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© Fraunhofer IWS © Fraunhofer IWS
TOM: December 2, 2015: „Potentials of Thermal Spraying with Aqueous Suspensions“
Demands, Potentials and Economic Aspects of
Thermal Spraying with Aqueous Suspensions
Filofteia-Laura Toma, Christoph Leyens
Fraunhofer Institute for Material and Beam Technology (IWS), Dresden, Germany
Annegret Potthoff
Fraunhofer Institute for Ceramic Technologies and Systems (IKTS), Dresden, Germany
© Fraunhofer IWS
Outlines
Introduction
Thermal spraying with suspensions
Demands and availability of suspensions
Hardware components
Properties of water-based suspension sprayed coatings (case applications)
Economical aspects
Conclusions and perspectives
© Fraunhofer IWS
Thermal spraying with suspensions
Use of know spray processes with liquid feedstock (SHVOF/HVSFS, SPS)
New opportunities: direct use of nano-sized and sub-µm materials
If the whole technological chain is considered, the spraying with suspensions
eliminates steps in the feedstock powder preparation
Raw materials for suspensions:
focus on oxide ceramics (YSZ, Al2O3, TiO2, Y2O3, Cr2O3, …)
Chemically prepared metal powders
Specific materials properties play a more significant role than in
conventional spraying
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SUSPENSION
Solid content
Impurities
Particle size,
morphology
Chemical composition
Surface charge
properties
pH value,
conductivity
Homogeneity,
sedimentation
Viscosity,
flowability
Safe production, environmental friendly, low health risks
Characteristics of suspensions
Toma et al., Journal of Thermal Spray Technology, 2015
© Fraunhofer IWS
Thermal spraying with suspensions
Requirements for industry
Long-term stability while spraying and long-time spraying
High deposition efficiencies and high coating qualities
Compatibility of suspensions with the hardware components (no corrosion,
abrasion or clogging)
Safety aspects (transport, handling, long-time storing)
Availability and reproducibility of batches
Price (!)
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Thermal spraying with suspensions
Aqueous suspensions for spraying process:
Dispersion in water (no organic solvents!) safety aspects, costs
High content of solids (up to 70 wt%) efficiency
Good flowability (low viscosity) stability of spraying process
High stability against sedimentation and high redispersability handling
Suspensions availability
Commercial available from industrial producers
Lab /on-site prepared suspensions
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Suspensions availability: from producers
What you can get:
Commercial ready-to-use water-based suspensions available for
various oxide ceramic materials
Solid content up to 50 wt. %
Stability for use up to 6 months
Often based on well-dispersed nanomaterials
Contain dispersant aids from production process and/or for stability reasons
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Suspensions availability: from producers
Advantages and disadvantages:
Easy handling, less equipment necessary
No contact with fine (nano)powders (healthy and safety aspects)
− Limited variety, for example regarding to particle size, or solid content of
raw material
− Limited information regarding suspension properties (i.e. composition,
particle size distribution, rheology, drying rate) “black box”
− Flowability? Process stability?
in most of cases, some characterization of the delivered suspensions
are recommended before spraying
© Fraunhofer IWS
Suspensions availability: on-site prepared suspensions
What you can get:
Raw materials with a great variety of properties regarding:
Chemical composition and crystallinity
Particle size (nm- or µm-scaled)
Impurities
Recipes transferable from
lab to the industrial users
© Fraunhofer IWS
Suspensions availability: on-site prepared suspensions
Advantages and disadvantages:
Large variety of raw materials available flexibility in preparation of tailored
suspensions
Storage of raw material possible
Suspensions with good flowability and high content of solids can be produced
Known ingredients only
− Need knowledge in manufacturing process for suspension preparation,
specific for each raw material
− Need dispersion equipment
− Safety precaution when handling of (nano)powders
© Fraunhofer IWS
Suspensions availability: on-site prepared suspensions
Suspension development:
© Fraunhofer IWS
Technological development of suspension spraying at
Fraunhofer IWS
S-HVOF (with axial injection of the suspension)
Injectors
Hardware components
S-HVOF of a YSZ-Suspension in slow motion
© Fraunhofer IWS
Technological development of suspension spraying at
Fraunhofer IWS
S-APS (with radial injection of the suspension)
Injectors
Hardware components
© Fraunhofer IWS
Automatic, mobile and free-standing system
Customizable to any spray facility
3 pressurized vessels
1x cleaning fluid
2x suspension
Pressure and flow rate adjustment
Touchscreen handling
Continuous operation
Development of multilayers and composite coatings
Technological development of suspension spraying at
Fraunhofer IWS
Industry-suitable pressurized suspension feeder
50 µm20 µm
© Fraunhofer IWS
Coating microstructures produced from aqueous
suspensions
Porous coating
Dense coating
Columnar-like Structure
with vertical cracks
Toma et al., Thermal Spray Bulletin 2010, 2013
Toma et al., Journal of Thermal Spray Technology 2015
Ganvir et al. Surface and Coatings Technology 2015
Thick coating
2 mm
2.75 mm
Thin coating
© Fraunhofer IWS
Very high purity of the
powder Majority -
Al2O3-content in the
coating
low -Al2O3-content in
the coating
B-Suspension
A-Suspension
Influence of the powder properties (purity)
Case applications: Al2O3 S-HVOF coatings as insulators
for electronic devices
© Fraunhofer IWS
Case applications: Al2O3 S-HVOF coatings as insulators
for electronic devices
0 10 20 30 40 50 60 70 80 90 100
107
108
109
1010
1011
1012
Ele
ctr
ical re
sis
tivity (
m)
Relative air humidity (%)
S-HVOF: 60 µm
S-HVOF: 120 µm
S-HVOF: 150µm
S-HVOF: 200µm
HVOF: 100 µm
Toma et al., Journal of Thermal Spray Technology, 2012
in cooperation with:
S-HVOF coatings preserve longer the electrical insulation in environments with
high RH than the conventional HVOF coatings
© Fraunhofer IWS
Case applications: Cr2O3 S-HVOF coatings for wear
protection
SHVOF Cr2O3 Coating APS Cr2O3 Coating
Hardness: 1100-1500 HV0,3 E-Modul (LaWave): 100-130 GPa
Hardness: 1100-1300 HV0,3 E-Modul (LaWave): 55-80 GPa
Toma et al., Proceedings of ITSC 2015, Long Beach, USA
© Fraunhofer IWS
Case applications: Cr2O3-TiO2 S-HVOF coatings for wear
protection
Preparation of binary suspensions Cr2O3-TiO2 (85/15) following two routes
Type A Type B
Powders
Cr2O3, 99
TiO2, 99
Cr2O3/TiO2 85/15
Aqueous
suspension
25 wt.% solid +
additive
25 wt.% solid +
additive
25 wt.% solid +
additive
Mixing of suspensions in proportion
85 /15
S-HVOF Suspension Type A Suspension Type B
© Fraunhofer IWS
Case applications: Cr2O3-TiO2 S-HVOF coatings for wear
protection
Microstructures of coatings from suspensions A and B are comparable
Dense and homogeneous coatings
Hardness: 1000 – 1250 HV0,3 (APS: 890 HV0,3; HVOF: 1150 HV0,3)
Typ A Typ B
© Fraunhofer IWS
Case applications: YSZ suspension sprayed coatings for
thermal barrier coatings
Ganvir et al., Surface & Coatings Technology, 2015
S-HVOF
SPS
© Fraunhofer IWS
Economical aspects of thermal spraying with aqueous
suspensions
Influence of:
Suspension formulation
Spray process and spray parameters
Characteristics Value
Materials Mostly Oxides. Non-oxides also possible
Particle size From nm up to 10 µm
Solid content 15 – 50 wt.% ; up to 70 wt.% possible
Suspension feed rate 20 – 120 ml/min (10 – 60 g/min)
Coating thickness Typically 10 µm – 150 µm; higher thicknesses possible
Layer thickness per pass 2 µm < d < 40 µm
Deposition rate 30 – 70 % for “low melting” ceramics (i.e. Al2O3, TiO2)
25-40 % for high melting ceramics (i.e. Cr2O3, YSZ)
Coating roughness Ra: <1 - 3 µm; Rz: 10 - 25 µm
© Fraunhofer IWS
Implementation of the suspension spraying in the current spray booths
The demands on coating quality and properties are determined by the
appropriate choice of raw material, suspension formulation and process
conditions
Properties of aqueous suspension sprayed coatings are comparable or better
than conventional sprayed coatings
Deposition efficiencies and coating build-up rates are similar to conventional
spray processes competitiveness
Thermal spraying with aqueous suspensions - Conclusions and perspectives
© Fraunhofer IWS
Costs, operational safety and economical efficiencies clearly speak for
use of aqueous suspensions for thermal spraying on industrial scale
Thermal spraying with aqueous suspensions - Conclusions and perspectives
© Fraunhofer IWS
Acknowledgement
Part of the presented results were obtained in the German AiF project
“High performance Cr2O3-Coatings by thermal spraying with
suspensions” (IGF-No. 18.154B / DVS-No. 02.094) of the German
Welding Society (DVS) Düsseldorf, funded via AiF by the German
Federal Ministry of Economic Affairs and Energy in the framework of
the program for promotion of “Industrial Joint Research (IGF)”. The
financial support is acknowledged.
To our colleagues from Fraunhofer IWS and IKTS:
Stefan Langner, Nick Kulissa, Richard Trache, Irina Shakhverdova, Beate Wolf,
Anja Mayer
© Fraunhofer IWS
Dr.–Ing. Filofteia-Laura Toma Fraunhofer Institut für Werkstoff- und Strahltechnik (IWS)
Winterbergstr. 28, 01277 Dresden, Deutschland
Telefon: +49 351 83391-3191
Thank you for your attention!
