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Microstructure and dry-sliding wear properties of TiC-reinforced composite coating prepared by plasma-transferred arc weld-surfacing process
Materials Science and Engineering A 458 (2007) 366–370
Microstructure and dry-sliding wear properties of TiC-reinforced composite coating prepared by plasma-transferred arc weld-surfacing process Y.F. Liu a,∗ , J.S. Mu a , X.Y. Xu a , S.Z. Yang b a
Institute of Materials Science and Engineering, Beijing Jiaotong University, No. 3 of Shangyuan Residence Haid, Beijing 100044, PR China b Institute of Physics, Chinese Academy of Science, Beijing 100083, PR China Received 13 September 2006; received in revised form 13 December 2006; accepted 19 December 2006
Abstract A wear resistant TiC titanium carbide-reinforced composite coating was fabricated on 1Cr18Ni9Ti austenitic stainless steel substrate by plasmatransferred arc (PTA) weld-surfacing process using Fe–Ti–C powder blends. The microstructure, microhardness and dry-sliding wear behavior of the composite coating were investigated using optical microscopy (OM), X-ray diffraction (XRD), scanning electron micrograph (SEM), energydispersive X-ray analysis (EDS), microhardness tester and ring-on-ring wear tester. The formation mechanism of the composite coating has been discussed. Results show that the composite coating consists of primary TiC carbide as the reinforcing phase and TiC/␥-Fe eutectics as the matrix. The composite coating is metallurgically bonded to the 1Cr18Ni9Ti austenitic stainless steel substrate. The TiC/␥-Fe composite coating has high hardness and excellent wear resistance under dry-sliding wear test condition. © 2007 Elsevier B.V. All rights reserved. Keywords: Plasma-transferred arc weld-surfacing; Microstructure; Microhardness; Wear
1. Introduction Austenitic stainless steel 1Cr18Ni9Ti is widely used in chemical, petrochemical, nuclear reactor and other corrosive conditions owing to its exceptional corrosion resistance. However, with a low hardness and an austenitic structure, which can not be hardened by heat treatment, the wear resistance of the austenitic stainless steel is very poor, which restricting it for application as engineering tribological components. Since wear start from the surface of the work pieces, from both economic and technological points of view, to deposit high-performance wear resistant coating on the surface of tribological components with appropriate surface engineering technology is one of the most efficient approaches to enhance the tribological performance for austenitic stainless steel and other materials with poor wear resistance. Among the widely used surface engineering technologies, the plasma-transferred arc weld-surfacing process has many good advantages such
∗
Corresponding author. Tel.: +86 10 51687034; fax: +86 10 51683300. E-mail address: [email protected] (Y.F. Liu).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.086
as extremely high temperature, excellent arc stability, low thermal distortion of the part, high weld-surfacing speeds, high energy exchanging efficiency, low cost in equipment and operation (comparing with electron alloying, laser alloying and laser cladding) [1–9]. The plasma-transferred arc weld-surfacing process attracted more and more attentions and many research works have been conducted in this area [10–18]. TiC carbide as one of common ceramics has many excellent advantages, such as very high melting point and thermal stability, high hardness and excellent wear resistance, low coefficient of friction, high electrical and thermal conductivities. TiC carbide is widely used as the reinforcing phase of the wear-resistant coating for cutting tools and as diffusion barriers in semiconductor technology [19–28]. Because of its high melting point, good resistance to thermal shock, and good chemical and physical sputtering properties, TiC is also a promising material to be used in fusion reactors. The iron base solid solution ␥-Fe has excellent ductility and good compatibility with TiC. The composite coating with hard and wear resistant primary TiC carbide as the reinforcing primary phase and the strong and ductile ␥-Fe/TiC eutectics as the
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matrix would be expected to present outstanding wear resistance under dry-sliding wear condition. To highly improve the poor wear resistance of austenitic stainless steel, in this paper, PTA weld-surfacing process was adopted to produce TiC carbide-reinforced composite coating on 1Cr18Ni9Ti austenitic stainless steel substrate using commercial pure Fe, Ti, C elemental powder blends. The microstructure and chemical composition of the composite coating was characterized by optical microscope, scanning electron microscope, X-ray diffraction and energy-dispersive X-ray analysis. The wear resistance of the composite coating was evaluated under dry-sliding wear test conditions using ring-on-ring wear tester at ambient temperature. Fig. 1. Schematic illustration of the ring-on-ring dry-sliding wear tester.
2. Experimental procedures The experiment of PTA weld-surfacing was performed on DRF-1 PTA processing system. The operating principle of the DRF-1 PTA weld-surfacing processing system is detailed described in our previous work [18]. PTA weld-surfacing processing parameters have a notable influence on the coating features. With a definite output power, if the PTA scanning speed is too low, the substrate will be melted excessively, leading to excessive dilution of the coating by the substrate. On the contrary, under a definite output power, if the PTA scanning speed is too high, the substrate will be inadequately melted, leading to no metallurgical bonding to substrate of the coating. Aiming at acquiring high quality coating, consulting our previous work [18], the PTA processing parameters was optimized and the optimized parameters were listed in Table 1. Commercial pure Fe, Ti, C elemental powder blends in chemical composition (wt.%) of Fe–38Ti–4.5C and with an average particle size ranging from 70 to 140 m was selected as the precursor materials for fabricating the TiC-reinforced composite coating by the PTA weld-surfacing process. A commercial 1Cr18Ni9Ti austenitic stainless steel (initial microhardness 200 HV) with chemical composition 0.12% C, 18% Cr, 9% Ni, 0.8% Ti and the balance iron, 100 mm × 20 mm × 20 mm in size, was selected as the substrate material. Metallographic and wear testing specimens were machined by electric discharging machine. Metallographic samples were prepared using standard mechanical polishing procedures and were chemically etched using the etchant of HF, HNO3 and H2 O in volume ratio of 1:6:7. Microstructure of the coating was characterized using the optical microscope Nephot II and the scanning electron microscope S-4200. Phases presented in the coating were identified by X-ray diffraction using the Rigaku D/max 2200 pc automatic Table 1 The PTA weld-surfacing process parameters Working electric current (A) Working voltage (V) Scanning speed (mm/min) Powder feeding rate (g/min) Feeding gas flux (Ar, l/min) Working gas flux (Ar, l/min)
180 30 300 50 6 6
X-ray diffractometer with Cu K␣ radiation and the chemical composition of the phases was analyzed by energy-dispersive spectrometer Noran Ventage DSI. The hardness profile along the depth direction of the coating was measured by Vickers hardness tester with a testing load of 1.96 N and a loading time of 15 s. Dry-sliding wear resistance of the coating was evaluated with ring-on-ring dry-sliding wear tester (as shown in Fig. 1). The hardened GCr15 bearing steel (790 HV) upper ring seats on the rotating lower ring. The mating surface of the lower ring was coated with the TiC-reinforced composite coating produced by PTA weld-surfacing process. The wear test parameters were: load 196 N, average relative linear speed 0.13 m/s, total sliding distance 458 m. The wear mass loss was measured by electronic balance with accuracy of 0.1 mg. The original 1Cr18Ni9Ti austenitic stainless steel was selected as the reference material. Relative wear resistance, i.e. the ratio of wear mass loss of the 1Cr18Ni9Ti austenitic stainless steel specimen to that of the TiC-reinforced composite coating was utilized to judge the wear resistance. Both the hardness test results and the wear test results were determined from three specimens. 3. Results and discussion 3.1. Microstructure Fig. 2 shows the XRD pattern of the composite coating produced by PTA weld-surfacing process. After PTA weldsurfacing with (wt.%) Fe–38Ti–4.5C elemental powder blends as the precursor material, a novel composite coating mainly composed of TiC and ␥-Fe phases was produced on 1Cr18Ni9Ti austenitic stainless steel substrate. Typical microstructure of the PTA weld-surfacing composite coating is shown in Fig. 3(a) and (b). It can be seen, the composite coating has a very fine microstructure consisting of predominant primary phases and a small amount of interdendritic eutectics. The primary phases are mainly in the form of “island-like”. Small fractions of the primary phases that distributed near the “island-like” primary phases are in the form of “dot-like”. Volume fraction of the primary phase is approximately 65%. EDS analysis indicates that the average chemical
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Fig. 2. XRD pattern of the composite coating produced by PTA weld-surfacing process.
composition (at.%) of the primary phase and the interdendritic eutectics are: 47.82C–48.31Ti–2.25Fe–0.28Ni–1.34Cr and 6.35Ti–6.21C–84.14Fe–3.28Cr–1.02Ni, respectively. According to both the XRD and EDS analysis results, the primary phases are identified as TiC titanium carbide. The interdendritic eutectics consist of TiC and iron base solid solution ␥-Fe. Because the TiC carbide has the highest melting point and the most negative free energy of formation in the Fe–Ti–C system, during the solidification process, the TiC carbide is separated first from the melt. As the solidification proceeds, the residual melt becomes progressively poor in titanium and carbon. When the chemical composition and the temperature of the melt reach the eutectic valley, eutectic reaction takes place to form the eutectic TiC/␥-Fe. With good heat conductivity of the austenitic stainless steel substrate, the solidification process is very fast leading to both the TiC primary and the TiC/␥-Fe eutectics very fine, as shown in Fig. 3. It can be rationally presumed that due to the very fine microstructure, the strength and toughness of the composite coating will be highly enhanced and the wear behavior of the coating will be highly improved. The XRD and EDS analysis results are consistent with the microstructural formation analysis. Fig. 4 shows the microstructural characteristics of the bonding zone near the fusion line. The ␥-Fe dendrites grow directionally and epitaxially from the austenitic stainless steel substrate into the TiC carbide-reinforced composite coating,
Fig. 4. SEM image showing the high quality metallurgical bonding of the TiC-reinforced composite coating to the 1Cr18Ni9Ti austenitic stainless steel substrate.
Fig. 5. Microhardness profile in a cross-section of the TiC-reinforced composite coating.
which indicates that the bonding of the composite coating to the 1Cr18Ni9Ti austenitic stainless steel substrate is of high quality. 3.2. Microhardness Fig. 5 indicates the microhardness profile along the depth direction of the TiC carbide-reinforced composite coating produced by PTA weld-surfacing process. Because of the presence of a high volume fraction of the hard primary TiC dendrites
Fig. 3. OM (a) and SEM (b) micrographs showing the typical microstructures of the TiC-reinforced composite coating produced by PTA weld-surfacing process.
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Fig. 6. SEM micrographs showing the worn surface morphologies of 1Cr18Ni9Ti austenitic stainless steel (a) and TiC-reinforced composite coating (b) after dry-sliding wear test.
which are uniformly distributed in the TiC/␥-Fe eutectic matrix, the composite coatings have an average hardness of approximately 850 HV and uniform hardness distributions within the coatings except in the coating/substrate bonding zone where the hardness decreases gradually to the substrate. 3.3. Wear resistance The dry-sliding wear test results indicate that the relative wear resistance of the TiC carbide-reinforced composite coating produced by PTA weld-surfacing is about 18 times higher than that of the austenitic stainless steel reference specimen. The morphologies of the worn surface for the austenitic stainless steel and the TiC carbide-reinforced composite coating are shown in Fig. 6(a) and (b), respectively. During dry-sliding wear, the austenitic stainless steel reference specimen suffered severe adhesive wear. Its worn surface is very rough with numerous adhesive craters, deep ploughing grooves, as shown in Fig. 6(a). On the contrary, the worn surface of the TiC carbide-reinforced composite coating is very smooth with only slight scratches and some “patch-like” transferred cover layer, as shown in Fig. 6(b). Fig. 7 indicates the variations of friction coefficient with test time of the TiC carbide-reinforced composite coating and the austenitic stainless steel under dry-sliding wear test condition. The friction coefficient of the TiC carbide-reinforced composite
coating oscillates around approximately 0.37, which is obvious smaller than that of the austenitic stainless steel specimen (oscillates around 0.53). It can be concluded that the TiC carbide-reinforced composite coating produced by PTA weld-surfacing has excellent friction and wear properties under dry-sliding wear test condition. In the TiC carbide-reinforced composite coating, the hard, wear-resistant primary TiC carbides are uniformly distributed in the fine, strong and ductile TiC/␥-Fe eutectic matrix. With novel microstructure characteristic, the composite coating has excellent wear resistance under dry-sliding test condition. This is firmly evidenced by the wear test results, the very smooth worn surface with little adhesive worn morphologies and the considerably low friction coefficient. In the TiC carbide-reinforced composite coating, the primary TiC carbides play the dominating wear resisting role in the dry-sliding wear process. In the meantime, the hard and wear resistant primary TiC carbides were firmly supported by the fine, strong and ductile TiC/␥-Fe eutectic matrix. 4. Conclusions Hard and wear resistant TiC carbide-reinforced composite coating was successfully fabricated on a substrate of 1Cr18Ni9Ti austenitic stainless steel by the PTA weld-surfacing process using (wt.%) Fe–38Ti–4.5C powder blends as the precursor materials. The composite coating consists of predominantly TiC primary phase and small amount of interdendritic eutectics TiC/␥-Fe. The composite coating is metallurgically bonded to the 1Cr18Ni9Ti austenitic stainless steel substrate. The microstructural characteristic of the coating is that large amount of primary TiC carbide uniformly distributed in the strong and ductile TiC/␥-Fe eutectics matrix. The composite coating has a high and uniform hardness distribution and exhibits excellent wear resistance under dry-sliding wear test condition. Acknowledgement
Fig. 7. Friction coefficient as a function of test time for the TiC carbidereinforced composite coating and the 1Cr18Ni9Ti austenitic stainless.
The research was supported by the National High Technology Research and Development Program of China (grant no. 2003AA331190).
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Microstructure and dry-sliding wear properties of TiC-reinforced composite coating prepared by plasma-transferred arc weld-surfacing process Y.F. Liu a,∗ , J.S. Mu a , X.Y. Xu a , S.Z. Yang b a
Institute of Materials Science and Engineering, Beijing Jiaotong University, No. 3 of Shangyuan Residence Haid, Beijing 100044, PR China b Institute of Physics, Chinese Academy of Science, Beijing 100083, PR China Received 13 September 2006; received in revised form 13 December 2006; accepted 19 December 2006
Abstract A wear resistant TiC titanium carbide-reinforced composite coating was fabricated on 1Cr18Ni9Ti austenitic stainless steel substrate by plasmatransferred arc (PTA) weld-surfacing process using Fe–Ti–C powder blends. The microstructure, microhardness and dry-sliding wear behavior of the composite coating were investigated using optical microscopy (OM), X-ray diffraction (XRD), scanning electron micrograph (SEM), energydispersive X-ray analysis (EDS), microhardness tester and ring-on-ring wear tester. The formation mechanism of the composite coating has been discussed. Results show that the composite coating consists of primary TiC carbide as the reinforcing phase and TiC/␥-Fe eutectics as the matrix. The composite coating is metallurgically bonded to the 1Cr18Ni9Ti austenitic stainless steel substrate. The TiC/␥-Fe composite coating has high hardness and excellent wear resistance under dry-sliding wear test condition. © 2007 Elsevier B.V. All rights reserved. Keywords: Plasma-transferred arc weld-surfacing; Microstructure; Microhardness; Wear
1. Introduction Austenitic stainless steel 1Cr18Ni9Ti is widely used in chemical, petrochemical, nuclear reactor and other corrosive conditions owing to its exceptional corrosion resistance. However, with a low hardness and an austenitic structure, which can not be hardened by heat treatment, the wear resistance of the austenitic stainless steel is very poor, which restricting it for application as engineering tribological components. Since wear start from the surface of the work pieces, from both economic and technological points of view, to deposit high-performance wear resistant coating on the surface of tribological components with appropriate surface engineering technology is one of the most efficient approaches to enhance the tribological performance for austenitic stainless steel and other materials with poor wear resistance. Among the widely used surface engineering technologies, the plasma-transferred arc weld-surfacing process has many good advantages such
∗
Corresponding author. Tel.: +86 10 51687034; fax: +86 10 51683300. E-mail address: [email protected] (Y.F. Liu).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.086
as extremely high temperature, excellent arc stability, low thermal distortion of the part, high weld-surfacing speeds, high energy exchanging efficiency, low cost in equipment and operation (comparing with electron alloying, laser alloying and laser cladding) [1–9]. The plasma-transferred arc weld-surfacing process attracted more and more attentions and many research works have been conducted in this area [10–18]. TiC carbide as one of common ceramics has many excellent advantages, such as very high melting point and thermal stability, high hardness and excellent wear resistance, low coefficient of friction, high electrical and thermal conductivities. TiC carbide is widely used as the reinforcing phase of the wear-resistant coating for cutting tools and as diffusion barriers in semiconductor technology [19–28]. Because of its high melting point, good resistance to thermal shock, and good chemical and physical sputtering properties, TiC is also a promising material to be used in fusion reactors. The iron base solid solution ␥-Fe has excellent ductility and good compatibility with TiC. The composite coating with hard and wear resistant primary TiC carbide as the reinforcing primary phase and the strong and ductile ␥-Fe/TiC eutectics as the
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matrix would be expected to present outstanding wear resistance under dry-sliding wear condition. To highly improve the poor wear resistance of austenitic stainless steel, in this paper, PTA weld-surfacing process was adopted to produce TiC carbide-reinforced composite coating on 1Cr18Ni9Ti austenitic stainless steel substrate using commercial pure Fe, Ti, C elemental powder blends. The microstructure and chemical composition of the composite coating was characterized by optical microscope, scanning electron microscope, X-ray diffraction and energy-dispersive X-ray analysis. The wear resistance of the composite coating was evaluated under dry-sliding wear test conditions using ring-on-ring wear tester at ambient temperature. Fig. 1. Schematic illustration of the ring-on-ring dry-sliding wear tester.
2. Experimental procedures The experiment of PTA weld-surfacing was performed on DRF-1 PTA processing system. The operating principle of the DRF-1 PTA weld-surfacing processing system is detailed described in our previous work [18]. PTA weld-surfacing processing parameters have a notable influence on the coating features. With a definite output power, if the PTA scanning speed is too low, the substrate will be melted excessively, leading to excessive dilution of the coating by the substrate. On the contrary, under a definite output power, if the PTA scanning speed is too high, the substrate will be inadequately melted, leading to no metallurgical bonding to substrate of the coating. Aiming at acquiring high quality coating, consulting our previous work [18], the PTA processing parameters was optimized and the optimized parameters were listed in Table 1. Commercial pure Fe, Ti, C elemental powder blends in chemical composition (wt.%) of Fe–38Ti–4.5C and with an average particle size ranging from 70 to 140 m was selected as the precursor materials for fabricating the TiC-reinforced composite coating by the PTA weld-surfacing process. A commercial 1Cr18Ni9Ti austenitic stainless steel (initial microhardness 200 HV) with chemical composition 0.12% C, 18% Cr, 9% Ni, 0.8% Ti and the balance iron, 100 mm × 20 mm × 20 mm in size, was selected as the substrate material. Metallographic and wear testing specimens were machined by electric discharging machine. Metallographic samples were prepared using standard mechanical polishing procedures and were chemically etched using the etchant of HF, HNO3 and H2 O in volume ratio of 1:6:7. Microstructure of the coating was characterized using the optical microscope Nephot II and the scanning electron microscope S-4200. Phases presented in the coating were identified by X-ray diffraction using the Rigaku D/max 2200 pc automatic Table 1 The PTA weld-surfacing process parameters Working electric current (A) Working voltage (V) Scanning speed (mm/min) Powder feeding rate (g/min) Feeding gas flux (Ar, l/min) Working gas flux (Ar, l/min)
180 30 300 50 6 6
X-ray diffractometer with Cu K␣ radiation and the chemical composition of the phases was analyzed by energy-dispersive spectrometer Noran Ventage DSI. The hardness profile along the depth direction of the coating was measured by Vickers hardness tester with a testing load of 1.96 N and a loading time of 15 s. Dry-sliding wear resistance of the coating was evaluated with ring-on-ring dry-sliding wear tester (as shown in Fig. 1). The hardened GCr15 bearing steel (790 HV) upper ring seats on the rotating lower ring. The mating surface of the lower ring was coated with the TiC-reinforced composite coating produced by PTA weld-surfacing process. The wear test parameters were: load 196 N, average relative linear speed 0.13 m/s, total sliding distance 458 m. The wear mass loss was measured by electronic balance with accuracy of 0.1 mg. The original 1Cr18Ni9Ti austenitic stainless steel was selected as the reference material. Relative wear resistance, i.e. the ratio of wear mass loss of the 1Cr18Ni9Ti austenitic stainless steel specimen to that of the TiC-reinforced composite coating was utilized to judge the wear resistance. Both the hardness test results and the wear test results were determined from three specimens. 3. Results and discussion 3.1. Microstructure Fig. 2 shows the XRD pattern of the composite coating produced by PTA weld-surfacing process. After PTA weldsurfacing with (wt.%) Fe–38Ti–4.5C elemental powder blends as the precursor material, a novel composite coating mainly composed of TiC and ␥-Fe phases was produced on 1Cr18Ni9Ti austenitic stainless steel substrate. Typical microstructure of the PTA weld-surfacing composite coating is shown in Fig. 3(a) and (b). It can be seen, the composite coating has a very fine microstructure consisting of predominant primary phases and a small amount of interdendritic eutectics. The primary phases are mainly in the form of “island-like”. Small fractions of the primary phases that distributed near the “island-like” primary phases are in the form of “dot-like”. Volume fraction of the primary phase is approximately 65%. EDS analysis indicates that the average chemical
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Fig. 2. XRD pattern of the composite coating produced by PTA weld-surfacing process.
composition (at.%) of the primary phase and the interdendritic eutectics are: 47.82C–48.31Ti–2.25Fe–0.28Ni–1.34Cr and 6.35Ti–6.21C–84.14Fe–3.28Cr–1.02Ni, respectively. According to both the XRD and EDS analysis results, the primary phases are identified as TiC titanium carbide. The interdendritic eutectics consist of TiC and iron base solid solution ␥-Fe. Because the TiC carbide has the highest melting point and the most negative free energy of formation in the Fe–Ti–C system, during the solidification process, the TiC carbide is separated first from the melt. As the solidification proceeds, the residual melt becomes progressively poor in titanium and carbon. When the chemical composition and the temperature of the melt reach the eutectic valley, eutectic reaction takes place to form the eutectic TiC/␥-Fe. With good heat conductivity of the austenitic stainless steel substrate, the solidification process is very fast leading to both the TiC primary and the TiC/␥-Fe eutectics very fine, as shown in Fig. 3. It can be rationally presumed that due to the very fine microstructure, the strength and toughness of the composite coating will be highly enhanced and the wear behavior of the coating will be highly improved. The XRD and EDS analysis results are consistent with the microstructural formation analysis. Fig. 4 shows the microstructural characteristics of the bonding zone near the fusion line. The ␥-Fe dendrites grow directionally and epitaxially from the austenitic stainless steel substrate into the TiC carbide-reinforced composite coating,
Fig. 4. SEM image showing the high quality metallurgical bonding of the TiC-reinforced composite coating to the 1Cr18Ni9Ti austenitic stainless steel substrate.
Fig. 5. Microhardness profile in a cross-section of the TiC-reinforced composite coating.
which indicates that the bonding of the composite coating to the 1Cr18Ni9Ti austenitic stainless steel substrate is of high quality. 3.2. Microhardness Fig. 5 indicates the microhardness profile along the depth direction of the TiC carbide-reinforced composite coating produced by PTA weld-surfacing process. Because of the presence of a high volume fraction of the hard primary TiC dendrites
Fig. 3. OM (a) and SEM (b) micrographs showing the typical microstructures of the TiC-reinforced composite coating produced by PTA weld-surfacing process.
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Fig. 6. SEM micrographs showing the worn surface morphologies of 1Cr18Ni9Ti austenitic stainless steel (a) and TiC-reinforced composite coating (b) after dry-sliding wear test.
which are uniformly distributed in the TiC/␥-Fe eutectic matrix, the composite coatings have an average hardness of approximately 850 HV and uniform hardness distributions within the coatings except in the coating/substrate bonding zone where the hardness decreases gradually to the substrate. 3.3. Wear resistance The dry-sliding wear test results indicate that the relative wear resistance of the TiC carbide-reinforced composite coating produced by PTA weld-surfacing is about 18 times higher than that of the austenitic stainless steel reference specimen. The morphologies of the worn surface for the austenitic stainless steel and the TiC carbide-reinforced composite coating are shown in Fig. 6(a) and (b), respectively. During dry-sliding wear, the austenitic stainless steel reference specimen suffered severe adhesive wear. Its worn surface is very rough with numerous adhesive craters, deep ploughing grooves, as shown in Fig. 6(a). On the contrary, the worn surface of the TiC carbide-reinforced composite coating is very smooth with only slight scratches and some “patch-like” transferred cover layer, as shown in Fig. 6(b). Fig. 7 indicates the variations of friction coefficient with test time of the TiC carbide-reinforced composite coating and the austenitic stainless steel under dry-sliding wear test condition. The friction coefficient of the TiC carbide-reinforced composite
coating oscillates around approximately 0.37, which is obvious smaller than that of the austenitic stainless steel specimen (oscillates around 0.53). It can be concluded that the TiC carbide-reinforced composite coating produced by PTA weld-surfacing has excellent friction and wear properties under dry-sliding wear test condition. In the TiC carbide-reinforced composite coating, the hard, wear-resistant primary TiC carbides are uniformly distributed in the fine, strong and ductile TiC/␥-Fe eutectic matrix. With novel microstructure characteristic, the composite coating has excellent wear resistance under dry-sliding test condition. This is firmly evidenced by the wear test results, the very smooth worn surface with little adhesive worn morphologies and the considerably low friction coefficient. In the TiC carbide-reinforced composite coating, the primary TiC carbides play the dominating wear resisting role in the dry-sliding wear process. In the meantime, the hard and wear resistant primary TiC carbides were firmly supported by the fine, strong and ductile TiC/␥-Fe eutectic matrix. 4. Conclusions Hard and wear resistant TiC carbide-reinforced composite coating was successfully fabricated on a substrate of 1Cr18Ni9Ti austenitic stainless steel by the PTA weld-surfacing process using (wt.%) Fe–38Ti–4.5C powder blends as the precursor materials. The composite coating consists of predominantly TiC primary phase and small amount of interdendritic eutectics TiC/␥-Fe. The composite coating is metallurgically bonded to the 1Cr18Ni9Ti austenitic stainless steel substrate. The microstructural characteristic of the coating is that large amount of primary TiC carbide uniformly distributed in the strong and ductile TiC/␥-Fe eutectics matrix. The composite coating has a high and uniform hardness distribution and exhibits excellent wear resistance under dry-sliding wear test condition. Acknowledgement
Fig. 7. Friction coefficient as a function of test time for the TiC carbidereinforced composite coating and the 1Cr18Ni9Ti austenitic stainless.
The research was supported by the National High Technology Research and Development Program of China (grant no. 2003AA331190).
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