3
process
I. Campos-Silva1,*, M. Flores-Jiménez 1, G. Rodríguez-Castro 1, E. Hernández-Sánchez 2, J. Martínez-Trinidad 1, R. Tadeo-Rosas 1
1
Instituto Politécnico Nacional, Grupo Ingeniería de Superficies, SEPI-ESIME, U.P. Adolfo López Mateos, Zacatenco, México D.F., 07738, México.
2
Universidad Autónoma Metropolitana-Azcapotzalco, Materials Department, San Pablo 180, México D.F., 02200, México.
Abstract. In this study, the fracture toughness of boride coatings formed at the surface of AISI 1045 steel was improved by means of a diffusion annealing process. First, the boriding of AISI 1045 steel was performed by the powder-pack method at a temperature of 1223 K and a range of exposure times (8 – 12 h). The diffusion annealing process was conducted on the borided steels at a temperature of 1223 K with 8 h of exposure using a diluent atmosphere of SiC powder and bentonite. To establish the mechanical behavior of the boride coatings developed by both treatments, properties such as the real hardness and the Young’s modulus were estimated at 50 m from the surface using Vickers and Knoop testing, respectively. The fracture toughness of the boride coatings was estimated using a universal crack equation applicable independently of the cracking mode. The boride coating obtained by the boriding process exhibited an intermediate cracking mode, while the coatings obtained by the diffusion annealing process showed a radial-median mode.
The effect of the diffusion annealing process on the fracture toughness of the boride coatings revealed an increase of approximately 50% in comparison with the coatings developed by the powder-pack boriding process.
Keywords: boriding; diffusion annealing process; boride coatings; fracture toughness; real hardness; Young´s modulus.
*
Corresponding author: Tel: (+52) (55) 57296000 ext. 54768, Fax: (+52) (55) 57296000 ext. 54589 e-mail: [email protected]
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Boriding is the process of surface boron saturation in metals and alloys, which is performed to increase the hardness, wear and corrosion resistance of these materials in engineering components for industrial applications that require those properties. The boriding of different steel grades results in the formation of either a single coating (Fe2B) or a double coating (FeB/Fe2B) [1]. Due to the difference in the expansion coefficients (parallel to the surface) of the boride layers in the range of 473 to 873 K (FeB = and Fe2B =
) [2], cooling after boriding leads to tensile stresses in the FeB layer and compressive stresses in the Fe2B layer (both stresses are parallel to the borided surface). When the percentage of FeB in the total thickness of coating is high, the difference in deformation may lead to cracks at the boundary between the two borides, reducing the mechanical properties at the surface of the borided steel. In this case, the presence of a FeB/Fe2B coating is not desirable in industrial applications.
Different methods have been proposed to obtain a single-boride (Fe2B) coating at the surface of different ferrous materials. Matuschka [2] (see references therein) proposed a phase change by homogenizing (diffusion annealing process) in a melt of a powder mixture consisting of NaCl and KCl with oxygen-binding additives; the FeB decomposed in favor of the Fe2B coating, which increased at the expense of the FeB coating. In stainless steels, the growth of the Fe2B coating as a consequence of the reduction of the FeB was achieved by diffusion treatment in an inert gas atmosphere at a temperature of 1273 K with 2 h of exposure. The consequence of the post-diffusion treatment was the formation of a homogeneous single Fe2B coating that was less prone to crack formation and flaking [3]. An interrupted boriding process was developed to prevent the generation of the FeB/Fe2B
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performed using the powder-pack and molten-salt methods, in which the boride needles of the Fe2B coating were more rounded, thicker and shorter than the saw-toothed Fe2B formed with the continuous boriding (traditional) process [4]. The morphology of the Fe2B coating formed by the interrupted method had significant effects due to the improved wear and corrosion resistance and thus reduced the brittleness at the surface of the borided steel. Recently, two methods consisting of boriding followed by a diffusion annealing process (DAP) have been proposed to allow the phase transformation from the FeB coating to the Fe2B coating [5,6].
The study of Kartal et al. [5] proposed an alternative method, called “phase
homogenization in electrochemical boriding”, in which the steel samples were exposed to a current density of 200 mA/cm2 in a molten electrolyte (90% borax and 10% Na2CO3) for 15 min at a temperature of 1223 K. The DAP after boriding was performed in the same bath without any polarization for 45 min of exposure; the resulting boride coating consisted only of the Fe2B phase with a layer thickness of 75 m. Furthermore, the adhesion of the borided steel exposed to the phase homogenization process was evaluated by the Daimler- Benz Rockwell C method and a HF1 rating was obtained.
In addition, to prevent the generation of the FeB/Fe2B coating formed at the surface of the pure iron, a DAP after the gas boriding was conducted in H2 atmosphere at a temperature of 1173 K with different exposure times. The results showed that the increased exposure time led to the total elimination of the FeB coating [6].
This study proposes a two-stage process (powder-pack boriding followed by a diffusion annealing process) to avoid generating the FeB coating at the surface of the AISI 1045
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across the boride coatings obtained by the two-stage process using a constant load of 0.98 N. In addition, the steels exposed to the boriding treatment and DAP were characterized by Knoop and Vickers indentation tests at 50 microns from the surface using loads ranging from 0.098 to 9.8 N. Namely, the indentation properties, such as the Young’s modulus (E), the real hardness (Ho), the fracture toughness (KC), and the brittleness (B), obtained after the boriding process were compared to those of borided specimens exposed to the diffusion annealing process.
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2.1 Powder-pack boriding and diffusion annealing processes
The powder-pack boriding process was conducted on commercial square samples of AISI 1045 steels at a temperature of 1223 K with 8, 10 and 12 h of exposure time. The samples were embedded in a closed, cylindrical case in contact with a mixture of powders consisted of 20% B4C as the donor, 10% KBF4 as an activator, and 70% SiC as the diluent. Boriding
was accomplished by placing the container in a furnace without the use of inert gases. Once the treatment was completed, the container was removed from the furnace and slowly cooled to room temperature. The samples were prepared for microscopic examinations by standard metallographic techniques using GX51 Olympus equipment to determine the microstructure of the boride coating (Figure 1). In addition, fifty measurements were performed using a fixed reference of borided samples to estimate the thicknesses of the FeB coating and the total (FeB+Fe2B) coating. Likewise, X-ray diffraction (XRD) was
conducted on the borided sample obtained after 12 h of exposure. For this purpose, GBC MMA equipment was used with CuKradiation at λ = 0.154 nm.
After the microscopic and XRD examinations, the DAP was conducted on the borided samples. The AISI 1045 borided steels were embedded in a closed, cylindrical box in contact with a powder mixture of SiC, which acted as a diluent, and bentonite. The container was heated at 1273 K for 8 h in the absence of inert gases in the furnace and was then cooled in air. The borided steels exposed to the DAP were prepared for metallographic preparation, and the evolution of the boride layer was observed in the clear field by optical microscopy with the aid of a GX51 Olympus instrument (Figure 2). The microstructure of
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using CuK radiation at λ = 0.154 nm.
2.2 Vickers microhardness testing
The specimens of AISI 1045 steels that were borided and exposed to DAP were indented on a commercial indenter (Wolpert 402, MVD equipment) according to the ASTM E384 and ASTM C1327 procedures. First, the hardness-depth profiles were obtained across the boride layers with an applied load of 0.98 N, as shown in Figure 3. For a particular distance from the surface of the borided steel, three indentations were performed for the set of experimental conditions. In addition, a set of applied loads ranging between 0.098 and 9.8 N were performed perpendicularly to the surface at a constant distance (50 m) from the surface of the steels exposed to the boriding process (Figure 4(a)), where the presence of the FeB coating was verified by microscopic examinations. In the same manner, the range of applied loads was established in the borided steels exposed to the DAP at the same distance and in the same zone in which the indentations were performed on the FeB coating.
For all sets of experimental conditions, five acceptable indentations were conducted for each applied load to establish the behavior of the hardness (H) as a function of the diagonal length (d) and to verify the presence of the indentation size effect (ISE) in the boride coatings. Figure 4(b) depicts the indentations performed at 50 m from the surface for the borided steels exposed to the DAP.
Finally, the fracture toughness (KC) of the boride coatings produced by the two-stage process (boriding and diffusion annealing process) was estimated in the range of applied
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9 corners at a constant distance from the surface (50 m). The crack lengths were measured using a GX51 Olympus instrument and Image-Pro Plus software. The KC values on the
boride coatings were estimated using the model proposed by Chicot et al. [7].
2.3 Knoop microhardness testing
The variations of the Young’s modulus (E) in the range of applied loads (0.98 to 9.8 N) were estimated by the Knoop microhardness for both processes (boriding and diffusion annealing process) at 50 m from the surface of the borided samples. Five acceptable indentations for each load were used to determine the elastic recovery of the diagonal dimensions of the surface impression. The elastic recovery is correlated with the ratio of the Knoop hardness (HK) to the Young’s modulus (
E
) as follows [8]:where b' and a' are the dimensions of the Knoop diagonals after elastic recovery, b a is a known value of 1 / 7.11 for the Knoop indenter geometry, and has a constant value of 0.45 [8]. The dimensions of the Knoop diagonals after indentation in the boride coatings were measured using an Olympus GX51 metallurgical microscope and Image-Pro Plus software, as shown in Figure 5. Eq. (1) is insensitive to the incidence of cracking, and it is employed for a wide range of materials with hardness values between 2 to 20 GPa. As indicated by Marshall et al. [8] for , a range that covers most brittle materials, the relative error in the estimation is .
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The FeB and Fe2B coatings with a saw-toothed morphology were developed at the surface of the AISI 1045 steels exposed to the powder-pack boriding process. The XRD analysis depicted in Figure 6(a) confirmed that the boride coating formed at a temperature of 1223 K with 12 h of exposure was composed of both FeB and Fe2B. Martini et al. [9] recognized that the FeB phase growth on the outermost part of the sample is a consequence of the transformation of Fe2B crystals over this zone (in addition to the high boron potential on the surface). When the treatment times and temperatures are increased, the FeB regions become much deeper, and they grow from compact and oriented Fe2B crystals.
When a treatment involving a phase change via homogenization (diffusion annealing process) is applied to borided steels in the presence of a FeB/Fe2B coating, the amount of FeB coating can be limited or completely avoided. As a consequence of the boron concentration gradient, boron diffuses from the FeB coating along the grain boundaries through the steel substrate during DAP, causing the dissolution (or gradual reduction) of this coating with an increase of the Fe2B coating thickness [10].
The effect of DAP for the AISI 1045 steel boriding at 8 h of exposure was the increased length of the Fe2B coating with the presence of porosity near the surface region, in which FeB disappeared completely, as shown in the XRD pattern presented in Figure 6(b). Furthermore, in the case of steel boriding with 10 h of exposure, the resulting microstructure consisted of the Fe2B coating with only insolated FeB teeth that were approximately 13 m thick; the presence of the FeB coating was confirmed by the XRD spectra depicted in Figure 6(c). Finally, in the case of the AISI 1045 steel boriding at 12 h
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11 of exposure, the DAP produced a FeB coating of 18 m with an increased Fe2B thickness; the presence of both coatings was verified by the XRD pattern, as shown in Figure 6(d). The results given above show that it is necessary to increase the temperature or the exposure time of the DAP in a solid diluent atmosphere for thicker FeB coatings. Moreover, the results of the mean thickness values of the boride coatings achieved by both processes are given in Figure 7. In industrial applications, boride layers of thickness > 150 m should be avoided due to their friable nature, except for very light contact loading situations.
Figure 8 shows the hardness-depth profile across the FeB/Fe2B boride coatings formed at the surface of AISI 1045 steels and those exposed to the DAP. A hardness value of 14 GPa (independent of the exposure time) was obtained near the surface region of samples exposed to the boriding process, and an increase in hardness of 15 GPa was estimated at the end of the FeB coating. In the Fe2B coating, the hardness decreased to 12 GPa, and near the boride layer/substrate interface the hardness tends to decrease (10 GPa) because the influence of the steel substrate. Finally, the hardness rapidly dropped to values that are typical of the steel substrate. Overall, the hardness results of the borided steels exposed to the DAP showed a decrease of 13 GPa near the surface region (approximately 15 microns), whereas in the region of the layer/substrate interface the hardness was of 10 GPa. Moreover, the hardness values along the depth of the Fe2B exhibited a more uniform and gradual decrease.
In addition, the evolution of the hardness as a function of the applied load of the boride coatings developed by both processes was estimated at a constant distance (50 m) from the surface. The indentation distance was first selected to prevent delamination near the top
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indentations closer to the pure regions of the FeB coating obtained by the powder-pack boriding method, and prevent the influence of the FeB isolated teeth and the FeB thin layer obtained by the DAP in the steels exposed to the boriding process at 10 and 12 h of exposure, respectively. The results shown in Figure 9(a) revealed that the hardness decreased as the applied load increased for both treatment conditions. This phenomenon, which is known as the indentation size effect (ISE), depends on the size of the indentation produced by an applied load. The ISE has been traditionally described using Meyer’s Law [11]:
where
P
is the load, d is the resulting indentation size, andA
andn
are constants derived from the curve fit of the plot on a bi-logarithmic scale. Then
value (or the Meyer index) that characterizes the curve is often called the ISE. This effect is especially apparent in hard, brittle ceramics at low indentation loads wheren
is significantly less than 2. Figures 9(b) and 9(c) display the results of the indentation load against the diagonal length at 50 m from the surface for both treatment conditions. The results of the linear regression, which are summarized in Table I, corroborate the assumption that the values ofn
were less than two, which confirms the ISE in the boride coatings obtained by both processes.Based on these results, the apparent or real hardness (H0) of the boride coatings in the
range of the applied loads (0.098 to 9.8 N) was estimated using the proportional resistance specimen (PSR) model proposed by Li and Bradt [12] as follows:
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where is related to the elastic resistance of the test specimen and the friction at the indenter facet-specimen interface, and H0 can be obtained directly from a2 producing the following expression:
The values of a1 and a2 can be estimated from the plots Pmax
d versus
d (Figures 10(a)
and 10(b)). The results, which are summarized in Table II, show that the H0 obtained at 50 m from the surface of the steels exposing to the boriding process at different exposure times fell within the range of 10.1 to 10.9 GPa, whereas values of 9.2 to 9.4 GPa were obtained for the borided steels exposed to the DAP.
The effect of the DAP on the fracture toughness of borided steels was evaluated by Vickers indentation at 50 m from the surface. The radial-median and Palmqvist crack geometry models have been used to determine the fracture toughness in boride coatings [13-16]. Instead of the traditional crack models, Chicot et al. [7] proposed an alternative universal crack equation that is applicable independent of the cracking (radial-median, Palmqvist or intermediate cracking modes) by adjusting the model proposed by Miranzo and Moya [17]. Their work employed the Vickers indentation fracture toughness to study the multi- cracking behavior of titania, alumina and zirconia ceramic oxide coatings obtained by plasma spraying. The fracture toughness equation is expressed as:
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a
c
indentation center ( where is the crack length measured from the indentation
corner), qis a linear-dependence function that is experimentally determined, and
E
is theYoung’s modulus of the coating in the set of applied loads. According to Miranzo and
Moya [17], can be expressed as:
The expression is related to the apparent or real hardness of boride
coatings obtained by the PSR model (see Table II) and the Young’s modulus values of the
boride coatings estimated by Knoop hardness testing represented in Eq. (1) for the set of applied loads, which ranged between 0.98 and 9.8 N.
The plots of E versus P depicted in Figure 11 for the borided steels and those exposed to
DAP showed that the Young’s modulus of the boride coatings did not vary with the applied
loads. In addition, the estimated value of E decreased for the borided steels exposed to the
DAP. The comparative values of E obtained for both treatments are summarized in Table III.
Eq. (5) shows that the cracking mode is equivalent to the radial-median system when q = 0,
but the mode is equivalent to Palmqvist cracking for a value of q = 1 and .
Moderate values of q correspond to the intermediate crack mode.
Given that the half-diagonal length of the indent is connected to the applied load through the ISE, Eq. (2) can be rewritten as follows:
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15 Thus, the cracking modes can be established by the following proportionalities (for more details see reference [7]):
To estimate the cracking behavior on the boride layers obtained by boriding and DAP, the Vickers half-diagonal length and the crack length measured from the indentation center were plotted as a function of the indentation load in bi-logarithmic coordinates.
According to the results presented in Figure 12 and summarized in Table IV, the AISI 1045 steels exposed to the boriding process at different exposure times showed the presence of an intermediate cracking mode at 50 microns from the surface (particularly in the region of the FeB coating) because none of the experimental slopes of
c
and versus P in bi-logarithmic coordinates suggested the usual cracking modes (radial-median or Palmqvist). In this case, the q value proposed in Eq. (5) can be calculated from Eq. (10) as follows [7]:
where s can be obtained from the slope of the plot ln c versus ln P. The results depicted in Table IV for the steels exposed to the boriding process displayed a q-value equal to 1; thus, ac1/2 could be plotted against P. Figure 13 shows the behavior between P and ac1/2 that is adequately represented by a straight line for the different boriding conditions.
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C
was estimated using Eq. (5) according to the slopes of the straight lines represented in
Figure 13; the KC values obtained at 50 m from the surface (in the FeB coating) were
approximately 0.8 MPa m, which varied slightly with the exposure time, as shown in
Table V. These results were compared with those obtained by Ozbek and Bindal [14] in the