Microstructure and formation mechanism of the Si-Cr dual-alloyed coating prepared by pack-cementation
Introduction
AISI 5140 steel is widely used as mechanical parts owing to its low-cost, good strength and plasticity [[1], [2], [3], [4], [5]]. However, these parts usually fail from services as a result of surface failures due to complex working conditions, such as high speed, high temperature, and heavy load. Therefore, finding proper methods to improve the material surface quality is crucially important. Among the surface modification techniques, surface coating has been considered as an effective method in strengthening the surface of alloys [[6], [7], [8], [9], [10], [11]]. The introduction of hard coatings or lubrication coatings is an important method for the surface modification [[12], [13], [14], [15], [16]]. As compared to bare steels, the hardness and wear resistance of coated steels can be significantly improved by new hard phases or lubricating phases.
For coating on the steel surfaces, siliconizing and/or chromizing coating is extensively used due to the good bonding and their excellent properties [[17], [18], [19], [20], [21]]. Popoola et al. [17] reported an anti-wear pack siliconizing coating was fabricated on the low-carbon steel surface through process parameters optimization. Fe3Si intermetallic compound coating was observed at the surface [[22], [23], [24]], which exhibited lower friction and specific wear rates [25]. However, the FeSi coating has low toughness due to the brittleness of Fe3Si [[19], [20], [21]]. Chromizing coating produces a large number of hard phases in the form of CrC, Cr2C3, (Cr,Fe)7C3, and (Cr,Fe)23C6 on the steel surface, which effectively enhanced the surface hardness and wear resistance [[26], [27], [28], [29], [30]]. However, a weak adhesive carbon-poor layer (CP-layer) between the chromizing coating and steel substrate makes it easy to fail [2,5]. In our previous study [1], it is found that the pre-boronizing treatment before chromizing is able to prevent the formation of such CP-layer and the obtained boronizing-then-chromizing coating exhibited excellent properties.
The combined coating of silicon and chromium on the surface can adopt the advantages of both coating techniques while avoiding the drawbacks of each. In the past few decades, SiCr coating has been investigated by a few researchers [[31], [32], [33]]. Protasevich et al. [33] prepared the SiCr coating on the sintered steel, which displayed high wear resistance, excellent oxidation and corrosion resistance. Halen et al. [34] found that simultaneous siliconizing and chromizing was very effective in protecting the 316L austenitic stainless steel. Moreover, Tang et al. [19] have pointed out that the Cr addition in FeSi alloy system was helpful to improve the intrinsic brittleness of Fe3Si, which means that the SiCr coating has lower brittleness than the single siliconizing coating. Considering our early work via boronizing-then-chromizing [1], it can be conjectured that there is a great possibility for the SiCr coating to avoid the CP-layer. In the present work, in order to improve the surface properties of the AISI 5140 steel and remove the CP-layer caused by chromizing, two types of SiCr coatings were explored and prepared on the AISI 5140 steel by pack-cementation based on different packed powders. Induction heating was utilized to activate the coating elements including Si and Cr (see more details in Ref. [5]). The phases and microstructure of the two coatings were characterized. The properties including microhardness, and wear resistance were tested and compared. In view of considerable differences in the microstructure and properties between the two coatings, coating growth mechanism was studied. Additionally, Kirkendall effect was introduced to explain the forming reason of micropores in the coatings, and the growth mode of micropores was investigated.
Section snippets
Sample preparation
Chemical composition of the AISI 5140 carbon steel used in this study was 0.40C, 0.80 Cr, 0.27 Mn, 0.23 Si, 0.03 Ni, and Fe-balance (in wt%). After quenching (860 °C for 1 h, water cooling) and tempering (580 °C for 1 h) treatment, the microstructure of as-tempered steel is presented in Fig. 1. It can be seen that the initial treated steel contains a large number of pearlite colonies [35], consisting of ferrite and fine carbides (tempered sorbite). As-tempered carbon steel rods were cut into
Phase identification
Fig. 2 shows the XRD patterns of the as-tempered and SiCr samples. The phase identification was strictly based on the International Diffraction Data Center (ICDD) database. For the as-tempered sample, the carbides are too fine to detect, so only ferrite peaks can be observed in the XRD patterns. After coating treatment, the results reveal that ferrite, Fe3Si, CrC, and CrFe8Si are detected in the SiCr-1 sample and SiCr-2 sample. This indicates that the coatings of the two samples are mainly
Coating growth mechanism
Based on the above results, there are remarkable differences in the microstructure and properties between the SiCr-1 and SiCr-2 samples. The differences are mainly as follows: coating thickness, second-phase, interface morphology, Si/Fe and Cr/Fe intensity ratio, pore, CP-layer, microhardness, and wear resistance. Therefore, it is worthy studying the reasons behind these differences.
Fig. 10 presents the growth mechanisms of the Fe-Si-Cr coatings of SiCr-1 and SiCr-2 samples. According to the
Conclusions
In the present work, two categories of SiCr coatings were prepared on the steel surface. The microstructure, properties and formation mechanisms of the SiCr coatings were investigated. The main conclusions can be summarized as follows:
- (1)
Both the SiCr-1 and SiCr-2 coatings contain the α-Fe (rich in Cr and Si atoms), Fe3Si, CrFe8Si, and a small amount of CrC. The SiCr-1 sample has more CrC particles distributing at the grain boundaries, while the SiCr-2 sample has thicker coating and higher content
CRediT authorship contribution statement
Jing Zeng:Data curation, Formal analysis, Methodology, Writing - original draft.Jianjun Hu:Conceptualization, Funding acquisition, Investigation, Resources, Writing - original draft.Xian Yang:Data curation.Hongbing Xu:Funding acquisition, Project administration, Resources, Supervision.Hui Li:Data curation.Ning Guo:Conceptualization, Formal analysis, Funding acquisition, Investigation, Validation, Writing - original draft, Writing - review & editing.Qingshan Dong:Validation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was co-supported by the Scientific and Technological Research Program of Chongqing (cstc2017jcyjBX0031), Fundamental Research Funds for the Central Universities of China (No. XDJK2019B066) and Natural Science Foundation of China (51575073).
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