Ceramic Coatings by Thermal Spraying - a Comparison between High Velocity Oxy-Fuel and Atmospheric Plasma Spraying
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
COMPONENTS TECHNOLOGY INSIGHTS
Ceramic Coatings by Thermal
Spraying – a Comparison between
High Velocity Oxy-Fuel and
Atmospheric Plasma Spraying
Metals, plastics and ceramics each have particular properties, which
make them suitable for different applications. Density, thermal and
electrical conductivity, thermal expansion, hardness, Young’s modulus,
optical properties, melting point, chemical stability, manufacturing cost
and environment-friendliness are only some of the aspects which must
be considered in the selection of a material class for a given application.
Despite the development of a great number of new materials, a single
material may not fulfil all the requirements of the intended application. In
many cases, simply the modification of the surface of the structural
material by means of a coating is sufficient to improve an existing
application or to enable new ones.
A common combination is the application of of a ceramic. Fig. 1 presents examples of While thermal energy is necessary for the
a ceramic coating onto a metallic substrate. ceramic-coated metal parts for engineering melting of the feedstock, kinetic energy is
Due to their high brittleness and hardness, applications. required to accelerate the molten particles
even at high temperatures, ceramics are Thermal spraying is a group of coat- towards the substrate with sufficient ve-
in many cases unsuitable as structural ing techniques based on the melting of a locity to obtain an adherent coating with a
materials. Moreover, shaping of complex solid feedstock, which is then accelerated suitable density for the intended applica-
3D-parts from ceramics is challenging towards a substrate onto which it re-solid- tion. The higher the particle temperature
and the manufacturing costs are generally ifies, hence forming thick coatings with a becomes, the lower the viscosity upon
higher than for metals. However, ceramics lamellar structure, composed of individual impact, facilitating spreading of the molten
provide the benefits of higher corrosion splats – flattened, re-solidified droplets. In material. In contrast, a high impact velocity
and wear resistance, higher hardness and techniques such as Flame Spraying (FS), with an insufficient temperature could lead
a lower thermal and electrical conductivity. High Velocity Oxy-Fuel (HVOF), High Veloc- to erosion of the substrate, due to the im-
By the application of a ceramic coating ity Air-Fuel (HVAF) and Detonation Spraying pact of unmelted particles. However, these
onto a metallic substrate, 3D-parts can be (DS), the thermal energy required to melt
manufactured with the structural proper- the feedstock originates from the combus-
ties of a metal and the surface properties tion of a mixture of oxygen – or air in the Gilvan Barroso
case of HVAF – with a combustible gas or a Rauschert Heinersdorf-Pressig GmbH
Keywords liquid fuel. Other techniques, such as wire 96332 Pressig
ceramic coatings, thermal spraying, high Germany
arc spraying (only suitable for metallic coat-
velocity oxy-fuel, atmospheric plasma
spraying, feedstock, metallic substrate, ings) and plasma spraying, make use of an E-mail: g.barroso@prg.rauschert.de
wear protection electric arc as an energy source.
CERAMICAPPLICATIONS 9 (2021) [2] 1
TECHNOLOGY INSIGHTS COMPONENTS
Fig. 1
Examples of ceramic-coated metal parts coated at Rauschert
two parameters are also competing to obtain ideal conditions, the plasma gas is vergent-divergent nozzle design, the ther-
some extent. A higher particle velocity leads usually a binary or even ternary mixture of mal energy and pressure in the combustion
to a shorter residence time in the hot zone these gases. Despite the use of oxygen-free chamber are converted to kinetic energy,
of the flame or plasma plume, reducing the gases, oxygen from the surrounding air can yielding supersonic particle velocities.
particle temperature, and vice versa. oxidize the particles before they cool down. The maximum gas temperature in HVOF
Among all the thermal spraying techniques, Consequently, oxide-free coatings can only is limited by the flame temperature of the
the most commonly used method for the be obtained under a controlled atmosphere, fuel/oxygen mixture, which ranges between
deposition of ceramic coatings today is the such as an inert gas or a vacuum. 2500–3000 °C, depending on the fuel type
Atmospheric Plasma Spraying (APS) tech- Because of the oxidation and decomposi- and the fuel-to-oxygen ratio. To provide
nique. Due to very high particle velocities, tion, at extremely high temperatures, of sufficient thermal energy to the feedstock,
Detonation Spraying (DS) was, for many some feedstocks containing metals and particles are injected axially inside the
years, the method of choice to obtain dense carbides, APS is mostly used to deposit ox- burner (i.e. before the nozzle), or in some
ceramic coatings. However, because of ide ceramic coatings. The high plasma tem- cases radially into the nozzle. Since the
market strategy and proprietary rights, DS peratures ensure a complete melting of the axial injection takes place much closer to
equipment is not easily available; and only feedstock, even with high feed rates, yield- the combustion chamber, where the com-
a small number of companies are able to ing a high productivity. Coatings with poros- bustion gases are the hottest, this design
manufacture such coatings. Alternatively, ity values down to 2 % can be achieved. is especially suited to feedstocks with high
HVOF has been gaining importance as a However, metal and carbide-based coat- melting temperatures, such as ceramics.
method to obtain dense ceramic coatings. ings for less demanding applications are The development of HVOF burners has been
also frequently deposited by APS using mainly oriented towards metal and carbide-
Atmospheric Plasma Spraying (APS) suitable parameters and gas combinations. based coatings. Hence, during recent years,
In a typical plasma spraying torch, a cath- the design of HVOF burners has been opti-
ode is positioned axially to the nozzle, High Velocity Oxy-Fuel (HVOF) mized to achieve the highest particle veloc-
which acts as the anode. Upon the applica- In High Velocity Oxy-Fuel (HVOF), a combus- ity possible with reduced temperatures, to
tion of an electric potential, an arc forms, tible substance is mixed with oxygen and avoid phase transformations and oxidation
leading to the dissociation and ionization ignited – hence the name oxy-fuel. Typical of the feedstock. Recently, interest in HVOF
of the gas streaming between cathode and fuels for HVOF are either gases, such as as a method for the manufacturing of ce-
anode, generating a plasma. As the ionized propane, ethylene or hydrogen, or a liquid, ramic coatings has increased, especially
gases leave the nozzle, the ions recombine such as kerosene. for more challenging applications, such as
to reform the original gas molecules, liber- One of the main applications of thermal wear protection and electrical insulation.
ating an abundance of heat, reaching tem- spraying has always been coatings for
peratures above 10 000 K. Plasma spraying wear protection, for which high density, Comparative studies
processes can be classified according to hardness and excellent adhesion, in ad- One important difference between APS and
the atmosphere in which the spray process dition to a smooth surface, are crucial HVOF is the type and amount of feedstock
takes place. VPS (Vacuum Plasma Spraying) properties. These characteristics can be used. Due to the higher gas temperatures,
and LPPS (Low Pressure Plasma Spraying) achieved by increasing the particle veloc- feedstocks with larger particle size and/
are carried out in a chamber under reduced ity during spraying. The higher the velocity or feed rate can be used in APS without
pressure, whereas APS is carried out under upon impact, the greater the compaction of quality loss. HVOF requires finer powders
atmospheric pressure in natural air [1]. coatings. While the thermal contribution is and lower feed rates to ensure complete
The most common plasma gases are ar- dominant in APS; HVOF is oriented towards melting of the particles. Therefore, during
gon, hydrogen, nitrogen and helium. To high kinetic energy levels. Owing to a con- the same processing time, APS can yield
2 CERAMICAPPLICATIONS 9 (2021) [2]
COMPONENTS TECHNOLOGY INSIGHTS Fig. 2 Comparison of the microstructure of APS- and HVOF-sprayed Al2O3 coatings from Rauschert thicker coatings than HVOF. Moreover, the a more viable technique. However, for ap- particle velocity upon impact, which also part cooling requirements during spraying plications requiring high hardness, density, reduces the surface roughness of the as- are usually higher for HVOF. adhesion and smooth surfaces, HVOF coat- sprayed coatings (Fig. 3). Another important aspect to be considered ings stand out due to their technical advan- Typically, roughness values of as-sprayed is the processing cost. Fauchais et al. [1] tages. Al2O3 APS coatings are in the range of presented an economic analysis of differ- Fig. 2 presents examples of APS and HVOF Ra = 3–4 µm and Rz = 22–23 µm, and ent thermal spraying processes. APS has an Al2O3 coatings sprayed at Rauschert with as-sprayed Al2O3 HVOF coatings have economic advantage, since the prices for the same feedstock. The micrographs Ra = 2–3 µm and Rz = 14–17 µm. How- gases required for HVOF, such as ethylene, exemplify the typical microstructural dif- ever, the higher speed upon impact in- are 3–4 times higher than those for argon, ferences of coatings deposited with both creases the residual stresses in the HVOF hydrogen and nitrogen used in APS. Moreo- processes. A higher number of larger sized coatings. A careful tuning of the process ver, the specific energy requirement (i.e. pores and interlamellar (between splats) parameters is required to obtain well ad- the amount of power necessary to deposit porosity and cracking characterize the mi- herent, crack-free coatings. a given mass of coating), is considerably crostructure of APS coatings. On the other The differences in the microstructures higher for HVOF than for APS. Thus, from an hand, the porosity of HVOF coatings is com- of APS and HVOF coatings also reflect economic point of view, the APS process is posed of smaller pores, due to the higher on the surface properties after finish- Fig. 3 Topography profile of as-sprayed APS and HVOF Al2O3 coatings from Rauschert CERAMICAPPLICATIONS 9 (2021) [2] 3
TECHNOLOGY INSIGHTS COMPONENTS
resulting in values in the range of 6–26 %,
depending on the feedstock and torch used.
The average hardness of APS coatings was
in the range of 700–800 HV 1,0, whilst an
average of 920 HV 1,0 was measured for
HVOF coatings. The toughness of the HVOF
coatings was considerably higher than the
values measured for the APS counterparts.
Moreover, the values measured on APS
coatings varied greatly, showing depend-
ence on the torch used. The mass loss of
the best performing APS coating during the
abrasion test was twice that of the HVOF
Fig. 4
Comparison of the surface roughness of APS- and HVOF-sprayed Al2O3 coatings from Rauschert coating, providing evidence of the HVOF
after finishing coating’s higher wear resistance under the
investigated conditions.
ing. Fig. 4 shows the surface profile and higher level of compressive stresses in the Toma et al. [5] compared the electrical
the roughness values of APS and HVOF- coatings. properties of Al2O3 and MgAl2O4 (spinel)
sprayed Al2O3 coatings after the same fin- The higher particle temperatures and the coatings deposited by HVOF and APS. It is
ishing process. As evidenced by the surface reducing characteristic of the plasma gas well known that the typical α-Al2O3 phase
topography, the surface roughness is a con- used in APS caused a loss of oxygen from mainly transforms into γ-Al2O3 during ther-
sequence of coating porosity, which results TiO2, resulting in the formation of subox- mal spraying, which usually has a negative
in cavities in the surface after the finishing ides. The porosity of the APS coatings was influence on the mechanical properties and
process. Thus, the denser microstructure of around 2,5 % and below 1 % for those of electrical insulation. The authors demon-
the HVOF coatings leads to a lower surface the HVOF counterparts. This reduced po- strated that alumina coatings deposited by
roughness after finishing. rosity of the HVOF coatings contributed to HVOF had a 5-fold higher volume fraction of
It should be borne in mind, however, that a significant improvement in performance α-Al2O3 compared to the APS counterparts.
a comparison between different thermal during abrasion tests – the material loss Coatings prepared by HVOF were consider-
spraying processes is difficult, due to the was about 40 % lower for the HVOF coating ably denser for both materials.
large number of factors influencing the than that of the APS. The HVOF-sprayed spinel coatings had
coating’s characteristics. A thorough opti- Interestingly, no significant difference in the highest dielectric breakdown strength,
mization of the APS process with a suitable coating hardness was measured – all coat- reaching values over 30 kV·mm–1. Inter-
torch could lead to a similar microstructure ings had a hardness of about 850 HV 0,3. estingly, the APS-sprayed spinel coatings
to the one obtained by HVOF. This, however, However, the crack propagation resistance reached the lowest values of dielectric
would probably lead to increased process- was about 13 % higher in HVOF coatings breakdown strength – below 20 kV·mm–1.
ing costs, to the point where HVOF becomes than in those of APS. The DC electrical resistance, at room tem-
a more interesting option. The images pre- Killakoski et al. [3] investigated the prop- perature and 30 % relative humidity (RH),
sented herein refer to the microstructures erties of Al2O3–ZrO2 coatings with approxi- was in the order of 1011 Ω for all coatings.
of typical industrial coatings, which were mately 40 % ZrO2 sprayed by APS and HVOF. However, at 95 % RH, values for the APS-
developed considering both technical and APS Coatings had a porosity of around 2 % sprayed alumina coatings were in the order
economic aspects. Although different in- against values down to 0,88 % for those of of 104 Ω, one order of magnitude lower
dustrial applications of HVOF ceramic coat- HVOF. The highest hardness values were than the other three variants.
ings are known, the number of academic measured for the HVOF coating, 917 HV 0,3 At 200 °C, HVOF coatings outperformed
studies comparing APS and HVOF coatings against approx. 800 VH 0,3 for the APS the APS counterparts of the same material,
is limited. coating. However, APS coatings produced but the resistance of all coatings was in
Lima and Marple [2] compared the proper- slightly higher values of critical flexural the order of 1010 Ω of magnitude, with the
ties of APS and HVOF coatings from TiO2 strength and strain tolerance, attributed to exception of spinel HVOF coatings, which
feedstocks. Despite cooling with air jets, the coarser particle size and the less dense reached slightly higher values (approx.
the substrate temperatures increased to microstructure. 1,3 x 1011 Ω).
approx. 150 °C during APS and 270 °C dur- Liu and colleagues [4], compared APS- and Berger and co-workers [6] compared APS
ing HVOF. As expected, the average parti- HVOF-sprayed coatings made from different and HVOF coatings from TiO2/Cr2O3 feed-
cle temperature, measured in flight, was feedstocks composed of Al2O3 with 13 % stocks with different TiO2 to Cr2O3 ratios.
higher in APS (2718 °C) than that in HVOF TiO2. Moreover, coatings from the same After spraying by APS, a significant amount
(1811 °C), whereas the average particle feedstock were sprayed with two different of amorphous phases was detected in the
speed was over twice as high in HVOF than APS torches. As expected, HVOF coatings coatings. Dense and homogeneous coat-
in APS – 751 m·s–1 against 302 m·s–1. The were highly dense (1,2 % porosity), where- ings were sprayed with both techniques
higher particle speed in HVOF resulted in a as APS coatings had small and large pores, for all compositions, although HVOF coat-
4 CERAMICAPPLICATIONS 9 (2021) [2]
COMPONENTS TECHNOLOGY INSIGHTS
ings seemed slightly denser, with a finer parts, especially for small series production coatings. Moreover, the choice of a suit-
microstructure than APS coatings. For and/or large parts. The most common ther- able feedstock for each process is crucial,
almost all the compositions, HVOF coat- mal spraying techniques for the deposition increasing the difficulty in making a direct
ings were harder. The highest values were of ceramic coatings are APS and HVOF, both comparison between coatings from both
obtained with pure Cr2O3 coating, reach- available at Rauschert. While the particle methods.
ing 1350 HV 0,3, compared to approx. temperatures are usually higher in APS, As a competent partner with more than 30
1200 HV 0,3 for the APS counterpart. The HVOF yields higher particle velocities. years of experience in the development and
electrical resistivity of the coatings was Even though APS is usually preferred from production of ceramic coatings, Rauschert
higher for HVOF with all feedstock com- an economic point of view, many studies offers its clients APS and HVOF solutions
positions. The results for dry sliding and show that HVOF has technical advantages. with customized properties for a wide range
abrasive wear resistance measurements HVOF-sprayed coatings have higher den- of applications, including wear protection,
correlated well with the measured hard- sity, hardness and better wear resistance. electrical and thermal insulation and anti-
ness values. However, it is important to keep in mind that adherent coatings.
thermal spraying has an exceptionally large
Conclusions number of parameters, thus the APS pro-
The deposition of ceramic coatings onto cess can be optimized to yield technically Acknowledgements
metals is an intelligent approach to obtain improved coatings, which usually leads to The author acknowledges the contributions
parts with the surface characteristics of increased processing costs. of the colleagues from Rauschert to this pa-
a ceramic, but with the structural proper- Conversely, HVOF can be optimized to re- per, especially Dietmar Neder, Raika Brück-
ties of metals. This method provides lower duce processing costs, which is likely to ner, Klaus Schneider, Jim Greenfield, Jack
manufacturing costs compared to ceramic reduce the technical performance of the Ellis, Anna Schardt and Ulrich Werr.
References
[1] Fauchais, P. L. ; Heberlein, J. V. R.; Bou- [3] Kiilakoski, J.; Musalek, R.; Lukac, F.; [5] Toma, F.-L.; Scheitz, S.; Berger, L.-M.;
los, M. I. : Thermal Spray Fundamentals Koivuluoto H.; Vuoristo, P.: Evaluating the Sauchuk, V.; Kusnezoff M.; Thiele, S.:
– From Powder to Part. Springer, New toughness of APS and HVOF-sprayed Comparative Study of the Electrical Prop-
York, Heidelberg, Dordrecht, London, Al2O3–ZrO2-coatings by in-situ- and mi- erties and Characteristics of Thermally
2014 croscopic bending. J. of the Europ. Cer- Sprayed Alumina and Spinel Coatings. J.
[2] Lima, R. S.; Marple, B. R.: From APS to am. Soc. 38 (2018) [4] 1908–1918 of Thermal Spray Technology 20 (2011)
HVOF spraying of conventional and nano- [4] Liu, Y.; Fischer, T. E.; Dent, A.: Compari- 195–204
structured titania feedstock powders a son of HVOF and plasma-sprayed alumi- [6] Berger, L.-M.; Saaro, S.; Stahr, C. C.;
study on the enhancement of the me- na/titania coatings – microstructure, me- Thiele, S.; Woydt, M.: Development of
chanical properties. Surface and Coat- chanical properties and abrasion behav- ceramic coatings in the Cr2O3–TiO2 sys-
ings Technology 200 (2006) [11] 3428– iour. Surface and Coatings Technology tem. Thermal Spray Bull. 61 (2009) [1]
3437 167 (2003) [1] 68–76 64–77
CERAMICAPPLICATIONS 9 (2021) [2] 5
You can also read
