Friction modification of WC-Co by ion implantation

Surface and Coatings Technology 128᎐129 Ž2000. 404᎐409

Friction modification of WC-Co by ion implantation L.D. Yua,U , G.W. Shuy b, T. Vilaithong c a

Institute for Science and Technology Research and De¨ elopment, Chiang Mai Uni¨ ersity, Chiang Mai 50200, Thailand b Ad¨ anced Materials Laboratory, MRL, Industrial Technology Research Institute, Chutung, Hsinchu, Taiwan c Fast Neutron Research Facility, Department of Physics, Chiang Mai Uni¨ ersity, Chiang Mai 50200, Thailand

Abstract This study focuses on the effects of ion implantation on the friction modification of WC-Co cermet. Samples were implanted with Ar, C, N, O and B ions at 120᎐140 keV, and underwent tribological tests, mainly within the elastic region, against the cermet itself using varied loads. The experimental results show that the C ion implantation most significantly reduces the friction coefficient, and that the effect increases with the ion dose, whereas N, O and B ion implantations increase microhardness. A modified friction model suggests that the contributors to the friction modification are attributed to the compromise between microhardness and toughness, depending on the microstructure or phase and the ion-implantation-induced surface compression stress, which depends on the ion size. 䊚 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Friction coefficient, WC-Co, Ion implantation, Microhardness, Wear rate

1. Introduction Ion implantation of WC-Co has been widely investigated w1᎐5x for the improvement of wear resistance, but rarely for friction behavior. Although friction is closely associated with wear, more factors influence friction than wear, and low wear does not generally lead to low friction w6x. Therefore, we focus our study on the effects of ion implantation on the modification of friction in WC-Co cermet, with particular interest in tool industry applications. Previous work on ion implantation of WC-Co has shown that N implantation can improve wear resistance w4x, B implantation can achieve an increase in hardness w7x, and Cl and In implantation increases tool life by a factor of 2 w5x. Some preliminary studies on friction modification of ion-implanted WC-Co w3x have suggested that implanted ions, e.g. C or N, squeeze out Co, so as to weaken the adherence of the carbide grains and hence ease the wear. Another mechanism, which has been investigated by tribology studies w8x, is that U

Corresponding author. Tel.: q66-53-943379; fax: q66-53-222776.

increasing the carbide size increases wear resistance. However, for the tool industry, questions still remain on what ion species can most effectively reduce friction, and to what extent, and why. This study examines the effects of implantation with various ion species, including C, N, and B, on WC-Co friction modification.

2. Experiment Discs of 18-mm diameter and 2-mm thick WC-6.5Co cermet were prepared and polished by diamond suspension polishing solutions to a mirror finish. Ion implantation was carried out at the Ion Beam Technology Center, Chiang Mai University, using the 150-kV, mass-analyzed heavy ion implantation facility w10,11x. The WC-Co samples were implanted with either lowdose or high-dose Ar, C, N, O and B ions at medium energies, as shown in Table 1. To achieve uniform ion implantation onto the sample surface, and to avoid a significant temperature elevation, a scanning focusedbeam technique that incorporated the translation of the target holder was employed. The instantaneous beam current densities ranged from a few tens of ␮A

0257-8972r00r$ - see front matter 䊚 2000 Published by Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 6 4 2 - 3

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405

Table 1 Conditions of ion implantation into the WC-Co samples a ⭈

Sample

Ionratomic radiusŽ A .

Energy ŽkeV.

Dose

RrRma x Žnm.

S0

᎐

᎐

none

᎐

S1 S2

Arr0.89 Arr0.89

130 130

1 = 1017 Ar ionsrcm2 1 = 1017 Ar ionsrcm2

43r92 Ž130-keV Ar.

S3 S4 S5

Cr1.00 Cr1.00 Cr1.00

120 120 120

5 = 1016 C ionsrcm2 1.5= 1017 C ionsrcm2 4.5= 1017 C ionsrcm2

102r230 Ž120 keV C.

S6 S7

N2r0.81 N2 r0.81

140 140

5 = 1016 N ionsrcm2 2 = 1017 N ionsrcm2

53r130 Ž70 keV N.

S8 S9

O2 r0.70 O2 r0.70

140 140

5 = 1016 O ionsrcm2 2 = 1017 O ionsrcm2

48r120 Ž70 keV O.

S10 S11

BFrB:1.34, F:0.60 BFrB:1.34, F:0.60

120 120

5 = 1016 B ionsrcm2 2 = 1017 B ionsrcm2

50r126 Ž44 keV B.

The listed atomic radii w9x of the ions are referred to in the text. R and R ma x represent the mean range and the maximum penetration range Žwhere the ion concentration starts to be lower than 1%. of the implanted ions Žspecified in the brackets., respectively Žcalculated by PROFILE w14x.. a

to more than 100 ␮Arcm2 , depending on the ion species. The working temperature at the target did not exceed 100⬚C, and the operating pressure in the target chamber was approximately 5 = 10y4 Pa. A pin-on-disk tribology test was performed, using an ISC-200 tribometer ŽImplant Sciences. with a WC ball Ž6.3 mm in diameter. at 500, 150 and 50 g loads. The sliding speed was 5 cmrs and the total number of rotations was 5000 for the 500-g loading and 2000 for the others in an environment of room temperature and dry air. An MXT-␣ digital microhardness tester ŽMatsuzawa Seiki. was used for microhardness testing, under the conditions of a 5-g load and a 10-s indent dwell time, using a Knoop indenter.

3. Results Fig. 1 shows the results of the measured friction coefficients as a function of loads, as well as the related wear rates and microhardness. As can be seen from the wear scar width w12x, the wear-tested depth is mostly within the maximum ion ranges ŽTable 1.; the data mainly coming from the ion-implanted regions. For the unimplanted sample, the friction coefficient ␮ increases while decreasing the load L, and the product of ␮ L1r3 is nearly constant Žthe meaning of ␮ L1r3 is discussed later.. Ar ion implantation, at either low or high doses, does not reduce the friction coefficient at loads of 500 and 150 g. At a load of 50 g, it shows a slight decrease, as shown in Fig. 2a. This reduction in friction coefficient at low loads could be due to ion-implantation-induced surface compression stress, because

only the lowest load wear is significantly affected by the very near-surface structure. Ar ion implantation decreases microhardness because of the radiation damage, but does not induce chemical effects. The friction coefficient ␮ varies little with the load L, and the product ␮ L1r3 decreases with a decrease in load. C ion implantation significantly reduces the friction coefficient by as much as four times for a testing load of 50 g, as shown in Fig. 2b, and the reduction increases with the ion dose. The friction reduction incorporates an increase in microhardness, which steadily increases with the ion dose. The friction coefficient ␮ is almost constant as a function of the load L, and the product ␮ L1r3 decreases with a decrease in load. N ion implantation at low doses reduces the friction coefficient, especially at the lower loads, but at a higher dose it does not have any effect on friction, as shown in Fig. 2c. The implantation results in an increase of the hardness with the ion dose. However, the very high hardness does not bring about friction reduction. The friction coefficient ␮ is almost constant for the low dose case, but it increases with the decreasing load L for the high dose case. The product ␮ L1r3 decreases with the decreasing load. O ion implantation at the low dose does not reduce the friction coefficient, except for wear at the lowest load, but with a high dose it reduces the coefficient, especially at lower loads, as shown in Fig. 2d. The friction coefficient ␮ varies only slightly with the load L, and the product ␮ L1r3 decreases with the decreasing load. B ion implantation at the low dose decreases the friction slightly, but at the high dose, it markedly in-

406

L.D. Yu et al. r Surface and Coatings Technology 128᎐129 (2000) 404᎐409

Fig. 1. Friction coefficient, microhardness and wear rate results of ion-implanted and unimplanted samples. All the data are average values. The ␮ stable is the average friction coefficient during the time it is stable; HK: the Knoop hardness number; WR: wear rate; and ␮ L1r 3 : the product of the friction coefficient and the Žload.1r 3 Descriptions on the numbered samples refer to Table 1. The units 500 g, 150 g and 50 g in the legend are the wear testing loads.

creases the friction, as shown in Fig. 2e. The low dose ion implantation produces super-high microhardness, but it does not significantly reduce the friction. This once again demonstrates a commonly accepted fact that hard material, which may have low toughness, may not guarantee a low friction coefficient. Modifications of the friction coefficient are more complicated than that of hardness, and even wear resistance, as many factors are involved. The friction coefficient ␮ increases with the decreasing load L, especially for the high dose case, and the product ␮ L1r3 decreases with

the decreasing load for the low dose case and remains nearly constant for the high dose case. A comparison among the high dose cases of Fig. 2a᎐e, shows that C ion implantation reduces the friction coefficient the most.

4. Discussion The tribology test was performed using loads in the elastic region for the tested material. This is due to the

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407

Fig. 2. Friction coefficient as a function of sliding distance, at 50 g of the testing load for both low-dose and high-dose ion implantation in WC-Co samples, compared with unimplanted data. Ža.: Ar ion implanted; Žb.: C ion implanted; Žc.: N ion implanted; Žd.: O ion implanted; Že. B ion implanted.

fact that the measured wear track widths are generally within the range of the calculated Hertzian contact diameters for the different loads w13x. The wear can be attributed to an adhesive wear mechanism, as the ball and disk materials are the same. According to the theory of friction w14x, friction behavior is different for metallic and ceramic materials. For metals, the friction

coefficient ␮ is independent of the load L; while for ceramics, the coefficient is proportional to Ly1 r3. In other words, the product ␮ L1r3 is constant for the ceramic wear, while it decreases with decreasing load for metallic wear. The reason is thought to stem from the difference in mechanical characteristics between metals and ceramics. The points that act as the interfa-

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L.D. Yu et al. r Surface and Coatings Technology 128᎐129 (2000) 404᎐409

cial junctions on ceramics are stronger than those on metal, so in the elastic region when the load increases on the ceramics, the points are only elastically compressed and the tips on the points are flattened, so that the wear is eased and friction is decreased. In the case of metals, increasing the load may break the weaker points, and then additional subpoints with sharp tips are produced, so that the friction increases. As WC-Co is a cermet, its friction coefficient can be expressed as a sum of contributions from both the metal mechanism and the ceramic mechanism: ␮ cermet s ␮ metal q ␮ ceramics Therefore, the pronounced increase in ␮ with a decreasing load, or the almost constant ␮ L1r3 term with varying loads, indicates that the ceramic mechanism dominates; such as is seen for unimplanted and high-dose B ion implanted samples. If ␮ is constant with the varying loads, then it indicates that the metal mechanism dominates; as seen in C ion implantation, low-dose N ion implantation and high-dose O ion implantation. For other implantation conditions, the fact that ␮ increases with the decreasing load while ␮ L1r3 does not change significantly, indicates that both mechanisms may play roughly equal roles. As stated above, high hardness may not lead to high wear resistance and low friction because of poor toughness, which very often accompanies the high hardness. Therefore, the most desirable requirement for a material to be in an ideal wear situation with optimum tribological presentation, is to possess both high hardness and high toughness w15x. C ion implantation meets this condition. Ion implantation-induced surface compression stress is another factor which can benefit surface strengthening and friction reduction. However, introducing proper surface compressive stress by ion implantation is not simple. Ion size and implantation dosages are two key parameters that influence the stress production. Small ions with a low dosage may not introduce enough stress, such as the case of the low-dose O ion implantation Žthe atomic radius of oxygen is the smallest among the species used., while large ions with a high dose may create too severe a surface degradation, which eventually leads to no useful stress at all, such as the case of the high-dose B ion implantation Žthe atomic radius of boron is the largest among the species used.. Ar ions are intermediate in size, and Ar ion implantation gives a pure induced surface compressive stress. However, the hardness cannot reflect this effect, because the testing indentation penetrates deeper than the near surface layer where the compressive stress is, so the measured hardness is a combined effect from the near surface region and the deep region. However, from the

friction coefficients, it can be seen that the surface compressive stress does improve friction. The resulting wear behavior does not always follow the trend of the friction change, as shown in Fig. 2. Considering the incorporating Ar ion implantation effect with the others, it is understandable that the tribological phenomena induced by implantation of the ion species, except Ar, must be caused by both physical and chemical changes. In the wear rates in the C ion implantation case, due to the large size of the carbon atom, the degraded surface causes more wear when the dose is low. As the dose increases, other factors, such as carbide formation, start to dominate the wear and then reduce the wear as well as friction. The case of N ion implantation is more complicated, supposedly due to characteristics of the nitride. Small amounts of nitride, induced by the low-dose N ion implantation, may be composed of small hard nitride grains or particles, which may be not strongly bonded with the substrate, and during wearing they can easily fall into the wear tracks and abrade the track surface, causing an increase in the wear rate. Since high-dose N ions are implanted, the nitride grains increase in size and bonding strength with the substrate; and other beneficial compounds, such as Co 3 O4 and W2 C, are also formed, and so the wear is reduced w4x. The friction reduction in the case of the low-dose N ion implantation is caused by mild abrasive wear, while the un-reduced friction in the case of the high-dose N ion implantation is thought to be attributed to the formed hard compound plowing. In O ion implantation, oxide purportedly plays the role in reducing the wear rate and friction coefficient. In B ion implantation, at the low dose, it is probably due to the hard and high wear-resistant boride w16x that reduces the wear. But at the high-dose B ion implantation, because of the large size of the boron atom, the near surface region is heavily damaged; therefore, the wear rate increases drastically as well as the friction coefficient.

5. Conclusion C ion implantation most significantly reduces the friction coefficient in comparison with the other ion species implantation. Although N, O and B ion implantations may increase microhardness, none achieve a comparable friction decrease as C. A modified friction model suggests that the contributors to the friction modification are attributed to the compromise between microhardness and toughness, depending on the microstructure or phase, and the ion-implantationinduced surface compression stress which depends on the ion size.

L.D. Yu et al. r Surface and Coatings Technology 128᎐129 (2000) 404᎐409

Acknowledgements w8x

We wish to thank Mr P. Vichaisirimongkol for his technical assistance in the ion implantation, and Mr S. Intarasiri for his assistance in the tribology testing. The work was supported in part by the National Metal and Materials Technology Center.

w10x

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