Sliding wear characteristics of low temperature plasma nitrided 316 austenitic stainless steel

WEAR ELSEVIER

Sliding wear characteristics of low temperature plasma nitrided 316 austenitic stainless steel Y. Sun *, T. Bell Schmd t~fMetalhlrgy and Materials. The Unil'crsity o/Birmintlham. PO Box 3h3, BirmhHdlam BI5 27"1" UK

Received 15 December 1997:accepted 27 March 1998

Abstract The sliding wear behaviour of plasma nitrided layers produced in AISI 316 type austenitic stainless steel at temperatures between 450°C and 550°C has been investigated in the present work. Both dry sliding test with bearing steel and alumina as the countefface and sliding test in a corrosive solution have been conducted. The results show that the sliding wear behaviour of plasma nitrided 316 steel depends on the nitriding temperature, countefface material and testing conditions. Under dry sliding conditions, plasma nitriding at various temperatures increases the wear resistance of 316 steel by more than two orders of magnitude when sliding against bearing steel. When sliding against alumina under dry conditions, the low temperature t 450°C ) nitrided layer, which comprises predominantly a single'S" phase, is more effective in improving the wear resistance of 316 steel. When sliding in a cnrrosive solution, no improvement in corrosion wear resistance has been observed tot the high temperature nitrided layers, whilst the low temperature nitrided layer, as compared with untreated 316 steel, exhibits not only improved com)sion resistance but also much increased corrosinn wear resistance under the present testing conditions. Accordingly. combined corrosion and wear resistance of 316 steel can be achieved by plasma nitriding at h)w temperatures to produce a thin and precipitationtree layer with high hardness and good corrosion resistance. ~D 1998 Elsevier Science S.A. All rights reserved. K('yword~: Stainle~,r,:Pla~,manitriding: Wear: Cnm)sion

1. Introduction A m o n g many surface engineering techniques, plasma nitriding is the most successfully and widely used technique to engineer the surfaces of austenitic stainless steels. During the plasma nitriding process, the sputtering effect of energetic positive ions can effectively remove the oxide film (Cr,Oa) on the surface and thus accelerate nitrogen mass transfer I I I. Accordingly, plasma nitriding can be carried out in a wider range of temperatures and at a faster nitriding rate than the conventional gaseous and salt bath nitriding processes I I I. In order to accelerate nitrogen diffusion in austenite, nitriding of austenitic stainless steels is normally carried out at temperatures higher than 500°C. producing a nitrided case up to 0.2-ram thick, which is characterised by the precipitation of chromium nitrides 12.3 I- This results in the hardening of the nitrided case and improvement in the abrasive and adhesive wear resistance of austenitic stainless steels 14.5 I- However. the precipitation of chromium nitrides in the nitrided case leads to the depletion of chromium content in the aus* ('tlrre~l~mdingaulht~r.Tel.: +441)21414 5220/5221; fax: +44O21414 5232.

tenite matrix, and thus a signilicant reduction in the corrosion resistance o f the nitrided layer 16,71. Accordingly, the improvement in surface hardness and wear resistance o f austenitic stainless steels by nitriding is usually accompanied by a loss in corrosion resistance. Attempts have thus been made in the past decade to improve the corrosion resistance of nitrided austenitic stainless steels. These attempts led to the development of a low temperature plasma nitriding technique in the mid 1980s 18 I, which was carried out ill relatively low temperatures, normally lower than 450°C, rather than at conventional nitriding temperatures between 55{FC and 650°C for austenitic stainless steels. II has been found that at low temperatures, plasma nitriding can produce a thin layer which has completely dilfcrent structures and properties from that produced at high temperatures 18- I 01. h has recently been established that the low temperature nitrided layer has a high hardness and excellent corrosion resistance 18,10,11 I. Although the resultant layer is relatively thin ( less than 20 ~nl ) and its structure has trot been fully characteriscd so far, this technique hits found increasing applications in industry [ 12.13 I. The sliding friction and wear behaviour of low temperature plasma nitrided austenitic 310 type stainless steel has been

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studied in the present work, under both dry and corrosi`.e environmental conditions. The influence of nitriding temperature, sliding countefface material and testing environment in the wear characteristics has been evaluated. The present paper describes the experimental results and discusses the potential of low temperature nitriding in intproving the corrosion wear resistance of austenitic stainless steels.

2. Experiments The material used in the present work was AISI 316 austenitic stainless steel with the following chemical compositions ( in wt.C/~): 19.2 Cr. I 1.3 Ni. 2.67 Mo, 1.86, Mn. 0.06 C and balance Fe. Disc type wear testing specimen.', of 50 mm in diameter and l0 mm in thickness were prepared from hot rolled bars of 55 mm in diameter. Before nitriding, the specimens were surf:ace ground, fl~llowed by manuall~ grinding and polishing using I ptm diamond paste to achieve a fine surface linish ( R,, = 0.01 run ). The original structures of the steel comprised austenite of equiaxed grains with traces o f t'errite. Plasma nitriding was carried out using a 25-kW KIochner plasma nitriding unit. Three processing temperatures, ,,sere employed, i.e., 450°C, 500°C and 550°C, `.shich produced typical layer structures representing low temperature ( 450°C ), and high temperature ( 5I)I)°C and 551.)"£ ~nitriding. A constant nitriding time o f 5 h, gas composition o f 25ch N, + 75CkH_, and pressure of 51~) Pa were employed in all the nitriding experiments. Sliding wear tests were carried out using a pin-on-disc type machine with friction data collecting and displaying facilities. Detailed description of the tribumeter has been given elsewhere [ 14 I. During testing, the stainless steel disc was rotating against a stationary slider. Two types of sliders were used in this work, i.e.. a 6.2-ram diameter bearing steel ball and an 8-ram diameter alumina ~Al_,O O ball. All the tests were carried out at a sliding speed of 0.2 m / s fi~r a sliding distance up to 1200 m. Two sets o f tests were conducted to evaluate the tribologieal behaviour of the nitrided steel. These include I I ) dry sliding test and ( 2 ) sliding test in a corrosive solution. in the corrosion wear test. the test specimen was immersed in the testing solution containing 10oh HCI + 5 q HNO~ + 85 c/; H_,O. which was made by diluting the etchant for etching

,,tainles,, ,,reel to reveal mierostructures. No attempts have been made to u,,e other corrosive solutions in the present `.`.ork. In order to minimise corrosion of the slider during testing, only the alumina ball was used as the slider. The `.`.ear `.olume was evaluated by measuring the surface prattle across the sliding track, using aTalysurt'type machine. Each prc',emed `.alue is the average of four protiles from the •,ame track. I11 the ca~;e of coffosion wear test. the specimen `.`.as `.`.eighed before and after testing, in conjunction with ~,urface prolile measurements, the wear volume from the sliding track :.ludcorrosion rate outside the track can be estimated. After `.`.ear te,,ting, optical microseope and scanning dectron microsct~pe equipped with EDX facilities were used to e\an'line the `.sorn ~,url'aces and subsurfaces.

3. Results and discussion 3. I. Strtn'tto'e~ ~?/low lelnperature uitrided 316 steel

Fig. I sho`.`,s the optical mierographs of the nitrided layers produced at 450 C. 500°C and 550°C in 316 stainless steel. A ".arieL`, of analytical techniques have been used to charaeteri,.e the ,,fracture,, and compositions oftbe layers. A detailed description of ~tructural analysis is given elsewhere 115 I. Table I summarises the phase compositions, thickness, surface hardness, and roughness of the nitrided layers produced. From Fig. I. it can be seen that nitriding at 450°C produced a nitridcd layer `.`.hich is resistant to the etchants ( the Marble reagent and the 5t)G NCI + 25cA HNO~ + 25c;f H.O solution ) used to re,. ca] the microstructure of austenitie stainless steels, such that a predominandy "while" layer was obtained. Small antount of "dark" phases were observed in the "white" layer ahutg the original austenite grain I~mndaries. due to the precipitation of chromium nitrides 115 I. Further optimisation of the nilriding process can effectively eliminate the formation of dark phases. X-my diffraction analysis showed that the low temperature layer mainly comprises a phase with a face centred structure. This p!:,~se was regarded as an expanded austenite due to the supersaturation of nitrogen up to l0 wt.C,f 19.10]. Several investigators also termed this phase the "S" phase since its crystal structure could not be fully characterised by X-ray diffraction 110.11 I. A recent study suggested that this low temperature nitrided phase does not have an

Fig. I. Optical micrograph~ ~howin E the n)orpholo~y of nitridcd la~ cr~ produced at 45(YC. 5(Xt°C and 55[)°C fi)r 5 h.

E Sml. Z Ih,II/ Wear 218 ¢IO0,~J 34-42

36 Table I Sun|mary o s rut"ures a d properlieso nitrided hl)ers Nitridingcondition Stractures Thickness ( ~111) Hardness ( It V.,,, ) Roughne:,~,. R., ( Ixm ) Roughness. J~nl.i, ( ~111}

45n C'/5 h "S'. trace:,of CrN 15

500C/5 h "S'. CrN. 7'. ?l N ! 35

55I)%'/5 h CrN. 7'. 7( N )

I 4no (I. 16

1500 0.19

f~() I I(X) 0.27

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Initial surlhce roughnes:, hvfi~re nitriding: R, = n.ol p.m: R,,,.,, = (I. I ltm. "S'. S phase: 7'- 3"-Fe~N: 7( N ). nitn)~en auslenite.

ideal cubic structure, but rather the lattices are highly stressed and distorted [ 15 ]. Accordingly. the term "S" phase is employed in this paper. Nitriding at 500°C produced a thicker layer which is predominantly "black" alter etching, with some "white' phases. A further increase in temperature (550°C) results in a completely "black" layer. X-ray diffraction analysis revealed that the 'black" layers contain significant amount of chromium nitrides. This morphology is typical of conventional nitriding of austenitic stainless steels to produce a relatively thick and hard nitrided layer, but with a loss in corrosion resistance 10.71. From Fig. I and Table I. it is c l e a r that the nitrided layer produced at 450°C is very thin. about 15 I~m, whilst those produced at 5(10°C and 550°C are much thicker. However, the low temperature layer has a very high hardness, up In 1400 HV,.,~. which is even higher than that of the 550°C nitrided layer (I 100 H V . . s ) . Anumg the three nitrided layers, the one produced at 500°C has the highest surface hardness (1500 HV..5). In addition, increasing nitriding temperature increases the surface roughness of the specimen,

nnder lhe present loading emlditions, the w e a r resistance increased with surthce hardness o f the nitrided layer, whilst layer thickness had no significant influence in the wear rate. Although the low temperature layer is much thinner than the high temperature layers, it can provide much improved wear resistance up to a load of50 N. the highest used in the present work.

Under the present testing conditions, wear o f the untreated specimen occurred in a severe mode. resulting in a very rough metallic surface and plate-like metallic wear debris. These are typical of the result of adhesive wear. a mechanism well established for austenitic stainless steels sliding against steels [ 1,6 ]. Fig. 4 shows typical morphologies of the wear tracks, from which it can he seen that the worn surl:ace was severely deformed and scored, with the formation of prows and cracks. Fig. 5 shows the microsection of a worn specimen, illustrating the plastically deformed nature of the surface and subsurface.

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3.2. DO" sliding w e a r against a hearing steel ball

Typical friction coefficient (traetitm eo~[ficient) vs. sliding distance curves for the untreated and nitrided specimens sliding against a bearing steel ball are given in Fig. 2. It can be seen that plasma nitriding increased the coeflicient of friction. For the untreated specimen, a steady state friction ctvet'ficient was reached after about I00 m sliding. Whilst the friction coefficient of the nitrided specimens initially increased with sliding distance until a peak value was reached, then it decreased with sliding distance and gradually approached the steady state alter about 6(10 m sliding. The friction curves of the 450°C and 5(10°(;' nitrided specimens show a similar paUem, whilst that of the 550°C nitrided specimen shows a larger scatter, which is probably associated with the rougher surface resulted from a higher nitriding temperature (Table I I. Despite the increased sliding friction coeflicient, the wear volume o f the nitrided specimens was more than two orders of magnitude less than that of the untreated specimen under the loads employed (Fig. 3). The wear resistance of the nitrided layer increased in the fi)llowing order in terms of nitriding temperature: 550°C. 45(FC. 5(XFC. It is obvious that

Sliding Distance (m) Fig. 2. T)pical Irit.'lion t.-oeflieit.'nl curves for lilt.' untreated and nilrided speCdilllells slidin~ ilgainM a hl..ilrill~ steel slider.

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Load(N) I'ig. 3. Wear ~Olulnu ;Is a funclitut of applied load Ibr ~;ll'iOas speCilllUns sliding a~2ainM a Mecl slider..'~tklil|~ diManee: ] 2(H) I1|.

Ft Sun. 7-. B c l l l W+'ar 21.'~ ~19t~,%34 42

37

Fig. 4. Scanning electron micrograph showing typical wear track prtv.lucedon untreated 316 steel by a at-el slider. L< .d

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Fig. 5. Optical lnicro~raph o t the micrl*~e,.:tiOll ak'ros~ lilt." x ~ ¢ a r tr~luk prod u c e d o n untreated 316 ~tccl h3 a ,loci ,lider ulldcr Zl ]~,~ltl2n N dnd h,r

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sliding distance 12110m. and the formation o f w e a r debris through shcarin~ and pn)\~ formation. O n the other hand, no measurable ~car x~as found

in the slider, where only s o m e materials transferred from the stainless steel surface were observed, indicating the formation o f strong adhesion junctions in the real contact areas. In fact. the adhesion between the stainless steel surface and the slider was so strong that stick-slip occurred throughout the test. so m u c h so that w e a r tracks with periodically "~,,idc and narrov, regions were produced t Fig. 4 ). W e a r of the nitrided surfaces under the prc,,cnl testing conditions took place i0 a different m o d e from |hat obser,, ed for the untreated surface described above. Onl.v mild v,'ear was o b s e r v e d in the nitrided surfaces, lrrespecti~c ofnitriding temperature, the role o f nilriding in reducing w e a r is to produce a hard layer to resist phtstic deformation attd to change the surhtce chemistry st) as to eliminate adhesion between the contact surfaces. Indeed. no adhesion and plastic, deli)rmalion were observed at the nitrided surfaces and subsurfaces,. Fig. 6 shows a typical m o r p h o l o g y of the worn surfaces of nitridcd specimens. T h e w e a r track is smooth with s o m e abrasion m a r k s and transfizrrcd materials. Under an optical nficroscope, the w e a r track exhibited a brov, n appearance, typical o f an oxidiscd surface. Detailed microscopic arid E D X analysis revealed that the oxidised appearance of the ','~car track was the result o f the wearing of the slider and the transfer of the resultant w e a r debris to the w e a r track, which "~a.', then oxidised under l'urthel sliding motion. Indeed. u signilicant a m o u n t o f w e a r occurred in the slider due to Ihe abrasion action o f the hard nitrided surl'aces. Table 2 lists the wear v o l u m e measured for the slider under various loads. A corn-

Fig. 6. Scanning elecmm micmgraph showing the wear Irack pnx.luced on ih¢ 451) (" mlrlded ,pCCilllCnb.~a ,reel slider under 2UN f~ a sliding distance I 21H~ m.

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parison bet,.,,cen the wear volumes o f the nitrided surfaces and the ,4eel ,,lidcr,, demonstrates thai during the dry sliding x~car proton',, x~cur mainly occurred in the slider. This is di ffcrcnt from Ihc untreated surface sliding against the steel slider. ~ h c r c wear mainly occurred in the test surl:aee with onl~ m m m m l t~car in the slider. Clearly. the i m p r o v e m e n t in v.ear resiMance o f the test specimens by nitriding is at the expense of Ihe rapid wear o f the steel counterface. 3.3. D I 3" ~lidink' , q a i n s t an u h m f f n a h a l l

When ,lidin+ against an alumina ball under dry conditions. the ++ear behax tour o f the untreated and nitrided .316 steel \+as quile different from that observed when sliding against a '4eel ball described al~)ve. Fig. 7 shows typical friction

38

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the slider. Fig. 9a shows the worn surface o f the untreated specimen, which has a different morphology from (hat produced by a steel slider. The worn surface prt;,.luced by the alumina slider is microscopically smoother and is not so heavily scored. Macroscopically. large peaks and valleys were formed (Fig. 10). Plastic deformation of the surface and subsurface is also evident, but to a less extend ( Fig. 10) as compared with that observed at the worn surface and subsur['ace produced by a steel slider ( Fig. 5 ). On the other hand.

°o too 200 3(30 4(30 5OO 6OO 7OO SlidingDistance(m) Fig. 7. Typical friction coefficient curves fiw the untreated and nitrided specimensslidin~against an alumina slider under dry conditions. coefficient curves. A comparison between Figs. 2 and 7 reveals that the untreated specimen exhibited similar friction curves when sliding against both a steel ball and an alumina ball. whilst the nitrided specimens exhibit a different friction behaviour in that when sliding against an alumina ball the friction curves become stable after about I 0 0 m sliding distance and the steady state friction coefficient is higher than when sliding against a steel ball. Again. the 550°C nitrided specimen exhibited a larger scatter in friction coefficient. The improvement in wear resistance o f 316 steel against an alumina slider by plasma nitdding is not so significant as against a steel slider. Fig. 8 compares the wear volumes produced from various test specimens, whilst the corresponding wear loss in the alumina slider is given in Table 2. A reduction in wear volume by 2 to 6 times was obtained by nitriding. Low temperature nitriding is more effective in improving the wear resistance of 316 steel when sliding against alumina. In addition. under similar testing conditions, the 316 steel-alumina tribosystem resulted in larger wear volumes both from the test specimen and from the slider than the 316 steel-beating steel system. This is true for both the untreated and nitrided specimens produced at different temperatures. Particularly when the untreated 316 steel was sliding against the alumina slider, severe wear not only occurred in the 316 steel specimen, but also occurred in the alumina slider. This is different from the steel slider, where no measurable wear loss was observed. Accordingly. the untreated 316 steel-aluminacombination represents the worst tribological situation where both the contact bodies are worn severely. This situation has been improved by plasma nitriding, particularly when the nitriding temperature was low ( e.g.. 450°C ) such that a predominantly "S" phase layer was produced. The low temperature layer and the corresponding alumina slider exhibited the lowest wear rate. With increasing nitdding temperature, the wear volume from both the nitrided surface and the alumina slider increased ( Fig. 8 and Table 2 ). Clearly. the tribosystem involving untreated and nitrided 316 steel and alumina as the counterface is characterised by the high wear rate of both the steel and the counteffacc. There is no correlation between wear rate and surl'ace hardness: the soft untreated surl'ace resulted in the largest wear volume in

uateim 4~rc 5otvc ~x.x. Fig. 8. Wear volumesproducedfrom various specimensby an alumina slider under dry conditions.

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Fig. 9, Scanningelectron micmgraphs showing typical morphologyof worn tracks pr~vducedon (a) untrealed and (h) 4.'iO"Cnilrided 316 ~lccl by an alunlina ~.lidcrunder dry conditions.

311

Y. ,",tin. £ Bell~ Wear 21S ~ I'~(:,%34 42

Fig. I(1. ()pli¢;ll nlit'rogr;iph t)l Ihc n)it.'lli,cclilln :l¢l'tl~ lilt.• ~c,tr Ir;lek prl,duced Illl unln.'~l(Cd316,1¢¢1 b~. ,111;ihllllHlll ,lidcr (lltdc( tit} ~'tlltdl[ltlll" the w e a r tracks prodtl¢cd on the nilrided ,,tlrlacc,, arc bolh microscopically and macroncopicall\ ,,mooth ( Fig. t)b). No

bulk plastic deformation v, as ohser\ed a! the ,,urface ~lmJ subsurface of the nitrided ~,pecimcn~. Ho~,.'~cr. q~nilicant anloont of wear was itlso prodnced l'l'Oln both the ililridcd surfaces and the sliders. It in believed thai during the ~ e a r process, ccrt;.lill chemical reactions ~)cctnrcd at the contzt¢l surfaces, probably through lhc diffusitm of ()\.\gen fn)m alumina tt) the chrt'wttitnn bcarinff nl~lilllc,,', ",loci ,,urlzlcc. resulting in the decomposition of alumina Zllltl Iornlation of c h r o m i u m oxide. A diffusion c(~uple ~ its Ihtl,, lorrncd zlt Ihc contact nnrfaces, resulting ill accelerated '.scar of both sttrfaces. This i ~ a situation where di ffusion ~ ear pre~ all,,. ~ hich has been observed during high speed metal cutting and machining processes 1171. A similar phcnt~mcnt~n has also been observed in the litanitml-alunlina tribo,,.~ nlcm ~ hich i,, also characlerised by high ~.~.car ralcn o f bolh t,'olllacl sar1:.lt.'c,,

1181.

No micr~:hemical analysis has been perl't~rmed in the present ;'.ork to delermine the elemental composition profiles in the ,,ubmicron or nanometer scale. Theoretically. under equilibrium condition,,, aluminium oxide is more stable than chrornium oxide, and thus transfer of oxygen from alumina to st:tinles,, steel i,, thermodynamically non-viable. H o w e v e r . during the ~ e a r process, frictional heating of the contact areas and bulk a n d / o r asperity plastic deformation are produced. ~ h i c h ma.~ change the system energy and activate the stainIcsn ~leel surface. This m a y also promote the diffusion of o~3 gcn from alumina to stainless steel. The fact that the free encrg 5 of formatitm of c h r o m i u m oxide is close to that of alunliniurn o~,ide also m a k e s the chemical reaction possible under the d 3 namic wearing process. Indeed. austenitic stainles~ stccN ha~c been used as the metallic material to bond ~lluntina through diffusion at elevated temperatures and under ,t high pressure 1191. More detailed studies are required to idcntifx the ~ c a r m e c h a n i s m s in the tribosystem involving ~lainlc~ ~tccl and alumina. 3.4. ('orrt,'~ir,t

wear

|qont mclallographic examination ( Fig. I ) it can be seen that the lo~ temperature nitrided layer is more corrosion rc,,i,tant than the high temperature nitrided layers, at least in Ihe ctchanl,, u',cd. E l e c t n ~ h e m i c a l testing in various solutions h:.P, ill~o dcnlon" :rated the g ( ~ corrosion resistance of the "S" pha,,c la'.cr I ,";- I I I. In order to demonstrate the potential of ]o,.,, temperatuz ," nitriding in improving the c o m b i n e d w e a r and corrosion resistance o f auslenitic stainless steels, sliding ~ e a r test ill a corrosi~.e solution I i.e., diluted HCI + HNO~ ,,oltttitm ) has aNo been carried out in the present work. Fig.

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40

}'. 3'mz. 7~ Bell / Wear 218 t 109H) 34--12

I I shows the recorded friction curves for the untreated and nitrided specimens. A steady friction curve was obtained for

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the untreated specimen throughout the test. On the other hand. the IYiction curves of the nitrided specimens are characterised by several distinct regions. The friction curve was initially very stable and smooth (region I). Alter certain critical sliding distance, a sudden increase in friction coefficient occurred, resulting in a fluctuating friction region ( region II ) with large scatter Between the values for the untreated specimen and fi)r region I of the nitridcd specimen. With a further increase in sliding distance, friction characteristics similar to that of the untreated specimen was gradually appn'oached ( region Ill ).

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Examination o f the worn specimens after terminating the test in various regions revealed that the peculiar friction bchaviour o f the nitrided specimens was associated w i t h the breaking through and the gradual removal o f the nitrided layer

during the wear process. The smooth region I was the restult of the contact between the nitrided layer and slider. The sudden increase in friction coel'licient at the end of region l was due to the wearing through of the nitrided layer in local areas, whilst region III was associated with the complete removal of the nitrided layer within the wear track. Accordingly. from examination of the friction curves the lifetime ol+the nitridcd layers under the present testing conditions can be evaluated. From Fig. I I. it can be seen that the 5(R)°C and 550°C nitrided layers ,.,.,ere worn through after 500 m and 600 m sliding distance, respectively+ whilst the low temperature 1450°C) nitrided layer was worn through :d'ter 8(X) m. Clearly. under the present corrosion wear condition. the thin "S" phase layer produced at a low temperature lasted hmger than the relatively thick layers produced at higher temperatures. The benelicial effect ol + low temperature nitriding on achieving good corrosion wear resistance is more clearly seen in Fig. 12. which shows the measured wear volumes in Ihe wear tracks and the corrosion depths outside the wear tracks Ior various specimens tested. Plasma nitriding at 500°C and 55t)°C signilicantly deteriorated the corrosion resistance o f 316 steel, resulting in a corrosion depth four times larger than that of the untreated specimen (Fig. 12b). As a result, the thick and hard nitrided layers produced at high temperatures did m~t improve the wear resistance of 316 steel in the tested solution ( Fig. 12a). On the other hand. the corrosion resistance of the nitrided layer produced al 450°C was smtilar to that of the untreated specimen (Fig. 12b). As a result, the thin. hard and corrosion resistant "S" phase layer produced at low temperatures signilicantly intproved the corrosion wear resistance o1"31~ stainless steel ( Fig. 12a). Corrosion o f the untreated surface outside the ,,,,ear track occurred uniformly, whilst the high temperature ( 5OtF(" and 55(FC) nitrided layers were corroded both unif~mnly and h~:ally, wilh the Ibrmation of pits penetrating down to the layer--core interface ( Fig. 13a). On the other hand. the 45(FC nitridcd layer was attacked hy the mlution only at the original austenile grain boundaries, where precipitation ~1"chromium

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2U 4() flu ~() I(X) 12n (D) l-c~l Time ImJnl Fig. 12. ('ormshm ~¢ar ~ohnme in II|e wear track as a ttlnetion of ~,liding diqancc g+l) and ¢orr(l~i(ll] depth Otllsid¢ Ihc wear Ir+t~$kLib tl I'tlnClion of testing lime (h) G~r various, "¢pccJlllClls le~,lcd,

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~

Fi~, 13. Optical micrograph~ ~hm~ing the ( a ~55()(' and ( h J45() (' nhridcd laycr~ I oul~idc the ~war Inlck ) allcr com)~ion w~,arIc~(iB~.

nitrides occurred ( Fig. 13b ). It is thus anticipated that further optimisation of the nitriding process to eliminate the t'ormation of grain boundary precipitates will further improve the corrosion resistance and corrosion wear resistance of low temperature nitrided layers. Examination of the wear scars in Ihe alumina sliders showed that the sizes of the scars were much smallcr than those produced under dry sliding conditions described previously, indicating that diffusion wear. as proposed I'~r dr',' sliding wear against an alumina slider, did not prevail in the corrosion wear situation. It is (~bviotis that the ,,uccessi,.e formation and then removal of corrosi(m products during sliding contact motion in the corrosive solution was responsible for material loss in the wear Iracks of the ilitrided specimens. The combination o f high h;.irdness and I0~,~, corrosion rate of the low temperature nitrided la~er accounted for the observed low corrosion wear rule. Practically, the corrosion beha~iour ~1" matcrialn in ~er~ much dependent upon the environment and ,,olution invol~ ed. Only one corrosive ~,oluliou hay been used in the present work. More detailed tests under various conditions are required to eharacterise the c~)rrosion wear heha~, iOtlr o f h)~.~, tenlperature nitrided austenilic stainless steeln. The present results arc promisiug indication, btlt may not trallSfe[" to other loading and environmcnlal condilions.

4. C o n c l u s i o n s ( I ) Plasma nitriding o f 316 austenitic stainless steel at d i f f e r e n t t e m p e r a t u r e s p r o d u c e s n i t r i d e d layer,, w i t h difl'e~enl structures a n d thickness. T h e s l i d i n g w e a r characteristic,, o f the n i t r i d e d l a y e r s d e p e n d s on n i t r i d i n g t e m p e r a t u r e , c o u n terface m a l e r i a l and [esl e n v i r e m m e n t . In g e n e r a l , l o w [enlperature nilriding provides the best results iu b o t h dr', and

corrosive conditions. ( 2 ) Under dry sliding against a steel slider, plasma nitriding at various temperatures hlcreases the ','.ear resislance of 316 type austenitic stainless steel by more than tv.o orders. ~1" magnitnde. Plasma nitriding provides a hard layer to resist pluslic defi)rmation and eliminate adhesion between the test surl~ce and the slider, which are the main reasons of severe w e a r o f the n n t r e a t e d 316 steel. H o w e v e r , the i m p r o \ elllen[ in wear resistance of 316 steel hy plasnta nitriding is at the expense of rapid wear of the connterface. (3) As compared with dry sliding asainsl a ,,teel ,,lider. dry sliding against an alumina slider results iu a Ia~,ter ,,..car rate not only in the umreated and nhrided 316 steel, but also in the corresponding alumina slider. Particularly. the untreated 316 sleel-alumintl tribo]ogical

silualion

trih(~systcm represents the w o r s t

w h e r e the h i g h e s t w e a r rate o c c u r s in

b o t h the test surface and the slider. T h i s s i t u a t i o n is i m p r w e e d by plasma nitriding. The degree of improvemenl depends very much on nitriding temperature.

t4)

W h e n s l i d i n g against an a l u m i n a s l i d e r in the tested

corro',i',e ',olution. the h i g h t e m p e r a t u r e

nitrided layers do

not p m ~ i d e an~ improvemem in corrosion wear resistance. due t(~ their deteriorated corrosion resistance. Whilst plasma nitridmg at Io~ temperatures can maintain or even improve the corrosion resistance of 316 steel, and thus signilicanlly intpro~ e the corrosion wear behaviour of the steel. ( 5 ) Combined corrosion and wear resistance of 316 austenitic ',lainles', ',teel can thus be achieved by low temperature pla,.ma nitridin.~ m produce a single "S" phase layer with high hardne,,,, and g ~ o d corrosion resistance.

Ackno,~ ledgements "l'hi~ ",,,~rk ",~a', ,.upported by the European Commission u n d e r ('onlract CI PA-CT-t)4-015 I.

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