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Fe composite coating on steel
Applied Surface Science 254 (2008) 6489–6494
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Laser cladding of in situ TiB2/Fe composite coating on steel Baoshuai Du, Zengda Zou, Xinhong Wang *, Shiyao Qu School of Materials Science and Engineering, Shandong University, Jinan 250061, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 December 2007 Received in revised form 25 February 2008 Accepted 6 April 2008 Available online 22 April 2008
To enhance the wear resistance of mechanical components, laser cladding has been applied to deposit in situ TiB2/Fe composite coating on steel using ferrotitanium and ferroboron as the coating precursor. The phase constituents and microstructure of the composite coating were investigated using X-ray diffraction (XRD), scanning electron micrograph (SEM) and electron probe microanalysis (EPMA). Microhardness tester and block-on-ring wear tester were employed to measure the microhardness and dry-sliding wear resistance of the composite coating. Results show that defect-free composite coating with metallurgical joint to the steel substrate can be obtained. Phases presented in the coating consist of TiB2 and a-Fe. TiB2 particles which are formed in situ via nucleation-growth mechanism are distributed uniformly in the aFe matrix with blocky morphology. The microhardness and wear properties of the composite coating improved significantly in comparison to the as-received steel substrate due to the presence of the hard reinforcement TiB2. ß 2008 Elsevier B.V. All rights reserved.
Keywords: In situ TiB2/Fe composite coating Laser cladding Ferrotitanium Ferroboron
1. Introduction Wear is one of the most frequently encountered failure modes for mechanical components, with the other two as corrosion and fatigue respectively. It attacks a component by removing material from the upper end, indicating that wear is often a surface related phenomenon. And it is well documented that materials with a high hardness and good toughness are expected to provide good wear resistance [1]. Particulate reinforced metal matrix composites (PMMCs) are desirable with respect to this property due to the synergetic effect caused by the combination of hard reinforcements and ductile metal matrix [2,3]. However, it is expensive, time-consuming and may even strategically unrealistic to produce bulk PMMCs. Thus, fabricating PMMC coating using surfacing technology comes as an alternative and more economic way to improve the wear resistance of components. Laser cladding is a unique process producing thick coating with metallurgical joint to the substrate that has found increasing application in the field of surface engineering. It utilizes laser beam with high energy density as the heat source to melt the cladding materials and a limited portion of the underlying substrate to form a coating. The cladding materials can be added either by preplacing powders on the substrate or by injecting them into the trailing edge of the molten pool. Compared with other surfacing
* Corresponding author. Tel.: +86 531 88392208; fax: +86 531 82616431. E-mail address: [email protected] (X. Wang). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.04.051
technology, laser cladding possesses have many advanced features. These include metallurgical bond between the coating and substrate, highly refined microstructure, low dilution ratio and limited heat affected zone (HAZ) [4–6]. Furthermore, laser cladding is an effective way to produce PMMC composite coatings because the desired coating can be gained by coupling the design of the precursor with the tailoring of the laser processing parameters. According to the literature, considerable studies have been conducted on the fabrication of PMMC coatings by laser cladding [3–8]. Among various ceramic particulates, titanium diboride is expected to be one of the best reinforcements for steel matrix due to its high hardness (3400 HV), high melting temperature (3225 8C), outstanding tribological properties and good compatibility with the steel matrices [9,10]. Thus, it is attractive to fabricate TiB2 reinforced Fe-based MMCs. Although some work [11–13] has been done to produce TiB2 reinforced composite coatings, most of them focus on adding TiB2 particles directly into the metal matrix. However, emphasis has been given to the in situ formation of reinforcing particles using laser-cladding technology recently for several important reasons. Compared with the ex situ route, this in situ process is more economical and has an intrinsic advantage that the surface of the particles is cleaner and hence the bond between the reinforcing particles and the matrix tends to be stronger [8]. Moreover, coating properties can be tailored to the application by varying the laser cladding process variables, such as laser traverse speed, power, beam size, overlapping ratio and composition of precursor. In the past, pure titanium and
B. Du et al. / Applied Surface Science 254 (2008) 6489–6494
6490 Table 1 Chemical composition of ferrotitanium and ferroboron Ferroalloy
Chemical composition (wt.%) Ti
B
Si
Al
C
Cu
P
S
Mn
Ferrotitanium Ferroboron
26.15 –
– 16.43
4.5 0.58
7.1 0.1
0.089 0.24
0.24 –
0.049 0.039
0.03 0.002
2.5 –
ferrotitanium with a high content of titanium (71 wt.%) were used as the donors of Ti element to produce in situ MMC coatings in our group. However, it is found that Ti element in such precursors is easily suffered from oxidation and burning during the cladding process. In this research, ferrotitanium with low content of titanium and ferroboron are employed as the coating precursor to produce TiB2 reinforced Fe-based composite coating. It is expected that employing such ferroalloy as the precursor can not only substantially reduce the cost of the cladding material but also benefit the cladding process by lowering the melting point of the cladding material by eutectic reactions of Fe and Ti as well as Fe and B. In this present investigation, systematic study was carried on laser clad precursor of ferrotitanium and ferroboron mixture on steel substrate to elucidate the phase constituents, microstructure and wear properties of the composite coating. 2. Experimental procedure AISI 1010 steel (0.08–0.13 wt.% C, 0.3–0.6 wt.% Mn, <0.04 wt.% S, <0.05 wt.% P, balance Fe) with dimensions 50 mm 30 mm 10 mm was chosen as the substrate. Prior to the laser cladding process, steel coupon was polished by abrasive paper and degreased in acetone to keep a clean surface. Ferrotitanium (100 mm), ferroboron (100 mm) powders are used as the precursor for laser cladding whose chemical compositions are listed in Table 1. As mentioned in the introduction, by employing these ferroalloy powders, the cost of cladding materials can be reduced substantially and the cladding process can be improved. The coating precursor with a composition of 60.73 wt.% ferrotitanium + 39.27 wt.% ferroboron was thoroughly mixed in a ball miller for three hours followed by mixing with an organic binder to form slurry for preplacing on the substrates. The atomic ratio of Ti and B in the precursor is 1/1.8, locating in the Ti-rich side of stoichiometric TiB2. The purpose of this composition design is to compensate the loss of Ti during the laser cladding process as well as to avoid the formation the other brittle borides, as will be
Fig. 1. Schematic of the block-on-ring wear tester.
discussed later. The thickness of the preplaced coating precursor was fixed to about 1.2 mm. A 5 kw DHL-5000 continuous wave CO2 laser (10.6 mm wavelength) was employed for the laser coating process. During the cladding process, laser beam diameter was kept constant at 3 mm. However, other processing parameters including laser power and scanning speed were optimized based on the criteria that defect-free coating with relatively smooth surface could be obtained. The optimized laser power and scanning speed under current investigation were 2.5 kw and 5 mm/s, respectively. The overlapping ratio was 30%. A side jet of argon with the gas flow rate of 20 L/min was used to prevent the sample from oxidation. The sample for metallography investigation were prepared by sectioning the coated steel block perpendicular to the laser track using an abrasive cutting machine under water cooling condition. Microstructure was examined by a JXA-840 scanning electron microscopy (SEM). The sample used for the SEM observation was deeply etched to reveal the morphology of TiB2 particles. The micro-zone composition was investigated using electron probe Xray microanalyzer (EPMA) (JXA-8800R). Sample used for EPMA was polished without etching. Phase identification was carried out on an X-ray diffractometer (XRD) (D/Max-Rc) with Cu Ka radiation operated at 30 kV and 40 mA. Microhardness measurements were performed by using a microhardness Vickers tester with a load of 200 g applied for 15 s. Wear resistance was evaluated by employing a block-on-ring wear tester shown schematically in Fig. 1. Wear test was performed under room temperature using a W18Cr4V (HRC62) steel wheel as
Fig. 2. Macrographs of the sample: (a) top view and (b) cross-sectional view.
B. Du et al. / Applied Surface Science 254 (2008) 6489–6494
Fig. 3. X-ray diffraction patterns of the precursor and the clad layer.
the counterbody. The rotating speed of the steel wheel is 400 rpm and the normal load applied on it was 150 N. The sample was weighed at regular intervals of every 10 min. Wear test time was 30 min, corresponding to a total sliding distance of 1507.2 m. Moreover, wear resistance of the substrate was also measured in order to provide a comparison. The friction coefficients of both the substrate and coating were recorded simultaneously. Surfaces of the wear tracks were observed using the SEM. 3. Results and discussion 3.1. Microstructure and phase constituents The top view of the sample after laser cladding is given in Fig. 2(a). It reveals that the surface of the specimen after laser
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cladding is relatively smooth and by employing overlapping technology large clad area can be obtained. Cross-sectional view is shown in Fig. 1(b). It was shown that the sample can be divided into three regions: clad layer, HAZ and substrate. During laser cladding the severity of thermal excursions experienced by the sample varied from region to region, resulting in these three distinct regions. The coating also demonstrates enough toughness, which can be proved by the fact that no cracking was shown in the coating. In addition, the modified layer revealed a sound metallurgical joint to the substrate. Fig. 3 shows the XRD spectrums of the precursor and the clad layer. It is clearly indicated that the precursor mainly consists of FeB, Fe2Ti and a-Fe. These are the phase constituents of ferrotitanium and ferroboron used in this investigation. However, after laser cladding, the phases detected in the coatings are a-Fe and TiB2, i.e. TiB2 can be synthesized in situ. It should be noted that no other diffraction peaks related to the precursor (ferrotitanium and ferroboron) have been identified in the coating, as can be seen from a direct comparison between the X-ray diffraction pattern of the precursor and that of the laser processed composite coating (Fig. 3). Another feature of the phase constituents of the clad layer is the absence of brittle phases such as Fe2B and FeB which have been found when using self-propagating high-temperature synthesis (SHS) to produce TiB2 reinforced iron-based MMCs [2]. The avoidance of such brittle phase is of importance for the cracking-resistance of laser-clad coating which undergoes severe thermal stress. To analyze the phases presented in this coating, the thermodynamic equilibrium of the composite coating was considered. It is generally accepted that a Fe–TiB2 pseudobinary phase diagram exists which shows a direct equilibrium between austenite or ferrite and TiB2 on the stoichiometric composition of Fe1 xTix/3B2x/3 [13,14]. However, in our case the influences of C and
Fig. 4. SEM micrographs of the composite coating: (a) upper part; (b) middle; (c) bottom and (d) near the interface.
B. Du et al. / Applied Surface Science 254 (2008) 6489–6494
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Fig. 5. EPMA backscattered electron image (a) and lining scanning analysis showing elements distribution of Ti, B and Fe (b).
O on the Fe–Ti–B system should be taken into consideration. C may come from the melting substrate as well as the ferroalloy, while O can be introduced into the melting pool from the precursor of ferroalloy and oxidation during the cladding process. By employing the Thermocalc software, Tanaka et al. [14] assessed the phase equilibrium of Fe–Ti–B with consideration of other impurities and it was found Fe–TiB2 region is located on the slightly Ti-rich side of the stoichiometric composition of Fe1 xTix/3B2x/3. Taken the chemical composition of the precursor as well as the dilution effect of steel substrate in to consideration, chemical composition of the composite coating falls into this area. Hence, only a-Fe and TiB2 presented in the coating. The micrographs of the clad layer are shown in Fig. 4. It can be seen that the clad layer presents a microstructure essentially consisting of reinforcing particles dispersed in a metal matrix. The TiB2 particles are faceted and exhibit blocky morphology. Moreover, the content and size of the reinforcements decrease gradually from the upper part to the bottom of the clad layer. When approaching the interface, reinforcing particles tends to be very sparse and the size of the particles becomes smaller. EPMA line scanning was performed to identify the elements distribution of the composite coating and the result is shown in Fig. 5. It is obvious that the black particles are rich in Ti and B while lack of Fe. Combing the result of the X-ray diffraction, conclusion can be made that the black particles are TiB2 and the metal matrix is a-Fe dendrites. Due to the interaction of the laser beam with the material, a homogeneous melt pool can be formed during the laser cladding process. TiB2 has the highest melting point, so during the cooling process of the laser cladding it is likely to separate first from the melt, i.e. TiB2 can be formed via the nucleation-growth mechanism. The TiB2 blocky morphology can be rationalized based on its C32 crystal structure. The TiB2 crystal is formed by trigonal prisms closely packed in different directions, with growth along the [0 0 0 1] and < 1 1¯ 0 0 > directions involving alternating Ti and B layers in both cases [15,16]. Crystal of TiB2 growing unconstrained from the melt tends to form the blocky morphology. Similar observations have been made on TiB2 crystals growing from TiAl melt [16]. The gradient distribution of the TiB2 particles is possibly due to the Maragoni convective fluid flow of the molten metal caused by the thermal gradient and the surface tension.
three regions which are characterized in Fig. 1 as clad layer, HAZ and substrate. Compared with the substrate, microhardness of the clad layer increases tremendously. This is due to the presence of a high volume fraction of TiB2 reinforcing particles. The coating shows a relatively uniform distribution of microhardness values. However, when approaching the interface microhardness values drop gradually because of the gradient distribution of the reinforcing particles. 3.3. Dry-sliding wear resistance The dry-sliding wear test results are shown in Fig. 7. At room temperature, the composite coating exhibits a dry-sliding wear resistance that is increased by about 25 times relative to the AISI 1010 steel substrate. Morphologies of the worn surfaces of the composite coating and substrate are illustrated in Fig. 8. It shows that there are craters and deep grooves for the steel substrate (Fig. 8(a,b)), which indicates the wear mechanism for the substrate is grooving and severe adhesive wear. In contrast, wear track of the composite coating ((Fig. 8(c,d)) shows a rather smooth surface in spite of some slight scratches. Moreover, it can be seen that the TiB2 are polished and particle pullout is absent. Fig. 9 shows the variations of the friction coefficient of the clad layer as well as the steel substrate as a function of time under the dry-sliding wear test condition. It illustrates that the friction coefficient is to be about 0.6 for the composite coating and 0.83 for the steel substrate. Besides, the friction coefficient of the in situ
3.2. Microhardness Fig. 6 shows the microhardness profile of the sample. It can be seen that the microhardness values are in good consistent with the
Fig. 6. Microhardness profile of the sample.
B. Du et al. / Applied Surface Science 254 (2008) 6489–6494
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Fig. 7. Cumulative weight losses over a 30-min period during dry-sliding wear block-on-ring test.
Fig. 9. Friction coefficient as a function of time for the in situ TiB2 reinforced clad layer and the mild steel substrate.
TiB2 reinforced composite coating exhibits much lower fluctuation in comparison to that of the steel substrate. Wear mechanism in the multiphase composites is very complex, depending upon several factors such as volume fraction, distribution, morphology of the ceramic particles, and wear condition [1,9,17]. Nevertheless, the dominant wear mechanism of the in situ synthesized TiB2/Fe under current wear test condition can be regarded as a mixed adhesion and micro-polishing mechanism. The binder phase a-Fe in the composite coating undergoes metal–metal contact with the mating wheel, indicating
adhesion can take place between the tribo-surfaces, which also results in the subsequent material transfer. This in turn leads to the exposure of TiB2 particles to the steel countersurface. Consequently, TiB2 can effectively carry the load and play an important role in resisting wear attacks when dry-sliding with the metallic counterpart. It is obvious that TiB2 has a very high hardness compared with the mating steel wheel. Moreover, since the TiB2 particles are formed in situ, the interfacial bonding between the aFe matrix and TiB2 reinforcing phase tends to be stronger, resulting in the absence of pullout of the TiB2 particles as indicated in
Fig. 8. SEM micrographs showing the worn surface morphology: (a) low magnification of substrate; (b) high magnification of substrate; (c) low magnification of clad layer and (d) high magnification of substrate.
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B. Du et al. / Applied Surface Science 254 (2008) 6489–6494
Fig. 8(b). Hence, the TiB2 particles are worn via micro-polishing mechanism. Taken together, the dominating wear mechanism for the composite coating is a mixed adhesive and micropolishing mechanism. The severe adhesive wear experienced by the steel substrate leads to a higher friction coefficient. However, the inherent strong atomic bonds of the TiB2 phases coupled with the relative smooth surface of the composite coating prevent the cold-welding to the metallic asperities on the contacting surface of the W18Cr4V metallic counterpart, so the composite coating demonstrated a lower friction coefficient and slighter fluctuation degree. 4. Conclusions Using ferrotitanium and ferroboron as precursor, a laser clad TiB2 reinforced composite coating on mild steel substrate free from cracks and pores has been obtained. Phase constituents of the coating include a-Fe and TiB2. TiB2 are formed in situ via the nucleation-growth mechanism during the cooling of the laser cladding process. The microstructure of the composite coating consists of blocky TiB2 particles distributed uniformly in the a-Fe metal matrix. Due to the synergetic effect of the in situ formed hard TiB2 particles with the ductile a-Fe matrix, microhardness and wear resistance of the composite coating improved dramatically compared with the steel substrate.
Acknowledgements This research is supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China (project No. 060422020) and the Natural Science Foundation of Shandong Province (No. Z2006F07).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
A. Agarwal, N.B. Dahotre, Wear 240 (2000) 144. A. Anal, T.K. Bandyopadhyay, K. Das, J. Mater. Process. Technol. 172 (2006) 70. J. Xu, Z. Li, W.H. Li, Z.L. Liu, W.J. Liu, Mater. Sci. Eng. A 447 (2007) 307. Y.T. Pei, T.C. Zuo, Mater. Sci. Eng. A 241 (1998) 259. S. Yang, W.J. Liu, M.L. Zhong, Z.J. Wang, Mater. Lett. 58 (2004) 2958. R. Anandkumar, A. Almeida, R. Colac¸o, R. Vilar, V. Ocelik, J.Th.M. De Hosson, Surf. Coat. Technol. 201 (2007) 9497. Q.W. Meng, L. Geng, B.Y. Zhang, Surf. Coat. Technol. 200 (2006) 4923. A. Singh, N.B. Dahotre, J. Mater. Sci. 39 (2004) 4553. C.C. Degnan, P.H. Shipway, Wear 252 (2002) 832. C.C. Degnan, P.H. Shipway, Metall. Mater. Trans. 33A (2002) 2973. A. Agarwal, N.B. Dahotre, Int. J. Refract. Metals Hard Mater. 17 (1999) 283. A. Agarwal, N.B. Dahotre, Mater. Character. 42 (1999) 31. M. Darabara, G.D. Papadimitriou, L. Bourithis, Surf. Coat. Technol. 201 (2006) 3518. K. Tanaka, T. Saito, J. Phase Equilib. 20 (3) (1998) 207. A.A. Abdel-Hamid, S.T. Hamar, R. Hamar, J. Cryst. Growth. 71 (1985) 744. M.E. Hyman, C. Mccullough, J.J. Valencia, C.G. Levi, R. Meherabian, Metall. Mater. Tran. 20A (1989) 1847. M. Darabara, G.D. Papadimitriou, L. Bourithis, Surf. Coat. Technol. 202 (2007) 246.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Laser cladding of in situ TiB2/Fe composite coating on steel Baoshuai Du, Zengda Zou, Xinhong Wang *, Shiyao Qu School of Materials Science and Engineering, Shandong University, Jinan 250061, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 December 2007 Received in revised form 25 February 2008 Accepted 6 April 2008 Available online 22 April 2008
To enhance the wear resistance of mechanical components, laser cladding has been applied to deposit in situ TiB2/Fe composite coating on steel using ferrotitanium and ferroboron as the coating precursor. The phase constituents and microstructure of the composite coating were investigated using X-ray diffraction (XRD), scanning electron micrograph (SEM) and electron probe microanalysis (EPMA). Microhardness tester and block-on-ring wear tester were employed to measure the microhardness and dry-sliding wear resistance of the composite coating. Results show that defect-free composite coating with metallurgical joint to the steel substrate can be obtained. Phases presented in the coating consist of TiB2 and a-Fe. TiB2 particles which are formed in situ via nucleation-growth mechanism are distributed uniformly in the aFe matrix with blocky morphology. The microhardness and wear properties of the composite coating improved significantly in comparison to the as-received steel substrate due to the presence of the hard reinforcement TiB2. ß 2008 Elsevier B.V. All rights reserved.
Keywords: In situ TiB2/Fe composite coating Laser cladding Ferrotitanium Ferroboron
1. Introduction Wear is one of the most frequently encountered failure modes for mechanical components, with the other two as corrosion and fatigue respectively. It attacks a component by removing material from the upper end, indicating that wear is often a surface related phenomenon. And it is well documented that materials with a high hardness and good toughness are expected to provide good wear resistance [1]. Particulate reinforced metal matrix composites (PMMCs) are desirable with respect to this property due to the synergetic effect caused by the combination of hard reinforcements and ductile metal matrix [2,3]. However, it is expensive, time-consuming and may even strategically unrealistic to produce bulk PMMCs. Thus, fabricating PMMC coating using surfacing technology comes as an alternative and more economic way to improve the wear resistance of components. Laser cladding is a unique process producing thick coating with metallurgical joint to the substrate that has found increasing application in the field of surface engineering. It utilizes laser beam with high energy density as the heat source to melt the cladding materials and a limited portion of the underlying substrate to form a coating. The cladding materials can be added either by preplacing powders on the substrate or by injecting them into the trailing edge of the molten pool. Compared with other surfacing
* Corresponding author. Tel.: +86 531 88392208; fax: +86 531 82616431. E-mail address: [email protected] (X. Wang). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.04.051
technology, laser cladding possesses have many advanced features. These include metallurgical bond between the coating and substrate, highly refined microstructure, low dilution ratio and limited heat affected zone (HAZ) [4–6]. Furthermore, laser cladding is an effective way to produce PMMC composite coatings because the desired coating can be gained by coupling the design of the precursor with the tailoring of the laser processing parameters. According to the literature, considerable studies have been conducted on the fabrication of PMMC coatings by laser cladding [3–8]. Among various ceramic particulates, titanium diboride is expected to be one of the best reinforcements for steel matrix due to its high hardness (3400 HV), high melting temperature (3225 8C), outstanding tribological properties and good compatibility with the steel matrices [9,10]. Thus, it is attractive to fabricate TiB2 reinforced Fe-based MMCs. Although some work [11–13] has been done to produce TiB2 reinforced composite coatings, most of them focus on adding TiB2 particles directly into the metal matrix. However, emphasis has been given to the in situ formation of reinforcing particles using laser-cladding technology recently for several important reasons. Compared with the ex situ route, this in situ process is more economical and has an intrinsic advantage that the surface of the particles is cleaner and hence the bond between the reinforcing particles and the matrix tends to be stronger [8]. Moreover, coating properties can be tailored to the application by varying the laser cladding process variables, such as laser traverse speed, power, beam size, overlapping ratio and composition of precursor. In the past, pure titanium and
B. Du et al. / Applied Surface Science 254 (2008) 6489–6494
6490 Table 1 Chemical composition of ferrotitanium and ferroboron Ferroalloy
Chemical composition (wt.%) Ti
B
Si
Al
C
Cu
P
S
Mn
Ferrotitanium Ferroboron
26.15 –
– 16.43
4.5 0.58
7.1 0.1
0.089 0.24
0.24 –
0.049 0.039
0.03 0.002
2.5 –
ferrotitanium with a high content of titanium (71 wt.%) were used as the donors of Ti element to produce in situ MMC coatings in our group. However, it is found that Ti element in such precursors is easily suffered from oxidation and burning during the cladding process. In this research, ferrotitanium with low content of titanium and ferroboron are employed as the coating precursor to produce TiB2 reinforced Fe-based composite coating. It is expected that employing such ferroalloy as the precursor can not only substantially reduce the cost of the cladding material but also benefit the cladding process by lowering the melting point of the cladding material by eutectic reactions of Fe and Ti as well as Fe and B. In this present investigation, systematic study was carried on laser clad precursor of ferrotitanium and ferroboron mixture on steel substrate to elucidate the phase constituents, microstructure and wear properties of the composite coating. 2. Experimental procedure AISI 1010 steel (0.08–0.13 wt.% C, 0.3–0.6 wt.% Mn, <0.04 wt.% S, <0.05 wt.% P, balance Fe) with dimensions 50 mm 30 mm 10 mm was chosen as the substrate. Prior to the laser cladding process, steel coupon was polished by abrasive paper and degreased in acetone to keep a clean surface. Ferrotitanium (100 mm), ferroboron (100 mm) powders are used as the precursor for laser cladding whose chemical compositions are listed in Table 1. As mentioned in the introduction, by employing these ferroalloy powders, the cost of cladding materials can be reduced substantially and the cladding process can be improved. The coating precursor with a composition of 60.73 wt.% ferrotitanium + 39.27 wt.% ferroboron was thoroughly mixed in a ball miller for three hours followed by mixing with an organic binder to form slurry for preplacing on the substrates. The atomic ratio of Ti and B in the precursor is 1/1.8, locating in the Ti-rich side of stoichiometric TiB2. The purpose of this composition design is to compensate the loss of Ti during the laser cladding process as well as to avoid the formation the other brittle borides, as will be
Fig. 1. Schematic of the block-on-ring wear tester.
discussed later. The thickness of the preplaced coating precursor was fixed to about 1.2 mm. A 5 kw DHL-5000 continuous wave CO2 laser (10.6 mm wavelength) was employed for the laser coating process. During the cladding process, laser beam diameter was kept constant at 3 mm. However, other processing parameters including laser power and scanning speed were optimized based on the criteria that defect-free coating with relatively smooth surface could be obtained. The optimized laser power and scanning speed under current investigation were 2.5 kw and 5 mm/s, respectively. The overlapping ratio was 30%. A side jet of argon with the gas flow rate of 20 L/min was used to prevent the sample from oxidation. The sample for metallography investigation were prepared by sectioning the coated steel block perpendicular to the laser track using an abrasive cutting machine under water cooling condition. Microstructure was examined by a JXA-840 scanning electron microscopy (SEM). The sample used for the SEM observation was deeply etched to reveal the morphology of TiB2 particles. The micro-zone composition was investigated using electron probe Xray microanalyzer (EPMA) (JXA-8800R). Sample used for EPMA was polished without etching. Phase identification was carried out on an X-ray diffractometer (XRD) (D/Max-Rc) with Cu Ka radiation operated at 30 kV and 40 mA. Microhardness measurements were performed by using a microhardness Vickers tester with a load of 200 g applied for 15 s. Wear resistance was evaluated by employing a block-on-ring wear tester shown schematically in Fig. 1. Wear test was performed under room temperature using a W18Cr4V (HRC62) steel wheel as
Fig. 2. Macrographs of the sample: (a) top view and (b) cross-sectional view.
B. Du et al. / Applied Surface Science 254 (2008) 6489–6494
Fig. 3. X-ray diffraction patterns of the precursor and the clad layer.
the counterbody. The rotating speed of the steel wheel is 400 rpm and the normal load applied on it was 150 N. The sample was weighed at regular intervals of every 10 min. Wear test time was 30 min, corresponding to a total sliding distance of 1507.2 m. Moreover, wear resistance of the substrate was also measured in order to provide a comparison. The friction coefficients of both the substrate and coating were recorded simultaneously. Surfaces of the wear tracks were observed using the SEM. 3. Results and discussion 3.1. Microstructure and phase constituents The top view of the sample after laser cladding is given in Fig. 2(a). It reveals that the surface of the specimen after laser
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cladding is relatively smooth and by employing overlapping technology large clad area can be obtained. Cross-sectional view is shown in Fig. 1(b). It was shown that the sample can be divided into three regions: clad layer, HAZ and substrate. During laser cladding the severity of thermal excursions experienced by the sample varied from region to region, resulting in these three distinct regions. The coating also demonstrates enough toughness, which can be proved by the fact that no cracking was shown in the coating. In addition, the modified layer revealed a sound metallurgical joint to the substrate. Fig. 3 shows the XRD spectrums of the precursor and the clad layer. It is clearly indicated that the precursor mainly consists of FeB, Fe2Ti and a-Fe. These are the phase constituents of ferrotitanium and ferroboron used in this investigation. However, after laser cladding, the phases detected in the coatings are a-Fe and TiB2, i.e. TiB2 can be synthesized in situ. It should be noted that no other diffraction peaks related to the precursor (ferrotitanium and ferroboron) have been identified in the coating, as can be seen from a direct comparison between the X-ray diffraction pattern of the precursor and that of the laser processed composite coating (Fig. 3). Another feature of the phase constituents of the clad layer is the absence of brittle phases such as Fe2B and FeB which have been found when using self-propagating high-temperature synthesis (SHS) to produce TiB2 reinforced iron-based MMCs [2]. The avoidance of such brittle phase is of importance for the cracking-resistance of laser-clad coating which undergoes severe thermal stress. To analyze the phases presented in this coating, the thermodynamic equilibrium of the composite coating was considered. It is generally accepted that a Fe–TiB2 pseudobinary phase diagram exists which shows a direct equilibrium between austenite or ferrite and TiB2 on the stoichiometric composition of Fe1 xTix/3B2x/3 [13,14]. However, in our case the influences of C and
Fig. 4. SEM micrographs of the composite coating: (a) upper part; (b) middle; (c) bottom and (d) near the interface.
B. Du et al. / Applied Surface Science 254 (2008) 6489–6494
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Fig. 5. EPMA backscattered electron image (a) and lining scanning analysis showing elements distribution of Ti, B and Fe (b).
O on the Fe–Ti–B system should be taken into consideration. C may come from the melting substrate as well as the ferroalloy, while O can be introduced into the melting pool from the precursor of ferroalloy and oxidation during the cladding process. By employing the Thermocalc software, Tanaka et al. [14] assessed the phase equilibrium of Fe–Ti–B with consideration of other impurities and it was found Fe–TiB2 region is located on the slightly Ti-rich side of the stoichiometric composition of Fe1 xTix/3B2x/3. Taken the chemical composition of the precursor as well as the dilution effect of steel substrate in to consideration, chemical composition of the composite coating falls into this area. Hence, only a-Fe and TiB2 presented in the coating. The micrographs of the clad layer are shown in Fig. 4. It can be seen that the clad layer presents a microstructure essentially consisting of reinforcing particles dispersed in a metal matrix. The TiB2 particles are faceted and exhibit blocky morphology. Moreover, the content and size of the reinforcements decrease gradually from the upper part to the bottom of the clad layer. When approaching the interface, reinforcing particles tends to be very sparse and the size of the particles becomes smaller. EPMA line scanning was performed to identify the elements distribution of the composite coating and the result is shown in Fig. 5. It is obvious that the black particles are rich in Ti and B while lack of Fe. Combing the result of the X-ray diffraction, conclusion can be made that the black particles are TiB2 and the metal matrix is a-Fe dendrites. Due to the interaction of the laser beam with the material, a homogeneous melt pool can be formed during the laser cladding process. TiB2 has the highest melting point, so during the cooling process of the laser cladding it is likely to separate first from the melt, i.e. TiB2 can be formed via the nucleation-growth mechanism. The TiB2 blocky morphology can be rationalized based on its C32 crystal structure. The TiB2 crystal is formed by trigonal prisms closely packed in different directions, with growth along the [0 0 0 1] and < 1 1¯ 0 0 > directions involving alternating Ti and B layers in both cases [15,16]. Crystal of TiB2 growing unconstrained from the melt tends to form the blocky morphology. Similar observations have been made on TiB2 crystals growing from TiAl melt [16]. The gradient distribution of the TiB2 particles is possibly due to the Maragoni convective fluid flow of the molten metal caused by the thermal gradient and the surface tension.
three regions which are characterized in Fig. 1 as clad layer, HAZ and substrate. Compared with the substrate, microhardness of the clad layer increases tremendously. This is due to the presence of a high volume fraction of TiB2 reinforcing particles. The coating shows a relatively uniform distribution of microhardness values. However, when approaching the interface microhardness values drop gradually because of the gradient distribution of the reinforcing particles. 3.3. Dry-sliding wear resistance The dry-sliding wear test results are shown in Fig. 7. At room temperature, the composite coating exhibits a dry-sliding wear resistance that is increased by about 25 times relative to the AISI 1010 steel substrate. Morphologies of the worn surfaces of the composite coating and substrate are illustrated in Fig. 8. It shows that there are craters and deep grooves for the steel substrate (Fig. 8(a,b)), which indicates the wear mechanism for the substrate is grooving and severe adhesive wear. In contrast, wear track of the composite coating ((Fig. 8(c,d)) shows a rather smooth surface in spite of some slight scratches. Moreover, it can be seen that the TiB2 are polished and particle pullout is absent. Fig. 9 shows the variations of the friction coefficient of the clad layer as well as the steel substrate as a function of time under the dry-sliding wear test condition. It illustrates that the friction coefficient is to be about 0.6 for the composite coating and 0.83 for the steel substrate. Besides, the friction coefficient of the in situ
3.2. Microhardness Fig. 6 shows the microhardness profile of the sample. It can be seen that the microhardness values are in good consistent with the
Fig. 6. Microhardness profile of the sample.
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Fig. 7. Cumulative weight losses over a 30-min period during dry-sliding wear block-on-ring test.
Fig. 9. Friction coefficient as a function of time for the in situ TiB2 reinforced clad layer and the mild steel substrate.
TiB2 reinforced composite coating exhibits much lower fluctuation in comparison to that of the steel substrate. Wear mechanism in the multiphase composites is very complex, depending upon several factors such as volume fraction, distribution, morphology of the ceramic particles, and wear condition [1,9,17]. Nevertheless, the dominant wear mechanism of the in situ synthesized TiB2/Fe under current wear test condition can be regarded as a mixed adhesion and micro-polishing mechanism. The binder phase a-Fe in the composite coating undergoes metal–metal contact with the mating wheel, indicating
adhesion can take place between the tribo-surfaces, which also results in the subsequent material transfer. This in turn leads to the exposure of TiB2 particles to the steel countersurface. Consequently, TiB2 can effectively carry the load and play an important role in resisting wear attacks when dry-sliding with the metallic counterpart. It is obvious that TiB2 has a very high hardness compared with the mating steel wheel. Moreover, since the TiB2 particles are formed in situ, the interfacial bonding between the aFe matrix and TiB2 reinforcing phase tends to be stronger, resulting in the absence of pullout of the TiB2 particles as indicated in
Fig. 8. SEM micrographs showing the worn surface morphology: (a) low magnification of substrate; (b) high magnification of substrate; (c) low magnification of clad layer and (d) high magnification of substrate.
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Fig. 8(b). Hence, the TiB2 particles are worn via micro-polishing mechanism. Taken together, the dominating wear mechanism for the composite coating is a mixed adhesive and micropolishing mechanism. The severe adhesive wear experienced by the steel substrate leads to a higher friction coefficient. However, the inherent strong atomic bonds of the TiB2 phases coupled with the relative smooth surface of the composite coating prevent the cold-welding to the metallic asperities on the contacting surface of the W18Cr4V metallic counterpart, so the composite coating demonstrated a lower friction coefficient and slighter fluctuation degree. 4. Conclusions Using ferrotitanium and ferroboron as precursor, a laser clad TiB2 reinforced composite coating on mild steel substrate free from cracks and pores has been obtained. Phase constituents of the coating include a-Fe and TiB2. TiB2 are formed in situ via the nucleation-growth mechanism during the cooling of the laser cladding process. The microstructure of the composite coating consists of blocky TiB2 particles distributed uniformly in the a-Fe metal matrix. Due to the synergetic effect of the in situ formed hard TiB2 particles with the ductile a-Fe matrix, microhardness and wear resistance of the composite coating improved dramatically compared with the steel substrate.
Acknowledgements This research is supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China (project No. 060422020) and the Natural Science Foundation of Shandong Province (No. Z2006F07).
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