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Microstructure and properties of Al2O3-13%TiO2 coatings sprayed using nanostructured powders
Available online at www.sciencedirect.com
RARE METALS Vol. 26, No. 4, Aug 2007, p . 391 E-mail: [email protected]
-##
=iO ScienceDirect
Microstructure and properties of A1203-13%Ti02 coatings sprayed using nanostructured powders ZHANG Jianxin, H E Jining, DONG Yanchun,LJ Xiangzhi, and YAN Dianran School of Materials Science and Engineering,Hebei University of Technology, Tianjin 300130, China
(Received 2006-04-05)
Abstract: The microstructure and wear performance of AI2O3-l3%Ti02 coatings prepared by plasma spraying of agglomerated nanoparticle powders were investigated. SEM analysis showed that the as-sprayed A1203-Ti02coatings comprise of two kinds of typical region: fully melted region and unmeltedpartially melted nanostructured region, which is different than the conventional coating with lamellar structure. It is shown that the microhardness of the nanostructured coatings was about 15%-30% higher than that of the conventional coating and the wear resistance is significantly improved, especially under a high wear load. The nanostructured coating sprayed at a lower power shows a lower wear resistance than the coatings produced at a higher power, because of the presence of pores and microstructural defects which are detrimental to the fracture toughness of the coatings. Key words: surface coating; nanostructured coating; plasma spray; microhardness; wear resistance
1. Introduction Oxide ceramic coatings are widely used in various fields to offer substrate materials excellent surface properties against wear, corrosion, erosion, and thermal impact. As significant advance has beer made in the various aspects of synthesis, processing and characterization of bulk nanoscale materials, growing interest has recently attracted in producing nanostructured ceramic coatings by thermal spray processes to achieve properties superior to their conventional coatings. Unlike conventional coatings sprayed using feedstock in micrometer size typically of 15-100 pm, nanostructured coatings are mostly created with specially agglomerated nanostructured powders [ 1-61. Nanostructured coatings of WC-Co, A1203-Ti02, Zr02-Y203,and other systems produced by thermal spraying have been investigated over the past years. It was shown that these nanostructured coatings possess a unique microstructure comprising of fully melted regions dnd partially melted regions, differCorresponding author: ZHANG Jianxin
E-mail: [email protected]
ent than the layered structure of completely melted regions in conventional coatings. Improved properties, such as higher hardness, greater wear and thermal resistance, and more reduced thermal conductivity were achieved with nanostructured coatings, compared with those of their conventional counterparts [7-121. However, previous studies of wear performance of nanostructured coatings were almost limited to a load of less than 100 N and their behavior under higher loads is not well understood. In this study, nanostructured A1203-13% Ti02 coatings were prepared by atmospheric plasma spraying of agglomerated nanoparticles powders. The microstructure of the coatings was characterized by X-ray diffraction and scanning electron microscope and their wear performance was investigated in the load range of 100-500 N, which were compared with those of the conventional counterpart. The wear differences between the conventional coating and the nanostructured coating, as well as, between the nanostructured coatings sprayed at different powers were explained in terms of their mi-
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392
crostructure.
2.
Experimental
Commercial A1203and Ti02powders of 50-110 nm in size from Shandong Xianguang Nanomaterials Company were used in this study as raw feedstock. The powders were well wet-mixed according to the weight ratio of Al203-13% TiO2, agglomerated into micrometer-sized granules by spray drying and then sintered at temperatures of 1100-1200°C. Substrate samples of 9 mm x 8 mm x 10 mm were cut from mild steel. The samples were first grit-blasted and then coated by plasma spraying Ni/Al powders
to form a bonding layer of about 100 pm in thickness. Nanostructured A1203-13% Ti02 coatings were deposited by plasma spraying the agglomerated powders and the coating thickness was about 300 pm. For comparison, conventional AI2O3-13% Ti02 coating was also deposited with traditional powders of 25-50 pm in size from Shenyang Abrasive Wheel Factory and in an optimized spray condition. The plasma spray processes were carried out on an atmosphere plasma spraying system (Model GP-80, Yeyuan Spray Equipment Factory, Jiangsu, China) with a BT-G3 gun. The processing parameters used for the nanostructured coatings and the optimized ones for the conventional coating are shown in Table I.
Table 1. Plasma spray parameters Coating
Spray power / kW Ar gas / (Lmin-l) Hz gas / (Lmin-l) Canier gas N2/ (Lmin-I) Gun distance I mm
Conventional
30
80
100
Nanostructured
30-40
80-85
100-200
The agglomerated nanostructured powders and plasma-sprayed coatings were characterized using a PHILIPS X-PertMPD X-ray difftactometer and a PHILIF'S XL30RMP scanning electron microscope. Microhardness measurements were performed under a load of 0.98 N on the cross section of the coatings. Wear tests were done without lubrication on a block-on-ring arrangement of an MM200 tribometer. The stationary block with deposited A1&13% Ti02 coatings was pressed against a rotating ring made of quenched bearing steel with a mean hardness of HRC 6 1. The sliding speed at the contact surface of specimens was 0.40 m / s and the applied load was in the range of 100-500 N. Wear rates were calculated using the mean measurement value of three samples in terms of the volume of the coating material removed per unit load and sliding distance, in unit of mm3.N'.rn-l.
3. Results and discussion 3.1. Characterization of the agglomerated powders The fabrication process of the nanostructured powders used in this study was to make A1203and
4 5-7
100 80-110
Ti02 nanoparticles be agglomerated into micrometer powders by spray-drying the slurry and subsequently sintering the spray-dried granules. Fig. 1 shows the morphology and X-ray diffraction patterns of the resultant powders after spraying and drying granulation, and sintering treatment. The agglomerated powders show granules and their size ranges from 20 pm to 50 pm. A high magnification scanning electron micrograph (Fig. l(b)) demonstrates that the grain size in the nanostructured granules is about 100 nm and slight necking exists between nanoparticles due to sintering effect, which improves the strength of connection between nanoparticles necessary for plasma spraying. The XRD pattern of the agglomerated A1203-13% Ti02 powders is shown in Fig. l(c). In the heat-treated nanostructured powders, A1203 has the same a structure as the raw feedstock and Ti02 changes from anatase to mtile, because of an irreversible phase transition occumng at 610OC.
3.2. Coating characteristics Fig. 2 presents SEM images of the cross-section of the conventional A1203-13% Ti02coating and the as-sprayed nanostructured coating. The conventional
Zhang J.X. et al., Microstructure and properties of A1203-13%Ti02coatings sprayed using.. .
coating shows a typical lamellar morphology, which results from impact, spreading, and rapid solidification of molten droplets on the previously deposited splats. In contrast, the nanostructured coatings show two distinct microstructure features. One is similar to that of the conventional coating formed from fully melted powders, as denoted as F in Figs. 2(b-d), and the other is characterized by the region marked as P, which comprises typically unmelted or partially melted nanosized particles. During plasma spraying, nanostructured powders behave differently in thermal response due to the distribution in the powder size and the temperature distribution in plasma jet.
393
Some powders are completely melted to form the splat lamellae and others melt to such a degree that either the powders become a liquid matrix with partly melted particles embedded in it or the outer part of powders melts, leaving an unmelted solid core. In these cases, impact of these powders on the substrate or on early deposited splats produces an unmelted or partially melted microstructure. It is obvious that increasing the plasma spray power makes more powders fully molten, hence reducing the proportion of the unmelted or partially melted regions in the nanostructured coatings, as shown in Figs. 2(b-d).
(C)
a-Al,O,
e
Rutrle TiO?
A
i ~
il
J& i Jfi,
20
30
40
50
60
70
::
J
80
281 (")
Fig. 1. (a) SEM micrograph of the agglomerated powders, (b) nanostructured feature, and (c) XRD pattern.
The XRD analysis of the as-prepared nanostructurd coatings showed that some a-Al2O3changedto y-Alz03after plasma spraying, as shown in Fig. 3, because y-A1203forms more easily from the melt than a-Alz03at a high cooling rate, because of the low interfacial energy between crystal and liquid [13]. It is also seen that the relative intensity of the
primary diffraction peak of y-Alz03 increases with increasing spray power, indicating a reduction in the fraction of the partially melted regions in the nanostructured coatings. The a-AI2O3diffraction in Fig. 3 is certainly due to the presence of unmelted or partially melted alumina.
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394
Fig. 2. SEM micrographs of (a) the conventional coating and the nanostructured coatings sprayed at (b)30 kW, (c) 35 kW, and (d) 40 kW.
20/(") Fig. 3. XRD patterns of the nanostructured coatings sprayed at 30 kW, 35 kW, and 40 kW.
3.3. Microhardness of the coating The microhardness of the AI2O3-TiO2coatings sprayed with the conventional micron-sized powders and the agglomerated nanostructured powders is
given in Table 2. It is seen that the nanostructured coatings have a higher microhardness than the conventional coating and a difference of about 15%-35% in microhardness was detected between the two types of coating. These results indicate that the nanostructure in the coating obtained using the agglomerated powders gives rise to a strengthening effect, which does not occur in conventional coating. In principle, the degree of the strengthening effect depends essentially on the amount of the nanostructure formed in the coatings. The proportion of unmelted or partially melted nanostructured region changes with plasma spray power and the higher the spray power, the less the amount of the nanostructured region in the coatings. The nanostructured coating deposited at a higher power shows a lower microhardness than the coating produced at a lower power. Fig. 4 shows the indentation impressions performed with a 0.98 N load on the cross-section of the coatings sprayed at 35 kW using conventional feedstock and the agglomerated nanostructured
Zhang J.X. et af., Microstructure and properties of AI2O3-13%Ti02coatings sprayed using.. .
powders, respectively. Compared with the conventional coating in which the indentation edges are seriously crashed, the nanostructured coating shows a well-defined indentation, indicating that it possesses a lower brittleness. The crack propagation resistance of the coatings can be estimated according to the relationship between the indentation load and crack
395
length [14]. The calculated results proved that the fracture toughness of the nanostructured coatings is about 30% higher than that of the conventional coating. The enhanced toughness of the nanostructured coatings is attributed to the presence of the nanostructured zones embedded in a fine-grain matrix, which provide more crack arrests [ 151.
Table 2. Microhardness of the A120~-Ti02coafings Coating
Spray power / kW
Microhardness, HVIm ~
Standard deviation, fa
~~
Conventional A1203-13wt.% Ti02
30
1083
+I57
Nanostructured A1203-13wt.% TiOz
30
1476
f163
Nanostructured AI2O3-13wt.% TiOa
35
I303
+174
Nanostructured A1,07-13wt.% Ti07
40
1271
5169
Fig. 4. Indentation of the coatings d e p i t e d at 35 kW using (a) conventional feedstock and (b) nanostructured powders.
3.4. Wear behavior of the coatings Wear rates of the different A1203-13% TiOz coatings are presented in Fig. 5. The wear resistance of the coatings is remarkably different. Under the condition of a constant load of 300 N, the conventional coating shows the lowest wear resistance. However, the wear resistance of the nanostructured coatings is much higher than that of the conventional coating and their wear difference tends to become larger as the wear time increases, as demonstrated in Fig. 5(a). According to Fig. 5(b), the wear behavior of the coatings is also closely related to the applied wear load. For the coatings sprayed with the nanostructured powders, the wear rate rises slowly and almost lineally with the increase in wear load. In contrast, the wear of the conventional coating grows fast with
increasing load, especially in the severe load range. In addition, amongst the nanostructured coatings obtained at different spray powers, the coating processed at 30 kW wears faster than the others during the test either under the constant load of 300 N or in the load range of 100-500 N. For the coatings sprayed using nanostructured powders and at spray powers of 35 kW and 40 kW, there is a small difference in wear rate in the two wear cases. Wear is a very complex process and many factors, such as hardness, toughness, microstructure, defect content, and so on, influence it. Of these factors, fracture toughness and hardness are the most important properties needed to improve the wear resistance of ceramic coatings [ 161. An increase in the fracture toughness andor hardness of those coatings will rationally enhance their wear property. In this
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1
study, both the microhardness and fracture toughness of the A1203-13% Ti02 coatings derived from nanostructured powders are higher than those of the coating deposited with conventional powders and therefore the as-sprayed nanostructured coatings have a better wear resistance than the conventional counterpart. The wear difference between the nanostructured coatings sprayed at a different power can be related to their different microstructures. It is rev d e d that in the coating deposited with a higher power (35 kW)the full melted zones and partially fl
35 (a) 30-
-
melted zones are densely pilled together and few microstructural defects can be seen, as shown in Fig. 6(a). However, in the coating produced at 30 kW,the partially melted zones are porous and certain defects are formed between the successively deposited splats, as pointed by the arrow in Fig. 6(b).Under applied load, cracks are easily nucleated and propagate at the limits of the areas near these defects, thus causing a significant decrease in the fracture toughness and wear resistance of the coatings.
--Conventional coating -0-Nanocoating, 30 kW -0-
5% 60 1
20
-0-
-A-
-v-
Conventional coating Nanocoating, 30 kW Nanocoating, 35 kW Nanocoating,40 kW
E 40
3
00
i
Ib
20 i5 Time / min 1;
3’0 Load / N
Fig. 5. Variation of wear rates of the coatings with (a) wear time (300N)and (b) applied load.
Fig. 6. SEM micrographs of the nanostructured coatings sprayed at (a) 35 k W and (b) 30 kW.
4.
Conclusion
In this study, nanostructured A1203-13% Ti02 coatings were deposited by plasma spraying using agglomerated nanoparticle powders. Compared with the conventional A1203-13%KOz counterpart with a lamellar structure, the nanostructured coatings are comprised of two typical regions: fully melted re-
gion and unmelted/partially melted region. Lncreasing plasma spay power leads to a decrease in the fraction of the unmelted/partly melted region. The nanostructured coatings have a microhardness by about 15%-30% higher than that of the coating made of conventional powders. Under the constant load of 300 N, the wear resistance of the nanostructured coatings is better than that of the conventional coating. As the applied load increases, the wear rate of
Zhang J.X. et aL, Microstructure and properties of A1203-13%Ti02coatings sprayed using..
the nanostructured coatings rises slowly, whereas the conventional coating wears fast, especially under a high load. Amongst the nanostructured coatings, the coating sprayed at 30 kW shows the worst wear resistance than the coating produced at 35 and 40 kW, because of the presence of pores and microstructural defects.
References Vepreka S. and Jilekb M., Super- and ultrahard nanocomposite coatings: generic concept for their preparation, properties and industrial applications, Vacuum,2002,67: 443. Ji G., Momiroli J.P., and Grosdidier T., Nanostructures in thermal spray coatings, Scripta Muter., 2003, 48: 1599. Stan V. and Argon AS., Mechanical properties of superhard nanocomposites, Sud Coat. Technol., 2001,146-147: 175. Liang X.B., Deng Z.C., Xu Y., and Xu B.S., Thermal spraying nanoscale powder materials and coatings, Muter: Eng., 2002, 11: 38. He J.H., and Schoenung J.M., Nanostructured coatings, Muter. Sci. Eng., 2002, A336: 274. Lavemia E.J. and Kabacoff L., Thermal spray processing of nanostructured coatings, J. Them Spray Technol., 1998.7: 291. Marple B.R., Voyer J., Bisson J.F., and Moreau C., Thermal spraying of nanostructured cermet coatings, J. Muter: Process. Technol.,200 1,117:41 8. Luo H., Goberman D., Shaw L., and Gell M., Indentation fracture behavior of plasma sprayed nanostructured A1203-13 wt.%TiO, coatings, Muter. Sci.
.
397
Eng., 2003, A M : 237. [9] Shaw L.L., Goberman D., Ren R.M., Gell M., Jiang S., Wang Y., Xiao T.D., and Strutt P.R., The dependency of microstructure and properties of nanostructured coatings on plasma spray condition, Sud Coat. Technol.,2000,130: 1. [lo] Goberman D., S o h Y.H., Shaw L., Jordan E., and Gel1 M., Microstructure development of &03-13 wt.% Ti02plasma sprayed coatings derived from nanocrystalline powder, Acta Muter::,2002,SO 1141. [ l l ] Wang Y., Jiang S., Wang M.D., Wang S.H., Xiao T.D., and Strutt P.R., Abrasive wear characteristics of plasma sprayed alumindtitania coatings, Wear,2000, 237: 176. [121 Lima R.S., Kucuk A., and Bermdt C.C., Integrity of nanostructured partially stabilized zirconia after plasma spray processing, Muter: Sci. Eng., 2001, A313 75. [13] Lin X.H., Zeng Y., Zhou X.M., and Ding C.X., Microstructure of alumina-3 wt.% titania coatings by plasma spraying with nanostructured powders, Muter: Sci. Eng., 2003, A357: 228. [I41 Anstis GR., Chantikul P.,Lawn B.R., and Marshall D.B., A critical evaluation of indentation techniques for measuring fracture toughness-direct crack measurements, J. Am. Ceram. Soc., 1981,64 (9): 533. [15] Lima R.S. and Marple B.R., Enhanced ductility in thermally sprayed titania coating synthesized using a nanostructured feedstock, Muter: Sci. Eng., 2005, A395 269. [16] Normand B., Fervel V., Coddet C., and Nikitine V., Tribological properties of plasma sprayed aluminatitania coatings: role and control of the microstructure, Sud Coat. Technol., 2000,123: 278-287.
RARE METALS Vol. 26, No. 4, Aug 2007, p . 391 E-mail: [email protected]
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=iO ScienceDirect
Microstructure and properties of A1203-13%Ti02 coatings sprayed using nanostructured powders ZHANG Jianxin, H E Jining, DONG Yanchun,LJ Xiangzhi, and YAN Dianran School of Materials Science and Engineering,Hebei University of Technology, Tianjin 300130, China
(Received 2006-04-05)
Abstract: The microstructure and wear performance of AI2O3-l3%Ti02 coatings prepared by plasma spraying of agglomerated nanoparticle powders were investigated. SEM analysis showed that the as-sprayed A1203-Ti02coatings comprise of two kinds of typical region: fully melted region and unmeltedpartially melted nanostructured region, which is different than the conventional coating with lamellar structure. It is shown that the microhardness of the nanostructured coatings was about 15%-30% higher than that of the conventional coating and the wear resistance is significantly improved, especially under a high wear load. The nanostructured coating sprayed at a lower power shows a lower wear resistance than the coatings produced at a higher power, because of the presence of pores and microstructural defects which are detrimental to the fracture toughness of the coatings. Key words: surface coating; nanostructured coating; plasma spray; microhardness; wear resistance
1. Introduction Oxide ceramic coatings are widely used in various fields to offer substrate materials excellent surface properties against wear, corrosion, erosion, and thermal impact. As significant advance has beer made in the various aspects of synthesis, processing and characterization of bulk nanoscale materials, growing interest has recently attracted in producing nanostructured ceramic coatings by thermal spray processes to achieve properties superior to their conventional coatings. Unlike conventional coatings sprayed using feedstock in micrometer size typically of 15-100 pm, nanostructured coatings are mostly created with specially agglomerated nanostructured powders [ 1-61. Nanostructured coatings of WC-Co, A1203-Ti02, Zr02-Y203,and other systems produced by thermal spraying have been investigated over the past years. It was shown that these nanostructured coatings possess a unique microstructure comprising of fully melted regions dnd partially melted regions, differCorresponding author: ZHANG Jianxin
E-mail: [email protected]
ent than the layered structure of completely melted regions in conventional coatings. Improved properties, such as higher hardness, greater wear and thermal resistance, and more reduced thermal conductivity were achieved with nanostructured coatings, compared with those of their conventional counterparts [7-121. However, previous studies of wear performance of nanostructured coatings were almost limited to a load of less than 100 N and their behavior under higher loads is not well understood. In this study, nanostructured A1203-13% Ti02 coatings were prepared by atmospheric plasma spraying of agglomerated nanoparticles powders. The microstructure of the coatings was characterized by X-ray diffraction and scanning electron microscope and their wear performance was investigated in the load range of 100-500 N, which were compared with those of the conventional counterpart. The wear differences between the conventional coating and the nanostructured coating, as well as, between the nanostructured coatings sprayed at different powers were explained in terms of their mi-
RARE METALS, Vol. 26, No. 4, Aug 2007
392
crostructure.
2.
Experimental
Commercial A1203and Ti02powders of 50-110 nm in size from Shandong Xianguang Nanomaterials Company were used in this study as raw feedstock. The powders were well wet-mixed according to the weight ratio of Al203-13% TiO2, agglomerated into micrometer-sized granules by spray drying and then sintered at temperatures of 1100-1200°C. Substrate samples of 9 mm x 8 mm x 10 mm were cut from mild steel. The samples were first grit-blasted and then coated by plasma spraying Ni/Al powders
to form a bonding layer of about 100 pm in thickness. Nanostructured A1203-13% Ti02 coatings were deposited by plasma spraying the agglomerated powders and the coating thickness was about 300 pm. For comparison, conventional AI2O3-13% Ti02 coating was also deposited with traditional powders of 25-50 pm in size from Shenyang Abrasive Wheel Factory and in an optimized spray condition. The plasma spray processes were carried out on an atmosphere plasma spraying system (Model GP-80, Yeyuan Spray Equipment Factory, Jiangsu, China) with a BT-G3 gun. The processing parameters used for the nanostructured coatings and the optimized ones for the conventional coating are shown in Table I.
Table 1. Plasma spray parameters Coating
Spray power / kW Ar gas / (Lmin-l) Hz gas / (Lmin-l) Canier gas N2/ (Lmin-I) Gun distance I mm
Conventional
30
80
100
Nanostructured
30-40
80-85
100-200
The agglomerated nanostructured powders and plasma-sprayed coatings were characterized using a PHILIPS X-PertMPD X-ray difftactometer and a PHILIF'S XL30RMP scanning electron microscope. Microhardness measurements were performed under a load of 0.98 N on the cross section of the coatings. Wear tests were done without lubrication on a block-on-ring arrangement of an MM200 tribometer. The stationary block with deposited A1&13% Ti02 coatings was pressed against a rotating ring made of quenched bearing steel with a mean hardness of HRC 6 1. The sliding speed at the contact surface of specimens was 0.40 m / s and the applied load was in the range of 100-500 N. Wear rates were calculated using the mean measurement value of three samples in terms of the volume of the coating material removed per unit load and sliding distance, in unit of mm3.N'.rn-l.
3. Results and discussion 3.1. Characterization of the agglomerated powders The fabrication process of the nanostructured powders used in this study was to make A1203and
4 5-7
100 80-110
Ti02 nanoparticles be agglomerated into micrometer powders by spray-drying the slurry and subsequently sintering the spray-dried granules. Fig. 1 shows the morphology and X-ray diffraction patterns of the resultant powders after spraying and drying granulation, and sintering treatment. The agglomerated powders show granules and their size ranges from 20 pm to 50 pm. A high magnification scanning electron micrograph (Fig. l(b)) demonstrates that the grain size in the nanostructured granules is about 100 nm and slight necking exists between nanoparticles due to sintering effect, which improves the strength of connection between nanoparticles necessary for plasma spraying. The XRD pattern of the agglomerated A1203-13% Ti02 powders is shown in Fig. l(c). In the heat-treated nanostructured powders, A1203 has the same a structure as the raw feedstock and Ti02 changes from anatase to mtile, because of an irreversible phase transition occumng at 610OC.
3.2. Coating characteristics Fig. 2 presents SEM images of the cross-section of the conventional A1203-13% Ti02coating and the as-sprayed nanostructured coating. The conventional
Zhang J.X. et al., Microstructure and properties of A1203-13%Ti02coatings sprayed using.. .
coating shows a typical lamellar morphology, which results from impact, spreading, and rapid solidification of molten droplets on the previously deposited splats. In contrast, the nanostructured coatings show two distinct microstructure features. One is similar to that of the conventional coating formed from fully melted powders, as denoted as F in Figs. 2(b-d), and the other is characterized by the region marked as P, which comprises typically unmelted or partially melted nanosized particles. During plasma spraying, nanostructured powders behave differently in thermal response due to the distribution in the powder size and the temperature distribution in plasma jet.
393
Some powders are completely melted to form the splat lamellae and others melt to such a degree that either the powders become a liquid matrix with partly melted particles embedded in it or the outer part of powders melts, leaving an unmelted solid core. In these cases, impact of these powders on the substrate or on early deposited splats produces an unmelted or partially melted microstructure. It is obvious that increasing the plasma spray power makes more powders fully molten, hence reducing the proportion of the unmelted or partially melted regions in the nanostructured coatings, as shown in Figs. 2(b-d).
(C)
a-Al,O,
e
Rutrle TiO?
A
i ~
il
J& i Jfi,
20
30
40
50
60
70
::
J
80
281 (")
Fig. 1. (a) SEM micrograph of the agglomerated powders, (b) nanostructured feature, and (c) XRD pattern.
The XRD analysis of the as-prepared nanostructurd coatings showed that some a-Al2O3changedto y-Alz03after plasma spraying, as shown in Fig. 3, because y-A1203forms more easily from the melt than a-Alz03at a high cooling rate, because of the low interfacial energy between crystal and liquid [13]. It is also seen that the relative intensity of the
primary diffraction peak of y-Alz03 increases with increasing spray power, indicating a reduction in the fraction of the partially melted regions in the nanostructured coatings. The a-AI2O3diffraction in Fig. 3 is certainly due to the presence of unmelted or partially melted alumina.
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Fig. 2. SEM micrographs of (a) the conventional coating and the nanostructured coatings sprayed at (b)30 kW, (c) 35 kW, and (d) 40 kW.
20/(") Fig. 3. XRD patterns of the nanostructured coatings sprayed at 30 kW, 35 kW, and 40 kW.
3.3. Microhardness of the coating The microhardness of the AI2O3-TiO2coatings sprayed with the conventional micron-sized powders and the agglomerated nanostructured powders is
given in Table 2. It is seen that the nanostructured coatings have a higher microhardness than the conventional coating and a difference of about 15%-35% in microhardness was detected between the two types of coating. These results indicate that the nanostructure in the coating obtained using the agglomerated powders gives rise to a strengthening effect, which does not occur in conventional coating. In principle, the degree of the strengthening effect depends essentially on the amount of the nanostructure formed in the coatings. The proportion of unmelted or partially melted nanostructured region changes with plasma spray power and the higher the spray power, the less the amount of the nanostructured region in the coatings. The nanostructured coating deposited at a higher power shows a lower microhardness than the coating produced at a lower power. Fig. 4 shows the indentation impressions performed with a 0.98 N load on the cross-section of the coatings sprayed at 35 kW using conventional feedstock and the agglomerated nanostructured
Zhang J.X. et af., Microstructure and properties of AI2O3-13%Ti02coatings sprayed using.. .
powders, respectively. Compared with the conventional coating in which the indentation edges are seriously crashed, the nanostructured coating shows a well-defined indentation, indicating that it possesses a lower brittleness. The crack propagation resistance of the coatings can be estimated according to the relationship between the indentation load and crack
395
length [14]. The calculated results proved that the fracture toughness of the nanostructured coatings is about 30% higher than that of the conventional coating. The enhanced toughness of the nanostructured coatings is attributed to the presence of the nanostructured zones embedded in a fine-grain matrix, which provide more crack arrests [ 151.
Table 2. Microhardness of the A120~-Ti02coafings Coating
Spray power / kW
Microhardness, HVIm ~
Standard deviation, fa
~~
Conventional A1203-13wt.% Ti02
30
1083
+I57
Nanostructured A1203-13wt.% TiOz
30
1476
f163
Nanostructured AI2O3-13wt.% TiOa
35
I303
+174
Nanostructured A1,07-13wt.% Ti07
40
1271
5169
Fig. 4. Indentation of the coatings d e p i t e d at 35 kW using (a) conventional feedstock and (b) nanostructured powders.
3.4. Wear behavior of the coatings Wear rates of the different A1203-13% TiOz coatings are presented in Fig. 5. The wear resistance of the coatings is remarkably different. Under the condition of a constant load of 300 N, the conventional coating shows the lowest wear resistance. However, the wear resistance of the nanostructured coatings is much higher than that of the conventional coating and their wear difference tends to become larger as the wear time increases, as demonstrated in Fig. 5(a). According to Fig. 5(b), the wear behavior of the coatings is also closely related to the applied wear load. For the coatings sprayed with the nanostructured powders, the wear rate rises slowly and almost lineally with the increase in wear load. In contrast, the wear of the conventional coating grows fast with
increasing load, especially in the severe load range. In addition, amongst the nanostructured coatings obtained at different spray powers, the coating processed at 30 kW wears faster than the others during the test either under the constant load of 300 N or in the load range of 100-500 N. For the coatings sprayed using nanostructured powders and at spray powers of 35 kW and 40 kW, there is a small difference in wear rate in the two wear cases. Wear is a very complex process and many factors, such as hardness, toughness, microstructure, defect content, and so on, influence it. Of these factors, fracture toughness and hardness are the most important properties needed to improve the wear resistance of ceramic coatings [ 161. An increase in the fracture toughness andor hardness of those coatings will rationally enhance their wear property. In this
RARE METALS, Vol. 26, No. 4, Aug 2007
396
1
study, both the microhardness and fracture toughness of the A1203-13% Ti02 coatings derived from nanostructured powders are higher than those of the coating deposited with conventional powders and therefore the as-sprayed nanostructured coatings have a better wear resistance than the conventional counterpart. The wear difference between the nanostructured coatings sprayed at a different power can be related to their different microstructures. It is rev d e d that in the coating deposited with a higher power (35 kW)the full melted zones and partially fl
35 (a) 30-
-
melted zones are densely pilled together and few microstructural defects can be seen, as shown in Fig. 6(a). However, in the coating produced at 30 kW,the partially melted zones are porous and certain defects are formed between the successively deposited splats, as pointed by the arrow in Fig. 6(b).Under applied load, cracks are easily nucleated and propagate at the limits of the areas near these defects, thus causing a significant decrease in the fracture toughness and wear resistance of the coatings.
--Conventional coating -0-Nanocoating, 30 kW -0-
5% 60 1
20
-0-
-A-
-v-
Conventional coating Nanocoating, 30 kW Nanocoating, 35 kW Nanocoating,40 kW
E 40
3
00
i
Ib
20 i5 Time / min 1;
3’0 Load / N
Fig. 5. Variation of wear rates of the coatings with (a) wear time (300N)and (b) applied load.
Fig. 6. SEM micrographs of the nanostructured coatings sprayed at (a) 35 k W and (b) 30 kW.
4.
Conclusion
In this study, nanostructured A1203-13% Ti02 coatings were deposited by plasma spraying using agglomerated nanoparticle powders. Compared with the conventional A1203-13%KOz counterpart with a lamellar structure, the nanostructured coatings are comprised of two typical regions: fully melted re-
gion and unmelted/partially melted region. Lncreasing plasma spay power leads to a decrease in the fraction of the unmelted/partly melted region. The nanostructured coatings have a microhardness by about 15%-30% higher than that of the coating made of conventional powders. Under the constant load of 300 N, the wear resistance of the nanostructured coatings is better than that of the conventional coating. As the applied load increases, the wear rate of
Zhang J.X. et aL, Microstructure and properties of A1203-13%Ti02coatings sprayed using..
the nanostructured coatings rises slowly, whereas the conventional coating wears fast, especially under a high load. Amongst the nanostructured coatings, the coating sprayed at 30 kW shows the worst wear resistance than the coating produced at 35 and 40 kW, because of the presence of pores and microstructural defects.
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