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A study of MoSi2MoS2 coatings fabricated by SHS casting route
Materials Science and Engineering A277 (2000) 266 – 273 www.elsevier.com/locate/msea
A study of MoSi2MoS2 coatings fabricated by SHS casting route Peiqing La *, Qunji Xue, Weimin Liu Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China Received 1 June 1999; received in revised form 14 June 1999
Abstract Coatings were fabricated on a steel substrate by a self-propagating high temperature synthesis (SHS) casting route. The processing is described in detail. The phases in the coating were examined using X-ray diffraction (XRD). The microstructures of the coatings were analyzed with optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalyzer (EPMA). Tribological properties and microhardness of the coatings with 6 – 12 wt.% MoS2 were measured. The experimental results show that the coatings were composed of MoSi2 and MoS2 phases and coatings with 6 – 12 wt.% MoS2 had dense microstructure. Metallurgical bonding had formed between the coatings and the substrate. Friction of the coatings with 6–12 wt.% MoS2 is lower than its steel substrate. Microhardness and wear resistance of the coatings with 6 – 12 wt.% MoS2 are higher than that of the steel substrate. Near the interface microhardness of the coating varies with the distance to the interface. © 2000 Elsevier Science S.A. All rights reserved. Keywords: MoSi2MoS2 coatings; Self-propagating high temperature synthesis (SHS) casting route; Microstructure; Mcrohardness; Tribological properties
1. Introduction The intermetallic compound molybdenum-disilicide (MoSi2) has been the focus of considerable attention as an attractive material at high temperature applications. Its properties provide a desirable combination of a high melting temperature (2293 K), high oxidation resistance and a relatively low density (6.25 g cm − 3) [1–7]. In addition, MoSi2 possesses high hardness and high elastic modulus, which ensures a good wear resistance [5]. Therefore, when MoSi2 coating on steel substrate is prepared wear properties and resistance to oxidation and corrosion of the steel substrate will be dramatically improved [1,8]. However, MoSi2 has low-temperature brittle behavior. A. Hidouci and J.M. Pelletier have fabricated MoSi2 coating on steel substrate from Mo and Si powders or MoSi2 powders by laser processing [1]. But the coatings contain a large number of cracks due to the brittleness of MoSi2 and the thermal stresses produced in the processing. Therefore, properties of the coatings are limited and application of the coatings is * Corresponding author. Tel.: +86-931-8417088. E-mail address: [email protected] (P. La)
restricted. It is well documented that an improved toughening for MoSi2 is possible through the incorporation of second phase reinforcements [2–7,9]. Therefore, MoS2 is attempted to be in situ synthesized in the MoSi2 coating as a reinforcement to increase its toughness and obtain a MoSi2 coating in this research. On the other hand, as a solid lubricant the addition of MoS2 in the coating is expected to bring a low friction to the coatings [10]. To our knowledge, there are few literatures about MoSi2MoS2 system and its coating [1–7]. SHS (self-propagating high temperature synthesis) casting route is a novel technology for fabricating coatings, the SHS coating method combines materials preparation with coating fabrication in one step [11– 13]. As a result, the cost for coating fabrication can be decreased. Furthermore, thick coatings could be fabricated by using this processing method [11–13]. Therefore, SHS casting coating method is used to fabricate MoSi2MoS2 coatings. The purpose of the present work is in an attempt to fabricate MoSi2MoS2 coatings by SHS casting coating method and examine the mechanism of the processing, microstructure and properties of the coatings.
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2. Experiments
2.1. Fabrication of the coatings Commercial carbon steel 1045 was machined to the size of F60 × 5 mm and used as the substrate. Reaction (1) and (2) are used to produce MoSi2 and MoS2, respectively. Reactant powders of reaction (1) and (2) were weighted according to the designing weight content of MoS2 in the MoSi2 coating. The powders were dry-mixed for 8 h in a ball-mixing mill with Al2O3 spheres in order to reduce the amount of agglomeration as well as to provide a homogeneous mixture of the reactants. MoO3 + 2Al+2Si= MoSi2 +Al2O3 DH °,f 298 K = − 1051.7 (kJ mol − 1), MoO3 + 2Al+2S= MoS2 +Al2O3
(1)
DH °,f 298 K = − 1170.2 (kJ mol − 1). (2) The characteristics of the reactant powders are given in Table 1. Before coating, the substrate was cleaned with acetone in an ultrasonic cleaner for 10 min. After drying with hot air, the substrate was positioned at the bottom of a steel tube with refractory surroundings. In order to generate a steady reaction wave in the SHS processing and reduce gases in the reactants, the reactant powder must be compacted [14,15]. For this purpose, the assembled reactants were cold-pressed under a uniaxial pressure of 20 MPa. An igniting agent composed of Al3 + , S2 − , Mn4 + compound was then put on the surface of the reactant [12]. This can be ignited at a temperature of 673 K with considerable heat release. This assembled sample was placed in a reactor as schematically illustrated in Fig. 1. Table 1 Properties of the raw material powders Powder
Size (mesh)
Purity (%)
Impurity
MoO3 Al S Si
B200 100–200 B200 B200
\99 \99 \99 \99
Pb, Cl, PO4, SO4, NO3 Fe, Si, Cu, H2O
Fig. 1. Sketch of the SHS coating fabrication apparatus.
267
After cleaning with argon gas at room temperature, the reactor was heated to 573 K and held there for 1 h to remove gases on the surface of the reactant powders. After 1 h of incubation time, the reactor was cleaned again by argon gas and then 5 MPa-argon gas was introduced into the reactor for the pressing of the products when the combustion synthesis reactions ended [11,12]. The reactor was then heated and when the temperature reached 673 K, the igniter reaction started. The reactants powder compact was then ignited by the heat released from the igniter. The gas pressure was then maintained for 60 min after which, the resultant sample was allowed to cool to ambient temperature in the reactor. The sample was then removed for examination. It was observed with the naked eye that a black layer was present on the top of the target sample. This layer was not bonded with the underlying material and could be removed by hand, to reveal a coating on the steel substrate that was determined to be 2–3 mm in thickness. Specimens with dimensions of 12× 12×6 mm were cut from the typical samples for XRD and microstructure investigation.
2.2. In6estigation of the microstructure The coating region of the specimens and material from the black layer on top of the coating were analyzed using Shimazu Dmax-RB X-ray diffraction (XRD). The microstructure of the coatings and the interface between the substrate and the coating were examined with scanning electron microcopy (SEM) and optical microcopy (OM). Element distribution at the interface was determined by using EPM-810Q electron probe microanalyzer (EPMA). Prior to the OM, SEM and EPMA examination, the specimens of the coatings were polished using colloidal silica (0.05 mm) and etched with a 10% HNO3 ethanol mixture.
2.3. Examination of the microhardness and tribological properties The microhardnesses of the coatings and the steel substrate were measured by using an AVKSHI MVK-1 hardness tester. The load was 20 gf, loading speed was 0.034 mm s − 1 and dwell time was 30 s. Three tests were made for each material and the test deviation was no more than 5%. The tribological tests were conducted at room temperature (23°C) using a ball-on-disc apparatus under reciprocating sliding conditions and lubricating with chemically pure liquid paraffin with a viscosity of 30.19 cSt (at 25°C) and a boiling point higher than 300°C. About 1 ml of the lubricant was added to the disk surface before test. The coatings samples were machined as discs with a diameter of 24 mm and thickness of 6 mm. Samples of
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Table 2 Chemical composition of the SAE52100 steel Elements
C
Si
Mn
Fe
Content
0.42–0.50
0.15–0.35
0.2–0.4
Balance
cross-section and L is the length of the stroke. The volumetric wear rate W was calculated using Eq. (4), where s is the total sliding distance. The friction coefficient is continuously recorded by an X-Y recorder system. The friction coefficient values reported in this paper are to be considered nominal values that are representative of the predominant behavior during the majority of each run. Three tests were made for each material and the test deviation is less than 6%. The wear property and friction coefficient of the steel substrate is measured at the same condition as a comparison. Vd = SL.
(3)
W= Vd s − 1.
(4)
3. Results Fig. 2. XRD result of coating with 10 wt.% MoS2.
the steel substrate were machined as discs as above, and served for comparative investigations of tribological properties. The counterpart was a SAE52100 steel ball of hardness HRC62-63 and surface roughness Ra about 0.01 mm with a diameter of 9.53 mm. The SAE 52100 steel has a hardness of HRC62-63 and its chemical composition is given in Table 2.The operating conditions were as follows. The stroke was 1 mm; frequency of oscillation was 25 Hz and the sliding time was 30 min. The load was 200 N. Prior to tribological testing, the surfaces of the discs were polished with 1200-grit emery paper. All discs were cleaned in an ultrasonic bath with acetone and then ethanol for 20 min, and then dried with hot air. The cross section of the worn scar of the discs was measured using a surface profilometer. The wear volume was calculated using formula in Eq. (3), where Vd is the wear volume, S is the area of
3.1. Microstructure MoSi2 coatings that have 6, 8, 10, 12 and 16 wt.% content of MoS2 are obtained. But coating that has zero MoS2 is not obtained due to the coating is broken and delaminated from the substrate by the thermal stress in the processing. This is coincident with the result in the literature [1]. Fig. 2 is the XRD result of the coating with 10 wt.% MoS2. It can be seen that the coating is composed of MoSi2 and MoS2 phase. MoSi2 phase has two crystal structure, one is tetragonal MoSi2 and the other is hexagonal (MoSi2)9H. (MoSi2)9H is a non-equilibrium phase. No other phases were detected in the coating using XRD, indicating that the coating prepared in the current work had high purity. Fig. 3 gives the XRD result of the top black layer on the coating with 10 wt.% MoS2, which shows the black layer, consist of u-Al2O3 and (Al2O3)d. These oxides are presumed to be the by-products in the coating fabrica-
Fig. 3. XRD result of the black layer on top of the coating with 10 wt.% MoS2.
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Fig. 4. OM micrographs of coating with 8 wt.% MoS2 (a), 10 wt.% MoS2 coating (b) and (c) and 12 wt.% MoS2 coating (d).
tion process, and since they did not bind with the coating, they would not have effect on the properties of the coating. OM micrographs of coatings with 8, 10 and 12 wt.% MoS2 are shown in Fig. 4, where the black phase is MoS2 and the white phase is MoSi2. The MoS2 phase is sphere or lathing in appearance and has a uniform distribution in the MoSi2 matrix. The size of the MoS2 phase is in the range of 5 – 30 mm. No pores and cracks are visible in these coatings. Phase size becomes large in the coating with MoS2 content increases. Two phases of the matrix can be seen in the Fig. 4 (c), the white is MoSi2 and the gray is (MoSi2)9H. SEM micrograph of coating with 10 wt.% is given in Fig. 5(a). It can be seen small MoS2 phase is in the MoSi2 grain and large MoSi2 phase is in the grain boundary [4]. The interface between the MoS2 phase and MoSi2 is clear in Fig. 5(b). It shows the interface bonding between the reinforcement and the matrix is well. However, cracks are observed in coating with 16 wt.% MoS2, which is shown in Fig. 6. For this reason, coatings that have more than 16 wt.% MoS2 are not continued to fabricate in these experiments. The interface morphology of coatings with 6 and 12 wt.% MoS2 from OM is shown in Fig. 7, where the section at the left refers to the coating; and the right side to the substrate. It shows no signs of cracks or pores near the
interface region. There is an interface phases region in which phases are different from in the bulk coating and the steel substrate. It is about 100 mm in the width. The interface region of coating with 12 wt.% MoS2 is wider than that of 6 wt.% MoS2. The interface microstructure of coating with 12 wt.% MoS2 and the distribution of elements at sides of the interface from EPMA are given in Fig. 8. It can be seen Fe element have diffused into the coating.
3.2. Microhardness and tribological properties The microhardness of the coatings with 6–12 wt.% MoS2 are given in Fig. 9. Hardness of the coatings with 6–12 wt.% MoS2 is three times larger than the steel substrate. The variation of the hardness of coating with 10 wt.% MoS2 with the distance to the interface is given in Fig. 10. It can be seen that hardness increases with the distance increases. The wear rate of the coatings with 6–12 wt.% MoS2 and the steel substrate is shown in Fig. 11. The friction of the coatings with 6–12 wt.% MoS2 is given in Fig. 12. It can be seen wear rate and friction of the coatings is great lower than the steel substrate. Since there are cracks in the coating hardness and tribological properties of the coating with 16 wt.% MoS2 is not measured in the experiment.
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4. Discussion
4.1. Mechanism of the processing When the temperature in the reactor reached 673 K the igniter was ignited and released thermal energy, which in turn ignited the reactants. Then the combus-
Fig. 7. Interface micrographs of coatings with 6 wt.% MoS2 (a) and 12 wt.% MoS2 (b) from OM.
tion wave of the reaction propagated from the top to the bottom of the reactant compact. The reactants transformed to the products where the combustion wave had passed. The adiabatic temperature (Tad) of the reaction (1) and (2) at 673 K is calculated to be approximately 3700 and 4500 K, respectively, using the Eqs. (5) and (6). Fig. 5. SEM micrograph of coating with 10 wt.% MoS2.
DH $f,673 K =
&
Tm
673
CpsdT +
&
Tad
CpldT +DHm,
(5)
Tm
DH $f,673 K = DH $f,298 K + 673 298DCpdT. (6)
Fig. 6. Cracks in coating with 16 wt.% MoS2.
In the equations, DCp is the difference between the heat capacity of the products and the reactants; Cps and Cpl are solid and liquid state heat capacities of the products respectively, and DHm is the melting enthalpy of the products [14–16]. In these calculations, it is assumed that all the heat generated by the reaction goes only to raise the temperature of the products and there is no loss of heat to the surroundings, meaning it is a ‘closed system’. It is also assumed that the difference between the heat capacity of the products and the reactants, due to the rise in temperature, is zero (Neumann-Kopp rule) [14,15]. In addition, it is assumed that the reaction goes to completion. Thus Tad is only a measure of the exothermicity of the reaction and defines the upper limit for any combustion system [14].
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Fig. 8. Interface micrographs of coating with 12 wt.% MoS2 and distribution of elements at the interface from EPMA.
According to the melting point of the products that is given in Table 3, the adiabatic temperature of the reaction (1) and (2) is considerably higher than the melting point of the products; which are therefore in a liquid state. Since liquid Al2O3 has a lower density, it will separate and float to the top of the liquid MoSi2 and MoS2 [11]. Therefore Al2O3 is not detected in the coatings by XRD. It is probable that liquid MoS2 is miscible liquid MoSi2 at the high temperature [17,18], thus liquid MoS2 is dispersed into liquid MoSi2. Therefore liquid MoSi2MoS2 composite is formed and MoS2 is not detected in the by-products by XRD. When the above processing is ended, the liquid MoSi2MoS2 is in contact with the steel substrate. Fe on the steel substrate surface (Tm 1807 K) will be melted by the superheated liquid MoSi2MoS2. As there is a difference between Fe atom concentration of the two liquids, Fe atoms will diffuse into the liquid MoSi2MoS2. Diffusion of atoms will improve the wettability between the liquid MoSi2MoS2 and the steel substrate. Therefore, the liquid drop will gradually spread on the steel substrate [11,12,21]. Diffusion of atoms at the interface will continue until the coating and the steel substrate surface start to solidify. Thus a metallurgical transitional zone between the steel substrate and the MoSi2MoS2 material is formed [12,13]. When the liquid material in the transitional zone solidified on the steel substrate surface and
Fig. 9. Microhardness of the coatings and the steel substrate.
Fig. 10. Variation of microhardness of coating with10 wt.% MoS2 with the distance to the interface.
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Fig. 11. Wear rate of the coatings and the steel substrate.
Fig. 12. Friction of the coatings and the steel substrate.
so did the liquid MoSi2MoS2 on the solid material in the transitional zone, the coating is fabricated. Thus the solid MoSi2MoS2 material is bonded with the steel substrate by the interface material in the metallurgical transitional zone and a metallurgically bonded interface is formed.
4.2. Microstructure The liquid MoSi2 solidified under gas pressure and this results in the coatings being dense and the interface free from pores. The hexagonal (MoSi2)9H phase is resulted by the rapid solidification of liquid MoSi2 in the processing [14,19]. When the temperature of the coatings is lower than the melting point of liquid MoSi2, it will crystallize first. MoS2 phase will be trapped or pushed by the crystallizing front according to the size [22]. Thus small MoS2 phase in the MoSi2
Fig. 13. Agglomeration of MoS2 phase in the coating with12 wt.% MoS2.
grain and large MoS2 phase in the MoSi2 grain boundary. When the temperature of the coatings is lower than 1508 K, MoS2 will crystallize. MoS2 phase in the coatings becomes large with the content is attributed to the agglomerate of MoS2 phase in the coatings. It can be seen in Fig. 13. Since the coatings are bonded with the substrate by strong metallurgical bonding, any thermal stress is less likely to generate cracks at the interface when the content of MoS2 in the coatings is 6–12 wt.%. The diffusion of Fe into the liquid MoSi2 adjacent to the interface had changed the composition of liquid MoSi2 and new phases of MoSiFe appears at the interface. The width of interface phases relates to the extent of the atom diffusion, which is dominated by the time how long the MoSi2MoS2 is in liquid state. The time of MoSi2MoS2 in the liquid state is determined by the exothermic energy of the reactants in the processing. The exothermic energy varies with the content of MoS2 in the coatings, which is shown in Fig. 14. It can be seen the exothermic energy in the fabrication of coating with 12 wt.% is larger than that of coating with 6 wt.% MoS2. Therefore coating with 12 wt.% MoS2 has the longest liquid keeping time and the coating with 6 wt.% MoS2 has the shortest. This will result the interface phases width of coating with 12 wt.% MoS2 is wider than that of coating with 6 wt.% MoS2.
Table 3 Density and melting point of the products [16] Products
MoS2
MoSi2
Al2O3
Density (kg m−3) Melting point (K)
4800 1458
6310 2073
3970 2228
Fig. 14. Variation of the reactant exothermic energy with the content of MoS2 in the coatings.
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4.3. Microhardness and tribological properties
5. Conclusions
Toughness of the pure MoSi2 is poor at low temperature [1,4]. A large thermal stress will be produced at the interface in the coating fabrication due to the difference of thermal expansion efficient between the coating and the steel substrate and phase transformation [1]. Therefore pure MoSi2 coating is broken and delaminated from the steel substrate by the thermal stress in the processing and the coating can not be obtained [1]. It is considered that the mechanical properties of composite material are determined by the volume, distribution and size of second phase in the matrix and interface bonding and mechanical properties of the matrix [2,4,20]. In the coatings with 6 –12 wt.% MoS2, MoS2 is uniformly distributed in the matrix and there is a good bonding between the second phase and the matrix due to in situ synthesizing in the coatings and MoS2 is a tough phase. Therefore, toughness of MoSi2 can be improved by adding MoS2. The coatings can not be broken and delaminated from the steel substrate by the thermal stress in the processing. Thus the coatings are fabricated. However, when the content of MoS2 in the coatings is higher than 12 wt.%, size of the second phase becomes large, the reinforcing effect of MoS2 to MoSi2 toughness is decreased. In coating with 16 wt.% MoS2, the second phase is so large that it has little reinforcing effect to the matrix and cracks can be resulted in the coating by the thermal stress in the processing. MoS2 is a soft phase and MoSi2 is a hard phase, therefore hardness of the coatings is lower than pure MoSi2 and higher than the steel substrate. Low friction of the coatings with 6 – 12 wt.% MoS2 is due to the low friction of the second phase in the coatings. Since the coatings with 6 – 12 wt.% MoS2 have high hardness, wear resistance of the coatings is better than that of the steel substrate. The tribological mechanism of the coatings will be investigated in detail in elsewhere. Hardness of the coating adjacent to the interface is determined by its microstructure. From the Fe element distribution, which is given in Fig. 8, it can be found the content of Fe in the coating decreases with the distance to the interface increase. Hardness of MoSi2Fe increases with the content of Fe decreasing [23]. Therefore hardness of the coating increases with the distance to the interface increase.
It has been shown in this study that MoSi2 coatings with 6–16 wt.% MoS2 can be successfully prepared on a steel substrate by the SHS casting route. Coatings with 6–12 wt.% MoS2 have a dense microstructure and were fully bonded with the substrate by the metallurgical transition zone. Microstructure of the coatings is related to the content of MoS2 in the coatings. The coatings with 6–12 wt.% MoS2 had low friction, good wear resistance and high hardness.
.
References [1] A. Hidouci, J.M. Pelletier, Mater. Sci. Eng. A252 (1998) 17–26. [2] A.H. Bartlett, R.G. Castro, J. Mater. Sci. 33 (1998) 1653–1660. [3] H. Kung, Y.C. Lu, A.H. Bartlett, R.G. Castro, J.J. Pectrovic, E. Shtessel, J. Mater. Res. 13 (6) (1998) 1522 – 1528. [4] D.G. Morris, M. Leboeuf, M.A. Morris, Mater. Sci. Eng. A251 (1998) 262 – 268. [5] J. Pan, M.K. Surappa, R.A. Saravanan, B.W. Liu, D.M. Yang, Mater. Sci. Eng. A244 (1998) 191 – 198. [6] I.J. Shon, Z.A. Munir, Mater. Sci. Eng. A202 (1995) 256–261. [7] G. Zhang, X.M. Yue, T. Watanabe, J. Mater. Sci. 34 (1999) 997 – 1001. [8] K.D. Cooper, Metals Handbook, 10th edition, The Materials Information Society, USA, 1990, pp. 861 – 877. [9] Y. Waku, N. Nakagawa, T. Wakamoto, H. Ohtsubo, K. Shimizu, Y. Kohtoku, Nature 389 (4) (1997) 49 – 52. [10] D.F. Moore, Principles and Applications of Tribology, Pergamon Press, Oxford, 1975, p. 88. [11] L. Lin, Q. Xue, Int. J. SHS 2 (1995) 171 – 181. [12] P. La, M. Bai, Q. Xue, W. Liu, Surf. Coat. Technol. 113 (1-2) (1999) 44 – 51. [13] V.I. Yukhvid, Pure Appl. Chem 7 (1992) 977 – 988. [14] J. Subrahmanyam, M. Vijayakumar, J. Mater. Sci. 27 (1992) 6249 – 6273. [15] J.P. Lebrat, A. Varma, A.E. Miller, Metall. Trans. A 1 (1992) 68 – 72. [16] J.A. Dean, Lange’s Handbook of Chemistry, 11th edition, McGraw-Hill, New York, 1972. [17] U. Anselmi-Tamburini, M. Arimondi, F. Maglia, G. Spinolo, J. Am. Ceram. Soc. 81 (7) (1998) 1765 – 1772. [18] L. Ratake, S. Diefenbach, Mater. Sci. Eng. R 15 (1995) 263–347. [19] P.S. Frankwicz, J.H. Perepezko, Mater. Sci. Eng. A246 (1998) 199 – 206. [20] P.K. Rohatgi, Y. Liu, S. Ray, Metals Handbook, 10th edition, The Materials Information Society, USA, 1990, pp. 801–811. [21] H.D. Li, J.M. Xiao, Surface and Interface of Material, Tsinghua University press, Beijing, 1990 In Chinese. [22] J.M Howe, Int. Mater. Rev. 38 (1993) 233. [23] C.T. Liu, J.O. Stiegler, F.H. (Sam) Froes, Metals Handbook, 10th ed., The Materials Information Society, USA, 1990, pp. 913–923.
A study of MoSi2MoS2 coatings fabricated by SHS casting route Peiqing La *, Qunji Xue, Weimin Liu Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China Received 1 June 1999; received in revised form 14 June 1999
Abstract Coatings were fabricated on a steel substrate by a self-propagating high temperature synthesis (SHS) casting route. The processing is described in detail. The phases in the coating were examined using X-ray diffraction (XRD). The microstructures of the coatings were analyzed with optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalyzer (EPMA). Tribological properties and microhardness of the coatings with 6 – 12 wt.% MoS2 were measured. The experimental results show that the coatings were composed of MoSi2 and MoS2 phases and coatings with 6 – 12 wt.% MoS2 had dense microstructure. Metallurgical bonding had formed between the coatings and the substrate. Friction of the coatings with 6–12 wt.% MoS2 is lower than its steel substrate. Microhardness and wear resistance of the coatings with 6 – 12 wt.% MoS2 are higher than that of the steel substrate. Near the interface microhardness of the coating varies with the distance to the interface. © 2000 Elsevier Science S.A. All rights reserved. Keywords: MoSi2MoS2 coatings; Self-propagating high temperature synthesis (SHS) casting route; Microstructure; Mcrohardness; Tribological properties
1. Introduction The intermetallic compound molybdenum-disilicide (MoSi2) has been the focus of considerable attention as an attractive material at high temperature applications. Its properties provide a desirable combination of a high melting temperature (2293 K), high oxidation resistance and a relatively low density (6.25 g cm − 3) [1–7]. In addition, MoSi2 possesses high hardness and high elastic modulus, which ensures a good wear resistance [5]. Therefore, when MoSi2 coating on steel substrate is prepared wear properties and resistance to oxidation and corrosion of the steel substrate will be dramatically improved [1,8]. However, MoSi2 has low-temperature brittle behavior. A. Hidouci and J.M. Pelletier have fabricated MoSi2 coating on steel substrate from Mo and Si powders or MoSi2 powders by laser processing [1]. But the coatings contain a large number of cracks due to the brittleness of MoSi2 and the thermal stresses produced in the processing. Therefore, properties of the coatings are limited and application of the coatings is * Corresponding author. Tel.: +86-931-8417088. E-mail address: [email protected] (P. La)
restricted. It is well documented that an improved toughening for MoSi2 is possible through the incorporation of second phase reinforcements [2–7,9]. Therefore, MoS2 is attempted to be in situ synthesized in the MoSi2 coating as a reinforcement to increase its toughness and obtain a MoSi2 coating in this research. On the other hand, as a solid lubricant the addition of MoS2 in the coating is expected to bring a low friction to the coatings [10]. To our knowledge, there are few literatures about MoSi2MoS2 system and its coating [1–7]. SHS (self-propagating high temperature synthesis) casting route is a novel technology for fabricating coatings, the SHS coating method combines materials preparation with coating fabrication in one step [11– 13]. As a result, the cost for coating fabrication can be decreased. Furthermore, thick coatings could be fabricated by using this processing method [11–13]. Therefore, SHS casting coating method is used to fabricate MoSi2MoS2 coatings. The purpose of the present work is in an attempt to fabricate MoSi2MoS2 coatings by SHS casting coating method and examine the mechanism of the processing, microstructure and properties of the coatings.
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 9 9 ) 0 0 5 4 6 - 8
P. La et al. / Materials Science and Engineering A277 (2000) 266–273
2. Experiments
2.1. Fabrication of the coatings Commercial carbon steel 1045 was machined to the size of F60 × 5 mm and used as the substrate. Reaction (1) and (2) are used to produce MoSi2 and MoS2, respectively. Reactant powders of reaction (1) and (2) were weighted according to the designing weight content of MoS2 in the MoSi2 coating. The powders were dry-mixed for 8 h in a ball-mixing mill with Al2O3 spheres in order to reduce the amount of agglomeration as well as to provide a homogeneous mixture of the reactants. MoO3 + 2Al+2Si= MoSi2 +Al2O3 DH °,f 298 K = − 1051.7 (kJ mol − 1), MoO3 + 2Al+2S= MoS2 +Al2O3
(1)
DH °,f 298 K = − 1170.2 (kJ mol − 1). (2) The characteristics of the reactant powders are given in Table 1. Before coating, the substrate was cleaned with acetone in an ultrasonic cleaner for 10 min. After drying with hot air, the substrate was positioned at the bottom of a steel tube with refractory surroundings. In order to generate a steady reaction wave in the SHS processing and reduce gases in the reactants, the reactant powder must be compacted [14,15]. For this purpose, the assembled reactants were cold-pressed under a uniaxial pressure of 20 MPa. An igniting agent composed of Al3 + , S2 − , Mn4 + compound was then put on the surface of the reactant [12]. This can be ignited at a temperature of 673 K with considerable heat release. This assembled sample was placed in a reactor as schematically illustrated in Fig. 1. Table 1 Properties of the raw material powders Powder
Size (mesh)
Purity (%)
Impurity
MoO3 Al S Si
B200 100–200 B200 B200
\99 \99 \99 \99
Pb, Cl, PO4, SO4, NO3 Fe, Si, Cu, H2O
Fig. 1. Sketch of the SHS coating fabrication apparatus.
267
After cleaning with argon gas at room temperature, the reactor was heated to 573 K and held there for 1 h to remove gases on the surface of the reactant powders. After 1 h of incubation time, the reactor was cleaned again by argon gas and then 5 MPa-argon gas was introduced into the reactor for the pressing of the products when the combustion synthesis reactions ended [11,12]. The reactor was then heated and when the temperature reached 673 K, the igniter reaction started. The reactants powder compact was then ignited by the heat released from the igniter. The gas pressure was then maintained for 60 min after which, the resultant sample was allowed to cool to ambient temperature in the reactor. The sample was then removed for examination. It was observed with the naked eye that a black layer was present on the top of the target sample. This layer was not bonded with the underlying material and could be removed by hand, to reveal a coating on the steel substrate that was determined to be 2–3 mm in thickness. Specimens with dimensions of 12× 12×6 mm were cut from the typical samples for XRD and microstructure investigation.
2.2. In6estigation of the microstructure The coating region of the specimens and material from the black layer on top of the coating were analyzed using Shimazu Dmax-RB X-ray diffraction (XRD). The microstructure of the coatings and the interface between the substrate and the coating were examined with scanning electron microcopy (SEM) and optical microcopy (OM). Element distribution at the interface was determined by using EPM-810Q electron probe microanalyzer (EPMA). Prior to the OM, SEM and EPMA examination, the specimens of the coatings were polished using colloidal silica (0.05 mm) and etched with a 10% HNO3 ethanol mixture.
2.3. Examination of the microhardness and tribological properties The microhardnesses of the coatings and the steel substrate were measured by using an AVKSHI MVK-1 hardness tester. The load was 20 gf, loading speed was 0.034 mm s − 1 and dwell time was 30 s. Three tests were made for each material and the test deviation was no more than 5%. The tribological tests were conducted at room temperature (23°C) using a ball-on-disc apparatus under reciprocating sliding conditions and lubricating with chemically pure liquid paraffin with a viscosity of 30.19 cSt (at 25°C) and a boiling point higher than 300°C. About 1 ml of the lubricant was added to the disk surface before test. The coatings samples were machined as discs with a diameter of 24 mm and thickness of 6 mm. Samples of
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Table 2 Chemical composition of the SAE52100 steel Elements
C
Si
Mn
Fe
Content
0.42–0.50
0.15–0.35
0.2–0.4
Balance
cross-section and L is the length of the stroke. The volumetric wear rate W was calculated using Eq. (4), where s is the total sliding distance. The friction coefficient is continuously recorded by an X-Y recorder system. The friction coefficient values reported in this paper are to be considered nominal values that are representative of the predominant behavior during the majority of each run. Three tests were made for each material and the test deviation is less than 6%. The wear property and friction coefficient of the steel substrate is measured at the same condition as a comparison. Vd = SL.
(3)
W= Vd s − 1.
(4)
3. Results Fig. 2. XRD result of coating with 10 wt.% MoS2.
the steel substrate were machined as discs as above, and served for comparative investigations of tribological properties. The counterpart was a SAE52100 steel ball of hardness HRC62-63 and surface roughness Ra about 0.01 mm with a diameter of 9.53 mm. The SAE 52100 steel has a hardness of HRC62-63 and its chemical composition is given in Table 2.The operating conditions were as follows. The stroke was 1 mm; frequency of oscillation was 25 Hz and the sliding time was 30 min. The load was 200 N. Prior to tribological testing, the surfaces of the discs were polished with 1200-grit emery paper. All discs were cleaned in an ultrasonic bath with acetone and then ethanol for 20 min, and then dried with hot air. The cross section of the worn scar of the discs was measured using a surface profilometer. The wear volume was calculated using formula in Eq. (3), where Vd is the wear volume, S is the area of
3.1. Microstructure MoSi2 coatings that have 6, 8, 10, 12 and 16 wt.% content of MoS2 are obtained. But coating that has zero MoS2 is not obtained due to the coating is broken and delaminated from the substrate by the thermal stress in the processing. This is coincident with the result in the literature [1]. Fig. 2 is the XRD result of the coating with 10 wt.% MoS2. It can be seen that the coating is composed of MoSi2 and MoS2 phase. MoSi2 phase has two crystal structure, one is tetragonal MoSi2 and the other is hexagonal (MoSi2)9H. (MoSi2)9H is a non-equilibrium phase. No other phases were detected in the coating using XRD, indicating that the coating prepared in the current work had high purity. Fig. 3 gives the XRD result of the top black layer on the coating with 10 wt.% MoS2, which shows the black layer, consist of u-Al2O3 and (Al2O3)d. These oxides are presumed to be the by-products in the coating fabrica-
Fig. 3. XRD result of the black layer on top of the coating with 10 wt.% MoS2.
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Fig. 4. OM micrographs of coating with 8 wt.% MoS2 (a), 10 wt.% MoS2 coating (b) and (c) and 12 wt.% MoS2 coating (d).
tion process, and since they did not bind with the coating, they would not have effect on the properties of the coating. OM micrographs of coatings with 8, 10 and 12 wt.% MoS2 are shown in Fig. 4, where the black phase is MoS2 and the white phase is MoSi2. The MoS2 phase is sphere or lathing in appearance and has a uniform distribution in the MoSi2 matrix. The size of the MoS2 phase is in the range of 5 – 30 mm. No pores and cracks are visible in these coatings. Phase size becomes large in the coating with MoS2 content increases. Two phases of the matrix can be seen in the Fig. 4 (c), the white is MoSi2 and the gray is (MoSi2)9H. SEM micrograph of coating with 10 wt.% is given in Fig. 5(a). It can be seen small MoS2 phase is in the MoSi2 grain and large MoSi2 phase is in the grain boundary [4]. The interface between the MoS2 phase and MoSi2 is clear in Fig. 5(b). It shows the interface bonding between the reinforcement and the matrix is well. However, cracks are observed in coating with 16 wt.% MoS2, which is shown in Fig. 6. For this reason, coatings that have more than 16 wt.% MoS2 are not continued to fabricate in these experiments. The interface morphology of coatings with 6 and 12 wt.% MoS2 from OM is shown in Fig. 7, where the section at the left refers to the coating; and the right side to the substrate. It shows no signs of cracks or pores near the
interface region. There is an interface phases region in which phases are different from in the bulk coating and the steel substrate. It is about 100 mm in the width. The interface region of coating with 12 wt.% MoS2 is wider than that of 6 wt.% MoS2. The interface microstructure of coating with 12 wt.% MoS2 and the distribution of elements at sides of the interface from EPMA are given in Fig. 8. It can be seen Fe element have diffused into the coating.
3.2. Microhardness and tribological properties The microhardness of the coatings with 6–12 wt.% MoS2 are given in Fig. 9. Hardness of the coatings with 6–12 wt.% MoS2 is three times larger than the steel substrate. The variation of the hardness of coating with 10 wt.% MoS2 with the distance to the interface is given in Fig. 10. It can be seen that hardness increases with the distance increases. The wear rate of the coatings with 6–12 wt.% MoS2 and the steel substrate is shown in Fig. 11. The friction of the coatings with 6–12 wt.% MoS2 is given in Fig. 12. It can be seen wear rate and friction of the coatings is great lower than the steel substrate. Since there are cracks in the coating hardness and tribological properties of the coating with 16 wt.% MoS2 is not measured in the experiment.
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4. Discussion
4.1. Mechanism of the processing When the temperature in the reactor reached 673 K the igniter was ignited and released thermal energy, which in turn ignited the reactants. Then the combus-
Fig. 7. Interface micrographs of coatings with 6 wt.% MoS2 (a) and 12 wt.% MoS2 (b) from OM.
tion wave of the reaction propagated from the top to the bottom of the reactant compact. The reactants transformed to the products where the combustion wave had passed. The adiabatic temperature (Tad) of the reaction (1) and (2) at 673 K is calculated to be approximately 3700 and 4500 K, respectively, using the Eqs. (5) and (6). Fig. 5. SEM micrograph of coating with 10 wt.% MoS2.
DH $f,673 K =
&
Tm
673
CpsdT +
&
Tad
CpldT +DHm,
(5)
Tm
DH $f,673 K = DH $f,298 K + 673 298DCpdT. (6)
Fig. 6. Cracks in coating with 16 wt.% MoS2.
In the equations, DCp is the difference between the heat capacity of the products and the reactants; Cps and Cpl are solid and liquid state heat capacities of the products respectively, and DHm is the melting enthalpy of the products [14–16]. In these calculations, it is assumed that all the heat generated by the reaction goes only to raise the temperature of the products and there is no loss of heat to the surroundings, meaning it is a ‘closed system’. It is also assumed that the difference between the heat capacity of the products and the reactants, due to the rise in temperature, is zero (Neumann-Kopp rule) [14,15]. In addition, it is assumed that the reaction goes to completion. Thus Tad is only a measure of the exothermicity of the reaction and defines the upper limit for any combustion system [14].
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Fig. 8. Interface micrographs of coating with 12 wt.% MoS2 and distribution of elements at the interface from EPMA.
According to the melting point of the products that is given in Table 3, the adiabatic temperature of the reaction (1) and (2) is considerably higher than the melting point of the products; which are therefore in a liquid state. Since liquid Al2O3 has a lower density, it will separate and float to the top of the liquid MoSi2 and MoS2 [11]. Therefore Al2O3 is not detected in the coatings by XRD. It is probable that liquid MoS2 is miscible liquid MoSi2 at the high temperature [17,18], thus liquid MoS2 is dispersed into liquid MoSi2. Therefore liquid MoSi2MoS2 composite is formed and MoS2 is not detected in the by-products by XRD. When the above processing is ended, the liquid MoSi2MoS2 is in contact with the steel substrate. Fe on the steel substrate surface (Tm 1807 K) will be melted by the superheated liquid MoSi2MoS2. As there is a difference between Fe atom concentration of the two liquids, Fe atoms will diffuse into the liquid MoSi2MoS2. Diffusion of atoms will improve the wettability between the liquid MoSi2MoS2 and the steel substrate. Therefore, the liquid drop will gradually spread on the steel substrate [11,12,21]. Diffusion of atoms at the interface will continue until the coating and the steel substrate surface start to solidify. Thus a metallurgical transitional zone between the steel substrate and the MoSi2MoS2 material is formed [12,13]. When the liquid material in the transitional zone solidified on the steel substrate surface and
Fig. 9. Microhardness of the coatings and the steel substrate.
Fig. 10. Variation of microhardness of coating with10 wt.% MoS2 with the distance to the interface.
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Fig. 11. Wear rate of the coatings and the steel substrate.
Fig. 12. Friction of the coatings and the steel substrate.
so did the liquid MoSi2MoS2 on the solid material in the transitional zone, the coating is fabricated. Thus the solid MoSi2MoS2 material is bonded with the steel substrate by the interface material in the metallurgical transitional zone and a metallurgically bonded interface is formed.
4.2. Microstructure The liquid MoSi2 solidified under gas pressure and this results in the coatings being dense and the interface free from pores. The hexagonal (MoSi2)9H phase is resulted by the rapid solidification of liquid MoSi2 in the processing [14,19]. When the temperature of the coatings is lower than the melting point of liquid MoSi2, it will crystallize first. MoS2 phase will be trapped or pushed by the crystallizing front according to the size [22]. Thus small MoS2 phase in the MoSi2
Fig. 13. Agglomeration of MoS2 phase in the coating with12 wt.% MoS2.
grain and large MoS2 phase in the MoSi2 grain boundary. When the temperature of the coatings is lower than 1508 K, MoS2 will crystallize. MoS2 phase in the coatings becomes large with the content is attributed to the agglomerate of MoS2 phase in the coatings. It can be seen in Fig. 13. Since the coatings are bonded with the substrate by strong metallurgical bonding, any thermal stress is less likely to generate cracks at the interface when the content of MoS2 in the coatings is 6–12 wt.%. The diffusion of Fe into the liquid MoSi2 adjacent to the interface had changed the composition of liquid MoSi2 and new phases of MoSiFe appears at the interface. The width of interface phases relates to the extent of the atom diffusion, which is dominated by the time how long the MoSi2MoS2 is in liquid state. The time of MoSi2MoS2 in the liquid state is determined by the exothermic energy of the reactants in the processing. The exothermic energy varies with the content of MoS2 in the coatings, which is shown in Fig. 14. It can be seen the exothermic energy in the fabrication of coating with 12 wt.% is larger than that of coating with 6 wt.% MoS2. Therefore coating with 12 wt.% MoS2 has the longest liquid keeping time and the coating with 6 wt.% MoS2 has the shortest. This will result the interface phases width of coating with 12 wt.% MoS2 is wider than that of coating with 6 wt.% MoS2.
Table 3 Density and melting point of the products [16] Products
MoS2
MoSi2
Al2O3
Density (kg m−3) Melting point (K)
4800 1458
6310 2073
3970 2228
Fig. 14. Variation of the reactant exothermic energy with the content of MoS2 in the coatings.
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4.3. Microhardness and tribological properties
5. Conclusions
Toughness of the pure MoSi2 is poor at low temperature [1,4]. A large thermal stress will be produced at the interface in the coating fabrication due to the difference of thermal expansion efficient between the coating and the steel substrate and phase transformation [1]. Therefore pure MoSi2 coating is broken and delaminated from the steel substrate by the thermal stress in the processing and the coating can not be obtained [1]. It is considered that the mechanical properties of composite material are determined by the volume, distribution and size of second phase in the matrix and interface bonding and mechanical properties of the matrix [2,4,20]. In the coatings with 6 –12 wt.% MoS2, MoS2 is uniformly distributed in the matrix and there is a good bonding between the second phase and the matrix due to in situ synthesizing in the coatings and MoS2 is a tough phase. Therefore, toughness of MoSi2 can be improved by adding MoS2. The coatings can not be broken and delaminated from the steel substrate by the thermal stress in the processing. Thus the coatings are fabricated. However, when the content of MoS2 in the coatings is higher than 12 wt.%, size of the second phase becomes large, the reinforcing effect of MoS2 to MoSi2 toughness is decreased. In coating with 16 wt.% MoS2, the second phase is so large that it has little reinforcing effect to the matrix and cracks can be resulted in the coating by the thermal stress in the processing. MoS2 is a soft phase and MoSi2 is a hard phase, therefore hardness of the coatings is lower than pure MoSi2 and higher than the steel substrate. Low friction of the coatings with 6 – 12 wt.% MoS2 is due to the low friction of the second phase in the coatings. Since the coatings with 6 – 12 wt.% MoS2 have high hardness, wear resistance of the coatings is better than that of the steel substrate. The tribological mechanism of the coatings will be investigated in detail in elsewhere. Hardness of the coating adjacent to the interface is determined by its microstructure. From the Fe element distribution, which is given in Fig. 8, it can be found the content of Fe in the coating decreases with the distance to the interface increase. Hardness of MoSi2Fe increases with the content of Fe decreasing [23]. Therefore hardness of the coating increases with the distance to the interface increase.
It has been shown in this study that MoSi2 coatings with 6–16 wt.% MoS2 can be successfully prepared on a steel substrate by the SHS casting route. Coatings with 6–12 wt.% MoS2 have a dense microstructure and were fully bonded with the substrate by the metallurgical transition zone. Microstructure of the coatings is related to the content of MoS2 in the coatings. The coatings with 6–12 wt.% MoS2 had low friction, good wear resistance and high hardness.
.
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