Elsevier

Tribology International

Volume 165, January 2022, 107322
Tribology International

Study on nanoscale friction and wear mechanism of nickel-based single crystal superalloy by molecular dynamics simulations

https://doi.org/10.1016/j.triboint.2021.107322Get rights and content

Highlights

  • The grinding ball repeatedly rubs the workpiece in all directions, which can better study the friction behavior of the workpiece material in all directions.

  • In the model, the coherent interface of materials is reproduced and other elements are added to make the simulation more realistic.

  • The friction fluctuates greatly with the coherent interface as the turning point after repeated friction.

  • The coherent interface inhibits the movement of atoms and blocks the extension of stacking faults.

  • At the same time friction, the γ phase wear scar depth is always higher than γ’ phase, and the γ' phase wear scars become deeper and deeper while the γ phase wear scars become shallower and shallower after repeated rubbing.

Abstract

The molecular dynamics method was used to simulate repeated nano-friction on a nickel-based single crystal (NBSC) superalloy. The friction force, atomic displacement, wear scar morphology, subsurface defects, potential energy, and other aspects were comprehensively studied to understand the friction mechanism of NBSC during working conditions. The frictional force fluctuated significantly and the coefficient of friction decreased during the repeated friction process. As the friction cycles increased, the wear scars of the γ' phase deepened, whereas those in the γ phase became shallower. Simultaneously, repeated friction reduced the volume and number of stacking faults. Additionally, the two phases softened to some extent at higher temperatures, but the phase boundary still had good wear resistance at high temperatures.

Introduction

To meet the increasing demand for high–performance engines in the aviation industry, the industrial and academic circles of various countries have continuously explored and developed materials with special properties. Nickel–based single crystal (NBSC) superalloys have become a key material for advanced aero–engine turbine working and guide blades owing to their excellent high–temperature mechanical properties [1], [2], [3], [4]. However, as a key hot–end component of aero engines, turbine blades often fail under the action of large thermal stress, high centrifugal force, and high–temperature alternating loads during service [5], [6], [7], [8]. Research on the specific failure causes shows that the friction and wear of the mortise and tenon at high temperatures are the direct causes of blade failure. Therefore, studies on the friction and wear of NBSC superalloys under high-temperature conditions are of great significance for understanding the friction behaviour and wear mechanism of advanced aerospace materials [9], [10], [11].

Investigations on friction and wear have traditionally been conducted through experimentation. The simulation of actual working conditions allows for the characteristics and changes in friction and wear to be obtained, and the mechanism or principle of various friction types to be studied from the perspective of wear products, wear surface state, and wear effects on the structure [12], [13], [14], [15]. However, traditional friction experiments cannot observe the dynamic migration and phase transformation process of materials, and research on friction behaviour can only be inferred from the friction products and related characterisation results. Moreover, the wear of solid materials is a continuous process of small accumulations, and the instantaneous wear amount can reach the micro/nano level, in which the objects of friction are several layers of molecules or atoms with discrete properties. Therefore, it is necessary to find a feasible method to reveal the micro nature of the physical and chemical changes in the friction and wear processes of NBSC superalloys from the basic unit of the material.

Molecular dynamics (MD) simulation, as a scientific algorithm, has proven to be a powerful tool for studying nanoscale machining processes [16], [17], [18]. Stoyanov et al. [19] studied the friction and wear behaviour of NBSC superalloys at high temperatures and found that the coefficient of friction (μ) of the NBSC superalloy is related to the crystallographic plane. Liu et al. [20] studied the effect of Al2O3 content on the mechanical and tribological properties of Ni–Cr alloys from room temperature to 1000 °C, and demonstrated that NiCr–40 wt%Al2O3 composites have good wear resistance. Xu et al. [21] used MD to study the process of diamond-rubbing Cu and Si and demonstrated that when the friction depth is close to the Cu–Si interface of the thinner Cu layer, the tangential force increases sharply, while the normal force remains almost unchanged. Rentsch et al. [22] first proposed the simulation results for the accumulation of chips in abrasive processing. They proposed a new method to improve the model representation and computational speed through a large–scale MD model. Li and Fang [23], [24] studied the subsurface damage of single-crystal Cu and Ni/Cu multilayers during nanoscale high–speed grinding using a MD simulation. They found that transformation of the deformation mechanism depended on the competition between dislocations and twins. Additionally, some researchers have also performed MD nano–processing simulations by considering the removal mechanism and deformation characteristics of different materials. Zhang [25] simulated the repeated processing of single-crystal Cu, which indicated that the residual defects and deformities of the workpiece after the first machining led to the nucleation and duplication of dislocations during the second machining. Pei et al. [26] studied the size effect of a simulation model by establishing a large–scale MD model, and found that when the model exceeded two million atoms, the boundary conditions had almost no effect on the simulation results. Recently, Wang et al. [27] used the MD method to systematically study the effects of cutting speed and depth, and abrasive shape on GaN single crystals in the nanofabrication process. Yin et al. [28] studied the nanofriction behaviour of SiC/Al–NC materials under the action of diamond grinding balls, and demonstrated that the frictional force and normal force increased with an increase in the scratch depth and grinding ball size.

To date, research on NBSC superalloys has mainly focused on elastoplasticity [29], microstructural evolution [30], fretting fatigue [31], and their corresponding failure mechanisms [32], [33]. Moreover, it is the basis and consensus of the above research that the lattice mismatch between the γ’ and γ phases generates a strengthening effect [34]. Few studies have considered the friction and wear behaviours and wear mechanisms. Hao et al. [35] applied MD methods to study the nanoscale cutting process of the Ni-Fe-Cr series of Ni-based superalloys with a SiC tool and studied the diffusion mechanism of the workpiece atoms. Ren et al. [36] studied the nano-machining of monocrystalline Ni by MD simulation and concluded that increasing the grinding speed can reduce the subsurface defects within a certain range, and the grinding depth is proportional to the subsurface defects. However, the true complex structure of NBSC alloys cannot be reflected by the above simulations, and the microscopic friction behaviour of NBSC alloys is still unknown.

In this study, an NBSC superalloy containing γ’ and γ phases and various alloying elements was established to determine the effect of grain boundary strengthening on the friction and wear. Additionally, the research was conducted with rotating friction to avoid the influence of material anisotropy on the research results. Meanwhile, the high-temperature strengthening mechanisms of NBSC superalloys were activated by introducing high-temperature and repeated friction conditions.

Section snippets

Simulation modelling

MD simulations were employed to investigate the nanofriction process of the NBSC superalloy. As shown in Fig. 1, the simulation model comprises two parts, namely a diamond grinding ball and a workpiece. Because the research focused on the friction behaviour of the NBSC superalloy and the diamond grinding ball hardly deforms during the friction process, the ball was set as a rigid body. Additionally, studies on the structure and composition of NBCS superalloys [37], [38] show that they are

Effect of friction on mechanical properties and atomic displacement of workpiece

To enable the energy state of the model to be more similar to the real material, the entire simulation system was fully relaxed before friction to achieve a balanced state. First, the atoms in the thermostat layer were maintained at 300 K using the Nosé–Hoover method to simulate heat dissipation during processing. Then, a 100,000 steps equilibrium was performed using the canonical ensemble (NVT), and the energy and atomic displacement of the system was transmitted every 500 steps. Fig. 2(a)

Conclusions

In this study, the MD method was used to systematically study the friction and wear mechanism of NBSC superalloys, and the detailed conclusions are as follows:

  • (1)

    At the initial stage of friction, the friction force increases with the increase of the depth of the wear scar. When the friction is repeated, the friction force fluctuates greatly, and reaches the extreme value before and after the grinding ball reaches the phase boundary. This is caused by the strong coherent interface hindering the

CRediT authorship contribution statement

Zongxiao Zhu: Modeling and simulation, Data curation, Writing – original draft, Formal analysis, collected and analyzed data, wrote the manuscript. Shi Jiao: Modeling and simulation, Data curation, Writing – original draft, Formal analysis, collected and analyzed data, wrote the manuscript. Hui Wang: Formal analysis, Data curation, collected and analyzed data. Linjun Wang: Formal analysis, Data curation, collected and analyzed data. Min Zheng: Formal analysis, Data curation, collected and

Declaration of Competing Interest

The authors declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51805513), the National Natural Science Foundation of China (51835012), and the Gansu Provincial Natural Science Foundation (20JR5RA462).

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