- Email: [email protected]
Effects of microstructure and properties on parameter optimization of boron carbide coatings prepared using a vacuum plasma-spraying process
Surface & Coatings Technology 206 (2012) 2673–2681
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effects of microstructure and properties on parameter optimization of boron carbide coatings prepared using a vacuum plasma-spraying process Chun-Ming Lin a,⁎, Hsien-Lung Tsai a, Cheng Yang b a b
Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei, 10673, Taiwan, ROC Materials and Electro-Optics Research Division, Chung-Shan Institute of Science and Technology, Lungtan, Taoyuan, 32544, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 17 September 2011 Accepted in revised form 14 November 2011 Available online 28 November 2011 Keywords: Vacuum plasma-spraying process Parameter optimization Boron carbide coatings Microanalysis Oxidation
a b s t r a c t This study employed artificial intelligence methods such as the Taguchi Method to determine the optimal parameters for boron carbide (B4C) coatings using a vacuum plasma spray technique. We determined the optimal parameters to produce coatings and investigated B4C coatings in an electromagnetic radiation environment by observing and analyzing changes in the microstructure and properties. These artificial intelligence methods comprised two stages. In the first stage, orthogonal arrays (OA) were used to distribute test parameters, and the Taguchi Method was used to optimize the parameters. In the second stage, a specimen of boron carbide coating was evenly exposed to radiation to investigate the microstructure and properties of the coating. The coating irradiated at a wavelength of 10.6 μm did not undergo any change in phase. The microstructure of the surface coating exhibited cracks, but these were insignificant. As the irradiation time increased, the number of cracks on the surface increased significantly. In addition, because the growth of surface cracks was governed by irradiation time, it was inferred that the growth was thermal efficiencycontrolled (i.e., only generated by heating effects). Additionally, under irradiation, the surface of the boron carbide coating became oxidized and formed B2O3, H3BO3, and amorphous carbon; the coating size increased proportionally with irradiation time and weight of oxidation. The initial oxidation of the coating was classified as surface controlled, and the oxidation that formed on the B2O3 surface gradually transformed into extended controlled oxidation. Both types of oxidation control were conducted simultaneously, causing the weight of B2O3 to increase linearly. Two processes were involved in the extended control of oxidation: the formation of B2O3 (the cause of weight gain), and the vaporization of B2O3 (the cause of weight loss). Published by Elsevier B.V.
1. Introduction Boron carbide (B4C) has many favorable properties, including a high melting point, a high degree of hardness, low density, high elastic modulus, good chemical stability, large cross-sectional area for neutron absorption, excellent thermoelectric performance, and a low coefficient of thermal expansion. As a result, it is widely used for coating purposes in the essential goods, aerospace, and nuclear industries [1–4]. Boron carbide is a high-temperature material that is ideal for use in plasma-facing materials (PFM) in nuclear reactors and high-temperature environments. It is also used in the control barrier layer in nuclear reactors, the inner walls of dry etching reaction chambers for semiconductors, and in the fuel propulsion system of aircrafts. Additionally, it is commonly employed in the form of a coating for industrial purposes and is typically produced as PVD, ⁎ Corresponding author at: Department of Mechanical Engineering, National Taiwan University of Science and Technology 43, Keelung Road, Section 4, Taipei, 10673, Taiwan, ROC. Tel.: + 886 2 2737 3141x7304; fax: + 886 2 2737 6460. E-mail address: [email protected] (C.-M. Lin). 0257-8972/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.surfcoat.2011.11.023
CVD, and plasma sprays. The implantation of vapor deposition and plasma ion produces a delicate thin coating, which has a low performance rating, and involves a complicated process. In general, the plasma spray technique has many favorable properties, including a high performance rating, a thick coating, and a high bonding strength. The technique involves the instantaneous melting and rapid solidification of film using a plasma flame generated using a high power electrical supply. In the spraying process, high-speed airflow is applied to accelerate and atomize the sprayed material so that the molten particulates strike the working surface at a very high speed. The desired coating thus gradually builds up through the continuous flattening and solidification of the particulate matter. Because the plasma flame temperature can exceed 15,000 °C, only materials with a high melting point (e.g., boron carbide) are suitable for plasma spray deposition. However, a large number of parameters need to be appropriately controlled if the quality of the coating is to be ensured. The processing parameters are typically chosen on a trial-and-error basis. However, such an approach is not only time-consuming and expensive, but also fails to guarantee the quality of the finished product [5–7]. The Taguchi design method, based on a quality loss function and a signal-to-noise
2674
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
The Taguchi design method was applied to determine the optimal processing parameters for the VPS process of boron carbide by establishing a preliminary solution for the optimal set of processing parameters. The study then used the optimal parameters to produce optimal coatings by observing the microstructure and properties of the specimens.
(i.e., without radiation) and on the boron carbide coating specimens after various irradiation times. The surfaces of the specimens that underwent microscopy observations were not mechanically polished. Cross-section specimens were sectioned and embedded in resin at room temperature and then mechanically polished with no chemical etching (note that the specimens were polished mechanically with #4000 grit emery paper and then buff-polished using SiO2 powder, before being polished with diamond paste to 0.3 μm). The microstructures and properties of the various coating specimens were analyzed using a field emission scanning electron microscope (FE-SEM, JSM-6390 LA). In addition, the chemical composition and element distribution of the specimens were examined via energy dispersive X-ray spectrometry (EDS) and electron probe X-ray microanalysis (EPMA, JEOL JXA8009R). Finally, the surface behavior of various specimens was examined using Raman spectra, which were recorded at room temperature using a Renishaw in a Via micro-Raman system equipped with a grating with 1800 grooves/mm and a microscope with a 50× objective lens. The radiation source was set at low power to avoid heating the surface. The slit width of the spectrometer was 30 μm. The Ar+ laser beam 514.5 nm at 5 mW was focused on a spot 5 μm in diameter, and the backscattered radiation was collected from these specimens at a scanning range of 200 to 2000 cm− 1.
2.2. Experimental procedure
3. Results and discussion
2.2.1. Boron carbide coatings production Commercial grade Boron carbide powder was purchased from Coulter. Spray coating tests were performed using pure aluminium substrates with dimensions of 100 mm (length)×50 mm (width)× 3 mm (thickness). Prior to the spray tests, the substrates were cleaned and degreased, and the surfaces roughened (Ra 6.45 μm) via a sandblasting abrasion treatment to enhance the adhesion of coatings. The VPS process was performed using an A-3000 vacuum plasma spraying system (Swiss Plasma-Technik Company) comprising an ABB IRB-L6-type robot and an F4-VB spray gun. In addition, the particle size distribution of the boron carbide powder was controlled to between 17 and 20 μm, and the thickness of the spray was controlled to between 260 and 270 μm.
3.1. Taguchi method analysis
(S/N) ratio, provides a robust and efficient means of determining the optimal conditions for manufacturing processes [8–10]. An analysis of variance (ANOVA) test is performed to determine the significance of the various factors and to rank them in order of importance. It may also be interesting to show the significance of multi-factor interactions [10–13].The present study investigates the microstructure and properties of boron carbide coatings with optimal spray parameters fabricated using a vacuum plasma spray (VPS) process (note that the composition has a powder-like composition). This investigation focuses specifically on the effect of electromagnetic radiation on the microstructure and properties of boron carbide coatings. 2. Experimental methods 2.1. Taguchi method
2.2.2. Radiation test Radiation tests were performed using specimens with dimensions of 1 mm× 1 mm prepared using a CO2 continuous laser (RofinSinar 860HF) with a maximum power of 2.5 kW and a central wavelength of 10.6 μm. During the radiation process, the laser beam was defocused on the coating surface to a beam diameter of 40 mm, evenly exposing the specimens to radiation. Irradiation parameters included a laser power of 2.0 kW and a scanning velocity of 6 m/min. The upper and lower surfaces of the plates were shielded with helium and argon gas, respectively, which were provided by nozzles with an orifice diameter of 4 mm. The helium flow rate was set at 10 to 20 L/min, while the argon flow rate was set at 5 to 15 L/min. The irradiation times (incident energy) were 5 s (7.96 MJ/m2), 10 s (15.92 MJ/m2), 15 s (23.87 MJ/m2), 20 s (31.83 MJ/m2), and 25 s (39.79 MJ/m 2). Note that the CO2 laser process was chosen as a radiation source primarily because of the similarity of the system radiation (i.e., electromagnetic radiation) to actual radiation environments. 2.3. Analysis of the microstructure and properties of boron carbide coatings The characterization focused on the original boron carbide powder (i.e., without radiation) and on specimens of boron carbide coating irradiated for various durations, which were investigated via X-ray diffraction (XRD, Rigaku D/Max-2500 diffractometer with Cu-Kα radiation) at a scanning speed of 1°/min, a voltage of 30 kV, and a current of 50 mA. For comparison, microstructural (i.e., surface and cross-section) observations were also performed on the original
As shown in Table 1, the Taguchi trials involved five control factors, namely spray distance (A), chamber pressure (B), current (C), argon gas flow rate (D), and hydrogen gas flow rate (E). Moreover, each control factor was assigned four different levels. As shown in Table 2, the experimental trials were configured using an L16 (4 5) orthogonal array (OA). In the spray coating experiments, lower porosity indicated a denser coating and thus a coating of better quality. Accordingly, the smaller-the-better quality loss function (QLF) was adopted in evaluating the experimental results; however, the quality characteristic was expressed as a percentage, meaning that the additive nature of the quality characteristic was very poor. Thus, the fol^ was performed to transform the non-additive lowing operation Ω data to the additive data [14–16]: ∧ 1−P Ω ¼ 10 log P
ð1Þ
where P is porosity. The porosity, average porosity, and signal-tonoise (S/N) ratio of the quality characteristic are summarized in Table 2 for each of the 16 experimental trials. Table 3 presents the S/N ratio response table obtained by applying the ANOM statistical technique to the experimental results given in Table 2. For convenience, the S/N ratio response of each control factor is also presented in graphical form in Fig. 1. It can be seen that the optimal control factor settings are as follows: Factor A: level 1; Factor B: level 1; Factor C: level 4; Factor D: level 4; and Factor E: level 3. In other words, the Taguchi analysis suggests that the optimal spray coating parameters (i.e., the parameters that minimize spray Table 1 Level settings of control factors. Factors
A B C D E
Process parameter
Spray distance (mm) Chamber pressure (mbar) Current (A) Argon gas flow rate (l/min) Hydrogen gas flow rate (l/min)
Levels 1
2
3
4
150 650 650 30 10.5
200 700 660 35 12
250 750 670 40 13.5
300 800 680 45 15
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681 Table 2 L16 (45) Orthogonal array experimental configuration and experimental results. S.N
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
A
B
1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4
C
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
1 2 3 4 2 4 4 3 3 4 1 2 4 3 2 1
D
1 2 3 4 3 4 1 2 4 3 2 1 2 1 4 3
E
1 2 3 4 4 3 2 1 2 1 4 3 3 4 1 2
Unit: %
S/N ratio
N1
N2
N3
N4
N5
Average
Ω(dB)
18.19 14.95 16.17 18.33 13.67 11.23 16.12 26.14 12.17 22.02 22.48 24.32 15.86 28.00 29.41 21.36
17.72 15.64 16.15 19.54 13.58 12.29 16.68 26.85 13.10 23.35 27.35 27.37 15.70 25.05 30.23 22.05
15.43 14.43 14.62 20.13 18.04 12.82 18.40 23.73 12.38 19.66 27.20 23.55 15.38 24.43 33.78 28.00
15.96 15.30 16.16 16.44 17.63 9.76 18.40 24.00 14.64 19.69 24.92 25.85 13.28 26.53 34.82 27.71
16.58 16.04 13.89 17.34 15.81 10.56 19.54 25.29 12.74 18.51 23.78 27.46 16.04 30.24 32.01 25.03
16.78 15.27 15.40 18.36 15.75 11.33 17.83 25.20 13.01 20.65 25.15 25.71 15.25 26.85 32.05 24.83
6.96 7.44 7.40 6.48 7.28 8.93 6.64 4.72 8.25 5.85 4.74 4.61 7.45 4.35 3.26 4.81
neff ¼
3.1.1. Experimental confirmation To confirm the reproducibility of the experiments, a spray coating test was performed using the optimal settings of the control factor level (i.e., A1B1C4D4E3). The optimal S/N ratio was predicted to be ^ Ωj −Ω
ð2Þ
j¼1
1
ð3Þ
^ Ω 10
1 þ 10
The predicted value of the coating porosity was found to be 7.88%. The porosity of the spray coating fabrication with optimal processing conditions was determined to be 8.51%. The difference between the predicted porosity value and the measured porosity value was only 0.63%. Thus, the reproducibility of the experiments is confirmed. 3.1.2. Analysis of variance To confirm all of the experiments, a spray coating test was performed using analysis of variance (ANOVA). Table 4 presents the
Table 3 S/N ratio response table.
Level Level Level Level
1 2 3 4
N 1 ¼ DOF opt
ð5Þ
where N is the total number of trials and DOFopt is the total degrees of freedom associated with items used in the ηopt estimate. At a confidence of 95% for the porosity, F0.05;1;6 = 5.99, Vep = 0.748 (from Table 4), N = 16, DOFopt = 16, and neff = 2.28. Thus, the CI is computed to be 1.4 dB. The experimental results (Table 5) confirm that VPS process parameters were initially optimized. Although the reproducibility of the experimental results was confirmed with an average porosity as high as 3.812%, the porosity of the specimens was influenced by the spraying process, which led to severely shortened life cycles and a reduction in quality. As seen from the ANOVA outcomes (Table 4), proper regulation of the spray distance, chamber pressure, and hydrogen gas flow rate are necessary to cope with the defects mentioned above. 3.2. Microstructure properties
^ is the average value of the S/N ratio, Ωjis the optimal level of where Ω each factor, and q is the factor affecting the quality characteristic. From Eq. (2), the optimal S/N ratio was estimated to be Ωopt = ^ was then converted to the equiva10.679 dB. The estimated value of Ω lent porosity value in accordance with P¼
ð4Þ
where the F-ratio is that required for risk α with a confidence equal to 1-risk, the degrees of freedom of the pooled error, Vep is the pooled error variance, γ is the sample size of the confirmation experiment, and neff is the effective specimen size given by:
coating porosity) are a spray distance of 150 mm, a chamber pressure of 650 mbar, a current of 680 A, an argon gas flow rate of 45 l/min, and a hydrogen gas flow rate of 13.5 l/min. In addition, Fig. 1 and Table 2 show that all five control factors have a significant effect on the coating porosity.
q X
ANOVA response table of the control factor with the least effect on the quality characteristic. The confidence interval (CI) is the range between the maximum and minimum value, and the true average value will fall within that range with a stated level of confidence. The confidence interval of the above estimation can be calculated using the following equation: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! u u 1 CI ¼ tF α;1;v V ep neff
^ = 6.23(dB). Average S/N ratio Ω
^þ Ωopt ¼ Ω
2675
A
B
C
D
E
Spray distance (mm)
Chamber pressure (mbar)
Current (A)
Argon gas flow rate (l/min)
Hydrogen gas flow rate (l/min)
7.069 6.895 5.862 4.969
7.485 6.644 5.509 5.156
6.360 5.649 6.182 6.603
5.638 6.088 6.335 6.733
5.198 6.785 7.098 5.714
We investigated the microstructure and properties of the boron carbide coating prepared by the VPS process under the optimal conditions (A1B1C4D4E3) that allowed the specimen to be evenly exposed to radiation. 3.2.1. X-ray diffraction analysis Fig. 2 presents the results of the XRD analysis for the aluminum substrate and the boron carbide powder, and the coating specimens (i.e., without radiation) (note that the XRD spectra of the coating were obtained under optimal conditions using pure boron carbide power as a reference). The results show that the crystalline phase of the aluminum substrate and the boron carbide of the powder coating were categorized into two main diffraction peaks, which are labeled Al and B4C. These exhibit no phase transformation in the structure of the coating because they have the same structure; the substrate and the B4C phase show diffraction peaks in the same positions. In addition to the diffraction of boron carbide coating (see Fig. 2 (c)), Al diffraction peaks were also observed. These may be caused by diffraction from the substrate signal due to the porous structure of the coating. In addition, the XRD pattern in Fig. 3 shows the various irradiation times of the specimens of boron carbide coating. The results show that, after various irradiation times, the boron carbide coatings could be categorized into four main diffraction peaks, which are labeled Al, B, B2O3, and B4C. Therefore, no phase transformation of the coating structure occurred for irradiation times of 5 to 25 s because the phases show diffraction peaks in the same positions. In particular, for coating structures with irradiation times of 5 to 25 s, the B2O3 phase intensity increased, as shown in Fig. 3 (a) to (e) (note that other phases remained unchanged). Thus, the irradiation of a coating structure may only lead to heating, thereby influencing the content of
2676
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
Fig. 1. S/N ratio response graph.
the B2O3 phase. The B2O3 phase is discussed later in relation to the observations of microstructure. 3.2.2. Microstructure analysis Fig. 4 presents SEM images of the boron carbide powder, and the coating specimens, prior to (i.e., without radiation) and after various irradiation times ranging from 5 to 25 s (i.e., cross-section). The results show that the original specimen produced by spraying had an average porosity of 6.15% with an average bore diameter of 8.6 μm. There was no significant difference between the original coating and the irradiated specimen. The radiation process was insufficient to change the coating structure (i.e., defects and phase transformation). The boron carbide powder produced by the VPS process formed a coating structure, whereby the powder was heated using plasma modes; hence, the powder formed a molten state after rapid pounding against the substrate while the boron carbide coating structure was mechanically bonded. In other words, the radiation coating process cannot cause a phase transformation because this only occurs if heat is accumulated. In addition, cracks formed and grew on the surface of the coating. Fig. 5 shows the original specimens (i.e., without radiation) and the specimens after irradiation times of 5 to 25 s. The surface of the original specimen exhibited shallow cracks, which bore no significance, as shown in Fig. 5(a). When the coating specimen was irradiated for less than 5 s, surface cracks tended to increase, as shown in Fig. 5(b). Fig. 5(c) shows irradiation times of 10 to 25 s, after which the number of cracks on the surface increased significantly. It can be seen that at a radiation wavelength of 10.6 μm, the surface of the boron carbide coating is the result of ionization and electronic excitation (i.e., heating) rather than an erosion process. Because the growth of surface cracks is governed by irradiation time, the growth is thermal efficiency-controlled. By contrast, the cross-sectional area of the cracks grew in the original boron carbide coating (i.e., without radiation) and in specimens irradiated for times of between 5 and 25 s. It was found that the original coating specimen tended to exhibit cracks, which bore no significance, as shown in Fig. 6 (a). When the irradiation time was less than 5 s, the cracks tended to grow in a horizontal direction rather than grow denser (Fig. 6 (b). Thus, as shown in Fig. 6 (c) and (f), for irradiation times of 10 to 25 s, the number of cracks on the surface increased significantly, and specific crack growth was observed in between layers or interfaces of the coating and substrate. In general, Table 4 Results of ANOVA for the porosity. Factors
Degree of freedom
Sum of square
Mean square
F-Test
Pure sun of square
Percent contribution
A B C D E Error(pooled) Total
3 3 3 3 3 6 15
11.477 13.664 1.966 2.524 9.556 4.490 39.19
3.826 4.555
5.11 6.09
9.23 11.42
23.56% 29.14%
3.185 0.748
4.26
7.31 11.23 31.88
18.66% 28.65% 100.00%
the growth of cracks in a horizontal direction results primarily from thermal effects occurring in unison and acting collaboratively (i.e., at various irradiation times). The observation of vertical cracks in the boron carbide coating of the original (i.e., without radiation) and in specimens irradiated for various durations means that these cracks are not formed in between coatings. The results suggest that these cracks occur because boron carbide powder forms a molten state following rapid pounding against the surface of the substrate, and is heaped between layers during solidification, which interferes with the growth of cracks (i.e., the vertical direction). By contrast, the growth of cracks in the horizontal direction was not restricted. A previous study [17] indicated that cracks from radiation reactions exist on coatings, and that the number of cracks increased because the radiation energy led to enhanced thermal diffusion efficiency during crack growth. For all coating structures, the number of cracks present reflects the competing effects of crack growth. When cracks form on the surface of the coating during thermal initiation, a relatively greater increase in the microcrack density distribution is caused by the reduced Young's modulus of the coating, which results in even greater crack growth. The results after various irradiation times (see Fig. 2) agree with the results of previous studies. In addition, it was reported in [18–20] that, following radiation, the coating has enhanced internal stress in between layers, and the cracks tended to grow in a horizontal direction. This result (see Fig. 6) is consistent with those of other similar studies. 3.3. Coatings properties analysis Fig. 7 shows images and the corresponding EPMA elemental mapping results for the original specimen (i.e., without radiation) and for specimens irradiated for times of 5 to 25 s. In the analysis of the original coating specimen, shown in Fig. 7 (a), it was found that most of the coating surface had C-B elements (purple), confirming the XRD analytic results, and that C-B elements were present in the form of chemical compounds. It was also found that O-C elements (light blue) and C elements (blue) were evenly distributed. It can be assumed that the O in the atmosphere entered the coatings as they cooled, causing the C in the coating and the O in the atmosphere to combine in oxidized form. Fig. 7 (b) to (f) display specimens treated for various irradiation durations. The results reveal that the C and O-C oxides on the surface of the coating decreased gradually, while O increased gradually. By contrast, the C-B chemical compound gradually became B (red), which in turn facilitated the formation of the B-O chemical compound (yellow), which increased the rate at which C-B chemical compounds formed B-O oxide in
Table 5 Results of the confirmation experiment. S.N
A
B
C
D
E
17
150
650
680
45
13.5
Unit: %
CI (95%)
E.V.P
P.V.P
Average
8.51
8.50
8.50
10.679 ± 1.4(dB)
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
2677
Fig. 2. X-ray diffraction diagram: (a) Al substrate; (b) B4C powder; and (c) B4C coating.
proportion with the irradiation time. The oxidation reaction equation between B4C and BxC (4≦X≦25) can be used to approximate the oxidation behavior [20]: B4 C þ 402 →2B2 O3 þ CO2
ð1Þ
Bx C þ ð1 þ 3x=4ÞO2 →x=2B2 O3 þ CO2
ð2Þ
As shown in Eqs. (1) and (2), when B4C and BxC are oxidized, they form B2O3 and CO2. Eq. (1) can also be used to estimate the weight of coatings after oxidation; this weight can increase by up to 152.2% as the O in the atmosphere combines with the C in the coatings to form CO2. The CO2 dispersed in the air by the radiation process caused some C to be reduced due to the formation of CO2 and altered the C–B chemical compound. In addition, based on observation using EPMA (see Fig. 7), radiation caused the C-B chemical compound to disintegrate into B. Furthermore, radiation occurred in the atmospheric environment, triggering reactions between the disintegrated B and the O in the atmosphere, forming the B-O chemical compound (the reaction of which accelerates with radiation duration). Therefore, the chemical compound predicted to be formed from the oxidation of coatings in Eq. (1) is B2O3. Table 6 shows the changes in weight of the boron carbide in coating specimens with irradiation times of 5 to 25 s. The results show that the there were no significant changes when the specimen was irradiated for 5 s (7.96 MJ/m 2); however, the weight increased by 0.10 mg/cm2 when the irradiation time was increased to 10 s (15.92 MJ/m2). The weight increased by 0.40 mg/cm 2 when the irradiation time was increased to
Fig. 3. X-ray diffraction diagrams of coating specimens at various irradiation times: (a) 5 s; (b) 10 s; (c) 15 s; (d) 20 s; and (e) 25 s.
15 s (23.87 MJ/m 2). The weight increased by 0.66 mg/cm 2 when the irradiation time was increased to 20 s (31.87 MJ/m 2) and the weight increased by 0.89 mg/cm 2 when the radiation time was
2678
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
Fig. 4. B4C microstructure: (a) B4C powder; (b) B4C coating cross-section; (c) 0 s(i.e., without radiation); and (d) various irradiation times ranging from 5 to 25 s. (Note that the irradiation results similar).
Fig. 5. B4C surface microstructure: (a) 0 s (i.e., without radiation); (b) 5 s; and (c) various irradiation times ranging from 10 to 25 s. (Note that the irradiation results are similar).
increased to 25 s (39.79 MJ/m 2). These figures demonstrate that coating weight increased as the irradiation time increased, which might be caused by oxidation. The XRD findings support previous studies that demonstrating that B2O3 formed on the surface of the coating after various irradiation times, indicating that these casting weights increased in proportion to irradiation time, which might be due to B2O3. In addition, Table 6 shows the correlation between different irradiation times (incident energy) and the boron carbide oxidized surface. After irradiation times of 5 to 25 s, only 0.0057% of the boron carbide coating surface was oxidized, and B2O3 occupied only 11% of the surface area. Therefore, the initial oxidation of the coating may be classified as surface protection, and B2O3 gradually transformed into extended control oxidation on the surface [21]. These two methods of oxidation control were conducted simultaneously, causing the weight of B2O3 to increase linearly, which proved that the gain in weight was caused by the oxidation of the boron carbide coating. Two processes were involved in the extended control of oxidation [22], namely the formation of B2O3 (the cause of weight gain) and the vaporization of B2O3 (the cause of weight loss). At relatively low temperatures, the oxidation rate of boron carbide (the formation of B2O3) far exceeded the rate of B2O3 vaporization. In these cases, the vaporization of the B2O3 did not have any significant impact on weight. However, at high temperatures, B2O3 and its vaporization rate increased with temperature, thereby influencing the fluctuations in weight, as characterized by nonlinear changes. Oxides of various forms could also arise in the vaporization process. Therefore, base the oxidation reaction (i.e., the initial oxidation of the coating as surface protection, and oxides gradually transformed into extended control oxidation mechanisms on the surface) of boron carbide at high temperatures caused by the irradiation process. Fig. 8 shows the results of using Raman spectrography to evaluate the B4C and B2O3 phase. The results indicate that the signal peaks of the untreated coatings at 478, 529, 731, 827, 1009, and 1078 cm − 1 after various irradiation times refer to boron carbide; these findings are in agreement with those of previous studies on boron carbide [21]. Treated specimens with irradiation times ranging from 20 to 120 s (to ensure the stability of the oxide created through B2O3, the
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
2679
Fig. 6. Impact of crack growth on B4C coating structure in irradiated environments: (a) 0 s (i.e., without radiation); (b) 5 s; (c) 10 s; (d) 15 s; (e) 20 s; and (f) 25 s.
radiation time was increased to facilitate observation) reached their signal peaks at 210, 498, 809, 880, 1123, 1167, 1360, and 1601 cm − 1. The measurements of 210, 498, 880, and 1167 cm − 1 refer to the signal peaks of H3BO3 [22–25], showing that through irradiation, B2O3 produced unstable oxides that easily reacted in the environment. According to previous studies [24], B2O3 has natural moistureabsorbing properties and reacts with the H2O generated through irradiation or during tests to form H3BO3, as shown in Eq. (3). 1=2B2 O3 þ 3=2H2 O ¼ H3 BO3
ð3Þ
The reaction in Eq. (3) shows that the heat involved in the formation of H3BO3 was negative, indicating that H3BO3 formed easily because B2O3 was exposed to the atmosphere. H3BO3 formed naturally when H2O was in the air and B2O3 was on the surface of the coating undergoing interaction. Furthermore, signal peaks at 1360
and 1601 cm − 1 were formed using amorphous carbon and relate to the D band and G band, respectively [21,25,26]. These were probably formed when the coatings were treated with radiation and the B4C structure broke down, causing the B-C connection to break, and forming a C band that included some B [21]. A unique B2O3 signal peak was also observed at 809 cm − 1 [25, 26], and a small signal peak was observed at 1123 cm − 1. These could have been caused by an extended coating surface formed by an increase in irradiation time. By comparing the Raman spectrogram before and after radiation, the B4C signal peak was determined to be no longer visible after irradiating the boron carbide coating for extended periods. This was most likely due to the fact that the coatings were unable to endure such extended radiation, thereby causing the structure to undergo changes. The results support those of previous studies [21], indicating that the oxidation of B4C coatings was not only forms B2O3, but also through H3BO3 and amorphous carbon. H3BO3
2680
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
Fig. 7. Effect of irradiation on EPMA surface elements in B4C coating structures: (a) 0 s (i.e., without radiation); (b) 5 s; (c) 10 s; (d) 15 s; (e) 20 s; and (f) 25 s.
Table 6 Weight changes and B4C content of oxidized specimens before and after irradiation. Irradiation time (sec)
incident energy (MJ/m2)
Weight Changes (mg/cm2)
Oxidized Content (%)
5 10 15 20 25
7.96 15.92 23.87 31.83 39.79
0 0.10 0.40 0.66 0.89
0 0.0007 0.0025 0.0041 0.0057
and amorphous carbon only form at high temperatures following long-term irradiation, while B2O3 only forms at high temperatures after short-term exposure to radiation. 4. Conclusions 1. This study developed an integrated artificial intelligence (AI) framework based on the Taguchi method for optimizing the processing parameters in the vacuum plasma spraying of boron
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
2681
gradually transformed into extended control oxidation. Both types of oxidation control were conducted simultaneously, causing the weight of B2O3 to increase linearly. Two processes were involved in the extended control of oxidation, namely the formation of B2O3 (the cause of weight gain) and the vaporization of B2O3 (the cause of weight loss). At lower temperatures, the rate of boron carbide oxidation (the formation of B2O3) was significantly greater than the rate of B2O3 vaporization. In such cases, the vaporization of B2O3 did not have any significant effect on weight. However, at high temperatures, the content of B2O3 and its vaporization rate increased with temperature, which probably influenced the fluctuations in weight, which were characterized by nonlinear changes. H3BO3 and amorphous carbon could also form during the vaporization process.
Fig. 8. Oxidation of B4C coatings treated with irradiation, as tested by Raman spectra.
carbide. According to the results obtained using the Taguchi design method alone, the optimal processing parameters are as follows: a spray distance of 150 mm, a chamber pressure of 650 mbar, a current of 680 A, an argon gas flow rate of 45 L/min, and a hydrogen flow rate of 13.5 l/min. Given these optimal parameter settings, the predicted value of the porosity is 5.2% while the measured value of the porosity is 5.56%. The coating porosity obtained when using the optimal parameter settings determined from the integrated Taguchi method was significantly lower than that obtained when using the parameter settings acquired from the Taguchi design method alone (i.e., 8.51%). Consequently, the effectiveness of the proposed AI framework is confirmed. 2. Under irradiation, the structure of the boron carbide coating specimens did not undergo a phase transformation following irradiation because the formation of boron carbide coatings involved mechanical bonding. In terms of the microstructure of the surface of the coating, the cracks that formed bore no significance. Thus, as irradiation time increased, the number of cracks on the surface increased significantly. In addition, the growth of surface cracks was governed by irradiation time, inferring that the growth was thermal efficiency-controlled. 3. Under irradiation, the surface of the boron carbide coating became oxidized and formed B2O3, H3BO3, and amorphous carbon; the coating increased proportionally with irradiation time and oxidation weight. The initial oxidation of the coating was classified as surface control, and the oxidation that formed on the B2O3 surface
References [1] S.J. Harris, G.G. Krauss, S.J. Simko, R.J. Baird, S.A. Gebremariam, G. Doll, Wear 252 (2002) 161. [2] S. Nanobashvili, J. Matĕíček, F. Žáček, J. Stőckel, P. Chráska, V. Brožek, J. Nucl. Mater. 307–311 (2002) 1334. [3] M. Bouchacourt, F. Thevenot, J. Mater. Sci. 20 (1985) 1237. [4] A.A. Gerossman, R.P. Doerner, S. Luckhardt, R. Seraydarian, A.k. Burnham, J. Nucl. Mater. 266–269 (1999) 819. [5] I. Caretti, J.M. Albella, I. Jiménez, Diamond Relat. Mater. 16 (2007) 63. [6] H.A. Castillo, E. Restrepo-Parra, J.M. Vélez, W. de la Cruz, Surf. Coat. Technol. 205 (2011) 3607. [7] H.A. Castillo, P.J. Arango, J.M. Vélez, E. Restrepo-Parra, G. Soto, W. De la Cruz, Surf. Coat. Technol. 204 (2010) 4051. [8] J.C. Yu, X.X. Chen, T.R. Hung, J. Internet Manuf. 15 (2004) 625. [9] J.T. Tasi, T.K. Liu, J.H. Chou, IEEE Trans. Evol. Comput. 8 (2004) 365. [10] H.L. Lin, C.P. Chou, Mater. Manuf. Processes 25 (2010) 631. [11] N. Yusoff, M. Ramasamy, S. Yusup, Chem. Eng. Res. Des. 89 (2011) 665. [12] J.M. Wang, H.J. Yan, J.M. Zhou, S.X. Li, G.C. Gui, Appl. Therm. Eng. 33–34 (2012) 33. [13] C.S. Chou, C.Y. Ho, C.I. Huang, Adv. Powder Technol. 20 (2009) 55. [14] X. Ni, Z. Zhou, X. Wen, L. Li, Opt. Lasers Eng. 49 (2011) 297. [15] C.S. Chou, C.Y. Wu, C.H. Yeh, R.Y. Yang, J.H. Chen, Adv. Powder Technol. (in press). [16] T. Nihat, C. Can, T. Gul, J. Mater. Process. Technol. 152 (2004) 316. [17] D. Schwingel, R. Taylor, T. Haubold, J. Wigren, C. Gualco, Surf. Coat. Technol. 108–109 (1998) 99. [18] Y. Zeng, C.X. Ding, S.W. Lee, J. Eur. Ceram. Soc. 21 (2001) 87. [19] P. Bengtsson, T. Johannesson, J. Spray Technol. 4 (1995) 245. [20] H. Bolt, M. Araki, J. Linke, W. Malléner, K. Nakamura, R.W. Steinbrech, S. Suzuki, J. Nucl. Mater. 233–237 (1996) 809. [21] Y.Q. Li, T. Qiu, Mater. Sci. Eng., A 444 (2007) 184. [22] G.A. Gogotsi, Y.G. Gogotsi, D.Y. Ostovoj, J. Mater. Sci. Lett. 7 (1988) 814. [23] D. Ge, V. Domnich, T. Juliano, E.A. Stach, Y. Gogotsi, Acta Mater. 52 (2004) 3921. [24] A. Erdemir, C. Bindal, C. Zuiker, E. Savrun, Surf. Coat. Technol. 86–87 (1996) 507. [25] A. Erdemir, G.R. Fenske, R.A. Erck, Surf. Coat. Technol. 43–44 (1990) 588. [26] P.V. Zinin, L.C. Ming, I. Kudryashov, N. Konishi, S.K. Sharma, J. Raman Spectrosc. 38 (2007) 1362.
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effects of microstructure and properties on parameter optimization of boron carbide coatings prepared using a vacuum plasma-spraying process Chun-Ming Lin a,⁎, Hsien-Lung Tsai a, Cheng Yang b a b
Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei, 10673, Taiwan, ROC Materials and Electro-Optics Research Division, Chung-Shan Institute of Science and Technology, Lungtan, Taoyuan, 32544, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 17 September 2011 Accepted in revised form 14 November 2011 Available online 28 November 2011 Keywords: Vacuum plasma-spraying process Parameter optimization Boron carbide coatings Microanalysis Oxidation
a b s t r a c t This study employed artificial intelligence methods such as the Taguchi Method to determine the optimal parameters for boron carbide (B4C) coatings using a vacuum plasma spray technique. We determined the optimal parameters to produce coatings and investigated B4C coatings in an electromagnetic radiation environment by observing and analyzing changes in the microstructure and properties. These artificial intelligence methods comprised two stages. In the first stage, orthogonal arrays (OA) were used to distribute test parameters, and the Taguchi Method was used to optimize the parameters. In the second stage, a specimen of boron carbide coating was evenly exposed to radiation to investigate the microstructure and properties of the coating. The coating irradiated at a wavelength of 10.6 μm did not undergo any change in phase. The microstructure of the surface coating exhibited cracks, but these were insignificant. As the irradiation time increased, the number of cracks on the surface increased significantly. In addition, because the growth of surface cracks was governed by irradiation time, it was inferred that the growth was thermal efficiencycontrolled (i.e., only generated by heating effects). Additionally, under irradiation, the surface of the boron carbide coating became oxidized and formed B2O3, H3BO3, and amorphous carbon; the coating size increased proportionally with irradiation time and weight of oxidation. The initial oxidation of the coating was classified as surface controlled, and the oxidation that formed on the B2O3 surface gradually transformed into extended controlled oxidation. Both types of oxidation control were conducted simultaneously, causing the weight of B2O3 to increase linearly. Two processes were involved in the extended control of oxidation: the formation of B2O3 (the cause of weight gain), and the vaporization of B2O3 (the cause of weight loss). Published by Elsevier B.V.
1. Introduction Boron carbide (B4C) has many favorable properties, including a high melting point, a high degree of hardness, low density, high elastic modulus, good chemical stability, large cross-sectional area for neutron absorption, excellent thermoelectric performance, and a low coefficient of thermal expansion. As a result, it is widely used for coating purposes in the essential goods, aerospace, and nuclear industries [1–4]. Boron carbide is a high-temperature material that is ideal for use in plasma-facing materials (PFM) in nuclear reactors and high-temperature environments. It is also used in the control barrier layer in nuclear reactors, the inner walls of dry etching reaction chambers for semiconductors, and in the fuel propulsion system of aircrafts. Additionally, it is commonly employed in the form of a coating for industrial purposes and is typically produced as PVD, ⁎ Corresponding author at: Department of Mechanical Engineering, National Taiwan University of Science and Technology 43, Keelung Road, Section 4, Taipei, 10673, Taiwan, ROC. Tel.: + 886 2 2737 3141x7304; fax: + 886 2 2737 6460. E-mail address: [email protected] (C.-M. Lin). 0257-8972/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.surfcoat.2011.11.023
CVD, and plasma sprays. The implantation of vapor deposition and plasma ion produces a delicate thin coating, which has a low performance rating, and involves a complicated process. In general, the plasma spray technique has many favorable properties, including a high performance rating, a thick coating, and a high bonding strength. The technique involves the instantaneous melting and rapid solidification of film using a plasma flame generated using a high power electrical supply. In the spraying process, high-speed airflow is applied to accelerate and atomize the sprayed material so that the molten particulates strike the working surface at a very high speed. The desired coating thus gradually builds up through the continuous flattening and solidification of the particulate matter. Because the plasma flame temperature can exceed 15,000 °C, only materials with a high melting point (e.g., boron carbide) are suitable for plasma spray deposition. However, a large number of parameters need to be appropriately controlled if the quality of the coating is to be ensured. The processing parameters are typically chosen on a trial-and-error basis. However, such an approach is not only time-consuming and expensive, but also fails to guarantee the quality of the finished product [5–7]. The Taguchi design method, based on a quality loss function and a signal-to-noise
2674
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
The Taguchi design method was applied to determine the optimal processing parameters for the VPS process of boron carbide by establishing a preliminary solution for the optimal set of processing parameters. The study then used the optimal parameters to produce optimal coatings by observing the microstructure and properties of the specimens.
(i.e., without radiation) and on the boron carbide coating specimens after various irradiation times. The surfaces of the specimens that underwent microscopy observations were not mechanically polished. Cross-section specimens were sectioned and embedded in resin at room temperature and then mechanically polished with no chemical etching (note that the specimens were polished mechanically with #4000 grit emery paper and then buff-polished using SiO2 powder, before being polished with diamond paste to 0.3 μm). The microstructures and properties of the various coating specimens were analyzed using a field emission scanning electron microscope (FE-SEM, JSM-6390 LA). In addition, the chemical composition and element distribution of the specimens were examined via energy dispersive X-ray spectrometry (EDS) and electron probe X-ray microanalysis (EPMA, JEOL JXA8009R). Finally, the surface behavior of various specimens was examined using Raman spectra, which were recorded at room temperature using a Renishaw in a Via micro-Raman system equipped with a grating with 1800 grooves/mm and a microscope with a 50× objective lens. The radiation source was set at low power to avoid heating the surface. The slit width of the spectrometer was 30 μm. The Ar+ laser beam 514.5 nm at 5 mW was focused on a spot 5 μm in diameter, and the backscattered radiation was collected from these specimens at a scanning range of 200 to 2000 cm− 1.
2.2. Experimental procedure
3. Results and discussion
2.2.1. Boron carbide coatings production Commercial grade Boron carbide powder was purchased from Coulter. Spray coating tests were performed using pure aluminium substrates with dimensions of 100 mm (length)×50 mm (width)× 3 mm (thickness). Prior to the spray tests, the substrates were cleaned and degreased, and the surfaces roughened (Ra 6.45 μm) via a sandblasting abrasion treatment to enhance the adhesion of coatings. The VPS process was performed using an A-3000 vacuum plasma spraying system (Swiss Plasma-Technik Company) comprising an ABB IRB-L6-type robot and an F4-VB spray gun. In addition, the particle size distribution of the boron carbide powder was controlled to between 17 and 20 μm, and the thickness of the spray was controlled to between 260 and 270 μm.
3.1. Taguchi method analysis
(S/N) ratio, provides a robust and efficient means of determining the optimal conditions for manufacturing processes [8–10]. An analysis of variance (ANOVA) test is performed to determine the significance of the various factors and to rank them in order of importance. It may also be interesting to show the significance of multi-factor interactions [10–13].The present study investigates the microstructure and properties of boron carbide coatings with optimal spray parameters fabricated using a vacuum plasma spray (VPS) process (note that the composition has a powder-like composition). This investigation focuses specifically on the effect of electromagnetic radiation on the microstructure and properties of boron carbide coatings. 2. Experimental methods 2.1. Taguchi method
2.2.2. Radiation test Radiation tests were performed using specimens with dimensions of 1 mm× 1 mm prepared using a CO2 continuous laser (RofinSinar 860HF) with a maximum power of 2.5 kW and a central wavelength of 10.6 μm. During the radiation process, the laser beam was defocused on the coating surface to a beam diameter of 40 mm, evenly exposing the specimens to radiation. Irradiation parameters included a laser power of 2.0 kW and a scanning velocity of 6 m/min. The upper and lower surfaces of the plates were shielded with helium and argon gas, respectively, which were provided by nozzles with an orifice diameter of 4 mm. The helium flow rate was set at 10 to 20 L/min, while the argon flow rate was set at 5 to 15 L/min. The irradiation times (incident energy) were 5 s (7.96 MJ/m2), 10 s (15.92 MJ/m2), 15 s (23.87 MJ/m2), 20 s (31.83 MJ/m2), and 25 s (39.79 MJ/m 2). Note that the CO2 laser process was chosen as a radiation source primarily because of the similarity of the system radiation (i.e., electromagnetic radiation) to actual radiation environments. 2.3. Analysis of the microstructure and properties of boron carbide coatings The characterization focused on the original boron carbide powder (i.e., without radiation) and on specimens of boron carbide coating irradiated for various durations, which were investigated via X-ray diffraction (XRD, Rigaku D/Max-2500 diffractometer with Cu-Kα radiation) at a scanning speed of 1°/min, a voltage of 30 kV, and a current of 50 mA. For comparison, microstructural (i.e., surface and cross-section) observations were also performed on the original
As shown in Table 1, the Taguchi trials involved five control factors, namely spray distance (A), chamber pressure (B), current (C), argon gas flow rate (D), and hydrogen gas flow rate (E). Moreover, each control factor was assigned four different levels. As shown in Table 2, the experimental trials were configured using an L16 (4 5) orthogonal array (OA). In the spray coating experiments, lower porosity indicated a denser coating and thus a coating of better quality. Accordingly, the smaller-the-better quality loss function (QLF) was adopted in evaluating the experimental results; however, the quality characteristic was expressed as a percentage, meaning that the additive nature of the quality characteristic was very poor. Thus, the fol^ was performed to transform the non-additive lowing operation Ω data to the additive data [14–16]: ∧ 1−P Ω ¼ 10 log P
ð1Þ
where P is porosity. The porosity, average porosity, and signal-tonoise (S/N) ratio of the quality characteristic are summarized in Table 2 for each of the 16 experimental trials. Table 3 presents the S/N ratio response table obtained by applying the ANOM statistical technique to the experimental results given in Table 2. For convenience, the S/N ratio response of each control factor is also presented in graphical form in Fig. 1. It can be seen that the optimal control factor settings are as follows: Factor A: level 1; Factor B: level 1; Factor C: level 4; Factor D: level 4; and Factor E: level 3. In other words, the Taguchi analysis suggests that the optimal spray coating parameters (i.e., the parameters that minimize spray Table 1 Level settings of control factors. Factors
A B C D E
Process parameter
Spray distance (mm) Chamber pressure (mbar) Current (A) Argon gas flow rate (l/min) Hydrogen gas flow rate (l/min)
Levels 1
2
3
4
150 650 650 30 10.5
200 700 660 35 12
250 750 670 40 13.5
300 800 680 45 15
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681 Table 2 L16 (45) Orthogonal array experimental configuration and experimental results. S.N
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
A
B
1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4
C
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
1 2 3 4 2 4 4 3 3 4 1 2 4 3 2 1
D
1 2 3 4 3 4 1 2 4 3 2 1 2 1 4 3
E
1 2 3 4 4 3 2 1 2 1 4 3 3 4 1 2
Unit: %
S/N ratio
N1
N2
N3
N4
N5
Average
Ω(dB)
18.19 14.95 16.17 18.33 13.67 11.23 16.12 26.14 12.17 22.02 22.48 24.32 15.86 28.00 29.41 21.36
17.72 15.64 16.15 19.54 13.58 12.29 16.68 26.85 13.10 23.35 27.35 27.37 15.70 25.05 30.23 22.05
15.43 14.43 14.62 20.13 18.04 12.82 18.40 23.73 12.38 19.66 27.20 23.55 15.38 24.43 33.78 28.00
15.96 15.30 16.16 16.44 17.63 9.76 18.40 24.00 14.64 19.69 24.92 25.85 13.28 26.53 34.82 27.71
16.58 16.04 13.89 17.34 15.81 10.56 19.54 25.29 12.74 18.51 23.78 27.46 16.04 30.24 32.01 25.03
16.78 15.27 15.40 18.36 15.75 11.33 17.83 25.20 13.01 20.65 25.15 25.71 15.25 26.85 32.05 24.83
6.96 7.44 7.40 6.48 7.28 8.93 6.64 4.72 8.25 5.85 4.74 4.61 7.45 4.35 3.26 4.81
neff ¼
3.1.1. Experimental confirmation To confirm the reproducibility of the experiments, a spray coating test was performed using the optimal settings of the control factor level (i.e., A1B1C4D4E3). The optimal S/N ratio was predicted to be ^ Ωj −Ω
ð2Þ
j¼1
1
ð3Þ
^ Ω 10
1 þ 10
The predicted value of the coating porosity was found to be 7.88%. The porosity of the spray coating fabrication with optimal processing conditions was determined to be 8.51%. The difference between the predicted porosity value and the measured porosity value was only 0.63%. Thus, the reproducibility of the experiments is confirmed. 3.1.2. Analysis of variance To confirm all of the experiments, a spray coating test was performed using analysis of variance (ANOVA). Table 4 presents the
Table 3 S/N ratio response table.
Level Level Level Level
1 2 3 4
N 1 ¼ DOF opt
ð5Þ
where N is the total number of trials and DOFopt is the total degrees of freedom associated with items used in the ηopt estimate. At a confidence of 95% for the porosity, F0.05;1;6 = 5.99, Vep = 0.748 (from Table 4), N = 16, DOFopt = 16, and neff = 2.28. Thus, the CI is computed to be 1.4 dB. The experimental results (Table 5) confirm that VPS process parameters were initially optimized. Although the reproducibility of the experimental results was confirmed with an average porosity as high as 3.812%, the porosity of the specimens was influenced by the spraying process, which led to severely shortened life cycles and a reduction in quality. As seen from the ANOVA outcomes (Table 4), proper regulation of the spray distance, chamber pressure, and hydrogen gas flow rate are necessary to cope with the defects mentioned above. 3.2. Microstructure properties
^ is the average value of the S/N ratio, Ωjis the optimal level of where Ω each factor, and q is the factor affecting the quality characteristic. From Eq. (2), the optimal S/N ratio was estimated to be Ωopt = ^ was then converted to the equiva10.679 dB. The estimated value of Ω lent porosity value in accordance with P¼
ð4Þ
where the F-ratio is that required for risk α with a confidence equal to 1-risk, the degrees of freedom of the pooled error, Vep is the pooled error variance, γ is the sample size of the confirmation experiment, and neff is the effective specimen size given by:
coating porosity) are a spray distance of 150 mm, a chamber pressure of 650 mbar, a current of 680 A, an argon gas flow rate of 45 l/min, and a hydrogen gas flow rate of 13.5 l/min. In addition, Fig. 1 and Table 2 show that all five control factors have a significant effect on the coating porosity.
q X
ANOVA response table of the control factor with the least effect on the quality characteristic. The confidence interval (CI) is the range between the maximum and minimum value, and the true average value will fall within that range with a stated level of confidence. The confidence interval of the above estimation can be calculated using the following equation: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! u u 1 CI ¼ tF α;1;v V ep neff
^ = 6.23(dB). Average S/N ratio Ω
^þ Ωopt ¼ Ω
2675
A
B
C
D
E
Spray distance (mm)
Chamber pressure (mbar)
Current (A)
Argon gas flow rate (l/min)
Hydrogen gas flow rate (l/min)
7.069 6.895 5.862 4.969
7.485 6.644 5.509 5.156
6.360 5.649 6.182 6.603
5.638 6.088 6.335 6.733
5.198 6.785 7.098 5.714
We investigated the microstructure and properties of the boron carbide coating prepared by the VPS process under the optimal conditions (A1B1C4D4E3) that allowed the specimen to be evenly exposed to radiation. 3.2.1. X-ray diffraction analysis Fig. 2 presents the results of the XRD analysis for the aluminum substrate and the boron carbide powder, and the coating specimens (i.e., without radiation) (note that the XRD spectra of the coating were obtained under optimal conditions using pure boron carbide power as a reference). The results show that the crystalline phase of the aluminum substrate and the boron carbide of the powder coating were categorized into two main diffraction peaks, which are labeled Al and B4C. These exhibit no phase transformation in the structure of the coating because they have the same structure; the substrate and the B4C phase show diffraction peaks in the same positions. In addition to the diffraction of boron carbide coating (see Fig. 2 (c)), Al diffraction peaks were also observed. These may be caused by diffraction from the substrate signal due to the porous structure of the coating. In addition, the XRD pattern in Fig. 3 shows the various irradiation times of the specimens of boron carbide coating. The results show that, after various irradiation times, the boron carbide coatings could be categorized into four main diffraction peaks, which are labeled Al, B, B2O3, and B4C. Therefore, no phase transformation of the coating structure occurred for irradiation times of 5 to 25 s because the phases show diffraction peaks in the same positions. In particular, for coating structures with irradiation times of 5 to 25 s, the B2O3 phase intensity increased, as shown in Fig. 3 (a) to (e) (note that other phases remained unchanged). Thus, the irradiation of a coating structure may only lead to heating, thereby influencing the content of
2676
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
Fig. 1. S/N ratio response graph.
the B2O3 phase. The B2O3 phase is discussed later in relation to the observations of microstructure. 3.2.2. Microstructure analysis Fig. 4 presents SEM images of the boron carbide powder, and the coating specimens, prior to (i.e., without radiation) and after various irradiation times ranging from 5 to 25 s (i.e., cross-section). The results show that the original specimen produced by spraying had an average porosity of 6.15% with an average bore diameter of 8.6 μm. There was no significant difference between the original coating and the irradiated specimen. The radiation process was insufficient to change the coating structure (i.e., defects and phase transformation). The boron carbide powder produced by the VPS process formed a coating structure, whereby the powder was heated using plasma modes; hence, the powder formed a molten state after rapid pounding against the substrate while the boron carbide coating structure was mechanically bonded. In other words, the radiation coating process cannot cause a phase transformation because this only occurs if heat is accumulated. In addition, cracks formed and grew on the surface of the coating. Fig. 5 shows the original specimens (i.e., without radiation) and the specimens after irradiation times of 5 to 25 s. The surface of the original specimen exhibited shallow cracks, which bore no significance, as shown in Fig. 5(a). When the coating specimen was irradiated for less than 5 s, surface cracks tended to increase, as shown in Fig. 5(b). Fig. 5(c) shows irradiation times of 10 to 25 s, after which the number of cracks on the surface increased significantly. It can be seen that at a radiation wavelength of 10.6 μm, the surface of the boron carbide coating is the result of ionization and electronic excitation (i.e., heating) rather than an erosion process. Because the growth of surface cracks is governed by irradiation time, the growth is thermal efficiency-controlled. By contrast, the cross-sectional area of the cracks grew in the original boron carbide coating (i.e., without radiation) and in specimens irradiated for times of between 5 and 25 s. It was found that the original coating specimen tended to exhibit cracks, which bore no significance, as shown in Fig. 6 (a). When the irradiation time was less than 5 s, the cracks tended to grow in a horizontal direction rather than grow denser (Fig. 6 (b). Thus, as shown in Fig. 6 (c) and (f), for irradiation times of 10 to 25 s, the number of cracks on the surface increased significantly, and specific crack growth was observed in between layers or interfaces of the coating and substrate. In general, Table 4 Results of ANOVA for the porosity. Factors
Degree of freedom
Sum of square
Mean square
F-Test
Pure sun of square
Percent contribution
A B C D E Error(pooled) Total
3 3 3 3 3 6 15
11.477 13.664 1.966 2.524 9.556 4.490 39.19
3.826 4.555
5.11 6.09
9.23 11.42
23.56% 29.14%
3.185 0.748
4.26
7.31 11.23 31.88
18.66% 28.65% 100.00%
the growth of cracks in a horizontal direction results primarily from thermal effects occurring in unison and acting collaboratively (i.e., at various irradiation times). The observation of vertical cracks in the boron carbide coating of the original (i.e., without radiation) and in specimens irradiated for various durations means that these cracks are not formed in between coatings. The results suggest that these cracks occur because boron carbide powder forms a molten state following rapid pounding against the surface of the substrate, and is heaped between layers during solidification, which interferes with the growth of cracks (i.e., the vertical direction). By contrast, the growth of cracks in the horizontal direction was not restricted. A previous study [17] indicated that cracks from radiation reactions exist on coatings, and that the number of cracks increased because the radiation energy led to enhanced thermal diffusion efficiency during crack growth. For all coating structures, the number of cracks present reflects the competing effects of crack growth. When cracks form on the surface of the coating during thermal initiation, a relatively greater increase in the microcrack density distribution is caused by the reduced Young's modulus of the coating, which results in even greater crack growth. The results after various irradiation times (see Fig. 2) agree with the results of previous studies. In addition, it was reported in [18–20] that, following radiation, the coating has enhanced internal stress in between layers, and the cracks tended to grow in a horizontal direction. This result (see Fig. 6) is consistent with those of other similar studies. 3.3. Coatings properties analysis Fig. 7 shows images and the corresponding EPMA elemental mapping results for the original specimen (i.e., without radiation) and for specimens irradiated for times of 5 to 25 s. In the analysis of the original coating specimen, shown in Fig. 7 (a), it was found that most of the coating surface had C-B elements (purple), confirming the XRD analytic results, and that C-B elements were present in the form of chemical compounds. It was also found that O-C elements (light blue) and C elements (blue) were evenly distributed. It can be assumed that the O in the atmosphere entered the coatings as they cooled, causing the C in the coating and the O in the atmosphere to combine in oxidized form. Fig. 7 (b) to (f) display specimens treated for various irradiation durations. The results reveal that the C and O-C oxides on the surface of the coating decreased gradually, while O increased gradually. By contrast, the C-B chemical compound gradually became B (red), which in turn facilitated the formation of the B-O chemical compound (yellow), which increased the rate at which C-B chemical compounds formed B-O oxide in
Table 5 Results of the confirmation experiment. S.N
A
B
C
D
E
17
150
650
680
45
13.5
Unit: %
CI (95%)
E.V.P
P.V.P
Average
8.51
8.50
8.50
10.679 ± 1.4(dB)
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
2677
Fig. 2. X-ray diffraction diagram: (a) Al substrate; (b) B4C powder; and (c) B4C coating.
proportion with the irradiation time. The oxidation reaction equation between B4C and BxC (4≦X≦25) can be used to approximate the oxidation behavior [20]: B4 C þ 402 →2B2 O3 þ CO2
ð1Þ
Bx C þ ð1 þ 3x=4ÞO2 →x=2B2 O3 þ CO2
ð2Þ
As shown in Eqs. (1) and (2), when B4C and BxC are oxidized, they form B2O3 and CO2. Eq. (1) can also be used to estimate the weight of coatings after oxidation; this weight can increase by up to 152.2% as the O in the atmosphere combines with the C in the coatings to form CO2. The CO2 dispersed in the air by the radiation process caused some C to be reduced due to the formation of CO2 and altered the C–B chemical compound. In addition, based on observation using EPMA (see Fig. 7), radiation caused the C-B chemical compound to disintegrate into B. Furthermore, radiation occurred in the atmospheric environment, triggering reactions between the disintegrated B and the O in the atmosphere, forming the B-O chemical compound (the reaction of which accelerates with radiation duration). Therefore, the chemical compound predicted to be formed from the oxidation of coatings in Eq. (1) is B2O3. Table 6 shows the changes in weight of the boron carbide in coating specimens with irradiation times of 5 to 25 s. The results show that the there were no significant changes when the specimen was irradiated for 5 s (7.96 MJ/m 2); however, the weight increased by 0.10 mg/cm2 when the irradiation time was increased to 10 s (15.92 MJ/m2). The weight increased by 0.40 mg/cm 2 when the irradiation time was increased to
Fig. 3. X-ray diffraction diagrams of coating specimens at various irradiation times: (a) 5 s; (b) 10 s; (c) 15 s; (d) 20 s; and (e) 25 s.
15 s (23.87 MJ/m 2). The weight increased by 0.66 mg/cm 2 when the irradiation time was increased to 20 s (31.87 MJ/m 2) and the weight increased by 0.89 mg/cm 2 when the radiation time was
2678
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
Fig. 4. B4C microstructure: (a) B4C powder; (b) B4C coating cross-section; (c) 0 s(i.e., without radiation); and (d) various irradiation times ranging from 5 to 25 s. (Note that the irradiation results similar).
Fig. 5. B4C surface microstructure: (a) 0 s (i.e., without radiation); (b) 5 s; and (c) various irradiation times ranging from 10 to 25 s. (Note that the irradiation results are similar).
increased to 25 s (39.79 MJ/m 2). These figures demonstrate that coating weight increased as the irradiation time increased, which might be caused by oxidation. The XRD findings support previous studies that demonstrating that B2O3 formed on the surface of the coating after various irradiation times, indicating that these casting weights increased in proportion to irradiation time, which might be due to B2O3. In addition, Table 6 shows the correlation between different irradiation times (incident energy) and the boron carbide oxidized surface. After irradiation times of 5 to 25 s, only 0.0057% of the boron carbide coating surface was oxidized, and B2O3 occupied only 11% of the surface area. Therefore, the initial oxidation of the coating may be classified as surface protection, and B2O3 gradually transformed into extended control oxidation on the surface [21]. These two methods of oxidation control were conducted simultaneously, causing the weight of B2O3 to increase linearly, which proved that the gain in weight was caused by the oxidation of the boron carbide coating. Two processes were involved in the extended control of oxidation [22], namely the formation of B2O3 (the cause of weight gain) and the vaporization of B2O3 (the cause of weight loss). At relatively low temperatures, the oxidation rate of boron carbide (the formation of B2O3) far exceeded the rate of B2O3 vaporization. In these cases, the vaporization of the B2O3 did not have any significant impact on weight. However, at high temperatures, B2O3 and its vaporization rate increased with temperature, thereby influencing the fluctuations in weight, as characterized by nonlinear changes. Oxides of various forms could also arise in the vaporization process. Therefore, base the oxidation reaction (i.e., the initial oxidation of the coating as surface protection, and oxides gradually transformed into extended control oxidation mechanisms on the surface) of boron carbide at high temperatures caused by the irradiation process. Fig. 8 shows the results of using Raman spectrography to evaluate the B4C and B2O3 phase. The results indicate that the signal peaks of the untreated coatings at 478, 529, 731, 827, 1009, and 1078 cm − 1 after various irradiation times refer to boron carbide; these findings are in agreement with those of previous studies on boron carbide [21]. Treated specimens with irradiation times ranging from 20 to 120 s (to ensure the stability of the oxide created through B2O3, the
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
2679
Fig. 6. Impact of crack growth on B4C coating structure in irradiated environments: (a) 0 s (i.e., without radiation); (b) 5 s; (c) 10 s; (d) 15 s; (e) 20 s; and (f) 25 s.
radiation time was increased to facilitate observation) reached their signal peaks at 210, 498, 809, 880, 1123, 1167, 1360, and 1601 cm − 1. The measurements of 210, 498, 880, and 1167 cm − 1 refer to the signal peaks of H3BO3 [22–25], showing that through irradiation, B2O3 produced unstable oxides that easily reacted in the environment. According to previous studies [24], B2O3 has natural moistureabsorbing properties and reacts with the H2O generated through irradiation or during tests to form H3BO3, as shown in Eq. (3). 1=2B2 O3 þ 3=2H2 O ¼ H3 BO3
ð3Þ
The reaction in Eq. (3) shows that the heat involved in the formation of H3BO3 was negative, indicating that H3BO3 formed easily because B2O3 was exposed to the atmosphere. H3BO3 formed naturally when H2O was in the air and B2O3 was on the surface of the coating undergoing interaction. Furthermore, signal peaks at 1360
and 1601 cm − 1 were formed using amorphous carbon and relate to the D band and G band, respectively [21,25,26]. These were probably formed when the coatings were treated with radiation and the B4C structure broke down, causing the B-C connection to break, and forming a C band that included some B [21]. A unique B2O3 signal peak was also observed at 809 cm − 1 [25, 26], and a small signal peak was observed at 1123 cm − 1. These could have been caused by an extended coating surface formed by an increase in irradiation time. By comparing the Raman spectrogram before and after radiation, the B4C signal peak was determined to be no longer visible after irradiating the boron carbide coating for extended periods. This was most likely due to the fact that the coatings were unable to endure such extended radiation, thereby causing the structure to undergo changes. The results support those of previous studies [21], indicating that the oxidation of B4C coatings was not only forms B2O3, but also through H3BO3 and amorphous carbon. H3BO3
2680
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
Fig. 7. Effect of irradiation on EPMA surface elements in B4C coating structures: (a) 0 s (i.e., without radiation); (b) 5 s; (c) 10 s; (d) 15 s; (e) 20 s; and (f) 25 s.
Table 6 Weight changes and B4C content of oxidized specimens before and after irradiation. Irradiation time (sec)
incident energy (MJ/m2)
Weight Changes (mg/cm2)
Oxidized Content (%)
5 10 15 20 25
7.96 15.92 23.87 31.83 39.79
0 0.10 0.40 0.66 0.89
0 0.0007 0.0025 0.0041 0.0057
and amorphous carbon only form at high temperatures following long-term irradiation, while B2O3 only forms at high temperatures after short-term exposure to radiation. 4. Conclusions 1. This study developed an integrated artificial intelligence (AI) framework based on the Taguchi method for optimizing the processing parameters in the vacuum plasma spraying of boron
C.-M. Lin et al. / Surface & Coatings Technology 206 (2012) 2673–2681
2681
gradually transformed into extended control oxidation. Both types of oxidation control were conducted simultaneously, causing the weight of B2O3 to increase linearly. Two processes were involved in the extended control of oxidation, namely the formation of B2O3 (the cause of weight gain) and the vaporization of B2O3 (the cause of weight loss). At lower temperatures, the rate of boron carbide oxidation (the formation of B2O3) was significantly greater than the rate of B2O3 vaporization. In such cases, the vaporization of B2O3 did not have any significant effect on weight. However, at high temperatures, the content of B2O3 and its vaporization rate increased with temperature, which probably influenced the fluctuations in weight, which were characterized by nonlinear changes. H3BO3 and amorphous carbon could also form during the vaporization process.
Fig. 8. Oxidation of B4C coatings treated with irradiation, as tested by Raman spectra.
carbide. According to the results obtained using the Taguchi design method alone, the optimal processing parameters are as follows: a spray distance of 150 mm, a chamber pressure of 650 mbar, a current of 680 A, an argon gas flow rate of 45 L/min, and a hydrogen flow rate of 13.5 l/min. Given these optimal parameter settings, the predicted value of the porosity is 5.2% while the measured value of the porosity is 5.56%. The coating porosity obtained when using the optimal parameter settings determined from the integrated Taguchi method was significantly lower than that obtained when using the parameter settings acquired from the Taguchi design method alone (i.e., 8.51%). Consequently, the effectiveness of the proposed AI framework is confirmed. 2. Under irradiation, the structure of the boron carbide coating specimens did not undergo a phase transformation following irradiation because the formation of boron carbide coatings involved mechanical bonding. In terms of the microstructure of the surface of the coating, the cracks that formed bore no significance. Thus, as irradiation time increased, the number of cracks on the surface increased significantly. In addition, the growth of surface cracks was governed by irradiation time, inferring that the growth was thermal efficiency-controlled. 3. Under irradiation, the surface of the boron carbide coating became oxidized and formed B2O3, H3BO3, and amorphous carbon; the coating increased proportionally with irradiation time and oxidation weight. The initial oxidation of the coating was classified as surface control, and the oxidation that formed on the B2O3 surface
References [1] S.J. Harris, G.G. Krauss, S.J. Simko, R.J. Baird, S.A. Gebremariam, G. Doll, Wear 252 (2002) 161. [2] S. Nanobashvili, J. Matĕíček, F. Žáček, J. Stőckel, P. Chráska, V. Brožek, J. Nucl. Mater. 307–311 (2002) 1334. [3] M. Bouchacourt, F. Thevenot, J. Mater. Sci. 20 (1985) 1237. [4] A.A. Gerossman, R.P. Doerner, S. Luckhardt, R. Seraydarian, A.k. Burnham, J. Nucl. Mater. 266–269 (1999) 819. [5] I. Caretti, J.M. Albella, I. Jiménez, Diamond Relat. Mater. 16 (2007) 63. [6] H.A. Castillo, E. Restrepo-Parra, J.M. Vélez, W. de la Cruz, Surf. Coat. Technol. 205 (2011) 3607. [7] H.A. Castillo, P.J. Arango, J.M. Vélez, E. Restrepo-Parra, G. Soto, W. De la Cruz, Surf. Coat. Technol. 204 (2010) 4051. [8] J.C. Yu, X.X. Chen, T.R. Hung, J. Internet Manuf. 15 (2004) 625. [9] J.T. Tasi, T.K. Liu, J.H. Chou, IEEE Trans. Evol. Comput. 8 (2004) 365. [10] H.L. Lin, C.P. Chou, Mater. Manuf. Processes 25 (2010) 631. [11] N. Yusoff, M. Ramasamy, S. Yusup, Chem. Eng. Res. Des. 89 (2011) 665. [12] J.M. Wang, H.J. Yan, J.M. Zhou, S.X. Li, G.C. Gui, Appl. Therm. Eng. 33–34 (2012) 33. [13] C.S. Chou, C.Y. Ho, C.I. Huang, Adv. Powder Technol. 20 (2009) 55. [14] X. Ni, Z. Zhou, X. Wen, L. Li, Opt. Lasers Eng. 49 (2011) 297. [15] C.S. Chou, C.Y. Wu, C.H. Yeh, R.Y. Yang, J.H. Chen, Adv. Powder Technol. (in press). [16] T. Nihat, C. Can, T. Gul, J. Mater. Process. Technol. 152 (2004) 316. [17] D. Schwingel, R. Taylor, T. Haubold, J. Wigren, C. Gualco, Surf. Coat. Technol. 108–109 (1998) 99. [18] Y. Zeng, C.X. Ding, S.W. Lee, J. Eur. Ceram. Soc. 21 (2001) 87. [19] P. Bengtsson, T. Johannesson, J. Spray Technol. 4 (1995) 245. [20] H. Bolt, M. Araki, J. Linke, W. Malléner, K. Nakamura, R.W. Steinbrech, S. Suzuki, J. Nucl. Mater. 233–237 (1996) 809. [21] Y.Q. Li, T. Qiu, Mater. Sci. Eng., A 444 (2007) 184. [22] G.A. Gogotsi, Y.G. Gogotsi, D.Y. Ostovoj, J. Mater. Sci. Lett. 7 (1988) 814. [23] D. Ge, V. Domnich, T. Juliano, E.A. Stach, Y. Gogotsi, Acta Mater. 52 (2004) 3921. [24] A. Erdemir, C. Bindal, C. Zuiker, E. Savrun, Surf. Coat. Technol. 86–87 (1996) 507. [25] A. Erdemir, G.R. Fenske, R.A. Erck, Surf. Coat. Technol. 43–44 (1990) 588. [26] P.V. Zinin, L.C. Ming, I. Kudryashov, N. Konishi, S.K. Sharma, J. Raman Spectrosc. 38 (2007) 1362.