October_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 0 1 5 biogenic carbon showed a lower wear when testing against an alumina ball. In addition, the friction coefficient also became stable during sliding, although the authors reported that designing PLA samples with a high volume of rein- forcement can lead to difficulty in pro- ducibility due to nozzle choking. Lin et al. reported that the tribo- logical properties of PEEK-carbon fiber (CF) composites are dependent on the orientation of the fibers [22] . External ad- ditions like nanosilica also affected the tribological behavior due to a rolling ef- fect. Zhang et al. showed that a 3D-print- ed Nylon 618-based gear showed better performance than injection-molded Ny- lon 66 gears while testing under low- medium torque [23] . Soundararajan et al. were able to manufacture polyamide 6 (PA6) filaments with up to 30 wt% TiO 2 additions [24] . They also showed that PA6 + 30 wt% TiO 2 has the lowest wear rate. Based on these results, the authors pro- posed that these materials can be used in automobile applications. Singh et al. developed filaments of Nylon 6 with Al 2 O 3 [25] . The authors concluded that the presence of Al 2 O 3 particles enhanced the tribological performance of these composites, and they performed better than ABS. CASE STUDY: PLA-BASED COMPOSITES A commercially available Maker- Bot Replicator 2X 3D printer was used to print test coupons with 0.1, 0.2, 0.3, and 0.4-mm layer heights (PLT). The PLA filaments were printed with a bed temperature of 40°C and an extrud- er temperature of 200°C. As listed in Table 1, in addition to a non-compos- ite PLA (MakerBot), other composite filaments were investigated that had powder additions of bronze (colorFabb), copper (colorFabb), and iron (Protopas- ta). Short strips were used for optical microscopy of polished cross sections. The tribological behavior of the samples was investigated with a block- on-disc tribometer (CSM Instruments SA, Switzerland) by using 3D-printed blocks (~4 x ~4 x ~3mm) against alumina discs (Ad Value Technology, Tucson, Ar- izona). All the 3D-printed samples were machined into the desired dimensions. The configuration where the 3D-print- ed layers were perpendicular to the substrate was used for tribological per- formance evaluation. A surface profilo- meter (Surfcom 480A, Tokyo Seimitsu Co. Ltd., Japan) was used to measure R a (arithmetic surface roughness). All the 3D-printed blocks were also polished to a Ra <1 µm, and alumina discs were pol- ished to Ra <3 µm. The experimental conditions used during these studies were 5 N, 31.4 cm/s linear speed, 10 mm track radius, and a sliding distance of 500 m, respectiv- ely. In this article, the µ mean of the re- sults is reported, which was calculated by taking the average of mean fric- tion coefficients of three data sets of similar compositions. The mass of the 3D-printed blocks and alumina were measured before and after the tribolo- gy testing with a weighing scale (mod- el XA82/220/2X, Radwag Balances and Scales, Poland). In all cases, the mass was transferred from the 3D-printed sample to alumina discs. Because of this, the specific wear rate (WR) report- ed is representative of the 3D-printed samples. The specific WR was calculat- ed from: WR = (m i – m f )/(ρNd) (Eq 1) where, m i is the initial mass, m f is the fi- nal mass, ρ is density of the composite, N is the applied load, and d is the total distance traversed by the sample during the tribology testing. All the blocks and discs were then coated with Au/Pd by using a Balzers SCD 030 sputter coat- er (BAL-TEC RMC, Tucson, Arizona) for microstructure evaluation [12] . The mi- crostructure evaluation of the polished samples was also performed by using this microscope in secondary (SE) and backscatter (BSE) mode. Figure 2 shows the SEM micro- graphs of PLA-bronze (Figs. 2a-b), PLA-Cu (Figs. 2c-d), and PLA-Fe (Figs. 2e-f) sam- ples after printing with printing layer thickness (PLT) of 0.2 mm. In all cases, the particles were uniformly distribut- ed in the microstructure, although de- fects like porosity were observed in the samples. For comparison, inset of Figs. 2a, 2c, and 2e show themicrostruc- ture of different filaments filled with bronze, Cu, and Fe particulates. The fil- aments also showed that particles are uniformly distributed in the filaments. Figure 3 plots friction coefficient (µ) vs. distance of PLA (Fig. 3a), PLA- bronze (Fig. 3b), PLA-Cu (Fig. 3c), and PLA-Fe (Fig. 3d) against alumina sub- strates. In the PLA-alumina tribo- couple, the profile of µ vs. distance showed fluctuations after the initial break-in period. Figure 4 shows the tri- bological performance of PLA-based composites. For non-composite PLA, µ mean decreased from 0.64 to 0.55 as the PLT was increased from 0.1 to 0.4 mm (Fig. 4a). Comparatively, the wear rate (WR) increased marginally from ~6.8 x 10 -4 to ~8.9 x 10 -4 mm 3 /Nm as the PLT was increased from 0.1 to 0.4 mm (Fig. 4b). Figure 5a shows the PLA sur- face where wear scars are visible, and Figs. 5b-c show the corresponding alu- mina surface where PLA was transferred due to abrasive wear. TABLE 1 – SPECIFICATIONS OF FILAMENT USED DURING THIS STUDY Company Type Color Diameter (mm) Density of Filaments (g/cc) MakerBot Industries LLC, Brooklyn, NY PLA True Red 1.75 mm 1.2 colorFabb, Belfeld, Bronze Fill PLA Bronze 3.9 The Netherlands Copper Fill PLA Copper 3.9 Protoplant, Vancouver, WA Magnetic Iron PLA Black 1.8