**1. Introduction**

Polyetheretherketone (PEEK) is a novel crystalline thermoplastic polymer material that is widely used in aerospace, electronics, and medical industries for its excellent properties, such as low density and superior machinability [1–3]. However, PEEK has a few shortcomings, such as lower strength and lower hardness than most metal materials. These make it difficult to meet the higher performance requirements of certain industries. Therefore, the study about the performance of modified PEEK is of great significance. The mechanical properties of PEEK can be further improved through modification, and fiber-reinforced modification is a commonly used method to modify PEEK. The material properties of the fiber-reinforced PEEK vary with different fiber types. The fibers commonly used for PEEK reinforcement include carbon fiber, glass fiber, graphite fiber, and polytetrafluoroethylene (PTFE) fiber [4–7]. Carbon fiber has certain advantages, such as high strength, high modulus, small thermal expansion coefficient, and superior machinability [8,9]. On the other hand, glass fiber is often preferred due to its high stiffness, elastic modulus, and good load-bearing capacity [10]. Both carbon and glass fibers are widely used for reinforcement purposes [11–13]. However, despite materials possessing excellent properties, they also need to obtain better surface integrity through mechanical processing to fulfill the high requirements of the manufacturing industry [14]. Therefore, to expand the application

**Citation:** Gao, S.; Qu, J.; Li, H.; Kang, R. Effect of Fiber Type and Content on Mechanical Property and Lapping Machinability of Fiber-Reinforced Polyetheretherketone. *Polymers* **2022**, *14*, 1079. https://doi.org/10.3390/ polym14061079

Academic Editors: Wei Wu, Hao-Yang Mi, Chongxing Huang, Hui Zhao and Tao Liu

Received: 22 February 2022 Accepted: 7 March 2022 Published: 8 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

fields of PEEK materials, it is crucial to study the material properties of the fiber-reinforced PEEK and analyze the influences of the change in the material properties on machinability.

The material properties and machinability of PEEK material change with fiber reinforcement, and the types and the mass fraction of fibers have diverse impacts on the PEEK matrix. The mechanical properties of PEEK can be enhanced with fiber reinforcing. Li et al. [9] compared the mechanical properties of PEEK and carbon-fiber-reinforced PEEK (CF/PEEK) and pointed out that carbon fiber can significantly improve the hardness, tensile, and compressive strength of PEEK. Zhang et al. [15] studied the material properties enhancement of the PEEK matrix by analyzing various fibers. The results showed that carbon fiber, glass fiber, and TiO2 could effectively increase the tensile strength of the PEEK matrix. Although the material properties of PEEK have been improved by fiber reinforcement, PEEK material still needs to be mechanically processed to meet the higher surface integrity requirements of some industries. Therefore, it is of great significance to study the machinability of the fiber-reinforced PEEK materials. The commonly applied machining methods of modified PEEK materials include turning and grinding. Davim et al. [14] studied the influence of turning parameters on cutting force and surface roughness of PEEK and glass-fiber–reinforced PEEK (GF/PEEK) materials with PCD tools turning. The results showed that the cutting force decreased with the increase in cutting velocity and feed rate. The surface roughness of PEEK and GF/PEEK decreased with increasing cutting velocity and increased with the feed rate. The surface quality of pure PEEK was better than GF/PEEK with the same turning parameters. Ji et al. [16,17] worked on the nanomechanical properties and machinability with the single-point diamond turning of PEEK, CF/PEEK, and GF/PEEK materials. The results demonstrated that PEEK was a single-phase material with constant values of nano-hardness and modulus, whereas CF/PEEK and GF/PEEK were fiber composite materials with superior hardness and elastic modulus. The processing surface had poor uniformity of force in turning, which led to poorer turning processability as compared to the pure PEEK. Khoran et al. [18] investigated the grinding machinability of PEEK materials. They concluded that the ground temperature has a great influence on grinding surface morphology and force. The ground surface quality was profoundly affected by the cryogenic cooling that led to superior surface quality.

Although micron surface roughness of PEEK materials can be achieved by turning and grinding, the surface roughness after processing was bad due to the poor uniformity of the cutting force. Therefore, ultra-precision lapping is suitable for PEEK material processing due to its good uniformity and controllability of the lapping force, and improved machinability is achieved on the surface of the workpiece. Furthermore, ultra-precision lapping is an effective method to obtain high surface quality and accuracy [19]. However, there are only a few studies on the ultra-precision lapping process of the fiber-reinforced PEEK. Therefore, the lapping process based on analyzing the material properties of fiber-reinforced PEEK should be further researched to fully understand the process.

In this paper, the material properties and lapping machinability of fiber-reinforced PEEK has been studied. The effects of the type and mass fraction of modified fibers on the mechanical properties of PEEK have been investigated by analyzing the mechanical properties of PEEK that have been reinforced by carbon fiber and glass fiber. Finally, the lapping experiment was conducted to analyze the influences of property changes of materials on the material removal rate and surface roughness with abrasive paper lapping.
