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Thermal fatigue mechanism of WC particles reinforced steel substrate surface composite at different thermal shock temperatures
Journal of Alloys and Compounds 596 (2014) 48–54
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Thermal fatigue mechanism of WC particles reinforced steel substrate surface composite at different thermal shock temperatures Zulai Li a, Yehua Jiang a, Rong Zhou a, Fan Gao a, Quan Shan a, Jun Tan a,b,c,⇑ a
School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China IFW Dresden, Institute for Complex Materials, P.O. Box 27 01 16, D-01171 Dresden, Germany c TU Dresden, Institute of Materials Science, D-01062 Dresden, Germany b
a r t i c l e
i n f o
Article history: Received 7 November 2013 Received in revised form 23 January 2014 Accepted 24 January 2014 Available online 2 February 2014 Keywords: Metal matrix composites Oxidation Surfaces and interfaces Scanning electron microscopy, SEM
a b s t r a c t In order to provide significant references and theoretic base for the design and practical application of surface composites with high thermal fatigue performance, WC particles reinforced steel substrate surface composites were fabricated using vacuum evaporative pattern casting. And thermal fatigue behaviors of WC particles in the composites were investigated by stereomicroscope, X-ray diffraction and scanning electron microscopy. The results showed that the thermal fatigue failure of the WC particles in the composite was influenced by the combination of thermal stress and oxidation at high temperatures. When the thermal shock temperature was low (500 °C), the thermal stress was the major factor to influence the thermal fatigue failure. However, the oxidation particles played an important role with the increasing thermal shock temperature. The results might supply significant guides to the design of particles reinforced surface composites. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Recently, researches and applications of ceramic particle reinforced metal matrix composites attracted extensive interest for their excellent overall performance [1–4]. Due to their advantages such as low cost, ability to produce composites with high volume fraction, high productivity and possibility to fabricate components with complex geometry, the selectively particle reinforced metal matrix composites (surface composites) was developed and applied as practical engineering materials [5,6]. At low temperature, the surface composites showed good abrasion resistance. At high temperature, however, due to the large difference in the thermal expansion coefficients, the thermal stress, generated at the interface between ceramic particles and matrix, is a dominant factor in limiting the thermal shock resistance [7]. The oxidation-fatigue often caused the failure of components and parts when served at high temperature and high-speed wear conditions [8]. The thermal fatigue behavior has a significant connection with the service lifetime of components and parts [1,2]. In addition, if the composites are exposed to high temperature in oxidizing environments, the
⇑ Corresponding author at: School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China. Tel.: +86 871 65107512; fax: +86 871 65107922. E-mail address: [email protected] (J. Tan). http://dx.doi.org/10.1016/j.jallcom.2014.01.190 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
ceramic particle or the interface between the matrix and particle of the composites can be easily oxidized. The oxidation leads to the degradation of the mechanical properties and it shortens the lifetime of the composite [9,10]. WC particle reinforced metal matrix surface composite has gained much attention and become one of the hottest topics among the studies of the surface composites [5,6,11–14]. Liu et al. prepared WC/metal matrix surface composites by laser melt injection and investigated the microstructure of the composites [12]. The influence of WC particle volume fractions in the surface composites on erosion wear resistance was studied by Zhou et al. [5]; and the research on three-body abrasive wear resistance of WC/iron matrix surface composites was also reported by Li et al. [5,6]. Lou et al. has reported the impact of matrices in WC/metal matrix surface composites on the interfacial reaction between WC particles and matrices as well as the structural stability of the tungsten carbide particle [11]. However, only few researchers discussed the thermal fatigue resistance of WC particle reinforced metal matrix composite. The thermal fatigue mechanism at different temperatures was still poor understood. Therefore, thermal fatigue mechanism of WC particle reinforced surface composites needs further investigations. In this work, WC particle reinforced steel substrate surface composites were fabricated using vacuum evaporative pattern casting (V-EPC). The thermal shock test was adopted to investigate the thermal fatigue resistance of WC particles reinforced steel
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Z. Li et al. / Journal of Alloys and Compounds 596 (2014) 48–54
substrate surface composites. The thermal fatigue mechanism of WC particles in the composites was studied in detail.
2. Experimental procedure 2.1. Composites fabrication The preparation of the surface composites using the V-EPC infiltration processing (Fig. 1) is summarized in three steps as follows: The first step was the fabrication of the expandable polystyrene (EPS) ‘‘cluster’’, which was a binding of EPS gating system and pattern using a plastering agent. One of the surfaces of the pattern was paved with a kind of uniform mixture layer-by-layer to form a certain thickness preform layer and then the pattern was dried at 50 °C. The preform was the mixture containing WC particles (the average particulate size was 0.38–0.55 mm) used as reinforcing agent and polyvinyl vinyl alcohol water solution as binding agent. After fabricating the ‘‘cluster’’, it was painted with selfmade casting coating in about 1 mm thickness and dried at 50 °C. The second step was the preparation of the EPC sand mold. The sand box containing the ‘‘cluster’’ was filled with dry quartz sand (0.2–0.3 mm), then executed on a shake table for good degree of ramming and gas permeability. The last step was the preparation of composites. In this work, the composites could be used at high temperature and highspeed wear. Thus the high Cr carbon steel, containing 1.2 mass% carbon elements and 15 mass% chromium elements, belonging to heat resistant steel, was selected as the substrate. Before and during the process of pouring the molten high chromium carbon steel into the mold, the vacuum pressure within the sand box was maintained at about 0.065 MPa and 0.060 MPa, separately, summarized in Table 1, which was beneficial to the discharge of the gas from the decomposition and vaporization of polystyrene patterns at high temperature. And the molten metal was infiltrated into the preform layer under vacuum to form the composite reinforced with WC particles. The thickness of the composite layer was about 3 mm. The amount of the WC particles was roughly determined as 48 ± 5% (vol.) from the area ratio of the WC and matrix extracted from optical microscopy (OM) pictures.
Substrate
Transition layer
Composite layer Cooling water
Fig. 2. A schematic diagram of cooling-down method in thermal shock tests.
3. Results and discussions 3.1. Thermal fatigue behaviors of WC particles in the surface of WC/ steel substrate composite
2.2. Thermal shock test Thermal shock tests were conducted to investigate the thermal fatigue behavior of the composites. Samples were cut into a cuboid (25 mm 20 mm 8 mm) and then put into the resistance furnaces at 500, 600, 700 and 800 °C, respectively, and held for 5 min. Then they were taken out and partly immersed into water to accelerate the quenching of the composite layer as shown in Fig. 2. This procedure was repeated until the emerging of obvious cracks on the composite layer.
2.3. Anti-oxidation test of WC particles Commercial pure WC particles with an average particulate size of 0.38– 0.55 mm were selected. Each sample with about 2 g was put in one ceramic boat separately. They were then put into high temperature box resistance furnaces and heated for a certain minutes (from 1 min to 25 min). Finally the samples were cooled down to room temperature in air. Mass gain of the samples was measured on an electronic balance with an accuracy of 0.0001 g. The anti-oxidation resistance of the WC particles was expressed as mass gain rate (l), given by the expression as follows:
l¼
Fig. 1. A schematic diagram of V-EPC infiltration principle: 1 – quartz sand, 2 – EPS, 3 – coating, 4 – preform, 5 – vacuum sucker.
Mt M0 M0
ð1Þ
where Mt and M0 represent mass of the samples before and after oxidation, respectively. A decrease in l indicated an increase in the anti-oxidation resistance of the WC particles. Stereo microscope and X-ray diffraction (XRD: D/max-3B, Ricoh Co.) analysis with Cu Ka radiation were employed to analyze the influence of the temperature on the anti-oxidation resistance of WC particles.
Table 1 Parameters of the process. Temperature, T (°C)
Vacuum pressure (MPa)
Drawing
Nourishing
Pouring
1680
1680
1650
Before pouring 0.065
During pouring 0.060
After pouring 0.062
The WC reinforced steel substrate surface composite prepared by V-EPC technique had a nearly defect-free macrostructure, shown in Fig. 3. The interface between the substrate and composite layer combined well with a good metallurgical bonding. The oxidation of WC particles in the composite decreased significantly with the increase of the distance from the composite surface, taking the macroscopic investigation of the composite layer after 6 thermal shocking cycles at 800 °C as an example, shown in Fig. 4. Two typical tracing lines were shown in Fig. 4a and b indicates the content of oxygen element during the line scanning traces decreased significantly with the increasing distance from the composite surface. Therefore, for the thermal fatigue resistance of WC particles, the oxidation of WC particles in the surface of WC/steel substrate composite was one of the most critical factors. Although WC particles in the surface of the composite were only partly exposed to the air, the influence of the oxidation of these particles on the thermal fatigue behaviors of the composites could not be ignored. Fig. 5 shows a typical morphological change of WC particles in the composites with the increasing thermal shock cycles at 800 °C. Fig. 5a illustrates that obvious cracks initiated and gradually propagated after three thermal shock cycles. After six thermal shock cycles, however, WC particles were further oxidized resulting in obvious holes on the surface of the composite, shown in Fig. 5b. Due to the existence of the efflorescence in the holes, an obvious trace of WC particle shedding could be observed, as shown in Fig. 6. Fig. 7 shows the morphological evolution of WC particle in the composite after different thermal shock cycles at 600 °C. Three cracks (Fig. 7b) appeared on the surface of WC particle after one thermal shock cycle. The more thermal shock cycles, the wider the cracks. Furthermore, the cracks had a trend to extend into the matrix and the interior of WC particles, and then emerged
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Z. Li et al. / Journal of Alloys and Compounds 596 (2014) 48–54
Matrix Composite layer WC particle
100µm
Substrate
Fig. 3. Cross-section optical microscope photograph of the WC particles reinforced steel substrate surface composite indicating a good metallurgical bonding between substrate and composite layer.
coalescence to form a close circuit (Fig. 7c–g). Due to the oxidation and propagation of the cracks, WC particle shed from the composite (Fig. 7h) after 36 thermal shock cycles. The fracture morphology (Fig. 8) shows that the fracture model was cleavage and tended to exist in the WC particle rather than the interface, which reveals that the fracture was caused by the oxidation and thermal stress of WC particle. Fig. 9 records the morphological evolution of WC particles in the composite after different thermal shock cycles at 500 °C, indicating that these cracks appeared on the surface of WC particle vertically along the interface between the WC particles and the matrix (Fig. 9b) after one thermal shock cycles. And after two thermal shock cycles, the cracks propagated rapidly to the interior of WC particles to form coalescence with T-shaped (Fig. 9c), which was similar with the crack propagation law in ceramic [15,16]. With
Fig. 6. The SEM photographs of the WC particles after 6 thermal shock cycles at 800 °C indicating the existence of the efflorescence in the holes.
the increasing thermal shock cycles, the width of the cracks basically maintained changeless. Therefore, the different morphological evolution at different temperatures descripted above might origin from the different oxidation behaviors of WC particles at corresponding temperature. 3.2. The oxidation behaviors of WC particles To further investigate the thermal fatigue mechanism of the WC particle reinforced steel substrate surface composite at different
160 140
2#
Intensity
120 1#
100 80 60 40 20 0
0.00
0.05
0.10
0.15
0.20
0.25
Distance, D / mm
(a)
(b)
Fig. 4. Oxidation depth investigation of the composite layer after 6 thermal shocking cycles at 800 °C: (a) the position of linear scanning, and (b) the line scanning traces of oxygen element indicating the oxidation of WC particles in the composite decreased significantly with the increase of the distance from the composite surface.
Fig. 5. A typical morphological change of WC particles in the composites with the increasing thermal shock cycles at 800 °C: (a) 3 times, and (b) 6 times.
Z. Li et al. / Journal of Alloys and Compounds 596 (2014) 48–54
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Fig. 7. The morphological evolution of WC particles in the composites with increasing thermal shock cycles at 600 °C: (a) 0, (b) 1, (c) 3, (d) 6, (e) 9, (f) 15, (g) 30, and (h) 36.
temperatures, the oxidation behaviors of commercial pure WC particles was investigated. Fig. 10 shows the surface topography of WC particles after oxidation, indicating that the sizes of WC particles increased with the increasing temperature. Meanwhile, the colors of WC particles changed from their characteristic colors to yellow-green, which was the same color as WO3. From Fig. 10d and e, one can see that at higher temperatures (above 600 °C), WO3 evaporation could occur to form pores. XRD patterns of the samples oxidized at 800 °C, shown in Fig. 11, convinced that WC particles were finally oxidized to orthorhombic WO3. Fig. 12 shows the mass gain rate (l) as a function of time at different temperatures. One can see that at low temperature, e.g. below 500 °C, the energy was hardly to activate the oxidation of WC
particles. When the temperature reached a certain temperature (between 500 and 600 °C), mass increment rate increased gradually, indicating that obvious oxidation occurred. With the increasing temperature, the speed of the oxidation increased. When the temperature was 600 °C, the mass increment rate of the sample reached a stable value after about 21 min. While the temperature reached 700 °C, 15 min is enough to stabilize the oxidation of WC particles. For 800 °C, the mass increment rate of the sample attaining about 11% needed only 7 min. 3.3. Discussions The thermal fatigue behaviors of WC particles in the composite were affected by the thermal shock temperature. Although both
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Fig. 8. The fracture morphology of the WC particle in the composite after 36 thermal shock cycles at 600 °C.
thermal stress and oxidation contributed to the thermal fatigue behaviors of the WC particles [17,18], they might play different roles at different thermal shock temperatures. When the thermal shock temperature was relatively high (above 700 °C), the surface of the WC particles in the composite
was mostly oxidized into WO3. After a couple of thermal shock cycles, the oxidized WC particles showed brittle fracture during the thermal shock process. Therefore, the oxidation played an important role in the thermal fatigue behaviors of WC particles at high temperature. When the thermal shock temperature was low (500 °C), the WC particles could only be oxidized slightly, thermal stress played the major role in the thermal fatigue behaviors of WC particles. Due to the difference of the thermal expansion coefficients, the residual stress from the preparation of the composite existed in the interface between the WC particles and the matrix and could lead to the crack initiation. The rapid increase or decrease of the temperature could lead to the matrix transmitting shrinkage stress or expansion stress to the particles. This alternating stress occurred on the interface between the WC particles and the matrix. After certain thermal shock cycles, the cracks appeared on the surface of WC particle under the influence of the alternating stress and the residual stress. The difference in thermal expansion coefficient between the WC particles and the matrix was the dominated inducement of the crack initiation. In this study, Crack propagation could not be further observed with the increasing thermal shock cycles at 500 °C. As mentioned above, at this temperature, the oxidation of WC particles could not been activated. Therefore, the thermal fatigue of the composite at low temperature (e.g.: 500 °C) could only attributed to the
Fig. 9. The morphological evolution of WC particles in the composites with increasing thermal shock cycles at 500 °C: (a) 0, (b) 1, (c) 2, (d) 3, (e) 6, and (f) 9.
Z. Li et al. / Journal of Alloys and Compounds 596 (2014) 48–54
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Fig. 10. The surface morphology of WC particles after oxidation at different temperatures: (a) room temperature, (b) 500 °C, (c) 600 °C, (d) 700 °C, and (e) 800 °C.
Fig. 11. XRD patterns of the oxidation products of WC particles.
12
700°C
800°C
600°C
10
Weight increment rate/%
Compared with the thermal fatigue behaviors of the composite at 500 °C, was the thermal fatigue behaviors of the WC particles at 600 °C similar? As descripted above, the crack propagation rate increased and the WC particle shed from the composite after several thermal shock cycles, indicating that the effect of oxidation in the thermal fatigue behaviors of WC particles at 600 °C should not be ignored. At the beginning of the thermal shock test, the WC particles were oxidized slightly, the cracks tended to initiate under the combination of the alternating stress and the residual stress. And then the cracks rapidly propagated to the interior of the WC particles. With more thermal shock cycles, the surface of WC particles was further oxidized deeply, and the oxidation product was brittle WO3. The thermal stress stimulated the brittle fracture of the WO3, and more fractions of WO3 oxidation products accelerated the form of the aperture of the WC particles. Thus, the air can go into the aperture of the WC particles easily, accelerating the oxidation rate. The repetition of thermal shock cycles caused the propagation of the thermal fatigue crack rapidly. Finally, the WC particles completely shed from the matrix.
4. Conclusions
8 6 4 2
500°C
0 0
5
10
15
20
25
Heating time/min Fig. 12. The diagram of the mass gain rate against time.
thermal stress. Although the composite was still affected by the thermal stress with the thermal shock cycle increase, the initiation and propagation of the cracks lessened under this situation. Meanwhile, the plastic deformation capacity of the matrix around the particles could partly absorb the energy produced by the thermal stress, and thus prevented the crack from further propagation.
In conclusion, WC particle reinforced steel substrate surface composites were fabricated successfully using V-EPC technique. The thermal fatigue failure of the WC particles in composite was caused by the combination of thermal stress and oxidation. When the thermal shock temperature was relatively low (500 °C), the thermal stress played the major role on the thermal fatigue behaviors of the composite. When the thermal shock temperature was intermediate, both effects could not be ignored. When the thermal shock temperature was elevated to 800 °C, both oxidation and thermal stress played an important role in the thermal fatigue behaviors of WC particles. The results might supply significant references for the design and practical application of WC particle reinforced steel substrate surface composites.
Acknowledgements The authors acknowledge the funding support from the National Science Foundation of China (51241002, 51361019).
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[10] C.-C. Chiu, J. Mater. Sci. 29 (1994) 2078–2082. [11] D. Lou, J. Hellman, D. Luhulima, J. Liimatainen, V.K. Lindroos, Mater. Sci. Eng., A 340 (2003) 155–162. [12] D.J. Liu, L.Q. Li, F.Q. Li, Y.B. Chen, Surf. Coat. Technol. 202 (2008) 1771–1777. [13] J.N. Li, C.Z. Chen, C.F. Zhang, W. Li, Mater. Res. Innovations 15 (2011) 344–348. [14] S.W. Huang, M. Samandi, M. Brandt, Wear 256 (2004) 1095–1105. [15] R. Ritchie, Mater. Sci. Eng., A 103 (1988) 15–28. [16] R. Dauskardt, W. Yu, R. Ritchie, J. Am. Ceram. Soc. 70 (1987) 248–252. [17] K. Sheffler, D. Gupta, J. Eng. Gas Turbines Power 110 (1988) 605. [18] G. Chang, W. Phucharoen, R. Miller, Surf. Coat. Technol. 32 (1987) 307–325.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Thermal fatigue mechanism of WC particles reinforced steel substrate surface composite at different thermal shock temperatures Zulai Li a, Yehua Jiang a, Rong Zhou a, Fan Gao a, Quan Shan a, Jun Tan a,b,c,⇑ a
School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China IFW Dresden, Institute for Complex Materials, P.O. Box 27 01 16, D-01171 Dresden, Germany c TU Dresden, Institute of Materials Science, D-01062 Dresden, Germany b
a r t i c l e
i n f o
Article history: Received 7 November 2013 Received in revised form 23 January 2014 Accepted 24 January 2014 Available online 2 February 2014 Keywords: Metal matrix composites Oxidation Surfaces and interfaces Scanning electron microscopy, SEM
a b s t r a c t In order to provide significant references and theoretic base for the design and practical application of surface composites with high thermal fatigue performance, WC particles reinforced steel substrate surface composites were fabricated using vacuum evaporative pattern casting. And thermal fatigue behaviors of WC particles in the composites were investigated by stereomicroscope, X-ray diffraction and scanning electron microscopy. The results showed that the thermal fatigue failure of the WC particles in the composite was influenced by the combination of thermal stress and oxidation at high temperatures. When the thermal shock temperature was low (500 °C), the thermal stress was the major factor to influence the thermal fatigue failure. However, the oxidation particles played an important role with the increasing thermal shock temperature. The results might supply significant guides to the design of particles reinforced surface composites. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Recently, researches and applications of ceramic particle reinforced metal matrix composites attracted extensive interest for their excellent overall performance [1–4]. Due to their advantages such as low cost, ability to produce composites with high volume fraction, high productivity and possibility to fabricate components with complex geometry, the selectively particle reinforced metal matrix composites (surface composites) was developed and applied as practical engineering materials [5,6]. At low temperature, the surface composites showed good abrasion resistance. At high temperature, however, due to the large difference in the thermal expansion coefficients, the thermal stress, generated at the interface between ceramic particles and matrix, is a dominant factor in limiting the thermal shock resistance [7]. The oxidation-fatigue often caused the failure of components and parts when served at high temperature and high-speed wear conditions [8]. The thermal fatigue behavior has a significant connection with the service lifetime of components and parts [1,2]. In addition, if the composites are exposed to high temperature in oxidizing environments, the
⇑ Corresponding author at: School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China. Tel.: +86 871 65107512; fax: +86 871 65107922. E-mail address: [email protected] (J. Tan). http://dx.doi.org/10.1016/j.jallcom.2014.01.190 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
ceramic particle or the interface between the matrix and particle of the composites can be easily oxidized. The oxidation leads to the degradation of the mechanical properties and it shortens the lifetime of the composite [9,10]. WC particle reinforced metal matrix surface composite has gained much attention and become one of the hottest topics among the studies of the surface composites [5,6,11–14]. Liu et al. prepared WC/metal matrix surface composites by laser melt injection and investigated the microstructure of the composites [12]. The influence of WC particle volume fractions in the surface composites on erosion wear resistance was studied by Zhou et al. [5]; and the research on three-body abrasive wear resistance of WC/iron matrix surface composites was also reported by Li et al. [5,6]. Lou et al. has reported the impact of matrices in WC/metal matrix surface composites on the interfacial reaction between WC particles and matrices as well as the structural stability of the tungsten carbide particle [11]. However, only few researchers discussed the thermal fatigue resistance of WC particle reinforced metal matrix composite. The thermal fatigue mechanism at different temperatures was still poor understood. Therefore, thermal fatigue mechanism of WC particle reinforced surface composites needs further investigations. In this work, WC particle reinforced steel substrate surface composites were fabricated using vacuum evaporative pattern casting (V-EPC). The thermal shock test was adopted to investigate the thermal fatigue resistance of WC particles reinforced steel
49
Z. Li et al. / Journal of Alloys and Compounds 596 (2014) 48–54
substrate surface composites. The thermal fatigue mechanism of WC particles in the composites was studied in detail.
2. Experimental procedure 2.1. Composites fabrication The preparation of the surface composites using the V-EPC infiltration processing (Fig. 1) is summarized in three steps as follows: The first step was the fabrication of the expandable polystyrene (EPS) ‘‘cluster’’, which was a binding of EPS gating system and pattern using a plastering agent. One of the surfaces of the pattern was paved with a kind of uniform mixture layer-by-layer to form a certain thickness preform layer and then the pattern was dried at 50 °C. The preform was the mixture containing WC particles (the average particulate size was 0.38–0.55 mm) used as reinforcing agent and polyvinyl vinyl alcohol water solution as binding agent. After fabricating the ‘‘cluster’’, it was painted with selfmade casting coating in about 1 mm thickness and dried at 50 °C. The second step was the preparation of the EPC sand mold. The sand box containing the ‘‘cluster’’ was filled with dry quartz sand (0.2–0.3 mm), then executed on a shake table for good degree of ramming and gas permeability. The last step was the preparation of composites. In this work, the composites could be used at high temperature and highspeed wear. Thus the high Cr carbon steel, containing 1.2 mass% carbon elements and 15 mass% chromium elements, belonging to heat resistant steel, was selected as the substrate. Before and during the process of pouring the molten high chromium carbon steel into the mold, the vacuum pressure within the sand box was maintained at about 0.065 MPa and 0.060 MPa, separately, summarized in Table 1, which was beneficial to the discharge of the gas from the decomposition and vaporization of polystyrene patterns at high temperature. And the molten metal was infiltrated into the preform layer under vacuum to form the composite reinforced with WC particles. The thickness of the composite layer was about 3 mm. The amount of the WC particles was roughly determined as 48 ± 5% (vol.) from the area ratio of the WC and matrix extracted from optical microscopy (OM) pictures.
Substrate
Transition layer
Composite layer Cooling water
Fig. 2. A schematic diagram of cooling-down method in thermal shock tests.
3. Results and discussions 3.1. Thermal fatigue behaviors of WC particles in the surface of WC/ steel substrate composite
2.2. Thermal shock test Thermal shock tests were conducted to investigate the thermal fatigue behavior of the composites. Samples were cut into a cuboid (25 mm 20 mm 8 mm) and then put into the resistance furnaces at 500, 600, 700 and 800 °C, respectively, and held for 5 min. Then they were taken out and partly immersed into water to accelerate the quenching of the composite layer as shown in Fig. 2. This procedure was repeated until the emerging of obvious cracks on the composite layer.
2.3. Anti-oxidation test of WC particles Commercial pure WC particles with an average particulate size of 0.38– 0.55 mm were selected. Each sample with about 2 g was put in one ceramic boat separately. They were then put into high temperature box resistance furnaces and heated for a certain minutes (from 1 min to 25 min). Finally the samples were cooled down to room temperature in air. Mass gain of the samples was measured on an electronic balance with an accuracy of 0.0001 g. The anti-oxidation resistance of the WC particles was expressed as mass gain rate (l), given by the expression as follows:
l¼
Fig. 1. A schematic diagram of V-EPC infiltration principle: 1 – quartz sand, 2 – EPS, 3 – coating, 4 – preform, 5 – vacuum sucker.
Mt M0 M0
ð1Þ
where Mt and M0 represent mass of the samples before and after oxidation, respectively. A decrease in l indicated an increase in the anti-oxidation resistance of the WC particles. Stereo microscope and X-ray diffraction (XRD: D/max-3B, Ricoh Co.) analysis with Cu Ka radiation were employed to analyze the influence of the temperature on the anti-oxidation resistance of WC particles.
Table 1 Parameters of the process. Temperature, T (°C)
Vacuum pressure (MPa)
Drawing
Nourishing
Pouring
1680
1680
1650
Before pouring 0.065
During pouring 0.060
After pouring 0.062
The WC reinforced steel substrate surface composite prepared by V-EPC technique had a nearly defect-free macrostructure, shown in Fig. 3. The interface between the substrate and composite layer combined well with a good metallurgical bonding. The oxidation of WC particles in the composite decreased significantly with the increase of the distance from the composite surface, taking the macroscopic investigation of the composite layer after 6 thermal shocking cycles at 800 °C as an example, shown in Fig. 4. Two typical tracing lines were shown in Fig. 4a and b indicates the content of oxygen element during the line scanning traces decreased significantly with the increasing distance from the composite surface. Therefore, for the thermal fatigue resistance of WC particles, the oxidation of WC particles in the surface of WC/steel substrate composite was one of the most critical factors. Although WC particles in the surface of the composite were only partly exposed to the air, the influence of the oxidation of these particles on the thermal fatigue behaviors of the composites could not be ignored. Fig. 5 shows a typical morphological change of WC particles in the composites with the increasing thermal shock cycles at 800 °C. Fig. 5a illustrates that obvious cracks initiated and gradually propagated after three thermal shock cycles. After six thermal shock cycles, however, WC particles were further oxidized resulting in obvious holes on the surface of the composite, shown in Fig. 5b. Due to the existence of the efflorescence in the holes, an obvious trace of WC particle shedding could be observed, as shown in Fig. 6. Fig. 7 shows the morphological evolution of WC particle in the composite after different thermal shock cycles at 600 °C. Three cracks (Fig. 7b) appeared on the surface of WC particle after one thermal shock cycle. The more thermal shock cycles, the wider the cracks. Furthermore, the cracks had a trend to extend into the matrix and the interior of WC particles, and then emerged
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Matrix Composite layer WC particle
100µm
Substrate
Fig. 3. Cross-section optical microscope photograph of the WC particles reinforced steel substrate surface composite indicating a good metallurgical bonding between substrate and composite layer.
coalescence to form a close circuit (Fig. 7c–g). Due to the oxidation and propagation of the cracks, WC particle shed from the composite (Fig. 7h) after 36 thermal shock cycles. The fracture morphology (Fig. 8) shows that the fracture model was cleavage and tended to exist in the WC particle rather than the interface, which reveals that the fracture was caused by the oxidation and thermal stress of WC particle. Fig. 9 records the morphological evolution of WC particles in the composite after different thermal shock cycles at 500 °C, indicating that these cracks appeared on the surface of WC particle vertically along the interface between the WC particles and the matrix (Fig. 9b) after one thermal shock cycles. And after two thermal shock cycles, the cracks propagated rapidly to the interior of WC particles to form coalescence with T-shaped (Fig. 9c), which was similar with the crack propagation law in ceramic [15,16]. With
Fig. 6. The SEM photographs of the WC particles after 6 thermal shock cycles at 800 °C indicating the existence of the efflorescence in the holes.
the increasing thermal shock cycles, the width of the cracks basically maintained changeless. Therefore, the different morphological evolution at different temperatures descripted above might origin from the different oxidation behaviors of WC particles at corresponding temperature. 3.2. The oxidation behaviors of WC particles To further investigate the thermal fatigue mechanism of the WC particle reinforced steel substrate surface composite at different
160 140
2#
Intensity
120 1#
100 80 60 40 20 0
0.00
0.05
0.10
0.15
0.20
0.25
Distance, D / mm
(a)
(b)
Fig. 4. Oxidation depth investigation of the composite layer after 6 thermal shocking cycles at 800 °C: (a) the position of linear scanning, and (b) the line scanning traces of oxygen element indicating the oxidation of WC particles in the composite decreased significantly with the increase of the distance from the composite surface.
Fig. 5. A typical morphological change of WC particles in the composites with the increasing thermal shock cycles at 800 °C: (a) 3 times, and (b) 6 times.
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Fig. 7. The morphological evolution of WC particles in the composites with increasing thermal shock cycles at 600 °C: (a) 0, (b) 1, (c) 3, (d) 6, (e) 9, (f) 15, (g) 30, and (h) 36.
temperatures, the oxidation behaviors of commercial pure WC particles was investigated. Fig. 10 shows the surface topography of WC particles after oxidation, indicating that the sizes of WC particles increased with the increasing temperature. Meanwhile, the colors of WC particles changed from their characteristic colors to yellow-green, which was the same color as WO3. From Fig. 10d and e, one can see that at higher temperatures (above 600 °C), WO3 evaporation could occur to form pores. XRD patterns of the samples oxidized at 800 °C, shown in Fig. 11, convinced that WC particles were finally oxidized to orthorhombic WO3. Fig. 12 shows the mass gain rate (l) as a function of time at different temperatures. One can see that at low temperature, e.g. below 500 °C, the energy was hardly to activate the oxidation of WC
particles. When the temperature reached a certain temperature (between 500 and 600 °C), mass increment rate increased gradually, indicating that obvious oxidation occurred. With the increasing temperature, the speed of the oxidation increased. When the temperature was 600 °C, the mass increment rate of the sample reached a stable value after about 21 min. While the temperature reached 700 °C, 15 min is enough to stabilize the oxidation of WC particles. For 800 °C, the mass increment rate of the sample attaining about 11% needed only 7 min. 3.3. Discussions The thermal fatigue behaviors of WC particles in the composite were affected by the thermal shock temperature. Although both
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Fig. 8. The fracture morphology of the WC particle in the composite after 36 thermal shock cycles at 600 °C.
thermal stress and oxidation contributed to the thermal fatigue behaviors of the WC particles [17,18], they might play different roles at different thermal shock temperatures. When the thermal shock temperature was relatively high (above 700 °C), the surface of the WC particles in the composite
was mostly oxidized into WO3. After a couple of thermal shock cycles, the oxidized WC particles showed brittle fracture during the thermal shock process. Therefore, the oxidation played an important role in the thermal fatigue behaviors of WC particles at high temperature. When the thermal shock temperature was low (500 °C), the WC particles could only be oxidized slightly, thermal stress played the major role in the thermal fatigue behaviors of WC particles. Due to the difference of the thermal expansion coefficients, the residual stress from the preparation of the composite existed in the interface between the WC particles and the matrix and could lead to the crack initiation. The rapid increase or decrease of the temperature could lead to the matrix transmitting shrinkage stress or expansion stress to the particles. This alternating stress occurred on the interface between the WC particles and the matrix. After certain thermal shock cycles, the cracks appeared on the surface of WC particle under the influence of the alternating stress and the residual stress. The difference in thermal expansion coefficient between the WC particles and the matrix was the dominated inducement of the crack initiation. In this study, Crack propagation could not be further observed with the increasing thermal shock cycles at 500 °C. As mentioned above, at this temperature, the oxidation of WC particles could not been activated. Therefore, the thermal fatigue of the composite at low temperature (e.g.: 500 °C) could only attributed to the
Fig. 9. The morphological evolution of WC particles in the composites with increasing thermal shock cycles at 500 °C: (a) 0, (b) 1, (c) 2, (d) 3, (e) 6, and (f) 9.
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Fig. 10. The surface morphology of WC particles after oxidation at different temperatures: (a) room temperature, (b) 500 °C, (c) 600 °C, (d) 700 °C, and (e) 800 °C.
Fig. 11. XRD patterns of the oxidation products of WC particles.
12
700°C
800°C
600°C
10
Weight increment rate/%
Compared with the thermal fatigue behaviors of the composite at 500 °C, was the thermal fatigue behaviors of the WC particles at 600 °C similar? As descripted above, the crack propagation rate increased and the WC particle shed from the composite after several thermal shock cycles, indicating that the effect of oxidation in the thermal fatigue behaviors of WC particles at 600 °C should not be ignored. At the beginning of the thermal shock test, the WC particles were oxidized slightly, the cracks tended to initiate under the combination of the alternating stress and the residual stress. And then the cracks rapidly propagated to the interior of the WC particles. With more thermal shock cycles, the surface of WC particles was further oxidized deeply, and the oxidation product was brittle WO3. The thermal stress stimulated the brittle fracture of the WO3, and more fractions of WO3 oxidation products accelerated the form of the aperture of the WC particles. Thus, the air can go into the aperture of the WC particles easily, accelerating the oxidation rate. The repetition of thermal shock cycles caused the propagation of the thermal fatigue crack rapidly. Finally, the WC particles completely shed from the matrix.
4. Conclusions
8 6 4 2
500°C
0 0
5
10
15
20
25
Heating time/min Fig. 12. The diagram of the mass gain rate against time.
thermal stress. Although the composite was still affected by the thermal stress with the thermal shock cycle increase, the initiation and propagation of the cracks lessened under this situation. Meanwhile, the plastic deformation capacity of the matrix around the particles could partly absorb the energy produced by the thermal stress, and thus prevented the crack from further propagation.
In conclusion, WC particle reinforced steel substrate surface composites were fabricated successfully using V-EPC technique. The thermal fatigue failure of the WC particles in composite was caused by the combination of thermal stress and oxidation. When the thermal shock temperature was relatively low (500 °C), the thermal stress played the major role on the thermal fatigue behaviors of the composite. When the thermal shock temperature was intermediate, both effects could not be ignored. When the thermal shock temperature was elevated to 800 °C, both oxidation and thermal stress played an important role in the thermal fatigue behaviors of WC particles. The results might supply significant references for the design and practical application of WC particle reinforced steel substrate surface composites.
Acknowledgements The authors acknowledge the funding support from the National Science Foundation of China (51241002, 51361019).
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