Surface & Coatings Technology

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1 Surface & Coatings Technology 291 (2016) Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: The effect of TiC/Al 2 O 3 composite ceramic reinforcement on tribological behavior of laser cladding Ni60 alloys coatings Yangchuan Cai, Zhen Luo, Mengnan Feng, Zuming Liu, Zunyue Huang, Yida Zeng a College of Materials Science and Technology, Tianjin University, Tianjin , China b School of Materials Science and Engineering, Tianjin University, Tianjin , China c Tianjin Key Laboratory of Advanced Joining Technology, Tianjin University, Tianjin , China article info abstract Article history: Received 10 December 2015 Revised 15 February 2016 Accepted in revised form 16 February 2016 Available online 17 February 2016 Keywords: Composite coating TiC Al 2 O 3 Laser cladding Ti-rich alloy compound The composite coating reinforced with TiC and Al 2 O 3 ceramic particles were fabricated on the Cr12MoV steel by laser cladding. The cross-section, microstructure, phase, microhardness and wear resistance of the composite coating were investigated by the optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), Vickers microhardness tester and dry sliding wear testing machine, respectively. An obvious crack occurs in the coating when the content of the ceramic reinforcement is 15%. Composite coatings have good metallurgical bonding with the substrate when the content of the ceramic is less 15%. The phases of the composite coatings are composed of solid solutions (Ni Cr Fe and γ-(fe,ni)), chromic carbides (Cr 3 C 2 and Cr 2 C), ferric carbide (Fe 3 C) and ceramics (TiC and Al 2 O 3 ). The composite ceramics that Tirich alloy compound was coating unfused Al 2 O 3 and TiC ceramics respectively were generated in the coating. A part of Ti element and C element, dissolved from the original TiC ceramic, precipitate from the matrix to form new TiC ceramic. These kinds of composite ceramic particles significantly improved the wear resistance of the coating via enhancing the bonding strength between the ceramic particles and the matrix Elsevier B.V. All rights reserved. 1. Introduction Cr12MoV is a traditional cold extrusion die steel, in order to resist the repeating impact and friction, some excellent surface properties such as high wear and high hardness are required [1,2]. Using traditional methods (e.g., carburization, nitridation and surfacing) to improve the surface properties has not only higher cost but also lower efficiency. Laser cladding technology can obtain the above special surface properties via cladding a thin coating on the surface of traditional metal [3,4]. At the same time, the cost and efficiency of the production process are reduced. Ceramic particles can significantly improve the wear resistance of the coating because it has very high hardness. However, pure ceramic coatings are difficult to obtain because cracks can be easily generated during the laser cladding process [5,6]. Thus, the mixtures of ceramic particles and alloy powders are usually used to obtain the composite coating that has excellent wear resistance [7,8]. Also, different kinds of ceramic have the different effect on the coating. Therefore, the influence of ceramic particles on the mechanical properties of the coating is one of the hotspots in laser cladding technology. Xiaolei Wu et al. [9] investigated that the Ni alloy composite coatings was reinforced by titanium Corresponding author at: 25-C-1201, School of Materials Science and Engineering, Tianjin University, No.92 Weijin Road, Tianjin, China. address: lz@tju.edu.cn (Z. Luo). carbide particle (TiCp). The original TiCp and in-situ reacted TiCp improved the tribological behavior significantly. Dongsheng Wang et al. [10] reported that the addition of TiO 2 enhanced the properties of Al 2 O 3 coating, and the composite coating had excellent wear resistance when the TiO 2 content was 13 wt.%. H.C.Man et al. [11] fabricated the TiC + WC composite ceramic reinforcement via using laser-induced reaction synthesis. The wear resistance of the composite coating was enhanced significantly. The application and effect of ceramic particles on the laser cladding layer are studied widely. The influence mechanism of the ceramic particles on the properties of the coating is also specific. However, the interaction between two different kinds of ceramics, during the laser cladding process, is rarely reported, and the influence mechanism of composite ceramic particles on the tribological behavior of the matrix is also not clear. Consequently, the TiC and Al 2 O 3 ceramics were selected to add into the Ni60 alloy powder. The evolvement rule and interaction mechanism of the two kinds of ceramics in the laser cladding process were studied systematically. 2. Experimental procedure 2.1. Materials and fabrication of laser cladding coating The material used in the experiment was Ni60 + TiC/Al 2 O 3 composite powder. The chemical composition of Ni60 is shown in Table 1, and / 2016 Elsevier B.V. All rights reserved.

2 Y. Cai et al. / Surface & Coatings Technology 291 (2016) Table 1 Nominal chemical composition of Ni60 powders (wt.%). C Cr B Si Fe Ni Bal. the average size was μm. The content of TiC/Al 2 O 3 composite ceramic reinforcement was 0%, 5%, 10% and 15% respectively, and the weight ratio of TiC and Al 2 O 3 was 1:1, as shown in Table 2. The TiC powder particles were irregular with the size of 5 10 μm, whereas Al 2 O 3 powder particles were spherical with the size of μm. The shape and size of the TiC and Al 2 O 3 are shown in Fig. 1. Normalized Cr12MoV cold extrusion die steel with dimensions of 300 mm 200 mm 10 mm was used as the substrate in the study. Substrates were grounded with sand paper and then cleaned with alcohol to remove any organic elements. Then, the composite powders, mixed with alcohol, were coated on the surface of the substrate, and the thickness was about 1.0 mm. The JK2003SM type Nd: YAG laser equipment was used to weld the powders on the substrate, the laser cladding parameters are shown in Table Microstructure characterization After the laser cladding, specimens were cut from the transversal cross-sections of the composite coating. The cross-section of the cladding layer was studied by the Olympus stereomicroscope, and the microstructure of the composite coating was characterized by the Olympus optical microscope, S4800 scanning electron microscopy (SEM) and JEM-2100F transmission electron microscope (TEM). The phase of the polished coating was identified by the X-ray diffractometer (XRD). The microhardness, distributes from the surface of the composite coating to the substrate, was measured by an MHV2000 type digital microhardness tester with a 100 g load, and 10 s dwell time; the distance between the two points was 0.2 mm Wear testing The wear behavior of the composite coating was tested on a MM-200 dry sliding wear tester. The material of the frication pair was GCr15. Table 4 shows the parameters of wear test. The wear volume was determined by the depth and width of wear scars. Therefore, the wear volume could be calculated by Eq. (1) [12]: 2 s V ¼ R 2 arcsin B B ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 R2 B 2 4 5:L ð1þ 2R Results and discussion Fig. 1. Particle shapes and sizes of powder Macroscopic feature of composite coating Fig. 2 is the cross-section of the composite coating with different content of ceramic reinforcements. Composite coatings have good metallurgical bonding with the substrate, and there are no obvious defects found in the juncture. In addition, the surface contour lines of cladding layers are all convex curve. It is due to the heat that absorbed by coating powder approximately matching the Gaussian distribution because the energy of the laser spot conforms to the Gaussian distribution. Furthermore, the surface tension has a linear relation with temperature, the higher the temperature, the smaller the surface tension. As a result, the surrounding of the liquid coating metal has higher surface tension than that of the center [13,14]. Under the function of surface tension, the liquid coating metal will gather to the center from the surrounding, the convex coating is then formed. Fig. 2 (e) is the high power morphology of the composite coating cross-section when the content of the ceramic reinforcement is 15%. There exists an obvious crack in the coating. Tan H et al. [15] found that the content of the ceramic reinforcement in the coating had optimum value. The enhancement of the ceramic reinforcement was not obvious under the lower value; when the value was excess, the agglomeration of the reinforcement occurred in the coating that became the potential crack source. where B is the width of wear scar (mm), L is the width of wear scar (mm), and R is the outer radius of wear ring (mm). Then, the weight loss was calculated via measuring the weight of specimens before and after the test by an analytical balance. After the wear test, the worn surface of the composite coating was studied by an S4800 scanning electron microscopy (SEM). Table 2 Chemical composition of composite powders (wt.%). Group Al 2 O 3 TiC Ni60 A1 coating A2 coating A3 coating A4 coating Wear resistance of the composite coating Fig. 3 shows the microhardness of the substrate and composite coatings with a different ceramic reinforcement content. The microhardness of the coating is significantly enhanced compared with the substrate, and the value is increased from HV to HV with the increase of ceramic reinforcement content. It is due to the reinforcement Table 3 Laser cladding parameters. Laser output power (W) Laser beam scanning velocity (mm/min) Laser beam diameter (mm) Argon flow rate (ml/min) Overlap ratio (%)

3 224 Y. Cai et al. / Surface & Coatings Technology 291 (2016) Table 4 Parameters of wear resistance test. Parameter Value Friction pair macro-hardness (HRC) 60 Friction pair radius (mm) 50 Friction pair width (mm) 10 Specimen size (mm) Wear load (N) 49 Sliding speed (r/min) 200 Sliding time (min) 120 with high hardness in the coating increased the microhardness of the substrate. The wear volume loss and wear weight loss are all decreased with the increase of ceramic reinforcement content, as shown in Fig. 4. The result indicates that the wear resistance of the coating has the same variation with the microhardness (i.e. the property is increased with the increase of ceramic reinforcement content). Singh R et al. [16] reported that the increase of the microhardness could effectively increase the wear resistance of the composite coating. Fig. 5 is the worn surface morphologies of the composite coating. The result shows that, when the content of ceramic reinforcement is 0%, there exist many deep and wide plowing grooves and couple with some plastic deformation on the worn surface, as shown in Fig. 5 (a). When the ceramic reinforcement content is 5%, the plowing grooves on the worn surface become shallow and narrow, and the plastic deformation reduced. When the ceramic reinforcement content is 10%, the shallow and narrow plowing grooves uniformly distribute on the worn surface Formation mechanism of the composite ceramic reinforcement The microstructure of composite coatings is the chromic carbides (Cr 3 C 2 and Cr 2 C) and ferric carbide (Fe 3 C) distribute in solid solutions (Ni Cr Fe and γ-(fe,ni)), as shown in Fig. 6. The coating, did not add ceramic reinforcements (i.e. A1 coating), has a certain wear resistance because the chromic carbide and ferric carbide enhanced the microhardness. A2 coating and A3 coating added ceramic reinforcements Fig. 3. Microhardness of the substrate and the composite coatings. (TiC and Al 2 O 3 ), thus the obvious crests of TiC and Al 2 O 3 are observed in XRD patterns. The hardness of the TiC and Al 2 O 3 is higher than that of chromic carbide and ferric carbide, which significantly increased the microhardness and wear resistance of the composite coating. The scanning microstructure and elements distribution of the A3 composite coating are shown in Fig. 7. The results show that Fe element and Ni element uniformly distribute in the coating to constitute the γ-(fe,ni) solid solution. A part of Cr element forms chromic carbide with C element; and the Ni Cr Fe solid solution is generated by the Ni element, Fe element and the balance of Cr element. Ti element and Al element mainly distribute at the boundary of the solid solution. The distribution area of Ti element presents scattered spots and it has various sizes. The Al element distributes in the large Ti element distribution area, but the small Ti element distribution area does not contain Al element. The amount of Al element distribution area is greater than the Ti element distribution area. In order to deeply study the distribution law of Ti element and Al element, (a) region and (b) region in Fig. 8 were selected to observe and analyze under the high power scanning microscope. There exists a large ceramic particle at the boundary of solid solution, as shown in Fig. 8, the core has a darker color than the outer sphere. Fig. 2. Cross-section of the composite coatings: (a) A1 coating; (b) A2 coating; (c) A3 coating; (d) and (e) A4 coating.

4 Y. Cai et al. / Surface & Coatings Technology 291 (2016) Fig. 4. Wear losses of the composite coating. Fig. 5. Worn surface morphologies of the composite coating: (a) A1 coating; (b) A2 coating; (c) A3 coating. The Ti element distributes in the ceramic particle, including the core and the outer sphere, and Al and O elements only distribute in the core; the ceramic particle and solid solution have the same low content of the C element. Li JN et al. [17] reported that, in the laser cladding process, the carbide ceramic constituted limited solid solution and eutectic alloy with matrix alloy since it had a certain solubility in the matrix alloy. It can be inferred that, the core of ceramic is Al 2 O 3,andtheouter sphere is the Ti-rich alloy compound, not TiC particle. During the laser cladding process, ceramic particles will be fused and decomposed because the surface is the irregularity. The TiC and Al 2 O 3 in Fig. 7 have the average size of 1 μm and 0.8 μm respectively. The original size of TiC is 5 10 μm, and Al 2 O 3 is μm. The size difference of ceramic particles illustrates that the dissolved amount of Al 2 O 3 is higher Fig. 6. XRD patterns of the composite coatings before wear test. than TiC. The thermal diffusivity of the ceramic particles can be calculated by the Eq. (2) to account for this fusion phenomenon [18]. α ¼ λ ρc where α is the thermal diffusivity (m 2 /s), λ is the thermal conductivity (W/(m K)), ρ is the density (kg/m 3 ), and c is the specific heatcapacity (J/(kg K)). The thermal conductivity, density and the specific heat capacity of TiC are W/(m K), 4.93 g/ml and 837 J/(kg K) respectively. And the value of corresponding physical parameters of Al 2 O 3 is W/(m K), 3.5 g/ml and 837 J/(kg K) respectively. The thermal diffusivity of TiC and Al 2 O 3 is m 2 /s and m 2 /s respectively after being calculated via Eq. (1). Akoshima M et al. [19] found that the higher of thermal conductivity, the more heat would be conducted under the same temperature gradient. The absorptive heat is less after the temperature is increased by 1 C when the value of ρc is smaller. The more heat can be remained to transmit to internal materials, which makes the temperature of every part of materials increase with the increase of interface temperature. Therefore, Al 2 O 3 needs less time than TiC after increasing the same temperature. The melting point of TiC (3140 C) is higher than Al 2 O 3 (2045 C) that making the melting quantity of Al 2 O 3 is higher than TiC in the same laser cladding process. The element distribution in the high power scanning microscope of the (b) region is shown in Fig. 9. There exist three kinds of ceramic particles in the microstructure; the first one is the composite ceramic that Al 2 O 3 is coated by the Ti-rich alloy compound (i.e. Fig. 9(1)); The second one is the large Ti-rich ceramic particles (i.e. Fig. 9(2)); The last one is the small Ti-rich ceramic particles (i.e. Fig. 9(3)). In the laser cladding process, the ceramic particles will absorb more laser energy than Ni-base alloy powder. As a result, the small ceramic particles are dissolved completely, the edge and surface of the large ð2þ

5 226 Y. Cai et al. / Surface & Coatings Technology 291 (2016) Fig. 7. Distribution of elements in the microstructure of A3 composite coating. ceramic particles are dissolved partly [20]. During the solidification process of the coating, the Ti element and C element, decompose from TiC ceramic particles, will first precipitate from the solid solution because the TiC ceramic has a higher melting point than Al 2 O 3 ceramic. In addition, the residual TiC and Al 2 O 3 ceramics can be used as nucleated points. Finally, the Ti-rich alloy compound will precipitate at the round of the residual ceramic particles to form the composite ceramic (i.e. 1 and 2 in Fig. 9). Some scholars [21] reported that a part of Ti element and C element could precipitate from the matrix to form the smaller TiC ceramic particle. Using the transmission electron microscope (TEM) observed the small Ti-rich ceramic particles (i.e. Fig. 9(3)), as shown in Fig. 10 (a). The diffraction pattern of the ceramic particle and its surrounding are shown in Fig. 10 (b and c). After computational analysis, the ceramic particle is TiC, and its surrounding is γ-[fe,ni] solid solution. Based on the data analysis above, the small Ti-rich ceramic particles are TiC ceramic that precipitate from the matrix. The research result is in accordance with Wang XH [21]. In conclusion, TiC and Al 2 O 3 ceramic particles with equal proportion were added in the Ni-based alloy powders, as shown in Fig. 11 (a); In Fig. 8. High power scanning microscope of (a) region.

6 Y. Cai et al. / Surface & Coatings Technology 291 (2016) Fig. 9. High power scanning microscope of (b) region. the laser cladding process, the edge and surface of TiC and Al 2 O 3 ceramic particles were dissolved under the laser energy, and the Al 2 O 3 ceramic particle had more dissolved quantity than TiC ceramic because Al 2 O 3 had higher thermal diffusion coefficient, as shown in Fig. 11 (b); the Ti element, dissolved from TiC ceramic particle, was easy to form the alloy compound with other alloy elements. The Ti-rich alloy compound was precipitated firstly on the surface of unfused ceramics, had a high surface energy to reduce the nucleation work, to form the composite ceramic particle (i.e. 1 and 2 in Fig. 11 (c and d)). Besides, a small amount of Ti element and C element, dissolved from TiC ceramic particle, was precipitated from the matrix again to form the new TiC ceramic particle (i.e. 1 in Fig. 11 (c and d)).

7 228 Y. Cai et al. / Surface & Coatings Technology 291 (2016) Table 5 Wettability and solubility of TiC and Al 2 O 3 ceramic phases in Nickel. Ceramic phase θ ( ) Adhesive power (J m 2 ) Solubility (%) TiC Al 2 O Fig. 10. TEM micrograph of ceramic particle Wear resistance mechanism of the composite coating Analyzing Table 5 (i.e. the wettability and solubility of TiC and Al 2 O 3 ceramics in the Ni-based alloy) and Fig. 12 (i.e. the crystal structure of TiC, Al 2 O 3 and Ni) find that the crystal structure of Al 2 O 3 is the closepacked hexagonal structure and that of TiC and Ni is face-centered cubic. As a result, the solubility, wettability and adhesive power of TiC ceramic in matrix metal are greater than Al 2 O 3 ceramic. Therefore, TiC ceramic has higher bonding strength with the matrix metal than that of Al 2 O 3 ceramic. Al 2 O 3 ceramic particles could be as the source of crack to make it easy to exfoliate from the coating. The Ti-rich alloy compound, in the self-developed composite coating, on the surface of Al 2 O 3 and TiC enhanced the bonding strength of the original ceramic with the matrix metal. The part of the TiC ceramic particle that precipitates from the matrix has a small size that has the effect of precipitation strengthening to the coating. Therefore, the self-developed composite ceramic reinforcements improved the wear resistance significantly. 4. Conclusions Fig. 12. Crystal structure of TiC, Al 2 O 3 and Ni. (1) Composite coatings have good metallurgical bonding with the substrate, and the surface contour lines of cladding layers are all convex curve. There exists an obvious crack in the coating when the content of the ceramic reinforcement is 15%. (2) The microstructure of three composite coatings, before the wear test, is composed of solid solutions (Ni Cr Fe and γ-(fe,ni)), chromic carbides (Cr 3 C 2 and Cr 2 C) and ferric carbide (Fe 3 C). The obvious crests of TiC and Al 2 O 3 are observed in XRD patterns after added ceramic reinforcements (TiC and Al 2 O 3 ). (3) Composite coatings mainly contain three kinds of ceramic reinforcements. The first one is the unfused Al 2 O 3 ceramic coated Fig. 11. Formation schematic diagram of the composite ceramic reinforcement.

8 Y. Cai et al. / Surface & Coatings Technology 291 (2016) with Ti-rich alloy compound; the second one is the unfused TiC ceramic coated with Ti-rich alloy compound; the last one is the TiC ceramic that dispersedly precipitate from the matrix. These three kinds of ceramic particles significantly improved the wear resistance of the coating via enhancing the bonding strength between the ceramic particles and the matrix. [11] Acknowledgments [12] This research is supported by the National Nature Science Foundation of China (Grant and ). References