has a very smooth morphology with
an average surface roughness of about 1 nm when prepared by a sputtering power more than
25W. A saturated value of 70 N was compatible with the adhesion capability of commercial
hard coating. The hardness of ZrAlNiCuSi thin film was greater than that of commercial tool
steel. The thin ZrAlNiCuSi films had different antimicrobial effects for different microbes.
For Escherichia coli, the thin films had good bactericidal effect for 96 hours. For the microbes
Staphylococcus aureus, Pseudomonas aeruginosa, Acinetobacter baumannii and Candida
albicans, ZrAlNiCuSi thin films exhibit good microbiostatic effect for at least 24 hours.
2
1. Introduction
In hospitals various microbes, including fungi, bacteria and viruses, flourish together
everywhere [1-3]. Hospital related infectious disease or nosocomial infection can result
from endogenous flora, reactivation of latent infectious agents, or exogenous environmental
microbes [3,6,9].
Nosocomial infection is frequently caused by transmission of pathogens from the
hospital environment (such as instrument or equipment) and by direct hand to hand contact
between health-care providers and patients [3]. The most common causative pathogens
have been reported as Staphylococcus aureus, Escherichia coli, Pseudomonas or
mixed-microbial infection [6-8]. In Taiwan, Acinetobacter baumannii has been another
important pathogen in recent years [11,12].
Stainless steel is a material widely used industrially for working surfaces and door
fittings (such as handles and push plates). In hospitals it is found in many different types of
medical apparatus, particularly surgical instruments [9,13,14].
The zirconium (Zr)-based bulk amorphous alloys have a high glass-forming ability and
also have an extremely wide super-cooled liquid region exceeding 100 K [16,19-24]. In
addition, Zr-based bulk amorphous alloys exhibit high tensile strength, high elastic
modulus and high corrosion resistance [21,25-29]. Moreover, Zr-based bulk amorphous
alloys exhibit relatively good thermal stability, homogeneous distribution of multiple
elements and easy fabrication [16,19,25,30].
3
A modification using thin amorphous metal films on 304 stainless steel door-contactor,
touched frequently by hospital patients and health care providers alike, may be an effective
way to control nosocomial infection. [6,9,10,13-15].
In this study, we focused on evaluating the effect of sputtering power on the
microstructure, mechanical, and antimicrobial properties of thin ZrAlNiCuSi films
fabricated onto a 304 stainless steel substrate. A bare 304 stainless steel substrate was used
as a control.
4
2. Experimental
The pre-prepared Zr61All7.5Ni10Cu17.5Si4 bulk metallic glass (ZrAlNiCuSi) was selected
as the test material. Using a vacuum direct current (DC) MDX1000 sputtering system with
Varian V550 turbo pump, a thin film was fabricated onto a 304 stainless steel substrate.
Sputtering involves firing ions toward a ZrAlNiCuSi bulk amorphous alloy target to
displace atoms, which were then deposited on a 304 stainless steel substrate to form a thin
ZrAlNiCuSi film. The operating condition of the DC sputtering system was set at a base
pressure 6 x 10-6 torr, working pressure of 4 mtorr, argon (Ar) flow of 5.4 sccm, sputtering
time of 30 minutes and varying sputtering powers of 15W, 20W, 25W, 30W, 35W and
40W.
The amorphous feature of the thin ZrAlNiCuSi films was confirmed by PANalytical
X'Pert Pro glancing incident x-ray diffraction (GIXRD). The composition & distribution of
the thin Zr-based alloy films was analyzed by the HORIBA energy dispersive spectrum
(EDS). The porosity and surface roughness were examined by a Hitachi S4700 field
emission scanning electron microscope (FESEM) and NT-MDTP7LS atomic force
microscopy (AFM), respectively. The alpha step was conducted to measure the thickness of
thin ZrAlNiCuSi films, which resulted from different sputtering powers. The adhesion
capability between the films and the 304 stainless steel substrate was evaluated using a
TEER 2330 scratch tester, under a maximum load of 3 Kg. Nano-indentation was applied
to measure the hardness of the thin ZrAlNiCuSi films produced at a sputtering power of
5
30W.
For evaluating the bactericidal, bacteriostatic and mycostatic effect [2,32] of the thin
ZrAlNiCuSi films, the organisms Staphylococcus aureus (ATCC 25923, Gram positive
cocci), Pseudomonas aeruginosa (ATCC 27853, Gram negative bacilli) , Escherichia coli
(ATCC 25922, Gram negative bacilli), Acinetobacter baumannii (isolated from patient,
Gram negative coccobacilli ) and Candida albicans (isolated from patient, fungus) were
isolated. A drop of 0.01 ml of each incubated microbe, which corresponds to a microbial
count 1.5 x 106 CFU (under Vitek colorimeter keeping 80-88% turbidity = 0.5 McFarland =
1.5 x 108 CFU/ml), was sampled and placed on the thin ZrAlNiCuSi film complex as the
test plate and on the 304 stainless steel plate alone, used as the control. The test and control
plates were placed on Mueller-Hinton (M-H) agar (beef soup 300gm, peptone 17.5gm,
starch 1.5gm and agar 17.0gm.) and they were incubated at 37.5oC. Microbial growth was
evaluated at 24, 48, and 96 hours. The areas of growth at each incubation period were
recorded by subtraction photography and then assessed using Image-pro-Plus software to
adjust for the bactericidal, bacteriostatic or mycostatic effect. Some of the tissue from the
surface of each of the infected plates was transferred for inoculation into blood agar (BAP)
and eosin-methylene blue agar (EMB) so that semi-quantitative colony counts under aseptic
conditions could be performed. The organisms were identified by the two agar mediums
(BAP and EMB) without favorable bias.
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3. Results and Discussion
3.1. Properties characterization of ZrAlNiCuSi thin film
The thickness of the ZrAlNiCuSi films is a function of sputtering power, as shown in
Figure 1 and it exhibits a reasonably linear relationship. A 550 nm thickness of
ZrAlNiCuSi film can be fabricated within 30 min under a 40 W sputtering power. In
parallel, the result of X-ray diffraction reveals that a typical amorphous feature for all of the
ZrAlNiCuSi films is produced by different sputtering powers, as shown in Figure 2.
Because these films are so thin, several sharp peaks caused by the crystalline structure of
the 304 stainless steel can be seen on the X-ray diffraction pattern, as shown in Figure 2.
The compositions of the ZrAlNiCuSi bulk metallic glass remains the same as sputtered thin
ZrAlNiCuSi films under energy dispersive spectrum (EDS) examination as shown in Table
1. This un-homogenous distribution [25,31] is only found in the low sputtering power
ranges, such as 15W.
The surface roughness, as measured by AFM, is a function of sputtering power. Figure 1
shows a decreasing trend in roughness with an increasing sputtering power.
Roughness of
about 1nm can be obtained for a thin film prepared by a sputtering power more than 25W.
The results of scratch test reveal that the adhesion capability [21,25,29,30] increases with
sputtering power, as shown in Figure 3. A saturated value of 70 N occurs when the thin
films are fabricated at a sputtering power over 35 W. This value is compatible with the
adhesion capability of commercial hard coatings. Manufacturing thin ZrAlNiCuSi films by
7
sputtering onto a 304 stainless steel substrate is definitely an acceptable procedure. [21,28]
The hardness of the thin ZrAlNiCuSi films produced at 30W sputtering power was
measured to be an average of 5.83±0.41 GPa and 6.01±0.17 GPa under loads of 2500μN
and 3000μN, respectively. [16,20,24,28]
3.2. Evaluation of the antimicrobial (bactericidal, bacteriostatic, or mycostatic) effects
Copper (Cu)-ions can kill bacteria by destroying cell walls and membranes. [6-8, 33-35].
The initial step of inoculation is good adhesion between the microbes and the host. Using
thin amorphous metal films on hospital equipment and fittings may alter the adhesion
capability of microbes. [1,4,17,18]Therefore, in ZrAlNiCuSi amorphous thin films, the
nano-structured surface (which has about 1 nm roughness) and homogenous distribution of
the atomic-scaled Cu-rich clusters [10,25,31,36-39] may play an important role in
bactericidal action.
Different microbes have different adherence factors (adhesins). A single adhesin may
have more than one receptor [1,10,13]. Adherence to host (or environment) is the initial
step in bacterial pathogenesis or infection [1,15,18]. Bacterial adhesion to solid surfaces is
due to the relationship between the adhesin quantity of bacteria and surface quality of
material [4-6,14,16]. In this control study of bactericidal, bacteriostatic or mycostatic effect
of the thin ZrAlNiCuSi films, different isolated organisms were incubated on
Mueller-Hinton (M-H) agar plates for 24, 48 and 96 hours at 37.5oC. Different microbes [2]
were placed on the thin ZrAlNiCuSi film and the 304 stainless steel control under aseptic
8
techniques.
Some of the tissue [2] from the surface of each of the infected materials on the
Mueller-Hinton agar was streaked onto blood agar (BAP) and eosin-methylene blue agar
(EMB) for semi-quantitative colony counts under aseptic procedure. The Staphylococcus
aureus, Pseudomonas aeruginosa, Escherichia coli, Acinetobacter baumannii and Candida
albicans [2,32] were identified by both agar mediums (BAP and EMB) without favorable
bias.
Image-pro-Plus software in serial photography was used to evaluate the bactericidal,
bacteriostatic or mycostatic effect. In a serial study, thin ZrAlNiCuSi films have better
bactericidal effect for Escherichia coli than the 304 stainless steel plate after 96 hours
incubation time as shown in
Figure 4 and Figure 6. For the organisms Staphylococcus
aureus, Pseudomonas aeruginosa, Acinetobacter baumannii and Candida albicans, thin
ZrAlNiCuSi films exhibit a good microbiostatic effect for the first 24 hours. For
Staphylococcus aureus and Candida albicans thin ZrAlNiCuSi films show better
microbiostatic effect than 304 stainless steel at 24 hours to 48 hours as illustrated in Figure
5 and Figure 6. In addition, there is no significant difference on antimicrobial (bactericidal,
bacteriostatic or mycostatic) effects for the thin ZrAlNiCuSi films prepared at different
sputtering powers (35W or 40W).
9
4. Conclusion
According to the results of X-ray diffraction, AFM examination, scratch test,
nano-indentation, bacterial incubation test, and bacterial identification test for the ZrAlNiCuSi
thin film coating on 304 stainless steel , the evaluation of microstructures and antimicrobial
capabilities of thin film can be summarized as:
(1) Microstructure analysis by X-ray diffraction confirms that a thin film of ZrAlNiCuSi
alloy in amorphous state can be fabricated by DC sputtering method. A 550 nm
thickness of thin ZrAlNiCuSi film can be fabricated within 30 min under 40W
sputtering power. The surface profile of the ZrAlNiCuSi film has a very smooth
morphology with surface roughness of about 1 nm and can be obtained at a sputtering
power more than 25 W.
(2) A saturated value of 70 N occurs when the thin film is fabricated at a sputtering power
more than 35W. This value is compatible with the adhesion capability of commercial
hard coating, such as CrN on tungsten carbide tools. In manufacturing, thin films of
ZrAlNiCuSi can easily be fabricated on 304 stainless steel by sputtering.
(3) The hardness of ZrAlNiCuSi thin film produced by 30W sputtering power is measured
to be an average of 5.83±0.41 GPa and 6.01±0.17 GPa under a loading pressure of
2500μN and 3000μN, respectively. This value is harder than commercial steel used in
tools.
(4) When comparing the antimicrobial properties of sputtered thin ZrAlNiCuSi films to
10
different pathogens, they can be seen to have a better bactericidal effect for Escherichia
coli than a 304 stainless steel plate after 96 hours incubation time. For the microbes of
Staphylococcus aureus, Pseudomonas aeruginosa, Acinetobacter baumannii and
Candida albicans, a good microbiostatic effect is seen within the first 24 hours. For
Staphylococcus aureus and Candida albicans thin ZrAlNiCuSi films have a better
microbiostatic effect than 304 stainless steel between 24 and 48 hours.
Acknowledgement
The grammar was checked by Dr. Victoria Rose from England. The authors are very
grateful for the assistance of Ms. Hsiu-Fang Lin and the instrument support from the
Division of Laboratory Medicine in E-Da Hospital as well as the assistance of the Metal
Industrial Research and Development Centre.
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