Wear 192 (1996) 241-247
Dry sliding wear of NiAl B .J. Johnson, F.E. Kennedy, I. Baker Thayer School of Engineering, Dartmouth College, Hanover, NH 03755, USA Received 26 June 1994; accepted
Abstract The dry sliding wear behavior of the B2-structured (ordered body-centered cubic) compound NiAl has been studied. Pin-on-disk experiments were conducted at room temperature in air using pins made from extruded NiAl with compositions of 50,48, and 45 at.% aluminum. Partially stabilized zirconia was the disk material. Wear rate measurements showed an inverse relation between wear and hardness. Hardness increased with increasing nickel content as the composition moved away from stoichiometric composition. Friction results indicate higher friction coefficients for Ni-5OAl than for either Ni-48Al or Ni-GAl. Surface examinations of the pin wear scars, by both optical microscopy and scanning electron microscopy, show the wear process of NiAl is dominated by plastic deformation in all compositions, with little evidence of material loss by brittle fracture. Keywords:
Sliding wear; NiAl; Plastic deformation;
1. Introduction The strongly ied extensively
over the past few decades
NiAl has been studas a candidate
those in aerospace which could benefit from its low density [ I]. Jet engine turbine blades, for example, are currently produced from a variety of Ni-based superalloys, the melting points of which determine the maximum operating temperature of the engines. By virtue of its high melting point, about 1640 “C, NiAl would theoretically allow higher operating temperatures than with currently used superalloys, and therefore higher engine efficiencies. The lower density and higher thermal conductivity of NiAl offer further potential for improvement in turbine performance. Ni,Al, another nickel aluminide intermetallic phase, has also received attention as a candidate material for jet engine components, but this material does not have as high a melting point nor as low a density as NiAl. There have been numerous studies of the mechanical properties of NiAl in recent years. The most significant aspect of the mechanical behavior of the material is its low roomtemperature ductility. 2-2.5% elongation seems to be the typical room-temperature tensile ductility of stoichiometric NiAl [ 241, whereas there have been no reports of any tensile elongation in off-stoichiometric compositions. Limited room-temperature ductility in compression tests has not been as difficult to achieve [ 51, even in slightly off-stoichiometric compounds [ 61. Failure occurs by intergranular fracture in high-temperature
0043-1648/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO43-1648(95)06735-3
stoichiometric polycrystals, apparently from strain incompatibility between grains. In contrast, off-stoichiometric alloys fracture mostly transgranularly [ 71. The lower ductility of off-stoichiometric alloys is believed to occur simply because the higher yield strength of these compositions lies above the inherent cleavage strength, or fractur stress [ 31. At some point between 300 “C and 600 “C (the exact temperature depends upon several factors) NiAl undergoes a ductile-to-brittle transition, apparently brought about by diffusion-assisted processes. Several factors appear to affect the ductile-to-brittle transition temperature (DBlT) , including composition and strain rate. Extremely small grain sizes may suppress crack propagation somewhat, but the ductility has been found to be relatively grain size independent below theDBTT . The yield strength of NiAl is characterized by a “plateau” in its temperature dependence around room temperature. From room temperature up to the DBTT, the strength is essentially independent of temperature. Above the DB’IT, presumably the same mechanisms which increase ductility also induce a temperature-dependent decline in strength [ 61. The stoichiometric composition exhibits relatively poor strength at most temperatures ( = 150 MPa at room temperature), while deviation from stoichiometry provides excellent low temperature defect strengthening [ 4,8]. A primary potential application of NiAl is in turbine engines. Turbine blade tips experience sliding-type wear when in contact with surrounding gas path seals, but to the
B.J. Johnson et cd /Wear
authors’ knowledge, sliding wear and friction of NiAl has not yet been studied. The purpose of this work was to conduct a preliminary investigation of the wear mechanisms acting in NiAl under dry sliding conditions in a room-temperature air atmosphere. Later work will expand upon this to include elevated-temperature and inert atmosphere testing. While the wear properties of NiAl have not been reported, several projects have investigated the sliding wear of Ni,Al. Blau and DeVore [9-l l] tested a range of commercially available Ni,Al alloys in air at both room and elevated temperatures, with alumina ( A1203) and tungsten carbide (WC) as counterfaces. They used Ni,AI disks in sliding contact with stationary ceramic balls. Ni,Al pins were tested in sliding against disks of the same material by Rao Bonda and Rigney [ 121. The alloys included Ni,Al with and without boron additions, and their tests were run both in air and vacuum at room temperature and at 400 “C. Marquardt and Wert [ 131 also subjected Ni,Al to sliding tests against itself. Serrated deformation structures ahead of, and occasionally within, the wear scars signified planar slip, as expected in ordered-fee (Ll,) metals. The studies of Ni,Al suggest several effects of ordering on wear behavior, and several mechanisms for material removal. Mechanically mixed surface layers, and their delamination by cracking along heavily used slip planes, have been proposed as a mechanism of material removal [ 131. This agrees with other well-known delamination theories for disordered metals based upon crack nucleation in regions of very high dislocation density [ 141. Through high-temperature testing, wear rates of Ni,Al have been shown to be dependent upon strength [ 91. Rao Bonda and Rigney’s results [ 121 raise the issue of oxidation in intermetallic wear, although its role with other intermetallics would be unpredictab!e. The same study found that changes in ductility, through boron additions, had no effect on the wear rate of polycrystalline Ni,Al [ 121. The role of ductility in the sliding wear of NiAl is studied in this work.
2. Experimental 2.1. Materials In this study, near stoichiometric (i.e. 49.8% Ni, 50.2% Al) NiAl was complemented by two Ni-rich compounds with 48.1% and 45.0% Al. Composition percentages are nominal atomic. Reference to various alloys in this text will be as follows: Ni-5OAl implies the near stoichiometric NiAl compound, with NiASAl and Ni-45Al designating the otherNiA1 compositions. All materials were cast from elemental constituents, and then extruded twice within mild steel jackets. The first extrusions were performed at 1000 “C with an area reduction ratio of 7: 1. The second took place at 750 “C with a reduction of 6:l. Each specimen subsequently received a homogenization heat treatment of 900 “C for 200 min, followed by an air cool.
I92 (1996) 241-247
Wear specimens were machined into cylindrical pins of 9.5 mm diameter, with hemispherical tips ground at one end. After grinding, the wear pins were heat treated, and then returned to the lathe for polishing of the hemispherical surface to a mirror finish using emery paper of decreasing grit size followed by wet rags covered with 0.3 p,rn alumina powder. Wear pins were used in multiple tests by simply re-grinding and polishing the hemispherical tip. There were several reasons for the selection of zirconia (ZrO,) as the counterface material. First, zirconia is used as a gas path seal in some jet engines, and so NiAl components might slide against it in actual application [ 151. Secondly, zirconia is quite chemically inert, limiting the influence of chemical interactions on wear. Finally, the high hardness of this ceramic should reduce wear of the counterface, to some extent isolating the wear processes of NiAl. The hot-isostatically pressed zirconia disk with zero porosity had been doped with yttria (Y,Oa) to fully stabilize its phase. The test surface of the zirconiadisk had an outside diameter of 100 mm. By varying the pin sliding radius over a range between 22.5 mm and 47.5 mm, approximately ten tests could be accommodated on each side of the disk before resurfacing. Resurfacing was performed with a diamond-embedded grinding wheel to give a surface roughness (R,) ranging from 0.0075 *rn to 0.010 pm. 2.2. Test methods Wear testing was performed with a pin-on-disk tester designed and built in house (see Fig. 1) . The pins were held against the disk with a normal load of 27 N, while sliding at a constant linear speed of 0.5 m s ~ ’for a total sliding distance of 500 m. Different sliding radii were accommodated by changing the rotational speed of the disk to maintain a constant sliding speed. Mass measurements of the pins before and after testing provided a total mass loss which was converted to volume loss using theoretical alloy densities. Wear rate results are presented in terms of a wear coefficient by normalizing the volume loss with load and sliding distance. A strain gage force transducer on the pin holder allowed continuous measurement of friction forces. The Rockwell hardness of the NiAl pins was measured on their rear (non-contacting) surface. At least six indentations were made on each of three specimens of a given material, and the values were averaged to obtain the reported specimen hardness. Several techniques were used to characterize the materials and determine the mechanisms of wear. Optical microscopy proved to be very useful, both for microstructural characterization and viewing of the wear scars. A scanning electron microscope (SEM), operated at 15 kV, provided higher resolution images of features which could not be resolved optically. The SEM was equipped with integrated energydispersive X-ray analysis (EDX) . EDX counts were analyzed with a semi-quantitative algorithm, and were taken primarily from wear surfaces to determine rough amounts of
B.J. Johnson et al. / Weur 192 (1996) 241-247
STRAIN GA6E ZIRCfflIA
Fig. 1. Diagram of pin-on-disk
ADJUSTABLE COUNTERME IGHT I -
wear tester used in this study.
zirconia transferred to the wear pins. Debris collected from the wear tests was held close to a permanent magnet to test for the magnetism which was noticed in N&Al debris by Rao Bonda and Rigney [ 121. Finally, Vickers microhardness measurements were taken for various features on the wear surfaces. Quantitative measurements of wear of the zirconia disk were not made.
3. Results Post-annealed microstructures of the NiAl alloys proved to be highly reproducible, with only a slight variation between the alloys. All compositions were single phase. Average grain diameters, as measured using a linear intercept method, were 22 p,m, 33 pm, and 37 pm for Ni-SOAl, NiA8A1, and Ni45A1, respectively. Fig. 2 shows a typical micrograph. Macrohardness results are plotted in Fig. 3. Hardness clearly increases with increasing ratio of Ni to Al, although the rate of increase declines as the Ni content rises. This is in agreement with earlier microhardness results for the same alloys [ 161. Since the grain size was larger for the materials with greater Ni content, the increase in hardness for those materials was accompanied by an increase in grain size. Average wear rates, for nine tests for each alloy, are presented in Fig. 4. Individual data points were fairly well scattered, as is typical for dry sliding wear results. Coefficients of variation for the wear data are given in the caption of Fig. 4. A few external effects could have enhanced the data scatter observed here. Although no correlation could be made to the recorded wear rates, humidity and counterface roughness did vary from test to test, and might have had an effect. Variation in sliding radius was shown to have a slight influence on wear rate, with generally higher volume losses recorded for tests at smaller radii. The total sliding distance
Optical micrograph showing grain structure of Ni-5OAI material after
heat treatment prior to wear testing.
Rockwell “A” Hardness
66 64 : 62 6058 ’
Fig. 3. Macrohardness of nial compositions. Error bars represent one standard deviation above and below the mean values, which were calculated from at least 18 indentations each.
and the sliding velocity were constant for these tests, so pins sliding at smaller radii accumulated more total traversals per unit time over the same wear track. This appeared to result in both a shorter “run-in” period and a greater overall wear
B.J. Johnson et al. / Weur 192 (1996) 241-247
2.5 2.0 : Wear Coefficient
[mm3 /(Nm)] x E-5 L
Fig. 4. Wear rates of NiAI. The values shown are the mean values of nine wear tests for each material. The coefficients of variation (standard deviation/mean) were 0.28, 0.31, and 0.22 for the Ni-SOAI, NiAIAI and Ni45A1, respectively.
0.35 Lj 0.30 c t 0.25 Friction Coefficient
straightforward material removal mechanism that almost certainly occurs in these alloys. Plowing of the wear surface is believed to be caused by hard third-body debris particles, because the width of the grooves is much larger than the surface asperities of the zirconiacounterface. Hard third-body particles could be formed from either dislodged counterface or intermetallic material. Several of the worn pins were sectioned through the wear scar, and the cross-sections were examined in an optical microscope to study subsurface damage. One such micrograph is shown in Fig. 7. It is apparent that considerable subsurface plastic deformation occurred in the top = 20 pm beneath the sliding surface. Microhardness measurements of the worn surfaces of the pins revealed that strain hardening had occurred during the wear process for all three alloys, and the results are presented in Table 1. The increase in microhardness values in the wear scar is further evidence of extensive near-surface plastic deformation. Besides plastic deformation, other features noticed on the worn surfaces were dark patches of worn material dotting the surface of the otherwise fairly reflective intermetallic. These
Fig. 5. Friction coefficient of NiAl pins in sliding tests against zirconia disk. The values shown are the mean values of nine tests for each material. The coefficients of variation (standard deviation/mean) were 0.07, 0.11, and 0.09 for the Ni-SOAI, Ni-48Al and Ni-45AI. respectively.
volume for tests run at smaller sliding radii. In the NiAlZr02 situation, the wear-sliding radius relation might indicate that wear rates were higher during steady-state conditions than during run-in. The average friction values displayed in Fig. 5 have been calculated from mean steady-state values, also taken from nine tests for each alloy. Unlike the wear rate data, recorded friction forces varied little from test to test. Stoichiometric NiAl produced significantly higher friction than either of the Ni-rich alloys, which differed only slightly from each other. Apparent on all worn specimens was evidence of a significant amount of plastic flow on the wear surfaces. Surface cracking was all but nonexistent (it was observed in only one SEM image), meaning that the brittle nature of these materials does not seem to be affecting wear. Plasticity appears in two forms: extrusion of material across and over the trailing edge of the wear scar, and deep grooving or microplowing of material across the wear scar from leading to trailing edge. These phenomena are shown in Fig. 6. Plastic flow can be quite extensive in the sliding direction. Tearing of extruded material fragments from the trailing edge is one fairly
Fig. 6. Worn surface of Ni-45AI material, showing microplowing in sliding direction and extruded material at trailing edge of wear scar (at right).
Fig. 7. Cross-section right to left.
of worn pin of Ni-tSAI.
Sliding direction was from
B.J. Johnson et al. /Wear 192 (1996) 241-247 Table 1 Microhardness
data from worn specimens
Indentation size mean value (std. deviation)
Ni-48Al Bulk material (unworn) Wear scar Wear debris
22.7 pm (0.76 pm) 18.9 cm (2.13 km) 15.4 km (1.35 km)
360 519 782
Ni-45Al Bulk material (unworn) Wear scar Wear debris
21.5 km (0.35 p.m) 20.9 pm (2.67 km) 15.6 pm (1.52 pm)
401 425 762
Hardness values are determined from the mean values from 10 indentations, all at 100 gf load. All hardness indentations were done on the pin surface, either on the unworn portion of the surface (bulk material), within the wear scar, or on compacted wear debris.
Fig. 8. Wear debris on worn surface of pin made from Ni45Al.
patches were seen to be concentrated in a band on the wear scar coinciding with the region of highest trailing edge extrusion. Patches of wear debris which seem to be adhered and smoothed out along with the surrounding surface can be seen at higher magnification in Fig. 8. It is believed that these dark patches of material are pieces of mechanically mixed wear debris stuck to the NiAl substrate, smoothed, and pressed against the surface by the continuing wear process. EDX analysis of the worn surfaces showed that the dark blotches were composed of mechanically mixed components from the nickel aluminide pin and the zirconia counterface. Microhardness indentations on the patches of compressed wear debris showed that the debris is significantly harder than the base material (Table 1) . Occasionally the NiAl wear surfaces showed signs of chunking, or separation of (relatively) large pieces of material followed by the formation of a groove (Fig. 9). The width of the groove is of the order of a grain diameter, raising the possibility of whole grains pulling out and gouging the material while being dragged across the surface. Subsequent deformation usually partially covers the leading ends of the
Fig. 9. Worn surface of pin made from Ni-SOAl, showing region of grain pull-out. Sliding direction was right to left.
trenches, making it difficult to discern exact causes of the particle release. Chunking formations were not observed very frequently, so they are not considered to be a major factor in wear of NiAl. Although tne wear scars of the three alloys were quite similar in appearance, several small differences could be discerned. Debris patches on Ni-5OAl surfaces were uniformly small, with no gross surface features to be found. The mechanically mixed areas were larger and less evenly distributed in Ni-48Al and NiASAl. There was little evidence of grain pullout on the latter two alloys, but a few instances of pullout were noted with the stoichiometric alloy, as shown in Fig. 9. A simple test using a hand-held permanent magnet showed that the wear debris collected from tests of all alloys was magnetic, One possible source of the magnetism is the antiferromagnetic oxide NiO, which probably forms during the wear process. Another possibility is metallic Ni, which is slightly ferromagnetic; its presence in the wear debris was indicated by X-ray diffractometry [ 171. Rao Bonda and Rigney [ 121 observed the same phenomenon with debris from Ni3Al sliding in air.
4. Discussion The alloys studied in this work are known to be very brittle at room temperature. Despite this, the evidence presented above shows that plastic deformation, not fracture, plays a primary role in the wear process for NiAl. Further evidence for this can be seen by plotting mean wear rate (or wear coefficient) vs. hardness for the three alloys, as in Fig. 10. Despite the scatter in the data that make up the average wear rates, there is a definite trend toward lower wear for higher pin hardness. In fact, the mean values of wear rate and hardness fall on a straight line. The materials follow the inverse relationship between wear rate and hardness that is typical of ductile materials for which plastic deformation is associated
B.J. Johnson et al. / Weur 192 (1996) 241-247
(mm3/N - m) x10-5 l.O-
00; * 55
Fig. 10. Relationship NiAl compositions.
for the three
with the wear process. It should be noted that the increase in hardness with increasing Ni content is also accompanied by an increase in grain size. Therefore, the lowest wear rates in this study were found for the alloys with the largest grain size, which were also the hardest materials. Fracture toughness was also determined for the three alloys, and toughness values (K,) of all three alloys were found to lie between 6 and 11 MPa ml’*. Statistical analysis of those data revealed that there was little statistical difference between the toughness of the three alloys; no correlation was found between toughness values and wear rates [ 171. The lack of correlation between fracture toughness and wear is not surprising, since microscopy of the worn surfaces revealed almost no surface cracking of the NiAl materials in the wear scars. Based on the evidence presented above, it is postulated that the wear mechanism of the NiAl alloys is plastic deformation dependent. Plastic flow itself can contribute to material removal in several ways. Some material can be extruded over the trailing edge of a wear scar (as in Fig. 6), where it loses the stability lent by the bulk material and most likely breaks away, accounting for a significant amount of debris creation. An alternative mechanism can explain the wear that occurred away from the trailing edge of the circular contact region. Johnson [ 181 models the effect of repeated asperity contact and the resulting accumulated plastic deformation (shakedown). His ideas, applied to the present situation, can be described in the following manner: comparatively soft intermetallic asperities come into contact with hard zirconiaasperities and also hardened third-body debris particles. After repeated impact, flattening, smearing, and extrusion off to the sides of the asperities, the intermetallic material probably reaches a point at which it breaks away from the bulk in very small particles, leading to micron-sized particles such as those found in these tests. It is likely that the presence of third-body debris particles in the wear track influences plastic flow and related material
removal. Wear rate and sliding radius appear to be inversely related, and this would tend to support the hypothesis of higher wear rates under steady state conditions than during run-in. This would mean that either a transfer of material to the counterface, or the build-up of third-body debris particles, has a detrimental effect on wear rate. Both would occur after a finite period of run-in time dependent upon sliding radius. Third-body particles seem plausible as an explanation for the scoring and grooving noticed on wear scars. They also might influence adhesion phenomena. Metals generally do not adhere well to zirconia, but debris particles might adhere strongly to the substrate material and remove some of it when sheared by another impacting particle or asperity. This would lead to the presence of zirconia in the wear debris, as was noted in these tests. For example, wear debris after a test of Ni-45Al was found to contain about 12 at.% Zr. As a final step in evaluating the results of this project, wear rate data have been compared with results from three studies of sliding wear of N&Al [ 9-131. In all the reported studies, N&Al was found to wear much more heavily than the NiAlbased alloys of this study. This is surprising not only because of the greater strength of N&Al, but also because the other investigations used lower normal loads: 0.98 N and 10 N in Blau and DeVore’s tests [9,11], and 3.9 N in Marquardt and Wert’s [ 131. The difference probably stems from the counterfaces. Marquardt and Wert studied Ni,Al worn against 304L stainless steel [ 131, and an intermetallic could be expected to adhere more strongly to another metal than to a ceramic. Some of Blau and DeVore’s tests used alumina as a counterface [ 91, which also might adhere more strongly to nickel aluminide than zirconia. Typical friction coefficients measured by Blau and DeVore were 0.5-0.7 [ 9,101, which is significantly higher than the 0.2-0.3 recorded in this study. Effects of the tungsten carbide counterface used in other tests by Blau and DeVore are not known. In addition to the differences in counterface materials in those studies, there were also differences in counterface roughness. All of the counterface surfaces used in the N&Al tests by the other investigators were rougher than the 0.00755 0.010 p,m R, surfaces used as counterfaces in the tests performed here. In a companion study, tests have been run on three NiAl alloys containing an incorporated ductile Fe phase [ 191. One of the alloys, containing 10% Fe, was single-phase, and its wear behavior obeyed the same trends noted in this work, i.e. wear rate was inversely proportional to hardness. Two other alloys, with 30% and 44% Fe, were found to be two-phase and for those materials the wear rate was found to be independent of hardness, but proportional to the amount of ductile phase present. As was the case in this work, wear of the ductile-phase toughened materials was found to be found to be dominated by plastic deformation processes. A follow-up study is now under way to investigate the influence of environment on wear of the NiAl alloys. The role of oxygen, humidity, and temperature are being studied; results will be reported in due course.
B.J. Johnson et al. /Wear
5. Conclusions This investigation involved wear testing of three NiAl alloys, containing 45, 48, and 50 at.% aluminum. The tests were conducted under dry sliding conditions in air against zirconia. The following conclusions were drawn from this experimental study: 1. Wear rate is hardness-dependent for these NiAl alloys, with higher hardness (and larger grain size) leading to lower wear. 2. Brittle cracking was not found on the worn surfaces, except in occasional large pits which were most likely caused by removal of whole grains. 3. Plastic deformation and mechanically mixed debris dominated the surface features of the wear pins. It can be concluded that the wear process of NiAl is dominated by plastic deformation for all compositions. 4. Friction coefficients for a given alloy sliding against zirconia varied little from test to test. Stoichiometric NiAl produced higher friction than either of the Ni-rich alloys, which differed only slightly from each other. 5. Dust collected from the wear tests was magnetic, possibly owing to the presence of metallic Ni and the antiferromagnetic oxide, NiO.
Acknowledgements The work reported here was supported by the National Science Foundation Surface Engineering and Tribology Program, grant #MSS-9215788. The authors were assisted by Robert Yi and Michael MacAvoy, undergraduates at Dartmouth College. Materials were provided by Dr. J.D. Wittenberger of the NASA Lewis Research Center and Dr. T. Lilley of Norton Company.
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