1. Introduction
The titanium Ti6Al4V alloy has the advantages of a small density, high specific strength, and excellent corrosion resistance [
1,
2], and is widely used in aerospace [
3,
4], chemical industry [
5], biomedical [
6,
7], and other fields. However, there are some restrictions for the applications of the Ti6Al4V alloy due to its low wear resistance and poor hardness properties [
8,
9]. Laser cladding is a promising method for the surface modification of material. The surface performance can be improved by laser cladding a coating on the Ti6Al4V alloy [
10]. Cobalt has a strong precipitation hardening effect, intrinsic high strength properties, and the ability to maintain the hardness over a wide temperature range [
11,
12]. The thermal expansion coefficient of cobalt is close to that of the Ti6Al4V alloy, so it is an ideal material for coating a titanium alloy surface [
13,
14,
15].
It has been found that the addition or formation of some carbides can effectively improve the performance of the coating. Hu et al. [
16] produced Ni-based composite coatings via mixing different contents of TaC by laser cladding on the Ti6Al4V alloy. It was found, through an analysis of the microstructure and corrosion behavior of the coating, that the addition of TaC has a positive effect on the corrosion resistance of the coating. Chen et al. [
17] prepared TiC bioinert coatings on the Ti6Al4V alloy surface by using mixed TiC and ZrO
2 powders as the cladding material. The microhardness and wear resistance of the coating were greatly improved compared with the substrate Ti6Al4V alloy. Li et al. [
18] prepared a micro- and nano-structured WC reinforced Co-based coating on the Ti6Al4V alloy by laser cladding. The results showed that the wear resistance of the coating was obviously improved compared with the substrate. Due to the accumulation of carbides or oxides, it is easy for the coating to harden and become brittle. Yang et al. [
19] prepared WC7Co/Ti6Al4V composite coatings on a pure Ti substrate by laser cladding. It was found that the mean hardness of different structures exhibits a significant gradient distribution in the coating. The abrasive mechanisms of the coating are mainly adhesive wear and oxidation wear during the dry sliding process. Li et al. [
20] prepared Ti + SiC coatings on the Ti6Al4V alloy by laser cladding. It was found that the hardness and wear resistance of the coating were significantly improved compared with the substrate.
Graphene oxide is an important derivative of graphene. The addition of graphene oxide (GO) can greatly improve the performance of the coating, which has been a research hotspot in recent years. Li et al. [
21] employed GO-filled sol-gel films of a plasma electrolytic oxidation layer on the Ti6Al4V alloy. It was found that the addition of GO significantly improved the corrosion resistance and wear resistance of the coating. The improvement of wear resistance of the coating may be due to the solid lubrication property of GO nanoflakes. Sadeghi et al. [
22] deposited double-layer coatings of GO-poly on the Ti6Al4V alloy by electrophoretic deposition. The friction coefficient was reduced from 0.5 to less than 0.03 due to the combination of the double-layer coatings. Zuo et al. [
23] prepared GO particle-inserted coatings with different GO concentrations on a titanium alloy by micro-arc oxidation treatment. The results showed that the addition of GO significantly improved the tribocorrosion resistance. When the GO concentration was 10 mL/L, the performance of the coating was the best. Liu et al. [
24] prepared ceramic coatings with GO addition on the Ti6Al4V alloy by plasma electrolytic oxidation technology. It was found that the wear resistance of the GO-added coating was significantly improved due to the physical barrier of GO. Wang et al. [
25] grafted GO on the surface of the Ti6Al4V alloy. The corrosion resistance and tribological mechanism of the coating were discussed. The results show that the relative slip of GO sheets and entrapment of wear particles of the surface topography are the main factors contributing to the improved tribological behavior. Palaniappan et al. [
26] used neodymium-decorated GO as the corrosion inhibiting barrier for the Ti6Al4V alloy. As a result, the corrosion resistance of the Ti6Al4V alloy was improved. Bulbul et al. [
27] prepared hydroxyapatite (HA) reinforced by reduced nano-GO using the sol-gel method. The results showed that a crack-free coating was formed on the surface of the Ti6Al4V alloy by replacing HA with GO. The adhesion strength was also significantly improved.
In the current literature, few people have used laser cladding to prepare coatings with GO addition. Due to the high temperature of the laser cladding process, most of the GO in the coating will be dissolved even under the protection of inert gas. However, the performance of the coating can be effectively improved by undissolved GO and formed carbides if the amount of GO added is appropriate. Therefore, in this study, a Co-based coating with GO addition was prepared by laser cladding on a titanium alloy, and the effect of the addition of GO on the coating performance was analyzed.
2. Materials and Methods
In this study, the Ti6Al4V alloy was taken as the substrate, and its chemical composition is shown in
Table 1. The cobalt powder (CoCrMo) used was self-fluxing alloy powder, and its chemical composition is shown in
Table 2. The GO used in this paper included single-layer films, which were provided by Suzhou Tanfeng Graphene Technology Co., Ltd. (Suzhou, China). The purity of GO powder was 99.0 wt.%. The CoCrMo and GO powders were divided into six groups and mixed. The content of GO was 0 wt.%, 0.2 wt.%, 0.5 wt.%, 0.8 wt.%, 1.1 wt.%, and 1.4 wt.%, respectively. In order to ensure the uniformity of powder dispersion, the mixed powder was ball-milled. The ball-milling speed was 250 r/min and the ball-milling time was 3 h. In order to ensure the consistency of the experiment, the cobalt powder without GO addition was treated in the same way. The process parameters of laser cladding are shown in
Table 3.
The substrate Ti6Al4V alloy was cleaned and kept at 403.15 K for 30 min. Then, the experiment of laser cladding was carried out immediately, in order to reduce the cracks and pores of the coating in the process of cladding. Argon was used as protective gas during the experiment. At the end of the experiment, the sample was cooled at constant temperature and sampled by wire electrical discharge machining (WEDM). Then, the etchant reagent (HF:HNO3:H2O = 2:3:15) was prepared, and the cross-section of the sample after grinding and polishing was etched for about 15 s. The microstructure of the sample was observed with a LEICA-DM-2700M optical microscope (Leica, Heerbrugg, Switzerland) and Sigma300 scanning electron microscope (Carl Zeiss, Oberkochen, Germany). The Vickers-hardness of the sample was measured with an HVS-1000ZCM-XY digital microhardness tester (Suoyan Testing Instrument, Shanghai, China). Taking the upper surface of the substrate as the starting point, the measuring points were taken every 0.15 mm along the depth direction. The hardness of each sample was measured at 12 points. The wear resistance of the sample was tested by an MFT-5000 friction wear testing machine (Rtec Company, Wilmington, DE, USA). The diameter of the ball was 5 mm, the normal load was 25 N, the loading time was 30 min, the frequency was 1 Hz, and the stroke was 10 mm. The wear marks were observed by an MFP-D white light interferometer (Rtec Company, Wilmington, DE, USA). The wear resistance of the coating was studied by analyzing the friction coefficient, wear morphology, and wear rate.
4. Conclusions
With the addition of GO, the formed carbides and undissolved film-shaped GO exist simultaneously in the coating. The existence of the carbides refined the grain and improved the hardness and wear resistance of the coating. The existence of the undissolved GO enhanced the lubrication performance of the coating.
When the content of GO is excessive, the ability of grain refinement decreases. The dissolution of GO consumed a lot of laser energy. Moreover, the existence of excessive undissolved film-shaped GO weakened the laser energy absorption of the coating. The hardness and wear resistance of the coating decreased instead.
When the GO content is 0.5 wt.%, the performance of the coating is the best. The hardness of the coating is 2.58 times that of the substrate, and 32.3% higher than that of the Co-based coating without GO addition. The friction coefficient is reduced by 40.1% compared with the substrate and is 27.2% lower than that of the Co-based coating without GO addition. The wear rate is 85.5% lower than that of the substrate and 66.5% lower than that of the Co-based coating without GO addition.