EFFECT OF HIGH-ENERGY XE ION IMPLANTATION ON TRIBOLOGICAL PROPERTIES OF CHROMIUM UNDER WET CONDITIONS

This paper presents the results of tribological wear resistance tests on pure chromium subjected to xenon ion implantation with an energy of 160 MeV. Chromium is a very popular engineering material used in many industries, including aerospace, automotive, power generation and electronics. It owes its popularity to its resistance to high temperatures, corrosion, and high mechanical strength. High-energy implantation of heavy ions causes a significant amount of radiation damage, which affects the crystalline structure of the material's surface layer and thus changes its mechanical properties. This is especially important in space and nuclear power applications. The research was aimed at demonstrating changes in frictional wear resistance as a result of exposure to Xe ions at different doses per unit area. The research was conducted using the pin-on-disc method in an aqueous environment. A statistically significant decrease in tribological wear of pure chromium subjected to Xe ion implantation with an energy of 160 MeV was observed.


INTRODUCTION
In the field of materials engineering, significant attention is paid to the investigation of tribological properties of surface layers of various materials, which are crucial for the functioning of many devices and systems in the industry [1].Tribological properties, such as friction, wear, and lubrication, play a key role in the context of industrial applications, particularly for materials used as moving parts in machines and devices.Understanding these properties allows for designing more efficient systems and extending their service life.In the case of chromium (Cr), optimizing tribological properties can contribute to increased reliability and performance of devices in which it is used, as well as reducing costs associated with the need to replace worn parts [2,3].
Pure chromium is characterized by high corrosion resistance, hardness, and the ability to form durable protective coatings on the surfaces of other metals [4,5].Due to these properties, chromium is widely used in the automotive, aerospace, energy, sports equipment, and medical industries [6].Any studies focus on refining and modifying the structure of pure chromium to improve its properties and applications.It is also worth noting that pure chromium has significant applications in the nuclear and space industries, where it is exposed to intense radiation [7].In these particularly demanding conditions, understanding the impact of Xe ion implantation on the tribological properties of chromium can contribute to the development of materials with better operational parameters, which demonstrate resistance to damage and long-term stability in challenging environmental conditions.
There is a wealth of information in scientific literature regarding the use of ion implantation to modify the surface layer properties of engineering materials.In many cases, through the use of appropriate elements such as nitrogen or carbon, improvements in tribological wear resistance, microhardness, and increased corrosion resistance are achieved [8][9][10].The parameters of the ion implantation process, such as the dose of implanted ions and implantation energy, play a significant role in the effects obtained.When irradiating material with heavy element ions at high energies, there is an increased contribution of radiation damage associated with the applied process.High doses and energies of implanted ions can cause amorphous changes in the surface layer structure and even ion etching of the implanted surface [11][12][13].
The current state of knowledge contains little information about the impact of high-energy Xe ion implantation on the tribological properties of pure chromium.Wang et al. [14] indicated the influence of Xe ion implantation with an energy of 5 MeV on the nano-hardness and structural changes of chromium coatings.Hoffman and Gaerttner [15] studied the influence of Xe ion interaction on stress in the surface layer of chromium, albeit at relatively low implantation energies reaching 11.5 keV.The researchers did not address changes in tribological wear resistance.Therefore, there is a need for studies aimed at understanding the impact of high-energy Xe ion implantation on pure chromium to better comprehend the effects of such surface modification on the tribological properties of this material.Considering that chromium is often used in environments with elevated humidity due to its high corrosion resistance, tribological tests were conducted in a wet environment in the presence of water.
The aluminium grip of the sample protruded outside from the vacuum chamber and was air-cooled.The ion beam fluence did not exceed (2-8) •10¹² ions/cm²•s, which limited the power introduced to the sample by the ion beam to a value of (100-400) W/m².As a result, the temperature of the sample during irradiation did not exceed 60 °C.The pressure in the chamber during irradiation was 1.1•10⁻⁹ Pa.
Tribological testing was carried out via the pin/ball-on-disk method under wet friction conditions using the Anton Paar nanotribometer (NTR2) in an aqueous environment.During the test, the countersample was a tungsten carbide ball with a diameter of 1.0 mm pressed with a force of 300 mN.The relative speed of the countersample to the sample was 1.4 cm/s.Wear of the tested samples was measured as the cross-sectional area of the wear track on the surface of the sample and the material loss was stated.The track was measured using the Taylor Hobson Form Talysurf Intra profilometer in 12 locations.

RESULTS
A theoretical distribution of implanted ions over the sample depth was calculated using the SRIM software [16].
The results are shown in Figure 1.Based on the distribution of implanted ions, it can be determined that the maximum implantation depth was approximately 8.55 µm, while the highest concentration of implanted xenon ions was reached at depths in the range of 7.95-8.1 µm.The decrease in the coefficient of friction of the sample subjected to implantation with the highest dose, may be due to increased degradation of the surface of the test sample caused by the interaction of a large number of high-energy ions.This is a common phenomenon when using high-energy implantation [17].The surface deformation may have influenced an increase in the amount of lubricant (in this case, water), which caused a decrease in the coefficient of friction [18].The counter-sample during the test induced deformation of the surface of the test sample and approximation of its morphology to that of the other samples, hence the approximation of the friction coefficient values during the test to those characteristics of the other samples.The wear results were subjected to statistical analysis using Statistica software ver.13.1.Based on the Kolmogorov-Smirnov test, the normality of the results distribution in each sample was confirmed.The variance among samples was compared using Levene's test, revealing consistency in variance across the samples.Statistically significant differences among the tested samples were demonstrated through ANOVA analysis.
A Scheffe (post-hoc) test was performed, revealing statistically significant differences between the samples implanted with Xe ions and the non-implanted sample.A statistically significant decrease in wear was also demonstrated for samples implanted with doses of 2.5•10¹⁴ and 5•10¹⁴ Xe²⁰⁺/cm² compared to the sample implanted with a dose of 1•10¹⁴ Xe²⁰⁺/cm².Increasing the dose of implanted ions from 2.5•10¹⁴ to 5•10¹⁴ Xe²⁰⁺/cm² did not result in statistically significant changes in the tribological wear of pure chromium.The deepest and simultaneously widest wear track can be observed in the sample that was not subjected to ion implantation.As a result of the interaction of xenon ions within the chromium structure, changes occurred that significantly contributed to the increase in its resistance to tribological wear under waterlubricated conditions.It is worth noting that the study refers only to the implanted layer, as the maximum depth of the wear track for implanted samples does not exceed 0.6 µm, while considering the simulation results presented in Figure 1, the range of interaction for implanted ions exceeds 8.0 µm.It is also worth emphasizing that for pure chromium, high-energy ion implantation improved the resistance to tribological wear, while in the case of studies on the titanium alloy Ti6Al4V, an increase in tribological wear was observed after applying Xe ion implantation [19].

Figure 4
Results of profilometric measurements of wear tracks on pure chromium samples after tribological testing

SUMMARY
High-energy ion implantation of Xe ions has been shown to improve the tribological properties of pure chromium, including its resistance to wear.This is important in various industries such as automotive, aerospace, and energy.Based on the obtained research results, the following conclusions were drawn: • High-energy Xe ion implantation allows for a reduction in tribological wear of pure chromium under wet friction conditions.
• When wear track depth does not exceed the depth of implanted ions, conclusions can be drawn regarding the resistance of the implanted layer without the influence of the non-implanted layer's resistance.

•
The reduction of tribological wear in chromium in a water environment, after high-energy xenon ion implantation, is very promising.Many previous studies indicate a decrease in resistance to tribological wear after the application of high-energy implantation.The results of this study contribute to a better understanding of the effects of high-energy Xe ion implantation on pure chromium, potentially leading to the development of materials with improved performance parameters and resistance to damage in harsh environmental conditions.

Figure 1
Figure 1 Range of 160 MeV Xe ions concentration in chromium after irradiation with a fluence of 5•10¹⁴ Xe/cm²

Figure 2 3 .
Figure2Friction coefficients of the irradiated samples