*2.7. Frictional Properties Measured by the Progressive Load Scratch Test and Drawing Friction Test*

A microtribometer (CETR-UMT-2; Bruker, Billerica, MA, USA) was used to characterize the frictional properties of each disk specimen by the progressive-load scratch test. A diamond stylus having a 12.5-μm tip radius was moved 5 mm over a specimen surface with linearly increasing normal load (0.5–20 gf) at a constant speed of 0.016 mm s−1, and the value of the friction coefficient (tangential force) was obtained (*n* = 5). The initial frictional force, average frictional force during the first 0.5-mm scratch and total frictional force and average frictional force during the entire 5-mm scratch were calculated. After the scratch test, each specimen was inspected with a stereoscopic microscope (SMZ1500; Nikon, Tokyo, Japan) to determine the distance for detachment.

The forces generated with each wire/bracket combination were measured under dry and wet (in artificial saliva) conditions at room temperature (25 ◦C) using a custom-fabricated drawing-friction testing device attached to a universal testing machine (EZ Test; Shimadzu, Kyoto, Japan) [9]. Each bracket was bonded to a stainless steel plate with a non-filled adhesive resin (Superbond; Sun Medical, Shiga, Japan), and a bracket-mounting device provided 10◦ angular positioning for the bracket. The stainless steel plate with the bracket was attached to a friction-testing device. A 5-cm wire segment was then bound to the bracket using an elastic ligature (Alastik Easy-To-Tie Ligatures, 3M Unitek). The upper end of the wire was fixed to a grip attached to the load cell, and the lower end of the wire was fixed to a 150-g weight. Each wire was drawn through the bracket at a crosshead speed of 10 mm min−<sup>1</sup> for a distance of 5 mm. The X axis was recorded for wire movement and the Y axis for the force. In the present study, the static frictional force was determined at the initial peak of movement, and the kinetic frictional force was calculated by averaging force values after the static friction peak [7,8]. The sample size for each condition was 10 (*n* = 10). After the drawing-friction test, a specimen was inspected with a stereoscopic microscope (SMZ1500; Nikon) to observe the detachment of the DLC layers.

#### *2.8. Statistical Analyses*

Statistical analyses were performed using SPSS Statistics software (ver. 23J for Windows; IBM, Armonk, NY, USA). The mean frictional forces, along with the standard deviation, were analyzed by two-way analysis of variance (ANOVA). The two factors were the coating procedure (non-coating, DLC-1, DLC-2) and test environment (dry, wet). Additionally, the mean hardness, elastic modulus and frictional force were compared using one-way ANOVA, followed by Tukey's or Games–Howell tests. The mean distance for detachment in the progressive-load scratch test was compared using Welch's *t*-test. For all statistical tests, significance was predetermined at *p* < 0.05.
