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Effects of the exposure to corrosive salts on the frictional behavior of gray cast iron and a titanium-based metal matrix composite
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Tribology International 40 (2007) 1335–1343 www.elsevier.com/locate/triboint
Effects of the exposure to corrosive salts on the frictional behavior of gray cast iron and a titanium-based metal matrix composite Peter J. Blaua,, John J. Truhan Jr.b, Edward A. Kenika a
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA b University of Tennessee, Knoxville, TN, USA
Received 8 September 2006; received in revised form 16 February 2007; accepted 22 February 2007 Available online 2 April 2007
Abstract The introduction of increasingly corrosive road-deicing chemicals has created significant and costly problems for the trucking industry. From a tribological perspective, corrosion of the sliding surfaces of brakes after exposure to road salts can create oxide scales that affect friction. This paper describes experiments on the effects of exposure to sodium chloride and magnesium chloride sprays on the transient frictional behavior of cast iron and a titanium-based composite sliding against a commercial brake lining material. Corrosion scales on cast iron, whose compositions were analyzed by several methods, initially act as abrasive third-bodies. Then they become crushed, spread out, and behave as a solid lubricant. Owing to its greater corrosion resistance, the titanium composite remained scale-free and its frictional response was markedly different. No corrosion scales were formed on the titanium composite after aggressive exposure to salts; however, a reduction in friction was still observed. Unlike the crystalline sodium chloride deposits that tended to remain dry, hygroscopic magnesium chloride deposits absorbed ambient moisture from the air, liquefied, and retained a persistent lubricating effect on the titanium surfaces. r 2007 Elsevier Ltd. All rights reserved. Keywords: Friction; Titanium alloys; Brakes; Cast iron; Corrosion; Metal matrix composites
1. Introduction Together, corrosion and wear exact a high cost on the trucking industry. In particular, the introduction of increasingly aggressive road-deicing chemicals containing magnesium chloride has created corrosion problems for truck fleet operators. A study commissioned by the Colorado Department of Transportation determined that magnesium chloride solutions were more corrosive to standard automotive materials than sodium chloride, particularly in cyclic exposure conditions [1]. Decorative trim, frames, and chassis components of vehicles that operate in Canada and the northern United States are especially affected by road de-icing practices. The corrosion of the highway infrastructure, like iron bridges and pre-stressed concrete with exposed re-bar, is accelerating as Corresponding author. Tel.: +1 865 574 5377; fax: +1 865 574 6918.
E-mail address: [email protected] (P.J. Blau). 0301-679X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2007.02.020
well. The American Trucking Associations, Technology and Maintenance Council, developed a major position paper on the impact of corrosion and featured the growing problem in national industry-wide meetings [2,3]. This corrosion issue has prompted actions to advocate changing the compositions of road de-icing compounds and to develop new strategies for protecting truck parts from the insidious effects of chemical attack. Not only do de-icing and anti-icing salts attack the undercarriage of the vehicles, but they can work their way into the space between brake linings and steel backing plates, causing corrosion product buildup and eventual lining fracture—a condition known as rust jacking. Discovery of cracked linings by inspectors can place a vehicle in a costly out-of-service condition. As corrosion product scales build up behind a lining, the resultant bulge reduces the effective lining contact area against the brake drum and promotes more wear and higher temperature rises during braking. Recently, one manufacturer has
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advertised a protective coating for its steel brake shoe tables that is intended to prevent under-lining scale formation [4]. In addition to rust jacking, the corrosion of the sliding surfaces of brakes by road salts has the potential to alter the transfer layer composition and the braking effectiveness. Considerable past work have addressed the characterization of transfer layers and their role in frictional stability [5–9], but there appear to be no systematic studies on the effects of exposure to road de-icers on friction. Furthermore, both disc and drum brakes are subject to such problems. Despite concerns over the effects of corrosion on the friction of brake sliding surfaces, there are few if any published studies about it. One approach to avoid the corrosion of braking surfaces is to replace the cast iron or steel with a more corrosion-resistant material, but that material must also have satisfactory braking characteristics. In recent years, the US Department of Energy, Office of FreedomCAR and Heavy Vehicle Technologies, has supported studies of alternative brake materials. Lower density, lighter-weight materials, like aluminum metal matrix composites, carbon-graphite, and ceramic composites have a potential to improve fuel efficiency, but they must also accommodate the higher braking demands, which arise from reduced aerodynamic drag and lower tire rolling resistance of next-generation vehicles. In recent years, Oak Ridge National Laboratory has conducted tribology and corrosion studies of titanium alloys and their composites as a possible light-weight, corrosion-resistant alternative to cast iron in disc brakes. Results were summarized in a technical report [10] and a subsequent journal paper [11]. It was the purpose of the current work to investigate the effects of exposure to sodium- and magnesium chloride salt
sprays on the frictional characteristics of gray cast iron and a titanium-based metal matrix composite. A commercial disc brake lining formulation was used as the pad material. The effects of salt exposure on the friction coefficient during repetitive drags and the ability of the braking interface to recover its pre-exposure characteristics were of particular interest. 2. Experimental method and materials Friction tests were conducted on a custom-designed SubScale Brake Testing (SSBT) machine that has been described in more detail elsewhere [12]. It is not an industry standard test method, but rather was designed specifically to conduct research on experimental brake rotor and friction materials using relatively simple specimens. A side view of the testing arrangement is shown in Fig. 1. The apparatus uses a variable-speed, belt-driven spindle to turn a 127 mm diameter disc while a square pad of lining material (12.7 12.7 mm) is pressed against the disc surface by a pneumatically actuated piston. A damping device is included in the air pressure line to avoid impacting the slider on disc, so the normal force is applied over an interval of approximately 0.25 s. Friction force is typically recorded at a data rate of 64 or 128 data/s, using the conditioned output of a load cell that is located below the lining specimen holder. For these experiments, the rotational speed of the disc was held constant at 6.0 m/s (1061 rpm) and five drags were made for each series of tests (20 s applied, 10 s off, repeatedly). Based on the dimensions of a typical heavy truck tire (1.0 m diameter) and a typical air disc brake sliding contact radius, the corresponding vehicle speed would be approximately 36 mph. The normal force was 15072 N. That contact force, acting on the pad area of 161.3 mm2, results in a nominal contact pressure of
Fig. 1. Schematic diagram of the friction testing apparatus showing its major parts.
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0.93 MPa, a value typical of relatively light braking (‘snub’) conditions, the situation where the presence of corrosion scales might have a more noticeable effect on friction. The specimens to be intentionally corroded consisted of gray cast iron and a titanium-based metal matrix composite containing 10 vol% TiC particles, known as CermeTi C10 [13]. The cast iron discs had a graphite flake morphology and an overall composition (3.5 wt% C) that is typical of that used in truck drum brakes (see Fig. 2(a)). The CermeTi C10 is a powder-reinforced metal matrix composite that is produced by a process involving cold isostatic pressing, sintering, and then hot isostatic pressing. The matrix alloy is essentially Ti–6Al–4V and its microstructure is shown in Fig. 3. The Vickers microindentation hardness (10 g-f) of the matrix phase was HV ¼ 3.48 GPa, and that of the TiC particles dispersed within it was HV ¼ 26.0 GPa. Immediately prior to testing, the turned sliding faces of the discs were abraded by hand using 120 grit alumina abrasive paper, and the resulting arithmetic average roughness (Ra) was 0.60 mm (SD 0.11 mm). The abrasion marks were in the circumferential direction, as they would be in a turned disc brake rotor. Measurements of commercial cast iron automobile rotors indicated typical Ra values ranging from about 0.7–1.2 mm, although, the Ra of worn rotors or transfer film-covered rotors may vary beyond this range depending on their operating conditions. The lining material selected for these experiments was Jurid 539TM, a commercial, non-asbestos-containing, semimetallic friction material that is formulated for use in heavy truck air disc brakes. Test specimens were cut from full-sized brake pads and machined into rectangular blocks (12.7 mm square faces with lengths of 15–19 mm). The microstructure of this material is shown in Fig. 2(b). It contains a proprietary mixture of metallic particles and various inorganic compounds in a polymer resin binder. Prior to exposure to various corrosive media, one side of each disc specimen was burnished and friction tested to produce a transfer layer. This initial sliding test also provided a baseline to compare the sliding response of the salt-exposed discs.
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3. Salt exposure and characterization of the corrosion products on cast iron Salt solutions were prepared using USP-grade sodium chloride and magnesium chloride crystals. Sufficient salt was added to tap water to create saturated solutions (35.7 g/100 ml water for NaCl and 54.3 g/100 ml water for MgCl2). After initial burnishing and friction testing to establish a baseline, the disc specimens were placed in a desiccator and suspended over water to maintain a high relative humidity environment. Using a hand atomizer, all specimens were periodically sprayed with a solution of NaCl, MgCl2, or both NaCl and MgCl2, one after the other. After an exposure of 72 h in the moist environment, specimens were removed and allowed to dry in a vertical position, as brake rotors would be oriented on a vehicle as they dried from exposure to splashes from the road. They were not rinsed in water or otherwise cleaned before friction testing, and therefore, their surfaces contained
Fig. 3. Microstructure of the CermeTiTM C10 showing light matrix and faint gray carbide particles. A granular-appearing, 3 mm thick transfer layer that formed during a baseline sliding test is faintly visible lining the top surface of this wear-track cross-section.
Fig. 2. Microstructure of the gray cast iron (a) and the commercial brake lining material (b) used in these experiments.
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deposits of crystallized salts in addition to any corrosion scales that may have formed. The titanium alloy showed no discoloration or scaling of any kind. The only evidence of their exposure to the salts was a few clusters of salt crystals where residual droplets of solution had evaporated. In contrast to the metallic CermeTi C10 surfaces, the cast iron surfaces exposed to either NaCl or both NaCl and MgCl2 were highly rusted (see Fig. 4). They contained frangible, botryoidal, and readily-delaminating crusts of colored scales. The cast iron specimen corroded by both solutions appeared to be somewhat blacker overall than the specimen corroded by NaCl spray alone. Multi-colored assemblages of iron oxide minerals (black, brown, yellow, orange, and red) were observed in addition to a few deposits of white cake, or transparent clear or yellowish, residual salt crystals that apparently formed during the drying process. The microstructures of the rust scales formed on cast iron discs as a result of 72-h exposure to either NaCl or MgCl2 intermittent sprays were analyzed with both X-ray diffraction (XRD) and scanning electron microscopy/ energy-dispersive X-ray spectrometry (SEM/EDS). XRD analysis was performed on the rusted surfaces away from the wear tracks produced by simulated brake testing. Continuous scans were performed on a Panalytical XpertTM diffractometer from nominally 10–901 2y in less than 2 min using CuKa radiation and the X’Celerator detector. A search match was conducted for each data set using Jade matching software and the International Center for Diffraction Data (ICDD) database. SEM/EDS was performed both on the rusted surfaces and the wear tracks, as well as on polished cross-sectional specimens mounted in epoxy to preserve the friable scale. An FEI XL30FEG SEM equipped with an Oxford STW X-ray detector was used in the microstructural and microchemical analysis, generally at 15 kV (or at 5 kV when higher spatial or surface sensitivity was required).
Fig. 4. Appearance of the disc corroded using a combination of magnesium chloride and sodium chloride sprays.
XRD scans for the iron oxide scales from exposure to either NaCl or MgCl2 were similar, and comparison with the ICDD database revealed three possible matches for the crystalline phase(s) present in the rust scale. These matches included the tetragonal and orthorhombic forms of iron oxide hydroxide (nominally FeOOH, ICDD cards #751594 and #73-2326, respectively) and the monoclinic mineral phase Akaganeite (nominally a Cl-modified iron oxide hydroxide, Fe8(O,OH)16Cl1.3). Such hydrated oxide phases would be expected for rust scales formed by exposure to aqueous solutions. Based on peak locations and relative intensities, it was judged that the tetragonal bFeOOH phase was the best match and, despite the varying colors suggestive of other species, it was the dominant phase in the scales. Backscattered electron (BSE) images of cross-sections of the scales from (a) the NaCl-corroded and (b) MgCl2corroded specimens are shown in Fig. 5. Both scales exhibit two major sub-layers that can vary locally in relative thickness and in degree of porosity. Although both scales
Fig. 5. BSE images of cross-sections of (a) NaCl-tested and (b) MgCl2tested specimens. Note the graphite flakes (black, crack-like features) in the bright metal matrix and the scale that has formed between the metal and the graphite in (b). Similar flakes and internal scale was observed below the thicker scale of the NaCl-tested specimen.
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are friable and tend to spall, the more porous, outer scale layer of the NaCl-corroded specimen was the most difficult to retain. The intact scales were 50 and 30 mm thick for the NaCl- and MgCl2-tested materials, respectively. As seen in Fig. 5(b) for the MgCl2-tested specimen, corrosion extended at least 35 mm into the Fe matrix along graphite flakes that intersected the surface. The outer layer of the MgCl2-tested specimen appears slightly brighter than the inner scale in the BSE image in Fig. 5(b). BSE imaging is sensitive to local atomic number, and that might indicate that the outer scale has a higher average atomic number than the inner scale. However, there is no reason to expect more metallic content in the outer scale and that slight difference in shading may also have arisen from instrumental factors, such as relative surface tilt of the inner scale due to polishing relief. No consistent contrast difference was observed for the scale of the NaCl-tested specimen (Fig. 5(a)). As a result of the fine structure, porosity and cracking of the scales, quantitative microanalysis from the crosssectional specimens was generally not feasible. However, semi-quantitative comparisons between the different regions of the scales were helpful in understanding the microstructure of the scales. The major elements present in all of the scales were O and Fe. As hydrogen is not detectable by X-ray microanalysis, these results are consistent with the XRD results. In the NaCl-corroded specimen, both Na and Cl were detected in the scale at levels that decreased from the outer to the inner scales and finally to the scale along the graphite flakes. Some of the Na and Cl detected could be associated with NaCl trapped inside of porosity in the scale. The Na levels detected in the scale were generally lower than that of Cl. In the dense regions of the scale, both Cl and Na were just above detection limits. In the MgCl2-corroded specimen, the detected Cl levels were low and decreased with depth into the scale similar to the NaCl-corroded specimen. However, significant Mg levels were detected in all three regions of the scale. As such, the Mg/Cl ratio varied significantly as a function of depth in the scale. The Mg/O signal ratios were the same for the regions; whereas, the Fe/O and Fe/Mg ratios for the outer scale were higher than the inner scale, consistent with its higher contrast in the BSE image (Fig. 5(b)). The Fe/Mg atomic ratio was estimated as 5/1 for the outer scale and 2.5/1 for the inner scale. The Mg cation can substitute for Fe in some structures, either directly for Fe2+ cations or in conjunction with H+ for Fe3+ cations in defective structures. This substitution appears to be the case for the scales produced by corrosion in the aqueous MgCl2 solution. Both SEM and X-ray microanalysis were performed on plan-view specimens, in order to examine the structure of the wear tracks. Although NaCl and MgCl residue were observed on the as-corroded surface of the respective specimens, no such residue was identified on the wear tracks. BSE images of the wear tracks from (a) the NaCl-
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corroded and (b) the MgCl2-corroded specimens are shown in Fig. 6. At least three contrast levels (with corresponding elemental signatures) are exhibited for wear tracks on either specimen. The brightest areas (B) are the bare iron matrix with only slight oxygen present, possibly from a native oxide formed after wear testing. Little or no Mg, Na or Cl was observed in these regions. The dark regions (D) are consistent with a relatively heavy oxide hydroxide layer on the iron substrate. These areas contained varying low levels of Cl and either Mg or Na, depending on salt exposure. Intermediate gray levels (I) indicate decreased thickness of that layer and lower levels of Cl and Na. In the MgCl2-tested specimen, the Mg levels depended on both remaining scale thickness and location. The cross-sectional specimens of the wear tracks confirmed these observations and indicated that portions of the scale were retained in certain regions. The loss of the scale during wear testing
Fig. 6. BSE images of wear tracks on the surface of (a) NaCl-tested and (b) MgCl2-tested specimens. Dark region at right edge of (a) is the scale at the edge of the wear track. Bright areas are bare iron metal, dark areas are thick scale, and intermediately-shaded areas are partially retained scale. Arrows indicate some graphite flakes exposed at the surface.
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revealed some graphite flakes in the iron substrates (arrows in Fig. 6). Since the scales from specimens tested in pure NaCl and MgCl2 were structurally and chemically similar, it was presumed that the scale from the NaCl plus MgCl2 tested specimen would also be similar. 4. Friction test results The various corrosion exposure conditions used for the cast iron discs in this study are summarized in Table 1. The average friction forces and standard deviations for each of these conditions, for the last 7 s of each of five sequential 10-s drags, are given in Table 2. To facilitate comparison, the average friction data are also shown normalized to the friction of the baseline, non-corroded case in Fig. 7. Each cluster of vertical bars represents the sequence of drags, in chronological order from left to right. Higher friction was observed on the first drag in the series for test conditions B, C, F and G. The conditions in which both salts were applied (F and G) exhibited the most pronounced reductions in friction coefficient overall. The average friction coefficient for five drags at constant speed and load are shown in Fig. 8 for tests on burnished and then corroded cast iron discs. Fig. 9 shows comparable data for titanium composite discs. Results indicate that the friction coefficient for the cast iron recovered its original value by the fourth drag interval, but the friction coefficient
Fig. 7. Average friction force per drag, relative to that for non-corroded cast iron.
Table 1 Sequence of corrosion exposure and testing for cast iron disks Code
Description
A
Tested in the as-finished condition without corrosion exposure (baseline) Corroded 72 h in sprayed sodium chloride solution Burnished then corroded 72 h in sprayed sodium chloride solution Corroded 72 h in sprayed magnesium chloride solution Burnished then corroded 72 h in sprayed magnesium chloride solution Corroded 72 h in sprayed sodium chloride and magnesium chloride solutions Burnished then corroded 72 h in sprayed sodium chloride and magnesium chloride solutions
B C D E F G
Table 2 Friction force data for non-corroded and corroded cast iron surfaces
Fig. 8. Average friction coefficient for each 20 s drag for Jurid 539 sliding on gray cast iron before and after corrosion in a mixture of sodium chloride and magnesium chloride.
for the titanium rotor did not recover its original level, even after five repeated drags. 5. Discussion
Condition
Drag 1
Drag 2
Drag 3
Drag 4
Drag 5
A B C D E F G
45.9 41.6 35.5 31.9 34.3 37.4 25.4
45.4 32.1 27.7 34.5 31.9 14.2 14.0
44.9 31.4 26.8 31.3 32.8 13.8 11.3
44.8 31.4 26.4 30.9 33.9 13.9 11.9
44.6 31.1 26.3 32.1 35.4 16.2 12.5
(3.2) (6.5) (4.7) (9.9) (11.5) (11.7) (4.8)
(3.3) (2.0) (1.4) (5.3) (5.1) (3.6) (5.2)
(3.3) (2.0) (0.7) (3.9) (5.9) (4.1) (3.0)
(3.3) (1.8) (1.2) (3.6) (6.2) (4.2) (3.4)
Average (SD) of friction force (N) for last 7 s of each drag.
(3.1) (2.0) (1.1) (4.3) (6.6) (4.5) (2.9)
As shown in Figs. 8 and 9, the friction coefficients measured on the non-corroded cast iron discs fell within the range of that for a range of typical brake materials (m0.35–0.55); however, those for the Ti tended to be somewhat lower. One of the advantages of the Ti materials is that they are more resistant to loss of friction at elevated temperatures, a phenomenon known as ‘fade’. Therefore, as previous work has shown [10,11], the potential for
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Fig. 9. Average friction coefficient for each 20 s drag for Jurid 539 sliding on the Ti composite before and after corrosion in a mixture of sodium chloride and magnesium chloride.
Ti-based rotor materials to be used in heavy vehicle applications involving long periods of repetitive braking or continual drags occur, seems attractive despite the lower friction coefficients under the less aggressive conditions investigated here. Furthermore, the current lining materials were not optimized for use with Ti alloy surfaces. Observations of the wear tracks on the corroded discs showed that sliding under the stated conditions did not fully remove the remnants of the corrosion scales. Fig. 10 is typical of the appearance of the wear tracks on all corroded discs, exhibiting distinct markings of corrosion products showing through the transfer film on the contact path. Third-bodies, created by the crushing and interfacial pulverization of corrosion scales could act as solid lubricants, and that effect can persist for several subsequent brake applications. To investigate the hypothesis that the frictional behavior upon first engagement was influenced by abrasion by the corrosion scale, a test was conducted on a cast iron disc upon which was applied an adhesive-backed piece of 220 grit silicon carbide abrasive paper. The friction traces in Fig. 11 shows how initial contact began with an erratic sliding period on the first drag, but there was a rapid decrease to a lower friction coefficient that persisted on subsequent drags. Opening the contact to allow the thirdbodies to escape between drags helped to lower friction as well. The momentarily high friction on the first of several drags can be caused by abrasive action from brittle corrosion scales (see Fig. 12). Unlike the influence of corrosion scales, the friction reduction seen in subsequent drags on SiC probably resulted from loading the abrasive paper with shaved lining material, which covered the hard particles. In view of the similarity between the friction traces of linings run against corroded specimens and
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Fig. 10. Image of a wear track on cast iron subjected to corrosion in NaCl spray. The remains of the corrosion products can be seen showing through the transfer film on the wear path.
Fig. 11. Sequence of frictional drags on a 220 grit silicon carbide abrasive paper-covered disc showing an initial rise and drop to a lower friction level.
against abrasive paper, initial abrasion by corrosion scales, followed by third-body layer formation, is a reasonable explanation for the frictional behavior of the intentionally corroded cast iron discs. The effects of sliding speed and contact pressure on persistent friction reduction remains a subject for future study. The fact that the titanium composite disc did not recover its original friction level after exposure to sodium and magnesium chloride salts in concert was likely to have arisen not from the properties of the composite, but rather from the characteristics of the magnesium chloride itself. Figs. 13(a) and (b) compare dried deposits of NaCl and MgCl2, respectively. The sodium chloride crystallized into individual
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Fig. 12. Edge of the wear track on the disc corroded under condition F (see Table 1). The brittle nature of the scales adjacent to the wear track is evident, and similar to Fig. 10, there are indications of residual corrosion products in the wear track.
cubic crystals while the magnesium chloride deposit spread across the surface in a series of crystalline fans. The higher surface area of the magnesium salt deposits, their propensity to absorb moisture, and the higher viscosity of the resulting brine [1] may be the reasons why the titanium composite disc did not recover its friction level as rapidly as the cast iron (see Figs. 8 and 9). While rapid loss of the brittle scales on the cast iron surface helped to take residual magnesium chloride deposits with them, the titanium surfaces had no similar scales that could work in this manner. Thus, the lubricative magnesium salt deposits could remain longer on disc and pad surfaces. Consequently, while Ti discs are expected to last longer due to their excellent corrosion resistance, it may be necessary to rinse or otherwise clean their surfaces to remove residual de-icing salts after exposure (especially those that contain magnesium chloride) in order to restore higher friction levels. 6. Summary and conclusions An investigation was conducted on the effects of corrosion by salt spray on the frictional characteristics of gray cast iron and a corrosion-resistant, titanium-based metal matrix composite (Ti–10 vol% TiC) discs sliding against blocks of a commercial brake lining material. A sub-scale brake materials testing system was used. A series of repeated frictional drags was applied to both sprayexposed and non-spray-exposed discs. The following conclusions were drawn: (1) Spraying the test discs with salt solutions badly corroded the cast iron, but had no observable effects on the Ti composite at equal levels of exposure. XRD and energy dispersive analysis of the scales on the iron,
Fig. 13. Dried deposits from sprays of NaCl(aq) solution (a) and MgCl2(aq) solutions (b) formed different crystal morphologies on the sliding surface.
revealed the presence of iron oxide hydroxide with varying concentrations of alkali metals Mg and Na. The corrosion products on magnesium chloride-exposed surfaces penetrated into the iron substrate along the graphite flakes by as much as 35 mm. (2) Compared with non-exposed discs, the friction coefficients of both the cast iron and Ti composite were reduced after salt spray exposure; however, the magnitude, persistence, and reasons for the observed effects of exposure differed between the two material combinations. (3) For the corroded cast iron, there was an initially-high frictional spike followed by a drop in friction relative to the non-corroded specimen. The lowered friction coefficient persisted for several drags but eventually returned to friction levels similar to those of the noncorroded cast iron even though evidence for iron oxide in the transfer film was observed. The initial spike is
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interpreted as the effect of abrasive scales and the return to normal, non-corroded values of friction could be due to the abrasion and ‘self-dressing’ of the surfaces by fragments of brittle oxide scales. (4) Friction of the exposed titanium composite was also reduced after exposure to salts, but the effects of salt exposure lasted longer. It is hypothesized that the Ti material had no scales to abrade the residual salts away and thus, the deposits remained longer on the surfaces. This suggests that while Ti was far more corrosion resistant than the cast iron, it did not possess the same self-cleaning capability to remove salt deposits. This persistent, friction-lowering effect could be more of a problem for magnesium chloride containing deposits, which tend to absorb moisture from the air and remain more lubricative than those of sodium chloride alone.
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Acknowledgments The authors wish to thank T.R. Watkins for X-ray diffraction characterization of the corrosion scales, T.S. Geer for specimen preparation, and Steve Pawel and Jim Keiser for their helpful comments on the manuscript. This research was sponsored by the US Department of Energy, Office of FreedomCAR and Vehicle Technologies, High Strength Weight Reduction Materials Program, under contract DE-AC05-00OR22725 with UT-Battelle LLC, Oak Ridge, TN. References [1] Xi Y, Xie Z. Corrosion effects of magnesium chloride and sodium chloride on automobile components. Colorado Department of
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Transportation Research Report CDOT-DTD-R-2002-4, Boulder, CO, 2002. Road Chemical Induced Corrosion. Information Report 2002-1 of the S.6. Chassis Study Group. Alexandria, VA: Technology and Maintenance Council, 2002; 19pp. (Contains reprints of trade publications describing the effects of corrosion on truck components.) The battle with corrosion: what’s being done to solve it? technical session #1 of the annual meeting and transportation technology exposition. Ft. Lauderdale, Florida; Technology and Maintenance Council; 2004. Galloway M. Corrosion-fighting coatings help rust jacking. Fleet Maintenance Technol Winter 2004;26. Wirth A, Whitaker R. An energy dispersive X-ray and imaging X-ray photoelectron spectroscopical study of transfer film chemistry and its influence on friction coefficient. J Phys D: Appl Phys 1992;25: A38–43. Eriksson M, Jacobson S. Tribological surfaces of organic brake pads. Tribol Int 2000;33:817–27. O¨sterle W, Griepentrog M, Gross Th, Urban I. Chemical and microstructural changes induced by friction and wear of brakes. Wear 2001;251:1469–76. Blau PJ. Microstructure and detachment mechanism of friction layers on the surface of brake shoes. J Mater Engr Perform 2003;12(1): 56–60. Osterle W, Urban I. Friction layers and friction films on PMC brake pads. Wear 2004;257(1–2):215–26. Blau PJ. Research on non-traditional materials for friction surfaces in heavy vehicle disc brakes. Oak Ridge National Laboratory, Tech. Report ORNL/TM-2004/265, 2004: 29 pp. Blau PJ, Jolly BC, Qu J, Peter WH, Blue CA. Tribological investigation of titanium-based materials for brakes. Wear 2007, in press. Published on-line March 19, 2007. Blau PJ, Meyer III HM. Characteristics of wear particles produced during friction tests of conventional and unconventional disc brake materials. Wear 2003;255(Part 2):1261–9. CermeTi C10 is a product of Dynamet Technology, Inc., Burlington, MA. It is produced by a combination of cold and hot isostatic pressing from powders. Further information provided on the Dynamet website: /http://www.dynamettechnology.com/S.
Tribology International 40 (2007) 1335–1343 www.elsevier.com/locate/triboint
Effects of the exposure to corrosive salts on the frictional behavior of gray cast iron and a titanium-based metal matrix composite Peter J. Blaua,, John J. Truhan Jr.b, Edward A. Kenika a
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA b University of Tennessee, Knoxville, TN, USA
Received 8 September 2006; received in revised form 16 February 2007; accepted 22 February 2007 Available online 2 April 2007
Abstract The introduction of increasingly corrosive road-deicing chemicals has created significant and costly problems for the trucking industry. From a tribological perspective, corrosion of the sliding surfaces of brakes after exposure to road salts can create oxide scales that affect friction. This paper describes experiments on the effects of exposure to sodium chloride and magnesium chloride sprays on the transient frictional behavior of cast iron and a titanium-based composite sliding against a commercial brake lining material. Corrosion scales on cast iron, whose compositions were analyzed by several methods, initially act as abrasive third-bodies. Then they become crushed, spread out, and behave as a solid lubricant. Owing to its greater corrosion resistance, the titanium composite remained scale-free and its frictional response was markedly different. No corrosion scales were formed on the titanium composite after aggressive exposure to salts; however, a reduction in friction was still observed. Unlike the crystalline sodium chloride deposits that tended to remain dry, hygroscopic magnesium chloride deposits absorbed ambient moisture from the air, liquefied, and retained a persistent lubricating effect on the titanium surfaces. r 2007 Elsevier Ltd. All rights reserved. Keywords: Friction; Titanium alloys; Brakes; Cast iron; Corrosion; Metal matrix composites
1. Introduction Together, corrosion and wear exact a high cost on the trucking industry. In particular, the introduction of increasingly aggressive road-deicing chemicals containing magnesium chloride has created corrosion problems for truck fleet operators. A study commissioned by the Colorado Department of Transportation determined that magnesium chloride solutions were more corrosive to standard automotive materials than sodium chloride, particularly in cyclic exposure conditions [1]. Decorative trim, frames, and chassis components of vehicles that operate in Canada and the northern United States are especially affected by road de-icing practices. The corrosion of the highway infrastructure, like iron bridges and pre-stressed concrete with exposed re-bar, is accelerating as Corresponding author. Tel.: +1 865 574 5377; fax: +1 865 574 6918.
E-mail address: [email protected] (P.J. Blau). 0301-679X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2007.02.020
well. The American Trucking Associations, Technology and Maintenance Council, developed a major position paper on the impact of corrosion and featured the growing problem in national industry-wide meetings [2,3]. This corrosion issue has prompted actions to advocate changing the compositions of road de-icing compounds and to develop new strategies for protecting truck parts from the insidious effects of chemical attack. Not only do de-icing and anti-icing salts attack the undercarriage of the vehicles, but they can work their way into the space between brake linings and steel backing plates, causing corrosion product buildup and eventual lining fracture—a condition known as rust jacking. Discovery of cracked linings by inspectors can place a vehicle in a costly out-of-service condition. As corrosion product scales build up behind a lining, the resultant bulge reduces the effective lining contact area against the brake drum and promotes more wear and higher temperature rises during braking. Recently, one manufacturer has
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advertised a protective coating for its steel brake shoe tables that is intended to prevent under-lining scale formation [4]. In addition to rust jacking, the corrosion of the sliding surfaces of brakes by road salts has the potential to alter the transfer layer composition and the braking effectiveness. Considerable past work have addressed the characterization of transfer layers and their role in frictional stability [5–9], but there appear to be no systematic studies on the effects of exposure to road de-icers on friction. Furthermore, both disc and drum brakes are subject to such problems. Despite concerns over the effects of corrosion on the friction of brake sliding surfaces, there are few if any published studies about it. One approach to avoid the corrosion of braking surfaces is to replace the cast iron or steel with a more corrosion-resistant material, but that material must also have satisfactory braking characteristics. In recent years, the US Department of Energy, Office of FreedomCAR and Heavy Vehicle Technologies, has supported studies of alternative brake materials. Lower density, lighter-weight materials, like aluminum metal matrix composites, carbon-graphite, and ceramic composites have a potential to improve fuel efficiency, but they must also accommodate the higher braking demands, which arise from reduced aerodynamic drag and lower tire rolling resistance of next-generation vehicles. In recent years, Oak Ridge National Laboratory has conducted tribology and corrosion studies of titanium alloys and their composites as a possible light-weight, corrosion-resistant alternative to cast iron in disc brakes. Results were summarized in a technical report [10] and a subsequent journal paper [11]. It was the purpose of the current work to investigate the effects of exposure to sodium- and magnesium chloride salt
sprays on the frictional characteristics of gray cast iron and a titanium-based metal matrix composite. A commercial disc brake lining formulation was used as the pad material. The effects of salt exposure on the friction coefficient during repetitive drags and the ability of the braking interface to recover its pre-exposure characteristics were of particular interest. 2. Experimental method and materials Friction tests were conducted on a custom-designed SubScale Brake Testing (SSBT) machine that has been described in more detail elsewhere [12]. It is not an industry standard test method, but rather was designed specifically to conduct research on experimental brake rotor and friction materials using relatively simple specimens. A side view of the testing arrangement is shown in Fig. 1. The apparatus uses a variable-speed, belt-driven spindle to turn a 127 mm diameter disc while a square pad of lining material (12.7 12.7 mm) is pressed against the disc surface by a pneumatically actuated piston. A damping device is included in the air pressure line to avoid impacting the slider on disc, so the normal force is applied over an interval of approximately 0.25 s. Friction force is typically recorded at a data rate of 64 or 128 data/s, using the conditioned output of a load cell that is located below the lining specimen holder. For these experiments, the rotational speed of the disc was held constant at 6.0 m/s (1061 rpm) and five drags were made for each series of tests (20 s applied, 10 s off, repeatedly). Based on the dimensions of a typical heavy truck tire (1.0 m diameter) and a typical air disc brake sliding contact radius, the corresponding vehicle speed would be approximately 36 mph. The normal force was 15072 N. That contact force, acting on the pad area of 161.3 mm2, results in a nominal contact pressure of
Fig. 1. Schematic diagram of the friction testing apparatus showing its major parts.
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0.93 MPa, a value typical of relatively light braking (‘snub’) conditions, the situation where the presence of corrosion scales might have a more noticeable effect on friction. The specimens to be intentionally corroded consisted of gray cast iron and a titanium-based metal matrix composite containing 10 vol% TiC particles, known as CermeTi C10 [13]. The cast iron discs had a graphite flake morphology and an overall composition (3.5 wt% C) that is typical of that used in truck drum brakes (see Fig. 2(a)). The CermeTi C10 is a powder-reinforced metal matrix composite that is produced by a process involving cold isostatic pressing, sintering, and then hot isostatic pressing. The matrix alloy is essentially Ti–6Al–4V and its microstructure is shown in Fig. 3. The Vickers microindentation hardness (10 g-f) of the matrix phase was HV ¼ 3.48 GPa, and that of the TiC particles dispersed within it was HV ¼ 26.0 GPa. Immediately prior to testing, the turned sliding faces of the discs were abraded by hand using 120 grit alumina abrasive paper, and the resulting arithmetic average roughness (Ra) was 0.60 mm (SD 0.11 mm). The abrasion marks were in the circumferential direction, as they would be in a turned disc brake rotor. Measurements of commercial cast iron automobile rotors indicated typical Ra values ranging from about 0.7–1.2 mm, although, the Ra of worn rotors or transfer film-covered rotors may vary beyond this range depending on their operating conditions. The lining material selected for these experiments was Jurid 539TM, a commercial, non-asbestos-containing, semimetallic friction material that is formulated for use in heavy truck air disc brakes. Test specimens were cut from full-sized brake pads and machined into rectangular blocks (12.7 mm square faces with lengths of 15–19 mm). The microstructure of this material is shown in Fig. 2(b). It contains a proprietary mixture of metallic particles and various inorganic compounds in a polymer resin binder. Prior to exposure to various corrosive media, one side of each disc specimen was burnished and friction tested to produce a transfer layer. This initial sliding test also provided a baseline to compare the sliding response of the salt-exposed discs.
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3. Salt exposure and characterization of the corrosion products on cast iron Salt solutions were prepared using USP-grade sodium chloride and magnesium chloride crystals. Sufficient salt was added to tap water to create saturated solutions (35.7 g/100 ml water for NaCl and 54.3 g/100 ml water for MgCl2). After initial burnishing and friction testing to establish a baseline, the disc specimens were placed in a desiccator and suspended over water to maintain a high relative humidity environment. Using a hand atomizer, all specimens were periodically sprayed with a solution of NaCl, MgCl2, or both NaCl and MgCl2, one after the other. After an exposure of 72 h in the moist environment, specimens were removed and allowed to dry in a vertical position, as brake rotors would be oriented on a vehicle as they dried from exposure to splashes from the road. They were not rinsed in water or otherwise cleaned before friction testing, and therefore, their surfaces contained
Fig. 3. Microstructure of the CermeTiTM C10 showing light matrix and faint gray carbide particles. A granular-appearing, 3 mm thick transfer layer that formed during a baseline sliding test is faintly visible lining the top surface of this wear-track cross-section.
Fig. 2. Microstructure of the gray cast iron (a) and the commercial brake lining material (b) used in these experiments.
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deposits of crystallized salts in addition to any corrosion scales that may have formed. The titanium alloy showed no discoloration or scaling of any kind. The only evidence of their exposure to the salts was a few clusters of salt crystals where residual droplets of solution had evaporated. In contrast to the metallic CermeTi C10 surfaces, the cast iron surfaces exposed to either NaCl or both NaCl and MgCl2 were highly rusted (see Fig. 4). They contained frangible, botryoidal, and readily-delaminating crusts of colored scales. The cast iron specimen corroded by both solutions appeared to be somewhat blacker overall than the specimen corroded by NaCl spray alone. Multi-colored assemblages of iron oxide minerals (black, brown, yellow, orange, and red) were observed in addition to a few deposits of white cake, or transparent clear or yellowish, residual salt crystals that apparently formed during the drying process. The microstructures of the rust scales formed on cast iron discs as a result of 72-h exposure to either NaCl or MgCl2 intermittent sprays were analyzed with both X-ray diffraction (XRD) and scanning electron microscopy/ energy-dispersive X-ray spectrometry (SEM/EDS). XRD analysis was performed on the rusted surfaces away from the wear tracks produced by simulated brake testing. Continuous scans were performed on a Panalytical XpertTM diffractometer from nominally 10–901 2y in less than 2 min using CuKa radiation and the X’Celerator detector. A search match was conducted for each data set using Jade matching software and the International Center for Diffraction Data (ICDD) database. SEM/EDS was performed both on the rusted surfaces and the wear tracks, as well as on polished cross-sectional specimens mounted in epoxy to preserve the friable scale. An FEI XL30FEG SEM equipped with an Oxford STW X-ray detector was used in the microstructural and microchemical analysis, generally at 15 kV (or at 5 kV when higher spatial or surface sensitivity was required).
Fig. 4. Appearance of the disc corroded using a combination of magnesium chloride and sodium chloride sprays.
XRD scans for the iron oxide scales from exposure to either NaCl or MgCl2 were similar, and comparison with the ICDD database revealed three possible matches for the crystalline phase(s) present in the rust scale. These matches included the tetragonal and orthorhombic forms of iron oxide hydroxide (nominally FeOOH, ICDD cards #751594 and #73-2326, respectively) and the monoclinic mineral phase Akaganeite (nominally a Cl-modified iron oxide hydroxide, Fe8(O,OH)16Cl1.3). Such hydrated oxide phases would be expected for rust scales formed by exposure to aqueous solutions. Based on peak locations and relative intensities, it was judged that the tetragonal bFeOOH phase was the best match and, despite the varying colors suggestive of other species, it was the dominant phase in the scales. Backscattered electron (BSE) images of cross-sections of the scales from (a) the NaCl-corroded and (b) MgCl2corroded specimens are shown in Fig. 5. Both scales exhibit two major sub-layers that can vary locally in relative thickness and in degree of porosity. Although both scales
Fig. 5. BSE images of cross-sections of (a) NaCl-tested and (b) MgCl2tested specimens. Note the graphite flakes (black, crack-like features) in the bright metal matrix and the scale that has formed between the metal and the graphite in (b). Similar flakes and internal scale was observed below the thicker scale of the NaCl-tested specimen.
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are friable and tend to spall, the more porous, outer scale layer of the NaCl-corroded specimen was the most difficult to retain. The intact scales were 50 and 30 mm thick for the NaCl- and MgCl2-tested materials, respectively. As seen in Fig. 5(b) for the MgCl2-tested specimen, corrosion extended at least 35 mm into the Fe matrix along graphite flakes that intersected the surface. The outer layer of the MgCl2-tested specimen appears slightly brighter than the inner scale in the BSE image in Fig. 5(b). BSE imaging is sensitive to local atomic number, and that might indicate that the outer scale has a higher average atomic number than the inner scale. However, there is no reason to expect more metallic content in the outer scale and that slight difference in shading may also have arisen from instrumental factors, such as relative surface tilt of the inner scale due to polishing relief. No consistent contrast difference was observed for the scale of the NaCl-tested specimen (Fig. 5(a)). As a result of the fine structure, porosity and cracking of the scales, quantitative microanalysis from the crosssectional specimens was generally not feasible. However, semi-quantitative comparisons between the different regions of the scales were helpful in understanding the microstructure of the scales. The major elements present in all of the scales were O and Fe. As hydrogen is not detectable by X-ray microanalysis, these results are consistent with the XRD results. In the NaCl-corroded specimen, both Na and Cl were detected in the scale at levels that decreased from the outer to the inner scales and finally to the scale along the graphite flakes. Some of the Na and Cl detected could be associated with NaCl trapped inside of porosity in the scale. The Na levels detected in the scale were generally lower than that of Cl. In the dense regions of the scale, both Cl and Na were just above detection limits. In the MgCl2-corroded specimen, the detected Cl levels were low and decreased with depth into the scale similar to the NaCl-corroded specimen. However, significant Mg levels were detected in all three regions of the scale. As such, the Mg/Cl ratio varied significantly as a function of depth in the scale. The Mg/O signal ratios were the same for the regions; whereas, the Fe/O and Fe/Mg ratios for the outer scale were higher than the inner scale, consistent with its higher contrast in the BSE image (Fig. 5(b)). The Fe/Mg atomic ratio was estimated as 5/1 for the outer scale and 2.5/1 for the inner scale. The Mg cation can substitute for Fe in some structures, either directly for Fe2+ cations or in conjunction with H+ for Fe3+ cations in defective structures. This substitution appears to be the case for the scales produced by corrosion in the aqueous MgCl2 solution. Both SEM and X-ray microanalysis were performed on plan-view specimens, in order to examine the structure of the wear tracks. Although NaCl and MgCl residue were observed on the as-corroded surface of the respective specimens, no such residue was identified on the wear tracks. BSE images of the wear tracks from (a) the NaCl-
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corroded and (b) the MgCl2-corroded specimens are shown in Fig. 6. At least three contrast levels (with corresponding elemental signatures) are exhibited for wear tracks on either specimen. The brightest areas (B) are the bare iron matrix with only slight oxygen present, possibly from a native oxide formed after wear testing. Little or no Mg, Na or Cl was observed in these regions. The dark regions (D) are consistent with a relatively heavy oxide hydroxide layer on the iron substrate. These areas contained varying low levels of Cl and either Mg or Na, depending on salt exposure. Intermediate gray levels (I) indicate decreased thickness of that layer and lower levels of Cl and Na. In the MgCl2-tested specimen, the Mg levels depended on both remaining scale thickness and location. The cross-sectional specimens of the wear tracks confirmed these observations and indicated that portions of the scale were retained in certain regions. The loss of the scale during wear testing
Fig. 6. BSE images of wear tracks on the surface of (a) NaCl-tested and (b) MgCl2-tested specimens. Dark region at right edge of (a) is the scale at the edge of the wear track. Bright areas are bare iron metal, dark areas are thick scale, and intermediately-shaded areas are partially retained scale. Arrows indicate some graphite flakes exposed at the surface.
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revealed some graphite flakes in the iron substrates (arrows in Fig. 6). Since the scales from specimens tested in pure NaCl and MgCl2 were structurally and chemically similar, it was presumed that the scale from the NaCl plus MgCl2 tested specimen would also be similar. 4. Friction test results The various corrosion exposure conditions used for the cast iron discs in this study are summarized in Table 1. The average friction forces and standard deviations for each of these conditions, for the last 7 s of each of five sequential 10-s drags, are given in Table 2. To facilitate comparison, the average friction data are also shown normalized to the friction of the baseline, non-corroded case in Fig. 7. Each cluster of vertical bars represents the sequence of drags, in chronological order from left to right. Higher friction was observed on the first drag in the series for test conditions B, C, F and G. The conditions in which both salts were applied (F and G) exhibited the most pronounced reductions in friction coefficient overall. The average friction coefficient for five drags at constant speed and load are shown in Fig. 8 for tests on burnished and then corroded cast iron discs. Fig. 9 shows comparable data for titanium composite discs. Results indicate that the friction coefficient for the cast iron recovered its original value by the fourth drag interval, but the friction coefficient
Fig. 7. Average friction force per drag, relative to that for non-corroded cast iron.
Table 1 Sequence of corrosion exposure and testing for cast iron disks Code
Description
A
Tested in the as-finished condition without corrosion exposure (baseline) Corroded 72 h in sprayed sodium chloride solution Burnished then corroded 72 h in sprayed sodium chloride solution Corroded 72 h in sprayed magnesium chloride solution Burnished then corroded 72 h in sprayed magnesium chloride solution Corroded 72 h in sprayed sodium chloride and magnesium chloride solutions Burnished then corroded 72 h in sprayed sodium chloride and magnesium chloride solutions
B C D E F G
Table 2 Friction force data for non-corroded and corroded cast iron surfaces
Fig. 8. Average friction coefficient for each 20 s drag for Jurid 539 sliding on gray cast iron before and after corrosion in a mixture of sodium chloride and magnesium chloride.
for the titanium rotor did not recover its original level, even after five repeated drags. 5. Discussion
Condition
Drag 1
Drag 2
Drag 3
Drag 4
Drag 5
A B C D E F G
45.9 41.6 35.5 31.9 34.3 37.4 25.4
45.4 32.1 27.7 34.5 31.9 14.2 14.0
44.9 31.4 26.8 31.3 32.8 13.8 11.3
44.8 31.4 26.4 30.9 33.9 13.9 11.9
44.6 31.1 26.3 32.1 35.4 16.2 12.5
(3.2) (6.5) (4.7) (9.9) (11.5) (11.7) (4.8)
(3.3) (2.0) (1.4) (5.3) (5.1) (3.6) (5.2)
(3.3) (2.0) (0.7) (3.9) (5.9) (4.1) (3.0)
(3.3) (1.8) (1.2) (3.6) (6.2) (4.2) (3.4)
Average (SD) of friction force (N) for last 7 s of each drag.
(3.1) (2.0) (1.1) (4.3) (6.6) (4.5) (2.9)
As shown in Figs. 8 and 9, the friction coefficients measured on the non-corroded cast iron discs fell within the range of that for a range of typical brake materials (m0.35–0.55); however, those for the Ti tended to be somewhat lower. One of the advantages of the Ti materials is that they are more resistant to loss of friction at elevated temperatures, a phenomenon known as ‘fade’. Therefore, as previous work has shown [10,11], the potential for
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Fig. 9. Average friction coefficient for each 20 s drag for Jurid 539 sliding on the Ti composite before and after corrosion in a mixture of sodium chloride and magnesium chloride.
Ti-based rotor materials to be used in heavy vehicle applications involving long periods of repetitive braking or continual drags occur, seems attractive despite the lower friction coefficients under the less aggressive conditions investigated here. Furthermore, the current lining materials were not optimized for use with Ti alloy surfaces. Observations of the wear tracks on the corroded discs showed that sliding under the stated conditions did not fully remove the remnants of the corrosion scales. Fig. 10 is typical of the appearance of the wear tracks on all corroded discs, exhibiting distinct markings of corrosion products showing through the transfer film on the contact path. Third-bodies, created by the crushing and interfacial pulverization of corrosion scales could act as solid lubricants, and that effect can persist for several subsequent brake applications. To investigate the hypothesis that the frictional behavior upon first engagement was influenced by abrasion by the corrosion scale, a test was conducted on a cast iron disc upon which was applied an adhesive-backed piece of 220 grit silicon carbide abrasive paper. The friction traces in Fig. 11 shows how initial contact began with an erratic sliding period on the first drag, but there was a rapid decrease to a lower friction coefficient that persisted on subsequent drags. Opening the contact to allow the thirdbodies to escape between drags helped to lower friction as well. The momentarily high friction on the first of several drags can be caused by abrasive action from brittle corrosion scales (see Fig. 12). Unlike the influence of corrosion scales, the friction reduction seen in subsequent drags on SiC probably resulted from loading the abrasive paper with shaved lining material, which covered the hard particles. In view of the similarity between the friction traces of linings run against corroded specimens and
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Fig. 10. Image of a wear track on cast iron subjected to corrosion in NaCl spray. The remains of the corrosion products can be seen showing through the transfer film on the wear path.
Fig. 11. Sequence of frictional drags on a 220 grit silicon carbide abrasive paper-covered disc showing an initial rise and drop to a lower friction level.
against abrasive paper, initial abrasion by corrosion scales, followed by third-body layer formation, is a reasonable explanation for the frictional behavior of the intentionally corroded cast iron discs. The effects of sliding speed and contact pressure on persistent friction reduction remains a subject for future study. The fact that the titanium composite disc did not recover its original friction level after exposure to sodium and magnesium chloride salts in concert was likely to have arisen not from the properties of the composite, but rather from the characteristics of the magnesium chloride itself. Figs. 13(a) and (b) compare dried deposits of NaCl and MgCl2, respectively. The sodium chloride crystallized into individual
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Fig. 12. Edge of the wear track on the disc corroded under condition F (see Table 1). The brittle nature of the scales adjacent to the wear track is evident, and similar to Fig. 10, there are indications of residual corrosion products in the wear track.
cubic crystals while the magnesium chloride deposit spread across the surface in a series of crystalline fans. The higher surface area of the magnesium salt deposits, their propensity to absorb moisture, and the higher viscosity of the resulting brine [1] may be the reasons why the titanium composite disc did not recover its friction level as rapidly as the cast iron (see Figs. 8 and 9). While rapid loss of the brittle scales on the cast iron surface helped to take residual magnesium chloride deposits with them, the titanium surfaces had no similar scales that could work in this manner. Thus, the lubricative magnesium salt deposits could remain longer on disc and pad surfaces. Consequently, while Ti discs are expected to last longer due to their excellent corrosion resistance, it may be necessary to rinse or otherwise clean their surfaces to remove residual de-icing salts after exposure (especially those that contain magnesium chloride) in order to restore higher friction levels. 6. Summary and conclusions An investigation was conducted on the effects of corrosion by salt spray on the frictional characteristics of gray cast iron and a corrosion-resistant, titanium-based metal matrix composite (Ti–10 vol% TiC) discs sliding against blocks of a commercial brake lining material. A sub-scale brake materials testing system was used. A series of repeated frictional drags was applied to both sprayexposed and non-spray-exposed discs. The following conclusions were drawn: (1) Spraying the test discs with salt solutions badly corroded the cast iron, but had no observable effects on the Ti composite at equal levels of exposure. XRD and energy dispersive analysis of the scales on the iron,
Fig. 13. Dried deposits from sprays of NaCl(aq) solution (a) and MgCl2(aq) solutions (b) formed different crystal morphologies on the sliding surface.
revealed the presence of iron oxide hydroxide with varying concentrations of alkali metals Mg and Na. The corrosion products on magnesium chloride-exposed surfaces penetrated into the iron substrate along the graphite flakes by as much as 35 mm. (2) Compared with non-exposed discs, the friction coefficients of both the cast iron and Ti composite were reduced after salt spray exposure; however, the magnitude, persistence, and reasons for the observed effects of exposure differed between the two material combinations. (3) For the corroded cast iron, there was an initially-high frictional spike followed by a drop in friction relative to the non-corroded specimen. The lowered friction coefficient persisted for several drags but eventually returned to friction levels similar to those of the noncorroded cast iron even though evidence for iron oxide in the transfer film was observed. The initial spike is
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interpreted as the effect of abrasive scales and the return to normal, non-corroded values of friction could be due to the abrasion and ‘self-dressing’ of the surfaces by fragments of brittle oxide scales. (4) Friction of the exposed titanium composite was also reduced after exposure to salts, but the effects of salt exposure lasted longer. It is hypothesized that the Ti material had no scales to abrade the residual salts away and thus, the deposits remained longer on the surfaces. This suggests that while Ti was far more corrosion resistant than the cast iron, it did not possess the same self-cleaning capability to remove salt deposits. This persistent, friction-lowering effect could be more of a problem for magnesium chloride containing deposits, which tend to absorb moisture from the air and remain more lubricative than those of sodium chloride alone.
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Acknowledgments The authors wish to thank T.R. Watkins for X-ray diffraction characterization of the corrosion scales, T.S. Geer for specimen preparation, and Steve Pawel and Jim Keiser for their helpful comments on the manuscript. This research was sponsored by the US Department of Energy, Office of FreedomCAR and Vehicle Technologies, High Strength Weight Reduction Materials Program, under contract DE-AC05-00OR22725 with UT-Battelle LLC, Oak Ridge, TN. References [1] Xi Y, Xie Z. Corrosion effects of magnesium chloride and sodium chloride on automobile components. Colorado Department of
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Transportation Research Report CDOT-DTD-R-2002-4, Boulder, CO, 2002. Road Chemical Induced Corrosion. Information Report 2002-1 of the S.6. Chassis Study Group. Alexandria, VA: Technology and Maintenance Council, 2002; 19pp. (Contains reprints of trade publications describing the effects of corrosion on truck components.) The battle with corrosion: what’s being done to solve it? technical session #1 of the annual meeting and transportation technology exposition. Ft. Lauderdale, Florida; Technology and Maintenance Council; 2004. Galloway M. Corrosion-fighting coatings help rust jacking. Fleet Maintenance Technol Winter 2004;26. Wirth A, Whitaker R. An energy dispersive X-ray and imaging X-ray photoelectron spectroscopical study of transfer film chemistry and its influence on friction coefficient. J Phys D: Appl Phys 1992;25: A38–43. Eriksson M, Jacobson S. Tribological surfaces of organic brake pads. Tribol Int 2000;33:817–27. O¨sterle W, Griepentrog M, Gross Th, Urban I. Chemical and microstructural changes induced by friction and wear of brakes. Wear 2001;251:1469–76. Blau PJ. Microstructure and detachment mechanism of friction layers on the surface of brake shoes. J Mater Engr Perform 2003;12(1): 56–60. Osterle W, Urban I. Friction layers and friction films on PMC brake pads. Wear 2004;257(1–2):215–26. Blau PJ. Research on non-traditional materials for friction surfaces in heavy vehicle disc brakes. Oak Ridge National Laboratory, Tech. Report ORNL/TM-2004/265, 2004: 29 pp. Blau PJ, Jolly BC, Qu J, Peter WH, Blue CA. Tribological investigation of titanium-based materials for brakes. Wear 2007, in press. Published on-line March 19, 2007. Blau PJ, Meyer III HM. Characteristics of wear particles produced during friction tests of conventional and unconventional disc brake materials. Wear 2003;255(Part 2):1261–9. CermeTi C10 is a product of Dynamet Technology, Inc., Burlington, MA. It is produced by a combination of cold and hot isostatic pressing from powders. Further information provided on the Dynamet website: /http://www.dynamettechnology.com/S.