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Formation of high hardness zirconia coatings by gas tunnel type plasma spraying
SURfaCE
ELSEVIER
Surface and CoatingsTechnology90 (1997) 197-202
#O/ITING8 f[tligOlO
Formation of high hardness zirconia coatings by gas tunnel type plasma spraying Akira Kobayashi 1 Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567, Japan
Received 2 November 1995; accepted 12 October 1996
Abstract
The high hardness zircohia (ZrO2) coatings could be obtained at an atmospheric pressure by using a gas tunnel type plasma spraying. The characteristics of these high hardness ZrO2 coatings were investigated. The Vickers hardness of the ZrO2 coating at a short spraying distance was very high; a high hardness of more than Hv = 1200 was achieved at the surface side of the coating. The microstructure of the obtained high hardness ZrO2 coating was also investigated by the microscopic method. And the characteristics of the high hardness ZrO2 coating was discussed in comparison with that of the coating formed by the conventional type plasma spraying. It was clarified that the ZrO2 coating of the gas tunnel type was not only much harder but also less porous than that of the conventional type. © 1997 Elsevier Science S.A. Keywords: high hardness coating; zirconla coating; Vickers hardness; gas tunnel type plasma spraying
1. Introduction
In a plasma-sprayed coating with ceramic powder which has a high melting point, one of the problems is that there are many pores within the coating [ 1], especially for wear resistance and/or corrosion resistance coatings. This causes the lowering of mechanical and chemical qualities of the coating, and also limits its application fields. In these circumstances, a gas tunnel-type plasma spraying was developed by the author [2]. With the gas tunnel-type plasma spraying, the sprayed particles can be melted et~ficiently and the bonding force between the particles becomes stronger than the conventional one. Therefore, the qualities of a ceramic sprayed coating obtained with this new type plasma spraying is higher than with a conventional type plasma spraying. Namely, the porosity is decreased and the dense coating is obtained. These results have been investigated in the former studies [3-5]. As to the formation of such a high functionality material, high quality coatings can be obtained by the gas tunnel-type plasma spraying [6]; in the case of a short spraying distance, an alumina coating produced 1Tel.: +81 6 8798651; fax: +81 6 8798689; e-mait: [email protected]. 0257-8972/97/$17.00 © 1997ElsevierScienceS.A. All fightsreserved PII S0257-8972(96) 03 I43-X
had a high Vickers hardness of Hv=1200-1600 [7]. (Alumina sprayed coating is normally /-/,=700-800.) This high hardness coating had a high hardness layer near the coating surface. Various types of ceramic coatings produced by plasma spraying have been used in many industrial fields [8] including electronics, automobile, aerospace, steel making, and other manufacturing because of their excellent functionality. Zirconia (ZrO2) coatings with plasma spraying are attracting the attention of many researchers because of their superior properties such as corrosion resistance, thermal resistance, and wear resistance, and so on. For example, ZrO2 coatings have been formed as a thermal barrier coating (TBC) by plasma spraying [9, 10]. But even for TBC, high density is needed at the coating surface to achieve corrosion and/or wear resistance at high temperature [11]. Another example of dense coating is the electrode of a solid oxide fuel cell (SOFC) [12]. Thus, it is important to produce a dense zirconia coating. In this study, the high hardness ZrO2 coating was formed by gas tunnel-type plasma spraying, at a short spraying distance in the case of a large power input. The characteristics of the coating were investigated. As one of the coating properties, the Vickers hardness was measured on the cross-section of this high hardness ZrO2 coating. The effect of spraying distance, etc., on
A. Kobayashi/ Surface and Coatings Technology90 (1997) 197-202
198
the hardness of the coating is discussed. The microstructure of an obtained ZrO2 coating was investigated by optical microscopy. Finally, the results with regard to the characteristics of the ZrO2 coating are compared with the case of conventional type plasma spraying.
2. Experimental
Table 1 Experimental conditions Power input Working gas Gas flow rate Powder feed rate Spraying distance Traverse speed
P= 33 kW Ar Q=2001 min -1 w=40-60 g min -1 L = 20-70 mm v= 120 c m r a i n - 1 v=80 cmmin -~ d=20 mm
Gas divertor nozzle diameter Fig. 1 shows the gas tunnel-type plasma spraying apparatus used in this study. This plasma torch was improved (see below), compared with the gas tunneltype plasma spraying which has been described in previous papers [2-4], in order to carry out the high power experiment. However, the mechanism and properties are essentially the same: i.e., the gas tunnel-type plasma jet is a high energy and high efficiency type, which is useful for achieving a high quality coating. (A) in Fig. 1 is a plasma torch for the initiation of a gas tunnel-type plasma jet; (B) is the gas tunnel-type plasma torch. For the polarity of an electrode, the gas diverter nozzle was used an anode. The configuration of the vortex generator was little changed from one the formerly used. The nearest distance between two electrodes, an inside nozzle electrode and the gas diverter nozzle, was 15 ram, and the diameter of the gas diverter nozzle was d = 20 mm. In this case, the inlet of sprayed powder was located between torch (A) and torch (B). The experiment to form ZrO2 coatings was carried out at atmospheric pressure using a gas tunnel-type plasma spraying. Pure argon was used as the working gas. The distance between the torch and the substrate is the spraying distance, L. The traverse speed v was chosen from two values: 120 and 80 cm rain -t. The experimental conditions for the ZrO2 coatings are shown in Table 1. Here, Q is the working gas flow
PS1
PS2
Table 2 Zircoma powder used in this study Chemical compsitions (wt.%) ZrOz Y203 A12Q
SiO2
Others
90.78
0.20
0.49
8.15
0.38
Size (txm) 10-44
rate, and P is the power input to the plasma torch. Each of them was a constant value. The spraying distance was the main difference in this experiment. Especially at a short spraying distance of L = 2 0 - 5 0 mm, a high hardness ZrO2 coating was formed, and the effect of the spraying distance on the property of the ZrO2 coating was studied. During the experiment, the temperature of the substrate was measured at various spraying distances. In this case the substrate was not preheated. Table 2 gives the composition and size of the zirconia powder used in this study. The average size of this powder was 26.3 gm. The powder feed rate, w, was 4 0 g r o i n -1. A SUS304 plate of 5 m m in thickness, 25 x 50 mm in width and length, was used as a substrate. The surface of the substrate was shot blasted. The Vickers hardness, Hv, was measured at the crosssection of the coating formed under the various spraying conditions. The measured conditions of Vickers hardness were: the loading weight was 100 g, the holding time was 25 s. More than 10 traverses were made over the whole thickness, i.e., the number of measuring points was more than 10 at the same distance from the coating surface. The hardness of coating was decided by the average of the 10-point measurement. The microstructure of the cross-section of the obtained zirconia coating was observed using an optical microscope. For comparison purposes, the zirconia coatings were formed under similar spraying conditions using the conventional type of plasma spraying. The hardness of the coating was measured and the cross-section was also observed using an optical microscope. 3. Results and discussion
t foo.or I
g.s} 2
Fig. 1. Schematicof a gas tuanel-typeplasma spraying apparatus. (A) Conventional type plasma torch; (B) gas tunnel-type plasma torch; PS1, PS2, power supply. L, spraying distance; v, traverse speed.
3.1. Characteristics of high hardness Zr02 coating Fig. 2 shows the relation between the Vickers hardness Hv of a zirconia ( Z r Q ) coating produced by the
A. Kobayashi/ Surface and Coatings Technology 90 (1997) 197-202
199
1500 ~. 1000
P = 3 3 kW
P =33 kW
[,..., 800
~
1000 600
•~
Lp
40(
SO0 T
do
I
4o
s;
~
If Jo
P
,o
gas tunnel-type plasma spraying and the spraying distance L. In this case, the hardness Was decided by the highest value of ten times the average inside the coating. The experimental conditions were as follows; the power input was P = 33 kW, the gas flow rate of pure argon was Q = 2 0 0 1 m i n -1, and the powder feed rate was w = 40 g rain-~. The traverse speeds of the substrate were v = 120 and 80 cm min - t , respectively. The spraying distance was changed f r o m L = 20 to 70 mm. The spalling between the sprayed coating and the substrate sometimes occurred at the shorter distance of L = 20 m m when v = 80 cm r a i n - 1. But, under other conditions in this experiment, the spalling was suppressed by using a thick substrate. The Vickers hardness is increased with decreasing spraying distance, as shown in Fig. 2. The hardness becomes m u c h greater at a shorter spraying distance than the critical spraying distance, L v [7], indicated in Fig. 2. The characteristics of the Vickers hardness are similar in each case of traverse speed, v = 1 2 0 and 80 cm r a i n - z. But in the case of the lower traverse speed, a 20% higher Vickers hardness could be obtained at every spraying distance. The highest Vickers hardness o f the ZrOz coating is more than H~=1000 at L = 2 0 and 30 mm, in the case of v = 120 c m m i n - L Meanwhile, the highest is more than H~ = 1200 at L = 30 mm, in the case of v = 80 cm r a i n - 1. In this way, a very high hardness ZrO2 coating can be obtained by the gas tunnel-type plasma spraying at a short spraying distance (L < L p ) , as well as the case of an alumina coating [7]. With a decrease in the traverse speed, the energy transfer to the coating must be higher, c o n s e q u e n t l y Lp is increased. In the case of v = 80 cm m i n - ~, Lp is about 60 ram. Also, when power is increased, the length of plasma jet is increased, and /Iv and Lp are increased. Fig. 3 shows the results of the measurement of the substrate temperature during the zirconia (ZrO2) plasma
20
do
F
r
Spraying distance, L (re.m)
Spraying distance, L (mm)
Fig. 2. Relationship between the Vickers hardness of the ZrOz coating and the spraying distance at P=33kW at v=120 (O) and 80 cm rain -~ (&). Lp, critical spraying distance.
'
Fig. 3. Relationship between the substrate temperature and spraying distance at P = 3 3 kW and v = 120 cm min -~.
spraying at each spraying distance L = 2 0 - 7 0 m m . In this case, the experimental conditions were the same as those in Fig. 2: P = 3 3 k W , Q = 2 0 0 1 m i n -~, and w = 40 g m i n - ~. The traverse speed of the substrate was v = 120 cm r a i n - ~. The substrate temperature was increased with the decreasing spraying distance as shown in Fig. 3. The substrate temperature becomes much higher at shorter spraying distances. At L = 20 mm, the substrate temperature was T = 900 K. The Vickers hardness of the ZrO2 coating reached Hv = 1000 or more. 3.2. Distribution o f Vickers hardness o f Z r O 2 coatings
Fig. 4 shows the distribution of Vickers hardness H~ on the cross-section of a high hardness ZrO2 coating. The measurement was carried out at each distance from the coating surface in the direction of thickness. The coating thickness was about 260 pm. In this case, the spraying conditions were _P=33kW, L = 3 0 m m , and 1500
Zr02 coating Surface ca~
~) =
1 ~ -
l-~gh hardness / layer
/
.........................
0
P = 33 kW L = 30 m m
Substrate \
i'i' ~a~
~%
IO0 200 300 Distance from coating surface, 1 (gin)
Fig. 4. Distribution of Vickers hardness in the direction of thickness, on the cross-section of a high hardness ZrO2 coating, for P = 33 kW
and L = 30 mm. (a) High hardness layer; (b) boundary region; (c) hard layer (first pass); (d) part near substrate.
A. Kobayashi/ Su,face and Coatings Technology90 (1997) 197-202
200
v = 1 2 0 cm rain -~. The other conditions were the same as for Fig. 2. The distribution of Vickers hardness shows a combination of two parabolic curves; this corresponds to the concept that the coating is formed by two traverse passes. The first pass formed a parabolic curve of Vickers hardness, the second parabolic curve was formed by the second pass. Each peak of the parabolic curve is located at the surface side of the coating. The Vickers hardness for the second pass is higher than that for the first pass. The peak value of Vickers hardness is more than H~ = 1000 at the distance of about 5 = 4 0 larn from the coating surface. This part (see Fig. 4(a)) formed a layer of high hardness in the coating, 30 gm thick. On the other hand, the hardness is lower, H ~ = 800, in the middle of the coating. This part (see Fig. 4(b)) corresponds to the boundary between the two passes. Therefore, the bonding force between the sprayed powder is assumed to be weak in this part. While the Vickers hardness for the first pass coating near the substrate (Fig. 4(d)) is also lower, H~ = 600, in spite of the existence of a hard part (Fig. 4(c)).
3.3. Microstructure of the high hardness
ZrO 2
coating
Fig. 5 shows the cross-section of the high hardness ZrOz coating taken using an optical microscope. This coating is the same as is shown in Fig. 2: the conditions were P = 33 kW, L = 30 ram, and v = 120 cm m i n - ~. Observing this cross section, where the coating thickness is about 260 lam, there are generally few pores; the porosity being about 2%. The upper part of the coating corresponds to the high hardness layer, (a) in Fig. 4, which is located about 40 tam from the surface. This part of coating was formed by the second pass
of the traverse, and was affected strongly by the plasma energy. Therefore, there are less pores and the pore size is very small compared with that in other parts. There are a few larger pores in the middle of the coating ((b) in Fig. 4). This part corresponds to the boundary region between the two passes. In the underneath part near the substrate ((c)-(d) in Fig. 4), which was formed by the first pass, the porosity is a little more than in the upper part. But other differences cannot be clearly realized all over the crosssection of the coating from the photograph in Fig. 5.
3.4. Comparison with the conventional-type ZrOz coating Fig. 6 shows the relationship between the Vickers hardness, /-/~, of the ZrO2 coating and the spraying distance, L, for both the coating produced by gas tunneltype plasma spraying and by conventional-type plasma spraying. In this case, the spraying conditions were chosen in order to obtain the same final coating thickness for a similar power input (see Table 3). The powder feed rate for both was w = 40 g m i n - i. For gas tunnel-type plasma spraying the conditions 1500
P
~
~1000
= 33-35 kW
s tunnel type
"e... t
Lp
Conventional type
500 fl ace
(a) .,.~,.. .
~.~ .
.
.
'"
,
. -
ZrO~ coating (b)
.
z0 30 40 50 60 70 Spraying distance, L (mm)
Table 3 Spraying conditions for both spraying methods Gas tunnel
100 ~am Fig. 5. Photograph of the cross-section of the same ZrO2 coating as shown in Fig. 3. The thickness of coating is 260 mm at P=33 kW, L= 30 mm, and v=120 cmmin -I. (a) High hardness layer; (b) boundary region.
90
Fig. 6. Comparison of the hardness characteristics of the ZrQ coatings produced by gas tunnel-type (©) and conventional-type(@) spraying for P=33-35 kW.
e
Substrate
80
Power input, P (kW) Gas flow rate Q (I rain -1) Ar H2 Powder feed rate, w (g min -1) Traverse speed, v (cm min -I)
Conventional
33
35
200 -40 80
45 7 40 2000
The gas tunnel divertor nozzle diameter was d=20 mm, the nozzle diameter of the conventional type was 8 nun. Spraying distance was changed in the range of L = 30-70 mm.
A. Kobayashi/ Surface and Coatings Technology90 (1997) ]97-202
were as follows: P = 3 3 kW, Q=2001 min -~ (the working gas was Ar only), and v=80 cm min -i. For conventional-type plasma spraying the following conditions were used: the power input was similar P = 35kW, the gas flow rate was Q = 4 5 1 m i n -~ for the main working gas (Ar) and 7 1 min - 1 for the secondary gas (H2), and the traverse speed was much higher value, v =2000 cm rain-1. These were the best conditions for conventional-type plasma spraying in order to form dense ZrO2 coatings. Non-spalling, high hardness and a less cracking coating could not be obtained under other conditions. For both methods, the Vickers hardness is increased with decreasing spraying distance, as shown in Fig. 6. However, the gas tunnel-type produces a more than 20% higher value compared with the conventional type. The highest Vickers hardness of the zirconia coating is more than H~=1200 at L = 3 0 m m . Lp, indicated in Fig. 6, was shorter than the case of gas tunnel type. In the case of the conventional spraying, in spite of the high hardness obtained at L
~ " ~"!,
~"" •
J,~. '" 3 , .
201
its cross-section in the direction of thickness. The coating produced by the conventional-type spraying at L = 40 mm also had similar cracks. As mentioned above, a dense and high hardness ZrO2 coating could be obtained with gas tunnel-type plasma spraying, at a short spraying distance. It was difficult to produce such a high hardness coating without cracking using conventional-type plasma spraying. With conventional-type spraying, it is difficult to control the spraying conditions, i.e., plasma energy, etc.
4. Conclusion
A high hardness ZrO2 coating formed by gas tunneltype plasma spraying was investigated, and the following results were obtained. The Vickers hardness of the ZrO2 coating was increased with decreasing spraying distance, and became very high at a short spraying distance of L
Acknowledgement 100/am
(a)
100 g m
(b)
Fig. 7. Comparison of the cross-section of the ZrO2 coatings at L = 30 m m (see Fig. 5): (a) gas tunnel; and (b) conventional.
The author would like to thank Mr. N. Bessho and M. Higuchi for their help during the experiment. This study was financially supported in part by the Grantin-Aid for Scientific Research from the Japanese Ministry of Education, Science and Culture.
202
A. Kobayashi / Surface and Coatings Technology 90 (1997) 197-202
References [I] Jpn. Inst. Thermal Spraying., Thermal Spraying Handbook, Shingijutsu Kaihatsu Center, 1987. [2] Y. Arata, A. Kobayashi and Y. tlabara, 3". AppL Phys., 62 (1987) 4884. [3] Y. Arata, A. Kobayashi and Y. Habara, 3. High Temp. Soc., 13 (1987) i16. [4] A. Kobayashi, S. Kurihara, Y. Habara and Y. Arata, (2. ,L Jpn. Weld. Soc., 8 (I990) 457. [5] Y. Arata, A. Kobayashi and S. Kurihara, 3". High Temp. Soc., 15 (1989) 210.
[6] A. Kobayashi, Y. Habara and Y. Arata, J. High Temp. Soc., 18 (1992) 89. [7] A. Kobayashi, Proc. ITSC, (1992) 47. [8] T. Araya, d. Weld. Soc. d'pn., 57 (1988) 216. [9] J. Hasui, 3". Weld. Soc. Jpn., 37 (1987) 52. [10] A. Kobayashi, Weld. Int., 4 (1990) 276. [11] T. Suzuki, M. Ito, M. Nakahashi. and H. Takeda, Z Jpn. Ceramic Assoe, 97 (1989) 571. [12] A. Notomi and N. Hisatome, Pure Appl. Chem., 68 (1996) 1101.
ELSEVIER
Surface and CoatingsTechnology90 (1997) 197-202
#O/ITING8 f[tligOlO
Formation of high hardness zirconia coatings by gas tunnel type plasma spraying Akira Kobayashi 1 Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567, Japan
Received 2 November 1995; accepted 12 October 1996
Abstract
The high hardness zircohia (ZrO2) coatings could be obtained at an atmospheric pressure by using a gas tunnel type plasma spraying. The characteristics of these high hardness ZrO2 coatings were investigated. The Vickers hardness of the ZrO2 coating at a short spraying distance was very high; a high hardness of more than Hv = 1200 was achieved at the surface side of the coating. The microstructure of the obtained high hardness ZrO2 coating was also investigated by the microscopic method. And the characteristics of the high hardness ZrO2 coating was discussed in comparison with that of the coating formed by the conventional type plasma spraying. It was clarified that the ZrO2 coating of the gas tunnel type was not only much harder but also less porous than that of the conventional type. © 1997 Elsevier Science S.A. Keywords: high hardness coating; zirconla coating; Vickers hardness; gas tunnel type plasma spraying
1. Introduction
In a plasma-sprayed coating with ceramic powder which has a high melting point, one of the problems is that there are many pores within the coating [ 1], especially for wear resistance and/or corrosion resistance coatings. This causes the lowering of mechanical and chemical qualities of the coating, and also limits its application fields. In these circumstances, a gas tunnel-type plasma spraying was developed by the author [2]. With the gas tunnel-type plasma spraying, the sprayed particles can be melted et~ficiently and the bonding force between the particles becomes stronger than the conventional one. Therefore, the qualities of a ceramic sprayed coating obtained with this new type plasma spraying is higher than with a conventional type plasma spraying. Namely, the porosity is decreased and the dense coating is obtained. These results have been investigated in the former studies [3-5]. As to the formation of such a high functionality material, high quality coatings can be obtained by the gas tunnel-type plasma spraying [6]; in the case of a short spraying distance, an alumina coating produced 1Tel.: +81 6 8798651; fax: +81 6 8798689; e-mait: [email protected]. 0257-8972/97/$17.00 © 1997ElsevierScienceS.A. All fightsreserved PII S0257-8972(96) 03 I43-X
had a high Vickers hardness of Hv=1200-1600 [7]. (Alumina sprayed coating is normally /-/,=700-800.) This high hardness coating had a high hardness layer near the coating surface. Various types of ceramic coatings produced by plasma spraying have been used in many industrial fields [8] including electronics, automobile, aerospace, steel making, and other manufacturing because of their excellent functionality. Zirconia (ZrO2) coatings with plasma spraying are attracting the attention of many researchers because of their superior properties such as corrosion resistance, thermal resistance, and wear resistance, and so on. For example, ZrO2 coatings have been formed as a thermal barrier coating (TBC) by plasma spraying [9, 10]. But even for TBC, high density is needed at the coating surface to achieve corrosion and/or wear resistance at high temperature [11]. Another example of dense coating is the electrode of a solid oxide fuel cell (SOFC) [12]. Thus, it is important to produce a dense zirconia coating. In this study, the high hardness ZrO2 coating was formed by gas tunnel-type plasma spraying, at a short spraying distance in the case of a large power input. The characteristics of the coating were investigated. As one of the coating properties, the Vickers hardness was measured on the cross-section of this high hardness ZrO2 coating. The effect of spraying distance, etc., on
A. Kobayashi/ Surface and Coatings Technology90 (1997) 197-202
198
the hardness of the coating is discussed. The microstructure of an obtained ZrO2 coating was investigated by optical microscopy. Finally, the results with regard to the characteristics of the ZrO2 coating are compared with the case of conventional type plasma spraying.
2. Experimental
Table 1 Experimental conditions Power input Working gas Gas flow rate Powder feed rate Spraying distance Traverse speed
P= 33 kW Ar Q=2001 min -1 w=40-60 g min -1 L = 20-70 mm v= 120 c m r a i n - 1 v=80 cmmin -~ d=20 mm
Gas divertor nozzle diameter Fig. 1 shows the gas tunnel-type plasma spraying apparatus used in this study. This plasma torch was improved (see below), compared with the gas tunneltype plasma spraying which has been described in previous papers [2-4], in order to carry out the high power experiment. However, the mechanism and properties are essentially the same: i.e., the gas tunnel-type plasma jet is a high energy and high efficiency type, which is useful for achieving a high quality coating. (A) in Fig. 1 is a plasma torch for the initiation of a gas tunnel-type plasma jet; (B) is the gas tunnel-type plasma torch. For the polarity of an electrode, the gas diverter nozzle was used an anode. The configuration of the vortex generator was little changed from one the formerly used. The nearest distance between two electrodes, an inside nozzle electrode and the gas diverter nozzle, was 15 ram, and the diameter of the gas diverter nozzle was d = 20 mm. In this case, the inlet of sprayed powder was located between torch (A) and torch (B). The experiment to form ZrO2 coatings was carried out at atmospheric pressure using a gas tunnel-type plasma spraying. Pure argon was used as the working gas. The distance between the torch and the substrate is the spraying distance, L. The traverse speed v was chosen from two values: 120 and 80 cm rain -t. The experimental conditions for the ZrO2 coatings are shown in Table 1. Here, Q is the working gas flow
PS1
PS2
Table 2 Zircoma powder used in this study Chemical compsitions (wt.%) ZrOz Y203 A12Q
SiO2
Others
90.78
0.20
0.49
8.15
0.38
Size (txm) 10-44
rate, and P is the power input to the plasma torch. Each of them was a constant value. The spraying distance was the main difference in this experiment. Especially at a short spraying distance of L = 2 0 - 5 0 mm, a high hardness ZrO2 coating was formed, and the effect of the spraying distance on the property of the ZrO2 coating was studied. During the experiment, the temperature of the substrate was measured at various spraying distances. In this case the substrate was not preheated. Table 2 gives the composition and size of the zirconia powder used in this study. The average size of this powder was 26.3 gm. The powder feed rate, w, was 4 0 g r o i n -1. A SUS304 plate of 5 m m in thickness, 25 x 50 mm in width and length, was used as a substrate. The surface of the substrate was shot blasted. The Vickers hardness, Hv, was measured at the crosssection of the coating formed under the various spraying conditions. The measured conditions of Vickers hardness were: the loading weight was 100 g, the holding time was 25 s. More than 10 traverses were made over the whole thickness, i.e., the number of measuring points was more than 10 at the same distance from the coating surface. The hardness of coating was decided by the average of the 10-point measurement. The microstructure of the cross-section of the obtained zirconia coating was observed using an optical microscope. For comparison purposes, the zirconia coatings were formed under similar spraying conditions using the conventional type of plasma spraying. The hardness of the coating was measured and the cross-section was also observed using an optical microscope. 3. Results and discussion
t foo.or I
g.s} 2
Fig. 1. Schematicof a gas tuanel-typeplasma spraying apparatus. (A) Conventional type plasma torch; (B) gas tunnel-type plasma torch; PS1, PS2, power supply. L, spraying distance; v, traverse speed.
3.1. Characteristics of high hardness Zr02 coating Fig. 2 shows the relation between the Vickers hardness Hv of a zirconia ( Z r Q ) coating produced by the
A. Kobayashi/ Surface and Coatings Technology 90 (1997) 197-202
199
1500 ~. 1000
P = 3 3 kW
P =33 kW
[,..., 800
~
1000 600
•~
Lp
40(
SO0 T
do
I
4o
s;
~
If Jo
P
,o
gas tunnel-type plasma spraying and the spraying distance L. In this case, the hardness Was decided by the highest value of ten times the average inside the coating. The experimental conditions were as follows; the power input was P = 33 kW, the gas flow rate of pure argon was Q = 2 0 0 1 m i n -1, and the powder feed rate was w = 40 g rain-~. The traverse speeds of the substrate were v = 120 and 80 cm min - t , respectively. The spraying distance was changed f r o m L = 20 to 70 mm. The spalling between the sprayed coating and the substrate sometimes occurred at the shorter distance of L = 20 m m when v = 80 cm r a i n - 1. But, under other conditions in this experiment, the spalling was suppressed by using a thick substrate. The Vickers hardness is increased with decreasing spraying distance, as shown in Fig. 2. The hardness becomes m u c h greater at a shorter spraying distance than the critical spraying distance, L v [7], indicated in Fig. 2. The characteristics of the Vickers hardness are similar in each case of traverse speed, v = 1 2 0 and 80 cm r a i n - z. But in the case of the lower traverse speed, a 20% higher Vickers hardness could be obtained at every spraying distance. The highest Vickers hardness o f the ZrOz coating is more than H~=1000 at L = 2 0 and 30 mm, in the case of v = 120 c m m i n - L Meanwhile, the highest is more than H~ = 1200 at L = 30 mm, in the case of v = 80 cm r a i n - 1. In this way, a very high hardness ZrO2 coating can be obtained by the gas tunnel-type plasma spraying at a short spraying distance (L < L p ) , as well as the case of an alumina coating [7]. With a decrease in the traverse speed, the energy transfer to the coating must be higher, c o n s e q u e n t l y Lp is increased. In the case of v = 80 cm m i n - ~, Lp is about 60 ram. Also, when power is increased, the length of plasma jet is increased, and /Iv and Lp are increased. Fig. 3 shows the results of the measurement of the substrate temperature during the zirconia (ZrO2) plasma
20
do
F
r
Spraying distance, L (re.m)
Spraying distance, L (mm)
Fig. 2. Relationship between the Vickers hardness of the ZrOz coating and the spraying distance at P=33kW at v=120 (O) and 80 cm rain -~ (&). Lp, critical spraying distance.
'
Fig. 3. Relationship between the substrate temperature and spraying distance at P = 3 3 kW and v = 120 cm min -~.
spraying at each spraying distance L = 2 0 - 7 0 m m . In this case, the experimental conditions were the same as those in Fig. 2: P = 3 3 k W , Q = 2 0 0 1 m i n -~, and w = 40 g m i n - ~. The traverse speed of the substrate was v = 120 cm r a i n - ~. The substrate temperature was increased with the decreasing spraying distance as shown in Fig. 3. The substrate temperature becomes much higher at shorter spraying distances. At L = 20 mm, the substrate temperature was T = 900 K. The Vickers hardness of the ZrO2 coating reached Hv = 1000 or more. 3.2. Distribution o f Vickers hardness o f Z r O 2 coatings
Fig. 4 shows the distribution of Vickers hardness H~ on the cross-section of a high hardness ZrO2 coating. The measurement was carried out at each distance from the coating surface in the direction of thickness. The coating thickness was about 260 pm. In this case, the spraying conditions were _P=33kW, L = 3 0 m m , and 1500
Zr02 coating Surface ca~
~) =
1 ~ -
l-~gh hardness / layer
/
.........................
0
P = 33 kW L = 30 m m
Substrate \
i'i' ~a~
~%
IO0 200 300 Distance from coating surface, 1 (gin)
Fig. 4. Distribution of Vickers hardness in the direction of thickness, on the cross-section of a high hardness ZrO2 coating, for P = 33 kW
and L = 30 mm. (a) High hardness layer; (b) boundary region; (c) hard layer (first pass); (d) part near substrate.
A. Kobayashi/ Su,face and Coatings Technology90 (1997) 197-202
200
v = 1 2 0 cm rain -~. The other conditions were the same as for Fig. 2. The distribution of Vickers hardness shows a combination of two parabolic curves; this corresponds to the concept that the coating is formed by two traverse passes. The first pass formed a parabolic curve of Vickers hardness, the second parabolic curve was formed by the second pass. Each peak of the parabolic curve is located at the surface side of the coating. The Vickers hardness for the second pass is higher than that for the first pass. The peak value of Vickers hardness is more than H~ = 1000 at the distance of about 5 = 4 0 larn from the coating surface. This part (see Fig. 4(a)) formed a layer of high hardness in the coating, 30 gm thick. On the other hand, the hardness is lower, H ~ = 800, in the middle of the coating. This part (see Fig. 4(b)) corresponds to the boundary between the two passes. Therefore, the bonding force between the sprayed powder is assumed to be weak in this part. While the Vickers hardness for the first pass coating near the substrate (Fig. 4(d)) is also lower, H~ = 600, in spite of the existence of a hard part (Fig. 4(c)).
3.3. Microstructure of the high hardness
ZrO 2
coating
Fig. 5 shows the cross-section of the high hardness ZrOz coating taken using an optical microscope. This coating is the same as is shown in Fig. 2: the conditions were P = 33 kW, L = 30 ram, and v = 120 cm m i n - ~. Observing this cross section, where the coating thickness is about 260 lam, there are generally few pores; the porosity being about 2%. The upper part of the coating corresponds to the high hardness layer, (a) in Fig. 4, which is located about 40 tam from the surface. This part of coating was formed by the second pass
of the traverse, and was affected strongly by the plasma energy. Therefore, there are less pores and the pore size is very small compared with that in other parts. There are a few larger pores in the middle of the coating ((b) in Fig. 4). This part corresponds to the boundary region between the two passes. In the underneath part near the substrate ((c)-(d) in Fig. 4), which was formed by the first pass, the porosity is a little more than in the upper part. But other differences cannot be clearly realized all over the crosssection of the coating from the photograph in Fig. 5.
3.4. Comparison with the conventional-type ZrOz coating Fig. 6 shows the relationship between the Vickers hardness, /-/~, of the ZrO2 coating and the spraying distance, L, for both the coating produced by gas tunneltype plasma spraying and by conventional-type plasma spraying. In this case, the spraying conditions were chosen in order to obtain the same final coating thickness for a similar power input (see Table 3). The powder feed rate for both was w = 40 g m i n - i. For gas tunnel-type plasma spraying the conditions 1500
P
~
~1000
= 33-35 kW
s tunnel type
"e... t
Lp
Conventional type
500 fl ace
(a) .,.~,.. .
~.~ .
.
.
'"
,
. -
ZrO~ coating (b)
.
z0 30 40 50 60 70 Spraying distance, L (mm)
Table 3 Spraying conditions for both spraying methods Gas tunnel
100 ~am Fig. 5. Photograph of the cross-section of the same ZrO2 coating as shown in Fig. 3. The thickness of coating is 260 mm at P=33 kW, L= 30 mm, and v=120 cmmin -I. (a) High hardness layer; (b) boundary region.
90
Fig. 6. Comparison of the hardness characteristics of the ZrQ coatings produced by gas tunnel-type (©) and conventional-type(@) spraying for P=33-35 kW.
e
Substrate
80
Power input, P (kW) Gas flow rate Q (I rain -1) Ar H2 Powder feed rate, w (g min -1) Traverse speed, v (cm min -I)
Conventional
33
35
200 -40 80
45 7 40 2000
The gas tunnel divertor nozzle diameter was d=20 mm, the nozzle diameter of the conventional type was 8 nun. Spraying distance was changed in the range of L = 30-70 mm.
A. Kobayashi/ Surface and Coatings Technology90 (1997) ]97-202
were as follows: P = 3 3 kW, Q=2001 min -~ (the working gas was Ar only), and v=80 cm min -i. For conventional-type plasma spraying the following conditions were used: the power input was similar P = 35kW, the gas flow rate was Q = 4 5 1 m i n -~ for the main working gas (Ar) and 7 1 min - 1 for the secondary gas (H2), and the traverse speed was much higher value, v =2000 cm rain-1. These were the best conditions for conventional-type plasma spraying in order to form dense ZrO2 coatings. Non-spalling, high hardness and a less cracking coating could not be obtained under other conditions. For both methods, the Vickers hardness is increased with decreasing spraying distance, as shown in Fig. 6. However, the gas tunnel-type produces a more than 20% higher value compared with the conventional type. The highest Vickers hardness of the zirconia coating is more than H~=1200 at L = 3 0 m m . Lp, indicated in Fig. 6, was shorter than the case of gas tunnel type. In the case of the conventional spraying, in spite of the high hardness obtained at L
~ " ~"!,
~"" •
J,~. '" 3 , .
201
its cross-section in the direction of thickness. The coating produced by the conventional-type spraying at L = 40 mm also had similar cracks. As mentioned above, a dense and high hardness ZrO2 coating could be obtained with gas tunnel-type plasma spraying, at a short spraying distance. It was difficult to produce such a high hardness coating without cracking using conventional-type plasma spraying. With conventional-type spraying, it is difficult to control the spraying conditions, i.e., plasma energy, etc.
4. Conclusion
A high hardness ZrO2 coating formed by gas tunneltype plasma spraying was investigated, and the following results were obtained. The Vickers hardness of the ZrO2 coating was increased with decreasing spraying distance, and became very high at a short spraying distance of L
Acknowledgement 100/am
(a)
100 g m
(b)
Fig. 7. Comparison of the cross-section of the ZrO2 coatings at L = 30 m m (see Fig. 5): (a) gas tunnel; and (b) conventional.
The author would like to thank Mr. N. Bessho and M. Higuchi for their help during the experiment. This study was financially supported in part by the Grantin-Aid for Scientific Research from the Japanese Ministry of Education, Science and Culture.
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A. Kobayashi / Surface and Coatings Technology 90 (1997) 197-202
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[6] A. Kobayashi, Y. Habara and Y. Arata, J. High Temp. Soc., 18 (1992) 89. [7] A. Kobayashi, Proc. ITSC, (1992) 47. [8] T. Araya, d. Weld. Soc. d'pn., 57 (1988) 216. [9] J. Hasui, 3". Weld. Soc. Jpn., 37 (1987) 52. [10] A. Kobayashi, Weld. Int., 4 (1990) 276. [11] T. Suzuki, M. Ito, M. Nakahashi. and H. Takeda, Z Jpn. Ceramic Assoe, 97 (1989) 571. [12] A. Notomi and N. Hisatome, Pure Appl. Chem., 68 (1996) 1101.