164 Thin Solid Films, 214 (1992) 164-168

The study of carbon film on the surface of N + B ion implanted layer

D e h u a Y a n g , X u s h o u Z h a n g a n d Q u n j i X u e Laborato O' of Solid Lubrieation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou (China)

(Received December 3, 1991; revised February 4, 1992; accepted February 10, 1992)

Abstract

Ion implantation induced carbon enrichment of the surface of GCrl5 bearing steel after N + B ion implantation has been observed by Auger electron spectroscopy and X-ray photoelectron spectroscopy combined with ion milling. Surface carbon film possessed a graphite-like structure, which reduced the friction coefficient of the implanted layer to a much lower value. However, the existence of N and B atoms in the implanted layer contributed to the increase of microhardness and wear resistance, but had no distinct effect on the friction coefficient of the implanted layer.

I. Introduction

Ion implantation has been used to modify friction and wear behaviors for many years. Dearnaley et al. and Hartley [1, 2] reported that ion implantation im- proved the tribological response of medium strength steels. Their pioneering studies quickly established that wear, fatigue, friction and corrosion could be substan- tially reduced by ion implantation [3]. The most signifi- cant recent advances in applying ion implantation to corrosion and wear protection have been the advances in understanding of the mechanisms by which ion im- plantation modifies corrosion and wear behavior and a more realistic assessment of the constraints of ion im- plantation (thin layer, sputtering, interaction with chamber atmosphere). These results have helped formu- late guidelines on how and where ion implantation can be usefully applied to define design parameters for prototype production facilities [4].

As for as the wear protection aspect is concerned, three approaches have been used to improve the wear resistance of metals by ion implantation: (i) implanta- tion to decrease the coefficient of friction; (ii) implanta- tion to increase the yield strength (or hardness); (iii) implantation to stabilize a wear resistant phase. All these are useful to help us to understand the wear process itself, but they are influenced by other factors. It is therefore important to know what the factors are and how they affect these three approaches. Several investigations of ion implanted samples [5-7] have shown that carbon enrichment of the surface layer has taken place as a consequence of ion implantation. The depth to which the carbon enrichment is detectable is comparable to the projected range of the implanted

ions. The surface carbon film appears to have an impor- tant effect on the friction and wear behavior of the implanted layer.

The objective of the present investigation was to study the characteristics of the surface carbon film and its effects on the mechanical properties of the N + B ion implanted layer. For this purpose, the X-ray photoelec- tron spectroscopy (XPS). Auger electron spectroscopy (AES) analysis and microhardness tests with different loads of the implanted layer were conducted. Sliding friction experiments were also carried out with the implanted plate sample in dry sliding contact with a bearing steel sphere at room temperature. Before tests, the N + B ion implanted layer was sputtered by Ar ion for different periods of time.

2. Experimental procedures

2.1. Preparation of the specimen The specimen to be used for implantation was GCr 15

bearing steel. Its microstructure was spheroidal pearlite and its hardness was 191 Hv. The chemical composition of GCrl5 bearing steel is shown in Table 1. GCrl5 bearing steel was machined into a 5 x 5 x 3 mm 3 flat piece and polished with 1000 grit abrasive paper and then cleaned ultrasonically in acetone. The ion implan- tation of N + B into the flat specimen was conducted using a ion implanter. All the implantation parameters are shown in Table 2.

2.2. Friction and wear experiment The friction and wear tests of implanted and unim-

planted samples were carried out on a dynamic friction

0040-6090/92/$5.00 (~'~ 1992 - - Elsevier Sequoia. All rights reserved

D. Yang et al. / Carbon film on the surface of N + B ion implanted layer

TABLE 1. Chemical composition of the GCrl5 bearing steel specimen

165

C Cr Si Mn P S Fe

0.95-1.05 1.30- 1.65 0.15 0.35 0.20-0.40 0.027 < 0.02 Remainder

TABLE 2. Test parameters of N + B ion implantation

Implantation Beam Temperature (°C) Beam density Doses energy (keV) current (p.A) (~tA cm 2) (ions cm 2)

N B N B

90 330 <150 7.9 6.6 3.1 X 1017 3.2 X 1017

A

w

Sliding

Direction

~Ixed Steel S~here ~ (Standard bearing

_ steel )

I ] I

Sliding Plate (GCr15 bearing steel)

Fig. 1. Schematic illustration of the friction couple contact.

precise measuring apparatus. A standard bearing steel sphere which had a hardness of 850 H v was selected to be the friction and wear couple of the testing samples. The schematic illustration of the contact of frictional pair is shown in Fig. 1. During testing, the upper specimen (sphere) was fixed and the lower specimen (plate) moved along in just one direction with a uni- form speed of 90 m m min 1 and a travel of 3.2 m m each time. The upper specimen was raised when the lower specimen went back to the initial position so as to start the next slide. The above steps were then repeated until the total sliding distance was completed. All the friction and wear tests were conducted in air at room temperature (23 °C). Before the test, the specimens were cleaned ultrasonically in acetone for 10min and weighed on a accurate scale balance with a sensitivity of 0.01 mg. After test, the above processes were repeated and the wear rate calculated according to sliding dis- tances and weight losses.

Results and discussions

3.1. A E S and X P S analysis

Figure 2 is the Auger sputter depth profile of N + B ion implanted GCr l5 bearing steel. It shows that N and B display Gaussian shape distributions and that their maximum atom concentration is almost at the same depth. There are no obvious changes of chromium atom concentration in the profile, which indicates that the N + B ion implantation has no effect on chromium. From the AES profile we can also see that there are very small amounts of oxygen in the implanted layer and oxygen peak is at C - F e interface. It is considered that the oxygen comes from surface oxide film which formed during the machining process. This shows that oxygen pollution in the implanting process is very slight. However, there is a certain depth of carbon film on the surface of the implanted samples, as in the results obtained by many other investigators of implan- tation. The possible reasons are as follows. First, before implanting some hydrocarbon was absorbed on the surface of the sample, which was carbonized on the surface of the sample during implanting. Second, the residual gases and vacuum pump oil caused pollution of the sample surface because of vacuum carbonization. The third reason is the preferential sputtering and segregation of the composition of the substrate caused by N + B ion implantation, i.e. some carbon atoms possibly came from the substrate. The Auger fine shapes [8, 9] provide information on the binding state of carbon in the implanted layer (Fig. 3). A graphite- like overlayer with up to about 70 at.% carbon covered the implanted layer. There are about 40 at.% carbon atoms at the C - F e interface in Auger sputter depth profile. At a deeper level, the carbon enrichment falls to not more than 10 at.% and only carbide-like binding is observed up to the most probable range of N and B

166 D. Yang et al. / Carbon film on the sur[ace of N + B ion implanted layer

I O0

8O

v

o .,-I -~ 60

@ o

o o 40 (1:,

o

2O

C IPo

r r " r - ' ~ ' - - I ~ T r ~ r

0 12 24 36 48 60

Sputter Ti=e ( min )

Fig. 2. AES sputter depth profile of the N + B implanted layer.

b

i I I

2OO 300 4 o o

Z ( o v ) Fig. 3. The carbon Auger spectra of the N + B ion implanted GCrl5 bearing steel: (a) the graphite-like spectrum shape, typical of the surface of implanted samples; (b) the typical carbide-like spectrum shape taken from the region where N and B ions are most concentrated.

ions. The XPS combined with ion mill ing reveal further in format ion abou t ca rbon b inding states (Fig. 4). There are b ind ing energy shifts of carbon in different depths of the implanted layer because of different chemical

I I

C1 s

Iz

I l I I |

290 286 282 278

Bindi~ Energy ( eV )

Fig. 4. The C~ XPS spectra of the N + B ion implanted GCrl5 bearing steel: (a) carbon on the surface of the implanted layer; (b) carbon on the interface of C Fe in Auger sputter depth profile; (c) carbon in the regions where N and B ions are most concentrated.

environments . Surface carbon is almost graphite-like, one part of ca rbon at the C - F e interface is graphite-like, another is carbide-like, carbon in the region where N and B ions have the largest concent ra t ion is carbide-like.

3.2. Microhardness and tribological exper imen t results

It is well known that ion implan ta t ion causes an increase in the hardness of the near surface region of

D. Yang et al . / Carbon film on the surface o f N + B ion implanted layer 167

600

400

200 v

0 i i I I

o 5o 100 15o 200

Fig. 5. Microhardness of the N + B ion implanted GCr l5 bearing steel vs. applied loads.

many materials [10, 11]. Therefore, to obtain an indica- tion of the effect of N + B ion implantation on surface hardness, microhardness measurements were made on the implanted layer with a variety of loads ranging from 200 to 2 gf (the results are shown in Fig. 5). The heavier the loads, the greater the depth of penetration of the Vickers indenter. As the depth of penetration decreases with decreasing load, the hardness values obtained are increasingly more representative of the implanted layer. So only small changes in the hardness are produced by load decreasing from 5 gf to 2 gf; hence, the hardness values obtained under these two loads are more nearly representative of the implanted layer.

Microhardness measurements were also made in the implanted layer sputtered by Ar ion for different times with a load of 5 gf. The results are shown in Table 3. It can be seen from Table 3 that the surface carbon film has obvious effects on the microhardness of the im- planted layer. After being sputtered for 20 min by Ar ion, the surface pure carbon film was eliminated, and then the elements of substrate and N, B were exposed, So the microhardness increased to the highest value. When the implanted layer was sputtered for 36 min to the position of the highest concentration of N and B elements, the microhardness value changed to become a

little lower than the highest. The reason is that the indenter penetrated a greater depth than that of N and B distribution. When the N and B were completely eliminated from the implanted layer by Ar ion sputter- ing for 54 min, the hardness was almost the same as that of the unimplanted substrate.

To obtain an indication of the influence of the sur- face carbon film on the tribological behavior of the implanted layer, the friction and wear experiments were conducted (the results are shown in Table 3). Before Ar ion sputtering, the graphite-like carbon film existed on the surface of the implanted layer and the implanted layer had a very low friction coefficient (0.22). After Ar ion sputtering for 20 min, the graphite-like carbon film was eliminated from the surface of the implanted layer; the friction coefficient increased to a value (0.58) which was almost the same as that of the unimplanted sample. It is thus implied that the surface carbon film signifi- cantly decreased the friction coefficient of the implanted sample, the existence of N + B ions in the implanted layer had no obvious influence on the friction co- efficient.

In wear processes, it was found that the wear mecha- nisms of the implanted and unimplanted samples were adhesive transfer and abrasion (the morphologies of the worn surfaces as shown in Fig. 6). In Table 3, the wear rate of the unimplanted sample is positive. This showed that the wear of the unimplanted sample was caused by adhesive transfer to the wear counterpart and abrasion of the wear counterpart. The transferring direction of materials was from the softer unimplanted sample to its counterpart. To the N + B ion implanted sample, the surface graphite-like carbon film decreased the friction coefficient, but it was too thin to bear all the load; at some points, the wear counterpart may penetrate this thin film, thus, the mating surfaces made contact and adhered to each other in these points. Because of N + B ion implantation, the hardness, anti-abrasive and anti- adhesive transfer abilities of the sample were highly improved.

The transferring direction of materials became from the counterpart to the implanted sample. As a result,

TABLE 3. The effects of carbon film on the properties of N + B ion implanted layer

Properties N + B ion implanted layer sputtered for different lengths of time (min) a

0 20 36 54

Unimplanted sample

H v 484.8 532.5 497.0 197.6 191.5 Friction coefficient 0.22 0.58 0.62 - 0.63

Wear rate (10_6g in_ l ) --4.2 --6.3 - 5 . 6 8.9

aTo facilitate evolution, the lengths of the sputtered time which represent the same depths as those shown in Fig. 2 are given relative values.

168 D. )rang et al. / Carbon film on the surface o f N + B ion implanted layer

very large changes in wear rates of the implanted sample before and after Ar ion sputtering. This illus- trates that the surface carbon film had only a slight effect on the wear rates of the implanted sample. The modification of wear behavior was mainly due to the existence of N and B ions in the implanted sample.

(a)

4. Conclusions

Ion implantation of N + B into GCr l5 bearing steel increased the microhardness and anti-adhesive transfer and anti-abrasive abilities of GCr I5 bearing steel. For various reasons, graphite-like carbon film formed on the surface of the implanted layer, which reduced the friction coefficient of the implanted layer. The exis- tence of N and B in the implanted layer increased the wear resistance, but had no obvious influence on the friction coefficient.

(b)

Fig. 6. Scanning electron micrograph of the wear track: (a) on the unimplanted N + B ion; (b) N + B ion implanted GCrl5 bearing steel ( x 200). (Single pass; sliding velocity 90 mm rain i: load, 300 gf: air and room temperature; sliding distance 2.5 mm).

the wear rate of the implanted sample is negative, which shows the weight of the sample increased in the wear process. From Table 3 it can be seen that there are no

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