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An investigation of the structure and properties of infiltrated layer on the surface of copper alloy
Materials Science and Engineering A 399 (2005) 206–215
An investigation of the structure and properties of infiltrated layer on the surface of copper alloy Gui-rong Yang ∗ , Yuan Hao, Wen-ming Song, Ying Ma State Key Loboratory of New Nonferrous Metal Materials, School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou, 730050 Gansu. PR China Received in revised form 1 March 2005; accepted 1 March 2005
Abstract This work reports about microstructure, hardness, adhesion strength, thermal fatigue and oxidation properties of the infiltrated layer on substrate of copper. The infiltrated layer was produced by vacuum infiltration casting technique using Ni-based powder as surface alloying particles. SEM observations indicated the formation of a compact infiltrated layer. The macro-hardness of the layer is about HRC 60 and the distribution of micro-hardness presents gradient change. The average micro-hardness of layer is about HV450. The transverse rupture strength (68–213 MPa) of the layer depends on layer thickness and layer orientation relative to loading direction. The layer appears to possess excellent oxidation resistance and superior thermal fatigue resistance at 800 ◦ C. © 2005 Elsevier B.V. All rights reserved. Keywords: Vacuum infiltration casting; Copper substrate; Microstructure; Properties of infiltrated layer
1. Introduction In a large number of engineering applications, service conditions vary with location. Consequently it is often useful to design the microstructure of a structural part to meet these varying requirements [1]. Copper with high electrical and thermal conductivity is widely used in optics and electronics; however, high strength and wear resistance are required in some applications. In order to improve wear resistance of copper, a number of methods have been developed, including internal oxidation [2], chemical vapor deposition [3,4], electroplating [5], thermal spray [6], infiltration [7] and other means [8,9]. Of these methods, infiltration technique has the advantages of simplicity and feasibility in changing the characteristic of copper surface. Infiltration of a liquid metal into a preform can be carried out by pressureless or pressure-assisted methods. The main advantages of pressureassisted infiltration include the following: (a) faster process; (b) near-net shape processing; (c) reduced contamination and (d) reduced porosity in comparison with spray techniques ∗
Corresponding author. Tel.: +86 931 2972809; fax: +86 931 2755806. E-mail address: [email protected] (G.-r. Yang).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.03.106
[10–21]. However, even in the case of pressure-assisted infiltration, choking of pores during infiltration can occur, which prematurely terminates the infiltration process [14]. This difficulty of achieving full infiltration decreases with increasing the particle size in the preform and increasing pouring temperature [22]. In this study, the bronze matrix surface alloying materials were fabricated by the vacuum infiltration casting technique. The present article deals with the performance evaluation of some infiltrated layers in terms of hardness, adhesion (bond) strength, thermal fatigue and oxidation resistance. In addition, the results of micro-structural analysis are reported. 2. Experimental procedures 2.1. Fabrication of infiltrated layer Casting alloy Ni60 particle of 120–160 mesh was chosen as the reinforcement, and bronze ZQAl 9-4 (Table 1), as the matrix of the surface alloying layer and the substrate material on which the infiltrated layer formed. The surface infiltrated layer was fabricated by a vacuum infiltration casting process. First, the Ni60 powder and some
G.-r. Yang et al. / Materials Science and Engineering A 399 (2005) 206–215 Table 1 The chemical composition of tested aluminum bronze (wt.%) Cu Al Fe Sb Si P As Sn Pb Mn Zn Total
8.0–10.0 2.0–4.0 ≤0.05 ≤0.20 ≤0.2 ≤0.1 ≤0.05 ≤0.2 ≤0.1 ≤0.5 ≤1.0 ≤1.0
Fig. 1. The procedure for forming infiltrated layer.
self-fabricated bond NJB (its main composition is boric acid) were mixed together according to desired proportion, paved on some inner surface of a casting mould or outer surface of mould cores, where strength and wear characteristics need improvement, then heated to harden so as to form preform layer. Second, molten bronze was poured at 1220 ◦ C into the mould under vacuum suction of the melt into the perform formed the surface infiltrated layer. The fabrication principle is shown in Fig. 1. 2.2. Microstructure and evaluation of properties 2.2.1. The microstructure of infiltrated layer The microstructure of surface infiltrated layer was observed by scanning electron microscope (SEM) of HITACHI
207
S–520. The specimens of surface layer for SEM observation were cut out along the direction from infiltrated layer surface to substrate, ground and polished using standard metallographic techniques. 2.2.2. Hardness and adhesion (bond) strength measurements The macro-hardness was measured using HRS-150 Rockwell hardness tester under 1471 N load. The micro-hardness was measured using HVS-1000 micro-hardness tester at the load of 9.8 N and loading time of 20 s. A three-point bend test was employed to evaluate the adhesion (bond) strength. The dimensions of the specimens used in the bend tests are 5 mm × 5 mm × 40 mm. The infiltrated layer was located either on the top or the bottom of the specimen. Fig. 2 shows the design of the specimens used for the three-point bend tests. Load is slowly applied to the specimen with the definite displacement. After loading, the stress–strain curve would be obtained, and it can be transformed to the load–displacement curve through software. This test was conducted on three specimens for each type of loading configuration. 2.2.3. Thermal cycling experiment The dimensions of the specimens used in the thermal cycling (thermal fatigue) experiment were 5 mm × 5 mm × 10 mm. For thermal cycling, the specimens were heated in a muffle furnace at 800 ◦ C for 15 min followed by water cooling for 5 min. After 5 min, the specimens were taken out of water when their temperature was same as that of water. A thorough visual scrutiny of the specimens was undertaken to determine the extent of damage they have undergone within the substrate, at the interface between the bond and the surface layer, and on the top surface of the infiltrated layer. A total of 130 thermal cycles were done. 2.2.4. Oxidation experiment The oxidation behavior of the infiltrated layer was evaluated by the weight changes. The six faces of rectangular test specimens (40 mm × 40 mm × 10 mm) were ground, ultrasonically cleaned in water, and then rinsed in ethanol prior to drying in air. The initial weight of specimens was measured using a photoelectric analytic balance with a precision of 0.1 mg. The specimens were heated in a muffle furnace kept
Fig. 2. The type of three-point bend test specimen for infiltrated layer: (a) infiltrated layer on the top of the specimen; (b) infiltrated layer at the bottom of the specimen.
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Fig. 3. A cross-sectional SEM view of infiltrated layer.
at 800 ◦ C for 2 h, followed by air cooling to room temperature, and weighing the specimens. This test was repeated until the weight change of specimens was near zero. The time of heating lasted for 4, 8, 12 and 24 h, respectively. The weight of the specimens was measured before and after each test to obtain the weight changes. The oxidation rate was calculated by dividing the weight change with the test duration. Three specimens were tested to get the average oxidation rate.
3. Results and discussion 3.1. Microstructure of infiltrated layer The cross-sectional views of the infiltrated layer are shown in Figs. 3–5. The micrographs reveal continuous and sound interface between the substrate and the infiltrated layer and no crack is present. No spalling or separation is even at high magnification. As seen in Fig. 3, the infiltrated layer consists of three parts: surface sintering layer (SSL), metallurgical fusing layer (MFL) and diffusion solid solution layer (DSSL). Figs. 4 and 5 show the morphology of SSL and DSSL, respectively. The formation of infiltrated layer is a kind of metallurgical fusion because the melting point of Ni60 powder is lower than that of copper alloy. After pouring the melt, the metal infiltrated into the pores and permeated the preform to the outer preform close to the mould surface. In the outermost layer, the Ni60 powder particles partially melt, and the copper can only sinter with the alloying powder at the original site. The composition of outer surface sintering layer is mainly composed of alloying powder with a small amount of copper. The temperature of the melt behind the infiltration on front is higher than that of the copper melt forming the SSL; therefore, the copper alloy and alloying powder formed metallurgical fusion layer (MFL) behind the SSL. The alloying powder located at the innermost surface of preform obtains
the most energy compared with the former two layers. The energy can melt the alloying powder, decompose and diffuse only to form many active atoms that may infiltrate directly into the substrate to form solid solution in the advantageous place. The distributions of Ni, Cr, and Cu across the substrate and infiltrated layer (Fig. 6) were continuous. The distribution of Ni and Cr gradually decreases from the outer layer to the inner layer. On the other hand, the distribution of Cu increases gradually toward the inner layer. The total layer thickness of DSSL and MFL is larger than that obtained through plating, spraying and vapor deposition because of faster diffusion in the liquid and longer diffusion distance [12–14]. The temperature of diffusion of other methods, such as heat treatment,
Fig. 4. The SEM view of outer layer.
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solid solution and Ni-based solid solution. The inner layer is mainly composed of Cu-based solid solution. 3.2. Hardness and adhesion (bond) strength 3.2.1. The macro-hardness The hardness of copper alloy is around HRB 130, while, the hardness of infiltrated layer is about HRC60.0. The hardness of infiltrated layer has been significantly improved. The reason of increasing hardness is the boride and Ni-based solid solution strengthened by Cr in the outer layer.
Fig. 5. The SEM view of the joint of substrate and infiltrated layer.
is lower than that of infiltration casting technique. The structure of the infiltrated surface material is promising from the point view of microstructure of structural materials because it has a transition layer (DSSL). Combining Figs. 6 and 7, the following conclusions can be drawn. The outer layer is composed of CrB and Ni-based solid solution. The metallurgical fusing layer (MFL) is composed of CrB, Cu-based
3.2.2. The micro-hardness The distribution of Ni, Cr, and Cu was displayed in Fig. 6. The contents of Ni and Cr are high in the outer and middle layer. The Ni content decreases gradually and the Cr content is very small in the inner layer because the diffusion coefficient of Cr is smaller than that of Ni [18,19]; therefore, Cr diffusion in infiltrated layer is imperceptible and the Ni content changes slowly from outer layer to the inner. The Ni60 alloy powder consists of Ni-based solid solution, boride and Ni–Cr alloy [15,20]. The surface layer containing these hard phases might be applied in the circumstances of wear, erosion, and oxidation-resistance, and of corrosion in air, seawater, steam and weak acid. Fig. 8 shows the variation of micro-hardness across infiltrated layer. The outer layer near the casting mould contains unavoidably shrinkage porosity because of chilling action that leads to imperfect fusion. The compactness of outer layer is not as good as that of sub-surface layer, and the hardness of the outer layer is not the highest. The hardness reaches the largest value in the compact sub-surface layer because its quality of fusion is better than the outer layer, and it contains
Fig. 6. Line scan of element distribution across infiltrated layer.
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Fig. 7. XRD of infiltrated layer.
more hard phases. The active atom decomposing from the melting of infiltration casting alloying powder located in the innermost part of infiltrated layer diffuses into the matrix to form the diffusion area with considerable thickness. The hard phase could not diffuse to this layer so its content drops in this region. The Cu-based solid solution is the main composition in this layer. That is the main factor for apparent decreasing in micro-hardness in this layer. 3.2.3. Adhesion strength Fig. 9 shows the bending strength of the specimen with infiltrated layer located at the bottom of the specimen. Two peak values of bending strength exist on the curve of Fig. 9
in contrast to the bending strength curve of the substrate (Fig. 10) which does not show a peak. Fig. 12 shows the SEM view of sample of Fig. 9 after the bend test. The diagonal line in the Fig. 12 is the inner edge of the infiltrated layer, and the arrow is the direction of load. The fracture plane of the infiltrated sample is only the outer layer or SSL. Its main composition is the hard phase (CrB), so the outer layer is more brittle than the inner two layers. The two load drops after the peaks suggest that the outer layer been destroyed completely. The bending strength has been improved by 46.3% under the same strain conditions as the substrate. Fig. 11 shows the bending strength of the specimen with infiltrated layer located on the top of the specimen. Unlike
Fig. 8. The variation of micro-hardness across the infiltrated layer.
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Fig. 9. The bending strength of specimen with infiltrated layer at the bottom.
Fig. 10. Bending strength of the substrate.
Fig. 9, there is evidence of yield point elongation or plastic deformation. The peak value occurs within a small range of strain. The SEM view of the fractured sample of Fig. 11 is shown in Fig. 13. The meaning of diagonal line and arrow in Fig. 13 is same to the Fig. 12. In Fig. 11, the yield point elongation appears due to the infiltrated layer. The fracture again occurred at outer layer when the infiltrated layer is on the top of the specimen (Fig. 13). At the beginning, the deformation of infiltrated layer can accommodate itself to that of substrate. As its accommodation reaches a limit, the yield
point elongation appears. With confined loading, the infiltrated layer tends to be destroyed because of the difference in strength and hardness between the infiltrated layer and the substrate. Table 2 shows the bending strength changing with the different thickness of infiltrated layer (SSL+MFL). The peak value increases to a different extent with the thickening of the layer when the infiltrated layer is at the bottom of the specimen. With the thicker infiltrated layer, the tensile strength of the infiltrated layer is higher. The stress state is tensile stress
Fig. 11. The bending strength of specimen with infiltrated layer on the top.
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G.-r. Yang et al. / Materials Science and Engineering A 399 (2005) 206–215 Table 2 The bend strength of specimen with infiltrated layer Thickness (mm)
σ(Pbottom ) (MPa)
σ(Ptop ) (MPa)
σ(etop ) (MPa)
1.4 1.6 1.8
67.80 76.84 77.97
213 195 181
75 75 75
σ(Pbottom ), the peak value with infiltrated layer on the bottom of the specimen; σ(Ptop ), the peak value with infiltrated layer on the top of the specimen; σ(etop ), the yielding value with infiltrated layer on the top of the specimen.
Fig. 12. Fracture SEM view of the specimen with infiltrated layer at the bottom (F = load).
when the infiltrated layer is at the bottom of the specimen. The bending strength of the specimen is mainly contributed by the infiltrated layer under the condition of the infiltrated layer being at the bottom of specimen. On the other hand,
Fig. 13. Fracture SEM view of the specimen with infiltrated layer on the top (F = load).
the peak value decreases to different extent with the thickening of the layer as the infiltrated layer is on the top of the specimen. The stress state is compressive stress as the infiltrated layer is on the top of the specimen. Because the infiltrated layer (mainly SSL) is composed of hard phases, it is brittle. When the infiltrated layer is thinner, the infiltrated layer can easily accommodate the smaller deformation with the bronze than that of the specimen with thicker infiltrated layer under the same deformation condition. Therefore, the bending strength of specimen with thicker infiltrated layer is somewhat lower than that of specimen with thinner infiltrated layer. The yield strength is constant for the infiltrated layer with different thickness. The yielding strength is the result of the interaction of the bronze and the infiltrated layer, which has nothing to do with the thickness of the infiltrated layer. During the test, each specimen was subjected to visual scrutiny after destroying. It was found that the failure mode was different for the infiltrated layer under tensile and compressive stress. The bending strength of specimen has been improved largely due to the existence of the infiltrated layer, especially for specimens with the layers on the top of substrate. During the three-point bending test, the infiltrated layer and the substrate interacted in a complex but positive way. 3.2.4. Thermal cycling The performance of the infiltrated layer under thermal cycling conditions is mainly governed by the stresses developed at the infiltrated layer/substrate interface because of the difference in the thermal expansion coefficients [23]. The response of the infiltrated layer to thermal cycling between 800 ◦ C and cool water temperature (10 ◦ C), is reported in Table 3. Figs. 14 and 15 are the SEM views of the surface and the interface of infiltrated layer showing appearance of crack. The endurance to thermal cycling for coatings fabricated using plasma spray technique is 50 cycles [6]. Therefore, it is interesting to note that the infiltrated layer offers good thermal fatigue resistance comparing with that of layer fabricated through plasma sprayed coatings [6]. The cycles of infiltrated layer with 1.2 mm thick is 82 as the micro-crack appeared on the surface of infiltrated layer and cycles being 89 as the cracks appeared in the interface, while the cycles of infiltrated layer with 1.8 mm thick is 79 as the micro-crack appeared on the surface of infiltrated layer and cycles being 83 as the cracks appeared in the interface. Therefore, the thermal fatigue resistance of thinner layer is somewhat better than that of thicker layer known from Table 3. The main problem
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Table 3 Results of the thermal cycling test Thickness of infiltrated layer (mm)
Cycles conducted
Observations
1.2
130
1.8
130
Substrate oxidation is detected after the 20th cycle; infiltrated layer remains fine. Test is conducted after the 35th; after the 82nd, the surface of infiltrated layer appears the micro-crack; cracks in the interface between the layer and substrate appear after the 89th; no obvious change after 130 times cycles. Substrate oxidation is detected after the 20th cycle; infiltrated layer remains fine. Test is conducted after the 35th; after the 79th, the surface of infiltrated layer appears the micro-crack; cracks in the interface between the layer and substrate appear after the 83rd; no obvious change after 130 times cycles.
arises due to the oxidation of the substrate. One thing to be considered is that the infiltrated layers are subjected to a very severe testing condition, as is normally done in accelerated tests, and these layers may not experience such severe conditions during working applications. It may be noted that all tests have been concluded because of a significant oxidation of the substrate. Even though the surface of the infiltrated layer cracks to a certain extent, they do not spall off. The outermost surface of the infiltrated layer is not fully compact, which may accommodate the thermal expansion coefficient mismatch more effectively [23] than compact layers. The Ni60 alloying powder contains Ni, Cr, B and Si. Their mean thermal expansion coefficient values [24] (Table 4) show that the thermal expansion coefficient of Ni is similar to that of the substrate Cu. The main element of the alloying powder Ni60 is Ni; the elements Cr, B and Si exist in state of intermetallic compounds. These compounds could not diffuse into the DSSL layer and only exist in the SFL layer and the outer MFL layer. From Fig. 6, it is noted that the main
Fig. 15. The crack on the surface of layer after thermal cycling of 92 times.
elements in DSSL are Ni and Cu, and therefore, the stress between the Ni and substrate is not large enough to cause cracks in the interface between the infiltrated layer and the substrate. Therefore, the micro-crack starts to appear on the surface of the infiltrated layer. 3.2.5. Oxidation Fig. 16 shows the weight gain at 800 ◦ C up to about 110 h for the substrate and infiltrated layer studied. It is clear that the oxidation rate of infiltrated layer is significantly slows. The mass gain curves can be divided in two parts, separated by a transition zone. The first stage, at very short oxidation Table 4 Mean thermal expansion coefficient α (◦ C × 10−6 ) [24]
Fig. 14. The crack in the interface after thermal cycling of 85 times.
Cu Ni Cr B Si
17.0 13.4 6.2 8.3 2.8–7.2
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three sub-layers of surface sintering layer (SSL), metallurgical fusing layer (MFL) and diffusion solid solution layer (DSSL). The macro-hardness of infiltrated hardness reaches about HRC60, and highest micro-hardness is HV520. The hardness of infiltrated layer has improved compared with the substrate. The three-point bending and thermal cycling tests suggest good bonding of the infiltrated layer and the substrate on account of the existence of MFL. The bending strength has been enhanced to a certain extent. The oxidation resistance of the infiltrated layer has been improved about three times compared to the substrate.
Acknowledgements Fig. 16. weight grain vs. oxidation time curves at 800 ◦ C.
times, is characterized by a rapid increase in the mass gain. For longer oxidation times, the oxidation rate tends to become constant. The main factors determining the oxidation behavior are the surface state and surface composition. The state of surface is the same for all the specimens. For the infiltrated layer, the main factor for the oxidation is the Ni and Cr contents. The oxidation resistance of Ni–Cr alloy is better than that of the pure Ni [25], and the oxidation kinetics of Ni is significantly slowed by the Cr alloying addition. This is because a rather porous NiO oxide layer develops on the surface of pure nickel, while the Cr2 O3 oxide layer forming on Ni–Cr alloys is inherently dense and coherent [26,27]. However, the infiltrated layer also contains Cu so that the weight gain of layer is somewhat higher than that of the pure Ni–Cr alloy. The high-temperature oxidation resistance of metals depends on a surface layer (scale) that acts as a barrier to oxygen transport. The dense oxides barrier would prevent the permeating of oxygen through the barrier to a certain extent which is effective to minimize the diffusion of oxygen to the underlying materials (substrate) [27]. The possible oxide species that can be developed on Ni–Cr alloys, include NiO, NiCr2 O4 , and Cr2 O3 , the prevalence of each varying with oxidation temperature and treatment [27]. For the infiltrated layer, the possible oxide species maybe include NiO, NiCr2 O4 , Cr2 O3 and CuO.
4. Conclusion A vacuum infiltration casting process can be used for the fabrication of infiltrated layer with excellent properties compared with substrate. Ni-based powder has been successfully used as infiltration alloying powder to obtain good infiltrated layer on copper substrate. The infiltrated layer has clearly excellent interfacial boundaries due to long-range diffusion of elements across the interface. The layer includes
The authors are pleased to acknowledge the financial support for this research provided by the National Natural Science Foundation of Gansu Project (ZS021A25-024-C), the Chun-Hui Plan of Education Department of China Project (Z2004-1-62013) and Young Teacher Startup Foundation project of Lan zhou University of Technology.
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An investigation of the structure and properties of infiltrated layer on the surface of copper alloy Gui-rong Yang ∗ , Yuan Hao, Wen-ming Song, Ying Ma State Key Loboratory of New Nonferrous Metal Materials, School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou, 730050 Gansu. PR China Received in revised form 1 March 2005; accepted 1 March 2005
Abstract This work reports about microstructure, hardness, adhesion strength, thermal fatigue and oxidation properties of the infiltrated layer on substrate of copper. The infiltrated layer was produced by vacuum infiltration casting technique using Ni-based powder as surface alloying particles. SEM observations indicated the formation of a compact infiltrated layer. The macro-hardness of the layer is about HRC 60 and the distribution of micro-hardness presents gradient change. The average micro-hardness of layer is about HV450. The transverse rupture strength (68–213 MPa) of the layer depends on layer thickness and layer orientation relative to loading direction. The layer appears to possess excellent oxidation resistance and superior thermal fatigue resistance at 800 ◦ C. © 2005 Elsevier B.V. All rights reserved. Keywords: Vacuum infiltration casting; Copper substrate; Microstructure; Properties of infiltrated layer
1. Introduction In a large number of engineering applications, service conditions vary with location. Consequently it is often useful to design the microstructure of a structural part to meet these varying requirements [1]. Copper with high electrical and thermal conductivity is widely used in optics and electronics; however, high strength and wear resistance are required in some applications. In order to improve wear resistance of copper, a number of methods have been developed, including internal oxidation [2], chemical vapor deposition [3,4], electroplating [5], thermal spray [6], infiltration [7] and other means [8,9]. Of these methods, infiltration technique has the advantages of simplicity and feasibility in changing the characteristic of copper surface. Infiltration of a liquid metal into a preform can be carried out by pressureless or pressure-assisted methods. The main advantages of pressureassisted infiltration include the following: (a) faster process; (b) near-net shape processing; (c) reduced contamination and (d) reduced porosity in comparison with spray techniques ∗
Corresponding author. Tel.: +86 931 2972809; fax: +86 931 2755806. E-mail address: [email protected] (G.-r. Yang).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.03.106
[10–21]. However, even in the case of pressure-assisted infiltration, choking of pores during infiltration can occur, which prematurely terminates the infiltration process [14]. This difficulty of achieving full infiltration decreases with increasing the particle size in the preform and increasing pouring temperature [22]. In this study, the bronze matrix surface alloying materials were fabricated by the vacuum infiltration casting technique. The present article deals with the performance evaluation of some infiltrated layers in terms of hardness, adhesion (bond) strength, thermal fatigue and oxidation resistance. In addition, the results of micro-structural analysis are reported. 2. Experimental procedures 2.1. Fabrication of infiltrated layer Casting alloy Ni60 particle of 120–160 mesh was chosen as the reinforcement, and bronze ZQAl 9-4 (Table 1), as the matrix of the surface alloying layer and the substrate material on which the infiltrated layer formed. The surface infiltrated layer was fabricated by a vacuum infiltration casting process. First, the Ni60 powder and some
G.-r. Yang et al. / Materials Science and Engineering A 399 (2005) 206–215 Table 1 The chemical composition of tested aluminum bronze (wt.%) Cu Al Fe Sb Si P As Sn Pb Mn Zn Total
8.0–10.0 2.0–4.0 ≤0.05 ≤0.20 ≤0.2 ≤0.1 ≤0.05 ≤0.2 ≤0.1 ≤0.5 ≤1.0 ≤1.0
Fig. 1. The procedure for forming infiltrated layer.
self-fabricated bond NJB (its main composition is boric acid) were mixed together according to desired proportion, paved on some inner surface of a casting mould or outer surface of mould cores, where strength and wear characteristics need improvement, then heated to harden so as to form preform layer. Second, molten bronze was poured at 1220 ◦ C into the mould under vacuum suction of the melt into the perform formed the surface infiltrated layer. The fabrication principle is shown in Fig. 1. 2.2. Microstructure and evaluation of properties 2.2.1. The microstructure of infiltrated layer The microstructure of surface infiltrated layer was observed by scanning electron microscope (SEM) of HITACHI
207
S–520. The specimens of surface layer for SEM observation were cut out along the direction from infiltrated layer surface to substrate, ground and polished using standard metallographic techniques. 2.2.2. Hardness and adhesion (bond) strength measurements The macro-hardness was measured using HRS-150 Rockwell hardness tester under 1471 N load. The micro-hardness was measured using HVS-1000 micro-hardness tester at the load of 9.8 N and loading time of 20 s. A three-point bend test was employed to evaluate the adhesion (bond) strength. The dimensions of the specimens used in the bend tests are 5 mm × 5 mm × 40 mm. The infiltrated layer was located either on the top or the bottom of the specimen. Fig. 2 shows the design of the specimens used for the three-point bend tests. Load is slowly applied to the specimen with the definite displacement. After loading, the stress–strain curve would be obtained, and it can be transformed to the load–displacement curve through software. This test was conducted on three specimens for each type of loading configuration. 2.2.3. Thermal cycling experiment The dimensions of the specimens used in the thermal cycling (thermal fatigue) experiment were 5 mm × 5 mm × 10 mm. For thermal cycling, the specimens were heated in a muffle furnace at 800 ◦ C for 15 min followed by water cooling for 5 min. After 5 min, the specimens were taken out of water when their temperature was same as that of water. A thorough visual scrutiny of the specimens was undertaken to determine the extent of damage they have undergone within the substrate, at the interface between the bond and the surface layer, and on the top surface of the infiltrated layer. A total of 130 thermal cycles were done. 2.2.4. Oxidation experiment The oxidation behavior of the infiltrated layer was evaluated by the weight changes. The six faces of rectangular test specimens (40 mm × 40 mm × 10 mm) were ground, ultrasonically cleaned in water, and then rinsed in ethanol prior to drying in air. The initial weight of specimens was measured using a photoelectric analytic balance with a precision of 0.1 mg. The specimens were heated in a muffle furnace kept
Fig. 2. The type of three-point bend test specimen for infiltrated layer: (a) infiltrated layer on the top of the specimen; (b) infiltrated layer at the bottom of the specimen.
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Fig. 3. A cross-sectional SEM view of infiltrated layer.
at 800 ◦ C for 2 h, followed by air cooling to room temperature, and weighing the specimens. This test was repeated until the weight change of specimens was near zero. The time of heating lasted for 4, 8, 12 and 24 h, respectively. The weight of the specimens was measured before and after each test to obtain the weight changes. The oxidation rate was calculated by dividing the weight change with the test duration. Three specimens were tested to get the average oxidation rate.
3. Results and discussion 3.1. Microstructure of infiltrated layer The cross-sectional views of the infiltrated layer are shown in Figs. 3–5. The micrographs reveal continuous and sound interface between the substrate and the infiltrated layer and no crack is present. No spalling or separation is even at high magnification. As seen in Fig. 3, the infiltrated layer consists of three parts: surface sintering layer (SSL), metallurgical fusing layer (MFL) and diffusion solid solution layer (DSSL). Figs. 4 and 5 show the morphology of SSL and DSSL, respectively. The formation of infiltrated layer is a kind of metallurgical fusion because the melting point of Ni60 powder is lower than that of copper alloy. After pouring the melt, the metal infiltrated into the pores and permeated the preform to the outer preform close to the mould surface. In the outermost layer, the Ni60 powder particles partially melt, and the copper can only sinter with the alloying powder at the original site. The composition of outer surface sintering layer is mainly composed of alloying powder with a small amount of copper. The temperature of the melt behind the infiltration on front is higher than that of the copper melt forming the SSL; therefore, the copper alloy and alloying powder formed metallurgical fusion layer (MFL) behind the SSL. The alloying powder located at the innermost surface of preform obtains
the most energy compared with the former two layers. The energy can melt the alloying powder, decompose and diffuse only to form many active atoms that may infiltrate directly into the substrate to form solid solution in the advantageous place. The distributions of Ni, Cr, and Cu across the substrate and infiltrated layer (Fig. 6) were continuous. The distribution of Ni and Cr gradually decreases from the outer layer to the inner layer. On the other hand, the distribution of Cu increases gradually toward the inner layer. The total layer thickness of DSSL and MFL is larger than that obtained through plating, spraying and vapor deposition because of faster diffusion in the liquid and longer diffusion distance [12–14]. The temperature of diffusion of other methods, such as heat treatment,
Fig. 4. The SEM view of outer layer.
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solid solution and Ni-based solid solution. The inner layer is mainly composed of Cu-based solid solution. 3.2. Hardness and adhesion (bond) strength 3.2.1. The macro-hardness The hardness of copper alloy is around HRB 130, while, the hardness of infiltrated layer is about HRC60.0. The hardness of infiltrated layer has been significantly improved. The reason of increasing hardness is the boride and Ni-based solid solution strengthened by Cr in the outer layer.
Fig. 5. The SEM view of the joint of substrate and infiltrated layer.
is lower than that of infiltration casting technique. The structure of the infiltrated surface material is promising from the point view of microstructure of structural materials because it has a transition layer (DSSL). Combining Figs. 6 and 7, the following conclusions can be drawn. The outer layer is composed of CrB and Ni-based solid solution. The metallurgical fusing layer (MFL) is composed of CrB, Cu-based
3.2.2. The micro-hardness The distribution of Ni, Cr, and Cu was displayed in Fig. 6. The contents of Ni and Cr are high in the outer and middle layer. The Ni content decreases gradually and the Cr content is very small in the inner layer because the diffusion coefficient of Cr is smaller than that of Ni [18,19]; therefore, Cr diffusion in infiltrated layer is imperceptible and the Ni content changes slowly from outer layer to the inner. The Ni60 alloy powder consists of Ni-based solid solution, boride and Ni–Cr alloy [15,20]. The surface layer containing these hard phases might be applied in the circumstances of wear, erosion, and oxidation-resistance, and of corrosion in air, seawater, steam and weak acid. Fig. 8 shows the variation of micro-hardness across infiltrated layer. The outer layer near the casting mould contains unavoidably shrinkage porosity because of chilling action that leads to imperfect fusion. The compactness of outer layer is not as good as that of sub-surface layer, and the hardness of the outer layer is not the highest. The hardness reaches the largest value in the compact sub-surface layer because its quality of fusion is better than the outer layer, and it contains
Fig. 6. Line scan of element distribution across infiltrated layer.
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Fig. 7. XRD of infiltrated layer.
more hard phases. The active atom decomposing from the melting of infiltration casting alloying powder located in the innermost part of infiltrated layer diffuses into the matrix to form the diffusion area with considerable thickness. The hard phase could not diffuse to this layer so its content drops in this region. The Cu-based solid solution is the main composition in this layer. That is the main factor for apparent decreasing in micro-hardness in this layer. 3.2.3. Adhesion strength Fig. 9 shows the bending strength of the specimen with infiltrated layer located at the bottom of the specimen. Two peak values of bending strength exist on the curve of Fig. 9
in contrast to the bending strength curve of the substrate (Fig. 10) which does not show a peak. Fig. 12 shows the SEM view of sample of Fig. 9 after the bend test. The diagonal line in the Fig. 12 is the inner edge of the infiltrated layer, and the arrow is the direction of load. The fracture plane of the infiltrated sample is only the outer layer or SSL. Its main composition is the hard phase (CrB), so the outer layer is more brittle than the inner two layers. The two load drops after the peaks suggest that the outer layer been destroyed completely. The bending strength has been improved by 46.3% under the same strain conditions as the substrate. Fig. 11 shows the bending strength of the specimen with infiltrated layer located on the top of the specimen. Unlike
Fig. 8. The variation of micro-hardness across the infiltrated layer.
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Fig. 9. The bending strength of specimen with infiltrated layer at the bottom.
Fig. 10. Bending strength of the substrate.
Fig. 9, there is evidence of yield point elongation or plastic deformation. The peak value occurs within a small range of strain. The SEM view of the fractured sample of Fig. 11 is shown in Fig. 13. The meaning of diagonal line and arrow in Fig. 13 is same to the Fig. 12. In Fig. 11, the yield point elongation appears due to the infiltrated layer. The fracture again occurred at outer layer when the infiltrated layer is on the top of the specimen (Fig. 13). At the beginning, the deformation of infiltrated layer can accommodate itself to that of substrate. As its accommodation reaches a limit, the yield
point elongation appears. With confined loading, the infiltrated layer tends to be destroyed because of the difference in strength and hardness between the infiltrated layer and the substrate. Table 2 shows the bending strength changing with the different thickness of infiltrated layer (SSL+MFL). The peak value increases to a different extent with the thickening of the layer when the infiltrated layer is at the bottom of the specimen. With the thicker infiltrated layer, the tensile strength of the infiltrated layer is higher. The stress state is tensile stress
Fig. 11. The bending strength of specimen with infiltrated layer on the top.
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G.-r. Yang et al. / Materials Science and Engineering A 399 (2005) 206–215 Table 2 The bend strength of specimen with infiltrated layer Thickness (mm)
σ(Pbottom ) (MPa)
σ(Ptop ) (MPa)
σ(etop ) (MPa)
1.4 1.6 1.8
67.80 76.84 77.97
213 195 181
75 75 75
σ(Pbottom ), the peak value with infiltrated layer on the bottom of the specimen; σ(Ptop ), the peak value with infiltrated layer on the top of the specimen; σ(etop ), the yielding value with infiltrated layer on the top of the specimen.
Fig. 12. Fracture SEM view of the specimen with infiltrated layer at the bottom (F = load).
when the infiltrated layer is at the bottom of the specimen. The bending strength of the specimen is mainly contributed by the infiltrated layer under the condition of the infiltrated layer being at the bottom of specimen. On the other hand,
Fig. 13. Fracture SEM view of the specimen with infiltrated layer on the top (F = load).
the peak value decreases to different extent with the thickening of the layer as the infiltrated layer is on the top of the specimen. The stress state is compressive stress as the infiltrated layer is on the top of the specimen. Because the infiltrated layer (mainly SSL) is composed of hard phases, it is brittle. When the infiltrated layer is thinner, the infiltrated layer can easily accommodate the smaller deformation with the bronze than that of the specimen with thicker infiltrated layer under the same deformation condition. Therefore, the bending strength of specimen with thicker infiltrated layer is somewhat lower than that of specimen with thinner infiltrated layer. The yield strength is constant for the infiltrated layer with different thickness. The yielding strength is the result of the interaction of the bronze and the infiltrated layer, which has nothing to do with the thickness of the infiltrated layer. During the test, each specimen was subjected to visual scrutiny after destroying. It was found that the failure mode was different for the infiltrated layer under tensile and compressive stress. The bending strength of specimen has been improved largely due to the existence of the infiltrated layer, especially for specimens with the layers on the top of substrate. During the three-point bending test, the infiltrated layer and the substrate interacted in a complex but positive way. 3.2.4. Thermal cycling The performance of the infiltrated layer under thermal cycling conditions is mainly governed by the stresses developed at the infiltrated layer/substrate interface because of the difference in the thermal expansion coefficients [23]. The response of the infiltrated layer to thermal cycling between 800 ◦ C and cool water temperature (10 ◦ C), is reported in Table 3. Figs. 14 and 15 are the SEM views of the surface and the interface of infiltrated layer showing appearance of crack. The endurance to thermal cycling for coatings fabricated using plasma spray technique is 50 cycles [6]. Therefore, it is interesting to note that the infiltrated layer offers good thermal fatigue resistance comparing with that of layer fabricated through plasma sprayed coatings [6]. The cycles of infiltrated layer with 1.2 mm thick is 82 as the micro-crack appeared on the surface of infiltrated layer and cycles being 89 as the cracks appeared in the interface, while the cycles of infiltrated layer with 1.8 mm thick is 79 as the micro-crack appeared on the surface of infiltrated layer and cycles being 83 as the cracks appeared in the interface. Therefore, the thermal fatigue resistance of thinner layer is somewhat better than that of thicker layer known from Table 3. The main problem
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Table 3 Results of the thermal cycling test Thickness of infiltrated layer (mm)
Cycles conducted
Observations
1.2
130
1.8
130
Substrate oxidation is detected after the 20th cycle; infiltrated layer remains fine. Test is conducted after the 35th; after the 82nd, the surface of infiltrated layer appears the micro-crack; cracks in the interface between the layer and substrate appear after the 89th; no obvious change after 130 times cycles. Substrate oxidation is detected after the 20th cycle; infiltrated layer remains fine. Test is conducted after the 35th; after the 79th, the surface of infiltrated layer appears the micro-crack; cracks in the interface between the layer and substrate appear after the 83rd; no obvious change after 130 times cycles.
arises due to the oxidation of the substrate. One thing to be considered is that the infiltrated layers are subjected to a very severe testing condition, as is normally done in accelerated tests, and these layers may not experience such severe conditions during working applications. It may be noted that all tests have been concluded because of a significant oxidation of the substrate. Even though the surface of the infiltrated layer cracks to a certain extent, they do not spall off. The outermost surface of the infiltrated layer is not fully compact, which may accommodate the thermal expansion coefficient mismatch more effectively [23] than compact layers. The Ni60 alloying powder contains Ni, Cr, B and Si. Their mean thermal expansion coefficient values [24] (Table 4) show that the thermal expansion coefficient of Ni is similar to that of the substrate Cu. The main element of the alloying powder Ni60 is Ni; the elements Cr, B and Si exist in state of intermetallic compounds. These compounds could not diffuse into the DSSL layer and only exist in the SFL layer and the outer MFL layer. From Fig. 6, it is noted that the main
Fig. 15. The crack on the surface of layer after thermal cycling of 92 times.
elements in DSSL are Ni and Cu, and therefore, the stress between the Ni and substrate is not large enough to cause cracks in the interface between the infiltrated layer and the substrate. Therefore, the micro-crack starts to appear on the surface of the infiltrated layer. 3.2.5. Oxidation Fig. 16 shows the weight gain at 800 ◦ C up to about 110 h for the substrate and infiltrated layer studied. It is clear that the oxidation rate of infiltrated layer is significantly slows. The mass gain curves can be divided in two parts, separated by a transition zone. The first stage, at very short oxidation Table 4 Mean thermal expansion coefficient α (◦ C × 10−6 ) [24]
Fig. 14. The crack in the interface after thermal cycling of 85 times.
Cu Ni Cr B Si
17.0 13.4 6.2 8.3 2.8–7.2
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three sub-layers of surface sintering layer (SSL), metallurgical fusing layer (MFL) and diffusion solid solution layer (DSSL). The macro-hardness of infiltrated hardness reaches about HRC60, and highest micro-hardness is HV520. The hardness of infiltrated layer has improved compared with the substrate. The three-point bending and thermal cycling tests suggest good bonding of the infiltrated layer and the substrate on account of the existence of MFL. The bending strength has been enhanced to a certain extent. The oxidation resistance of the infiltrated layer has been improved about three times compared to the substrate.
Acknowledgements Fig. 16. weight grain vs. oxidation time curves at 800 ◦ C.
times, is characterized by a rapid increase in the mass gain. For longer oxidation times, the oxidation rate tends to become constant. The main factors determining the oxidation behavior are the surface state and surface composition. The state of surface is the same for all the specimens. For the infiltrated layer, the main factor for the oxidation is the Ni and Cr contents. The oxidation resistance of Ni–Cr alloy is better than that of the pure Ni [25], and the oxidation kinetics of Ni is significantly slowed by the Cr alloying addition. This is because a rather porous NiO oxide layer develops on the surface of pure nickel, while the Cr2 O3 oxide layer forming on Ni–Cr alloys is inherently dense and coherent [26,27]. However, the infiltrated layer also contains Cu so that the weight gain of layer is somewhat higher than that of the pure Ni–Cr alloy. The high-temperature oxidation resistance of metals depends on a surface layer (scale) that acts as a barrier to oxygen transport. The dense oxides barrier would prevent the permeating of oxygen through the barrier to a certain extent which is effective to minimize the diffusion of oxygen to the underlying materials (substrate) [27]. The possible oxide species that can be developed on Ni–Cr alloys, include NiO, NiCr2 O4 , and Cr2 O3 , the prevalence of each varying with oxidation temperature and treatment [27]. For the infiltrated layer, the possible oxide species maybe include NiO, NiCr2 O4 , Cr2 O3 and CuO.
4. Conclusion A vacuum infiltration casting process can be used for the fabrication of infiltrated layer with excellent properties compared with substrate. Ni-based powder has been successfully used as infiltration alloying powder to obtain good infiltrated layer on copper substrate. The infiltrated layer has clearly excellent interfacial boundaries due to long-range diffusion of elements across the interface. The layer includes
The authors are pleased to acknowledge the financial support for this research provided by the National Natural Science Foundation of Gansu Project (ZS021A25-024-C), the Chun-Hui Plan of Education Department of China Project (Z2004-1-62013) and Young Teacher Startup Foundation project of Lan zhou University of Technology.
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