Optimization of Deposition Parameters for α-Al 2 O 3 Coatings by Double Glow Plasma Technique

The parameters to prepare α-Al2O3 coatings by double glow plasma technique were optimized to obtain dense and thick α-Al2O3 coatings on 316 L stainless steel at low temperature of 580 °C. The parallel distance between the source electrode and the substrate, working pressure, source electrode voltage, and workpiece electrode voltage were designed by L9 Orthogonal array design. The thickness, microstructure, chemical composition and phase components of the sputter-deposited Al/α-Al2O3 coatings were analysed by means of 3-D non-contact surface profiler analysis, scanning electron microscopy equipped with energy dispersive X-ray spectrometry and glancing-angle X-ray diffraction, respectively. The results indicated that the parallel distance between the source electrode and the substrate played a dominant role in determining the thickness of the films. The coatings prepared at the optimum parameters exhibited a very dense, uniform, and compact microstructure without any cracks and defects. It was found that iron aluminide formed at the interface between the coatings and the substrates. The thickness of the coatings reached about 11.6 μm at the deposition rate of ~64.4 nm min–1. The oxidized coatings with the lowest porosity about 0.13 % had relative content of α-Al2O3 as high as 66.5 % when oxygen flow was 2 sccm.


Introduction
Alumina (Al 2 O 3 ) has been of great interest in the past few years owing to its excellent properties.Al 2 O 3 is widely used in numerous applications, such as wear resistant coatings for cutting tools, 1 corrosion resistance for magnesium alloy, 2 an insulating layer for semiconductor devices, 3 moisture permeation barrier coatings for porous papers, 4 oxygen permeation barrier coatings for aeronautics and astronautics, 5 or a hydrogen permeation barrier coating for nuclear fusion reactors. 6 2 O 3 has a number of polymorphs, including κ-phase, γ-phase, θ-phase, α-phase etc.Among the various crystal structures of alumina, the metastable phase tends to transform into the stable one.Thermodynamically stable α-Al 2 O 3 with corundum structure is considered as protective coating on substrates to be used at high temperatures.Generally, it can be deposited using chemical vapour deposition (CVD), physical vapour deposition (PVD), pulsed laser deposition (PLD) as well as magnetron sputtering (MS).8][9] However, this causes unwanted effects limiting the application of substrate materials, such as steel and glass, 7 and cracks owing to thermal expansion mismatch, 10 which can be avoided by reactive diffusion through post-deposition annealing.Meanwhile, post deposition oxidation after direct Al deposition on 316 L stainless is an efficient way to form (Al 2 O 3 + Fe 2 O 3 ) interface layers. 11 is possible to lower substrate temperature even more by increasing the kinetic energy and controlling flux of the depositing species.α-Al 2 O 3 films on silicon or graphite substrate were deposited by filtered vacuum arc method at substrate temperature of 780 °C according to Y-Yamada-Takamura's study. 12With a negative bias voltage of 200 V, it was possible to deposit films containing α-Al 2 O 3 with pre-heating the substrate to a lower temperature of 460 °C.A similar result was obtained by Wallin's research, 13 in which α-Al 2 O 3 coatings were deposited directly onto the hard alloy and molybdenum substrates at low temperature (650 °C) using high energetic ion bombardment (20 eV -100 eV).The above-discussed experiments demonstrated that the crystalline growth temperature of α-Al 2 O 3 could be reduced through energetic ion bombardment.Another interesting approach to synthesize the α-Al 2 O 3 at lower temperatures is based on the utilization of additive elements.Jin et al. 14 and Andersson et al. 15 both reported the use of an α-Cr 2 O 3 template layer for the low temperature growth of α-Al 2 O 3 films, and the crystallization temperatures decreased to 600 and 500 °C, respectively.The existence of α-Cr 2 O 3 can provide more nucleation sites and decrease the surface energy in the nucleation and growth processes owing to the greatly similar crystalline character.
The double glow plasma technique, a hybrid plasma surface treatment technique, is the combination of the plasma and sputtering processes developed for the deposition of high quality alloyed layers on the surface of inexpensive materials.In this study, double glow plasma technique 16 was used to prepare the high content and well-bonded α-Al 2 O 3 coatings on 316 L stainless steel at low temperature of 580 °C.The effects of different factors on the formation of Al 2 O 3 coatings are discussed in detail.

Experimental Materials and methods
A complex target (the composition was Al 90 %, α-Al 2 O 3 10 % in weight percent) plate (50 mm × 6 mm) was used as the source electrode for supplying the alloying elements.316 L stainless steel plate of size 10 mm × 10 mm × 3 mm was used as the substrate material and its nominal composition is shown in Table 1.The specimens were first polished by 1000 grit SiC paper and then cleaned in acetone prior to double-glow plasma surface alloying treatments.A detailed description of double glow plasma apparatus is reported elsewhere. 17The apparatus had three electrodes: the anode (the shelter) and two negatively cathodes.One cathode was the source cathode (referring to the complex target), and the other cathode was the substrate material.
The source electrode voltage was set at 600-800 V and workpiece electrode voltage was set at 300-400 V.When two different voltages were applied to the two cathodes, glow discharge could be produced.The chamber was evacuated to a base pressure of 2 • 10 -4 Pa.During the deposition process, the flow rates of argon were kept at 40 sccm, where the working pressure was maintained at 20-40 Pa.The parallel distance between the substrate and target was about 10-20 mm with deposition time of 3 h.
Orthogonal array design is a highly efficient method to investigate the relationship between the effects of different factors. 18As working pressure, source electrode, workpiece electrode and parallel distance would likely influence the deposition rate, three levels were selected for each factor (Table 2), which were fixed in the probable working range.These factors did not play an independent role in the process of the sputtering deposition.Therefore, it is a better choice to study the deposition parameters by orthogonal array design, 19,20 which was applied to evaluate the influences of different factors on the deposition rate.After deposition, the coatings with the optimize parameters were further plasma oxidized through different oxidation environment.The flow rates of argon were kept at 40 sccm while the oxygen flow rates were set in the range of 0.5-4 sccm.The source electrode voltage was set at −700 V and workpiece electrode voltage was set at −300 V to keep the desired temperature of 580 °C.By controlling the mechanical pump, the total pressure of Ar + O 2 was kept at 40 Pa and the oxidization time was 1 h.

Characterizations
The phase composition of the prepared coatings was characterized with glancing-angle (1°) X-ray diffractometry (GAXRD, UltimaIV, Rigaku Company, CuKα radiation, Japan) operating at 50 kV and 40 mA.X-ray data were collected using a 0.02° step scan with a count time of 8 s.The morphology and chemical composition of the as-deposited coatings were studied by scanning electron microscopy (SEM, Quanta250, FEI Company, US) with an X-ray energy-dispersive spectroscopy (EDS, EDAX Inc. US).In order to measure the thickness of the coatings, one step was prepared by partly blocking the atoms sputtering on the substrate.A micro-XAM surface mapping microscope (Micro-XAM, ADE Phase-Shift, USA), which could present the three-dimensional shape of the coatings was used to measure the thickness of the coatings.Finally, the electrochemical corrosion behaviours of the composite coatings and 316 L stainless steel were characterized by potentiodynamic polarization, which was carried out on a CHI660d electrochemical analyser (Shanghai, China) in aqueous solution, w(NaCl) = 3.5 %.

Results and discussion
Thickness and the morphology of the coatings The L9(3 4 ) table of orthogonal design was used to test the deposition rate under the assigned conditions (Table 3).The thickness of the coatings is taken as measured responses.The range analysis values are presented in Table 4.
K i is the mean value around the corresponding index of the thickness column factor.In other words, each cell in this table is of average thickness with one of the deposition parameters kept constant.For example, K 11 is average value of thickness when working pressure is 20 Pa, while other parameters are varied.Then, K 11 = (4.4+ 11.8 + 2) / 3 = 6.1.Theoretically, the R value reflects the impact of the corresponding factor on the film properties.The greater the R value, the larger the impact degree. 19It can be obtained from Table 4 that the parallel distance between the source electrode and the substrate plays the dominant role in the thickness of the films.The optimum parameters are working pressure, 30 Pa; source electrode voltage, 700 V; workpiece electrode voltage, 300 V; the parallel distance between the source electrode and the substrate, 15 mm.Fig. 1 shows typical surface micrographs of ion sputtering coating samples (unoxidized coatings).It can be seen that sample S1 has some cracks and epidermal desquamation scrape.Sample S3 has some defects by suffering from spalling, so does sample S6.The surface micrographs of the samples S5 and S9 are similar to S1, which results from the mismatch of different thermal expansion coefficients of the materials caused by hollow cathode effect. 21When the parallel distance between the source electrode and the substrate is too small (10 mm), the overlapping glow current increases sharply, which will result in a high temperature of substrate or sputtering target in the local area.
Owing to the temperature gradient existing in the different area of the coatings, 22 cracks and epidermal desquamation scrape can be found.The EDS analysis of points 1 # and 2# in Fig. 2 shows that the coatings are peeled off.These results indicate that the ion bombardment under high source electrode voltage of 800 V can produce compressive stress, which plays an important role in the process of spalling. 23wever, the coatings of S2, S4, S7 and S8 are very dense, uniform, and compact to the substrate without any cracks and defects.The above-discussed experiments demonstrate that two main factors play important roles in the deposition progress: one is the source electrode voltage resulting in the increase in ion quantity and velocity; the other is the parallel distance between the source electrode and the substrate.Al/α-Al 2 O 3 prepared with optimum parameters Fig. 3 shows the phase and morphology of the coating prepared with optimum parameters.From Fig. 3(a), it can be found that the FeAl and Fe 3 Al intermetallic compounds are the predominant phases in the unoxidized specimen, mainly caused by the diffusion of Al element into substrate promoted by heating the workpiece.Besides FeAl and Fe 3 Al, a small amount of α-Al 2 O 3 is detected, which means that the α-Al 2 O 3 doped in the target is deposited on the substrate owing to energetic ion bombardment.
The coating is continuous and compact with no visible metallurgy flaw as shown in Fig. 3(b).From Fig. 3(c), the thickness of unoxidized coating was measured at about 11.6 μm.Considering the total deposition time of 3 h, the deposition rate was calculated to be about 64 nm min −1 .Compared with P. Jin's study, 14 the coatings in this study have faster growth speed.It is beneficial to promote depositing efficiency and surface quality of the coatings by supplying different bias voltage between the target and substrates. 24

Oxidation
The coatings with the optimum parameters were further plasma oxidized under different oxygen flow.The GAXRD patterns of the oxidized coatings are shown in Fig.The surface morphology of the coatings oxidized with different oxygen flow is shown in Fig. 5.The surface morphology of the coating has no obvious change when the oxygen flow is 0.5 sccm (Fig. 5(a)).As shown in the Fig. 5(b) and (c), the dense and well-bonded coatings are obtained with the increase in oxygen flow.According to the Volmer-Weber mode, 27 stable nucleus can capture atoms both diffused from the surface of substrate and sputtered from the target, leading to the increment of the grain size.Numerous growing grains make isolated α-Al 2 O 3 island grow into continuous coatings.From the magnified image of Fig. 4(b) and (c), the α-Al 2 O 3 island consists of many circular particles.When further increasing the oxygen flow to 4 sccm, the coating is partly spalling.

Corrosion property
The polarization curves of 316 L and oxidized layers with different oxygen flow in 3.5 % NaCl solution are shown in Fig. 6.The corrosion potential and corrosion current are calculated in Table 4.The porosity of the coatings P is calculated as the corrosion current ratio of coatings and substrates. 28e i corr is the corrosion current of the oxidized coating and i o corr is the corrosion current of the 316 L stainless steel.The higher the corrosion potential is, the stronger is the resistance to corrosion.At the same time, corrosion current is in proportion to the corrosion rate of materials.The smaller the corrosion current is, the better is the corrosion resistance performance.According to the curves, the polarization curves of alumina coatings shift upward to a more notable potential, suggesting that the alumina coatings are effective in the corrosion protection of the stainless steel substrate by reducing the corrosion current density and increasing the passive region.
In comparison with previously reported work, 24 in which alumina coatings with fraction of w = 60 % exhibit the corrosion potential of −0.291 V, we obtain the minimum corrosion potential of sample 3 of −0.206 V which is better due to the following two reasons.Firstly, a relatively high content of α-Al 2 O 3 in the coating is beneficial to improving corrosion resistance of the substrate. 29Secondly, the dense alumina coating with millions of tiny particles may act as a barrier to block the diffusion of the aggressive media.
With the calculation formula, 30 the lowest porosity of the coatings is 0.13 %.

Conclusions
The influence of deposition parameters (including working pressure, source electrode, workpiece electrode and parallel distance) on the deposition rate were investigated by orthogonal array design.The parallel distance between the source electrode and the substrate was the most prominent factor which significantly influenced the thickness of the films.It was found that iron aluminide formed at the interface between the coatings and the substrates.When applying further plasma oxidation, the formation of α-Al 2 O 3 was promoted as oxygen flow increased.The oxidized coatings with the lowest porosity of about 0.13 % had the relative content of α-Al 2 O 3 as high as 66.5 % when the oxygen flow was 2 sccm.