Ref. Ares(2017)6114712 - 13/12/2017
D3.5
Version: 1.0 Date: 10.07.2017 Scheduled for: month 30 Author: LITHOZ Dissemination level: PU Document reference: D3.5
Provide characterization of sintered
ceramic and cermet parts Project acronym: ToMax
Project name: Toolless Manufacturing of Complex Structures
Call: H2020-FoF-2014
Grant agreement no.: 633192
Project duration: 36 months (2015-01-01 – 2017-12-31)
Project coordinator: TUW (Technische Universität Wien)
Partners: (lead) Partner LITHOZ
Project no. 633192 "Toolless Manufacturing of Complex Structures"
ToMax H2020-633192 ToMax D3.5v1.0
FoF-02-2014 "Manufacturing processes for complex structures and geometries with efficient use of material"
D3.5 Provide characterization of sintered ceramic and cermet parts
Due date of deliverable: 2017-06-30
Actual submission date: 2017-07-10
Start date of project: 2015-01-01
Duration: 36 months
Lead partner for this deliverable: Partner LITHOZ
Project founded by the European Commission within the Horizon 2020 Programme
Dissemination Level
PU Public X
CO Confidential, only for members of the consortium (including the Commission Services)
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History chart
Version Author Date Status Reviewer 1.0 Martin 2017-06-22 Schwentenwein 2017-07-10 final Koch
Abstract
The subject of this deliverable is the characterization of the 3D printed ceramic (alumina and sialon) and cermet (alumina-molybdenum) parts that have been developed in the course of Tomax. For the sintered alumina ceramics it could be demonstrated that it is possible to fabricate fully dense objects with maximum wall thickness of > 10 mm in the green and 10 mm in the sintered state without any defects.
Regarding sialon ceramics it was possible to realize fully dense parts with the same properties as known from traditional processes. This is the first time that it could be demonstrated that it is possible to obtain dense and strong silicon nitride-based parts by lithographic 3D printing. For the cermet system it could also be shown that for a 20 wt% of metal defect-free parts could be realised that exhibited the same thermal properties is modelled by simulations. As for sialons, this was also the first proof for successful lithographic 3D printing of such materials.
The document is proprietary of the TOMAX consortium members. No copying or distributing, in any form or by any means, is allowed without the prior written agreement of the owner of the property rights. This document reflects only the authors' view. The European Commission is not liable for any use that may be made of the information contained herein.
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Content
Introduction ...... 5 Technical Approach and Results ...... 6 A – Background ...... 6 B – LCM process ...... 6 C – 3D Printed Alumina ...... 7 D – 3D printed Sialon ...... 8 E – 3D printed Cermet ...... 10 Summary ...... 12
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Introduction
The aim of this deliverable was the characterization of 3D printed alumina, sialon and cermet parts. The printed parts have to be defect-free and to fulfil other criteria depending on the evaluated material.
The two major aims of this work were: - Printing of dense and massive alumina objects - Printing of dense and strong silicon nitride-based objects - Printing of thermally conductive cermet parts
All discussed parts and designs were fabricated using the lithography-based ceramic manufacturing (LCM) by Tomax-partner Lithoz. Chracterization was done by Lithoz as well as by the involved end-users.
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Technical Approach and Results
A – Background For the experiments described within this report different photocurable suspensions based on alumina, a sialon system as well as a cermet (alumina-molybdenum with 20 wt% molybdenum) were employed. The suspensions for this work were developed in the course of Tomax. Details to the chemistry and properties of these materials are described in the previous deliverables. All experiments were conducted on a CeraFab 7500 system, which has been developed and commercialized by Lithoz. The working principle of this is system is based on digital light processing (DLP).
B – LCM process This section discusses the actual printing of the alumina samples using a lithographic process. Figure 1 shows a schematic drawing of the working principle of the above mentioned CeraFab 7500.
Figure 1: Schematic principle of the LCM process as implemented in the CeraFab 7500: light engine (1), rotating vat (2), building platform (3), and wiper blade (4)
The key elements of this process are that it - uses a rotating vat in combination with a static wiper blade in order to apply the ceramic suspension prior to each curing sequence, - is based on DLP and hence uses a simultaneous illumination of the whole layer information, and - uses a bottom-up based printing approach; this means that the parts are pulled out of the vat upwards the z-axis while the printing process.
The building envelope of the CeraFab-system is 76 x 43 x 150 mm. The resolution in the x/y- plane is 40 x 40 µm. The used layer thickness varied between the different materials and is thus mentioned in the respective sections.
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C – 3D Printed Alumina For printing of the alumina sample parts a layer thickness of 25 µm in the green state was chosen. The printed parts were subsequently cleaned and underwent thermal post-treatment for debinding and sintering to full density; sintering was done at 1600°C with a dwelling time of 2 hours. The average density of the produced test objects (cylinders, discs, cuboids, bending bars) was 3.967 ± 0.008 g/cm³ and corresponds to 99.54 % of the theoretical density. For characterization of the mechanical strength only preliminary tests without grinding and polishing of the surface were conducted so far (‘as fired’). This test setup typically leads to significantly lower than values compared to experiments according to standards with polished surfaces due to the presence of micro-grooves from the layer-by- layer printing procedure. First tests gave an average strength value of 395 MPa for the alumina samples. Thus, it is expected that for polished surfaces strength values of more than 450 MPa will be obtained in the next step; these advanced tests are subject of the last 6 months of Tomax. The high density of the alumina material is also confirmed by microscopy of the fracture surfaces and polished specimens as depicted in Figure 2.
100 µm
Figure 2: Fracture surface of printed alumina object after testing (left), and a light microscopy image of a polished surface (right)
Both the achieved density and the strength (as fired) show a slight improvement compared to the prior state of the art before Tomax. The main achievement was however the increase in geometrical flexibility of the printed alumina parts; for the first time dense and defect-free alumina samples with a wall thickness of or beyond 10 mm could be realized by LCM. Figure 3 shows such exemplary parts; details on this work can be found in the report for D3.4.
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Figure 3: Photographs of printed alumina objects after sintering, a cylinder with a diameter of 11.2 cm (left) and a cuboid with an edge length of 20 mm (right)
D – 3D printed Sialon The photocurable suspensions used for making the samples described here were discussed in the report for D3.2. By developing a dedicated thermal post-processing protocol it could be shown that it was possible to produce fully dense and defect-free silicon-nitride-based components by lithographic 3D printing. The measured densities of the sintered parts were typically 3.24 g/m³ to 3.25 g/m³ which correspond to 99 – 100 % of the theoretical density as obtained according to the Archimedes method. Preliminary characterization of the mechanical strength was done using biaxial bending tests in a ball-on-3 balls (B3B)-setup. The number of tested specimens was only 7 in this first iteration which is why there was no possibility to deduce any reasonable statistics or Weibull modulus. The results of the biaxial strength experiments are depicted in Figure 4.
Figure 4: Stress/strain curves of the B3B-bending tests (left) and the printing orientation of the tested samples
It should be noted that all biaxial bending discs were printed standing on their cylinder jacket so that during testing the load could be applied in direction of the layer boundaries. Thus, the measured values give an indication on the quality of the printed construct. The average D3.5 - Characterization of sintered parts Page 8 of 12
H2020-633192 ToMax D3.5v1.0 diameter of the tested samples was 21.60 mm and their thickness after grinding and polishing at one side 1.67 mm. A comparison with traditionally manufactured samples tested in the same fashion is shown in Table 1.
Table 1: Comparison between LCM-printed and isostatically pressed samples
Process Density Hardness (HV10) Biaxial bending strength [g/cm³] [MPa] LCM 3.25 1500 764 Iso-pressing 3.25 1500 770
It can be seen, that both samples are at the same level in terms of the bending strength; also in terms of hardness (Vickers hardness HV10) the performance of the printed and the iso- pressed samples were at eye-level. The value of 760 MPa significantly exceeds the targeted value of 550 MPa as specified in MS7 which is due at the end of the project in month 36. To confirm these findings fractography of the tested samples was also conducted. Figure 5 shows once more a comparison between printed and traditionally manufactured samples without giving any indications of significant differences of the two processing methods.
LCM Iso- pressed
LCM Iso- pressed
Figure 5: Fracture surfaces of biaxial bending discs in different magnifications: from LCM-printed sample (left) and isostatically pressed sample (right)
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A first simple thermal shock experiment was also conducted. A printed sialon impeller was heated to 800°C in air and then quenched by immersing it in a water bath at room temperature. Subsequent optical inspection did not show any effects of the quenching on the integrity and quality of the parts; the described experiment can be watched under https://www.youtube.com/watch?v=lxcVGufLsLs . Figure 6 shows a picture from the tested impeller immediately after the experiment.
Figure 6: Sialon impeller after thermal shock experiment
In addition to these tests, tactile surface roughness measurements were also conducted on the Sialon-parts to determine the surface quality in different orientations. These experiments were done according to ISO 1997 on the as-fired samples without any prior grinding or treatment of the surfaces. The measured results Ra were 0.76 µm (on top of the printed layer), 0.50 µm (alongside a printed layer) and 0.70 µm (perpendicular to a printed layer in building direction). These findings are well in line with the typical surface roughness values that are obtained from dense oxide ceramics manufactured using LCM. All these findings and results clearly demonstrate that lithographic printing of sialon-materials is an option to fabricate ceramic components that exhibit basically the same properties as the traditionally manufactured analogues. Further experiments to be able provide statistically relevant results on the properties of printed sialons will be the subject of the last year of Tomax.
E – 3D printed Cermet As an exemplary cermet material the system aluminum oxide/molybdenum was investigated. A possible application of such a cermet is for example in heat sinks due to the increased thermal conductivity coming from the metallic phase. The preparation and printing of the used suspensions are described in the report for D3.2. There a mixture with 20 wt% Mo- content (8.7 vol%) was found as best candidates in offering both reasonable metal loading and proper photoreactivity. Thermal post-processing of the parts printed from this system could be realized under hydrogen atmosphere to give the sintered cermet parts. Figure 7 show polished surfaces of the sintered cermet.
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20 µm
100 µm
Figure 7: Polished cross-section of a sintered alumina/molybdenum-cermet in 2 different magnifications
It can be seen that the molybdenum is homogenously dispersed within the alumina matrix, thus no sedimentation occurred during the printing process. One can also see an interlaminar defect at the bottom side of the figure, this indicates that further optimization of the thermal post-processing procedure of the cermet material is still necessary. However, in the intact region the material looks very homogeneous without any indication of artifacts from individual layers (eg pore concentrations at the layer boundaries). Thus, for defect-free samples, the same thermal conductivity was measured as predicted by model for a cermet of this composition as shown in Figure 8.
modeled measured
Temp. rise [V] Temp.
Time [ms]
Figure 8: Xenon Flash Analysis of the cermet: comparison between modeled and experimental data D3.5 - Characterization of sintered parts Page 11 of 12
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This indicates that there is no influence from the printing process on the thermal properties and that the fabrication of cermet materials by lithographic printing is feasible.
Summary
In the course of Tomax lithographic printing of ceramic materials was further advanced, both regarding quality of already existing materials (defect alumina parts with dimensions >10 mm in the sintered state at full density) and regarding new developments. Especially the development of a silicon nitride-based material shows great promise; the obtained material properties are at eye-level to parts made from traditional processes and allow for the first time the fabrication of dense and defect-free sialons by a lithographic AM process, which already lead to the commercialization of this new material by Lithoz. Moreover, printing of cermets by LCM was also demonstrated for the first time. Here the obtained results also indicate that the printed cermet parts should completely functional at full maturity of the process. However for this approach further optimization is still necessary, especially the thermal post-processing (debinding) of the printed parts needs some further improvement.
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