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Tribological properties of NiAl produced by pressure-assisted combustion synthesis
Wear 265 (2008) 979–985
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Wear journal homepage: www.elsevier.com/locate/wear
Tribological properties of NiAl produced by pressure-assisted combustion synthesis O. Ozdemir, S. Zeytin, C. Bindal ∗ Sakarya University, Engineering Faculty, Department of Metallurgical and Materials Engineering, Esentepe Campus, 54187 Sakarya, Turkey
a r t i c l e
i n f o
Article history: Received 29 September 2006 Received in revised form 16 January 2008 Accepted 18 February 2008 Available online 15 April 2008 Keywords: Friction Wear Intermetallics NiAl-based alloy Pressure-assisted combustion synthesis
a b s t r a c t In this study, aluminum powder with 15 m size and carbonyl-nickel powder with 4–7 m size having 99% and 99.8% purity, respectively were used in order to produce NiAl by volume combustion synthesis. The production of intermetallic compound was carried out in an electrical resistance furnace in open air under a uniaxial pressure of 150 MPa at 1050 ◦ C for 1 h. The formation temperature of NiAl intermetallic compound was determined to be 655 ◦ C by differential scanning calorimeter (DSC) analysis. The presence of NiAl was confirmed by X-ray diffraction (XRD) analysis. Optical and scanning electron microscope (SEM) analysis revealed that NiAl phase has very low porosity. The relative density of the samples measured according to based on Archimedes principle was 99.6%. The microhardness of the samples measured by Vickers indenter was approximately 367 ± 17 HV1.0 . The effect of load on wear rate in the case of using Al2 O3 ball as a counterface material was also investigated using a ball-on-disc test method. The coefficient of friction for NiAl intermetallic compound run under 2 N load was calculated as 0.73 while it was 0.53 run under 10 N load. Depending on load, the wear rate of NiAl sliding against alumina ball was 0.016 mm3 /N m for 2 N, 0.017 mm3 /N m for 5 N and 0.009 mm3 /N m for 10 N, respectively. The distribution of alloying elements within intermetallic compound was determined by energy-dispersive X-ray spectroscopy (EDS). © 2008 Elsevier B.V. All rights reserved.
1. Introduction The strongly ordered B2 intermetallic NiAl has a number of unique physical and mechanical properties that make it attractive for use at high temperature and in aggressive environments. Due to their high-melting point (1640 ◦ C), low density ( = 5.89 g/cm3 ), excellent corrosion and oxidation resistance, NiAl intermetallics have several potential applications including turbochargers, hightemperature dies and moulds, furnace fixtures, rollers in steel slab heating furnaces, hydroturbines, cutting tools, pistons and valves and various components within gas turbines. The primary potential application on NiAl is in turbine engines which turbine blade tips suffer from sliding type wear when in contact with surrounding gas path seals [1–5]. Intermetallics have been prepared by a variety of methods including casting, mechanical alloying gas atomization, sintering, combustion synthesis, etc. [6–8]. Combustion synthesis (CS) is a process used to form ceramics, intermetallics and their composites at operating temperatures much lower than their melting points and in very short-processing times [7,9–11]. The synthesis from
∗ Corresponding author. Tel.: +90 264 295 57 59; fax: +90 264 295 56 01. E-mail address: [email protected] (C. Bindal). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.02.005
compacted powders can be performed under the explosion mode or the propagation mode. In the former, one end of a reactant sample is heated with an igniter at a high-heating rate (hundreds of degrees per second), and once ignited, the combustion wave self-propagates thorough the sample. Whereas in the later, a reactant sample is wholly heated in a furnace equipped with resistance heating at a low-heating rate (tens of degrees per minute), and once heated to its ignition temperature, the combustion reaction becomes spontaneous and takes place everywhere in the sample [8,12]. One of the major problems reported for this technique is the high-porosity levels in the final product. It is now recognized that the presence of the transient liquid phase, by itself, is insufficient to density the product, and the CS process needs to be augmented by densification steps [11]. The main objective of present study is to conduct a preliminary investigation of the wear mechanisms acting in NiAl bulk material produced by means of pressure-assisted explosion mode under dry sliding conditions at room temperature in air atmosphere. In order to characterize the morphological features of sintered NiAl intermetallic and wear tracks, scanning electron microscope (SEM) was employed. In order to determine hardness and density of the test materials, a Vickers indenter and Archimedes technique were utilized. The effect of load on wear rate in the case of using Al2 O3 ball as a counterface material was also investigated using a ball-on-disc
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test method. It was shown that the wear rate increased linearly with the increase in load. The distribution of alloying elements within intermetallic compound was determined by energy-dispersive Xray spectroscopy (EDS). 2. Experimental procedures 2.1. Material processing and characterization NiAl compounds were produced using pressure-assisted volume combustion synthesis. The reactant powder mixtures consisted of commercial powders: Ni (99.8% purity and 3–7 m particle size); Al (99% purity and <15 m particle size). Ni and Al powders in the required stoichiometry (1:1 Ni to Al molar ratio) were mixed using a small ball mill containing Ni balls at a speed of 150 cycle/min for 10 min under Ar + 3% H2 gas medium with the addition of a small amount of ethanol (0.1%). By applying a uniaxial pressure of 150 MPa the mixture was cold pressed into a cylindrical compact in a metal mold which coated with a thin layer of boron nitride. The compact pressed in metal mold was inserted in an open-air furnace and heated to 1050 ◦ C at a heating rate of 20 ◦ C/min. The process temperature was kept constant for 60 min under pressure without using vacuum or inert gas. The diameter and height of the sintered samples were 15 and 5 mm, respectively. Further details of process are given in Ref. [13]. Scanning electron microscopy was used to examine the microstructures of the samples polished using standard metallographic techniques. In order to determine phases formed, X-ray diffraction analysis was utilized. Density measurements were made using the Archimedes (water immersion) method and microhardness measurement was made with a Vickers hardness tester. 2.2. Wear tests Friction and wear test was performed with a ball-on-disc type tribometer according to the G66 ASTM standard test method. The ball-on-disc tribometer is as shown schematically in Fig. 1. The counterface material was made of alumina ball, 9.5 mm in diameter, with a highly polished surface finish of better than 0.05 m CLA roughness. NiAl intermetallic disks were run against the ball under the loads of 2, 5 and 10 N. The wear parameters in the test were given in Table 1. The ball was firmly fixed to a stationary holder for the ball-on-disc configuration (Fig. 1). Discs were attached to a horizontal chuck driven by a variable-speed electric motor. The frictional force, monitored by a load cell attached to the ball holder, was recorded continuously. The friction coefficient was measured when steady state was reached in the wear test. The maximum compressive contact pressure in central point of the contact area was calculated from the Hertzian equation [14]. According to the Hertzian equation, maximum contact pressures of 771.1, 1046.6 and 1318.6 MPa (for disc NiAl E = 235 GPa, = 0.3; for Al2 O3 ball E = 420 GPa, = 0.24 [15,16]) were obtained at normal loads of 2, 5 and 10 N, respectively. Wear rate was measured
Fig. 1. Schematic diagram of ball-on-disc tribometer used.
primarily by volumetric (volume loss) means. Volumetric wear of the spherical ball specimens was also determined by measuring the diameter of the wear track and calculated by means of volume loss of the ball. Wear rates of samples were obtained using following equation. Width of wear scar was measured in the optical microscopy: W=
2(R + (L/2))(r 2 /2)( − sin ) sliding distance
where W is the wear rate (mm3 /m), L the wear track width (mm), R the radius of the wear scar (mm), r the radius of ball (mm) and = 2 arcsin(L/2r) (radian). 3. Experimental results 3.1. Material characterization Fig. 2 shows the differential scanning calorimeter (DSC) graph of Ni and Al powder mixture (1:1 molar ratio) under a constant heating rate of 20 ◦ C/min. These data show that the formation
Table 1 Test parameters Parameters
Selected value
Applied load (N) Velocity (m/s) Environment Temperature (◦ C) Humidity (%) Duration (min) Roughness, Ra (m) Test ball diameter (mm)
2, 5 and 10 0.1 Air 20 ± 2 65 ± 5 53 0.05 9.5
(1)
Fig. 2. DSC analysis of Ni–Al powder mixed in a molar proportion of 1:1.
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Fig. 3. (a) Optical and (b) SEM images of NiAl produced by pressure-assisted combustion synthesis.
Fig. 4. X-ray diffraction patterns of NiAl processed at 1050 ◦ C for 60 min.
Fig. 6. Variation of coefficient friction and wear rate for NiAl as a function of applied load.
of NiAl is a one step reaction with the ignition temperature of approximately 654 ◦ C which is lower than the Al melting point (Tm : 660 ◦ C) but higher than the Al-rich eutectic temperature (640 ◦ C) in the Ni–Al phase diagram [7]. The formation mechanism of intermetallic nickel aluminides strongly depends on heating rate. Several exothermic and endothermic peaks were observed at lower heating rates (5–20 ◦ C/min). This results in solid-state pre-combustion reaction (two-step reaction) between Ni and Al as well as intermediate reactions in the process of formation of nickel aluminides such as NiAl3 and Ni2 Al3 forming prior to the final products. However, both slow heating and effective heat transfer from the sample to the massive metal mold prevented the reaction from becoming self-sustaining and allowed it to remain as solid state diffusion-controlled phases. The heat loss
resulting from the heat exchange between the sample and metal mold may prevent the combustion reaction [8,17,18]. After 1 h at 1050 ◦ C the reaction is completed and the samples having a uniform NiAl structure with small amount of porosity were produced (Fig. 3). Considering the reaction that given NiAl from Ni and Al that is, Ni + Al → NiAl, the formation enthalpy of NiAl intermetallic was calculated using data of heat capacities Cp,NiAl , Cp,Ni and Cp,Al [19,6]. Using heat contents we obtained a heat formation enthalpy of −64.11 kJ/mol for NiAl which is in good agreement with literature. No phase transformation for the reactants is taken into account because the ignition temperature of reaction is lower than the melting temperature of aluminum (660 ◦ C) and nickel (1453 ◦ C). Xray diffraction (XRD) technique was used to identify the phases in the samples. The XRD pattern indicates that only the NiAl phase
Fig. 5. The variation of friction coefficient of test materials as a function of cycles for 2 N load.
Fig. 7. Specific wear rate and wear resistance vs. applied load.
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Fig. 8. SEM image and concerning profilometry traces of wear tracks for (a) 2 N, (b) 5 N and (c) 10 N.
Fig. 9. SEM image of wear tracks formed on NiAl depending on load: (a) 2 N, (b) 5 N and (c) 10 N.
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Fig. 10. (a) SEM image of worn surface of NiAl and EDS spectrum of (b) mark 1, (c) mark 2 and (d) mark 3 for sample sliding under a load of 2 N.
was present in the sample (Fig. 4). The hardness of the specimen measured by using Vickers indentation technique with a load of 10 N, was approximately 367 ± 17 HV1.0 . The density of NiAl intermetallic compound calculated by Archimede’s technique was 5.632 ± 0.0145 g/cm3 which is in good agreement with theoretical density of 5.654 g/cm3 .
3.2. Friction and wear Friction and wear tests were performed in a ball-on-disc tribometer. Wear tests applied on the test materials produced by pressure-assisted combustion synthesis revealed that the friction coefficient was changing between 0.5 and 0.8 values for long-
Fig. 11. (a) SEM image of worn surface of NiAl and EDS spectrum of (b) mark 1, (c) mark 2 and (d) mark 3 for sample sliding under a load of 5 N.
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Fig. 12. (a) SEM image of worn surface of NiAl and EDS spectrum of (b) mark 1 and (c) mark 2 for sample sliding under a load of 10 N.
duration testing. Fig. 5 shows the variation of friction coefficient of NiAl intermetallic as a function of cycle number for a load of 2 N. Fig. 6 reveals the variation of friction coefficients as a function of applied load. It’s clear that the friction coefficient decreases with increasing in load. The wear rate of NiAl sliding against alumina ball ranged from 0.031 to 0.093 mm3 /m, depending on applied load as shown in Fig. 6. The difference between wear rate and specific wear rate is coming from using of load in calculations of specific wear rate. Also, it was observed that the specific wear rate value of NiAl as a function of load was 0.016 mm3 /N m for 2 N, 0.017 mm3 /N m for 5 N and 0.009 mm3 /N m for 10 N. These results verify that the increase in loads result in decrease in the specific wear rate (Fig. 7). Very small amount of debris around the rubbing surfaces was observed. Depending on load the wear resistance of test material increases (Fig. 7). Probably, this is a result of plastic deformation due to high-load value. Figs. 8 and 9 show SEM image of wear tracks formed on the test material during wear test. Fig. 8 shows the wear tracks as well as the surface profiles of wear tracks in the NiAl intermetallic. As it can be seen in Fig. 8 wear track of the specimens run for 2 N resulted in a regular surface profile, whereas for applied load 5 and 10 N, some scattering was observed in surface profiles formed on the NiAl intermetallic. The distribution of alloying elements in rubbed surface was determined by means of EDS (Figs. 10–12). Using some variables of test materials and experimental parameters such as thermal conductivity, K, diffusivity, D, etc., bulk temperature (dTBulk ), flash temperature (dTFlash ), real contact area (Ar ), nominal area (An ), ratio (Ar /An ) and maximum contact pressure (Pmax ) were calculated and are given in Table 2. In image analysis, dark areas denote the regions which including oxygen, while bright areas show Ni–Al intermetallic compound. The variation of nickel, aluminum and oxygen detected by EDS analysis are given in Table 3. As it can be seen in Table 3 the presence of oxidation was confirmed by EDS obviously.
Table 2 The values of bulk, flash temperature, real contact area over nominal contact area and maximum contact pressure depending on applied load and some physical parameters of NiAl and testing dTBulk (◦ C) dTFlash (◦ C) Ar /An
Load (N) Variables 2 5 10
35.02 × 10−3
: 0.73; r: m 0.0019 0.0025 : 0.61; r: 47.7 × 10−3 m : 0.53; r: 60.2 × 10−3 m 0.0031
79.45 66.39 57.68
1.483 1.998 2.51
Pmax (MPa) 771.13 1046.59 1318.62
v: 0.1 m/s; DNiAl : 21.11 m2 /s(calculated) ; KAl2 O3 : 5.43 W/m K; KNiAl : 76 W/m K; HB: 3.5 GPa.
Table 3 The variation Ni, Al and O at marked points confirmed by EDS analysis Load (N)
Compositions (at.%) Marks
Ni
Al
O
2
1 2 3
54.099 52.087 44.209
45.901 47.913 38.305
– – 17.486
5
1 2 3
34.631 25.327 20.474
24.101 21.242 17.498
41.268 53.431 62.028
10
1 2
19.261 29.502
16.015 23.565
64.724 46.933
4. Discussion DSC studies revealed that the enthalpy of formation of NiAl is −64.11 kJ/mol at exothermic reaction temperature. Both optical and SEM examinations of the cross-section of NiAl intermetallic compound was dense which could be used for structural applications (Fig. 3(a) and (b)). As a matter of fact that a dense NiAl intermetallic was obtained which has relative density of 99.6% confirmed by density measurements and the hardness of the sample was approximately 367 ± 17 HV1.0 .
O. Ozdemir et al. / Wear 265 (2008) 979–985
In the samples sliding against alumina ball under different loads, steady-state wear is established for the intermetallic. The friction coefficient decreases and wear rate increases with increasing of load (Fig. 5). The above results revealed that the friction coefficient for NiAl intermetallic compound under the load of 2 N was 0.73, while it was 0.53 under the load of 10 N. Depending on the applied load, the specific wear rate of NiAl sliding against alumina ball was 0.016 mm3 /N m for 2 N, 0.017 mm3 /N m for 5 N and 0.009 mm3 /N m for 10 N, respectively. When the applied load was increased from 2 to 10 N, the specific wear rate was decreased approximately 45%. Around the rubbing surfaces small amount of debris was observed and wear track includes some abrasive scratch lines (Fig. 8). As it can be seen in Figs. 10–12, EDS analysis of the wear track formed on the worn sample showed that oxygen concentration increased on the worn surfaces. EDS analysis performed on the worn track run under 2 N revealed that bright regions exhibit the Ni and Al, while dark regions denote the oxygen in addition to Ni and Al peaks (Fig. 10(b) and (c)). It was found that the intensity of oxygen peak increases with increasing of applied load, also, the formation of cracks was detected in dark regions which rich in oxygen (Figs. 11 and 12). This indicate that wear mode of the system was oxidative and abrasive. The presence of oxygen in rubbing surface confirms that the formation of aluminum oxide owing to its lower free energy of formation compared to nickel oxide. As a matter of fact that previous study performed by Sierra and Vazquez [2] revealed that the composition of wear debris and track surface allow us to suggest a wear mechanism. During the sliding motion of NiAl against Al2 O3 ball, the NiAl suffer plastic deformation (Fig. 8) and some particles are detached from the surface. These particles remain between the NiAl and Al2 O3 surfaces. Because of the high-surface temperatures originated from the friction force, they became oxidized. Some cracks were observed in wear track (Figs. 11(a) and 12(a)). It is possible to claim that these are the consequences of hard oxides due to their inherent brittle nature. Stott [20] has claimed that the oxidation layer occurred in surface decreases the wear rate. Also, it is very important to have information about Hertzian pressure developed during wear test in this type process. Using a computer diagram and some parameters of disk and counterface material the maximum contact pressures were calculated and it was seen that as the applied load increases the maximum contact pressure increases which can cause cracks in worn surface. Additionally, as it can be seen in Table 2 as the applied load increases the dTBulk temperature, Ar /An ratio and maximum contact pressure also increases. 5. Conclusions Pressure-assisted combustion synthesis being an effective production method has been proposed in order to produce NiAl intermetallic compound. Pressure-assisted combustion synthesis performed by holding Ni–Al powder mixture at 1050 ◦ C under a pressure of 150 MPa for 60 min without using any controlled atmosphere results in formation of dense and homogeneous of single NiAl compound which this indicate that pressure-assisted combustion synthesis is a promising manufacture method for NiAl.
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The hardness and relative density of NiAl intermetallic compound were 367 HV1.0 and 99.6%, respectively and the formation exothermic temperature of the NiAl materials was calculated as 655 ◦ C. Wear studies revealed that wear mode of the system is oxidative and abrasive. The formation of aluminum oxide compared to nickel oxide is attributed to its lower free energy of formation. Acknowledgements This work was supported by Scientific Researches Project Commission of Sakarya University (contact no. 2002/29). The authors thank Sakarya University Rectorate for his help. The special appreciations are extended to technician Ersan Demir for experimental assistance at Sakarya University. References [1] J.A. Hawk, D.E. Alman, Abrasive wear behavior of NiAl and NiAl–TiB2 composites, Wear 225–229 (1999) 544–556. [2] C. Sierra, A.J. Vazquez, Dry sliding wear behavior of nickel aluminides coatings produced by self-propagating high-temperature synthesis, Intermetallics 14 (2000) 848–852. [3] J.-H. Jim, D.J. Stephenson, The sliding wear behavior of reactively hot pressed nickel aluminides, Wear 217 (1998) 200–207. [4] B.J. Johnson, F.E. Kennedy, I. Baker, Dry sliding wear of NiAl, Wear 192 (1996) 241–247. [5] A. Michalski, J. Jaroszewicz, M. Rosinski, D. Siemiaszko, NiAl–Al2 O3 composites produced by pulse plasma sintering with the participation of the SHS reaction, Intermetallics 14 (2006) 603–606. [6] L. Plazanet, F. Nardou, Reaction process during relative sintering of NiAl, J. Mater. Sci. 33 (1998) 2129–2136. [7] K. Morsi, Review: reaction synthesis processing of Ni–Al intermetallic materials, Mater. Sci. Eng. A299, Elsevier Science B.V., 2001, pp. 1–15. [8] Q. Fan, H. Chai, Z. Jin, Dissolution–precipitation mechanism of self-propagating high-temperature synthesis of mononickel aluminide, Intermetallics 9 (2001) 609–619. [9] J.H. Jin, D.J. Stephenson, The sliding wear behavior of respectively hot pressed nickel aluminides, Wear 217 (1998) 200–207. [10] D.E. Alman, Reactive sintering, Powder Metal Technologies and Application, ASM Handbook, 1998, pp. 516–521. [11] T.S. Hussey, M.J. Koczak, R.W. Smith, S.R. Kalidindi, Synthesis of nickel aluminides by vacuum plasma spraying and exothermic in-situ reactions, Mater. Sci. Eng. A229 (1997) 137–146. [12] A. Varma, A.S. Mukasyan, Combustion synthesis of advanced materials, Powder Metal Technologies and Application, Vol. 7, ASM Handbook, 1988, pp. 523–540. [13] O. Ozdemir, The Investigation of The Effect of Cobalt Addition to Ni–Al Intermetallic Materials Produced by Pressure Assisted Combustion Synthesis, PhD Thesis, Sakarya University, Institute of Science and Technology, Sakarya-Turkey, 2004. [14] http://www.tribology-abc.com/calculators/e2 1.htm. [15] G. Sauthoff, Structure and properties of nonferrous alloys, Intermetall. Mater. Sci. Technol. 8 (1996) 643–805. [16] H. Choo, M.A.M. Bourke, P.G. Nash, M.R. Daymond, Evolution of thermal residual stress in intermetallic matrix composites during heating, Ceram. Eng. Sci. Proc. ACERS 21 (3) (2000) 627–634. [17] H.X. Zhu, R. Abbaschian, Reactive processing of nickel-aluminide intermetallic compound, J. Mater. Sci. 38 (2003) 3861–3870. [18] D. Zhong, J.J. Moore, J. Disam, S. Thiel, I. Dahan, Deposition of NiAl thin films from NiAl compound target fabricated via combustion synthesis, Surf. Coat. Technol. 120–121 (1999) 22–27. [19] R. Hu, P. Nash, The enthalpy of formation of NiAl, J. Mater. Sci. 40 (2005) 1067–1069. [20] F.H. Stott, The-role of oxidation in the wear of alloys, Tribol. Int. 31 (1–3) (1998) 61–71.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Tribological properties of NiAl produced by pressure-assisted combustion synthesis O. Ozdemir, S. Zeytin, C. Bindal ∗ Sakarya University, Engineering Faculty, Department of Metallurgical and Materials Engineering, Esentepe Campus, 54187 Sakarya, Turkey
a r t i c l e
i n f o
Article history: Received 29 September 2006 Received in revised form 16 January 2008 Accepted 18 February 2008 Available online 15 April 2008 Keywords: Friction Wear Intermetallics NiAl-based alloy Pressure-assisted combustion synthesis
a b s t r a c t In this study, aluminum powder with 15 m size and carbonyl-nickel powder with 4–7 m size having 99% and 99.8% purity, respectively were used in order to produce NiAl by volume combustion synthesis. The production of intermetallic compound was carried out in an electrical resistance furnace in open air under a uniaxial pressure of 150 MPa at 1050 ◦ C for 1 h. The formation temperature of NiAl intermetallic compound was determined to be 655 ◦ C by differential scanning calorimeter (DSC) analysis. The presence of NiAl was confirmed by X-ray diffraction (XRD) analysis. Optical and scanning electron microscope (SEM) analysis revealed that NiAl phase has very low porosity. The relative density of the samples measured according to based on Archimedes principle was 99.6%. The microhardness of the samples measured by Vickers indenter was approximately 367 ± 17 HV1.0 . The effect of load on wear rate in the case of using Al2 O3 ball as a counterface material was also investigated using a ball-on-disc test method. The coefficient of friction for NiAl intermetallic compound run under 2 N load was calculated as 0.73 while it was 0.53 run under 10 N load. Depending on load, the wear rate of NiAl sliding against alumina ball was 0.016 mm3 /N m for 2 N, 0.017 mm3 /N m for 5 N and 0.009 mm3 /N m for 10 N, respectively. The distribution of alloying elements within intermetallic compound was determined by energy-dispersive X-ray spectroscopy (EDS). © 2008 Elsevier B.V. All rights reserved.
1. Introduction The strongly ordered B2 intermetallic NiAl has a number of unique physical and mechanical properties that make it attractive for use at high temperature and in aggressive environments. Due to their high-melting point (1640 ◦ C), low density ( = 5.89 g/cm3 ), excellent corrosion and oxidation resistance, NiAl intermetallics have several potential applications including turbochargers, hightemperature dies and moulds, furnace fixtures, rollers in steel slab heating furnaces, hydroturbines, cutting tools, pistons and valves and various components within gas turbines. The primary potential application on NiAl is in turbine engines which turbine blade tips suffer from sliding type wear when in contact with surrounding gas path seals [1–5]. Intermetallics have been prepared by a variety of methods including casting, mechanical alloying gas atomization, sintering, combustion synthesis, etc. [6–8]. Combustion synthesis (CS) is a process used to form ceramics, intermetallics and their composites at operating temperatures much lower than their melting points and in very short-processing times [7,9–11]. The synthesis from
∗ Corresponding author. Tel.: +90 264 295 57 59; fax: +90 264 295 56 01. E-mail address: [email protected] (C. Bindal). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.02.005
compacted powders can be performed under the explosion mode or the propagation mode. In the former, one end of a reactant sample is heated with an igniter at a high-heating rate (hundreds of degrees per second), and once ignited, the combustion wave self-propagates thorough the sample. Whereas in the later, a reactant sample is wholly heated in a furnace equipped with resistance heating at a low-heating rate (tens of degrees per minute), and once heated to its ignition temperature, the combustion reaction becomes spontaneous and takes place everywhere in the sample [8,12]. One of the major problems reported for this technique is the high-porosity levels in the final product. It is now recognized that the presence of the transient liquid phase, by itself, is insufficient to density the product, and the CS process needs to be augmented by densification steps [11]. The main objective of present study is to conduct a preliminary investigation of the wear mechanisms acting in NiAl bulk material produced by means of pressure-assisted explosion mode under dry sliding conditions at room temperature in air atmosphere. In order to characterize the morphological features of sintered NiAl intermetallic and wear tracks, scanning electron microscope (SEM) was employed. In order to determine hardness and density of the test materials, a Vickers indenter and Archimedes technique were utilized. The effect of load on wear rate in the case of using Al2 O3 ball as a counterface material was also investigated using a ball-on-disc
980
O. Ozdemir et al. / Wear 265 (2008) 979–985
test method. It was shown that the wear rate increased linearly with the increase in load. The distribution of alloying elements within intermetallic compound was determined by energy-dispersive Xray spectroscopy (EDS). 2. Experimental procedures 2.1. Material processing and characterization NiAl compounds were produced using pressure-assisted volume combustion synthesis. The reactant powder mixtures consisted of commercial powders: Ni (99.8% purity and 3–7 m particle size); Al (99% purity and <15 m particle size). Ni and Al powders in the required stoichiometry (1:1 Ni to Al molar ratio) were mixed using a small ball mill containing Ni balls at a speed of 150 cycle/min for 10 min under Ar + 3% H2 gas medium with the addition of a small amount of ethanol (0.1%). By applying a uniaxial pressure of 150 MPa the mixture was cold pressed into a cylindrical compact in a metal mold which coated with a thin layer of boron nitride. The compact pressed in metal mold was inserted in an open-air furnace and heated to 1050 ◦ C at a heating rate of 20 ◦ C/min. The process temperature was kept constant for 60 min under pressure without using vacuum or inert gas. The diameter and height of the sintered samples were 15 and 5 mm, respectively. Further details of process are given in Ref. [13]. Scanning electron microscopy was used to examine the microstructures of the samples polished using standard metallographic techniques. In order to determine phases formed, X-ray diffraction analysis was utilized. Density measurements were made using the Archimedes (water immersion) method and microhardness measurement was made with a Vickers hardness tester. 2.2. Wear tests Friction and wear test was performed with a ball-on-disc type tribometer according to the G66 ASTM standard test method. The ball-on-disc tribometer is as shown schematically in Fig. 1. The counterface material was made of alumina ball, 9.5 mm in diameter, with a highly polished surface finish of better than 0.05 m CLA roughness. NiAl intermetallic disks were run against the ball under the loads of 2, 5 and 10 N. The wear parameters in the test were given in Table 1. The ball was firmly fixed to a stationary holder for the ball-on-disc configuration (Fig. 1). Discs were attached to a horizontal chuck driven by a variable-speed electric motor. The frictional force, monitored by a load cell attached to the ball holder, was recorded continuously. The friction coefficient was measured when steady state was reached in the wear test. The maximum compressive contact pressure in central point of the contact area was calculated from the Hertzian equation [14]. According to the Hertzian equation, maximum contact pressures of 771.1, 1046.6 and 1318.6 MPa (for disc NiAl E = 235 GPa, = 0.3; for Al2 O3 ball E = 420 GPa, = 0.24 [15,16]) were obtained at normal loads of 2, 5 and 10 N, respectively. Wear rate was measured
Fig. 1. Schematic diagram of ball-on-disc tribometer used.
primarily by volumetric (volume loss) means. Volumetric wear of the spherical ball specimens was also determined by measuring the diameter of the wear track and calculated by means of volume loss of the ball. Wear rates of samples were obtained using following equation. Width of wear scar was measured in the optical microscopy: W=
2(R + (L/2))(r 2 /2)( − sin ) sliding distance
where W is the wear rate (mm3 /m), L the wear track width (mm), R the radius of the wear scar (mm), r the radius of ball (mm) and = 2 arcsin(L/2r) (radian). 3. Experimental results 3.1. Material characterization Fig. 2 shows the differential scanning calorimeter (DSC) graph of Ni and Al powder mixture (1:1 molar ratio) under a constant heating rate of 20 ◦ C/min. These data show that the formation
Table 1 Test parameters Parameters
Selected value
Applied load (N) Velocity (m/s) Environment Temperature (◦ C) Humidity (%) Duration (min) Roughness, Ra (m) Test ball diameter (mm)
2, 5 and 10 0.1 Air 20 ± 2 65 ± 5 53 0.05 9.5
(1)
Fig. 2. DSC analysis of Ni–Al powder mixed in a molar proportion of 1:1.
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Fig. 3. (a) Optical and (b) SEM images of NiAl produced by pressure-assisted combustion synthesis.
Fig. 4. X-ray diffraction patterns of NiAl processed at 1050 ◦ C for 60 min.
Fig. 6. Variation of coefficient friction and wear rate for NiAl as a function of applied load.
of NiAl is a one step reaction with the ignition temperature of approximately 654 ◦ C which is lower than the Al melting point (Tm : 660 ◦ C) but higher than the Al-rich eutectic temperature (640 ◦ C) in the Ni–Al phase diagram [7]. The formation mechanism of intermetallic nickel aluminides strongly depends on heating rate. Several exothermic and endothermic peaks were observed at lower heating rates (5–20 ◦ C/min). This results in solid-state pre-combustion reaction (two-step reaction) between Ni and Al as well as intermediate reactions in the process of formation of nickel aluminides such as NiAl3 and Ni2 Al3 forming prior to the final products. However, both slow heating and effective heat transfer from the sample to the massive metal mold prevented the reaction from becoming self-sustaining and allowed it to remain as solid state diffusion-controlled phases. The heat loss
resulting from the heat exchange between the sample and metal mold may prevent the combustion reaction [8,17,18]. After 1 h at 1050 ◦ C the reaction is completed and the samples having a uniform NiAl structure with small amount of porosity were produced (Fig. 3). Considering the reaction that given NiAl from Ni and Al that is, Ni + Al → NiAl, the formation enthalpy of NiAl intermetallic was calculated using data of heat capacities Cp,NiAl , Cp,Ni and Cp,Al [19,6]. Using heat contents we obtained a heat formation enthalpy of −64.11 kJ/mol for NiAl which is in good agreement with literature. No phase transformation for the reactants is taken into account because the ignition temperature of reaction is lower than the melting temperature of aluminum (660 ◦ C) and nickel (1453 ◦ C). Xray diffraction (XRD) technique was used to identify the phases in the samples. The XRD pattern indicates that only the NiAl phase
Fig. 5. The variation of friction coefficient of test materials as a function of cycles for 2 N load.
Fig. 7. Specific wear rate and wear resistance vs. applied load.
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Fig. 8. SEM image and concerning profilometry traces of wear tracks for (a) 2 N, (b) 5 N and (c) 10 N.
Fig. 9. SEM image of wear tracks formed on NiAl depending on load: (a) 2 N, (b) 5 N and (c) 10 N.
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Fig. 10. (a) SEM image of worn surface of NiAl and EDS spectrum of (b) mark 1, (c) mark 2 and (d) mark 3 for sample sliding under a load of 2 N.
was present in the sample (Fig. 4). The hardness of the specimen measured by using Vickers indentation technique with a load of 10 N, was approximately 367 ± 17 HV1.0 . The density of NiAl intermetallic compound calculated by Archimede’s technique was 5.632 ± 0.0145 g/cm3 which is in good agreement with theoretical density of 5.654 g/cm3 .
3.2. Friction and wear Friction and wear tests were performed in a ball-on-disc tribometer. Wear tests applied on the test materials produced by pressure-assisted combustion synthesis revealed that the friction coefficient was changing between 0.5 and 0.8 values for long-
Fig. 11. (a) SEM image of worn surface of NiAl and EDS spectrum of (b) mark 1, (c) mark 2 and (d) mark 3 for sample sliding under a load of 5 N.
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Fig. 12. (a) SEM image of worn surface of NiAl and EDS spectrum of (b) mark 1 and (c) mark 2 for sample sliding under a load of 10 N.
duration testing. Fig. 5 shows the variation of friction coefficient of NiAl intermetallic as a function of cycle number for a load of 2 N. Fig. 6 reveals the variation of friction coefficients as a function of applied load. It’s clear that the friction coefficient decreases with increasing in load. The wear rate of NiAl sliding against alumina ball ranged from 0.031 to 0.093 mm3 /m, depending on applied load as shown in Fig. 6. The difference between wear rate and specific wear rate is coming from using of load in calculations of specific wear rate. Also, it was observed that the specific wear rate value of NiAl as a function of load was 0.016 mm3 /N m for 2 N, 0.017 mm3 /N m for 5 N and 0.009 mm3 /N m for 10 N. These results verify that the increase in loads result in decrease in the specific wear rate (Fig. 7). Very small amount of debris around the rubbing surfaces was observed. Depending on load the wear resistance of test material increases (Fig. 7). Probably, this is a result of plastic deformation due to high-load value. Figs. 8 and 9 show SEM image of wear tracks formed on the test material during wear test. Fig. 8 shows the wear tracks as well as the surface profiles of wear tracks in the NiAl intermetallic. As it can be seen in Fig. 8 wear track of the specimens run for 2 N resulted in a regular surface profile, whereas for applied load 5 and 10 N, some scattering was observed in surface profiles formed on the NiAl intermetallic. The distribution of alloying elements in rubbed surface was determined by means of EDS (Figs. 10–12). Using some variables of test materials and experimental parameters such as thermal conductivity, K, diffusivity, D, etc., bulk temperature (dTBulk ), flash temperature (dTFlash ), real contact area (Ar ), nominal area (An ), ratio (Ar /An ) and maximum contact pressure (Pmax ) were calculated and are given in Table 2. In image analysis, dark areas denote the regions which including oxygen, while bright areas show Ni–Al intermetallic compound. The variation of nickel, aluminum and oxygen detected by EDS analysis are given in Table 3. As it can be seen in Table 3 the presence of oxidation was confirmed by EDS obviously.
Table 2 The values of bulk, flash temperature, real contact area over nominal contact area and maximum contact pressure depending on applied load and some physical parameters of NiAl and testing dTBulk (◦ C) dTFlash (◦ C) Ar /An
Load (N) Variables 2 5 10
35.02 × 10−3
: 0.73; r: m 0.0019 0.0025 : 0.61; r: 47.7 × 10−3 m : 0.53; r: 60.2 × 10−3 m 0.0031
79.45 66.39 57.68
1.483 1.998 2.51
Pmax (MPa) 771.13 1046.59 1318.62
v: 0.1 m/s; DNiAl : 21.11 m2 /s(calculated) ; KAl2 O3 : 5.43 W/m K; KNiAl : 76 W/m K; HB: 3.5 GPa.
Table 3 The variation Ni, Al and O at marked points confirmed by EDS analysis Load (N)
Compositions (at.%) Marks
Ni
Al
O
2
1 2 3
54.099 52.087 44.209
45.901 47.913 38.305
– – 17.486
5
1 2 3
34.631 25.327 20.474
24.101 21.242 17.498
41.268 53.431 62.028
10
1 2
19.261 29.502
16.015 23.565
64.724 46.933
4. Discussion DSC studies revealed that the enthalpy of formation of NiAl is −64.11 kJ/mol at exothermic reaction temperature. Both optical and SEM examinations of the cross-section of NiAl intermetallic compound was dense which could be used for structural applications (Fig. 3(a) and (b)). As a matter of fact that a dense NiAl intermetallic was obtained which has relative density of 99.6% confirmed by density measurements and the hardness of the sample was approximately 367 ± 17 HV1.0 .
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In the samples sliding against alumina ball under different loads, steady-state wear is established for the intermetallic. The friction coefficient decreases and wear rate increases with increasing of load (Fig. 5). The above results revealed that the friction coefficient for NiAl intermetallic compound under the load of 2 N was 0.73, while it was 0.53 under the load of 10 N. Depending on the applied load, the specific wear rate of NiAl sliding against alumina ball was 0.016 mm3 /N m for 2 N, 0.017 mm3 /N m for 5 N and 0.009 mm3 /N m for 10 N, respectively. When the applied load was increased from 2 to 10 N, the specific wear rate was decreased approximately 45%. Around the rubbing surfaces small amount of debris was observed and wear track includes some abrasive scratch lines (Fig. 8). As it can be seen in Figs. 10–12, EDS analysis of the wear track formed on the worn sample showed that oxygen concentration increased on the worn surfaces. EDS analysis performed on the worn track run under 2 N revealed that bright regions exhibit the Ni and Al, while dark regions denote the oxygen in addition to Ni and Al peaks (Fig. 10(b) and (c)). It was found that the intensity of oxygen peak increases with increasing of applied load, also, the formation of cracks was detected in dark regions which rich in oxygen (Figs. 11 and 12). This indicate that wear mode of the system was oxidative and abrasive. The presence of oxygen in rubbing surface confirms that the formation of aluminum oxide owing to its lower free energy of formation compared to nickel oxide. As a matter of fact that previous study performed by Sierra and Vazquez [2] revealed that the composition of wear debris and track surface allow us to suggest a wear mechanism. During the sliding motion of NiAl against Al2 O3 ball, the NiAl suffer plastic deformation (Fig. 8) and some particles are detached from the surface. These particles remain between the NiAl and Al2 O3 surfaces. Because of the high-surface temperatures originated from the friction force, they became oxidized. Some cracks were observed in wear track (Figs. 11(a) and 12(a)). It is possible to claim that these are the consequences of hard oxides due to their inherent brittle nature. Stott [20] has claimed that the oxidation layer occurred in surface decreases the wear rate. Also, it is very important to have information about Hertzian pressure developed during wear test in this type process. Using a computer diagram and some parameters of disk and counterface material the maximum contact pressures were calculated and it was seen that as the applied load increases the maximum contact pressure increases which can cause cracks in worn surface. Additionally, as it can be seen in Table 2 as the applied load increases the dTBulk temperature, Ar /An ratio and maximum contact pressure also increases. 5. Conclusions Pressure-assisted combustion synthesis being an effective production method has been proposed in order to produce NiAl intermetallic compound. Pressure-assisted combustion synthesis performed by holding Ni–Al powder mixture at 1050 ◦ C under a pressure of 150 MPa for 60 min without using any controlled atmosphere results in formation of dense and homogeneous of single NiAl compound which this indicate that pressure-assisted combustion synthesis is a promising manufacture method for NiAl.
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The hardness and relative density of NiAl intermetallic compound were 367 HV1.0 and 99.6%, respectively and the formation exothermic temperature of the NiAl materials was calculated as 655 ◦ C. Wear studies revealed that wear mode of the system is oxidative and abrasive. The formation of aluminum oxide compared to nickel oxide is attributed to its lower free energy of formation. Acknowledgements This work was supported by Scientific Researches Project Commission of Sakarya University (contact no. 2002/29). The authors thank Sakarya University Rectorate for his help. The special appreciations are extended to technician Ersan Demir for experimental assistance at Sakarya University. References [1] J.A. Hawk, D.E. Alman, Abrasive wear behavior of NiAl and NiAl–TiB2 composites, Wear 225–229 (1999) 544–556. [2] C. Sierra, A.J. Vazquez, Dry sliding wear behavior of nickel aluminides coatings produced by self-propagating high-temperature synthesis, Intermetallics 14 (2000) 848–852. [3] J.-H. Jim, D.J. Stephenson, The sliding wear behavior of reactively hot pressed nickel aluminides, Wear 217 (1998) 200–207. [4] B.J. Johnson, F.E. Kennedy, I. Baker, Dry sliding wear of NiAl, Wear 192 (1996) 241–247. [5] A. Michalski, J. Jaroszewicz, M. Rosinski, D. Siemiaszko, NiAl–Al2 O3 composites produced by pulse plasma sintering with the participation of the SHS reaction, Intermetallics 14 (2006) 603–606. [6] L. Plazanet, F. Nardou, Reaction process during relative sintering of NiAl, J. Mater. Sci. 33 (1998) 2129–2136. [7] K. Morsi, Review: reaction synthesis processing of Ni–Al intermetallic materials, Mater. Sci. Eng. A299, Elsevier Science B.V., 2001, pp. 1–15. [8] Q. Fan, H. Chai, Z. Jin, Dissolution–precipitation mechanism of self-propagating high-temperature synthesis of mononickel aluminide, Intermetallics 9 (2001) 609–619. [9] J.H. Jin, D.J. Stephenson, The sliding wear behavior of respectively hot pressed nickel aluminides, Wear 217 (1998) 200–207. [10] D.E. Alman, Reactive sintering, Powder Metal Technologies and Application, ASM Handbook, 1998, pp. 516–521. [11] T.S. Hussey, M.J. Koczak, R.W. Smith, S.R. Kalidindi, Synthesis of nickel aluminides by vacuum plasma spraying and exothermic in-situ reactions, Mater. Sci. Eng. A229 (1997) 137–146. [12] A. Varma, A.S. Mukasyan, Combustion synthesis of advanced materials, Powder Metal Technologies and Application, Vol. 7, ASM Handbook, 1988, pp. 523–540. [13] O. Ozdemir, The Investigation of The Effect of Cobalt Addition to Ni–Al Intermetallic Materials Produced by Pressure Assisted Combustion Synthesis, PhD Thesis, Sakarya University, Institute of Science and Technology, Sakarya-Turkey, 2004. [14] http://www.tribology-abc.com/calculators/e2 1.htm. [15] G. Sauthoff, Structure and properties of nonferrous alloys, Intermetall. Mater. Sci. Technol. 8 (1996) 643–805. [16] H. Choo, M.A.M. Bourke, P.G. Nash, M.R. Daymond, Evolution of thermal residual stress in intermetallic matrix composites during heating, Ceram. Eng. Sci. Proc. ACERS 21 (3) (2000) 627–634. [17] H.X. Zhu, R. Abbaschian, Reactive processing of nickel-aluminide intermetallic compound, J. Mater. Sci. 38 (2003) 3861–3870. [18] D. Zhong, J.J. Moore, J. Disam, S. Thiel, I. Dahan, Deposition of NiAl thin films from NiAl compound target fabricated via combustion synthesis, Surf. Coat. Technol. 120–121 (1999) 22–27. [19] R. Hu, P. Nash, The enthalpy of formation of NiAl, J. Mater. Sci. 40 (2005) 1067–1069. [20] F.H. Stott, The-role of oxidation in the wear of alloys, Tribol. Int. 31 (1–3) (1998) 61–71.