Selective laser sintering of alumina-molybdenum nanocomposites

Ceramics International 48 (2022) 29540–29545 Available online 11 August 2022 0272-8842/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Selective laser sintering of alumina-molybdenum nanocomposites C. G´ omez-Rodríguez a , b , L.V. García-Qui˜ nonez c , L.F. Verdeja b , G.A. Castillo-Rodríguez d , J.A. Aguilar-Martínez d , A.E. Mari˜ no-G´ amez d , D. Fern´ andez-Gonz´ alez e , * a Departamento de Mec´ anica, Facultad de Ingeniería, Campus Coatzacoalcos, Universidad Veracruzana, Av. Universidad Km 7.5 Col. Santa Isabel, Coatzacoalcos, 6535, Veracruz, Mexico b Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica, Escuela de Minas, Energía y Materiales, Universidad de Oviedo, 33004, Oviedo/Uvi´ eu, Asturias, Spain c CONACYT-Centro de Investigaci´ on Científica y de Educaci´ on Superior de Ensenada, B.C (CICESE), Parque de Innovaci´ on e Investigaci´ on, Carretera Aeropuerto, 66612, Apodaca, Mexico d Facultad de Ingeniería Mec´ anica y El´ ectrica, Universidad Aut´ onoma de Nuevo Le´ on, Av. Pedro de Alba s/n, 66455, San Nicol´ as de los Garzas, Mexico e Nanomaterials and Nanotechnology Research Center (CINN-CSIC), Universidad de Oviedo (UO), Principado de Asturias (PA), Avda. de la Vega, 4-6, 33940, El Entrego, Spain ARTICLE INFO Keywords: Selective laser sintering Molybdenum Alumina Refractories Nanocomposite Toughness ABSTRACT Alumina/molybdenum nanocomposites were obtained from alumina and molybdenum particles. Alumina with different molybdenum contents (0, 1, 2.5, 5, 10 and 20 wt %) was first uniaxially pressed at 100 MPa to obtain green compacts that were later sintered using a carbon dioxide (CO 2 ) laser. Samples were characterized by X-ray diffraction and Scanning Electron Microscope to evaluate the morphological and microstructural characteristics of the composites. SEM results show that for mixtures with 1, 2.5, 5 and 10 wt % Mo, metallic Mo appears dispersed within the alumina grains (grain boundaries and triple points). Nevertheless, for the mixture with 20 wt % Mo, it also appears in the alumina matrix as Mo in triple points and grain boundaries, although MoO 3 is also identified (as inclusion). The presence of these phases was confirmed by X-ray Diffraction technique. These metallic molybdenum particles distributed in the alumina matrix at triple points and grain boundaries promote the densification of the composite. Metallic molybdenum has also a pinning effect, which drastically affects the microstructural evolution during the sintering, mainly on the grain size of alumina. The best results were observed for the composite Al 2 O 3 -10 wt % Mo, with an average alumina grain size <10 μ m and few pores. 1. Introduction High alumina bricks are widely used in industrial furnaces due to the great characteristics of high temperature performance, great corrosion and wear resistances, among others. However, fracture toughness is sometimes a problem for certain engineering applications. For that reason, several methods have been proposed to improve the fracture toughness in ceramics, mainly based on the crack deflection, which can be made with fibers or whiskers, nacre-like structure or hard second phases. The incorporation of a second phase, such as metallic particles, to the alumina matrix has been subject of research for many years [1]. This way, Matteazzi and Le Ca¨ er synthesized nanometric α -Al 2 O 3 -M composites, where M was Fe, V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, W, Si and Fe alloys [2], which were sintered by aluminothermic reduction. Waku and co-workers also investigated Al 2 O 3 –Mo, -Ta, -Nb composites prepared by hot pressing [3]. Other composites Al 2 O 3 -metal have been proposed to improve the fracture toughness: Al 2 O 3 –Ni [4–7] and Al 2 O 3 –W [4,8]. However, molybdenum appears as the metal with the greatest interest to reinforce alumina ceramics when fracture toughness is considered. It has been added in the form of metallic powders, MoO 3 or Mo fibers [9–11]. Nawa and collaborators sintered Al 2 O 3 –Mo composites by hot pressing under vacuum conditions [12]. Wang and coauthors prepared Al 2 O 3 –Mo composites by different methods: dissolution of MoO 3 powder in ammonia solution followed by spray-drying or hot-plate drying before hot pressing under hydrogen reductant atmosphere; mechanical mixing and hot pressing [13]. Lada and others prepared Al 2 O 3 -10 vol % Mo composites by aqueous gel casting method and sintering in argon atmosphere at 1600 ◦C for 2 h with very slow heating and cooling rates [14]. Broniszewski et al. sintered alumina-molybdenum composites from powders by hot pressing at * Corresponding author. E-mail address: [email protected] (D. Fern´ andez-Gonz´ alez). Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint https://doi.org/10.1016/j.ceramint.2022.08.058 Received 27 April 2022; Received in revised form 1 July 2022; Accepted 6 August 2022 Ceramics International 48 (2022) 29540–29545 29541 1350 ◦C for 1 h under a pressure of 20 MPa in argon atmosphere [15]. Zygmuntowicz and researchers obtained Al 2 O 3 –Mo composites by centrifugal slip casting method and sintering at 1400 ◦C for 2 h under 20 vol % H 2 -80 vol % N 2 atmosphere [16]. Several questions arise from the above-mentioned research: the improvement of the fracture toughness resulted from both the second phase (molybdenum) on the composite and the inhibition of the alumina grain growth, and the requirement of long sintering processes under special atmosphere (in general reductant) to avoid the oxidation of the molybdenum (to form molybdenum (VI) oxide (MoO 3 )), whose boiling point is at 1155 ◦C. On the other hand, selective laser sintering (SLS) is a technique that allows the consolidation of powder layers, sintering layer after layer, to obtain three-dimensional parts with fine finishes and complex shapes, being an efficient, economical, high-precision and fast process. Two different types of lasers are commonly used Nd:YAG and carbon dioxide (CO 2 ). The type of material that can be studied with each laser mainly depends on both the wavelength that each laser generates (1.64 μ m and 10.64 μ m, respectively) and the wavelength that each material can absorb. Different materials have been treated using laser as heat source for the selective sintering of metallic, ceramic and polymer powders, or mixes between them [17–20]. Great part of the studies in the sintering using laser have focused on the variables that depend on the laser and have direct influence in the properties of the obtained parts: scanning speed [21], scan spacing [22,23], energy density [24,25], laser scanning strategy [26], layer thickness [22,27], laser power [24], particle size and distribution [28]. There is little research about the selective laser sintering of alumina parts. Part of this research focuses on the preparation of powders for later sintering in furnace [29,30]. Regarding direct sintering of alumina (or alumina with additives) using selective laser sintering, it is possible to report the research of Fayed et al. [31] that employed Nd:YAG laser to sinter alumina parts; Shishkovsjy and collaborators reported alumina-zirconium porous ceramics manufactured by selective laser sintering using Nd-YAG laser [32]; Subramanian and Marcus sintered alumina with aluminum by Nd-YAG selective laser sintering with subsequent treatment in furnace at different temperatures and times to study the effect [33]. Alumina-molybdenum cermet manufactured using selective laser sintering was not yet reported in the literature. Despite the significant interest of the alumina-metal composites to obtain alumina-base refractories with improved the fracture toughness (and flexural strength), and the potential advantages of laser sintering technique related with the economization of the processing time, this technique has not been already applied to manufacture such composites. This way, considering the high melting points of the considered materials (Al 2 O 3 , 2072 ◦C; Mo, 2623 ◦C), a heating source as the CO 2 laser was employed. Therefore, we present in this manuscript novel research about the utilization of the selective laser sintering to obtain aluminaMo composites. The influence of the molybdenum content (0, 1, 2.5, 5, 10 and 20 wt %) on the microstructure and morphology of the composite was studied when the green compacts were irradiated with carbon dioxide laser. 2. Experimental 2.1. Raw materials and sample preparation Powders of aluminum oxide (Al 2 O 3 , 99.9% purity, J.T. Baker) and molybdenum (Mo, 99.9% trace metals basis, Sigma Aldrich) were used as raw materials. SEM images of the initial powders are reported in Fig. 1. These images indicate that most of the molybdenum initial particles are nanometric while the size of the alumina initial particles is in the range 20–100 μ m. Six compositions of Al 2 O 3 and Mo samples were prepared with the objective of studying the influence of the molybdenum content on the microstructure of the ceramic-metal composite. The samples were prepared considering the following relation: (100−X) wt. % Al 2 O 3 +X wt. % of Mo, where X =0, 1, 2.5, 5, 10 and 20). Once weighted the starting materials in the above-indicated proportion, raw materials were loaded in an attrition mill with alumina balls (3 mm in diameter) in a ratio of 10 to 1 and using isopropyl alcohol as milling medium. Blending was carried out for 5 h in order to ensure a homogeneous mixture. Powder mixtures were treated at 120 ◦C for 24 h to remove any traces of alcohol and dry powders were sieved by 180 μ m. Powders mixture (1 g) were uniaxially pressed in a metallic mold to obtain cylindrical samples of 7 mm in height and 12 mm in diameter using a pressure of 100 MPa for 2 min. A SEM image of the green Fig. 1. SEM images of the initial powders of alumina (a) and molybdenum (b). Fig. 2. SEM image of the green compacts. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) C. G´ omez-Rodríguez et al. Ceramics International 48 (2022) 29540–29545 29542 compacts is reported in Fig. 2 to check the difference with the specimen sintered using laser of CO 2 . 2.2. Laser sintering Green compacts were sintered by means of a CO 2 laser equipment (Coherent, Diamond k-150 model with a wave length λ =10.64 μ m), where the laser irradiation was perpendicular to the circular surface of the samples. The laser nozzle is fixed in this equipment and the sample was placed on the translation system, which was programmed to move longitudinally, so a parallel radiation was achieved on the diameter of the samples. Subsequently, two more irradiation routes were assigned one next to the other. The sintering resulted on morphological and microstructural changes of the sample. The following selective laser sintering parameters were considered in this investigation: laser power of 79.5 W, spot diameter of the laser beam of 3 mm and scanning distance of 12 mm. Moreover, the irradiation power density is Q =70.29 W/cm 2 and the translation speed was 1.25 mm/s. There is a distance between the laser nozzle and the surface of the sample of 35 mm. Green compacts are located on the surface of the translation equipment and experiments were carried out at room temperature and under ambient atmosphere. 2.3. Characterization X-ray diffraction technique was used to identify the crystalline phases at the end of the selective laser sintering process. The equipment where the diffraction patterns were obtained was a PANalytical EMPYREAN diffractometer with a target of metal cobalt (radiation K α = 1.5406 Å) operated at 40 kV, 40 mA and an X’Celerator detector in Bragg-Brentano geometry. The scans were carried out over a 2θ range from 5◦to 120◦with a step scan of 0.013◦and 25.5 s per step in a continuous mode. Peak fitting was carried out with the commercial software XPowder12. 3. Results 3.1. X-ray diffraction Results of the X-ray diffraction analyses of laser sintered samples are summarized in Fig. 3. The oxidation of molybdenum has been subject of significant research throughout the years [34,35], because metallic molybdenum exhibits good mechanical properties at high temperatures but the resistance to oxidation is really poor. This way, at temperatures greater than 400 ◦C oxidizes to form MoO 3 , which easily volatilizes (1155 ◦C). This is a problem in the manufacture of composites with molybdenum as this element is lost during their manufacture, as for instance in the case of graphite-molybdenum composites sintered at high temperatures (2600 ◦C [36]). X-ray diffraction analyses suggest that oxidation of molybdenum for contents <10 wt % is not relevant and peaks correspond to only molybdenum and aluminum oxide, which might be associated to the sintering speed. X-ray diffraction analysis of alumina-20 wt % molybdenum composite indicates the presence of molybdenum (VI) oxide, which as it is later observed in SEM-EDX analyses, tends to agglomerate and the pinning effect of the molybdenum microparticles disappear. 3.2. SEM-EDX The morphological characterization was performed by Field Emission Scanning Electron Microscopy (FESEM) using a FEI Nova Nano SEM 200, with an acceleration voltage of 10-15 kV in low vacuum mode and an Helix detector; additionally, energy dispersive X-ray microanalysis, EDX (Oxford, model INCA X-Sight) was also carried out for semiquantitative chemical microanalysis. Images are collected in Fig. 4. It is possible to see that molybdenum clearly appears at grain boundaries and triple points, particularly for the composite Al 2 O 3 -10 wt % Mo. It is precisely in this sample where few pores are identified in the micrographs, which suggest high densification rates. In other cases, there are empty spaces at triple points and grain boundaries that suggest that the sintering of the composite was not complete. It is also possible to see in these cases that grain has grown during the selective laser sintering process, probably due to the insufficient quantity of molybdenum in some cases (when Mo content is <10 wt %) and to the significant oxidation of the molybdenum (when Mo content is >10 wt %). The sample with 10 wt % has the smallest grain size (<10 μ m), precisely promoted by the fine molybdenum particles located at triple points and grain boundaries that have a pinning effect. As it was observed in the Xray diffraction analyses, molybdenum (VI) oxide appears as inclusions in several points of the microstructure for the greatest molybdenum contents (20 wt %). This question is highly relevant for the mechanical properties of the composite as these inclusions appear as stress concentration sites in the composite. In the other cases, metallic molybdenum particles clearly appear at triple points and grain boundaries (see Fig. 4). 3.3. Discussion The reinforcement of ceramic materials, and in particular alumina ceramics, has always attracted significant interest in an attempt to promote the mechanical properties (as well as the slag corrosion resistance in other cases) by second phases located at triple points and grain boundaries that could divert the crack during its propagation. These second phases also avoid the grain growth and, in this way, a fine grained structure has a significant length of grain boundary, which is beneficial to avoid crack propagation. This manuscript collects a novel research about the application of selective laser sintering to manufacture Al 2 O 3 –Mo ceramic materials with the aim of studying the microstructure and analyzing how this could have influence on the mechanical properties, in particular on the fracture toughness. The main problem of the molybdenum is that it easily oxidizes under sintering conditions and this habitually requires sintering the molybdenum-based composites, and Al 2 O 3 –Mo, under special atmospheres as Ar/H 2 [37], vacuum [12,38], argon [14,15] or H 2 /N 2 [16]. In this way, the utilization of fast sintering techniques, as laser, might be useful to minimize the losses of molybdenum as molybdenum (VI) oxide during the sintering process and, at the same time, obtain dense composites. It was possible to see for molybdenum contents <10 wt % that molybdenum (VI) oxide does not appear during the X-ray diffraction analyses (this phase appears in the sample with 20 wt % molybdenum), although this is not indicative of potential minimum losses of this Fig. 3. XRD patterns of: (a) Aluminium oxide (Al 2 O 3 ) sample and (b–f) samples containing 1, 2.5, 5, 10 and 20 wt % of Mo, respectively. At the bottom, the diffraction patterns of Al 2 O 3 , MoO 3 and Mo are observed. C. G´ omez-Rodríguez et al. Ceramics International 48 (2022) 29540–29545 29543 element during the sintering process of the composites with <10 wt % Mo. SEM technique suggests pining effect promoted by the metallic molybdenum and few pores, which indicates that sintering of the Al 2 O 3 –Mo composite could be achieved without special atmosphere for these contents of molybdenum. This is potentially beneficial because near-net shape sintered samples could be obtained in very short times without using any special atmosphere, which otherwise could complicate the process. Therefore, laser sintering of ceramic materials arises as a technology that could compete with other novel high energy density technologies as solar energy [39–46] or spark plasma sintering [47,48]. This way, competitiveness of ceramic industry could increase because sintered ceramic materials might be obtained in shorter times. Selective laser sintering technique is a good option to sinter ceramics because it is a versatile technique, which is one of the aspects that is being searched by the industry and, particularly by the additive manufacturing industry. The reason is that this technology allows the rapid fabrication of functional parts, in addition to having the ability to process a wide range of materials (polymers, ceramics and metals) [49], although the manufacture of dense alumina ceramics is still to be studied. Therefore, this manuscript could be a starting point to study the obtaining of dense ceramics using laser technology. There is currently a wide variety of manufacturing techniques, but the most attractive and of greatest importance to the industrial sector are, naturally, those that provide high efficiency in production output without neglecting quality standards. Therefore, one of the most cost-effective methods for manufacturing complex 3D parts is the selective laser sintering technique, which has achieved the highest market share growth since 1997 [50]. Based on several studies [51–56], selective laser sintering could provide a number of advantages as the selection of any specific area, sintering at room temperature (with the sintering temperature being reached only in the surface of the beam), sintering of layers of microns in thickness, selection of the area that is to be irradiated, among others as the possibility of operating under two methods of irradiation, punctual or translational, where in this last case, it is possible to adjust the speed of the beam and the direction of irradiation. Moreover, it is possible to process parts of considerable size. Anyway, the greatest interest of this technology arises from the possibility of obtaining microstructures and morphologies of sintered refractories in seconds, which are comparable with those obtained by means of other sintering methods (as the sintering in furnace) that require hours. This could relate with the productivity but also with the savings in manufacturing costs, the reduction in energy consumption, and, therefore, this process could be environmentally friendly and could attract interest of the industry of ceramics in general. There are two laser sources used for the Selective Laser Sintering of materials which are, continuous wave CO 2 with a wavelength of 10.6 μ m, and Nd:YAG laser of wavelength 1.06 μ m with pulsed or continuous mode [57]. The use of the laser source for each material is mainly governed by the percentage of energy absorption of each material. In the research presented in this manuscript, laser of CO 2 was chosen because we know that the matrix of the composite (Al 2 O 3 ) has high absorption (96% of wavelength of 10.6 μ m with CO 2 laser) with long wavelengths [50]. In the case of metals, as molybdenum, it is known that they absorb shorter wavelengths. This way, a sufficient amount of energy was supplied to induce high local temperature leading the beginning of the solid-state sintering. Thus, when the Al 2 O 3 atoms were irradiated, they had sufficient energy to make that the vacancies of the Al 2 O 3 , which are in the hexagonal structure of the aluminum oxide, were replaced at high temperature by other cations that are inside of the volume of the crystal of the aluminum oxide. Therefore, there is a migration towards the grain boundaries, making the grain boundaries to start to move and grow due to the cationic movement, which takes place in the same direction. This growth associated to the movement of cations is drastically affected by the molybdenum particles, located at triple points and grain boundaries, avoiding the growth of the grains and the movement of the grain boundaries. 4. Conclusions Alumina-molybdenum composites sintered by selective laser sintering were proposed in this manuscript. Different molybdenum contents were studied: 0, 1, 2.5, 5, 10 and 20 wt %. It was possible to see that increasing the molybdenum content resulted in composites with less percentage of porosity. It is possible, for that reason, to say that increasing the molybdenum content results in greater densification rates, which could ensure the good characteristics of the alumina. The best results were observed in the case of the composite Al 2 O 3 -10 wt % Mo, with an average alumina grain size <10 μ m and few pores. It is also observed for this molybdenum content that this element drastically affects the microstructural evolution during the sintering, mainly the final size of alumina, as this element, located at triple points and grain boundaries, inhibits the grain growth. On the other hand, results suggest that laser sintering could be an attractive method to sinter Al 2 O 3 –Mo ceramics in short times. This is particularly relevant when the molybdenum content is <10 wt % Mo as greater contents involve the appearance of significant quantities of molybdenum (VI) oxide, which, apart from its potential volatilization and, thus losses of molybdenum, is Fig. 4. SEM images of irradiated samples, where: (a) corresponds to Al 2 O 3 particles and (b–f) corresponds to 1, 2.5, 5, 10 and 20 wt% of Mo, respectively. C. G´ omez-Rodríguez et al. Ceramics International 48 (2022) 29540–29545 29544 detrimental for the mechanical properties of the composite when it appears as inclusion. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Daniel Fern´ andez-Gonz´ alez acknowledges the grant (Juan de la Cierva-Formaci´ on program) FJC2019-041139-I funded by MCIN/AEI/ 10.13039/501100011033 (Ministerio de Ciencia e Innovaci´ on, Agencia Estatal de Investigaci´ on). We thank Dra. Nayely Pineda Aguilar for her support in the utilization of the Scanning Electron Microscope equipment. Cristian G´ omez-Rodríguez acknowledges the Universidad Veracruzana (Vicerrectoría Coatzacoalcos and Facultad de Ingeniería) for allowing him to make a short stay in the Nanomaterials and Nanotechnology Research Center (CINN-CSIC) and the University of Oviedo (Asturias, Spain) for research activities. References [1] J.S. Moya, S. L´ opez-Esteban, C. 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