**1. Introduction**

Since 20Cr2Ni4A alloy steel has excellent surface properties after carburization, it has been widely used for gears and bearings in heavy-duty equipment that is frequently exposed to harsh conditions, such as heavy loads, elevated working temperatures, high frequency vibrations, and high rotational speeds [1,2]. Hence, failures such as wear, pitting, and contact fatigue damage, are more likely to occur on the surface of gears and bearings [3–5]. Vacuum carburization, as a surface strengthening method, plays an important role in the process of obtaining excellent comprehensive mechanical properties of 20Cr2Ni4A alloy steel. Vacuum carburization is a rapid process in which acetylene is injected into a high-temperature furnace in pulsed mode under low pressure and vacuum [6–8]. However, vacuum carburizing still has its disadvantages, such as a high carburizing temperature, a long carburizing cycle, an uneven structure, and coarse carbide particles. Studies showed that the

introduction of rare earth (RE) ions in the carburizing heat treatment of steel or other parts reduces the treatment temperature and increases the carbon activity and the carbon di ffusion coe fficient during the carburization process; it can also improve the structure of the carburized layer [9,10]. Yan et al. [11,12] studied the e ffect of an RE catalyst on the gas carburizing kinetics of steel and found that compared to the conventional gas carburizing process, the incorporation of an RE led to an obvious increase in layer depth and carbon concentration profile, under the same carburizing conditions. The di ffusion coe fficient and transfer coe fficient increased by 50% and 117%, respectively. Dong et al. [13] used RE ion implantation to assist vacuum carburizing of 12Cr2Ni4A steels. The results showed that the addition of RE elements obviously improved the hardness of the carburized layer and decreased the hardness gradient. Moreover, the structure of the carburized layer was compact, and the carbides were fine and dispersed.

Ion implantation is a modern material surface modification technology that has become increasingly common for changing the characteristics and properties of metal materials [14]. It can selectively improve the wear resistance, corrosion resistance, oxidation resistance, and fatigue resistance of the material surface by implanting a small amount of metal ions without changing the original properties of the material substrate [15–17]. In recent years, ion implantation of lanthanum and yttrium has been mainly focused on enhancing the corrosion resistance, high-temperature oxidation resistance, and biocompatibility of pure metals or alloys [18–20]. Xu [21] discovered a significant improvement in the aqueous corrosion resistance of zircaloy-4 by yttrium ion implantation, compared to that of the as-received zircaloy-4. Wang [22] obtained the same conclusion upon exploring the lanthanum ion implantation into a zirconium alloy. Darwin et al. [23] found that ion implantation of lanthanum significantly improved the high-temperature oxidation resistance of an Fe80Cr20 alloy and developed a promising interconnection metal material for planar-type solid oxide fuel cells, namely, Fe80Cr20 60h-La. The co-implantation of yttrium and carbon ions enhanced the wear resistance of H13 steel, which was shown by an increase in the microhardness of 60%–170% and a wear resistance of 2–3 times [24].

However, there have been only a few studies on the application of REs for vacuum carburizing of alloy steel, and the detailed structure and surface properties of the carburizing layer have seldom been analyzed. Thus, to meet the surface property requirements of 20Cr2Ni4A alloy carburized steel, ion-implanted lanthanum and yttrium were used to improve the hardness and surface properties. Due to the large radius of REs, substantial lattice damage and high-density structural defects are produced in the alloy steel matrix during the process of ion implantation. After carburizing, the surface analysis was studied in detail by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), X-ray di ffraction (XRD), scanning electron microscopy (SEM), and optical microscopy (OM). In addition, the Rockwell and Vickers hardness tests were used to evaluate the hardness of the carburized layer on three di fferent treated samples. Lastly, the catalysis mechanism and the strengthening mechanism of REs during carburization are discussed.

### **2. Materials and Methods**

The parent material of the 20Cr2Ni4A alloy steel (0.17 wt% C, 0.25 wt% Si, 0.45 wt% Mn, 1.45 wt% Cr, 3.48 wt% Ni, and 0.01 wt% Mo) was cut into samples with dimensions of 15 mm × 15 mm × 15 mm. Before ion implantation, these samples were polished with a series of silicon carbide (SiC) abrasive papers with grits of 600# to 2000#, polished with a W3.5 diamond paste and finally ultrasonically cleaned with anhydrous alcohol, for 10 min. Lanthanum and yttrium with 99.99% purity were implanted with a metal vapor vacuum arc (MEVVA) ion source (Shanghai Kingstone Semiconductor Joint Stock Limited Company, Shanghai, China) at the Institute of Low Energy Nuclear Physics in Beijing Normal University (China). The parameters of the ion implantation process are shown in Table 1. Vacuum carburization was carried out after ion implantation of the REs. Vacuum carburization was carried out for 5 hours in a vacuum environment (1 × 10−<sup>5</sup> Pa) with a carbon potential of 1.2% and a temperature of 920 ◦C. To promote the uniform di ffusion of carbon atoms, C2H2 and N2 were injected alternately for 25 pulse cycles. Five hours after carburizing at 920 ◦C, high temperature tempering at 650 ◦C for 3 hours promoted the precipitation of carbides and the decomposition of retained austenite; then oil quenching at 820 ◦C was done to improve the hardness and toughness of the material; finally, low temperature tempering was carried out at 180 ◦C for 3 hours, to eliminate the internal stress of the sample. The whole heat treatment process was completed on the vacuum carburization automatic production line (ECM Technologies, Grenoble, France).


**Table 1.** Parameters of ion implantation before carburization.

The chemical characterization of the implanted surface and the depth profile of the element content distribution of the samples, with depth, were performed by XPS (Thermo Scientific Escalab EI+) (Thermo Fisher Scientific, Waltham, MA, America) using a monochromatic Al radiation source (energy = 55 eV). The acceleration voltage of argon ion etching was 4 kV, and the rate was 5 nm/min. The binding energy of elements was normalized to 284.8 eV of carbon. To illustrate the crystal structure defects of the implantation area after ion implantation, the surfaces of the implanted samples were observed using TEM (JEM-2010) on an instrument equipped with energy dispersive spectroscopy (EDS) (JEOL, Tokyo, Japan). The TEM samples for surface observation by ion implantation were prepared according to the following procedure. Thinning samples were bonded to silicon wafers using epoxy resin glue. Then, the sample were sliced vertically along the plane of the implanted surface and machined to a thickness of about 10 μm. Finally, the samples were ground by Gatan 691 ion thinner (Gatan, Pleasanton, CA, USA) at a small incident angle between 8◦and 3◦.

The phase composition was studied by XRD (D8 Advance, Bruker AXS, Karlsruhe, Germany, Cu K α1 = 1.5406 Å) at 40 kV and 40 mA, with an incident angle of 0.5◦, after ion implantation and vacuum carburizing. The scanning speed and angle range were 1◦/min and 10◦–90◦, respectively. The penetration depths of X-ray was estimated to be 26 nm at incident angles of 0.5◦ [25]. Considering the maximum concentration distribution depth of the two implanted ions at 20–25 nm, 0.5◦ was chosen as the incident angle of the XRD test after carburization. The content of the retained austenite was determined with an X-350A X-ray stress analyzer (ST Stress Technology Co., Ltd., Handan, China). The surface morphology and phase structure of the samples were observed by Optical Microscope (OM) (Olympus, Tokyo, Japan) and a Nova NanoSEM50 environmental SEM (FEI, Hillsboro, OR, USA) equipped with EDS (OXFORD, Oxford, England) was also used. The corrosive fluid for OM comprised an alcohol solution containing 4% nitric acid (volume fraction). To obtain the e ffective case depth of the carburized layer, according to the GB/T 9450-2005 standard, Vickers hardness testing was used to determine the longitudinal section hardness of the samples under a 1 kgf (HV1) load, and each hardness value was the average of three measurements. In the Vickers hardness testing, the distance (S) between two adjacent indentation centers should not be less than 2.5 times of the indentation diagonal and the distance di fference between successive adjacent indentation centers and part surfaces (a2–a1,) should not exceed 0.1 mm, as shown in Figure 1. The load time of the apparatus was 15 seconds. The Rockwell hardness of the surface and the core of the samples was measured by a Rockwell hardness tester (Shjingmi, Shanghai, China), and each hardness value was the average of five measurements. For the carburized surface, the depth of Rockwell indenter was about 0.08 mm; for the center, the depth of the Rockwell indenter was about 0.14 mm. The carbon element distribution on the surface of the carburized layer was determined by an electron probe microanalyzer (EPMA-1720, Shimadzu, kyoto, Japan). The test parameters were as follows—acceleration voltage, 10 kV; electron beam current, 200 nA; beam spot, 20 um; and test time, 20s.

**Figure 1.** Locations of the hardness indentations.
