- Email: [email protected]
Effect of oxides on wear resistance and surface roughness of ferrous coated layers fabricated by atmospheric plasma spraying
Materials Science and Engineering A335 (2002) 268– 280 www.elsevier.com/locate/msea
Effect of oxides on wear resistance and surface roughness of ferrous coated layers fabricated by atmospheric plasma spraying Byoungchul Hwang a, Sunghak Lee a,*, Jeehoon Ahn b b
a Center for Ad6anced Aerospace Materials, Pohang Uni6ersity of Science and Technology, Pohang 790 -784, South Korea Materials & Processes Research Center, Research Institute of Industrial Science and Technology, Pohang 790 -600, South Korea
Received 29 August 2001; received in revised form 14 November 2001
Abstract The objective of this study is to investigate the correlation of microstructure, wear resistance, and surface roughness in ferrous coated layers applicable to cylinder bores. Four kinds of ferrous spray powders, two of which were STS 316 steel powders and the others were blend powders of ferrous powders mixed with 20 wt.% Al2O3 –ZrO2 powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the coated layers showed that iron oxides were formed in the austenitic matrix by oxidation during spraying for the STS 316 coated layers, while Al2O3 –ZrO2 oxides were mainly formed in the martensitic matrix for the blend coated layers. The wear test results revealed that the blend coated layers showed the better wear resistance than the STS 316 coated layers because they contained a number of hard Al2O3 –ZrO2 oxides. However, they had rough worn surfaces because of the preferential removal of the matrix and the cracking of oxides during the wear process. The STS 316 coated layers showed the slightly worse wear resistance than the blend coated layers, but they showed the excellent surface roughness resulting from homogeneous wear in oxides and matrix due to the smaller hardness difference between them. In order to improve the overall wear properties with consideration of the wear resistance of a counterpart material and the surface roughness, the hardness difference between oxides and matrix should be minimized, while the hardness should be maintained up to a certain level by forming an appropriate amount of oxides. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Atmospheric plasma spraying; Ferrous coated layer; Wear resistance; Surface roughness; Oxide
1. Introduction Physical and chemical properties such as resistance to erosion, wear, oxidation, and heat largely depend on surface properties. In order to improve these properties, coating with metals, oxides, and organic materials is widely applied to the surface of structural materials and machinery parts. Coating methods include electro-coating, physical vapor deposition (PVD), chemical vapor deposition (CVD), ion plating, hardfacing, thermal spraying, etc. [1]. Among them, thermal spraying is most widely used for industrial applications since, (1) it does not have any limitations on the shape and kind of a substrate; (2) it is relatively easy to control the thickness of a coated layer; * Corresponding author. Tel.: + 82-562-279-2715; fax: + 82-562279-2399. E-mail address: [email protected] (S. Lee).
(3) a coated layer can be formed quickly, and (4) it is less limited in application scopes and coating materials [2,3]. Among thermal spraying methods, plasma spraying is intensively applied to automotive industry to improve engine efficiency and wear resistance of engine parts such as cylinder bore, crankshaft, and piston ring. Spraying powders are first melted by plasma heat source, and then quickly sprayed onto a substrate to form a coated layer on it [1–9]. Particularly, cylinder blocks which have been entirely made of a gray cast iron are now manufactured by thermal spraying of Al –Si alloys, and it is expected that about half of the cylinder blocks produced worldwide will be replaced with aluminum alloys within a couple of years [4]. Since the economic benefit from lighter body and less fuel consumption is of great importance, many automotive companies are encouraged to undertake active studies on ferrous coating materials for cylinder bores [5].
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 9 3 7 - 2
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
As the wear process occurring inside a diesel engine depends on the interaction of cylinder, piston ring, and engine lubricant, it is imperative to strike a balance among these in order to reduce cylinder wear and oil consumption. A suitable coating material for this end should meet following requirements, (1) low friction coefficient and wear rate against a piston ring in a lubricated condition; (2) lower wear rate than a cast iron liner for a same tribosystem; (3) good resistance to thermal shock; and (4) consistent coating properties to provide high surface finish reliability [7– 9]. In order to provide cylinder bores with adequate wear and friction properties, it is thus necessary to promote a good lubrication functioning with piston rings by forming a proper amount of pores and reinforced phases in the coated layer as well as to select a suitable spray powder and to optimize spraying conditions. The present study is concerned with the fabrication of ferrous coated layers by atmospheric plasma spraying applicable to cylinder bores and with the investigation of the effect of oxides on hardness, wear resistance, and surface roughness. For these purposes, four kinds of ferrous coated layers were fabricated by changing ferrous powders which determine the total oxide volume fraction. Microstructure, hardness, and wear properties of the fabricated ferrous coated layers were comparatively analyzed to understand the mechanisms involved in the wear and surface roughening processes.
2. Experimental
2.1. Spray powders Spray powders used for the fabrication of ferrous coated layers are four kinds of ferrous powders, i.e. Metco 41C, Diamalloy 1003, 91, and Metco 42C which are commercial brand names of Sulzer Metco Inc., New York, USA [10], and commercialized Al2O3 – ZrO2 powders (60 wt.% Al2O3 +40 wt.% ZrO2). Chemical compositions of the ferrous powders are provided in Table 1. Metco 41C and Diamalloy 1003 are made of an STS
269
316 stainless steel, 91 is of a plain carbon steel, and Metco 42C is of an STS 431 stainless steel. These spray powders are observed using a scanning electron microscope (SEM) as shown in Fig. 1(a through e), and their properties are summarized in Table 1. Metco 41C powders processed by water atomization have an irregular shape (Fig. 1(a)), and Diamalloy 1003 powders processed by gas atomization are fine and spheroidal (Fig. 1(b)). 91 powders show an irregular morphology of a spongy shape with a number of pores (Fig. 1(c)). Metco 42C powders processed by water atomization look alike with Metco 41C powders in their size and shape (Fig. 1(d)). Al2O3 –ZrO2 powders have an irregular blocky shape (Fig. 1(e)), and their size is ranged from 45 to 75 mm. SEM observation of the inner cross-section of Al2O3 –ZrO2 powders reveals that the dark gray area (arrow-marked) is surrounded by the light gray area (Fig. 2(a)). Fig. 2(b through d) are EDS (energy dispersive spectroscopy) mappings of the crosssection of Al2O3 –ZrO2 powders. These gray areas show oxides composed of Al2O3 (the inner dark area) and ZrO2 (the outer light area).
2.2. Fabrication of ferrous coated layer Two kinds of STS 316 coated layers were fabricated using Metco 41C and Diamalloy 1003 powders, and two kinds of blend coated layers were fabricated by mixing 91 and Metco 42C powders with Al2O3 –ZrO2 powders to increase the volume fraction of oxides. For convenience, the STS 316 coated layers sprayed by Metco 41C and Diamalloy 1003 powders are hereinafter referred to as ‘A1’ and ‘A2’, respectively, while the coated layers sprayed by blend powders of 91 and Metco 42C powders mixed with 20 wt.% Al2O3 – ZrO2 powders as ‘B1’ and ‘B2’, respectively. A plain carbon steel (chemical composition: Fe–0.45C–0.3Si– 0.75Mo–0.03P–0.035S (wt.%)) was used as a substrate, which was cut into 30× 30× 5 mm in size for a wear test specimen. Its surface was polished and blasted with Al2O3 grits (0.6–1.4 mm in diameter) to improve the adhesive bond strength between the coated layer and
Table 1 Compositions and characteristics of ferrous powders used for atmospheric plasma spray coating Powder
Metco 41Ca
Diamalloy 1003a
91a
Metco 42Ca
Chemical Composition Particle size (mm) Manufacturing method Particle shape
Fe-17Cr–12Ni–2.5Mo–1Si–0.1C (STS 316) 45–106
Fe–17Cr–12Ni– 2.5Mo–1Si–0.1C (STS 316) 11–45
Fe–0.2C–0.5Mn (Plain Carbon Steel) 45–106
Fe–16Cr–2Ni–0.2C (STS 431) 45–106
Water atomized
Gas atomized
Agglomerated and sintered
Water atomized
Irregular
Spherical
Irregular spongy and blocky
Irregular
a
Commercial brand names of ferrous powders obtained from Sulzer Metco Inc. [10].
270
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
Fig. 1. SEM micrographs of (a) Metco 41C, (b) Diamalloy 1003, (c) 91, (d) Metco 42C, and (e) Al2O3 – ZrO2 powders.
the substrate, and was ultrasonically cleansed with acetone and alcohol. Plasma spraying was conducted on the substrate using a Sulzer Metco 9MB spray system, and hydrogen combined with argon was used as fuel gas. To prevent overheating in specimens during spraying, a specimen holder was cooled with compressed air, and the traverse speed of a spraying gun was maintained constant. Detailed plasma spraying conditions are listed in Table 2.
A gray cast iron (FCH2D), which was completely finished as a diesel engine cylinder block (chemical composition: Fe–3.54C– 2.21Si–0.674Mn–0.025Cr– 0.013Cu–0.056P– 0.031S (wt.%)), was chosen as a standard to compare the coated layers with. It contains flake graphites, pores, and MnS inclusions in the pearlitic matrix. Pores are typical defects formed during casting, and MnS inclusions hardly affect properties of the cast iron [11].
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
271
Fig. 2. (a) SEM image of Al2O3 –ZrO2 powders and (b) through (d) EDS mappings of O, Al, and Zr, respectively.
2.3. Microstructural analysis and hardness test The ferrous coated layers were sectioned perpendicular to the coated surface, polished, and observed by an optical microscope and an SEM. Phases present in the coated layers and their chemical compositions were analyzed by X-ray diffraction (XRD) and EDS analyses, respectively. Hardness was measured by a Vickers hardness tester under a 300 g load, and microhardness of matrix and oxides was measured under a 10 g load by an ultra micro-vickers hardness tester.
hardness level of a commercial piston ring. According to the ASTM G99-95a condition (surface roughness, Ra 5 0.8 mm), the spray coated layer was ground using a grinder to make the surface roughness constant at about 0.1 mm, and then was used as the disc specimen. The disc specimen is worn in contact with the upper pin as the rotating axis rotates, while being pressed from the upper body of the wear tester, under both unlubricated and lubricated conditions. In the unlubricated condition, the test was performed under a 3 kgf load at room temperature for 30 min, whereas in the lubricated condition, it was done under a 20 kgf load at 100 °C
2.4. West test Wear test was conducted on the coated layers using a pin-on-disc type wear tester (Model; EFM-III-EN/F, Orientec Co., Japan) in accordance with ASTM G9995a [12]. An SAE 9254V spring steel (chemical composition: Fe –0.55C– 1.4Si–0.7Mn – 0.015Ni – 0.7C – 0.035P – 0.04S (wt.%)) was used for the pin (diameter; 5 mm). It was subjected to a nitriding treatment at 470 °C to raise the surface hardness over 700 VHN so that it matches the
Table 2 Atmospheric plasma spraying conditions Arc flow rate Arc pressure Auxiliary gas flow rate Auxiliary gas pressure Spray rate Arc voltage Arc current Spray distance
80–100 l min−1 0.689 MPa 5–15 l min−1 0.345 MPa 2.7–6.8 kg h−1 61–68 V 400–500 A 70–100 mm
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
272
Fig. 3. Optical micrographs of the (a) A1-, (b) A2-, (c) B1-, and (d) B2-specimens. Not etched.
for 30 min using an SAE 10W30 automotive engine oil as a lubricant. The coated layers would be all worn out if exposed to too a heavy load or for a long time, whereas the weight loss after the wear test would be too small if under too a small load. This would make proper evaluation of the wear resistance quite difficult. Thus, adequate loading and time were selected for both the unlubricated and lubricated conditions in this study. Rotation velocity and wear distance were 200 rpm and 450 m, respectively. In order to evaluate the wear resistance more objectively, the worn depth (H) of the disc and the reduced length (h) of the pin were measured after the wear test, and the volume loss of the disc and pin was calculated in accordance with the following (Eq. (1)) and (Eq. (2)):
Disc volume loss= 2yR r 2Sin − 1 −
D 2r
D (4r 2 −D 2)1/2 4
n
(1)
where R=12 mm, wear track radius; D = 2[2RH − H 2]1/2: wear track width;
Pin volume loss =
yh 6
n
3d 2 +h2 4
(2)
where r= 2.5 mm: pin end radius; d= 2[2rh −h 2]1/2: wear scar diameter. The surface and cross-section of the wear specimens were observed by an SEM, and surface roughness was measured by a roughness gage.
3. Results
3.1. Microstructure Fig. 3(a–d) are optical micrographs of the ferrous coated layers. In all the specimens, elongated splats form a curved lamellar structure, which is typical to spray coated layers [3]. Comparing the A1-specimen with the A2-specimen, many unmelted particles are observed in the A1-specimen, as indicated by arrows in Fig. 3(a), due to its larger spray powders, whereas the number of unmelted particles in the A2-specimen is reduced than in the A1-specimen (Fig. 3(b)). In the B1and B2-specimens having blend coated layers, coarse
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
oxides are dispersed in the matrix, together with a small amount of unmelted particles (Fig. 3(c and d)). The thickness of the coated layers ranges from 270 to 570 mm as indicated in Table 3. The surface roughness of the A1-specimen is the highest, followed by the B1-, B2-, and A2-specimens. The A2-specimen shows the lowest surface roughness because its spray powder size is the smallest. SEM micrographs of the coated layers are shown in Fig. 4(a–d). The gray and white areas indicate oxides and matrix, respectively. In the A1-specimen, a number of large pores are found due to the accumulation of incompletely melted splats, whereas the other specimens show a dense structure with a few pores (Fig. 4(a and
273
b)). More oxides but less pores are observed in the A2-specimen than in the A1-specimen, and the distance between splats is narrower. Oxides in the A1- and A2-specimens are relatively small, and fine oxides are also found in some areas. Large oxides are formed in a liquid state and are distorted during impact to the substrate, while fine oxides are indicative of precipitates developed in a solid state upon cooling. Thin oxide films located between Fe splats, which are believed to be formed by oxidation of the Fe splat surface exposed to atmosphere, and some spheroidal Fe precipitates are also found inside oxides [13– 15]. In the B1- and B2specimens, the size and volume fraction of oxides are larger than in the A1- and A2-specimens since Al2O3 –
Table 3 Properties of the ferrous coated specimens Specimen
Thickness of coated layer (mm)
Surface roughness Ra (mm)
Total oxide volume fraction (%)
Porosity (%)
A1 A2 B1 B2
572 275 311 380
23.0 8.3 17.2 15.5
2.8 16.6 21.0 23.2
8.5 2.3 1.5 1.7
Fig. 4. SEM micrographs of the (a) A1-, (b) A2-, (c) B1-, and (d) B2-specimens. Not etched.
274
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
Fig. 5. SEM micrograph of the B1-specimen, showing the lath martensitic matrix. Nital etched.
ZrO2 oxides are present in addition to iron oxides formed (Fig. 4(c and d)). The A1- and A2-specimens have an austenitic matrix, whereas the B1- and B2-specimens have a lath martensitic matrix which was formed when spray powders were rapidly solidified during coating (Fig. 5) [16]. Table 3 summarizes the quantitative analysis data of total oxide volume fraction and porosity in the coated layers. The total oxide volume fractions were measured to be 2.8, 16.6, 21.0, and 23.2% for the A1-, A2-, B1-, and B2-specimens, respectively. The B1- and B2-specimens show the higher oxide volume fraction than the A1- and A2-specimens. Porosity of the A1-specimen is quite high at 8.5%, whereas those of other specimens are low at 1.5–2.5%. Fig. 6(a–d) are XRD patterns of the coated layers. In the A1- and A2-specimens, austenite and various iron oxide phases such as FeO (hematite), Fe2O3 (wu¨ stite), and g-Fe2O3 are detected. A small amount of martensite (or ferrite) is also observed because phase transformation to ferrite or martensite can occur during rapid solidification in some areas with less Ni content, although most of STS 316 spray powders form austenite due to an austenite stabilizer, Ni. In the B1- and B2-specimens, martensite (or ferrite), FeO, a-Al2O3, g-Al2O3, are tetragonal-ZrO2 (t-ZrO2) are detected as shown in Fig. 6(c and d). a-Fe detected here would be mostly martensite as shown in Fig. 5. A small amount of austenite is also observed in the B2-specimen.
3.2. Hardness Table 4 lists the hardness data of the gray cast iron and the coated specimens, together with the microhardness data of oxides and matrix. The coated layers, except the A1-specimen, show the higher hardness than the gray cast iron. Hardness shows the highest in the
B2-specimen, and decreases in the order of the B1-, A2-, and A1-specimens. Ferrous oxides in the A1- and A2-specimens show some 500 VHN, while Al2O3 –ZrO2 oxides in the B1- and B2-specimens about 900 VHN [3]. Since the A1-specimen has a low oxide volume fraction with many pores, its hardness is very low. Despite the closeness between the B1- and B2-specimens in the hardness and volume fraction of oxides, the B2-specimen shows the higher hardness than the B1-specimen because the former has the harder matrix than the latter. The B1- and B2-specimens have the higher hardness than the A2-specimen because of their higher hardness of matrix and oxides as well as the higher oxide volume fraction. Overall bulk hardness of all the coated layers is lower than the matrix hardness. This might be because the coated structure is less densified than the matrix due to the presence of many pores in them. Also, the difference between indentation loads could be taken into consideration.
3.3. Wear resistance and surface roughness The wear test data obtained under the lubricated and unlubricated conditions are shown in Table 5. All the coated disc specimens, except the A1-specimen, show better wear resistance than the gray cast iron under both conditions. Precisely speaking, the wear resistance of the coated disc specimens improves over the gray cast iron by three to five times under the unlubricated condition, while over ten times under the lubricated condition. Volume loss of the disc specimens under both conditions increases in the order of the B2-, B1-, and A2-specimens. Evaluating the wear resistance of the disc specimens only in terms of the disc volume loss, the B1- and B2-specimens seem to have the more excellent wear resistance. However, the volume loss of the pin is extremely small in the A2-specimen, particularly under the lubricated condition, although its volume loss of the disc is slightly higher than those of the B1- and B2-specimen. According to the surface roughness data of the worn surface measured after the wear test (Table 6), the gray cast iron shows the highest surface roughness, followed by the B1-, B2-, and A2-specimens. The overall tendency in both conditions is about the same. These results indicate that the surface roughness deteriorates when the hardness difference between oxides and matrix becomes large (Table 4). Thus, the A2-specimen, showing the smallest hardness difference, has the best surface roughness in both the lubricated and unlubricated conditions. Fig. 7(a through d) are SEM micrographs of the cross-section of the worn surface in the lubricated condition. Same as in the surface roughness data of Table 6, the surface smoothness improves in the order of the gray cast iron, B1-, B2-, and A2-specimens. The
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
surface of the gray cast iron is very rough, and cracks are formed at flake graphite/matrix interfaces or at flake graphites (Fig. 7(a)). On the contrary, the A2-
275
specimen does not have any cracks near the surface, and shows almost a flat surface (Fig. 7(b)). In the B1and B2-specimens, a number of cracks are formed at
Fig. 6. XRD patterns of the (a) A1-, (b) A2-, (c) B1-, and (d) B2-specimens.
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
276
Table 4 Vickers hardness data of the ferrous coated specimens Specimen
Table 6 Surface roughness of the ferrous coated specimens measured after the wear test
Vickers hardness (VHN) Specimen
A1 A2 B1 B2 Gray cast iron a b
Matrixa
Oxidea
Overall bulkb
223 248 338 430 296
493 515 906 913 –
197 293 305 368 271
Vickers microhardness values measured under a load of 10 g. Vickers hardness values measured under a load of 300 g.
oxides exposed to the worn surface and at oxide/matrix interfaces, some of which might be fallen off as the cracks further develop (Fig. 7(c and d)). This falling-off deteriorates the surface roughness as this worsens the prominence and depression around oxides.
4. Discussion
4.1. Microstructural e6olution and oxidation beha6ior of ferrous coated layers When spray powders are melted by a high-temperature heat source and then solidified during plasma spraying, they react with oxygen in air to form a larger amount of oxides than in other thermal spraying methods [3]. This oxidation reaction affects phases, compositions, microstructures, and properties of the coated layers. The major oxidation reaction occurring in ferrous powders is Fe(s)+1/2O2(g) FeO(s), and other iron oxides such as Fe2O3, and g-Fe2O3 are also formed by other reactions [14]. These oxides exhibit several characteristic morphologies as observed in Fig. 4(a through d). Oxides in the A1- and A2-specimens are all ferrous oxides, but their volume fraction differs considerably. In the A1-specimen whose spray powders are larger than those of the A2-specimen, splats are incompletely formed because some powders are not melted or par-
A2 B1 B2 Gray cast iron
Surface roughness, Ra (mm) Unlubricated condition
Lubricated condition
0.42 0.68 0.60 1.90
0.36 1.70 1.18 1.80
tially melted as they travel amid the plasma flame, and many pores are formed in the coated layer, thereby resulting in a rougher surface and an inhomogeneous microstructure (Fig. 4(a)). Thus, it can be predicted that the adhesive bond strength between splats or between splats and substrate is weak. On the other hand, in the A2-specimen whose powers are much smaller (larger surface area), a considerable amount of Fe is oxidized to form various ferrous oxides, and a dense structure with lower porosity and better surface roughness can be obtained (Fig. 4(b)). Since ferrous powders of the B1- and B2-specimens are mainly composed of ferrite, which is mostly transformed to martensite during cooling after melting, the matrix hardness considerably increases. Although the carbon content in ferrous powders of the B2-specimen is the same as that of the B1-specimen (0.2 wt.%), the B2-specimen forms a hard martensitic matrix as its hardenability improves over the B1-specimen due to a considerable amount Cr in it. Al2O3 oxides formed in the B1- and B2-specimens are mainly of g-Al2O3 because quasi-stable g-Al2O3 is more readily nucleated than stable a-Al2O3 during melting and cooling (Fig. 6(c and d)) [17,18]. However, a small amount of aAl2O3 exists because of the presence of unmelted powders, whose amount depends on the flame and substrate temperatures during spraying. ZrO2 oxides also coarsely dispersed in the coated layer are transformed from monoclinic ZrO2 (m-ZrO2) to t-ZrO2 during cooling (Fig. 6(c) and (d)) [18].
Table 5 Wear test results of the ferrous coated specimens Specimen
A1 A2 B1 B2 Gray cast iron
Unlubricated condition
Lubricated condition
Volume loss of disc (mm3)
Volume loss of pin (mm3)
Volume loss of disc (mm3)
Volume loss of pin (mm3)
39.32 1.98 1.62 0.96 4.51
31.9×10−3 47.5×10−3 153.2×10−3 115.0×10−3 10.7×10−3
78.31 0.22 0.20 0.13 2.76
0.9×10−3 0.6×10−3 349.2×10−3 85.2×10−3 0.3×10−3
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
277
Fig. 7. SEM micrographs of the cross-section of the (a) gray cast iron, (b) A2-, (c) B1-, and (d) B2-specimens after the wear test in the lubricated condition.
Fig. 8. Vickers microhardness of the ferrous coated specimens vs. volume loss of the disc.
278
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
4.2. Effect of oxides on wear resistance and surface roughness Fig. 8 illustrates the relationship between the volume loss of the coated disc specimen and the hardness. The A1-specimen, whose total oxide volume fraction is very low (2.8%) and porosity is very high (8.5%), was excluded in this figure because of its extremely low hardness and poor wear resistance, whereas other specimens show about the similar level of total oxide volume fraction and hardness. The disc volume loss tends to be inversely proportional to the hardness of the coated layer in both the lubricated and unlubricated conditions. This indicates that the wear resistance of the disc specimen can be enhanced by containing a lot of hard oxides and by increasing the matrix hardness. Consequently, the B1- and B2specimens show the more excellent wear resistance than the A2-specimen since they contain many hard oxides such as Al2O3 and ZrO2 and form the martensitic matrix by phase transformation. However, in simultaneous consideration of the volume loss of the pin as well as the disc, hard oxides could rather result in a negative effect on the overall wear properties. This is because abrasive wear can occur on the pin during the wear process, since hard oxides or protuberances in the coated disc play a ploughing role on the softer surface of the pin [19]. According to Table 5, the disc volume loss of the A2-specimen is slightly larger than those of the B1and B2-specimens because of its lower hardness, but the volume loss of the pin is smaller. Particularly in the lubricated condition, the wear resistance of the pin against the A2-specimen disc is much better than the cases of the B1- and B2-specimen. Comparing the volume losses of the pin and disc between the B1- and B2-specimens, the harder B2specimen shows the smaller volume losses in both the pin and disc. This can be explained by the hardness difference between oxides and matrix. Although the B1- and B2-specimens show about the same level of hardness and volume fraction of oxides, the hardness difference is larger in the B1-specimen than in the B2-specimen because of the higher matrix hardness of the B2-specimen (430 VHN) over the B1-specimen’s (338 VHN) (Table 4). Thus, the pin volume loss of the B1-specimen is larger than that of the B2-specimen because fretting wear occurs in the pin during its contact with the disc as it repeatedly passes over hard oxides and soft matrix of the disc [19– 21]. The relationship between the oxide/matrix hardness difference and the pin volume loss is shown in Fig. 9(a). In both conditions, the pin volume loss increases linearly with the oxide/matrix hardness difference. Although the pin volume loss against the disc cannot be fully interpreted in terms of the hardness difference,
Fig. 9(a) indicates that it is an important factor affecting the pin volume loss as well as the hardness of the coated layer. This hardness difference also greatly influences the surface roughness. According to Fig. 9(b), the hardness difference is in a proportional relation with the surface roughness. With too large a hardness difference, cracks are initiated at oxides and at oxides/matrix interfaces beneath the surface (preexisting pores and cracks play a similar role), and parts of oxides are fallen off as wear proceeds [20], causing large protuberances on the worn surface and seriously deteriorating the surface roughness (Fig. 7(c and d)). The B2-specimen shows the lower surface roughness than the B1-specimen, because the matrix of the former, which can hold hard oxides in it, is harder than that of the latter. It can also be explained that the matrix of the B1-specimen is preferentially removed in comparison with the B2-specimen. On the other hand, very few oxides have fallen off in the A2-specimen as shown in Fig. 7(b) [22], which leads to a homogeneous wear of oxides and matrix and to the better surface roughness than in the B1and B2-specimens because of the reduced oxide/matrix hardness difference. This improvement of the surface roughness in the A2-specimen can also reduce the pin wear due to the effect of fretting wear. Thus, the A2-specimen seems more appropriate in view of the practical applications requiring excellent wear properties in both cylinder bores and piston rings in the lubricated condition, although the B1- and B2specimens show the better wear resistance of the coated layer since they contain more hard oxides than the A2-specimen. These wear test data indicate that the surface roughness as well as the volume loss of the pin and disc should be simultaneously considered in order to enhance the overall wear properties of the ferrous coated layers. In this case, a different interpretation from conventional wear concepts is required. Although the wear resistance improves with more oxides formed in the coated layer, the larger oxides/matrix hardness difference can cause the serious pin wear and poor surface roughness, thereby diminishing the overall wear properties. In this sense, the overall wear properties of the A2-specimen turns out better, despite its lower hardness than the B1- and B2-specimens, since the volume loss of the pin is smaller and the surface roughness is enhanced due to the smaller oxide/matrix hardness gap. Therefore, to improve the wear properties of ferrous coated layers, it is required to minimize the oxides/matrix hardness difference as well as to raise the hardness of the coated layer up to a certain level by forming an appropriate amount of oxides.
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
279
Fig. 9. Hardness difference between oxides and matrix in the ferrous coated specimens vs. (a) volume loss of the pin and (b) surface roughness of the worn surface.
5. Conclusions Four kinds of ferrous coated layers were fabricated by atmospheric plasma spraying, and the effects of microstructure and oxides on wear resistance and surface roughness were investigated with the following conclusions. Microstructural analysis of the ferrous coated layers showed that various iron oxides such as FeO, Fe2O3, and g-Fe2O3 were formed in the austenitic matrix of the STS 316 coated layers as a result of the reaction with oxygen in air. The blend coated layers were composed of the martensitic matrix transformed from ferrite and of g-Al2O3 and t-ZrO2 oxides which
were formed as Al2O3 –ZrO2 powders were rapidly solidified during plasma spraying. Wear resistance of the ferrous coated layers was improved over that of the gray cast iron by three to five times in the unlubricated condition, and over ten times in the lubricated condition. Particularly, the blend coated layers containing a lot of hard Al2O3 and ZrO2 oxides showed the better wear resistance. However, these layers had many surface prominences as the matrix was preferentially worn out and the oxides was fallen off during the wear process, thereby seriously deteriorating both the wear resistance of the counterpart material and the surface roughness.
280
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
Although the STS 316 coated layers showed slightly lower hardness than the blend coated layers, they showed excellent surface roughness resulting from homogeneous wear in oxides and matrix due to the smaller hardness difference between them. In consideration of the wear resistance of the counterpart material and the surface roughness, the overall wear properties of the STS 316 coated layers would be better. Therefore, in order to simultaneously reduce the volume loss of the counterpart material as well as the coated layer as required in both cylinder bores and piston rings, it is necessary to increase the hardness of the coated layer up to a certain level by forming an appropriate amount of oxides and to minimize the hardness difference between oxides and matrix.
Acknowledgements The work was supported by the Ministry of Commerce, Industry, and Energy (MOCIE) of Korea. The authors would like to thank Ju Wan Kim of University of Ulsan and Jungseok Oh of Hyundai Motor Company for their help of the plasma spraying experiments.
References [1] K. Holmberg, A. Matthews, Coatings Tribology, Tribology Series 28, Elsevier, 1994 (chapter 7).
[2] G. Barbezat, Schweissen Schneiden 48 (1996) 127. [3] L. Pawlowsky, The Science and Engineering of Thermal Spray Coatings, Wiley, New York, 1995 (chapter 6 – 8). [4] Automotive News Market Data Book. [5] R.C. McCune, Weld. J. 24 (1995) 41. [6] R. Smith, Proc. of the 2nd Plasma-Technik Symposium, 1 (1991) 17. [7] M.R. Kim, Ph.D. thesis, Drexel Univ., 1998, pp. 1. [8] V.D.N. Rao, D.M. Kabat, H.A. Cikanek, C.A. Fucinari, G. Wuest, Material Systems for Cylinder Bore Applications Plasma Spray Technology, SAE, 1997, p. 107. [9] G. Barbezat, G. Wuest, Surf. Eng. 14 (1998) 113. [10] Materials Guide, Sulzer Metco Inc., 1998, pp. 10. [11] S.H. Choo, S. Lee, S.J. Kwon, Metall. Trans. A 30A (1999) 1211. [12] ASTM Standard Test Method for Wear Testing with a Pin-onDisk Apparatus, ASTM G99-95a, ASTM, Philadelphia, PA. [13] A. Rabiei, D.R. Mumm, J.W. Hutchinson, R. Schweinfest, M. Ru¨ hle, A.G. Evans, Mater. Sci. Eng. A269 (1999) 152. [14] M.L. Lau, E.J. Lavernia, Mater. Sci. Eng. A272 (1999) 222. [15] R.A. Neiser, M.F. Smith, R.C. Dykhulzen, J. Thermal Spray Tech. 7 (1998) 537. [16] D.A. Porter, K.E. Easterling, Phase Transformations in Metals and Alloys, Chapman & Hall, London, 1992, p. 382. [17] L.L. Shaw, D. Goberman, R. Ren, M. Gell, S. Jiang, Y. Wang, T.D. Xiao, P.R. Strutt, Surf. Coat. Tech. 130 (2000) 1. [18] M. Vural, S. Zeytin, A.H. Vcisik, Surf. Coat. Tech. 97 (1997) 347. [19] K.C. Ludema, Friction, lubrication, and wear technology, in: ASM Handbook, vol. 18, ASM International, Materials Park, OH, 1992, p. 184. [20] D.A. Rigney, Fundamentals of Friction and Wear of Materials, ASM International, Metals Park, OH, 1981, p. 291. [21] Z.R. Zhou, L. Vincent, Wear 225 – 229 (1999) 962. [22] J.W. Park, H.C. Lee, S. Lee, Metall. Trans. A 30A (1999) 399.
Effect of oxides on wear resistance and surface roughness of ferrous coated layers fabricated by atmospheric plasma spraying Byoungchul Hwang a, Sunghak Lee a,*, Jeehoon Ahn b b
a Center for Ad6anced Aerospace Materials, Pohang Uni6ersity of Science and Technology, Pohang 790 -784, South Korea Materials & Processes Research Center, Research Institute of Industrial Science and Technology, Pohang 790 -600, South Korea
Received 29 August 2001; received in revised form 14 November 2001
Abstract The objective of this study is to investigate the correlation of microstructure, wear resistance, and surface roughness in ferrous coated layers applicable to cylinder bores. Four kinds of ferrous spray powders, two of which were STS 316 steel powders and the others were blend powders of ferrous powders mixed with 20 wt.% Al2O3 –ZrO2 powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the coated layers showed that iron oxides were formed in the austenitic matrix by oxidation during spraying for the STS 316 coated layers, while Al2O3 –ZrO2 oxides were mainly formed in the martensitic matrix for the blend coated layers. The wear test results revealed that the blend coated layers showed the better wear resistance than the STS 316 coated layers because they contained a number of hard Al2O3 –ZrO2 oxides. However, they had rough worn surfaces because of the preferential removal of the matrix and the cracking of oxides during the wear process. The STS 316 coated layers showed the slightly worse wear resistance than the blend coated layers, but they showed the excellent surface roughness resulting from homogeneous wear in oxides and matrix due to the smaller hardness difference between them. In order to improve the overall wear properties with consideration of the wear resistance of a counterpart material and the surface roughness, the hardness difference between oxides and matrix should be minimized, while the hardness should be maintained up to a certain level by forming an appropriate amount of oxides. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Atmospheric plasma spraying; Ferrous coated layer; Wear resistance; Surface roughness; Oxide
1. Introduction Physical and chemical properties such as resistance to erosion, wear, oxidation, and heat largely depend on surface properties. In order to improve these properties, coating with metals, oxides, and organic materials is widely applied to the surface of structural materials and machinery parts. Coating methods include electro-coating, physical vapor deposition (PVD), chemical vapor deposition (CVD), ion plating, hardfacing, thermal spraying, etc. [1]. Among them, thermal spraying is most widely used for industrial applications since, (1) it does not have any limitations on the shape and kind of a substrate; (2) it is relatively easy to control the thickness of a coated layer; * Corresponding author. Tel.: + 82-562-279-2715; fax: + 82-562279-2399. E-mail address: [email protected] (S. Lee).
(3) a coated layer can be formed quickly, and (4) it is less limited in application scopes and coating materials [2,3]. Among thermal spraying methods, plasma spraying is intensively applied to automotive industry to improve engine efficiency and wear resistance of engine parts such as cylinder bore, crankshaft, and piston ring. Spraying powders are first melted by plasma heat source, and then quickly sprayed onto a substrate to form a coated layer on it [1–9]. Particularly, cylinder blocks which have been entirely made of a gray cast iron are now manufactured by thermal spraying of Al –Si alloys, and it is expected that about half of the cylinder blocks produced worldwide will be replaced with aluminum alloys within a couple of years [4]. Since the economic benefit from lighter body and less fuel consumption is of great importance, many automotive companies are encouraged to undertake active studies on ferrous coating materials for cylinder bores [5].
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 9 3 7 - 2
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
As the wear process occurring inside a diesel engine depends on the interaction of cylinder, piston ring, and engine lubricant, it is imperative to strike a balance among these in order to reduce cylinder wear and oil consumption. A suitable coating material for this end should meet following requirements, (1) low friction coefficient and wear rate against a piston ring in a lubricated condition; (2) lower wear rate than a cast iron liner for a same tribosystem; (3) good resistance to thermal shock; and (4) consistent coating properties to provide high surface finish reliability [7– 9]. In order to provide cylinder bores with adequate wear and friction properties, it is thus necessary to promote a good lubrication functioning with piston rings by forming a proper amount of pores and reinforced phases in the coated layer as well as to select a suitable spray powder and to optimize spraying conditions. The present study is concerned with the fabrication of ferrous coated layers by atmospheric plasma spraying applicable to cylinder bores and with the investigation of the effect of oxides on hardness, wear resistance, and surface roughness. For these purposes, four kinds of ferrous coated layers were fabricated by changing ferrous powders which determine the total oxide volume fraction. Microstructure, hardness, and wear properties of the fabricated ferrous coated layers were comparatively analyzed to understand the mechanisms involved in the wear and surface roughening processes.
2. Experimental
2.1. Spray powders Spray powders used for the fabrication of ferrous coated layers are four kinds of ferrous powders, i.e. Metco 41C, Diamalloy 1003, 91, and Metco 42C which are commercial brand names of Sulzer Metco Inc., New York, USA [10], and commercialized Al2O3 – ZrO2 powders (60 wt.% Al2O3 +40 wt.% ZrO2). Chemical compositions of the ferrous powders are provided in Table 1. Metco 41C and Diamalloy 1003 are made of an STS
269
316 stainless steel, 91 is of a plain carbon steel, and Metco 42C is of an STS 431 stainless steel. These spray powders are observed using a scanning electron microscope (SEM) as shown in Fig. 1(a through e), and their properties are summarized in Table 1. Metco 41C powders processed by water atomization have an irregular shape (Fig. 1(a)), and Diamalloy 1003 powders processed by gas atomization are fine and spheroidal (Fig. 1(b)). 91 powders show an irregular morphology of a spongy shape with a number of pores (Fig. 1(c)). Metco 42C powders processed by water atomization look alike with Metco 41C powders in their size and shape (Fig. 1(d)). Al2O3 –ZrO2 powders have an irregular blocky shape (Fig. 1(e)), and their size is ranged from 45 to 75 mm. SEM observation of the inner cross-section of Al2O3 –ZrO2 powders reveals that the dark gray area (arrow-marked) is surrounded by the light gray area (Fig. 2(a)). Fig. 2(b through d) are EDS (energy dispersive spectroscopy) mappings of the crosssection of Al2O3 –ZrO2 powders. These gray areas show oxides composed of Al2O3 (the inner dark area) and ZrO2 (the outer light area).
2.2. Fabrication of ferrous coated layer Two kinds of STS 316 coated layers were fabricated using Metco 41C and Diamalloy 1003 powders, and two kinds of blend coated layers were fabricated by mixing 91 and Metco 42C powders with Al2O3 –ZrO2 powders to increase the volume fraction of oxides. For convenience, the STS 316 coated layers sprayed by Metco 41C and Diamalloy 1003 powders are hereinafter referred to as ‘A1’ and ‘A2’, respectively, while the coated layers sprayed by blend powders of 91 and Metco 42C powders mixed with 20 wt.% Al2O3 – ZrO2 powders as ‘B1’ and ‘B2’, respectively. A plain carbon steel (chemical composition: Fe–0.45C–0.3Si– 0.75Mo–0.03P–0.035S (wt.%)) was used as a substrate, which was cut into 30× 30× 5 mm in size for a wear test specimen. Its surface was polished and blasted with Al2O3 grits (0.6–1.4 mm in diameter) to improve the adhesive bond strength between the coated layer and
Table 1 Compositions and characteristics of ferrous powders used for atmospheric plasma spray coating Powder
Metco 41Ca
Diamalloy 1003a
91a
Metco 42Ca
Chemical Composition Particle size (mm) Manufacturing method Particle shape
Fe-17Cr–12Ni–2.5Mo–1Si–0.1C (STS 316) 45–106
Fe–17Cr–12Ni– 2.5Mo–1Si–0.1C (STS 316) 11–45
Fe–0.2C–0.5Mn (Plain Carbon Steel) 45–106
Fe–16Cr–2Ni–0.2C (STS 431) 45–106
Water atomized
Gas atomized
Agglomerated and sintered
Water atomized
Irregular
Spherical
Irregular spongy and blocky
Irregular
a
Commercial brand names of ferrous powders obtained from Sulzer Metco Inc. [10].
270
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
Fig. 1. SEM micrographs of (a) Metco 41C, (b) Diamalloy 1003, (c) 91, (d) Metco 42C, and (e) Al2O3 – ZrO2 powders.
the substrate, and was ultrasonically cleansed with acetone and alcohol. Plasma spraying was conducted on the substrate using a Sulzer Metco 9MB spray system, and hydrogen combined with argon was used as fuel gas. To prevent overheating in specimens during spraying, a specimen holder was cooled with compressed air, and the traverse speed of a spraying gun was maintained constant. Detailed plasma spraying conditions are listed in Table 2.
A gray cast iron (FCH2D), which was completely finished as a diesel engine cylinder block (chemical composition: Fe–3.54C– 2.21Si–0.674Mn–0.025Cr– 0.013Cu–0.056P– 0.031S (wt.%)), was chosen as a standard to compare the coated layers with. It contains flake graphites, pores, and MnS inclusions in the pearlitic matrix. Pores are typical defects formed during casting, and MnS inclusions hardly affect properties of the cast iron [11].
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
271
Fig. 2. (a) SEM image of Al2O3 –ZrO2 powders and (b) through (d) EDS mappings of O, Al, and Zr, respectively.
2.3. Microstructural analysis and hardness test The ferrous coated layers were sectioned perpendicular to the coated surface, polished, and observed by an optical microscope and an SEM. Phases present in the coated layers and their chemical compositions were analyzed by X-ray diffraction (XRD) and EDS analyses, respectively. Hardness was measured by a Vickers hardness tester under a 300 g load, and microhardness of matrix and oxides was measured under a 10 g load by an ultra micro-vickers hardness tester.
hardness level of a commercial piston ring. According to the ASTM G99-95a condition (surface roughness, Ra 5 0.8 mm), the spray coated layer was ground using a grinder to make the surface roughness constant at about 0.1 mm, and then was used as the disc specimen. The disc specimen is worn in contact with the upper pin as the rotating axis rotates, while being pressed from the upper body of the wear tester, under both unlubricated and lubricated conditions. In the unlubricated condition, the test was performed under a 3 kgf load at room temperature for 30 min, whereas in the lubricated condition, it was done under a 20 kgf load at 100 °C
2.4. West test Wear test was conducted on the coated layers using a pin-on-disc type wear tester (Model; EFM-III-EN/F, Orientec Co., Japan) in accordance with ASTM G9995a [12]. An SAE 9254V spring steel (chemical composition: Fe –0.55C– 1.4Si–0.7Mn – 0.015Ni – 0.7C – 0.035P – 0.04S (wt.%)) was used for the pin (diameter; 5 mm). It was subjected to a nitriding treatment at 470 °C to raise the surface hardness over 700 VHN so that it matches the
Table 2 Atmospheric plasma spraying conditions Arc flow rate Arc pressure Auxiliary gas flow rate Auxiliary gas pressure Spray rate Arc voltage Arc current Spray distance
80–100 l min−1 0.689 MPa 5–15 l min−1 0.345 MPa 2.7–6.8 kg h−1 61–68 V 400–500 A 70–100 mm
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
272
Fig. 3. Optical micrographs of the (a) A1-, (b) A2-, (c) B1-, and (d) B2-specimens. Not etched.
for 30 min using an SAE 10W30 automotive engine oil as a lubricant. The coated layers would be all worn out if exposed to too a heavy load or for a long time, whereas the weight loss after the wear test would be too small if under too a small load. This would make proper evaluation of the wear resistance quite difficult. Thus, adequate loading and time were selected for both the unlubricated and lubricated conditions in this study. Rotation velocity and wear distance were 200 rpm and 450 m, respectively. In order to evaluate the wear resistance more objectively, the worn depth (H) of the disc and the reduced length (h) of the pin were measured after the wear test, and the volume loss of the disc and pin was calculated in accordance with the following (Eq. (1)) and (Eq. (2)):
Disc volume loss= 2yR r 2Sin − 1 −
D 2r
D (4r 2 −D 2)1/2 4
n
(1)
where R=12 mm, wear track radius; D = 2[2RH − H 2]1/2: wear track width;
Pin volume loss =
yh 6
n
3d 2 +h2 4
(2)
where r= 2.5 mm: pin end radius; d= 2[2rh −h 2]1/2: wear scar diameter. The surface and cross-section of the wear specimens were observed by an SEM, and surface roughness was measured by a roughness gage.
3. Results
3.1. Microstructure Fig. 3(a–d) are optical micrographs of the ferrous coated layers. In all the specimens, elongated splats form a curved lamellar structure, which is typical to spray coated layers [3]. Comparing the A1-specimen with the A2-specimen, many unmelted particles are observed in the A1-specimen, as indicated by arrows in Fig. 3(a), due to its larger spray powders, whereas the number of unmelted particles in the A2-specimen is reduced than in the A1-specimen (Fig. 3(b)). In the B1and B2-specimens having blend coated layers, coarse
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
oxides are dispersed in the matrix, together with a small amount of unmelted particles (Fig. 3(c and d)). The thickness of the coated layers ranges from 270 to 570 mm as indicated in Table 3. The surface roughness of the A1-specimen is the highest, followed by the B1-, B2-, and A2-specimens. The A2-specimen shows the lowest surface roughness because its spray powder size is the smallest. SEM micrographs of the coated layers are shown in Fig. 4(a–d). The gray and white areas indicate oxides and matrix, respectively. In the A1-specimen, a number of large pores are found due to the accumulation of incompletely melted splats, whereas the other specimens show a dense structure with a few pores (Fig. 4(a and
273
b)). More oxides but less pores are observed in the A2-specimen than in the A1-specimen, and the distance between splats is narrower. Oxides in the A1- and A2-specimens are relatively small, and fine oxides are also found in some areas. Large oxides are formed in a liquid state and are distorted during impact to the substrate, while fine oxides are indicative of precipitates developed in a solid state upon cooling. Thin oxide films located between Fe splats, which are believed to be formed by oxidation of the Fe splat surface exposed to atmosphere, and some spheroidal Fe precipitates are also found inside oxides [13– 15]. In the B1- and B2specimens, the size and volume fraction of oxides are larger than in the A1- and A2-specimens since Al2O3 –
Table 3 Properties of the ferrous coated specimens Specimen
Thickness of coated layer (mm)
Surface roughness Ra (mm)
Total oxide volume fraction (%)
Porosity (%)
A1 A2 B1 B2
572 275 311 380
23.0 8.3 17.2 15.5
2.8 16.6 21.0 23.2
8.5 2.3 1.5 1.7
Fig. 4. SEM micrographs of the (a) A1-, (b) A2-, (c) B1-, and (d) B2-specimens. Not etched.
274
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
Fig. 5. SEM micrograph of the B1-specimen, showing the lath martensitic matrix. Nital etched.
ZrO2 oxides are present in addition to iron oxides formed (Fig. 4(c and d)). The A1- and A2-specimens have an austenitic matrix, whereas the B1- and B2-specimens have a lath martensitic matrix which was formed when spray powders were rapidly solidified during coating (Fig. 5) [16]. Table 3 summarizes the quantitative analysis data of total oxide volume fraction and porosity in the coated layers. The total oxide volume fractions were measured to be 2.8, 16.6, 21.0, and 23.2% for the A1-, A2-, B1-, and B2-specimens, respectively. The B1- and B2-specimens show the higher oxide volume fraction than the A1- and A2-specimens. Porosity of the A1-specimen is quite high at 8.5%, whereas those of other specimens are low at 1.5–2.5%. Fig. 6(a–d) are XRD patterns of the coated layers. In the A1- and A2-specimens, austenite and various iron oxide phases such as FeO (hematite), Fe2O3 (wu¨ stite), and g-Fe2O3 are detected. A small amount of martensite (or ferrite) is also observed because phase transformation to ferrite or martensite can occur during rapid solidification in some areas with less Ni content, although most of STS 316 spray powders form austenite due to an austenite stabilizer, Ni. In the B1- and B2-specimens, martensite (or ferrite), FeO, a-Al2O3, g-Al2O3, are tetragonal-ZrO2 (t-ZrO2) are detected as shown in Fig. 6(c and d). a-Fe detected here would be mostly martensite as shown in Fig. 5. A small amount of austenite is also observed in the B2-specimen.
3.2. Hardness Table 4 lists the hardness data of the gray cast iron and the coated specimens, together with the microhardness data of oxides and matrix. The coated layers, except the A1-specimen, show the higher hardness than the gray cast iron. Hardness shows the highest in the
B2-specimen, and decreases in the order of the B1-, A2-, and A1-specimens. Ferrous oxides in the A1- and A2-specimens show some 500 VHN, while Al2O3 –ZrO2 oxides in the B1- and B2-specimens about 900 VHN [3]. Since the A1-specimen has a low oxide volume fraction with many pores, its hardness is very low. Despite the closeness between the B1- and B2-specimens in the hardness and volume fraction of oxides, the B2-specimen shows the higher hardness than the B1-specimen because the former has the harder matrix than the latter. The B1- and B2-specimens have the higher hardness than the A2-specimen because of their higher hardness of matrix and oxides as well as the higher oxide volume fraction. Overall bulk hardness of all the coated layers is lower than the matrix hardness. This might be because the coated structure is less densified than the matrix due to the presence of many pores in them. Also, the difference between indentation loads could be taken into consideration.
3.3. Wear resistance and surface roughness The wear test data obtained under the lubricated and unlubricated conditions are shown in Table 5. All the coated disc specimens, except the A1-specimen, show better wear resistance than the gray cast iron under both conditions. Precisely speaking, the wear resistance of the coated disc specimens improves over the gray cast iron by three to five times under the unlubricated condition, while over ten times under the lubricated condition. Volume loss of the disc specimens under both conditions increases in the order of the B2-, B1-, and A2-specimens. Evaluating the wear resistance of the disc specimens only in terms of the disc volume loss, the B1- and B2-specimens seem to have the more excellent wear resistance. However, the volume loss of the pin is extremely small in the A2-specimen, particularly under the lubricated condition, although its volume loss of the disc is slightly higher than those of the B1- and B2-specimen. According to the surface roughness data of the worn surface measured after the wear test (Table 6), the gray cast iron shows the highest surface roughness, followed by the B1-, B2-, and A2-specimens. The overall tendency in both conditions is about the same. These results indicate that the surface roughness deteriorates when the hardness difference between oxides and matrix becomes large (Table 4). Thus, the A2-specimen, showing the smallest hardness difference, has the best surface roughness in both the lubricated and unlubricated conditions. Fig. 7(a through d) are SEM micrographs of the cross-section of the worn surface in the lubricated condition. Same as in the surface roughness data of Table 6, the surface smoothness improves in the order of the gray cast iron, B1-, B2-, and A2-specimens. The
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
surface of the gray cast iron is very rough, and cracks are formed at flake graphite/matrix interfaces or at flake graphites (Fig. 7(a)). On the contrary, the A2-
275
specimen does not have any cracks near the surface, and shows almost a flat surface (Fig. 7(b)). In the B1and B2-specimens, a number of cracks are formed at
Fig. 6. XRD patterns of the (a) A1-, (b) A2-, (c) B1-, and (d) B2-specimens.
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
276
Table 4 Vickers hardness data of the ferrous coated specimens Specimen
Table 6 Surface roughness of the ferrous coated specimens measured after the wear test
Vickers hardness (VHN) Specimen
A1 A2 B1 B2 Gray cast iron a b
Matrixa
Oxidea
Overall bulkb
223 248 338 430 296
493 515 906 913 –
197 293 305 368 271
Vickers microhardness values measured under a load of 10 g. Vickers hardness values measured under a load of 300 g.
oxides exposed to the worn surface and at oxide/matrix interfaces, some of which might be fallen off as the cracks further develop (Fig. 7(c and d)). This falling-off deteriorates the surface roughness as this worsens the prominence and depression around oxides.
4. Discussion
4.1. Microstructural e6olution and oxidation beha6ior of ferrous coated layers When spray powders are melted by a high-temperature heat source and then solidified during plasma spraying, they react with oxygen in air to form a larger amount of oxides than in other thermal spraying methods [3]. This oxidation reaction affects phases, compositions, microstructures, and properties of the coated layers. The major oxidation reaction occurring in ferrous powders is Fe(s)+1/2O2(g) FeO(s), and other iron oxides such as Fe2O3, and g-Fe2O3 are also formed by other reactions [14]. These oxides exhibit several characteristic morphologies as observed in Fig. 4(a through d). Oxides in the A1- and A2-specimens are all ferrous oxides, but their volume fraction differs considerably. In the A1-specimen whose spray powders are larger than those of the A2-specimen, splats are incompletely formed because some powders are not melted or par-
A2 B1 B2 Gray cast iron
Surface roughness, Ra (mm) Unlubricated condition
Lubricated condition
0.42 0.68 0.60 1.90
0.36 1.70 1.18 1.80
tially melted as they travel amid the plasma flame, and many pores are formed in the coated layer, thereby resulting in a rougher surface and an inhomogeneous microstructure (Fig. 4(a)). Thus, it can be predicted that the adhesive bond strength between splats or between splats and substrate is weak. On the other hand, in the A2-specimen whose powers are much smaller (larger surface area), a considerable amount of Fe is oxidized to form various ferrous oxides, and a dense structure with lower porosity and better surface roughness can be obtained (Fig. 4(b)). Since ferrous powders of the B1- and B2-specimens are mainly composed of ferrite, which is mostly transformed to martensite during cooling after melting, the matrix hardness considerably increases. Although the carbon content in ferrous powders of the B2-specimen is the same as that of the B1-specimen (0.2 wt.%), the B2-specimen forms a hard martensitic matrix as its hardenability improves over the B1-specimen due to a considerable amount Cr in it. Al2O3 oxides formed in the B1- and B2-specimens are mainly of g-Al2O3 because quasi-stable g-Al2O3 is more readily nucleated than stable a-Al2O3 during melting and cooling (Fig. 6(c and d)) [17,18]. However, a small amount of aAl2O3 exists because of the presence of unmelted powders, whose amount depends on the flame and substrate temperatures during spraying. ZrO2 oxides also coarsely dispersed in the coated layer are transformed from monoclinic ZrO2 (m-ZrO2) to t-ZrO2 during cooling (Fig. 6(c) and (d)) [18].
Table 5 Wear test results of the ferrous coated specimens Specimen
A1 A2 B1 B2 Gray cast iron
Unlubricated condition
Lubricated condition
Volume loss of disc (mm3)
Volume loss of pin (mm3)
Volume loss of disc (mm3)
Volume loss of pin (mm3)
39.32 1.98 1.62 0.96 4.51
31.9×10−3 47.5×10−3 153.2×10−3 115.0×10−3 10.7×10−3
78.31 0.22 0.20 0.13 2.76
0.9×10−3 0.6×10−3 349.2×10−3 85.2×10−3 0.3×10−3
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
277
Fig. 7. SEM micrographs of the cross-section of the (a) gray cast iron, (b) A2-, (c) B1-, and (d) B2-specimens after the wear test in the lubricated condition.
Fig. 8. Vickers microhardness of the ferrous coated specimens vs. volume loss of the disc.
278
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
4.2. Effect of oxides on wear resistance and surface roughness Fig. 8 illustrates the relationship between the volume loss of the coated disc specimen and the hardness. The A1-specimen, whose total oxide volume fraction is very low (2.8%) and porosity is very high (8.5%), was excluded in this figure because of its extremely low hardness and poor wear resistance, whereas other specimens show about the similar level of total oxide volume fraction and hardness. The disc volume loss tends to be inversely proportional to the hardness of the coated layer in both the lubricated and unlubricated conditions. This indicates that the wear resistance of the disc specimen can be enhanced by containing a lot of hard oxides and by increasing the matrix hardness. Consequently, the B1- and B2specimens show the more excellent wear resistance than the A2-specimen since they contain many hard oxides such as Al2O3 and ZrO2 and form the martensitic matrix by phase transformation. However, in simultaneous consideration of the volume loss of the pin as well as the disc, hard oxides could rather result in a negative effect on the overall wear properties. This is because abrasive wear can occur on the pin during the wear process, since hard oxides or protuberances in the coated disc play a ploughing role on the softer surface of the pin [19]. According to Table 5, the disc volume loss of the A2-specimen is slightly larger than those of the B1and B2-specimens because of its lower hardness, but the volume loss of the pin is smaller. Particularly in the lubricated condition, the wear resistance of the pin against the A2-specimen disc is much better than the cases of the B1- and B2-specimen. Comparing the volume losses of the pin and disc between the B1- and B2-specimens, the harder B2specimen shows the smaller volume losses in both the pin and disc. This can be explained by the hardness difference between oxides and matrix. Although the B1- and B2-specimens show about the same level of hardness and volume fraction of oxides, the hardness difference is larger in the B1-specimen than in the B2-specimen because of the higher matrix hardness of the B2-specimen (430 VHN) over the B1-specimen’s (338 VHN) (Table 4). Thus, the pin volume loss of the B1-specimen is larger than that of the B2-specimen because fretting wear occurs in the pin during its contact with the disc as it repeatedly passes over hard oxides and soft matrix of the disc [19– 21]. The relationship between the oxide/matrix hardness difference and the pin volume loss is shown in Fig. 9(a). In both conditions, the pin volume loss increases linearly with the oxide/matrix hardness difference. Although the pin volume loss against the disc cannot be fully interpreted in terms of the hardness difference,
Fig. 9(a) indicates that it is an important factor affecting the pin volume loss as well as the hardness of the coated layer. This hardness difference also greatly influences the surface roughness. According to Fig. 9(b), the hardness difference is in a proportional relation with the surface roughness. With too large a hardness difference, cracks are initiated at oxides and at oxides/matrix interfaces beneath the surface (preexisting pores and cracks play a similar role), and parts of oxides are fallen off as wear proceeds [20], causing large protuberances on the worn surface and seriously deteriorating the surface roughness (Fig. 7(c and d)). The B2-specimen shows the lower surface roughness than the B1-specimen, because the matrix of the former, which can hold hard oxides in it, is harder than that of the latter. It can also be explained that the matrix of the B1-specimen is preferentially removed in comparison with the B2-specimen. On the other hand, very few oxides have fallen off in the A2-specimen as shown in Fig. 7(b) [22], which leads to a homogeneous wear of oxides and matrix and to the better surface roughness than in the B1and B2-specimens because of the reduced oxide/matrix hardness difference. This improvement of the surface roughness in the A2-specimen can also reduce the pin wear due to the effect of fretting wear. Thus, the A2-specimen seems more appropriate in view of the practical applications requiring excellent wear properties in both cylinder bores and piston rings in the lubricated condition, although the B1- and B2specimens show the better wear resistance of the coated layer since they contain more hard oxides than the A2-specimen. These wear test data indicate that the surface roughness as well as the volume loss of the pin and disc should be simultaneously considered in order to enhance the overall wear properties of the ferrous coated layers. In this case, a different interpretation from conventional wear concepts is required. Although the wear resistance improves with more oxides formed in the coated layer, the larger oxides/matrix hardness difference can cause the serious pin wear and poor surface roughness, thereby diminishing the overall wear properties. In this sense, the overall wear properties of the A2-specimen turns out better, despite its lower hardness than the B1- and B2-specimens, since the volume loss of the pin is smaller and the surface roughness is enhanced due to the smaller oxide/matrix hardness gap. Therefore, to improve the wear properties of ferrous coated layers, it is required to minimize the oxides/matrix hardness difference as well as to raise the hardness of the coated layer up to a certain level by forming an appropriate amount of oxides.
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
279
Fig. 9. Hardness difference between oxides and matrix in the ferrous coated specimens vs. (a) volume loss of the pin and (b) surface roughness of the worn surface.
5. Conclusions Four kinds of ferrous coated layers were fabricated by atmospheric plasma spraying, and the effects of microstructure and oxides on wear resistance and surface roughness were investigated with the following conclusions. Microstructural analysis of the ferrous coated layers showed that various iron oxides such as FeO, Fe2O3, and g-Fe2O3 were formed in the austenitic matrix of the STS 316 coated layers as a result of the reaction with oxygen in air. The blend coated layers were composed of the martensitic matrix transformed from ferrite and of g-Al2O3 and t-ZrO2 oxides which
were formed as Al2O3 –ZrO2 powders were rapidly solidified during plasma spraying. Wear resistance of the ferrous coated layers was improved over that of the gray cast iron by three to five times in the unlubricated condition, and over ten times in the lubricated condition. Particularly, the blend coated layers containing a lot of hard Al2O3 and ZrO2 oxides showed the better wear resistance. However, these layers had many surface prominences as the matrix was preferentially worn out and the oxides was fallen off during the wear process, thereby seriously deteriorating both the wear resistance of the counterpart material and the surface roughness.
280
B. Hwang et al. / Materials Science and Engineering A335 (2002) 268–280
Although the STS 316 coated layers showed slightly lower hardness than the blend coated layers, they showed excellent surface roughness resulting from homogeneous wear in oxides and matrix due to the smaller hardness difference between them. In consideration of the wear resistance of the counterpart material and the surface roughness, the overall wear properties of the STS 316 coated layers would be better. Therefore, in order to simultaneously reduce the volume loss of the counterpart material as well as the coated layer as required in both cylinder bores and piston rings, it is necessary to increase the hardness of the coated layer up to a certain level by forming an appropriate amount of oxides and to minimize the hardness difference between oxides and matrix.
Acknowledgements The work was supported by the Ministry of Commerce, Industry, and Energy (MOCIE) of Korea. The authors would like to thank Ju Wan Kim of University of Ulsan and Jungseok Oh of Hyundai Motor Company for their help of the plasma spraying experiments.
References [1] K. Holmberg, A. Matthews, Coatings Tribology, Tribology Series 28, Elsevier, 1994 (chapter 7).
[2] G. Barbezat, Schweissen Schneiden 48 (1996) 127. [3] L. Pawlowsky, The Science and Engineering of Thermal Spray Coatings, Wiley, New York, 1995 (chapter 6 – 8). [4] Automotive News Market Data Book. [5] R.C. McCune, Weld. J. 24 (1995) 41. [6] R. Smith, Proc. of the 2nd Plasma-Technik Symposium, 1 (1991) 17. [7] M.R. Kim, Ph.D. thesis, Drexel Univ., 1998, pp. 1. [8] V.D.N. Rao, D.M. Kabat, H.A. Cikanek, C.A. Fucinari, G. Wuest, Material Systems for Cylinder Bore Applications Plasma Spray Technology, SAE, 1997, p. 107. [9] G. Barbezat, G. Wuest, Surf. Eng. 14 (1998) 113. [10] Materials Guide, Sulzer Metco Inc., 1998, pp. 10. [11] S.H. Choo, S. Lee, S.J. Kwon, Metall. Trans. A 30A (1999) 1211. [12] ASTM Standard Test Method for Wear Testing with a Pin-onDisk Apparatus, ASTM G99-95a, ASTM, Philadelphia, PA. [13] A. Rabiei, D.R. Mumm, J.W. Hutchinson, R. Schweinfest, M. Ru¨ hle, A.G. Evans, Mater. Sci. Eng. A269 (1999) 152. [14] M.L. Lau, E.J. Lavernia, Mater. Sci. Eng. A272 (1999) 222. [15] R.A. Neiser, M.F. Smith, R.C. Dykhulzen, J. Thermal Spray Tech. 7 (1998) 537. [16] D.A. Porter, K.E. Easterling, Phase Transformations in Metals and Alloys, Chapman & Hall, London, 1992, p. 382. [17] L.L. Shaw, D. Goberman, R. Ren, M. Gell, S. Jiang, Y. Wang, T.D. Xiao, P.R. Strutt, Surf. Coat. Tech. 130 (2000) 1. [18] M. Vural, S. Zeytin, A.H. Vcisik, Surf. Coat. Tech. 97 (1997) 347. [19] K.C. Ludema, Friction, lubrication, and wear technology, in: ASM Handbook, vol. 18, ASM International, Materials Park, OH, 1992, p. 184. [20] D.A. Rigney, Fundamentals of Friction and Wear of Materials, ASM International, Metals Park, OH, 1981, p. 291. [21] Z.R. Zhou, L. Vincent, Wear 225 – 229 (1999) 962. [22] J.W. Park, H.C. Lee, S. Lee, Metall. Trans. A 30A (1999) 399.