*2.2. Methods*

## 2.2.1. Material Property Tests

Carbon fiber and glass fiber have excellent mechanical properties. The machinability and application of the fiber-reinforced PEEK are influenced by changing the material properties. Therefore, the tensile strength, hardness, and modulus of fiber-reinforced PEEK were tested in this paper to further analyze the influence of material properties on lapping machinability.

The tensile test was conducted at 298K with a tensile rate of 1mm/min using a material testing machine (5500A, Instron, Norwood, MA, USA) to measure the tensile stress curves of the fiber-reinforced PEEK. Flat dog-bone-shaped tensile specimens with a gauge length of 90 mm were made by wire cutting. The tensile specimens were ground on each side with SiC paper, resulting in a final specimen thickness of 4 mm and a gauge section width of 5 mm.

The nanoindenter (TI950, Hysitron, Eden Prairie, MN, USA) with a standard Berkovich indenter was used in this work to measure the load–displacement curves of the fiberreinforced PEEK materials. The sample was a 15 mm × 15 mm × 5 mm square block made by wire cutting, and the sample surface was polished before the nanoindentation tests. To ensure that the data of the experiment are reliable, three points in a random site of the PEEK matrix on the samples were checked during the test for each indentation depth. The maximum load was 8 mN, with the loading for 10 s, holding for 5 s, and unloading for 10 s. Furthermore, nanoindentation was conducted on the pure PEEK material, carbon fiber, and glass fiber, respectively, with the same conditions.
