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Densification of complex shape ceramics parts by SPS
Journal Pre-proof Densification of complex shape ceramics parts by SPS S. Hocquet, V. Dupont, F. Cambier, F. Ludewig, N. Vandewalle
PII:
S0955-2219(19)30709-5
DOI:
https://doi.org/10.1016/j.jeurceramsoc.2019.10.038
Reference:
JECS 12800
To appear in:
Journal of the European Ceramic Society
Received Date:
28 August 2019
Revised Date:
3 October 2019
Accepted Date:
17 October 2019
Please cite this article as: Hocquet S, Dupont V, Cambier F, Ludewig F, Vandewalle N, Densification of complex shape ceramics parts by SPS, Journal of the European Ceramic Society (2019), doi: https://doi.org/10.1016/j.jeurceramsoc.2019.10.038
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Densification of complex shape ceramics parts by SPS S. Hocquet, V. Dupont, F. Cambier* BCRC, Belgian Ceramic Research Centre, 4 Avenue Gouverneur Cornez 7000 Mons (Belgium) – www.bcrc.be F. Ludewig, N. Vandewalle University of Liège – GRASP Laboratory, B5a Sart Tilman, 4000 Liège (Belgium) – www.grasp.uliege.be * corresponding author: Dr F. Cambier, e-mail [email protected], phone +32 65403421
Abstract
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Spark Plasma Sintering (SPS) equipments, with higher productivity and hybrid heating modes, allow reducing thermal gradients in large samples and broaden their application potential. Furthermore, strong shape limitations remain: only simple shapes can be obtained because of the use of uniaxial pressure.
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The aim of this paper is to propose a SPS densification method for complex shape ceramic or powder metallurgy parts, without any modification of the tools and equipment. The samples to be sintered are placed in a powder bed in the classical die used in SPS. Different powder beds have been tested.
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A numerical model was developed in order to allow a rapid simulation of the force field around a part surrounded by a powder bed in a SPS tool. The optimization of the operating conditions was carried out, first, on small simple-shape samples, then, on complex geometry parts which were densified to demonstrate the feasibility of the proposed sintering process.
Keywords
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SPS; complex shape ceramic parts, powder bed
1. Introduction
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Spark Plasma Sintering (SPS) technology is an ultra-fast method of sintering ceramic and metallic powders, combining the application of uniaxial pressure and a Joule effect heat treatment (passage of a high intensity pulsed electric current - several thousands of amperes: in the mold and possibly in the powder to be sintered if the latter is conductive). This technology has many advantages: the ability to sinter quickly, sometimes without additives, a wide range of materials including difficult to sinter materials at lower temperatures (leading to improved end-properties thanks to better microstructure control). Moreover, the thermal cycle is shorter, leading to energy savings per unit of material produced [1]. Recently, improved productivity equipment with hybrid heating modes allow to reduce temperature gradients in large parts, broadening the potential for applications in the market [24]. However, there is a limitation in the shapes that can be produced (simple forms, achievable by uniaxial pressing), which limits the scope of SPS technology. Although solutions already exist for sintering complex shapes by SPS [5-7], they require, in some cases, the use of specific 1
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tools [8,9] sometimes difficult to implement and results in additional costs like development of tools providing heating with minimal temperature gradient across the sample. The “electroconsolidation” techniques applicable for ceramic were first published by Lange in early 1970s [10-12], then appeared sporadically in the scientific literature [13-16]. This technique consists of the consolidation of samples placed within a conductive granular media, with the help of a uniaxial pressure and the application of an electric current. The granular media is not only used for the transfer of the pressure onto the sample but also for heating the system. Until now, this technique, which is very similar to SPS, was used with a conductive media. However, it has been shown that the densification of insulating materials, like alumina, is improved by SPS. The notion of “plasma” generated by the electrical discharges in SPS, although discussed in the literature [17], is not mentioned in the publications concerning electroconsolidation. SPS can use a pulsed electric current, which is not possible in electroconsolidation.
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More recently, Manière et al. [18,19] developed an innovative approach called “deformed interface approach” where an assembly of powders containing a deformable interface allows the post-sintering separation of complex shape parts and the matching of sacrificial shapes. With this method, Manière et al. have also integrated the possibility to produce multiple parts using specifically designed graphite deformable sub-mold. Alumina powder was inserted at the interface “sample/punches”, and nickel was the material to be sintered.
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In the present work, the aim is to propose a process to rapidly sinter a part with complex geometry, shaped in very short time, e.g. by conventional machining of presintered cylinders or by laser green machining. The method consists in embedding the complex ceramic parts in a granular medium that will transmit both the heat and the load. The force in the granular medium and at the surface of the part to be sintered is simulated by a software developed by the authors [20-23], based on the Janssen’s model [24] classically used to measure the granular pressure in a container.
2. Materials and methods
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Two ceramic powders were chosen for the densification of parts by SPS: thermally and electrically insulating Al2O3 (Grade P172SB, 99.8%, D50 = 0.4 um, Alteo) and thermally and electrically conductive WC-12%Co (grade CTF24E, Ceratizit) cermet.
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Al2O3 powder was uniaxially pressed (30 MPa), followed by cold isostatic pressing (195 MPa). Al2O3 samples were then pre-sintered at 1100°C for 3 hours in order to give them sufficient mechanical strength to withstand subsequent machining (green density: 61%). Conventional spindle machining and laser machining were used to shape Al2O3 samples into cylinders (20 mm height and diameter) (Fig. 1a) and into more complex shape, as illustrated in Fig. 2.
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The WC-12%Co samples were only uniaxially and isostatically pressed, and subsequently directly machined without any heat pre-treatment (green density: 57%) (Fig.1b). A complex part was also prepared by laser machining (Fig. 3). Fig. 1. Al2O3 (a) and WC-12%Co (b) cylinders after conventional machining. Fig. 2. Al2O3 complex part obtained at the green state by laser machining (a: CAD file, b: pre-sintered). Fig. 3. WC-12%Co complex part obtained by laser machining (a: CAD file, b: green state).
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Two graphite and two silicon carbide powders were selected as granular media (hereafter cited as “powder bed”) surrounding the ceramic green parts to be densified. The choice was made taking into account their higher sintering temperature (more than 1900°C for pure SiC and more than 3000°C for graphite), compared with typical sintering temperature of Al2O3 and WC12%Co. Moreover, graphite powders conduct heat and electricity, while SiC powders are semiconductive and more thermally insulating. SiC powders also present a high hardness. The two graphite powders are: TIMREX M150-96 grade (Natural Graphite Flake, IMERYS, France), with a mean grain size lower than 150 um and a synthetic graphite powder (d < 50 um, Merck Millipore, Belgium). The two grades of SiC powders (grade “thin”, d < 200 µm and grade “abrasion”, d = 300 - 850 µm) were provided by H.C. Starck (Germany). All SPS experiments were carried out on FAST/SPS HPD10 furnace and on hybrid heated FAST/SPS H-HP D 125 furnace (both from FCT System GmbH, Germany).
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Uniaxial compaction curves of graphite and SiC powders were obtained on an instrumented hydraulic press (MAVO 1500 kN, Max Voggenreiter GmbH, Germany), equipped with steel press toolings. Density and porosity of densified ceramic parts were measured by the Archimedes’ method in water. The theoretical densities were considered: 3.98 g/cm3 for alumina and 14.30 g/cm3 for WC-12%Co cermet.
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Vickers hardness tests were carried out on a Matsuzawa MXT70 system (Japan using a load of 9.8 kN for 15 s).
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The compaction ratio (Cr) of the powders were obtained from Eq. 1, where i and f are the densities of the powder placed in a tube, before and after tapping, respectively (GranuPaQ, Aptis, Belgium). (1)
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In order to simulate (in 2D) the force field inside the SPS toolings, during the application of a load from the SPS punch transmitted to the granular medium embedding the ceramic parts to be densified, a software called oriented stress linearity (OSL) was developed. OSL uses a suite of softwares developed for the study of granular “gases” in microgravity and the compaction of anisotropic grains [20-23].
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The developed protocol allows placing a part in the middle of the SPS matrix, inside a granular medium. Contact forces between grains, matrix and ceramic part were modeled through a dynamic molecular code where the normal repulsion and tangent forces are generated by a damped spring (Fig. 4). The stiffness, kn, of spring represents the hardness of the grain whereas the damping, ν, makes it possible to take into account the dissipation during the collisions. The simulation consists of the integration of Newton's equations. These are divided into stages: (i) contact detection (solid interaction), (ii) force calculation and (iii) equation integration. This cycle is performed at each time step. The time step is chosen to ensure proper integration of forces. 3
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The parts to be sintered consist of rectangular planes and discs as well as cones. These different primitives are then merged to form a single object. In order to keep a limited number of grains, the simulations were carried out in two dimensions. For ease, the walls of the matrix are considered fixed during the sintering process. The objective of the simulations is to study the distribution of forces within the granular medium and, more particularly, on the piece to be sintered. The part is placed in the middle of the granular bed and the pressure is exerted through the lower punch. Using this model, several parameters have been investigated such as grain size, presence of friction and orientation of the part. The simulations can contain up to 90 000 grains with determined radius. Fig. 4. Model for normal interaction between two grains i and j.
3. Results and discussion 3.1.
Powder beds characteristics
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The results of simulation provide the exerted forces of the granular medium on the part to be sintered.
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Table 1 indicates the value of the compaction rate (Cr) for the selected powder beds, resulting from the GranuPaQ tests.
Table 1. Compaction rates (Cr) of SiC and graphite powders powder beds.
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As expected, graphite powders have a higher compaction rate compared to SiC powders. The compaction rate of the graphite powder from Merck was considered as too high to be suitable as powder bed. Indeed, when the load is applied during a SPS cycle on such a graphite powder, its grains will flow around the ceramic part, leaving considerably less powder at the interface between the SPS punches and the ceramic.
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These compaction rates were confirmed by recording the force vs. punch displacement during the uniaxial compaction test in a press (the upper punch movement is considered to be constant until the force reaches of 25 kN), as presented in Fig. 5.
Fig. 5. Compaction curves (force vs. upper punch displacement) of graphite and SiC powder beds.
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The maximal force (here defined at 25 kN) is reached earlier in the case of the two SiC powders (after only 4 mm of upper punch displacement), while a displacement of 9 mm is needed to compact the graphite powder. In the used equipment the lower punch is motionless. Therefore, from the load applied for the demoulding of the sample from the die one can estimate the friction between the sample and graphite mould. This force is higher for SiC powders (around 12 – 17 kN) due to intense friction between the grains and the walls of the matrix. As a lubricant powder, the graphite powder only applies a very small force at the end of the compaction process.
3.2.
Modeling using the OSL software 4
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Fig. 6 illustrates the evolution of forces exerted on a cylinder (lying or standing inside the granular medium, related to the direction of applied load) vs. the angle of direction.
Fig. 6. Evolution of forces exerted on a cylinder (normalized by the force of the plunger Fp) vs. angle of direction (red: lying; blue: standing).
The notion of force is closely linked to the notion of pressure. In granular media, the standard model is Janssen's one [24]. This model consists of the saturation of weight supported by the bottom of a cylindrical container, depending on the total weight poured into this container. The theoretical model is based on the hypothesis of a continuous medium and the ability of the medium to deflect a constraint in the perpendicular direction. The hypothesis of continuity of the medium imposes that the size of the grains is small compared to that of the system. A deviation coefficient K (<1) is introduced in order to represent the deviation capacity.
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The Janssen’s model has been adapted to take into account the applied load in SPS from the lower punch. In this case, the forces of friction between grains oppose to the force exerted by the lower punch. The static equilibrium of a layer is given by:
A.dpv = K.µs.pv.dh.P + .g.A.dh
(2)
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where
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A: section of the SPS matrix P: perimeter of the SPS matrix pv: vertical pressure K: coefficient of deviation µs: coefficient of friction h: height of the matrix .g.A.dh: the weight of the layer
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With the limit condition: pv(H) = pp (where pp is the pressure applied by the lower punch), the solution is: 𝜌𝑔𝐴 −𝐾𝜇𝑠 𝑃(𝐻−ℎ) 𝜌𝑔𝐴 )𝑒 𝐴 − 𝐾𝜇𝑠 𝑃 𝐾𝜇𝑠 𝑃
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𝑝𝑣 (ℎ ) = (𝑝𝑝 +
(3)
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Considering a system with no friction: 𝑝𝑣 (ℎ) = 𝑝𝑝 + 𝜌𝑔(𝐻 − ℎ )
(4)
This last equation indicates that the pressure at the bottom of the matrix (for h = H) is that of the punch while the pressure at the top (h = 0, upper punch) corresponds to the pressure of the lower punch minus the hydrostatic pressure of the medium. Fig. 7 shows the behaviour of this equation for different values of coefficient K.
Fig. 7. Pressure vs. height inside SPS matrix filled with granular medium for various values of parameter K.
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Considering the pressure exerted by the lower punch, the hydrostatic pressure can be neglected. The Janssen’s equation is then reduced to: 𝐾𝜇𝑆𝑃 (𝐻−ℎ) 𝐴
𝑃 (ℎ) = 𝑝𝑝 𝑒 −
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Although it produces concordant results between numerical and experimental measurements, the Janssen's model assumes homogeneous pressure in the direction perpendicular to the main direction (gravity or piston pressure). In addition, this model is difficult to adapt to all types of shape. The generalization of this model has been carried out more recently [25,26]. Thus, the OSL model describes the evolution of constraints in a granular medium by propagating a vector in place of the scalar used by the previous models.
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In the 2D version, the OSL model is defined as the propagation of a force on a network. The force F divides in two parts: Ft (vertical) and Fx (horizontal). This is illustrated in Fig. 8.
Fig. 8. Scheme of propagation of the force on a network.
The parameter p (whose ideal value is ½) appears as the tendency of the medium to deviate or not the force. It can be assimilated to the parameter K of the Janssen’s model. The components of the force (constraint) are dependent during the propagation process. The transmission to the site (i, j) is given by:
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1 [𝐹 (𝑖 − 1, 𝑗 − 1) − 𝐹𝑥 (𝑖 + 1, 𝑗 − 1)] 2 𝑥 1 + (1 − 𝑝) tan ψ [𝐹𝑡 (𝑖 − 1, 𝑗 − 1) − 𝐹𝑡 (𝑖 + 1, 𝑗 − 1)] 2
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𝐹𝑥 (𝑖, 𝑗 ) =
(6)
where
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0: the weight of a grain
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1 𝐹𝑡 (𝑖, 𝑗 ) = 𝜔0 + 𝑝𝐹𝑡 (𝑖, 𝑗 − 1) + (1 − 𝑝)[𝐹𝑡 (𝑖 − 1, 𝑗 − 1) + 𝐹𝑡 (𝑖 + 1, 𝑗 − 1)] 2 1 [𝐹 (𝑖 − 1, 𝑗 − 1) − 𝐹𝑥 (𝑖 + 1, 𝑗 − 1)] + 2 tan 𝜓 𝑥
: a structural angle of the medium that can be associated with the angle of a natural cone of grains at rest.
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The previous equations in their continuous forms become: 𝜕𝐹𝑡 𝜕𝐹𝑥 + =𝜌 𝜕𝑡 𝜕𝑥 𝜕𝐹𝑥 𝜕𝐶02 𝐹𝑡 + =0 𝜕𝑡 𝜕𝑥
(7)
They express the propagation of constraints in the form of a wave equation where time is replaced by the vertical component. The OSL model has been implemented in a C/C++ program. This one allows reproducing the Janssen effect in an empty system without a sintered part. Fig. 9 and 10 (for p = 1/2) illustrate 6
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the results obtained in the static case and in the "SPS case" (pressure of 25 N/m applied from the bottom), respectively. The images (Figs. 9a and 10a) give a representation of the field of pressure as modelised by the OSL model. The graphs (Figs. 9b and 10b) express the evolution of the pressure, according to the position (the green-black strokes curves corresponds to the hydrostatic model and the red to the OSL model).
Fig. 9. Field of pressure (a) and evolution of pressure (b) in the powder bed for an empty system (p = ½) in the “static case”. Fig. 10. Field of pressure (a) and evolution of pressure (b) in the powder bed for an empty system (p = ½) in the “SPS case” (pressure of 25 kN applied from the bottom).
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The OSL model has been adapted to manage the presence of a piece to be sintered within the powder bed. The case of a lying cylinder (shown as a circle in 2D projection) was performed as the first test. Fig. 11 illustrates the results obtained by applying the OSL model in the presence of the lying cylinder. As for the two previous figures, the image (Fig. 11a) gives a representation of the pressure field obtained; the graph (Fig. 11b) shows the evolution of the pressure as a function of the height (the green-black strokes curve corresponds to the hydrostatic model and the red one to the OSL model).
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Fig. 11. Pressure field distribution (a) of a laying cylinder in the SPS matrix and pressure as a function of height (b) surrounded by graphite medium (applied load: 25 kN).
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The pressure field distribution shows (Fig. 11a) that a strong stress appears above the cylinder. This one is surrounded by weak lines constraints. It is important to note that this type of cone was observed experimentally during demolding at the end of SPS process. Indeed, when demolding, a part of the compacted powder bed, of conical shape, is detached above the sintered part (see Fig. 12).
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Fig. 12. Illustration of density variations after compaction of the powder bed (graphite TIMREX M150-96) - SPS 1400°C/10 min. dwell time - standing cylinder – tooling diameter 40 mm.
3.3.
Densities and shrinkage/deformation of ceramic cylinders SPSed in powder bed
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Al2O3 cylinders embedded in graphite and SiC powders inside the SPS tooling were sintered at 1400°C (heating rate: 100°C/min). Preliminary tests showed that a dwell time of 10 min is necessary to densify Al2O3 cylinders above 98%. The load (from 5 to 25 kN) was applied progressively from 1100°C up to the sintering temperature. Densities and shrinkages/deformations for Al2O3 cylinders lying perpendicularly to the direction of pressing in graphite TIMREX M150-96 are presented in Fig. 13.
Fig. 13. Densities and shrinkages of Al2O3 cylinders vs. applied load, sintered at 1400°C, dwell time 10 min, powder bed: TIMREX M150-96.
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Applying a load of 5 kN, the shrinkages are very close to those obtained by conventional sintering in a box furnace (around 14%). The measured density is around 97% (error bars are drawn from the sintering of 5 cylinders). When the applied load is increased, the density reached more than 99%, but the measured shrinkages have moved further and further away from the mean value, with an anisotropic effect. The diameter of the cylinder in the direction of pressing increased (to reach around 27% under 25 kN), as the diameter and height in the perpendicular direction decreased (around 10% and 3% respectively). The cylinders underwent a “squeezing” deformation due to the applied load during the SPS sintering cycle.
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The same behavior was observed for WC-12%Co. Fig. 14 shows the densities and shrinkages for densification of WC-12%Co at sintering temperature ranging from 1150°C to 1200°C (dwell time for 20 min, heating rate of 100°C/min, load ranging from 5 to 25 kN progressively applied from the beginning of sintering at 950°C).
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Fig. 14. Relative densities and shrinkages of WC-12%Co cylinders lying in TIMREX M150-96 graphite vs. applied load from 5 to 25 kN - sintering temperature from 1150°C to 1400°C, dwell time 20 min, heating rate 100°C/min (H: height of the sample; D: diameter of the sample perpendicular to the direction of the applied load; D Press: diameter of the sample parallel to the direction of the applied load).
97% density was reached for WC-12%Co samples at 1200°C, under 25 kN. The same experiments were performed using the two SiC powders selected as powder beds.
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Fig. 15 presents the shrinkage measured on Al2O3 and WC-12%Co cylinders lying in the 3 different powder beds, sintered in the same conditions (1400°C under 10 kN for Al2O3 cylinders, 1150°C under 10 kN for WC-12%Co cylinders). The shrinkages are much more “homogeneous” in the case of SiC powder beds, compared with graphite. It is likely that a better repartition of forces around the ceramic samples due to higher hardness and frictions between powder grains, lead to less compressibility and then to a stronger isostatic effect.
Fig. 15. Shrinkages measured after SPS sintering on Al2O3 (a) and WC-12%Co (b) cylinders, lying in SiC and graphite powder beds.
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If less deformation was observed with SiC powder beds, the higher hardness leads to indentation of grains in the ceramic samples due to the applied pressure (see Fig. 16).
Fig. 16. WC-12%Co cylinders before (a) and after (b) SPS sintering (1125°C, 25 kN) in powder bed SiC 300-850 µm.
3.4.
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Two ways to improve the process, avoiding indentation and large deformation can be envisaged: 1) using a powder bed composed of a mixture of SiC and graphite powders; 2) adapting the process of a deformed interfaces approach to sinter complex shapes materials by SPS, as developed by C. Manière et al. [18,19]
Recycling of graphite powder bed
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After the SPS cycle the dense ceramic part embedded in a compact powder bed was easily demoulded out of the SPS tooling, as the powder bed was not sintered.
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Successive SPS sintering cycles (1400°C / 10 min / 25 kN load) were carried out to sinter different Al2O3 cylinders in the same conditions and using the same TIMREX M150-96 powder bed that was ground manually in an agate mortar between every cycles. The shrinkages measured on each sample are reported in Fig. 17. After the first SPS cycle, the shrinkages were found to be more homogenous and closer to the 14% corresponding to the shrinkage of a similar alumina sample sintered in a conventional furnace. The lower compressibility of the graphite powder bed after the first SPS cycle explains this observation, as shown in Fig. 18 for the force vs. displacement curves during the uniaxial pressing of grinded TIMREX M150-96 powder bed under 25 kN between two SPS cycles.
Fig. 17. Shrinkage of alumina sample after SPS cycle (1400°C - 10 min dwell time; applied load 25 kN), re-using the same TIMREX M150-96 powder bed (manual grinding between 2 consecutive cycles).
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Fig. 18. Force vs. displacement - uniaxial pressing of the same TIMREX M150-96 powder bed after successive SPS cycles (1400°C, dwell time 10 min, 25 kN).
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3.5.
Influence of heating rate
In the case of Al2O3 cylinders also higher heating rates (400°C/min) combined with higher sintering temperature (up to 1600°C) were tested. To allow electrical conduction, a minimal load of 3 kN had to be applied. Table 2 gives the shrinkage and densities measured for the SPSed samples under various conditions (sintering temperature (T); dwell time (t), applied load (F); shrinkage measured along the diameter in the direction of pressing (Dpress); shrinkage measured along the diameter in the perpendicular direction (D); shrinkage measured along the height (H); density (: Table 2. Al2O3 cylinders sintered in SPS (in TIMREX M150-96 powder bed) under different conditions of sintering temperature, dwell time and applied load.
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The densities of Al2O3 cylinders sintered at 1400°C for 10 min and under an applied load of 10 and 25 kN are higher than 98.5%, but with a heterogeneous shrinkage and larger deformation compared to conventional sintering. A series of tests at 1600°C under a minimal load of 3 kN lead to dense parts with deformations close to the conventional 14% measured after sintering in a box furnace. Despite the higher sintering temperature (1600°C), but lower load, a dwell time of 20 min was necessary to reach a density of 97%.
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3.6. Upscaling to SPS toolings of 80mm (H-HPD125 hybrid SPS furnace)
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Preliminary tests were performed on Al2O3 and WC-12%Co cylinders embedded in TIMREX M150-96 powder bed. To reach similar densities of samples, compared with SPS sintering in the 40 mm diameter tooling, it was necessary to increase the temperature (Figs. 19 and 20) by 100°C, i.e. up to 1280°C for WC-12%Co and up to 1500°C for Al2O3. The applied load was adjusted to obtain the same pressure, taking into account the larger diameter of the SPS toolings.
Fig. 19. Densities of Al2O3 cylinders sintered in TIMREX M150-96: comparison between SPS tooling with 40 mm diameter (applied load of 10 kN) and with 80 mm diameter (applied load of 40 kN).
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Fig. 20. Densities of WC-12%Co cylinders sintered in TIMREX M150-96 powder bed: comparison between SPS tooling with 40 mm diameter (applied load of 25 kN) and with 80 mm diameter (applied load of 100 kN).
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A benefit of using a wider tool is that a larger amount of powder bed can be used to fill the die surrounding the sample and therefore a better repartition of the force around it is expected. This is proven by the better homogeneity of the measured shrinkage on dense parts (Table 3 for Al2O3 cylinders and Table 4 for WC-12%Co cylinders).
Table 3. Shrinkage and density of sintered Al2O3 cylinders in 40 and 80 mm SPS toolings (powder bed: TIMREX M150-96); T, Dpress, D, H and have the same meaning as in table 2 (cylinders lying in the powder bed). Table 4. Shrinkage and density of sintered WC-12%Co cylinders in 40 and 80 mm SPS toolings (powder bed: TIMREX M150-96) – T, Dpress, D, H and have the same meaning as in Tables 2 and 3.
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In Table 3, the densities of Al2O3 cylinders sintered at 1400°C in the 40 mm diameter SPS tools and at 1500°C in the 80 mm diameter SPS tools are very close (more than 98%), but a significant difference appears between the shrinkage of the two sets of samples. Indeed, the shrinkages (in all directions) for samples sintered in the 80 mm diameter tools are closer to the conventional value (i.e. 14%). The same results were obtained for WC-12%Co samples, with even a larger density reached for a sintering temperature of 1280°C (Table 4).
3.7. Sintering of complex ceramic parts by SPS
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The feasibility of densification by SPS of ceramic parts embedded in a granular media was proved by the previous tests, and “upscaling” to the larger hybrid furnace SPS resulted in even better density and more homogeneous shrinkage. With the optimal SPS conditions to sinter Al2O3 and WC-12%Co parts, highly complex geometry parts (presented in Figs. 2 and 3) were prepared by laser machining and sintered in the hybrid SPS furnace (toolings diameter 80 mm) in TIMREX M150-96 powder bed. The bushhammer part in WC-12%Co was sintered at 1280°C for 20 min, and under 100 kN. Fig. 21 shows the part before (left) and after (right) sintering. The density after sintering is close to 98%.
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Fig. 21. The bushhammer part in WC-12%Co, before (left) and after (right) sintering by SPS in TIMREX M15096 powder bed (1280°C-20min-100kN).
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After sintering, the part was cut transversally, embedded in resin and polished (Fig. 22). Then, the hardness of “spikes” and of the “base” of the part was evaluated (Vickers Hv1000).
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Fig. 22. Transversal cut of sintered WC-12%Co part for microhardness measurements.
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The theoretical value of hardness for this WC-12%Co material is 14 GPa. Fig. 23 clearly indicates a significant difference of hardness between the two sets of data: the hardness of the base is lower than that measured on spikes. Fig. 23. Microhardness measured on base and spikes of the sintered bushhammer part.
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This observation can be explained by the results of computer simulation using the OSL software which shows (Fig. 24) that an important concentration of forces in the granular media is transferred onto the spikes (in Fig. 24, the yellow color corresponds to larger forces). As a consequence, there is a higher reinforcement of the spikes, compared to the base of sintered part. In view of the preliminary tests on simple cylinders (showing that the higher the applied load, the larger the density of the final part), it means that there is a reinforcement of the spike, compared with the base of the part. Therefore, also the density and hardness of spikes should be higher. Fig. 24. 2D simulation of the force around a part embedded in a granular media (OSL simulation) during the application of the load in a SPS furnace.
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The complex shape Al2O3 part (gear shown at the pre-sintered state in Fig. 2b) was also successfully sintered in the hybrid SPS furnace (tooling diameter 80 mm), at 1500°C, for 10 min, under 40 kN (Fig. 25). The final density was 96.1%, what is less than expected. Most probably the graphite particles (TIMREX M150-96 powder bed) filling the holes in the structure of alumina-based gear could not transmit effectively the load on the lateral wall, resulting in less densification. Fig. 25. Al2O3 gear sintered by SPS in TIMREX M150-96 at 1500°C for 10 min and under load of 40 kN.
4. Conclusion A method to sinter ceramic parts by SPS with complex geometries, surrounded by a granular medium transferring heat and load was studied. Alumina and tungsten carbide were selected as ceramic materials to demonstrate the feasibility of the method.
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The powder beds (graphite and SiC) were first characterized in terms of compaction ability.
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Alumina and tungsten carbide cylinders were sintered by SPS in order to determine the relation between SPS parameters (temperature, dwell time and applied load) on the resulting deformations of the dense parts. SiC powder used as powder bed is less compactable compared to graphite, and allows to reduce the deformation after sintering. The recycling of the graphite powder bed after one SPS cycle, and after manual grinding, leads to less deformation as the recycled powder bed present a lower ability of compaction. Alumina parts were successfully sintered with minimum deformations using high heating rate (400°C/min) with low applied load (3 kN) and graphite as powder bed.
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The possibility of upscaling the novel method was successfully demonstrated using larger SPS tooling with a diameter of 80 mm on gear made of alumina (d = 43 mm) and bushhammer made of WC-12%Co cermet (d = 40 mm). Both were densified with low deformations, using graphite powder bed.
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A numerical model was developed to simulate the force in the granular medium and on the part to be densified, to determine how the load applied by the SPS punch will be transmitted through the granular medium to the surface of the green ceramic sample. The simulation was used to explain why the spikes of the “boucharde” part presented higher hardness compared with the base of the same part.
Acknowledgement
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The SHAPESPS project was supported by the “Public Service of Wallonia”, SPF-DGO6 and CRIBC in the framework of the convention n°1217839
References
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9. X. Wei, O. Izhanov, C. Back, C.D. Haines, D.G. Martin, K.S. Vecchio, E.A. Olevsky, Spark plasma sintering of structure‐tailored ultrahigh‐temperature components: First step to complex net shaping, J. Am. Cer. Soc., 102 [2] (2018). https://doi.org/10.1111/jace.15752
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Captions for figures
Fig. 1. Al2O3 (a) and WC-12%Co (b) cylinders after conventional machining. Fig. 2. Al2O3 complex part obtained at the green state by laser machining (a: CAD file, b: pre-sintered).
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Fig. 3. WC-12%Co complex part obtained by laser machining (a: CAD file, b: green state). Fig. 4. Model for normal interaction between two grains i and j. Fig. 5. Compaction curves (force vs. upper punch displacement) of graphite and SiC powder beds. Fig. 6. Evolution of forces exerted on a cylinder (normalized by the force of the plunger Fp) vs. angle of direction (red: lying; blue: standing). Fig. 7. Pressure vs. height inside SPS matrix filled with granular medium for various values of parameter K. Fig. 8. Scheme of propagation of the force on a network. Fig. 9. Field of pressure (a) and evolution of pressure (b) in the powder bed for an empty system (p = ½) in the “static case”.
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Fig. 10. Field of pressure (a) and evolution of pressure (b) in the powder bed for an empty system (p = ½) in the “SPS case” (pressure of 25 kN applied from the bottom). Fig. 11. Pressure field distribution (a) of a laying cylinder in the SPS matrix and pressure as a function of height (b) surrounded by graphite medium (applied load: 25 kN). Fig. 12. Illustration of density variations after compaction of the powder bed (graphite TIMREX M15096) - SPS 1400°C/10 min. dwell time - standing cylinder – tooling diameter 40 mm.
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Fig. 13. Densities and shrinkages of Al 2O3 cylinders vs. applied load, sintered at 1400°C, dwell time 10 min, powder bed: TIMREX M150-96.
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Fig. 14. Relative densities and shrinkages of WC-12%Co cylinders lying in TIMREX M150-96 graphite vs. applied load from 5 to 25 kN - sintering temperature from 1150°C to 1400°C, dwell time 20 min, heating rate 100°C/min (H: height of the sample; D: diameter of the sample perpendicular to the direction of the applied load; DPress: diameter of the sample parallel to the direction of the applied load).
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Fig. 15. Shrinkages measured after SPS sintering on Al2O3 (a) and WC-12%Co (b) cylinders, lying in SiC and graphite powder beds. Fig. 16. WC-12%Co cylinders before (a) and after (b) SPS sintering (1125°C, 25 kN) in powder bed SiC 300-850 µm.
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Fig. 17. Shrinkage of alumina sample after SPS cycle (1400°C - 10 min dwell time; applied load 25 kN), re-using the same TIMREX M150-96 powder bed (manual grinding between 2 consecutive cycles).
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Fig. 18. Force vs. displacement - uniaxial pressing of the same TIMREX M150-96 powder bed after successive SPS cycles (1400°C, dwell time 10 min, 25 kN). Fig. 19. Densities of Al2O3 cylinders sintered in TIMREX M150-96: comparison between SPS tooling with 40 mm diameter (applied load of 10 kN) and with 80 mm diameter (applied load of 40 kN).
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Fig. 20. Densities of WC-12%Co cylinders sintered in TIMREX M150-96 powder bed: comparison between SPS tooling with 40 mm diameter (applied load of 25 kN) and with 80 mm diameter (applied load of 100 kN). Fig. 21. The bushhammer part in WC-12%Co, before (left) and after (right) sintering by SPS in TIMREX M150-96 powder bed (1280°C-20min-100kN). Fig. 22. Transversal cut of sintered WC-12%Co part for microhardness measurements. Fig. 23. Microhardness measured on base and spikes of the sintered bushhammer part.
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Fig. 24. 2D simulation of the force around a part embedded in a granular media (OSL simulation) during the application of the load in a SPS furnace. Fig. 24. 2D simulation of the force around a part embedded in a granular media (OSL simulation) during the application of the load in a SPS furnace. Fig. 25. Al2O3 gear sintered by SPS in TIMREX M150-96 at 1500°C for 10 min and under load of 40 kN.
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Fig. 1. Al2O3 (a) and WC-12%Co (b) cylinders after conventional machining.
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Fig. 2. Al2O3 complex part obtained at the green state by laser machining (a: CAD file, b: pre-sintered).
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Fig. 3. WC-12%Co complex part obtained by laser machining (a: CAD file, b: green state).
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Fig. 4. Model for normal interaction between two grains i and j.
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Fig. 5. Compaction curves (force vs. upper punch displacement) of graphite and SiC powder beds.
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Fig. 6. Evolution of forces exerted on a cylinder (normalized by the force of the plunger Fp) vs. angle of direction (red: lying; blue: standing).
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Fig. 8. Scheme of propagation of the force on a network.
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Fig. 7. Pressure vs. height inside SPS matrix filled with granular medium for various values of parameter K.
Fig. 9. Field of pressure (a) and evolution of pressure (b) in the powder bed for an empty system (p = ½) in the “static case”.
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Fig. 10. Field of pressure (a) and evolution of pressure (b) in the powder bed for an empty system (p = ½) in the “SPS case” (pressure of 25 kN applied from the bottom).
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Fig. 11. Pressure field distribution (a) of a laying cylinder in the SPS matrix and pressure as a function of height (b) surrounded by graphite medium (applied load: 25 kN).
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Fig. 12. Illustration of density variations after compaction of the powder bed (graphite TIMREX M15096) - SPS 1400°C/10 min. dwell time - standing cylinder – tooling diameter 40 mm.
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Fig. 13. Densities and shrinkages of Al2O3 cylinders vs. applied load, sintered at 1400°C, dwell time 10 min, powder bed: TIMREX M150-96.
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DPress
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Fig. 14. Relative densities and shrinkages of WC-12%Co cylinders lying in TIMREX M150-96 graphite vs. applied load from 5 to 25 kN - sintering temperature from 1150°C to 1400°C, dwell time 20 min, heating rate 100°C/min (H: height of the sample; D: diameter of the sample perpendicular to the direction of the applied load; DPress: diameter of the sample parallel to the direction of the applied load).
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Fig. 15. Shrinkages measured after SPS sintering on Al2O3 (a) and WC-12%Co (b) cylinders, lying in SiC and graphite powder beds.
Fig. 16. WC-12%Co cylinders before (a) and after (b) SPS sintering (1125°C, 25 kN) in powder bed SiC 300-850 µm.
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Fig. 17. Shrinkage of alumina sample after SPS cycle (1400°C - 10 min dwell time; applied load 25 kN), re-using the same TIMREX M150-96 powder bed (manual grinding between 2 consecutive cycles).
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Fig. 18. Force vs. displacement - uniaxial pressing of the same TIMREX M150-96 powder bed after successive SPS cycles (1400°C, dwell time 10 min, 25 kN).
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Fig. 19. Densities of Al2O3 cylinders sintered in TIMREX M150-96: comparison between SPS tooling with 40 mm diameter (applied load of 10 kN) and with 80 mm diameter (applied load of 40 kN).
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Fig. 20. Densities of WC-12%Co cylinders sintered in TIMREX M150-96 powder bed: comparison between SPS tooling with 40 mm diameter (applied load of 25 kN) and with 80 mm diameter (applied load of 100 kN).
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Fig. 21. The bushhammer part in WC-12%Co, before (left) and after (right) sintering by SPS in TIMREX M150-96 powder bed (1280°C-20min-100kN).
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Fig. 22. Transversal cut of sintered WC-12%Co part for microhardness measurements.
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Fig. 23. Microhardness measured on base and spikes of the sintered bushhammer part.
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Fig. 24. 2D simulation of the force around a part embedded in a granular media (OSL simulation) during the application of the load in a SPS furnace.
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Fig. 25. Al2O3 gear sintered by SPS in TIMREX M150-96 at 1500°C for 10 min and under load of 40 kN.
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Table 1. Compaction rates (Cr) of SiC and graphite powders powder beds.
Powder
Grade
Particle size (um)
Cr (%)
Merck
< 50
71.2
TIMREX M150-96
< 150
40.0
Graphite 34
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SiC
0-200
27.3
SiC
300-850
19.5
SiC
Table 2. Al2O3 cylinders sintered in SPS (in TIMREX M150-96 powder bed) under different conditions of sintering temperature, dwell time and applied load.
t (min)
F (kN)
Heating rate (°C/min)
Dpress (%)
D (%)
H (%)
(%)
1400
10
10
100
22.7+0.5
11.8+1.5
6.7+0.5
98.7+0.1
1400
10
25
100
27.0+0.0
9.0+0.0
2.3+1.2
99.2+0.1
1600
2
3
400
13.5+0.7
13.5+0.7
12.0+0.0
93.8+0.4
1600
20
3
400
16.5+0.7
12.5+0.7
10.0+1.4
97.2+0.1
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T (°C)
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Table 3. Shrinkage and density of sintered Al2O3 cylinders in 40 and 80 mm SPS toolings (powder bed: TIMREX M150-96); T, Dpress (cylinders lying in the powder bed). Tooling (mm)
Dpress (%)
D (%)
H (%)
(%)
1400
40
22.7+0.5
11.8+1.5
6.7+0.5
98.7+0.1
1400
80
13.0+0.1
8.0+0.0
8.0+0.5
84.4+0.1
1500
80
16.0+0.3
13.0+0.7
11.0+0.0
98.3+0.2
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T (°C)
35
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Table 4. Shrinkage and density of sintered WC-12%Co cylinders in 40 and 80 mm SPS toolings (powder bed: TIMREX M150-96) – T, Dpress, D, H and have the same meaning as in Tables 2 and 3. Tooling (mm)
Dpress (%)
D (%)
H (%)
(%)
1180
40
36.0+0.5
6.3+1.0
-1.3+0.2
92.2+0.1
1180
80
16.5+0.4
4.5+0.1
7.0+0.5
76.6+0.5
1230
80
21.0+0.5
8.0+0.4
11.0+0.2
87.0+0.1
1280
80
28.0+0.1
8.0+0.3
7.0+0.0
97.7+0.3
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T (°C)
36
36
PII:
S0955-2219(19)30709-5
DOI:
https://doi.org/10.1016/j.jeurceramsoc.2019.10.038
Reference:
JECS 12800
To appear in:
Journal of the European Ceramic Society
Received Date:
28 August 2019
Revised Date:
3 October 2019
Accepted Date:
17 October 2019
Please cite this article as: Hocquet S, Dupont V, Cambier F, Ludewig F, Vandewalle N, Densification of complex shape ceramics parts by SPS, Journal of the European Ceramic Society (2019), doi: https://doi.org/10.1016/j.jeurceramsoc.2019.10.038
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Densification of complex shape ceramics parts by SPS S. Hocquet, V. Dupont, F. Cambier* BCRC, Belgian Ceramic Research Centre, 4 Avenue Gouverneur Cornez 7000 Mons (Belgium) – www.bcrc.be F. Ludewig, N. Vandewalle University of Liège – GRASP Laboratory, B5a Sart Tilman, 4000 Liège (Belgium) – www.grasp.uliege.be * corresponding author: Dr F. Cambier, e-mail [email protected], phone +32 65403421
Abstract
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Spark Plasma Sintering (SPS) equipments, with higher productivity and hybrid heating modes, allow reducing thermal gradients in large samples and broaden their application potential. Furthermore, strong shape limitations remain: only simple shapes can be obtained because of the use of uniaxial pressure.
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The aim of this paper is to propose a SPS densification method for complex shape ceramic or powder metallurgy parts, without any modification of the tools and equipment. The samples to be sintered are placed in a powder bed in the classical die used in SPS. Different powder beds have been tested.
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A numerical model was developed in order to allow a rapid simulation of the force field around a part surrounded by a powder bed in a SPS tool. The optimization of the operating conditions was carried out, first, on small simple-shape samples, then, on complex geometry parts which were densified to demonstrate the feasibility of the proposed sintering process.
Keywords
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SPS; complex shape ceramic parts, powder bed
1. Introduction
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Spark Plasma Sintering (SPS) technology is an ultra-fast method of sintering ceramic and metallic powders, combining the application of uniaxial pressure and a Joule effect heat treatment (passage of a high intensity pulsed electric current - several thousands of amperes: in the mold and possibly in the powder to be sintered if the latter is conductive). This technology has many advantages: the ability to sinter quickly, sometimes without additives, a wide range of materials including difficult to sinter materials at lower temperatures (leading to improved end-properties thanks to better microstructure control). Moreover, the thermal cycle is shorter, leading to energy savings per unit of material produced [1]. Recently, improved productivity equipment with hybrid heating modes allow to reduce temperature gradients in large parts, broadening the potential for applications in the market [24]. However, there is a limitation in the shapes that can be produced (simple forms, achievable by uniaxial pressing), which limits the scope of SPS technology. Although solutions already exist for sintering complex shapes by SPS [5-7], they require, in some cases, the use of specific 1
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tools [8,9] sometimes difficult to implement and results in additional costs like development of tools providing heating with minimal temperature gradient across the sample. The “electroconsolidation” techniques applicable for ceramic were first published by Lange in early 1970s [10-12], then appeared sporadically in the scientific literature [13-16]. This technique consists of the consolidation of samples placed within a conductive granular media, with the help of a uniaxial pressure and the application of an electric current. The granular media is not only used for the transfer of the pressure onto the sample but also for heating the system. Until now, this technique, which is very similar to SPS, was used with a conductive media. However, it has been shown that the densification of insulating materials, like alumina, is improved by SPS. The notion of “plasma” generated by the electrical discharges in SPS, although discussed in the literature [17], is not mentioned in the publications concerning electroconsolidation. SPS can use a pulsed electric current, which is not possible in electroconsolidation.
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More recently, Manière et al. [18,19] developed an innovative approach called “deformed interface approach” where an assembly of powders containing a deformable interface allows the post-sintering separation of complex shape parts and the matching of sacrificial shapes. With this method, Manière et al. have also integrated the possibility to produce multiple parts using specifically designed graphite deformable sub-mold. Alumina powder was inserted at the interface “sample/punches”, and nickel was the material to be sintered.
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In the present work, the aim is to propose a process to rapidly sinter a part with complex geometry, shaped in very short time, e.g. by conventional machining of presintered cylinders or by laser green machining. The method consists in embedding the complex ceramic parts in a granular medium that will transmit both the heat and the load. The force in the granular medium and at the surface of the part to be sintered is simulated by a software developed by the authors [20-23], based on the Janssen’s model [24] classically used to measure the granular pressure in a container.
2. Materials and methods
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Two ceramic powders were chosen for the densification of parts by SPS: thermally and electrically insulating Al2O3 (Grade P172SB, 99.8%, D50 = 0.4 um, Alteo) and thermally and electrically conductive WC-12%Co (grade CTF24E, Ceratizit) cermet.
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Al2O3 powder was uniaxially pressed (30 MPa), followed by cold isostatic pressing (195 MPa). Al2O3 samples were then pre-sintered at 1100°C for 3 hours in order to give them sufficient mechanical strength to withstand subsequent machining (green density: 61%). Conventional spindle machining and laser machining were used to shape Al2O3 samples into cylinders (20 mm height and diameter) (Fig. 1a) and into more complex shape, as illustrated in Fig. 2.
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The WC-12%Co samples were only uniaxially and isostatically pressed, and subsequently directly machined without any heat pre-treatment (green density: 57%) (Fig.1b). A complex part was also prepared by laser machining (Fig. 3). Fig. 1. Al2O3 (a) and WC-12%Co (b) cylinders after conventional machining. Fig. 2. Al2O3 complex part obtained at the green state by laser machining (a: CAD file, b: pre-sintered). Fig. 3. WC-12%Co complex part obtained by laser machining (a: CAD file, b: green state).
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Two graphite and two silicon carbide powders were selected as granular media (hereafter cited as “powder bed”) surrounding the ceramic green parts to be densified. The choice was made taking into account their higher sintering temperature (more than 1900°C for pure SiC and more than 3000°C for graphite), compared with typical sintering temperature of Al2O3 and WC12%Co. Moreover, graphite powders conduct heat and electricity, while SiC powders are semiconductive and more thermally insulating. SiC powders also present a high hardness. The two graphite powders are: TIMREX M150-96 grade (Natural Graphite Flake, IMERYS, France), with a mean grain size lower than 150 um and a synthetic graphite powder (d < 50 um, Merck Millipore, Belgium). The two grades of SiC powders (grade “thin”, d < 200 µm and grade “abrasion”, d = 300 - 850 µm) were provided by H.C. Starck (Germany). All SPS experiments were carried out on FAST/SPS HPD10 furnace and on hybrid heated FAST/SPS H-HP D 125 furnace (both from FCT System GmbH, Germany).
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Uniaxial compaction curves of graphite and SiC powders were obtained on an instrumented hydraulic press (MAVO 1500 kN, Max Voggenreiter GmbH, Germany), equipped with steel press toolings. Density and porosity of densified ceramic parts were measured by the Archimedes’ method in water. The theoretical densities were considered: 3.98 g/cm3 for alumina and 14.30 g/cm3 for WC-12%Co cermet.
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Vickers hardness tests were carried out on a Matsuzawa MXT70 system (Japan using a load of 9.8 kN for 15 s).
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The compaction ratio (Cr) of the powders were obtained from Eq. 1, where i and f are the densities of the powder placed in a tube, before and after tapping, respectively (GranuPaQ, Aptis, Belgium). (1)
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In order to simulate (in 2D) the force field inside the SPS toolings, during the application of a load from the SPS punch transmitted to the granular medium embedding the ceramic parts to be densified, a software called oriented stress linearity (OSL) was developed. OSL uses a suite of softwares developed for the study of granular “gases” in microgravity and the compaction of anisotropic grains [20-23].
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The developed protocol allows placing a part in the middle of the SPS matrix, inside a granular medium. Contact forces between grains, matrix and ceramic part were modeled through a dynamic molecular code where the normal repulsion and tangent forces are generated by a damped spring (Fig. 4). The stiffness, kn, of spring represents the hardness of the grain whereas the damping, ν, makes it possible to take into account the dissipation during the collisions. The simulation consists of the integration of Newton's equations. These are divided into stages: (i) contact detection (solid interaction), (ii) force calculation and (iii) equation integration. This cycle is performed at each time step. The time step is chosen to ensure proper integration of forces. 3
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The parts to be sintered consist of rectangular planes and discs as well as cones. These different primitives are then merged to form a single object. In order to keep a limited number of grains, the simulations were carried out in two dimensions. For ease, the walls of the matrix are considered fixed during the sintering process. The objective of the simulations is to study the distribution of forces within the granular medium and, more particularly, on the piece to be sintered. The part is placed in the middle of the granular bed and the pressure is exerted through the lower punch. Using this model, several parameters have been investigated such as grain size, presence of friction and orientation of the part. The simulations can contain up to 90 000 grains with determined radius. Fig. 4. Model for normal interaction between two grains i and j.
3. Results and discussion 3.1.
Powder beds characteristics
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The results of simulation provide the exerted forces of the granular medium on the part to be sintered.
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Table 1 indicates the value of the compaction rate (Cr) for the selected powder beds, resulting from the GranuPaQ tests.
Table 1. Compaction rates (Cr) of SiC and graphite powders powder beds.
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As expected, graphite powders have a higher compaction rate compared to SiC powders. The compaction rate of the graphite powder from Merck was considered as too high to be suitable as powder bed. Indeed, when the load is applied during a SPS cycle on such a graphite powder, its grains will flow around the ceramic part, leaving considerably less powder at the interface between the SPS punches and the ceramic.
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These compaction rates were confirmed by recording the force vs. punch displacement during the uniaxial compaction test in a press (the upper punch movement is considered to be constant until the force reaches of 25 kN), as presented in Fig. 5.
Fig. 5. Compaction curves (force vs. upper punch displacement) of graphite and SiC powder beds.
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The maximal force (here defined at 25 kN) is reached earlier in the case of the two SiC powders (after only 4 mm of upper punch displacement), while a displacement of 9 mm is needed to compact the graphite powder. In the used equipment the lower punch is motionless. Therefore, from the load applied for the demoulding of the sample from the die one can estimate the friction between the sample and graphite mould. This force is higher for SiC powders (around 12 – 17 kN) due to intense friction between the grains and the walls of the matrix. As a lubricant powder, the graphite powder only applies a very small force at the end of the compaction process.
3.2.
Modeling using the OSL software 4
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Fig. 6 illustrates the evolution of forces exerted on a cylinder (lying or standing inside the granular medium, related to the direction of applied load) vs. the angle of direction.
Fig. 6. Evolution of forces exerted on a cylinder (normalized by the force of the plunger Fp) vs. angle of direction (red: lying; blue: standing).
The notion of force is closely linked to the notion of pressure. In granular media, the standard model is Janssen's one [24]. This model consists of the saturation of weight supported by the bottom of a cylindrical container, depending on the total weight poured into this container. The theoretical model is based on the hypothesis of a continuous medium and the ability of the medium to deflect a constraint in the perpendicular direction. The hypothesis of continuity of the medium imposes that the size of the grains is small compared to that of the system. A deviation coefficient K (<1) is introduced in order to represent the deviation capacity.
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The Janssen’s model has been adapted to take into account the applied load in SPS from the lower punch. In this case, the forces of friction between grains oppose to the force exerted by the lower punch. The static equilibrium of a layer is given by:
A.dpv = K.µs.pv.dh.P + .g.A.dh
(2)
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where
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A: section of the SPS matrix P: perimeter of the SPS matrix pv: vertical pressure K: coefficient of deviation µs: coefficient of friction h: height of the matrix .g.A.dh: the weight of the layer
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With the limit condition: pv(H) = pp (where pp is the pressure applied by the lower punch), the solution is: 𝜌𝑔𝐴 −𝐾𝜇𝑠 𝑃(𝐻−ℎ) 𝜌𝑔𝐴 )𝑒 𝐴 − 𝐾𝜇𝑠 𝑃 𝐾𝜇𝑠 𝑃
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𝑝𝑣 (ℎ ) = (𝑝𝑝 +
(3)
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Considering a system with no friction: 𝑝𝑣 (ℎ) = 𝑝𝑝 + 𝜌𝑔(𝐻 − ℎ )
(4)
This last equation indicates that the pressure at the bottom of the matrix (for h = H) is that of the punch while the pressure at the top (h = 0, upper punch) corresponds to the pressure of the lower punch minus the hydrostatic pressure of the medium. Fig. 7 shows the behaviour of this equation for different values of coefficient K.
Fig. 7. Pressure vs. height inside SPS matrix filled with granular medium for various values of parameter K.
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Considering the pressure exerted by the lower punch, the hydrostatic pressure can be neglected. The Janssen’s equation is then reduced to: 𝐾𝜇𝑆𝑃 (𝐻−ℎ) 𝐴
𝑃 (ℎ) = 𝑝𝑝 𝑒 −
(5)
Although it produces concordant results between numerical and experimental measurements, the Janssen's model assumes homogeneous pressure in the direction perpendicular to the main direction (gravity or piston pressure). In addition, this model is difficult to adapt to all types of shape. The generalization of this model has been carried out more recently [25,26]. Thus, the OSL model describes the evolution of constraints in a granular medium by propagating a vector in place of the scalar used by the previous models.
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In the 2D version, the OSL model is defined as the propagation of a force on a network. The force F divides in two parts: Ft (vertical) and Fx (horizontal). This is illustrated in Fig. 8.
Fig. 8. Scheme of propagation of the force on a network.
The parameter p (whose ideal value is ½) appears as the tendency of the medium to deviate or not the force. It can be assimilated to the parameter K of the Janssen’s model. The components of the force (constraint) are dependent during the propagation process. The transmission to the site (i, j) is given by:
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1 [𝐹 (𝑖 − 1, 𝑗 − 1) − 𝐹𝑥 (𝑖 + 1, 𝑗 − 1)] 2 𝑥 1 + (1 − 𝑝) tan ψ [𝐹𝑡 (𝑖 − 1, 𝑗 − 1) − 𝐹𝑡 (𝑖 + 1, 𝑗 − 1)] 2
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𝐹𝑥 (𝑖, 𝑗 ) =
(6)
where
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0: the weight of a grain
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1 𝐹𝑡 (𝑖, 𝑗 ) = 𝜔0 + 𝑝𝐹𝑡 (𝑖, 𝑗 − 1) + (1 − 𝑝)[𝐹𝑡 (𝑖 − 1, 𝑗 − 1) + 𝐹𝑡 (𝑖 + 1, 𝑗 − 1)] 2 1 [𝐹 (𝑖 − 1, 𝑗 − 1) − 𝐹𝑥 (𝑖 + 1, 𝑗 − 1)] + 2 tan 𝜓 𝑥
: a structural angle of the medium that can be associated with the angle of a natural cone of grains at rest.
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The previous equations in their continuous forms become: 𝜕𝐹𝑡 𝜕𝐹𝑥 + =𝜌 𝜕𝑡 𝜕𝑥 𝜕𝐹𝑥 𝜕𝐶02 𝐹𝑡 + =0 𝜕𝑡 𝜕𝑥
(7)
They express the propagation of constraints in the form of a wave equation where time is replaced by the vertical component. The OSL model has been implemented in a C/C++ program. This one allows reproducing the Janssen effect in an empty system without a sintered part. Fig. 9 and 10 (for p = 1/2) illustrate 6
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the results obtained in the static case and in the "SPS case" (pressure of 25 N/m applied from the bottom), respectively. The images (Figs. 9a and 10a) give a representation of the field of pressure as modelised by the OSL model. The graphs (Figs. 9b and 10b) express the evolution of the pressure, according to the position (the green-black strokes curves corresponds to the hydrostatic model and the red to the OSL model).
Fig. 9. Field of pressure (a) and evolution of pressure (b) in the powder bed for an empty system (p = ½) in the “static case”. Fig. 10. Field of pressure (a) and evolution of pressure (b) in the powder bed for an empty system (p = ½) in the “SPS case” (pressure of 25 kN applied from the bottom).
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The OSL model has been adapted to manage the presence of a piece to be sintered within the powder bed. The case of a lying cylinder (shown as a circle in 2D projection) was performed as the first test. Fig. 11 illustrates the results obtained by applying the OSL model in the presence of the lying cylinder. As for the two previous figures, the image (Fig. 11a) gives a representation of the pressure field obtained; the graph (Fig. 11b) shows the evolution of the pressure as a function of the height (the green-black strokes curve corresponds to the hydrostatic model and the red one to the OSL model).
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Fig. 11. Pressure field distribution (a) of a laying cylinder in the SPS matrix and pressure as a function of height (b) surrounded by graphite medium (applied load: 25 kN).
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The pressure field distribution shows (Fig. 11a) that a strong stress appears above the cylinder. This one is surrounded by weak lines constraints. It is important to note that this type of cone was observed experimentally during demolding at the end of SPS process. Indeed, when demolding, a part of the compacted powder bed, of conical shape, is detached above the sintered part (see Fig. 12).
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Fig. 12. Illustration of density variations after compaction of the powder bed (graphite TIMREX M150-96) - SPS 1400°C/10 min. dwell time - standing cylinder – tooling diameter 40 mm.
3.3.
Densities and shrinkage/deformation of ceramic cylinders SPSed in powder bed
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Al2O3 cylinders embedded in graphite and SiC powders inside the SPS tooling were sintered at 1400°C (heating rate: 100°C/min). Preliminary tests showed that a dwell time of 10 min is necessary to densify Al2O3 cylinders above 98%. The load (from 5 to 25 kN) was applied progressively from 1100°C up to the sintering temperature. Densities and shrinkages/deformations for Al2O3 cylinders lying perpendicularly to the direction of pressing in graphite TIMREX M150-96 are presented in Fig. 13.
Fig. 13. Densities and shrinkages of Al2O3 cylinders vs. applied load, sintered at 1400°C, dwell time 10 min, powder bed: TIMREX M150-96.
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Applying a load of 5 kN, the shrinkages are very close to those obtained by conventional sintering in a box furnace (around 14%). The measured density is around 97% (error bars are drawn from the sintering of 5 cylinders). When the applied load is increased, the density reached more than 99%, but the measured shrinkages have moved further and further away from the mean value, with an anisotropic effect. The diameter of the cylinder in the direction of pressing increased (to reach around 27% under 25 kN), as the diameter and height in the perpendicular direction decreased (around 10% and 3% respectively). The cylinders underwent a “squeezing” deformation due to the applied load during the SPS sintering cycle.
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The same behavior was observed for WC-12%Co. Fig. 14 shows the densities and shrinkages for densification of WC-12%Co at sintering temperature ranging from 1150°C to 1200°C (dwell time for 20 min, heating rate of 100°C/min, load ranging from 5 to 25 kN progressively applied from the beginning of sintering at 950°C).
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Fig. 14. Relative densities and shrinkages of WC-12%Co cylinders lying in TIMREX M150-96 graphite vs. applied load from 5 to 25 kN - sintering temperature from 1150°C to 1400°C, dwell time 20 min, heating rate 100°C/min (H: height of the sample; D: diameter of the sample perpendicular to the direction of the applied load; D Press: diameter of the sample parallel to the direction of the applied load).
97% density was reached for WC-12%Co samples at 1200°C, under 25 kN. The same experiments were performed using the two SiC powders selected as powder beds.
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Fig. 15 presents the shrinkage measured on Al2O3 and WC-12%Co cylinders lying in the 3 different powder beds, sintered in the same conditions (1400°C under 10 kN for Al2O3 cylinders, 1150°C under 10 kN for WC-12%Co cylinders). The shrinkages are much more “homogeneous” in the case of SiC powder beds, compared with graphite. It is likely that a better repartition of forces around the ceramic samples due to higher hardness and frictions between powder grains, lead to less compressibility and then to a stronger isostatic effect.
Fig. 15. Shrinkages measured after SPS sintering on Al2O3 (a) and WC-12%Co (b) cylinders, lying in SiC and graphite powder beds.
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If less deformation was observed with SiC powder beds, the higher hardness leads to indentation of grains in the ceramic samples due to the applied pressure (see Fig. 16).
Fig. 16. WC-12%Co cylinders before (a) and after (b) SPS sintering (1125°C, 25 kN) in powder bed SiC 300-850 µm.
3.4.
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Two ways to improve the process, avoiding indentation and large deformation can be envisaged: 1) using a powder bed composed of a mixture of SiC and graphite powders; 2) adapting the process of a deformed interfaces approach to sinter complex shapes materials by SPS, as developed by C. Manière et al. [18,19]
Recycling of graphite powder bed
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After the SPS cycle the dense ceramic part embedded in a compact powder bed was easily demoulded out of the SPS tooling, as the powder bed was not sintered.
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Successive SPS sintering cycles (1400°C / 10 min / 25 kN load) were carried out to sinter different Al2O3 cylinders in the same conditions and using the same TIMREX M150-96 powder bed that was ground manually in an agate mortar between every cycles. The shrinkages measured on each sample are reported in Fig. 17. After the first SPS cycle, the shrinkages were found to be more homogenous and closer to the 14% corresponding to the shrinkage of a similar alumina sample sintered in a conventional furnace. The lower compressibility of the graphite powder bed after the first SPS cycle explains this observation, as shown in Fig. 18 for the force vs. displacement curves during the uniaxial pressing of grinded TIMREX M150-96 powder bed under 25 kN between two SPS cycles.
Fig. 17. Shrinkage of alumina sample after SPS cycle (1400°C - 10 min dwell time; applied load 25 kN), re-using the same TIMREX M150-96 powder bed (manual grinding between 2 consecutive cycles).
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Fig. 18. Force vs. displacement - uniaxial pressing of the same TIMREX M150-96 powder bed after successive SPS cycles (1400°C, dwell time 10 min, 25 kN).
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3.5.
Influence of heating rate
In the case of Al2O3 cylinders also higher heating rates (400°C/min) combined with higher sintering temperature (up to 1600°C) were tested. To allow electrical conduction, a minimal load of 3 kN had to be applied. Table 2 gives the shrinkage and densities measured for the SPSed samples under various conditions (sintering temperature (T); dwell time (t), applied load (F); shrinkage measured along the diameter in the direction of pressing (Dpress); shrinkage measured along the diameter in the perpendicular direction (D); shrinkage measured along the height (H); density (: Table 2. Al2O3 cylinders sintered in SPS (in TIMREX M150-96 powder bed) under different conditions of sintering temperature, dwell time and applied load.
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The densities of Al2O3 cylinders sintered at 1400°C for 10 min and under an applied load of 10 and 25 kN are higher than 98.5%, but with a heterogeneous shrinkage and larger deformation compared to conventional sintering. A series of tests at 1600°C under a minimal load of 3 kN lead to dense parts with deformations close to the conventional 14% measured after sintering in a box furnace. Despite the higher sintering temperature (1600°C), but lower load, a dwell time of 20 min was necessary to reach a density of 97%.
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3.6. Upscaling to SPS toolings of 80mm (H-HPD125 hybrid SPS furnace)
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Preliminary tests were performed on Al2O3 and WC-12%Co cylinders embedded in TIMREX M150-96 powder bed. To reach similar densities of samples, compared with SPS sintering in the 40 mm diameter tooling, it was necessary to increase the temperature (Figs. 19 and 20) by 100°C, i.e. up to 1280°C for WC-12%Co and up to 1500°C for Al2O3. The applied load was adjusted to obtain the same pressure, taking into account the larger diameter of the SPS toolings.
Fig. 19. Densities of Al2O3 cylinders sintered in TIMREX M150-96: comparison between SPS tooling with 40 mm diameter (applied load of 10 kN) and with 80 mm diameter (applied load of 40 kN).
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Fig. 20. Densities of WC-12%Co cylinders sintered in TIMREX M150-96 powder bed: comparison between SPS tooling with 40 mm diameter (applied load of 25 kN) and with 80 mm diameter (applied load of 100 kN).
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A benefit of using a wider tool is that a larger amount of powder bed can be used to fill the die surrounding the sample and therefore a better repartition of the force around it is expected. This is proven by the better homogeneity of the measured shrinkage on dense parts (Table 3 for Al2O3 cylinders and Table 4 for WC-12%Co cylinders).
Table 3. Shrinkage and density of sintered Al2O3 cylinders in 40 and 80 mm SPS toolings (powder bed: TIMREX M150-96); T, Dpress, D, H and have the same meaning as in table 2 (cylinders lying in the powder bed). Table 4. Shrinkage and density of sintered WC-12%Co cylinders in 40 and 80 mm SPS toolings (powder bed: TIMREX M150-96) – T, Dpress, D, H and have the same meaning as in Tables 2 and 3.
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In Table 3, the densities of Al2O3 cylinders sintered at 1400°C in the 40 mm diameter SPS tools and at 1500°C in the 80 mm diameter SPS tools are very close (more than 98%), but a significant difference appears between the shrinkage of the two sets of samples. Indeed, the shrinkages (in all directions) for samples sintered in the 80 mm diameter tools are closer to the conventional value (i.e. 14%). The same results were obtained for WC-12%Co samples, with even a larger density reached for a sintering temperature of 1280°C (Table 4).
3.7. Sintering of complex ceramic parts by SPS
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The feasibility of densification by SPS of ceramic parts embedded in a granular media was proved by the previous tests, and “upscaling” to the larger hybrid furnace SPS resulted in even better density and more homogeneous shrinkage. With the optimal SPS conditions to sinter Al2O3 and WC-12%Co parts, highly complex geometry parts (presented in Figs. 2 and 3) were prepared by laser machining and sintered in the hybrid SPS furnace (toolings diameter 80 mm) in TIMREX M150-96 powder bed. The bushhammer part in WC-12%Co was sintered at 1280°C for 20 min, and under 100 kN. Fig. 21 shows the part before (left) and after (right) sintering. The density after sintering is close to 98%.
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Fig. 21. The bushhammer part in WC-12%Co, before (left) and after (right) sintering by SPS in TIMREX M15096 powder bed (1280°C-20min-100kN).
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After sintering, the part was cut transversally, embedded in resin and polished (Fig. 22). Then, the hardness of “spikes” and of the “base” of the part was evaluated (Vickers Hv1000).
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Fig. 22. Transversal cut of sintered WC-12%Co part for microhardness measurements.
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The theoretical value of hardness for this WC-12%Co material is 14 GPa. Fig. 23 clearly indicates a significant difference of hardness between the two sets of data: the hardness of the base is lower than that measured on spikes. Fig. 23. Microhardness measured on base and spikes of the sintered bushhammer part.
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This observation can be explained by the results of computer simulation using the OSL software which shows (Fig. 24) that an important concentration of forces in the granular media is transferred onto the spikes (in Fig. 24, the yellow color corresponds to larger forces). As a consequence, there is a higher reinforcement of the spikes, compared to the base of sintered part. In view of the preliminary tests on simple cylinders (showing that the higher the applied load, the larger the density of the final part), it means that there is a reinforcement of the spike, compared with the base of the part. Therefore, also the density and hardness of spikes should be higher. Fig. 24. 2D simulation of the force around a part embedded in a granular media (OSL simulation) during the application of the load in a SPS furnace.
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The complex shape Al2O3 part (gear shown at the pre-sintered state in Fig. 2b) was also successfully sintered in the hybrid SPS furnace (tooling diameter 80 mm), at 1500°C, for 10 min, under 40 kN (Fig. 25). The final density was 96.1%, what is less than expected. Most probably the graphite particles (TIMREX M150-96 powder bed) filling the holes in the structure of alumina-based gear could not transmit effectively the load on the lateral wall, resulting in less densification. Fig. 25. Al2O3 gear sintered by SPS in TIMREX M150-96 at 1500°C for 10 min and under load of 40 kN.
4. Conclusion A method to sinter ceramic parts by SPS with complex geometries, surrounded by a granular medium transferring heat and load was studied. Alumina and tungsten carbide were selected as ceramic materials to demonstrate the feasibility of the method.
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The powder beds (graphite and SiC) were first characterized in terms of compaction ability.
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Alumina and tungsten carbide cylinders were sintered by SPS in order to determine the relation between SPS parameters (temperature, dwell time and applied load) on the resulting deformations of the dense parts. SiC powder used as powder bed is less compactable compared to graphite, and allows to reduce the deformation after sintering. The recycling of the graphite powder bed after one SPS cycle, and after manual grinding, leads to less deformation as the recycled powder bed present a lower ability of compaction. Alumina parts were successfully sintered with minimum deformations using high heating rate (400°C/min) with low applied load (3 kN) and graphite as powder bed.
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The possibility of upscaling the novel method was successfully demonstrated using larger SPS tooling with a diameter of 80 mm on gear made of alumina (d = 43 mm) and bushhammer made of WC-12%Co cermet (d = 40 mm). Both were densified with low deformations, using graphite powder bed.
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A numerical model was developed to simulate the force in the granular medium and on the part to be densified, to determine how the load applied by the SPS punch will be transmitted through the granular medium to the surface of the green ceramic sample. The simulation was used to explain why the spikes of the “boucharde” part presented higher hardness compared with the base of the same part.
Acknowledgement
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The SHAPESPS project was supported by the “Public Service of Wallonia”, SPF-DGO6 and CRIBC in the framework of the convention n°1217839
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21. E. Opsomer, M. Noirhomme, N. Vandewalle, F. Ludewig, How dynamical clustering triggers Maxwell's demon in microgravity, Phys. Rev. E 88 (2013) 012202. https://doi.org/10.1103/PhysRevE.88.012202
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23. F. Ludewig, N. Vandewalle Strong interlocking of nonconvex particles in random packings, Phys. Rev. E 85 (2012) 051307. https://doi.org/10.1103/PhysRevE.85.051307 24. H.A. Janssen Experiments on grain pressure in silos, Z. Vereins Deutsch. Ing., Dusseldorf 39 (1895) 1045-1049.
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Captions for figures
Fig. 1. Al2O3 (a) and WC-12%Co (b) cylinders after conventional machining. Fig. 2. Al2O3 complex part obtained at the green state by laser machining (a: CAD file, b: pre-sintered).
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Fig. 3. WC-12%Co complex part obtained by laser machining (a: CAD file, b: green state). Fig. 4. Model for normal interaction between two grains i and j. Fig. 5. Compaction curves (force vs. upper punch displacement) of graphite and SiC powder beds. Fig. 6. Evolution of forces exerted on a cylinder (normalized by the force of the plunger Fp) vs. angle of direction (red: lying; blue: standing). Fig. 7. Pressure vs. height inside SPS matrix filled with granular medium for various values of parameter K. Fig. 8. Scheme of propagation of the force on a network. Fig. 9. Field of pressure (a) and evolution of pressure (b) in the powder bed for an empty system (p = ½) in the “static case”.
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Fig. 10. Field of pressure (a) and evolution of pressure (b) in the powder bed for an empty system (p = ½) in the “SPS case” (pressure of 25 kN applied from the bottom). Fig. 11. Pressure field distribution (a) of a laying cylinder in the SPS matrix and pressure as a function of height (b) surrounded by graphite medium (applied load: 25 kN). Fig. 12. Illustration of density variations after compaction of the powder bed (graphite TIMREX M15096) - SPS 1400°C/10 min. dwell time - standing cylinder – tooling diameter 40 mm.
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Fig. 13. Densities and shrinkages of Al 2O3 cylinders vs. applied load, sintered at 1400°C, dwell time 10 min, powder bed: TIMREX M150-96.
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Fig. 14. Relative densities and shrinkages of WC-12%Co cylinders lying in TIMREX M150-96 graphite vs. applied load from 5 to 25 kN - sintering temperature from 1150°C to 1400°C, dwell time 20 min, heating rate 100°C/min (H: height of the sample; D: diameter of the sample perpendicular to the direction of the applied load; DPress: diameter of the sample parallel to the direction of the applied load).
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Fig. 15. Shrinkages measured after SPS sintering on Al2O3 (a) and WC-12%Co (b) cylinders, lying in SiC and graphite powder beds. Fig. 16. WC-12%Co cylinders before (a) and after (b) SPS sintering (1125°C, 25 kN) in powder bed SiC 300-850 µm.
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Fig. 17. Shrinkage of alumina sample after SPS cycle (1400°C - 10 min dwell time; applied load 25 kN), re-using the same TIMREX M150-96 powder bed (manual grinding between 2 consecutive cycles).
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Fig. 18. Force vs. displacement - uniaxial pressing of the same TIMREX M150-96 powder bed after successive SPS cycles (1400°C, dwell time 10 min, 25 kN). Fig. 19. Densities of Al2O3 cylinders sintered in TIMREX M150-96: comparison between SPS tooling with 40 mm diameter (applied load of 10 kN) and with 80 mm diameter (applied load of 40 kN).
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Fig. 20. Densities of WC-12%Co cylinders sintered in TIMREX M150-96 powder bed: comparison between SPS tooling with 40 mm diameter (applied load of 25 kN) and with 80 mm diameter (applied load of 100 kN). Fig. 21. The bushhammer part in WC-12%Co, before (left) and after (right) sintering by SPS in TIMREX M150-96 powder bed (1280°C-20min-100kN). Fig. 22. Transversal cut of sintered WC-12%Co part for microhardness measurements. Fig. 23. Microhardness measured on base and spikes of the sintered bushhammer part.
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Fig. 24. 2D simulation of the force around a part embedded in a granular media (OSL simulation) during the application of the load in a SPS furnace. Fig. 24. 2D simulation of the force around a part embedded in a granular media (OSL simulation) during the application of the load in a SPS furnace. Fig. 25. Al2O3 gear sintered by SPS in TIMREX M150-96 at 1500°C for 10 min and under load of 40 kN.
Captions for figures a
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Fig. 1. Al2O3 (a) and WC-12%Co (b) cylinders after conventional machining.
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Fig. 2. Al2O3 complex part obtained at the green state by laser machining (a: CAD file, b: pre-sintered).
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Fig. 3. WC-12%Co complex part obtained by laser machining (a: CAD file, b: green state).
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Fig. 4. Model for normal interaction between two grains i and j.
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Fig. 5. Compaction curves (force vs. upper punch displacement) of graphite and SiC powder beds.
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Fig. 6. Evolution of forces exerted on a cylinder (normalized by the force of the plunger Fp) vs. angle of direction (red: lying; blue: standing).
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Fig. 8. Scheme of propagation of the force on a network.
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Fig. 7. Pressure vs. height inside SPS matrix filled with granular medium for various values of parameter K.
Fig. 9. Field of pressure (a) and evolution of pressure (b) in the powder bed for an empty system (p = ½) in the “static case”.
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Fig. 10. Field of pressure (a) and evolution of pressure (b) in the powder bed for an empty system (p = ½) in the “SPS case” (pressure of 25 kN applied from the bottom).
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Fig. 11. Pressure field distribution (a) of a laying cylinder in the SPS matrix and pressure as a function of height (b) surrounded by graphite medium (applied load: 25 kN).
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Fig. 12. Illustration of density variations after compaction of the powder bed (graphite TIMREX M15096) - SPS 1400°C/10 min. dwell time - standing cylinder – tooling diameter 40 mm.
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Fig. 13. Densities and shrinkages of Al2O3 cylinders vs. applied load, sintered at 1400°C, dwell time 10 min, powder bed: TIMREX M150-96.
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DPress
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Fig. 14. Relative densities and shrinkages of WC-12%Co cylinders lying in TIMREX M150-96 graphite vs. applied load from 5 to 25 kN - sintering temperature from 1150°C to 1400°C, dwell time 20 min, heating rate 100°C/min (H: height of the sample; D: diameter of the sample perpendicular to the direction of the applied load; DPress: diameter of the sample parallel to the direction of the applied load).
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Fig. 15. Shrinkages measured after SPS sintering on Al2O3 (a) and WC-12%Co (b) cylinders, lying in SiC and graphite powder beds.
Fig. 16. WC-12%Co cylinders before (a) and after (b) SPS sintering (1125°C, 25 kN) in powder bed SiC 300-850 µm.
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Fig. 17. Shrinkage of alumina sample after SPS cycle (1400°C - 10 min dwell time; applied load 25 kN), re-using the same TIMREX M150-96 powder bed (manual grinding between 2 consecutive cycles).
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Fig. 18. Force vs. displacement - uniaxial pressing of the same TIMREX M150-96 powder bed after successive SPS cycles (1400°C, dwell time 10 min, 25 kN).
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Fig. 19. Densities of Al2O3 cylinders sintered in TIMREX M150-96: comparison between SPS tooling with 40 mm diameter (applied load of 10 kN) and with 80 mm diameter (applied load of 40 kN).
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Fig. 20. Densities of WC-12%Co cylinders sintered in TIMREX M150-96 powder bed: comparison between SPS tooling with 40 mm diameter (applied load of 25 kN) and with 80 mm diameter (applied load of 100 kN).
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Fig. 21. The bushhammer part in WC-12%Co, before (left) and after (right) sintering by SPS in TIMREX M150-96 powder bed (1280°C-20min-100kN).
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Fig. 22. Transversal cut of sintered WC-12%Co part for microhardness measurements.
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Fig. 23. Microhardness measured on base and spikes of the sintered bushhammer part.
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Fig. 24. 2D simulation of the force around a part embedded in a granular media (OSL simulation) during the application of the load in a SPS furnace.
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Fig. 25. Al2O3 gear sintered by SPS in TIMREX M150-96 at 1500°C for 10 min and under load of 40 kN.
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Table 1. Compaction rates (Cr) of SiC and graphite powders powder beds.
Powder
Grade
Particle size (um)
Cr (%)
Merck
< 50
71.2
TIMREX M150-96
< 150
40.0
Graphite 34
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SiC
0-200
27.3
SiC
300-850
19.5
SiC
Table 2. Al2O3 cylinders sintered in SPS (in TIMREX M150-96 powder bed) under different conditions of sintering temperature, dwell time and applied load.
t (min)
F (kN)
Heating rate (°C/min)
Dpress (%)
D (%)
H (%)
(%)
1400
10
10
100
22.7+0.5
11.8+1.5
6.7+0.5
98.7+0.1
1400
10
25
100
27.0+0.0
9.0+0.0
2.3+1.2
99.2+0.1
1600
2
3
400
13.5+0.7
13.5+0.7
12.0+0.0
93.8+0.4
1600
20
3
400
16.5+0.7
12.5+0.7
10.0+1.4
97.2+0.1
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T (°C)
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Table 3. Shrinkage and density of sintered Al2O3 cylinders in 40 and 80 mm SPS toolings (powder bed: TIMREX M150-96); T, Dpress (cylinders lying in the powder bed). Tooling (mm)
Dpress (%)
D (%)
H (%)
(%)
1400
40
22.7+0.5
11.8+1.5
6.7+0.5
98.7+0.1
1400
80
13.0+0.1
8.0+0.0
8.0+0.5
84.4+0.1
1500
80
16.0+0.3
13.0+0.7
11.0+0.0
98.3+0.2
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T (°C)
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Table 4. Shrinkage and density of sintered WC-12%Co cylinders in 40 and 80 mm SPS toolings (powder bed: TIMREX M150-96) – T, Dpress, D, H and have the same meaning as in Tables 2 and 3. Tooling (mm)
Dpress (%)
D (%)
H (%)
(%)
1180
40
36.0+0.5
6.3+1.0
-1.3+0.2
92.2+0.1
1180
80
16.5+0.4
4.5+0.1
7.0+0.5
76.6+0.5
1230
80
21.0+0.5
8.0+0.4
11.0+0.2
87.0+0.1
1280
80
28.0+0.1
8.0+0.3
7.0+0.0
97.7+0.3
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T (°C)
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