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Effect of annealing and Fe2O3 addition on the high temperature tribological behavior of the plasma sprayed yttria-stabilized zirconia coating
Surface and Coatings Technology 133᎐134 Ž2000. 403᎐410
Effect of annealing and Fe 2 O 3 addition on the high temperature tribological behavior of the plasma sprayed yttria-stabilized zirconia coating Jong-Han Shina , Dae-Soon Lima,U , Hyo-Sok Ahnb a
b
Department of Material Science and Engineering, Korea Uni¨ ersity, Seoul 136-701, South Korea Tribology Research Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea
Abstract The annealing process of plasma-sprayed 3 mol% yttria stabilized zirconia Ž3-YSZ. coatings with and without Fe 2 O 3 addition was investigated to evaluate the high temperature tribological performance. The wear experiments were carried out on a ring-on-plate type reciprocating wear tester at selected temperatures within the range of 25᎐600⬚C. In addition, plasma-sprayed zirconia-based coatings were annealed at 500⬚C up to 10 cycles. The results show that annealing treatment decreases the wear rate due to the release of tensile residual stress. The stress relaxation significantly influences the maximum value of monoclinic intensity. The peak shifts to lower temperature and its values are reduced. The stress relaxation improves the tribological properties by reducing the influence of low temperature degradation. The addition of Fe 2 O 3 to the 3-YSZ coating leads to a reduction of wear and friction. This decrease is believed to be the stabilization of the tetragonal phase and the increase of microhardness. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Plasma spray coating; Annealing; Fe 2 O 3 additive; 3-YSZ; Friction and wear
1. Introduction The plasma spraying process is a surface treatment technique in which a coating material is passed through the electric plasma where it becomes molten and propelled to a base material, forming a dense and reasonably uniform layer. This technique has been applied to various mechanical components and structures that require wear-resistance, corrosion-inhibition, heat-resistance, and thermal insulation. This technique also has many other advantages. It is easy to: Ž1. control the thickness of coatings; Ž2. to coat complexly shaped components; and Ž3. to coat worn components for extended service life w1᎐3x. Zirconia has been considered to be a good candidate U
Corresponding author. Tel.: q82-2-929-5344; fax: q82-2-9295344. E-mail address: [email protected] ŽD. Lim..
coating material for high temperature tribological applications because of its superior properties such as a low thermal conductivity, high thermal expansion coefficient similar to metal and high toughness w4x. In particular, 3 mol% yttria stabilized zirconia Ž3-YSZ. is one of the most attractive tribological materials because of its good combination of high toughness, strength and hardness w5x. However, the effect of phase transformation on tribological properties of 3-YSZ should be further studied for high temperature tribological uses, since the significant loss of fracture strength and wear resistance occurs in the temperature ranges of 200᎐400⬚C, in particular, for environments containing water or its vapor w5᎐7x. Previous studies showed that the wear loss increased with the increase in the test temperature and reaching its maximum loss at approximately 400⬚C. This unique behavior was explained by the tetragonal to monoclinic transformation w8,9x. Lange et al. found the presence of YŽOH. 3 crys-
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 9 6 5 - 8
J. Shin et al. r Surface and Coatings Technology 133᎐134 (2000) 403᎐410
404
tallites and suggested that dissolution of Y elements from the surface into H 2 O was responsible for the low temperature transformation w10x. However, further studies are necessary to understand the high temperature behavior of the plasma sprayed 3-YSZ coating, since the tribological behavior is strongly dependent on material characteristics and test conditions. In order to gain a better understanding of high temperature tribological behavior of 3-YSZ, in this work, the effect of annealing treatment and secondary additive material was studied as these two variables are expected to strongly influence the phase transformation and tribological behavior of the zirconia. Fe 2 O 3 powder was selected as a secondary additive material since it has been known to provide low wear and friction at a low cost w11x. 2. Experimental procedure The ring and plate of FC25 cast iron specimens were machined to the dimension of 30 Žoutside diameter. = 5 = 5 mm3 and 30 = 20 = 5 mm3, respectively. Prior to plasma spray coating, a Ni᎐Cr᎐Al᎐Co᎐Y composite was applied as a bond coat to enhance adhesion and reduce thermal expansion mismatch between the substrate and coating layer. Both the ring and plate specimens were coated with various zirconia based ceramic powders, as listed in Table 1. Ceramic powders were granulated by spray drying method after wet milling for 24 h. As shown in Fig. 1, most of the powders had spherical shape and relatively uniform size for good flowability during plasma spraying. The surfaces of the specimens after plasma spray coating were ground with diamond wheel and 6 m size diamond paste. As annealing treatment, the specimens were subjected to thermal cycling between 25 and 500⬚C for up to 10 cycles. The specimen were placed in the furnace and the furnace was then activated to heat
Table 1 The compositions of Fe 2 O 3-added 3-YSZ powders
3-YSZ 2.5 mol% Fe2 O3 added 3-YSZ 5.0 mol% Fe2 O3 added 3-YSZ 7.5 mol% Fe2 O3 added 3-YSZ 10.0 mol% Fe2 O3 added 3-YSZ
ZrO2 Žmol%.
Y2 O3 Žmol%.
Fe2 O3 Žmol%.
97.00 94.58 92.15 98.72 87.30
3.00 2.92 2.85 2.78 2.70
᎐ 2.50 5.00 7.50 10.00
to 500⬚C at heating rate of 5⬚rmin. The specimens held at 500⬚C for 30 min to stabilize the temperature. The furnace was then switched off and the specimens allowed to cool at the furnace cooling rate of 2.5⬚rmin before removal to cooling to room temperature. The annealing temperature of 500⬚C was selected since the temperature of the top ring reversal position of the liner and the top ring on the piston of adiabatic engine could reach approximately 500⬚C w12x. A wear test was conducted at room temperature to 600⬚C using a high temperature wear tester. The plate specimen was placed on a reciprocating holder moved by a DC motor. The specimen was loaded and slid against a ring type counterpart. Detailed features of the test machine were described elsewhere w8x. For each specimen, wear test was conducted for 1 h after 30 min soaking at the desired temperature. The sliding velocity and stroke length were fixed to 1 = 10y2 mrs and 8 = 10y3 m, respectively. The applied load was 2 N. Fig. 2 shows the schematic diagrams of the ring and plate specimens. The relative wear loss was determined from the width of worn area of the ring specimens as indicated in Fig. 2. The microhardness of the coatings was measured using a Vickers microhardness tester. The indentation was made with a 0.98 N load for 10 s along the axis of the coating thickness and the microhardness was
Fig. 1. SEM images of zirconia-based powders made by spray drying methods; Ža. 3-YSZ and Žb. 10 mol% Fe 2 O 3 -added 3-YSZ.
J. Shin et al. r Surface and Coatings Technology 133᎐134 (2000) 403᎐410
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confirmed by the XRD patterns. Fig. 5 shows the XRD patterns of 3-YSZ coating as a function of the annealing temperature. The tetragonal Ž220. peak of the as-sprayed coating is shifted to a higher angle compared to the zirconia-based powders, which indicates the existence of tensile residual stress on the surface in a longitudinal direction w14x. The specimens annealed at 500⬚C has similar peak positions as the zirconia based powders. This implies that annealing treatment releases the tensile residual stress. The relaxation of tensile residual stress on the surface influences the
Fig. 2. Schematic diagram showing contact geometry of ring and plate specimens: Ža. before wear test; and Žb. after wear test.
observed. Charles and Evans equation was used to calculate the toughness of coatings by measuring the crack length w13x. X-Ray diffraction ŽXRD. and scanning electron microscopy ŽSEM. were used for the structure identification and damage observation, respectively.
3. Results and discussion 3.1. Tribological properties of the 3-YSZ coatings after annealing treatment Fig. 3 shows that the friction coefficient and wear loss of the 3-YSZ coating before and after annealing. It reveals that the friction coefficient and wear increases gradually until 400⬚C and decreases at 600⬚C. On the other hand, the friction coefficient and wear loss of annealed 3-YSZ coating exhibits the maximum value at 200⬚C and decreases gradually beyond this temperature. In all temperature ranges, the wear loss decreases after annealing. A significant decrease in both friction coefficient and wear loss after annealing is noticeable at 400⬚C. Fig. 4 shows the monoclinic intensity fraction formed in the 3-YSZ coating as a function of the test temperature. The monoclinic intensity fraction was evaluated from the ratio between the XRD peak height of the two monoclinic peak Ž111.mq Ž111.m and the tetragonal peak Ž111.t w6x. With as-sprayed specimen, phase transformation from tetragonal to monoclinic phase occurs approximately below 400⬚C, whereas the amount of monoclinic phase of 3-YSZ coating after annealing has the maximum value at 200⬚C. This tendency is consistent to the wear results as shown in Fig. 3. The shift of the monoclinic intensity fraction may be explained by the stress relaxation. The stress relaxation is
Fig. 3. Tribological behavior of 3-YSZ coating before and after annealing treatment at various temperatures: Ža. friction coefficient; and Žb. width of the worn area of the ring.
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J. Shin et al. r Surface and Coatings Technology 133᎐134 (2000) 403᎐410
Fig. 4. Monoclinic intensity fraction Ž%. as function of annealing process for as-sprayed 3-YSZ coating at different annealing temperature.
Fig. 5. XRD patterns of 3-YSZ coating in 2 s 33᎐38⬚ region as a function of the annealing temperature.
monoclinic intensity fraction. It has been reported that tensile stress tends to promote phase transformation from tetragonal to monoclinic w15x. The tensile residual stress was released by annealing and this stress relaxation restricted the phase transformation from tetragonal to monoclinic, causing a shift of the maximum value of the monoclinic intensity fraction. The stress relaxation seems to also lower the monoclinic intensity fraction in all temperature ranges. Fig. 6 shows the cross-sectional microhardness changes of the annealed specimen as a function of the annealing cycles. The cross-sectional microhardness of the 3-YSZ coating is increased by approximately 25% through the annealing treatment. However, the microhardness is not markedly increased with the increase in the number of annealing cycles. The transformation from the tetragonal to monoclinic phase is accompanied by severe microcracking on the surface due to volume increase w6x. Severe microcracking will increase the wear loss and the friction coefficient. Several reports supporting this explanation have been published w8,16x. Therefore, the wear rate may be increased with the increase in the monoclinic transformation rate. The similar behavior of the tribological performance with the monoclinic transformation rate, as shown in Figs. 3 and 4, can be explained by the mechanism just mentioned.
annealing treatment as a function of the test temperature. It can be noted that the temperature for the maximum value of the friction coefficient and wear loss is shifted from 400 to 200⬚C after annealing treatment. A significant reduction of friction and wear loss is noticeable at relatively low temperature for the unannealed coating. The effect of Fe 2 O 3 addition as a low friction material is noticeable in the low temperature range Žup to 200⬚C. compared to the unannealed coating. However, the friction coefficient measured at room temperature and 200⬚C is increased by annealing. Fur-
3.2. Tribological properties of the Fe 2 O3-added 3-YSZ coatings after annealing treatment Fig. 7 shows the tribological properties of the 5.0 mol% Fe 2 O 3-added 3-YSZ coating before and after
Fig. 6. The cross-sectional microhardness changes of 3-YSZ coating before and after annealing treatment with different annealing cycles.
J. Shin et al. r Surface and Coatings Technology 133᎐134 (2000) 403᎐410
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Fig. 8. XRD patterns of zirconia based coatings in 2 s 20᎐70⬚ region.
with ZrO 2 ŽY2 O 3 . which causes low temperature degradation w17x. The stabilization of tetragonal phase is expected to improve the tribological properties of the Fe 2 O 3-added 3-YSZ coating. Fig. 9 is the XRD patterns obtained with the 5.0 mol% Fe 2 O 3-added 3-YSZ coating before and after the wear test at 200⬚C. Before the wear test ŽFig. 9a., the monoclinic phase is not
Fig. 7. Tribological behavior of 5.0 mol% Fe 2 O 3 -added 3-YSZ coating before and after annealing treatment at different temperatures: Ža. friction coefficient; and Žb. width of the worn area of the ring.
ther study is needed to explain this behavior. After annealing, the shift of the temperature for the maximum friction coefficient as well as its stabilization in all temperature ranges can be noticed. Fig. 8 shows XRD patterns of the as-ground 3-YSZ coating with the addition of various amounts of Fe 2 O 3 . It shows that the tetragonal phase is stabilized by Fe 2 O 3 addition. This stabilization may have occurred as Fe 2 O 3 prevents zirconia from reacting with water vapor in the same fashion as Al 2 O 3 w17x. Li et al. suggest that hydroxidation of Al 2 O 3 protects from further interaction of H 2 O
Fig. 9. XRD patterns of 5.0 mol% Fe 2 O 3 added 3-YSZ coating after annealing in 2 s 20᎐70⬚ region: Ža. before wear test; and Žb. after wear test at 200⬚C.
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in the microhardness and the fracture toughness of Fe 2 O 3-added 3-YSZ coatings as a function of Fe 2 O 3 addition, probably due to the localized residual stress distribution. Fig. 12 shows the area of the polished surface due to the friction sliding seems to increase as the amount of Fe 2 O 3 is increased. The SEM micrographs clearly show that the area of the well-polished surface due to the friction sliding increased by the addition of Fe 2 O 3 . This may be attributed to the stabilization and improved mechanical properties. Increased area of the polished surface will contribute to the lowering of the friction coefficient. However, regardless of the Fe 2 O 3 addition, the low temperature degradation observed in 3-YSZ can still be observed. This results implies that the monoclinic transformation, as shown in Fig. 9, of the Fe 2 O 3 added 3-YSZ is still responsible for maximum wear at 200⬚C. Fig. 13 shows SEM micrographs of the wear track of the 5.0 mol% Fe 2 O 3 added 3-PSZ coating after annealing treatment. It reveals that microcrackings, probably due to monoclinic transformation, are dominantly observed on the worn surface of the test at 200⬚C. The highest friction coefficient and wear loss at 200⬚C may be attributed to the microcrackings as shown in Fig. 13b.
4. Conclusions The experimental study on tribological properties of plasma sprayed zirconia-based coatings shows that the test temperature significantly influences the friction coefficient and wear characteristics. The friction coefficient and the wear loss were correlated with the fraction of monoclinic transformation. The friction co-
Fig. 10. Tribological behavior of Fe 2 O 3 -added 3-YSZ coatings after annealing as function of the Fe 2 O 3 addition; Ža. friction coefficient and Žb. width of the worn area of the ring.
found. However, the monoclinic phase is detected after the wear test at 200⬚C probably due to the local heating and the frictional stress ŽFig. 9b.. When compared to the results obtained with the 3-YSZ coating, the tribological performance of Fe 2 O 3-added 3-YSZ coatings is improved as indicated by Fig. 10. Fig. 10 shows the friction coefficient and the wear loss generally decreases by the addition of Fe 2 O 3 . Two explanations may be considered to be related to this observation. First, suppression of the phase transformation due to Fe 2 O 3 , as shown in Fig. 8, can contribute to less wear. Second, tribological properties can also be enhanced by improved mechanical properties. Fig. 11 shows changes
Fig. 11. The microhardness and the toughness changes of the zirconia-based coatings as function of the Fe 2 O 3 addition.
J. Shin et al. r Surface and Coatings Technology 133᎐134 (2000) 403᎐410
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efficient and the wear loss reached maximum at approximately 400⬚C. The temperature for the maximum values of friction coefficient and wear loss is shifted down to approximately 200⬚C by the annealing treat-
Fig. 13. SEM microimages of the wear track of the 5.0 mol% Fe 2 O 3 added 3-YSZ coating at various test temperature: Ža. room temperature; Žb. 200⬚C; and Žc. 600⬚C.
Fig. 12. SEM microimages of the wear track of the zirconia-based coating tested at room temperature: Ža. 3-YSZ; Žb. 5.0 mol% Fe 2 O 3 added 3-YSZ; and Žc. 10.0 mol% Fe 2 O 3 added 3-YSZ.
ment at 500⬚C. This behavior is explained by the restriction of phase transformation due to stress relief. The addition of Fe 2 O 3 to the 3-YSZ coating im-
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proves tribological properties in all test temperature ranges. The observed suppression of the monoclinic transformation as well as the increase in microhardness and toughness is related to the improvement of the tribological characteristics by the addition of Fe 2 O 3 . Acknowledgements The authors would like to thank the Ministry of Science and Technology and the Critical Technology 21 Program ŽMachinery Design Technology Enhancement. for financial support and interest in this work. This research was also partially supported by the National Research Laboratory Program of the same Ministry. References w1x L. Pawlowski, The Science and Engineering of Thermal Spray Coatings, John Wiley & Sons Ltd, Chichester, 1st edn, 1995, p. 28
w2x T.A. Wolfla, R.C. Tucker, Thin Solid Films 53 Ž1978. 253. w3x H.S. Ahn, J.Y. Kim, D.S. Lim, Wear 203r204 Ž1997. 77. w4x T.A. Taylor, D.L. Appleby, Surf. Coat. Technol. 43r44 Ž1990. 470. w5x T. Sato, S. Ohtaki, M Shimada, J. Mater. Sci. 20 Ž1985. 1466. w6x D.J. Kim, J. Eur. Ceram. Soc. 17 Ž1997. 897. w7x J.D. Lin, J.G. Duh, J. Mater. Sci. 32 Ž1997. 4901. w8x D.S. Lim and J.Y. Kim, Proc. Int. Symp. on Tribology, Bejing, Ž1993. 748. w9x J.Y. Kim, D.S. Lim, J. Kor. Ceram. Soc. 31 Ž10. Ž1994. 1176. w10x J.F. Li, R. Watanabe, Mater. Trans. 37 Ž1996. 1171. w11x R. Kamo, L. Kamo, A. Kleyman, W. Bryzik and Z. Schwarz, Proc. Int. Tribology Conf., Yokohama, Ž1995. 645 w12x B.J. Taylor and T.S. Eyre, Tribology International, 4 Ž1979. 79 w13x A.G. Evans, E.A. Charles, J. Am. Ceram. Soc. 59 Ž7r8. Ž1976. 371. w14x A. Mehner, H. Klumper-Westkamp, F. Hoffmann, P. Mayr, Thin Solid Films 308r309 Ž1997. 363. w15x J.G. Duh, Y.S. Wu, J. Mater. Sci. 26 Ž1991. 6522. w16x I. Birby, P. Harrison, R. Stevens, J. Eur. Ceram. Soc. 8 Ž1989. 37. w17x J.F. Li, R. Watanabe, J. Mater. Sci. 32 Ž1997. 1149.
Effect of annealing and Fe 2 O 3 addition on the high temperature tribological behavior of the plasma sprayed yttria-stabilized zirconia coating Jong-Han Shina , Dae-Soon Lima,U , Hyo-Sok Ahnb a
b
Department of Material Science and Engineering, Korea Uni¨ ersity, Seoul 136-701, South Korea Tribology Research Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea
Abstract The annealing process of plasma-sprayed 3 mol% yttria stabilized zirconia Ž3-YSZ. coatings with and without Fe 2 O 3 addition was investigated to evaluate the high temperature tribological performance. The wear experiments were carried out on a ring-on-plate type reciprocating wear tester at selected temperatures within the range of 25᎐600⬚C. In addition, plasma-sprayed zirconia-based coatings were annealed at 500⬚C up to 10 cycles. The results show that annealing treatment decreases the wear rate due to the release of tensile residual stress. The stress relaxation significantly influences the maximum value of monoclinic intensity. The peak shifts to lower temperature and its values are reduced. The stress relaxation improves the tribological properties by reducing the influence of low temperature degradation. The addition of Fe 2 O 3 to the 3-YSZ coating leads to a reduction of wear and friction. This decrease is believed to be the stabilization of the tetragonal phase and the increase of microhardness. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Plasma spray coating; Annealing; Fe 2 O 3 additive; 3-YSZ; Friction and wear
1. Introduction The plasma spraying process is a surface treatment technique in which a coating material is passed through the electric plasma where it becomes molten and propelled to a base material, forming a dense and reasonably uniform layer. This technique has been applied to various mechanical components and structures that require wear-resistance, corrosion-inhibition, heat-resistance, and thermal insulation. This technique also has many other advantages. It is easy to: Ž1. control the thickness of coatings; Ž2. to coat complexly shaped components; and Ž3. to coat worn components for extended service life w1᎐3x. Zirconia has been considered to be a good candidate U
Corresponding author. Tel.: q82-2-929-5344; fax: q82-2-9295344. E-mail address: [email protected] ŽD. Lim..
coating material for high temperature tribological applications because of its superior properties such as a low thermal conductivity, high thermal expansion coefficient similar to metal and high toughness w4x. In particular, 3 mol% yttria stabilized zirconia Ž3-YSZ. is one of the most attractive tribological materials because of its good combination of high toughness, strength and hardness w5x. However, the effect of phase transformation on tribological properties of 3-YSZ should be further studied for high temperature tribological uses, since the significant loss of fracture strength and wear resistance occurs in the temperature ranges of 200᎐400⬚C, in particular, for environments containing water or its vapor w5᎐7x. Previous studies showed that the wear loss increased with the increase in the test temperature and reaching its maximum loss at approximately 400⬚C. This unique behavior was explained by the tetragonal to monoclinic transformation w8,9x. Lange et al. found the presence of YŽOH. 3 crys-
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 9 6 5 - 8
J. Shin et al. r Surface and Coatings Technology 133᎐134 (2000) 403᎐410
404
tallites and suggested that dissolution of Y elements from the surface into H 2 O was responsible for the low temperature transformation w10x. However, further studies are necessary to understand the high temperature behavior of the plasma sprayed 3-YSZ coating, since the tribological behavior is strongly dependent on material characteristics and test conditions. In order to gain a better understanding of high temperature tribological behavior of 3-YSZ, in this work, the effect of annealing treatment and secondary additive material was studied as these two variables are expected to strongly influence the phase transformation and tribological behavior of the zirconia. Fe 2 O 3 powder was selected as a secondary additive material since it has been known to provide low wear and friction at a low cost w11x. 2. Experimental procedure The ring and plate of FC25 cast iron specimens were machined to the dimension of 30 Žoutside diameter. = 5 = 5 mm3 and 30 = 20 = 5 mm3, respectively. Prior to plasma spray coating, a Ni᎐Cr᎐Al᎐Co᎐Y composite was applied as a bond coat to enhance adhesion and reduce thermal expansion mismatch between the substrate and coating layer. Both the ring and plate specimens were coated with various zirconia based ceramic powders, as listed in Table 1. Ceramic powders were granulated by spray drying method after wet milling for 24 h. As shown in Fig. 1, most of the powders had spherical shape and relatively uniform size for good flowability during plasma spraying. The surfaces of the specimens after plasma spray coating were ground with diamond wheel and 6 m size diamond paste. As annealing treatment, the specimens were subjected to thermal cycling between 25 and 500⬚C for up to 10 cycles. The specimen were placed in the furnace and the furnace was then activated to heat
Table 1 The compositions of Fe 2 O 3-added 3-YSZ powders
3-YSZ 2.5 mol% Fe2 O3 added 3-YSZ 5.0 mol% Fe2 O3 added 3-YSZ 7.5 mol% Fe2 O3 added 3-YSZ 10.0 mol% Fe2 O3 added 3-YSZ
ZrO2 Žmol%.
Y2 O3 Žmol%.
Fe2 O3 Žmol%.
97.00 94.58 92.15 98.72 87.30
3.00 2.92 2.85 2.78 2.70
᎐ 2.50 5.00 7.50 10.00
to 500⬚C at heating rate of 5⬚rmin. The specimens held at 500⬚C for 30 min to stabilize the temperature. The furnace was then switched off and the specimens allowed to cool at the furnace cooling rate of 2.5⬚rmin before removal to cooling to room temperature. The annealing temperature of 500⬚C was selected since the temperature of the top ring reversal position of the liner and the top ring on the piston of adiabatic engine could reach approximately 500⬚C w12x. A wear test was conducted at room temperature to 600⬚C using a high temperature wear tester. The plate specimen was placed on a reciprocating holder moved by a DC motor. The specimen was loaded and slid against a ring type counterpart. Detailed features of the test machine were described elsewhere w8x. For each specimen, wear test was conducted for 1 h after 30 min soaking at the desired temperature. The sliding velocity and stroke length were fixed to 1 = 10y2 mrs and 8 = 10y3 m, respectively. The applied load was 2 N. Fig. 2 shows the schematic diagrams of the ring and plate specimens. The relative wear loss was determined from the width of worn area of the ring specimens as indicated in Fig. 2. The microhardness of the coatings was measured using a Vickers microhardness tester. The indentation was made with a 0.98 N load for 10 s along the axis of the coating thickness and the microhardness was
Fig. 1. SEM images of zirconia-based powders made by spray drying methods; Ža. 3-YSZ and Žb. 10 mol% Fe 2 O 3 -added 3-YSZ.
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confirmed by the XRD patterns. Fig. 5 shows the XRD patterns of 3-YSZ coating as a function of the annealing temperature. The tetragonal Ž220. peak of the as-sprayed coating is shifted to a higher angle compared to the zirconia-based powders, which indicates the existence of tensile residual stress on the surface in a longitudinal direction w14x. The specimens annealed at 500⬚C has similar peak positions as the zirconia based powders. This implies that annealing treatment releases the tensile residual stress. The relaxation of tensile residual stress on the surface influences the
Fig. 2. Schematic diagram showing contact geometry of ring and plate specimens: Ža. before wear test; and Žb. after wear test.
observed. Charles and Evans equation was used to calculate the toughness of coatings by measuring the crack length w13x. X-Ray diffraction ŽXRD. and scanning electron microscopy ŽSEM. were used for the structure identification and damage observation, respectively.
3. Results and discussion 3.1. Tribological properties of the 3-YSZ coatings after annealing treatment Fig. 3 shows that the friction coefficient and wear loss of the 3-YSZ coating before and after annealing. It reveals that the friction coefficient and wear increases gradually until 400⬚C and decreases at 600⬚C. On the other hand, the friction coefficient and wear loss of annealed 3-YSZ coating exhibits the maximum value at 200⬚C and decreases gradually beyond this temperature. In all temperature ranges, the wear loss decreases after annealing. A significant decrease in both friction coefficient and wear loss after annealing is noticeable at 400⬚C. Fig. 4 shows the monoclinic intensity fraction formed in the 3-YSZ coating as a function of the test temperature. The monoclinic intensity fraction was evaluated from the ratio between the XRD peak height of the two monoclinic peak Ž111.mq Ž111.m and the tetragonal peak Ž111.t w6x. With as-sprayed specimen, phase transformation from tetragonal to monoclinic phase occurs approximately below 400⬚C, whereas the amount of monoclinic phase of 3-YSZ coating after annealing has the maximum value at 200⬚C. This tendency is consistent to the wear results as shown in Fig. 3. The shift of the monoclinic intensity fraction may be explained by the stress relaxation. The stress relaxation is
Fig. 3. Tribological behavior of 3-YSZ coating before and after annealing treatment at various temperatures: Ža. friction coefficient; and Žb. width of the worn area of the ring.
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Fig. 4. Monoclinic intensity fraction Ž%. as function of annealing process for as-sprayed 3-YSZ coating at different annealing temperature.
Fig. 5. XRD patterns of 3-YSZ coating in 2 s 33᎐38⬚ region as a function of the annealing temperature.
monoclinic intensity fraction. It has been reported that tensile stress tends to promote phase transformation from tetragonal to monoclinic w15x. The tensile residual stress was released by annealing and this stress relaxation restricted the phase transformation from tetragonal to monoclinic, causing a shift of the maximum value of the monoclinic intensity fraction. The stress relaxation seems to also lower the monoclinic intensity fraction in all temperature ranges. Fig. 6 shows the cross-sectional microhardness changes of the annealed specimen as a function of the annealing cycles. The cross-sectional microhardness of the 3-YSZ coating is increased by approximately 25% through the annealing treatment. However, the microhardness is not markedly increased with the increase in the number of annealing cycles. The transformation from the tetragonal to monoclinic phase is accompanied by severe microcracking on the surface due to volume increase w6x. Severe microcracking will increase the wear loss and the friction coefficient. Several reports supporting this explanation have been published w8,16x. Therefore, the wear rate may be increased with the increase in the monoclinic transformation rate. The similar behavior of the tribological performance with the monoclinic transformation rate, as shown in Figs. 3 and 4, can be explained by the mechanism just mentioned.
annealing treatment as a function of the test temperature. It can be noted that the temperature for the maximum value of the friction coefficient and wear loss is shifted from 400 to 200⬚C after annealing treatment. A significant reduction of friction and wear loss is noticeable at relatively low temperature for the unannealed coating. The effect of Fe 2 O 3 addition as a low friction material is noticeable in the low temperature range Žup to 200⬚C. compared to the unannealed coating. However, the friction coefficient measured at room temperature and 200⬚C is increased by annealing. Fur-
3.2. Tribological properties of the Fe 2 O3-added 3-YSZ coatings after annealing treatment Fig. 7 shows the tribological properties of the 5.0 mol% Fe 2 O 3-added 3-YSZ coating before and after
Fig. 6. The cross-sectional microhardness changes of 3-YSZ coating before and after annealing treatment with different annealing cycles.
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Fig. 8. XRD patterns of zirconia based coatings in 2 s 20᎐70⬚ region.
with ZrO 2 ŽY2 O 3 . which causes low temperature degradation w17x. The stabilization of tetragonal phase is expected to improve the tribological properties of the Fe 2 O 3-added 3-YSZ coating. Fig. 9 is the XRD patterns obtained with the 5.0 mol% Fe 2 O 3-added 3-YSZ coating before and after the wear test at 200⬚C. Before the wear test ŽFig. 9a., the monoclinic phase is not
Fig. 7. Tribological behavior of 5.0 mol% Fe 2 O 3 -added 3-YSZ coating before and after annealing treatment at different temperatures: Ža. friction coefficient; and Žb. width of the worn area of the ring.
ther study is needed to explain this behavior. After annealing, the shift of the temperature for the maximum friction coefficient as well as its stabilization in all temperature ranges can be noticed. Fig. 8 shows XRD patterns of the as-ground 3-YSZ coating with the addition of various amounts of Fe 2 O 3 . It shows that the tetragonal phase is stabilized by Fe 2 O 3 addition. This stabilization may have occurred as Fe 2 O 3 prevents zirconia from reacting with water vapor in the same fashion as Al 2 O 3 w17x. Li et al. suggest that hydroxidation of Al 2 O 3 protects from further interaction of H 2 O
Fig. 9. XRD patterns of 5.0 mol% Fe 2 O 3 added 3-YSZ coating after annealing in 2 s 20᎐70⬚ region: Ža. before wear test; and Žb. after wear test at 200⬚C.
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in the microhardness and the fracture toughness of Fe 2 O 3-added 3-YSZ coatings as a function of Fe 2 O 3 addition, probably due to the localized residual stress distribution. Fig. 12 shows the area of the polished surface due to the friction sliding seems to increase as the amount of Fe 2 O 3 is increased. The SEM micrographs clearly show that the area of the well-polished surface due to the friction sliding increased by the addition of Fe 2 O 3 . This may be attributed to the stabilization and improved mechanical properties. Increased area of the polished surface will contribute to the lowering of the friction coefficient. However, regardless of the Fe 2 O 3 addition, the low temperature degradation observed in 3-YSZ can still be observed. This results implies that the monoclinic transformation, as shown in Fig. 9, of the Fe 2 O 3 added 3-YSZ is still responsible for maximum wear at 200⬚C. Fig. 13 shows SEM micrographs of the wear track of the 5.0 mol% Fe 2 O 3 added 3-PSZ coating after annealing treatment. It reveals that microcrackings, probably due to monoclinic transformation, are dominantly observed on the worn surface of the test at 200⬚C. The highest friction coefficient and wear loss at 200⬚C may be attributed to the microcrackings as shown in Fig. 13b.
4. Conclusions The experimental study on tribological properties of plasma sprayed zirconia-based coatings shows that the test temperature significantly influences the friction coefficient and wear characteristics. The friction coefficient and the wear loss were correlated with the fraction of monoclinic transformation. The friction co-
Fig. 10. Tribological behavior of Fe 2 O 3 -added 3-YSZ coatings after annealing as function of the Fe 2 O 3 addition; Ža. friction coefficient and Žb. width of the worn area of the ring.
found. However, the monoclinic phase is detected after the wear test at 200⬚C probably due to the local heating and the frictional stress ŽFig. 9b.. When compared to the results obtained with the 3-YSZ coating, the tribological performance of Fe 2 O 3-added 3-YSZ coatings is improved as indicated by Fig. 10. Fig. 10 shows the friction coefficient and the wear loss generally decreases by the addition of Fe 2 O 3 . Two explanations may be considered to be related to this observation. First, suppression of the phase transformation due to Fe 2 O 3 , as shown in Fig. 8, can contribute to less wear. Second, tribological properties can also be enhanced by improved mechanical properties. Fig. 11 shows changes
Fig. 11. The microhardness and the toughness changes of the zirconia-based coatings as function of the Fe 2 O 3 addition.
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efficient and the wear loss reached maximum at approximately 400⬚C. The temperature for the maximum values of friction coefficient and wear loss is shifted down to approximately 200⬚C by the annealing treat-
Fig. 13. SEM microimages of the wear track of the 5.0 mol% Fe 2 O 3 added 3-YSZ coating at various test temperature: Ža. room temperature; Žb. 200⬚C; and Žc. 600⬚C.
Fig. 12. SEM microimages of the wear track of the zirconia-based coating tested at room temperature: Ža. 3-YSZ; Žb. 5.0 mol% Fe 2 O 3 added 3-YSZ; and Žc. 10.0 mol% Fe 2 O 3 added 3-YSZ.
ment at 500⬚C. This behavior is explained by the restriction of phase transformation due to stress relief. The addition of Fe 2 O 3 to the 3-YSZ coating im-
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proves tribological properties in all test temperature ranges. The observed suppression of the monoclinic transformation as well as the increase in microhardness and toughness is related to the improvement of the tribological characteristics by the addition of Fe 2 O 3 . Acknowledgements The authors would like to thank the Ministry of Science and Technology and the Critical Technology 21 Program ŽMachinery Design Technology Enhancement. for financial support and interest in this work. This research was also partially supported by the National Research Laboratory Program of the same Ministry. References w1x L. Pawlowski, The Science and Engineering of Thermal Spray Coatings, John Wiley & Sons Ltd, Chichester, 1st edn, 1995, p. 28
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