Tribological properties of ion implanted Aluminum alloys

Tribological properties of ion implanted Aluminum alloys

\ PERGAMON Vacuum 41 "0888# 076Ð081 Tribological properties of ion implanted Aluminum alloys RJ Rodr(guez\ A Sanz\ A Medrano\ JA Garcia!Lorente AI...

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\ PERGAMON

Vacuum 41 "0888# 076Ð081

Tribological properties of ion implanted Aluminum alloys RJ Rodr(guez\ A Sanz\ A Medrano\ JA Garcia!Lorente AIN!Centre of Advanced Surface Engineering\ 20080 Cordovilla\ Pamplona\ Spain

Abstract Ion implantation of Aluminum is a useful technique to improve hardness and tribological properties[ This paper reports on a comparative study of modi_cations produced by Nitrogen implantations on three di}erent Al commercial alloys[ Di}erent Nitrogen doses have been implanted\ hardness and wear tests have been carried out to investigate the in~uence of implantation parameters[ Tribological changes are referred to implantation dose and to thermal post!treatments\ as well as to testing conditions[ Applications are relevant in aeronautical and moulding sectors[ Þ 0887 Elsevier Science Ltd[ All rights reserved[

0[ Introduction Properties of Aluminum alloys make them specially useful for many technological applications like aero! nautical\ moulding\ low weight mechanics and bio! medical[ Their low density\ high thermal and electrical conductivity as well as their good behaviour against cor! rosion lead to choose Al alloys in an increasing number of industrial _elds which seemed to be reserved to other metals[ Unfortunately\ the relatively low hardness and wear resistance of most of Aluminum alloys is still an obstacle for many applications[ Ion implantation has emerged in the last years as a powerful technique to improve hardness\ wear resistance\ friction coe.cient and corrosion behaviour of many interesting industrial alloys ð0Ð4Ł\ mainly steels\ stainless steels and Titanium alloys ð5\ 6Ł[ Industrial applications of ion implantation have became very common in plastic injection moulds\ can manufacturing tools\ paper and food processing industry and surgical prosthesis[ð7\ 8Ł Ion implantation subcontracting centres are routinely working in commercial jobs in the USA\ Japan and Europe\ where the ion implantation market is increasing at 14) per year[ Recently ion implantation into Al has been inves! tigated\ and it has been shown that this treatment pro! duces a non!homogeneous hard layer of AlN on the surface of the Aluminium ð09Ð01Ł[ In most of these inves! tigations it has been shown that nucleation of these AlN

 Corresponding author[ E!mail] rrodriguezÝain[es[

layers produce a clear increase in hardness and a decrease in wear rate[ The great interest produces by the good behaviour of the N!implanted Al is leading to further detailed research on implantation of particular com! mercial Al alloys\ the study of the dependence of the implanted dose and the e}ect of thermal post!treatments[ These are the main topics covered by the research reported in this paper[ 1[ Experimental details Sample preparation] Typical aluminum alloys com! mercially available such as 1906 A^ 6964 and 5971 have been used as substrates[ 09 mm thick samples of 49 mm in diameter were prepared[ They were mirror polished with 2 micron diamond paste and then 9[94 micron of alumina paste[ Table 0 summarises the composition "expressed in at[)# of these alloys[ Implantation] The samples were implanted by means of a Whickham 199 keV\ mass analysed\ high current implanter ð02\ 03Ł[ The target chamber was evacuated by a cryogenic pump^ the base pressure was 0×09−5 mbar[ A mechanical scanning system was used for ensuring homogeneity of an implanted dose[ Three alloys were implanted with 1×0906\ 3×0906 and 7×0906 doses of atomic Nitrogen ions "N¦# at a beam energy of 049 keV[ The current density was maintained below 9[93 mA:cm1 in order to keep the substrates at low temperatures[ The sample holder was water cooled and the measured sub! strate temperature "monitored by a IR thermocouple# was below 39>C[ Figure 0\ obtained by using the PROFILE code\ shows

9931!196X:87:, ! see front matter Þ 0887 Elsevier Science Ltd[ All rights reserved[ PII] S 9 9 3 1 ! 1 9 6 X " 8 7 # 9 9 1 0 3 ! 9

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R[ Rodr(`uez et al[:Vacuum 41 "0888# 076Ð081 Table 0 Composition of three Aluminum alloys showing the percentage limits of alloying elements

Alloy A 5971 Alloy B 6964 Alloy C 1906 A

) Low ) High ) Low ) High ) Low ) High

Si

Fe

Cu

9[6 0[2

9[4

9[0 0[1 1 2[4 3[4

9[3 9[1 9[7

9[4 9[6

Mn

Mg

9[3 0

9[5 0[1 1[0 1[8 9[3 0

9[2 9[3 0

Cr

Zn

9[14 9[07 9[17

9[0 4[0 5[0

9[0

9[14

Ti

Others

9[0 9[1

9[3 9[3

Fig[ 0[ Implantation pro_les of atomic Nitrogen ions "N¦# in Aluminum\ at\ 3×0906 and 7×0906 doses and 049 keV of energy "simulation made by the PROFILE CODE#[

the implantation pro_les for the three doses[ It is worth while to note that the highest dose 7×0906 implies a peak of atomic Nitrogen concentration of about 49 at)[ Microhardness] A Fischercope H099VP microindenter was used to measure the microhardness ð04Ł[ Samples were placed on a computer controlled XY table\ which allows to determine the indentation coordinates[ The equipment software allows to control all the relevant indentation parameters such as depth of indentation\ _nal load or the numbers of steps of loading and unload! ing[ A Vickers indenter was used and three _nal loads were tested] 1 mN\ 3 mN\ and 7 mN[ These loads make the indentation to go beyond the 09) of the implanted layer depth\ which implies that the measured hardness is a mixture of the layer and the substrate ð05Ł[ For each sample\ 09 indentations were done in order to obtain a statistical average[ The results show very little straggling\ mainly at 7 mN[ Friction[ The friction tests were conducted by employ! ing a pin:ball!on!disk tribometer FALEX 219 PC\ with humidity controller unit ð06\ 07Ł[ This equipment is also controlled by computer\ with a software which allows to see the evolution of friction coe.cient[ The humidity controller allows us to carry out the test to any chosen

humidity between 4) and 84)[ The parameters chosen for making this test were 49 g and 09 g of applied load^ with 29) and 69) of humidity[ The angular speed was kept about 59 rpm i[e[ a linear speed of about 9[0 m:s[ The friction was against a bearing steel ball of 9[4 inch in diameter "099Cr5#[ The tests duration was about 1 h "6499 cycles#[ Thermal Treatments[ Samples of three alloys were annealed in a furnace\ making di}erent cycles[ The rise of the ramp up was 49>C:min in all cycles[ The alloys were annealed between 04 and 079 min\ and between 099>C and 199>C[ In all tests it was placed in an unim! planted sample for taking it as reference[

2[ Results and discussion Figure 1[ represents the load:unload curves for one of the alloys "6964# at 1 mN\ and Fig[ 2[ represents the Corrected Hardness at 7 mN\ showing the e}ect of implantation dose on the increase in hardness[ Similar curves were obtained for the other alloys and loads[ It is observed that an increase in the hardness with the implanted doses for the three di}erent alloys[ The ratio

R[ Rodr(`uez et al[:Vacuum 41 "0888# 076Ð081

Fig[ 1[ Load:unload curves for one of the alloys "6964# at 1 mN of maximum indentation load for three di}erent implantation doses] "0#Unimplanted\ "1#1×0906 ions:cm1\ "2#3×0906 ions:cm1\ "3#7×0906ions:cm1[

Fig[ 2[ Depth evolution of the hardness for the N!implanted 6964 Aluminium alloy\ at the three implantation doses "maximum load 7 mN#] "3#Unimplanted\ "2#1×0906 ions:cm1\ "1#3×0906 ions:cm1\ "0#7×0906 ions:cm1[

Table 1 Hardness of three Aluminum alloys at 1 mN load\ showing the depen! dence of the implanted dose DOSE

A"N:mm#

B"N:mm#

C"N:mm#

UNIMPLAN[ 1[99E¦06 3[99E¦06 7[99E¦06

0149 0571 0641 0885

0622 1963 1266 1487

0439 1928 1023 1356

of this increase is a little bit less important than the increase found in other investigations ð09Ð01\ 07Ł\ because the depth of indentation it is larger than 09) of the layer thickness\ i[e[ it is not the microhardness of the pure implanted layer[ In some samples it was observed that for a higher dose of implanted Nitrogen the hardness decreased slightly with respect to the middle dose[ The reason of this decrease is because when AlN is saturate\ other compounds are formed\ being the critical saturation dose at 049 keV about 6×0906 ð01Ł[ Indentation results are summarised in Table 1\ and in Fig[ 3[

078

A _rst ball!on!disk test with the 6964 Aluminum alloy samples\ at 49 g of indentation load\ showed a reduction of the friction coe.cient produced by the hard layers of AlN[ However\ as these layers were worn in a few minutes\ it was decided to carry out the tests at 09 g of indentation load to allow a better observation of changes produced by the implantation[ At this load it was possible to appreciate a quite big decrease of the friction coe.cient as well as the wear rate[ This behaviour is much clear in low humidity than in high humidity conditions\ being the reason of this evolution related to some tribooxidation e}ect[ The samples of 6964 alloys exhibited a minor fric! tion coe.cient and less wear than the other two alloys[ In particular\ in the test at 09 g of load\ and 29) of humidity\ the wear is practically negligible[ It was also possible to observe that some samples had a better friction behaviour for middle dose than for the highest dose[ It is appreciable in the plots the fast increase and decrease of the curves\ which corresponds to tran! sitions between severe wear periods\ with abrasive particles\ and periods of mild wear[ A set of implanted samples was annealed in air during 29 minutes at 099>C[ It was observed that the tribological characteristics improved with reference to the non! annealed samples[ At temperatures over 049>C or for annealing cycles longer than an hour\ the mechanical properties start to decrease[ The loosing of mechanical properties is because of the grain growing of the substrat\ which reduces the hardness of the aluminium ð07Ł[ Com! parison between annealed and non annealed indentation curves is shown in Fig[ 5\ and the friction behaviour is displayed in Fig[ 6[

3[ Conclusions 0[ High dose Nitrogen "N¦# implantation at 049 keV produces signi_cant improvements in some tribo! mechanical properties of commercial Aluminium alloys[ In particular\ the microhardness increases from 29 at) to 59 at) at 1 mN load\ correlatively to the implanted dose[ 1[ The friction coe.cient decreases[ The wear at low loads was also consistently lower than in unimplanted samples[ These e}ects diminished when implanted doses approached a critical dose "about 6×0906 ions: cm1#[ 2[ Annealing cycle at 099>C during 29 min\ improves the hardness for two alloys but do not change the pro! perties of the 5971 alloy[ 3[ The three alloys have important uses[ 1996 A alloys are used in the aeronautical sector\ and 6964 is employed as a high strength alloy[ So\ it is expected that improvements made by the ion implantation\ and post!annealing\ can be extensively useful[

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R[ Rodr(`uez et al[:Vacuum 41 "0888# 076Ð081

Fig[ 3[ Summary of the indentation test results for the alloys A] 5971\ B] 6964 and C] 1906A\ at 1 mN\ showing the dose dependence[ Figures in table correspond to hardness "MPa# at maximum load "1 mN#[

Fig[ 4[ Friction coe.cient evolution at 09 g of load and 29) humidity for the N!implanted 6964 Aluminium alloy[ "0#7×0906 ions:cm1\ "1#3×0906 ions:cm1\ "2#1×0906ions:cm1\ "3#Unimplanted[

R[ Rodr(`uez et al[:Vacuum 41 "0888# 076Ð081

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Fig[ 5[ E}ect of 099>C\ 29 min annealing in the hardness "at 3 mN load# of 7×0906 N!implanted 6964 alloy[ "0#Without annealing\ "1#After annealing[

Fig[ 6[ E}ect of 099>C\ 29 min annealing in the friction evolution "at 09 g load\ 29) humidity# of 7×0906 N!implanted 1906A alloy] "0#After annealing\ "1#Without annealing[

Acknowledgements Authors would like to thank the support received from CICYT "project MAT83:9847# and MINER "project TECMA 026:85#[ Authors also thank the suggestions and discussions held with Mr[ Esteban de Frutos and Mr[ Segundo Sanchez from the aeronautical company CASA[

References ð0Ł ð1Ł ð2Ł ð3Ł ð4Ł ð5Ł

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R[ Rodr(`uez et al[:Vacuum 41 "0888# 076Ð081

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