Effect of heat treatment on microstructure and ... - Springer Link

J. Coat. Technol. Res., 6 (3) 401–406, 2009 DOI 10.1007/s11998-008-9133-5

Effect of heat treatment on microstructure and mechanical properties of cold sprayed Ti coatings with relatively large powder particles W.-Y. Li, C. Zhang, H. Liao, C. Coddet

FSCT and OCCA 2008 Abstract In this study, the effect of post-spray heat treatment on the microstructure, microhardness, and adhesive strength of the cold-sprayed Ti coating was investigated. It was found that a thick and relatively porous Ti coating was deposited by cold spraying. The coating surface layer presented a more porous structure. The microhardness of the as-sprayed Ti coating was slightly higher compared to pure Ti bulk, owing to the work hardening effect during deposition. After annealing at 850C for 4 h under vacuum condition, the Ti coating also presented a porous structure with more uniformly distributed small pores. A metallurgical bonding between the deposited particles was formed through annealing treatment. The adhesive strength of coating was significantly improved after annealing. The microhardness of the annealed Ti coating was also increased. Keywords Cold spraying, Vacuum heat treatment, Titanium, Microstructure, Microhardness, Adhesive strength

Introduction Cold spraying process has been widely investigated, owing to its high deposition efficiency and rate for volume production of many metallic, composite, and W.-Y.Li (&) Shaanxi Key Laboratory of Friction Welding Technologies, Northwestern Polytechnical University, 127, West Youyi Road, Xi’an, Shaanxi 710072, P.R. China e-mail: [email protected] C. Zhang, H. Liao, C. Coddet LERMPS, Universite´ de Technologie de Belfort-Montbe´liard, Site de Se´venans, 90010 Belfort Cedex, France

nanostructured coatings.1–8 In this process, the deposition of particles takes place through intensively impacting in a solid state at a temperature well below the melting point of the sprayed material. Consequently, the deleterious effects of oxidation, phase transformation, grain growth, and other problems inherent to conventional thermal spraying processes can be minimized or eliminated. Cold spray has potential applications in aerospace, automobile, marine engineering, chemical engineering, biotechnology, corrosion and wear resistance, spray forming, and so on. Previous studies found that it was difficult to form dense Ti and Ti alloy coatings by cold spraying.9–13 It was considered that the accumulative tamping effect resulted from the successive impacting particles takes an important role in the formation of the porous structure.10 This tamping effect could be influenced mainly by the strength and momentum of the particles.10 However, many recent experimental results demonstrated that metal reactivity and surface oxide films of powder particles could significantly influence the microstructure of the cold-sprayed coatings.13 Nevertheless, this kind of porous structure may be used to fabricate metal foams and biocompatible materials. On the other hand, the post-spray heat treatment has been shown to be an effective method to modify the microstructure and properties of coldsprayed coatings.14–17 In this article, the effect of post-spray heat treatment on the microstructure, microhardness, and adhesive strength of cold-sprayed Ti coating is investigated.

Experimental procedures A cold spray system with a commercial cold spray gun (CGT GmbH, Germany) was used to deposit Ti coatings. A home-made optimized nozzle was employed in deposition of Ti coating, which had an

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J. Coat. Technol. Res., 6 (3) 401–406, 2009

Fig. 2: Schematic diagram of the tensile test

Force Clamp Fig. 1: SEM morphology of the used Ti powder

Coating Substrate

Clamp

expansion ratio of about 4.9 and a divergent section length of 170 mm. The high-pressure compressed air was used as the accelerating gas and argon was used as powder carrier gas. To obtain high deposition efficiency, an air pressure of 2.7 MPa and a temperature of about 560C were adopted. The standoff distance from the nozzle exit to the substrate surface was 30 mm. A commercial Ti powder made by H.C. Starck GmbH (Amperit155.090, +45–160 lm) was used as feedstock. Figure 1 shows the morphology of titanium feedstock powder which has an irregular shape and porous structure. Mild steel plates (XC10, France) were used as substrates and sandblasted using alumina prior to spraying. After spraying, some samples were annealed at 850C for 4 h under a vacuum condition, taking into account the a ﬁ b phase transformation temperature of 882C. The coating microstructure was examined by optical microscope (OM) (Nikon, Japan), scanning electron microscope (SEM) (JSM5800LV, JEOL, USA), and X-ray diffraction (XRD) analysis (Shimazu XRD6000, Japan) with Cu Ka1 radiation. For a better observation of the coating microstructure, the polished coating was etched by a solution of 100 mL H2O + 3 mL HF + 6 mL HNO3. The porosity of the coating was estimated by the image analysis with Scion Image software (NIH, USA) using 10 OM micrographs. It should be pointed out that the obtained porosity is the two-dimensional projection of the three-dimensional porosity by this estimation method. The microhardness of the coating was measured by a Vickers hardness tester (Leitz, Germany) with a load of 100 g and holding time of 15 s. More than 10 hardness values were obtained to calculate the average. The adhesive strength of the coating was characterized using the ASTM standard C633-01. Coatings of a thickness of about 400 lm were produced on the mild steel disks with 25.4 mm diameter and 10 mm thickness. The uncoated surface of the substrate was

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Observation direction

Fig. 3: Schematic diagram of obtaining the cross-sectional fracture of coating by bending the substrate

sand-blasted, and then the coating with substrate was sandwiched between two threaded holders using an adhesive (FM1000, MOSAVIA Corporation, USA, tensile strength: 50–60 MPa) as illustrated in Fig. 2. The cross-sectional fracture surface of the coating obtained by bending the substrate, as illustrated in Fig. 3, was also examined by SEM.

Results and discussion Effect of annealing treatment on microstructure of cold-sprayed Ti coating A thick Ti coating (1.4 mm) was deposited with a high deposition rate and efficiency of about 60%. Figure 4 shows the OM micrograph of the as-sprayed Ti coating and the XRD results of the coating and feedstock. It is clear that the Ti coating deposited with relatively large powder presents a relatively porous structure with a porosity of about 4.2% ± 1.4 as shown in Fig. 4a. In addition, the surface layer is more porous than the inner layers of the coating. These results are consistent with the previous results.10,13 Generally speaking, a porous Ti coating is desirable for biomaterials applications. A relatively large Ti powder was chosen in this study to produce porous coatings based on two


(a)

(b)

(101)

Ti powder

Intensity

(002) (100)

(112) (102) (110) (103)

(201)

Ti coating

20

40

60

80

2θ (°) Fig. 4: OM micrograph of the cross-section of as-sprayed Ti coating (a) and XRD results of Ti coating and feedstock (b)

arguments. First, large particles will travel at a lower velocity during spraying, and thus less plastic deformation.13 Second, the formed pores (gaps between the deposited particles) in the coating from large particles will be larger than those from small particles. However, a comparison of porosity levels and pore sizes in the Ti coatings formed by large particles and fine particles (5–45 lm)13 did not fully prove the above-mentioned hypothesis. This point of view needs further investigation. Some micropores are also present in the particle– particle interfaces, which can be considered as the microcracks. This fact has been reported in literature3,4,14 and we also observed these kind of cracks in the as-sprayed Ti13,18 and other metallic coatings.13,16–18 The reasons are as follows. First, the adhesive strengths of common cold-sprayed metallic coatings are in a range of 15–50 MPa, which is similar to the conventional thermally sprayed coatings. This means the bonding mechanism of cold-sprayed coatings is mainly mechanical-locking as that for thermally sprayed coatings. Second, the etched cross-sections of cold-sprayed coatings present the obviously weak particle–particle interfaces (preferential corrosion). In addition, the XRD result (Fig. 4b) shows that the coldsprayed Ti coating has the same a-Ti structure as Ti feedstock, suggesting that cold spraying has no effect on changing Ti crystal structure. Taking into account the potential applications of Ti coatings as biomaterials,19 the post-spray annealing treatment on the as-sprayed coating was conducted to improve coating microstructure and properties. Figure 5 shows the OM microstructure of the annealed Ti coating. It is found that the porosity of the coating has increased slightly to about 5.7% ± 1.1. When the polished cross-section of the annealed Ti coating was etched and characterized using SEM, it was found that interfaces between the deposited particles have disappeared except for large pores, as shown in Fig. 6. The annealed Ti coating demonstrates an equiaxed a-Ti phase, which is typical of an annealed Ti bulk material. These facts suggest a metallurgical bonding between the deposited particles has formed during

Fig. 5: OM micrograph of the cross-section of annealed Ti coating at 850°C for 4 h

Fig. 6: SEM micrograph of the cross-section of annealed Ti coating (etched)

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annealing. This is in agreement with findings in other previous studies.14–17 On the other hand, besides the relatively large pores, many small pores in the size of sub-micrometers are still present in the annealed Ti coating, as seen in Fig. 6. These pores were resulted from the incomplete interfaces between the deposited particles and the subparticles through atomic diffusion during annealing treatment.16 Therefore, the porosity of the annealed coating is underestimated. Because in the process of image analysis, the small pores (submicrometer pores or unbonded interfaces between the particles) could not be distinguished due to the limited image resolution and these small pores or unbonded interfaces could not be accounted for the as-sprayed coating. In the annealed coating, the small pores or unbonded interfaces aggregated and grew and the aggregated pores could be easily distinguished by the image analysis software. In this way, it seems that the coating porosity increases during annealing. But the fact is that the porosity changes little. Figures 7 and 8 show the fractured surface morphologies of both the as-sprayed and annealed Ti coatings. It is seen from Fig. 7 that the fracture occurred mainly at the particle interface, which is

weak and usually becomes the crack path. On the other hand, evidence of interface melting is found at local regions, as indicated by the arrows in Fig. 7b. While Ti has a relatively high melting point, localized melting during particle deposition could be attributed to temperature rise caused mainly by the oxidation of particles.13 The melting mechanism of interfaces for cold-sprayed particles was explained in our previous work.18 From Fig. 8, it is clearly seen that a ductile fracture pattern, which contains dimples as shown in Fig. 8b, occurs at local interfaces in the annealed coating. This fact further proves the formation of metallurgical bonding between the deposited particles. In addition, it can be observed from Fig. 8c that small pores are present near large pores and this agrees with the result shown in Fig. 6. Effect of annealing treatment on adhesive strength of cold-sprayed Ti coating Three tensile samples were prepared for both as-sprayed and annealed Ti coatings for adhesive strength measurement. The adhesive strength is 19 ± 5 MPa for the as-sprayed Ti coatings. The adhesive strength of the annealed coating was not measured successfully because fracture of all three annealed samples occurred in the adhesives. However, we can infer that the adhesive strength of the annealed coating is at least higher than the bonding strength of the adhesive, which measures 50 MPa by the test. The bonding strength of the coating was significantly improved through annealing treatment. This result suggests that an effective bonding between the deposited particles and coating/substrate interface is formed, which is consistent with microstructural observation. Effect of annealing treatment on microhardness of cold-sprayed Ti coating

Fig. 7: SEM morphology of the fractured surface of as-sprayed Ti coating

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The hardness test results show that the microhardness of the as-sprayed Ti coating is 72 ± 28 Hv0.1. The coating hardness is slightly higher compared to pure Ti bulk (60 HV20), owing to the work hardening effect during deposition.3,14,16 However, the coating microhardness increases significantly to 143 ± 13 Hv0.1 after annealing treatment. This trend is inconsistent with previous results obtained for other annealed coldsprayed coatings.14,16,17 Previous results concluded that annealing eliminated the work-hardening effect of Ti coating, resulting in a decrease in microhardness value. A closer study at the surface imprints caused by microhardness testing revealed cracking around indentations in as-sprayed coatings. This indicates weak bonding of the as-sprayed coating, and thus the lower hardness. However, for the annealed coating, no crack was found around the indentations, which may suggest an improved bonding after annealing, and thus an increase in hardness. This result is also consistent with


Fig. 8: SEM morphology of the fractured surface of annealed Ti coating

the observation of microstructure shown in Figs. 6 and 8. However, the underlying mechanism still needs further study.

Conclusion From the results obtained in this study, the following conclusions can be drawn: 1.

2.

3. 4.

A thick and relatively porous Ti coating can be deposited by cold spraying without changing the original crystalline structure in the feedstock. Surface layer of the coating presents a more porous structure. After annealing at 850C for 4 h under vacuum condition, cold-sprayed Ti coating also demonstrates a porous structure with more uniformly distributed small pores. It is believed that a metallurgical bonding between the deposited particles is formed through atomic diffusion in annealing treatment. The adhesive strength of Ti coating is significantly improved from about 19 MPa to higher than 50 MPa through heat treatment. The microhardness of the as-sprayed Ti coating is slightly higher than pure bulk Ti owing to the work hardening effect during deposition. The microhardness of the annealed Ti coating is significantly increased with an improved adhesion of the coating.

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