Hybridizing Lithography-Based Ceramic Additive Manufacturing with Two-Photon-Polymerization

Round 1

Reviewer 1 Report

The manuscript entitled “Lithography-Based Ceramic Additive Manufacturing Hybridizing with Two-photon-polymerization”, authored by Johanna C. Sänger, Martin Schwentenwein, Raul Bermejo and Jens Günster has been reviewed.

In this work, the authors demonstrate the combination of Lithography-Based Ceramic Manufacturing (LCM) and two-photon-polymerization 2PP techniques, employing a transparent ceramic resin serving as the feedstock. The hybridization of the two light-based additive manufacturing processes results in YSZ structures up to centimeter size range. The authors claim that this kind of hybridization will bring the desired precision to the region of interest without escalating printing time and costs.  For this research, a transparent feedstock solution of yttria stabilized zirconia (YSZ) particles (~ 5 nm at 70 wt.%) and a standard resin that originally suit to 2PP and capable to print with LCM were used. The work also demonstrates the sintering of the respective 3D printed YSZ hybrid parts to full densification. However, the end product after sintering showed a linear shrinkage of ~38%, which is a bit higher side. Furthermore, the washing conditions after LCM-printing passivate the sample surface and therefore hinder a good connection to the 2PP print. Based on these first results, the best printing parameters are yet to be found.

This paper outlines a very interesting and novel approach to fabricate fairly large structures with highest geometric accuracy by combining LCM and 2PP manufacturing techniques. This kind of hybridization techniques will benefit to achieve the desired precision to the region of interest on reasonably large parts at moderate printing time and costs, though the printing strategy needs careful planning.

The scientific content of the paper is adequate for publication after making the following corrections. 

Q1. P1, L-33-36, “Additive Manufacturing (AM) of ceramics in the micrometer scale opens new applications especially as ceramics inhere unique and desirable properties [1]. They are thermally stable, almost chemical inert, abrasion resistant, biocompatible and have special electric properties”.

Please correct the sentence.

Q2. P2, L-48-51, “Even though the opportunities opened by this new feedstock are thrilling, the achievement comes with a major drawback. Printing millimeter or even centimeter sized features with 2PP-accuracy takes tremendously long processing time, making this approach excessively expensive for the manufacture of macroscopic parts”.

Based on these results what do you think the best approach to reduce the processing time? How can you reduce the cost? Explain briefly.

Q3. P2, L-57-58, “In order to apply the 2PP process to ceramic slurries a certain level of transparency is required”.

How do you prepare the ceramic nanoparticles in aqueous or nonaqueous medium without agglomeration? And how do you sustain the consistency of the transparent ceramic stock solution printer friendly? What will be your preferred range of size and shape of the particles to achieve best results?

Q4. P2, L-77-78, “The feedstock for both processes was a photocurable poly-(ethylenglycol) diacrylate (PEGDA) resin (average Mn = 250, Sigma–Aldrich) with 70 wt% of yttria stabilized zirconia”

Please check the spelling.

Q5. P3, L-117-118, “All structures were printed with a speed of 10.000 μm s−1 and a power of 8 mW”.

Please correct the unit “case sensitive”.

Q6. P3, L-125-126, “Printed structures were washed using ethanol (absolute water free min. = 99.5%, 125 Sigma–Aldrich) to remove uncured material.

What if you eliminate the ethanol washing? Why don’t we assume that the uncured material will evaporate during binder burnout prior sintering.

Q7. P4-5, L-128-131, “Thermal debinding and sintering was performed with the following temperature schedule: 0 → 150 °C (1 K min−1) → 250 °C (0.1 K min−1) → 265 (1 K min−1) → 300 °C (0.1 K min−1) → 350 °C (1 K min−1) → 400 °C (0.1 K min−1) → 800°C (1 K min−1, dwell 2 h) → 0 °C (5 K min−1)”.

Please correct (case sensitive) the units 1 K min−1 every where throughout the sentnence.

Q8. P4, L-152-153, “I(x) = I0 exp(-αx), following the Lambert-Beer law of absorption, with I0 being

Please edit the Lambert-Beer law of absorption with I0 and I(x) as I₀ and I(x) respectively. correct the sentence.

Q9. How do you accomplish the thermal debinding and sintering of the hybrid ceramic structures? Especially at lowest heating rates of 1K min^-1 and 0.1 K min^-1? Briefly explain the sintering method/furnace you used in this work. What was the accuracy of your equipment? What is your opinion about laser heating? Is that more effective and economical?

Q10. P6, L188-189, “Figure 3 c) Overview of 2PP-printed parts from the same resin on one substrate after sintering 188 to 800 °C

The ceramic parts are looking very precise and great. But are they dense enough at 800 °C? What about the mechanical properties? Have you conducted a nanoindentation or similar studies to measure the strength, hardness or elastic modulus of your sintered hybrid structures?

Q11. P8, L253-258, “Topological effects, that is, rough surface and too harsh washing of the LCM structure or aging of the resin in general may result in a poor connectivity between LCM and 2PP structures.

Have you studied any topological studies such as AFM or XPS to study the surface morphology and contaminants?

Author Response

Comments and Suggestions for Authors

The manuscript entitled “Lithography-Based Ceramic Additive Manufacturing Hybridizing with Two-photon-polymerization”, authored by Johanna C. Sänger, Martin Schwentenwein, Raul Bermejo and Jens Günster has been reviewed.

In this work, the authors demonstrate the combination of Lithography-Based Ceramic Manufacturing (LCM) and two-photon-polymerization 2PP techniques, employing a transparent ceramic resin serving as the feedstock. The hybridization of the two light-based additive manufacturing processes results in YSZ structures up to centimeter size range. The authors claim that this kind of hybridization will bring the desired precision to the region of interest without escalating printing time and costs.  For this research, a transparent feedstock solution of yttria stabilized zirconia (YSZ) particles (~ 5 nm at 70 wt.%) and a standard resin that originally suit to 2PP and capable to print with LCM were used. The work also demonstrates the sintering of the respective 3D printed YSZ hybrid parts to full densification. However, the end product after sintering showed a linear shrinkage of ~38%, which is a bit higher side. Furthermore, the washing conditions after LCM-printing passivate the sample surface and therefore hinder a good connection to the 2PP print. Based on these first results, the best printing parameters are yet to be found.

This paper outlines a very interesting and novel approach to fabricate fairly large structures with highest geometric accuracy by combining LCM and 2PP manufacturing techniques. This kind of hybridization techniques will benefit to achieve the desired precision to the region of interest on reasonably large parts at moderate printing time and costs, though the printing strategy needs careful planning.

The scientific content of the paper is adequate for publication after making the following corrections. 

Q1. P1, L-33-36, “Additive Manufacturing (AM) of ceramics in the micrometer scale opens new applications especially as ceramics inhere unique and desirable properties [1]. They are thermally stable, almost chemical inert, abrasion resistant, biocompatible and have special electric properties”.

Please correct the sentence.

• Additive Manufacturing (AM) of ceramics in the micrometer scale opens new applications, as ceramics inhere unique properties [1]: thermally stable, chemical inert, abrasion resistant, biocompatible and various functional properties. Due to the large surface volume ratio of microstructures, corrosion and chemical resistance play an important role.

Q2. P2, L-48-51, “Even though the opportunities opened by this new feedstock are thrilling, the achievement comes with a major drawback. Printing millimeter or even centimeter sized features with 2PP-accuracy takes tremendously long processing time, making this approach excessively expensive for the manufacture of macroscopic parts”.

Based on these results what do you think the best approach to reduce the processing time? How can you reduce the cost? Explain briefly.

• A sentence was added, which deals with speeding up the 2PP-Process and the context in which 2PP and LCM stand:

“There are technologies in development to speed up 2PP-printing, e.g. using parallel beams or increasing the scanning speed, but presumably those will not reach the printing speed of larger Vat-Photopolymerization (VPP) techniques, like the Lithography-Based Ceramic Manufacturing (LCM). Combining the accuracy of 2PP with the printing speed of stereolithography LCM appears in this context very appealing.”

Q3. P2, L-57-58, “In order to apply the 2PP process to ceramic slurries a certain level of transparency is required”.

How do you prepare the ceramic nanoparticles in aqueous or nonaqueous medium without agglomeration? And how do you sustain the consistency of the transparent ceramic stock solution printer friendly? What will be your preferred range of size and shape of the particles to achieve best results?

• A stable nanoparticle suspension is provided by the mentioned CeraNovis GmbH. The authors prepare from this suspension a photocurable resin (PEG-DA) suitable for the respective printing technology, that is printer friendly. The particle size is below 100nm (5nm with TEM, 11,1nm with SAXS), which ensures the transmission of up to 90% depending on particle fraction and path length. For more information please check the preliminary study in reference 5

Q4. P2, L-77-78, “The feedstock for both processes was a photocurable poly-(ethylenglycol) diacrylate (PEGDA) resin (average Mn = 250, Sigma–Aldrich) with 70 wt% of yttria stabilized zirconia”

Please check the spelling.

• The spelling of PEG-DA was corrected

Q5. P3, L-117-118, “All structures were printed with a speed of 10.000 μm s−1 and a power of 8 mW”.

Please correct the unit “case sensitive”.

• It is corrected.

Q6. P3, L-125-126, “Printed structures were washed using ethanol (absolute water free min. = 99.5%, 125 Sigma–Aldrich) to remove uncured material.

What if you eliminate the ethanol washing? Why don’t we assume that the uncured material will evaporate during binder burnout prior sintering.

• Yes, the uncured material would also evaporate during debinding. But it will leave behind unwanted nanoparticles, especially inside holes. The uncured material must be removed to remove also unwanted particles.

Q7. P4-5, L-128-131, “Thermal debinding and sintering was performed with the following temperature schedule: 0 → 150 °C (1 K min−1) → 250 °C (0.1 K min−1) → 265 (1 K min−1) → 300 °C (0.1 K min−1) → 350 °C (1 K min−1) → 400 °C (0.1 K min−1) → 800°C (1 K min−1, dwell 2 h) → 0 °C (5 K min−1)”.

Please correct (case sensitive) the units 1 K min−1 every where throughout the sentnence.

• It is corrected.

Q8. P4, L-152-153, “I(x) = I0 exp(-αx), following the Lambert-Beer law of absorption, with I0 being

Please edit the Lambert-Beer law of absorption with I0 and I(x) as I₀ and I(x) respectively. correct the sentence.

• It is corrected.

Q9. How do you accomplish the thermal debinding and sintering of the hybrid ceramic structures? Especially at lowest heating rates of 1K min^-1 and 0.1 K min^-1? Briefly explain the sintering method/furnace you used in this work. What was the accuracy of your equipment? What is your opinion about laser heating? Is that more effective and economical?

• Angaben angefragt Andrea Spitzer
• Laser heating?

The method section has been amended accordingly:

“Thermal debinding and sintering was performed in a high temperature oven (HTF 17/10, Carbolite Gero GmbH, Germany) with a controller (Eurotherm 2416, Eurotherm Germany GmbH, Germany) with the following temperature schedule”

and

“The Eurotherm-controller allows a programming of the temperature ramps in a wide range. Absolute precision of the furnace at the sample position is approximately in the range of ±10°C.”

High heating rates as provided by laser annealing are generally not desirable for debinding.

Q10. P6, L188-189, “Figure 3 c) Overview of 2PP-printed parts from the same resin on one substrate after sintering 188 to 800 °C

The ceramic parts are looking very precise and great. But are they dense enough at 800 °C? What about the mechanical properties? Have you conducted a nanoindentation or similar studies to measure the strength, hardness or elastic modulus of your sintered hybrid structures?

• Definitely the parts aren’t as dense as if they would be sintered at 1200 or even 1450°C (typical for YSZ). The mechanical properties are described in the preliminary study (see Reference 5) with nano-indentation tests. There we found the compressive strength is similar to bulk zirconia material. We refrain from calculating the Youngs modulus from the stress strain curves. Though, the present study is focusing on the hybridization of LCM and 2PP. In a yet unpublished study we describe the mechanical behavior of the material sintered at different temperatures even further. Stay tuned!

Q11. P8, L253-258, “Topological effects, that is, rough surface and too harsh washing of the LCM structure or aging of the resin in general may result in a poor connectivity between LCM and 2PP structures.

Have you studied any topological studies such as AFM or XPS to study the surface morphology and contaminants?

• Unfortunately, not yet.

Reviewer 2 Report

This communication demonstrates the combination of Lithography-Based Ceramic Manufacturing LCM and two-photon-polymerization 2PP techniques, employing a transparent ceramic resin serving as feedstock.It is demonstrated that the YSZ feedstock can be used for 2PP printing, crosslinking substrates with a UV lamp and the LCM process.In future applications, it looks promising.However, there are several problems in the article: (1) typo error: "Figure 2a" and "Figure 2b" appear on lines 174 and 175, “micro-busses” in line 178but there is only one figure in Figure 2. (2) “A comparison between non-sintered and sin- 200 tered state shows a linear shrinkage of ~38%.” in line 200-201. I am wondering how to control the shrinkage of the volume. Is it unique planar shrinkage or 3-dimentional shrinkage?

Author Response

Comments and Suggestions for Authors

This communication demonstrates the combination of Lithography-Based Ceramic Manufacturing LCM and two-photon-polymerization 2PP techniques, employing a transparent ceramic resin serving as feedstock.It is demonstrated that the YSZ feedstock can be used for 2PP printing, crosslinking substrates with a UV lamp and the LCM process.In future applications, it looks promising.

However, there are several problems in the article: 

• typo error: "Figure 2a" and "Figure 2b" appear on lines 174 and 175, “micro-busses” in line 178but there is only one figure in Figure 2. 
• Is corrected.

(2) “A comparison between non-sintered and sin- 200 tered state shows a linear shrinkage of ~38%.” in line 200-201. I am wondering how to control the shrinkage of the volume. Is it unique planar shrinkage or 3-dimentional shrinkage?

• It is three-dimensional shrinkage, which also tends to deform the specimen. 2PP-structures are attached to a surface and if the surface doesn’t shrink at the same ratio the structures tend to crack, deform or even detach. That’s why we chose UV-cured substrates and LCM structures from the same composition to ensure a homogeneous co-shrinkage.