Tensile strength and wear resistance of glass-reinforced PA1212 fabricated by selective laser sintering

ABSTRACT Glass fibre (GF) and glass bead (GB)–reinforced polyamide1212 (PA1212) was additively manufactured by selective laser sintering. The effects of laser power and GF content on the tensile and tribological properties of the printed specimens with a base GB weight fraction of 40 wt.% were investigated. The strengthening mechanism of GFs/GBs was illustrated by analyzing the interfacial adhesion between the fillers and the PA1212 matrix. The specimens with 40 wt.% GBs and 10 wt.% GFs fabricated at a laser power of 30 W exhibited a strength of 52 MPa, a friction coefficient of 0.23, and a wear rate of 0.0011 mm3/N·m. The selected optimal laser power and GF addition contributed to the strong interfacial adhesion, which realised flat surface morphology and an adequate encapsulation of fillers in the specimen. The reinforcement of GBs/GFs in PA1212 can serve as a reference for a deeper understanding of the strengthening mechanisms for other additively manufactured engineering plastics.


Introduction
Additive manufacturing (AM), also known as 3D printing, has attracted much interest recently due to its design freedom and near-net shape forming capability (Ameen et al. 2018).Physical components can be constructed directly from layers of polymer, metal, or ceramic materials by AM (Han et al. 2020;Wu et al. 2020;Zhang and Liu 2021).Selective laser sintering (SLS), which involves the use of a laser heat source to sinter powder materials (Bracaglia et al. 2017;Tan et al. 2016), is one of the most popular AM techniques featuring high forming accuracy, high material utilisation ratio, and no support structure requirements.Material design for SLS is a research and development hotspot that plays a decisive role in the physical and mechanical properties of the formed parts.Theoretically, all powders that can form atomic bonds after heating can be used as the molding materials.A variety of SLS materials have been developed, including polymers (Shi et al. 2015;Pan et al. 2016;Yuan et al. 2019), metals (Chen et al. 2019;Wu et al. 2021), ceramics (Liu et al. 2016), etc.Compared with metals and ceramics, polymers have advantages such as having a low forming temperature, low sintering laser power consumption, high precision, and high melting viscosity (Kruth et al. 2008).Moreover, because of their low surface energy, high melting viscosity, and the absence of the spheroidization effect that is commonly observed in metal powder sintering, polymers were the first materials applied for SLS and are presently the most widely used and successful SLS materials (Espera et al. 2019).
Even as the types of suitable polymer materials increase with the development of SLS, polyamide (PA) is still the best choice for the direct fabrication of functional plastic parts by this technique, accounting for more than 95% of the SLS material market to date (Yang et al. 2019).Polyamide powder materials have been widely used in SLS for their advantages in corrosion resistance, chemical stability, and cost effectiveness (Goodridge, Tuck, and Hague 2012;Uddin, Williams, and Blencowe 2021;Li et al. 2021;Bai et al. 2017).Kozior (2020) investigated the effects of layer thickness and printing orientation on the stress relaxation of the SLS-printed parts via a five-parameter rheological model of the so-called Maxwell-Wiechert model for PA2200.The rheological analysis indicated that the optimisation of SLS processing parameters can produce parts with a combination of light weight and decent strength.Adding inorganic fillers, metal powders, or polymers to the PA matrix and adjusting the process parameters can improve the mechanical properties of the as-sintered parts for wider functional applications.There are only a few metal/PA materials used in SLS mainly due to the poor compatibility between metal and polymer materials.Adding polymer materials to the PA matrix usually requires complicated preparation methods and has a limited reinforcement effect.Currently, inorganic fillers such as glass beads (GBs) (Chung and Das 2006;Cano, Salazar, and Rodríguez 2018), carbon fibres (CFs) (Jing et al. 2017), glass fibres (GFs) (Gebretsadik, Hardell, and Prakash 2020), nano silica (Xu et al. 2016), and nanotubes (Bai et al. 2014;Yuan et al. 2018) are the main choices to enhance the mechanical properties of PA in varying degrees.Nano fillers are usually added to PA through in situ polymerisation or surface coating processes, which could be cumbersome, costly, and inconsistent (Chen et al. 2022).
GB fillers have little effect on the viscosity of the material and can offer good flowability, uniform stress distribution, great dimensional stability, no orientation effect, and weak anisotropy, which can reduce the shrinkage and warpage of the molded material.At the same time, GBs are cheaper and easier to obtain than the other fillers mentioned above, which can reduce the material cost significantly (Guo et al. 2021).Mousa, Pham, and Soe (2014) studied the optimisation of SLS process parameters and prepared GB-filled PA12 composites to increase the printing accuracy, demonstrating that GBs can improve the molding effect of the PA composite.Guo et al. (2021) prepared GB/PA12 composites with lower surface roughness, greater surface hardness, better dimensional accuracy, and a higher modulus of 2.74 GPa than neat PA12 (1.18 GPa) through multi jet fusion.Heather et al. (O'Connor and Dowling 2020) studied the GB reinforcement on PA12 composites, which showed an 85% improvement in the tensile modulus but a 39% decrease in the tensile strength when 40 vol.%GBs were added.The stiffness of GBs improved the tensile modulus, while the excessive GB addition of 40 vol.%led to poor interfacial adhesion between the GBs and matrix.The observation of exposed GBs in the fracture morphology suggested an insufficient encapsulation of the PA matrix to GBs, which explained the reduction of strength.
GFs have the characteristics of high strength and stiffness, which can significantly enhance the mechanical properties of PA (Salazar et al. 2014;Liu et al. 2021;Zarybnicka et al. 2022;Persson, Hinrichsen, and Andreassen 2020).Holmström, Hopperstad, and Clausen (2020) used injection molding to prepare PA6 reinforced with different loading fractions of short GFs and found that the PA6 composite was comparable to metal in loadbearing applications.In a study of GF reinforcement on PA12 (Lanzl, Wudy, and Drummer 2020), the addition of 30 vol.% GFs led to a three-fold improvement in the stiffness of the as-printed parts but reduction in their elongation.To achieve better strengthening by GFs, the GF distribution within the polymer matrix should be homogenous, especially in thin-walled SLS polymer parts (Bochnia, Blasiak, and Kozior 2020).In general, the coefficient of friction between PA and the reinforcement is inevitably high, which limits the applications of PA composite materials.Thus, reducing the coefficient of friction and improving the wear resistance is essential to expanding the tribological applications of such materials.
GFs improve not only the mechanical properties of the polyamide composites but also their wear resistance (Panda, Bijwe, and Pandey 2019;Chabaud et al. 2019;Demirci and Düzcükoğlu 2014;Kunishima et al. 2020).Nandhini Ravi et al. (2020) studied the strengthening effect of GFs at the welding interface of PA66 and found that the addition of GFs enhanced both its tensile and tribological properties.Sung et al. (Kim, Shin, and Jang 2012) prepared short GF-reinforced PA12 composites by twin-screw extrusion and studied the effects of fibre content, fibre orientation, and applied load on the tribological properties of the composites.The results showed that the fibre content had a great influence on the friction level and wear resistance of the composites, and the GF plaques generated on the sliding surface played an important role in improving the wear resistance of the composites.
PA1212 is an important engineering plastic material in the even-even polyamide series and has excellent physical and chemical properties (Cai et al. 2015).Compared to traditional polyamides with long carbon chains such as PA12 and PA11, PA1212 attracts attention in SLS applications due to its wider sintering window and better dimensional stability (Zhou et al. 2020).However, the mechanical and tribological properties of neat PA1212 material limit its mechanical applications.In this paper, GBs and GFs were used to strengthen PA1212, and the effects of varied GF content and laser power on the tensile strength and wear resistance of the printed parts were studied.In consideration of the strength reduction caused by 40 vol.%GBs in PA12 (O'Connor and Dowling 2020), a more appropriate GB proportion of 40 wt.% was introduced to PA1212 for subsequent GF addition, by which better mechanical and tribological properties were expected to be achieved.It is expected that the results can provide meaningful guidelines for improving the properties of PA1212 to promote a wide range of applications.

Materials preparation
In this study, spherical PA1212 powder particles (Hunan Farsoon High-Technology Co., LTD) were used, which were prepared through the dissolution-precipitation method (average particle size range: 60-80 μm).The average particle size and theoretical density of the reinforcement filler solid GB (Jinyi New Material Technology Co., LTD) powders were ∼18 μm and ∼2.5 g/cm 3 , respectively.The length and diameter of the GFs (Jinyi New Material Technology Co., LTD) were ∼150 μm and ∼15 μm, respectively.PA1212 powder, GBs, and GFs were added into a mixer according to the proportion shown in Table 1.The mixture was stirred at room temperature (23 °C) at a speed of 60 r/min for 30 min then sifted through a 60-mesh sieve to obtain the raw composite powders for SLS printing.The specific composition of each powder is shown in Table 1.PA1212 composite powder (0GB-10GF), mixed with only 10 wt.% GFs, was prepared for X-ray diffraction (XRD) and underwent flowability characterisation.

SLS processing of the PA1212 composites
As shown in Figure 1, PA1212 and its composite specimens were fabricated by an industrial-scale SLS apparatus (HT252, Hunan Farsoon High-Technology, China) with a continuous-wave CO 2 laser, which offers up to a maximum adjustable laser power of 60 W with a beam diameter of 380 μm.The powder bed was preheated at 170 °C to prevent the curling of the specimens during the SLS process.The hatch distance should be smaller than the laser beam diameter to obtain a proper overlap for good formability.A hatch distance of 250 μm realised a good combination of high specimen density and smooth surface morphology due to suitable overlapping.The built-in galvanometer speed of HT252 is 10 m/s.Therefore, the hatch distance and scanning speed were set at 0.25 mm and 10 m/s, respectively.The powder layer thickness was 0.1 mm.The laser energy density (ED), as defined by Equation (1) (Hong et al. 2019), is commonly employed to calculate the energy input per unit area of the laser.
where P is the laser power, V is the scanning speed, and S is the hatch distance.Laser power of 20, 25, 30, and 35 W were set to explore its influence on the performance of the sintered specimens.

Characterisation
The morphology of the powder and as-sintered specimens were observed using a scanning electron microscope (SEM, Quanta FEG 250) equipped with energy dispersive spectroscopy at an accelerating voltage of 2 kV.The size distribution of the powders was examined by laser light scattering with a particle size analyzer (TopSizer, Zhuhai Omec Instrument Co., LTD) with water as the dispersion medium.The powder particle size distribution was determined by the span value, where a smaller span value indicates a narrower powder particle size distribution.The flow properties of the powders, such as the dynamic density (g/cc) and avalanche angle (deg), were measured using a revolution powder analyzer (RPA, Mercury scientific-Newton, CT, USA) before the SLS processing.
The crystal structure of the 0GB-0GF, 40GB-0GF, 0GB-10GF, and 40GB-10GF powders were observed by using a Rigaku D/Max 500 X-ray diffractometer with Cu K α radiation operated at 40 kV and 40 mA.XRD measurements were performed by step scanning (2θ ranged from 10°to 80°with a 0.02°step size).A count time of 0.3 s per step was used, giving a total scanning time of 18 min.Samples weighing 7-8 mg were placed into aluminum pans and measured by a Seiko 220 (DSC-200F3, Nestal, Germany) differential scanning calorimeter (DSC) to evaluate the melting and crystallization behaviour of the raw powders.The DSC measurements were performed under a dry nitrogen atmosphere.The samples were first heated from 30 °C to 220 °C at a rate of 10 °C/min then held at 220 °C for 10 min to eliminate the effect of the thermal history.Subsequently, the samples were cooled to 100 °C at a rate of 10 °C/min to obtain non-isothermal crystallization curves.Finally, the samples were heated from 100 °C to 220 °C at a rate of 10 °C/min to obtain non-isothermal melting curves.
The density and shore hardness of the as-sintered specimens were measured by an electronic balance (AE5003, Hunan Changsha Xiangyi testing equipment Co., Ltd) using the drainage method with a shore hardness tester (LX-D, Zhejiang Yueqing Aidebao Instrument Co., Ltd).According to the ISO 527 standard, tensile tests of the SLS specimens were conducted at a room temperature of 23 °C using a Zwick/Roell Z010 universal testing machine.The fracture morphology of the tensile specimens was observed by an SEM.The tribological properties of the cylindrical specimens were evaluated by a HT-1000 pin-on-disk high temperature friction and wear tester (Lanzhou Kaihua Central Technology) in ambient environmental conditions (20-25 °C, ∼55% humidity).The upper pin was a GCr15 stainless ball with a diameter of 6 mm.The force applied on the specimen was 10 N, and the test was performed for a duration of 60 min at a friction radius of 5 mm and a velocity of 1000 r/min.The schematic diagram of the friction and wear test is shown in Figure 3.The sample was fixed onto a rotating wheel by screws and a fixed plate that rotated with the rotating wheel, and strong friction was generated between the friction pair and the sample by the applied normal pressure.The worn surfaces were measured by an ultra-depth three-dimensional microscope (VHX-5000).The wear rate k (mm 3 / N•m) can be defined as where V (mm 3 ) is the wear volume, N (N) is the applied normal force, and D (mm) is the sliding length.The morphology of the worn surface was observed by an SEM.

Characteristics of the selective laser sintering raw powders
The morphology of the 0GB-0GF, 40GB-0GF, 40GB-10GF, 40GB-20GF powders is shown in Figure 4(a-d).The original PA1212 powder had an irregular spherical shape, a rough surface, and many pores, which aided the adsorption of filler particles.GBs, which possess good sphericity and are smaller than PA1212 powder particles, were mixed with PA1212 powders uniformly as shown in Figure 4(b).As shown in Figure 4(c,d), spherical GBs and rod-like GFs were well mixed with PA1212 powder and formed composite powders.As SLS is a layer-by-layer method with minimal shear flow and pressure, it is possible to keep the composite powders dispersed uniformly.
The quantitative statistics of the powder particle size and their respective distributions is illustrated in Figure 4 (a 1 -d 1 ).The particle size distribution of neat PA1212 powder was narrow.When adding 40 wt.%GBs, the median diameter D(50) decreased, and the particle size distribution width increased slightly.When 10 wt.% and 20 wt.% GFs were added, the median diameter D (50) decreased slightly, and the change in the particle size distribution was not obvious.The laser spot diameter of the SLS machine is 380 μm, which is larger than the diameter of all the powder particles in this study.Therefore, the PA1212 composite powders used are suitable for the SLS process.
PA is a hetero-chain polymer containing an amide group (-NHCO-) in the main chain, which has a complex crystal structure.Figure 4(g) shows the XRD patterns of neat PA1212, PA1212-GB, PA1212-GF, and PA1212-GB-GF.The typical α-crystal morphology of PA1212 powders is ( 110) and ( 010)/(110) at 2θ = 20°a nd (010)/(110) at 2θ = 24°.The (100) crystal plane diffraction peak represents the intramolecular hydrogen bond formed during the folding of polymer molecular chains into molecular layers, and the (010)/(110) crystal plane diffraction peak indicates the hydrogen bond formed between each molecular layer.With the addition of GBs and GFs, the PA1212 content was reduced, which led to reduction in the peak value.
The flowability of the powders can be determined by comparing their dynamic densities and avalanche angles (the maximum angle of surface free powder prior to an avalanche) in the drum.The dynamic density was calculated by the RPA software and represents the average density of the powder during the volume expanding period within the drum, which is equivalent to the bulk density.Generally, a higher dynamic density represents better powder flowability, but under the circumstance that the density of GBs and GFs is higher than the nominal density of PA1212, the dynamic density is not suitable to characterise the powder flowability.Therefore, only the avalanche angle was used to measure the flowability of the composite powders.The powder flowability negatively correlates with the avalanche angle, i.e. a relatively low avalanche angle corresponds to a better powder flowability.
Table 2 shows the dynamic densities and avalanche angles of PA1212, 0GB-10GF, 40GB-0GF, 40GB-10GF, and 40GB-20GF.When only GFs or GBs were added in the cases of 0GB-10GF and 40GB-0GF, the avalanche angles were 56.2°and 40.7°, respectively, which were higher than that for neat PA1212 (37.8°), indicating that the addition of both GBs and GFs would reduce the flowability of the PA1212 powder.The impact of GFs was more significant, which was likely due to their high aspect ratio.When adding 40 wt.%GBs to the 10 wt.% GF composite powder, the avalanche angle of 40GB-10GF was 44.9°, which was smaller than that of the 0GB-10GF composite powder, indicating that the addition of GBs helped to improve the flowability of the GF/PA1212 powder.This increase may be attributed to the spherical GB particles filling the gap between the GFs and PA1212, thereby improving the powder flowability.When the GF content was increased to 20 wt.%, the avalanche angle of the 40GB-20GF composite powder was 59.3°, indicating that the powder flowability became worse.To sum up, it was expected that the sintering process of 40GB-10GF would be the smoothest, and its material forming properties would be the best among all the tested samples.
To further understand the influence of GBs and GFs on PA1212, the thermal properties of 0GB-0GF, 40GB-0GF, 40GB-10GF, and 40GB-20GF powders were examined by a DSC.As shown in Figure 5(a), the heating curves of the four raw powders exhibit a melting peak.When GBs and GFs were added, the melting peak temperature of the composite powder decreased, and the area under the melting peak gradually decreased.
Similarly in Figure 5(b), the cooling curves of the powders exhibit a trough, and the trough temperature increased gradually while the area above the trough decreased gradually when GBs and GFs were added.With this addition, the melting point of the composite powder decreased while the crystallization point increased slightly.The overall thermal performance of the powder degraded, and the enthalpy value and energy demand for the SLS sintering process decreased.Only one melting and crystallization peak was observed because GBs and GFs have high melting points and did not melt during the test process.During cooling, GFs and GBs acted as nucleating agents that promoted the crystallization of the PA1212 matrix, thereby raising the initial crystallization temperature.In the SLS process, the composites could be sintered better, which is consistent with the density results.

Sintering degree of polyamide1212 composites
The density distribution of the as-sintered specimens is presented in Figure 6(a).When the laser power was 20 W, the density of neat PA1212 was the lowest at 0.9395 g/cm 3 , which was even lower than that of water.This low density stemmed from the pores caused by the inadequate energy density.The results showed that the material density significantly increased with the GF content.When the GF content ≥ 10 wt.%, the highest density achieved was 1.4723 g/cm 3 .The densities of GBs and GFs were both measured as 2.5 g/ cm 3 , which is higher than the nominal density of PA1212.This discrepancy in the inherent density significantly affected the density of the as-sintered specimens.The densities of specimens with the same GB and GF content exhibited an increasing tendency with the laser power, as shown in Figure 6(a).In general, the  density of SLS specimens positively correlates with the energy density (Hong et al. 2019).
The shore hardness of all the composites produced by SLS is shown in Figure 6(b).It was observed that the addition of GFs led to an increase in the hardness.At 10 wt.% GFs, the hardness of the specimens reached a maximum of 83.8 HD, which was 10.3% higher than that of 0GB-0GF under the same laser power.In general, the increase in the filler content led to an increase in the hardness.However, excessive GF addition decreased the hardness.The decrease in the weight fraction of PA1212 reduced the matrix encapsulation of the fillers, resulting in poor interfacial adhesion between the composite materials.The increase in the laser power from 20 to 30 W led to a continuous increase in the shore hardness.The decrease in the shore hardness occurred as the laser power was increased to 35 W. This laser power provided an excessively high energy density, which caused overheating of powder and rough surface morphology, resulting in the decrease in the shore hardness (Czelusniak andAmorim 2020, 2021).
As shown in Figure 7(a), the surface exhibited porous morphology under the inadequate energy density caused by a low laser power of 20 W. During the SLS process, the inadequate energy density led to fast cooling of the molten pool, which resulted in a lack of fusion and the formation of pores.The interfacial adhesion between fillers and the matrix was consequently poor.Similar defects caused by inadequate energy density are shown in Figure 7(b).As the laser power increased to 30 W, defects were eliminated, and the surface exhibited flatter morphology.The adequate energy density led to an appropriate  cooling rate of the molten pool, thereby ensuring the full fusion of powders and realising a more compact fillermatrix interface as shown in Figure 7(c).However, as the laser power increased to 35 W, the surface became rougher and showed slight overheating of powders.The excessive energy density led to a lower cooling rate and fluctuated molten pool surface, hence resulting in rougher surface morphology as shown in Figure 7(d).The evolution of the as-sintered surface was consistent with the effect of laser power on the density and hardness of the specimens.The increase in the laser power provided adequate energy density, by which the pores and lack of fusion were eliminated, which contributed to the increase in the density and hardness.The reduction in the hardness resulted from the increase in the laser power from 30 to 35 W was related to the rough surface morphology caused by the excessive energy density.
By studying the density and hardness of all the printed specimens, the 40GB-10GF specimen exhibited the optimal shore hardness across all the laser power, and the hardness was the highest at a laser power of 30 W. Therefore, this laser power was selected to investigate the influence of different GF content on the tensile and tribological properties of the materials, and the 40GB-10GF specimens were selected to study the effects of laser power on the tensile properties and tribological properties of the printed parts.

Mechanical properties of the as-sintered specimens
Figure 8 shows the mechanical properties of the SLS-fabricated specimens with different GF content and laser power.The effect of different GF content on the tensile properties of SLS specimens under the same laser power (30 W) was considered.At the same GB content (40 wt.%), the tensile strength increased from 44 to 52 MPa, and the tensile modulus increased from 2359 to 4068 MPa when the GF content increased from 0 to 10 wt.% (Figure 8(a,b)).However, at 20 wt.% GFs, the tensile strength and tensile modulus decreased to 47 and 3972 MPa, respectively.The tensile strength and modulus of neat PA1212 were 35 and 1065 MPa, respectively.The elongation of neat PA1212 at break was the highest at 28%.When GBs and GFs were added, the elongation at break decreased significantly, and more reduction was observed as the GF content increased.In conclusion, 10 wt.% GFs offer the largest improvement in the mechanical properties of the material.
As seen in Figure 8(c), with the increase in the laser power from 20 to 35 W, the tensile strength and elongation of 40GB-10GF increased from 28 to 61 MPa and 2.76 to 4.5%, respectively.The higher laser power provided larger energy density, leading to the full melting of powder particles, which resulted in high tensile strength.
The mechanical properties of the SLS-fabricated composites were mainly affected by the intrinsic factors of each component and the laser power.The movement of molecular chains was limited by the interaction between GFs/GB particles and the polymer matrix.Therefore, the tensile strength and modulus of the as-built composites improved with the GF content while the elongation at break decreased.For the SLS process, the strength and elongation positively correlate with the energy density.As shown in Figure 9(ad), the higher energy density caused by the increase in the laser power helped to eliminate porosity.The pores stemmed from a lack of fusion and represented the poor interfacial adhesion between the fillers and matrix, which resulted in the exposure of GFs and GBs that was also consistent with the lower specimen strength and elongation.The poor surface and fracture morphology of the 40GB-20GF specimens are shown in Figure 9(e,f).The excessive GF addition lowered the weight fraction of the PA1212 matrix and thus reduced the encapsulation of fillers.The GF aggregation and the extruded GBs indicated the weakened interfacial adhesion between the fillers and matrix, which accounted for the decreased strength.

Tribological performance of the as-sintered specimens
Figure 10(a,c) show the friction curves and average coefficients of friction of the SLS-fabricated PA1212 composites with different GF content printed at a laser power of 30 W. It can be clearly seen that after the addition of GBs and GFs into the PA1212 matrix, the coefficient of friction decreased significantly.The coefficient of friction of neat PA1212 increased rapidly to about 0.45 in the first 5 min and then decreased slowly to about 0.42 in the rest of the test, ending up with an average coefficient of friction of 0.43.Compared with the coefficient of friction of PA1212, that of GB/GF/PA1212 composites increased rapidly to about 0.25 in the first 5 min of the test and then leveled off for the rest of the test.The coefficients of friction of 40GB-0GF and 40GB-10GF were 0.26 and 0.23, respectively.When the GF content was too high, i.e. 20 wt.%, the coefficient of friction stabilised at about 0.29, and the coefficient of friction increased compared to the one with a GF content of 10 wt.%. Figure 10  (b,d) show the friction curves and the average coefficient of friction of the 40GB-10GF specimens printed under different laser power.These specimens showed a significant difference in the friction and wear test.When the laser power was either too low or too high, the friction curve became unstable, and the coefficient of friction became larger.When the laser power was 30 W, the coefficient of friction of the 40GB-10GF specimens was 0.23, which was the lowest in the group.
Figure 10(e) shows the wear rates of PA1212 composites with different GF content at a laser power of 30 W. The wear rate of PA1212 was 0.0153 mm 3 /N•m.When the GB content was 40 wt.% without the addition of GFs, the wear rate decreased significantly to 0.0036 mm 3 /N•m.At 10 wt.% GFs, the wear rate of the specimen decreased to 0.0011 mm 3 /N•m.As the GF content increased to 20 wt.%, the wear rate increased instead, being lower than that of neat PA1212 only.The addition of both GFs and GBs led to the decrease in the wear rate of the PA1212 composite material, but excessive addition of GFs led to a larger wear rate.Figure 10 (f) shows the wear rates of 40GB-10GF specimens fabricated with 20, 25, 30, and 35 W laser power.The wear rate of the 40GB-10GF specimen was 0.0163 mm 3 /N•m at a laser power of 20 W. With an increase in the laser power, the wear rate of the 40GB-10GF specimen decreased significantly (25 W: 0.0034 mm 3 /N•m; 30 W: 0.0011 mm 3 /N•m).When the laser power was 35 W, the wear rate of the specimen increased to 0.0029 mm 3 /N•m.
The wear resistance of the specimen correlates to its surface morphology and formability, which are affected by the energy density.In an SLS process where other printing parameters are kept constant, the laser power directly defines the energy density, which affects the formability of the material.The inadequate energy density caused a lack of fusion in the molten pool, which explained the poor formability shown in Figure 7 (a,b).The excessive energy density led to overheating of the molten pool and thus resulted in low viscosity and poor formability, as shown in Figure 7(d).
The cross-sectional area of the wear track was analyzed by an ultra-depth three-dimensional microscope, from which the wear volume was calculated.The wear rate was calculated using equation (2). Figure 11(a) shows the wear profile of composites with different GF content.The wear surface profile of each material was observed to be smooth without any excessively rough or discontinuous surfaces, which is due to the self-lubricating nature of PA1212.The wear track of neat PA1212 was obviously deeper, and the wear profile of the 40GB-10GF specimen was the shallowest, indicating that the addition of GFs and GBs enhanced the adhesion between the materials in the composite, and the interface between the GFs and matrix was well bonded, thereby reducing the material loss in the friction process.
Figure 11(b) shows the wear profile of composites with 40GB-10GF at different laser power.Too little or too much laser power was not conducive to obtaining desirable tribological properties of the composites.When the laser power was 30 W, the wear profile of the composites was closest to a straight line, indicating that the selection of this power optimised the tribological properties of the composites, which was consistent with the coefficient of friction and wear rate results.

Effect of GFs on the tensile properties of the PA1212 composites
As shown in Figure 8(a), specimens with 40 wt.%GBs showed a strength of 44 MPa, which is 26% higher than that of neat PA1212.To further strengthen the PA1212 composites, GFs were added.The specimens with 40 wt.%GBs and 10 wt.% GFs exhibited a strength of 52 MPa, which was 18% higher than that of 40GB-0GF and 49% higher than that of neat PA1212.This reinforcement stemmed from the intrinsic high strength of rigid GFs.The addition of 40 wt.%GBs realised a good interfacial adhesion between the GBs and the matrix, by which GBs preferentially bore the load and consequently improved the tensile strength of the composite.The addition of GFs improved the strength but also reduced the powder flowability.Therefore, the addition of 40 wt.%GBs was necessary to maintain the powder flowability, and the addition of GFs to 40 wt.%GB-filled PA1212 can further enhance its tensile strength.When the part was subjected to tensile stress, the rigid GFs played a significant role in load bearing, and the tensile strength was thus improved.The pulling of fibres also consumed energy, thereby improving the tensile strength of the specimen.GFs are a rigid material with poor plasticity.Therefore, with the increase in the GF content, the plasticity of the material became worse, and the elongation at break decreased.The distribution of GFs in the matrix is illustrated in Figure 12.Appropriate addition of 10 wt.% GFs resulted in their dispersive distribution, which ensured a strong bonding interface between the individual GFs and the PA1212 matrix.The addition of 20 wt.% GFs produced crosswise tangled GF clusters in the matrix, thereby reducing the interface between a single GF and PA1212 and degrading the tensile properties of the composite.There is a trade-off between fibre reinforcement and porosity (Chen et al. 2022), where the increase in the GF content led to the poor fluidity of the powder and the corresponding increase in the porosity of the printed specimens, thus degrading the mechanical properties.

Effect of GFs on the tribological properties of PA1212
The wear surfaces of the GF-filled PA1212 specimens are shown in Figure 13.Cracks and melting traces were distributed in the crack edges of neat PA1212 as shown in Figure 13(a,e), which was an indication of typical fatigue wear.Slight abrasive wear was confirmed by the shallow furrow embedded in the wear surface.The pits, which were left by adhesive shedding and the dust that re- crushed on the wear surface after repeated actions of friction, indicated the occurrence of adhesive wear.Under the action of friction heat, the surface of PA1212 softened, migrated to the dual surface, and shed during the friction process.Subsequently, another new layer was generated, which further promoted the wear of PA1212.Figure 13(b,f) show the wear morphology of a 40GB-0GF specimen, where distributed small unbroken spheres of GBs were observed.The boundary between the GBs and matrix is clearly shown, and the melting traces indicated the occurrence of adhesive wear.Figure 13(c,g) show the wear morphology of PA1212 filled with 10 wt.% GFs.The distribution of obvious plastic deformation traces without noticeable peeling indicated excellent friction and wear performance.The optimised fatigue wear resistance stemmed from the addition of GFs, which strengthened the composites and prevented the propagation of fatigue cracks.The GFs on the worn surface preferentially bore the load and transferred stress to the surrounding matrix, bearing much more stress overall than the nominal PA1212 based on the actual contact.As more GFs were added, the interfacial adhesion of PA1212/GF deteriorated, which resulted in exposed broken fibres and cracks in furrows shown in Figure 13(d,h).In PA1212/GF composites, fatigue cracks were easy to initiate from the interface between PA1212 and GFs.The cracks grew rapidly as the interface interaction was weak, which consequently led to the tearing of PA1212 and GFs and the acceleration of wear for PA1212.
Mechanical properties correlate to tribological properties to a certain extent (Li et al. 2010).The addition of 40 wt.%GBs significantly improved interfacial adhesion between GBs and the matrix, thereby enhancing the tensile strength and tensile modulus of the composites and leading to further improvement in their tribological properties.The 40GB-10GF specimen exhibited a wear rate of 1.14 × 10 −3 mm 3 /N•m, which was 68.2% lower than that of the 40GB-0GF specimen.However, the subsequent increase in the GF content consequently led to an increase in the wear rate, which was determined by the interfacial adhesion  between GFs and the matrix.A proper addition of GFs optimised the load distribution by improving the loading ratio of the matrix.The load dispersion partially offset the plowing effect of dual surface micro-convex bodies on the PA1212 matrix, consequently reducing the wear rate of the material.During the friction process, the inconsistent force stemmed from GF misorientation, resulting in a varying contribution to the friction behaviour.
The wear of fibre-reinforced composites mainly comes from the following aspects (Shiao and Wang 1996): (1) matrix wear: adhesion, fracture, and plough; (2) fibre wear: fibre peeling, fracture, and its own thinning wear.Under friction, the combined wear of both the fibre and matrix constitutes the wear of composite materials.At 10 wt.% GFs, the interface between GFs and PA1212 was strong, and GFs bore the load in the PA1212 matrix.The addition of GFs improved the thermal conductivity and deformation resistance of the PA1212 matrix (Vaggar, Kamate, and Badyankal 2021), thus reducing the matrix wear and improving the wear resistance of the composite.The excessive addition of GFs reduced dispersion in the matrix.The heterogeneous distribution led to the exposure of GFs, which weakened the protection of the matrix.The broken parts from exposed GFs acted as abrasive particles on the friction surface and thus increased wear.The friction force of GF-filled polymers under friction is composed of the following four components, i.e.(a) the matrix adhesion force F a related to the shear strength of the matrix, (b) furrow force F p related to the roughness of the dual surface and number of hard micro-convex bodies; (c) interfacial peeling force F i related to the interfacial adhesion between fibres and the matrix, (d) fibre breaking force F f determined by the nature of the fibres.Among these four components, F f is a constant, and F a is slightly affected by the crystallization of PA1212 that stems from the effect of fibre content on the shear force.Thus, the friction force is mainly determined by F p and F i .In the case of 10 wt.% GFs, the interfacial adhesion was strong because of the adequate matrix covering the GFs, which increased F i , and the corresponding friction it caused.As the GF content exceeded 10 wt.%, the inadequate matrix covering the GFs weakened the interfacial adhesion and thus decreased F i , resulting in the falling of GFs from the matrix, which became micro-convex bodies.The furrow effect of hard micro convex bodies on the polymer friction surface and the dual part during the friction process increased the roughness of the dual surface.The rougher dual surface and hard particles increased F p , which was the reason for the increased coefficient of friction under the condition of 20 wt.% GFs.

Conclusions
In this work, based on the investigation of the influence of GF content and laser power on the tensile and tribological behaviours of SLS PA1212 composites, the following main conclusions are drawn: . The addition of 10 wt.% GFs and 40 wt.%GBs significantly strengthened the PA1212 matrix.The 40GB-10GF specimens exhibited a good combination of shore hardness and tensile strength of 83.8 HD and 52 MPa, respectively, at a laser power of 30 W. .The tribological properties of PA1212 composites improved with an increase in the laser power and GF content from 20 to 30 W and 0 to 10 wt.%, respectively.An excessively high laser power of 35 W led to a rougher surface, and an excessive GF content of 20 wt.% degraded the tribological properties by weakening the furrow force and interfacial peeling force.The 40GB-10GF specimens exhibited the lowest coefficient of fiction and wear rate at a laser power of 30 W, which were 0.23 and 0.0011 mm 3 /N•m, respectively. .Proper addition of 40 wt.%GBs and 10 wt.% GFs resulted in strong interfacial adhesion between the PA1212 matrix and GBs/GFs, which contributed to the improvement of tensile and tribological properties.GBs/GFs preferentially bore the load owing to the strong interfacial adhesion, which improved the deformation and wear resistance of the composites.Excessive addition of GFs significantly decreased the interfacial adhesion and led to a poor surface, which explained the increased coefficient of friction and wear rate.

Declaration of competing interest
The authors declared that they do not have any commercial or associative interest that represents conflicts of interest in connection with the work submitted.

Figure 2 .
Figure 2. Schematic illustration of (a) the selective laser sintering process and (b) sintered specimens.

Figure 3 .
Figure 3. Schematic diagram of a friction and wear test: (a) front view; (b) top view.

Figure 6 .
Figure 6.(a) Density and (b) shore hardness of the composites printed with different GF content and laser power.

Figure 8 .
Figure 8. Tensile properties of the specimens sintered at 30 W: (a) tensile strength and elongation at break; (b) tensile modulus; tensile properties of 40GB-10GF specimens sintered at different laser power: (c) tensile strength, elongation at break, and (d) tensile modulus.

Figure 11 .
Figure 11.Three-dimensional contour: (a) SLS specimens with different GF content at 30 W laser power, (b) 40GB-10GF specimens at different laser power.

Figure 12 .
Figure 12.Schematic of the mechanism of GFs enhancing the mechanical properties of PA1212.

Table 1 .
Content ratios of powders sintered in this work.