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 4 For FFF, acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) are themost studied polymers [6,9-10] . ABS is derived from petrochemicals, while PLA is renewable and biodegradable. In addition, other polymers like polyvinyl alcohol (PVA), poly-caprolactone (PCL), polyamide (Nylon), polyether ether ke- tone (PEEK), polyetherimide (PEI), high- impact polystyrene, and poly (oxymeth- ylene) can be used for FFF [6-7] . Figure 1 shows the schematics of a customized filament-based FFF prin- ter [12] . Hall et al. have customized a commercially available extruder (Fila- bot EX2 extruder; Filabot Lab) to de- sign polylactic acid (PLA) filaments reinforced with up to 5 wt% ceram- ic particles like MAX phases (Cr 2 AlC, Ti 3 SiC 2 , and Ti 3 AlC 2 ) and MoAlB phases (Fig. 1a) [12] . They used a desktop 3D printer (H400 3D printer; Afinia 3D) to 3D print polymer-ceramic composites. This study shows that FFF has the po- tential to be commercially deployed as a low-cost system for entrepreneurs and small businesses. It is also possible to supplement a single printing head with a dual head to accelerate the print- ing process [7] . The top right of Fig. 1 shows the schematics of a pristine polymer base filament that is being fed into the ex- trusion head. These filaments can be filled with tailored particles. Additional- ly, fibers can be added separately to the filament during the extrusion process (in-nozzle impregnation method) [7] . The intent of the additions is to modify the properties of the polymeric matrix to improve its strength and/or to incorpo- rate functional properties. The printing process can be fur- ther engineered through different para- meters (Fig. 1 bottom), including: orien- tation, usually horizontal (x-axis), vertical (z-axis), or lateral (y-axis); lay- er height (material deposited along the z-axis during a single cycle); ras- ter width (width of the extruded layer); air gap (gap between extruded layers); raster angle (angle of deposition mea- sured with respect to the x-axis); print- ing speed; infill density and pattern; nozzle temperature and diameter; and contour [9] . Based on a brief survey, the ad- vantages of FFF include simple and tai- lorable process, flexible printing speed for different applications, entrepre- neur and small business friendly, and environmentally benign if bioplastics are used. In addition, the microstruc- ture can be engineered by filling in dif- ferent additives such as metals (Cu, Fe, and others) and ceramics (SrTiO 3 , MAX phases). Daminabo et al. outlined some of the challenges of FFF, including lay- er trails due to deposition kinetics (stepped layers), sample collapse due to insufficient scaffold (also referred to as “overhang effect”), leak/loss due to localizedmelting of the filament (string- ing), distortion/warpage due to internal stresses and/or thermal gradients, and nonuniform samples [6] . In their review, Solomon et al. summarized that the biggest limitations of this process are the inadequate availability of filament materials for FFF [9] . Popescu et al. sum- marized that the mechanical behavior of FFF components are anisotropic and dependent on the bonding between the printed layers [11] . This bonding can be further enhanced by tailoring layer thickness and raster width where thin- ner print layers enable better bonding. However, there is a trade-off of longer print times. TRIBOLOGICAL BEHAVIOR Mohamed et al. investigated the effect of layer thickness, fill gap, fill an- gle, build direction, raster width, and number of walls on the tribological be- havior of PC-ABS (proprietary high-per- formance ABS developed by Stratasys Inc.) using pin-on-disk testing against EN31 steel discs [13] . They observed that wear rate decreased when the layer thickness was decreased. Comparative- ly, wear rate increased as raster angle and air gap were increased. Srinivasan et al. investigated the effect of infill density, layer thickness, and infill pattern on the tribological behavior of ABS with the pin-on-disc method during dry sliding [14] . Their team concluded that the property of these solids can be enhanced by lower- ing layer thickness and increasing infill density; they also recommended using a grid pattern. Sood et al. outlined that scratching, fatigue, crack formation, and breakage of adhesive bonds were responsible for pit formation and wear in FFF-based ABS parts [15] . They also recommended that lower distortion is needed to prevent wear. Prusinowski and Kaczy´nski inves- tigated the tribological behavior of ABS and continuous fiber-reinforced ABS composites by using pin (ABS-based material)-on-disc (40 HM steel) [16] . They studied the effect of layer thickness and filler content as part of this study. They observed that fibers were effective in enhancing the wear resistance of ABS composites during dry sliding. Hall et al. used FFF to print PLA- based samples reinforced with MoAlB, Ti 3 SiC 2 , Ti 3 AlC 2 , and Cr 2 AlC by using an infill of 99% and a layer thickness of 0.2 mm [12] . They reported that the ad- dition of these phases can enhance the triboactive behavior of these compos- ites. For example, the friction coefficient (µ mean ) decreased by approximately 76% of µ mean in PLA after the addition of 1 wt% additives, but there was marginal or no decrease in wear rate. Abdelaal et al. reported that lay- er thickness and impact angle had the most influence on 3D-printed PLA samples during water-silica impact test- ing [17] . Srinivasan et al. compared the tribological performance of ABS with PLA-20% carbon fiber composite [18] . They reported PLA-20% carbon has a better response than ABS. In addition, they also reported that infill density and layer thickness had a significant ef- fect on the results. Bustillos et al. reported a 14% en- hancement in wear performance of PLA after the addition of graphene [19] . Ha- non et al. reported that the addition of bronze improved the wear performance of PLA but had limited effect on the fric- tion coefficient when testing against polished steel using a cylinder-on-plate reciprocating tribometer [20] . Ertane et al. also reported that the wear rate of PLA can be improved by using biogen- ic carbon (biochar generated by pyroly- sis of wheat stem at 800˚C for 2 h) [21] . For example, PLA reinforced with 30 vol%