Tribological implantation I. Kanno,
properties of aluminum and plasma treatment
(1991) 920-924 North-Holland
and T. Okada
ISIR, Osaka Unioersit~y.Ibaraki, Osaka 567, Jupan
K. Katagiri Iwatr Unicersity. Morioku, Irate 020, Japan
H. Mori UH VEM Center, Osaka Unicersity, Suita, Osaka 565, Jupun
K. Iwamoto Hiruno Koh-on Co.. Ltd., Hirano, Osaka 547. Jupun
Surface modification of Al by a combination of ion implantation and plasma treatment has been investigated from the tribological viewpoint. Nitrogen ions were implanted into pure Al specimens at 90 keV and then ion-nitrided at a few kV for 6 h. The wear test was carried out using a wheel-on-plate-type testing machine. It was shown that these treatments reduce the friction coefficient, the wear rate and increase the hardness. The surface of the Al specimen was observed by optical microscopy and SEM. A lot of blisters emerged on the surface of the treated Al specimen. The TEM observation revealed the detailed surface structure of the modified layer. These results suggest that the combination of the treatments is effective for the modification of tribological properties of the Al surface.
Aluminum nitride (AIN) has been investigated because of its interesting features such as high hardness, electrical insulating property, high thermal conductivity and so on. The AlN films have been produced by several processes. i.e., ion implantation [l]. ion nitriding . chemical vapor deposition , and reactive sputtering . It was reported that- the AlN layer became thicker by the combination process of plasma nitriding after ion implantation . In this work, an AlN layer was formed on a bulk Al surface by nitrogen ion implantation and ion nitriding. The specimens treated by such processes were investigated from the tribological viewpoint, that is, the characteristics of friction, wear and hardness. The nearsurface microstructure was examined by scanning electron microscopy @EM) and transmission electron microscopy (TEM). The crystal structures were identified by their electron diffraction patterns. 0168-583X/91/$03.50
6 1991 - Elsevier Science Publishers
2. I. Specimen The 10 mm diameter 99.99% pure Al bar was rolled to 2 mm thickness and cut into pieces of 10 X 30 mm’. The samples were annealed at 225 o C for 4 h and then electro-polished using an ethanol-perchloric acid electrolyte. Then we conducted three kinds of modification treatments: implantation alone, ion nitriding alone and ion nitriding after implantation. Non-mass-analyzed nitrogen ion (the composition is N+(60%) + N+(40%)) implantation was conducted using a Zymet Z-100 ion implanter at an accelerating energy of 90 keV at room temperature. The implanted doses were 1 X 10”. 5 x 10” and 1 x 10’s N/cm’-. Ion nitriding was performed by generating a nitrogen plasma with dc glow discharge. In order to remove the surface oxide layer. argon pre-sputtering was carried out in a vacuum of 0.12 Torr, applying 2.0 kV with the
I. Kanno et al. / Tribological properties of mvdijied Al current density of 13 PA/cm2 for 10 min in the implanted Al and for 1 h in nontreated Al respectively. After replacing argon by nitrogen gas, ion nitriding was successively conducted in a vacuum of about 0.13 Torr, applying 2.5 kV with the current density of 17 &A/cm’ for 6 h. The measurement of the temperature with a thermocouple showed that the temperature of the sample was around 500 o C throughout the procedure. _1._1,Surface ana!vsis and wear test The surface morphology of ion-implanted and/or ion-nitrided Al specimens was observed using both an optical microscope and SEM. In order to examine the subsurface layer structure, a TEM observation on the cross section was performed on the ion-implanted Al and the crystal structure was also determined by the selected area diffraction (SAD} pattern. In order to prepare the cross-sectional thin foil for TEM sample, two specimens with implanted surfaces were glued with epoxy, and then thinned perpendicular to the surfaces. The Vickers hardness of the specimens was measured using an Akashi MVK-G2 microhardness tester. with the indentation load varied from 2 to 25 gf. The friction and wear test were made using a wheelon-plate-type testing machine . The curved surface of the 304 stainless steel wheel, the diameter being 40 mm, was finished with no. 1000 abrasive paper in every experiment. The tests were performed putting a load of 100 gf on the samples, immersing in a motor oil lubricant (SAE low-30 Grade). The sliding speed of the wheel was 90 cm/min. The frictional force was detected using a torque sensor of the strain gauge type. The wear volume was evaluated from the tracks in the samples observed with an optical microscope.
Fig. 1. Surface morphology of the Al nitrogen ion implanted up to a dose of 1 x 10’s N/cm’ (optical micrograph).
revealed that the surface had lost its smoothness and a lot of cracks emerged on the surface. It is thought that the difference in thermal expansion coefficient between the subsurface layer (AIN) formed by the implantation and the substrate (Al) causes those cracks when the sample was heated during ion nittiding. In order to observe the cross-sectional view of the surface layer of the implanted (1 X 10’s N/cm”) and/or ion nitrided Al, the samples were strongly bent. Then the surface morphology including the cross section was observed by SEM. The deformation induced the peel-off of the surface layers from the substrate (fig. 2). Although no specific surface layer could be found in the untreated Al, the formation of the brittle nitrided layer, which is OS-l.0 urn in thickness, was confirmed in all the treated samples. A TEN observation for the cross section in nitrogen-implanted Al at 1 x 10” N/cm2 was also performed (fig. 3). In this specimen, it was found that the
3. Results and discussion 3.1. Surface lqvers The surface morphology of the specimens implanted to the doses of 1 X IO” and 5 X 10” N/cm” was almost similar to that of the unimplanted one, and no changes could be observed. On the other hand, the implantation up to a dose of 1 x 10’s N/cm’ formed a lot of blisters on the Al surface which are almost uniform in size, and are about 0.5-1.0 urn in diameter. The density of the blisters varied with the grain (fig, 1). The emergence of the blisters on the Al surface induced by the high-dose nitrogen ion implantation has already been reported [7,8]. Although the blistering also occurred on ion nitriding, the number was small, the size was not uniform and mainly concentrated on the grain boundary. The optical microscope observation of Al which was ion nitrided after the jmplantation up to various doses
Fig. 2. Surface morphology of the ion-nitrided Al after nitrogen implantation to 1 X 10’” N/cm2. In order to observe the cross section, a specimen was strongly bent (SEM ~crograph). VII. METALS ,‘TRIBOLOGY
I. Kanno et al. / Trthological propertresof modified Al
Fig. 3. Cross section of the Al nitrogen ion implanted
up to a dose of 1 X 10” the AIN).
modified layer consists of three layers. The outmost layer is thought to be the Al oxide , and the second layer is composed primarily of the AIN judging from its SAD pattern. Although the third is estimated to be the Al from its SAD pattern, it is considered that a lot of defects exist in this region because of the featureless different contrast from the inner substrate.
A Ion-Nitriding 0 A 0 l
Implantation 1 El7 5E17 1 El8 lElB+ I.N.
inset shows SAD pattern of
3.2. Hurdness The Vickers hardness in the nitrogen-implanted and/or the ion-nitrided Al surface was evaluated. The results are shown in fig. 4. In the case of the nitrogen implanted specimens, the implantation to 1 x 10” N/cm* increased the hardness most significantly and the implantation to 5 X 10” and 1 x 10” N/cm2 hardened the Al up to almost the same level as the ion nitrided one. The Vickers hardness of the Al specimens which were ion-nitrided after nitrogen implantation at the doses from 1 X 10” to 1 x 10” N/cm* was also measured. Although an increase of the hardness was not observed in the case of preimplantation up to 1 x lOI the preimplantation up to higher doses inN/cm2, creased the hardness on post-ion-nitriding. This is shown in fig. 4 for a preimplantation of 1 x 10” N/cm’. It is postulated from these results that if a certain degree of defects was introduced in the substrate Al by the nitrogen ion implantation, the damage promotes the formation of the AIN layer by the ion nitriding after the implantation. 3.3. Friction und wear
hardness of the treated Al surface.
The samples for friction and wear tests were the untreated Al, ion nitrided Al, nitrogen-implanted (1 x 10” and 1 X 10” N/cm’) Al. and ion-nitrided Al after nitrogen implantation. The friction coefficient between samples and the stainless wheel under the load of 100 gf, up to 2500 cm, was measured (fig. 5). It can be seen that the friction coefficient p in the nitrogen-implanted
0 25 0
E 0.15 .v r’ d ” 010 6 ;
lA. 0 .AW
In 011 Load
Slldmg distance (m) Fig. 5. The friction coefficient vs the sliding distance and ion-nitrided Al was smaller than in the unimplanted sample. In the case of the implantation treatment, the p decreased with the dose. Although the implantation to 1 X 10 ” N/cm’ hardly changed the p compared with the untreated one, the implantation to 1 x 10” N/cm’ drastically decreased it. Judging from the wear tracks. which will be mentioned later, the modified surface layer in the implanted specimens should have been worn away. The possible reason for the decrease in the p by the implantation to 1 X 10”
N/cm’ is that either the AlN still exists on the surface of the wear track, or the structure changes have taken place in addition to the deformation in the substrate Al. The p falls down in the same way in the case of ion nitriding. whether preimplantation was performed or not. This indicates that the modified layers of nearly the same quality concerning the IJ are formed by ion nitriding in every specimen. The wear volume of the each specimen at the load of 100 gf after sliding distance of 2500 cm and typical wear tracks are shown in fig. 6. The wear volume was reduced by each treatment. The ion nitriding especially decreased the wear volume. In the ion-nitrided Al after implantation up to 1 x 10” N/cm’. the wear scarcely occurred. Wear tests at a load of 300 gf in the three ion nitrided samples of fig. 6 were also carried out. Although the wear volumes increased in comparison with those of 100 gf, the decrease in the wear volume was manifest in ion nitriding after implantation to 1 x 10” N/cm’. These results indicate that the ion nitriding is effective to form the modified layer which has better wear property than nitrogen implantation and the combined treatment is the most effective process of all. It is considered that the improvement on wear property by ion nitriding after implantation to 1 x 10” N/cm’ is related with the mechanism of the hardness increase.
(1) The blisters (of size 0.5P1.0 pm in diameter) emerged
distance 25m In oil Load
gin I lE17
IBlFUl IonlE17 lE18 Nitriding + I. N. +l.N.
Fig. 6. The wear volume and wear tracks of the Al specimens
on the nitrogen ion implanted pure Al surface at the dose of 1 x 10” N/cm”. Their density varies from one grain to another. The SEM observation on the subsurface layer revealed that a modified layer of 0.5-1.0 pm in thickness was formed by the nitrogen implantation and ion nitriding. The surface was hardened by ion implantation or ion nitriding. The specimen implanted alone at 1 x 10” N/cm’ is the hardest of the implanted ones. When ion nitriding is superposed on that specimen. a further increase of hardness was found. The friction coefficient decreased by the implantation and ion nitriding processes. The wear property was improved by the implantation and ion nitriding. Ion nitriding is more effective than nitrogen implantation. A combined treatment, that is ion nitriding after implantation is the most effective.
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